JP4866880B2 - Electrode, gas sensor and manufacturing method thereof - Google Patents

Description

  The present invention relates to a gas sensor using a semiconductor material and a manufacturing method thereof, and more particularly to a gas sensor suitable for detection of hydrogen gas and having high reliability and high sensitivity, and a manufacturing method thereof.

  In 1975, Landstrom et al. Found that a Si-MOSFET (metal-oxide-semiconductor field-effect transistor) using Pd (palladium) as a gate electrode works effectively as a hydrogen gas sensor. The gas sensor was born. The principle of this hydrogen gas sensor is as follows. That is, hydrogen gas molecules are dissociated into hydrogen atoms (hydrogen radicals) by the catalytic action of palladium on the surface of palladium which is a catalytic metal. The dissociated hydrogen atoms move to the interface between the gate electrode (gate metal) made of palladium and the gate insulating film by diffusion. As a result, hydrogen atoms are polarized in a form having a dipole moment at the interface between the gate electrode and the gate insulating film, and positive charges accumulate on the gate insulating film. As a result, the threshold voltage Vth of the Si-MOSFET shifts, and the hydrogen gas concentration is detected by utilizing the phenomenon that the threshold voltage Vth shifts. After that, many researches and developments such as elucidation of the sensor mechanism and applicability to gas sensors that detect gases other than hydrogen gas using palladium, platinum (Pt) (platinum) and other platinum group metals as gate electrodes were made. For example, Applied Physics Letters, Vol 26, No 2, 15 January, 1975, pp. 55-57 (Non-Patent Document 1). As a review, for example, Sensors and Actuators, B1 (1990) 15-20 (Non-Patent Document 1) See 2)). Device structures other than Si-MOSFETs (field effect transistors) include MIS capacitors ((metal-insulator-semiconductor) capacitors), Schottky diodes, PN junction diodes, etc. The operation of these gas sensors has been investigated. Yes. In addition to Si (silicon) as a semiconductor material, application to SiC semiconductor material, GaAs semiconductor material, GaN semiconductor material, and the like has been studied.

  However, Si-MOSFET type hydrogen gas sensors that currently use the mainstream palladium as the gate electrode have the problem of hydrogen poisoning that the hydrogen response sensitivity is saturated when irradiated with a high concentration of hydrogen gas. There is a risk that it will be judged as low concentration in spite of being there. For this reason, the Si-MOSFET type hydrogen gas sensor using palladium as a gate electrode cannot correctly detect the hydrogen gas concentration in a high concentration region, and the hydrogen gas concentration may be several percent or more, possibly causing a hydrogen gas explosion. There remains a problem in ensuring certain dangers by detecting a certain concentration.

  On the other hand, the Si-MOSFET type hydrogen gas sensor using platinum as the gate electrode has lower hydrogen response sensitivity than the Si-MOSFET type hydrogen gas sensor using palladium as the gate electrode, and detects low concentration hydrogen gas. It is known that the response speed becomes slow (for example, it may be 500 seconds when the concentration of hydrogen gas is 100 ppm). In a Si-MOSFET type hydrogen gas sensor using platinum as a gate electrode, the maximum thickness of a platinum film that can respond to hydrogen is about 60 nm, and when the platinum film thickness is 100 nm or more, it hardly responds to hydrogen gas. . Thus, it is highly reliable that the Si-MOSFET type hydrogen gas sensor using platinum as the gate electrode responds only with a thin film thickness as compared with the Si-MOSFET type hydrogen gas sensor using palladium as the gate electrode. This is a major obstacle to manufacturing a gas sensor. However, the Si-MOSFET type hydrogen gas sensor using platinum as the gate electrode has the advantage that there is no exposure in high-concentration hydrogen gas and that the response speed is as fast as 2 to 5 seconds at a high concentration of 10%. It is very attractive in terms of detecting explosion hazard.

The biggest reason why a Si-MOSFET type hydrogen gas sensor using platinum as a gate electrode cannot be commercialized is adhesion to an oxide film such as a silicon oxide (SiO ₂ ) film or a semiconductor film such as silicon or gallium arsenide (GaAs). It is inferior in nature and long-term reliability could not be guaranteed. The problem of the peeling of the platinum film is an extremely important problem in terms of practical use and guarantee of the life of the gas sensor that exposes the gate electrode portion to the outside air.

  Further, in the processing process in the FET manufacturing process, partial film peeling occurs in the platinum film, and in a practical manufacturing process, a technique for stably bonding the platinum film directly on the gate insulating film has not been established. As for the film peeling of the platinum film, as a manufacturing method, contamination of the process equipment by the peeled platinum film becomes a problem. Therefore, when a platinum film is used in the manufacture of an electronic device using a semiconductor material such as silicon or gallium arsenide, a platinum film and a semiconductor such as silicon or gallium arsenide are formed between the platinum film and an oxide film such as a silicon oxide film. A barrier metal film made of titanium (Ti), molybdenum (Mo), tungsten (W), or the like is inserted between the materials to maintain adhesion to the platinum film, thereby preventing contamination of the platinum film due to film peeling. Technology is established. Platinum is a noble metal and is more agglomerated by platinum itself than by bonding to oxygen or other constituent atoms of solid materials (oxides such as silicon oxide or semiconductor materials such as silicon or gallium arsenide (GaAs)). There is a tendency to be stable. This is an intrinsic property of platinum, and it is an inevitable method to insert a barrier metal film in order to improve the adhesion between the platinum film and the film made of a solid material.

  On the other hand, from the viewpoint of operating the hydrogen gas sensor, when these barrier metal films are present, the hydrogen gas is blocked or occluded by the barrier metal film, and the hydrogen gas sensor does not react to the hydrogen gas at all, or the hydrogen response sensitivity. Is extremely low and cannot be used as a hydrogen gas sensor (see, for example, IEEE Electron Device Letters, EDL-5 (1984) 14-15 (Non-patent Document 3)).

  Here, when a platinum group metal such as platinum or palladium is formed on the gate insulating film, the platinum group metal may be partially peeled off from the gate insulating film without being sufficiently adhered. There is a prior application technique (Japanese Patent Application Laid-Open No. 2002-107322 (Patent Document 1)) for the purpose of avoiding variations in initial characteristics of a gas sensor or a gradual deterioration of the characteristics of the gas sensor as the use period becomes longer. The target gas is hydrogen, carbon monoxide (CO), alcohol, hydrocarbon, ammonia, and amine. According to the disclosure of the technology, the essence of the invention for avoiding film peeling is titanium (Ti), tantalum (Ta), chromium (between a gate electrode made of palladium, platinum, iridium (Ir), and the like and a gate insulating film. It is described that the problem of film peeling can be avoided by realizing a structure in which a metal oxide film of Cr), tungsten (W), niobium (Nb), or molybdenum (Mo) is sandwiched.

  A Si-MOSFET type hydrogen gas sensor using a platinum film composed of a thin film having a thickness of about 30 nm to 45 nm as a gate electrode does not respond to ammonia other than hydrogen gas, ethanemethanol or the like. However, a Si-MOSFET type hydrogen gas sensor (for example, Sensors and Actuators, B1 (1990) 15-20) that uses a platinum ultrathin film (up to 6 nm) with an extremely thin platinum film as a gate electrode (non-patent literature) In 2)), the gas selectivity is different. In this study, the platinum thin film is not uniformly deposited on the gate insulating film, but is formed in a striped pattern, resulting in a gap area without platinum on the surface of the gate insulating film. Gas sensors that can detect ammonia, ethanemethanol, etc. have been manufactured. This is a gas sensor that actively uses the property that the platinum film is easily peeled off on the gate insulating film. The response mechanism of the gas sensor is as shown below as discussed in the literature Sensors and Actuators, B1 (1990) 15-20 (non-patent literature 2) and its reference literature. That is, ammonia gas or the like adheres to the platinum surface formed in stripes, and changes the surface potential φs of the platinum surface. At this time, the capacitance between the platinum-free gap region and the platinum microcrystal on the surface of the gate insulating film, and the capacitance between the platinum-free gap region and the channel formed in the semiconductor substrate (Si substrate) This is a principle that is essentially different from the Landstrom mechanism in which the threshold value Vth changes. However, even in gas sensors with strong gas selectivity using this ultra-thin platinum film (up to 6 nm), problems of reliability such as platinum film peeling are still unsolved and will be put into practical use as they are. It is difficult.

  In the method of manufacturing a hydrogen gas sensor using a platinum group as a gate electrode, the metal film constituting the gate electrode is often formed using a sputtering method, and even in the response characteristics of the hydrogen gas sensor, after the element manufacturing process is completed, For example, it has been reported that the response characteristic of a hydrogen gas sensor is remarkably recovered by annealing with 1% diluted hydrogen gas (see, for example, Sensors and Actuators, B33 (1996) 96-99). ).

Although it is an example of a Si-MOSFET type hydrogen gas sensor using palladium as a gate electrode, a sensor FET that responds to hydrogen gas and a reference FET that does not respond to hydrogen gas are formed on the same silicon chip (semiconductor substrate). Thus, a differential amplification method has been studied in which the output difference between the sensor FET and the reference FET is extracted as a signal when hydrogen gas is irradiated onto the silicon chip. In this technique, a metal multilayer film (silver (Ag)) that does not respond to hydrogen gas on the gate electrode of the reference FET as a technique for aligning the threshold voltages Vth of both when there is no hydrogen gas (sensor FET and reference FET) / Cu (Cu) / Palladium (Pd) / Platinum (Pt)) is described (see, for example, SYChoi et al. Sensors and Actuators, 9 (1986) 353-361).
JP 2002-107322 A Landstrom et al. Applied Physics Letters, Vol 26, No 2, 15 January, 1975 55-57 Landstrom Sensors and Actuators, B1 (1990) 15-20 SYChoi et al. IEEE Electron Device Letters, EDL-5 (1984) 14-15 Y. Morita et al. Sensors and Actuators, B33 (1996) 96-99 SYChoi et al. Sensors and Actuators, 9 (1986) 353-361

  An object (first object) of the present invention is to suppress hydrogen poisoning even in high-concentration hydrogen gas while maintaining the adhesion between the platinum film and the gate insulating film in a hydrogen gas sensor using a platinum film as a gate electrode. Another object of the present invention is to provide a gas sensor and a method for manufacturing the same that realizes a hydrogen response strength equal to or higher than that of a hydrogen gas sensor using palladium as a gate electrode even if a platinum film is used as a gate electrode.

  Another object (second object) of the present invention is a gas sensor having a platinum film as a gate electrode, and a gas sensor having high gas selectivity while maintaining the adhesion between the platinum film and the gate insulating film, and a method for manufacturing the gas sensor. It is to provide.

  Furthermore, another object (third object) of the present invention is to provide a reproducibility of the threshold voltage Vth and uniformity of the threshold voltage Vth within the wafer surface in a hydrogen gas sensor having a catalytic metal such as platinum or palladium as a gate electrode. In the state where there is no gas to observe the threshold voltage Vth of the sensor FET and the threshold voltage Vth of the reference FET that does not respond to a gas such as hydrogen gas. The object is to provide a technique capable of providing means for aligning.

  Another object (fourth object) of the present invention is to improve the reliability of a sensor chip itself mounted with a gas sensor. Usually, in LSI (Large Scale Integrated Circuit) formed on a silicon chip, there is a technology to prevent deterioration from various external environments by molding (sealing) the surface of the silicon chip with resin in order to realize long-term reliability. Established. Similarly, in the Si-MOSFET type gas sensor using a semiconductor material, it is necessary to open at least only the sensing region of the gate electrode to the outside air from the principle of operation. For this reason, the product life is inevitably shorter than LSIs (Large Scale Integrated Circuits) formed on silicon chips, and is necessarily positioned as a consumable item. Since it is directly linked, it is an extremely important purpose in practical use.

  The above and other objects and novel features of the present invention will be apparent from the description of this specification and the accompanying drawings.

  Of the inventions disclosed in the present application, the outline of typical ones will be briefly described as follows.

  A gas sensor according to a typical embodiment includes: (a) a semiconductor substrate; (b) a gate insulating film formed on the semiconductor substrate; (c) a gate electrode formed on the gate insulating film; d) a source region formed on the semiconductor substrate; and (e) a drain region formed on the semiconductor substrate. The gate electrode has (c1) an oxygen-doped titanium film formed on the gate insulating film and containing oxygen, and (c2) a platinum film formed on the oxygen-doped titanium film. At this time, the platinum film is composed of a plurality of crystal grains, and oxygen and titanium exist in a grain boundary region between the plurality of crystal grains.

  A gas sensor according to a representative embodiment includes (a) a lower electrode made of a semiconductor substrate, (b) a capacitive insulating film formed on the lower electrode, and (c) formed on the capacitive insulating film. And a gas sensor including a capacitive element including an upper electrode. The upper electrode includes (c1) an oxygen-doped titanium film formed on the capacitor insulating film and containing oxygen, and (c2) a platinum film formed on the oxygen-doped titanium film. At this time, the platinum film is composed of a plurality of crystal grains, and oxygen and titanium exist in a grain boundary region between the plurality of crystal grains.

  In addition, the gas sensor according to the representative embodiment includes (a) a semiconductor substrate, (b) a gate insulating film formed on the semiconductor substrate, and (c) a titanium oxide film formed on the gate insulating film. And (d) a gate electrode formed on the titanium oxide film, (e) a source region formed on the semiconductor substrate, and (f) a drain region formed on the semiconductor substrate. Here, the gate electrode is characterized in that a plurality of platinum crystal grains have a striped structure, and voids or insulating materials exist between the plurality of platinum crystal grains. Here, the striped structure means a striped form in which a gap region without platinum microcrystals exists on the surface of the gate insulating film and the interval is branched, or a structure in which the gap is formed by an infinite number of holes. To do.

  Further, a gas sensor manufacturing method according to a representative embodiment includes: (a) a step of forming a gate insulating film on a semiconductor substrate; and (b) a film thickness on the gate insulating film after the step (a). A step of forming a titanium film having a thickness of 1 nm to 10 nm, and a step of (c) forming a platinum film on the titanium film after the step (b). At this time, the platinum film is composed of a plurality of crystal grains, and the platinum film is formed to have a thickness of 1 nm to 30 nm. (D) After the step (c), the method includes a step of heating the semiconductor substrate at a temperature in the range of 300 ° C. to 800 ° C. in an atmosphere containing oxygen gas.

  Among the inventions disclosed in the present application, effects obtained by typical ones will be briefly described as follows.

  A hydrogen gas sensor that uses a platinum film as a gate electrode, while maintaining the adhesion between the platinum film and the gate insulating film, it can suppress hydrogen poisoning even in high-concentration hydrogen gas. The platinum film is used as the gate electrode. However, it is possible to provide a gas sensor that realizes a hydrogen response intensity equal to or higher than that of a hydrogen gas sensor that uses palladium as a gate electrode, and a method for manufacturing the same.

  In the following embodiments, when it is necessary for the sake of convenience, the description will be divided into a plurality of sections or embodiments. However, unless otherwise specified, they are not irrelevant to each other. There are some or all of the modifications, details, supplementary explanations, and the like.

  Further, in the following embodiments, when referring to the number of elements (including the number, numerical value, quantity, range, etc.), especially when clearly indicated and when clearly limited to a specific number in principle, etc. Except, it is not limited to the specific number, and may be more or less than the specific number.

  Further, in the following embodiments, the constituent elements (including element steps and the like) are not necessarily indispensable unless otherwise specified and apparently essential in principle. Needless to say.

  Similarly, in the following embodiments, when referring to the shape, positional relationship, etc., of components, etc., unless otherwise specified, and in principle, it is considered that this is not clearly the case, it is substantially the same. Including those that are approximate or similar to the shape. The same applies to the above numerical values and ranges.

  In all the drawings for explaining the embodiments, the same members are denoted by the same reference symbols in principle, and the repeated explanation thereof is omitted. In order to make the drawings easy to understand, even a plan view may be hatched.

(Embodiment 1)
First, a technique studied by the present inventors will be described. As explained in the background art, it is known that when a titanium film is inserted between a gate electrode made of a platinum film and a gate insulating film, it does not respond to hydrogen gas, and the present inventors also conducted a confirmation experiment. . Prototype Si-MOSFET type hydrogen gas sensor in which a 5 nm titanium film calculated from the deposition speed of the titanium film is inserted between the platinum film and the gate insulating film, and a gate electrode made of a 45 nm platinum film is formed on the titanium film. However, this hydrogen gas sensor did not respond to hydrogen gas. On the other hand, a Si-MOSFET type hydrogen gas sensor in which a titanium film is not inserted between a platinum film and a gate insulating film and a gate electrode made of a platinum film having a film thickness of 45 nm is formed on the gate insulating film is a hydrogen gas sensor. Responded to. The film thickness of the titanium film was selected as the film thickness that can guarantee the adhesion of the platinum film because the response deterioration of the hydrogen gas sensor is expected to be as small as possible. The thickness of the platinum film used was a standard value used in a Si-MOSFET type hydrogen gas sensor using the platinum film as a gate electrode.

  The present inventors further evaluated the structure of platinum / metal oxide / insulating film / semiconductor substrate that is disclosed as most suitable in the prior application technique in accordance with the description of the prior application technique (Japanese Patent Laid-Open No. 2002-107322). . Specifically, after forming a titanium oxide film on a silicon oxide film that is a gate insulating film using platinum instead of palladium described in Example 1 of the prior application technique, a gate electrode made of a platinum film is formed. Experiments were performed to deposit 45 nm. In this experiment, as in the case where the gate electrode made of a platinum film is formed directly on the gate insulating film (silicon oxide film) by lift-off, a partial film peeling occurs in the platinum film, and the effect of improving the adhesion is not observed. I couldn't see it. That is, it was found that when the platinum film was directly laminated on the titanium oxide film, the adhesion was poor as in the case where the platinum film was directly formed on the silicon oxide film. At least for the platinum film, the simple multilayer structure of platinum / metal oxide / insulating film / semiconductor substrate disclosed in the prior application technique (Japanese Patent Laid-Open No. 2002-107322) can improve the poor adhesion. I found it difficult.

  Example 4 of the prior application technique (Japanese Patent Laid-Open No. 2002-107322) describes an example of forming a Si-MIS type Schottky diode. Specifically, although the film thickness is not disclosed, a titanium film is formed in an argon (Ar) atmosphere using a sputtering method, and then a palladium film (the film thickness is not disclosed) is continuously formed. Yes. It was reported that the film was heated at 400 ° C. for 60 minutes in an argon atmosphere, and the titanium film was oxidized by this heat treatment to form a titanium oxide film and showed a sufficient hydrogen gas response. Furthermore, it is described that the same effect can be obtained in the MOSFET.

  Therefore, a Si-MOSFET type hydrogen gas sensor (titanium film and platinum) sandwiched between a gate electrode made of a titanium film (5 nm) and a platinum film (45 nm) and a gate insulating film (silicon oxide film), which were prototyped earlier. According to Example 4 of the prior application technique (Japanese Patent Laid-Open No. 2002-107322), a heat treatment was performed by heating in an argon atmosphere at 400 ° C. for 60 minutes, and the hydrogen gas response was examined. However, it did not show any response. Then, this time, the atmosphere gas was changed, and the hydrogen gas response was examined by performing a heat treatment in which the hydrogen gas sensor was heated at 400 ° C. for 60 minutes in the air. However, no response was shown.

  According to the prior application technique, in a laminated structure composed of a platinum group such as platinum and palladium / a metal film such as titanium / insulating film / semiconductor substrate, a method of oxidizing a metal film such as titanium is insulating. Although it is disclosed that oxygen in a film is used or oxygen in a heat treatment atmosphere is used, according to experiments by the present inventors, at least a gate electrode made of a platinum film, a gate electrode made of a platinum film Oxidizing the titanium film while covering the titanium film can be realized by extracting oxygen from the insulating film (silicon oxide film) in contact with the film, or oxidizing using oxygen in the atmospheric gas, I came to think that the platinum film would be in the way and it would be difficult in general. From the consideration of thermodynamic stability, it was determined that it is not easy to oxidize a metal film such as a titanium film by depriving oxygen from the silicon oxide film of the thermal oxide film.

  On the other hand, as shown in the prior application technique, even if the titanium film could be oxidized into a titanium oxide film, it was predicted from the following thought experiment that the morphology of the platinum film was drastically changed by the volume expansion of the titanium film.

Consider an experiment in which a sample in which a titanium film having a uniform thickness is formed on a substrate is heated in an oxygen atmosphere for thermal oxidation. It is assumed that there is no influence such as atomic diffusion from the substrate to the titanium film, and the titanium film is oxidized to be a microcrystalline thin film of rutile titanium oxide (TiO ₂ ), which is typical of titanium oxide. The volume occupied per one titanium metal is about 17.6Å ^3, the volume occupied by one per titanium oxide (TiO ₂₎ of the rutile structure is 31.2Å ^3. That is, the volume of titanium oxide (TiO ₂ ) is 1.77 times larger than the volume of titanium, and if the titanium film is changed to a titanium oxide (TiO ₂ ) film having a rutile structure, the film thickness is 1 It will swell 77 times. This volume expansion is an unavoidable phenomenon in the process in which titanium is oxidized to titanium oxide (including titanium oxide (TiO) other than TiO _{2 and the} like). This phenomenon is a well-known phenomenon in the process of partially covering the silicon substrate surface with a silicon nitride film (SiN film) and thermally oxidizing silicon in an oxygen atmosphere. In this case, silicon in a place covered with the silicon nitride film is not oxidized, and silicon in a region not covered with the silicon nitride film becomes a silicon oxide film. In this process, it is known that a part of the silicon oxide film is embedded inside as viewed from the original silicon surface position, and a part protrudes outward from the silicon surface position.

Next, consider a case where a stacked structure of platinum (Pt) / titanium (Ti) / silicon oxide (SiO ₂ ) / silicon (Si) is thermally oxidized. If all of the titanium film is changed from oxygen in the atmosphere gas to a titanium oxide (TiO ₂ ) film having a rutile structure, the platinum film is subjected to very large stress due to the volume expansion of the titanium oxide (TiO ₂ ). It was predicted that the platinum film that had been uniformly deposited on the film would break and peel, and that the morphology of the platinum film would change drastically. Volume expansion in the process of forming this titanium oxide (including titanium oxide TiO other than TiO ₂ ) is an unavoidable phenomenon, and it is considered that the drastic change in the morphology of the platinum film is common. That is, by heating in an oxygen gas atmosphere, a simple structure such as a platinum (Pt) / titanium oxide / silicon oxide (SiO ₂ ) / Si structure disclosed in the prior application (Japanese Patent Laid-Open No. 2002-107322). It is concluded that it is impossible to realize a laminated structure.

  Against this background, assuming that the platinum film is used as a gate electrode, the greatest object of the present invention is to increase the adhesion between the platinum film and the gate insulating film by using titanium, molybdenum, tungsten, etc. In a situation where a metal film must be inserted between the gate insulating film and the silicon semiconductor process, a gas sensor that is resistant to deterioration of reliability, such as film peeling of a Si-MOSFET type gas sensor using a platinum film as the gate electrode, is used. It is how it is realized in a form that is compatible with technology. This purpose is the same for a gas sensor with a strong gas selectivity using a platinum ultrathin film.

  Further, from the viewpoint of application of the gas sensor to the hydrogen detection system, when the threshold voltage Vth of the Si-MOSFET type hydrogen gas sensor is not uniform within a wafer or between wafers, a signal is taken out from the hydrogen gas sensor. It becomes difficult to design the interface circuit required for However, when an Si-MOSFET type hydrogen gas sensor in which a metal film made of a platinum film or a palladium film is formed by sputtering is actually manufactured and evaluated, sputtering damage to the gate insulating film that occurs during sputtering occurs. Many electron trap levels are formed in the gate insulating film, the threshold voltage Vth of the hydrogen gas sensor varies greatly, and the threshold of the hydrogen gas sensor is sufficiently high at the diluted hydrogen annealing level described at the end of the background art. There is a problem that the variation in the value voltage Vth cannot be solved. However, conventionally, there is no report example that addresses the problem of uniformity and reproducibility of the threshold voltage Vth of the hydrogen gas sensor in the Si-MOSFET type hydrogen gas sensor.

  Further, in a differential amplification type gas sensor, as a technique for aligning the threshold voltage Vth of the sensor FET and the threshold voltage Vth of the reference FET, a metal that does not respond to the gas to be observed such as hydrogen gas at the gate electrode of the reference FET The technique of forming a film is not compatible with a mass production process of a gas sensor because at least one photomask process for forming a gate electrode must be added and the process process becomes long and complicated.

Based on the above, further discussion will be made. First, returning to the principle of hydrogen response, the structure of platinum (Pt) / titanium (Ti) / silicon oxide (SiO ₂ ) / Si was reconsidered. As is well known from the principle of operation of the Si-MOSFET type hydrogen gas sensor, the threshold voltage Vth is defined as φ _Si as the silicon work function and φ _{M as} the work function of the metal film constituting the gate electrode. Then, the difference φ _MS between them becomes a parameter that determines the threshold voltage Vth, and is added to the expression of the threshold voltage Vth in the form of φ _MS = φ _M −φ _Si . Titanium has a work function of 4.3 eV and platinum has a work function of 5.5 eV. Therefore, when a titanium film is formed on the same gate insulating film and when a platinum film is formed, 1. A difference in threshold voltage Vth of 2 eV (5.5 eV-4.3 eV) occurs.

  As long as titanium is a metal, the hydrogen atoms dissociated by the platinum film are trapped in the titanium film as soon as they reach the interface between the titanium film and the platinum film, and the positive charge is screened by the electrons in the titanium (neutralization) Therefore, the hydrogen atoms do not contribute to the change in the threshold voltage Vth of the hydrogen gas sensor. From the principle of operation of the hydrogen gas sensor, if the electron number sheet concentration that contributes to this screening (the value obtained by integrating the electron concentration in titanium in the film thickness direction) is made smaller than the surface density of the number of hydrogen atoms reaching the titanium surface, screening The effect can be expected to disappear.

Even at a hydrogen gas concentration of 10%, the maximum change in threshold voltage Vth is about 1000 mV. That is, the charge number density per area due to the hydrogen atoms adsorbed on the gate insulating film is about 10 ¹² / cm ² and is not necessarily large. By doping oxygen into the titanium film, structural defects can be formed and electrons in the titanium can be trapped. Therefore, if the doping amount of oxygen into the titanium film is 10 ²¹ atoms / cm ³ or more, the screening effect is suppressed. Can be realized.

  For this reason, it was not always necessary to use titanium oxide for the entire titanium film, and it was considered that an oxygen-doped titanium film in which oxygen is doped into titanium may be used. As long as the titanium film is doped with oxygen, as shown in the literature (Journal of Applied Physics, Vol 63, (1988) 3431-2434), Si-- which uses a platinum film as a gate electrode also against oxygen. The MOSFET type hydrogen gas sensor responds if the platinum film thickness is reduced, so oxygen is several times larger in size and molecule size than hydrogen, but the platinum film thickness and titanium film thickness We thought that it would be possible to dope the titanium film with oxygen that passed through the platinum film by choosing the appropriate film thickness, and conducted the following experiments. It is generally considered difficult to incorporate oxygen in the atmosphere gas into the titanium film in the state where the platinum film which is a metal film is present. However, based on the above consideration, in the case of platinum film, if the property of being difficult to oxidize is utilized well, the surface of the platinum film is not oxidized, that is, the catalytic function of the platinum film is not deteriorated, and the volume expansion of the titanium film is also increased. I thought that it would be possible to reduce the impact on the platinum film.

  Platinum has a weaker interaction with atoms of other materials than other platinum group metals, and when a thin film made of platinum is formed on a silicon oxide film on a silicon substrate, a microcrystal having an fcc (face-centered cubic lattice) crystal structure. Is well known from X-ray diffraction experiments, and the present inventors have confirmed.

Several types of film thicknesses of the titanium film and the platinum film were prepared, and a laminated structure of platinum (Pt) / titanium (Ti) / silicon oxide (SiO ₂ ) / Si was prepared and heated in air. As a result of analyzing this laminated structure, a number of unexpected phenomena were discovered, and almost all of the above problems were solved by properly selecting the thickness of the titanium film, the thickness of the platinum film, and the annealing temperature (heat treatment temperature). It turns out that it can be solved.

  The formation method of the titanium film and the platinum film is an EB (electron beam) evaporation method for the reason described later. A thin film is formed on a silicon oxide film formed on a silicon substrate. First, when only a titanium film was formed and evaluated as a thin film, the titanium film with a film thickness of 1 nm to 10 nm was amorphous, crystal grains were not observed, the surface was uniform, and unevenness was hardly seen. Even when the thickness of the titanium film was increased to 45 nm, most of the film was amorphous and there was a tendency for the titanium metal crystal grains to partially exist, but the surface was uniform and almost no irregularities were seen. . This was greatly different from the form (morphology) of the platinum film formed on the silicon oxide film formed on the silicon substrate as described above. Therefore, several kinds of titanium films were formed on the silicon oxide film formed on the silicon substrate, and platinum films were formed by continuous film formation with different film thicknesses. This platinum film was also a thin film in which fine crystals having an fcc (face centered cubic lattice) crystal structure were oriented in the (111) direction.

  First, an outline of the experimental results that triggered the discovery of a new phenomenon is given. A titanium film with a film thickness of 5 nm and a platinum film with a film thickness of 15 nm are formed on a silicon oxide film formed on a silicon substrate, annealed in air at a temperature of 400 ° C. for 2 hours, and 30 at a temperature of 800 ° C. An experiment was conducted in which annealing was performed for a minute.

(1) Results of an annealing experiment at 400 ° C. for 2 hours Titanium atoms are transferred to the platinum crystal grain boundary and the vicinity thereof, the process of escaping to the platinum surface, and oxygen is transferred to the platinum crystal grain boundary and the vicinity thereof to form a titanium film. The process of intruding into the inside appeared. Therefore, the titanium film is transformed into an oxygen-doped titanium film (a film in which oxygen-doped amorphous titanium, amorphous titanium oxide, or titanium oxide microcrystals are mixed). The fine crystal grains oriented in the (111) direction of platinum are sunk into the TiO _x film, and the dent depth varies depending on the fine crystal grains oriented in the (111) direction of each platinum (Pt). Unevenness occurs at the interface between the platinum and the TiO _x film. On the other hand, a structure in which a Pt-Ti-O region composed of oxygen-doped amorphous titanium and a platinum-titanium diffusion layer is formed in the grain boundary region between the microcrystals of platinum through which titanium and oxygen have passed. I understood it. That is, titanium and oxygen are present in the grain boundary region between the platinum microcrystals, and in this specification, the titanium and oxygen are present in the platinum grain boundary region. It is assumed that a Pt—Ti—O region is formed in the grain boundary region. The analysis of the experiment supporting this phenomenon will be disclosed later, and the film thickness of the titanium film, which was 5 nm at the time of film formation, seems to be slightly thin rather than thick after this annealing (heat treatment).

  This result was very different from the hypothesis that when the titanium film was oxidized with the platinum film covered, the volume expanded and the morphology of the platinum film changed significantly. The reason for this is explained by the discovery that there is a process in which titanium atoms pass through the crystal grain boundaries of platinum and its vicinity and escape to the surface of platinum.

(2) Results of annealing experiment at 800 ° C. for 30 minutes The titanium film is changed to a titanium oxide film (rutile structure), the film thickness is doubled from 5 nm to 10 nm of the titanium film, and is uniformly coated on the silicon oxide film. . The platinum microcrystal formed on the titanium oxide film (rutile structure) is a thin film oriented in the (111) direction, but the plane dimension changes to several hundred nm, and the film thickness is 88 nm and 5.9. The platinum crystallites appear to have melted and recrystallized once, and the crystallite surface is flat. This platinum has an island-like morphology with a gap of about 100 nm to 300 nm between microcrystals. The melting points of bulk platinum metal and titanium metal are 1768.3 ° C. and 1668 ° C., respectively, and the platinum crystallites appear to have melted despite the very low experimental temperature of 800 ° C. On the other hand, it is a new discovery that the titanium film is uniformly coated instead of the titanium oxide film (rutile structure).

  It was as expected that the thickness of the titanium film increased and the morphology of the platinum film changed drastically, but it was unexpected that the platinum film melted and turned into microcrystals at such a low temperature.

  Taking this experiment as an opportunity, detailed examination of the oxidation process as shown below was conducted. The outline of the experimental range is as follows.

Titanium film thickness range: 1 nm to 30 nm
Platinum film thickness range: 2 nm to 100 nm
Annealing temperature range: 100 ° C to 900 ° C
Annealing time range: 5 minutes to 3000 hours Types of annealing atmosphere: air, 100% oxygen, Ar, nitrogen Annealing experiments were performed in the range of the above conditions.

  The main experimental results related to the present invention are summarized as follows. It has been found that the phenomena occurring in three types of ranges of 300 ° C. or lower, 300 to 500 ° C., and 500 to 900 ° C. are different when roughly classified. The experimental results will be described below.

  (1) There is no oxidation of the titanium film in Ar or nitrogen gas. Almost no oxygen is supplied from the silicon oxide film formed in the lower layer of the titanium film.

(2) At 300 ° C. or lower, annealing for a long time allows the titanium film to be doped with 10 ²¹ atoms / cm ³ or more, but in practice, it is necessary to reduce the thickness of the titanium film and the platinum film.

  (3) In the temperature range of 350 ° C. to 475 ° C., when the thickness of the platinum film is increased to 45 nm or more, the doping of oxygen into the titanium film rapidly decreases, the change of the titanium film is almost eliminated, and the hydrogen gas response is Disappear. This is consistent with the fact that in the confirmation experiment of the prior application technique, when a heat treatment was performed in air at a temperature of 400 ° C. for 60 minutes, no hydrogen gas response was made. When the thickness of the platinum film is increased, the crystal grain boundary length in the direction perpendicular to the platinum film is increased, which has the effect of increasing the size of the crystal grains themselves, and oxygen intrusion can be suppressed rapidly. Accordingly, the outflow of titanium through the platinum crystal grain boundary is suppressed, and the desired structure in the present invention is not achieved.

  (4) In the temperature range of 350 ° C. to 475 ° C., when the thickness of the platinum film becomes 1 nm or more and 30 nm or less, the following phenomenon starts to be observed and can be applied to a hydrogen gas sensor. Titanium atoms are transmitted through the crystal grain boundary of platinum and the vicinity thereof, and a process of escape to the surface of platinum and a process of oxygen entering the titanium film through the crystal grain boundary of platinum and the vicinity thereof appear. For this reason, the titanium film is transformed into an oxygen-doped titanium film (a film in which oxygen-doped amorphous titanium, amorphous titanium oxide, or titanium oxide microcrystals are mixed), The fine crystal grains oriented in the (111) direction of platinum dig into the oxygen-doped titanium film, and the depth of penetration varies depending on the fine crystal grains oriented in the (111) direction of individual platinum. Irregularities occur at the interface between the oxygen-doped titanium film. On the other hand, a structure in which an oxygen-doped amorphous titanium and a Pt—Ti—O region composed of a platinum-titanium diffusion layer are formed in a grain boundary region between titanium and platinum microcrystals through which oxygen has passed. Is done.

  (5) When the temperature was further raised to about 500 ° C. to 800 ° C., the form of the platinum film / titanium film shown in (4) began to change. In particular, in an island structure of platinum found at 800 ° C., the distance between platinum microcrystals is too wide as 100 nm to 300 nm, and there is a high possibility that the sensitivity is lowered as a sensor having strong gas selectivity. For this reason, it is desirable to lower the heating temperature to form a striped form with a gap of several nanometers to several tens of nanometers of platinum. With such high temperature annealing, platinum microcrystals begin to fuse. In the process, when small gaps begin to occur between the platinum microcrystals, the penetration of oxygen into the titanium film dramatically accelerates, and the titanium film gradually changes from an oxygen-doped state to a titanium oxide film. . Once a small gap begins to occur between the platinum microcrystals, titanium atoms cannot travel through the grain boundary and go out to the platinum film surface, so the thickness of the titanium oxide film is the same as the initial titanium film thickness. Compared to increase. Depending on the heating temperature conditions and the thickness of the titanium film or platinum film, the gap between the platinum microcrystals and the extent to which the titanium film is oxidized are determined.

  In order to realize that the platinum crystallite interval changes from a few nanometers to a striped form with a gap of several tens of nanometers, an annealing time of 20 minutes at a temperature of about 650 ° C., a titanium film thickness of 5 nm, a platinum film It has been found that the film thickness of about 10 nm is good. The target device structure can also be realized by doping the titanium film with oxygen by lowering the temperature to about 400 ° C. under conditions where a titanium film or platinum film is made thicker and a gap of several nanometers to several tens of nanometers begins to be formed. .

  When the gap length between the platinum microcrystals is widened, the capacitance Cs between the platinum-free gap region on the surface of the gate insulating film and the platinum microcrystals is reduced, and the strength of the gas response is reduced. Therefore, preferable conditions for the thickness of the titanium film, the thickness of the platinum film, the annealing temperature, and the oxygen partial pressure are disclosed in detail below.

  As described above, the method and means for realizing the object of the present invention can be summarized as follows. A configuration for realizing the first object of the present invention is a Si-MOSFET type gas sensor shown below. That is, the Si-MOSFET type gas sensor according to the present invention uses the property that the platinum film is a (111) -oriented microcrystal on the flat insulating film, so that the grain boundary region between the platinum microcrystals is highly doped with oxygen. A structure in which a Pt—Ti—O region composed of amorphous titanium and platinum-titanium diffusion layers is disposed. A TiOx film (a film in which oxygen-doped amorphous titanium, amorphous titanium oxide, or titanium oxide microcrystals are mixed) is provided under the Pt-Ti-O region, and a gate is further provided thereunder. An insulating film (including a silicon oxide film) is included. Then, silicon is disposed below the gate insulating film.

  Next, as a configuration for realizing the second object of the present invention, there is a Si-MOSFET type gas sensor shown below. That is, the Si-MOSFET type gas sensor of the present invention has a striped form with a spacing of several nanometers to several tens of nanometers of platinum microcrystals, and the titanium film under the platinum microcrystals is a titanium oxide film (depending on the fabrication conditions, A gate insulating film (silicon oxide) is a thin film of a rutile titanium oxide microcrystal, or a thin film of a titanium oxide microcrystal mixed with an anatase structure or a titanium oxide microcrystal thin film mixed with oxygen-doped titanium in addition to a rutile structure. It is formed on a film etc.).

  Subsequently, a technique for realizing the third object of the present invention is an Si-MOSFET type gas sensor having a step of forming a titanium film and a platinum film on a gate insulating film by an EB (electron beam) evaporation method. This is to form a titanium film and a platinum film. At this time, for example, the thin film growth rate is 0.8 to 3 cm / s.

  Furthermore, in the technology for realizing the fourth object of the present invention, in the Si-MOSFET type gas sensor having a step of forming a titanium film and a platinum film on the gate insulating film, after forming the channel layer of the Si-MOSFET, A titanium film and a platinum film are formed on the gate insulating film, and then annealed in a gas atmosphere containing oxygen before forming the interlayer insulating film. In order to realize a gas sensor having a long target life, gold wiring is used for the source electrode and drain electrode of the Si-MOSFET, the extraction wiring, and the wiring of the heater for chip heating. In this case, the method of connecting the chip and the mounting substrate is desirable because bonding with a gold wire prevents rust and improves adhesion to the pad portion.

A feature of the present invention is that a structure of platinum (Pt) / titanium (Ti) / silicon oxide (SiO ₂ ) / silicon (Si) is heated in an oxygen gas atmosphere. As a result, the titanium film begins to escape to the outside through the crystal grain boundary of platinum, and oxygen enters the titanium film through the crystal grain boundary of platinum. As a result, the titanium film, which was a metal, is doped with oxygen to increase the electrical resistance and partially become titanium oxide, thereby substantially eliminating the carrier (electrons) of the titanium film. It decreases so much that it does not disturb the response of hydrogen gas. On the other hand, the titanium film only needs to be a metal, and the penetration of titanium into the grain boundary improves the mutual adhesion between platinum microcrystals. Furthermore, oxygen passes between platinum microcrystals (grain boundary region) and a part of oxygen stays in the vicinity of the grain boundary region, so that the hydrogen gas passage (grain boundary region) becomes thicker. The response sensitivity is improved and the response speed is also increased.

Next, a basic configuration of the hydrogen gas sensor according to the first embodiment will be described. FIG. 1 is a schematic diagram showing a gate electrode and a laminated structure immediately below it in a Si-MOSFET type hydrogen gas sensor using a platinum film as a gate electrode. As shown in FIG. 1, a silicon oxide (SiO ₂ ) film 2 is formed on a semiconductor substrate 1 made of silicon. The film formed on the silicon oxide film 2 is an oxygen-doped titanium film 3 indicating a titanium film containing oxygen. The oxygen-doped titanium film 3 is often mixed with titanium oxide microcrystals 4 and the oxygen-doped titanium film 3 is often amorphous. A platinum film is formed on the oxygen-doped titanium film 3. The platinum film is a film made of a plurality of platinum microcrystals 5 oriented in the (111) direction, and crystal grain boundaries 6 are formed between the platinum microcrystals 5. Titanium and oxygen are present in the grain boundary vicinity region 7 formed along the crystal grain boundary 6. That is, in the first embodiment, the grain boundary vicinity region 7 which is the vicinity region of the crystal grain boundary 6 formed between the platinum microcrystals 5 is the Pt—Ti—O region. Depending on the formation conditions, a microcrystal 8 made of oxygen-doped titanium or titanium oxide may be formed on the grain boundary region 6. However, the presence of the microcrystal 8 is not necessarily required for the operation of the hydrogen gas sensor itself.

FIG. 2 is a cross-sectional TEM (transmission electron microscope) photograph showing a gate electrode and a laminated structure immediately below it in a Si-MOSFET type hydrogen gas sensor using a platinum film as a gate electrode. In forming the structure shown in FIG. 2, heat treatment is performed in air at a heat treatment temperature of 400 ° C. and a heat treatment time of 2 hours. Before this heat treatment, the thickness of the titanium film was 5 nm and the thickness of the platinum film was 15 nm. The gate insulating film formed under the titanium film is a silicon oxide (SiO ₂ ) film, and a semiconductor substrate made of silicon (Si) is formed under the gate insulating film. As shown in FIG. 2, after the heat treatment, platinum crystal grains enter the titanium film so as to sink, and platinum fine crystals oriented in the (111) direction of platinum are well captured. As a result of structural analysis of the titanium film, oxygen-doped amorphous titanium and partially crystallized titanium oxide (circle 2) are seen in the titanium film, which corresponds well with the schematic diagram of FIG. I understand that.

  The (circle 1), (circle 2), (circle 5), (circle 6), and (circle 7) portions shown in FIG. 2 were subjected to EDX (energy dispersive X-ray spectroscopy) analysis, and platinum microcrystals were analyzed. In the central region (circle 6), the region near the grain boundary (circle 5), and the platinum surface vicinity region (circle 7), we have obtained an analysis result in which a large amount of titanium and oxygen are present. A result corresponding to the formation of a structure in which a Pt—Ti—O region composed of amorphous titanium and platinum-titanium diffusion layers is formed is obtained.

  FIG. 3 is a photograph showing the result of observation of the morphology of the surface after heat treatment by SEM (scanning electron microscope). In FIG. 3, the granular material that appears white on the surface is TiOx, which corresponds to the portion (circle 7) shown in FIG. In FIG. 3, the striped region that appears black indicates a Pt—Ti—O region formed in the region near the grain boundary.

4 and 5 are diagrams showing distribution profiles in the cross-sectional direction of Pt, Ti, O, Si, and Si (Oxide) by Auger analysis of the gate structure before and after the heat treatment (the gate electrode and the laminated structure immediately below it). is there. FIG. 4 shows the analysis results after the heat treatment, and FIG. 5 shows the analysis results before the heat treatment. Both have a laminated structure corresponding to FIG. The distribution of titanium (Ti), oxygen (O), and platinum (Pt) in the vicinity of the surface where the sputtering time (corresponding to the surface depth) indicated by the horizontal axis in FIGS. It can be said that the structure in Form 1 is well reflected. In FIG. 5 showing the gate structure before the heat treatment, almost no oxygen is seen in the titanium film or the platinum film, and it can be seen that the gate structure is a Pt / Ti / SiO ₂ structure. However, in FIG. 4 showing the gate structure after the heat treatment, it can be seen that the titanium film is doped with a large amount of oxygen, and titanium and oxygen enter the platinum film. Since the thickness of the titanium film does not change before and after the heat treatment, it is one proof that after the heat treatment, the titanium itself flows out to the platinum surface through the platinum grain boundary. Since the Auger analysis is the result of averaging the information in the film, the situation where titanium and oxygen are visible in the platinum film is considered to be that titanium and oxygen are gathered in the vicinity of the grain boundary region of platinum. As described above, it can be said that the gate structure in the first embodiment is supported from the results of EDX analysis and Auger analysis.

  FIG. 6 shows a threshold voltage Vth and a hydrogen response strength ΔVg of an Si-MOSFET type gas sensor in an Si-MOSFET type gas sensor using a palladium film as a gate electrode and formed by sputtering. It is a figure which shows the result measured with the several element (Si-MOSFET type gas sensor). A large variation of the threshold voltage Vth of 2400 mV occurs in 32 elements (Si-MOSFET type gas sensor). The variation of the threshold voltage Vth is about the diluted hydrogen anneal described in the background art. There was a problem that it could not be solved sufficiently. This result is generated in the Si-MOSFET type gas sensor using the palladium film as the gate electrode, but the same tendency is obtained for the Si-MOSFET type gas sensor using the platinum film as the gate electrode. . This problem is due to the influence of unavoidable sputtering damage that occurs when the metal film is formed on the gate insulating film by the sputtering method, and the variation in the threshold voltage Vth is sufficiently suppressed in the heat treatment such as hydrogen annealing. There are limits to this.

Therefore, in the present invention, for the purpose of reducing damage when forming the metal film, a Pt / Ti / SiO ₂ / Si laminated structure is formed by EB (electron beam) evaporation instead of sputtering. Yes. In this case, a Si-MOSFET type gas sensor in which titanium or platinum is continuously formed by the EB vapor deposition method is manufactured, and the distribution of the threshold voltage Vth of the Si-MOSFET type gas sensor on the wafer surface and the lot-to-lot dependency are investigated. saw. Then, the variation of the threshold voltage Vth is smaller when the titanium film or the platinum film is formed by the EB vapor deposition method than when the titanium film or the platinum film is formed by the sputtering method, and the threshold voltage Vth varies depending on the growth rate. I have found that the variation is different. For example, FIG. 7 is a diagram showing the relationship between the deposition rate and the variation in threshold voltage when a metal film such as a titanium film or a platinum film is formed by the EB vapor deposition method. As shown in FIG. 7, when a metal film constituting a gate electrode such as a titanium film or a platinum film is formed by the EB vapor deposition method, the variation in the threshold voltage Vth is small, and the variation in the threshold voltage Vth depends on the deposition rate. Are different. As described above, in the present invention, by using the EB vapor deposition method for forming the titanium film or platinum film constituting the gate electrode, the variation in the threshold voltage Vth after the heat treatment actually used can be extremely reduced. Heading.

  Then, the manufacturing method of the Si-MOSFET type gas sensor in this Embodiment 1 is demonstrated. Since the manufacturing method itself of the Si-MOSFET type gas sensor itself is a well-understood technique, in the first embodiment, a process for manufacturing a gate structure, which is a main part of the present invention, and annealing in an oxygen atmosphere are performed. The process will be described mainly. In the first embodiment, an n-channel MOSFET having a gate length (Lg) of 20 μm and a gate width (Wg) of 300 μm is manufactured. This manufacturing method will be described below with reference to FIG.

First, as shown in FIG. 8, local isolation regions 11 and 11a are formed in a semiconductor substrate 10 into which p-type impurities are introduced. The local isolation regions 11 and 11a are formed from a silicon oxide film having a thickness of, for example, 250 nm by performing local oxidation in order to define a gate electrode formation region. Next, in order to form an n-type channel region on the surface of the semiconductor substrate 10, impurity ion implantation is performed at a dose of 10 × 10 ¹¹ / cm ² . Thereafter, ion implantation for forming n ⁺ type semiconductor regions to be the source region 12 and the drain region 13 is performed in the semiconductor substrate 10 to form an active layer of the Si-MOSFET.

Subsequently, after pre-processing the semiconductor substrate 10 (wafer), a gate insulating film 14 having a film thickness of 18 nm is formed on the surface of the semiconductor substrate 10. The gate insulating film 14 is formed of, for example, a silicon oxide film, and can be formed by a thermal oxidation method in an oxygen atmosphere. Thereafter, the gate electrode 15 is formed on the gate insulating film 14 by, for example, a lift-off method. At this stage, the gate electrode 15 is composed of a laminated film of a titanium film formed on the gate insulating film 14 and a platinum film formed on the titanium film. The thickness of the titanium film is, for example, 5 nm, and the thickness of the platinum film is, for example, 15 nm. At this time, as shown in FIG. 8, n ⁺ type semiconductor regions constituting the source region 12 and the drain region 13 are formed in accordance with the local isolation region 11 that defines the formation region of the gate electrode 15. The gate electrode 15 is formed so as to cover not only the gate insulating film 14 but also the top of the local isolation region 11, and is arranged so that the end of the gate electrode 15 overlaps the end of the n ⁺ type semiconductor region. Is done. This is because the first embodiment cannot use a technique for forming an n ⁺ -type semiconductor region in a self-aligned manner with respect to the main gate electrode 15 as a Si-MOSFET formation method. The titanium film and the platinum film constituting the gate electrode 15 are formed by an electron beam irradiation vapor deposition method (EB vapor deposition method), and the deposition rate is 1 Å / s.

  Next, in the high-purity air (in an atmosphere containing oxygen), the annealing process (heat treatment) in which the heat treatment temperature is 400 ° C. and the heat treatment time is 2 hours is performed, so that the first embodiment shown in FIG. The gate structure (laminated structure) that is the feature of the above can be realized. That is, the heat treatment causes the titanium to escape to the outside through the crystal grain boundary of platinum and the phenomenon that oxygen enters the titanium film through the crystal grain boundary of platinum. It is formed. As a result, the titanium film, which was a metal, is doped with oxygen to increase the electrical resistance and partially become titanium oxide, thereby substantially eliminating the carrier (electrons) of the titanium film. It decreases so much that it does not disturb the response of hydrogen gas. On the other hand, the titanium film only needs to be a metal, and the penetration of titanium into the grain boundary improves the mutual adhesion between platinum microcrystals. Furthermore, oxygen passes between platinum microcrystals (grain boundary region) and a part of oxygen stays in the vicinity of the grain boundary region, so that the hydrogen gas passage (grain boundary region) becomes thicker. The response sensitivity is improved and the response speed is also increased.

  Thereafter, as shown in FIG. 8, an insulating film 16 made of PSG (phosphorus doped glass) is formed on the semiconductor substrate 10 including the gate electrode 15. Then, a contact hole penetrating the insulating film 16 is formed, and a process such as surface treatment is performed. The film thickness of the insulating film 16 was 500 nm. Usually, the thickness of the insulating film 16 is often selected in the range of 400 nm to 1000 nm. Then, a source electrode 17 and a drain electrode 18 made of an aluminum (Al) film containing silicon are formed on the insulating film 16 including the inside of the contact hole. The film thickness of the source electrode 17 and the drain electrode 18 is, for example, 500 nm. Although not shown in FIG. 8, an aluminum wiring formed of an aluminum film containing the same silicon as the source electrode 17 and the drain electrode 18 is also formed as a lead electrode for the gate electrode 15 and a heater for heating the chip. The wiring width of the aluminum wiring is, for example, 10 μm, and the wiring length is 29000 μm.

  Subsequently, an insulating film 19 functioning as a protective film is formed on the semiconductor substrate 10 on the aluminum wiring. The insulating film 19 is made of, for example, a silicon nitride film and can be formed by a low temperature plasma CVD method. The film thickness of the insulating film 19 is 700 nm, for example. Finally, an opening is formed on an electrode pad (not shown) for connection to the bonding wire, and an opening is formed so as to expose the gate electrode 15 as the sensor as shown in FIG. . In this way, the Si-MOSFET type gas sensor according to the first embodiment can be formed.

In the first embodiment, a silicon oxide film is used as the gate insulating film 14, but a tantalum oxide (Ta ₂ O ₅ ) film, an aluminum oxide (Al ₂ O ₃ ) film, or silicon nitride is formed on the silicon oxide film. An insulating film such as a (Si ₃ N ₄ ) film may be formed. After this process, a laminated film composed of a titanium film and a platinum film constituting the gate electrode 15 is formed, and thereafter, the Si-MOSFET type gas sensor in the first embodiment is formed through the same process as described above. May be. For example, by forming a tantalum oxide film or the like between the gate insulating film 14 and the gate electrode 15, it is possible to speed up the process of removing hydrogen from the gas sensor after the hydrogen gas irradiation is turned off. That is, by forming a tantalum oxide film or the like on the gate insulating film 14, an effect can be obtained that the time constant at which hydrogen escapes from the trap level existing in the gate insulating film (silicon oxide film) can be increased. As a result, the tail residue of the hydrogen response in the gas sensor can be sufficiently removed.

  Further, in high-purity air, the annealing process at a heat treatment temperature of 400 ° C. and a heat treatment time of 2 hours may not be performed immediately after the formation of the gate electrode 15 but may be performed after the gas sensor manufacturing process is completed. . However, when annealing is performed at 400 ° C. for 2 hours after the gas sensor manufacturing process is completed, the surface of the electrode pad made of aluminum wiring is oxidized, or aluminum itself reacts with silicon oxide. For this reason, the problem that the adhesiveness with the wire connected to the electrode pad is weakened easily occurs, and further, the insulating film 19 as a protective film may be cracked to deteriorate the reliability. For example, in a long-term (2400 hours) temperature heating test or high-temperature and high-humidity (85 ° C. and 95% humidity) test, after the gas sensor manufacturing process is completed, annealing is performed at a heat treatment temperature of 400 ° C. and a heat treatment time of 2 hours. As a result, cracks occurred in the insulating film 19 serving as a protective film, and defects derived from the electrode pads occurred. On the other hand, when the annealing treatment described above was performed immediately after the formation of the gate electrode 15, no gas sensor defect occurred. Therefore, it is desirable to perform the annealing process at a heat treatment temperature of 400 ° C. and a heat treatment time of 2 hours immediately after the formation of the gate electrode 15 as described in the first embodiment.

  In the Si-MOSFET type gas sensor shown in the first embodiment, an example in which wirings and electrodes (source electrode 17 and drain electrode 18) are formed using an aluminum film has been described. In the example, for the purpose of improving the reliability of the wiring, an example in which the ohmic contact to the wiring using the gold film and the silicon is surely described will be described with reference to FIG. In this modification, the process until the contact hole is formed in the insulating film 16 is the same as that of the first embodiment shown in FIG.

  Subsequently, a molybdenum film (100 nm), a gold film (500 nm), and a molybdenum film (10 nm) are sequentially formed on the insulating film 16 including the inside of the contact hole. At this time, the molybdenum film (100 nm) is formed by EB vapor deposition, and finally, a gold film (500 nm) and a molybdenum film (10 nm) are formed by sputtering. Then, by patterning these laminated films, the source electrode 17 (the drain electrode 18 is not shown) and the wiring 20 shown in FIG. 9 are formed. Since gold has an effect of diffusing in silicon even at a relatively low temperature, a molybdenum film (100 nm) is used as a barrier metal. In this case, when heat treatment is performed after the wiring 20 or the like is formed, for example, a molybdenum silicide (MoSi) film 21 that is an alloy film of molybdenum and silicon may be formed in a contact region between the source electrode 17 and the semiconductor substrate 10. it can. The molybdenum film (100 nm) functions as a barrier film that suppresses diffusion of gold into silicon. The molybdenum film (10 nm) is inserted in order to improve the adhesiveness with the insulating film 19. Since the wiring 20 comprising the molybdenum film (100 nm), the gold film (500 nm), and the molybdenum film (10 nm) is also used for the pad portion, the molybdenum film (10 nm) that appears on the surface is removed when the pad portion is formed. In this case, the method of connecting the chip and the mounting substrate uses a gold wire because bonding with the gold wire prevents rust and improves the adhesion to the pad portion.

  By forming the molybdenum silicide film 21 between the source electrode 17 and the semiconductor substrate 10, an ohmic contact between the source electrode 17 and the semiconductor substrate 10 can be ensured. As described above, the structure using the gold film as the wiring material is more expensive than the structure using the aluminum film as the wiring material, but is excellent in moisture resistance and oxidation resistance. From this, it is possible to use different wiring materials, such as using a gold film for the wiring from the viewpoint of ensuring the reliability of the wiring and using an aluminum film for the wiring from the viewpoint of reducing the cost.

  Next, the annealing temperature dependence of the hydrogen response strength in the Si-MOSFET type gas sensor disclosed in the first embodiment and its modification will be described. FIG. 10 shows the relationship between the hydrogen response intensity ΔVg and the annealing temperature in a state where the thickness of the titanium film is fixed to 5 nm and the thickness of the platinum film is fixed to 15 nm, and the annealing time in air is fixed to 2 hours. It is a figure which shows a relationship. FIG. 11 is a diagram showing the relationship between the hydrogen response intensity ΔVg and the annealing time in a state where the annealing temperature is fixed at 400 ° C. As shown in FIG. 10, it can be seen that a good hydrogen response strength ΔVg can be obtained in the temperature range from 350 ° C. to 500 ° C. as the annealing temperature. In FIG. 10, it is understood that the data from room temperature to 300 ° C. produced variations between wafers in different thermal processes in the element manufacturing process. Further, as shown in FIG. 11, when the annealing temperature is set to 400 ° C., it can be seen that a good hydrogen response strength ΔVg can be obtained by setting the annealing time to, for example, 20 minutes or more.

  Subsequently, for four different wafers (# 1- # 4), the hydrogen response voltages for three types of hydrogen concentration gases diluted with air were examined. FIG. 12 is a diagram showing the relationship between the hydrogen concentration and the hydrogen response voltage in four different wafers. From the results shown in FIG. 12, the Si-MOSFET type hydrogen gas sensor using the conventional palladium as the gate electrode is much more sensitive than the Si-MOSFET type hydrogen gas sensor using the usual platinum as the gate electrode. It surpasses the performance of. In particular, in the hydrogen gas sensor according to the first embodiment, it has been found that the response speed at hydrogen concentrations of 1000 ppm and 10000 ppm is as high as 1 to 2 seconds.

  Next, in the gas sensor of the first embodiment and the modified example, the distribution of MOSFET characteristics in the wafer surface, and the reproducibility and uniformity of the MOSFET characteristics between lots and between wafers will be described. FIG. 13 is a diagram showing current-voltage characteristics at 100 ° C. of a MOSFET constituting the hydrogen gas sensor. FIG. 13 shows the characteristics of two MOSFETs in which the threshold voltage Vth is far apart in high-purity air in a 5-inch wafer in a state before heat treatment. The difference in threshold voltage Vth between them is about 300 mV.

  FIG. 14 is a diagram showing in-plane distribution and reproducibility of the threshold voltage Vth in two wafers (# 2, # 3) having different lots after the heat treatment. As can be seen from FIG. 14, the variation of the threshold voltage Vth is as small as 178 mV, and the reproducibility is very good. It can be seen that the variation in the threshold voltage Vth is more than twice as great as the variation in the threshold voltage before annealing shown in FIGS. This is understood to be the effect that the thermally stable platinum can control the threshold voltage Vth by changing the titanium film to the oxygen-doped titanium film by the annealing treatment. FIG. 15 is a diagram showing the in-plane distribution of the normalized hydrogen response output with respect to 1000 ppm of air diluted hydrogen gas. As shown in FIG. 15, according to the hydrogen gas sensor in the first embodiment, it can be seen that a very uniform in-plane distribution can be obtained.

  Until now, the thickness of the titanium film was fixed to 5 nm and the thickness of the platinum film was fixed to 15 nm, and the characteristics of the hydrogen gas sensor were evaluated. Subsequently, the thickness of the titanium film and the platinum film was changed to change the thickness of the hydrogen gas sensor. The result of evaluating the characteristics will be described. 16 and 17 are diagrams showing the results of examining the hydrogen response voltage with respect to 1000 ppm air-diluted hydrogen gas while changing the film thickness of the titanium film and the platinum film and changing the annealing time at 400 ° C. FIG. As shown in FIGS. 16 and 17, it can be seen that the hydrogen response is 1 nm to 10 nm in the thickness of the titanium film and 1 nm to 30 nm in the thickness of the platinum film.

  As described above, the first embodiment describes the Si-MOSFET type hydrogen gas sensor in which the titanium film is formed under the platinum film constituting the gate electrode during the manufacturing process. Even if a tungsten (W) film, a molybdenum (Mo) film, a tantalum (Ta) film, a niobium (Nb) film, a chromium (Cr) film, or the like is used, the same method as described in the first embodiment is used. Needless to say, a highly reliable and highly sensitive hydrogen gas sensor can be formed.

(Embodiment 2)
In the second embodiment, Si-- having high gas selectivity that reacts with ammonia or ethanol gas other than hydrogen gas by utilizing the fact that gaps are actually formed between platinum microcrystals, not crystal grain boundaries. A MOSFET type gas sensor will be described.

  FIG. 18 is a diagram schematically showing a gate structure for realizing a gas sensor with high gas selectivity in a Si-MOSFET type gas sensor using platinum as a gate electrode. In FIG. 18, a silicon oxide film 2 is formed on a semiconductor substrate 1 made of silicon. A titanium oxide film 22 is formed on the silicon oxide film 2. The titanium oxide film 22 may be a thin film made of a titanium oxide microcrystal having a rutile structure, a thin film made of a titanium oxide microcrystal mixed with an anatase structure in addition to a rutile structure, or a titanium oxide fine film mixed with an oxygen-doped titanium film, depending on manufacturing conditions. It is a thin film of crystals.

  The feature of the gate structure of the gas sensor in the second embodiment is that the grain boundary region between the platinum microcrystals 23 oriented in the (111) direction disappears and a gap 23 a is formed between the platinum microcrystals 23. . Practically, the width of the gap 23a formed between the platinum microcrystals 23 is desirably several nm to several tens of nm.

FIG. 19 shows a cross-sectional TEM photograph of the structure of Pt / Ti / SiO ₂ / Si when annealed in air at a heat treatment temperature of 800 ° C. for a heat treatment time of 30 minutes, and Pt / Ti / before annealing. it is a diagram showing a structure of SiO ₂ / Si in the right area of the photograph. It can be seen from the cross-sectional TEM photograph shown in FIG. 19 that the film thickness of the titanium film and the film thickness of the platinum film change greatly after the annealing treatment. At this time, it is known from the X-ray diffraction that the titanium film is almost a TiO ₂ (rutile structure) crystal. It can be seen that the morphology of the platinum film is also significantly different from the gate structure after annealing with a heat treatment time of 2 hours at a heat treatment temperature of 400 ° C. shown in FIG.

Next, the operation principle of the gas sensor according to the second embodiment will be described with reference to FIG. As discussed in Sensors and Actuators, B1 (1990) 15-20 (Non-Patent Document 2) and its references, the mechanism of sensor response is that ammonia gas or the like adheres to the surface of island-like platinum microcrystals 23. The surface potential φs of the platinum microcrystal 23 is changed. As a result, the capacitance C _S between the platinum microcrystal 23 and the gap region (gap 23a) where platinum does not exist on the surface of the silicon oxide film 2, the capacitance C _E between the gap region and the channel region 1a, platinum With respect to the capacitance system of the capacitance C _M between the microcrystal 23 and the channel region 1a, the capacitance C _S and the capacitance C _E enter in series, and the capacitance C _M is configured in parallel. Therefore, there is a relationship of ΔV = Δφs C _S · C _E / [C _S · C _E + C _M (C _S + C _E )] between the change amount Δφs of the surface potential φs, and the gate potential with respect to gas adsorption. Change ΔV can be observed. In this way, the gas sensor according to the second embodiment can detect the gas concentration as the change ΔV in the gate potential. In other words, the gas sensor according to the second embodiment means that any gas that changes the surface potential of platinum microcrystals can be detected from the operation principle described above, and it can be seen that the gas sensor has high gas selectivity. Note that an insulating material may be formed in the gap 23a shown in FIG. 18 in the manufacturing process, but even in that case, the gas concentration can be detected by the above-described operation as the gas sensor.

  Next, a manufacturing method of the Si-MOSFET type gas sensor according to the second embodiment will be described. The portions common to the first embodiment and the second embodiment will not be described in detail, and the portions directly related to the second embodiment will be mainly described. In the second embodiment, similar to the first embodiment, an n-channel MOSFET having a gate length (Lg) of 20 μm and a gate width (Wg) of 300 μm is manufactured. This manufacturing method will be described below with reference to FIGS. 20 to 22, the same reference numerals as those in FIG. 8 indicate the same components.

  First, in FIG. 20, the steps up to the step of forming local isolation regions 11 and 11a on a semiconductor substrate 10 and forming active layers of MOSFETs as source region 12 and drain region 13 by ion implantation are the same as those in the first embodiment. It is.

  Subsequently, after pre-processing the semiconductor substrate 10 (wafer), a gate insulating film 14 having a film thickness of 18 nm is formed on the surface of the semiconductor substrate 10. The gate insulating film 14 is formed of, for example, a silicon oxide film, and can be formed by a thermal oxidation method in an oxygen atmosphere. Thereafter, a laminated film 24 composed of a titanium film and a platinum film is formed on the entire surface of the semiconductor substrate 10 by EB vapor deposition by electron beam irradiation. At this time, the thickness of the titanium film is 5 nm, and the thickness of the platinum film is 15 nm. The deposition rate of the titanium film and the platinum film is 1 Å / s.

  Next, as shown in FIG. 21, annealing is performed at a heat treatment temperature of 650 ° C. and a heat treatment time of 20 minutes in high-purity air to change the laminated film 24 into a thin film 25. The thin film 25 has a structure in which gaps between platinum microcrystals (about 5 nm) and innumerable holes (about 5 nm) are formed. When a dummy solid film formed under the same conditions as those for forming the thin film 25 is evaluated by X-ray, the titanium film is changed to a structure in which a titanium oxide having an anatase structure is slightly mixed with a titanium oxide having a rutile structure. Thereafter, as shown in FIG. 22, while leaving the gate electrode 26 and the extraction electrode (not shown) by photolithography, unnecessary platinum and titanium oxide films are removed by ion milling. Although there is a method in which the titanium oxide film is left as a surface protective film when the electrical conductivity is extremely low, in Embodiment 2, unnecessary titanium oxide film other than the formation region of the gate electrode 26 is removed. ing. Thereby, the gate structure shown in FIG. 18 can be realized.

  Thereafter, as shown in FIG. 22, an insulating film 16 made of PSG (phosphorus-doped glass) is formed on the semiconductor substrate 10 including the gate electrode 26. Then, a contact hole penetrating the insulating film 16 is formed, and a process such as surface treatment is performed. Then, a source electrode 17 and a drain electrode 18 made of an aluminum (Al) film containing silicon are formed on the insulating film 16 including the inside of the contact hole. The film thickness of the source electrode 17 and the drain electrode 18 is, for example, 500 nm. Although not shown in FIG. 22, an aluminum wiring formed from an aluminum film containing the same silicon as the source electrode 17 and the drain electrode 18 is also formed as a lead electrode of the gate electrode 26 and a heater for heating the chip. The wiring width of the aluminum wiring is, for example, 20 μm, and the wiring length is 29000 μm.

  Subsequently, an insulating film 19 functioning as a protective film is formed on the semiconductor substrate 10 on the aluminum wiring. The insulating film 19 is made of, for example, a silicon nitride film and can be formed by a low temperature plasma CVD method. The film thickness of the insulating film 19 is 700 nm, for example. Finally, an opening is formed on an electrode pad (not shown) for connection to the bonding wire, and an opening is formed so as to expose the gate electrode 26 as the sensor as shown in FIG. . In this manner, the Si-MOSFET type gas sensor according to the second embodiment can be formed.

  In this case, the insulating film (PSG) 16 may remain in the gap 23a shown in FIG. 18, but there is no problem because it operates as a sensor. As in the modification of the first embodiment, a gas sensor is also formed in which the wiring is formed from a laminated structure of Mo / Au / Mo.

In the second embodiment, a silicon oxide film is used as the gate insulating film 14. On this silicon oxide film, a tantalum oxide (Ta ₂ O ₅ ) film, an aluminum oxide (Al ₂ O ₃ ) film, or silicon nitride is used. As in the first embodiment, an insulating film such as a (Si ₃ N ₄ ) film may be formed. However, in the second embodiment, when it is desired to surely remove the insulating film (PSG) 16 remaining in the gap 23a shown in FIG. 18, tantalum oxide (Ta ₂ O ₅ ) having etching selectivity with respect to the silicon oxide film. It is effective to form an insulating film such as a film, an aluminum oxide (Al ₂ O ₃ ) film, or a silicon nitride (Si ₃ N ₄ ) film on the silicon oxide film.

  In the second embodiment, the same values as those of the first embodiment are used such that the thickness of the titanium film is 5 nm and the thickness of the platinum film is 15 nm. The annealing temperature dependency and annealing time dependency of the gas sensor according to the second embodiment configured as described above are examined. FIG. 23 is a diagram showing the relationship between the hydrogen response intensity ΔVg and the annealing temperature with the annealing time fixed at 30 minutes. FIG. 24 is a diagram showing the relationship between the hydrogen response intensity ΔVg and the annealing time when the annealing temperature is fixed at 650 ° C.

  As shown in FIG. 23, when the annealing temperature approaches 800 ° C., the distance between the platinum microcrystals increases, the gap where the platinum microcrystals are separated from each other widens, and the hydrogen response characteristic (hydrogen response intensity ΔVg) is poor. Become. On the other hand, when the annealing temperature is lower than 500 ° C., the platinum microcrystal begins to take a striped structure having a gap, but the area of the gap in the plane of the gate electrode is small and the hydrogen response characteristic is deteriorated. As a result, in the structure where the thickness of the titanium film is 5 nm and the thickness of the platinum film is 15 nm, the hydrogen response characteristics are improved with the structure having a striped structure by setting the annealing temperature to around 650 ° C. I understand that I can do it. Subsequently, as shown in FIG. 24, it is shown that the annealing time dependency at an annealing temperature of 650 ° C. is preferably about 15 to 20 minutes.

  However, when the temperature is changed from 650 ° C. and the annealing time dependency is taken, a unique tendency as shown in FIG. 25 becomes apparent. FIG. 25 is a diagram showing the relationship between the annealing time and gas response voltage at various annealing temperatures. As shown in FIG. 25, when the annealing temperature is as high as 800 ° C., the gas response characteristic (gas response voltage) is improved by reducing the annealing time. On the other hand, it can be seen that long-term annealing is effective at a low annealing temperature of 550 ° C.

  Next, the relationship between the gas response voltage and the titanium (Ti) / platinum (Pt) film thickness ratio was examined by changing the film thickness of the titanium film and the film thickness of the platinum film. FIG. 26 is a diagram showing the relationship between the gas response voltage and the titanium (Ti) / platinum (Pt) film thickness ratio at three types of annealing temperatures with the annealing time fixed at 30 minutes. As shown in FIG. 26, when the annealing temperature is 800 ° C., the thickness of the titanium film and the thickness of the platinum film are set such that, for example, the thickness of the titanium film is 10 nm and the thickness of the platinum film is 20 nm. Even if the thickness is increased, the gas response characteristic (gas response voltage) is not yet saturated. In the case of the gas sensor having high gas selectivity described in the second embodiment, there is a wide range in the optimum film thickness ratio between the titanium film thickness and the platinum film thickness, the optimum annealing temperature, and the optimum annealing time. Yes, the conditions for optimizing the gas response characteristics can be found without a particularly thick titanium and platinum film. For example, the gas response characteristics can be optimized even when the thickness of the titanium film is 1 nm to 10 nm and the thickness of the platinum film is 1 nm to 30 nm.

(Embodiment 3)
Next, the hydrogen gas sensor according to the third embodiment will be described. FIG. 27 is an optical micrograph of the semiconductor chip forming the hydrogen gas sensor according to the third embodiment. As shown in FIG. 27, a sensor FET 27, a reference FET 28, a heater 29 made of metal wiring, and a PN junction diode 30 for measuring the chip temperature are formed on a 2 mm × 2 mm semiconductor chip (silicon chip) CHP. Yes. In this case, the wiring heater has a wiring width of 30 μm and a wiring length of 19000 μm. The sensor FET 27 and the reference FET 28 use the Si-MOSFET described in the first embodiment. The configurations of the sensor FET 27 and the reference FET 28 that are basically different from those of the first embodiment will be mainly described.

  In the hydrogen gas sensor according to the third embodiment, since two types of FETs (sensor FET 27 and reference FET 28) and PN junction diode 30 are integrated on the semiconductor chip CHP, element separation is required to separate these devices. In the third embodiment, the most well-known PN junction isolation technique is used to isolate elements from each other. FIG. 28 shows a cross-sectional structure of the sensor FET 27 formed on the n-type semiconductor substrate, and FIG. 29 shows a cross-sectional structure of the reference FET 28 formed on the n-type semiconductor substrate. 28 to 29, the same reference numerals as those in FIG. 8 indicate the same components.

As shown in FIG. 28, in the sensor FET 27, a p-type well 31 that defines a formation region of the sensor FET is formed in an n-type semiconductor substrate 10a. The p-type well 31 is composed of, for example, a p-type semiconductor region formed by ion implantation. In this case, in order to fix the potential of the p-type well 31, a p ⁺ -type semiconductor region 34 is formed in the n-type semiconductor substrate 10a by, for example, an ion implantation method. Then, like the source electrode 17 and the drain electrode 18, a p-type well electrode 32 formed of an aluminum film is formed so as to be connected to the p ⁺ -type semiconductor region 34. Similarly, in order to fix the potential of the n-type semiconductor substrate 10a, an n ⁺ -type semiconductor region 35 is formed in the n-type semiconductor substrate 10a by, for example, an ion implantation method. Then, the substrate electrode 33 formed of an aluminum film is formed so as to be connected to the n ⁺ type semiconductor region 35.

As shown in FIG. 29, also in the reference FET 28, a p-type well 31 that defines a formation region of the reference FET 28 is formed in the n-type semiconductor substrate 10a. The p-type well 31 is composed of, for example, a p-type semiconductor region formed by ion implantation. In this case, in order to fix the potential of the p-type well 31, a p ⁺ -type semiconductor region 34 is formed in the n-type semiconductor substrate 10a by, for example, an ion implantation method. Then, like the source electrode 17 and the drain electrode 18, a p-type well electrode 32 formed of an aluminum film is formed so as to be connected to the p ⁺ -type semiconductor region 34. Similarly, in order to fix the potential of the n-type semiconductor substrate 10a, an n ⁺ -type semiconductor region 35 is formed in the n-type semiconductor substrate 10a by, for example, an ion implantation method. Then, the substrate electrode 33 formed of an aluminum film is formed so as to be connected to the n ⁺ type semiconductor region 35.

  The difference between the sensor FET 27 and the reference FET 28 thus configured is that the gate electrode 15 is exposed from the insulating film 19 in the sensor FET 27, whereas the gate electrode 15 is different from the insulating film 16 in the reference FET 28. It is a point covered with 19. This difference is because the sensor FET 27 needs to be in contact with hydrogen gas, whereas the reference FET 28 is configured not to be in contact with hydrogen gas.

  Although a sectional view of a PN junction diode (not shown) is omitted, the junction area is 60 μm × 200 μm. It has been found from the temperature calibration condition that a state in which a current of 10 μA flows at a forward bias rising voltage Vf = 0.3 V, which is a diode characteristic, corresponds to a chip temperature of 100 ° C.

  The wiring of the hydrogen gas sensor according to the third embodiment employs, for example, a Mo / Au / Mo structure. Similarly, the wiring of the heater 29 and the source electrode 17 and the drain electrode 18 also adopt a Mo / Au / Mo structure. On the other hand, for the wiring of the heater 29 and the source electrode 17 and the drain electrode 18, a prototype using the aluminum wiring used in the first embodiment is also manufactured. In the third embodiment, the reliability test is performed using such two kinds of wiring materials. As a result, in the high-temperature thermal acceleration test and the high-temperature and high-humidity test, the gold wiring has a longer life than the aluminum wiring, but the manufacturing cost is high.

  Here, after forming the gate electrode, an insulating film (PSG) 16 is formed. Thereafter, a wiring is formed on the insulating film 16, and an insulating film (silicon nitride film) 19 is formed on the entire surface of the semiconductor substrate covering the wiring. Thereafter, in the formation region of the sensor FET 27, the insulating film 16 and the insulating film 19 on the gate electrode 15 (sensor sensitive portion) made of a platinum film are removed to form the sensor FET 27. On the other hand, in the formation region of the reference FET 28, the insulating film 16 and the insulating film 19 on the gate electrode 15 made of a platinum film are left without being removed. With this configuration, both the sensor FET 27 and the reference FET 28 realize substantially the same threshold voltage Vth = 1.02V.

  If only the sensor FET 27 is formed alone, the annealing process in which the annealing temperature is 400 ° C. and the annealing time is 2 hours in high-purity air may not be performed during the gas sensor manufacturing process. It can also be performed when the manufacturing process is completed. By this annealing treatment (heat treatment), the work function of titanium before the heat treatment is the dominant factor of the threshold voltage Vth, but the work function of platinum is changed to the dominant factor of the threshold voltage Vth. The threshold voltage Vth becomes shallow near 1.2V.

  However, if the reference FET 28 is also integrated on the semiconductor chip CHP at the same time and the threshold voltage Vth is made uniform with the sensor FET 27, the insulating film 16 and the insulating film 19 that protect the gate electrode 15 of the reference FET 28 do not pass oxygen. As for the threshold voltage Vth of the reference FET 28, the work function of the original titanium remains the dominant factor of the threshold voltage Vth.

  FIG. 30 is a diagram showing that the relationship between the gate voltage Vgs and the source / drain current Ids changes depending on the presence or absence of the insulating film 16 and the insulating film 19 (PSG / SiN). As shown in FIG. 30, it can be seen that the threshold voltage Vth differs by about 1.2 V between the adjacent sensor FET 27 and the reference FET 28 separated by about 1 mm depending on the presence or absence of the insulating film 16 and the insulating film 19 on the gate electrode 15. . Therefore, when the reference FET 28 and the sensor FET 27 are subjected to heat treatment after the gas sensor manufacturing process is completed and the threshold voltages Vth are equalized, both FETs (the sensor FET 27 and the reference FET 28 are processed during the gas sensor manufacturing process). It is necessary to remove the insulating film 16 and the insulating film 19 (PSG / SiN) on the gate electrode 15 (sensor sensitive part). Both FETs (the sensor FET 27 and the reference FET 28) were formed on the gate electrode 15 of the reference FET 28 after performing an annealing process in high-purity air with an annealing temperature of 400 ° C. and an annealing time of 2 hours. It is necessary to cover the opening with a new insulating film so as not to respond to hydrogen gas.

(Embodiment 4)
Next, in the fourth embodiment, a Si-MOSFET type hydrogen gas sensor in which a palladium film is formed on a platinum film constituting a gate electrode will be described. Since the Si-MOSFET type hydrogen gas sensor using a palladium film as a gate electrode has a feature of high-speed response to a low concentration of hydrogen gas, the Si-MOSFET type hydrogen sensor having the structure described in the first embodiment is used. Improve the hydrogen gas sensor and try to form a palladium film on the platinum film.

  In a Si-MOSFET type hydrogen gas sensor in which a palladium film is formed on a gate insulating film, a phenomenon occurs in which the lattice constant of palladium is expanded due to occluded hydrogen due to hydrogen exposure or ethanol gas exposure. It is known that this phenomenon causes mechanical stress in the palladium film, and a phenomenon called a blister occurs where the palladium film partially swells at the interface between the palladium film and the gate insulating film (silicon oxide film). It has been. As a method for avoiding this blister, it is known that the gate electrode formed on the gate insulating film has a two-layer structure of a palladium film and a platinum film. That is, it is known that a platinum film is formed on a gate insulating film and a palladium film is formed on the platinum film.

However, the structure in which the platinum film is directly formed on the gate insulating film is not practical because the film peeling of the platinum film itself occurs. Therefore, a Pd / Pt / Ti / SiO ₂ / Si structure is formed as a gate structure, and this structure is subjected to heat treatment at a heat treatment temperature of about 400 ° C. in an oxygen atmosphere. Then, since the surface of the palladium film is oxidized, the catalytic action of the palladium film is reduced. Therefore, the above-described heat treatment is applied in a state where the gate structure is formed of a Pt / Ti / SiO ₂ / Si structure, and after this heat treatment is finished, a palladium film is formed on the platinum film. That is, after the platinum film is processed into the gate electrode shape and the above-described heat treatment is performed, the palladium film is formed so as to cover the entire platinum film. Thus, by forming the palladium film after the above-described heat treatment is applied, it is possible to suppress the surface of the palladium film from being oxidized and the catalytic function from being lowered.

In the fourth embodiment, the hydrogen gas sensor is manufactured in accordance with the method for manufacturing the hydrogen gas sensor in the first embodiment. Also in the fourth embodiment, after the Pt / Ti / SiO ₂ / Si structure is formed, heat treatment is performed in high-purity air at a heat treatment temperature of 400 ° C. and a heat treatment time of 2 hours. The gate structure shown in FIG. Thereafter, a palladium film (palladium electrode) having a thickness of 70 nm is formed by a lift-off method so as to cover the platinum film constituting the gate electrode of the sensor FET that is necessary again. The thickness of the palladium electrode is preferably in the range of 30 nm to 300 nm. At this time, the film thickness of the platinum film is in the range of 5 nm to 15 nm, and the film thickness of the titanium film is often used in the range of 3 nm to 5 nm.

  After forming the gate electrode including the platinum film and the palladium film, the surface of the palladium film is annealed (heat treatment) at a relatively low temperature (250 ° C. to 300 ° C.) for about 30 minutes in the air. The layer can be converted to PdO (Pd oxide). In this case, the hydrogen response intensity is somewhat reduced, but a hydrogen gas sensor with high long-term stability can be obtained. This is because, unlike platinum, palladium forms a relatively stable oxide. By realizing such a structure, it is possible to realize a highly reliable hydrogen gas sensor that responds at high speed in a low concentration (about 500 ppm or less) hydrogen gas concentration region and suppresses film peeling of the gate electrode. . Therefore, by forming the hydrogen gas sensor according to the first embodiment and the hydrogen gas sensor according to the fourth embodiment in a semiconductor chip (silicon chip), a hydrogen gas having a low concentration (100 ppm) is changed to a hydrogen gas having a high concentration (10%). A hydrogen gas sensor that detects hydrogen gas in a wide range up to gas can be manufactured. That is, the hydrogen gas sensor according to the fourth embodiment that positively utilizes the catalytic action of the palladium film functions effectively for low-concentration hydrogen gas, and the above-described implementation is performed for high-concentration hydrogen gas. The hydrogen gas sensor in the form 1 functions effectively.

(Embodiment 5)
In the first to fourth embodiments, the example in which the MOSFET is applied to the hydrogen gas sensor has been described. However, the present invention can be realized by using a MIS type capacitor or a Schottky diode instead of the MOSFET. In particular, in the MIS type capacitor shown in FIG. 8 described in the first embodiment, the planar shape of the gate electrode 15 is, for example, a circular shape having a diameter of 50 μm to 100 μm, and the source electrode 17 and the drain electrode 18 are concentric with the gate electrode 15. The shape can be surrounded by In this MIS capacitor, for example, the voltage in the flat band state is defined as the threshold voltage Vth, and the response of hydrogen gas can be measured. In the MIS type capacitor, the gate electrode 15 functions as an upper electrode, and the semiconductor substrate 10 immediately below the gate electrode 15 functions as a lower electrode. The semiconductor substrate 10 is configured to be electrically connected to the source electrode 17 and the drain electrode 18, so that the extraction electrode of the lower electrode becomes the source electrode 17 and the drain electrode 18. Furthermore, the gate insulating film 14 between the gate electrode 15 and the semiconductor substrate 10 functions as a capacitive insulating film. According to the MIS type capacitor configured as described above, a channel region formed on the surface of the semiconductor substrate 10 is formed or disappears depending on the presence or absence of hydrogen gas (corresponding to the threshold voltage Vth being changed by hydrogen gas). To use). That is, since the electric capacity of the MIS type capacitor changes depending on the presence or absence of the channel region formed in the semiconductor substrate 10, the presence of hydrogen gas can be detected indirectly by detecting the change in electric capacity. .

  In the examples so far, the Si-MOSFET is applied to the hydrogen gas sensor, but other semiconductor materials such as silicon carbide (SiC), gallium arsenide (GaAs), and gallium nitride (GaN) are used. The present invention can also be applied to an element. For example, the present invention can also be applied to an MIS capacitor, a Schottky gate FET, and a PN junction gate FET using such a semiconductor material.

(Embodiment 6)
In the first to fifth embodiments, the example in which the present invention is applied to the gas sensor having the MOS structure has been described. However, the technical idea of the present invention is a gas sensor based on another principle that actively uses a platinum thin film. Alternatively, the present invention is effective when applied to an electrode formed by using platinum on an insulator (insulating film) or a semiconductor film. In the sixth embodiment, an example in which the present invention is applied to a gas sensor or an electrode of another principle will be disclosed.

First, FIG. 31 discloses an example in which the present invention is applied to a hydrogen gas detection electrode in an EMF type hydrogen gas sensor (Surface Technology 2006, Vol. 57, N04, pages 15-18). FIG. 31 shows a plan view of the sensor, and FIG. 32 shows a sectional view of the sensor (a sectional view taken along line AA in FIG. 31). As shown in FIGS. 31 and 32, phosphotungstic acid (P ₂ O ₅ .24WO ₃ .nH ₂ O) as a solid electrolyte 38 was dissolved in ion-exchanged water in an alumina substrate 41 so as to have a concentration of 25 g / 10 ml. 1 ml of the solution was added dropwise and dried to fix. After forming the detection electrode 36 composed of a 5 nm titanium film and a 15 nm platinum film so as to be connected to the solid electrolyte 38, an annealing process was performed in air at a heat treatment temperature of 400 ° C. and a heat treatment temperature of 2 hours. . Thereafter, a nickel film having a thickness of 300 nm was formed as the reference electrode 37, and an aluminum film was formed as the extraction electrodes 39 and 40 with a thickness of 700 nm. Glass wool is used as a reinforcing agent for the solid electrolyte 38 dried and solidified.

  In the EMF type hydrogen gas sensor configured as described above, the hydrogen gas is obtained by utilizing the fact that the potential of the detection electrode 36 including the platinum film is changed by the hydrogen gas with respect to the potential of the reference electrode 37 formed of the nickel film. Is detected.

  At this time, for example, if a platinum film constituting the detection electrode 36 is directly formed on the alumina substrate 41 or the solid electrolyte 38, there is a concern that film peeling of the platinum film may occur, but this will be described in the sixth embodiment. With the structure, the effect of the platinum film with respect to film peeling is significantly improved. That is, the feature of the sixth embodiment is that the structure of platinum (Pt) / titanium (Ti) / alumina substrate 41 and the structure of platinum (Pt) / titanium (Ti) / solid electrolyte 38 are heated in an oxygen gas atmosphere. There is. As a result, when titanium enters the platinum grain boundary, the mutual adhesion between the platinum microcrystal and the alumina substrate 41 (or the platinum microcrystal and the solid electrolyte 38) is improved. As the annealing treatment (heat treatment), when the heat treatment temperature is 300 ° C., the heat treatment time is 240 hours. Further, when palladium is used for the hydrogen gas detection electrode 36, the detection electrode 36 has a laminated structure of palladium (Pd) / platinum (Pt) / titanium (Ti) as in the fourth embodiment. The adhesion of the detection electrode 36 can be improved.

Next, platinum electrodes are widely used in industrial applications. The platinum thin film structure of the present invention (the structure described in the first embodiment) can be applied to, for example, a platinum electrode used in an electrolytic crystal growth method or the like. In the electrolytic crystal growth method, a platinum wire (Pt line) of about 1φ is often used as the platinum electrode. For example, in the field crystal growth method of the FA ₂ PF ₆ crystal, which is an electrolytically mobile organic crystal, the FA ₂ PF ₆ crystal is obtained by dissolving the electrolyte in an organic solvent and energizing it. The electrolytic crystal growth method is a method in which a current is passed through a solution in which a crystal raw material is dissolved to grow a crystal on an anode. As shown in FIG. 33, the crystal cell used in the electrolytic crystal growth method has an anode 43 made of nickel in a glass triangular flask 47 and a cathode 44 in a cylindrical test tube 48. An anode 42 is formed at the tip of the anode 43, and the anode 42 and the cathode 44 are connected by an electrolytic solution. At this time, the filter 45 is formed so that the foreign matter generated from the cathode 44 by the electrolytic reaction does not diffuse to the anode 42 side. Further, a vacuum cock 46 is provided so that the crystal cell can be evacuated and purged with nitrogen ("Chemistry of Conductive Low Dimensional Materials", edited by the Chemical Society of Japan, Chemistry General No. 42, 1983, Academic Press reference).

Here, the anode 42 provided at the tip of the anode 43 is usually formed of a simple platinum wire of about 1φ. In this case, since platinum in a bulk state called platinum wire is used, a large amount of platinum is necessary and expensive. Therefore, it is conceivable to use a platinum thin film as the anode 42 instead of a bulk platinum wire. For example, FIG. 34 is a diagram illustrating a configuration example in which a platinum thin film is used as the anode 42. As shown in FIG. 34, a platinum thin film 49 is formed over the entire surface including both surfaces of the ceramic substrate 50. The anode 43 is electrically connected to the platinum thin film 49. As shown in FIG. 34, the anode 42 is composed of a platinum thin film 49, so that the amount of platinum used can be saved as compared with the case where the anode 42 uses a platinum wire in a bulk state. Therefore, for example, when it is desired to manufacture a large amount of FA ₂ PF ₆ crystal at a low cost, the amount of platinum used can be remarkably saved by using the anode 42 having the structure shown in FIG.

  However, when the structure shown in FIG. 34 is used as the anode 42, there is a concern about film peeling between the ceramic substrate 50 and the platinum thin film 49. Therefore, similarly to the structure shown in the first embodiment, a titanium film having a thickness of 5 nm and a platinum film having a thickness of 15 nm are formed on the ceramic substrate 50, and then the heat treatment temperature is 400 in air. Annealing treatment is performed at a temperature of 2 ° C. for 2 hours. That is, the structure of platinum (Pt) / titanium (Ti) / ceramics substrate 50 is heated in an oxygen gas atmosphere. Thereby, titanium penetrates into the grain boundaries of platinum, thereby improving the mutual adhesion between the platinum microcrystals and the ceramic substrate 50. In this case, since the titanium film is an oxygen-doped titanium film as in the first embodiment, the deterioration of the titanium film due to energization in the electrolytic solution can be suppressed by using the oxygen-doped titanium film. . Note that the electrode structure in the sixth embodiment not only is used as an electrode by an electrolytic crystal growth method but also works effectively as an electrode used in a fuel cell or the like.

  Note that in the sixth embodiment, a tungsten (W) film, a molybdenum (Mo) film, a tantalum (Ta) film, a niobium (Nb) film, a chromium (Cr) film, or the like is used instead of the titanium film. Needless to say, a highly reliable electrode can be formed by the same method as described above.

  As described above, as a common technical idea, the structure and the manufacturing method of the present invention are very effective in a gas sensor or electrode that uses the catalyst electrode Pt by forming a thin film on an insulator or semiconductor. Although all the descriptions of the above embodiments and the like have been described in the case of the N-channel type MISFET, it is needless to say that the present invention can be similarly applied to the case of the P-channel type MISFET.

  As described above, the present invention has been described using the first to sixth embodiments. The effects of the present invention are summarized as follows.

(1) The grain boundary region between the platinum microcrystals is filled with the Pt—Ti—O region and is connected to TiOx formed immediately below the platinum film. . That is, the adhesive strength of the platinum film is improved, and the reliability of the gas sensor can be improved.

(2) The platinum crystal grain boundary is annealed in an oxygen atmosphere, so that titanium escapes through the platinum crystal grain boundary, and further, oxygen enters through the platinum crystal grain boundary so that the grain boundary region ( Pt—Ti—O region) can be newly formed, and hydrogen gas can easily pass through compared with a simple platinum crystal grain boundary. Therefore, even in a Si-MOSFET type gas sensor that uses a platinum film as a gate electrode, Si-- that uses palladium as a gate electrode.
High sensitivity and high-speed response equivalent to MOSFET type gas sensors can be realized.

(3) To realize this structure, the platinum film is formed after the titanium film is formed. Therefore, the titanium film serves as an adhesive and functions to prevent peeling of the platinum film during the element manufacturing process. . Furthermore, by using a platinum film as a mask and annealing in a gas atmosphere in which oxygen gas is dominant, such as oxygen gas mixed with an inert gas in the air, titanium is exposed to the outside through the crystal grain boundary (platinum). It precipitates on the surface of the crystal grains) and oxygen can enter from the platinum grain boundaries to form Pt—Ti—O regions.

(4) By increasing the annealing temperature in oxygen, the interval between platinum microcrystals is several nanometers to several tens of nanometers.
Striped platinum microcrystals with a gap of m are formed on a titanium oxide film (mainly rutile type titanium dioxide,
Anatase crystals or oxygen-doped amorphous titanium may be mixed), so that ammonia (NH ₃ ) other than hydrogen gas, ethanol (C ₂ H ₅ OH), CO, A highly selective gas sensor that also responds to C ₂ H ₄ can be realized. To provide a highly reliable gas sensor that is a Pt / titanium oxide structure that is originally formed from a highly adhesive Pt / Ti structure, and that the platinum film is thick, and is resistant to platinum film peeling. Can do.

  (5) When the EB (electron beam) vapor deposition method is employed, a film having a smaller threshold voltage Vth variation than the sputtering method can be formed, and the variation in the threshold voltage Vth after the heat treatment actually used is reduced. It can be made extremely small.

  Thus, by devising the gate structure and manufacturing method of the Si-MOSFET, it is possible to realize a long life of 3 to 5 years in the Si-MOSFET type gas sensor, which has been said to be difficult to extend the life. ing. Furthermore, the present invention devised a special gate structure of platinum film used for the gate electrode and a manufacturing method thereof, so that the reliability of the gas sensor is remarkably increased, and the hydrogen gas having a high concentration of 1000 ppm or more necessary for danger prediction is obtained. On the other hand, a very fast response speed (for example, 1 second) can be realized. Furthermore, this structure is applied not only to Si-MOSFETs that use platinum films as gate electrodes, but also to Si-MOSFETs that use platinum and palladium films as gate electrodes to form gas sensors that combine both Si-MOSFETs. Thus, for example, highly sensitive detection can be realized in a wide concentration range of 100 ppm to 10%.

  The newly developed hydrogen gas sensor is not toxic by HMDS (hexamethyldisiloxane) gas in operation at a low temperature of 100 ° C. to 110 ° C., and is also a measure of gas interference, such as methane gas, ethane, isooctane, and CO gas. An excellent property of not reacting has been found. This hydrogen gas sensor is large as a semiconductor chip (silicon chip) having a chip size of 2 mm × 2 mm, but realizes an extremely small power consumption of 100 mW. Further, the response region of the hydrogen gas sensor is extremely wide from 100 ppm to 10%, and the result is that the response speed is as high as 2 seconds for hydrogen gas having a high concentration of 1% or more.

  As mentioned above, the invention made by the present inventor has been specifically described based on the embodiment. However, the present invention is not limited to the embodiment, and various modifications can be made without departing from the scope of the invention. Needless to say.

  The present invention can be widely used in manufacturing industries that manufacture gas sensors formed using semiconductor materials.

In the Si-MOSFET type hydrogen gas sensor using the platinum film of the present invention as a gate electrode, it is a schematic diagram showing a gate electrode and a laminated structure immediately below the gate electrode. 5 is a cross-sectional TEM (transmission electron microscope) photograph showing a gate electrode and a laminated structure immediately below the gate electrode in a Si-MOSFET type hydrogen gas sensor using a platinum film as a gate electrode. It is a photograph which shows the result of having observed the morphology observation of the surface after heat processing by SEM (scanning electron microscope). It is a figure which shows the distribution profile of the cross-sectional direction of Pt, Ti, O, Si, and Si (Oxide) by the Auger analysis about the gate structure (gate electrode and laminated structure immediately under it) after heat processing. It is a figure which shows the distribution profile of the cross-sectional direction of Pt, Ti, O, Si, and Si (Oxide) by the Auger analysis about the gate structure (gate electrode and laminated structure immediately under it) before heat processing. In a Si-MOSFET type gas sensor in which a palladium film is used as a gate electrode and this palladium film is formed by a sputtering method, a threshold voltage Vth and a hydrogen response strength ΔVg of the Si-MOSFET type gas sensor are respectively set to a plurality of elements ( It is a figure which shows the result measured with the Si-MOSFET type gas sensor. It is a figure which shows the relationship between the film-forming speed | rate and the variation in threshold voltage, when forming metal films, such as a titanium film and a platinum film, by EB vapor deposition. 5 is a diagram showing a process of manufacturing the Si-MOSFET type gas sensor in the first embodiment. FIG. FIG. 10 is a diagram showing a process of manufacturing a Si-MOSFET type gas sensor according to a modification of the first embodiment. The figure shows the relationship between the hydrogen response strength and the annealing temperature with the titanium film thickness fixed at 5 nm and the platinum film thickness fixed at 15 nm and the annealing time in air fixed at 2 hours. is there. It is a figure which shows the relationship between the hydrogen response intensity | strength in the state which fixed the annealing temperature at 400 degreeC, and annealing time. It is a figure which shows the relationship between hydrogen concentration and a hydrogen response voltage in four different wafers, respectively. It is a figure which shows the current voltage characteristic in 100 degreeC of MOSFET which comprises a hydrogen gas sensor. It is a figure which shows in-plane distribution and reproducibility of the threshold voltage Vth in two wafers from which the lot after heat processing differs. It is a figure which shows in-plane distribution of the normalized hydrogen response output with respect to 1000 ppm air dilution hydrogen gas. It is a figure which shows the result of having investigated the hydrogen response voltage with respect to 1000 ppm air dilution hydrogen gas while changing the annealing time at 400 degreeC while changing the film thickness of a titanium film and a platinum film | membrane. It is a figure which shows the result of having investigated the hydrogen response voltage with respect to 1000 ppm air dilution hydrogen gas while changing the annealing time at 400 degreeC while changing the film thickness of a titanium film and a platinum film | membrane. It is a figure which shows typically the gate structure which implement | achieves a gas sensor with high gas selectivity in the Si-MOSFET type gas sensor which uses platinum for the gate electrode. A cross-sectional TEM photograph of the structure of Pt / Ti / SiO ₂ / Si when annealed at a heat treatment temperature of 800 ° C. for 30 minutes in the air, and Pt / Ti / SiO ₂ / Si before annealing. It is a figure which shows this structure in the right area | region of a photograph. FIG. 10 is a diagram illustrating a process of manufacturing the Si-MOSFET type gas sensor in the second embodiment. It is a figure which shows the process of manufacturing the gas sensor of a Si-MOSFET type following FIG. It is a figure which shows the process of manufacturing the Si-MOSFET type gas sensor following FIG. It is a figure which shows the relationship between the hydrogen response intensity | strength in the state which fixed the annealing time to 30 minutes, and annealing temperature. It is a figure which shows the relationship between the hydrogen response intensity | strength in the state which fixed annealing temperature at 650 degreeC, and annealing time. It is a figure which shows the relationship between the annealing time in various annealing temperature, and a gas response voltage. It is a figure which shows the relationship between the gas response voltage in three types of annealing temperature, and a titanium (Ti) / platinum (Pt) film thickness ratio in the state which fixed annealing time for 30 minutes. 5 is an optical micrograph of a semiconductor chip forming a hydrogen gas sensor in a third embodiment. It is a figure which shows the cross-section of sensor FET formed on the n-type semiconductor substrate. It is a figure which shows the cross-section of reference FET formed on the n-type semiconductor substrate. It is a figure which shows that the relationship between a gate voltage and a source-drain current changes with the presence or absence of the laminated | stacked insulating film (PSG / SiN). It is a top view which shows the typical structure of an EMF type | mold hydrogen gas sensor. It is sectional drawing cut | disconnected by the AA line of FIG. It is a figure which shows the structure of the crystal cell used with the electrolytic crystal growth method. It is a figure which shows the structural example which uses the thin film of platinum as an anode.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Semiconductor substrate 1a Channel region 2 Silicon oxide film 3 Oxygen doped titanium film 4 Titanium oxide microcrystal 5 Platinum microcrystal 6 Crystal grain boundary 7 Grain boundary region 8 Microcrystal 10 Semiconductor substrate 10a N-type semiconductor substrate 11 Local isolation region 11a Local Isolation region 12 Source region 13 Drain region 14 Gate insulating film 15 Gate electrode 16 Insulating film 17 Source electrode 18 Drain electrode 19 Insulating film 20 Wiring 21 Molybdenum silicide film 22 Titanium oxide film 23 Platinum microcrystal 23a Gap 24 Laminated film 25 Thin film 26 Gate Electrode 27 Sensor FET
28 Reference FET
29 heater 30 PN junction diode 31 p-type well 32 p-type well electrode 33 substrate electrode 34 p ⁺ -type semiconductor region 35 n ⁺ -type semiconductor region 36 detection electrode 37 reference electrode 38 solid electrolyte 39 extraction electrode 40 extraction electrode 41 alumina substrate 42 anode 43 Anode 44 Cathode 45 Filter 46 Vacuum cock 47 Glass triangular flask 48 Cylindrical test tube 49 Platinum thin film 50 Ceramic substrate CHP Semiconductor chip Vth Threshold voltage ΔVg Hydrogen response strength Vgs Gate voltage Id Source drain current

Claims (23)

(A) a semiconductor substrate;
(B) a gate insulating film formed on the semiconductor substrate;
(C) an adhesion film formed on the gate insulating film;
(D) a gate electrode formed on the adhesion film;
(E) a source region formed in the semiconductor substrate;
(F) a drain region formed in the semiconductor substrate;
The adhesion film has an oxygen-doped titanium film which is a film formed by mixing oxygen-doped amorphous titanium and amorphous titanium oxide or titanium oxide microcrystals,
The gate electrode has a platinum film formed on the oxygen-doped titanium film,
The platinum film is composed of a plurality of crystal grains, and oxygen and titanium exist in a grain boundary region between the plurality of crystal grains. The gas sensor according to claim 1,
The gas sensor characterized in that the amount of oxygen is introduced into said oxygen-doped titanium film is 10 ²¹ / cm ³ or more. The gas sensor according to claim 1,
The platinum film has a thickness of 1 nm to 30 nm, and the oxygen-doped titanium film has a thickness of 1 nm to 10 nm. The gas sensor according to claim 1,
The gate sensor further includes a palladium film on the platinum film. (A) a lower electrode made of a semiconductor substrate;
(B) a capacitive insulating film formed on the lower electrode;
(C) an adhesion film formed on the capacitive insulating film;
(D) a gas sensor including a capacitive element including an upper electrode formed on the adhesion film,
The adhesion film has an oxygen-doped titanium film which is a film formed by mixing oxygen-doped amorphous titanium and amorphous titanium oxide or titanium oxide microcrystals,
The upper electrode has a platinum film formed on the oxygen-doped titanium film,
The platinum film is composed of a plurality of crystal grains, and oxygen and titanium exist in a grain boundary region between the plurality of crystal grains. The gas sensor according to claim 5, wherein
The gas sensor characterized in that the amount of oxygen is introduced into said oxygen-doped titanium film is 10 ²¹ / cm ³ or more. The gas sensor according to claim 5, wherein
The platinum film has a thickness of 1 nm to 30 nm, and the oxygen-doped titanium film has a thickness of 1 nm to 10 nm. The gas sensor according to claim 5, wherein
The upper electrode further includes a palladium film on the platinum film. Having a first gas sensor and a second gas sensor formed on the same semiconductor substrate;
The first gas sensor
(A) a first gate insulating film formed on the semiconductor substrate;
(B) a first adhesion film formed on the first gate insulating film;
(C) a first gate electrode formed on the first adhesion film;
(D) a first source region formed in the semiconductor substrate;
(E) a first drain region formed in the semiconductor substrate;
The first adhesion film includes a first oxygen-doped titanium film which is a film formed by mixing oxygen-doped amorphous titanium and amorphous titanium oxide or titanium oxide microcrystals,
The first gate electrode has a first platinum film formed on the first oxygen-doped titanium film,
The first platinum film is composed of a plurality of crystal grains, and oxygen and titanium exist in a grain boundary region between the plurality of crystal grains,
The second gas sensor
(F) a second gate insulating film formed on the semiconductor substrate;
(G) a second adhesion film formed on the second gate insulating film;
(H) a second gate electrode formed on the second adhesion film;
(I) a second source region formed in the semiconductor substrate;
(J) a second drain region formed in the semiconductor substrate;
The second adhesion film includes a second oxygen-doped titanium film which is a film formed by mixing oxygen-doped amorphous titanium and amorphous titanium oxide or titanium oxide microcrystals,
The second gate electrode is
(H1) a second platinum film formed on the second oxygen-doped titanium film;
(H2) a palladium film formed on the second platinum film,
The second platinum film is composed of a plurality of crystal grains, and a gas sensor in which oxygen and titanium are present in a grain boundary region between the plurality of crystal grains,
The gas sensor detects a gas having a concentration range of 100 ppm to 10%. (A) a semiconductor substrate;
(B) a gate insulating film formed on the semiconductor substrate;
(C) a titanium oxide film formed on the gate insulating film;
(D) a gate electrode formed on the titanium oxide film;
(E) a source region formed in the semiconductor substrate;
(F) a drain region formed in the semiconductor substrate;
The gate electrode has a plurality of platinum crystal grains in a striped structure,
A gas sensor, wherein a void or an insulating material exists between the plurality of platinum crystal grains. The gas sensor according to claim 10, wherein
The titanium oxide film is a film composed of a rutile titanium oxide microcrystal, a titanium oxide microcrystal film mixed with an anatase structure in addition to a rutile structure, or a titanium oxide microcrystal film mixed with oxygen-doped titanium containing oxygen. A gas sensor comprising any one of the above. The gas sensor according to claim 10, wherein
The gas sensor according to claim 1, wherein the gate electrode has a thickness of 1 nm to 30 nm, and the titanium oxide film has a thickness of 1 nm to 10 nm. (A) a lower electrode made of a semiconductor substrate;
(B) a capacitive insulating film formed on the lower electrode;
(C) a titanium oxide film formed on the capacitive insulating film;
(D) a gas sensor including a capacitive element including an upper electrode formed on the titanium oxide film,
The upper electrode has a striped structure with a plurality of platinum crystal grains,
A gas sensor, wherein a void or an insulating material exists between the plurality of platinum crystal grains. The gas sensor according to claim 13, wherein
The titanium oxide film is a film composed of a rutile titanium oxide microcrystal, a titanium oxide microcrystal film mixed with an anatase structure in addition to a rutile structure, or a titanium oxide microcrystal film mixed with oxygen-doped titanium containing oxygen. A gas sensor comprising any one of the above. The gas sensor according to claim 13, wherein
The gas sensor according to claim 1, wherein the upper electrode has a thickness of 1 nm to 30 nm, and the titanium oxide film has a thickness of 1 nm to 10 nm. The gas sensor according to claim 1,
A heater for heating the gas sensor is further formed on the semiconductor substrate. The gas sensor according to claim 16, wherein
An extraction electrode connected to the source region or the drain region;
The extraction electrode is formed from a laminated film laminated in the order of gold film / molybdenum film from the upper layer,
Further, the heater for heating the gas sensor is formed of a laminated film of molybdenum film / gold film / molybdenum film. (A) forming a gate insulating film on the semiconductor substrate;
(B) after the step (a), forming a titanium film having a thickness of 1 nm or more and 10 nm or less on the gate insulating film;
(C) after the step (b), a step of forming a platinum film on the titanium film,
The platinum film is composed of a plurality of crystal grains, and is a method of manufacturing a gas sensor, wherein the thickness of the platinum film is 1 nm or more and 30 nm or less,
(D) A method of manufacturing a gas sensor, comprising a step of heating the semiconductor substrate at a temperature in the range of 350 ° C. to 475 ° C. in an atmosphere containing oxygen gas after the step (c). A method of manufacturing a gas sensor according to claim 18,
A method of manufacturing a gas sensor, wherein an electron beam evaporation method is used for forming the titanium film in the step (b) and forming the platinum film in the step (c). (A) a semiconductor substrate;
(B) a gate insulating film formed on the semiconductor substrate;
(C) an adhesion film formed on the gate insulating film;
(D) a gate electrode formed on the adhesion film;
(E) a source region formed in the semiconductor substrate;
(F) a drain region formed in the semiconductor substrate;
The adhesion layer comprises an amorphous metal oxygen doping, and fine crystals of amorphous metal oxides or metal oxide, oxygen doping metal film is a film formed by commingled is,
The gate electrode has a platinum film formed on the oxygen-doped metal film,
The platinum film is composed of a plurality of crystal grains, and oxygen and a metal constituting the oxygen-doped metal film exist in a grain boundary region between the plurality of crystal grains,
The gas sensor according to claim 1, wherein the metal is any one of tungsten, molybdenum, tantalum, niobium, and chromium. (A) a lower electrode made of a semiconductor substrate;
(B) a capacitive insulating film formed on the lower electrode;
(C) an adhesion film formed on the capacitive insulating film;
(D) a gas sensor including a capacitive element including an upper electrode formed on the adhesion film,
The adhesion layer comprises an amorphous metal oxygen doping, and fine crystals of amorphous metal oxides or metal oxide, oxygen doping metal film is a film formed by commingled is,
The upper electrode has a platinum film formed on the oxygen-doped metal film,
The platinum film is composed of a plurality of crystal grains, and oxygen and a metal constituting the oxygen-doped metal film exist in a grain boundary region between the plurality of crystal grains,
The gas sensor according to claim 1, wherein the metal is any one of tungsten, molybdenum, tantalum, niobium, and chromium. An electrode formed on a base made of an insulating material or a semiconductor material,
The electrode is
(A) The film formed on the base and formed by mixing oxygen-doped amorphous metal and amorphous metal oxide or metal oxide microcrystal An oxygen-doped metal film,
(B) having a platinum film formed on the oxygen-doped metal film,
The platinum film is composed of a plurality of crystal grains, and oxygen and a metal constituting the oxygen-doped metal film exist in a grain boundary region between the plurality of crystal grains,
The electrode is characterized in that the metal is any one of titanium, tungsten, molybdenum, tantalum, niobium, and chromium. A gas sensor manufacturing method for forming a sensor FET that responds to hydrogen gas and a reference FET that does not respond to hydrogen gas on the same semiconductor substrate,
(A) forming a gate insulating film on the semiconductor substrate;
(B) after the step (a), forming a titanium film having a thickness of 1 nm or more and 10 nm or less on the gate insulating film;
(C) after the step (b), a step of forming a platinum film composed of a plurality of crystal grains and having a film thickness of 1 nm to 30 nm on the titanium film;
(D) after the step (c), patterning the platinum film to form a first gate electrode of the sensor FET and a second gate electrode of the reference FET;
(E) after the step (d), heating the semiconductor substrate in an atmosphere containing oxygen gas at a temperature in the range of 350 ° C. to 475 ° C .;
(F) After the step (e), a step of forming an insulating film covering the first gate electrode and the second gate electrode;
(G) After the step (f), patterning the insulating film to expose the first gate electrode while leaving the insulating film covering the second gate electrode remaining. A method of manufacturing a gas sensor.