JP4906032B2 - Semiconductor gas sensor - Google Patents

Description

  The present invention relates to a semiconductor gas sensor that is small and has low power consumption.

  Conventionally, a Schottky diode type gas sensor element based on SiC has been proposed (see, for example, Patent Document 1).

US Pat. No. 6,291,838

  In general, a semiconductor gas sensor is operated at a high temperature in order to activate a desorption / adsorption reaction of a detection gas.

  The gas sensor element of Patent Document 1 must be operated at a high temperature during actual operation. However, since the heater function is not given to the gas sensor element of Patent Document 1, in the gas detection system using the gas sensor element of Patent Document 1, A separate heater must be arranged in the vicinity of the gas sensor element, and the entire system must be large.

  In addition, in the conventional gas detection system, the loss in heat transfer due to the positional relationship between the gas sensor element and the heater is large, and the temperature rise efficiency is good for the electric power for heating the heater. It ’s not good.

  Accordingly, an object of the present invention is to provide a semiconductor gas sensor capable of reducing power consumption.

  According to the present invention: a heater made of a first semiconductor single crystal; first and second electrodes connected to the heater; made of a second semiconductor single crystal and formed on at least a part of the surface of the heater A semiconductor comprising: a stacked semiconductor thin film portion; a third electrode made of a catalytic metal connected to the semiconductor thin film portion; and a fourth electrode connected to the semiconductor thin film portion. A gas sensor is obtained.

As a material of the heater (first semiconductor single crystal), for example, Si, GaAs, SiGe, GaN, ZnO, or Al ₂ O ₃ can be used. Among these, considering the ease of processing and thermal conductivity, Si is preferable as the heater material.

As the semiconductor thin film portion (second semiconductor single crystal), for example, SiC, GaAs, SiGe, GaN, or ZnO can be used. The condition required for the second semiconductor single crystal is that it has a large band gap, can be operated at a high temperature, and is chemically stable. More specifically, it has a band gap of 2.0 eV or more, can be used at 300 ° C. or more, and is compared with a metal oxide that causes a composition change by reacting a detection gas such as NO _X or SO _X with the base material itself. The reaction between the gas and the solid is preferably a single crystal limited to the extreme surface layer. In addition, it is desirable that a single crystal film serving as a high-quality reaction film can be formed on silicon having excellent workability. Considering such conditions, SiC is appropriate as a material for the semiconductor thin film portion.

  That is, the first and second semiconductor single crystals may be made of the same material, but in the application of the present invention, the combination of the first and second semiconductor single crystals is preferably Si and SiC.

  According to the present invention, the semiconductor thin film portion is laminated on the heater, and the gas sensor element portion is configured by the semiconductor thin film portion, the third electrode, and the fourth electrode. The gas sensor element portion can be heated almost directly, and high temperature rise efficiency can be obtained.

  In particular, when the heater is made into a beam-like structure using a support member that supports the heater, the heater is in a state of being thermally separated from the support member and others except for the support point of the heater by the support member. This heat can be transferred to the gas sensor element portion more efficiently.

  Hereinafter, embodiments of the present invention will be specifically described with reference to the drawings.

  As shown in FIGS. 1 to 3, the gas sensor according to the embodiment of the present invention is processed using an SOI (Silicon-on-Insulator) substrate composed of a silicon substrate 1, an insulating layer 2, and a silicon single crystal layer 3 as a base material. Is obtained. Among these, the silicon single crystal layer 3 is also called an SOI layer. The SOI substrate in this embodiment has a buried oxide film (BOX layer) as the insulating layer 2, and the conductivity types of the silicon substrate 1 and the silicon single crystal layer 3 are p-type. The silicon single crystal layer 3 has a thickness of about 1 μm and a resistivity of about 10 Ωcm.

  In the central portion of the SOI substrate in the present embodiment, a hole is formed that passes from the back surface of the silicon substrate 1 to the silicon single crystal layer 3. More specifically, a relatively large hole 30 is formed from the back surface of the silicon substrate 1 to the insulating layer 2, and the two holes 31 and 32 are formed so as to communicate with the hole 30. It is formed in the crystal layer 3. The two holes 31 and 32 are arranged substantially symmetrically with a line (II-II line in FIG. 1) passing through the center of the SOI substrate interposed therebetween, and the above-described “line passing through the center ( A heater 4 extending on the hole 30 along the line II-II) ”is provided. That is, in the gas sensor according to the present embodiment, the heater 4 has a beam-like structure. Therefore, the heater 4 is effectively in a state of being thermally separated from the SOI substrate.

  The heater 4 is obtained by processing an n-type region formed in the p-type silicon single crystal layer 3 and has a resistivity of about 6 Ωcm. Since the heater 4 is n-type and the silicon single crystal layer 3 is p-type, when the heater 4 is energized, the heater 4 and the silicon single crystal layer 3 are separated from each other by a pn junction.

  Low resistance regions 5, 6, and 7 are formed inside the heater 4. Among these, the low resistance regions 5 and 6 are located near the end of the heater 4 region, and the low resistance region 7 is located in the center.

  First and second electrodes 9 and 10 for flowing current to the heater 4 are formed on the low resistance regions 5 and 6. In other words, the low resistance regions 5 and 6 are regions for ohmically connecting the first and second electrodes 9 and 10 to the heater 4.

  On the low resistance region 7, a Schottky diode that functions as a gas sensor element portion is formed. In the present embodiment, the semiconductor side of the Schottky diode and the heater 4 use the second electrode 10 in common. At this time, the low resistance region 7 makes an ohmic junction between the semiconductor side of the Schottky diode and the second electrode 10. The semiconductor-side electrode of the Schottky diode may be formed separately from the second electrode 10 of the heater 4.

  Specifically, a semiconductor thin film portion 11 is formed on the low resistance region 7, and a third electrode 12 that forms a Schottky junction with the semiconductor thin film portion 11 is formed on the semiconductor thin film portion 11. The semiconductor thin film portion 11 in the present embodiment is made of a non-metal oxide single crystal, specifically, a SiC single crystal. The film thickness of the entire semiconductor thin film portion 11 is about 0.5 μm, and the conductivity type is n-type. The semiconductor thin film portion 11 in the present embodiment has a two-layer structure of a lower layer portion and an upper layer portion. The lower layer is a low resistivity layer (about 0.02Ω) having a thickness of about 0.1 μm, and the upper layer is a high resistivity layer (about 10 Ωcm). The resistivity of the lower layer is preferably 0.1 Ωcm or more and 0.01 Ωcm or less in order to form an ohmic junction with the second electrode 10, and the resistivity of the upper layer constitutes a Schottky junction with the third electrode 12. Therefore, it is preferably 1 Ωcm or more and 100 Ωcm or less.

  Here, the film thickness of the semiconductor thin film portion 11 is desirably in the range of 10 nm or more and 500 μm or less. The thinner the thin film portion 11 is, the easier it is to raise the temperature of the gas sensor element portion, so that the power consumption can be reduced. In particular, when a gas sensor that can be used for a long time by battery driving is to be constructed, it is desirable that the film thickness of the semiconductor thin film portion 11 be 500 μm or less in view of power versus temperature rise efficiency. On the other hand, if the film thickness of the semiconductor thin film portion 11 is smaller than 10 nm, it becomes impossible to operate as a sensor due to the tunnel effect, so the film thickness of the semiconductor thin film portion 11 must be 10 nm or more.

  The third electrode 12 formed on the semiconductor thin film portion 11 is made of a catalytic metal that promotes dissociation of the gas to be detected and oxidation-reduction reaction. The catalyst metal in the present embodiment, that is, the material of the third electrode 12 is Pt, and the thickness of the third electrode 12 is about 30 nm. As a material (catalyst metal) of the third electrode 12, in addition to Pt, Pd, Ni which is a transition metal, or a mixture thereof can be used. The thickness of the third electrode 12 is preferably 10 nm or more and 100 nm or less in order to improve the gas detection sensitivity.

  In the present embodiment, a base film is formed between the semiconductor thin film portion 11 and the third electrode 12 to improve the adhesion between them. The material of the base film in the present embodiment is Ti. This Ti can also be used as a base film material when Ni is used as the catalyst metal. Instead of Ti, TiW can also be used as a base film.

In the gas sensor described above, all of the top surface, side surface, and bottom surface are thermally oxidized films (specifically, SiO ₂ films) except for the first, second, and third electrodes 9, 10, 12 and the holes 30, 31, 32. 8 is covered. Although the thermal oxide film 8 is omitted in FIG. 1, the lead line 13 and the electrode pad 14 connected to the third electrode 12 are formed on the thermal oxide film 8. For easy understanding of the drawings, the lead lines 13 and the electrode pads 14 are omitted in FIGS. 2 and 3.

  In the gas sensor described above, since the semiconductor thin film portion 11 constituting the gas sensor element portion is directly formed on the heater 4, the gas sensor element portion can be efficiently heated by the heater 4. Therefore, the efficiency of raising the temperature of the gas sensor element portion with electric power for heating the heater 4 (power versus temperature raising efficiency) is excellent.

  In addition, since the film thickness of the semiconductor thin film portion 11 is about 0.5 μm and extremely thin, it is possible to suppress the amount of heat required for raising the temperature of the gas sensor element portion, and as a result, further excellent power versus temperature raising efficiency is achieved. can do.

  In addition, as the semiconductor thin film portion 11 of the gas sensor element portion, SiC, which is a material that has a large band gap and can be operated at a high temperature and is extremely stable even in a strong oxidation-reduction atmosphere, is easy to process as the heater 4 and has a thermal conductivity of SiC. Since Si, which is a high material, is used, a relatively inexpensive and high-performance gas sensor can be obtained.

  Note that in a gas sensor including a gas sensor element and a heater, the temperature of the heater 4 must be measured when the gas sensor is operating. In the present embodiment, the temperature sensor for measuring the temperature of the heater 4 is configured using the temperature dependence of the resistor of the heater 4. That is, the heater 4 and the temperature sensor are the same as an object, and while the heater 4 is energized to generate heat, the temperature of the heater 4 is specified by checking the resistance value of the heater 4 that changes according to the heat change. To do. As described above, in the present embodiment, among the main components of the gas sensor system, the gas sensor element, the heater, and the temperature sensor are integrally configured to form a very small element of 100 μm □ or less. it can.

  In addition, it can replace with what was mentioned above and can also be set as the resistance temperature measuring body which consists of Pt thin films. Even in that case, the temperature sensor is preferably provided at least in the vicinity of the semiconductor thin film portion 11. As another example of the temperature sensor, for example, there is a method in which a pn junction is formed in a region close to the semiconductor thin film portion 11 inside the heater 4 and the temperature dependency of the voltage-current characteristics of the pn junction is used. . As a further application of the temperature sensor, it is possible to arrange a plurality of temperature sensors, measure the detailed temperature distribution of the heater 4, and detect the gas from the change in the temperature rise pattern.

  In the semiconductor gas sensor according to the present embodiment described above, the Schottky diode that is the gas sensor element portion is electrically connected to the substantially central portion of the heater 4 that is a resistor. Accordingly, the heater 4 is partially interposed between the second electrode 10 which is a common electrode of the Schottky diode and the heater 4 and the Schottky diode. However, in order to further improve the gas detection sensitivity, theoretically, it is better not to electrically interpose the heater 4 (resistor) between the second electrode 10 and the Schottky diode. Therefore, when there are margins in conditions such as manufacturing costs, the heater 4 may be electrically separated from the path connecting the semiconductor thin film portion 11 of the Schottky diode and the second electrode 10. Examples of the separation method include pn separation and a method of drawing a line from the semiconductor thin film portion 11 onto the thermal oxide film 8 and connecting the drawn line to the second electrode 10 on the thermal oxide film 8. is there.

  Hereinafter, the manufacturing method of the semiconductor gas sensor according to the above-described embodiment will be specifically described with reference to FIGS. 4 to 11 are sectional views taken along the line II-II in FIG. 1 as in FIG. 2, and FIGS. 12 and 13 are taken along the line III-III in FIG. 1 as in FIG. FIG.

  First, with respect to a p-type SOI substrate (silicon substrate 1, insulating layer 2, and silicon single crystal layer 3) having a plane orientation (100) as shown in FIG. 1, as shown in FIG. An n-type heater 4 region is formed by an implantation method.

  Next, as shown in FIG. 6, an n-type high concentration region is formed as a low resistance region 7 in the heater 4. The low resistance region 7 is a region slightly wider than the region where the SiC single crystal thin film as the semiconductor thin film portion 11 is patterned.

  Thereafter, as shown in FIG. 7, a SiC single crystal thin film 11a is formed on the heater 4 by molecular beam epitaxy (MBE). As a technique for forming a SiC single crystal thin film on a silicon single crystal, a technique disclosed in “Method for forming a silicon carbide single crystal thin film (Japanese Patent Laid-Open No. 2000-264792)” developed by one of the inventors of the present invention is available. is there. As a result of research on this technology, it was found that a good quality ultra-thin film of SiC single crystal that could not be realized by other methods can be directly formed on the silicon single crystal. did.

Specifically, the substrate shown in FIG. 6 was placed in a vacuum vessel (reaction vessel), and SiC single crystal thin film 11a was grown using monomethylsilane (MMS) as a reaction gas. At that time, as the main growth conditions of the SiC single crystal thin film 11a, the partial pressure of monomethylsilane was about 5 × 10 ⁻⁵ Torr, and the substrate temperature was about 950 ° C. Further, a predetermined amount of nitrogen gas was introduced into the vacuum vessel at a predetermined timing as an n-type impurity dopant during growth, and the resistivity of the lower layer portion and the upper layer portion of the SiC single crystal thin film 11a was controlled. Here, the SiC single crystal thin film 11a produced on the Si substrate is cubic 3C-SiC, the surface defect density is about 10 ⁵ / cm ² or less, and the surface roughness is Ra: about 100 nm or less. It was a thin film. The SiC single crystal thin film 11a may have a hexagonal 4H—SiC or 6H—SiC crystal structure, but cubic 3C—SiC having no micropipes that cause a leakage current is preferable.

Regarding the growth conditions, for example, the substrate temperature is preferably in the range of about 600 to 1100 ° C. Further, instead of monomethylsilane, for example, a compound containing at least carbon atoms and silicon atoms such as tetramethylsilane and dimethylsilane can be used. Further, N × 10 ^−X Torr (1 ≦ N <10, 4 ≦ X ≦ 6 (X is a natural number)) can be selected as the partial pressure of the compound. The purpose of controlling the resistivity of the SiC single crystal thin film 11a by introducing the impurity dopant is to set the resistivity of the upper layer portion and the lower layer portion of the semiconductor thin film portion 11 to the appropriate values described above. Therefore, the SiC single crystal thin film 11 a is controlled so as to have a low resistivity at a position close to the heater 4 and to have a high resistivity at a position close to the third electrode 12. The SiC single crystal thin film 11a in the present embodiment has a two-layer structure of an upper layer portion and a lower layer portion, but forms an ohmic junction with the heater 4 and a Schottky junction with the third electrode. The SiC single crystal thin film 11a may have, for example, a structure in which the resistivity gradually changes.

Next, as shown in FIG. 8, in order to obtain a semiconductor thin film portion 11 having a desired shape from the SiC single crystal thin film 11a, a mask is selectively formed with a photoresist, and unnecessary portions are formed by RIE (Reactive-Ion-Etching). Remove. As the etching gas, a mixed gas of SF ₆ and O ₂ can be used.

  Thereafter, as shown in FIG. 9, heat treatment is performed in a WET or DRY oxygen atmosphere to form a thermal oxide film 8 of about 0.1 μm.

  Further, as shown in FIG. 10, first and second electrodes 9 and 10 are formed. Specifically, first, the thermal oxide film 8 on the low resistance regions 5 and 6 is removed by HF or the like using a photoresist as a mask. Next, using the holes formed in the thermal oxide film 8, the low resistance regions 5 and 6 are formed by a thermal diffusion method or an ion implantation method. Further, an electrode material such as an aluminum or nickel thin film having a thickness of about 0.5 μm is deposited by vacuum vapor deposition or magnetron sputtering. Here, in order to stabilize the low resistance regions 5 and 6, it is desirable to perform a heat treatment at about 450 ° C. Thereafter, the electrode material is patterned using an appropriate etchant using a photoresist as a mask to obtain first and second electrodes 9 and 10 as shown in FIG.

  Subsequently, the upper portion of the semiconductor thin film portion 11 in the thermal oxide film 8 is removed by etching, and the third electrode 12 is formed on the exposed semiconductor thin film portion 11. Specifically, Ti is first deposited as a base film on the semiconductor thin film portion 11 by a magnetron sputtering method or the like, and then Pt or Ni which is a catalyst metal is deposited by a magnetron sputtering method or the like. Thereafter, etching is performed and patterning is performed to form the third electrode 12.

  Here, a so-called lift-off method can be applied to the patterning and formation of the third electrode 12. That is, it is possible to apply a method in which a photoresist is patterned in advance, a Pt thin film is deposited thereon, and then the excess Pt thin film is peeled off together with the photoresist, that is, a lift-off method.

  Next, in order to thermally separate the heater 4 and the SOI substrate that is a support member of the heater 4, a desired region of the thermal oxide film 8 is removed by HF using a photoresist mask, and the thermal oxide film 8. Then, the silicon single crystal layer 3 is removed by a wet etching method such as KOH or a dry etching method such as RIE (see FIG. 12).

Subsequently, as shown in FIG. 13, the insulating layer 2 is removed by HF. Thereafter, anisotropic etching of Si using the insulating layer (SiO ₂ ) 2 as an etching stopper is performed from the back surface of the silicon substrate 1 to obtain a heater 4 having a beam structure as shown in FIGS.

  In the semiconductor gas sensor according to the present embodiment described above, a Schottky diode (gas detector: gas sensor element) made of Pt / SiC, a heater 4 and a temperature sensor are built in the sensor substrate. For this reason, the power required to raise the temperature of the semiconductor gas sensor to an operating temperature of 450 ° C. or higher at which the desorption / adsorption reaction of the gas to be detected is activated is about several milliwatts. The semiconductor type gas sensor can be realized.

1 is a perspective view schematically showing a semiconductor gas sensor according to an embodiment of the present invention. It is II-II sectional drawing of the semiconductor type gas sensor shown by FIG. It is III-III sectional drawing of the semiconductor type gas sensor shown by FIG. It is II-II sectional drawing which shows the manufacturing process of the semiconductor type gas sensor shown by FIG. It is II-II sectional drawing which shows the manufacturing process of the semiconductor type gas sensor shown by FIG. It is II-II sectional drawing which shows the manufacturing process of the semiconductor type gas sensor shown by FIG. It is II-II sectional drawing which shows the manufacturing process of the semiconductor type gas sensor shown by FIG. It is II-II sectional drawing which shows the manufacturing process of the semiconductor type gas sensor shown by FIG. It is II-II sectional drawing which shows the manufacturing process of the semiconductor type gas sensor shown by FIG. It is II-II sectional drawing which shows the manufacturing process of the semiconductor type gas sensor shown by FIG. It is II-II sectional drawing which shows the manufacturing process of the semiconductor type gas sensor shown by FIG. FIG. 3 is a III-III sectional view showing a manufacturing process of the semiconductor gas sensor shown in FIG. 1. FIG. 3 is a III-III sectional view showing a manufacturing process of the semiconductor gas sensor shown in FIG. 1.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Silicon substrate 2 Insulating layer 3 Silicon single crystal layer 4 Heater 5 Low resistance area 6 Low resistance area 7 Low resistance area 8 Thermal oxide film 9 1st electrode 10 2nd electrode 11 Semiconductor thin film part 12 3rd electrode 13 Drawer Wire 14 Electrode pad 30 holes 31 holes 32 holes

Claims (12)

A heater composed of a first semiconductor single crystal;
First and second electrodes connected to the heater;
A semiconductor thin film portion made of a second semiconductor single crystal and laminated on at least a part of the surface of the heater;
A third electrode made of a catalytic metal connected to the semiconductor thin film portion;
A semiconductor gas sensor comprising a fourth electrode connected to the semiconductor thin film portion ,
A temperature sensor for measuring the temperature of the heater, which is integrally formed with the heater;
The semiconductor type gas sensor according to claim 1, wherein the temperature sensor is configured using temperature dependency of an electric resistance value of the heater. 2. The semiconductor gas sensor according to claim 1 , wherein a Schottky barrier is formed at an interface between the third electrode and the semiconductor thin film portion. 3. The semiconductor gas sensor according to claim 1 , wherein the semiconductor thin film portion is made of SiC. In the semiconductor type gas sensor according to any one of claims 1 to 3, a semiconductor type gas sensor, wherein the first semiconductor single crystal is a silicon single crystal. The semiconductor type gas sensor according to any one of claims 1 to 4 ,
A support member that is integrally formed with the heater and supports the heater;
The semiconductor gas sensor, wherein the heater has a beam-like structure. The semiconductor gas sensor according to claim 5 , wherein
The support member is an SOI (Silicon-on-Insulator) substrate composed of a substrate having at least an insulating layer and a silicon single crystal layer formed on the insulating layer,
The heater may be obtained by processing the silicon single crystal layer of the SOI substrate so as to have the beam-like structure with respect to the SOI substrate having a hole penetrating from the back surface of the substrate to the silicon single crystal layer. A semiconductor type gas sensor characterized by being a thing. In the semiconductor type gas sensor according to any one of claims 1 to 6, a semiconductor type gas sensor, wherein the thickness of the semiconductor thin film portion is 10nm or more 500μm or less. In the semiconductor type gas sensor according to any one of claims 1 to 7, a semiconductor gas sensor in which the first or second electrode is characterized in that also serves as the fourth electrode. The semiconductor gas sensor according to any one of claims 1 to 8 , wherein the semiconductor thin film portion includes a low resistivity portion and a high resistivity portion, and the low resistivity portion is formed on the heater surface. The semiconductor gas sensor, wherein the high resistivity part is formed on the low resistivity part, and the third electrode is formed on the high resistivity part. In the semiconductor type gas sensor according to any one of claims 1 to 9, the base film to improve the adhesion between the third electrode and the semiconductor thin film portion and the semiconductor thin film portion and the third electrode A semiconductor gas sensor, further comprising a gap therebetween. A heater composed of a first semiconductor single crystal;
First and second electrodes connected to the heater;
A semiconductor thin film portion made of a second semiconductor single crystal and laminated on at least a part of the surface of the heater;
A third electrode made of a catalytic metal connected to the semiconductor thin film portion;
A fourth electrode connected to the semiconductor thin film portion;
In a semiconductor gas sensor characterized by comprising:
A support member that is integrally formed with the heater and supports the heater;
The heater has a beam-like structure,
The support member is an SOI (Silicon-on-Insulator) substrate composed of a substrate having at least an insulating layer and a silicon single crystal layer formed on the insulating layer,
The heater may be obtained by processing the silicon single crystal layer of the SOI substrate so as to have the beam-like structure with respect to the SOI substrate having a hole penetrating from the back surface of the substrate to the silicon single crystal layer. Is a thing
A semiconductor type gas sensor. A heater composed of a first semiconductor single crystal;
First and second electrodes connected to the heater;
A semiconductor thin film portion made of a second semiconductor single crystal and laminated on at least a part of the surface of the heater;
A third electrode made of a catalytic metal connected to the semiconductor thin film portion;
A fourth electrode connected to the semiconductor thin film portion;
In a semiconductor gas sensor characterized by comprising:
The semiconductor gas sensor, wherein the first or second electrode also serves as the fourth electrode.

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