JPH0358060B2 - - Google Patents

Description

[Detailed description of the invention] The present invention uses monosilane (SiH ₄ ), phosphine (PH ₃ ), arsine (AsH ₃ ), carbon monoxide (CO),
The present invention relates to a reducing gas sensor that detects so-called reducing gases such as hydrogen (H ₂ ). For example, many gases used in semiconductor manufacturing processes are generally highly toxic or explosive to the human body. Reducing gas sensors are mainly used as detection means for warning of leakage of these gases.

Conventionally, this type of gas detection means utilizes the principles of photoionization detection method, flame photometry method, and atomic absorption method, but all of these methods are extremely expensive, and if Installing such detection means in a computer requires a considerable investment in equipment, and the response is generally slow (20 to 120
Sec.), which also has the disadvantage of being insufficient in terms of risk prevention. On the other hand, recently, inexpensive detection sensors using semiconductor sintered bodies have been commercially available, but they generally have low selectivity to gases, and the PH ₃ ,
It has not been applied to semiconductor process gases such as AsH ₃ and SiH ₄ , and since its basic principle essentially utilizes the properties of semiconductors, it is easily affected by temperature and humidity. There are many flaws.

In view of these points, the present invention provides an extremely useful reducing gas sensor that can be mass-produced at low cost, has a quick response, and is not easily affected by temperature and humidity.

Next, embodiments of the present invention will be described based on the drawings.
FIG. 1 is an overall plan view of the reducing gas sensor, FIG. 2 is a side sectional view, and FIG. 3 is a sectional view taken along line A-A in FIG. A metal thin film 2 for forming an electrode, an insulating thin film or a semiconductor thin film 3, and a metal thin film 4 for forming an electrode are formed in a layered manner. A bonding pad 5 is formed by extending the uppermost metal thin film 4 to one end of the base 1, and an electrode lead wire 6 is bonded to this portion. 7 is an electrode lead wire bonded to the metal thin film 2 on the bottom layer; 8
is a molding resin that covers the entire structure except for the uppermost metal thin film 4 that comes into contact with the reducing gas.

The substrate 1 functions to facilitate the formation of a thin film and to increase the mechanical strength of the sensor. Therefore, from this point of view, the thickness of the base 1 is preferably about 0.3 to 3 mm. Specifically, it is preferable to use glass plates, quartz plates, alumina ceramics, silicon crystals, copper, nickel, gold, platinum, titanium, tantalum, molybdenum, niobium, aluminum, palladium, alloys, etc. as materials constituting the base 1. . The metal thin film 2 can be made of, for example, gold, platinum, titanium, tantalum, molybdenum, niobium, aluminum, palladium, an alloy, etc., and preferably has a thickness of several microns.

The same metals listed for the metal thin film 2 can be used for the uppermost metal thin film 4 as well. In that case, it may be made of the same kind of metal as the metal thin film 2,
Alternatively, it may be made of a different metal. but,
The thickness must be such that the work function of the metal film changes upon contact with the reducing gas, and as a result, changes in the potential barrier of the metal-insulator junction can be sensitively detected. Usually, the thickness is preferably about 100 Å to 1 micron.

The insulating thin film or semiconductor thin film 3 is made of TiO ₂ ,
SiO ₂ , Al ₂ O ₃ , Ta ₂ O ₅ , Si ₃ N ₄ , plasma polymerized film,
It can be made of an insulating material such as phthalocyanine or a semiconductor material, but the film thickness needs to be thin enough to produce a tunnel effect. That is, the tunnel effect is a phenomenon in which particles with kinetic energy lower than the height of a potential barrier penetrate through the potential barrier and move to the other side. The degree of particle movement is related to the height and width of the potential barrier, as discussed below. Therefore, in the case of the present invention, the thickness of the insulating thin film or semiconductor thin film must be made sufficiently thin in order to obtain a sufficient tunneling current value and to sensitively detect changes in the work function. This film thickness may normally be in the range of 20 Å to 1 μm. The reason why not only an insulating thin film but also a semiconductor thin film can be used as the thin film 3 is that semiconductors can also be used as long as they meet the necessary conditions to produce a tunnel effect. , it is not intended to utilize the general characteristics of semiconductors. In other words, it is sufficient that the potential barrier peak rises sharply and the height thereof can be changed sharply depending on the gas concentration. The bonding pad 5 can be made of the same metal as the metal thin film 4 described above. However, the thickness must be such that bonding of the lead wire 6 is possible and the current loss can be sufficiently reduced, usually several microns. The insulating thin film or semiconductor thin film portion 9 located below the bonding pad 5 can be made of the same material as the thin film 3, but it needs to be sufficiently thick, usually several μm. This is to prevent unnecessary current from flowing in areas other than the gas contact surface due to the tunnel effect. As the molding resin 8, epoxy resin or the like can be used. By covering the parts other than the gas contact surface, the resin 8 acts to prevent parts other than the sensing part from being affected by atmospheric gas or humidity. As the lead wires 6 and 7, conductive wires such as gold or copper that can be easily bonded and have low current loss are used.

Next, the operation of the gas sensor will be explained. First of all,
When gas does not come into contact with the metal thin film 4 and there is no bias between the metal thin films 2 and 4,
The energy state is as shown in FIG. In the diagram
Ef is the Fermi level, ₁ is the height of the potential barrier of the metal thin film 4, and ₂ is the height of the potential barrier of the metal thin film 2. Note that this figure shows the energy state when the metal thin films 2 and 4 are of different metals, but when they are of the same type of metal, it can be considered in the same way as ₁ = ₂ .
Next, when a bias is applied between the metal thin films 2 and 4 in the above state as shown in FIG. 5a, the fermi level Ef on the metal thin film 4 side is pushed up, resulting in an energy state as shown in FIG. 5b.

The tunnel current J ₁ flowing between the two electrodes in this state is given by the following equation.

J ₁ = (e/4πhs ² ) {( ₁ + ₂ − eV) exp [
-4πsm ^1/2 /h) ( ₁ + ₂ -eV) ^1/2 ] - ( ₁ + ₂
+eV) exp[-(4πsm ^1/2 /h)( ₁ + ₂ +e
V) ^1/2 ]}...(1) Next, in this state, when the top metal thin film 4 is brought into contact with a reducing gas as shown in FIG. Since oxygen is taken away by the reducing gas, the work function of the metal thin film 4 decreases, and the potential barrier increases as shown in Figure b.
It goes down from ₁ to ₁ ′. If the current at this time is _J2 , it is expressed by the following equation.

J ₂ = (e/4πhs ² ) {( ₁ ′+ ₂ −eV) × ex
p [(-4πsm ^1/2 /h) ( ₁ '+ ₂ -eV) ^1/2 ]-(
₁ ′+ ₂ +eV) ×exp[−(4πsm ^1/2 /h)( ₁ ′+
₂ +eV) ^1/2 ]}...(2) However, it is assumed that the bias voltage V is in the range of 0<V< ₂ /e. Further, e is the charge of the electron, h is Planck's constant, S is the thickness of the insulating layer, and m is the mass of the electron. In the above equation, everything other than ₁ ' is considered to be a constant; on the other hand, ₁ ' changes due to the contact of the reducing gas with the metal thin film 4, and the more the amount of gas in contact with the metal thin film 4, the more the change in ₁ ' increases. Since this is large, the concentration of the reducing gas can be detected by detecting the difference ΔJ=J ₁ −J ₂ (3) between equation (1) and equation (2). Although a detailed explanation will be omitted, even if the bias voltage is applied with a polarity opposite to that described above, the gas concentration can be similarly detected by the tunnel effect.

Figures 7 to 9 show measured data showing the characteristics of the gas sensor. FIG. 7 shows the relationship between the bias voltage and the low resistivity of the insulating thin film in the tunnel effect. This data includes titanium (several μm thick) as the metal thin film 2, gold (200 Å thick) as the metal thin film 4,
Using TiO ₂ as the insulating thin film 3, at 25℃ in air
This is what was actually measured under. FIG. 8 shows actual measured current values when monosilane (SiH ₄ ) gas was varied from 0 to 100 ppm in air. Figure 9 shows a sensor placed in a container filled with air and monosilane applied.
This shows the response characteristics of the sensor when an amount equivalent to 100ppm was collected with a syringe and injected into a container. However, the inside of the container was stirred using a fan.

FIG. 10 shows the configuration when the above sensor is used as an alarm; in the figure, 11 is a reducing gas sensor;
2 is a bias power supply for the sensor, 16 is a preamplifier,
17 is a meter driver, and 18 is a level meter consisting of an LED. The gas concentration detected by the sensor 11 is roughly indicated by this level meter 18. 19 is a reference voltage set to an alarm level, 20 is this reference voltage 19 and sensor 11
21 is a timer, 22 is a matching circuit, 23 is a buzzer, and 24 is an external circuit such as a relay contact. This prevents alarms from being issued by mistake. After the signal appears once, if the signal continues to appear after a certain period of time set by the timer 21, it is determined to be a true signal and an alarm is issued.

11 and 12 each show an example of installing a gas sensor, and FIG. 11 is an example of use as a duct alarm. That is, in this case, the gas in the duct 25 is sampled by the duct probe 26 and supplied to the gas sensor 11 through the protective filter 27. In the figure, 28 is a capillary for flow rate control, and 29 is a suction pump. Figure 12 shows an example of installing a gas sensor in a monosilane line system.
is a monosilane cylinder, 31 is a doping device, 3
2 is an exhaust gas treatment device. The gas sensor 11 detects the gas concentration in the cylinder store 33, the clean room 34, and the duct 25, respectively.

In the above embodiment, on the substrate, (a) a metal thin film several μm thick (forming an electrode), (b) an insulating thin film or semiconductor thin film 20 Å to 1 μm thick, and (c) gold or platinum or A thin metal film (forming an electrode) made of palladium with a thickness of 100 Å to 1 μm is formed in layers, but if the base is metal and is used as an electrode, then (a ) An insulating or semiconductor thin film with a thickness of 20 Å to 1 μm (b) A metal thin film (forming an electrode) with a thickness of 100 Å to 1 μm made of gold, platinum, or palladium may be formed in a layered manner. One example is shown in the 13th
As shown in the figure. In the figure, 100 is a metal that serves as a base and a second electrode, and is equipped with a lead 101;
102 is an insulating thin film or a semiconductor thin film, and 103 is a metal thin film constituting the first electrode.

Since the present invention has the above-described configuration, it has the following effects.

Since a base material such as an insulating material is used and a thin film is formed on the base material, the sensor can be manufactured by a so-called IC manufacturing process, and mass production becomes possible.

Unlike conventional semiconductor type detection sensors, variations in characteristics and deterioration phenomena do not occur.

As shown in FIG. 9, since the response is quick, danger prevention measures can be taken effectively, and this is extremely useful as a detection means for a gas leak alarm.

It is very easy to use because gas detection can be performed even if the bias voltage applied between two metal thin films is of opposite polarity.

[Brief explanation of drawings]

The drawings show one embodiment of the present invention, in which Fig. 1 is an overall plan view, Fig. 2 is an overall side sectional view, Fig. 3 is a sectional view taken along line A-A in Fig. 1, and Figs. 4 to 6 are Figures 7 to 9 are diagrams explaining the operating principle, Figures 7 to 9 are diagrams showing each characteristic of the sensor, Figure 10 is a block diagram showing an example of the configuration of an alarm device using the sensor of the present invention, Figures 11 and 12
The figure shows an installation example of the sensor of the present invention, and FIG. 13 is a sectional view of a main part showing another embodiment of the present invention. 1,100...Substrate, 2...Metal thin film, 3,102
...Insulating thin film or semiconductor thin film, 4,103...
Top layer metal thin film.

Claims (1)

[Scope of Claims] 1. An insulating material, a semiconductor, or a metal as a base, and on top of it, in order: (a) an electrode made of a metal thin film with a thickness of several μm; (b) an insulating thin film or a semiconductor with a thickness of 20 Å to 1 μm; Thin film (c) Consists of a layered structure of electrodes made of metal thin films of gold, platinum, or palladium with a thickness of 100 Å to 1 μm. A reducing gas sensor that detects the concentration of gas. 2. Using metal as a substrate and electrode, on top of it, (a) an insulating thin film or semiconductor thin film with a thickness of 20 Å to 1 μm, and (b) an electrode made of a metal thin film of gold, platinum, or palladium with a thickness of 100 Å to 1 μm A reducing gas sensor is characterized by having a layered structure and detecting the concentration of gas from changes in work function caused by contact of the reducing gas with the thin metal film of the top layer.