JP2012189537A - Gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To constitute a gas sensor whose selectivity of detection object gas is high, and which is small, low power consumption, and at low cost.SOLUTION: A cavity ES is vacuum or decompression environment. When there is hydrogen gas around a gas sensor 101, the hydrogen gas transmits glass layers 2 and 3 to infiltrate into the cavity ES. On the other hand, neither nitrogen gas nor oxygen gas does not transmit the glass layers 2 and 3. A Q value of a vibrator 11 in the cavity ES varies according to hydrogen gas partial pressure. The vibrator 11 is driven by a vibrator drive/detection circuit and the Q value is detected on the basis of its amplitude. Gas partial pressure is detected by variation of the Q value.

Description

  The present invention relates to a gas sensor that detects a predetermined detection target gas such as hydrogen gas, and more particularly to a gas sensor including a vibrator.

  Conventionally, as a gas sensor used for a combustible gas leak alarm or a gas concentration meter, a catalytic combustion type gas sensor that detects a temperature rise due to catalytic combustion on a catalyst surface has been widely used. As disclosed in Patent Document 1, this type of gas sensor measures the temperature increase of the platinum wire coil as a resistance change by causing the gas to contact and burn on the surface of a catalyst such as a platinum wire coil.

  In addition, a semiconductor type gas sensor that measures a change in electrical conductivity due to gas adsorption on the surface of a metal oxide semiconductor such as NiO as a resistance change is also used.

  A quartz vibrator hydrogen gas sensor that detects a change in weight due to a hydrogen storage reaction of palladium as a change in frequency has also been proposed (see Patent Document 2).

JP-A-10-90210 Japanese Patent Laid-Open No. 2-110341

  The above-mentioned catalytic combustion type gas sensor that is currently in widespread use of a chemical reaction using a catalyst such as platinum or palladium, and accordingly has the following problems to be solved.

(A) It reacts with various gases in the test gas, and the selectivity for the specific gas is low.

(B) Since the operating temperature of the gas sensor generally requires a high temperature of 200 to 500 ° C., large power consumption is required. Further, the deterioration (time-dependent change) of the sensor element is remarkable due to the high temperature.

(C) Since noble metal materials such as platinum and palladium are required, the cost is increased.

  The present invention has been made in view of these circumstances, and an object of the present invention is to provide a gas sensor having high gas type selectivity, small size, low power consumption, and low cost.

The gas sensor of the present invention is configured as follows.
(1) A cavity that forms an internal space that is isolated from the external environment that is the object to be detected, and a vibrator disposed in the cavity,
The internal space is evacuated or depressurized, and at least a part of the cavity is made of a gas selective permeable material having different permeability depending on the gas species.

(2) In (1), when the detection target gas is hydrogen gas or helium gas, the gas selective permeable material is preferably glass.

(3) In (1) or (2), the vibrator is formed by processing a silicon substrate, and a drive electrode for driving the vibrator on the silicon substrate and a detection electrode for detecting displacement of the vibrator are provided. Preferably it is formed.

(4) In any one of (1) to (3), a measurement circuit that measures a signal corresponding to the resonance Q value of the vibrator or a signal determined according to the resonance Q value and outputs the measurement result as a detection signal It is preferable to provide.

(5) In (4), the measurement circuit includes a self-excited oscillation circuit that oscillates at a resonance frequency of the vibrator, and an oscillation signal amplitude detection that detects an oscillation signal amplitude in at least one of the self-excited oscillation circuits. And a circuit.

(6) In any one of (1) to (5), it is preferable that a plurality of thin wall portions dispersed in a part of the cavity made of the gas selective permeable material is formed.

  According to the present invention, gas selectivity is high, chemical reaction is not used, deterioration due to aging is extremely small, no noble metal material for gas reaction is unnecessary, and sensor cost can be suppressed.

FIG. 1 is a cross-sectional view of a main part of a gas sensor 101 according to the first embodiment. FIG. 2 shows a vibrator driving / detecting circuit using the vibrator 11. FIG. 3 is a cutaway perspective view of the gas sensor 101. FIG. 4 is a diagram showing the relationship between the resonance Q (Q value) of the vibrator 11 and the internal pressure P in the cavity ES. FIG. 5 is a cross-sectional view of three types of gas sensors according to the second embodiment. FIG. 6 is a plan view of the vibrator portion of the silicon layer in the gas sensor of the third embodiment. FIG. 7 is an equivalent circuit that models the mechanism of the vibrator. FIG. 8 is a cross-sectional view of three types of gas sensors according to the fourth embodiment. FIG. 9 is a block diagram of the vibrator driving / detecting circuit of the fifth embodiment. FIG. 10 is a block diagram of another transducer driving / detecting circuit of the fifth embodiment. FIG. 11 is a sectional view of a gas sensor according to the sixth embodiment.

<< First Embodiment >>
FIG. 1 is a cross-sectional view of a main part of a gas sensor 101 according to the first embodiment. The gas sensor 101 includes a silicon layer 1 and insulating glass layers 2 and 3 sandwiching the silicon layer 1 from above and below. The upper and lower glass layers 2 and 3 constitute a sealed space (hereinafter referred to as “cavity”) ES surrounded by the glass layers 2 and 3. A vibrator 11 is formed in a predetermined region of the silicon layer 1. The vibrator 11 is supported by the silicon layer 1 and the glass layers 2 and 3 so as to vibrate in the cavity ES without contacting the glass layers 2 and 3.

  The silicon layer 1 is composed of a Si wafer, and the vibrator 11 is formed by a fine processing technique (MEMS technique) for the Si wafer. The glass layers 2 and 3 are bonded to the silicon layer 1 by a bonding technique such as anodic bonding.

  A via opening 6 is formed in the area around the vibrator of the glass layers 2 and 3. An external electrode 7 is formed by depositing a conductive material (for example, aluminum) on the inner surface of the via opening 6 and outside the via opening. The electric signal of the vibrator 11 is taken out by the external electrode 7.

Here, the following three points are important as gas sensors.
(1) A vibrator is formed in a sealed cavity ES.
(2) The cavity ES is in a vacuum or reduced pressure environment.
(3) The wall of the cavity ES, that is, the member that partitions the cavity ES and the external environment has gas selective permeability.

  Since glass has a random structure, it has larger voids than crystals. In particular, silica glass has a relatively high gas permeability to low-molecular gases (for example, hydrogen gas and helium gas), and in general air The permeability of the existing gas (nitrogen, oxygen, argon, carbon dioxide) is very low. That is, it has a low molecular gas selective permeability.

  FIG. 2 shows a vibrator driving / detecting circuit using the vibrator 11. In FIG. 2, the vibrator 11 is a MEMS element, and when a voltage is applied between the ground electrode (common electrode) and the drive electrode, the movable part is displaced by electrostatic attraction. Therefore, the movable part vibrates when an alternating drive voltage is applied to the drive electrode. When the frequency of the drive voltage coincides with the resonance frequency of the movable part, the movable part efficiently vibrates with a large amplitude. Further, a capacitance change corresponding to the vibration of the movable part occurs between the detection electrode of the vibrator 11 and the ground electrode.

  The oscillation circuit 12 detects a change in capacitance generated at the detection electrode of the vibrator 11 and applies a drive voltage to the drive electrode according to the detection voltage signal. That is, the vibrator 11 and the oscillation circuit 12 constitute a self-excited oscillation circuit.

  The oscillation circuit amplitude detection circuit 13 detects the oscillation amplitude of the oscillation circuit 12, and outputs a voltage signal that changes according to the oscillation amplitude as a sensor output signal. Since the oscillation amplitude of the oscillation circuit 12 is proportional or strongly correlated with the vibration amplitude of the vibrator 11, the sensor output signal is a signal related to the amplitude of the vibrator 11.

  Here, the principle of gas detection will be described. FIG. 3 is a cutaway perspective view of the gas sensor 101. As schematically shown in FIG. 3, the cavity ES is a vacuum or a reduced pressure environment. If there is hydrogen gas around the gas sensor 101, the hydrogen gas permeates the glass layers 2 and 3 and enters the cavity ES. On the other hand, nitrogen gas and oxygen gas do not permeate.

  The amount of gas that enters the cavity ES is expressed by the following equation.

q = K ・ S ・ t ・ ΔP / d
here,
q: Permeated gas amount
K: Transmission coefficient
S: Cavity surface area
d: Glass layer thickness
t: Time ΔP: Pressure difference inside and outside the cavity ES.

  As the hydrogen gas enters, the hydrogen gas partial pressure in the cavity ES increases. When the hydrogen gas partial pressure inside the cavity ES becomes the same as the hydrogen gas partial pressure around the gas sensor 101 (when ΔP becomes zero), the penetration into the cavity ES stops.

  FIG. 4 is a diagram showing the relationship between the resonance Q (Q value) of the vibrator 11 and the internal pressure P in the cavity ES. The viscous resistance that the vibrator 11 receives increases as the internal pressure of the cavity ES increases. As the viscous resistance increases, the Q value of the vibrator 11 decreases and the oscillation amplitude decreases as shown in FIG. Therefore, the hydrogen gas partial pressure in the atmosphere around the gas sensor 101 can be detected by checking the level of the sensor output signal.

  Since hydrogen gas permeates in a direction in which the hydrogen gas partial pressure difference ΔP inside and outside the cavity ES becomes zero, if the hydrogen gas partial pressure in the atmosphere around the gas sensor 101 is lower than the hydrogen gas partial pressure in the cavity ES, The hydrogen gas partial pressure also decreases. That is, in a steady state, the hydrogen gas partial pressure in the cavity ES becomes equal to the hydrogen gas partial pressure in the ambient atmosphere of the gas sensor 101, and the gas sensor 101 is a sensor corresponding to the hydrogen gas partial pressure in the ambient atmosphere with a constant response. An output signal is obtained.

<< Second Embodiment >>
In the second embodiment, examples of some structures of the gas sensor are shown. FIG. 5 is a cross-sectional view of three types of gas sensors according to the second embodiment.
In the first embodiment, as shown in FIG. 1, recesses for forming the cavity ES are formed in the upper and lower glass layers 2 and 3 sandwiching the silicon layer 1, but the cavity surrounding the vibrator is as shown in FIG. It can take various forms.

  In the example of FIG. 5A, a recess for forming the upper cavity ES1 is formed in the upper glass layer 3, and a recess for forming the lower cavity ES2 is formed on the lower surface of the silicon layer 1. In this way, a flat glass plate may be used for the lower glass layer 2.

FIG. 5B shows an example in which the glass layer 3 is attached to the surface of the SOI substrate 10. The SOI substrate 10 is a substrate in which a SiO ₂ film 10B is formed on the surface of the Si substrate 10A, and a Si film 10C is formed on the surface. The vibrator 11 is formed on the Si film on the surface, and the lower cavity is formed in the SiO ₂ film 10B. An etching removal portion for forming ES2 is formed.

  FIG. 5C is also an example in which the glass layer 3 is attached to the surface of the SOI substrate 10. In this example, a recess for forming the upper cavity ES1 is formed on the surface of the Si film 10C. The external electrode may be taken out on the lower surface of the SIO substrate 10 as shown in FIG.

<< Third Embodiment >>
In the third embodiment, an example of a specific structure of the vibrator 11 is shown. FIG. 6 is a plan view of the vibrator portion of the silicon layer. The overall structure of the gas sensor is as shown in the first embodiment.

  Of the silicon layer 1, the portion that is cross-hatched is the fixed portion, and the portion that is not cross-hatched is the vibrator portion 110. The vibrator portion 110 is elastically held by the fixed portion with a beam 111 at the center thereof. The vibrator unit 110 is displaced in the left-right direction in the direction shown in FIG. The vibrator unit 110 has a plurality of comb-like electrodes at positions facing drive electrodes and detection electrodes described later.

  A drive fixed electrode 120 and a detection fixed electrode 130 are arranged on the left and right of the vibrator unit 110. The vibrator driving voltage Vd is applied to the driving fixed electrode 120. An electrostatic attractive force acts between the driving fixed electrode and the comb teeth of the vibrator due to the voltage difference between the vibrator driving voltage Vd and the potential Vcom of the vibrator portion 110, and this electrostatic attraction becomes a driving force. Then, according to the vibration of the vibrator unit 110, the capacitance Cm between the comb teeth of the detection fixed electrode 130 and the vibrator unit 110 changes.

  FIG. 7 is an equivalent circuit modeling the above mechanism. The vibrator unit 110 is displaced according to the drive voltage Vd applied to the drive fixed electrode 120, and the capacitance Cm between the vibrator unit 110 and the detection fixed electrode 130 changes.

  The internal pressure in the cavity ES greatly affects the resonance characteristics of the vibrator by the vibrator portion 110 and the beam 111. This is because the viscosity of the gas existing in the cavity ES becomes a factor of the viscosity between the comb teeth, and this viscosity inhibits the vibration of the vibrator unit 110. This characteristic appears as a PQ correlation curve shown in FIG. Since the change in the viscosity of the gas with respect to the change in the internal pressure in the cavity ES is increased by the comb teeth of the vibrator unit 110, the drive fixed electrode 120, and the detection fixed electrode 130, the gas detection sensitivity can be increased.

<< Fourth Embodiment >>
The fourth embodiment shows several packaging examples of the gas sensor. FIG. 8 is a cross-sectional view of three types of gas sensors according to the fourth embodiment.
In the first and second embodiments, the gas sensor chip is provided with the glass layer, but in the fourth embodiment, a glass cover is provided in the package, and the gas sensor chip is accommodated in the package.

In the example of FIG. 8A, the gas sensor chip 100 is bonded to the ceramic package 8 via a die bond material, and the opening of the ceramic package 8 is sealed with the glass cover 4. The gas sensor chip 100 corresponds to the SOI substrate 10 shown in FIG. That is, the gas sensor chip 100 is configured by a substrate in which the SiO ₂ film 10B is formed on the surface of the Si substrate 10A and the Si film 10C is formed on the surface. The vibrator 11 is formed on the Si film on the surface, and the etching removal portion for forming the lower cavity ES2 is formed on the SiO ₂ film 10B. Aluminum electrode pads are formed on the upper surface of the gas sensor chip 100, and are electrically connected to the electrodes of the ceramic package 8 by wire bonding.

  The glass cover 4 is a gas selective permeable material, and is bonded with a glass frit or the like. The inside of the ceramic package 8 is sealed under vacuum or reduced pressure. The operation as a hydrogen gas sensor / helium gas sensor is as described above.

  In the example of FIG. 8B, the opening of the ceramic package 8 is sealed with a metal cover 5 with a glass cover (window). The metal cover 5 is, for example, a Kovar material, and is joined to the ceramic package 8 by brazing, welding, or the like.

  In the example of FIG. 8C, not only the gas sensor chip 100 but also the signal processing circuit chip 200 is accommodated in the ceramic package 8. The signal processing circuit chip 200 includes a vibrator driving / detecting circuit that drives the vibrator of the gas sensor chip 100 and detects the amplitude. This signal processing circuit chip outputs, for example, a voltage signal proportional to the gas partial pressure to the outside.

<< Fifth Embodiment >>
In the fifth embodiment, two specific configuration examples of the vibrator driving / detecting circuit are shown.
9 and 10 are block diagrams of the vibrator driving / detecting circuit. In both figures, the detection electrode of the vibrator 11 is connected to a CV conversion circuit, and the CV conversion circuit 12A converts the capacitance change accompanying the vibration of the vibrator 11 into a voltage change signal. Necessary signal components are extracted from this voltage signal by a filter (LPF, HPF, BPF, etc.). The output signal of the filter circuit is subjected to necessary phase adjustment by the phase control circuit 12C, and is input to an AGC circuit (Auto Gain Control Amp) 12D. In the AGC circuit 12D, a drive signal subjected to automatic gain control is formed and input to the drive electrode of the vibrator 11. Thus, a closed-loop self-excited oscillation circuit is configured, and the vibrator 11 vibrates at the resonance frequency.

9 and 10 differ in the automatic gain control function of the AGC circuit.
FIG. 9 shows a vibrator driving / detecting circuit having a constant driving signal amplitude type, and performs automatic gain control so that the driving signal always has a constant amplitude. In this case, the amplitude of the detection signal of the vibrator 11 responds to the atmospheric gas partial pressure, that is, the Q value of the vibrator 11. Therefore, for example, the gas partial pressure can be detected by detecting the amplitude of the output signal of the CV conversion circuit 12A.

  FIG. 10 illustrates a transducer drive / detection circuit with a constant detection signal amplitude, and performs automatic gain control so that the detection signal always has a constant amplitude. In this case, the amplitude of the drive signal of the vibrator 11 responds to the atmospheric gas partial pressure, that is, the Q value of the vibrator 11. Therefore, for example, the gas partial pressure can be detected by detecting the amplitude of the output signal of the AGC circuit 12D.

  9 and 10, signal processing of the oscillation circuit amplitude detection circuit will be described. The output signal of the oscillation circuit 12 is rectified by the rectifier circuit 13A and converted to a DC signal corresponding to the amplitude by the filter circuit (LPF) 13B. Further, the signal is amplified to a necessary level by the amplifier circuit 13C and input to the correction circuit 13D. The correction circuit 13D corrects temperature characteristics and the like, and outputs a DC voltage signal corresponding to the atmospheric gas partial pressure.

  Thus, although the detection signals are different between FIG. 9 and FIG. 10, it is possible to detect the change of the PQ curve in the sensor element resonance due to the atmospheric gas partial pressure by detecting the amplitude of a specific part of the oscillation circuit. it can.

<< Sixth Embodiment >>
FIG. 11 is a sectional view of a gas sensor according to the sixth embodiment.
In order to increase the responsiveness and sensitivity of the gas sensor, it is important to increase the gas permeation speed. For this purpose, the following method is effective.

・ Make the gas permeable layer thinner.
-Increase the gas permeable layer area.
• Increase operating temperature.

  However, when the gas permeable layer is thin and the area is widened, there is a concern that the mechanical strength cannot be maintained due to the pressure difference between the vacuum / depressurized environment of the sealed space in the sensor and the outside air. Therefore, by providing a plurality of non-penetrating cavities NC in the glass material as shown in FIG. 11, the gas permeable layer can be made thin while maintaining the strength.

<< Other embodiments >>
In addition to glass, a metal material or polymer material having the same selective gas permeability may be used as the gas selective permeability material. Examples of metal materials include Pd-based alloys and niobium-based alloys.

  The glass of the gas selective permeable material is not limited to glass mainly composed of silicate such as silica glass, but may be organic glass or metal glass.

  The detection target gas is not limited to hydrogen or helium gas, and if there is a gas selective permeable material corresponding to the gas type of the detection target, the predetermined gas sensor can be configured.

ES ... cavity ES1 ... upper cavity ES2 ... lower cavity NC ... non-penetrating cavity 1 ... silicon layer 2, 3 ... glass layer 4 ... glass cover 5 ... metal cover 6 ... via opening 7 ... external electrode 8 ... ceramic package 10 ... SOI Substrate 10A ... Si substrate 10B ... SiO ₂ film 10C ... Si film 11 ... oscillator 12 ... oscillation circuit 12A ... CV conversion circuit 12C ... phase control circuit 12D ... AGC circuit 13 ... oscillation circuit amplitude detection circuit 100 ... gas sensor chip 101 ... gas sensor DESCRIPTION OF SYMBOLS 110 ... Vibrator part 111 ... Beam 120 ... Drive fixed electrode 130 ... Detection fixed electrode 200 ... Signal processing circuit chip

Claims (6)

A cavity forming an internal space isolated from the external environment to be detected, and a vibrator disposed in the cavity,
The gas sensor according to claim 1, wherein the internal space is vacuumed or depressurized, and at least a part of the cavity is made of a gas selective permeable material having different permeability depending on the gas type.   The gas sensor according to claim 1, wherein the gas selective permeable material is glass, and the detection target gas is hydrogen gas or helium gas.   The said vibrator | oscillator is comprised by the process of the silicon substrate, The drive electrode which drives the said vibrator | oscillator, and the detection electrode which detects the displacement of the said vibrator | oscillator were formed in the said silicon substrate. Gas sensor.   The measurement circuit according to claim 1, further comprising a measurement circuit that measures a signal corresponding to the resonance Q value of the vibrator or a signal determined according to the resonance Q value, and outputs the measurement result as a detection signal. Gas sensor.   The measurement circuit includes a self-excited oscillation circuit that oscillates at a resonance frequency of the vibrator, and an oscillation signal amplitude detection circuit that detects an oscillation signal amplitude of at least one of the self-excited oscillation circuits. The gas sensor according to claim 4.   The gas sensor according to any one of claims 1 to 5, wherein a plurality of thin wall portions dispersed in a part of the cavity made of the gas selective permeable material are formed.

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