JP2014025869A - Aging method of gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance sensitivity of a MEMS gas sensor related to fuel gas by aging.SOLUTION: Aging is applied to a MEMS gas sensor while applying heater power intermittently to a heater film in an atmosphere of room temperature with high humidity. Thereby, methane sensitivity is improved.

Description

  The present invention relates to aging of a MEMS gas sensor.

  It is known to provide a bridge or diaphragm on a cavity in a Si substrate and to provide a gas sensor on the bridge or diaphragm. This type of gas sensor is called a MEMS gas sensor, and there is one using a metal oxide semiconductor such as SnO2 and one using an oxidation catalyst such as Pt-γAl2O3. The operating condition of the MEMS gas sensor is that it is heated to the operating temperature periodically for a short time, and the rest is at room temperature.

  The prior art regarding aging is shown. Patent Document 1 (JP2003-294668A) proposes aging in which a gas sensor is left in an atmosphere such as 95% relative humidity (hereinafter “RH”) at 10 ° C. The type of gas sensor is not a MEMS type, and it is said that the aging of the gas sensor can be reduced by aging in a high humidity atmosphere for a long time. Patent Document 2 (JPH09-138209A) reports a result of aging a gas sensor in which a heater film and a metal oxide semiconductor film are provided on insulating glass in high temperature and high humidity. According to Patent Document 2, when a detection voltage is applied to a metal oxide semiconductor film at high temperature and high humidity and aging occurs, Mg ions diffuse from the insulating glass into the metal oxide semiconductor film and segregate to the cathode side due to the detection voltage. Yes.

JP2003-294668A JPH09-138209A

  This invention makes it a subject to raise the sensitivity to the fuel gas of a MEMS gas sensor by aging.

The present invention relates to an aging method for a gas sensor in which a metal oxide semiconductor film and a heater film are provided on a bridge of an insulating film provided in a cavity of a Si substrate or a diaphragm of an insulating film,
Aging is performed in a high-humidity atmosphere at room temperature while intermittently applying heater power to the heater film.

  Aging a MEMS gas sensor in a high humidity atmosphere with RH (relative humidity) near 100% improves sensitivity to fuel gases such as methane and LPG, and does not increase hydrogen sensitivity. Relative sensitivity is also improved. Therefore, the methane detection characteristic or LPG detection characteristic of the MEMS gas sensor can be easily improved. Aging does not need to be performed in an environment where the temperature is controlled, and can be easily aged by placing the gas sensor in an atmosphere in which RH is near 100%. The effect of aging increases as the relative humidity approaches 100%, but the relative humidity does not need to be 100% and may be, for example, 90% or more. Further, it is sufficient that the metal oxide semiconductor in the gas sensor is placed in an atmosphere with a relative humidity of about 100%, and the entire aging chamber does not need to be set to a relative humidity of 100%.

The room temperature is, for example, a temperature of 0 ° C. or higher and 40 ° C. or lower, preferably 10 ° C. or higher and 30 ° C. or lower. In this invention, since it is not necessary to accurately manage the aging temperature, a constant temperature and humidity chamber or the like is not required. The detection target gas is a fuel gas such as methane or LPG, but hydrogen is not included in the detection target. Of the fuel gases, it is particularly important to increase sensitivity to methane, which has a low sensitivity of the gas sensor.

Cross section of gas sensor A characteristic diagram when the gas sensor is aged while heating at 25 ° C. and 100% RH at a rate of 0.1 second once per second, the horizontal axis indicates the aging period, and the vertical axis indicates the ratio of the resistance value between the gas and the air. FIG. 2 is a characteristic diagram showing the aging results with the vertical axis representing the resistance value. It is a characteristic diagram showing the change in gas sensitivity due to the difference in aging atmosphere. The gas sensor is heated by 0.1 second every second, and the aging period is 5 days. It is a characteristic diagram showing the change in gas sensitivity due to the heating cycle during aging, and the aging atmosphere is 25 ° C. and RH 100%.

  In the following, an optimum embodiment for carrying out the present invention will be shown.

  1 to 5 show an embodiment. FIG. 1 shows the structure of the MEMS gas sensor 2 used, 4 is a Si substrate, a cavity 6 is provided by etching, and an insulating film 8 such as SiO2, Si3N4, Ta2O5 is provided in a bridge shape on the cavity 6. The insulating film 8 may be formed in a diaphragm shape that covers the cavity of the through hole instead of the bridge shape. A heater film 10 such as Pt is provided on the insulating film 8, and a thick SnO2 film 14 is provided via an interlayer insulating film 12 such as SiO2, Si3N4, Ta2O5. The SnO2 film 14 may be a thin film, may be another metal oxide semiconductor film such as an In2O3 film, and the interlayer insulating film 12 may not be provided, and the heater film 10 may be used as one electrode of the SnO2 film 14. A pair of electrode films (not shown) are connected to the SnO 2 film 14, and the electrode film and the heater film 10 are connected to the stem 18 through lead wires 20. A catalyst film that covers the SnO2 film 14 may be provided. Reference numeral 16 denotes a base that supports the Si substrate 4, 22 is a cap, and accommodates a filter 24 such as activated carbon or zeolite, 26 is a nonwoven fabric, prevents powder from spilling out from the filter 24, and 28 is a nonwoven fabric 26. The holding ring 30 and 31 are openings, and 32 is a wire mesh. The structure, material, etc. of the MEMS gas sensor 2 are arbitrary.

  The gas sensor 2 is for detecting methane, for example, and the driving condition is to heat the pulse sensor to 470 ° C. for 0.1 second once every 30 seconds. Then, for example, a detection voltage is applied in synchronization with the pulse heating, and the gas is detected from the resistance value of the SnO2 film 14 at the end of the pulse heating period. The driving conditions themselves are arbitrary.

  In order to age the gas sensor 2, for example, the gas sensor is set on a pallet (not shown), stored in a humidified room, and aged in an atmosphere having an RH of about 100% at room temperature. Humidification is performed, for example, by supplying water vapor from a humidifier while monitoring the indoor relative humidity with a humidity sensor. Alternatively, water may be filled in the aging container. The room temperature is, for example, a temperature of 0 ° C. or higher and 40 ° C. or lower, preferably a temperature of 10 ° C. or higher and 30 ° C. or lower, and the temperature need not be constant. Further, the relative humidity is, for example, 90% or more and 100% or less, and the condition for dew condensation on the SnO2 film 14 is not preferable. When the relative humidity is 100% and the water vapor is not supersaturated, the gas sensor 2 is periodically heated, so that the SnO2 film 14 does not condense. During aging, the gas sensor 2 is driven under the condition that it is heated to 470 ° C. once per second for 0.1 second, and the cycle for applying the heater power is shorter than the normal drive cycle of the gas sensor 2. During aging, the detection voltage may or may not be applied to the SnO2 film 14, and the preferred aging period is 3 days or more and 10 days or less.

  2 to 5 show the results of aging, and in each case, the number of sensors is five. FIG. 2 shows how the ratio of the resistance value between gas and air changes with the aging period. The aging atmosphere is 25 ° C. and the relative humidity is about 100% air. Heater power was applied to the heater once per second for 0.1 second, and the maximum temperature was heated to 470 ° C. The resistance values in the gas and air were measured before the start of aging, after 1 day, after 2 days, and after 5 days.

  As can be seen from FIG. 2, aging increased methane sensitivity, and this effect occurred with 1 day aging and almost saturated after 5 days. FIG. 6 shows the data of the measurement of FIG. 2 as resistance values. The resistance value in the air increases by aging, and the resistance value in methane decreases, so the methane sensitivity increases. Since the resistance value in hydrogen does not change or slightly increases, the relative sensitivity of methane to hydrogen is also improved by aging.

  FIG. 4 shows the influence of the aging atmosphere. The left side is the same as the data on the fifth day in FIG. 2 at 25 ° C. relative humidity 100%, and the right side is 25 ° C. RH 55%. In other respects, the test conditions are the same as those in FIGS. In aging near RH 100%, methane sensitivity is higher than aging at RH 55%.

  Figure 5 shows the effect of the heating cycle during aging: 0.1 second heating once per second, 0.1 second heating once every 5 seconds, 0.1 second heating once every 30 seconds, 0.1 second once every 60 seconds Four types of heating conditions were shown. The maximum temperature of the SnO2 film during heating is 470 ° C. The aging atmosphere is 25 ° C., the RH is about 100%, and the aging period is 5 days. When the heating cycle is shortened and the number of pulse heating is increased, the methane sensitivity is greatly increased. A preferable heating cycle is 0.5 seconds to 5 seconds, and one heating time is, for example, 20 milliseconds to 0.5 seconds.

  In addition to this, the inventor performed aging at RH 95% at 40 ° C. and aging at RH 80% at 35 ° C. The aging effect was similar between RH95% at 40 ° C and RH100% at 25 ° C, and the aging effect was higher at 25 ° C RH100%. The aging effect was small at RH 80% at 35 ° C. From these facts, it can be seen that the aging effect is determined by the relative humidity, the influence of the temperature is small, and the temperature itself is not important. In addition, the higher the relative humidity, the higher the aging effect means that it is important for water to condense in the pores inside the SnO2 film. The shorter the heating period, the higher the aging effect means that the condensed water This suggests that aging proceeds in the process of evaporating rapidly.

Although the specification describes the sensitivity of methane and the relative sensitivity of methane and hydrogen, aging in high humidity at room temperature can similarly increase the sensitivity to LPG.

2 Gas sensor 4 Si substrate 6 Cavity 8 Insulating film 10 Heater film 12 Interlayer insulating film 14 SnO2 film 16 Base 18 Stem 20 Lead wire 22 Cap 24 Filter 26 Non-woven fabric 28 Press ring 30, 31 Opening 32 Metal mesh

Claims (3)

In an aging method of a gas sensor in which a metal oxide semiconductor film and a heater film are provided on an insulating film bridge or an insulating film diaphragm provided in a cavity of a Si substrate,
An aging method for a gas sensor, characterized by aging in a high humidity atmosphere at room temperature while intermittently applying heater power to the heater film.   2. The gas sensor aging method according to claim 1, wherein the methane sensitivity of the gas sensor is improved by aging.   The gas sensor aging method according to claim 1, wherein the room temperature is a temperature of 0 ° C. or more and 40 ° C. or less.

Images