JPH0611472A - Gas sensor - Google Patents

Abstract

PURPOSE:To sense the remaining lifetime precisely by furnishing two sensors having uniform shape and characteristics, using one of them for usual measuring and using the other as reference sensor for judging the remaining lifetime. CONSTITUTION:Overhangs 12a, 12b extending in the form of cantilever are furnished in recesses 11a, 11b of a base board 10, and thereover resistance heat emitting elements 13a, 13b and gas sensor elements 14a, 14b of respective identical structure are provided. In this manner, two sensors A, B of uniform shape and characteristics are installed adjacently, and one of them, for example A, is used as sensor for measuring while the other, B, is used as reference sensor. Because the characteristics of the two elements are uniform, it is expected that the measurements by the two are equal at the initial stage. Accordingly the difference between the measurements after a certain runs of measuring is going to represent the degree of deterioration of the sensor for measuring, and when this value of difference exceeds a certain threshold, judgement is passed that the life of the sensor for measuring A has gone out.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to gas sensors, and more particularly to detecting the life of semiconductor gas sensors.

[0002]

2. Description of the Related Art A semiconductor gas sensor is designed to detect various gases by utilizing the fact that a resistance value decreases when a heated metal oxide semiconductor such as SnO ₂ is reacted with a reducing gas. Since this sensor does not operate unless it is heated, a heater is required, and a considerable amount of electric power is required to heat this heater, and an AC power source is indispensable in the conventional sensor.

In order to improve this point, an overhanging portion such as a bridge structure or a cantilever structure which is overhanged in the air is provided, and a metal oxide semiconductor is formed on this overhanging portion.
Therefore, the heat capacity is reduced as much as possible to improve the response characteristics,
Attempts have also been made to reduce power consumption.

FIG. 5 is a configuration diagram for explaining an example of a gas detector formed in a cantilever structure. FIG. 5A is a plan view and FIG. 5B is a BB line in FIG. 1 is a line cross-sectional view
Reference numeral 0 is a substrate, 11 is a recess formed in the substrate 10, 12 is a thin film insulator of a silicon chip formed on the substrate 10, and the thin film insulator 12 is a cantilever type on the recess 11. A bridge 12 that is bridged in a cantilever manner is provided on the overhanging portion or the above-mentioned concave portion, and the resistance heating element 13 and the resistance heating element 13 are provided on the overhanging portion or the bridge 12. The gas detection element 14 is provided. The gas detection element 14 is, as described above, specifically made of SnO.
_{2 is} a metal oxide semiconductor. The concave portion 11, the projecting portion 12, the resistance heating element portion 13, and the gas detecting portion 14 are preferably formed on the same substrate 10.

With the above-mentioned conventional technique, the metal oxide semiconductor of SnO ₂ can be made thin and the heat capacity of the metal oxide semiconductor supporting member can be made small, so that it can be heated by a minute heater. Furthermore, since the response speed is very fast at 0.5 to 0.6 ms, the heater can be used in pulse drive. As a result, the power consumption is 1/30 to 1/50 of that of the conventional one.
I'm done now.

With the above-described structure, it is possible to greatly reduce the power consumption by pulse-driving the heater, but on the other hand, the performance of the sensor itself deteriorates. However, when the measurement operation is repeated, the sensitivity of the semiconductor gas sensor gradually decreases in proportion to the number of measurements, and when the sensor performance deteriorates, the difference between the resistance value in air and the resistance value in gas is decreased. Is getting smaller. Further, the semiconductor gas sensor has variations in characteristics due to manufacturing lots, but in a sensor that reacts when the gas concentration exceeds a certain value, such variations in characteristics or deterioration of the characteristics described above is fatal. Target.

[0007]

As described above, the performance of semiconductor gas sensors is deteriorated during use, or there are variations in the manufacturing process.
Since no gas can be detected at all, conventionally, a usable number of times is set in advance for safety, and the sensor is replaced when the number of times of measurement reaches the set value. However, in that case, since the usable number of times has to be adjusted to a sensor having a poor characteristic (the life is short), the sensor having a good characteristic is sacrificed, and the useful life of the sensor is substantially shortened.

[0008]

In order to solve the above problems, the present invention has (1) two sensors having the same shape and characteristics, one of which is used for normal measurement, The other is characterized by being used as a reference sensor for determining the life of the measurement sensor, and further, (2)
When the difference between the output of the measurement sensor and the output of the reference sensor is equal to or more than a threshold value, the notification means notifies, or (3) when the measurement sensor senses a predetermined number of times, the reference sensor is used. This sensor senses the difference between the outputs of both sensors.

[0009]

[Function] Two sensors which are made adjacent to each other or have substantially the same characteristics are used as a pair, and one sensor is used as a reference sensor, and the reference sensor is not normally used (= the characteristics are not deteriorated).
Each time the measurement sensor is used a certain number of times (for example, 10
Measurements are made every time (00 times), the change in the difference between the two measured values is monitored, and an alarm is issued when the difference exceeds a threshold value.

[0010]

1 is a diagram showing an example of a gas sensor to which the present invention is applied. FIG. 1A is a plan view, FIG. 1B is a sectional view taken along line bb in FIG. 10 is a substrate, 11a, 11
b is a recess formed in the substrate, 12 is a thin-film insulator formed on the substrate 10, and the thin-film insulator 12 is a cantilever-shaped projecting portion protruding above the recesses 11a and 11b. A bridge portion 12 which is bridged in a double-supported beam type on the concave portion.
a and 12b, and the projecting portion or bridge portion 12a, 1
2b has the same resistance heating elements 13a, 1
3b and gas detection elements 14a and 14b are provided in proximity to the resistance heating elements 13a and 13b, respectively.

As described above, in the present invention, the shape,
Two sensors with uniform characteristics are provided adjacent to each other, while
For example, the A side is used as a sensor for measurement and the B side is used as a reference sensor. However, when elements of the same shape are formed on the same chip or close to each other, the characteristics of the two elements are the same. Can be expected to have approximately the same measurements for the two sensors. Therefore, the difference between the measured values after a certain number of times of measurement can be considered to represent the degree of deterioration of the measuring sensor, and when the value of the difference exceeds a certain threshold value, the life of the measuring sensor is It can be determined that it has come.

FIG. 2 is a block diagram for explaining the operation of the present invention, and FIGS. 3 and 4 are flowcharts. In FIG. 2, Eh is a heater pulse current drive source and Es is a measurement constant current pulse drive source. , SW1 to SW3 are analog switches (multiplexers), A / D is an AD converter, R is a gas detection resistor (tin oxide thin film resistor) on the measurement block A side,
H is a heater on the measurement block A side, Rref is a gas detection resistor on the reference block B side, Href is a heater on the reference block B side, 20 is a calculation unit, 21 is a storage unit, and the storage unit 21 has a measurement counter CNTR. , A storage register REG of the measurement frequency interval for instructing the use of the reference block
n, a register REGdif for storing a threshold value obtained in advance for determining the life of the sensor, and the like are provided. Further, 23 is a control unit, 24 is an operation unit, 25 is a reporter, and the operation unit 24 has a detection switch operation unit 24a and an adjustment switch operation unit 24b.

(1) Adjustment before shipment of sensor The sensor is placed in the gas to be detected. Adjustment SW according to an instruction (not shown) from the control unit 23
The operation unit 24b is turned on to connect the terminals 1 and 2 of SW1 to SW3. A pulse current having a predetermined current value and a time width is applied to Href and Rref of the reference block B, and Rr
The detection voltage Vref ₀ generated at both ends of ef is measured by A / D. Similarly, the switches SW1 to SW3 are switched, the terminals 1 and 3 of these SW1 to SW3 are connected, and the voltage V0 across the R of the measurement block A is measured. In the calculation unit 20, ΔV0 = | V0−Vref ₀ |
Is calculated and stored in the storage unit 21. (2) Measurement operation The initial value 0 is set in CNTR and the following operations are repeated. Switch SW by operating the detection switch operation unit 24b
1 to SW3 are controlled, and switches SW1 to SW3 for gas detection are turned on to the 3 side in the procedure of (1) to perform measurement, and the result is read by the calculation unit 20 via A / D. At the same time, the contents of CNTR are incremented by 1, and the value is REGn.
It is determined whether it is equal to the content of. If the judgments at are not equal, the operation at is repeated.
When they are equal, ΔV0 = | V−Vref | is calculated from the measured voltage V across R and the voltage Vref measured across Rref in the same manner as in (1), and is calculated and stored in (1). | ΔV−ΔV0 | is calculated between and, and the magnitude of REGdif is compared. If the comparison result of the above is | ΔV−ΔV0 |> REGdif, it is judged that the characteristic deterioration of the gas sensor R has reached the limit, and the reporter 25 issues a signal to warn the user of that. When the comparison result is other than the above, the measurement of is repeated.

In the above (2), after it is determined that the characteristic deterioration of R has reached the limit, Rref (reference sensor), which has a small number of measurement times, is used as a measurement sensor within a certain allowable number of times. By controlling to use, the reference sensor can be used more effectively.

[0015]

As is apparent from the above description, according to the present invention, the service life of the sensor can be extended,
Increasing the service life is particularly effective in usage situations where it is difficult to replace the sensor. In addition, because the method determines the life by comparing measured values of elements with uniform characteristics, it is possible to change due to changes in the environmental conditions used, such as changes in gas flow rate, changes in gas concentration, changes in ambient temperature, etc. You can reduce the effect of minutes. Therefore, there is an advantage that the life of the gas sensor can be detected more accurately.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a gas sensor to which the present invention is applied.

FIG. 2 is a block diagram for explaining the operation of the present invention.

FIG. 3 is a flowchart for explaining the operation of the present invention.

FIG. 4 is a flowchart for explaining the operation of the present invention.

FIG. 5 is a diagram for explaining an example of a conventional gas sensor.

[Explanation of symbols]

10 ... Substrate, 11a, 11b ... Recessed part, 12 ... Thin film insulator, 12a, 12b ... Overhang part, 13a, 13b ... Resistance heating element, 14a, 14b ... Gas detection member, A ... Sensor part for measurement, B ... Reference Sensor section, 20 ... Calculation section, 2
1 ... memory | storage part, 23 ... control part, 24 ... operation part, 25 ... reporting device.

Claims (3)

[Claims] 1. A sensor having two sensors of uniform shape and characteristics, one of which is used for normal measurement, and the other of which is used as a reference sensor for determining the life of the measurement sensor. A gas sensor characterized in that. 2. The gas sensor according to claim 1, wherein when the difference between the output of the measurement sensor and the output of the reference sensor becomes equal to or more than a threshold value, the notification means notifies. 3. The gas sensor according to claim 1, wherein when the measurement sensor senses a predetermined number of times, the reference sensor senses and the difference between the outputs of both sensors is taken.