JP2005134250A - Method for adjusting resistance value of heater for heating membrane gas sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To simply and inexpensively adjust the electric resistance of a membrane heater of a membrane gas sensor without having to build a new part in a sensor circuit or trim a heat pattern. <P>SOLUTION: The membrane heater 3 is formed via a support film 2, a sensitive film electrode 6 is formed via an insulating film 4, and a sensitive film 7 are formed on it on a substrate formed by hollowing out one side surface of an Si substrate 1 into a diaphragm shape. The membrane heater 3 of the membrane gas sensor is heated by the impression of a voltage greater than that at normal use to change its resistance value. By adjusting the impressing voltage in such a way that the value takes a desired value, it is possible to simply and inexpensively adjust the electrical value. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a low power consumption thin film gas sensor with battery driving in mind.

In general, a gas sensor is a device that is used for applications such as a gas leak alarm and is selectively sensitive to a specific gas, such as CO, CH ₄ , C ₃ H ₈ , CH ₃ OH, etc. High sensitivity, high selectivity, high response, high reliability, and low power consumption are indispensable.
By the way, gas leak alarms that are widely used for household use include those for the purpose of detecting flammable gases for city gas and propane gas, and those for the purpose of detecting incomplete combustion gases in combustion equipment, or However, the penetration rate is not so high due to cost and installation problems. Under such circumstances, it is desired to improve the installation property, specifically, to be battery-driven and cordless in order to improve the diffusion rate.

Low power consumption is the most important for realizing battery driving. However, in catalytic combustion and semiconductor type gas sensors, it is necessary to detect by heating to a high temperature of 100 ° C. to 500 ° C. Thus, in the conventional method in which powder such as SnO ₂ is sintered, there is a limit to reducing the thickness even if a method such as screen printing is used, and the heat capacity is too large to be used for battery driving. Therefore, the realization of a thin film gas sensor in which a heater and a sensing film are formed with a thin film of 1 μm or less and a low heat capacity structure such as a diaphragm structure by a fine processing process has been awaited.

In the catalytic combustion type and semiconductor type thin film gas sensors, a heater is required to control the temperature of the sensor because a chemical reaction is used under the condition that the surface temperature of the sensitive film is 50 ° C to 100 ° C. Greatly affects variations in sensor characteristics.
On the other hand, a metal thin film having a thickness of several microns or less is used as a heater used in the thin film gas sensor. When the resistance value of the thin film heater is adjusted after film formation or patterning, for example, a correction resistor as in Patent Document 1 is used. There is a trimming method in which the pattern is attached separately or the pattern is cut with a laser or the like.

However, it is expensive to apply fine processing to each sensor using a laser or the like. In the first place, because of the structure of the thin film gas sensor, a sensitive layer and a selective combustion layer are often formed on the upper part of the heater, so that it is difficult to adjust the final sensor resistance value by trimming.
Further, as in Patent Document 2, for a thin film heater used for a flow sensor, by incorporating a feedback control circuit so that the heater power consumption is constant, variations in the heater and temperature resistance coefficient are corrected to predetermined values. There is a method.

JP 2000-180406 A (page 3, FIG. 8) Japanese Patent Laid-Open No. 11-037818 (19th page, FIG. 1)

However, the method disclosed in Patent Document 2 can eliminate or simplify the screening process, but there is a problem that an increase in the number of parts leads to high costs for the device body.
Accordingly, an object of the present invention is to make it possible to adjust the electric resistance of a thin film heater without newly incorporating parts into the sensor circuit or trimming the heater pattern.

  In order to solve the above-mentioned problems, the present invention is characterized in that annealing is performed by increasing the voltage applied to the heater until a predetermined resistance value is obtained while measuring the resistance of the heater. That is, by applying a voltage to the heater and heating it, the electric resistance value changes as a result of the arrangement of atoms constituting the heater changing. The resistance value once changed utilizes an annealing function (effect) that no longer changes even when a voltage equal to or lower than the voltage (heater temperature) applied during adjustment is applied.

  According to the present invention, since the resistance value is adjusted by applying a voltage to the heater and heating it, the predetermined value can be obtained without newly incorporating parts or applying external fine processing to the heater resistance. Can be adjusted to As a result, not only the cost is reduced, but also there is an advantage that a gas sensor with uniform characteristics can be provided.

FIG. 1 is a flowchart for explaining the present invention. This is performed, for example, using a microcomputer for the thin film gas sensor having the structure shown in FIG. First, the thin film gas sensor of FIG. 2 will be described.
Si ₃ N ₄ and SiO ₂ films are sequentially formed by plasma CVD on a Si wafer 1 having a thermal oxide film on both sides as a support layer having a diaphragm structure and a thermal insulating layer 2. Next, the heater layer 3 and the SiO ₂ insulating layer 4 are formed in this order by sputtering. A bonding layer 5 and a sensing layer electrode 6 are formed thereon. Film formation is performed by an ordinary sputtering method using an RF magnetron sputtering apparatus. Next, SnO ₂ as a sensing layer is formed. Film formation is performed by a reactive sputtering method using an RF magnetron sputtering apparatus. SnO ₂ having 0.5 wt% Sb is used as the target. Finally, silicon is removed from the back surface of the Si substrate 1 by etching to form a diaphragm structure.

A direct current voltage is applied to the heater layer 3 of the sensor obtained as described above by giving an instruction from the above-mentioned microcomputer, and the resistance value at that time is measured while gradually increasing the voltage value (FIG. 1). Step S1).
Now, when electric power Q is input to the heater with the heat capacity C, the temperature T is constantly
T = Q / C
The relationship is established. That is, when power is applied for a sufficiently long time to the heat capacity C of the heater, the temperature is proportional to the input power Q to the heater, that is, the power consumption P of the heater. From the temperature measurement by the thermoviewer, it is known that the heater is 100 ° C. when the power consumption is 5 mW, and 450 ° C. when the power consumption is 30 mW. Therefore, the heater temperature at other power consumption can be estimated. Essentially, the change in the heater resistance is considered to be caused by the change in the heater temperature. Therefore, it is necessary to associate the power consumption and the heater temperature in each case depending on the material and pattern of the heater.

FIG. 3 shows the heater power consumption and the heater resistance value when a voltage is applied. As shown in the figure, when the power consumption is changed between 0 to 30 mW in the case of 30 mW or less, the resistance value is determined one-to-one with respect to the power consumption. On the other hand, when power consumption is applied exceeding 30 mW, the heater resistance value increases in accordance with the excess power consumption α as shown in FIG.
FIG. 5 shows an increase in the heater resistance value with respect to the excess α. It can be seen that the change in the heater resistance increases at a rate of approximately square with respect to the increase of the excess α.

FIG. 6 is a plot of the resistance value when the voltage is turned off after the DC voltage is continuously applied to the heater of the same sensor for 1 minute 30 seconds against the power consumption of the heater. It can be seen that the heater resistance value when the voltage is turned off is constant regardless of the applied voltage.
FIG. 7 is a graph showing an increase in the heater resistance value with respect to the excess α as in FIG. 5 and 7, the relationship that the heater resistance change increases at a rate of approximately square with respect to the excess power consumption both when the heater is heated and when the voltage is OFF. In addition, the heater resistance value which changed by such a process does not show a history with respect to heating at 30 mW or less, and the power consumption and the resistance value have a one-to-one relationship.

  Actually, since the range in which the history is generated differs depending on the heater, an adjustment device that automatically performs a cycle of voltage application (step S1) → heater resistance measurement (steps S1, S2) → voltage application (step S4) →. Make adjustments efficiently. Thereby, the heater can be adjusted to a target resistance value. For this purpose, it is effective to form a loop through steps S1 to S4 and then back to step S1 as in the flowchart of FIG. Note that step S2 is a step of measuring the heater resistance with the voltage turned OFF, step S3 is a step of determining whether or not the measured resistance value has reached the target value, and step S4 is not set to the target value (N). This is a step of increasing the voltage.

Flowchart showing an embodiment of the present invention Structural sectional view showing a thin film gas sensor to which the present invention is applied A graph showing the relationship between heater power consumption and heater resistance when voltage is applied within a range where no history of heater resistance occurs A graph showing the relationship between heater power consumption and heater resistance when voltage is applied to a range where a history of heater resistance occurs. A graph showing the relationship between the excess power consumption and the increase in heater resistance when a voltage is applied to a range where a history of heater resistance occurs. A graph showing the relationship between the heater power consumption and the heater resistance value when the voltage is off when voltage is applied to the range where the history of the heater resistance value occurs. A graph showing the relationship between the excess power consumption and the increase in heater resistance when the voltage is turned off when a voltage is applied to the range where the history of the heater resistance occurs.

Explanation of symbols

1 ... Si wafer (silicon substrate), 2 ... support layer and heat insulating layer, 3 ... heater layer, 4 ... SiO ₂ insulating layer, 5 ... bonding layer, 6 ... sensing layer electrode, 7 ... sensitive layer.

Claims (1)

A thin-film gas sensor in which a thin film heater is formed on a substrate surface in which one side of a Si substrate is hollowed out like a diaphragm via a support film, a sensing film electrode is formed via an electrical insulating film, and a sensing film is formed. A heating heater for a thin film gas sensor, wherein a voltage exceeding a voltage applied during normal use is applied to the thin film heater to increase the temperature of the thin film heater, and the electric resistance value of the thin film heater is adjusted. Resistance value adjustment method.

Images