JP2001091486A - Thermal type sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thermal type sensor preventing a heater from peeled off and a diaphragm from breaking and reducing the aging variation even used under a high temperature or heated intermittently. SOLUTION: This thermal type sensor having a thin film heater on a diaphragm formed of a silicon oxide film and/or a silicon nitride film is provided with a hafnium oxide layer between the diaphragm formed of the silicon oxide film and/or the silicon nitride film, and the thin type heater.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to thermal sensor technology.

[0002]

2. Description of the Related Art Various sensors have been manufactured using a silicon wafer as a substrate. Since these are manufactured by applying a thin film manufacturing technology or a semiconductor fine processing technology, they are relatively low in cost, can be miniaturized, and are suitable for mass production at the same time.

[0003] Among such sensors using a silicon wafer, a flow sensor, a temperature sensor, a humidity sensor, a contact combustion type combustible gas sensor, a semiconductor type gas sensor, a hot wire type semiconductor gas sensor, a solid electrolyte type gas sensor, and the like have a detection mechanism. , It is necessary to heat the detection unit. In a sensor requiring such heating (in the present invention, such a sensor is referred to as a "thermal sensor"), a thin film heater (micro heater) is provided near the detection unit. This thin film heater is usually made of platinum from the required performance such as durability (oxidation resistance) and performance stability.

[0004] The detection section is usually formed of a silicon oxide film and / or a silicon nitride film, and
In order to reduce the heat capacity of the portion to be heated by the heater and at the same time to reduce the heat conduction to other portions, it is formed in a portion having a reduced thickness (referred to as a "diaphragm").

FIG. 6A shows a model cross-sectional view of an example of a thermal sensor (combustible gas sensor) having such a thin film heater made of platinum. Reference numeral 1 denotes a silicon substrate, on which a silicon oxide film 2, a silicon nitride film 3, and a silicon oxide film 2 'are formed on a diaphragm 8, and a platinum heater 5 and a gas-sensitive film 6 are formed as detecting portions.

FIG. 6B is a model sectional view of another example of a thermal sensor (combustible gas sensor). This is obtained by omitting the silicon oxide film 2 'from the sensor of FIG. The top views of these sensors are shown in FIG.
(C) shows a model.

In such a thermal sensor having a thin-film heater on a diaphragm formed of such a silicon oxide film and / or a silicon nitride film, the thin-film heater peels off when the operating temperature of the heater is high, or the heater is removed. There is a problem that the diaphragm is broken (generation of cracks, cracks, etc.) at the time of intermittent driving. Even in the case where such destruction does not occur, there is a temporal change in sensor characteristics (a change in electric resistance of the heater or a change in sensor output) which is considered to be caused by the operation of the heater. FIG. 7 (a)
FIG. 7B is a scanning electron microscope photograph showing the heater peeled off in the intermittent drive test and the state in the vicinity thereof, and FIG. 7B is a trace diagram of the heater.

[0008]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned conventional problems, that is, there is no peeling of the heater and no destruction of the diaphragm even with use at high temperature or intermittent heating, and there is little change with time. An object is to provide a stable thermal sensor.

[0009]

The present inventors have studied the peeling of the thin film heater, the destruction of the diaphragm, or the change over time of the sensor characteristics.
The present inventors have found out that this is caused by poor adhesion between a thin film heater made of platinum and a diaphragm formed of a silicon oxide film and / or a silicon nitride film, and reached the present invention.

That is, in order to solve the above-mentioned problems, a thermal sensor according to the present invention has a thin-film heater on a diaphragm formed of a silicon oxide film and / or a silicon nitride film. , A thermal sensor having a hafnium oxide layer between the above-mentioned diaphragm formed of a silicon oxide film and / or a silicon nitride film and a thin heater.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In a thermal sensor according to the present invention, a thin film heater is formed between a diaphragm formed of a silicon oxide film and / or a silicon nitride film (hereinafter also referred to as a "base film") and a thin film heater. Layer must be made of hafnium oxide.

That is, the coefficient of thermal expansion of the layer made of hafnium oxide is an intermediate value between the coefficient of thermal expansion of the thin-film heater made of platinum and the coefficient of thermal expansion with the underlying film. Relaxes the thermal stress generated between the thin film heater and the underlying layer caused by a heat cycle, etc., so that the heater does not peel off or the diaphragm is destroyed.
A stable thermal sensor with little change over time can be obtained.

Here, the hafnium oxide layer can be formed by applying a vacuum application technique such as sputtering or electron beam evaporation. In the present invention, the thickness of the hafnium oxide layer is variously selected depending on the intended use condition (temperature, etc.) of the thermal sensor, required durability, and the like, but usually 100 to 500 ° is sufficient. However, if necessary, the thickness may be about several thousand mm. At this time, cracks and cracks do not occur in the hafnium oxide layer.

The conductivity of hafnium oxide is 10 ^-14.
(Ω ^-1 · cm ^-1 ), which is extremely small, and there is almost no current leakage from the heater, enabling accurate measurement. In addition, the sensor has excellent chemical resistance (water resistance, strong acid resistance, strong alkali resistance). Since high resistance can be obtained even in the wet etching process of another film or silicon substrate in the manufacturing process, there is an advantage that the manufacturing cost is increased, or there is no adverse effect such as limitation of manufacturing conditions.

When the layer between the diaphragm and the thin film heater is tantalum pentoxide, titanium oxide, aluminum oxide (alumina), or zirconium oxide,
In a normal process, production is difficult, or even if it can be produced, the yield is extremely low, or sufficient durability cannot be obtained, resulting in a drawback such as a shortened life.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An example of a thermal sensor according to the present invention will be specifically described below with reference to the drawings. FIG. 1A is a top view of a flammable gas sensor that is a thermal sensor according to the present invention, and FIG. 1B is a cross-sectional view of the model along AA.

This sensor is a thermal sensor having a silicon wafer as a substrate (silicon 1 in the figure) and a thin platinum heater 5 on a diaphragm 8 formed of a silicon oxide film 2 and a silicon nitride film 3. Numeral 5 is covered with a gas-sensitive film 6, and these two constitute a detection unit.

The electric power to the platinum heater 5 is supplied from the platinum pad 7 by a gold lead wire bonded to the platinum pad. In this sensor, a hafnium oxide layer 4 is provided between a platinum heater 5 and a diaphragm 8 formed of a silicon oxide film 2 and a silicon nitride film 3.

This sensor can be formed, for example, as follows. First, a silicon wafer is thermally oxidized, and a silicon oxide layer (thickness: 100 to 10000 °) is formed on its surface.
(Normal), 6000 ° in this example) (FIG. 2)
(A)).

Next, the silicon nitride film 2 (thickness: 100 to 5000 ° (normal), 25
00Å) (FIG. 2A), and the silicon oxide film 2 and the silicon nitride film 3 in the diaphragm forming portion on the back side of the detecting portion.
By combining the photolithography process and the wet etching method, or by the dry etching method,
Etching into a predetermined pattern (FIG. 2C).

Further, a hafnium oxide layer (thickness: 100 to 500 ° (normal), 500 ° in this example) is formed on the silicon nitride film 3 on the detecting portion formation side, and a thin platinum heater (thickness) is formed on the hafnium oxide layer. Sa: 100 ~ 10000Å
(Normally, 5000 ° in this example) is formed by a vacuum application technique such as sputtering or electron beam evaporation (FIG. 2).
(D) and (e)). In addition, as shown in FIG. 1A, the thin film heater 5 made of platinum is usually formed in a zigzag shape or the like.

Next, as shown modelly in FIG. 2 (f), the gas-sensitive film 6 (a palladium-supported alumina layer or a tin (IV) oxide layer (Pd / Al ₂ O ₃ or SnO ₂ )) and the like are usually of a thickness. 1 to 15 μm, and in this example, Pd /
An Al ₂ O ₃ layer is formed to cover the thin-film heater 5, and finally, the silicon 1 is etched from the back surface with TMAH (tetramethylammonium hydroxide) or the like to form a diaphragm 8 (FIG. 2G).

Next, the characteristics of the combustible gas sensor thus manufactured were examined. A thermal sensor having a thin film heater made of platinum on a diaphragm formed of a silicon oxide film and a silicon nitride film, wherein a sensor having a 500 ° hafnium oxide layer between the diaphragm and the thin film heater (“hafnium oxide on”) The present invention relates to a thermal sensor having a thin film heater made of platinum on a diaphragm formed of a silicon oxide film and a silicon nitride film, wherein there is no hafnium oxide layer between the diaphragm and the thin film heater. A sensor obtained in exactly the same manner except that a thin film heater is directly disposed on a silicon nitride film (“on silicon nitride film”: a comparative example according to the prior art), and platinum on a diaphragm formed from a silicon oxide film. A thermal sensor having a thin-film heater consisting of hafnium oxide between a diaphragm and a thin-film heater. And no silicon nitride film, a silicon oxide film directly thin-film sensor heater is obtained in the same manner is in addition being arranged ( "on the silicon oxide film": Comparative Example according to the prior art) 3
We prepared the following types of sensors and studied the following.

FIG. 3 shows these three types of sensors under the same conditions.
The result of a change with time of the electric resistance of the heater when heated to a relatively high temperature is shown. In the results of the sensor according to the present invention shown by the solid line, the electric resistance of the heater did not change even after 500 hours from the start of the experiment, whereas the electric resistance of the two types of sensors according to the prior art increased. You can see that Note that an increase in the electric resistance of the heater causes destruction of the sensor due to fusing of the heater.

FIG. 4 shows the results of an investigation of the change over time in the electrical resistance of the heaters of these three types of sensors when pulsed driving in which heating is intermittently repeated is performed. The sensor according to the present invention shown by a solid line shows no change in the electric resistance of the heater even after 500 hours from the start of the experiment, whereas the electric resistance of the two sensors according to the prior art is different from this. It can be seen that has soared.

From these experimental results, it can be seen that the thermal sensor according to the present invention is less deteriorated even when used at a relatively high temperature, and is less damaged during intermittent driving. The production yield of the thermal sensor according to the present invention at the time of manufacturing the sensor was 99%.
%, Which is almost equal to the production yield of the other two types of sensors having no hafnium oxide layer. On the other hand, a study was conducted to provide an aluminum oxide layer for the same purpose as the hafnium oxide layer, but the manufacturing yield (non-defective product rate) at that time was 40%.
It was about 50%. This is considered to be because the film stress of hafnium oxide is extremely small, so that the residual stress of the thin film diaphragm can be kept small, and as a result, the breakage of the thin film diaphragm is small, and as a result, the production yield is improved.

Here, a silicon nitride film (symbol “●” (SIN)) and a silicon oxide film (symbol “O” (S
IO2)) or hafnium oxide film (symbol “△” (H
fO2)) were formed to have the same size and thickness, respectively, and the results obtained when the film stresses were measured by a warpage measuring device are shown in FIG. The result shown in FIG. 5 supports the above assumption.

As a further secondary effect of the present invention, the power consumption of the heater is 25% less than the power consumption of the other two sensors.
It was possible to reduce the degree. This is presumably because the thermal conductivity of hafnium oxide is extremely small, so that heat diffusion to other parts such as the diaphragm has decreased. From this, a secondary effect such as an increase in sensor sensitivity can also be obtained in application to a catalytic combustion sensor or the like.

[0029]

The thermal sensor of the present invention has a stable production with a high production yield, low power consumption, and little aging even when used at a high temperature or subjected to intermittent heating without peeling of the heater or destruction of the diaphragm. It is an excellent thermal sensor.

[Brief description of the drawings]

FIG. 1A is a diagram illustrating a model top view of an example of a thermal sensor according to the present invention. (B) It is a figure showing the model sectional view of the thermal sensor of (a).

2 (a) to 2 (g) are views showing a method for manufacturing the thermal sensor of FIG. 1 (a).

FIG. 3 is a diagram showing a result of examining a change in electric resistance of a heater when the thermal sensor of the present invention is driven at a relatively high temperature.

FIG. 4 is a diagram showing a result of examining a change in electric resistance of a heater when the thermal sensor of the present invention is intermittently driven.

FIG. 5 is a diagram showing the results of measuring the film stress of a silicon nitride film, a silicon oxide film, and a hafnium oxide film.

FIG. 6 is a model diagram showing an example of a conventional thermal sensor. (A) It is a figure showing the sectional view of an example of a thermal sensor. (B) It is a figure showing the sectional view of other examples. (C) It is a top view of the thermal sensor of (a) or (b).

FIG. 7 is a diagram showing a state in which a heater is peeled off in a conventional thermal sensor. (A) It is a scanning electron microscope photograph. (B) It is the figure which traced the heater part of the photograph of (a).

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Silicon 2 Silicon oxide film 3 Silicon nitride film 4 Hafnium oxide layer 5 Platinum heater 6 Gas sensitive film 7 Platinum bad 8 Diaphragm

Continued on front page F term (reference) 2G004 BE12 BE22 BF02 BJ02 BK04 BL08 BL17 BL18 BM07 2G046 AA01 AA02 BA01 BB04 BC04 BE02 BF04 DA05 DB05 DC11 DD01 DD03 EA07 EA11 EB06 FB01 FB02 FB06 FE14 FE38 2G060 AFA18 BA01 BA03 BA05 BB02 HA01 HA03 HB06 HE01 HE03

Claims (1)

[Claims] 1. A thermal sensor having a thin film heater on a diaphragm formed of a silicon oxide film and / or a silicon nitride film, wherein the thin film heater and the thin film heater formed of the silicon oxide film and / or the silicon nitride film are connected to each other. A thermal sensor having a hafnium oxide layer between them.