JP2002131106A - Microheater and thermal air flowmeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve long-term stability of a thermal air flow sensor. SOLUTION: The microheater is provided with a thin film heating section established over a cavity section 3 formed on a monocrystal silicon substrate 2 to heat flowing gas to be measured. The above thin film heating section comprises heating resistors 4 and 5, a resistance temperature sensor 6, an air resistance temperature sensor 7, and an upper thin film 8/a lower thin film 9 vertically sandwiching these resistance temperature sensors. At least either of the upper thin film 8 or the lower thin film 9 is formed so as to contain a tensile stress film 10 and also a water-resistant compression stress film 11 is laminatedly formed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a micro-heater and a thermal air flow meter having a thin-film heating portion provided so as to bridge a cavity formed in a substrate.

[0002]

2. Description of the Related Art In recent years, thermal air flow meters, humidity sensors, and the like formed on a semiconductor chip have been provided, and these are provided with a microheater necessary for the principle of detection. This micro heater has a heating element having a thickness of several μm to prevent heat from escaping to the substrate when the temperature is raised.
A heat insulation structure formed in a thin film structure of about m, for example, a diaphragm structure is adopted. Furthermore, by adopting these structures and reducing the size of the sensor, thermal responsiveness and power consumption are reduced.

The principle of detecting the air flow rate with a thermal air flow meter is as follows. When the air flows, the temperature T of the air is first detected by an air temperature measuring resistor. A heating resistor that is exposed to the air flow is disposed downstream of the air temperature measuring resistor in the air flow, and the heating resistor generates heat according to a current supplied. The energizing current is controlled so as to maintain a state higher than the temperature T of the air detected by the heating resistor by a constant temperature ΔT. At this time, since the amount of heat taken from the surface of the heating resistor differs according to the flow rate of air flowing through the surface of the heating resistor, the current supplied to the heating resistor also changes according to the flow rate of air. Therefore, by collecting data of the current value and the air flow rate necessary to keep the temperature of the heating resistor higher by ΔT than the air temperature T detected by the air temperature measuring resistor, The air flow rate can be known from the current value supplied to the heating resistor. Also,
A temperature measuring resistor is arranged adjacent to the heating resistor to detect the temperature of the heating resistor.

In general, the air temperature measuring resistor, the heating resistor and the temperature measuring resistor are formed in a thin film structure to constitute a micro-heater. In the thin film structure, a thin film of a heating element material is provided on both upper and lower sides. This is a structure formed so as to be sandwiched between provided thin films. The heating element material is usually Pt (platinum),
Si (silicon), NiCr (nickel chrome), Ta
Conductive materials such as N (tantalum nitride), SiC (silicon carbide), and W (tungsten) are used. The thin films provided above and below the heating element include MgO (magnesium oxide), SiO ₂ (silicon dioxide), Six
Ny (silicon nitride, x and y are positive integers), Ta ₂ O
₅ (tantalum oxide), Al ₂ O ₃ (alumina), TiN
A thin film of an insulating material such as (titanium nitride) is used.

[0005] In such a thin film structure, if a compressive stress remains as an average stress of the thin film after the film is formed, the thin film diaphragm bends, so that the air flow on the diaphragm may be disturbed and the air flow measurement may vary. In order to avoid this, as disclosed in JP-A-11-271123, Si
₃ N ₄ employs a tensile stress film, such as film by the mean stress tensile stress of the thin film after the film formation, thereby preventing deflection of the film diaphragm. The Si ₃ N ₄ film is employed as a strength member of the thin film diaphragm because of its high rigidity.
Further, the Si ₃ N ₄ film is water-resistant and has a function of protecting the inner film from moisture.

[0006]

In a sensor using a microheater having a thin-film heating section, a tensile stress film such as a Si ₃ N ₄ film is employed as a strength member of a thin-film diaphragm as described above. In addition, the average stress of the thin film is set to a tensile stress to suppress the bending of the film.

A sensor equipped with such a micro heater
(Air flow meter) measures, for example, the intake air of a car
And exposed to high humidity environments. Therefore,
Si ₃N₄Sensor with micro heater using membrane
(Air flow meter) was tested in a high humidity environment.
At this time, the thin film diaphragm of the sensor breaks after a certain period of use.
We have found that breaking is new. As a result of the survey,
In the sensor, the Si formed on the outermost surface₃N₄Membrane exposed to moisture
Si under tensile stress₃N₄Micro in membrane
It can be seen that crack development is promoted by the coexistence of moisture.
won. As described above, the strength member of the thin film diaphragm is used.
Si₃N₄If the film is subjected to static fatigue, the thin film structure will be destroyed.
You. In other words, the function of the micro-heater may fail,
Measurement using a sensor using an micro heater, for example, an air flow meter
Accuracy decreases. With conventional sensors, high humidity
No countermeasures were taken against static fatigue.

The technique disclosed in Japanese Patent Application Laid-Open No. 11-271123 is a method for preventing a characteristic change of a thin film material due to intrusion of moisture by forming a Si ₃ N ₄ film on the outermost side of a thin film portion. It is. As described above, by forming the water-resistant Si ₃ N ₄ film on the outermost side of the thin film portion, the inner Si ₃ N ₄ film is formed.
The O ₂ film has a function of protecting the O ₂ film from moisture, thereby suppressing the deformation of the thin film due to the swelling stress of the SiO ₂ film. However, in this configuration, since the water-resistant Si ₃ N ₄ film has a large tensile stress, the problem that the Si ₃ N ₄ film itself is broken by static fatigue has not been solved.

FIG. 4 shows a thin film diagram of a conventional sensor.
6 shows a cross section of another example of a flam. Heating resistors 4, 5 and temperature measurement
The resistors 6 and 7 are a substrate-side thin film (hereinafter, referred to as a lower thin film).
8 and the thin film on the side opposite to the substrate (hereinafter referred to as upper thin film 9).
More sandwiched. The lower thin film 8 is a tensile stress film 10.
Si₃N₄SiO 2 film and water absorbing compressive stress film 12 ₂
It is formed in a structure in which films are stacked. The upper thin film 8
Is SiO 2 as the water-absorbing compressive stress film 12.₂Formed by membrane
ing. SiO₂The membrane has a compressive residual stress
However, because of the water absorption, moisture is transmitted. Therefore,
Environmental moisture is SiO₂Si through the film₃N₄Reach the membrane
This causes static fatigue failure of the diaphragm. In FIG.
The relationship between fracture stress and fracture time is shown. Of thin film diaphragm
Water resistant compressive stress film on the upper and lower parts of tensile stress film as a strength member
The conventional sensor that has no formation has long standing time
In this case, the fracture stress decreases, and so-called static fatigue occurs. But
Therefore, the fracture stress due to static fatigue reaches the film deposition residual stress value.
At which point the diaphragm breaks.

An object of the present invention is to maintain the function of a microheater and suppress a decrease in measurement accuracy of a sensor using the microheater, for example, an air flow meter.

[0011]

The present invention solves the above problems by the following means. As described above, the reason why the function of the microheater is reduced is that moisture penetrates into the tensile stress film, and this moisture promotes static fatigue of the tensile stress film. The inventors have succeeded in suppressing the progress of static fatigue of the tensile stress film by preventing moisture from penetrating into the tensile stress film.

That is, in a microheater having at least one end supported by a substrate and having at least one surface made of a thin film in contact with a gas to be heated and heating the gas to be heated, A heat-generating resistor that generates heat when energized, a substrate-side thin film sandwiching the heat-generating resistor from both sides, and a non-substrate-side thin film, wherein the anti-substrate-side thin film is formed to include a tensile stress film; A surface of the film facing the gas to be heated is covered with a water-resistant compressive stress film.

When the substrate-side thin film is formed to include a tensile stress film, similarly, the surface of the tensile stress film on the side facing the gas to be heated may be covered with a water-resistant compressive stress film.

With such a structure, the tensile stress film
The surrounding environment, that is, the thin film on the side adjacent to the gas to be heated is a water-resistant compressive stress film. Therefore, even if moisture adheres to the compressive stress film, static fatigue failure does not occur. further,
The tensile stress film, which is the strength member of the thin film diaphragm such as the Si ₃ N ₄ film inside, is shielded from moisture from the outside air, so that static fatigue fracture does not occur.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, a first embodiment in which the present invention is applied to a thermal air flow meter disposed at a portion for detecting an intake air amount of an internal combustion engine such as an automobile or a ship will be described below. I will explain it.

FIG. 1 is a schematic plan view of a measuring element 1 of a thermal air flow meter according to the present embodiment, and FIG.
FIG. 4 is a cross-sectional view taken along line -A. The illustrated measuring element 1 is formed into a rectangular single-crystal silicon substrate 2 which is a substrate having a hollow portion 3 in the center portion, and is formed so as to bridge the hollow portion 3. And a thin film diaphragm which is a supported thin film heating section. In the following description, for convenience, the lower side of the figure is described as the lower side, and the upper side of the figure is described as the upper side. However, this does not define the upper and lower sides in the use state of the micro heater.

The thin-film diaphragm includes a tensile stress film 10 which is a strength member of the thin-film diaphragm, a resistor disposed on one surface of the tensile stress film 10, and the resistive stress film 10. A water-resistant compressive stress film 11 formed so as to sandwich the body, and a water-resistant compressive stress film 11 formed in contact with the surface of the tensile stress film 10 opposite to the surface on which the resistor is arranged. It is composed of Water resistant compressive stress film 11 and tensile stress film 1 on single crystal silicon substrate 2 side
0 is referred to as a substrate-side thin film (hereinafter, referred to as a lower thin film) 8, and the water resistant compressive stress film 11 on the resistor side is referred to as an anti-substrate-side thin film (hereinafter, referred to as an upper thin film) 9.

The resistors are disposed between the two heating resistors 4 and 5 that generate heat when energized, and are disposed between the two heating resistors 4 and 5 to measure the temperature of the heating resistors 4 and 5. , And an air temperature measuring resistor 7 for measuring the temperature of the air whose flow rate is to be measured. Each of these resistors is formed in a linear shape and arranged in parallel with each other. ing. Terminal electrodes 16 are connected to the heating resistors 4 and 5, the temperature measuring resistor 6, and the air temperature measuring resistor 7, respectively.

The lower thin film 8 is formed, for example, on the lowermost surface as a water-resistant compressive stress film 11 by Six CVD using a plasma CVD method.
A Ny film (x and y are arbitrary positive integers, except for a Si ₃ N ₄ film) is formed, and its upper surface side (the side facing the heating resistors 4 and 5 and the temperature measuring resistors 6 and 7). ) Is formed by laminating a Si ₃ N ₄ film as a tensile stress film 10 using a CVD method. The upper thin film 9 is formed by forming a SixNy film as a water-resistant compressive stress film 11 on the side of the tensile stress film 10 in contact with the heating resistors 4 and 5 and the temperature measuring resistors 6 and 7. That is, the upper and lower outermost portions adjacent to the surrounding environment are formed of SixNy as the compressive stress film 11. When the lower surface of the thin film diaphragm is not exposed to the surrounding environment,
That is, when the cavity is closed, the water-resistant compressive stress film 11 disposed on the lowermost surface of the drawing need not be formed.

As shown in FIG. 3, the thermal air flow meter provided with the measuring element 1 having the above-described configuration is installed in an intake passage 22 of an electronic control fuel injection device, and detects an intake air flow passing through the intake passage 22. I do. The thermal air flow meter includes a measuring element 1, a support 20 supporting the measuring element 1, and an external circuit 21. The measuring element 1 and the external circuit 21
Between each terminal electrode 16 of the measuring element 1 and the external circuit 21;
They are electrically connected by wiring (not shown) protected by the support 20. The measuring element 1 includes an intake passage 22
The external circuit 21 is disposed in the sub passage 23 inside,
It is installed on the outer wall surface of the intake passage 22. Measuring element 1
Are arranged so that the air temperature measuring resistor 7 is located on the upstream side of the airflow to be measured with respect to the heating resistors 4 and 5 and in a direction orthogonal to the airflow.

The principle of detecting the intake flow rate is as follows. When the air flow 24 flows from the direction indicated by the white arrow in FIG. 3, when the air flow passes through the measuring element 1, the temperature T of the air is detected by the air temperature measuring resistor 7. ,
The current value supplied to the heating resistors 4 and 5 is controlled so that the heating resistors 4 and 5 maintain a state higher than the detected temperature T of the air by a constant temperature ΔT. At this time, since the amount of heat taken from the surfaces of the heating resistors 4 and 5 varies depending on the flow rate of the air flowing through the surfaces, the current supplied to the heating resistors also changes according to the flow rate of the air. Therefore, data of the current value and the air flow rate necessary to keep the temperature of the heating resistors 4 and 5 higher than the air temperature T detected by the air temperature measuring resistor 7 by ΔT are taken in advance. Thus, the flow rate of intake air can be detected from the current value supplied to the heating resistors 4 and 5 at this time. The direction of the air flow can be detected by comparing the magnitude of the current supplied to the heating resistors 4 and 5 using the characteristic that the temperature of the heating resistor on the upstream side decreases.

In an electronically controlled fuel injection device for an internal combustion engine of an automobile or the like, since the outside air is sucked, the air whose flow rate is to be measured contains water in addition to sand, salt, and other dust. In an internal combustion engine of an automobile or the like, an air filter is provided to remove inhaled dust in the outside air. However, since moisture passes through the air filter, the sensor is exposed to a high humidity environment. Furthermore, when dust having a small particle diameter that has passed through the air filter adheres to the surface of the measurement element, dew condensation is likely to occur in the vicinity thereof. As described above, when the thin film diaphragm is exposed to a high-humidity environment and moisture adheres, the tensile stress film, which is a strength member of the thin film diaphragm, may be broken by static fatigue due to moisture. This is because the development of microcracks existing in the tensile stress film is promoted by the coexistence of moisture.

In the present embodiment, in order to cope with such a situation, the water-resistant compressive stress films 11 are formed on the upper and lower surfaces of the tensile stress film 10 which is the strength member of the thin-film diaphragm as described above, Is prevented from directly contacting the tensile stress film 10, it is possible to prevent the tensile stress film 10 from breaking due to static fatigue.

This will be described with reference to the results of a static fatigue test of a thin film diaphragm. FIG. 5 shows the relationship between the breaking stress and the breaking time. In the sensor of the present embodiment in which the water-resistant compressive stress film is formed on the upper and lower portions of the tensile stress film, which is the strength member of the thin film diaphragm, the fracture stress is constant and does not decrease regardless of the standing time, and the static fatigue No destruction will occur.

As described above, in the sensor according to the present embodiment,
Each of the upper thin film 8 and the lower thin film 9 is formed by laminating one or more thin films, and the heating resistors 4 and 5 and the temperature measuring resistor 6,
The air temperature measuring resistor 7 is formed on a tensile stress film 10 which is a strength member of the thin film diaphragm.
A water-resistant compressive stress film 11 is formed below. However, since the heating resistors 4 and 5, the temperature measuring resistor 6, and the air temperature measuring resistor 7 are patterned, an etching stop thin film may be formed between layers. For example, as in the second embodiment shown in FIG. 6, when polycrystalline silicon is used as the heating resistors 4 and 5, the temperature measuring resistor 6, and the air temperature measuring resistor 7, it is used to stop etching of SiO ₂ . Thin film 1
3 may be formed on the lower surface (the upper surface of the tensile stress film 10) of the heating resistors 4, 5 and the temperature measuring resistor 6, and the air temperature measuring resistor 7.

Next, a method of manufacturing the thermal air flowmeter will be described with reference to FIG. FIG. 7 shows a cross section taken along line BB of FIG. First, as shown in FIG. 7A, a SixNy film is deposited as a water-resistant compressive stress film 11 on the surface of the single crystal silicon substrate 2 by a plasma CVD method or the like. In contrast to the CVD method in which the substrate temperature is set high to promote the chemical reaction of the reaction gas, in the plasma CVD method, the substrate temperature can be kept low because the atmosphere is turned into plasma to accelerate the chemical reaction. The water-resistant compressive stress film can also be formed using polycrystalline silicon, Al ₂ O _3, TiN, or diamond. Subsequently, a Si ₃ N ₄ film is formed on the water-resistant compressive stress film 11 as a tensile stress film 10 which is a strength member of the thin film diaphragm by CV.
It is deposited by the D method.

Next, a polycrystalline silicon film to be the heating resistors 4 and 5, the temperature measuring resistor 6, and the air temperature measuring resistor 7 is formed on the tensile stress film 10 by CVD. Thereafter, unnecessary portions of the formed polycrystalline silicon film are removed by etching to form patterns of a heating resistor, a temperature measuring resistor, and an air temperature measuring resistor (see FIG. 7B). .

After the above-mentioned etching removal is completed, next, in order to form a water-resistant compressive stress film 11 serving as an upper thin film,
Six is formed by a plasma CVD method so as to cover the heating resistors 4 and 5, the temperature measuring resistor 6, and the air temperature measuring resistor 7.
Ny is deposited. A portion of the water-resistant compressive stress film 11 corresponding to the heat generating resistors 4 and 5 and the temperature measuring resistor 6 and the terminal electrode 16 of the air temperature measuring resistor 7 is connected to an upper thin film (water-resistant) for connecting a lead wire. It is necessary to form an electrode opening 15 in the compressive compressive stress film 11).
Is formed in a predetermined shape by etching a predetermined portion after forming the upper thin film. At this time, patterning for forming a cavity 3 having a predetermined shape is performed from the back surface side of the single crystal silicon substrate 2 (see FIG. 7C).

Finally, in order to form the cavity 3, anisotropic etching is performed from the back side of the single crystal silicon substrate 2 which is patterned into a predetermined shape, and the heating resistors 4, 5 and the temperature measuring resistor are formed. 6, the air temperature measuring resistor 7
It is formed on a thin film diaphragm sandwiched between an upper thin film and a lower thin film (see FIG. 4D).

FIG. 8 shows a third embodiment of the present invention.
The second embodiment is different from the first embodiment in that the upper thin film 9 is formed of a water-resistant compressive stress film 11 of SixNy and a low-pressure film such as SiO ₂ formed on the water-resistant compressive stress film 11. This is a point formed by the rigid protective film 14.

In a thin film diaphragm having a low toughness SixNy film on the outermost side (the surface in contact with the measurement air), the SixNy film is destroyed by collision of dust mixed in the measurement air, and the moisture in the measurement air is subjected to tensile stress. Lower Si film
Up to ₃ N ₄ film which may cause the static fatigue fracture of the tensile reaches stress film. However, in the measuring element 1 according to the present embodiment, the impact due to the dust is prevented by the SiO ₂ of the outermost protective film 14.
The SixNy film that is absorbed by the film and is the water-resistant compressive stress film 11 can be prevented from being damaged. Although moisture permeates the SiO ₂ film but does not permeate the SixNy film, it is possible to prevent static fatigue fracture of the further inner Si ₃ N ₄ film.

[0032]

According to the present invention, a thin film diaphragm which is a thin film heating portion is constituted by sandwiching a heating resistor which generates heat by energization between thin films, and the thin film is formed of a tensile stress film which is a strength member of the thin film diaphragm; The surface of the tensile stress film facing the flowing gas to be heated is covered with a waterproof compressive stress film.
For this reason, moisture contained in the gas to be heated does not enter the tensile stress film, and the progress of destruction due to static fatigue of the tensile stress film can be suppressed. As a result, it is possible to prevent the function of the thin film heating section from being reduced.

[Brief description of the drawings]

FIG. 1 is a schematic plan view showing a first embodiment of the present invention.

FIG. 2 is a sectional view taken along line AA of FIG. 1;

FIG. 3 is a plan view showing a schematic configuration of a thermal air flow meter.

FIG. 4 is a cross-sectional view showing an example of the related art.

FIG. 5 is a conceptual diagram showing a result of a static fatigue fracture test of a thin film diaphragm in a high humidity environment, comparing a conventional example and an embodiment of the present invention.

FIG. 6 is a sectional view of a second embodiment of the present invention.

FIG. 7 is a cross-sectional view showing a step of manufacturing the measuring element shown in FIG.

FIG. 8 is a sectional view of a third embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Measuring element 2 Single crystal silicon substrate 3 Cavity part 4 Heating resistor 5 Heating resistor 6 Side temperature resistor 7 Air temperature side temperature resistor 8 Lower thin film 9 Upper thin film 10 Tensile stress film 11 Water resistant compressive stress film 12 Water absorption Compressive stress film 13 Etch stop thin film 14 Protective film 15 Electrode opening 16 Terminal electrode 20 Support 21 External circuit 22 Intake passage 23 Sub passage 24 Air flow

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Akio Hokawa 502, Kandachicho, Tsuchiura-shi, Ibaraki Machinery Research Laboratory, Hitachi, Ltd. (72) Shinya Igarashi 2477 Takaba, Hitachinaka City, Ibaraki Prefecture Within engineering (72) Inventor Masamichi Yamada 7-1-1, Omika-cho, Hitachi City, Ibaraki Prefecture F-term in Hitachi Research Laboratory, Hitachi Ltd. 2F035 AA02 EA05 EA08

Claims (7)

[Claims] 1. A micro-heater for heating a gas to be heated having a thin-film heating portion comprising at least both ends supported by a substrate and a thin film in contact with a gas to be heated flowing through at least one surface, and heating the gas to be heated. The portion is configured to include a heating resistor that generates heat by energization, a substrate-side thin film and an anti-substrate-side thin film sandwiching the heating resistor from both sides, and the anti-substrate-side thin film includes a tensile stress film. Wherein the surface of the tensile stress film on the side facing the gas to be heated is covered with a water-resistant compressive stress film. 2. A micro-heater which has at least one end supported by a substrate and has a thin-film heating portion made of a thin film having at least one surface in contact with a gas to be heated and heats the gas to be heated, wherein the thin-film heating portion comprises: A heating resistor that generates heat when energized, and a substrate-side thin film and an anti-substrate-side thin film sandwiching the heating resistor from both sides;
Wherein the substrate-side thin film is formed to include a tensile stress film, and the surface of the tensile stress film facing the gas to be heated is covered with a water-resistant compressive stress film. . 3. A micro-heater comprising a thin-film heat-generating portion comprising a heat-generating resistor that generates heat by energization sandwiched between thin films, and heating a gas flowing through the surface of the thin-film heat-generating portion. A water-resistant compressive stress film covering a surface of the tensile stress film on a side facing the flowing gas. 4. The microheater according to claim 1, wherein the water-resistant compressive stress film is made of polycrystalline silicon, Al ₂ O _3, SixNy, TiN, or diamond. Micro heater. 5. The micro-heater according to claim 1, wherein the heating resistor is formed in a linear shape, and is parallel to the heating resistor formed in a linear shape. A micro-heater, comprising: a temperature-measuring resistor arranged in a direction to measure the temperature of the heat-generating resistor; and the temperature-measuring resistor is sandwiched by the thin film together with the heat-generating resistor from both sides. 6. The micro-heater according to claim 5, wherein the heating element is disposed in a direction parallel to the heating resistor and upstream of the heating resistor formed in a linear shape. A micro heater provided with a gas temperature measuring resistor for measuring the temperature of a heated gas, wherein the temperature measuring resistor is sandwiched by thin films from both sides together with the heating resistor and the temperature measuring resistor. 7. A thermal air flow meter, which is disposed in an intake pipe connected to a combustion chamber of an internal combustion engine and includes a microheater for heating air to be measured and measures an air flow passing through the intake pipe. A thermal air flow meter, wherein the micro heater is the micro heater according to any one of claims 1 to 6.