JP3601993B2 - Thermal sensor and method of manufacturing the same - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a thermal sensor having a heat generating portion and a method for manufacturing the same.
[0002]
[Prior art]
The thermal type sensor detects acceleration, pressure, and flow rate of a fluid based on a heat transfer phenomenon that occurs in accordance with changes in acceleration, pressure, fluid, and the like. Or, it measures the flow rate, and is used, for example, when measuring the acceleration of an automobile, the pressure applied to an internal combustion engine, and the intake air amount of the internal combustion engine.
[0003]
As an example of a conventional thermal sensor, a bridge type heat-sensitive flow sensor disclosed in Japanese Patent Publication No. 5-7659 will be described with reference to FIGS. 4 and 5, 1 is a substrate made of a silicon semiconductor, 2 is an insulating support film made of silicon nitride, 3 is an insulating protective film made of silicon nitride, and 4 is a heat-sensitive resistive film. A heating resistance made of a certain permalloy, 5 and 6 each represent a temperature measuring resistance made of a permalloy which is a heat-sensitive resistance film, and 7 shows a comparative resistance made of a permalloy which is a heat-sensitive resistance film. The two temperature measuring resistors 5 and 6 are disposed so as to be perpendicular to the flow direction of the fluid 10 to be measured with the heating resistor 4 interposed therebetween. An air space 9 is provided immediately below the portion where the heat generating resistor 4 and the temperature measuring resistors 5 and 6 are formed to form a bridge structure. The air space 9 is formed by removing a part of the substrate 1 from the opening 8 formed in the substrate 1 using an etchant that does not etch the silicon nitride film.
[0004]
Next, the operation of the flow sensor will be described. In the flow sensor, a current is supplied to the heating resistor 4 by a control circuit (not shown) so that the temperature of the heating resistor 4 is maintained at, for example, 200 ° C. higher than the temperature of the substrate 1 detected by the comparison resistor 7. . Since the air space 9 exists below the heating resistor 4 as described above, the heat generated by the heating resistor 4 is not transmitted to the comparison resistor 7 and is detected by the comparison resistor 7. The temperature is approximately equal to the temperature of the surrounding air.
[0005]
The heat generated by the heat generating resistor 4 is transmitted to the temperature measuring resistors 5 and 6 via the support film 2, the protective film 3 or the heat-sensitive resistance film. As shown in FIG. 4, the temperature measuring resistors 5 and 6 are arranged at positions symmetrical with respect to the heating resistor 4. Therefore, when there is no air flow, the temperatures of the two temperature measuring resistors 5 and 6 are There is no difference between the resistance values of the temperature measuring resistors 5 and 6. However, when there is a flow of air, the upstream temperature measuring resistor is cooled by air, but the downstream temperature measuring resistor is heated by the heat transmitted from the heating resistor to the air. Not as cool as resistance. For example, when the air flow in the direction indicated by the arrow 10 occurs, the temperature measuring resistor 5 on the upstream side has a lower temperature than the temperature measuring resistor 6 on the downstream side, and the difference between the two resistance values is the flow rate or flow rate of the air. The larger the is, the more it spreads. As described above, by detecting the difference between the resistance values of the temperature measuring resistors 5 and 6, the flow velocity or the flow rate of the air can be measured. When the direction of the air flow is opposite to the direction of the arrow 10, the temperature of the temperature measuring resistor 6 becomes lower than that of the temperature measuring resistor 5, so that the direction of the air flow can also be detected.
[0006]
In addition to the bridge type described above, a diaphragm type heat-sensitive flow sensor is widely used. Hereinafter, a conventional diaphragm type thermal flow sensor will be described with reference to FIGS. 6 and 7, the components 1 to 10 are substantially the same as the components of the bridge type flow detecting element having the same reference numerals. An opening formed by removing a part of the substrate 1 by etching from the surface opposite to the surface on which it is formed. That is, in the diaphragm type thermal flow sensor, the substrate 1 immediately below the diaphragm region 14 in which the heating resistor 4 and the two temperature measuring resistors 5 and 6 are sandwiched between the support film 2 and the protective film 3 is removed. With such a configuration, higher strength can be obtained as compared with a bridge type flow sensor. The principle of detecting the flow velocity or flow rate of air is the same as that of the bridge type flow rate detection element.
[0007]
[Problems to be solved by the invention]
However, such a diaphragm type flow rate sensor has problems as described below.
[0008]
In a conventional flow sensor, when a pinhole is present in the support film, when removing the substrate, the etchant penetrates through the pinhole and the surface of the substrate supporting the support film is etched to form a pit or a diaphragm. Since the shape becomes irregular, there has been a problem that the reliability of the flow sensor is reduced. If a dense support film without pinholes is formed to cope with this, the support film itself bends after the substrate is removed, so that it has been difficult to maintain the shape of the diaphragm flat.
[0009]
Further, when the heating resistor is energized and the temperature of the heating resistor rises, the heating resistor expands thermally and the diaphragm region is bent, so that the reliability of the flow sensor decreases and the response characteristics of the flow sensor vary. There was a problem that occurs. In addition, when the bending change of the diaphragm area is repeated during the operation, the durability of the flow rate sensor is deteriorated.
[0010]
A first object of the present invention is to provide a thermal sensor with excellent detection accuracy and a method for manufacturing the same.
[0011]
A second object of the present invention is to provide a thermal sensor having a diaphragm region having excellent detection accuracy, high reliability, and no deflection, and a method for manufacturing the same.
[0012]
[Means for Solving the Problems]
Thermal type sensor according to the present invention, a silicon substrate having an opening,
A first support film formed on the silicon substrate and made of silicon dioxide by thermal oxidation of the silicon substrate surface;
A second support film formed on the first support film and constituting a diaphragm at the opening;
A heat-generating portion formed on the diaphragm of the second support film and made of a heat-sensitive resistance material;
The first support membrane has been removed at the opening;
The second support film is made of a material different from that of the first support film so that a tensile stress is set on the diaphragm.
[0013]
Further, the method for manufacturing a thermal sensor according to the present invention includes the steps of: forming a first support film made of silicon dioxide on a silicon substrate by thermal oxidation of the surface of the silicon substrate;
Forming a second support film made of a different material from the first support film on the first support film;
Forming a heat generating portion made of a heat-sensitive resistance material on the second support film;
A first etching step of removing a part of the silicon substrate by wet etching to form an opening reaching the first support film from the back surface of the silicon substrate;
A second etching step of removing a portion of the first support film exposed at the opening to form a diaphragm composed of the second support film.
[0014]
In the present invention, it is preferable that the second support film is formed of silicon nitride.
[0015]
In the present invention, it is preferable that a protective film made of silicon nitride is formed on the second support film and the heat generating portion.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, a flow sensor which is a thermal sensor of the present invention will be described with reference to the drawings.
[0018]
Embodiment 1 FIG.
First, a flow sensor 30 according to the first embodiment will be described with reference to FIG. The flow sensor 30 includes the heat generating portion 11 formed on the insulating second support film 16. More specifically, the second support film 16 is formed on the substrate with the insulating first support film interposed therebetween, and the heat generating portion 11 is formed of a heat-sensitive resistance film. It includes temperature resistance and heat generation resistance. Further, the substrate located below the diaphragm region 14 around the heat generating portion 11 has been removed, and a protective film (not shown) is provided on the heat generating portion 11.
[0019]
Next, a method of manufacturing the flow sensor 30 will be described with reference to FIGS. First, as shown in FIG. 2A, an insulating film having a thickness of about 0.5 μm is laminated as a first supporting film 15 on a substrate 1 of a silicon wafer having a thickness of about 400 μm. An insulating film having a thickness of about 1 μm is laminated thereon as the second support film 16. Specifically, it is preferable that the first support film 15 be formed of an oxide film.
[0020]
Subsequently, platinum or the like having a thickness of 0.2 μm is formed as a heat-sensitive resistive film on the second support film 16 by an evaporation method, a sputtering method, or the like. Alternatively, patterning is performed using a dry etching method or the like to form the heat generating portion 11 having a temperature measuring resistor and a heat generating resistor as shown in FIG. Further, as shown in FIG. 2C, a 1 μm-thick insulating protective film 3 is laminated so as to cover the heat generating portion 11 and the second support film 16.
[0021]
Further, a back surface protection film (not shown) is provided on the surface of the substrate 1 opposite to the surface on which the first support film 15 is arranged, and an etching hole is formed in the back surface protection film by using a photolithography method or the like. I do. Subsequently, as shown in FIG. 3A, a part of the substrate 1 is removed by alkali etching or the like by using a back surface protective film having an etching hole, an opening 12 is provided, and a diaphragm region 14 is formed.
[0022]
Thereafter, as shown in FIG. 3B, only the first support film 15 immediately below the diaphragm region 14 is removed by wet etching or the like. At this time, by shortening the etching time to 10 minutes or less, the first support film 15 is etched only in the thickness direction, so that only the first support film 15 immediately below the diaphragm region 14 can be removed. it can.
[0023]
As described above, by removing the first support film 15 located immediately below the diaphragm region 14, the diaphragm region 14 can be formed thin. That is, after a support film having a two-layer structure is formed on the substrate 1, the first support film 15 protects the surface of the substrate 1 other than immediately below the diaphragm region 14 while removing the substrate 1 to remove the diaphragm region. By forming only the first support film 15 immediately below the diaphragm region 14, it is possible to reduce the thickness of the diaphragm region 14 without generating pits or the like on the surface of the substrate 1. it can. In this manner, by improving the thermal conductivity of the diaphragm region 14, the flow rate detection accuracy of the flow rate sensor 30 can be improved.
[0024]
Further, when the substrate 1 immediately below the diaphragm region 15 is removed, the etching solution penetrates into a pinhole or the like existing in the second support film 16 or the protection film 3, and the etching solution reaches the surface of the substrate 1. In order to prevent this, it is preferable to use the first support film 15 having no pinhole. When the first support film having no pinhole is used, generation of etching pits on the surface of the substrate 1 can be more reliably prevented. Furthermore, since the second support film 16 prevents the diaphragm region 14 from being bent, abnormal shape of the diaphragm region can be prevented. That is, the flow sensor 30 manufactured by the above-described method has a small variation in response characteristics and is highly reliable.
[0025]
Embodiment 2 FIG.
In the flow sensor according to the second embodiment of the present invention, the second support film and the protective film of the flow sensor according to the first embodiment are configured to be capable of setting a tensile stress. That is, by setting the tensile stress of the second support film and the protective film, when the heat generating portion is energized and thermally expanded, the stress generated by the thermal expansion is canceled by the tensile stress of the second support film and the protective film. This can prevent the diaphragm region from bending. By doing so, the reliability of the flow sensor can be improved. It is sufficient that the tensile stress of the protective film and the second support film is such that the thermal stress overcomes the thermal expansion generated in accordance with the current actually supplied to the heat generating portion. Further, the flow sensor according to the second embodiment exhibits the same operation and effect as the flow sensor according to the first embodiment.
[0026]
In the above embodiment, the tensile stress of the second support film and the protective film is set, but the present invention is not limited to this, and only the tensile stress of one of the second support film and the protective film is set. May be set.
[0027]
Embodiment 3 FIG.
The flow sensor according to the third embodiment of the present invention is a flow sensor in which the first support film of the flow sensor according to the first embodiment is formed of a silicon dioxide film. Since the silicon dioxide film can be easily formed and has few pinholes, the surface of the substrate 1 can be more reliably protected. In addition, the first support film is formed by forming a first support film with a silicon dioxide film, further forming a second support film with a hydrofluoric acid resistant film, and using an aqueous solution of hydrofluoric acid. Since only the flow sensor can be easily removed, the flow sensor can be manufactured easily and at low cost.
[0028]
Further, the silicon dioxide film can be formed by a CVD method, a thermal oxidation method, or the like. In particular, when using a thermal oxidation method of thermally oxidizing a silicon substrate to form a silicon dioxide film, a silicon dioxide film without pinholes can be easily formed on both surfaces of the silicon substrate. By using the silicon dioxide film as the first support film and the silicon dioxide film formed on the other surface as the back surface protective film, the step of forming the back surface protective film can be omitted. That is, by using the thermal oxidation method, the steps can be omitted, and the manufacturing cost of the flow sensor can be reduced.
[0029]
Embodiment 4 FIG.
The flow sensor according to the fourth embodiment of the present invention is such that the protective film and the second support film of the flow sensor according to the second embodiment are formed of a silicon nitride film. Specifically, a silicon nitride film is formed by a sputtering method, a CVD method, or the like. Since the silicon nitride film can easily control the tensile stress of the film, by using such a silicon nitride film as the protective film and the second support film, the film stress in the diaphragm region is controlled as the tensile stress. Therefore, even when the heat generating portion thermally expands, the stress due to the thermal expansion is canceled by the tensile stress of the protective film and the second support film, and the deflection of the diaphragm region can be prevented.
[0030]
Further, since the silicon nitride film is a hydrogen fluoride-resistant film, by using a silicon dioxide film as the first support film, only the first support film can be easily removed with an aqueous solution of hydrofluoric acid. be able to. That is, by forming the first support film with a silicon dioxide film and the second support film with a silicon nitride film, the method of manufacturing the flow sensor becomes easy, and the manufacturing cost can be reduced.
[0031]
In the above embodiment, both the protective film and the second support film are formed of the silicon nitride film. However, the present invention is not limited to this, and any one of the protective film and the second support film may be used. One may be formed of an insulating nitride film.
[0032]
In the above-described first to fourth embodiments, the thermal sensor is used for the flow rate sensor. However, the present invention is not limited to this, and the thermal sensor of the present invention may be used for an acceleration sensor, a pressure sensor, or the like. The function and effect can be obtained.
[0033]
【The invention's effect】
As described above, in the thermal sensor according to the present invention, the tensile stress is set on the second support film constituting the diaphragm. As a result, it is possible to prevent the diaphragm from being bent and deformed due to the thermal expansion of the heat generating portion, and as a result, it is possible to improve the reliability of the thermal sensor. Further, since a diaphragm having excellent heat conductivity can be obtained, the detection accuracy of the thermal sensor can be improved. Further, by using a silicon substrate as the substrate and forming the first support film made of silicon dioxide by thermal oxidation of the surface of the silicon substrate, a film having few pinholes can be easily obtained.
[0034]
Further, according to the method of manufacturing a thermal sensor according to the present invention, a pinhole is reduced by using a silicon substrate as the substrate and forming the first support film made of silicon dioxide by thermal oxidation of the silicon substrate surface. A membrane can be easily obtained. Further, after forming a second support film made of a material different from the first support film on the first support film, an opening of the silicon substrate is formed by wet etching in a first etching step, In the second etching step, the exposed portion of the first support film is removed to form a diaphragm composed of the second support film. Since the diaphragm can be formed thin by such two etchings, the detection accuracy of the thermal sensor can be improved.
[0035]
Further, by forming the second support film of silicon nitride, it is easy to control the tensile stress of the film. Further, since silicon nitride has acid resistance to hydrogen fluoride, an aqueous solution of hydrofluoric acid can be used as an etchant for the first support film made of silicon dioxide, so that the first support film can be easily etched. And the manufacturing cost of the sensor can be reduced.
[0036]
In addition, by forming a protective film made of silicon nitride on the second support film and the heat generating portion, both the second support film and the protective film become silicon nitride films, thereby controlling the tensile stress of the film. Can be easily prevented, the flexural deformation of the diaphragm can be prevented, and the reliability of the sensor can be improved.
[Brief description of the drawings]
FIG. 1 is a plan view of a flow sensor according to a first embodiment of the present invention.
FIGS. 2A and 2B are cross-sectional views illustrating a manufacturing process of the flow sensor according to the first embodiment of the present invention, wherein FIG. 2A illustrates a process of forming a first support film and a second support film on a substrate, and FIG. Shows a step of forming a heat generating portion, and (c) shows a step of forming a protective film.
FIGS. 3A and 3B are cross-sectional views illustrating a manufacturing process of the flow sensor according to the first embodiment of the present invention. FIG. 3A illustrates a process of removing a substrate, and FIG. 3B illustrates a process of removing a first support film. .
FIG. 4 is a plan view of a conventional bridge-type thermal flow sensor.
FIG. 5 is a cross-sectional view of a conventional bridge-type thermal flow sensor.
FIG. 6 shows a plan view of a conventional diaphragm-type thermal flow sensor.
FIG. 7 is a cross-sectional view of a conventional diaphragm-type thermal flow sensor.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Substrate, 3 Protective film, 11 Heating part, 12 Opening, 14 Diaphragm area, 15 First supporting film, 16 Second supporting film.

Claims (6)

A silicon substrate having an opening,
A first support film formed on the silicon substrate and made of silicon dioxide by thermal oxidation of the silicon substrate surface;
A second support film formed on the first support film and constituting a diaphragm at the opening;
A heat-generating portion formed on the diaphragm of the second support film and made of a heat-sensitive resistance material;
The first support membrane has been removed at the opening;
A thermal sensor, wherein the second support film is formed of a material different from that of the first support film so that a tensile stress is set on the diaphragm. 2. The thermal sensor according to claim 1, wherein the second support film is formed of silicon nitride. The thermal sensor according to claim 2, wherein a protective film made of silicon nitride is formed on the second support film and the heat generating portion. Forming a first support film made of silicon dioxide on the silicon substrate by thermal oxidation of the silicon substrate surface;
Forming a second support film made of a different material from the first support film on the first support film;
Forming a heat generating portion made of a heat-sensitive resistance material on the second support film;
A first etching step of removing a part of the silicon substrate by wet etching to form an opening reaching the first support film from the back surface of the silicon substrate;
A second etching step of removing a portion of the first support film exposed at the opening to form a diaphragm formed of the second support film. . 5. The method according to claim 4, wherein the second support film is formed of silicon nitride. 6. The method for manufacturing a thermal sensor according to claim 5, further comprising a step of forming a protective film made of silicon nitride on the second support film and the heat generating portion.

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