An object of the present invention is to provide a thermal airflow sensor that prevents moisture absorption by a silicon oxide film formed closest to a surface (formed to be located on an uppermost portion), and that reduces a measuring error. In order to attain the foregoing object, the thermal airflow sensor according to the present invention applies an ion implantation to a silicon oxide film 4, formed closest to a surface (formed to be located on an uppermost portion), by using an atom or molecule selected from at least any one of silicon, oxygen, and an inert element such as argon or nitrogen, in order to increase a concentration of an atom contained in the silicon oxide film 4 more than that before the ion implantation.

TECHNICAL FIELD

The present invention relates to a thermal airflow sensor that is a measuring element used for an air flowmeter, includes a resistance heating element and a resistance temperature detector for measuring temperature, and measures quantity of airflow.

BACKGROUND ART

A thermal air flowmeter that can directly detect quantity of airflow has become mainstream of air flowmeters. In particular, a thermal air flowmeter including a measuring element produced by a semiconductor machining technique has gained attention, since it can reduce cost, and it can be driven with low electric power. Japanese Unexamined Patent Publication No. 10-311750 describes a measuring element (thermal airflow sensor) used for the thermal airflow meter described above. In the thermal air flowmeter described in this Publication, an electric insulating film is formed on a semiconductor substrate, a resistance heating element and a resistance temperature detector are formed on the electric insulating film, and an electric insulator is formed on the resistance heating element and the resistance temperature detector. The region where the resistance heating element and the resistance temperature detector are formed has a diaphragm structure that is formed by removing a part of the semiconductor substrate through anisotropic etching from the backside of the semiconductor substrate.

CITATION LIST

Patent Literature

Patent Literature 1: Japanese Unexamined Patent Application Publication No. Hei10(1998)-311750

SUMMARY OF INVENTION

Technical Problem

In the thermal airflow sensor proposed in Japanese Unexamined Patent Application Publication No. Hei10(1998)-311750, the region where the resistance heating element and the resistance temperature detector are formed has the diaphragm structure, so that the surface is directly exposed to environment. The surface of the thermal air flowmeter is covered by the electric insulating film, and a silicon oxide film formed by a chemical vapor deposition (CVD) method is generally used as the electric insulating film. In general, the density of atoms of the silicon oxide film formed by the CVD method is coarser than that of a thermal oxide film formed by heating an oxide film, so that the silicon oxide film is easy to absorb moisture. When the silicon oxide film formed on the surface by the CVD method absorbs moisture, its volume expands to change a film stress. When the film stress of the silicon oxide film on the surface changes, the shape of the diaphragm where the semiconductor substrate is partly removed changes in the film thickness direction. When the shape of the diaphragm changes, the resistance temperature detector formed in the diaphragm region is distorted, resulting in that an error is caused in the measurement result of the air flowmeter.

An object of the present invention is to provide a thermal airflow sensor that prevents moisture absorption by the silicon oxide film formed closest to the surface (located on the uppermost portion) in order to reduce a measuring error.

Solution to Problem

In order to attain the foregoing object, in the thermal airflow sensor according to the present invention, an ion implantation is applied to the silicon oxide film formed closest to the surface (located on the uppermost layer), in order to increase the concentration (density) of atoms, contained in the silicon oxide film, more than that before the ion implantation.

More specifically, the thermal airflow sensor according to the present invention includes a semiconductor substrate; an electric insulator formed on the semiconductor substrate and including a resistance heating element, a resistance temperature detector, and a silicon oxide film; and a diaphragm formed by removing apart of the semiconductor substrate, the resistance heating element and the resistance temperature detector being formed on the diaphragm, and the silicon oxide film formed as the electric insulator being formed on the resistance heating element and the resistance temperature detector, wherein an ion implantation is applied to the silicon oxide film located on the uppermost layer in order to increase the concentration of an atom, contained in the silicon oxide film on at least the region covering the diaphragm, more than that of the silicon oxide film before the ion implantation.

In this case, it is preferable that an ion implantation layer may be formed on at least a part of the surface of the silicon oxide film in the thickness direction.

Preferably, the thermal airflow sensor may include a thermal oxide film formed on the semiconductor substrate by thermally oxidizing silicon, the resistance heating element and the resistance temperature detector formed on the thermal oxide film, and the silicon oxide film formed on the resistance heating element and the resistance temperature detector and exposed to the surface, wherein the concentration of the atom contained in the ion implantation layer increases more than the concentration of the atom contained in the thermal oxide film.

Preferably, the silicon oxide film may be formed by a CVD method, the ion implantation layer may be formed on the surface of the silicon oxide film, and a silicon oxide film that retains a composition before the ion implantation may be present on an interface with a layer under the silicon oxide film.

Preferably, an ion implanted into the ion implantation layer may contain an atom or molecule of at least one of silicon, oxygen, and inert element.

Preferably, the inert element may contain at least either one of argon or nitrogen.

Preferably, the ion implantation layer may be formed on the region covering the diaphragm, and a region where the ion implantation is not applied may be formed on the outside of the diaphragm.

Advantageous Effects of Invention

According to the present invention, the moisture absorption by the silicon oxide film can be prevented, whereby a change in a detection property of the air flowmeter when environment such as moisture changes can be suppressed. This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2010-165449, the entire contents of which are incorporated herein by reference.

DESCRIPTION OF EMBODIMENTS

First Embodiment

A thermal airflow sensor according to the first embodiment of the present invention will be described with reference toFIGS. 1 and 2.FIG. 1is a schematic plan view of the thermal airflow sensor, andFIG. 2is an enlarged sectional view along A-A′ line inFIG. 2.

As illustrated inFIG. 1, the thermal airflow sensor (a measuring element used for a thermal air flowmeter) according to the present embodiment includes a silicon substrate1serving as a semiconductor substrate, a resistance heating element10, resistances temperature detectors9and11for measuring air temperature, a terminal electrode12, and a diaphragm portion6. Numeral8denotes an end of the diaphragm portion6.

A manufacturing method according to the present embodiment will be described with reference toFIG. 2.

The silicon substrate1is thermally oxidized to form a thermal oxide film2serving as a lower electric insulating film. Polycrystalline silicon (Si) is deposited on the thermal oxide film2, and patterned to form the resistance heating element10and the resistance temperature detectors9and11. Phosphor or the like is doped into the polycrystalline silicon to adjust a resistance value. The resistance heating element10and the resistance temperature detectors9and11may be made of a metal such as platinum or molybdenum, instead of the polycrystalline silicon. A silicon nitride (SiN) film3and a silicon oxide film4, serving as an upper electric insulating film, are deposited on the resistance heating element10and the resistance temperature detectors9and11. Thereafter, silicon (Si) or oxygen (O or O2) alone or both of them are implanted into the silicon oxide film4by an ion implantation, and then, an annealing process is performed for about 30 to 60 minutes at 700 to 850° C. to form an ion implantation layer5. The terminal electrode12illustrated inFIG. 1is formed by depositing aluminum or gold after the deposition of the polycrystalline silicon. Finally, the diaphragm portion6is formed from the back surface by using etching solution such as KOH with the silicon oxide film being used as a mask. The diaphragm portion6may be formed by a dry etching process. Numeral8inFIG. 2denotes a position of the end of the etching mask serving as a mask material. The portion outside the end of the etching mask indicated by numeral8is covered by the mask material, and with this state, the etching is executed, whereby the silicon substrate corresponding to the diaphragm portion6is removed.

An operation and effect of the present embodiment will be described below.

In the present embodiment, the upper isolated electrode film includes two layers, which are the silicon nitride (SiN) film3and the silicon oxide film4. However, the upper film may include more layers. In any cases, the silicon oxide film4located on the uppermost layer of the upper isolated electrode film is formed by the CVD method, so that the density (concentration) of atom (or molecule, hereinafter merely referred to as atom) of this film is coarse, compared to the thermal oxide film. Therefore, the silicon oxide film4is easy to absorb moisture, and moisture absorption is easy to occur. When the silicon oxide film4formed by the CVD method and located on the uppermost layer absorbs moisture, the film expands, and hence, the film stress changes. Since a part of the silicon substrate1is removed on the diaphragm portion6, the shape of the diaphragm portion6changes because of the change in the film stress of the silicon oxide film4located on the uppermost layer.

FIG. 3is a graph illustrating an amount of warpage of the diaphragm portion6, and this graph illustrates the amount of warpage between both ends of the diaphragm portion6on A-A′ line inFIG. 1. It is general that the diaphragm portion6warps in the film thickness direction as illustrated inFIG. 3. The resistance temperature detectors9and11are formed on the diaphragm portion6. Therefore, when the shape of the diaphragm portion6changes, the resistance temperature detectors9and11are distorted, and the amount of distortion of the resistance temperature detectors9and11varies due to the change in the film stress caused by the change in the amount of absorbed moisture. When the amount of distortion of the resistance temperature detectors9and11varies, the resistance value changes due to a piezoresistive effect, which generates an error in the measured quantity of airflow.

The change in the film stress caused by the moisture absorption is caused because the density (concentration) of atom of the silicon oxide film4, formed by the CVD method and located on the uppermost layer, is low. Therefore, in order to increase the density (concentration) of the atom of the silicon oxide film4located on the uppermost layer, an atom or molecule of at least one of silicon, oxygen, and an inert element such as argon or nitrogen is implanted into the silicon oxide film4after the deposition of the silicon oxide film4, and then, an annealing process is performed at about 700 to 850° C. for defect recovery.

In the present embodiment, the ion implantation is applied to the silicon oxide film4formed by the CVD method to form the ion implantation layer5. The silicon oxide film4is made of silicon dioxide (SiO2). Accordingly, when the ion implantation is applied, the ion implantation layer5includes at least a molecule, and may include silicon, oxygen, and other impurity atoms together with the molecule in some cases. In the present embodiment, the concentration (density) of the atoms that are the total of all atoms contained in the film and the atoms constituting the molecule is important. Therefore, in the specification of the present invention, the concentration (density) of the atoms that are the total of all atoms contained in the film and the atoms constituting the molecule is merely referred to as “atom concentration” or “concentration of atom”.

In the present embodiment, silicon and oxygen are implanted into the silicon oxide film4by the ion implantation. Therefore, the ion implantation layer5is also the layer of the silicon oxide film, like the other portion (layer)4awhere the ion implantation is not applied, although the concentration of silicon dioxide (SiO2) is different. When the inert element such as argon or nitrogen is implanted by the ion implantation, the ion implantation layer5becomes the layer of the silicon oxide film containing impurity, as described later. In any cases, the ion implantation layer5is formed on a part4bof the silicon oxide film4, formed by the CVD method, on the upper surface, and a part4aon the lower surface (on the depth layer) is left as the layer of the silicon oxide film having the concentration same as that of the silicon oxide film4before the ion implantation.

When the silicon oxide film4is thin, or a significant moisture-absorption preventing effect is required, the whole silicon oxide film4can be the ion implantation layer5.

FIG. 4is a graph illustrating a ratio of a concentration (density) of silicon dioxide (SiO2) in the ion implantation layer5to the concentration (density) of silicon dioxide (SiO2) formed by the CVD method, the concentration (density) of silicon dioxide (SiO2) formed by the CVD method being defined as a reference. An abscissa axis indicates a distance between the surface of the silicon oxide film4and an interface of a base film (in the present embodiment, SiN film3) in the thickness direction. As illustrated inFIG. 4, the concentration (density) of silicon dioxide (SiO2) near the surface can be increased more than the concentration (density) of silicon dioxide (SiO2) near the interface (the silicon oxide film4where the ion implantation is not applied) with the base film. Accordingly, the moisture absorption can be reduced, and the change in the film stress can be prevented, whereby the measuring error in the quantity of airflow can be reduced.

Subsequently, the amount of implanted ion will be described with reference toFIG. 5.FIG. 5illustrates a ratio of the density of silicon dioxide (SiO2) in the silicon oxide film4to the density of silicon dioxide (SiO2) in the thermal oxide film2, wherein the density of silicon dioxide in the thermal oxide film2is defined as a reference. An abscissa axis indicates a distance between the surface of the silicon oxide film4and the interface of the base film (in the present embodiment, SiN film3) in the thickness direction.

The concentration (density) of the atom contained in the ion implantation layer5increases by implanting an atom or molecule of at least one of silicon, oxygen, and an inert element such as argon or nitrogen into the silicon oxide film4formed by the CVD method. Therefore, even a small amount of implantation is naturally effective. According to our experiment, the change in the film stress due to the moisture absorption by the thermal oxide film2was not observed. Therefore, the density of the atoms constituting the ion implantation layer5is preferably set to be equal to or higher than the density of the atoms of the thermal oxide film2.

As illustrated inFIG. 5, silicon and oxygen are implanted by the ion implantation in such a manner that the concentration (density) of silicon dioxide (SiO2) in a certain range of the ion implantation layer5in the thickness direction is larger than the concentration (density) of silicon dioxide (SiO2) of the thermal oxide film2.

FIG. 6illustrates an example in which the distribution of the concentration ratio (density ratio) of silicon dioxide (SiO2) inFIG. 5is improved. Since the moisture absorption occurs on the surface first, the concentration (density) of the atoms of the ion implantation layer5is preferably set to be higher in the vicinity of the surface as illustrated inFIG. 6.FIG. 6illustrates the case of the silicon oxide film formed by the CVD method. Similarly, the atom concentration of the thermal oxide film is preferably set to be higher in the vicinity of the surface. The object is attained by the enhancement of the atom concentration (atom density). Therefore, instead of increasing the atom concentration of silicon and atom concentration of oxygen simultaneously, silicon or oxygen alone may be implanted by the ion implantation.

Not only the concentration (density) of silicon or oxygen contained in the silicon oxide film4but also the concentration (density) of other elements contained in the silicon oxide film4can be increased. When an active element is implanted, an insulating property might be deteriorated. In view of this, the inert element (e.g., argon, nitrogen) is preferably implanted as the other impurity element. The concept for the concentration ratio (density ratio) in this case is the same as described with reference toFIGS. 4,5, and6.

The change in the shape of the diaphragm due to the change in the film stress occurs when a film whose stress changes is arranged on the diaphragm portion6. The silicon substrate1is thick and has high rigidity, so that the shape of the diaphragm portion6does not change even if the film stress of the silicon oxide film4changes on the silicon substrate1. Therefore, the ion implantation layer5may be formed on the region including the diaphragm portion6, and a region where the ion implantation is not applied may be formed at the outside of the diaphragm portion6, as illustrated inFIG. 7. Alternatively, the amount of ion implantation may be reduced on the region at the outside of the diaphragm portion6, compared to the region including the diaphragm portion6.

The concentration (density) of atoms such as silicon or oxygen constituting the silicon oxide film4a, to which the ion implantation is not applied, and the ion implantation layer5can be measured according to SIMS analysis.

In the present embodiment, the thermal oxide film2is used as the electric insulating film serving as the layer under the resistance heating element10and the resistance temperature detectors9and11. However, it is obvious that the effect of the present invention can be provided by a composite film of the thermal oxide film and SiN film. Specifically, when the diaphragm portion is made of a film in the diaphragm structure, the dependence of the diaphragm shape on the film stress is noticeable, so that the method described above is effective for this structure.

In the present embodiment, the ion implantation layer is formed on the silicon oxide film. However, the ion implantation layer is effectively formed on the other films, so long as the film absorbs moisture, and its film stress changes. Under present circumstances, a film formed by the CVD method and a plasma TEOS oxide film are popular as the silicon oxide film4. However, the ion implantation to the silicon oxide film4in the present embodiment and to the film used as being arranged like the silicon oxide film4is effective, when a method of forming the silicon oxide film4located on the uppermost layer or the film used as being arranged like the silicon oxide film4is used in such a manner that the concentration (density) of atoms contained in these films becomes smaller than the thermal oxide film2to allow these films to easily absorb moisture.

There may be the case in which, as illustrated inFIG. 8, a protection film such as PIQ film13or the like is deposited on the end of the diaphragm portion6in order to protect the measuring element from dust. Since the deposition of the PIQ film13is only in the vicinity of the end of the diaphragm portion6, the method according to the present invention is also effective for this case.

LIST OF REFERENCE SIGNS