STRAIN RESISTANCE FILM, PHYSICAL QUANTITY SENSOR, AND METHOD FOR MANUFACTURING THE STRAIN RESISTANCE FILM

A strain resistance film containing a composition represented by Cr100-x-y-zAlxNyOz, satisfying 5≤x≤50, 0.1≤y≤20, 0.1≤z≤17, and y+z≤25.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority from Japanese Patent Application No. 2022-161188 filed on Oct. 5, 2022, the entire contents of which are hereby incorporated by reference.

BACKGROUND

The disclosure relates to a strain resistance flint in which resistance value changes according to strain, and a physical quantity sensor using thereof.

CrAl film or CrAlN film is a material that exhibits a high k-factor up to high temperatures, and is noted as a detection material for pressure sensors and strain sensors that can with stand high temperatures. Patent Document 1 describes an example thereof.

According to this type of CrAl film or CrAlN film, the conductive properties change from metal to semiconductor at a certain Al content ratio. It becomes possible to control the temperature coefficient of resistance (TCR) by controlling this conduction property.

TCR, which is one of the above conductive properties, has been controlled by the annealing, temperature at the same Cr—Al ratio. Also, k-factor is controlled by Changing, Cr—Al composition ratio and the annealing temperature. Therefore, when adjusting TCR for each product, it was necessary to adjust Cr—Al composition ratio of the material for each product.Patent Document 1: Japanese unexamined patent publication 2018-91848

SUMMARY

It is desirable to provide a strain resistance film that maintains a relatively high k-factor and is easy to control TCR, and a physical quantity sensor having the strain resistance film.

As a result of intensive studies on strain resistance films containing Cr, Al and N, the inventors have found that a relatively high k-factor can be maintained and TCR can be easily controlled by further containing “0” and particularly controlling the composition ratio of “N” and “0”.

That is, the strain resistance film has a composition represented by Cr100-x-y-zAlxNyOzwhich satisfies 5≤x≤50, 0.1≤y≤20, 0.1≤z≤17, and y+z≤25.

The physical quantity sensor has the strain resistance film described above. The physical quantity sensor is not particularly limited, and examples thereof include such as strain sensors and pressure sensors.

A manufacturing method for a strain resistance film isa method for manufacturing a strain resistance film, in whichk-factor and TCR of the strain resistance film are controlled by controlling amounts of oxygen and nitrogen in an atmosphere gas, which is used to manufacture the strain resistance film by a thin film method.

DETAILED DESCRIPTION

Hereinafter, the disclosure will be described in detail based on embodiments.

FIG.1is a schematic cross-sectional view of pressure sensor10using a strain resistance film32according to an embodiment. As shown inFIG.1, the pressure sensor10has a membrane22that deforms in response to pressure. According toFIG.1, the membrane22is configured with an end wall formed at one end of a hollow cylindrical stem20, however, it can also be configured with a flat Si substrate122(SeeFIG.3B) as in Example. The other end of the stem20is an open end of a hollow, and the hollow of the stem20communicates with the flow path12bof the connecting member12.

In the pressure sensor10, the fluid introduced into the flow path12bis guided from the hollow of the stem20to the inner surface22aof the membrane22to apply fluid pressure to the membrane22. Stem20is composed of metal such as stainless steel. However, the shape corresponding to the stem20may be formed by etching a silicon substrate or bonding the flat silicon substrate to another member.

A flange21is formed around the open end of the stem20so as to protrude outward from the axis of the stem20. The flange21is between the connecting member12and the suppressing member14so that the channel12bleading to the inner surface22aof the membrane22is sealed.

The connecting member12has a thread groove12afor fixing the pressure sensor10. The pressure sensor10is fixed via the thread groove12ato a pressure chamber or the like in which a fluid to be measured is enclosed. As a result, the channel12bformed inside the connecting member12and the inner surface22aof the membrane22of the stem20are airtightly communicated with the pressure chamber in which the fluid to be measured exists.

A circuit board16is attached to the upper surface of the suppressing member14. The circuit board16has a ring shape surrounding the stem20, but the shape of the circuit board16is not limited thereto. The circuit board16incorporates, for example, a circuit to which the detection signal from the strain resistance film32is transmitted.

As shown inFIG.1, a strain resistance film32and the like are provided on the outer surface of the membrane22. The strain resistance film32and the circuit board16are electrically connected via an intermediate wiring72or the like by wire bonding or the like.

FIG.2is a schematic cross-sectional view showing an enlarged view of a part of the strain resistance film32included in the pressure sensor10shown inFIG.1and its surroundings.

As shown inFIG.2, the strain resistance film32is provided on the outer surface22bof the membrane22via the base insulating layer52or the like. The base insulating layer52is formed to cover substantially the entire outer surface22bof the membrane22, and is made of silicon oxide such as SiO2, silicon nitride, or silicon oxynitride and the like. The thickness of the base insulating layer52is preferably 10 μm or less, more preferably 1 to 5 μm. The base insulating layer52can be formed on the outer surface22bof the membrane22by a vapor deposition of CVD and the like.

If the outer surface22bof the membrane22has insulating properties, the strain resistance film32may be formed directly on the outer surface22bof the membrane22without forming the base insulating layer52. For example, when the membrane22is made of an insulating material such as alumina, the strain resistance film32may be provided directly on the membrane22.

As shown inFIG.2, an electrode36is provided on the strain resistance film32.FIG.3Ais a schematic plan view of the strain resistance film30with electrodes shown inFIGS.1and2as viewed from above, and shows the pattern arrangement of the strain resistance film30with electrodes.

As shown inFIG.3A, the strain resistance film32has a first resistor R1, a second resistor R2, a third resistor R3and a fourth resistor R4formed in a predetermined pattern. The first to fourth resistors R1, R2, R3, and R4generate strain according to the deformation of the membrane22, and the resistance value changes according to the deformation of the membrane22. These first to fourth resistors R1to R4are connected by electrical wiring34so as to form a Wheatstone bridge circuit.

The pressure sensor10shown inFIG.1detects the fluid pressure acting on the membrane22from the output of the Wheatstone bridge circuit by the first to fourth resistors R1to R4shown inFIG.3A. That is, the first to fourth resistors R1to R4are provided at positions where the membrane22shown inFIGS.1and2is deformed and strained by the fluid pressure, and the resistance value changes according to the amount of strain.

The strain resistance film32having the first to fourth resistors R1to R4can be manufactured by such as patterning a conductive thin film of a predetermined material. In this embodiment, the strain resistance film32has a composition represented by the composition Cr100-x-y-zAlxNyOzwhere 5≤x≤50, 0.1≤y≤20, 0.1≤z≤17 which satisfies y+z≤25.

The lower limit of “x” may be preferably 12.7 or more, 23 or more, or 25 or more. The upper limit of “x” may be 40 or less, or 30 or less. “y” is preferably 1.1 to 12.2. “z” may be preferably 0.4 to 1.3, more preferably 0.4 to 0.7 or 2.6 to 9.5. “y+z” may be preferably 12.0 to 19.6, more preferably 14.0 to 19.6, or 1.8 to 10.7.

Although the thickness of the strain resistance film32is not particularly limited, it is preferably one nm or more, more preferably 10 nm or more. Although upper limit of the thickness is not particularly limited, it may be 1000 nm or less or 500 nm or less.

An oxide film or the like may be formed on the surface of the strain resistance film32. Examples of the oxide film include a chromium oxide film, a silicon oxide film, a silicon nitride film, a silicon oxynitride film, an aluminum oxide film, an aluminum nitride film, an aluminum oxynitride film, and the like.

The composition analysis of “x”, “y”, “z”, etc. in the strain resistance film32may be performed using XRF (X-ray fluorescence) method, XPS (X-ray photoelectron spectroscopy), etc. In the embodiment, it is preferable that the result of composition analysis at an intermediate position along the depth direction in the strain resistance film32except for its surface oxide film is within the above range.

The strain resistance film32can be formed by a thin film method such as sputtering or vapor deposition. The first to fourth resistors R1to R4shown inFIG.3Acan be formed by such as patterning a thin film into a meandering shape. Note that the electrical wiring34, as shown inFIG.3A, may be formed by patterning the strain resistance film32or by a conductive film or layer different from the strain resistance film32.

“O” and “N” contained in the strain resistance film32are preferably intentionally introduced into the strain resistance film32by being used as an atmosphere gas during film formation of the strain resistance film, however, may be intentionally introduced into the strain resistance film32by used as atmosphere gases for annealing. Alternatively, it may be a substance that was not completely removed from the reaction chamber when the strain resistance film32was formed, and was taken into the strain resistance film32.

In any case, when the strain resistance film32of the embodiment is manufactured by a thin film method, k-factor and TCR (Temperature Coefficient of Resistance) can be controlled by controlling the amounts of oxygen and nitrogen in the atmosphere gas for manufacturing the film. Also, TCS (Temperature Coefficient of Sensitivity) of the strain resistance film32can also be controlled by controlling the amounts of oxygen and nitrogen in the atmosphere gas for manufacturing the film.

For example, when the strain resistance film32is manufactured using a sputtering method, the conditions in the sputtering apparatus are preferably 0.08 to 0.2 Pa, and the atmosphere gas for manufacturing contains an inert gas, nitrogen gas and oxygen gas. Further, when the total pressure of the atmosphere gas is 100%, the partial pressure of nitrogen gas is preferably 0 to 20%, the partial pressure of oxygen gas is preferably 0 to 15%, and the rest is the partial pressure of inert gas. Ar gas, Ne gas, or the like can be used as the inert gas.

After manufacturing the strain resistance film32by the thin film method, the strain resistance film32may be heat treated. Although the heat treatment temperature is not particularly limited, it may be such as 50° C. to 550° C., preferably 350° C. to 550° C.

As shown inFIGS.2and3A, the electrode36is provided on top of the electrical wiring34made of the same material as the strain resistance film32, and electrically connected to the strain resistance film32. As shown inFIG.2, the electrode36is formed on a part of the upper surface of the electrical wiring34made of the strain resistance film32. The output of the Wheatstone bridge circuit of strain resistance film32is transmitted to circuit board16shown inFIG.1via the electrode36and an intermediate wiring72. The materials of the electrode36include conductive metals, alloys, and the like, and more specifically, Cr, Ti, Ni, Mo, platinum group elements, Au, and the like. Further, the electrode36may have a multi-layer structure made of different materials.

The base insulating layer52and the electrode36shown inFIG.2can also be formed by a thin film method such as sputtering or vapor deposition, similarly to the strain resistance film32, however, the base insulating layer52and the electrode36may be formed by a method other than the thin film method. For example, the base insulating layer52of SiO2film may be provided on the surface of the Si substrate by heating the Si substrate surface and forming a thermal oxide film.

According to the strain resistance film32of the embodiment, the TCR can be easily controlled while maintaining a high k-factor of four or more. The k-factor corresponds to the gauge factor of the pressure sensor. For example, according to the strain resistance film satisfying the above composition ratio, it becomes possible to make an absolute value of the temperature coefficient of resistance (TCR) to 2000 ppm/° C. or less, more preferably 1000 ppm/° C. or less, while maintaining k-factor high to four or more. By setting the absolute value of the temperature resistance coefficient (TCR) to a predetermined value or less, it becomes possible to suppress errors in strain detection, pressure detection, etc. due to the influence of temperature.

In addition, the strain resistance film that satisfies the above composition ratio is possible to maintain a high k-factor of four or more, while making an absolute value of the temperature coefficient of sensitivity (TCS) to preferably 2000 ppm/° C. or less, more preferably 1000 ppm/° C. or less in the temperature range of −50° C. or more to 450° C. or less. By setting the absolute value of the temperature coefficient of sensitivity (TCS) to a predetermined value or less, it becomes possible to suppress errors in strain detection, pressure detection, etc. due to the influence of temperature.

Such strain resistance film32has a small resistance value change (and/or sensitivity change) accompanying temperature change over a wide range from low temperature to high temperature. Thus, the strain resistance film32is preferably used as the strain resistance film32of the pressure sensor10used in a wide temperature range, and can reduce temperature correction error and perform highly accurate detection.

In order to manufacture the strain resistance film32of the embodiment, a target having a fixed ratio of Cr and Al is used, and by such as controlling the amounts of nitrogen and oxygen in the atmosphere gas used for manufacturing the film, the nitrogen and/or oxygen contents in the obtained resistance film can be controlled and the k-factor and TCR of the strain resistance film can be controlled.

That is, without preparing targets with different ratios of Cr and Al, using a target with a fixed ratio of Cr and Al, it becomes easy to manufacture multiple types of strain resistance films with controlled k-factor and TCR. As a result, the production speed as well as the production efficiency of the strain resistance film are improved.

Furthermore, according to the strain resistance film32satisfying the above composition ratio, it is easy to control the resistivity (φ of the strain resistance film32to 1 to 15 Ω·μm. By setting the resistivity to a specific range, for instance, a circuit for sensor such as a Wheatstone bridge circuit can be easily manufactured using the strained resistance film32.

In this embodiment, the strain resistance film32has a film thickness of 10 nm or more. By using the strain resistance film32having a predetermined film thickness or more, the k-factor and TCR of the strain resistance film32can be easily controlled.

Further, according to the manufacturing method of the strain resistance film32of the embodiment, for example, by using a target having a fixed ratio of Cr and Al and controlling the amount of nitrogen and/or oxygen in the atmosphere gas for manufacturing the film, the nitrogen and the oxygen contents in the obtained strain resistance film can be controlled and the k-factor and TCR (and/or TCS) of the strain resistance film can also be controlled.

That is, without preparing targets with different ratios of Cr and Al, it becomes easy to manufacture multiple types of strain resistance films with controlled k-factor and TCR (and/or TCS) by using targets with a fixed ratio of Cr and Al. As a result, the production speed as well as the production efficiency of the strain resistance film are improved.

The disclosure is not limited to the above-described embodiments, and can be modified in various ways within the scope of the disclosure.

For example, the pressure sensor10is not limited to the sensor having the stem20shown inFIG.1. The pressure sensor10may be a pressure sensor manufactured by etching the substrate, or may be a sensor manufactured by bonding a flat substrate, on which a strain resistance film is formed, to the other member. The materials of the substrate may be any of metal, semiconductor, and insulator, and specifically, alumina (Al2O3) and the like can be used in addition to Si mentioned in the examples.

In addition, the strain resistance film of the embodiment can be used as a physical quantity sensor other than the pressure sensor10. Other physical quantity sensors include such as strain sensors, angle sensors, movement amount sensors, and acceleration sensors.

EXAMPLE

The disclosure will be described below based on more detailed examples, but the disclosure is not limited thereto.

First, as shown inFIG.3B, the Si substrate122was prepared and its surface was heated to form the SiO2film152as a thermal oxide film on the surface of the Si substrate122. After that, the strain resistance film132was formed on the surface of the SiO2film152using a DC sputtering apparatus. The atmosphere gas in the sputtering apparatus for manufacturing the film was Ar gas containing trace amounts of nitrogen and oxygen, and the pressure of the atmosphere gas was within the range of 0.08 to 0.2 Pa. When the total pressure of the atmosphere gas was 100%, partial pressure of nitrogen gas was 0 to 20%, partial pressure of oxygen gas was 0 to 15%, and the rest was the partial pressure of inert gas.

Strain resistance film samples 1 to 10 with different N contents (y value) and O contents (z values) were manufactured by changing the oxygen partial pressure and/or nitrogen partial pressure in the atmosphere gas during film formation, while fixing the ratio of the Cr target and the Al target used in the sputtering apparatus. The result confirmed that the higher the oxygen partial pressure in the atmosphere gas, the greater the0content (z value) in the central position along the thickness direction of the finally obtained strain resistance film.

Furthermore, the strained resistance film132after the film formation at 350° C. was annealed, then, resistors constituting a Wheatstone bridge circuit were formed in the strained resistance film132by microfabrication. Finally, an electrode layer was formed on the surface of the strain resistance film132by electron deposition to obtain samples. The average film thickness of the strain resistance film132in each sample was within the range of 300 nm±30 nm. The film thickness of the strain resistance film132was measured using a stylus profiler.

The following analysis and measurements were performed for each sample 1 to 10.

The composition of the strain resistance film132for each sample was analyzed by the XRF (X-ray fluorescence) method. Table 1 shows the results.

For each sample, the resistance value was measured by the four-terminal measurement method at 25° C. The specific resistance was calculated from the measured value of the sample, and the area and the thickness of the strain resistance film samples. Results are shown in Table 1.

[Measurement of Temperature Coefficient of Resistance (TCR)]

For each sample, the resistance value was measured while changing the environmental temperature from −50° C. to 450° C., and the relationship between the resistance value and temperature was linearly approximated by the least squares method to obtain a slope. TCR (ppm/° C.) for each sample was calculated from the slope. Reference temperature of the calculated TCR was 25° C. Results are shown in Table 1.

[Measurement of k-Factor and Temperature Coefficient of Resistance (TCS)]

For each sample, the gauge factor was measured while changing the environmental temperature from −50° C. to 450° C., and the relationship between the gauge factor and the temperature was linearly approximated by the least squares method to obtain a slope. TCS (ppm/° C.) for each sample was calculated from the slope. Reference temperature of the calculated TCS was 25° C., and the measured value was determined as k-factor. Results are shown in Table 1.

Samples 11 to 15 were prepared in the same manner as in Example 1, except for reducing the number of Al targets used in the sputtering apparatus relative to Example 1, fixing the ratio of the Cr target and the Al target, and adjusting the partial pressure of the nitrogen gas in the sputtering apparatus to reduce the nitrogen content. Then, analysis was performed to the samples in the same manner as in Example 1. Results are shown in Table 2.

Samples 21 to 23 were prepared in the same manner as in Example 1, except the atomic % of N+O in the finally obtained strain resistance film was about the same as that of sample number7in Example 1, which was around 14.0 atomic %, and the partial pressures of the nitrogen gas and the oxygen gas in the sputtering apparatus were adjusted so as to change the ratio of N atomic % and O atomic %. Then, analysis was performed to the samples in the same manner as in Example 1. Results are shown in Table 3.

The composition of the finally obtained strain resistance film was the same as that of sample number5in Example 1, and the sputtering time of the sputtering device was controlled so that the film thickness varies to obtain strain resistance films of different thicknesses.

Evaluation

Results from Table 1,FIG.4,FIG.5A, andFIG.5Bconfirm that the resistivity, k-factor, TCR, and TCS can be controlled simply by changing the content ratios of “N” and “O” while using targets with a fixed ratios of “Cr” and “Al”, and without a need for preparing targets with different ratios of Cr and Al. Each ofFIGS.4,5A, and5Bwas a graph obtained by sampling the data shown in Table 1.

According to Example 1, results from Table 1 andFIG.4confirm that the k-factor can be changed in the range of 2 to 8 by changing the content ratio (y+z) of N+O in the strain resistance film. In addition, results from Table 1 andFIG.4also confirm that the content ratio (y+z) of N+O in the strain resistance film is preferably 25 atomic % or less in order to make k-factor four or more, and the content ratio (y+z) of N+O in the strain resistance film is preferably 20 atomic % or less in order to make k-factor six or more.

Results from Table 1 andFIG.5Aconfirm that the absolute value of TCR (and TCS) is preferably 2000 or less, more preferably 1000 or less by changing the content ratio (y+z) of N+O in the strain resistance film. In addition, results from Table 1 andFIG.5Bconfirm that the absolute value of TCR (and TCS) is preferably 2000 or less, more preferably 1000 or less by changing the content ratio (z) of “O” in the strain resistance film.

Also, results from Tables 1 and 3 confirm that “x” is preferably 23 to 30, “z” is preferably 0.9 to 9.5, more preferably 2.6 to 9.5, and “y+z” is preferably 12.0 to 20.0, more preferably 14.0 to 20.0, in order to make the absolute value of TCR (and TCS) to 2000 or less, more preferably 1200 or less, and further preferably 1000 or less, while maintaining the resistivity of four S2 μm or more and the k-factor of four or more, preferably six or more, more preferably eight or more.

Further, results from Table 2 confirm that “x” is preferably 12 to 14, “z” is preferably 0.4 to 0.7, and “y+z” is preferably 1.8 to 10.7, in order to maintain the resistivity to three Ω·μm or more, the k-factor to eight or more, and the absolute value of TCR (and TCS) to 2000 or less, preferably 1200 or less, and more preferably 1000 or less.

Also, results from Tables 1 to 3 confirm that the content ratio (y) of “N” in the strain resistance film is 0.1 to 20, preferably 1.1 to 12.2.

Furthermore, the results shown in Table 3 confirm that, by adjusting the content ratio of “O” compared to “N”, the resistivity, k-factor, TCR, and TCS can be improved, when “x” and “y+z” are substantially constant.

It was confirmed that preferable results can be obtained when the thickness of the strain resistance film was preferably 10 nm or more, more preferably 30 nm or more.