JP2022074104A - Strain gauge and manufacturing method thereof - Google Patents

Abstract

To provide a strain gauge and its manufacturing method that can improve a film formation yield and the uniformity of characteristics.SOLUTION: A strain gauge includes: a thin plate-shaped substrate 1; and a thin film element 2 formed on one main surface 101 of a pair of main surfaces 101 and 102 of the substrate 1 and disposed in a designated manner. The substrate 1 is made of a resin having a rigidity ratio in a range of 200 to 1000×103 Pa m and a coefficient of thermal expansion in a range of 0 ppm/°C to 30 ppm/°C. The thin film element 2 is made of a Cr-N thin film having a nitrogen (N) content in a range of 2 to 8 at%, a temperature coefficient of resistance (TCR) in 0±400 ppm/°C, and a gauge ratio of 3 to 20.SELECTED DRAWING: Figure 1

Description

The present invention relates to a strain gauge used by adhering to the surface of a strain-causing structure to be measured.

The Cr-N thin film has a large gauge ratio of about 14 indicating sensitivity to strain, and the temperature coefficient of resistance (TCR) can be set to near zero (<± 50 ppm / ° C.) by adding a small amount of nitrogen and heat treatment. This is a new strain sensor material characterized by the fact that the resistance can be increased by several tens of kΩ (see Patent Document 1).

The conventional strain gauge (adhesive strain sensor element) has a structure in which a metal foil such as a CuNi-based or NiCr-based alloy formed in a grid pattern, which is a sensor material, is attached to a resin base (base) such as polyimide. .. When it is used for measuring strain and various dynamic quantities, it is further adhered to the surface of the strain-causing structure to be measured. The base is then required to provide ease of handling, including electrical insulation and shape retention. It is also important for the base to transmit strain correctly, which requires a material with a low Young's modulus and a large elongation, and resins are often used today.

When the strain sensor thin film is used as a dynamic quantity sensor, the sensor element is formed directly on the strain-causing structure (via an insulator film when the strain-causing structure is a conductor such as metal) without using a base. It is possible. In conventional strain gauges, "adhesion" is a manual operation, so misalignment is likely to occur, and there is concern about the effects of creep due to the base and adhesive, but in the case of thin films formed directly on the measurement target, these problems are taken into consideration. There is no need to do it. However, when the strain sensor is installed in a place where thin film formation is impossible, such as a hole, a tube, an inner part of a complicated shape, etc. due to the structure of the measurement target, it is necessary to select the adhesive method. Therefore, in order to develop an element that can be used for the Cr—N thin film by the adhesive method, the base substrate material was studied.

Polyimide used in conventional strain gauges has the highest heat resistance among resin films, but has a larger coefficient of thermal expansion and heat shrinkage than inorganic materials. Therefore, when polyimide is used as the base material, there is a problem that the influence of local stress on the base material becomes remarkable and cracks are likely to occur. As a result of research on this problem, it was found that it is effective to reduce the film forming gas pressure that promotes the strengthening of the thin film itself by densifying the Cr—N thin film structure (film quality), and succeeded in reducing cracks. However, the thermal effect on polyimide has not been completely eliminated, and the manufactured device is prone to warpage of the base material and partial deformation of the base material around the thin film, which has problems with its characteristics and its stability. It has become.

There, the coefficient of thermal expansion is close to that of the Cr-N thin film, it has excellent heat resistance, no thermal shrinkage, sufficient strength and high insulation properties, and it can be made thin to have strain transferability. Focusing on zirconia as a thin plate material, as a result of trial production and evaluation of a strain gauge using it as a base material, if the thickness of the base material is 80 μm or less, it is bonded while having almost the same function as the conventional one. It has been clarified that a highly sensitive strain gauge to be used can be provided (see Patent Document 2).

Japanese Patent No. 6159613 Japanese Patent No. 6022881

When the Cr-N thin film strain gauge is used as an adhesive type, the zirconia substrate element is very strong against bending, but weak against linear tension and compression in the inward direction of the substrate, and is fragile. Is. Examples of the substrate material that is hard to break include a conventionally used resin material. Therefore, we examined the resin material again, but since heat treatment is required in the film formation process, polyimide with the highest heat resistance is effective. , There was a problem in film formation yield and uniformity of characteristics.

When using a strain gauge, a Wheatstone bridge structure is used as the measurement circuit, but if there are variations in the characteristics of the four strain gauge elements used at that time, the zero balance of the midpoint potential of the current line will be lost and correct measurement will be performed. It also causes problems such as increased output drift due to external factors such as temperature. Therefore, it is extremely important that each of the plurality of (many) strain gauge elements manufactured has uniform characteristics without variation. Therefore, an object of the present invention is to provide a strain gauge and a method for manufacturing the strain gauge, which can improve the film formation yield and the uniformity of characteristics when a resin substrate is used.

As a result of repeated studies to solve the above problems, the present inventor has an adhesive strain gauge composed of a resin substrate having a predetermined characteristic, a Cr—N thin film having a predetermined composition and characteristics, and a film forming method thereof. It has been found that the film formation yield and the uniformity of characteristics can be improved by using a manufacturing method in which heat treatment is performed at a predetermined temperature. The strain gauge of the present invention has a substrate made of a resin having a rigidity ratio in the range of 200 to 1000 × 10 ³ Pa · m and a thermal expansion coefficient in the range of 0 ppm / ° C. to 30 ppm / ° C. A strain gauge comprising a thin film element formed on the substrate, wherein the thin film element has a nitrogen (N) content in the range of 2 to 8 at% and has a temperature coefficient of resistance (1). It is composed of a Cr—N thin film having a TCR) of 0 ± 400 ppm / ° C. and a gauge ratio of 3 to 20.

The method for manufacturing a strain gauge of the present invention described above includes a rigidity ratio in the range of 200 to 1000 × 10 ³ Pa · m and a coefficient of thermal expansion in the range of 0 ppm / ° C. to 30 ppm / ° C. It includes a step of forming a thin film element made of a Cr—N thin film, which is arranged in a designated manner on the main surface of a substrate made of a resin, and a step of heat-treating the thin film element at a temperature in the range of 180 to 200 ° C. I'm out.

INDUSTRIAL APPLICABILITY According to the present invention, there is provided an adhesive strain gauge and a method for manufacturing the same, which can improve the film formation yield and the uniformity of characteristics when a resin substrate is used.

Explanatory drawing about the method of reducing the heat treatment temperature in the sputtering film formation of a Cr—N thin film (the method by the film formation under the condition that the input power is moderately low). Explanatory drawing about the method of reducing the heat treatment temperature in the sputtering film formation of a Cr—N thin film (the method by the film formation under the condition that the nitrogen content is low). The figure which showed the measurement result of TCR in the sample which was heat-treated at the temperature of 200 degreeC or less using the heat treatment temperature reduction method. The figure which showed the measurement result of the gauge ratio in the sample which was heat-treated at the temperature of 200 degreeC or less using the heat treatment temperature reduction method. Explanatory drawing about the structure of the strain gauge of this invention. The figure which showed the pattern shape of the actually manufactured thin film element. The figure which showed the thin film element arrangement pattern formed in one film formation. The figure which showed the relationship between the heat shrinkage rate and the disconnection rate about the production example of Table 2. The figure which showed the relationship between the heat shrinkage rate and TCR non-uniformity about the production example of Table 2. The figure which showed the relationship between the heat shrinkage rate and Gf non-uniformity about the production example of Table 2. The figure which showed the relationship between the heat treatment temperature and the disconnection rate about a 1st substrate and a 2nd substrate. The figure which showed the relationship between the heat treatment temperature and the TCR non-uniformity about a 1st substrate and a 2nd substrate. The figure which showed the relationship between the heat treatment temperature and Gf non-uniformity about a 1st substrate and a 2nd substrate. The figure which showed the relationship between the rigidity ratio and the disconnection ratio about the sample which was heat-treated at 180 degreeC and 200 degreeC. FIG. 14 is an enlarged view of the rigidity ratio range of 200 to 300 kPa · m. The explanatory view about the relationship between the thermal expansion coefficient and the disconnection rate of the sample heat-treated at 180 ° C. and the sample heat-treated at 200 ° C., respectively. Explanatory drawing about the relationship between the stiffness ratio and TCR non-uniformity of the sample heat-treated at 180 ° C. and the sample heat-treated at 200 ° C., respectively. An enlarged view of the rigidity ratio range of 200 to 300 kPa · m in FIG. Explanatory drawing about the relationship between the coefficient of thermal expansion and TCR non-uniformity of a sample heat-treated at 180 ° C. and a sample heat-treated at 200 ° C., respectively. Explanatory drawing which concerns on the relationship between the stiffness ratio and Gf non-uniformity of a sample heat-treated at 180 degreeC and a sample heat-treated at 200 degreeC. Explanatory drawing which concerns on relationship of thermal expansion coefficient and Gf non-uniformity of the sample heat-treated at 180 degreeC and the sample heat-treated at 200 degreeC, respectively. Explanatory drawing about the characteristic of the base material constituting a strain gauge. Explanatory drawing which concerns on the relationship between nitrogen content and temperature coefficient of resistance (TCR) of a thin film element formed on a synthetic resin film. The explanatory view about the relationship between the nitrogen content and the gauge ratio (Gf) of the thin film element formed on the synthetic resin film.

It is considered that the important point for solving the problem is that the "heat resistance" of the resin material is low. Therefore, in the present invention, improvements have been made from the following two aspects. One is to reduce the heat treatment temperature of the strain sensor thin film, and the other is to search for a resin substrate that does not cause any problem in the reduced heat treatment temperature range.

The heat treatment of the Cr—N thin film as a strain sensor is performed for adjusting the TCR to 0, and conventionally, heat treatment temperatures of 200 to 300 ° C. have been used for substrates such as glass, ceramics, and metals. rice field. However, it was found from the test results shown in the latter stage that the heat treatment temperature should be 200 ° C. or lower even for a resin-based material having heat resistance.

The following two methods can be mentioned as a method for reducing the heat treatment temperature at the time of forming a thin film by sputtering or the like.

(1) Film formation under moderately low input power (see Fig. 1) (Reference: Niwa et al., Proceedings of the 32nd "Sensor Micromachines and Applied Systems" Symposium, 28pm1-A-1 (2015)) ..

(2) To prepare a thin film having a low nitrogen content (see FIG. 2) (Reference: Japanese Patent No. 6159613).

By these means, the TCR of the produced Cr—N thin film before the heat treatment (as-deposited film) becomes a negative small value, and the heat treatment temperature for making the TCR zero can be lowered.

X First, even when heat-treated at a temperature of 200 ° C or lower using the existing heat treatment temperature reduction method, the TCR is actually close to zero (within ± 400 ppm / ° C), the gauge ratio is sufficiently large, and it falls within a good value. Borosilicate glass (samples other than 0.2 mm thick, nitrogen content 2.09% and 4.20%) and zirconia substrates (samples 0.1 mm thick, nitrogen content 2.09% and 4.20%). It was confirmed by a film formation test using. The results are shown in FIGS. 3 and 4. All nitrogen contents were analyzed using a wavelength dispersive X-ray analyzer (WDS) for a non-patterned (solid film) Cr—N thin film prepared using borosilicate glass (0.2 mm thick) as a substrate under the same conditions. .. Next, in order to investigate the resin substrate element, a thin film was prepared on each substrate under the same conditions as the zirconia substrate element of FIGS. 3 and 4, and heat-treated at temperatures of 180 ° C, 200 ° C, and 220 ° C to prepare a sample. From the evaluation of the film formation yield and the uniformity of the characteristics, we searched for a resin substrate that does not cause any problems.

Considering the problems from the background so far, the zirconia substrate that has already been put into practical use has the advantage of being excellent in heat resistance and shape stability because it does not deform due to heat even during heat treatment at 300 ° C. It is conceivable that there is no heat shrinkage, the coefficient of thermal expansion is relatively small, and the Young's modulus is large. On the other hand, when polyimide is used as a substrate, deformation due to heat is observed at the heat treatment temperature, and it is difficult to form a film such as cracks in the formed thin film, and the characteristics of the sensor thin film vary widely. Therefore, when this is compared with the case of the zirconia substrate, it is considered that there is a problem in heat resistance and shape stability. In fact, in the case of resin, there is thermal shrinkage, the coefficient of thermal expansion is relatively large, and Young's modulus is small.

Therefore, we prepared a sample using a resin with a high heat-resistant temperature and small heat shrinkage as a substrate, and evaluated the manufacturing yield and characteristic variation of the thin film element by paying attention to the thermal expansion coefficient and Young's modulus of the resin substrate. Resin materials that can be formed into a film having a heat resistance of 200 ° C. or higher are limited, and even among them, the coefficient of thermal expansion and the coefficient of linear expansion often take relatively large values. We investigated several types of resin film materials, from polyimide (PI), which has the highest heat resistance, and polyamide (PA), which is said to have heat resistance next to it. Table 1 shows the substrate materials used in the study of the present invention and their characteristics. Among the characteristics shown in the table, the nominal values were used for the thickness, heat resistant temperature, heat shrinkage rate, thermal expansion coefficient, and Young's modulus.

Further, regarding shape stability, it is considered that a large thickness also has an advantageous effect. In fact, from the examples described later, even if the resin of the same material is thin, poor results are shown, and when the thickness is thick, good results are shown. Therefore, we considered that not only Young's modulus but also the thickness factor was important, and examined the factor of "rigidity" including them.

In general, the rigidity k _N at the time of tensile deformation in a plate material is given by the following equation.

k _N = N / δ _N = E ・ A / L = E ・ t ・ w / L.

Here, N is the tensile force acting on the plate material, δ _N is the deformation in the tensile direction that occurs in the plate material, E is the tensile elastic modulus of the plate material, A is the cross-sectional area (= t × w) perpendicular to the tensile direction, and L. Is the length of the plate material in the tensile direction, t is the thickness of the plate material, and w is the width of the plate material (the length in the direction perpendicular to the tensile direction). As will be described later, since the substrate shape, film formation region, and thin film pattern shape of the test sample are all the same, their w and L are the same for all the samples, and the difference in the rigidity of the substrate is Et. It is determined only by the term (tensile elastic modulus x thickness), and in the present invention, the value represented by that term is referred to as the rigidity ratio, and the rigidity ratio is evaluated together with the thermal expansion coefficient.

(Structure of strain gauge)
The strain gauge as an embodiment of the present invention shown in FIG. 5 is a designated embodiment formed on a thin plate-shaped substrate 1 and one of the main surfaces 101 and 102 of the substrate 1. It is composed of a thin film element 2 arranged in. The substrate 1 is made of a resin having a rigidity ratio in the range of 200 to 1000 × 10 ³ Pa · m and a coefficient of thermal expansion in the range of 0 ppm / ° C. to 30 ppm / ° C. The thin film element 2 has a nitrogen (N) content in the range of 2 to 8 at%, a temperature coefficient of resistance (TCR) of 0 ± 400 ppm / ° C., and a gauge ratio of 3 to 20. -Consists of N thin film.

(Manufacturing method of strain gauge)
A method for manufacturing a strain gauge as an embodiment of the present invention includes (1) a film forming step and (2) a heat treatment step.

A reactive sputtering method is used to form a Cr-N thin film on a substrate by introducing a small amount of nitrogen gas together with Ar to form a film, and the device is for general metals (for non-ferromagnetic materials, that is, for low magnetic force). ) A magnetron-type high-frequency sputtering device using a magnet was used. The amount of nitrogen added was controlled by adjusting the flow rate of nitrogen gas to be introduced. A Cr disk (3 inches in diameter) with a nominal purity of 99.9% is used as the target, and the degree of vacuum before film formation (background vacuum degree), the distance between the target and the substrate (TS distance), the film formation gas pressure, and the input power. The film was formed with a nitrogen flow rate ratio of 2 × 10 ^-5 Pa, 43 mm, 5 mTorr, 10 W and 0.02 to 0.12%, respectively.

The sensitive portion of the prototype Cr—N thin film strain gauge element was formed into a grid consisting of eight folds, the line width of the grid was 40 μm, the line spacing was 50 μm, and the length (sensing portion length) was 1 mm. Photolithography technology and corrosion shaping technology using Cr etching solution were used to form the element pattern. The thickness of the thin film was about 100 nm. FIG. 6 shows the pattern shape of the actually manufactured thin film element.

FIG. 7 shows a thin film element arrangement pattern. In one film formation, a Cr—N thin film is formed within a range of 30 mm × 30 mm in the central portion on a substrate having a size of 50 mm × 50 mm, and the patterned elements are laterally 1 to 8 from the Cr—N thin film. A total of 40 pieces are obtained, which consist of an array of 8 rows and 5 columns of A to E vertically.

The heat treatment was carried out in the air at a predetermined temperature for 30 minutes. A Ni (nickel) thin film was superposed on the prepared thin film at a predetermined position by the lift-off method, and this was used as an electrode for resistance measurement. Lead wires connected to the power supply and the voltmeter were soldered to this electrode, but before that, the elements were individually cut out from the 40 arrangements. Since the Ni thin film as the electrode film needs to be formed by superimposing an electrode film having a small resistivity in order not to include strain detection information in the electrode or the lead portion, the electrode other than the sensitive portion is formed. It was formed by superimposing it on a Cr—N thin film of a tab and a lead portion. The methods, methods, shapes, materials, conditions, etc. of thin film formation, pattern formation, heat treatment, etc. according to the present invention are not limited to the embodiment.

(1. Film formation process)
In the film forming step, the Cr—N thin film arranged in the designated manner on the main surface 101 of the substrate 1 is the main surface 101 of the substrate 1 by sputtering the main surface 101 of the substrate 1 using a Cr target. Formed directly on the surface. During sputtering, the nitrogen flow rate ratio is adjusted to, for example, in the range of 0.02 to 0.05%. The arrangement of the designated embodiment of the thin film element 2 on the main surface 101 of the substrate 1 is formed by an existing method such as masking and / or etching.

(2. Heat treatment process)
In the heat treatment step, the Cr—N thin film formed on the main surface 101 of the substrate 1 is heat-treated in a temperature range of 180 to 200 ° C. The heat treatment time is adjusted to, for example, in the range of 0.5 to 4 hr so that the desired characteristics of the Cr—N thin film are realized. As a result, the thin film device 2 has a nitrogen content in the range of 2.09 to 4.20 at% and a temperature coefficient of resistance (TCR) in the range of 45.1 ppm / ° C. to 370.1 ppm / ° C. Moreover, a thin film element made of a Cr—N thin film having a gauge coefficient in the range of 16.6 to 17.9 is formed.

(Examples and comparative examples)
A thin plate-shaped member of polyamide (trade name: Mictron (model number: ML)) was prepared as the first substrate. As the second substrate, a thin plate-shaped member of polyimide (trade name: Utopirex (model number: 25S)) was prepared. As the third substrate, a thin plate-shaped member of polyimide (trade name: Kapton (model number: 300V)) was prepared. As the fourth substrate, a thin plate-shaped member of polyimide (trade name: Kapton (model number: 100V)) was prepared. As the fifth substrate, a thin plate-shaped member of polyimide (trade name: apical (model number: NPI)) was prepared. As the sixth substrate, a thin plate-shaped member of polyimide (trade name: apical (model number: AH)) was prepared. As the seventh substrate, a thin plate-shaped member of polyimide (trade name: Upirex (model number: 75S)) was prepared. As a reference substrate, a thin plate-shaped member of zirconia (trade name: Ceraflex (model number: A)) was prepared. Table 1 shows the thickness, heat resistant temperature, and Young's modulus (tensile modulus) of the first substrate, the second substrate, the third substrate, the fourth substrate, the fifth substrate, the sixth substrate, the seventh substrate, and the reference substrate, respectively. ), Rigidity ratio, coefficient of thermal expansion and coefficient of thermal contraction are shown together.

ひずみゲージの第１作製条件として、成膜工程に際して窒素流量比が０．０２％に調節され、かつ、熱処理工程に際してＣｒ－Ｎ薄膜の熱処理温度が１８０℃に調節された。ひずみゲージの第２作製条件として、成膜工程に際して窒素流量比が０．０２～０．１２％、より好ましくは０．０２～０．０５％に調節され、かつ、熱処理工程に際してＣｒ－Ｎ薄膜の熱処理温度が２００℃に調節された。ひずみゲージの第３作製条件として、成膜工程に際して窒素流量比が０．０５％に調節され、かつ、熱処理工程に際してＣｒ－Ｎ薄膜の熱処理温度が２２０℃に調節された。窒素流量比は、スパッタリングチャンバにおける窒素ガスの流量Ｆ１およびスパッタリングガスであるアルゴンガスの流量Ｆ２の合計に対する窒素ガスの流量Ｆ１の比Ｆ１／（Ｆ１＋Ｆ２）を意味する。成膜工程においてスパッタリングチャンバの気圧が５ｍＴｏｒｒに調節された。

As the first preparation condition of the strain gauge, the nitrogen flow rate ratio was adjusted to 0.02% in the film forming step, and the heat treatment temperature of the Cr—N thin film was adjusted to 180 ° C. in the heat treatment step. As the second preparation condition of the strain gauge, the nitrogen flow rate ratio is adjusted to 0.02 to 0.12%, more preferably 0.02 to 0.05% in the film forming process, and the Cr—N thin film is adjusted in the heat treatment process. The heat treatment temperature was adjusted to 200 ° C. As the third preparation condition of the strain gauge, the nitrogen flow rate ratio was adjusted to 0.05% in the film forming step, and the heat treatment temperature of the Cr—N thin film was adjusted to 220 ° C. in the heat treatment step. The nitrogen flow rate ratio means the ratio F1 / (F1 + F2) of the flow rate F1 of the nitrogen gas to the total flow rate F1 of the nitrogen gas and the flow rate F2 of the argon gas which is the sputtering gas in the sputtering chamber. In the film forming process, the air pressure in the sputtering chamber was adjusted to 5 mTorr.

Each of the first substrate, the second substrate, the third substrate, the fourth substrate and the reference substrate is used, and according to the first production conditions, Example 1, Example 3, Example 5, Comparative Example 4 and Reference Example 1 are used. A strain gauge group was created. Each of the first substrate, the second substrate, the third substrate, the fifth substrate, the sixth substrate, the seventh substrate, and the reference substrate was used, and Example 2, Example 4, and Comparative Example 3 were used according to the second production conditions. Strain gauge groups of Example 6, Comparative Example 5, Example 7 and Reference Example 2 were prepared. Each of the first substrate and the second substrate was used, and the strain gauge groups of Comparative Example 1 and Comparative Example 2 were prepared according to the third fabrication condition. The strain gauge group is composed of 40 strain gauges arranged in 8 rows and 5 columns.

Table 2 summarizes the production conditions of the strain gauge groups of Examples 1 to 7, Comparative Examples 1 to 5 and Reference Examples 1 to 2 (hereinafter referred to as “each production example”) and the evaluation results described later. Has been done.

（ひずみゲージの評価）
１回の成膜で同時に同条件にしたがって形成された４０個全てのＣｒ－Ｎ薄膜ひずみセンサ素子（図７参照）の抵抗値を２０ＭΩまで測定可能なテスターを用いて測定した。その際、薄膜にクラックが生じて測定不能だった素子の個数を全素子数である４０で除算した値の百分率を「断線率」とし、成膜歩留まりを評価する指標とした。

(Evaluation of strain gauge)
The resistance values of all 40 Cr—N thin film strain sensor elements (see FIG. 7) formed simultaneously under the same conditions in one film formation were measured using a tester capable of measuring up to 20 MΩ. At that time, the percentage of the value obtained by dividing the number of elements that could not be measured due to cracks in the thin film by 40, which is the total number of elements, was defined as the "disconnection rate" and used as an index for evaluating the film formation yield.

The DC four-terminal method using a digital multimeter is used for resistance measurement for resistance temperature counting (TCR) measurement, and the TCR value is calculated from the resistance value of the thin film element measured at different temperatures in a temperature-controllable constant temperature bath. I asked. Here, TCR means a value in the temperature range of 0 to 50 ° C.

The gauge ratio (Gf) is positive to negative by applying strain using a continuous cantilever method in which a sample is bonded and bent on a plate made of SUS304 with dimensions of 50 mm x 250 mm x 1.6 mm as a strain-causing body. It was obtained from the resistance change when strain was applied up to about 600 με (= 0.06%). The amount of strain required to calculate Gf is a commercially available strain gauge (Kyowa Electric Co., Ltd., KFG-2-350-C1-11) bonded to a position on the same SUS plate-raised strain body where an equal amount of strain enters. Was measured using. A commercially available instant adhesive for general use was used for bonding.

Of the 40 elements manufactured, the TCR and Gf are basically 1A, 2A, 1E, 2E, 3B, 4B, 3D, 4D, 5B, 6B, 5D, 6D, 7A in the arrangement of FIG. , 8A, 7E, 8E, a total of 16 elements were measured. If there was an element that could not be measured due to disconnection among these, the adjacent element was measured instead, and the total number of measurements was 16. As an index for evaluating the variation in the values of TCR and Gf, the "non-uniformity" given by the following equation was obtained from the maximum value, the minimum value, and the average value in each of the 16 measurement results. Here, | f (x) | represents the absolute value of f (x).

(Non-uniformity) = | {(maximum value)-(minimum value)} / (average value) |.

From the above-mentioned trial production test, a resin substrate material having excellent heat resistance, which showed predetermined characteristics and was suitable for producing a strain gauge with a small variation with a good yield, was investigated. As can be seen from FIG. 23, the TCR is within ± 400 ppm / ° C., the Gf is 3 to 20, and the film formation yield is in the range of 2 to 8 at% of the nitrogen content of the Cr—N thin film. It is preferably 90% or more, that is, the disconnection rate is 10% or less, and the non-uniformity of the characteristics of the Cr—N thin film is twice or less the value of the reference example (existing zirconia substrate element in practical use). Is preferable. The disconnection refers to a state in which the thin film element is cracked due to deformation of the substrate or the like and the resistance value cannot be measured.

Regarding the measurement of TCR and Gf, the number of non-disconnected samples is less than 16 for the sample with a large number of disconnections, and the total number of non-uniformity tests is different, and the sample with such a large number of disconnections is extremely off. Since it may not be possible to prepare 16 valid samples because some of them show bad measurement results, appropriate evaluation results cannot be obtained, so they were not included in the evaluation results.

FIG. 8 shows the disconnection rate with respect to the heat shrinkage rate. FIG. 9 shows the non-uniformity of TCR. FIG. 10 shows the non-uniformity of Gf. All showed the worst results when the heat shrinkage rate was 0.05%, and did not show a uniform tendency. In particular, the non-uniformity of TCR showed a rather good value at 0.5%, which has the largest heat shrinkage rate. From these results, it was found that the heat shrinkage in the range of at least 0.5% or less does not affect the disconnection rate, the non-uniformity of TCR and the non-uniformity of Gf.

FIG. 11 shows the disconnection rate with respect to the heat treatment temperature for each of the first substrate (polyamide / Mixtron) and the second substrate (polyimide / Upirex) having a small coefficient of thermal expansion and a large Young's modulus. In FIG. 11, the upper limit of the preferable numerical range with respect to the disconnection rate is shown by a broken line. FIG. 12 shows the non-uniformity of the TCRs of the first substrate and the second substrate, respectively. In FIG. 12, the upper limit of the preferable numerical range with respect to the non-uniformity of TCR is shown by a broken line. FIG. 13 shows the non-uniformity of Gf of each of the first substrate and the second substrate. In FIG. 13, the upper limit of the preferable numerical range with respect to the non-uniformity of Gf is shown by a broken line.

As can be seen from FIG. 13, there was no problem with the non-uniformity of Gf. On the other hand, as can be seen from FIG. 11, the disconnection rate of the first substrate sharply increased at 220 ° C., which was out of the preferable numerical range. As can be seen from FIG. 12, when the heat treatment temperature of the second substrate is 200 ° C. or lower, the non-uniformity of TCR is included in the preferable numerical range, but when the heat treatment temperature is 220 ° C., it is out of the preferable numerical range.

FIG. 14 shows the disconnection rate with respect to the rigidity ratio of each of the sample heat-treated at 180 ° C. and the sample heat-treated at 200 ° C. In FIG. 15, the rigidity ratio range of 200 to 300 kPa · m in FIG. 14 is enlarged and shown. FIG. 16 shows the disconnection rate for each of the sample heat-treated at 180 ° C. and the sample heat-treated at 200 ° C. with respect to the coefficient of thermal expansion. As can be seen from FIGS. 14 to 16, the disconnection rate of the sample of the fourth substrate heat-treated at 180 ° C. and the sample of the third substrate heat-treated at 200 ° C. exceeded the upper limit of the preferable numerical range. As can be seen from FIG. 16, although the fourth substrate and the third substrate heat-treated at 180 ° C. have the same coefficient of thermal expansion, the disconnection rate of the former is 97.5%, whereas that of the latter is 97.5%. The disconnection rate was 0%.

It is considered that the disconnection rate of the sample of the fourth substrate heat-treated at 180 ° C. is not due to the influence of the coefficient of thermal expansion but due to the small rigidity ratio as can be seen from FIG. Therefore, as can be seen from FIG. 15, it was found that a substrate having a rigidity ratio of less than 200 kPa · m is not preferable. Further, from FIGS. 14 and 15, it can be seen that the disconnection rate of the sample heat-treated at 200 ° C. does not show a uniform change with respect to the rigidity ratio, and the rigidity ratio is not the cause of the unfavorable large disconnection rate. On the other hand, in FIG. 16, the disconnection rate of the sample heat-treated at 200 ° C. increased with the increase of the coefficient of thermal expansion, and at 27 ppm / ° C., it became a large value to the extent that it exceeded the upper limit of the preferable numerical range. Therefore, it was found that the disconnection rate of the sample heat-treated at 200 ° C. depends on the coefficient of thermal expansion, and when it exceeds about 15 ppm / ° C., it becomes a large value to the extent that it exceeds the upper limit of the preferable numerical range.

FIG. 17 shows the non-uniformity of TCR with respect to the stiffness ratio for each of the sample heat-treated at 180 ° C. and the sample heat-treated at 200 ° C. In FIG. 18, the rigidity ratio range of 200 to 300 kPa · m in FIG. 17 is enlarged and shown. FIG. 19 shows the non-uniformity of TCR with respect to the coefficient of thermal expansion for each of the sample heat-treated at 180 ° C. and the sample heat-treated at 200 ° C. From FIGS. 17 to 19, the same results as those shown in FIGS. 14 to 16 were obtained for the samples heat-treated at 200 ° C. Further, as can be seen from FIGS. 20 and 21, regarding the non-uniformity of Gf, both the sample heat-treated at 180 ° C. and the sample heat-treated at 200 ° C. have a relative rigidity ratio and a coefficient of thermal expansion. However, there was no problem in these ranges.

From the above results, the conditions for the substrate characteristics for strain gauges are that the disconnection coefficient is 10% or less and the non-uniformity of the thin film characteristics is less than twice the value of the reference example (existing zirconia substrate element in practical use). It was found that when the heat treatment was performed in the temperature range of 180 to 200 ° C., the coefficient of thermal expansion was 0 to 30 ppm / ° C. and the rigidity ratio was in the range of 200 to 1000 × 10 ³ Pa · m. The characteristics of each substrate and the range boundaries are shown in FIG.

FIG. 22 is a plot showing the combination of the rigidity ratio and the coefficient of thermal expansion of each of the first substrate, the second substrate, the third substrate, the fourth substrate, the fifth substrate, the sixth substrate, the seventh substrate, and the reference substrate. It is indicated by a circled number from 1 to 7 and a white circle. The first designated range S1 in the rigidity ratio-coefficient of thermal expansion shown in FIG. 22 includes a rigidity ratio in the range of 200 to 1000 × 10 ³ Pa · m and a coefficient of thermal expansion of 0 ppm / ° C. to 30 ppm. It means that it is included in the range of / ° C. The second designated range S2 in the rigidity ratio-coefficient of thermal expansion shown in FIG. 22 is included in the range of the rigidity ratio of 227.5 to 682.5 × 10 ³ Pa · m and has a coefficient of thermal expansion of 3. It means that it is contained in the range of about 27 ppm / ° C. From FIG. 22, plots showing the combination of the rigidity ratio and the coefficient of thermal expansion of each of the first substrate, the second substrate, the third substrate, the fifth substrate, and the seventh substrate are shown in the first designated range S1 and the second designated range S2. It can be seen that it is contained in. On the other hand, from FIG. 22, the plot showing the combination of the rigidity ratio and the coefficient of thermal expansion of the fourth substrate, the sixth substrate and the reference substrate is out of the first designated range S1 and the second designated range S2. Understand.

By using the resin substrate under the above conditions, the problem that the substrate is fragile in the existing zirconia substrate Cr—N thin film strain gauge element is solved, and further, as can be seen from Table 3, the first substrate and the second substrate are As with the reference substrate (zirconia substrate), it has been clarified that a Cr—N thin film can be formed with good yield without disconnection, and that the non-uniformity (variation) of TCR and Gf is improved to the same level or higher. rice field. Practically, the present invention has made a great improvement, and it is possible to provide a strain gauge that is easier to handle than the conventional one, has less problems of characteristic variation and drift, has high sensitivity, and is stable to temperature.

If the heat treatment temperature exceeds 200 ° C., the disconnection rate increases, the film formation yield deteriorates, and the non-uniformity of TCR increases, which causes instability of the characteristics, which is not preferable. The lower limit of the heat treatment temperature is not particularly limited because the influence of the temperature is reduced, but 160 ° C., which does not significantly differ in characteristics, is preferable, and 180 ° C. is more preferable. Further, if the coefficient of thermal expansion exceeds 30 ppm / ° C., the disconnection rate also increases, the film formation yield deteriorates, and the non-uniformity of TCR increases, which causes instability of the characteristics, which is not preferable. Further, when the rigidity ratio is less than 200 × 10 ³ Pa · m, the disconnection rate sharply increases and the film formation yield becomes extremely poor, which is not preferable. On the other hand, if the rigidity ratio is too large, there is no problem in the disconnection rate and the uniformity of the characteristics, but in the case of a resin film substrate having a small Young's modulus, the thickness increases and the gauge rate cannot be measured correctly. In the present invention, it is considered unfavorable if the Young's modulus of 13 GPa, which is the maximum among the resin films tested, and the film thickness of 75 μm, which is the maximum, exceeds about 1000 × 10 ³ Pa · m.

As the thin film characteristics of the resin substrate, the values of the temperature coefficient of resistance and the gauge ratio with respect to the nitrogen content are shown in FIGS. 23 and 24, respectively. In the resin substrate as well, as in the case of the zirconia substrate (reference example) and the glass substrate shown in FIGS. 3 and 4, the temperature coefficient of resistance shows a value near zero over the total nitrogen content investigated, and the gauge ratio is the nitrogen content. There was a tendency to decrease with increasing. When the Cr—N thin film according to the heat treatment temperature reduction method described in the present invention is followed, the Cr—N thin film exhibits a resistance temperature coefficient (TCR) within ± 400 ppm / ° C. and a gauge ratio (Gf) of 3 to 20 at a nitrogen content of 2 to 8 at%. By using this method and conditions and a substrate having the above-mentioned characteristics, it is possible to provide the above-mentioned target strain gauge.

表２には、各作製例のひずみゲージ群の断線率の評価結果が示されている。ひずみゲージ群を構成する複数個（４０個のうち断線していないもの）の薄膜素子の電気抵抗値がテスターにより計測され、すべての薄膜素子の個数に対する、当該電気抵抗値が測定不能であった薄膜素子の個数の比が断線率として評価された。表２から、実施例１～５のそれぞれのひずみゲージ群の断線率は、比較例１～４のそれぞれのひずみゲージ群よりも著しく低く、かつ、参考例１および参考例２のそれぞれのひずみゲージ群の断線率と同程度以下であることがわかる。

Table 2 shows the evaluation results of the disconnection rate of the strain gauge group of each production example. The electric resistance values of a plurality of thin film elements (out of 40 pieces that were not broken) constituting the strain gauge group were measured by a tester, and the electric resistance values could not be measured for all the thin film elements. The ratio of the number of thin film elements was evaluated as the disconnection rate. From Table 2, the disconnection rate of each strain gauge group of Examples 1 to 5 is significantly lower than that of each strain gauge group of Comparative Examples 1 to 4, and the strain gauges of Reference Example 1 and Reference Example 2 are respectively. It can be seen that it is less than or equal to the disconnection rate of the group.

Table 2 shows the resistance temperature coefficient TCR (mean value) of the strain gauge group of each production example and the evaluation results of its non-uniformity. From Table 2, the TCR of each strain gauge group of Examples 1 to 5 is included in the range of -186.1 to 370.1 [ppm / ° C.], and in this respect, Comparative Example 2, Comparative Example 3, and Reference Example. It can be seen that it is common to each strain gauge group of 1 and Reference Example 2. On the other hand, the TCR non-uniformity of each strain gauge group of Examples 1 to 5 is included in the range of 0.263 to 1.916 as in each strain gauge group of Reference Example 1 and Reference Example 2, and is compared. It can be seen that it is lower than the respective strain gauge groups of Examples 2 and 3.

Table 2 shows the gauge ratio Gf (mean value) of the strain gauge group of each production example and the evaluation results of the non-uniformity thereof. From Table 2, it can be seen that the Gf of each strain gauge group of Examples 1 to 5 is included in the range of 16.6 to 19.0, which is similar to that of the respective strain gauge groups of Comparative Examples 2 and 3. .. Further, the non-uniformity of Gf of each strain gauge group of Examples 1 to 5 is included in the range of 0.076 to 0.121, which is lower than that of the strain gauge groups of Comparative Examples 2 and 3, and further. It can be seen that it is lower than the strain gauge groups of Reference Example 1 and Reference Example 2.

FIG. 23 shows the relationship between the nitrogen content and the temperature coefficient of resistance (TCR) of the thin film element 2 formed on the film of the synthetic resin (first substrate) as the substrate 1. From FIG. 23, it can be seen that the temperature coefficient of resistance of the thin film element 2 shows a value near zero in the range of the nitrogen content of 2 to 10.5%.

FIG. 24 shows the relationship between the nitrogen content and the gauge ratio (Gf) of the thin film element 2 formed on the film of the synthetic resin (first substrate) as the substrate 1. From FIG. 24, it can be seen that the gauge ratio of the thin film element 2 tends to decrease as the nitrogen content increases in the range of the nitrogen content of 2 to 10.5%.

1 ... Substrate, 2 ... Thin film element, 101 ... Top surface of the substrate (main surface), 102 ... Bottom surface of the substrate (main surface).

Claims (3)

A substrate made of a resin having a rigidity ratio in the range of 200 to 1000 × 10 ³ Pa · m and a coefficient of thermal expansion in the range of 0 ppm / ° C to 30 ppm / ° C.
The thin film element formed on the substrate and
Is a strain gauge equipped with
Cr in which the thin film element has a nitrogen (N) content in the range of 2 to 8 at%, a temperature coefficient of resistance (TCR) of 0 ± 400 ppm / ° C., and a gauge ratio of 3 to 20. -A strain gauge characterized by being composed of an N thin film. In the strain gauge according to claim 1,
The substrate is made of a resin having a rigidity ratio in the range of 227.5 to 682.5 × 10 ³ Pa · m and a coefficient of thermal expansion in the range of 3 to 27 ppm / ° C.
The thin film element has a nitrogen content in the range of 2.09 to 4.20 at%, a temperature coefficient of resistance (TCR) in the range of -186.1 to 370.1 ppm / ° C., and a gauge ratio. Is a strain gauge comprising a Cr—N thin film contained in the range of 16.6 to 19.0. Arranged in a designated manner on the main surface of a resin substrate having a rigidity ratio in the range of 200 to 1000 × 10 ³ Pa · m and a coefficient of thermal expansion in the range of 0 ppm / ° C to 30 ppm / ° C. The process of forming a thin film element made of a Cr—N thin film,
A step of heat-treating the thin film element at a temperature in the range of 180 to 200 ° C.
1. The method for manufacturing a strain gauge according to claim 1.

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