Strain gauge and its manufacturing method

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

[Technical Field]

[0001] The present invention relates to a strain gauge that is attached to the surface of a strain-flexible structure to be measured. [Background technology]

[0002] Cr-N thin film is a new strain sensor material characterized by a large gauge factor of approximately 14, which indicates sensitivity to strain; the temperature coefficient of resistance (TCR) can be reduced to near zero (<±50 ppm / °C) by adding a small amount of nitrogen and heat treatment; and the resistance can be increased to several tens of kΩ (see Patent Document 1).

[0003] Conventional strain gauges (bonded strain sensor elements) are constructed by attaching a metal foil, such as a CuNi or NiCr alloy, formed into a grid pattern as the sensor material to a base (substrate) made of resin such as polyimide. When used to measure strain and various mechanical quantities, they are further bonded to the surface of the strain-generating structure to be measured. In this case, the base is required to provide electrical insulation and ease of handling, including shape retention. It is also important that the base transmits strain correctly, which requires a material with a small Young's modulus and large elongation, and resin is commonly used today.

[0004] When using a strain sensor thin film as a mechanical quantity sensor, it is possible to form the sensor element directly on the strain-sensitive structure without using a base (via an insulating film if the strain-sensitive structure is a conductor such as a metal). With conventional strain gauges, the "bonding" is done manually, which makes it prone to misalignment, and there are also concerns about the effects of creep due to the base or adhesive. However, with a thin film formed directly on the object to be measured, these issues do not need to be considered. However, when installing a strain sensor in a location where thin film formation is impossible due to the structure of the object to be measured, such as deep inside a hole, pipe, or complex shape, it is necessary to use an adhesive method. Therefore, in order to develop an element that can be used with a Cr-N thin film by the adhesive method, the base substrate material was also investigated.

[0005] Polyimide, which is used in conventional strain gauges, has the highest heat resistance among resin films, but its thermal expansion coefficient and thermal shrinkage are larger than those of inorganic materials. Therefore, when polyimide is used as a substrate, the effects of localized stress on the substrate become more pronounced, making it prone to cracking. Research into this issue has revealed that reducing the deposition gas pressure, which strengthens the thin film itself by densifying the Cr-N thin film structure (film quality), is effective in reducing cracking. However, the thermal effects of polyimide are not completely eliminated, and the fabricated elements are prone to substrate warping and partial deformation of the substrate around the thin film, which poses problems regarding the characteristics and stability of the device.

[0006] In this study, they focused on zirconia as a free-standing thin plate material that has a thermal expansion coefficient close to that of Cr-N thin film, excellent heat resistance with no thermal shrinkage, sufficient strength and high insulation, and can be made thin to provide strain transmission. As a result of prototyping and evaluating strain gauges using this as the base material, they found that if the thickness of the base material is 80 μm or less, it is possible to provide a highly sensitive strain gauge that can be bonded and used while maintaining almost the same functionality as conventional ones (see Patent Document 2). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 6159613 [Patent Document 2] Patent No. 6022881 Summary of the Invention [Problem to be solved by the invention]

[0008] When using a Cr-N thin film strain gauge as an adhesive, the zirconia substrate element is very strong against bending, but is weak against linear tension or compression in the in-plane direction of the substrate, and is therefore prone to breakage. A conventionally used resin material is cited as a substrate material that is less likely to break. Therefore, we again examined resin materials, and found that polyimide, which has the highest heat resistance, is effective because heat treatment is required in the film formation process. However, when using conventional, general polyimide as a substrate, there were still problems with the film formation yield and uniformity of properties.

[0009] When using strain gauges, a Wheatstone bridge structure is used as the measurement circuit. However, if there is variation in the characteristics of the four strain gauge elements used, for example, the zero balance of the midpoint potential of the current line is lost, making accurate measurements impossible. Furthermore, it can also cause problems such as increased output drift due to external factors such as temperature. Therefore, it is extremely important that the multiple (numerous) strain gauge elements manufactured each have uniform characteristics without variation. Therefore, the present invention aims to provide a strain gauge and a manufacturing method thereof that can improve film formation yield and uniformity of characteristics when using a resin substrate. [Means for solving the problem]

[0010] The inventors of the present invention have conducted extensive research to solve the above problems and have found that by using a bonded strain gauge consisting of a resin substrate with predetermined characteristics and a Cr-N thin film with predetermined composition and characteristics, as well as a film formation method and a manufacturing method involving heat treatment at a predetermined temperature, it is possible to improve the film formation yield and the uniformity of characteristics. 227.5×10 3 Pa·m~682.5×10 3 Pa·m range and the thermal expansion coefficient is 3ppm / ℃~27 A strain gauge comprising a substrate made of a resin having a nitrogen (N) content in the range of ppm / °C and a thin film element formed on the substrate, wherein the thin film element has a nitrogen (N) content in the range of 2.09at%~4.20at% and the temperature coefficient of resistance (TCR) is within the range -186.1 ppm / ℃~370.1 ppm / ℃and the gauge factor is 16.6~19.0 It consists of a Cr-N thin film.

[0011] The method for manufacturing a strain gauge of the present invention is as follows: 227.5×10 3 Pa·m~682.5×10 3 Pa·m range and the thermal expansion coefficient is 3ppm / ℃~27 The method includes the steps of forming a thin film element made of a Cr-N thin film arranged in a specified manner on the main surface of a substrate made of a resin having a Cr-N content in the range of ppm / °C, and heat-treating the thin film element at a temperature in the range of 180 to 200°C. [Effects of the Invention]

[0012] According to the present invention, there are provided a bonded strain gauge and a method for manufacturing the same that can improve the film formation yield and the uniformity of characteristics when a resin substrate is used. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is an explanatory diagram of a method for reducing the heat treatment temperature in sputtering deposition of a Cr—N thin film (a method for deposition under conditions where the input power is appropriately low). [Figure 2] FIG. 1 is an explanatory diagram of a method for reducing the heat treatment temperature in sputtering deposition of a Cr—N thin film (a method for depositing the film under conditions that reduce the nitrogen content). [Figure 3] This figure shows the TCR measurement results for samples heat-treated at temperatures below 200°C using the heat treatment temperature reduction method. [Figure 4] This figure shows the results of measuring the gauge factor for samples heat-treated at temperatures below 200°C using the heat treatment temperature reduction method. [Figure 5] 1 is an explanatory diagram illustrating the configuration of a strain gauge according to the present invention. [Figure 6] A diagram showing the pattern shape of an actually fabricated thin-film element. [Figure 7] FIG. 1 is a diagram showing a thin-film element array pattern formed in one film formation. [Figure 8]1 is a graph showing the relationship between the thermal shrinkage rate and the disconnection rate for the fabrication examples in Table 2. [Figure 9] Graph showing the relationship between thermal shrinkage rate and TCR nonuniformity for the fabrication examples in Table 2. [Figure 10] FIG. 1 is a graph showing the relationship between thermal shrinkage and Gf nonuniformity for the fabrication examples in Table 2. [Figure 11] FIG. 10 is a graph showing the relationship between the heat treatment temperature and the disconnection rate for the first substrate and the second substrate. [Figure 12] FIG. 1 is a graph showing the relationship between the heat treatment temperature and the TCR nonuniformity for the first and second substrates. [Figure 13] FIG. 10 is a graph showing the relationship between the heat treatment temperature and the Gf nonuniformity for the first substrate and the second substrate. [Figure 14] A graph showing the relationship between rigidity ratio and breakage rate for samples heat-treated at 180°C and 200°C. [Figure 15] Enlarged view of the stiffness ratio range of 200–300 kPa·m in Figure 14 . [Figure 16] FIG. 1 is an explanatory diagram showing the relationship between the thermal expansion coefficient and the breakage rate of a sample heat-treated at 180° C. and a sample heat-treated at 200° C. [Figure 17] FIG. 1 is an explanatory diagram of the relationship between stiffness ratio and TCR nonuniformity for samples heat-treated at 180°C and samples heat-treated at 200°C. [Figure 18] Enlarged view of the stiffness ratio range of 200–300 kPa·m in Figure 17 . [Figure 19] FIG. 1 is an explanatory diagram of the relationship between the thermal expansion coefficient and the TCR nonuniformity of a sample heat-treated at 180°C and a sample heat-treated at 200°C. [Figure 20] FIG. 1 is an explanatory diagram of the relationship between stiffness ratio and Gf nonuniformity for samples heat-treated at 180°C and samples heat-treated at 200°C. [Figure 21] FIG. 1 is a graph illustrating the relationship between the thermal expansion coefficient and Gf nonuniformity for a sample heat-treated at 180° C. and a sample heat-treated at 200° C. [Figure 22] FIG. 2 is an explanatory diagram illustrating the characteristics of a substrate that constitutes a strain gauge. [Figure 23]FIG. 1 is an explanatory diagram showing the relationship between the nitrogen content and the temperature coefficient of resistance (TCR) of a thin-film element formed on a synthetic resin film. [Figure 24] FIG. 1 is an explanatory diagram showing the relationship between the nitrogen content and gauge factor (Gf) of a thin-film element formed on a synthetic resin film. DETAILED DESCRIPTION OF THE INVENTION

[0014] The key to solving this problem is thought to be the low "heat resistance" of the resin material. Therefore, in this invention, we attempted to improve this from the following two aspects. One was to reduce the heat treatment temperature of the strain sensor thin film, and the other was to search for a resin substrate that would not cause problems within the reduced heat treatment temperature range.

[0015] Heat treatment of Cr-N thin films used as strain sensors is carried out to adjust the TCR to 0, and traditionally, heat treatment temperatures of 200 to 300°C have been used for substrates such as glass, ceramics, and metals. However, test results shown later indicate that for resin-based materials, even those with heat resistance, the heat treatment temperature must be 200°C or less.

[0016] When forming a thin film by sputtering or the like, there are two methods for reducing the heat treatment temperature.

[0017] (1) Film deposition under conditions of moderately low input power (see Figure 1) (Reference: Niwa et al., Proceedings of the 32nd Symposium on Sensors, Micromachines and Application Systems, 28pm1-A-1 (2015)).

[0018] (2) To prepare a thin film with a low nitrogen content (see Figure 2) (Reference: Japanese Patent No. 6159613).

[0019] By these means, the TCR of the fabricated Cr-N thin film before heat treatment (as-deposited film) becomes a small negative value, and the heat treatment temperature for making the TCR zero can be lowered.

[0020] First, we confirmed through film deposition tests using borosilicate glass (0.2 mm thick, samples with nitrogen contents other than 2.09% and 4.20%) and zirconia substrates (0.1 mm thick, samples with nitrogen contents of 2.09% and 4.20%) that even when heat treatment was performed at temperatures below 200°C using the existing heat treatment temperature reduction method, the TCR was actually near zero (within ±400 ppm / °C) and the gauge factor was sufficiently large and remained at a good value. The results are shown in Figures 3 and 4. The nitrogen content was analyzed using a wavelength dispersive X-ray analyzer (WDS) for unpatterned (plain) Cr-N thin films fabricated under the same conditions on borosilicate glass (0.2 mm thick) substrates. Next, to investigate resin substrate elements, thin films were fabricated on each substrate under the same conditions as for the zirconia substrate elements in Figures 3 and 4, and heat-treated at temperatures of 180°C, 200°C, and 220°C to prepare samples. Based on evaluations of film formation yield and uniformity of properties, a search was conducted for a resin substrate that would not cause any problems.

[0021] Considering the issues from the past, zirconia substrates, which are already in practical use, have the advantage of excellent heat resistance and shape stability, as they do not deform due to heat even during heat treatment at 300°C. The properties that contribute to this are thought to be the lack of thermal shrinkage, a relatively small coefficient of thermal expansion, and a large Young's modulus. On the other hand, when using polyimide as a substrate, thermal deformation occurs at the heat treatment temperature, making film formation difficult due to cracks in the formed thin film, and there is also considerable variation in the characteristics of the sensor thin film. Therefore, when comparing this with zirconia substrates, it is thought that there are problems with heat resistance and shape stability. In fact, resins undergo thermal shrinkage, have a relatively large coefficient of thermal expansion, and a small Young's modulus.

[0022] Therefore, we prepared samples using resin substrates with high heat resistance and low thermal shrinkage, and evaluated the manufacturing yield and characteristic variability of thin-film devices by focusing on the thermal expansion coefficient and Young's modulus of the resin substrate. Only a limited number of resin materials can be made into films with heat resistance above 200°C, and even among these, many have relatively large thermal shrinkage and linear expansion coefficients. We investigated several types of polyimide (PI), which has the highest heat resistance among resin film materials, as well as polyamide (PA), which is said to have the second highest heat resistance. Table 1 shows the substrate materials used in the study of this invention and their properties. Of the properties listed in the table, nominal values ​​were used for thickness, heat resistance, thermal shrinkage, thermal expansion coefficient, and Young's modulus.

[0023] Furthermore, it is believed that a large thickness also has an advantageous effect in terms of shape stability. In fact, as will be seen in the examples described later, even with the same resin material, a thin thickness showed poor results, while a thick thickness showed good results. Therefore, we considered that not only Young's modulus but also thickness factors were important, and investigated the factor of "rigidity," which includes both.

[0024] Generally, the stiffness k of a plate during tensile deformation N is given by the following formula:

[0025] k N =N / δ N =E·A / L=E·t·w / L .

[0026] where N is the tensile force acting on the plate, δ N is the deformation in the tensile direction that occurs in the plate, E is the tensile modulus of elasticity of the plate, A is the cross-sectional area perpendicular to the tensile direction (= t × w), L is the length of the plate in the tensile direction, t is the thickness of the plate, and w is the width of the plate (length perpendicular to the tensile direction). As will be described later, the substrate shape, film formation area, and thin film pattern shape of the test samples were all identical, so w and L were the same for all samples, and differences in substrate rigidity were determined only by the term E·t (tensile modulus of elasticity × thickness). In this invention, the value expressed by this term is referred to as the rigidity ratio, and this rigidity ratio was evaluated along with the thermal expansion coefficient.

[0027] (Structure of strain gauge) The strain gauge according to one embodiment of the present invention shown in Fig. 5 is composed of a thin plate-shaped substrate 1 and thin film elements 2 formed on one of a pair of main surfaces 101, 102 of the substrate 1 and arranged in a specified manner. The substrate 1 has a stiffness ratio of 200 to 1000 x 10 3 The thin-film element 2 is made of a resin having a thermal expansion coefficient in the range of 0 ppm / °C to 30 ppm / °C, a nitrogen (N) content in the range of 2 to 8 at%, a temperature coefficient of resistance (TCR) within 0±400 ppm / °C, and a gauge factor of 3 to 20.

[0028] (Strain gauge manufacturing method) A method for manufacturing a strain gauge according to one embodiment of the present invention includes (1) a film forming step and (2) a heat treatment step.

[0029] To fabricate the Cr-N thin film on the substrate, a reactive sputtering method was used, in which a small amount of nitrogen gas was introduced along with Ar to form the film. A magnetron-type high-frequency sputtering device with a magnet for general metals (non-ferromagnetic, i.e., low magnetic force) was used. The amount of nitrogen added was controlled by adjusting the flow rate of the introduced nitrogen gas. A Cr disk (3 inches in diameter) with a nominal purity of 99.9% was used as the target, and the degree of vacuum before film formation (background vacuum), target-substrate distance (TS distance), film formation gas pressure, input power, and nitrogen flow rate ratio were all set at 2 x 10 -5 The film was deposited under the conditions of Pa, 43 mm, 5 mTorr, 10 W, and 0.02 to 0.12%.

[0030] The sensitive part of the prototype Cr-N thin film strain gauge element was a lattice-shaped element consisting of eight folds, with a line width of 40 μm, line spacing of 50 μm, and a length (sensitive part) of 1 mm. The element pattern was formed using photolithography and etching shaping techniques using a Cr etching solution. The thickness of the thin film was approximately 100 nm. The pattern shape of the thin film element that was actually fabricated is shown in Figure 6.

[0031] The thin-film element array pattern is shown in Figure 7. In one deposition run, a Cr-N thin film is formed within a 30 mm x 30 mm area in the center of a 50 mm x 50 mm substrate, from which a total of 40 patterned elements are obtained, arranged in eight horizontal rows (1 to 8) and five vertical columns (A to E).

[0032] The heat treatment was carried out by holding the device at a predetermined temperature in air for 30 minutes. A Ni (nickel) thin film was formed by lift-off at a predetermined position on the prepared thin film, which served as an electrode for resistance measurement. Lead wires connected to a power supply and a voltmeter were soldered to this electrode, but before that, elements were individually cut out from the 40-element array. The Ni thin film used as the electrode film was formed by overlaying a Cr-N thin film on the electrode tab and lead portions other than the sensing portion, since it was necessary to overlay an electrode film with low resistivity to avoid including strain detection information in the electrode and lead portions. The methods, formats, shapes, materials, and conditions for thin film formation, pattern formation, heat treatment, etc., according to the present invention are not limited to those of the present embodiment.

[0033] (1. Film formation process) In the film formation process, a Cr—N thin film arranged in a specified pattern on one main surface 101 of the substrate 1 is formed directly on the main surface by sputtering using a Cr target. During sputtering, the nitrogen flow rate is adjusted to, for example, a range of 0.02 to 0.05%. The thin-film elements 2 are arranged in a specified pattern on the main surface 101 of the substrate 1 by known techniques such as masking and / or etching.

[0034] (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 at a temperature in the range of 180 to 200°C. The heat treatment time is adjusted to, for example, a range of 0.5 to 4 hours so that the Cr-N thin film achieves the target characteristics. As a result, the thin film element 2 is formed, which is made of a Cr-N thin film having a nitrogen content in the range of 2.09 to 4.20 at%, a temperature coefficient of resistance (TCR) in the range of 45.1 ppm / °C to 370.1 ppm / °C, and a gauge factor in the range of 16.6 to 17.9.

[0035] Examples and Comparative Examples A thin plate-shaped member made of polyamide (trade name: Mictron (model number: ML)) was prepared as the first substrate. A thin plate-shaped member made of polyimide (trade name: Utopirex (model number: 25S)) was prepared as the second substrate. A thin plate-shaped member made of polyimide (trade name: Kapton (model number: 300V)) was prepared as the third substrate. A thin plate-shaped member made of polyimide (trade name: Kapton (model number: 100V)) was prepared as the fourth substrate. A thin plate-shaped member made of polyimide (trade name: Apical (model number: NPI)) was prepared as the fifth substrate. A thin plate-shaped member made of polyimide (trade name: Apical (model number: AH)) was prepared as the sixth substrate. A thin plate-shaped member made of polyimide (trade name: Upirex (model number: 75S)) was prepared as the seventh substrate. A thin plate-shaped member made of zirconia (trade name: Ceraflex (model number: A)) was prepared as the reference substrate. Table 1 summarizes the thickness, heat resistance temperature, Young's modulus (tensile modulus of elasticity), rigidity ratio, thermal expansion coefficient, and thermal shrinkage rate of each of the first substrate, second substrate, third substrate, fourth substrate, fifth substrate, sixth substrate, seventh substrate, and reference substrate.

[0036] [Table 1] The first strain gauge fabrication condition was that the nitrogen flow rate was adjusted to 0.02% during the film formation process, and the heat treatment temperature of the Cr-N thin film was adjusted to 180°C during the heat treatment process. The second strain gauge fabrication condition was that the nitrogen flow rate was adjusted to 0.02-0.12%, preferably 0.02-0.05%, during the film formation process, and the heat treatment temperature of the Cr-N thin film was adjusted to 200°C during the heat treatment process. The third strain gauge fabrication condition was that the nitrogen flow rate was adjusted to 0.05% during the film formation process, and the heat treatment temperature of the Cr-N thin film was adjusted to 220°C during the heat treatment process. The nitrogen flow rate ratio refers to the ratio F1 / (F1+F2) of the nitrogen gas flow rate F1 to the sum of the nitrogen gas flow rate F1 and the argon gas flow rate F2 in the sputtering chamber. The air pressure in the sputtering chamber was adjusted to 5 mTorr during the film formation process.

[0037] The strain gauge groups of Example 1, Example 3, Example 5, Comparative Example 4, and Reference Example 1 were fabricated using the first, second, third, fourth, and reference substrates, respectively, according to first fabrication conditions. The strain gauge groups of Example 2, Example 4, Comparative Example 3, Example 6, Comparative Example 5, Example 7, and Reference Example 2 were fabricated using the first, second, third, fifth, sixth, seventh, and reference substrates, respectively, according to second fabrication conditions. The strain gauge groups of Comparative Examples 1 and 2 were fabricated using the first and second substrates, respectively, according to third fabrication conditions. The strain gauge groups consist of 40 strain gauges arranged in 8 rows and 5 columns.

[0038] Table 2 summarizes the fabrication conditions for the strain gauge groups of Examples 1 to 7, Comparative Examples 1 to 5, and Reference Examples 1 and 2 (hereinafter referred to as "each fabrication example") and the evaluation results described below.

[0039] [Table 2] (Strain gauge evaluation) The resistance values ​​of all 40 Cr-N thin film strain sensor elements (see Figure 7), which were formed simultaneously under the same conditions in one deposition run, were measured using a tester capable of measuring up to 20 MΩ. 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 the total number of elements, 40, was taken as the "breakage rate," and was used as an index to evaluate the deposition yield.

[0040] The resistance of the thin-film element was measured at different temperatures in a temperature-controlled thermostatic chamber using the DC four-terminal method with a digital multimeter to determine the temperature coefficient of resistance (TCR). Here, TCR refers to the value in the temperature range of 0 to 50°C.

[0041] The gauge factor (Gf) was determined by applying strain to a 50mm x 250mm x 1.6mm thick SUS304 plate bonded to the specimen and bending it using a continuous cantilever beam. The strain was measured from the change in resistance when strain was applied from positive to negative, approximately 600με (=0.06%). The strain required to calculate Gf was measured using a commercially available strain gauge (Kyowa Electronics, KFG-2-350-C1-11) bonded to the same SUS plate at a position where the same amount of strain was applied. A commercially available general-purpose instant adhesive was used for bonding.

[0042] Of the 40 devices fabricated, the TCR and Gf measurements were performed on a total of 16 devices, essentially consisting of 1A, 2A, 1E, 2E, 3B, 4B, 3D, 4D, 5B, 6B, 5D, 6D, 7A, 8A, 7E, and 8E in the arrangement shown in Figure 7. If a device was unmeasurable due to a disconnection, an adjacent device was measured instead, bringing the total number of measurements to 16. As an index for evaluating the variation in the TCR and Gf values, the "non-uniformity" was calculated using the maximum, minimum, and average values ​​of the 16 measurement results, as given by the following equation: where |f(x)| represents the absolute value of f(x).

[0043] (Heterogeneity) = |{(Maximum value) - (Minimum value)} / (Average value)|.

[0044] From the prototype testing described above, we investigated a resin substrate material with excellent heat resistance that exhibits the required characteristics and is suitable for producing strain gauges with low variation with a high yield. As shown in Figure 23, the required characteristics are as follows: when the nitrogen content of the Cr-N thin film is in the range of 2 to 8 at%, the TCR is within ±400 ppm / °C, the Gf is 3 to 20, the film formation yield is preferably 90% or more, i.e., the breakage rate is 10% or less. It is also preferable that the non-uniformity of the Cr-N thin film characteristics is less than twice the value of the reference example (an existing zirconia substrate element in practical use). A breakage refers to a state in which cracks occur in the thin film element due to substrate deformation or other reasons, making it impossible to measure the resistance value.

[0045] Regarding the measurements of TCR and Gf, samples with a large number of breaks were not included in the evaluation results because the total number of non-break samples would be less than 16, which would result in a difference in the heterogeneity test results, and because it may not be possible to gather 16 valid samples with such a large number of breaks, resulting in some samples showing extremely poor measurement results.

[0046] Figure 8 shows the disconnection rate as a function of heat shrinkage. Figure 9 shows the non-uniformity of TCR. Figure 10 shows the non-uniformity of Gf. In both cases, the worst results were observed at a heat shrinkage rate of 0.05%, and no uniform trend was observed. In particular, the TCR non-uniformity showed rather good values ​​at the highest heat shrinkage rate of 0.5%. These results demonstrate that a heat shrinkage rate of at least 0.5% or less does not affect the disconnection rate, TCR non-uniformity, or Gf non-uniformity.

[0047] FIG. 11 shows the disconnection rate versus heat treatment temperature for the first substrate (Polyamide Mictron) and the second substrate (Polyimide Upilex), which have a small thermal expansion coefficient and a large Young's modulus. In FIG. 11, the upper limit of a preferred numerical range for the disconnection rate is indicated by a dashed line. FIG. 12 shows the TCR non-uniformity for the first substrate and the second substrate. In FIG. 12, the upper limit of a preferred numerical range for the TCR non-uniformity is indicated by a dashed line. FIG. 13 shows the Gf non-uniformity for the first substrate and the second substrate. In FIG. 13, the upper limit of a preferred numerical range for the Gf non-uniformity is indicated by a dashed line.

[0048] As can be seen from Figure 13, there were no problems with the non-uniformity of Gf. On the other hand, as can be seen from Figure 11, the open circuit rate for the first substrate increased sharply at 220°C, falling outside the preferred range. As can be seen from Figure 12, the non-uniformity of TCR for the second substrate was within the preferred range when the heat treatment temperature was 200°C or lower, but fell outside the preferred range when the heat treatment temperature was 220°C.

[0049] Figure 14 shows the wire breakage rate versus stiffness ratio for samples heat-treated at 180°C and samples heat-treated at 200°C. Figure 15 shows an enlarged view of the stiffness ratio range of 200 to 300 kPa·m in Figure 14. Figure 16 shows the wire breakage rate versus thermal expansion coefficient for samples heat-treated at 180°C and samples heat-treated at 200°C. As can be seen from Figures 14 to 16, the wire breakage rates for the fourth substrate heat-treated at 180°C and the third substrate heat-treated at 200°C exceeded the upper limit of the preferred range. As can be seen from Figure 16, even though the fourth and third substrates heat-treated at 180°C have the same thermal expansion coefficient, the wire breakage rate for the former was 97.5%, while the wire breakage rate for the latter was 0%.

[0050] The failure rate of the fourth substrate sample, heat-treated at 180°C, is not due to the influence of the thermal expansion coefficient, but rather to the low rigidity ratio, as can be seen in Figure 14. Therefore, as can be seen in Figure 15, substrates with rigidity ratios below 200 kPa·m are undesirable. Furthermore, Figures 14 and 15 show that the failure rate of the sample heat-treated at 200°C does not change uniformly with the rigidity ratio, indicating that the rigidity ratio is not the cause of the undesirable high failure rate. On the other hand, Figure 16 shows that the failure rate of the sample heat-treated at 200°C increases with increasing thermal expansion coefficient, reaching a value that exceeds the upper limit of the desirable numerical range at 27 ppm / °C. Therefore, it was found that the failure rate of the sample heat-treated at 200°C depends on the thermal expansion coefficient, and that it exceeds the upper limit of the desirable numerical range when the thermal expansion coefficient exceeds approximately 15 ppm / °C.

[0051] Figure 17 shows the TCR nonuniformity versus stiffness ratio for the samples heat-treated at 180°C and 200°C. Figure 18 shows an expanded view of the stiffness ratio range of 200–300 kPa m in Figure 17. Figure 19 shows the TCR nonuniformity versus thermal expansion coefficient for the samples heat-treated at 180°C and 200°C. Figures 17–19 show similar results for the samples heat-treated at 200°C as shown in Figures 14–16. Furthermore, as can be seen from Figures 20 and 21, there were no problems with Gf nonuniformity in either the 180°C or 200°C samples for either stiffness ratio or thermal expansion coefficient within these ranges.

[0052] From the above results, the conditions for the substrate characteristics for strain gauges, where the disconnection rate 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), are a thermal expansion coefficient of 0 to 30 ppm / °C and a rigidity ratio of 200 to 1000 × 10 when heat treated in the temperature range of 180 to 200°C. 3 The properties of each substrate and the range boundaries are shown in Figure 22.

[0053] In Fig. 22, plots showing combinations of stiffness ratio and thermal expansion coefficient for each of the first, second, third, fourth, fifth, sixth, seventh, and reference substrates are indicated by circled numbers from 1 to 7 and white circles. The first specified range S1 of stiffness ratio-thermal expansion coefficient shown in Fig. 22 is a range of stiffness ratio from 200 to 1000 × 10 3 This means that the stiffness ratio is within the range of 227.5 to 682.5×10 Pa·m and the thermal expansion coefficient is within the range of 0 ppm / °C to 30 ppm / °C. The second specified range S2 in the stiffness ratio-thermal expansion coefficient relationship shown in FIG. 22 is the stiffness ratio of 227.5 to 682.5×10 3 This means that the stiffness ratio and thermal expansion coefficient are within the range of 3 to 27 ppm / °C. From FIG. 22, it can be seen that the plots representing the combinations of stiffness ratio and thermal expansion coefficient for the first, second, third, fifth, and seventh substrates are within the first specified range S1 and the second specified range S2. On the other hand, from FIG. 22, it can be seen that the plots representing the combinations of stiffness ratio and thermal expansion coefficient for the fourth, sixth, and reference substrates are outside the first specified range S1 and the second specified range S2.

[0054] By using a resin substrate satisfying the above conditions, the problem of easily broken substrates in existing zirconia-substrate Cr-N thin-film strain gauge elements is solved. Furthermore, as can be seen from Table 3, the first and second substrates enable the deposition of Cr-N thin films with good yield and without breakage, almost as in the case of the reference substrate (zirconia substrate), and the non-uniformity (variation) of TCR and Gf is improved to an equal or greater extent. From a practical standpoint, the present invention brings about significant improvements, making it possible to provide strain gauges that are easier to handle than conventional ones, have fewer problems with characteristic variation and drift, and are highly sensitive and temperature-stable.

[0055] A heat treatment temperature exceeding 200°C is not preferable because the rate of wire breakage increases, the film formation yield deteriorates, and the non-uniformity of TCR increases, causing instability of the characteristics. There is no particular lower limit for the heat treatment temperature because the effect of temperature decreases, but 160°C, at which the characteristics do not change significantly, is also preferable, and 180°C is even more preferable. Furthermore, a thermal expansion coefficient exceeding 30 ppm / °C is also not preferable because the rate of wire breakage increases, the film formation yield deteriorates, and the non-uniformity of TCR increases, causing instability of the characteristics. Furthermore, a rigidity ratio of 200×10 3 If the stiffness ratio is below Pa·m, the breakage rate will rise sharply and the film formation yield will be extremely poor, which is not desirable. On the other hand, if the stiffness ratio is too large, there will be no problems with the breakage rate or uniformity of properties, but in the case of a resin film substrate with a small Young's modulus, the thickness will increase and the gauge factor will not be able to be measured correctly. In this invention, the maximum Young's modulus of 13 GPa and the maximum film thickness of 75 μm among the resin films tested were approximately 1000 × 10 3 Anything above Pa·m is considered undesirable.

[0056] Figures 23 and 24 show the temperature coefficient of resistance and gauge factor values ​​as a function of nitrogen content as thin-film properties of resin substrates. As with the zirconia substrate (reference example) and glass substrate shown in Figures 3 and 4, the temperature coefficient of resistance for resin substrates showed values ​​near zero across all nitrogen contents investigated, and the gauge factor tended to decrease as the nitrogen content increased. When the heat treatment temperature reduction method adopted in this invention is followed, Cr-N thin films exhibit a temperature coefficient of resistance (TCR) within ±400 ppm / °C and a gauge factor (Gf) of 3 to 20 at nitrogen contents of 2 to 8 at%. By using this method, conditions, and a substrate with the above-mentioned properties, it is possible to provide the desired strain gauge.

[0057] [Table 3] Table 2 shows the results of evaluation of the wire breakage rate of the strain gauge group of each fabrication example. The electrical resistance values ​​of multiple thin-film elements (those of 40 that were not broken) that made up the strain gauge group were measured using a tester, and the wire breakage rate was evaluated as the ratio of the number of thin-film elements whose electrical resistance value was unmeasurable to the total number of thin-film elements. Table 2 shows that the wire breakage rate of each of the strain gauge groups of Examples 1 to 5 was significantly lower than that of each of the strain gauge groups of Comparative Examples 1 to 4, and was at the same level as or lower than that of each of the strain gauge groups of Reference Example 1 and Reference Example 2.

[0058] Table 2 shows the temperature coefficient of resistance (TCR) (average value) and the evaluation results of its non-uniformity for the strain gauge groups of each fabrication example. Table 2 shows that the TCR of each of the strain gauge groups of Examples 1 to 5 falls within the range of -186.1 to 370.1 ppm / °C, which is common to the strain gauge groups of Comparative Example 2, Comparative Example 3, Reference Example 1, and Reference Example 2. On the other hand, the TCR non-uniformity of each of the strain gauge groups of Examples 1 to 5 falls within the range of 0.263 to 1.916, similar to the strain gauge groups of Reference Example 1 and Reference Example 2, and is lower than the strain gauge groups of Comparative Examples 2 and 3.

[0059] Table 2 shows the results of evaluation of the gauge factor Gf (average value) and its non-uniformity of the strain gauge group of each fabrication example. Table 2 shows that the Gf of each of the strain gauge groups of Examples 1 to 5 falls within the range of 16.6 to 19.0, which is comparable to that of each of the strain gauge groups of Comparative Examples 2 and 3. It also shows that the non-uniformity of Gf of each of the strain gauge groups of Examples 1 to 5 falls within the range of 0.076 to 0.121, which is lower than that of each of the strain gauge groups of Comparative Examples 2 and 3, and further lower than that of each of the strain gauge groups of Reference Examples 1 and 2.

[0060] Fig. 23 shows the relationship between the nitrogen content and the temperature coefficient of resistance (TCR) of the thin-film element 2 formed on a synthetic resin (first substrate) film as the substrate 1. Fig. 23 shows that the temperature coefficient of resistance of the thin-film element 2 exhibits a value close to zero when the nitrogen content is in the range of 2 to 10.5%.

[0061] Fig. 24 shows the relationship between the nitrogen content and gauge factor (Gf) of thin-film element 2 formed on a synthetic resin (first substrate) film as substrate 1. Fig. 24 shows that, in the nitrogen content range of 2 to 10.5%, the gauge factor of thin-film element 2 tends to decrease as the nitrogen content increases. [Explanation of symbols]

[0062] 1...substrate, 2...thin film element, 101...upper surface (main surface) of the substrate, 102...lower surface (main surface) of the substrate.

Claims

1. a substrate made of a resin having a stiffness ratio in the range of 227.5×10 3 Pa·m to 682.5×10 3 Pa·m and a thermal expansion coefficient in the range of 3 ppm / °C to 27 ppm / °C; a thin film element formed on the substrate; A strain gauge comprising: The thin film element is made of a Cr-N thin film having a nitrogen (N) content in the range of 2.09 at% to 4.20 at%, a temperature coefficient of resistance (TCR) of -186.1 ppm / °C to 370.1 ppm / °C, and a gauge factor of 16.6 to 19.

0. A strain gauge characterized by:

2. forming a thin-film element made of a Cr—N thin film disposed in a specified manner on a main surface of a substrate made of a resin having a stiffness ratio in the range of 227.5×10 3 Pa·m to 682.5×10 3 Pa·m and a thermal expansion coefficient in the range of 3 ppm / °C to 27 ppm / °C; heat-treating the thin-film element at a temperature in the range of 180 to 200°C to adjust the nitrogen (N) content of the Cr-N thin film to a range of 2.09 at% to 4.20 at%, the temperature coefficient of resistance (TCR) of the Cr-N thin film to a range of -186.1 ppm / °C to 370.1 ppm / °C, and the gauge factor of the Cr-N thin film to a range of 16.6 to 19.0; 2. The method of claim 1, further comprising:

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