Strain Gauges and Strain Sensors

Abstract

The strain gauge 10 according to an embodiment of the present invention comprises a first laminate LB1 having a strain resistance layer 13 that is provided to a resin-based substrate 30 and has a Cr-based body-centered cubic structure, a first layer 11 that is provided between the resin-based substrate 30 and the strain resistance layer 13, and a second layer 12 that is provided between the strain resistance layer 13 and the first layer 11 and serves as a base layer during formation of the strain resistance layer 13, the first layer 11 suppressing diffusion of oxygen from the resin-based substrate 30 to the strain resistance layer 13, and the second layer 12 inhibiting crystal growth of the strain resistance layer 13 due to the crystal structure of the first layer 11. By this configuration, high adhesion to the resin-based substrate is obtained while maintaining gauge factor.

Description

[Technical Field]

[0001] The present invention relates to a strain gauge and a strain sensor. [Background technology]

[0002] Patent Document 1 discloses a strain gauge having a flexible resin substrate, a functional layer formed from a metal, alloy, or metal compound directly on one surface of the substrate, and a resistor formed from a film containing Cr, CrN, and CrN directly on one surface of the functional layer, wherein the resistor is mainly composed of α-Cr, and the functional layer has the function of promoting crystal growth of the α-Cr and forming a film mainly composed of α-Cr.

[0003] In the invention disclosed in Patent Document 1, the functional layer is provided to stabilize the gauge characteristics of the strain gauge, and Patent Document 1 describes that it has the function of promoting crystal growth of the resistor, the function of suppressing oxidation of the resistor due to oxygen or moisture, and the function of improving adhesion between the substrate and the resistor. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6793103 DISCLOSURE OF THE INVENTION [Problem to be solved by the invention]

[0005] The "functional layer" disclosed in Patent Document 1 is preferable from the viewpoint of improving the adhesion of the strain-resistant layer to a resin-based substrate made of a polyimide resin or the like. However, the inventors' investigations have revealed that forming a strain-resistant layer on a functional layer as a base reduces the gauge factor of the strain-resistant layer (details will be described later).

[0006] In light of the above circumstances, the present invention aims to provide a strain gauge that can improve adhesion to a resin-based substrate while maintaining the gauge factor, and a strain sensor that can measure strain with high accuracy even when the strain-generating element is small, using the strain gauge. [Means for solving the problem]

[0007] In order to solve the above-mentioned problems, one embodiment of the present invention provides a strain gauge comprising a first laminate including: a strain-resistant layer provided on a resin-based substrate, the strain-resistant layer being Cr-based and having a body-centered cubic structure; a first layer provided between the resin-based substrate and the strain-resistant layer; and a second layer provided between the strain-resistant layer and the first layer, the second layer serving as a base layer when forming the strain-resistant layer, wherein the first layer inhibits oxygen diffusion from the resin-based substrate to the strain-resistant layer, and the second layer inhibits crystal growth in the strain-resistant layer due to the crystalline structure of the first layer.

[0008] The present inventors have investigated the reason why the "functional layer" disclosed in Patent Document 1 reduces the gauge factor of the strain resistance layer, and have come to the following findings.

[0009] The decrease in adhesion is caused by oxygen diffusing from the resin-based substrate to the strain-resistant layer. The "functional layer" disclosed in Patent Document 1 has a higher atomic density than the strain-resistant layer, which promotes the crystal growth of the body-centered cubic structure of the strain-resistant layer and also functions as an oxygen barrier layer.

[0010] However, as disclosed in Patent Document 1, when a strain-resistant layer is formed directly on a layer with a high atomic density, the crystallinity of the strain-resistant layer increases. When the crystallinity of the strain-resistant layer increases, the responsiveness of the resistance change to strain decreases, and the gauge factor decreases. Thus, when a strain-resistant layer is formed on a "functional layer" as a base, the crystal growth of the strain-resistant layer is promoted due to the crystalline structure of the "functional layer."

[0011] Based on the above findings, further investigation was conducted and a layer (first layer) with oxygen barrier properties was provided between the resin-based substrate and the strain-resistant layer. However, rather than using this first layer as a base layer when forming the strain-resistant layer, a layer (second layer) that inhibits the crystal growth of the body-centered cubic structure in the strain-resistant layer was used as the base layer, leading to the new finding that the decrease in the gauge factor of the strain gauge can be suppressed.

[0012] In the above strain gauge, it is preferable that the crystallinity of the strain resistance layer in the first laminate is lower than the crystallinity of the strain resistance layer formed using the first layer as a base layer, from the viewpoint of ensuring a sufficient gauge factor.

[0013] In the above strain gauge, it may be preferable that the film thickness of the second layer is 1.5 nm or more and 10 nm or less.

[0014] In the above strain gauge, the second layer may be made of a Cr-based material, and in this case, the second layer may contain a second element that is an element other than Cr.

[0015] When the second layer contains the second element, the second element may include a typical element, or may include one or more elements selected from the group consisting of elements of Groups 12 to 15. It may be preferable that the second element has a Pauling electronegativity higher than that of Cr, and may further preferably satisfy a value of 2.6 or less.

[0016] In the above strain gauge, it may be preferable that the second layer can be wet-etched with an etchant used to wet-etch the strain resistance layer.

[0017] In the above strain gauge, the first layer may contain a first element having a Pauling electronegativity greater than that of Cr. In this case, it may be preferable that the first layer be made of a metal or alloy based on the first element.

[0018] In the above strain gauge, the first layer preferably has a composition that forms a face-centered cubic lattice structure or a hexagonal close-packed structure.

[0019] In the above strain gauge, it may be preferable that the first layer can be wet-etched with an etchant for wet-etching the strain resistance layer.

[0020] In another aspect, the present invention provides a strain sensor including the above strain gauge and an electrode for applying current to the strain gauge. [Effects of the Invention]

[0021] According to the present invention, a strain gauge is provided that can improve adhesion to a resin-based substrate while maintaining the gauge factor, and further, a strain sensor is provided that can measure strain with high accuracy even if the strain generating element is small. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a diagram illustrating an example of a strain sensor according to an embodiment of the present invention. [Figure 2] FIG. 2 is a conceptual diagram showing a cross section taken along line AA' in FIG. [Figure 3] 1 is a cross-sectional view illustrating the structure of a strain gauge according to an embodiment of the present invention. [Figure 4] FIG. 10 is a diagram showing an X-ray diffraction spectrum of the strain gauge according to the example. [Figure 5] 10 is a graph showing the dependency of the gauge factor Gf of the strain gauge on the thickness of the second layer according to the example. BEST MODE FOR CARRYING OUT THE INVENTION

[0023] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In the following description, the same components are denoted by the same reference numerals, and the description of components that have already been described will be omitted as appropriate.

[0024] FIG. 1 is a diagram illustrating an example of a strain sensor according to an embodiment of the present invention. As shown in FIG. 1, the strain sensor 100 according to this embodiment includes a strain gauge 10 having a meandering pattern and an electrode 20 for passing current through the strain gauge 10. Both the strain gauge 10 and the electrode 20 are formed on a resin-based substrate 30. Non-limiting examples of the material for the electrode 20 include Cu, Au, and alloys containing these. The electrode 20 includes a first electrode 201 connected to one end of the strain gauge 10 and a second electrode 202 connected to the other end. Each of the first electrode 201 and the second electrode 202 is provided with a plating layer 21 for the purpose of increasing solder adhesion strength. The material for the resin-based substrate 30 is not limited as long as it has a predetermined flexibility. A specific example of the material for the resin-based substrate 30 is polyimide resin.

[0025] Fig. 2 is a conceptual diagram showing a cross section taken along line A-A' in Fig. 1. Fig. 3 is a cross-sectional view illustrating the structure of a strain gauge according to one embodiment of the present invention. As shown in Fig. 2, strain gauge 10 includes a first laminate LB1 having a strain-resistant layer 13, a first layer 11 provided between resin-based substrate 30 and the strain-resistant layer, and a second layer 12 provided between strain-resistant layer 13 and first layer 11. Fig. 3 is intended to explain the structure of first laminate LB1 in detail and corresponds to a partially enlarged view of Fig. 2.

[0026] The strain-resistant layer 13 of the first laminate LB1 is made of a Cr-based metallic material having a body-centered cubic structure, specifically a Cr-Ta alloy containing a few atomic percent of Ta. The strain-resistant layer 13 may be made of a material containing Cr as the main component and a non-metallic element, specifically a typical element (e.g., N).

[0027] The first layer 11 of the first laminate LB1 is made of a metallic material that has the function of suppressing oxygen diffusion from the resin-based substrate 30 to the strain-resistant layer 13. The first layer functions as an oxygen barrier layer, thereby preventing a decrease in adhesion of the strain-resistant layer 13 to the resin-based substrate 30. From the viewpoint of functioning as an oxygen barrier layer, the first layer 11 preferably has a higher atomic density than the strain-resistant layer 13, and more specifically, preferably has a composition that forms a face-centered cubic (fcc) structure or a hexagonal close-packed (hcp) structure.

[0028] In order to properly function as an oxygen barrier layer, the first layer 11 may contain a first element having a Pauling electronegativity greater than that of Cr (1.66). In this case, it may be preferable that the first layer be made of a metal or alloy based on the first element.

[0029] Specific examples of materials constituting such first layer 11 include Ni (electronegativity: 1.91) and Ni-based alloys (fcc structure) such as Ni-Cr alloys, Cu (electronegativity: 1.90) and Cu-Ni alloys (fcc structure), and Co-based alloys (hcp structure) such as Co (electronegativity: 1.88) and Co-Cr alloys.

[0030] It may be advantageous in terms of manufacturing if the first layer 11 can be wet-etched with an etching solution (cerium ammonium nitrate is a specific example) used to wet-etch the strain-resistant layer 13.

[0031] The second layer 12 of the first laminate LB1 serves as a base layer when the strain-resistant layer 13 is formed and functions to inhibit the crystal growth of the strain-resistant layer 13. If the strain-resistant layer 13 is formed directly on the first layer 11, which functions as an oxygen barrier layer, the high atomic density of the first layer 11 will result in high crystallinity of the strain-resistant layer 13. For example, in the case of a Cr-based strain-resistant layer 13, the (110) orientation of the body-centered cubic structure of the strain-resistant layer 13 will be excessively strong. As the crystallinity of the strain-resistant layer 13 increases, the responsiveness of the resistance change to strain in the strain-resistant layer 13 decreases, resulting in a decrease in the gauge factor Gf. The decrease in the gauge factor Gf is thought to be due to the fact that the magnetovolume effect (a phenomenon in which magnetic properties and volume change while influencing each other), which is the reason why Cr-based materials have a relatively large gauge factor Gf, becomes less effective as the atomic density increases.

[0032] Therefore, a layer (first layer 11) having an oxygen barrier function is provided between the resin-based substrate 30 and the strain-resistant layer 13. However, instead of using this first layer 11 as a base layer when forming the strain-resistant layer 13, a layer (second layer 12) that inhibits excessive crystal growth of the body-centered cubic structure in the strain-resistant layer 13 is used as the base layer, thereby appropriately adjusting the crystallinity of the strain-resistant layer 13 and suppressing a decrease in the gauge factor Gf of the strain gauge 10. In other words, the second layer 12 is a layer that inhibits crystal growth of the strain-resistant layer that is caused by the crystal structure of the first layer.

[0033] From the viewpoint of stably suppressing the decrease in the gauge factor Gf, it may be preferable that the film thickness of the second layer 12 be 1.5 nm or more and 10 nm or less.

[0034] The second layer 12 may be made of a Cr-based material. In this case, the second layer 12 may contain a second element other than Cr. When the second layer 12 contains the second element, the second element may contain a typical element or one or more elements selected from the group consisting of Groups 12 to 15. It may be preferable that the Pauling electronegativity of the second element is higher than that of Cr (1.66). For manufacturing reasons, it may be preferable that the Pauling electronegativity of the second element is 2.6 or less. Also, for manufacturing reasons, it is preferable that the second layer 12 can be wet-etched with an etchant used to wet-etch the strain-resistant layer 13.

[0035] A specific example of a material constituting such second layer 12 is CrB containing B (electronegativity: 2.04) as a second element, and from the viewpoint of maintaining metallic properties, the B content may preferably be 40 atomic % or less, and more preferably 30 atomic % or less.

[0036] The above-described embodiments have been described to facilitate understanding of the present invention, and are not intended to limit the present invention. Therefore, each element disclosed in the above embodiments is intended to include all design modifications and equivalents that fall within the technical scope of the present invention.

[0037] The present invention will be specifically described below using examples. [Example]

[0038] A strain gauge was fabricated with the structure shown in Table 1. In Table 1, "NiCr" means a Ni-Cr alloy with a Cr content of 25 atomic %, "CuNi" means a Cu-Ni alloy with a Ni content of 45 atomic %, and "Co" means Co metal.

[0039] [Table 1]

[0040] Specifically, the strain gauge according to Example 1 has a structure in which the strain resistance layer 13 is provided directly on the resin-based substrate 30. The strain gauge according to Example 2 has a structure in which the strain resistance layer 13 is provided on the first layer 11 provided on the resin-based substrate 30 as a base.

[0041] In Examples 2 to 8, a first layer 11 made of the material shown in Table 1 was formed on a resin-based substrate 30 made of a polyimide film to a thickness shown in Table 1. A second layer 12 made of CrB with a B content of 30 atomic % and having a thickness shown in Table 1 was formed on the first layer 11 provided on the resin-based substrate 30. A strain-resistant layer 13 made of CrTa with a Ta content of 2 atomic % was formed on this second layer 12 as a base, thereby obtaining a strain gauge 10 in which a first laminate LB1 made of the first layer 11, the second layer 12, and the strain-resistant layer 13 was formed on the resin-based substrate 30.

[0042] X-ray diffraction measurement was performed on the obtained strain gauge 10. Because the first layer 11 and the second layer 12 were thin, peaks due to these layers were not observed in the spectrum obtained by X-ray diffraction measurement, and only the peak due to the strain resistance layer 13 was observed.

[0043] 4 is a graph showing the X-ray diffraction spectrum of the strain gauge according to the examples. As shown in FIG. 4, in Example 1, in which the strain-resistant layer 13 was directly provided on the resin-based substrate 30, a peak of (110) orientation in the body-centered cubic structure of the strain-resistant layer 13 was weakly detected in Example 4, in which the strain-resistant layer 13 was provided on the second layer 12 as a base, confirming that the strain-resistant layer 13 of Example 4 has lower crystallinity than the strain-resistant layer 13 of Example 1. On the other hand, in Example 2, in which the strain-resistant layer 13 was provided on the first layer 11 as a base, a peak of (110) orientation in the body-centered cubic structure of the strain-resistant layer 13 was strongly detected in Example 1, confirming that the strain-resistant layer 13 of Example 2 has higher crystallinity than the strain-resistant layer 13 of Example 2 and the strain-resistant layer 13 of Example 1.

[0044] A strain sensor 100 was fabricated from the resulting strain gauge, and the gauge factor Gf was measured. Furthermore, a high-temperature, high-humidity environmental test (100 hours) was conducted at 85°C and a relative humidity of 85%. After the test, the strain gauge was observed to evaluate whether the first laminate LB1 had peeled off from the resin-based substrate 30 (film peeling test). The results of the gauge factor Gf measurement and the film peeling test are shown in Table 1 and FIG. 5. The dotted line in FIG. 5 indicates, for comparison, the gauge factor Gf (11.7) of Example 1, in which the strain resistance layer 13 was provided directly on the resin-based substrate 30.

[0045] 5, the gauge factor Gf was higher when the second layer 12 was provided than when the second layer 12 was not provided. In particular, when the film thickness of the gauge factor Gf was in the range of 1.5 nm or more and 10 nm or less, the results were equivalent to those when the strain resistance layer 13 was provided directly on the resin-based substrate 30 (Example 1). [Explanation of symbols]

[0046] 100: Strain sensor 10: Strain gauge 11: 1st layer 12: 2nd layer 13: Strain resistance layer 20: Electrode 21: Plating layer 30: Resin base material 201: 1st electrode 202:Second electrode Gf: Gauge factor LB1: First laminate

Claims

1. a strain-resistant layer provided on a resin-based substrate and having a Cr-based body-centered cubic structure; a first layer provided between the resin-based substrate and the strain-resistant layer; a second layer provided between the strain-resistant layer and the first layer, the second layer serving as a base layer when the strain-resistant layer is formed; A strain gauge comprising a first laminate having the first layer inhibits oxygen from diffusing from the resin-based substrate to the strain-resistant layer; The second layer inhibits crystal growth of the strain-resistant layer due to the crystal structure of the first layer. A strain gauge characterized by:

2. The strain gauge according to claim 1 , wherein the crystallinity of the strain resistance layer in the first laminate is lower than the crystallinity of the strain resistance layer when formed using the first layer as a base layer.

3. 2. The strain gauge according to claim 1, wherein the second layer has a thickness of 1.5 nm to 10 nm.

4. The strain gauge according to claim 1 , wherein the second layer is made of a Cr-based material and contains a second element that is an element other than Cr.

5. The strain gauge according to claim 4 , wherein the second element includes a typical element.

6. 5. The strain gauge according to claim 4, wherein the second element includes one or more elements selected from the group consisting of Group 12 elements to Group 15 elements.

7. The strain gauge according to claim 5 , wherein the second element has a Pauling electronegativity higher than that of Cr.

8. The strain gauge according to claim 7 , wherein the second element has a Pauling electronegativity of 2.6 or less.

9. 2. The strain gauge according to claim 1, wherein the second layer can be wet-etched with an etchant for wet-etching the strain resistance layer.

10. The strain gage of claim 1 , wherein the first layer includes a first element having a Pauling electronegativity greater than that of Cr.

11. The strain gauge according to claim 10 , wherein the first layer is made of a metal or alloy based on the first element.

12. The strain gauge of claim 1 , wherein the first layer has a composition that forms a face-centered cubic lattice structure or a hexagonal close-packed structure.

13. 2. The strain gauge according to claim 1, wherein the first layer can be wet-etched with an etchant for wet-etching the strain resistance layer.

14. A strain sensor comprising: the strain gauge according to any one of claims 1 to 13; and an electrode for applying current to the strain gauge.

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