strain gauge

Abstract

To provide a strain gage having improved strain resistance.SOLUTION: This strain gauge comprises a flexible substrate, a resistor formed on the substrate, and a pair of electrodes formed on the substrate and electrically connected to the resistor via wiring. The wiring includes a first metal layer, and a second metal layer formed on the top of the first metal layer. The second metal layer is formed in a different pattern from the first metal layer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] There is known a strain gauge that is attached to an object to be measured to detect strain of the object. The strain gauge includes a resistor that detects strain, and the resistor is formed on, for example, an insulating resin. The resistor is connected to an electrode via, for example, a wiring (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-74934 Summary of the Invention [Problem to be solved by the invention]

[0004] Strain gauges are attached to a strain-generating body and detect the amount of strain in the body by expanding and contracting in response to the body's movements. Therefore, in order to detect larger amounts of strain, the strain gauge itself must not be damaged during the expansion and contraction process, and higher strain resistance is required.

[0005] The present invention has been made in view of the above points, and has an object to provide a strain gauge that can improve strain resistance. [Means for solving the problem]

[0006] This strain gauge has a flexible substrate, a resistor formed on the substrate, and a pair of electrodes formed on the substrate and electrically connected to the resistor via wiring, the wiring including a first metal layer and a second metal layer formed on an upper surface of the first metal layer, the second metal layer being formed in a pattern different from that of the first metal layer. In a plan view, the second metal layer has a pattern including a portion extending obliquely with respect to the extending direction of the first metal layer, a wavy pattern, and / or a discrete pattern. . [Effects of the Invention]

[0007] According to the disclosed technology, it is possible to provide a strain gauge that can improve strain resistance. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 2 is a plan view illustrating the strain gauge according to the first embodiment. [Figure 2] 1 is a cross-sectional view (part 1) illustrating a strain gauge according to a first embodiment. FIG. [Figure 3] FIG. 2 is a cross-sectional view (part 2) illustrating the strain gauge according to the first embodiment. [Figure 4] FIG. 10 is a diagram showing experimental results of strain limits. [Figure 5] FIG. 4 is a cross-sectional view (part 3) illustrating the strain gauge according to the first embodiment. [Figure 6] FIG. 2 is a plan view illustrating a strain gauge according to a first modified example of the first embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following describes the preferred embodiments of the present invention with reference to the accompanying drawings. In the drawings, the same components are designated by the same reference numerals, and redundant explanations may be omitted.

[0010] First Embodiment Fig. 1 is a plan view illustrating a strain gauge according to a first embodiment. Fig. 2 is a cross-sectional view illustrating the strain gauge according to the first embodiment, taken along line AA in Fig. 1. Fig. 3 is a cross-sectional view illustrating the strain gauge according to the first embodiment, taken along line BB in Fig. 1. Referring to Figs. 1 to 3, the strain gauge 1 has a substrate 10, a resistor 30, wiring 40, and electrodes 50.

[0011] In this embodiment, for convenience, the side of the strain gauge 1 on which the resistor 30 of the substrate 10 is provided is referred to as the upper side or one side, and the side on which the resistor 30 is not provided is referred to as the lower side or the other side. Furthermore, the surface on which the resistor 30 of each portion is provided is referred to as the one side or upper side, and the surface on which the resistor 30 is not provided is referred to as the other side or lower side. However, the strain gauge 1 can be used upside down or positioned at any angle. Furthermore, a planar view refers to viewing an object from the normal direction of the upper surface 10a of the substrate 10, and a planar shape refers to the shape of the object viewed from the normal direction of the upper surface 10a of the substrate 10.

[0012] The substrate 10 is a flexible member that serves as a base layer for forming the resistor 30 and the like. The thickness of the substrate 10 is not particularly limited and can be appropriately selected depending on the purpose, but can be, for example, about 5 μm to 500 μm. In particular, a thickness of 5 μm to 200 μm is preferable in terms of the transferability of strain from the surface of the strain generator bonded to the lower surface of the substrate 10 via an adhesive layer or the like and dimensional stability against the environment, and a thickness of 10 μm or more is even more preferable in terms of insulation properties.

[0013] The substrate 10 can be formed from an insulating resin film such as PI (polyimide) resin, epoxy resin, PEEK (polyether ether ketone) resin, PEN (polyethylene naphthalate) resin, PET (polyethylene terephthalate) resin, PPS (polyphenylene sulfide) resin, polyolefin resin, etc. The film refers to a flexible member having a thickness of about 500 μm or less.

[0014] Here, "formed from an insulating resin film" does not prevent the base material 10 from containing fillers, impurities, etc. in the insulating resin film. The base material 10 may be formed from an insulating resin film containing fillers such as silica or alumina, for example.

[0015] Materials other than resin for the substrate 10 include, for example, crystalline materials such as SiO2, ZrO2 (including YSZ), Si, Si2N3, Al2O3 (including sapphire), ZnO, perovskite ceramics (CaTiO3, BaTiO3), and amorphous glass. Alternatively, metals such as aluminum, aluminum alloys (duralumin), and titanium may be used as the material for the substrate 10. In this case, an insulating film, for example, is formed on the metal substrate 10.

[0016] The resistor 30 is a thin film formed in a predetermined pattern on the substrate 10, and is a sensing element that generates a resistance change when strain is applied. The resistor 30 may be formed directly on the upper surface 10a of the substrate 10, or may be formed on the upper surface 10a of the substrate 10 via another layer. For convenience, the resistor 30 is shown in FIG. 1 with a dark matte pattern.

[0017] The resistor 30 has a structure in which multiple elongated portions are arranged at predetermined intervals with their longitudinal directions in the same direction (the direction of line AA in Figure 1), and the ends of adjacent elongated portions are alternately connected, resulting in a zigzag folded structure as a whole. The longitudinal direction of the multiple elongated portions is the grid direction, and the direction perpendicular to the grid direction is the grid width direction (the direction of line BB in Figure 1).

[0018] One longitudinal end of each of the two elongated portions located outermost in the grid width direction is bent in the grid width direction to form terminal ends 30e1 and 30e2 of the resistor 30 in the grid width direction. Each of the terminal ends 30e1 and 30e2 of the resistor 30 in the grid width direction is electrically connected to an electrode 50 via a wiring 40. In other words, the wiring 40 electrically connects each of the terminal ends 30e1 and 30e2 of the resistor 30 in the grid width direction to each of the electrodes 50.

[0019] The resistor 30 can be formed from, for example, a material containing Cr (chromium), a material containing Ni (nickel), or a material containing both Cr and Ni. That is, the resistor 30 can be formed from a material containing at least one of Cr and Ni. An example of a material containing Cr is a Cr mixed phase film. An example of a material containing Ni is Cu-Ni (copper-nickel). An example of a material containing both Cr and Ni is Ni-Cr (nickel-chromium).

[0020] Here, the Cr mixed phase film is a film containing a mixture of Cr, CrN, Cr2N, etc. The Cr mixed phase film may contain inevitable impurities such as chromium oxide.

[0021] The thickness of the resistor 30 is not particularly limited and can be appropriately selected depending on the purpose, but can be, for example, approximately 0.05 μm to 2 μm. In particular, a thickness of 0.1 μm or more is preferable because it improves the crystallinity of the crystals constituting the resistor 30 (e.g., the crystallinity of α-Cr). Furthermore, a thickness of 1 μm or less is even more preferable because it reduces film cracks and warpage from the substrate 10 caused by internal stress in the film constituting the resistor 30. The width of the resistor 30 can be optimized for required specifications such as resistance value and lateral sensitivity, and can be set to, for example, approximately 10 μm to 100 μm, taking into consideration measures against disconnection.

[0022] For example, when the resistor 30 is a Cr mixed-phase film, the stability of the gauge characteristics can be improved by using α-Cr (alpha chromium), which has a stable crystalline phase, as the main component. Furthermore, by using α-Cr as the main component of the resistor 30, the gauge factor of the strain gauge 1 can be 10 or more, and the temperature coefficient of gauge factor (TCS) and temperature coefficient of resistance (TCR) can be within the range of −1000 ppm / °C to +1000 ppm / °C. Here, “main component” means that the target substance accounts for 50% by weight or more of all materials constituting the resistor. From the viewpoint of improving the gauge characteristics, the resistor 30 preferably contains 80% by weight or more, and more preferably 90% by weight or more, of α-Cr. Note that α-Cr is Cr with a bcc structure (body-centered cubic lattice structure).

[0023] Furthermore, when the resistor 30 is a Cr mixed phase film, the Cr mixed phase film preferably contains 20 wt % or less of CrN and Cr2N, which can suppress a decrease in the gauge factor.

[0024] Furthermore, the ratio of Cr2N in CrN and Cr2N is preferably 80% by weight or more and less than 90% by weight, and more preferably 90% by weight or more and less than 95% by weight. When the ratio of Cr2N in CrN and Cr2N is 90% by weight or more and less than 95% by weight, the Cr2N has semiconducting properties, which leads to a more significant decrease in TCR (negative TCR). Furthermore, by reducing the amount of ceramic formation, brittle fracture is reduced.

[0025] On the other hand, if trace amounts of N2 or atomic N are mixed into or present in the film, they will escape to the outside of the film due to external conditions (such as high temperature environments), causing changes in film stress.By creating chemically stable CrN, the unstable N mentioned above will not be generated, and a stable strain gauge can be obtained.

[0026] The wiring 40 is formed on the substrate 10. The wiring 40 includes a first metal layer 41 and a second metal layer 42 formed on the upper surface of the first metal layer 41. For convenience, the second metal layer 42 and the electrode 50 are shown in FIG. 1 with a matte finish that is thinner than the resistor 30 and the first metal layer 41.

[0027] According to the inventors' investigations, it was found that the narrower the width of the first metal layer 41 of the wiring 40, the less likely cracks or breaks will occur when the wiring 40 is subjected to strain. Specifically, the inventors fabricated four types of test strain gauges with first metal layer 41 widths of 10 μm, 100 μm, 345 μm, and 560 μm, and applied strain to each to investigate the occurrence of cracks or breaks. In this experiment, the first metal layer 41 was a 0.2 μm thick Cr mixed phase film, and no second metal layer 42 was laminated on the first metal layer 41.

[0028] As a result of the experiment, it was confirmed that cracks and breaks tend to decrease as the width of the first metal layer 41 becomes narrower, and it was found that the strain limit depends on the width of the first metal layer 41. The strain limit is the value of mechanical strain at which cracks or breaks begin to occur when strain is applied to the strain gauge.

[0029] FIG. 4 shows the results of strain limit experiments, plotting the minimum strain limit values ​​for multiple test strain gauges. As shown in FIG. 4 , the inventors' experimental results showed that the strain limit was 5500 με or more when the width of the first metal layer 41 was 560 μm, while the strain limit was 8500 με or more when the width of the first metal layer 41 was 10 μm. In other words, when the width of the first metal layer 41 was 10 μm, the strain limit was approximately 1.5 times larger than when the width of the first metal layer 41 was 560 μm. Furthermore, when the width of the first metal layer 41 was between 560 μm and 10 μm, the strain limit improved almost linearly.

[0030] This result is thought to be due to the fact that brittle fracture is more likely to occur when the width of the first metal layer 41, which is made of a Cr mixed phase film with a high elastic modulus, is wide, and that narrowing the width of the first metal layer 41 increases the apparent fracture resistance. In actual use of the strain gauge 1, a strain limit of approximately 8000 με is required, so the width of the first metal layer 41 is preferably 100 μm or less. However, due to manufacturing process considerations, it is difficult to make the width of the first metal layer 41 less than 10 μm. Considering this point, it can be said that the width of the first metal layer 41 is preferably 10 μm or more and 100 μm or less.

[0031] Further, according to another study by the inventors, it was found that when a 3 μm thick copper layer is formed solidly on the first metal layer 41 as the second metal layer 42, the strain limit is improved by a further 1.5 to 2 times compared to when the second metal layer is not formed, regardless of the width of the first metal layer 41. It is believed that the strain limit is further improved by laminating a copper layer on the Cr mixed phase film, because the copper layer has better elasticity than the Cr mixed phase film.

[0032] That is, from the viewpoint of further improving the strain limit, it is preferable that the second metal layer 42 be formed from a material with greater elasticity than the first metal layer 41. In addition to copper, examples of materials with greater elasticity than the Cr mixed phase film include gold, silver, and aluminum. It is believed that similar results can be obtained when these materials are used for the second metal layer 42.

[0033] Further investigation by the inventors has revealed that, in the wiring 40, the second metal layer 42 is preferably not formed solidly so as to cover the entire upper surface of the first metal layer 41, but is preferably formed in selected regions of the upper surface of the first metal layer 41. This will be described in detail below.

[0034] After extensive investigation, the inventors discovered that when strain gauge 1 is subjected to strain, stress tends to concentrate at the interface between first metal layer 41 and second metal layer 42, and that this stress concentration is likely to cause breakage of wiring 40 and, consequently, of resistor 30. Breakage of resistor 30 leads to a decrease in strain limit.

[0035] Generally, in the wiring 40, the second metal layer 42 is formed in a solid state so as to cover the entire upper surface of the first metal layer 41. However, in this structure, the first metal layer 41 and the second metal layer 42 have the same pattern, and therefore the direction of the stress generated at the interface between the first metal layer 41 and the second metal layer 42 is uniformly determined by the pattern of the wiring 40. This results in strict design restrictions, such as the need to position the resistor 30 so as to avoid the influence of the stress generated at the interface between the first metal layer 41 and the second metal layer 42.

[0036] 1, in the present application, the second metal layer 42 in the wiring 40 is not formed solidly so as to cover the entire upper surface of the first metal layer 41, but is formed in selected regions on the upper surface of the first metal layer 41. In other words, the second metal layer 42 is formed in a pattern different from that of the first metal layer 41.

[0037] 1, as an example, the first metal layer 41 has a linear pattern, and the second metal layer 42 has a wavy pattern. However, the first metal layer 41 is not limited to a linear pattern and can have any pattern. The first metal layer 41 can have any length. The first metal layer 41 may have a shape that is narrowest on the resistor 30 side and gradually becomes wider as it approaches the electrode 50.

[0038] In the structure of the present application, the second metal layer 42 is formed in a pattern different from that of the first metal layer 41, and therefore, in a plan view, a portion of the upper surface of the first metal layer 41 is exposed from the second metal layer 42. In addition, in the structure of the present application, the second metal layer 42 includes a portion that extends obliquely with respect to the extension direction of the first metal layer 41 in a plan view. For example, the first metal layer 41 extends in the direction of line AA in FIG. 1, but the second metal layer 42 includes a portion that extends obliquely with respect to the direction of line AA in FIG. 1. The pattern shape of the second metal layer 42 is neither identical nor similar to the pattern shape of the first metal layer 41.

[0039] In this way, in wiring 40 in which a second metal layer 42 is stacked on a first metal layer 41, by forming the second metal layer 42 in a pattern different from that of the first metal layer 41, it is possible to control the direction of stress generated at the interface between the first metal layer 41 and the second metal layer 42.

[0040] That is, when the first metal layer 41 and the second metal layer 42 have the same pattern, the direction of the stress generated at the interface between the first metal layer 41 and the second metal layer 42 is uniformly determined by the pattern of the wiring 40 (=first metal layer 41). However, in the structure of the present application, by devising the pattern of the second metal layer 42, it is possible to generate stress in a direction different from the extension direction of the pattern of the first metal layer 41.

[0041] 1, if the first metal layer 41 and the second metal layer 42 were to have the same pattern, stress would occur in the direction of the line AA along which the wiring 40 extends, and the portion of the resistor 30 that extends parallel to the line AA would be more susceptible to the effects of this stress. In this case, it would be necessary to change the position of the resistor 30 or the routing of the wiring 40 so that the resistor 30 is less susceptible to the effects of the stress, which reduces the degree of freedom in design.

[0042] In contrast, the second metal layer 42 includes a portion that extends obliquely relative to the extension direction of the first metal layer 41, so that stress is generated in a direction oblique to the AA line. This reduces the effect of stress on the portion of the resistor 30 that extends parallel to the AA line, and improves the strain limit while maintaining design freedom.

[0043] In other words, by controlling the direction of the stress generated at the interface between the first metal layer 41 and the second metal layer 42 using the pattern of the second metal layer 42 so that the effect of the stress on the resistor 30 is less likely to be felt, it is possible to improve the strain limit while maintaining design freedom.

[0044] That is, the strain gauge 1 is attached to a strain-generating body and detects the strain of the body by expanding and contracting in accordance with the body's movement. Therefore, in order to detect a larger amount of strain, the strain gauge 1 itself must not be damaged (such as by a break) during the expansion and contraction process, and higher strain resistance is required. In the strain gauge 1, by forming the second metal layer 42 in a pattern different from that of the first metal layer 41, it is possible to improve the strain limit (high strain resistance) while maintaining design freedom.

[0045] The second metal layer 42 may be formed in a wave pattern closer to a sine wave, or may be formed in a wave pattern closer to a triangular wave or a sawtooth wave. The wave period may be adjusted as needed. The width of the second metal layer 42 may be adjusted as needed. The width of the second metal layer 42 does not have to be constant.

[0046] The electrodes 50 are formed on the substrate 10 and electrically connected to the resistor 30 via the wiring 40, and are formed, for example, in a substantially rectangular shape wider than the wiring 40. The electrodes 50 are a pair of electrodes for outputting to the outside a change in the resistance value of the resistor 30 caused by strain, and are connected, for example, to lead wires for external connection.

[0047] The electrode 50 has a pair of first metal layers 51 and a second metal layer 52 laminated on the upper surface of each of the first metal layers 51. The first metal layers 51 are electrically connected to the terminations 30e1 and 30e2 of the resistor 30 via the first metal layers 41 of the wiring 40. The first metal layers 51 are formed in a substantially rectangular shape in a plan view. The first metal layers 51 may be formed to have the same width as the first metal layers 41 of the wiring 40.

[0048] Although the resistor 30, the first metal layer 41, and the first metal layer 51 are given different reference numerals for convenience, they can be integrally formed in the same process using the same material. Therefore, the resistor 30, the first metal layer 41, and the first metal layer 51 have approximately the same thickness. Furthermore, although the second metal layer 42 and the second metal layer 52 are given different reference numerals for convenience, they can be integrally formed in the same process using the same material. Therefore, the second metal layer 42 and the second metal layer 52 have approximately the same thickness.

[0049] The second metal layers 42 and 52 are preferably formed of a material with a lower resistance than the resistor 30 (the first metal layers 41 and 51). For example, if the resistor 30 is a Cr mixed-phase film, materials for the second metal layers 42 and 52 with a lower resistance than the Cr mixed-phase film include Cu, Ni, Al, Ag, Au, Pt, etc., alloys of any of these metals, compounds of any of these metals, and laminated films in which any of these metals, alloys, and compounds are appropriately laminated. Among these, Cu, Au, and Ag are preferred as the material for the second metal layer 42 because of their excellent elasticity, as described above. The thickness of the second metal layers 42 and 52 is not particularly limited and can be appropriately selected depending on the purpose, but can be, for example, approximately 3 μm to 5 μm.

[0050] The second metal layer 52 may be formed on a portion of the upper surface of the first metal layer 51, or may be formed on the entire upper surface of the first metal layer 51. One or more other metal layers may be laminated on the upper surface of the second metal layer 52. For example, the second metal layer 52 may be a copper layer, with a gold layer laminated on the copper layer. Alternatively, the second metal layer 52 may be a copper layer, with a palladium layer and a gold layer laminated in this order on the copper layer. By using a gold layer as the top layer of the electrode 50, the solder wettability of the electrode 50 can be improved.

[0051] The wiring 40 has a structure in which a second metal layer 42 is stacked on a first metal layer 41 made of the same material as the resistor 30. Therefore, the resistance of the wiring 40 is lower than that of the resistor 30, and the wiring 40 can be prevented from functioning as a resistor. As a result, the accuracy of strain detection by the resistor 30 can be improved.

[0052] In other words, by providing the wiring 40 with a lower resistance than the resistor 30, the actual sensitive part of the strain gauge 1 can be limited to the local area where the resistor 30 is formed, thereby improving the accuracy of strain detection by the resistor 30.

[0053] In particular, in a highly sensitive strain gauge with a gauge factor of 10 or more that uses a Cr mixed-phase film as the resistor 30, making the wiring 40 lower in resistance than the resistor 30 and limiting the actual sensitive part to the local region where the resistor 30 is formed has a significant effect on improving strain detection accuracy. Also, making the wiring 40 lower in resistance than the resistor 30 has the effect of reducing lateral sensitivity.

[0054] A cover layer 60 (insulating resin layer) may be provided on the upper surface 10a of the substrate 10 so as to cover the resistor 30 and the wiring 40 and expose the electrodes 50. By providing the cover layer 60, it is possible to prevent mechanical damage, etc. from occurring to the resistor 30 and the wiring 40. Furthermore, by providing the cover layer 60, it is possible to protect the resistor 30 and the wiring 40 from moisture, etc. Note that the cover layer 60 may be provided so as to cover the entire portion except for the electrodes 50.

[0055] The cover layer 60 can be formed from an insulating resin such as PI resin, epoxy resin, PEEK resin, PEN resin, PET resin, PPS resin, or composite resin (e.g., silicone resin or polyolefin resin). The cover layer 60 may contain a filler or a pigment. There are no particular restrictions on the thickness of the cover layer 60 and it can be appropriately selected depending on the purpose, but it can be, for example, about 2 μm to 30 μm.

[0056] To manufacture the strain gauge 1, first, a substrate 10 is prepared, and a metal layer (for convenience, referred to as metal layer A) is formed on the upper surface 10a of the substrate 10. Metal layer A is a layer that will ultimately be patterned to become the resistor 30, first metal layer 41, and first metal layer 51. Therefore, the material and thickness of metal layer A are the same as those of the resistor 30, first metal layer 41, and first metal layer 51 described above.

[0057] The metal layer A can be formed by, for example, magnetron sputtering using a target made of a raw material capable of forming the metal layer A. Instead of magnetron sputtering, the metal layer A may also be formed by reactive sputtering, vapor deposition, arc ion plating, pulsed laser deposition, or the like.

[0058] From the viewpoint of stabilizing the gauge characteristics, it is preferable to vacuum-deposit a functional layer of a predetermined thickness as a base layer on the upper surface 10a of the substrate 10 by, for example, conventional sputtering before depositing the metal layer A.

[0059] In the present application, the functional layer refers to a layer having the function of promoting the crystal growth of at least the upper layer, metal layer A (resistor 30). The functional layer preferably also has the function of preventing oxidation of metal layer A due to oxygen and moisture contained in the substrate 10, and the function of improving adhesion between the substrate 10 and metal layer A. The functional layer may also have other functions.

[0060] The insulating resin film that constitutes the substrate 10 contains oxygen and moisture, and since Cr forms a self-oxidized film, it is effective for the functional layer to have the function of preventing oxidation of the metal layer A, especially when the metal layer A contains Cr.

[0061] The material of the functional layer is not particularly limited as long as it has the function of promoting the crystal growth of at least the upper layer, the metal layer A (resistor 30), and can be appropriately selected depending on the purpose. For example, Cr (chromium), Ti (titanium), V (vanadium), Nb (niobium), Ta (tantalum), Ni (nickel), Y (yttrium), Zr (zirconium), Hf (hafnium), Si (silicon), C (carbon), Zn (zinc), Cu (copper), Bi (bismuth), Examples of the metals include one or more metals selected from the group consisting of copper (Ru), Fe (iron), Mo (molybdenum), W (tungsten), Ru (ruthenium), Rh (rhodium), Re (rhenium), Os (osmium), Ir (iridium), Pt (platinum), Pd (palladium), Ag (silver), Au (gold), Co (cobalt), Mn (manganese), and Al (aluminum), an alloy of any of the metals in this group, and a compound of any of the metals in this group.

[0062] Examples of the alloys include FeCr, TiAl, FeNi, NiCr, CrCu, etc. Examples of the compounds include TiN, TaN, Si3N4, TiO2, Ta2O5, SiO2, etc.

[0063] When the functional layer is made of a conductive material such as a metal or alloy, the thickness of the functional layer is preferably 1 / 20 or less of the thickness of the resistor, which can promote the crystal growth of α-Cr and prevent a portion of the current flowing through the resistor from flowing through the functional layer, thereby preventing a decrease in strain detection sensitivity.

[0064] When the functional layer is made of a conductive material such as a metal or alloy, the thickness of the functional layer is preferably 1 / 50 or less of the thickness of the resistor, which can promote the crystal growth of α-Cr and further prevent a portion of the current flowing through the resistor from flowing through the functional layer, thereby preventing a decrease in strain detection sensitivity.

[0065] When the functional layer is made of a conductive material such as a metal or alloy, the thickness of the functional layer is preferably 1 / 100 or less of the thickness of the resistor, which further prevents a portion of the current flowing through the resistor from flowing through the functional layer, thereby further preventing a decrease in strain detection sensitivity.

[0066] When the functional layer is made of an insulating material such as an oxide or nitride, the thickness of the functional layer is preferably 1 nm to 1 μm, which promotes the crystal growth of α-Cr and allows the functional layer to be easily formed without cracks.

[0067] When the functional layer is made of an insulating material such as an oxide or nitride, the thickness of the functional layer is preferably 1 nm to 0.8 μm, which not only promotes the crystal growth of α-Cr but also makes it easier to form the functional layer without cracking.

[0068] When the functional layer is made of an insulating material such as an oxide or nitride, the thickness of the functional layer is preferably 1 nm to 0.5 μm, which not only promotes the crystal growth of α-Cr but also makes it easier to form the functional layer without cracking.

[0069] The planar shape of the functional layer is patterned to be substantially the same as the planar shape of the resistor shown in FIG. 1, for example. However, the planar shape of the functional layer is not limited to being substantially the same as the planar shape of the resistor. If the functional layer is made of an insulating material, it does not need to be patterned to be the same as the planar shape of the resistor. In this case, the functional layer may be formed in a solid state at least in the area where the resistor is formed. Alternatively, the functional layer may be formed in a solid state over the entire upper surface of the substrate 10.

[0070] Furthermore, when the functional layer is made of an insulating material, the thickness of the functional layer is made relatively thick, between 50 nm and 1 μm, and the functional layer is made solid. This increases the thickness and surface area of ​​the functional layer, allowing heat generated by the resistor to be dissipated to the substrate 10. As a result, the deterioration of measurement accuracy in the strain gauge 1 due to self-heating of the resistor can be suppressed.

[0071] The functional layer can be formed in vacuum by conventional sputtering, for example, using a target made of a material capable of forming the functional layer and introducing Ar (argon) gas into a chamber. By using conventional sputtering, the functional layer is formed while etching the upper surface 10a of the substrate 10 with Ar, thereby minimizing the amount of the functional layer formed and achieving an improvement in adhesion.

[0072] However, this is just one example of a method for forming the functional layer, and the functional layer may be formed by other methods. For example, a method may be used in which the upper surface 10a of the substrate 10 is activated by plasma treatment using Ar or the like before forming the functional layer, thereby improving adhesion, and then the functional layer is vacuum-formed by magnetron sputtering.

[0073] There are no particular restrictions on the combination of the material of the functional layer and the material of the metal layer A, and it can be selected appropriately depending on the purpose. For example, it is possible to use Ti as the functional layer and form a Cr mixed phase film with α-Cr (alpha chromium) as the main component as the metal layer A.

[0074] In this case, for example, the metal layer A can be formed by magnetron sputtering using a target made of a material capable of forming a Cr mixed-phase film and introducing Ar gas into the chamber. Alternatively, the metal layer A can be formed by reactive sputtering using pure Cr as the target and introducing an appropriate amount of nitrogen gas into the chamber together with Ar gas. In this case, the ratios of CrN and CrN contained in the Cr mixed-phase film, and the ratio of CrN in CrN and CrN can be adjusted by changing the amount and pressure (nitrogen partial pressure) of the nitrogen gas introduced or by adjusting the heating temperature in a heating step.

[0075] In these methods, the Ti functional layer defines the growth plane of the Cr mixed-phase film, allowing the formation of a Cr mixed-phase film primarily composed of α-Cr, which has a stable crystal structure. Furthermore, the Ti constituting the functional layer diffuses into the Cr mixed-phase film, improving the gauge characteristics. For example, the gauge factor of the strain gauge 1 can be set to 10 or more, and the temperature coefficient of gauge factor (TCS) and temperature coefficient of resistance (TCR) can be set within the ranges of -1000 ppm / °C to +1000 ppm / °C. When the functional layer is made of Ti, the Cr mixed-phase film may contain Ti or TiN (titanium nitride).

[0076] When the metal layer A is a Cr mixed phase film, the functional layer made of Ti has all of the following functions: promoting the crystal growth of the metal layer A, preventing oxidation of the metal layer A due to oxygen and moisture contained in the substrate 10, and improving adhesion between the substrate 10 and the metal layer A. The same applies when Ta, Si, Al, or Fe is used as the functional layer instead of Ti.

[0077] In this way, by providing a functional layer below the metal layer A, it is possible to promote crystal growth in the metal layer A, and to produce a metal layer A consisting of a stable crystalline phase. As a result, it is possible to improve the stability of the gauge characteristics of the strain gauge 1. Furthermore, by diffusing the material that constitutes the functional layer into the metal layer A, it is possible to improve the gauge characteristics of the strain gauge 1.

[0078] Next, the second metal layer 42 and the second metal layer 52 are formed on the upper surface of the metal layer A. The second metal layer 42 and the second metal layer 52 can be formed by, for example, photolithography.

[0079] Specifically, first, a seed layer is formed by, for example, sputtering or electroless plating so as to cover the upper surface of metal layer A. Next, a photosensitive resist is formed on the entire upper surface of the seed layer, and is exposed and developed to form openings that expose the regions where second metal layer 42 and second metal layer 52 will be formed. At this time, by adjusting the shape of the openings in the resist, the pattern of second metal layer 42 can be formed into any shape. For example, a dry film resist or the like can be used as the resist.

[0080] Next, for example, by electrolytic plating using the seed layer as a power supply path, the second metal layer 42 and the second metal layer 52 are formed on the seed layer exposed in the opening. Electrolytic plating is advantageous in that it has high tact time and can form low-stress electroplated layers as the second metal layer 42 and the second metal layer 52. By forming a thick electroplated layer with low stress, warping of the strain gauge 1 can be prevented. Note that the second metal layer 42 and the second metal layer 52 may also be formed by electroless plating.

[0081] Next, the resist is removed by, for example, immersing the resist in a solution that can dissolve the resist material.

[0082] Next, a photosensitive resist is formed on the entire upper surface of the seed layer, and is exposed and developed to be patterned into a planar shape similar to that of the resistor 30, wiring 40, and electrode 50 in Fig. 1. For example, a dry film resist or the like can be used as the resist. Then, using the resist as an etching mask, the metal layer A and the seed layer exposed from the resist are removed to form the resistor 30, wiring 40, and electrode 50 in the planar shape of Fig. 1.

[0083] For example, unnecessary portions of the metal layer A and the seed layer can be removed by wet etching. If a functional layer is formed below the metal layer A, the functional layer is patterned by etching into the planar shape shown in Fig. 1, similar to the resistor 30, the wiring 40, and the electrode 50. At this point, the seed layer is formed on the resistor 30, the first metal layer 41, and the first metal layer 51.

[0084] Next, the second metal layer 42 and the second metal layer 52 are used as an etching mask to remove unnecessary seed layer exposed from the second metal layer 42 and the second metal layer 52, thereby forming the second metal layer 42 and the second metal layer 52. Note that the seed layer directly below the second metal layer 42 and the second metal layer 52 remains. For example, the unnecessary seed layer can be removed by wet etching using an etching solution that etches the seed layer but does not etch the functional layer, resistor 30, wiring 40, and electrode 50.

[0085] Thereafter, if necessary, a cover layer that covers the resistor 30 and the wiring 40 and exposes the electrodes 50 is provided on the upper surface 10a of the substrate 10, thereby completing the strain gauge 1. The cover layer can be produced, for example, by laminating a semi-cured thermosetting insulating resin film on the upper surface 10a of the substrate 10 so as to cover the resistor 30 and the wiring 40 and expose the electrodes 50, and then heating and curing the film. The cover layer may also be produced by applying a liquid or paste-like thermosetting insulating resin to the upper surface 10a of the substrate 10 so as to cover the resistor 30 and the wiring 40 and expose the electrodes 50, and then heating and curing the resin.

[0086] When a functional layer is provided on the upper surface 10a of the substrate 10 as an underlayer for the resistor 30, the first metal layer 41, and the first metal layer 51, the strain gauge 1 has a cross-sectional shape as shown in FIG. 5. The layer indicated by the reference numeral 20 is the functional layer. When the functional layer 20 is provided, the planar shape of the strain gauge 1 will be the same as that shown in FIG. 1, for example. However, as mentioned above, the functional layer 20 may be formed solidly on part or all of the upper surface of the substrate 10. Note that the functional layer 20 is extremely thin compared to the resistor 30 and the first metal layer 41, and therefore the presence or absence of the functional layer 20 is thought to have no effect on the magnitude of the strain limits of the resistor 30 and the first metal layer 41.

[0087] <Modification 1 of the First Embodiment> In Modification 1 of the first embodiment, an example is shown in which the pattern of the second metal layer is different from that of Embodiment 1. Note that in Modification 1 of the first embodiment, the description of the same components as those in the already described embodiments may be omitted.

[0088] 6 is a plan view illustrating a strain gauge according to Modification 1 of the first embodiment. Note that the cross-sectional structure of the strain gauge according to Modification 1 of the first embodiment is similar to that of FIGS.

[0089] Referring to FIG. 6, the strain gauge 1A differs from the strain gauge 1 (see FIG. 1, etc.) in that the wiring 40 is replaced with wiring 40A. In the strain gauge 1A, the wiring 40A is formed on a substrate 10. The wiring 40A has a first metal layer 41 and a second metal layer 42A laminated on the upper surface of the first metal layer 41. The wiring 40A is not limited to being linear and can have any pattern. The wiring 40A can also have any length. The wiring 40A may also have a shape that is narrowest on the resistor 30 side and gradually widens toward the electrode 50. Note that in FIG. 6, for convenience, the second metal layer 42A and the electrode 50 are shown with a matte finish that is thinner than the resistor 30 and the first metal layer 41.

[0090] In the wiring 40A, similarly to the wiring 40, the second metal layer 42A is not formed solidly so as to cover the entire upper surface of the first metal layer 41, but is formed in selected regions of the upper surface of the first metal layer 41. In other words, the second metal layer 42A is formed in a pattern different from that of the first metal layer 41. Specifically, in FIG. 6, as an example, the first metal layer 41 has a linear pattern, and the second metal layer 42A has a discrete pattern.

[0091] In other words, the second metal layer 42A has a plurality of wiring components that are spaced apart from one another. In the wiring 40A, the first metal layer 41 extends in the same direction as the direction of line AA in Fig. 1, but the second metal layer 42A includes a plurality of wiring components that extend obliquely with respect to the direction of line AA in Fig. 1. In other words, each wiring component that makes up the second metal layer 42A is disposed obliquely with respect to the extension direction of the first metal layer 41.

[0092] The inclination angle of each wiring component constituting second metal layer 42A with respect to the extension direction of first metal layer 41 is, for example, 45 degrees, but may be any angle depending on the direction in which stress is desired to be generated. Furthermore, second metal layer 42A may include wiring components with different inclination angles with respect to the extension direction of first metal layer 41.

[0093] In this way, in wiring 40A in which second metal layer 42A is stacked on first metal layer 41, by forming second metal layer 42A in a pattern different from that of first metal layer 41, it becomes possible to control the direction of stress generated at the interface between first metal layer 41 and second metal layer 42A, thereby achieving the same effects as in the first embodiment.

[0094] Note that the respective portions constituting the second metal layer 42A may have a wavy pattern, and the respective wavy patterns may be arranged discretely. In other words, the first embodiment and Modification 1 can be combined as necessary.

[0095] Although the preferred embodiments have been described in detail above, the present invention is not limited to the above-described embodiments, and various modifications and substitutions can be made to the above-described embodiments without departing from the scope of the claims.

[0096] For example, the second metal layer according to the present invention does not have to be a wavy pattern or an intermittent pattern, and can be any pattern as long as it is different from the first metal layer.

[0097] The present invention is also applicable to strain gauges in which a plurality of resistors are formed on a substrate. For example, the present invention is also applicable to strain gauges in which two resistors formed on a substrate form a half-bridge circuit. Alternatively, the present invention is also applicable to strain gauges in which four resistors formed on a substrate form a full-bridge circuit. [Explanation of symbols]

[0098] 1, 1A strain gauge, 10 substrate, 10a upper surface, 20 functional layer, 30 resistor, 30e1, 30e2 termination, 40, 40A wiring, 50 electrode, 41, 51 first metal layer, 42, 42A, 52 second metal layer, 60 cover layer

Claims

1. a flexible substrate; a resistor formed on the substrate; a pair of electrodes formed on the substrate and electrically connected to the resistor via wiring; the wiring includes a first metal layer and a second metal layer formed on an upper surface of the first metal layer; the second metal layer is formed in a pattern different from that of the first metal layer, A strain gauge, wherein, in a plan view, the second metal layer has a pattern including a portion extending obliquely with respect to the extension direction of the first metal layer, a wavy pattern, and / or a discrete pattern.

2. The strain gauge according to claim 1 , wherein a partial region of an upper surface of the first metal layer is exposed from the second metal layer in a plan view.

3. 3. The strain gauge according to claim 1, wherein the first metal layer is made of the same material as the resistor and is integral with the resistor.

4. The strain gauge according to claim 1 , wherein the second metal layer is made of a material having greater elasticity than the first metal layer.

5. The strain gauge according to claim 1 , wherein the second metal layer is made of a material having a lower resistance than the first metal layer.

6. The strain gauge according to claim 1 , wherein the first metal layer has a width of 10 μm to 100 μm.

7. 7. The strain gauge according to claim 1, wherein the wiring electrically connects each end of the resistor in the grid width direction to each of the electrodes.

8. The resistor is made of Cr, CrN, and Cr ２ 8. The strain gauge according to claim 1, which is formed from a film containing N.

9. 9. The strain gauge according to claim 8, wherein the gauge factor is 10 or more.

10. CrN and Cr contained in the resistor ２ 10. The strain gauge according to claim 8, wherein N is 20% by weight or less.

11. The CrN and the Cr ２ The Cr in N ２ 11. The strain gauge according to claim 10, wherein the proportion of N is 80% by weight or more and less than 90% by weight.

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