Strain gauge

Abstract

To provide a strain gauge capable of improving strain detection accuracy. [Solution] The 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, wherein the wiring electrically connects each end of the resistor in the grid width direction to each of the electrodes, and the wiring includes a first metal layer and a second metal layer formed on the first metal layer from a material having lower resistance than the first metal layer.

Description

Technical Field

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

Background Art

[0002] A strain gauge that is attached to a measurement object to detect the strain of the measurement object is known. 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 a wiring (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, when a wiring is connected to the resistor, the wiring functions as a part of the resistor, and the strain detection accuracy may decrease.

[0005] The present invention has been made in view of the above points, and an object thereof is to provide a strain gauge capable of improving strain detection accuracy.

Means for Solving the Problems

[0006] The present strain gauge includes a flexible base material, a resistor formed on the base material, and a pair of electrodes formed on the base material and electrically connected to the resistor via a wiring. The wiring electrically connects each end in the grid width direction of the resistor to each of the electrodes, and the wiring includes a first metal layer and a second metal layer formed of a material having a lower resistance than the first metal layer on the first metal layer.

Effects of the Invention

[0007] According to the disclosed technology, it is possible to provide a strain gauge capable of improving strain detection accuracy. [Brief explanation of the drawing]

[0008] [Figure 1] This is a plan view illustrating a strain gauge according to the first embodiment. [Figure 2] This is a cross-sectional view (part 1) illustrating a strain gauge according to the first embodiment. [Figure 3] This is a cross-sectional view (part 2) illustrating a strain gauge according to the first embodiment. [Figure 4] This figure shows the experimental results of the strain limit. [Figure 5] This is a cross-sectional view (part 3) illustrating a strain gauge according to the first embodiment. [Figure 6] This is a plan view illustrating a strain gauge according to a modified example 1 of the first embodiment. [Figure 7] This is a plan view illustrating a strain gauge according to a modified example 2 of the first embodiment. [Figure 8] This is a plan view illustrating a strain gauge according to a modified example 3 of the first embodiment. [Modes for carrying out the invention]

[0009] The embodiments for carrying out the invention will be described below with reference to the drawings. In each drawing, the same reference numerals are used for identical components, and redundant explanations may be omitted.

[0010] <First Embodiment> Figure 1 is a plan view illustrating a strain gauge according to the first embodiment. Figure 2 is a cross-sectional view illustrating a strain gauge according to the first embodiment, showing a cross-section along line AA in Figure 1. Figure 3 is a cross-sectional view illustrating a strain gauge according to the first embodiment, showing a cross-section along line BB in Figure 1. Referring to Figures 1 to 3, the strain gauge 1 includes a base material 10, a resistor 30, wiring 40, and electrodes 50.

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

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

[0013] The base material 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, or polyolefin resin. The term "film" refers to a flexible material with a thickness of approximately 500 μm or less.

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

[0015] Examples of materials other than the resin of the base material 10 include crystalline materials such as SiO2, ZrO2 (including YSZ), Si, Si2N3, Al2O3 (including sapphire), ZnO, perovskite ceramics (CaTiO3, BaTiO3), etc. Further, amorphous glass, etc. are also included. Also, as the material of the base material 10, metals such as aluminum, aluminum alloy (duralumin), and titanium may be used. In this case, for example, an insulating film is formed on the metal base material 10.

[0016] The resistor 30 is a thin film formed in a predetermined pattern on the base material 10, and is a sensing part that undergoes strain and causes a resistance change. The resistor 30 may be formed directly on the upper surface 10a of the base material 10, or may be formed on the upper surface 10a of the base material 10 via another layer. In FIG. 1, for convenience, the resistor 30 is shown as a dark textured pattern.

[0017] 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. Examples of the material containing Cr include, for example, a Cr mixed-phase film. Examples of the material containing Ni include, for example, Cu-Ni (copper nickel). Examples of the material containing both Cr and Ni include, for example, Ni-Cr (nickel chromium).

[0018] Here, the Cr mixed-phase film is a film in which Cr, CrN, Cr2N, etc. are mixed. The Cr mixed-phase film may contain unavoidable impurities such as chromium oxide.

[0019] The thickness of the resistor 30 is not particularly limited and can be appropriately selected according to the purpose. For example, it can be set to about 0.05 μm to 2 μm. In particular, when the thickness of the resistor 30 is 0.1 μm or more, it is preferable in terms of improving the crystallinity of the crystal constituting the resistor 30 (for example, the crystallinity of α-Cr), and when it is 1 μm or less, it is more preferable in terms of reducing film cracks caused by internal stress of the film constituting the resistor 30 and warping from the base material 10. The width of the resistor 30 can be optimized according to the required specifications such as resistance value and lateral sensitivity, and considering measures against disconnection, for example, it can be set to about 10 μm to 100 μm.

[0020] For example, when the resistor 30 is a Cr mixed-phase film, by using α-Cr (alpha chromium), which is a stable crystal phase, as the main component, the stability of the gauge characteristics can be improved. Also, when the resistor 30 has α-Cr as the main component, the gauge factor of the strain gauge 1 can be 10 or more, and the gauge factor temperature coefficient TCS and the resistance temperature coefficient TCR can be within the range of -1000 ppm / °C to +1000 ppm / °C. Here, the main component means that the target substance occupies 50% by weight or more of all the substances constituting the resistor. From the perspective of improving the gauge characteristics, the resistor 30 preferably contains 80% by weight or more of α-Cr, and more preferably 90% by weight or more. Note that α-Cr is Cr with a bcc structure (body-centered cubic lattice structure).

[0021] Also, when the resistor 30 is a Cr mixed-phase film, it is preferable that CrN and Cr2N contained in the Cr mixed-phase film are 20% by weight or less. By having CrN and Cr2N contained in the Cr mixed-phase film be 20% by weight or less, a decrease in the gauge factor can be suppressed.

[0022] Also, the proportion 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 proportion of Cr2N in CrN and Cr2N is 90% by weight or more and less than 95% by weight, due to Cr2N having semiconductor properties, the decrease in TCR (negative TCR) becomes more prominent. Furthermore, by reducing the ceramization, brittle fracture is reduced.

[0023] On the other hand, if trace amounts of N2 or atomic N are mixed into the film, external environmental factors (such as high temperatures) can cause them to escape from the film, resulting in changes in film stress. By creating chemically stable CrN, the generation of the aforementioned unstable N is avoided, and a stable strain gauge can be obtained.

[0024] The wiring 40 is formed on the substrate 10. The wiring 40 has a first metal layer 41 and a second metal layer 42 laminated on the upper surface of the first metal layer 41. The wiring 40 is not limited to being linear and can be in any pattern. Also, the wiring 40 can be of any length. For convenience, in Figure 1, the wiring 40 and electrodes 50 are shown with a textured surface that is thinner than the resistor 30.

[0025] The resistor 30 has a structure in which multiple elongated sections are arranged at predetermined intervals with their longitudinal directions aligned in the same direction (the direction of line AA in Figure 1), and the ends of adjacent elongated sections are connected alternately, resulting in a zigzag folding pattern overall. The longitudinal direction of the multiple elongated sections becomes the grid direction, and the direction perpendicular to the grid direction becomes the grid width direction (the direction of line BB in Figure 1).

[0026] The longitudinal ends of the two elongated portions located on the outermost side in the grid width direction are bent in the grid width direction, forming the respective ends 30e1 and 30e2 of the resistor 30 in the grid width direction. The respective ends 30e1 and 30e2 of the resistor 30 in the grid width direction are electrically connected to the electrodes 50 via the wiring 40. In other words, the wiring 40 electrically connects the respective ends 30e1 and 30e2 of the resistor 30 in the grid width direction to the respective electrodes 50.

[0027] The electrodes 50 are formed on the substrate 10 and are electrically connected to the resistor 30 via the wiring 40. For example, they are wider than the wiring 40 and are formed in a roughly rectangular shape. The electrodes 50 are a pair of electrodes for outputting the change in the resistance value of the resistor 30 caused by strain to the outside, and for example, lead wires for external connection are attached to them.

[0028] The electrode 50 has a pair of first metal layers 51 and a second metal layer 52 laminated on the upper surface of each first metal layer 51. The first metal layers 51 are electrically connected to the terminals 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 plan view. The first metal layers 51 may be formed to the same width as the wiring 40.

[0029] 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 from the same material in the same process. Therefore, the resistor 30, the first metal layer 41, and the first metal layer 51 have approximately the same thickness. Similarly, although the second metal layer 42 and the second metal layer 52 are given different reference numerals for convenience, they can be integrally formed from the same material in the same process. Therefore, the second metal layer 42 and the second metal layer 52 have approximately the same thickness.

[0030] The second metal layers 42 and 52 are formed from a material with lower resistance than the resistor 30 (first metal layers 41 and 51). The material of the second metal layers 42 and 52 is not particularly limited as long as it has lower resistance than the resistor 30, and can be appropriately selected according to the purpose. For example, if the resistor 30 is a Cr multiphase film, the material of the second metal layers 42 and 52 can be Cu, Ni, Al, Ag, Au, Pt, etc., or an alloy of any of these metals, a compound of any of these metals, or a laminated film in which any of these metals, alloys, or compounds are appropriately stacked. The thickness of the second metal layers 42 and 52 is not particularly limited and can be appropriately selected according to the purpose, but for example it can be about 3 μm to 5 μm.

[0031] The second metal layers 42 and 52 may be formed on a portion of the upper surface of the first metal layers 41 and 51, or on the entire upper surface of the first metal layers 41 and 51. One or more additional 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, and a gold layer may be laminated on top of the copper layer. Alternatively, the second metal layer 52 may be a copper layer, and a palladium layer and a gold layer may be sequentially laminated on top of the copper layer. By making the uppermost layer of the electrode 50 a gold layer, the solder wettability of the electrode 50 can be improved.

[0032] A cover layer 60 (insulating resin layer) may be provided on the upper surface 10a of the base material 10 so as to cover the resistor 30 and wiring 40 and expose the electrode 50. Providing the cover layer 60 prevents mechanical damage to the resistor 30 and wiring 40. In addition, the cover layer 60 protects the resistor 30 and wiring 40 from moisture and other elements. The cover layer 60 may be provided so as to cover the entire portion excluding the electrode 50.

[0033] 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, polyolefin resin). The cover layer 60 may contain fillers or pigments. There are no particular restrictions on the thickness of the cover layer 60, and it can be appropriately selected depending on the purpose, but for example, it can be about 2 μm to 30 μm.

[0034] Thus, the wiring 40 has a structure in which a second metal layer 42 is laminated on a first metal layer 41 made of the same material as the resistor 30. Therefore, since the wiring 40 has lower resistance than the resistor 30, it is possible to suppress the wiring 40 from functioning as a resistor. As a result, the accuracy of strain detection by the resistor 30 can be improved.

[0035] In other words, by providing wiring 40 with lower resistance than the resistor 30, the effective sensitive area of ​​the strain gauge 1 can be limited to the local region where the resistor 30 is formed. Therefore, the accuracy of strain detection by the resistor 30 can be improved.

[0036] In particular, in a highly sensitive strain gauge with a gauge factor of 10 or higher using a Cr multiphase film as the resistor 30, reducing the resistance of the wiring 40 to that of the resistor 30 and limiting the effective sensing area to the local region where the resistor 30 is formed has a remarkable effect on improving the accuracy of strain detection. Furthermore, reducing the resistance of the wiring 40 to that of the resistor 30 also has the effect of reducing lateral sensitivity.

[0037] Furthermore, the length L of the wiring 40 connecting the resistor 30 and the electrode 50 is preferably 5 mm or more along the length of the wiring 40, regardless of whether the wiring 40 is straight or not. By setting the length L to 5 mm or more, the heat generated when soldering lead wires etc. to the electrode 50 is less likely to be transferred to the resistor 30 or the cover layer 60 covering the resistor 30, thereby reducing the thermal load on the gauge characteristics.

[0038] Furthermore, considering the strain limit, there is an optimal range for the width of the wiring 40. This will be explained below.

[0039] According to the inventors' investigation, it was found that the narrower the width of the first metal layer 41 of the wiring 40, the less likely cracks or disconnections are to occur when the wiring 40 is subjected to strain. Specifically, the inventors prepared multiple test strain gauges of four types, each 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 and disconnections. In this experiment, the first metal layer 41 was a 0.2 μm thick Cr multiphase film, and the second metal layer 42 was not laminated on the first metal layer 41.

[0040] The experimental results showed that as the width of the first metal layer 41 narrowed, the number of cracks and disconnections tended to decrease, indicating 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 disconnections begin to occur when strain is applied to the strain gauge.

[0041] Figure 4 shows the experimental results of the strain limit, plotting the minimum strain limit values ​​for multiple test strain gauges. As shown in Figure 4, the inventors' experimental results showed that the strain limit was 5500 με or higher when the width of the first metal layer 41 was 560 μm, while the strain limit was 8500 με or higher when the width of the first metal layer 41 was 10 μm. In other words, the strain limit is approximately 1.5 times higher when the width of the first metal layer 41 is 10 μm compared to when it is 560 μm. Furthermore, the strain limit improves almost linearly between the width of the first metal layer 41 from 560 μm to 10 μm.

[0042] This result is thought to be obtained because a wider width of the first metal layer 41, which is made of a Cr multiphase film with a high elastic modulus, makes brittle fracture more likely, and 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 about 8000 με is required, so the width of the first metal layer 41 is preferably 100 μm or less. On the other hand, it is difficult to make the width of the first metal layer 41 less than 10 μm due to the manufacturing process. Considering this point, it can be said that the width of the first metal layer 41 is preferably between 10 μm and 100 μm.

[0043] Furthermore, according to another study by the inventors, when a 3 μm thick copper layer is formed as a solid second metal layer 42 on the first metal layer 41, the strain limit is improved by 1.5 to 2 times, regardless of the width of the first metal layer 41, compared to when the second metal layer is not formed. Since the copper layer has better elasticity than the Cr multiphase film, it is thought that the strain limit is further improved by laminating the copper layer on the Cr multiphase film.

[0044] In other words, from the viewpoint of further improving the strain limit, it is preferable that the second metal layer 42 is formed from a material with better elasticity than the first metal layer 41. That is, it is preferable that the second metal layer is formed from a material that has lower resistance than the first metal layer and better elasticity than the first metal layer 41. When the first metal layer is a Cr multiphase film, materials that have lower resistance than the first metal layer and better elasticity than the first metal layer 41 include gold, silver, and aluminum in addition to copper. It is believed that similar results can be obtained when these materials are used as the second metal layer 42.

[0045] Strain gauge 1 can also be used for weighing purposes. When strain gauge 1 is used for weighing purposes, it must satisfy the standards related to creep. Examples of standards related to creep include accuracy class C1 (hereinafter referred to as the C1 standard) based on OIML R60 and accuracy class C2 (hereinafter referred to as the C2 standard) based on OIML R60.

[0046] The C1 standard requires that the creep amount and creep recovery amount be ±0.0735% or less. The C2 standard requires that the creep amount and creep recovery amount be ±0.0368% or less. Note that when strain gauge 1 is used for sensor applications, the creep amount and creep recovery amount specifications are approximately ±0.5%.

[0047] According to the inventors' studies, the amount of creep and the amount of creep recovery are influenced by the viscoelastic term of the material constituting the resistor 30. That is, in a strain-generating body used for weighing purposes, tension and compression occur in the longitudinal direction of the resistor 30 (the direction of line AA in Figure 1). When the resistor 30 expands and contracts in the longitudinal direction due to the tension and compression of the strain-generating body, the amount of creep and the amount of creep recovery increase, so it is preferable to suppress the expansion and contraction of the resistor 30 in the longitudinal direction.

[0048] The wiring 40 has a structure in which a second metal layer 42 is laminated on a first metal layer 41, and by increasing the volume of metal constituting the wiring 40, it is possible to suppress the expansion and contraction of the resistor 30 in the longitudinal direction. As a result, the amount of creep and creep recovery can be suppressed, and the C1 and C2 standards can be satisfied.

[0049] Furthermore, by making the electrode 50 a structure in which a second metal layer 52 is laminated on a first metal layer 51, and increasing the volume of metal constituting the electrode 50, it becomes possible to further suppress the expansion and contraction of the resistor 30 in the longitudinal direction. As a result, the amount of creep and creep recovery can be further suppressed.

[0050] The effect of suppressing creep and creep recovery can be adjusted by the thickness of the second metal layers 42 and 52. When the strain gauge 1 is used for weighing purposes, as mentioned above, it is preferable that the second metal layers 42 and 52 are formed from a material with lower resistance than the first metal layers 41 and 51 (i.e., the resistor 30).

[0051] Furthermore, increasing the volume of the resistor 30 itself would lower its resistance, which is undesirable from an application standpoint. However, by laminating the second metal layers 42 and 52 on the first metal layers 41 and 51, the rigidity of the strain gauge 1 can be increased without lowering the resistance of the resistor 30.

[0052] Furthermore, the creep amount and creep recovery amount are the amounts by which the amount of elastic deformation (strain) of the surface on which the resistor 30 is provided in the strain gauge 1 changes over time, and can be measured by monitoring the strain voltage calculated based on the output between the pair of electrodes 50.

[0053] To manufacture the strain gauge 1, first, a base material 10 is prepared, and a metal layer (for convenience, referred to as metal layer A) is formed on the upper surface 10a of the base material 10. Metal layer A is the layer that will ultimately be patterned to become the resistor 30, the first metal layer 41, and the first metal layer 51. Therefore, the material and thickness of metal layer A are the same as those of the resistor 30, the first metal layer 41, and the first metal layer 51.

[0054] Metal layer A can be deposited, for example, by a magnetron sputtering method targeting a raw material capable of forming metal layer A. Alternatively, metal layer A may be deposited using reactive sputtering, evaporation, arc ion plating, pulsed laser deposition, or other methods instead of magnetron sputtering.

[0055] From the viewpoint of stabilizing gauge characteristics, it is preferable to vacuum-deposit a functional layer of a predetermined thickness as an underlayer on the upper surface 10a of the substrate 10, for example, by conventional sputtering, before depositing the metal layer A.

[0056] In this application, the functional layer refers to a layer that has the function of promoting crystal growth of at least the upper metal layer A (resistor 30). Preferably, the functional layer further has the function of preventing oxidation of the metal layer A by oxygen and moisture contained in the substrate 10, and the function of improving the adhesion between the substrate 10 and the metal layer A. The functional layer may further have other functions.

[0057] Since the insulating resin film that makes up the base material 10 contains oxygen and moisture, and especially when the metal layer A contains Cr, Cr forms an oxidized film, it is effective for the functional layer to have a function that prevents oxidation of the metal layer A.

[0058] The material of the functional layer is not particularly limited as long as it is a material that has the function of promoting crystal growth of at least the upper metal layer A (resistor 30), and can be appropriately selected according to 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 (bismodium) Examples include one or more metals selected from the group consisting of (S), 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 these metals, or a compound of any of these metals.

[0059] Examples of the alloys mentioned above include FeCr, TiAl, FeNi, NiCr, and CrCu. Examples of the compounds mentioned above include TiN, TaN, Si3N4, TiO2, Ta2O5, and SiO2.

[0060] When the functional layer is formed from 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. Within this range, the crystal growth of α-Cr can be promoted, and a portion of the current flowing through the resistor can flow into the functional layer, preventing a decrease in strain detection sensitivity.

[0061] When the functional layer is formed from a conductive material such as a metal or alloy, it is more preferable that the thickness of the functional layer be 1 / 50 or less of the thickness of the resistor. Within this range, the crystal growth of α-Cr can be promoted, and a portion of the current flowing through the resistor flows into the functional layer, further preventing a decrease in strain detection sensitivity.

[0062] When the functional layer is formed from a conductive material such as a metal or alloy, it is even more preferable that the thickness of the functional layer be 1 / 100 or less of the thickness of the resistor. Within this range, it is possible to further prevent a decrease in strain detection sensitivity due to some of the current flowing through the resistor flowing into the functional layer.

[0063] When the functional layer is formed from an insulating material such as an oxide or nitride, the thickness of the functional layer is preferably 1 nm to 1 μm. Within this range, the crystal growth of α-Cr can be promoted, and the functional layer can be easily formed without cracking.

[0064] When the functional layer is formed from an insulating material such as an oxide or nitride, the thickness of the functional layer is more preferably 1 nm to 0.8 μm. Within this range, the crystal growth of α-Cr can be promoted, and the functional layer can be formed more easily without cracking.

[0065] When the functional layer is formed from an insulating material such as an oxide or nitride, it is even more preferable that the thickness of the functional layer be 1 nm to 0.5 μm. Within this range, the crystal growth of α-Cr can be promoted, and the film can be formed more easily without cracking in the functional layer.

[0066] The planar shape of the functional layer is patterned to be substantially the same as the planar shape of the resistor shown in Figure 1. However, the planar shape of the functional layer is not limited to being substantially the same as the planar shape of the resistor. When the functional layer is formed from 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 as a solid block in at least the region where the resistor is formed. Alternatively, the functional layer may be formed as a solid block over the entire upper surface of the substrate 10.

[0067] Furthermore, when the functional layer is formed from an insulating material, forming the functional layer relatively thick, such as 50 nm to 1 μm, and forming it in a solid form increases the thickness and surface area of ​​the functional layer, allowing the heat generated by the resistor to be dissipated towards the substrate 10. As a result, the decrease in measurement accuracy due to self-heating of the resistor can be suppressed in the strain gauge 1.

[0068] The functional layer can be deposited using a conventional sputtering method, for example, by targeting a raw material capable of forming a functional layer and introducing Ar (argon) gas into a chamber. By using the conventional sputtering method, the functional layer is deposited while etching the upper surface 10a of the substrate 10 with Ar, thus minimizing the amount of functional layer deposited and achieving improved adhesion.

[0069] However, this is just one example of a method for forming a functional layer, and the functional layer may be formed by other methods. For example, the upper surface 10a of the substrate 10 may be activated by plasma treatment using Ar or the like before forming the functional layer to improve adhesion, and then the functional layer may be formed in a vacuum by magnetron sputtering.

[0070] There are no particular restrictions on the combination of materials for the functional layer and the metal layer A, and they can be appropriately selected according to the purpose. For example, it is possible to use Ti as the functional layer and deposit a Cr multiphase film mainly composed of α-Cr (alpha-chromium) as the metal layer A.

[0071] In this case, for example, metal layer A can be formed by targeting a raw material capable of forming a Cr multiphase film and using a magnetron sputtering method with Ar gas introduced into the chamber. Alternatively, metal layer A may be formed by targeting pure Cr and using a reactive sputtering method with an appropriate amount of nitrogen gas introduced into the chamber along with Ar gas. In this case, the ratio of CrN and Cr2N in the Cr multiphase film, as well as the ratio of Cr2N within CrN and Cr2N, can be adjusted by changing the amount and pressure (partial pressure of nitrogen) of nitrogen gas introduced or by adjusting the heating temperature by providing a heating step.

[0072] In these methods, a functional layer made of Ti dictates the growth surface of the Cr multiphase film, enabling the formation of a Cr multiphase film primarily composed of α-Cr, which has a stable crystalline structure. Furthermore, the diffusion of Ti constituting the functional layer into the Cr multiphase film improves the gauge characteristics. For example, the gauge factor of strain gauge 1 can be set to 10 or higher, and the gauge factor temperature coefficient TCS and resistance temperature coefficient TCR can be set within the range of -1000 ppm / °C to +1000 ppm / °C. Note that when the functional layer is formed from Ti, the Cr multiphase film may contain Ti or TiN (titanium nitride).

[0073] Furthermore, when metal layer A is a Cr multiphase film, the functional layer made of Ti has all of the following functions: promoting crystal growth of metal layer A, preventing oxidation of metal layer A by oxygen and moisture contained in the substrate 10, and improving adhesion between the substrate 10 and metal layer A. The same applies when Ta, Si, Al, or Fe are used instead of Ti as the functional layer.

[0074] Thus, by providing a functional layer beneath the metal layer A, crystal growth in the metal layer A can be promoted, and a metal layer A consisting of a stable crystalline phase can be fabricated. As a result, the stability of the gauge characteristics in the strain gauge 1 can be improved. Furthermore, the diffusion of the material constituting the functional layer into the metal layer A can improve the gauge characteristics in the strain gauge 1.

[0075] Next, a second metal layer 42 and a second metal layer 52 are formed on the upper surface of metal layer A. The second metal layer 42 and the second metal layer 52 can be formed, for example, by photolithography.

[0076] Specifically, first, a seed layer is formed to cover the upper surface of metal layer A, for example, by sputtering or electroless plating. Next, a photosensitive resist is formed over the entire upper surface of the seed layer, and exposure and development are performed to create openings that expose the areas for forming the second metal layer 42 and the second metal layer 52. As the resist, for example, a dry film resist can be used.

[0077] Next, for example, a second metal layer 42 and a second metal layer 52 are formed on the seed layer exposed in the opening by an electroplating method using the seed layer as the power supply path. The electroplating method is preferable because it has a high cycle time and can form low-stress electroplated layers as the second metal layer 42 and the second metal layer 52. By making the thick electroplated layer low-stress, warping of the strain gauge 1 can be prevented. The second metal layer 42 and the second metal layer 52 may also be formed by an electroless plating method.

[0078] Next, remove the resist. The resist can be removed, for example, by immersing it in a solution that can dissolve the resist material.

[0079] Next, a photosensitive resist is formed on the entire upper surface of the seed layer, exposed and developed to pattern it into a planar shape similar to the resistor 30, wiring 40, and electrode 50 in Figure 1. For example, a dry film resist can be used as the resist. Then, the resist is used as an etching mask to remove the metal layer A and seed layer exposed from the resist, forming the resistor 30, wiring 40, and electrode 50 with the planar shape shown in Figure 1.

[0080] For example, unwanted portions of metal layer A and seed layer can be removed by wet etching. If a functional layer is formed beneath metal layer A, etching will pattern the functional layer into the planar shape shown in Figure 1, similar to the resistor 30, wiring 40, and electrode 50. At this point, the resistor 30, the first metal layer 41, and the seed layer are formed on the first metal layer 51.

[0081] Next, the second metal layer 42 and the second metal layer 52 are used as etching masks, and the unwanted seed layer exposed from the second metal layer 42 and the second metal layer 52 is removed to form the second metal layer 42 and the second metal layer 52. Note that the seed layer directly beneath the second metal layer 42 and the second metal layer 52 remains. For example, the unwanted seed layer can be removed by wet etching using an etching solution that etches the seed layer but not the functional layer, resistor 30, wiring 40, and electrode 50.

[0082] Subsequently, if necessary, a cover layer is provided on the upper surface 10a of the base material 10 to cover the resistor 30 and wiring 40 and expose the electrode 50, thereby completing the strain gauge 1. The cover layer can be made, for example, by laminating a semi-cured thermosetting insulating resin film onto the upper surface 10a of the base material 10 to cover the resistor 30 and wiring 40 and expose the electrode 50, and then heating and curing it. Alternatively, the cover layer may be made by applying a liquid or paste-like thermosetting insulating resin to the upper surface 10a of the base material 10 to cover the resistor 30 and wiring 40 and expose the electrode 50, and then heating and curing it.

[0083] Furthermore, when a functional layer is provided on the upper surface 10a of the substrate 10 as a base layer for the resistor 30, the first metal layer 41, and the first metal layer 51, the strain gauge 1 will have the cross-sectional shape shown in Figure 5. The layer indicated by reference numeral 20 is the functional layer. The planar shape of the strain gauge 1 when the functional layer 20 is provided will be the same as, for example, as shown in Figure 1. However, as mentioned above, the functional layer 20 may also be formed as a solid layer on part or all of the upper surface of the substrate 10.

[0084] <Variation 1 of the First Embodiment> Modification 1 of the first embodiment shows an example where the width of the wiring is not constant. In Modification 1 of the first embodiment, descriptions of components that are the same as those described in the previously described embodiment may be omitted.

[0085] Figure 6 is a plan view illustrating a strain gauge according to Modification 1 of the First Embodiment. In the strain gauge according to Modification 1 of the First Embodiment, the cross-sectional structure is the same as that in Figures 2 and 3.

[0086] Referring to Figure 6, strain gauge 1A differs from strain gauge 1 (see Figure 1, etc.) in that wiring 40 is replaced with wiring 40A. In strain gauge 1A, the width of wiring 40A is narrowest on the resistor 30 side and gradually widens as it approaches the electrode 50.

[0087] In this way, by making the width of the wiring 40A narrowest on the resistor 30 side and gradually widening as it approaches the electrode 50, there is no part where the width changes abruptly at the connection between the resistor 30 and the wiring 40A. Therefore, patterning of the resistor 30 and wiring 40 using etching becomes easier. However, as mentioned above, considering the strain limit, it is preferable to vary the width of the wiring 40A within a range of 10 μm to 100 μm.

[0088] In the example shown in Figure 6, the width of the wiring 40A is narrowest on the resistor 30 side and gradually widens as it approaches the electrode 50. However, it is not limited to this, and the wiring 40A only needs to have a section that gradually widens as it approaches the electrode 50 from the resistor 30 side. For example, the width of the wiring 40A may be narrowest on the resistor 30 side, gradually widening as it approaches the electrode 50 up to a certain length, and then maintaining a constant width up to the electrode 50.

[0089] <Modification 2 of the First Embodiment> Modification 2 of the first embodiment shows an example of a strain gauge in which a plurality of resistors are formed on a substrate. In Modification 2 of the first embodiment, descriptions of components that are the same as those described in the previously described embodiment may be omitted.

[0090] Figure 7 is a plan view illustrating a strain gauge according to Modification 2 of the First Embodiment. In the strain gauge according to Modification 2 of the First Embodiment, the cross-sectional structure is the same as in Figures 2 and 3.

[0091] Referring to Figure 7, strain gauge 1B differs from strain gauge 1 (see Figures 1 to 3) in that it has resistors 301 and 302. The material and film thickness of resistors 301 and 302 are the same as those of resistor 30 in the first embodiment.

[0092] In strain gauge 1B, one end of resistor 301 is electrically connected to electrode 501 via wiring 401, and is also electrically connected to one end of resistor 302. The other end of resistor 301 is electrically connected to electrode 502 via wiring 402. The other end of resistor 302 is electrically connected to electrode 503 via wiring 403.

[0093] In other words, in strain gauge 1B, resistors 301 and 302 constitute a half-bridge circuit.

[0094] Wires 401, 402, and 403 have a structure similar to wiring 40 in the first embodiment, in which a second metal layer 42 is laminated on a first metal layer 41 made of the same material as resistors 301 and 302. Therefore, since wirings 401, 402, and 403 have lower resistance than resistors 301 and 302, it is possible to suppress wirings 401, 402, and 403 from functioning as resistors. As a result, the accuracy of strain detection by resistors 301 and 302 can be improved.

[0095] In other words, by providing wiring 401, 402, and 403 with lower resistance than resistors 301 and 302, the effective sensitive area of ​​the strain gauge 1B can be limited to the local region where resistors 301 and 302 are formed. Therefore, the accuracy of strain detection by resistors 301 and 302 can be improved. The effects of using a Cr multiphase film as the resistor and the effects of the length of the wiring connecting the resistor and the electrode are the same as in the first embodiment.

[0096] Furthermore, a portion of the wiring 401 connects resistor 301 and resistor 302. In this way, low-resistance wiring may be used not only between resistors and electrodes, but also in the parts connecting resistors to each other. This improves the accuracy of strain detection by resistors 301 and 302, as described above.

[0097] <Modification 3 of the First Embodiment> Modification 3 of the first embodiment shows another example of a strain gauge equipped with multiple resistors. In Modification 3 of the first embodiment, descriptions of components that are the same as those described in the previously described embodiment may be omitted.

[0098] Figure 8 is a plan view illustrating a strain gauge according to Modification 3 of the First Embodiment. In the strain gauge according to Modification 3 of the First Embodiment, the cross-sectional structure is the same as that in Figures 2 and 3.

[0099] Referring to Figure 8, strain gauge 1C differs from strain gauge 1 (see Figures 1 to 3) in that it has resistors 301, 302, 303, and 304. The materials and film thickness of resistors 301, 302, 303, and 304 are the same as those of resistor 30 in the first embodiment.

[0100] In strain gauge 1C, one end of resistor 301 is electrically connected to electrode 501 via wiring 401 and is also electrically connected to one end of resistor 302. The other end of resistor 301 is electrically connected to electrode 502 via wiring 402 and is also electrically connected to one end of resistor 303.

[0101] Furthermore, the other end of resistor 302 is electrically connected to electrode 504 via wiring 404, and is also electrically connected to one end of resistor 304. The other end of resistor 303 is electrically connected to electrode 503 via wiring 403, and is also electrically connected to the other end of resistor 304.

[0102] In other words, in strain gauge 1C, resistors 301, 302, 303, and 304 constitute a full bridge circuit.

[0103] Wires 401, 402, 403, and 404, like the wiring 40 in the first embodiment, have a structure in which a second metal layer 42 is laminated on a first metal layer 41 made of the same material as the resistors 301, 302, 303, and 304. Therefore, since the resistance of wirings 401, 402, 403, and 404 is lower than that of resistors 301, 302, 303, and 304, it is possible to suppress the wirings 401, 402, 403, and 404 from functioning as resistors. As a result, the accuracy of strain detection by resistors 301, 302, 303, and 304 can be improved.

[0104] In other words, by providing wiring 401, 402, 403, and 404 with lower resistance than resistors 301, 302, 303, and 304, the effective sensitive area of ​​the strain gauge 1C can be limited to the local region where resistors 301, 302, 303, and 304 are formed. Therefore, the accuracy of strain detection by resistors 301, 302, 303, and 304 can be improved. The effects of using a Cr multiphase film as the resistor and the effects of the length of the wiring connecting the resistor and the electrode are the same as in the first embodiment.

[0105] Furthermore, a portion of wiring 401 connects resistor 301 and resistor 302. Also, a portion of wiring 402 connects resistor 301 and resistor 303. Furthermore, a portion of wiring 403 connects resistor 303 and resistor 304. Also, a portion of wiring 404 connects resistor 302 and resistor 304. In this way, low-resistance wiring may be used not only between resistors and electrodes, but also in the parts connecting resistors to each other. This improves the accuracy of strain detection by resistors 301, 302, 303, and 304, as described above.

[0106] Although preferred embodiments have been described in detail above, the invention is not limited to the embodiments described above, and various modifications and substitutions can be made to the embodiments described above without departing from the scope of the claims. [Explanation of symbols]

[0107] 1, 1A, 1B, 1C Strain gauges, 10 Substrate, 10a Top surface, 20 Functional layer, 30, 301, 302, 303, 304 Resistors, 30e1, 30e2 Termination, 40, 401, 402, 403, 404 Wiring, 50, 501, 502, 503, 504 Electrodes, 41, 51 First metal layer, 42, 52 Second metal layer, 60 Cover layer

Claims

1. A flexible base material, A resistor formed on the aforementioned substrate, It has a pair of electrodes formed on the substrate and electrically connected to the resistor via wiring, The wiring electrically connects each end of the resistor in the grid width direction to each of the electrodes. The wiring comprises a strain gauge including a first metal layer and a second metal layer formed on the first metal layer from a material having lower resistance than the first metal layer.

2. The strain gauge according to claim 1, wherein the wiring comprises a portion that gradually widens as it approaches the electrode side from the resistor side.

3. Multiple resistors are formed on the substrate, The strain gauge according to claim 1 or 2, wherein the aforementioned wiring is also used in the portion connecting the resistors.

4. The strain gauge according to any one of claims 1 to 3, wherein the first metal layer is integrally formed from the same material as the resistor.

5. The strain gauge according to any one of claims 1 to 4, wherein the electrode comprises the first metal layer and the second metal layer formed on the first metal layer from a material having lower resistance than the first metal layer.

6. The strain gauge according to any one of claims 1 to 5, wherein the second metal layer is formed from a material having better elasticity than the first metal layer.

7. The strain gauge according to any one of claims 1 to 6, wherein the width of the first metal layer is 10 μm or more and 100 μm or less.

8. The resistor is made of Cr, CrN, and Cr ２ A strain gauge according to any one of claims 1 to 7, which is formed from a film containing N.

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

10. The resistor contains CrN and Cr ２ The strain gauge according to claim 8 or 9, wherein N is 20% by weight or less.

11. The CrN and the Cr ２ The Cr in N ２ 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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