Strain gauge

Abstract

To provide a strain gauge capable of reducing creep. [Solution] This strain gauge comprises a resin base material and a resistor formed on one side of the base material from a film containing Cr, CrN, and Cr2N, wherein the elastic modulus of the base material is greater than 9.8 GPa.

Description

Technical Field

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

Background Art

[0002] Conventionally, a strain gauge that is attached to a measurement object and used is known. For example, a strain gauge may be used as a sensor for detecting the strain of a material or a sensor for detecting the ambient temperature (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] In the strain gauge as described above, a metal base material or a resin base material is used. However, in a strain gauge using a resin base material, creep may occur. Creep is a phenomenon in which strain changes with time when a constant load acts on the strain gauge under certain temperature conditions. In a strain gauge, creep becomes a factor of measurement error.

[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 reducing creep.

Means for Solving the Problems

[0006] A strain gauge according to one embodiment of the present disclosure comprises a resin substrate, a resistor formed on one side of the substrate from a film containing at least one of Cr and Ni, and a protective layer covering the resistor on the one side of the substrate, wherein the substrate has an elastic modulus greater than 9.8 GPa, the surface side of the protective layer is carbonized and / or graphitized, and the protective layer has an elastic modulus greater than 9.8 GPa. [Effects of the Invention]

[0007] According to the disclosed technology, a strain gauge capable of reducing creep can be provided. [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 diagram illustrates the method for measuring creep amount and creep recovery amount. [Figure 4] This figure shows the results of the investigation into creep amount and creep recovery amount. [Figure 5] This is a cross-sectional view (part 2) illustrating a strain gauge according to 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, identical components may be denoted by the same reference numeral. In each drawing, mutually orthogonal X, Y, and Z directions may be defined. In this case, in the X direction, the starting point (root) of the arrow may be referred to as the X- side, and the ending point (arrowhead) of the arrow may be referred to as the X+ side. The same applies to the Y and Z directions. In addition, in the description of each drawing, the description of components that are the same as those already described 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 (part 1) illustrating a strain gauge according to the first embodiment, showing a cross-section along line AA in Figure 1.

[0011] Referring to Figures 1 and 2, the strain gauge 1 comprises a base material 10, a resistor 30, wiring 40, electrodes 50, and a cover layer 60. The cover layer 60 can be provided as needed. For convenience, in Figures 1 and 2, only the outer edge of the cover layer 60 is shown with a dashed line. First, the various parts constituting the strain gauge 1 will be described in detail.

[0012] In this embodiment, for convenience, the side of the base material 10 on which the resistor 30 is provided in the strain gauge 1 is referred to as the "upper side," and the side on which the resistor 30 is not provided is referred to as the "lower side." Furthermore, the surface located above each part is referred to as the "upper surface," and the surface located below each part is referred to as the "lower surface." However, the strain gauge 1 can also be used upside down. Furthermore, the strain gauge 1 can be positioned at any angle. Moreover, a planar view refers to viewing the object in the direction normal to the upper surface 10a of the base material 10 from top to bottom. And the planar shape refers to the shape of the object when viewed in the aforementioned normal direction.

[0013] The base material 10 is a component that serves as a base layer for forming the resistor 30, etc. The base material 10 is flexible. The thickness of the base material 10 is not particularly limited and may be appropriately determined according to the intended use of the strain gauge 1, etc. For example, the thickness of the base material 10 may be about 5 μm to 500 μm. A strain generating body may be joined to the lower surface of the strain gauge 1 via an adhesive layer or the like. From the viewpoint of strain transmission from the surface of the strain generating body to the sensing part and dimensional stability against environmental changes, the thickness of the base material 10 is preferably in the range of 5 μm to 200 μm. Also, from the viewpoint of insulation, the thickness of the base material 10 is preferably 10 μm or more.

[0014] The base material 10 is made of resin and has an elastic modulus greater than 9.8 GPa. Examples of such base material 10 include LCP (liquid crystal polymer) resin, which can have an elastic modulus of up to approximately 30 GPa. Alternatively, the base material 10 may be a resin material whose elastic modulus is improved by surface treatment to a level greater than 9.8 GPa.

[0015] For example, by irradiating a commercially available polyimide film with energy to heat it and carbonizing at least the upper surface 10a side (the side on which the resistor 30 is to be formed) of the substrate 10, the elastic modulus can be increased to more than 9.8 GPa. Note that the elastic modulus of commercially available polyimide films is 9.8 GPa or less. The energy irradiated onto the substrate 10 is, for example, ultraviolet light. The energy irradiated onto the substrate 10 may also be a laser beam.

[0016] Alternatively, the elastic modulus may be increased to more than 9.8 GPa by heating a commercially available polyimide film in a heating furnace or the like to graphitize at least the upper surface 10a side (the side on which the resistor 30 is to be formed) of the substrate 10 (region 10s). The heating temperature when graphitizing a commercially available polyimide film is, for example, around 2000°C to 3000°C. Depending on the heating temperature and other conditions, carbonized and graphitized portions may be present in the polyimide film.

[0017] To increase the elastic modulus of the substrate 10, only the region 10s on the upper surface 10a side of the substrate 10 may be carbonized and / or graphitized, or the entire substrate 10 in the thickness direction may be carbonized and / or graphitized. When the substrate 10 is completely carbonized and / or graphitized, the elastic modulus of the substrate 10 will be approximately 200 GPa.

[0018] Furthermore, if carbonizing and / or graphitizing the entire thickness of the base material 10 reduces its toughness, it is preferable to carbonize and / or graphitize only the upper surface 10a of the base material 10. In this case, it is preferable that the thickness of the region 10s to be carbonized and / or graphitized is 10% or more of the thickness of the base material 10.

[0019] Further, the degree of carbonization and / or graphitization of the base material 10 may be lower from the side of the upper surface 10a toward the side of the lower surface 10b. At this time, the degree of carbonization and / or graphitization of the base material 10 may change continuously from the side of the upper surface 10a toward the side of the lower surface 10b.

[0020] The insulating resin film for carbonization and / or graphitization is not limited to a polyimide film. For example, it may be an insulating resin film such as an epoxy resin, a PEEK (polyether ether ketone) resin, a PEN (polyethylene naphthalate) resin, a PET (polyethylene terephthalate) resin, a PPS (polyphenylene sulfide) resin, or a polyolefin resin. Here, the film refers to a member having a thickness of about 500 μm or less and flexibility.

[0021] The base material 10 may be irradiated with energy and heated, for example, a commercially available LCP resin to flatten the surface. By flattening the surface, the resistor 30 can be uniformly formed on the base material 10, and the electrical stability of the resistor 30 can be increased. Here, flattening means that the surface roughness (Ra) is 30 nm or less.

[0022] The insulating resin film constituting the base material 10 may contain fillers, impurities, or the like. For example, the base material 10 may be formed from an insulating resin film containing fillers such as silica or alumina.

[0023] The elastic modulus can be measured by dynamic viscoelasticity measurement. The elastic modulus referred to in the present application is the elastic modulus of the entire base material 10, not the partial elastic modulus of the carbonized and / or graphitized region.

[0024] The resistor 30 is a thin film formed in a predetermined pattern on one side of the substrate 10 (the upper side in Figures 1 and 2). In the strain gauge 1, the resistor 30 is a sensitive part that changes resistance when strain is applied. The resistor 30 may be formed directly on the upper surface 10a of the substrate 10, or it may be formed on the upper surface 10a of the substrate 10 via another layer. In Figure 1, for convenience, the resistor 30 is shown with a dense, textured pattern.

[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 X direction in the example of Figure 1), and the ends of adjacent elongated sections are connected alternately, so that the whole structure is folded in a zigzag pattern. 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 Y direction in the example of Figure 1).

[0026] In the resistor 30, the X-side end of the elongated portion located furthest to the Y+ side bends in the Y+ direction and reaches one end 30e1 in the grid width direction of the resistor 30. Similarly, the X-side end of the elongated portion located furthest to the Y- side bends in the Y- direction and reaches the other end 30e2 in the grid direction of the resistor 30. Each end 30e1 and 30e2 is electrically connected to the electrode 50 via the wiring 40. In other words, the wiring 40 electrically connects each end 30e1 and 30e2 in the grid width direction of the resistor 30 to each electrode 50.

[0027] 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 multiphase 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).

[0028] Here, a Cr multiphase film is a film in which Cr, CrN, and Cr2N are mixed. The Cr multiphase film may contain unavoidable impurities such as chromium oxide.

[0029] The thickness of the resistor 30 is not particularly limited and may be determined appropriately depending on the intended use of the strain gauge 1. For example, the thickness of the resistor 30 may be approximately 0.05 μm to 2 μm. In particular, when the thickness of the resistor 30 is 0.1 μm or more, the crystallinity of the crystal constituting the resistor 30 (for example, the crystallinity of α-Cr) is improved. Also, when the thickness of the resistor 30 is 1 μm or less, (i) cracks in the film and (ii) warping of the film from the substrate 10, caused by internal stress in the film constituting the resistor 30, are reduced.

[0030] Considering the need to minimize lateral sensitivity and prevent wire breakage, the width of the resistor 30 is preferably 10 μm or more and 100 μm or less. More specifically, the width of the resistor 30 is preferably 10 μm or more and 70 μm or less, and more preferably 10 μm or more and 50 μm or less.

[0031] For example, if the resistor 30 is a Cr multiphase film, the stability of the gauge characteristics can be improved by making α-Cr (alpha-chromium), a stable crystalline phase, the main component. Also, for example, if the resistor 30 is a Cr multiphase film, by making α-Cr the main component of the resistor 30, the gauge factor of strain gauge 1 can be set to 10 or more, and the gauge factor temperature coefficient TCS and resistance temperature coefficient TCR can be set within the range of -1000 ppm / ℃ to +1000 ppm / ℃. Here, "main component" means a component that accounts for 50% by weight or more of the total material constituting the resistor. From the viewpoint of improving gauge characteristics, it is preferable that the resistor 30 contains 80% by weight or more of α-Cr. Furthermore, from the same viewpoint, it is even more preferable that the resistor 30 contains 90% by weight or more of α-Cr. Note that α-Cr is Cr with a bcc structure (body-centered cubic lattice structure).

[0032] Furthermore, if the resistor 30 is a Cr multiphase film, it is preferable that the amount of CrN and Cr2N contained in the Cr multiphase film be 20% by weight or less. By having CrN and Cr2N contained in the Cr multiphase film be 20% by weight or less, the decrease in the gauge factor of the strain gauge 1 can be suppressed.

[0033] Furthermore, in the Cr multiphase film, it is preferable that the ratio of CrN to Cr2N is such that the proportion of Cr2N is 80% or more and less than 90% by weight relative to the total weight of CrN and Cr2N. More preferably, the ratio is such that the proportion of Cr2N is 90% or more and less than 95% by weight relative to the total weight of CrN and Cr2N. Cr2N has semiconducting properties. Therefore, by setting the proportion of Cr2N to 90% or more and less than 95% by weight as described above, the decrease in TCR (negative TCR) becomes even more pronounced. Moreover, by setting the proportion of Cr2N to 90% or more and less than 95% by weight as described above, the ceramicization of the resistor 30 is reduced, making brittle fracture of the resistor 30 less likely to occur.

[0034] On the other hand, CrN has the advantage of being chemically stable. By including more CrN in the Cr multiphase film, the possibility of unstable nitrogen generation can be reduced, thus enabling the creation of a stable strain gauge. Here, "unstable nitrogen" refers to trace amounts of N2 or atomic nitrogen that may be present in the Cr multiphase film. These unstable nitrogen atoms may escape from the film depending on the external environment (e.g., high temperature environment). When unstable nitrogen atoms escape from the film, the film stress of the Cr multiphase film may change.

[0035] In strain gauge 1, using a Cr multiphase film as the material for the resistor 30 enables higher sensitivity and miniaturization. For example, while the output of a conventional strain gauge was approximately 0.04mV / 2V, using a Cr multiphase film as the material for the resistor 30 allows for an output of 0.3mV / 2V or higher. Furthermore, while the size (gauge length × gauge width) of a conventional strain gauge was approximately 3mm × 3mm, using a Cr multiphase film as the material for the resistor 30 allows for miniaturization to approximately 0.3mm × 0.3mm.

[0036] The wiring 40 is provided on the substrate 10. The wiring 40 is electrically connected to the resistor 30 and the electrode 50. The wiring 40 is not limited to a straight line and can be in any pattern. Also, the wiring 40 can have any width and any length. For convenience, in Figure 1, the wiring 40 is shown with a matte pattern that is less dense than the resistor 30.

[0037] The electrode 50 is provided on the substrate 10. The electrode 50 is electrically connected to the resistor 30 via the wiring 40. In a plan view, the electrode 50 is wider than the wiring 40 and is formed in a substantially rectangular shape. The electrode 50 is a pair of electrodes for outputting to the outside the change in the resistance value of the resistor 30 caused by strain. For example, lead wires for external connection are joined to the electrode 50. A low-resistance metal layer such as copper, or a metal layer with good solderability such as gold, may be laminated on the upper surface of the electrode 50. For convenience, the resistor 30, wiring 40, and electrode 50 are given different reference numerals, but both can be formed integrally from the same material in the same process. In Figure 1, for convenience, the electrode 50 is shown with a textured pattern of the same density as the wiring 40.

[0038] The cover layer 60 (protective layer) is provided on the upper surface 10a of the substrate 10 as needed, covering the resistor 30 and wiring 40 and exposing the electrodes 50. Examples of materials for the cover layer 60 include insulating resins such as PI resin, epoxy resin, PEEK resin, PEN resin, PET resin, PPS resin, and composite resins (e.g., silicone resin, polyolefin resin). The cover layer 60 may also contain fillers and pigments. The thickness of the cover layer 60 is not particularly limited and can be appropriately selected according to the purpose. For example, the thickness of the cover layer 60 can be about 2 μm to 30 μm. By providing the cover layer 60, mechanical damage to the resistor 30 can be suppressed. In addition, by providing the cover layer 60, the resistor 30 can be protected from moisture and other elements.

[0039] [Reducing creep] The strain gauge 1 preferably has excellent creep characteristics. That is, it is preferable that the amount of creep and the amount of creep recovery of the strain gauge 1 are small. For example, if the amount of creep and the amount of creep recovery can be reduced to below a predetermined value, the strain gauge 1 can be used not only for sensor applications but also for weighing applications.

[0040] When strain gauges are used for weighing purposes, they must satisfy creep standards. Creep standards include, for example, accuracy grades C1 (hereinafter referred to as C1 standard), C2 (hereinafter referred to as C2 standard), and C3 (hereinafter referred to as C3 standard) based on OIML R60.

[0041] 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. The C3 standard requires that the creep amount and creep recovery amount be ±0.016% 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%.

[0042] The creep amount and creep recovery amount of a strain gauge are affected by the viscoelasticity of the constituent material. Generally, creep does not occur in metal materials, which are elastic materials, but it does occur in resins, which are viscous materials. Since strain gauge 1 uses a resin base material 10, the viscosity of the base material 10 cannot be ignored.

[0043] The creep amount and creep recovery amount are determined by the change in the amount of elastic deformation (strain) of the surface of the base material 10 on which the resistor 30 is provided, over time in the strain gauge 1. Therefore, the creep amount and creep recovery amount can be measured by monitoring the strain voltage calculated based on the output between the pair of electrodes 50 of the strain gauge 1. This will be explained in detail with reference to Figure 3.

[0044] Figure 3 illustrates the method for measuring creep and creep recovery. In Figure 3, the horizontal axis represents time, and the vertical axis represents strain voltage [mV].

[0045] First, after powering on the measuring device for 10 seconds, a 150% load is applied to strain gauge 1 attached to the strain generating body for 10 seconds, and then the load is removed. After 20 minutes have passed since the load was removed, a 100% load is applied to strain gauge 1 attached to the strain generating body for 20 minutes, and then the load is removed. Then, wait for another 20 minutes to pass after the load has been removed.

[0046] The strain voltage changes, for example, as shown in Figure 3. In Figure 3, the absolute value B of the difference in strain voltage is measured between 20 minutes after the 150% load is removed and immediately after the 100% load is applied. Also, the absolute value ΔA of the difference in strain voltage is measured between immediately after the 100% load is applied and 20 minutes after the 100% load is applied. In this case, ΔA / B is the creep amount. Next, the absolute value ΔC of the difference in strain voltage is measured between immediately after the 100% load is removed and 20 minutes after the 100% load is removed. In this case, ΔC / B is the creep recovery amount.

[0047] Note that 100% load is 3 kg, and 150% load is 1.5 times the load of 100% load.

[0048] Figure 4 shows the results of the investigation into creep amount and creep recovery amount, and summarizes the measured results as follows.

[0049] First, two types of measurement samples, Measurement Sample A and Measurement Sample B, were prepared as measurement samples for strain gauges. The only difference between Measurement Sample A and Measurement Sample B is whether or not the polyimide film used as the base material has been surface-treated. Specifically, this is as follows.

[0050] In measurement sample A, a 25 μm thick polyimide film with no surface treatment was used as the substrate 10. A Cr multiphase film was used as the resistor 30. A 15 μm thick polyimide film with no surface treatment was used as the cover layer 60. The elastic modulus of the substrate in measurement sample A was measured to be 9.8 GPa.

[0051] In measurement sample B, a 25 μm thick polyimide film with surface treatment was used as the substrate 10. That is, at least the upper surface of the substrate used in measurement sample B was carbonized and / or graphitized. A Cr multiphase film was used as the resistor 30. A 15 μm thick polyimide film without surface treatment was used as the cover layer 60. The elastic modulus of the substrate in measurement sample B was measured to be 11.8 GPa.

[0052] Next, measurement samples A and B were attached to separate strain-generating bodies made of SUS304, and the creep amount and creep recovery amount were measured using the measurement method shown in Figure 3. Based on the inventor's finding that the elastic modulus of the substrate is approximately proportional to the creep amount and creep recovery amount, and the measurement results for measurement samples A and B, the results shown in Figure 4 were obtained. In Figure 4, the data indicated by circles are from measurement sample A, and the data indicated by squares are from measurement sample B.

[0053] As shown in Figure 4, creep and creep recovery decrease as the elastic modulus of the substrate increases. For example, if the elastic modulus of the substrate is 10.4 GPa or higher, the creep and creep recovery amounts of the C1 standard can be satisfied. Furthermore, if the elastic modulus of the substrate is 12.6 GPa or higher, the creep and creep recovery amounts of the C2 standard can be satisfied. Furthermore, if the elastic modulus of the substrate is 14.7 GPa or higher, the creep and creep recovery amounts of the C3 standard can be satisfied.

[0054] Thus, by using a polyimide film with an elastic modulus greater than 9.8 GPa as the base material 10 in the strain gauge 1, it is possible to reduce creep compared to when using commercially available polyimide films with an elastic modulus of 9.8 GPa or less. In particular, by using a polyimide film with an elastic modulus greater than 10.4 GPa as the base material 10 in the strain gauge 1, the strain gauge 1 can also be used for weighing applications.

[0055] Here, we have shown experimental results using polyimide film, but as mentioned above, the insulating resin film subjected to carbonization and / or graphitization is not limited to polyimide film, and other insulating resin films may be used. In this case as well, if the elastic modulus of the substrate is 10.4 GPa or higher, the creep amount and creep recovery amount of the C1 standard can be satisfied; if it is 12.6 GPa or higher, the C2 standard; and if it is 14.7 GPa or higher, the C3 standard can be satisfied. For example, with LCP (liquid crystal polymer), the elastic modulus can be increased to a maximum of about 30 GPa, thereby reducing creep.

[0056] Furthermore, in the case of highly sensitive strain gauges with a gauge factor of 10 or higher (for example, when a Cr multiphase film is used for the resistor 30), the high sensitivity makes them highly susceptible to the influence of material properties, and the creep characteristics may be significantly reduced. Therefore, in highly sensitive strain gauges with a gauge factor of 10 or higher, it is extremely important to improve the creep characteristics by controlling the elastic modulus of the substrate.

[0057] [Method of manufacturing strain gauges] The manufacturing method of the strain gauge 1 is described below. To manufacture the strain gauge 1, first, an insulating resin film with an elastic modulus greater than 9.8 GPa is prepared as the base material 10. Then, 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, wiring 40, and electrode 50. Therefore, the material and thickness of metal layer A are the same as those of the resistor 30, wiring 40, and electrode 50 described above.

[0058] The metal layer A can be deposited, for example, by a magnetron sputtering method targeting a raw material capable of forming the metal layer A. Alternatively, the metal layer A may be deposited using reactive sputtering, vapor deposition, arc ion plating, or pulsed laser deposition instead of magnetron sputtering. After depositing the metal layer A on the upper surface 10a of the substrate 10, the metal layer A is patterned into a planar shape similar to the resistor 30, wiring 40, and electrode 50 in Figure 1 using a well-known photolithography method.

[0059] Alternatively, a base layer may be formed on the upper surface 10a of the substrate 10 before forming the metal layer A. For example, a functional layer of a predetermined thickness may be vacuum-deposited on the upper surface 10a of the substrate 10 by conventional sputtering. By providing a base layer in this way, the gauge characteristics of the strain gauge 1 can be stabilized.

[0060] 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 or moisture contained in the substrate 10, and / or the function of improving the adhesion between the substrate 10 and the metal layer A. The functional layer may further have other functions.

[0061] The insulating resin film constituting the base material 10 may contain oxygen and moisture, and Cr may form an oxidized film. Therefore, especially when metal layer A contains Cr, it is preferable to form a functional layer that has the function of preventing oxidation of metal layer A.

[0062] In this way, 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 is improved. Furthermore, the diffusion of the material constituting the functional layer into the metal layer A improves the gauge characteristics in the strain gauge 1.

[0063] Examples of materials for the functional layer include one or more metals selected from the group consisting of 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), 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.

[0064] Figure 5 is a cross-sectional view (part 2) illustrating a strain gauge according to the first embodiment. Figure 5 shows the cross-sectional shape of the strain gauge 1 when a functional layer 20 is provided as a base layer for the resistor 30, wiring 40, and electrode 50.

[0065] The planar shape of the functional layer 20 may be patterned to be substantially the same as the planar shapes of, for example, the resistor 30, the wiring 40, and the electrodes 50. However, the planar shapes of the functional layer 20 and the resistor 30, the wiring 40, and the electrodes 50 do not have to be substantially the same. For example, if the functional layer 20 is formed from an insulating material, the functional layer 20 may be patterned to be different from the planar shapes of the resistor 30, the wiring 40, and the electrodes 50. In this case, the functional layer 20 may be formed as a solid in the region where the resistor 30, the wiring 40, and the electrodes 50 are formed. Alternatively, the functional layer 20 may be formed as a solid over the entire upper surface of the substrate 10.

[0066] After forming the resistor 30, wiring 40, and electrode 50, a cover layer 60 is formed on the upper surface 10a of the base material 10 as needed. The cover layer 60 covers the resistor 30 and wiring 40, but the electrode 50 may be exposed from the cover layer 60. For example, the cover layer 60 can be formed by laminating a semi-cured thermosetting insulating resin film onto the upper surface 10a of the base material 10 so as to cover the resistor 30 and wiring 40 and expose the electrode 50, and then heating and curing the insulating resin film. Through the above steps, the strain gauge 1 is completed.

[0067] <Variation 1 of the First Embodiment> Modification 1 of the first embodiment shows an example in which a material with a high elastic modulus is used for the cover layer. 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.

[0068] In the first embodiment, a general resin material was used as the cover layer 60. In this case, the elastic modulus of the cover layer 60 is 9.8 GPa or less. However, it is preferable that the elastic modulus of the cover layer 60, in addition to that of the base material 10, is greater than 9.8 GPa. This further improves the creep characteristics of the strain gauge 1.

[0069] For example, an insulating resin film such as a polyimide film with an elastic modulus greater than 9.8 GPa can be used as the cover layer 60. In this case, the elastic modulus can be made greater than 9.8 GPa by applying the surface treatment shown in the first embodiment to the surface of a commercially available insulating resin film such as a polyimide film. Alternatively, for example, an LCP (liquid crystal polymer) resin with an elastic modulus of up to about 30 GPa can be used as the cover layer 60.

[0070] Furthermore, an inorganic material may be used as the cover layer 60. Examples of inorganic materials include oxides, nitrides, and nitrogen oxides of metals such as Cu, Cr, Ni, Al, Fe, W, Ti, and Ta, and alloys containing them. Semiconductors such as Si and Ge, and their oxides, nitrides, and nitrogen oxides may also be used as inorganic materials. When the cover layer 60 is an inorganic material, it can be formed using dipping, screen printing, sputtering, or CVD.

[0071] Preferred embodiments have been described in detail above. However, the strain gauges relating to this disclosure are not limited to the embodiments and modifications described above. For example, various modifications and substitutions can be made to the strain gauges relating to the embodiments described above without departing from the scope described in the claims. [Explanation of Symbols]

[0072] 1 Strain gauge, 10 Substrate, 10a Top surface, 10b Bottom surface, 10s Region, 20 Functional layer, 30 Resistor, 40 Wiring, 50 Electrode, 60 Cover layer

Claims

1. A resin base material, A resistor is formed on one side of the substrate from a film containing at least one of Cr and Ni, The substrate has a protective layer covering the resistor on one of its surfaces, The aforementioned substrate has an elastic modulus greater than 9.8 GPa. The surface side of the protective layer is carbonized and / or graphitized. The aforementioned protective layer is a strain gauge with an elastic modulus greater than 9.8 GPa.

2. The strain gauge according to claim 1, wherein at least one of the surfaces of the substrate is carbonized and / or graphitized.

3. The strain gauge according to claim 2, wherein the degree of carbonization and / or graphitization in the substrate decreases as you move from one side of the substrate to the other side.

4. The strain gauge according to claim 3, wherein the degree of carbonization and / or graphitization in the substrate changes continuously from one side of the substrate to the other side.

5. The strain gauge according to any one of claims 1 to 4, wherein the base material has an elastic modulus of 10.4 GPa or more.

6. The strain gauge according to any one of claims 1 to 5, wherein the base material has an elastic modulus of 12.6 GPa or more.

7. The strain gauge according to any one of claims 1 to 6, wherein the base material has an elastic modulus of 14.7 GPa or more.

8. A strain gauge according to any one of claims 1 to 7, wherein the gauge factor is 10 or more.

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