Strain gauge

Abstract

To provide a strain gauge with a resistor formed on a flexible substrate, which features reduced warping.SOLUTION: A strain gauge provided herein comprises a flexible resin substrate, a functional layer formed of a metal, alloy, or metal compound directly on one surface of the substrate, a resistor principally made up of α-Cr and formed of a film containing Cr, CrN, and Cr2N directly on one surface of the functional layer, and an insulation layer formed on the resistor, where the functional layer features a function of promoting α-Cr crystal growth to form a film principally made up of α-Cr. The resistor has a thickness of 0.05 to 2 μm, inclusive, and the functional layer has a thickness of 1 to 100 nm, inclusive. The insulation layer has an expansion coefficient that is identical to that of the substrate.SELECTED DRAWING: Figure 3

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 a measurement object to detect the strain of the measurement object. The strain gauge includes a resistor that detects the strain, and the resistor is made of a material that contains, for example, Cr (chromium) or Ni (nickel). The resistor is formed on a base material made of, for example, insulating resin (see Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2016-74934 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, unlike the case of using a substrate made of a material with high mechanical strength such as ceramics, the use of a flexible substrate can cause a problem of warping in the strain gauge. If the strain gauge warps, cracks may occur in the resistor, deteriorating the characteristics of the gauge or causing it to no longer function as a strain gauge.

[0005] The present invention has been made in view of the above-mentioned points, and has an object to reduce warping in a strain gauge having a resistor formed on a flexible substrate. [Means for solving the problem]

[0006] This strain gauge comprises a flexible resin substrate, a functional layer formed directly on one side of the substrate from a metal, alloy, or metal compound, a resistor whose main component is α-Cr and formed from a film containing Cr, CrN, and Cr2N directly on one side of the functional layer, and an insulating layer formed on the resistor, wherein the functional layer has the function of promoting crystal growth of the α-Cr and forming a film whose main component is α-Cr, the resistor has a thickness of 0.05 μm or more and 2 μm or less, the functional layer has a thickness of 1 nm or more and 100 nm or less, and the insulating layer has an expansion coefficient identical to that of the substrate. Effect of the Invention

[0007] According to the disclosed technique, it is possible to reduce warping in a strain gauge having a resistor formed on a flexible substrate. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a plan view illustrating a strain gauge according to a first embodiment. [Diagram 2] 1 is a cross-sectional view (part 1) illustrating a strain gauge according to a first embodiment. [Diagram 3] 4 is a cross-sectional view (part 2) illustrating the strain gauge according to the first embodiment. FIG. [Figure 4] 3A to 3C are diagrams illustrating a manufacturing process of the strain gauge according to the first embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In the drawings, the same components are denoted by the same reference numerals, and duplicated 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, showing a cross section along line AA in Fig. 1. Fig. 3 is a cross-sectional view illustrating the strain gauge according to the first embodiment, showing a cross section along line BB in Fig. 1. With reference to Figs. 1 to 3, the strain gauge 1 has a substrate 10, a resistor 30, a terminal portion 41, and an insulating layer 50.

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

[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.

[0015] The resistor 30 is a thin film formed in a predetermined pattern on the substrate 10, and is a sensing part that generates a resistance change when strained. 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 matte pattern.

[0016] The resistor 30 can be formed, for example, from 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 Ni-Cu (nickel copper). An example of a material containing both Cr and Ni is Ni-Cr (nickel chromium).

[0017] Here, the Cr mixed phase film is a film in which Cr, CrN, Cr2N, etc. are mixed. The Cr mixed phase film may contain inevitable impurities such as chromium oxide.

[0018] The thickness of resistor 30 is not particularly limited and can be appropriately selected depending on the purpose, and can be, for example, about 0.05 μm to 2 μm. In particular, a thickness of resistor 30 of 0.1 μm or more is preferable in that the crystallinity of the crystals constituting resistor 30 (for example, the crystallinity of α-Cr) is improved, and a thickness of 1 μm or less is even more preferable in that film cracks and warping from substrate 10 caused by internal stress of the film constituting resistor 30 can be reduced.

[0019] For example, when the resistor 30 is a Cr mixed phase film, the stability of the gauge characteristics can be improved by making α-Cr (alpha chromium) which is a stable crystal phase the main component. Furthermore, by making α-Cr the main component of the resistor 30, 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 in the range of -1000 ppm / °C to +1000 ppm / °C. Here, the main component means that the target substance accounts for 50 mass% or more of the total substance constituting the resistor, but from the viewpoint of improving the gauge characteristics, it is preferable that the resistor 30 contains α-Cr at 80 weight% or more. Note that α-Cr is Cr with a bcc structure (body-centered cubic lattice structure).

[0020] The terminal portions 41 extend from both ends of the resistor 30 and are formed in a substantially rectangular shape wider than the resistor 30 in a plan view. The terminal portions 41 are a pair of electrodes for outputting a change in the resistance value of the resistor 30 caused by strain to the outside, and for example, a lead wire for external connection is joined to the terminal portions 41. For example, the resistor 30 extends from one of the terminal portions 41 while folding back in a zigzag manner and is connected to the other terminal portion 41. The upper surface of the terminal portion 41 may be covered with a metal having better solderability than the terminal portion 41. Although the resistor 30 and the terminal portion 41 are denoted by different reference numerals for convenience, the two can be integrally formed from the same material in the same process.

[0021] The insulating layer 50 is formed on the substrate 10 so as to cover the upper surfaces (the surfaces opposite the substrate 10) of the resistor 30 and the terminal portion 41 and to expose the side surfaces. The insulating layer 50 has an opening 50x, and a part of the upper surface of the terminal portion 41 is exposed in the opening 50x. However, the entire upper surface of the terminal portion 41 may be exposed in the opening 50x. The planar shape of the insulating layer 50 is substantially the same as the planar shape of the substrate 10.

[0022] The material and thickness of the insulating layer 50 are selected so that the coefficient of expansion of the insulating layer 50 is the same as that of the substrate 10. The material of the insulating layer 50 can be appropriately selected from, for example, the materials exemplified as the materials of the substrate 10. However, the insulating layer 50 does not necessarily have to be made of the same material as the substrate 10. For example, the substrate 10 may be made of polyimide resin, and the insulating layer 50 may be made of epoxy resin. The coefficient of expansion of the insulating layer 50 may be adjusted by including a filler in the insulating layer 50 and selecting the material of the filler included in the insulating layer 50 and adjusting the content thereof.

[0023] In this application, the expansion coefficient of the insulating layer 50 being the same as the expansion coefficient of the substrate 10 includes not only the case where the expansion coefficient of the insulating layer 50 is completely the same as the expansion coefficient of the substrate 10, but also the case where the expansion coefficient of the insulating layer 50 is substantially the same as the expansion coefficient of the substrate 10.

[0024] Here, the case where the expansion coefficient of the insulating layer 50 is substantially the same as that of the substrate 10 refers to the case where the expansion coefficient of the insulating layer 50 is within a range of ±100 ppm / K relative to the expansion coefficient of the substrate 10. This range was experimentally determined by the inventors, and within this range, the internal stress of the resistor 30 can be reduced and the warpage of the strain gauge 1 can be reduced to the limit value at which the strain gauge 1 can function or less.

[0025] FIG. 4 is a diagram illustrating a manufacturing process of the strain gauge according to the first embodiment, where FIG. 4(a) to FIG. 4(c) show a cross section corresponding to FIG. 3, and FIG. 4(d) shows a cross section corresponding to FIG. 2.

[0026] 4(a), a substrate 10 is prepared, and a metal layer 300 is formed on the upper surface 10a of the substrate 10. The metal layer 300 is a layer that is finally patterned to become the resistor 30 and the terminal portion 41. Therefore, the material and thickness of the metal layer 300 are the same as those of the resistor 30 and the terminal portion 41 described above.

[0027] The metal layer 300 can be formed by, for example, a magnetron sputtering method using a target made of a raw material capable of forming the metal layer 300. Instead of the magnetron sputtering method, the metal layer 300 may be formed by a reactive sputtering method, a vapor deposition method, an arc ion plating method, a pulsed laser deposition method, or the like.

[0028] From the viewpoint of stabilizing the gauge characteristics, it is preferable to vacuum-form a functional layer having a thickness of about 1 nm to 100 nm on the upper surface 10a of the base material 10 by, for example, conventional sputtering as a base layer before forming the metal layer 300.

[0029] In the present application, the functional layer refers to a layer having a function of promoting crystal growth of at least the upper layer, the metal layer 300 (resistor 30). The functional layer preferably further has a function of preventing oxidation of the metal layer 300 due to oxygen and moisture contained in the base material 10, and a function of improving adhesion between the base material 10 and the metal layer 300. The functional layer may further have other functions.

[0030] Since the insulating resin film that constitutes the substrate 10 contains oxygen and moisture, and since Cr forms a self-oxidized film, particularly when the metal layer 300 contains Cr, it is effective for the functional layer to have the function of preventing oxidation of the metal layer 300.

[0031] 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 metal layer 300 (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), etc. Examples of the metals that can be used include one or more metals selected from the group consisting of copper (II), iron (Fe), molybdenum (Mo), tungsten (W), ruthenium (Ru), Rh (Rhodium), Re (Rhenium), Os (Osmium), Ir (Iridium), Pt (Platinum), Pd (Palladium), Ag (Silver), Au (Gold), Co (Cobalt), Mn (Manganese), and aluminum (Aluminum), an alloy of any of the metals in this group, or a compound of any of the metals in this group.

[0032] Examples of the alloy include FeCr, TiAl, FeNi, NiCr, CrCu, etc. Examples of the compound include TiN, TaN, Si3N4, TiO2, Ta2O5, SiO2, etc.

[0033] The functional layer can be formed in vacuum by conventional sputtering, for example, using a raw material capable of forming the functional layer as a target 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, so that the amount of the functional layer formed can be minimized and the effect of improving adhesion can be obtained.

[0034] 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 a 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.

[0035] The combination of the material of the functional layer and the material of the metal layer 300 is not particularly limited and 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 containing α-Cr (alpha chromium) as the metal layer 300.

[0036] In this case, for example, the metal layer 300 can be formed by magnetron sputtering using a raw material capable of forming a Cr mixed phase film as a target and introducing Ar gas into a chamber. Alternatively, the metal layer 300 may be formed by reactive sputtering using pure Cr as a target and introducing an appropriate amount of nitrogen gas together with Ar gas into a chamber.

[0037] In these methods, the growth surface of the Cr mixed-phase film is determined by the functional layer made of Ti, and a Cr mixed-phase film containing α-Cr, which has a stable crystal structure, as a main component, can be formed. Furthermore, the Ti constituting the functional layer is diffused into the Cr mixed-phase film, thereby improving the gauge characteristics. For example, the gauge factor of the strain gauge 1 can be set to 10 or more, and the gauge factor temperature coefficient TCS and the resistance temperature coefficient TCR can be set within the range of -1000 ppm / °C to +1000 ppm / °C. When the functional layer is made of Ti, the Cr mixed-phase film may contain Ti and TiN (titanium nitride).

[0038] When the metal layer 300 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 300, preventing oxidation of the metal layer 300 due to oxygen and moisture contained in the base material 10, and improving adhesion between the base material 10 and the metal layer 300. The same applies when Ta, Si, Al, or Fe is used as the functional layer instead of Ti.

[0039] In this way, by providing a functional layer below the metal layer 300, it is possible to promote crystal growth of the metal layer 300, and to produce a metal layer 300 consisting of a stable crystal phase. As a result, it is possible to improve the stability of the gauge characteristics of the strain gauge 1. In addition, the material constituting the functional layer diffuses into the metal layer 300, thereby improving the gauge characteristics of the strain gauge 1.

[0040] 4(b), an insulating layer 50 is formed on the base material 10 to cover the metal layer 300. The material and thickness of the insulating layer 50 are as described above. The insulating layer 50 can be produced, for example, by laminating a semi-cured thermosetting insulating resin film on the base material 10 so as to cover the metal layer 300, and then heating and curing the film. The insulating layer 50 may also be produced by applying a liquid or paste thermosetting insulating resin on the base material 10 so as to cover the metal layer 300, and then heating and curing the film.

[0041] 4(c), the metal layer 300 is patterned to form the functional layer, resistor 30, and terminal portion 41 having the planar shape of FIG. 1. For example, unnecessary portions of the metal layer 300 can be removed by a laser processing method in which laser light having a wavelength that is transmitted through the insulating layer 50 and easily absorbed by the metal layer 300 is irradiated onto the metal layer 300 through the insulating layer 50.

[0042] Patterning the metal layer 300 by laser processing eliminates the need for a step of etching the metal layer 300 with an etching solution, and therefore it is possible to prevent the resistor 30 and the substrate 10 from being corroded by the etching solution.

[0043] 4(d), an opening 50x is formed in the insulating layer 50, and a part of the upper surface of the terminal portion 41 is exposed in the opening 50x. However, the entire upper surface of the terminal portion 41 may be exposed in the opening 50x. The opening 50x can be formed by, for example, a laser processing method using a laser beam having a wavelength that is easily absorbed by the insulating layer 50. A photosensitive resin may be used as the insulating layer 50, and the opening 50x may be formed by a photolithography method.

[0044] Alternatively, in the step shown in Fig. 4(b), an insulating resin film in which the openings 50x have been formed in advance may be laminated and heated to harden, forming the insulating layer 50 having the openings 50x. In this case, the step shown in Fig. 4(d) is unnecessary. The strain gauge 1 is completed by the above steps.

[0045] In this way, by sandwiching the resistor 30 between the substrate 10 and the insulating layer 50 and making the expansion coefficient of the insulating layer 50 the same as that of the substrate 10, it is possible to reduce warping of the strain gauge 1. As a result, it is possible to prevent cracks from occurring in the resistor 30 due to warping of the strain gauge 1. Furthermore, since warping of the strain gauge 1 is reduced, the strain gauge 1 can function stably while maintaining good gauge characteristics.

[0046] Furthermore, since the insulating layer 50 is formed on the resistor 30, handling is made easy.

[0047] In addition, by forming the insulating layer 50 and then patterning the metal layer 300 by laser processing to form the resistor 30, the heat generated during the laser processing is dissipated to the substrate 10 and the insulating layer 50, so that it is possible to prevent the formation of protrusions or the like on the resistor 30 after processing.

[0048] Furthermore, by forming the resistor 30 by patterning the metal layer 300 by a laser processing method, the process of etching the metal layer 300 with an etching solution is not required, so that the resistor 30 and the substrate 10 can be prevented from being corroded by the etching solution. As a result, the resistor 30 can be prevented from being broken due to corrosion.

[0049] 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.

[0050] For example, even if the resistor 30 is formed by patterning the metal layer 300 by a method other than that described in the process shown in Figure 4 (c), it is possible to reduce warping of the strain gauge 1 by sandwiching the resistor 30 between the substrate 10 and the insulating layer 50 and making the expansion coefficient of the insulating layer 50 the same as the expansion coefficient of the substrate 10. [Explanation of symbols]

[0051] 1 strain gauge, 10 substrate, 10a top surface, 30 resistor, 41 terminal portion, 50 insulating layer, 50x opening

Claims

1. A flexible resin base material; A functional layer formed directly on one surface of the substrate from a metal, an alloy, or a metal compound; Cr, CrN, and Cr are directly attached to one surface of the functional layer. ２ A resistor mainly composed of α-Cr and formed from a film containing N; an insulating layer formed on the resistor, the functional layer has a function of promoting crystal growth of the α-Cr and forming a film containing the α-Cr as a main component, The resistor has a thickness of 0.05 μm or more and 2 μm or less, The thickness of the functional layer is 1 nm or more and 100 nm or less, A strain gauge, wherein the coefficient of expansion of the insulating layer is the same as the coefficient of expansion of the substrate.

2. The strain gauge according to claim 1 , wherein a side surface of the resistor is exposed from the substrate and the insulating layer.

3. 3. The strain gauge according to claim 1, wherein each of the substrate and the insulating layer is formed from an insulating resin film selected from the group consisting of polyimide resin, epoxy resin, polyether ether ketone resin, polyethylene naphthalate resin, polyethylene terephthalate resin, polyphenylene sulfide resin, and polyolefin resin.

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