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

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

[0004] However, unlike when a substrate made of a material with high mechanical strength such as ceramic is used, when a flexible substrate is used, warping of the strain gauge becomes a problem. Warping of the strain gauge can cause cracks in the resistor, deteriorating the gauge characteristics and causing the strain gauge to cease functioning.

[0005] The present invention has been made in view of the above points, and has an object to reduce warpage 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 CrN 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 the 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 the same expansion coefficient as the substrate. [Effects of the Invention]

[0007] According to the disclosed technology, warping can be reduced in a strain gauge having a resistor formed on a flexible substrate. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a plan view illustrating a strain gauge according to a first embodiment. [Figure 2] 1 is a cross-sectional view (part 1) illustrating a strain gauge according to a first embodiment. FIG. [Figure 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 INVENTION

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

[0010] First Embodiment Fig. 1 is a plan view illustrating a strain gauge according to a first embodiment. Fig. 2 is a cross-sectional view illustrating the strain gauge according to the first embodiment, taken along line AA in Fig. 1. Fig. 3 is a cross-sectional view illustrating the strain gauge according to the first embodiment, taken along line BB in Fig. 1. Referring to Figs. 1 to 3, the strain gauge 1 has a substrate 10, a resistor 30, terminal portions 41, and an insulating layer 50.

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

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

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

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

[0015] 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 strain is applied. The resistor 30 may be formed directly on the upper surface 10a of the substrate 10, or may be formed on the upper surface 10a of the substrate 10 via another layer. For convenience, the resistor 30 is shown in FIG. 1 with a matte finish.

[0016] The resistor 30 can be formed from, for example, a material containing Cr (chromium), a material containing Ni (nickel), or a material containing both Cr and Ni. That is, the resistor 30 can be formed from a material containing at least one of Cr and Ni. An example of a material containing Cr is a Cr mixed phase film. An example of a material containing Ni is 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 containing a mixture of Cr, CrN, Cr2N, etc. 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, but 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 because it improves the crystallinity of the crystals constituting resistor 30 (for example, the crystallinity of α-Cr), and a thickness of 1 μm or less is even more preferable because it reduces cracks in the film constituting resistor 30 and warpage from substrate 10 caused by internal stress in the film.

[0019] For example, when the resistor 30 is a Cr mixed-phase film, the stability of the gauge characteristics can be improved by using α-Cr (alpha chromium), which has a stable crystalline phase, as the main component. Furthermore, by using α-Cr as the main component of the resistor 30, the gauge factor of the strain gauge 1 can be 10 or more, and the temperature coefficient of gauge factor (TCS) and temperature coefficient of resistance (TCR) can be within the range of −1000 ppm / °C to +1000 ppm / °C. Here, “main component” means that the target substance accounts for 50 mass% or more of all materials constituting the resistor. From the viewpoint of improving the gauge characteristics, however, it is preferable that the resistor 30 contains α-Cr at 80 wt% 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 generally 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 are connected to, for example, lead wires for external connection. For example, the resistor 30 extends from one of the terminal portions 41 while folding back in a zigzag pattern and is connected to the other terminal portion 41. The upper surface of the terminal portion 41 may be coated with a metal that has better solderability than the terminal portion 41. Although the resistor 30 and the terminal portion 41 are denoted by different reference numerals for convenience, they 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 openings 50x, and a portion of the upper surface of the terminal portion 41 is exposed in the openings 50x. However, the entire upper surface of the terminal portion 41 may be exposed in the openings 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 the coefficient of expansion of the substrate 10. The material of the insulating layer 50 can be selected as appropriate from, for example, the materials exemplified as materials for 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 incorporating a filler into the insulating layer 50 and adjusting the material and content of the filler contained in the insulating layer 50.

[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 of the expansion coefficient of the substrate 10. This range was determined experimentally 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 a value below the limit at which the strain gauge 1 can function.

[0025] 4A to 4C are diagrams illustrating the manufacturing process of the strain gauge according to the first embodiment, where FIG. 4A to FIG. 4C show cross sections corresponding to FIG. 3, and FIG. 4D 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 will eventually be 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, magnetron sputtering using a target made of a raw material capable of forming the metal layer 300. Instead of magnetron sputtering, the metal layer 300 may be formed by reactive sputtering, vapor deposition, arc ion plating, pulsed laser deposition, or the like.

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

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

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

[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), Examples of the metals include one or more metals selected from the group consisting of copper (iron), iron (Fe), molybdenum (Mo), tungsten (W), ruthenium (Ru), rhodium (Rh), re (rhenium), osmium (Os), iridium (Ir), platinum (Pt), palladium (Pd), silver (Ag), gold (Au), cobalt (Co), manganese (Mn), and aluminum (Al), alloys of any of the metals in this group, and compounds of any of the metals in this group.

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

[0033] The functional layer can be formed in vacuum by conventional sputtering, for example, using a target made of a material capable of forming the functional layer and introducing Ar (argon) gas into a chamber. By using conventional sputtering, the functional layer is formed while etching the upper surface 10a of the substrate 10 with Ar, thereby minimizing the amount of the functional layer formed and achieving an improvement in adhesion.

[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 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] There are no particular restrictions on the combination of the material of the functional layer and the material of the metal layer 300, and they can be selected appropriately depending on the purpose. For example, it is possible to use Ti as the functional layer and form a Cr mixed phase film with α-Cr (alpha chromium) as the main component as the metal layer 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 can 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 Ti functional layer defines the growth plane of the Cr mixed-phase film, allowing the formation of a Cr mixed-phase film primarily composed of α-Cr, which has a stable crystal structure. Furthermore, the Ti constituting the functional layer diffuses into the Cr mixed-phase film, improving the gauge characteristics. For example, the gauge factor of the strain gauge 1 can be set to 10 or more, and the temperature coefficient of gauge factor (TCS) and temperature coefficient of resistance (TCR) can be set within the ranges of -1000 ppm / °C to +1000 ppm / °C. When the functional layer is made of Ti, the Cr mixed-phase film may contain Ti or TiN (titanium nitride).

[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 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 in the metal layer 300, and to produce a metal layer 300 consisting of a stable crystalline phase. As a result, it is possible to improve the stability of the gauge characteristics of the strain gauge 1. Furthermore, by diffusing the material that constitutes the functional layer into the metal layer 300, it is possible to improve the gauge characteristics of the strain gauge 1.

[0040] Next, in the step shown in FIG. 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-like thermosetting insulating resin on the base material 10 so as to cover the metal layer 300, and then heating and curing the resin.

[0041] 4(c), the metal layer 300 is patterned to form the functional layer, resistor 30, and terminal portion 41 having the planar shape shown in 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, the resistor 30 and the substrate 10 can be prevented from being corroded by the etching solution.

[0043] 4(d), an opening 50x is formed in the insulating layer 50, and a portion 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, for example, by a laser processing method using laser light with 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 with openings 50x formed therein may be laminated and heated to harden, thereby forming insulating layer 50 having openings 50x. In this case, the step shown in Fig. 4(d) is unnecessary. Through the above steps, strain gauge 1 is completed.

[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 warpage of the strain gauge 1. As a result, it is possible to prevent cracks from occurring in the resistor 30 due to warpage of the strain gauge 1. Furthermore, since warpage 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 becomes easy.

[0047] Furthermore, 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, thereby preventing 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 laser processing, the step of etching the metal layer 300 with an etching solution is not required, and therefore 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] The above describes in detail preferred embodiments, but 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), the resistor 30 can be sandwiched between the substrate 10 and the insulating layer 50, and the expansion coefficient of the insulating layer 50 can be made the same as the expansion coefficient of the substrate 10, thereby reducing warping of the strain gauge 1. [Explanation of symbols]

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

Claims

1. a flexible resin substrate; a functional layer formed directly on one surface of the substrate from a metal, alloy, or metal compound; Cr, CrN, and Cr are directly applied to one surface of the functional layer. ２ a resistor formed from a film containing N and containing α-Cr as a main component; 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 the substrate and the insulating layer are each 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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