Strain gauges

Abstract

To increase stability of gauge characteristics in a strain gauge having a resistor on a flexible base material.SOLUTION: A strain gauge includes: a flexible base material; a functional layer on one surface of the base material made of metal, an alloy, or a metal compound; and a resistor on one surface of the functional layer made of a material containing at least one of chromium and nickel.SELECTED DRAWING: Figure 2

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, when a flexible substrate is used, it is difficult to form a stable resistor on the substrate, and there is a problem in that the gauge characteristics (gauge factor, temperature coefficient of gauge factor TCS, and temperature coefficient of resistance TCR) lack stability.

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

[0006] This strain gauge is flexible. Made of resin A substrate and a substrate on one surface of the substrate. directly A functional layer formed of a metal, an alloy, or a metal compound, and a directly , Cr, CrN, and Cr 2 N-containing membrane formed from , mainly composed of α-Cr A resistor and The functional layer has a function of promoting the crystal growth of the α-Cr and forming a film mainly composed of the α-Cr, the resistor has a thickness of 0.05 μm or more and 2 μm or less, and the functional layer has a thickness of 1 nm or more and 100 nm or less. . Effect of the Invention

[0007] According to the disclosed technique, it is possible to improve the stability of the gauge characteristics 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 illustrating a strain gauge according to a first embodiment. [Diagram 3] 3A to 3C are diagrams illustrating a manufacturing process of the strain gauge according to the first embodiment. [Figure 4] FIG. 2 shows the results of fluorescent X-ray analysis of the functional layer. [Diagram 5] FIG. 1 is a diagram showing the results of X-ray diffraction of a resistor. 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. Referring to Fig. 1 and Fig. 2, the strain gauge 1 has a substrate 10, a functional layer 20, a resistor 30, and a terminal portion 41.

[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 functional layer 20 is formed on the upper surface 10a of the substrate 10 as a lower layer of the resistor 30. That is, the planar shape of the functional layer 20 is substantially the same as the planar shape of the resistor 30 shown in Fig. 1. The thickness of the functional layer 20 can be, for example, about 1 nm to 100 nm.

[0016] 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 resistor 30. The functional layer 20 preferably further has a function of preventing oxidation of the resistor 30 due to oxygen and moisture contained in the substrate 10, and a function of improving adhesion between the substrate 10 and the resistor 30. The functional layer 20 may further have other functions.

[0017] Since the insulating resin film that constitutes the substrate 10 contains oxygen and moisture, it is effective for the functional layer 20 to have the function of preventing oxidation of the resistor 30, particularly when the resistor 30 contains Cr (chromium), since Cr forms a self-oxidization film.

[0018] The material of the functional layer 20 is not particularly limited as long as it has the function of promoting the crystal growth of at least the upper layer, the 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 metal include one or more metals selected from the group consisting of Fe (iron), Mo (molybdenum), W (tungsten), Ru (ruthenium), Rh (rhodium), Re (rhenium), Os (osmium), Ir (iridium), Pt (platinum), Pd (palladium), Ag (silver), Au (gold), Co (cobalt), Mn (manganese), and Al (aluminum), an alloy of any of the metals in this group, or a compound of any of the metals in this group.

[0019] Examples of the alloy include FeCr, TiAl, FeNi, NiCr, CrCu, etc. Examples of the compound include TiN, TaN, Si 3 N4 , TiO 2 , Ta 2 O 5 , SiO 2 etc.

[0020] The resistor 30 is a thin film formed in a predetermined pattern on the upper surface of the functional layer 20, and is a sensing part that generates a resistance change when strained. For convenience, the resistor 30 is shown in Fig. 1 with a matte finish.

[0021] 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).

[0022] Here, the Cr mixed phase film is composed of Cr, CrN, Cr 2 The Cr mixed-phase film may contain unavoidable impurities such as chromium oxide. In addition, a part of the material constituting the functional layer 20 may be diffused into the Cr mixed-phase film. In this case, the material constituting the functional layer 20 may form a compound with nitrogen. For example, when the functional layer 20 is formed of Ti, the Cr mixed-phase film may contain Ti and TiN (titanium nitride).

[0023] 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.

[0024] By forming the resistor 30 on the functional layer 20, the resistor 30 can be formed using a stable crystalline phase, thereby improving the stability of the gauge characteristics (gauge factor, temperature coefficient of gauge factor TCS, and temperature coefficient of resistance TCR).

[0025] For example, when the resistor 30 is a Cr mixed phase film, the resistor 30 can be formed with α-Cr (alpha chromium) as the main component by providing the functional layer 20. Since α-Cr is a stable crystal phase, the stability of the gauge characteristics can be improved.

[0026] Here, the term "main component" means that the target substance accounts for 50% by mass or more of the total substances constituting the resistor. When the resistor 30 is a Cr mixed phase film, it is preferable that the resistor 30 contains 80% by weight or more of α-Cr from the viewpoint of improving the gauge characteristics. Note that α-Cr is Cr with a bcc structure (body-centered cubic lattice structure).

[0027] Furthermore, the gauge characteristics can be improved by diffusing the metal (e.g., Ti) constituting the functional layer 20 into the Cr mixed phase film. Specifically, 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.

[0028] 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.

[0029] 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 expose the terminal portion 41. By providing the cover layer 60, mechanical damage to the resistor 30 can be prevented. Furthermore, by providing the cover layer 60, the resistor 30 can be protected from moisture and the like. The cover layer 60 may be provided so as to cover the entire portion except for the terminal portion 41.

[0030] 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, composite resin (e.g., silicone resin, polyolefin resin), etc. The cover layer 60 may contain a filler or a pigment. There is no particular limit to the thickness of the cover layer 60 and it can be appropriately selected depending on the purpose, but it can be, for example, about 2 μm to 30 μm.

[0031] Fig. 3 is a diagram illustrating the manufacturing process of the strain gauge according to the first embodiment, showing a cross section corresponding to Fig. 2. To manufacture the strain gauge 1, first, in the step shown in Fig. 3(a), a substrate 10 is prepared, and a functional layer 20 is formed on an upper surface 10a of the substrate 10. The materials and thicknesses of the substrate 10 and the functional layer 20 are as described above.

[0032] The functional layer 20 can be formed in a vacuum by conventional sputtering, for example, using a raw material capable of forming the functional layer 20 as a target and introducing Ar (argon) gas into a chamber. By using conventional sputtering, the functional layer 20 is formed while etching the upper surface 10a of the substrate 10 with Ar, so that the amount of the functional layer 20 formed can be minimized to obtain an effect of improving adhesion.

[0033] However, this is just one example of a method for forming the functional layer 20, and the functional layer 20 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 20, thereby improving adhesion, and then the functional layer 20 is vacuum-formed by magnetron sputtering.

[0034] Next, in the step shown in FIG. 3(b), the resistor 30 and the terminal portion 41 are formed on the entire upper surface of the functional layer 20, and then the functional layer 20, the resistor 30, and the terminal portion 41 are patterned by photolithography into the planar shape shown in FIG. 1. The material and thickness of the resistor 30 and the terminal portion 41 are as described above. The resistor 30 and the terminal portion 41 can be integrally formed from the same material. The resistor 30 and the terminal portion 41 can be formed by, for example, a magnetron sputtering method using a raw material capable of forming the resistor 30 and the terminal portion 41 as a target. The resistor 30 and the terminal portion 41 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 instead of the magnetron sputtering method.

[0035] The combination of the material of the functional layer 20 with the materials of the resistor 30 and the terminal portion 41 is not particularly limited and can be selected appropriately depending on the purpose. For example, it is possible to use Ti for the functional layer 20 and form a Cr mixed phase film with α-Cr (alpha chromium) as the main component for the resistor 30 and the terminal portion 41.

[0036] In this case, for example, the resistor 30 and the terminal portion 41 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 resistor 30 and the terminal portion 41 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 defined by the functional layer 20 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. In addition, the Ti constituting the functional layer 20 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 the temperature coefficient of resistance TCR can be set within the range of -1000 ppm / °C to +1000 ppm / °C.

[0038] When the resistor 30 is a Cr mixed phase film, the functional layer 20 made of Ti has all of the following functions: promoting crystal growth of the resistor 30, preventing oxidation of the resistor 30 due to oxygen and moisture contained in the substrate 10, and improving adhesion between the substrate 10 and the resistor 30. The same applies when Ta, Si, Al, or Fe is used as the functional layer 20 instead of Ti.

[0039] 3(b), the strain gauge 1 is completed by providing a cover layer 60 on the upper surface 10a of the substrate 10 as necessary, the cover layer 60 covering the resistor 30 and exposing the terminal portion 41. The cover layer 60 can be produced, for example, by laminating a semi-cured thermosetting insulating resin film on the upper surface 10a of the substrate 10 so as to cover the resistor 30 and expose the terminal portion 41, and then heating and curing the film. The cover layer 60 may also be produced by applying a liquid or paste thermosetting insulating resin to the upper surface 10a of the substrate 10 so as to cover the resistor 30 and expose the terminal portion 41, and then heating and curing the film.

[0040] In this way, by providing the functional layer 20 under the resistor 30, it is possible to promote crystal growth of the resistor 30, and the resistor 30 made of a stable crystal phase can be manufactured. As a result, the stability of the gauge characteristics of the strain gauge 1 can be improved. In addition, the material constituting the functional layer 20 diffuses into the resistor 30, thereby improving the gauge characteristics of the strain gauge 1.

[0041] [Example 1] First, as a preliminary experiment, a Ti film was vacuum-formed as the functional layer 20 by conventional sputtering on the upper surface 10a of a substrate 10 made of a polyimide resin with a thickness of 25 μm. At this time, five samples were fabricated by forming the Ti film with different thicknesses.

[0042] Next, X-ray fluorescence (XRF) analysis was performed on the five samples produced, obtaining the results shown in Figure 4. The presence of Ti was confirmed from the X-ray peaks in Figure 4, and the X-ray intensity of each sample at the X-ray peak confirmed that the thickness of the Ti film could be controlled in the range of 1 nm to 100 nm.

[0043] Next, as Example 1, a Ti film having a thickness of 3 nm was vacuum-formed as the functional layer 20 by conventional sputtering on the upper surface 10a of the substrate 10 made of polyimide resin having a thickness of 25 μm.

[0044] Next, a Cr mixed phase film was formed as resistor 30 and terminal portion 41 on the entire upper surface of functional layer 20 by magnetron sputtering, and then functional layer 20, resistor 30, and terminal portion 41 were patterned by photolithography as shown in FIG.

[0045] As Comparative Example 1, without forming the functional layer 20 on the upper surface 10a of the substrate 10 made of a 25 μm-thick polyimide resin, a Cr mixed phase film was formed as the resistor 30 and the terminal portion 41 by magnetron sputtering, and patterned by photolithography as shown in Fig. 1. Note that the film formation conditions for the resistor 30 and the terminal portion 41 are all the same in the sample of Example 1 and the sample of Comparative Example 1.

[0046] Next, the sample of Example 1 and the sample of Comparative Example 1 were evaluated by X-ray diffraction (XRD), and the results shown in Fig. 5 were obtained. Fig. 5 shows X-ray diffraction patterns in the 2θ diffraction angle range of 36 to 48 degrees, and the diffraction peak of Example 1 is shifted to the right side compared to the diffraction peak of Comparative Example 1. In addition, the diffraction peak of Example 1 is higher than the diffraction peak of Comparative Example 1.

[0047] The diffraction peak of Example 1 is located near the diffraction line of α-Cr (110), and it is believed that the provision of functional layer 20 made of Ti promoted the crystal growth of α-Cr, resulting in the formation of a Cr mixed phase film mainly composed of α-Cr.

[0048] Next, a plurality of samples of Example 1 and Comparative Example 1 were prepared, and the gauge characteristics were measured. As a result, the gauge factor of each sample of Example 1 was 14 to 16, whereas the gauge factor of each sample of Comparative Example 1 was less than 10.

[0049] Furthermore, while the gauge factor temperature coefficient TCS and the temperature coefficient of resistance TCR of each sample in Example 1 were within the range of -1000 ppm / °C to +1000 ppm / °C, the gauge factor temperature coefficient TCS and the temperature coefficient of resistance TCR of each sample in Comparative Example 1 were not within the range of -1000 ppm / °C to +1000 ppm / °C.

[0050] In this way, by providing the functional layer 20 made of Ti, the crystal growth of α-Cr is promoted to form a Cr mixed phase film mainly composed of α-Cr, and a strain gauge was fabricated with a gauge factor of 10 or more, and a gauge factor temperature coefficient TCS and a resistance temperature coefficient TCR in the range of -1000 ppm / °C to +1000 ppm / °C. It is considered that the diffusion effect of Ti into the Cr mixed phase film contributes to the improvement of the gauge characteristics.

[0051] 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. [Explanation of symbols]

[0052] 1 strain gauge, 10 substrate, 10a upper surface, 20 functional layer, 30 resistor, 41 terminal portion, 60 cover layer

Claims

[Claim 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; a resistor mainly composed of α-Cr, which is formed from a film containing Cr, CrN, and Cr 2 N directly on one surface of the functional layer; 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 thickness of the resistor is 0.05 μm or more and 2 μm or less, A strain gauge, wherein the functional layer has a thickness of 1 nm or more and 100 nm or less.

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