Strain gauge

Abstract

To provide a strain gauge having a resistor formed on a base material having flexibility, which reduces the number of pinholes.SOLUTION: A strain gauge has a resin base material having flexibility, a functional layer formed of metal, an alloy or a metal compound directly on one surface of the base material, and a resistor which is formed of a film containing Cr, CrN and Cr2 N directly on one surface of the functional layer and contains α-Cr as a main component, wherein 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, the thickness of the functional layer is 1 nm or more and 100 nm or less, and surface unevenness of the one surface of the base material is 15 nm or less.SELECTED DRAWING: Figure 3

Description

Technical Field

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

Background Art

[0002] A strain gauge that is attached to a measurement object to detect the strain of the measurement object is known. The strain gauge includes a resistor that detects strain, and as the material of the resistor, for example, a material containing Cr (chromium) or Ni (nickel) is used. Further, the resistor is formed on a base material made of, for example, an insulating resin (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, when a resistor is formed on a flexible base material, pinholes may occur in the resistor. If the number of pinholes generated in the resistor exceeds a predetermined value, there is a risk that the gauge characteristics deteriorate or the strain gauge stops functioning.

[0005] The present invention has been made in view of the above points, and an object thereof is to reduce the number of pinholes in a strain gauge having a resistor formed on a flexible base material.

Means for Solving the Problems

[0006] This strain gauge has a flexible resin substrate, a functional layer formed directly on one surface of the substrate from metal, alloy, or a metal compound, and a resistor mainly composed of α-Cr formed from a film containing Cr, CrN, and Cr2N 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 mainly composed of the α-Cr. The thickness of the resistor is 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, and the surface unevenness of one surface of the substrate is 15 nm or less.

Advantages of the Invention

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

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

Modes for Carrying Out the Invention

[0009] Hereinafter, modes for carrying out the invention will be described with reference to the drawings. In each drawing, the same reference numerals are given to the same components, and redundant descriptions may be omitted.

[0010] 〈First Embodiment〉 FIG. 1 is a plan view illustrating a strain gauge according to the first embodiment. FIG. 2 is a cross-sectional view illustrating a strain gauge according to the first embodiment, showing a cross-section along line A-A in FIG. 1. Referring to FIGS. 1 and 2, the strain gauge 1 has a substrate 10, a resistor 30, and a terminal portion 41.

[0011] In this embodiment, for the sake of convenience, in the strain gauge 1, the side where the resistor 30 of the base material 10 is provided is defined as the upper side or one side, and the side where the resistor 30 is not provided is defined as the lower side or the other side. Also, the surface on the side where the resistor 30 of each part is provided is defined as one surface or the upper surface, and the surface on the side where the resistor 30 is not provided is defined as the other surface or the lower surface. However, the strain gauge 1 can be used in an upside-down state or arranged at an arbitrary angle. Also, the plan view means viewing the object from the normal direction of the upper surface 10a of the base material 10, and the planar shape means the shape of the object viewed from the normal direction of the upper surface 10a of the base material 10.

[0012] The base material 10 is a member serving as a base layer for forming the resistor 30 and the like, and has flexibility. The thickness of the base material 10 is not particularly limited and can be appropriately selected according to the purpose. For example, it can be about 5 μm to 500 μm. In particular, when the thickness of the base material 10 is 5 μm to 200 μm, it is preferable in terms of the transmission of strain from the surface of the strain-generating body joined to the lower surface of the base material 10 via an adhesive layer or the like and the dimensional stability against the environment. When it is 10 μm or more, it is more preferable in terms of insulation.

[0013] The base material 10 can be formed from an insulating resin film such as a PI (polyimide) resin, an epoxy resin, a PEEK (polyetheretherketone) 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 having flexibility.

[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 portion that undergoes strain and causes a resistance change. 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. In FIG. 1, for convenience, the resistor 30 is shown in a matte finish pattern.

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

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

[0018] The thickness of the resistor 30 is not particularly limited and can be appropriately selected according to the purpose. For example, it can be about 0.05 μm to 2 μm. In particular, when the thickness of the resistor 30 is 0.1 μm or more, it is preferable in terms of improving the crystallinity of the crystals constituting the resistor 30 (for example, the crystallinity of α-Cr), and when it is 1 μm or less, it is more preferable in terms of reducing film cracks caused by internal stress of the film constituting the resistor 30 and warping from the substrate 10.

[0019] For example, when the resistor 30 is a Cr mixed-phase film, by using α-Cr (alpha chromium), which is a stable crystal phase, as the main component, the stability of the gauge characteristics can be improved. Also, when the resistor 30 has α-Cr as the main component, the gauge factor of the strain gauge 1 can be 10 or more, and the gauge factor temperature coefficient TCS and the resistance temperature coefficient TCR can be within the range of -1000 ppm / °C to +1000 ppm / °C. Here, the main component means that the target substance occupies 50 mass% or more of all the substances constituting the resistor. From the perspective of improving the gauge characteristics, it is preferable that the resistor 30 contains 80 wt% or more of α-Cr. Note that α-Cr is Cr with a bcc structure (body-centered cubic lattice structure).

[0020] By the way, when the resistor 30 is formed on the base material 10, pinholes may occur in the resistor 30. If the number of pinholes generated in the resistor 30 exceeds a predetermined value, the gauge characteristics may deteriorate or the strain gauge may stop functioning. The inventors have found that one of the causes of pinholes occurring in the resistor 30 is a filler protruding from the upper surface 10a of the base material 10.

[0021] That is, when the base material 10 contains a filler, a part of the filler protrudes from the upper surface 10a of the base material 10, increasing the surface unevenness of the upper surface 10a of the base material 10. As a result, the number of pinholes generated in the resistor 30 formed on the upper surface 10a of the base material 10 increases, which becomes a factor in the deterioration of the gauge characteristics and the like.

[0022] The inventors have found that when the thickness of the resistor 30 is 0.05 μm or more, if the surface unevenness of the upper surface 10a of the base material 10 is 15 nm or less, the number of pinholes generated in the resistor 30 can be suppressed and the gauge characteristics can be maintained.

[0023] That is, when the thickness of the resistor 30 is 0.05 μm or more, from the viewpoint of reducing the number of pinholes generated in the resistor 30 formed on the upper surface 10a of the base material 10 and maintaining the gauge characteristics, the surface unevenness of the upper surface 10a of the base material 10 is preferably 15 nm or less. If the surface unevenness is 15 nm or less, even if the base material 10 contains a filler, it will not lead to deterioration of the gauge characteristics. Note that the surface unevenness of the upper surface 10a of the base material 10 may be 0 nm.

[0024] The surface unevenness of the upper surface 10a of the base material 10 can be reduced, for example, by heating the base material 10. Alternatively, instead of heating the base material 10, a method of irradiating laser light substantially perpendicular to the upper surface 10a of the base material 10 to cut off the convex portions, a method of moving a water cutter or the like parallel to the upper surface 10a of the base material 10 to cut off the convex portions, a method of polishing the upper surface 10a of the base material 10 using a grindstone, or a method of applying pressure while heating the base material 10 (heat press) may be used.

[0025] Note that the surface unevenness refers to the arithmetic mean roughness, and is generally denoted as Ra. The surface unevenness can be measured, for example, by a three-dimensional optical interference method.

[0026] The terminal portions 41 extend from both ends of the resistor 30 and are formed in a substantially rectangular shape that is wider than the resistor 30 in a plan view. The terminal portions 41 are 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 or the like are joined thereto. The resistor 30 extends, for example, while being folded back in a zigzag manner from one of the terminal portions 41 and is connected to the other terminal portion 41. The upper surface of the terminal portion 41 may be coated with a metal having better solderability than the terminal portion 41. Note that although the resistor 30 and the terminal portions 41 are given different reference numerals for convenience, the two can be integrally formed of the same material in the same process.

[0027] 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 or the like to the resistor 30 can be prevented. Also, by providing the cover layer 60, the resistor 30 can be protected from moisture and the like. Note that the cover layer 60 may be provided so as to cover the entire portion except the terminal portion 41.

[0028] The cover layer 60 can be formed of an insulating resin such as, for example, a PI resin, an epoxy resin, a PEEK resin, a PEN resin, a PET resin, a PPS resin, a composite resin (for example, a silicone resin, a polyolefin resin), etc. The cover layer 60 may contain a filler or a pigment. The thickness of the cover layer 60 is not particularly limited and can be appropriately selected according to the purpose, but can be, for example, about 2 μm to 30 μm.

[0029] In order to manufacture the strain gauge 1, first, the base material 10 is prepared, and the resistor 30 and the terminal portion 41 having the planar shape shown in FIG. 1 are formed on the upper surface 10a of the base material 10. The materials and thicknesses 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 of the same material.

[0030] The resistor 30 and the terminal portion 41 can be formed, for example, by forming a film of a raw material capable of forming the resistor 30 and the terminal portion 41 by magnetron sputtering using the raw material as a target and patterning by photolithography. Instead of the magnetron sputtering method, the resistor 30 and the terminal portion 41 may be formed by forming a film using a reactive sputtering method, an evaporation method, an arc ion plating method, a pulsed laser deposition method, or the like.

[0031] From the perspective of stabilizing the gauge characteristics, before forming the resistor 30 and the terminal portion 41, it is preferable to vacuum deposit a functional layer with a film thickness of about 1 nm to 100 nm on the upper surface 10a of the base material 10 as an underlayer by, for example, the conventional sputtering method. Note that after forming the resistor 30 and the terminal portion 41 over the entire upper surface of the functional layer, the functional layer is patterned into the planar shape shown in FIG. 1 together with the resistor 30 and the terminal portion 41 by photolithography.

[0032] 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 resistor 30. The functional layer preferably further has a function of preventing oxidation of the resistor 30 due to oxygen or moisture contained in the base material 10 and a function of improving the adhesion between the base material 10 and the resistor 30. The functional layer may further have other functions.

[0033] Since the insulating resin film constituting the base material 10 contains oxygen and moisture, particularly when the resistor 30 contains Cr, Cr forms a self-oxidized film, so it is effective for the functional layer to have a function of preventing oxidation of the resistor 30.

[0034] The material of the functional layer is not particularly limited as long as it has a function of promoting the crystal growth of at least the upper-layer resistor 30 and can be appropriately selected according to the purpose. For example, Cr (chromium), Ti (titanium), V (vanadium), Nb (niobium), Ta (tantalum), Ni (nickel), Y (yttrium), Zr (zirconium), Hf (hafnium), Si (silicon), C (carbon), Zn (zinc), Cu (copper), Bi (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), Al (aluminum), one or more metals selected from the group consisting of these, an alloy of any of these metals in this group, or a compound of any of these metals in this group can be mentioned.

[0035] Examples of the above alloy include FeCr, TiAl, FeNi, NiCr, CrCu, etc. Examples of the above compound include TiN, TaN, Si3N4, TiO2, Ta2O5, SiO2, etc.

[0036] The functional layer can be formed by vacuum deposition, for example, by a conventional sputtering method in which a raw material capable of forming the functional layer is used as a target and Ar (argon) gas is introduced into the chamber. By using the conventional sputtering method, the functional layer is formed while etching the upper surface 10a of the substrate 10 with Ar, so that the deposition amount of the functional layer can be minimized and the effect of improving adhesion can be obtained.

[0037] However, this is only an example of the method for forming the functional layer, and the functional layer may be formed by other methods. For example, an adhesion improvement effect can be obtained by activating the upper surface 10a of the substrate 10 by plasma treatment using Ar or the like before forming the functional layer, and then a method of forming the functional layer by vacuum deposition by the magnetron sputtering method may be used.

[0038] There is no particular limitation on the combination of the material of the functional layer and the materials of the resistor 30 and the terminal portion 41, and they can be appropriately selected according to the purpose. For example, Ti can be used as the functional layer, and a Cr mixed-phase film mainly composed of α-Cr (alpha chromium) can be formed as the resistor 30 and the terminal portion 41.

[0039] In this case, for example, the resistor 30 and the terminal portion 41 can be formed by a magnetron sputtering method in which a raw material capable of forming a Cr mixed-phase film is used as a target and Ar gas is introduced into the chamber. Alternatively, pure Cr can be used as a target, an appropriate amount of nitrogen gas can be introduced into the chamber together with Ar gas, and the resistor 30 and the terminal portion 41 may be formed by a reactive sputtering method.

[0040] In these methods, the growth surface of the Cr mixed-phase film is defined by the functional layer made of Ti, and a Cr mixed-phase film mainly composed of α-Cr with a stable crystal structure can be formed. Also, the gauge characteristics are improved by the diffusion of Ti constituting the functional layer into the Cr mixed-phase film. For example, 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. Note that when the functional layer is formed of Ti, the Cr mixed-phase film may contain Ti or TiN (titanium nitride).

[0041] In addition, when the resistor 30 is a Cr mixed-phase film, the functional layer made of Ti has all of the functions of promoting the crystal growth of the resistor 30, preventing the oxidation of the resistor 30 by oxygen and moisture contained in the base material 10, and improving the adhesion between the base material 10 and the resistor 30. The same applies when Ta, Si, Al, or Fe is used instead of Ti as the functional layer.

[0042] In this way, by providing a functional layer under the resistor 30, it becomes possible to promote the crystal growth of the resistor 30, and a resistor 30 composed of a stable crystal phase can be manufactured. As a result, in the strain gauge 1, the stability of the gauge characteristics can be improved. Also, by the diffusion of the material constituting the functional layer into the resistor 30, the gauge characteristics can be improved in the strain gauge 1.

[0043] After forming the resistor 30 and the terminal portion 41, if necessary, a cover layer 60 that covers the resistor 30 and exposes the terminal portion 41 is provided on the upper surface 10a of the base material 10, and the strain gauge 1 is completed. 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 base material 10 so as to cover the resistor 30 and expose the terminal portion 41, and then heating and curing it. The cover layer 60 may also be produced by applying a liquid or paste-like thermosetting insulating resin on the upper surface 10a of the base material 10 so as to cover the resistor 30 and expose the terminal portion 41, and then heating and curing it.

[0044] [Example 1] In Example 1, a plurality of base materials 10 made of a polyimide resin with a thickness of 25 μm containing a filler were prepared. Then, three samples each without heat treatment, samples subjected to heat treatment at 100°C, samples subjected to heat treatment at 200°C, and samples subjected to heat treatment at 300°C were produced. After returning to room temperature, the surface unevenness of the upper surface 10a of each base material 10 was measured by three-dimensional optical interference method.

[0045] Next, on the upper surface 10a of each base material 10, a resistor 30 with a film thickness of 0.05 μm was formed by magnetron sputtering, and after patterning as shown in FIG. 1 by photolithography, the number of pinholes generated in the resistor 30 was measured by an optical transmission method in which light was transmitted from the back surface of the sample.

[0046] Next, based on the measurement results, the relationship between the surface unevenness of the upper surface 10a of the base material 10 and the number of pinholes generated in the resistor 30 was summarized in FIG. 3. Note that the bar graph shown in FIG. 3 indicates the surface unevenness, and the line graph indicates the number of pinholes. Also, 100°C, 200°C, and 300°C on the horizontal axis indicate the temperature when the base material 10 was heat-treated, and untreated indicates that it was not heat-treated.

[0047] FIG. 3 shows that by heat-treating the base material 10 at 100°C or higher and 300°C or lower, the surface unevenness of the upper surface 10a of the base material 10 becomes 15 nm or less, which is about half of that in the untreated state. As a result, the number of pinholes generated in the resistor 30 decreases sharply to about 1 / 7. However, considering the heat resistance temperature of the polyimide resin, there is a risk of alteration or deterioration if heat treatment is performed at a temperature exceeding 250°C. Therefore, the heat treatment is preferably performed at a temperature of 100°C or higher and 250°C or lower. It is considered that the reduction of surface unevenness by heat treatment is due to the polyimide resin constituting the base material 10 entrapping the filler inside during thermal contraction by heat treatment.

[0048] According to the inventors' studies, the number of untreated pinholes shown in FIG. 3 (about 140) is at a level that deteriorates the gauge characteristics, but the number of pinholes after heat treatment (about 20) is at a level that does not adversely affect the gauge characteristics. That is, when the resistor 30 with a film thickness of 0.05 μm is used, by making the surface unevenness of the upper surface 10a of the base material 10 15 nm or less, it was confirmed that the number of pinholes generated in the resistor 30 can be reduced to a level that does not adversely affect the gauge characteristics.

[0049] Needless to say, even when the resistor 30 with a film thickness thicker than 0.05 μm is used, by making the surface unevenness of the upper surface 10a of the base material 10 15 nm or less, the number of pinholes generated in the resistor 30 can be reduced to a level that does not adversely affect the gauge characteristics. That is, by making the surface unevenness of the upper surface 10a of the base material 10 15 nm or less, when the resistor 30 with a film thickness of 0.05 μm or more is used, the number of pinholes generated in the resistor 30 can be reduced to a level that does not adversely affect the gauge characteristics.

[0050] In this way, by subjecting the base material 10 to heat treatment, it is possible to make the surface unevenness of the upper surface 10a of the base material 10 15 nm or less, and as a result, the number of pinholes generated in the resistor 30 with a film thickness of 0.05 μm or more can be significantly reduced. As a result, the strain gauge 1 can function stably while maintaining good gauge characteristics.

[0051] Note that in order to reduce the number of pinholes generated in the resistor 30, it is important to reduce the surface unevenness of the upper surface 10a of the base material 10, and the method of reducing the surface unevenness is not important. In the above, a method of reducing the surface unevenness by performing heat treatment was shown, but it is not limited to this, and any method may be used as long as the surface unevenness of the upper surface 10a of the base material 10 can be reduced.

[0052] The surface unevenness of the upper surface 10a of the base material 10 can be reduced, for example, by irradiating the upper surface 10a of the base material 10 with a laser beam substantially perpendicular to the upper surface 10a to cut off the convex portions, by moving a water cutter or the like parallel to the upper surface 10a of the base material 10 to cut off the convex portions, by polishing the upper surface 10a of the base material 10 using a grindstone, or by a method of applying pressure while heating the base material 10 (heat press), etc.

[0053] Also, in order to reduce the number of pinholes generated in the resistor 30, it is important to reduce the surface unevenness of the upper surface 10a of the base material 10, and it is not necessarily limited to the surface unevenness caused by the presence of the filler. It is also effective to reduce the surface unevenness not caused by the presence of the filler by the various methods described above. For example, when the surface unevenness of the base material 10 not containing a filler is larger than 15 nm, by the various methods described above, by making the surface unevenness of the upper surface 10a of the base material 10 15 nm or less, the number of pinholes generated in the resistor 30 having a film thickness of 0.05 μm or more can be reduced to a level that does not adversely affect the gauge characteristics.

[0054] [Example 2] In Example 2, a plurality of strain gauges 1 were produced using the base material 10 that had been subjected to a heat treatment at 200°C.

[0055] First, the base material 10 made of a polyimide resin with a thickness of 25 μm was subjected to a heat treatment at 200°C. Then, Ti with a film thickness of 3 nm was vacuum-deposited as a functional layer on the upper surface 10a of the base material 10 by a conventional sputtering method.

[0056] Subsequently, after forming a Cr mixed-phase film with a film thickness of 0.05 μm as the resistor 30 and the terminal portion 41 on the entire upper surface of the functional layer by magnetron sputtering, the functional layer, the resistor 30, and the terminal portion 41 were patterned as shown in FIG. 1 by photolithography.

[0057] Next, the gauge characteristics of each sample of Example 2 were measured. As a result, the gauge ratios of each sample of Example 2 were 14 to 16. Also, the temperature coefficient of gauge ratio TCS and the temperature coefficient of resistance TCR of each sample of Example 2 were within the range of -1000 ppm / °C to +1000 ppm / °C.

[0058] Thus, it was confirmed that the strain gauge 1 having good gauge characteristics can be produced by using the base material 10 heat-treated at 200°C. It is considered that the significant reduction in the number of pinholes generated in the resistor 30 is one of the reasons for obtaining good gauge characteristics. Note that the presence of the functional layer is not considered to be a factor increasing the number of pinholes generated in the resistor 30.

[0059] As described above, the preferred embodiments and the like have been described in detail. However, the present invention is not limited to the above-described embodiments and the like, and various modifications and substitutions can be made to the above-described embodiments and the like without departing from the scope described in the claims.

Explanation of Reference Numerals

[0060] 1 Strain gauge, 10 Base material, 10a Upper surface, 30 Resistor, 41 Terminal portion, 60 Cover layer

Claims

1. A flexible resin substrate, A functional layer formed directly on one surface of the substrate from a metal, an alloy, or a metal compound, On one surface of the functional layer, directly, a resistor mainly composed of α-Cr formed from a film containing Cr, CrN, and Cr ２ N, and has The functional layer has a function of promoting crystal growth of the α-Cr and forming a film mainly composed of the α-Cr, The thickness of the resistor is 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 in which the surface unevenness of one surface of the substrate is 15 nm or less.

2. The strain gauge according to claim 1, wherein the substrate contains a filler.

3. The strain gauge according to claim 1 or 2, wherein the substrate is formed from a polyimide resin.

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