Strain gauge

Abstract

To provide a strain gauge capable of increasing signal to noise ratio (S / N).SOLUTION: A strain gauge includes: a base material 10 made of resin having flexibility; a functional layer formed from metal, alloy, or a metal compound directly on both the surfaces of the base material 10; and a resistor 30. The resistor 30 is formed from a film containing Cr, CrN, and Cr2 N, and includes sensing sections 31S, 32S for causing a resistance change by receiving strain with α-Cr as a main constituent. The functional layer has a function of forming a film with the α-Cr as the main constituent while accelerating crystal growth of the α-Cr. The thickness of the resistor 30 is equal to or more than 0.05 μm and is equal to or less than 2 μm. The thickness of the functional layer is equal to or more than 1 nm and is equal to or less than 100 nm. The functional layer is in nearly the same pattern as that of the sensing sections 31S, 32S and is disposed in an opposing position while sandwiching the base material 10.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 strain of the measurement 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). Also, for example, one resistor is formed on one surface of a substrate made of insulating resin (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, when a strain gauge is attached to a strain generating element to detect changes in the resistance value of a resistor, external noise caused by electromagnetic waves or power supply noise may be superimposed on the output of the strain gauge, which reduces the signal-to-noise ratio (S / N) of the strain gauge and reduces the sensor sensitivity.

[0005] The present invention has been made in view of the above points, and has an object to provide a strain gauge that can improve the signal-to-noise ratio (S / N). [Means for solving the problem]

[0006] This strain gauge has a substrate made of flexible resin, a first functional layer formed directly on one surface of the substrate from a metal, alloy, or metal compound, a second functional layer formed directly on the other surface of the substrate from a metal, alloy, or metal compound, and a resistor, and the resistor has a first sensitive part formed directly on one surface of the first functional layer from a film containing Cr, CrN, and CrN, and mainly composed of α-Cr, which changes in resistance when subjected to strain, and a second sensitive part formed directly on the other surface of the second functional layer from a film containing Cr, CrN, and Cr and a second sensing part formed from a film containing 2N and having α-Cr as its main component, which undergoes a resistance change when subjected to strain, wherein the first functional layer and the second functional layer have the function of promoting crystal growth of the α-Cr and forming a film having the α-Cr as its main component, the thickness of the resistor is 0.05 μm or more and 2 μm or less, the thickness of the first functional layer and the second functional layer is 1 nm or more and 100 nm or less, the first sensing part and the second sensing part have approximately the same pattern and are arranged in opposing positions across the substrate. [Effects of the Invention]

[0007] According to the disclosed technology, it is possible to provide a strain gauge that can improve the signal-to-noise ratio (S / N). [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] FIG. 2 is a bottom view illustrating the strain gauge according to the first embodiment. [Figure 3] 1 is a cross-sectional view illustrating a strain gauge according to a 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, showing the strain gauge as viewed from the top side of the substrate. FIG. 2 is a bottom view illustrating the strain gauge according to the first embodiment, showing the strain gauge as viewed from the bottom side of the substrate. FIG. 3 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 FIGS. 1 to 3, strain gauge 1 has a substrate 10, a resistor 30 (resistance portions 31 and 32), and terminal portions 41 and 42.

[0011] In this embodiment, for convenience, in the strain gauge 1, the side of the substrate 10 on which the resistance portion 31 is provided is referred to as the upper side or one side, and the side on which the resistance portion 32 is provided is referred to as the lower side or other side. Furthermore, the surface on which the resistance portion 31 is provided in each portion is referred to as the one side or upper side, and the surface on which the resistance portion 32 is 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 the 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 the substrate 10 of 5 μm to 200 μm is preferable in that it can reduce strain sensitivity errors of the resistor portions 31 and 32.

[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 formed on the substrate 10. The resistor 30 includes resistive portions 31 and 32 laminated via the substrate 10. In other words, the resistor 30 is a general term for the resistive portions 31 and 32, and will be referred to as the resistor 30 when there is no need to particularly distinguish between the resistive portions 31 and 32. For convenience, the resistive portions 31 and 32 are shown with a matte finish in FIGS. 1 and 2.

[0016] The resistance portion 31 is a thin film formed in a predetermined pattern on the upper surface 10a side of the substrate 10. The resistance portion 31 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. The resistance portion 31 includes a sensing portion 31S that undergoes a resistance change when subjected to strain. The portion of the resistance portion 31 other than the sensing portion 31S functions as a wiring pattern connected to the terminal portion 41.

[0017] The resistance portion 32 is a thin film formed in a predetermined pattern on the lower surface 10b side of the substrate 10. The resistance portion 32 may be formed directly on the lower surface 10b of the substrate 10, or may be formed on the lower surface 10b of the substrate 10 via another layer. The resistance portion 32 includes a sensing portion 32S that undergoes a resistance change when subjected to strain. The portion of the resistance portion 32 other than the sensing portion 32S functions as a wiring pattern connected to the terminal portion 42.

[0018] The sensing portion 31S and the sensing portion 32S have approximately the same pattern and are arranged at positions facing each other across the substrate 10. In other words, the sensing portion 31S and the sensing portion 32S have approximately the same pattern and are arranged at positions that overlap in a plan view.

[0019] Here, the phrase "the sensing parts 31S and 32S have substantially the same pattern" means that they are manufactured based on the same design, and as a result, the patterns of both parts are almost the same, and a degree of manufacturing error is acceptable.

[0020] The wiring pattern portion of resistor portion 31 and the wiring pattern portion of resistor portion 32 may be disposed in positions facing each other across the substrate 10, or may not be disposed in positions facing each other across the substrate 10. Furthermore, terminal portion 41 and terminal portion 42 may be disposed in positions facing each other across the substrate 10, or may not be disposed in positions facing each other across the substrate 10.

[0021] The resistor 30 (resistance portions 31 and 32) 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 Cu-Ni (copper-nickel). An example of a material containing both Cr and Ni is Ni-Cr (nickel-chromium).

[0022] 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.

[0023] 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.

[0024] 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).

[0025] The terminal portions 41 extend from both ends of the resistor portion 31 on the upper surface 10a of the substrate 10 and are formed in a generally rectangular shape in plan view, wider than the resistor portion 31. The terminal portions 41 are a pair of electrodes for outputting a change in the resistance value of the resistor portion 31 caused by strain to the outside, and are joined to, for example, lead wires for external connection. For example, the resistor portion 31 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. Note that, although the resistor portion 31 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.

[0026] The terminal portions 42 extend from both ends of the resistor portion 32 on the lower surface 10b of the substrate 10 and are formed in a generally rectangular shape in plan view, wider than the resistor portion 32. The terminal portions 42 are a pair of electrodes for outputting a change in the resistance value of the resistor portion 32 caused by strain to the outside, and are joined to, for example, lead wires for external connection. For example, the resistor portion 32 extends from one of the terminal portions 42 while folding back in a zigzag pattern and is connected to the other terminal portion 42. The upper surface of the terminal portion 42 may be coated with a metal that has better solderability than the terminal portion 42. Note that, although the resistor portion 32 and the terminal portion 42 are denoted by different reference numerals for convenience, they can be integrally formed from the same material in the same process.

[0027] It is also possible to provide through-holes that penetrate the base material 10, and to concentrate the terminal portions 41 and 42 on the upper surface 10a side or the lower surface 10b side of the base material 10.

[0028] A cover layer 61 (insulating resin layer) may be provided on the upper surface 10a of the substrate 10 so as to cover the resistor portion 31 and expose the terminal portion 41. Also, a cover layer 62 (insulating resin layer) may be provided on the lower surface 10b of the substrate 10 so as to cover the resistor portion 32 and expose the terminal portion 42. By providing the cover layers 61 and 62, it is possible to prevent mechanical damage, etc. from occurring to the resistor portions 31 and 32. Furthermore, by providing the cover layers 61 and 62, it is possible to protect the resistor portions 31 and 32 from moisture, etc. Note that the cover layers 61 and 62 may be provided so as to cover the entire portion excluding the terminal portions 41 and 42.

[0029] The cover layers 61 and 62 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 layers 61 and 62 may contain a filler or a pigment.

[0030] The thickness of the cover layers 61 and 62 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 for the cover layer 62 is preferable in terms of the transmission of strain from the surface of the strain generator joined to the underside of the cover layer 62 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. Note that the cover layers 61 and 62 may be formed from different materials, or may be formed to different thicknesses.

[0031] To manufacture the strain gauge 1, first, a substrate 10 is prepared, and the planar resistive portion 31 and terminal portion 41 shown in Fig. 1 are formed on the upper surface 10a of the substrate 10. The material and thickness of the resistive portion 31 and terminal portion 41 are as described above. The resistive portion 31 and terminal portion 41 can be integrally formed from the same material.

[0032] The resistor portion 31 and the terminal portion 41 can be formed, for example, by forming a film by magnetron sputtering using a target made of a raw material capable of forming the resistor portion 31 and the terminal portion 41, and then patterning the film by photolithography. Instead of magnetron sputtering, the resistor portion 31 and the terminal portion 41 may also be formed by reactive sputtering, vapor deposition, arc ion plating, pulsed laser deposition, or the like.

[0033] From the viewpoint of stabilizing the gauge characteristics, it is preferable to vacuum-form a functional layer having a film thickness of about 1 nm to 100 nm by, for example, conventional sputtering as a base layer on the upper surface 10a of the substrate 10 before forming the resistance portion 31 and the terminal portion 41. After the resistance portion 31 and the terminal portion 41 are formed on the entire upper surface of the functional layer, the functional layer is patterned together with the resistance portion 31 and the terminal portion 41 into the planar shape shown in FIG.

[0034] 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 section 31. The functional layer preferably also has a function of preventing oxidation of the resistor section 31 due to oxygen or moisture contained in the substrate 10, and a function of improving adhesion between the substrate 10 and the resistor section 31. The functional layer may also have other functions.

[0035] The insulating resin film that constitutes the substrate 10 contains oxygen and moisture, and since Cr forms a self-oxidized film, particularly when the resistance portion 31 contains Cr, it is effective for the functional layer to have the function of preventing oxidation of the resistance portion 31.

[0036] 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 layer, the resistor portion 31, 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 suitable metals 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), alloys of any of the metals in this group, and compounds of any of the metals in this group.

[0037] Examples of the alloys include FeCr, TiAl, FeNi, NiCr, CrCu, etc. Examples of the compounds include TiN, TaN, Si3N4, TiO2, Ta2O5, SiO2, etc.

[0038] 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.

[0039] 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.

[0040] There are no particular restrictions on the combination of the material of the functional layer with the materials of the resistance portion 31 and the terminal portion 41, and it 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 resistance portion 31 and the terminal portion 41.

[0041] In this case, for example, the resistor portion 31 and the terminal portion 41 can be formed by magnetron sputtering using a target made of a material capable of forming a Cr mixed phase film and introducing Ar gas into a chamber. Alternatively, the resistor portion 31 and the terminal portion 41 can be formed by reactive sputtering using pure Cr as a target and introducing an appropriate amount of nitrogen gas into a chamber together with Ar gas.

[0042] 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).

[0043] When the resistor portion 31 is a Cr mixed phase film, the functional layer made of Ti has all of the following functions: promoting crystal growth of the resistor portion 31, preventing oxidation of the resistor portion 31 due to oxygen and moisture contained in the substrate 10, and improving adhesion between the substrate 10 and the resistor portion 31. The same applies when Ta, Si, Al, or Fe is used as the functional layer instead of Ti.

[0044] In this way, by providing a functional layer below the resistance portion 31, it is possible to promote crystal growth in the resistance portion 31, and to produce a resistance portion 31 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 resistance portion 31, it is possible to improve the gauge characteristics of the strain gauge 1.

[0045] Next, the resistor portion 32 and the terminal portion 42 having the planar shape shown in Fig. 2 are formed on the lower surface 10b of the substrate 10. The resistor portion 32 and the terminal portion 42 can be formed by the same method as the resistor portion 31 and the terminal portion 41. It is also preferable to form a functional layer as a base layer on the lower surface 10b of the substrate 10 before forming the resistor portion 32 and the terminal portion 42.

[0046] After forming the resistor portion 31 and the terminal portion 41, and the resistor portion 32 and the terminal portion 42, if necessary, a cover layer 61 that covers the resistor portion 31 and exposes the terminal portion 41 may be provided on the upper surface 10a of the substrate 10, and a cover layer 62 that covers the resistor portion 32 and exposes the terminal portion 42 may be provided on the lower surface 10b of the substrate 10. In this way, the strain gauge 1 is completed.

[0047] The cover layer 61 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 portion 31 and expose the terminal portion 41, and then heating and curing the film. The cover layer 62 can be produced, for example, by laminating a semi-cured thermosetting insulating resin film on the lower surface 10b of the base material 10 so as to cover the resistor portion 32 and expose the terminal portion 42, and then heating and curing the film. The cover layers 61 and 62 may be produced by applying a liquid or paste-like thermosetting insulating resin and then heating and curing it, instead of laminating an insulating resin film.

[0048] Thus, in strain gauge 1, sensing portion 31S and sensing portion 32S have substantially the same pattern and are positioned opposite each other across substrate 10. Therefore, for example, when bending stress occurs with sensing portion 31S on the tensile side and sensing portion 32S on the compressive side, the output of terminal 41 connected to sensing portion 31S and the output of terminal 42 connected to sensing portion 32S will be substantially equal in absolute value but opposite in sign. In contrast, external noise and power supply noise due to electromagnetic waves and the like are applied substantially equally to sensing portion 31S and sensing portion 32S. Therefore, by taking the differential signal between the output of terminal 41 and the output of terminal 42, the output is approximately doubled, thereby reducing noise. As a result, the signal-to-noise ratio (S / N) of strain gauge 1 can be improved, thereby improving sensor sensitivity.

[0049] Although it is possible to separately manufacture two strain gauges with sensing parts of approximately the same pattern and attach them to opposite positions across the strain generating element, this would be an inefficient attachment process and it would be difficult to ensure attachment accuracy.This problem can be solved by using a single strain gauge 1 with two sensing parts of approximately the same pattern arranged in opposite positions across the substrate 10.

[0050] 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. [Explanation of symbols]

[0051] 1 strain gauge, 10 substrate, 10a upper surface, 10b lower surface, 30 resistor, 31, 32 resistor portion, 31S, 32S sensing portion, 41, 42 terminal portion, 61, 62 cover layer

Claims

1. a flexible resin substrate; a first functional layer formed of a metal, alloy, or metal compound directly on one surface of the substrate; a second functional layer formed of a metal, alloy, or metal compound directly on the other surface of the substrate; a resistor; The resistor is Cr, CrN, and Cr are directly formed on one surface of the first functional layer. ２ a first sensing part formed from a film containing N and containing α-Cr as a main component, which undergoes a resistance change when subjected to strain; Cr, CrN, and Cr directly on the other surface of the second functional layer. ２ a second sensing part formed from a film containing N and containing α-Cr as a main component, the second sensing part undergoing a resistance change when subjected to strain; the first functional layer and the second functional layer have 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 first functional layer and the second functional layer is 1 nm or more and 100 nm or less; The first and second sensitive portions have substantially the same pattern and are strain gauges arranged at positions facing each other across the substrate.

2. 2. The strain gauge according to claim 1, further comprising an insulating resin layer that covers at least one of the first sensitive portion and the second sensitive portion.

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