Strain gauges, sensor modules

Abstract

To improve stability of gauge characteristics in a strain gauge having a resistor formed on a substrate having flexibility.SOLUTION: A strain gauge has: a resin-made substrate having flexibility; a functional layer formed of metal, an alloy, or a compound of metal directly on one face of the substrate; a resistor having α-Cr as a main component and formed of a film including Cr, CrN, and Cr2N directly on one face of the functional layer; and an insulating resin layer covering the resistor. The functional layer has a function to promote crystal growth of the α-Cr and deposit a film having 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. The coefficient of expansion of the substrate is within a range of 7 ppm / K to 20 ppm / K.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a strain gauge and a sensor module. [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, when a flexible substrate is used, it is difficult to form a stable resistor on the substrate, and there is a problem that the gauge characteristics (gauge factor, temperature coefficient of gauge factor TCS, and temperature coefficient of resistance TCR) are not stable.

[0005] The present invention has been made in view of the above points, and has as its 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 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 resin layer covering 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 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 expansion coefficient of the substrate is within the range of 7 ppm / K to 20 ppm / K. [Effects of the Invention]

[0007] According to the disclosed technology, it is possible to improve the stability of the gauge characteristics 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 illustrating a strain gauge according to a first embodiment. [Figure 3] 3A to 3C are diagrams illustrating a manufacturing process of the strain gauge according to the first embodiment. [Figure 4] 1 is a cross-sectional view illustrating a strain gauge according to a first modified example of the first embodiment. [Figure 5] FIG. 10 is a plan view illustrating a strain gauge according to a second embodiment. [Figure 6] FIG. 10 is a cross-sectional view illustrating a strain gauge according to a second embodiment. [Figure 7] 10A to 10C are diagrams (part 1) illustrating a manufacturing process of the strain gauge according to the second embodiment. [Figure 8] 10A to 10C are diagrams (part 2) illustrating the manufacturing process of the strain gauge according to the second embodiment. [Figure 9] FIG. 10 is a cross-sectional view illustrating a strain gauge according to a first modified example of the second embodiment. [Figure 10] FIG. 10 is a cross-sectional view illustrating a strain gauge according to a second modification of the second embodiment. [Figure 11] FIG. 10 is a cross-sectional view illustrating a sensor module according to a third embodiment. [Figure 12] FIG. 10 is a diagram showing the results of fluorescent X-ray analysis of the functional layer. [Figure 13] FIG. 10 is a diagram showing the results of X-ray diffraction of a resistor. [Figure 14] FIG. 10 is a diagram showing the relationship between the expansion coefficient of the substrate and the internal stress of the resistor. [Figure 15] FIG. 10 is a diagram showing the relationship between the surface irregularities of a substrate and the number of pinholes in a resistor. 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, showing a cross section along line AA in Fig. 1. Referring to Figs. 1 and 2, the strain gauge 1 has a substrate 10, a functional layer 20, a resistor 30, terminal portions 41, and a cover layer 60. In Fig. 1, for convenience of illustrating the resistor 30, only the outer edge of the cover layer 60 is shown by a dashed line.

[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 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 also 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 also have other functions.

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

[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, and a compound of any of the metals in this group.

[0019] Examples of the alloys include FeCr, TiAl, FeNi, NiCr, CrCu, etc. Examples of the compounds include TiN, TaN, Si3N4, TiO2, Ta2O5, SiO2, 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 subjected to strain. For convenience, the resistor 30 is shown in Fig. 1 with a matte finish.

[0021] 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 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 in which Cr, CrN, CrN, etc. are mixed. The Cr mixed phase film may contain inevitable impurities such as chromium oxide. Furthermore, a portion 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, if the functional layer 20 is formed from Ti, the Cr mixed phase film may contain Ti or TiN (titanium nitride).

[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] 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 crystalline phase, the stability of the gauge characteristics can be improved.

[0026] Here, "main component" means that the target substance accounts for 50% by mass or more of all 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 in order to improve 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 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.

[0028] From the viewpoint of reducing warpage of the substrate 10 by making the internal stress of the resistor 30 close to zero, it is preferable that the expansion coefficient of the substrate 10 is 7 ppm / K to 20 ppm / K. The expansion coefficient of the substrate 10 can be adjusted, for example, by selecting the material of the substrate 10, selecting the material of the filler contained in the substrate 10, and adjusting the content thereof.

[0029] However, when resistor 30 is formed on substrate 10, pinholes may occur in resistor 30. If the number of pinholes occurring in resistor 30 exceeds a predetermined value, the gauge characteristics may deteriorate and the resistor may no longer function as a strain gauge. The inventors have discovered that one of the causes of pinholes occurring in resistor 30 is filler protruding from top surface 10a of substrate 10.

[0030] That is, when the substrate 10 contains a filler, a part of the filler protrudes from the upper surface 10a of the substrate 10, increasing the surface unevenness of the upper surface 10a of the substrate 10. As a result, the number of pinholes generated in the resistor 30 formed on the upper surface 10a of the substrate 10 increases, which causes deterioration of the gauge characteristics, etc.

[0031] The inventors have found that when the thickness of resistor 30 is 0.05 μm or more, if the surface roughness of upper surface 10a of substrate 10 is 15 nm or less, the number of pinholes that occur in resistor 30 can be suppressed and gauge characteristics can be maintained.

[0032] That is, when the thickness of resistor 30 is 0.05 μm or more, from the viewpoint of reducing the number of pinholes that occur in resistor 30 formed on upper surface 10a of substrate 10 and maintaining the gauge characteristics, the surface roughness of upper surface 10a of substrate 10 is preferably 15 nm or less, and even if substrate 10 contains a filler, the gauge characteristics will not deteriorate if the surface roughness is 15 nm or less. Note that the surface roughness of upper surface 10a of substrate 10 may be 0 nm.

[0033] The surface irregularities of the upper surface 10a of the substrate 10 can be reduced, for example, by heating the substrate 10. Alternatively, instead of heating the substrate 10, other methods may be used, such as a method of irradiating the upper surface 10a of the substrate 10 with laser light substantially perpendicularly to remove the convex portions, a method of moving a water cutter or the like parallel to the upper surface 10a of the substrate 10 to remove the convex portions, a method of polishing the upper surface 10a of the substrate 10 with a grindstone, or a method of applying pressure to the substrate 10 while heating it (heat press).

[0034] The surface roughness refers to the arithmetic mean roughness, and is generally expressed as Ra. The surface roughness can be measured, for example, by three-dimensional optical interferometry.

[0035] 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.

[0036] The cover layer 60 is an insulating resin layer provided on the upper surface 10a of the substrate 10 so as to cover the resistor 30 and expose the terminal portions 41. By providing the cover layer 60, it is possible to prevent mechanical damage to the resistor 30. Furthermore, by providing the cover layer 60, it is possible to protect the resistor 30 from moisture and the like. Note that the cover layer 60 may be provided so as to cover the entire portion excluding the terminal portions 41.

[0037] 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, or composite resin (e.g., silicone resin or polyolefin resin). The cover layer 60 may contain a filler or a pigment. There are no particular restrictions on 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.

[0038] 3A and 3B are diagrams illustrating the manufacturing process of the strain gauge according to the first embodiment, showing a cross section corresponding to that of FIG. 2. To manufacture the strain gauge 1, first, in the step shown in FIG. 3A, a substrate 10 is prepared, and a functional layer 20 is formed on the upper surface 10a of the substrate 10. The materials and thicknesses of the substrate 10 and the functional layer 20 are as described above.

[0039] The functional layer 20 can be formed in 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 and an effect of improving adhesion can be obtained.

[0040] 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 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.

[0041] 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 using the same material. The resistor 30 and the terminal portion 41 can be formed as films by, for example, magnetron sputtering using a target material capable of forming the resistor 30 and the terminal portion 41. The resistor 30 and the terminal portion 41 may also be formed as films using reactive sputtering, vapor deposition, arc ion plating, pulsed laser deposition, or the like instead of magnetron sputtering.

[0042] There are no particular restrictions on the combination of the material of the functional layer 20 with the material of the resistor 30 and the terminal portion 41, and they 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.

[0043] In this case, for example, the resistor 30 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 30 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.

[0044] In these methods, the Ti functional layer 20 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 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 temperature coefficient of resistance (TCR) can be set within the ranges of -1000 ppm / °C to +1000 ppm / °C.

[0045] 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.

[0046] Next, in the step shown in FIG. 3(c), a cover layer 60 is formed on the upper surface 10a of the substrate 10 to cover the resistor 30 and expose the terminal portions 41. The material and thickness of the cover layer 60 are as described above. 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 portions 41, and then heating and curing the film. The cover layer 60 may also be produced by applying a liquid or paste-like thermosetting insulating resin to the upper surface 10a of the substrate 10 so as to cover the resistor 30 and expose the terminal portions 41, and then heating and curing the resin. The strain gauge 1 is completed through the steps described above.

[0047] In this way, by providing the functional layer 20 below the resistor 30, it is possible to promote crystal growth of the resistor 30, and to produce a resistor 30 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 20 into the resistor 30, it is possible to improve the gauge characteristics of the strain gauge 1.

[0048] <Modification 1 of the First Embodiment> In the first modification of the first embodiment, an example of a strain gauge in which an insulating layer is provided under a cover layer is shown. Note that in the first modification of the first embodiment, the description of the same components as those in the already described embodiments may be omitted.

[0049] Fig. 4 is a cross-sectional view illustrating a strain gauge according to Modification 1 of the first embodiment, showing a cross section corresponding to Fig. 2. Referring to Fig. 4, strain gauge 1A differs from strain gauge 1 (see Figs. 1, 2, etc.) in that an insulating layer 50 is provided below cover layer 60. Note that cover layer 60 may be provided so as to cover the entire portion excluding terminal portions 41.

[0050] The insulating layer 50 is provided on the upper surface 10a of the substrate 10 so as to cover the resistor 30 and expose the terminal portion 41. The cover layer 60 can be provided so as to cover, for example, part of the side surfaces and the upper surface of the insulating layer 50.

[0051] The material of the insulating layer 50 is not particularly limited and can be selected appropriately depending on the purpose as long as it has a higher resistance than the resistor 30 and the cover layer 60, and can be, for example, an oxide or nitride of Si, W, Ti, Ta, etc. The thickness of the insulating layer 50 is not particularly limited and can be selected appropriately depending on the purpose, and can be, for example, about 0.05 μm to 1 μm.

[0052] The method for forming the insulating layer 50 is not particularly limited and can be selected appropriately depending on the purpose, but for example, vacuum processes such as sputtering and chemical vapor deposition (CVD) methods, or solution processes such as spin coating and sol-gel methods can be used.

[0053] In this way, by providing the insulating layer 50 below the cover layer 60, it is possible to improve the insulating properties and environmental sealing properties compared to the case where only the cover layer 60 is provided. Therefore, the insulating layer 50 can be provided as appropriate according to the required specifications of insulating properties and environmental sealing properties.

[0054] Second Embodiment In the second embodiment, an example of a strain gauge in which electrodes have a laminated structure is shown. Note that in the second embodiment, the description of the same components as those in the already described embodiments may be omitted.

[0055] Fig. 5 is a plan view illustrating a strain gauge according to the second embodiment. Fig. 6 is a cross-sectional view illustrating the strain gauge according to the second embodiment, taken along line BB in Fig. 5. Referring to Figs. 5 and 6, the strain gauge 2 includes an electrode 40A formed by stacking multiple layers. Note that the cover layer 60 may be provided so as to cover the entire portion excluding the electrode 40A.

[0056] Electrode 40A has a laminated structure in which multiple metal layers are stacked. Specifically, electrode 40A has terminal portions 41 extending from both ends of resistor 30, metal layer 42 formed on the upper surface of terminal portion 41, metal layer 43 formed on the upper surface of metal layer 42, and metal layer 44 formed on the upper surface of metal layer 43. Metal layer 43 is a typical example of a first metal layer according to the present invention, and metal layer 44 is a typical example of a second metal layer according to the present invention.

[0057] The material of the metal layer 42 is not particularly limited and can be selected appropriately depending on the purpose, for example, Cu (copper). The thickness of the metal layer 42 is not particularly limited and can be selected appropriately depending on the purpose, for example, it can be about 0.01 μm to 1 μm.

[0058] The material of the metal layer 43 is preferably Cu, a Cu alloy, Ni, or a Ni alloy. The thickness of the metal layer 43 is determined taking into consideration the solderability to the electrode 40A, and is preferably 1 μm or more, more preferably 3 μm or more. By using Cu, a Cu alloy, Ni, or a Ni alloy as the material of the metal layer 43 and making the thickness of the metal layer 43 1 μm or more, solder erosion is improved. Furthermore, by using Cu, a Cu alloy, Ni, or a Ni alloy as the material of the metal layer 43 and making the thickness of the metal layer 43 3 μm or more, solder erosion is further improved. Note that, for ease of electroplating, the thickness of the metal layer 43 is preferably 30 μm or less.

[0059] Here, solder erosion refers to the material that constitutes electrode 40A dissolving in the solder that is joined to electrode 40A, causing a decrease in the thickness of electrode 40A or even the disappearance of electrode 40A. If solder erosion occurs, there is a risk that the adhesive strength and tensile strength between electrode 40A and the lead wire or the like joined to electrode 40A may decrease, so it is preferable that solder erosion does not occur.

[0060] The material of the metal layer 44 can be selected from materials having better solder wettability than the metal layer 43. For example, if the material of the metal layer 43 is Cu, a Cu alloy, Ni, or a Ni alloy, Au (gold) can be used as the material of the metal layer 44. By coating the surface of Cu, a Cu alloy, Ni, or a Ni alloy with Au, oxidation and corrosion of the Cu, Cu alloy, Ni, or Ni alloy can be prevented and good solder wettability can be obtained. The same effect can be achieved by using Pt (platinum) instead of Au as the material of the metal layer 44. There are no particular limitations on the thickness of the metal layer 44 and it can be appropriately selected depending on the purpose, but it can be, for example, about 0.01 μm to 1 μm.

[0061] In plan view, the terminal portion 41 is exposed around the laminated portion of the metal layers 42, 43, and 44, but the terminal portion 41 may have the same planar shape as the laminated portion of the metal layers 42, 43, and 44.

[0062] 7 and 8 are diagrams illustrating the manufacturing process of a strain gauge according to the second embodiment, showing a cross section corresponding to FIG. 6. To manufacture the strain gauge 2, first, a process similar to that of the first embodiment shown in FIG. 3(a) is performed, and then, in the process shown in FIG. 7(a), a metal layer 300 is formed on the upper surface of the functional layer 20. The metal layer 300 is a layer that will ultimately 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.

[0063] 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.

[0064] Next, in the step shown in FIG. 7(b), a seed layer 420 that will become the metal layer 42 is formed so as to cover the upper surface of the metal layer 300 by, for example, sputtering or electroless plating.

[0065] 7(c), a photosensitive resist 800 is formed on the entire upper surface of the seed layer 420, and is exposed and developed to form openings 800x that expose regions where the electrodes 40A will be formed. As the resist 800, for example, a dry film resist or the like can be used.

[0066] 7(d), for example, a metal layer 43 is formed on the seed layer 420 exposed in the opening 800x by electroplating using the seed layer 420 as a power supply path, and a metal layer 44 is further formed on the metal layer 43. Electroplating is advantageous in that it has a high tact time and can form a low-stress electroplated layer as the metal layer 43. By forming a thick electroplated layer with low stress, warping of the strain gauge 2 can be prevented. Alternatively, the metal layer 44 may be formed on the metal layer 43 by electroless plating.

[0067] When forming the metal layer 44, the side surfaces of the metal layer 43 are covered with the resist 800, so the metal layer 44 is formed only on the top surface of the metal layer 43, and not on the side surfaces.

[0068] Next, in the step shown in Fig. 8(a), the resist 800 shown in Fig. 7(d) is removed. The resist 800 can be removed, for example, by immersing it in a solution that can dissolve the material of the resist 800.

[0069] 8(b), a photosensitive resist 810 is formed on the entire upper surface of the seed layer 420, and is exposed to light and developed to be patterned into a planar shape similar to that of the resistor 30 and the terminal portion 41 in Fig. 5. For example, a dry film resist or the like can be used as the resist 810.

[0070] 8(c), the resist 810 is used as an etching mask to remove the functional layer 20, metal layer 300, and seed layer 420 exposed from the resist 810, thereby forming the functional layer 20, resistor 30, and terminal portion 41 having the planar shapes shown in FIG. 5. For example, unnecessary portions of the functional layer 20, metal layer 300, and seed layer 420 can be removed by wet etching. At this point, the seed layer 420 is formed on the resistor 30.

[0071] 8(d), the metal layer 43 and the metal layer 44 are used as an etching mask to remove unnecessary seed layer 420 exposed from the metal layer 43 and the metal layer 44, thereby forming the metal layer 42. For example, the unnecessary seed layer 420 can be removed by wet etching using an etching solution that etches the seed layer 420 but does not etch the functional layer 20 and the resistor 30.

[0072] After the step shown in Figure 8(d), in the same manner as the step shown in Figure 3(c), a cover layer 60 is formed on the upper surface 10a of the substrate 10 to cover the resistor 30 and expose the electrode 40A, thereby completing the strain gauge 2.

[0073] In this way, as electrode 40A, metal layer 43 made of a thick film (1 μm or more) of Cu, Cu alloy, Ni, or Ni alloy is formed on terminal portion 41, and further metal layer 44 made of a material (Au or Pt) with better solder wettability than metal layer 43 is formed on the outermost layer, thereby preventing solder erosion and improving solder wettability.

[0074] <Modification 1 of the Second Embodiment> In Modification 1 of the second embodiment, an example of an electrode having a layer structure different from that of the second embodiment is shown. Note that in Modification 1 of the second embodiment, the description of the same components as those in the already described embodiments may be omitted.

[0075] Fig. 9 is a cross-sectional view illustrating a strain gauge according to Modification 1 of the second embodiment, showing a cross section corresponding to Fig. 6. Referring to Fig. 9, strain gauge 2A differs from strain gauge 2 (see Fig. 6, etc.) in that electrode 40A is replaced with electrode 40B. Also, strain gauge 2A differs from strain gauge 2 (see Fig. 6, etc.) in that cover layer 60 is provided so as to cover substantially the entire portion except for electrode 40B.

[0076] Electrode 40B has a laminated structure in which multiple metal layers are stacked. Specifically, electrode 40B has terminal portions 41 extending from both ends of resistor 30, metal layer 42 formed on the upper surface of terminal portion 41, metal layer 43 formed on the upper surface of metal layer 42, metal layer 45 formed on the upper surface of metal layer 43, and metal layer 44 formed on the upper surface of metal layer 45. In other words, electrode 40B has a structure in which metal layer 45 is provided between metal layer 43 and metal layer 44 of electrode 40A.

[0077] The material of the metal layer 45 is not particularly limited and can be selected appropriately depending on the purpose, but for example, Ni can be used. NiP (nickel phosphorus) or Pd may be used instead of Ni. The metal layer 45 may also be Ni / Pd (a metal layer formed by laminating a Ni layer and a Pd layer in this order). The thickness of the metal layer 45 is not particularly limited and can be selected appropriately depending on the purpose, but can be, for example, about 1 μm to 2 μm.

[0078] The metal layer 45 can be formed on the metal layer 43 in the step shown in FIG. 7(d) by, for example, electrolytic plating using the seed layer 420 as a power supply path.

[0079] In this way, the number of layers of the electrodes is not particularly limited, and may be increased as necessary. In this case, too, a metal layer 43 made of a thick film (1 μm or more) of Cu, a Cu alloy, Ni, or a Ni alloy is formed on the terminal portion 41, and further, a metal layer 44 made of a material (Au or Pt) having better solder wettability than the metal layer 43 is formed on the outermost layer. Therefore, as in the second embodiment, it is possible to prevent solder erosion and improve solder wettability.

[0080] <Modification 2 of the Second Embodiment> Modification 2 of the second embodiment shows another example of an electrode having a layer structure different from that of the second embodiment. Note that in Modification 2 of the second embodiment, the description of the same components as those in the already described embodiment may be omitted.

[0081] Fig. 10 is a cross-sectional view illustrating a strain gauge according to Modification 2 of the second embodiment, showing a cross section corresponding to Fig. 6. Referring to Fig. 10, strain gauge 2B differs from strain gauge 2A (see Fig. 9) in that electrode 40B is replaced with electrode 40C. Also, strain gauge 2B differs from strain gauge 2 (see Fig. 6, etc.) in that cover layer 60 is provided so as to cover substantially the entire portion except for electrode 40C.

[0082] The electrode 40C has a laminated structure in which multiple metal layers are stacked. Specifically, the electrode 40C includes terminal portions 41 extending from both ends of the resistor 30, a metal layer 42 formed on the upper surface of the terminal portion 41, a metal layer 43 formed on the upper surface of the metal layer 42, a metal layer 45A formed on the upper and side surfaces of the metal layer 43 and the side surfaces of the metal layer 42, and a metal layer 44A formed on the upper and side surfaces of the metal layer 45A. The material and thickness of the metal layers 44A and 45A may be the same as those of the metal layers 44 and 45, for example. The metal layer 44A is a representative example of a second metal layer according to the present invention.

[0083] To form the electrode 40C, first, in the step shown in Fig. 7(d), for example, a metal layer 43 is formed by electrolytic plating using the seed layer 420 as a power supply path, and then the resist 800 is removed in the same manner as in the step shown in Fig. 8(a) without forming a metal layer 44. Thereafter, the same steps as in Figs. 8(b) to 8(d) are performed. Thereafter, for example, a metal layer 45A can be formed on the upper and side surfaces of the metal layer 43 and the side surfaces of the metal layer 42 by electroless plating. Furthermore, for example, a metal layer 44A can be formed on the upper and side surfaces of the metal layer 45A by electroless plating.

[0084] In this way, the electrode can be fabricated by appropriately combining electrolytic plating and electroless plating. In the structure of electrode 40C, metal layer 43 made of a thick film (1 μm or more) of Cu, Cu alloy, Ni, or Ni alloy is formed on terminal portion 41, and metal layer 44A made of a material (Au or Pt) with better solder wettability than metal layer 43 is further formed as the outermost layer. However, outermost metal layer 44A is formed not only on the top surface of metal layer 43 but also on the side surfaces of metal layers 42 and 43 via metal layer 45A. Therefore, compared to electrodes 40A and 40B, the effect of preventing oxidation and corrosion of Cu, Cu alloy, Ni, or Ni alloy constituting metal layer 43 can be further improved, and solder wettability can be further improved.

[0085] The same effect can be obtained by forming metal layer 44A directly on the upper and side surfaces of metal layer 43 and the side surfaces of metal layer 42 without forming metal layer 45A. That is, it is sufficient that metal layer 44A directly or indirectly covers the upper and side surfaces of metal layer 43 and the side surfaces of metal layer 42.

[0086] Third Embodiment In the third embodiment, an example of a sensor module using a strain gauge will be described. Note that in the third embodiment, the description of the same components as those in the embodiments already described may be omitted.

[0087] Fig. 11 is a cross-sectional view illustrating a sensor module according to the third embodiment, showing a cross section corresponding to Fig. 2. Referring to Fig. 11, the sensor module 5 has a strain gauge 1, a flexure element 110, and an adhesive layer 120. The cover layer 60 may be provided so as to cover the entire portion except for the terminal portion 41.

[0088] In the sensor module 5, the upper surface 110a of the flexure element 110 is fixed to the lower surface 10b of the substrate 10 via an adhesive layer 120. The flexure element 110 is made of a metal such as Fe, SUS (stainless steel), or Al, or a resin such as PEEK, and is an object that deforms (generates strain) in response to an applied force. The strain gauge 1 can detect the strain generated in the flexure element 110 as a change in the resistance value of the resistor 30.

[0089] The adhesive layer 120 is not particularly limited and can be selected appropriately depending on the purpose as long as it has the function of bonding the strain gauge 1 and the flexure element 110 together, and can be made of, for example, epoxy resin, modified epoxy resin, silicone resin, modified silicone resin, urethane resin, modified urethane resin, etc. Materials such as bonding sheets can also be used. The thickness of the adhesive layer 120 is not particularly limited and can be selected appropriately depending on the purpose, and can be, for example, about 0.1 μm to 50 μm.

[0090] To manufacture the sensor module 5, after fabricating the strain gauge 1, for example, one of the above materials to become the adhesive layer 120 is applied to the lower surface 10b of the substrate 10 and / or the upper surface 110a of the flexure element 110. Then, the lower surface 10b of the substrate 10 is placed opposite the upper surface 110a of the flexure element 110, and the strain gauge 1 is placed on the flexure element 110 with the applied material sandwiched between them. Alternatively, a bonding sheet may be sandwiched between the flexure element 110 and the substrate 10.

[0091] Next, the strain gauge 1 is heated to a predetermined temperature while being pressed against the flexure element 110, and the applied material is cured to form the adhesive layer 120. This bonds the upper surface 110a of the flexure element 110 to the lower surface 10b of the substrate 10 via the adhesive layer 120, completing the sensor module 5. The sensor module 5 can be used to measure, for example, load, pressure, torque, acceleration, etc.

[0092] In the sensor module 5, the strain gauge 1 may be replaced with the strain gauge 1A, 2, 2A, or 2B.

[0093] [Example 1] First, as a preliminary experiment, a Ti film was vacuum-deposited as the functional layer 20 on the upper surface 10a of a substrate 10 made of a 25 μm-thick polyimide resin by conventional sputtering. At this time, five samples were prepared by depositing Ti films with different film thicknesses.

[0094] Next, X-ray fluorescence (XRF) analysis was performed on the five samples produced, and the results shown in Figure 12 were obtained. The presence of Ti was confirmed from the X-ray peaks in Figure 12, and the X-ray intensity of each sample at the X-ray peak confirmed that the film thickness of the Ti film could be controlled in the range of 1 nm to 100 nm.

[0095] Next, in Example 1, a 3 nm thick Ti film was vacuum-formed as the functional layer 20 by conventional sputtering on the upper surface 10a of the substrate 10 made of polyimide resin and having a thickness of 25 μm.

[0096] Next, a Cr mixed phase film was formed as the resistor 30 and terminal portion 41 on the entire upper surface of the functional layer 20 by magnetron sputtering, and then the functional layer 20, resistor 30, and terminal portion 41 were patterned by photolithography as shown in FIG.

[0097] In Comparative Example 1, a Cr mixed phase film was formed as a resistor 30 and terminal portion 41 by magnetron sputtering on the upper surface 10a of a substrate 10 made of a 25 μm thick polyimide resin without forming a functional layer 20, and then patterned by photolithography as shown in Fig. 1. Note that the film formation conditions for the resistor 30 and terminal portion 41 were all the same in the sample of Example 1 and the sample of Comparative Example 1.

[0098] Next, X-ray diffraction (XRD) evaluation was performed on the sample of Example 1 and the sample of Comparative Example 1, and the results shown in Fig. 13 were obtained. Fig. 13 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 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.

[0099] The diffraction peak of Example 1 is located near the diffraction line of α-Cr (110), and it is thought that the provision of the functional layer 20 made of Ti promoted the crystal growth of α-Cr, resulting in the formation of a Cr mixed phase film containing α-Cr as the main component.

[0100] 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.

[0101] 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.

[0102] 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 temperature coefficient of gauge factor TCS and temperature coefficient of resistance TCR in the range of -1000 ppm / °C to +1000 ppm / °C. It is believed that the diffusion effect of Ti into the Cr mixed phase film contributes to the improvement of the gauge characteristics.

[0103] [Example 2] In Example 2, multiple substrates 10 made of 25 μm thick polyimide resin with different expansion coefficients were prepared, and when a Cr mixed phase film was formed as the resistor 30, the relationship between the expansion coefficient of the substrate 10 and the internal stress of the resistor 30 was investigated, and the results shown in Figure 14 were obtained.

[0104] The internal stress of resistor 30 was estimated by measuring the warpage of the evaluation sample and using Stoney's equation shown in formula (1). As can be seen from formula (1), the internal stress of resistor 30 shown in Fig. 14 is a value per unit thickness and does not depend on the thickness of resistor 30.

[0105]

number

[0106] 14, by setting the expansion coefficient of the substrate 10 within the range of 7 ppm / K to 20 ppm / K, the internal stress of the resistor 30 can be kept within the range of ±0.4 GPa. Here, ±0.4 GPa is the value at which the strain gauge 1 is warped to the limit of its functionality, and was determined experimentally by the inventors.

[0107] In other words, if the expansion coefficient of the substrate 10 is outside the range of 7 ppm / K to 20 ppm / K, the internal stress of the resistor 30 will exceed the range of ±0.4 GPa, causing the warping of the strain gauge 1 to increase and preventing it from functioning as a strain gauge. Therefore, the expansion coefficient of the substrate 10 must be within the range of 7 ppm / K to 20 ppm / K. Note that the material of the substrate 10 does not necessarily have to be polyimide resin.

[0108] By selecting the material of the substrate 10, selecting the material of the filler contained in the substrate 10 and adjusting the content, the expansion coefficient of the substrate 10 can be set within the range of 7 ppm / K to 20 ppm / K.

[0109] In this way, by setting the expansion coefficient of the substrate 10 within the range of 7 ppm / K to 20 ppm / K, the difference in the expansion rates between the substrate 10 and the resistor 30 and other factors can be absorbed, and the internal stress of the resistor 30 can be kept within the range of ±0.4 GPa. As a result, warping of the strain gauge 1 is reduced, and the strain gauge 1 can function stably while maintaining good gauge characteristics.

[0110] [Example 3] In Example 3, multiple substrates 10 were prepared, each made of a 25 μm thick polyimide resin containing a filler. Three samples were prepared: one unheated sample, one heat-treated at 100° C., one heat-treated at 200° C., and one heat-treated at 300° C. After returning to room temperature, the surface irregularities of the upper surface 10a of each substrate 10 were measured by three-dimensional optical interferometry.

[0111] Next, a resistor 30 having a thickness of 0.05 μm was formed on the upper surface 10 a of each substrate 10 by magnetron sputtering, and patterned by photolithography as shown in FIG. 1. Thereafter, 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.

[0112] Next, based on the measurement results, the relationship between the surface irregularities of the upper surface 10a of the substrate 10 and the number of pinholes that occur in the resistor 30 is summarized in Fig. 15. Note that the bar graph in Fig. 15 shows the surface irregularities, and the line graph shows the number of pinholes. Also, 100°C, 200°C, and 300°C on the horizontal axis show the temperatures at which the substrate 10 was heat-treated, and "untreated" indicates that no heat treatment was performed.

[0113] FIG. 15 shows that by heat-treating the substrate 10 at a temperature between 100°C and 300°C, the surface irregularities on the upper surface 10a of the substrate 10 are reduced to 15 nm or less, approximately half of the untreated value, and as a result, the number of pinholes in the resistor 30 is dramatically reduced to approximately one-seventh of the original value. However, considering the heat resistance temperature of polyimide resin, heat treatment at temperatures above 250°C may cause deterioration or degradation. Therefore, it is preferable to perform the heat treatment at a temperature between 100°C and 250°C. The reduction in surface irregularities due to heat treatment is thought to be due to the polyimide resin constituting the substrate 10 incorporating the filler during thermal shrinkage caused by the heat treatment.

[0114] 15 (approximately 140) is at a level that deteriorates the gauge characteristics, but the number of pinholes after heat treatment (approximately 20) is at a level that does not adversely affect the gauge characteristics. In other words, when resistor 30 having a film thickness of 0.05 μm is used, it has been confirmed that the number of pinholes generated in resistor 30 can be reduced to a level that does not adversely affect the gauge characteristics by setting the surface roughness of upper surface 10a of substrate 10 to 15 nm or less.

[0115] It goes without saying that even when a resistor 30 having a film thickness of more than 0.05 μm 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 by setting the surface roughness of the upper surface 10a of the substrate 10 to 15 nm or less. That is, when a resistor 30 having a film thickness of 0.05 μm or more is used, by setting the surface roughness of the upper surface 10a of the substrate 10 to 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.

[0116] In this way, by subjecting the substrate 10 to heat treatment, it is possible to reduce the surface roughness of the upper surface 10a of the substrate 10 to 15 nm or less, thereby significantly reducing the number of pinholes that occur in resistor 30 with a film thickness of 0.05 μm or more. As a result, the strain gauge 1 can function stably while maintaining good gauge characteristics.

[0117] In order to reduce the number of pinholes that occur in resistor 30, it is important to reduce the surface irregularities of upper surface 10a of substrate 10, but the method for reducing the surface irregularities is not important. Although the method for reducing the surface irregularities by performing a heat treatment has been described above, the present invention is not limited to this, and any method may be used as long as it can reduce the surface irregularities of upper surface 10a of substrate 10.

[0118] The surface unevenness of the upper surface 10a of the substrate 10 can be reduced by, for example, irradiating the upper surface 10a of the substrate 10 with laser light approximately perpendicular to the substrate 10 to remove the convex portions, moving a water cutter or the like parallel to the substrate 10 to remove the convex portions, polishing the upper surface 10a of the substrate 10 with a grinding stone, or applying pressure to the substrate 10 while heating it (heat press).

[0119] Furthermore, reducing the surface unevenness of the upper surface 10a of the substrate 10 is important to reduce the number of pinholes that occur in the resistor 30. This is not necessarily limited to surface unevenness caused by the presence of filler; reducing surface unevenness that is not caused by the presence of filler using the various methods described above is also effective. For example, if the surface unevenness of the substrate 10 that does not contain filler is greater than 15 nm, the number of pinholes that occur in a resistor 30 with a film thickness of 0.05 μm or more can be reduced to a level that does not adversely affect the gauge characteristics by using the various methods described above to reduce the surface unevenness of the upper surface 10a of the substrate 10 to 15 nm or less.

[0120] [Example 4] 7 and 8 was modified to resemble Variation 1 of the second embodiment to fabricate a strain gauge 2A equipped with an electrode 40B, and the presence or absence of solder erosion was confirmed. Specifically, Cu was used for metal layers 42 and 43, NiP was used for metal layer 45, and Au was used for metal layer 44, and 10 types of samples were fabricated (Samples No. 1 to No. 10) with different thicknesses of each metal layer, and the presence or absence of solder erosion was confirmed.

[0121] The results are shown in Table 1. In Table 1, a film thickness of "0" indicates that the metal layer was not formed. Also, "x" indicates that solder erosion occurred during the first soldering. Also, "o" indicates that no solder erosion occurred during the first soldering, but some solder erosion occurred during the second soldering (assuming solder rework, etc.). Also, "◎" indicates that no solder erosion occurred during either the first or second soldering.

[0122] [Table 1] As shown in Table 1, it was confirmed that solder erosion was improved by increasing the Cu thickness to 1 μm or more, and that solder erosion was further improved by increasing the Cu thickness to 3 μm or more. Furthermore, the results of Samples 1 and 5 confirmed that the presence or absence of solder erosion depends only on the Cu thickness, and not on the presence or absence of NiP or Au. However, as mentioned above, a metal layer made of Au or an equivalent material (such as Pt) is necessary to prevent solder erosion and improve solder wettability.

[0123] 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]

[0124] 1, 1A, 2, 2A, 2B strain gauge, 5 sensor module, 10 substrate, 10a upper surface, 20 functional layer, 30 resistor, 41 terminal portion, 40A, 40B, 40C electrodes, 42, 43, 44, 44A, 45, 45A metal layer, 50 insulating layer, 60 cover layer, 110 strain element, 120 adhesive layer

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 resin layer that covers 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 expansion coefficient of the substrate is within the range of 7 ppm / K to 20 ppm / K.

2. 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 resin layer that covers 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 surface roughness of one surface of the substrate is 15 nm or less.

3. 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 electrode electrically connected to the resistor; an insulating resin layer that covers 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, The electrode is a terminal portion extending from an end of the resistor; a first metal layer formed on the terminal portion from copper, a copper alloy, nickel, or a nickel alloy; a second metal layer formed on the first metal layer from a material having better solder wettability than the first metal layer.

4. 4. The strain gauge according to claim 1, further comprising an insulating layer formed below the insulating resin layer, the insulating layer being made of a material having a higher resistance than the resistor and the insulating resin layer, and covering the resistor.

5. 5. The strain gauge according to claim 1, wherein the resistor is mainly composed of alpha chromium.

6. 6. The strain gauge according to claim 5, wherein the resistor contains 80% by weight or more of alpha chromium.

7. 7. The strain gauge according to claim 5, wherein the resistor comprises chromium nitride.

8. The strain gauge of claim 7 , wherein the functional layer comprises titanium.

9. 9. The strain gauge of claim 8, wherein the resistor comprises titanium.

10. 10. The strain gauge according to claim 8, wherein the resistor comprises titanium nitride.

11. A strain gauge according to any one of claims 1 to 10; a strain-generating element provided on the other surface side of the base material.

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