JP7577907B2 - Strain gauges - Google Patents

Description

The present invention relates to a strain gauge.

There is known a strain gauge that is attached to a measurement object to detect the strain of the measurement object. The strain gauge has a resistor that detects strain, and the resistor is formed, for example, on a substrate. The resistor is connected to an electrode, for example, via wiring. The strain gauge also has an insulating resin layer on the substrate that covers, for example, the resistor and wiring (see, for example, Patent Document 1).

JP 2016-74934 A

However, for example, when a strain gauge is used in a high-temperature, high-humidity environment, water droplets may adhere to the top surface of the insulating resin layer and evaporate, collecting and concentrating dirt on the top surface of the insulating resin layer. When the dirt is concentrated, base points that are inclined to acid or alkali are generated, deteriorating the insulating resin layer, and ultimately causing corrosion of the wiring covered by the insulating resin layer due to the penetration of moisture.

The present invention has been made in consideration of the above points, and aims to provide a strain gauge that can suppress the penetration of moisture through the insulating resin layer that covers the wiring.

This strain gauge has a flexible substrate, a resistor formed on the substrate, wiring formed on the substrate and electrically connected to the resistor, and an insulating resin layer formed on the substrate and covering the resistor and the wiring, wherein the insulating resin layer has a plurality of recesses recessed from the top surface toward the substrate , and the recesses have a bottom surface and four inner sides connected to the bottom surface .

The disclosed technology makes it possible to provide a strain gauge that can suppress the penetration of moisture through the insulating resin layer that covers the wiring.

FIG. 2 is a plan view illustrating the strain gauge according to the first embodiment. FIG. 1 is a cross-sectional view (part 1) illustrating a strain gauge according to a first embodiment. FIG. 2 is a cross-sectional view (part 2) illustrating the strain gauge according to the first embodiment. FIG. 2 is a partial perspective view illustrating a cover layer of the strain gauge according to the first embodiment. FIG. 4 is a cross-sectional view (part 3) illustrating the strain gauge according to the first embodiment;

Below, a description will be given of a mode for carrying out the invention with reference to the drawings. In each drawing, the same components are given the same reference numerals, and duplicated explanations may be omitted.

First Embodiment
FIG. 1 is a plan view illustrating the strain gauge according to the first embodiment. FIG. 2 is a cross-sectional view illustrating the strain gauge according to the first embodiment, showing a cross section along the line A-A in FIG. 1. FIG. 3 is a cross-sectional view illustrating the strain gauge according to the first embodiment, showing a cross section along the line B-B in FIG. 1. FIG. 4 is a partial perspective view illustrating a cover layer of the strain gauge according to the first embodiment. With reference to FIGS. 1 to 4, the strain gauge 1 has a substrate 10, a resistor 30, wiring 40, electrodes 50, and a cover layer 60. Note that in FIG. 1, for convenience, only the outer edge of the cover layer 60 is shown by a dashed line.

In this embodiment, for convenience, in the strain gauge 1, the side of the substrate 10 on which the resistor 30 is provided is referred to as the upper side or one side, and the side on which the resistor 30 is not provided is referred to as the lower side or the other side. Also, the surface on which the resistor 30 is provided in each portion is referred to as the one side or upper surface, and the surface on which the resistor 30 is not provided is referred to as the other side or lower surface. However, the strain gauge 1 can be used upside down or placed at any angle. Also, 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.

The substrate 10 is a flexible member that serves as a base layer for forming the resistor 30 and the like. There are no particular limitations on the thickness of the substrate 10, and it can be selected appropriately depending on the purpose, but it 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 transmission of strain from the surface of the strain generator that is joined to the underside 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.

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, LCP (liquid crystal polymer) resin, polyolefin resin, etc. Note that a film refers to a flexible material with a thickness of about 500 μm or less.

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.

Examples of materials other than _resin for the substrate 10 include crystalline materials such _{as SiO2} , _ZrO2 (including YSZ), Si, _Si2N3 , _Al2O3 (including sapphire), ZnO, perovskite ceramics ( _CaTiO3 , _BaTiO3 ), and amorphous glass. Metals such as aluminum, aluminum alloys (duralumin), and titanium may also be used as the material for the substrate 10. In this case, for example, an insulating film is formed on the metal substrate 10.

The resistor 30 is a thin film formed in a predetermined pattern on the substrate 10, and is a sensing part that generates a resistance change when strained. The resistor 30 may be formed directly on the upper surface 10a of the substrate 10, or may be formed on the upper surface 10a of the substrate 10 via another layer. For convenience, the resistor 30 is shown in FIG. 1 with a dark matte pattern.

The resistor 30 has multiple elongated parts arranged at regular intervals with their longitudinal direction in the same direction (the direction of line A-A in Figure 1), and the ends of adjacent elongated parts are alternately connected, so that the resistor 30 is folded back in a zigzag pattern as a whole. The longitudinal direction of the multiple elongated parts is the grid direction, and the direction perpendicular to the grid direction is the grid width direction (the direction of line B-B in Figure 1).

One end in the longitudinal direction of the two elongated portions located at the outermost sides in the grid width direction is bent in the grid width direction to form respective ends _30e1 and _30e2 in the grid width direction of the resistor 30. The respective ends _30e1 and _30e2 in the grid width direction of the resistor 30 are electrically connected to the electrodes 50 via the wiring 40. In other words, the wiring 40 electrically connects the respective ends _30e1 and _30e2 in the grid width direction of the resistor 30 to the respective electrodes 50.

The resistor 30 can be formed, for example, from a material containing Cr (chromium), a material containing Ni (nickel), or a material containing both Cr and Ni. That is, the resistor 30 can be formed from a material containing at least one of Cr and Ni. An example of a material containing Cr is a Cr mixed phase film. An example of a material containing Ni is Cu-Ni (copper-nickel). An example of a material containing both Cr and Ni is Ni-Cr (nickel-chromium).

Here, the Cr mixed phase film is a film in which Cr, CrN, Cr ₂ N, etc. are mixed together. The Cr mixed phase film may contain inevitable impurities such as chromium oxide.

The thickness of the 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 0.1 μm or more is preferable in that the crystallinity of the crystals constituting the resistor 30 (for example, the crystallinity of α-Cr) is improved. Furthermore, a thickness of 1 μm or less is even more preferable in that film cracks caused by internal stress in the film constituting the resistor 30 and warping from the substrate 10 can be reduced. The width of the resistor 30 can be optimized for the required specifications such as resistance value and lateral sensitivity, and can be, for example, about 10 μm to 100 μm, taking into consideration measures against disconnection.

For example, when the resistor 30 is a Cr mixed phase film, the stability of the gauge characteristics can be improved by making the main component α-Cr (alpha chromium), which is a stable crystal phase. In addition, by making the resistor 30 mainly composed of α-Cr, the gauge factor of the strain gauge 1 can be set to 10 or more, and the gauge factor temperature coefficient TCS and the temperature coefficient of resistance TCR can be set within the range of -1000 ppm/°C to +1000 ppm/°C. Here, the main component means that the target substance accounts for 50% by weight or more of the total substance constituting the resistor, but from the viewpoint of improving the gauge characteristics, it is preferable that the resistor 30 contains α-Cr at 80% by weight or more, and more preferably at 90% by weight or more. Note that α-Cr is Cr with a bcc structure (body-centered cubic lattice structure).

Furthermore, when the resistor 30 is a Cr mixed-phase film, the Cr mixed-phase film preferably contains 20% by weight or less of CrN and Cr ₂ N. By containing 20% by weight or less of CrN and Cr ₂ N in the Cr mixed-phase film, a decrease in the gauge factor can be suppressed.

In addition, the ratio of _Cr2N in CrN and _Cr2N is preferably 80% by weight or more and less than 90% by weight, and more preferably 90% by weight or more and less than 95% by weight. When the ratio of _Cr2N in CrN and _Cr2N is 90% by weight or more and less than 95% by weight, the decrease in TCR (negative TCR) becomes more significant due to the _Cr2N having semiconducting properties. Furthermore, by reducing the formation of ceramics, brittle fracture is reduced.

On the other hand, if a small amount of _N2 or atomic N is mixed in or present in the film, it will escape to the outside of the film due to the external environment (for example, a high temperature environment), causing a change in the film stress. By creating chemically stable CrN, it is possible to obtain a stable strain gauge without generating the unstable N mentioned above.

The wiring 40 is formed on the substrate 10 and is electrically connected to the resistor 30 and the electrode 50. The wiring 40 has a first metal layer 41 and a second metal layer 42 laminated on the upper surface of the first metal layer 41. The wiring 40 is not limited to being linear, and can be in any pattern. The wiring 40 can also be of any width and length. For convenience, in FIG. 1, the wiring 40 and the electrode 50 are shown with a matte finish that is thinner than the resistor 30.

The electrodes 50 are formed on the substrate 10 and are electrically connected to the resistor 30 via the wiring 40, and are formed, for example, in a substantially rectangular shape wider than the wiring 40. The electrodes 50 are a pair of electrodes for outputting the change in resistance value of the resistor 30 caused by distortion to the outside, and for example, lead wires for external connection are joined to the electrodes 50.

The electrode 50 has a pair of first metal layers 51 and a second metal layer 52 laminated on the upper surface of each of the first metal layers 51. The first metal layers 51 are electrically connected to the ends _30e1 and _30e2 of the resistor 30 via the first metal layers 41 of the wiring 40. The first metal layers 51 are formed in a substantially rectangular shape in a plan view. The first metal layers 51 may be formed to have the same width as the wiring 40.

Note that although the resistor 30, the first metal layer 41, and the first metal layer 51 are given different reference numbers for convenience, they can be formed integrally from the same material in the same process. Therefore, the resistor 30, the first metal layer 41, and the first metal layer 51 have approximately the same thickness. Also, although the second metal layer 42 and the second metal layer 52 are given different reference numbers for convenience, they can be formed integrally from the same material in the same process. Therefore, the second metal layer 42 and the second metal layer 52 have approximately the same thickness.

The second metal layers 42 and 52 are formed from a material with a lower resistance than the resistor 30 (the first metal layers 41 and 51). There are no particular limitations on the material of the second metal layers 42 and 52, as long as it is a material with a lower resistance than the resistor 30, and it can be appropriately selected according to the purpose. For example, when the resistor 30 is a Cr mixed phase film, the material of the second metal layers 42 and 52 can be Cu, Ni, Al, Ag, Au, Pt, etc., or an alloy of any of these metals, a compound of any of these metals, or a laminated film in which any of these metals, alloys, and compounds are appropriately laminated. There are no particular limitations on the thickness of the second metal layers 42 and 52, and it can be appropriately selected according to the purpose, but it can be, for example, about 3 μm to 5 μm.

The second metal layers 42 and 52 may be formed on a portion of the upper surface of the first metal layers 41 and 51, or may be formed on the entire upper surface of the first metal layers 41 and 51. One or more other metal layers may be laminated on the upper surface of the second metal layer 52. For example, the second metal layer 52 may be a copper layer, and a gold layer may be laminated on the upper surface of the copper layer. Alternatively, the second metal layer 52 may be a copper layer, and a palladium layer and a gold layer may be sequentially laminated on the upper surface of the copper layer. By making the top layer of the electrode 50 a gold layer, the solder wettability of the electrode 50 can be improved.

In this way, the wiring 40 has a structure in which the second metal layer 42 is laminated on the first metal layer 41 made of the same material as the resistor 30. Therefore, the resistance of the wiring 40 is lower than that of the resistor 30, and the wiring 40 can be prevented from functioning as a resistor. As a result, the accuracy of strain detection by the resistor 30 can be improved.

In other words, by providing wiring 40 with a lower resistance than resistor 30, the actual sensing area of strain gauge 1 can be limited to the local area where resistor 30 is formed. This improves the accuracy of strain detection by resistor 30.

In particular, in a highly sensitive strain gauge with a gauge factor of 10 or more that uses a Cr mixed-phase film as the resistor 30, making the wiring 40 lower in resistance than the resistor 30 and limiting the actual sensing area to the local area where the resistor 30 is formed has a significant effect on improving strain detection accuracy. In addition, making the wiring 40 lower in resistance than the resistor 30 also has the effect of reducing lateral sensitivity.

The cover layer 60 (insulating resin layer) is formed on the substrate 10, covers the resistor 30 and the wiring 40, and exposes the electrodes 50. A portion of the wiring 40 may be exposed from the cover layer 60. By providing the cover layer 60 that covers the resistor 30 and the wiring 40, mechanical damage to the resistor 30 and the wiring 40 can be prevented. Furthermore, by providing the cover layer 60, the resistor 30 and the wiring 40 can be protected from moisture and the like. The cover layer 60 may be provided so as to cover the entire portion except for the electrodes 50.

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

The cover layer 60 has multiple recesses 60x recessed from the top surface toward the substrate 10. Therefore, the roughness of the top surface of the cover layer 60 is greater than the roughness of the side surfaces of the cover layer 60. By providing the recesses 60x that open into the top surface of the cover layer 60, the area of the top surface of the cover layer 60, including the surface area of the recesses 60x, can be increased. This allows the top surface of the cover layer 60 to exhibit high water repellency and prevent moisture from penetrating.

As mentioned above, when conventional strain gauges are used in a high-temperature, high-humidity environment, water droplets may adhere to the top surface of the cover layer and evaporate, collecting and concentrating dirt on the top surface of the cover layer. When the dirt is concentrated, base points that are inclined to acid or alkali are generated, deteriorating the cover layer, and ultimately causing corrosion of the wiring covered by the cover layer due to the penetration of moisture.

In the present application, the cover layer 60 having multiple recesses 60x exhibits high water repellency and prevents the penetration of moisture, thereby suppressing corrosion of the wiring 40 covered by the cover layer 60. In particular, when the second metal layer 42 of the wiring 40 contains copper, copper is susceptible to corrosion, so providing recesses 60x on the upper surface of the cover layer 60 to prevent the penetration of moisture is effective.

In order for the upper surface of the cover layer 60 to exhibit high water repellency, it is necessary to make the area of the upper surface of the cover layer 60 as large as possible, so it is preferable to provide a large number of fine recesses 60x on the upper surface of the cover layer 60. If the area of the upper surface of the cover layer 60 when no recesses 60x are formed is 1, the area of the upper surface of the cover layer 60 including the surface area of the recesses 60x when recesses 60x are formed is preferably 1.36 or more, and more preferably 2.00 or more. With such a surface area, the upper surface of the cover layer 60 exhibits high water repellency and can sufficiently suppress the penetration of moisture from the upper surface of the cover layer 60.

For example, if the recesses 60x are cubes with a width of 10 μm and a pitch of 20 μm (aspect ratio 1:2), and if the area of the top surface of the cover layer 60 when the recesses 60x are not formed is set to 1, then the area of the top surface of the cover layer 60 including the surface area of the recesses 60x when the recesses 60x are formed will be 1.36. Also, if the recesses 60x are cubes with a width of 10 μm and a pitch of 10 μm (aspect ratio 1:1), and if the area of the top surface of the cover layer 60 when the recesses 60x are not formed is set to 1, then the area of the top surface of the cover layer 60 including the surface area of the recesses 60x when the recesses 60x are formed will be 2.00.

The recess 60x can be any shape, such as a cylinder, an elliptical cylinder, or a square prism, but from the viewpoint of increasing the surface area, a square prism shape is preferable. That is, the recess 60x preferably has one bottom surface and four inner side surfaces connected to the bottom surface. When the recess 60x is a square prism shape, the recess 60x can be, for example, about 5 to 15 μm in length, 5 to 15 μm in width, and 5 to 15 μm in depth. The distance between adjacent recesses 60x can be, for example, about 5 to 15 μm.

The number density of the recesses 60x on the upper surface of the cover layer 60 is preferably 900/ ^mm2 or more, and more preferably 2500/mm2 or ^more . With such a number density, the upper surface of the cover layer 60 exhibits high water repellency, and the penetration of moisture from the upper surface of the cover layer 60 can be sufficiently suppressed.

For example, if the recesses 60x are cubes with a width of 10 μm and a pitch of 20 μm (aspect ratio 1:2), the number density of the recesses 60x on the upper surface of the cover layer 60 is 900 pieces/ ^mm2 . If the recesses 60x are cubes with a width of 10 μm and a pitch of 10 μm (aspect ratio 1:1), the number density of the recesses 60x on the upper surface of the cover layer 60 is 2500 pieces/ ^mm2 .

When viewed from the normal direction of the upper surface of the cover layer 60, the recesses 60x are regularly arranged in a predetermined pattern on the upper surface of the cover layer 60. Here, "regularly arranged" refers to an arrangement that is intentionally arranged so that the same pattern appears repeatedly. In this embodiment, the multiple recesses 60x are arranged in a matrix, but this is not limited to this, and they may be arranged, for example, in a staggered pattern, or in any other arrangement as long as it is regular.

By arranging the recesses 60x in a regular pattern, the entire upper surface of the cover layer 60 has a substantially uniform water repellency. The upper surface of the cover layer 60 preferably has a contact angle with water of 90 degrees or more, and more preferably 120 degrees or more. With such a contact angle, the upper surface of the cover layer 60 exhibits high water repellency and can sufficiently suppress the penetration of water from the upper surface of the cover layer 60.

To manufacture the strain gauge 1, first, a substrate 10 is prepared, and a metal layer (for convenience, referred to as metal layer A) is formed on the upper surface 10a of the substrate 10. Metal layer A is a layer that will ultimately be patterned to become the resistor 30, the first metal layer 41, and the first metal layer 51. Therefore, the material and thickness of metal layer A are the same as those of the resistor 30, the first metal layer 41, and the first metal layer 51 described above.

Metal layer A can be formed, for example, by magnetron sputtering using a raw material capable of forming metal layer A as a target. Instead of magnetron sputtering, metal layer A may also be formed using reactive sputtering, vapor deposition, arc ion plating, pulsed laser deposition, or the like.

From the viewpoint of stabilizing the gauge characteristics, it is preferable to vacuum-deposit a functional layer of a predetermined thickness as a base layer on the upper surface 10a of the substrate 10, for example, by conventional sputtering, before depositing the metal layer A.

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

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

The material of the functional layer is not particularly limited as long as it has the function of promoting the crystal growth of at least the upper metal layer A (resistor 30), and can be appropriately selected according to the purpose. For example, one or more metals selected from the group consisting of 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), and Al (aluminum), an alloy of any of the metals in this group, or a compound of any of the metals in this group can be mentioned.

Examples of the alloy include FeCr, TiAl, FeNi, NiCr, CrCu, _etc. Examples of the compound include TiN, TaN, _Si3N4 , _TiO2 , _Ta2O5 , _SiO2 , _etc.

When the functional layer is made of a conductive material such as a metal or alloy, the thickness of the functional layer is preferably 1/20 or less of the thickness of the resistor. In this range, the crystal growth of α-Cr can be promoted, and a part of the current flowing through the resistor can be prevented from flowing through the functional layer, which would reduce the sensitivity of strain detection.

When the functional layer is made of a conductive material such as a metal or alloy, it is more preferable that the thickness of the functional layer is 1/50 or less of the thickness of the resistor. In this range, the crystal growth of α-Cr can be promoted, and it is also possible to further prevent a portion of the current flowing through the resistor from flowing through the functional layer, thereby reducing the sensitivity of strain detection.

When the functional layer is made of a conductive material such as a metal or alloy, it is even more preferable that the thickness of the functional layer is 1/100 or less of the thickness of the resistor. In this range, it is possible to further prevent a portion of the current flowing through the resistor from flowing through the functional layer, thereby preventing a decrease in the strain detection sensitivity.

When the functional layer is made of an insulating material such as an oxide or nitride, the thickness of the functional layer is preferably 1 nm to 1 μm. This range promotes the crystal growth of α-Cr and allows the functional layer to be easily formed without cracking.

When the functional layer is made of an insulating material such as an oxide or nitride, it is more preferable that the thickness of the functional layer is 1 nm to 0.8 μm. This range promotes the crystal growth of α-Cr and makes it easier to form the functional layer without cracking.

When the functional layer is made of an insulating material such as an oxide or nitride, it is even more preferable that the thickness of the functional layer is between 1 nm and 0.5 μm. This range promotes the crystal growth of α-Cr and makes it easier to form the functional layer without cracking.

The planar shape of the functional layer is patterned to be approximately the same as the planar shape of the resistor shown in FIG. 1, for example. However, the planar shape of the functional layer is not limited to being approximately the same as the planar shape of the resistor. If the functional layer is made of an insulating material, it does not have to be patterned to be the same as the planar shape of the resistor. In this case, the functional layer may be formed in a solid shape at least in the area where the resistor is formed. Alternatively, the functional layer may be formed in a solid shape over the entire top surface of the substrate 10.

In addition, when the functional layer is made of an insulating material, the functional layer is formed relatively thick, at a thickness of 50 nm to 1 μm, and is formed in a solid shape, so that the thickness and surface area of the functional layer are increased, and the heat generated by the resistor can be dissipated to the substrate 10. As a result, the deterioration of measurement accuracy due to self-heating of the resistor can be suppressed in the strain gauge 1.

The functional layer can be formed in a vacuum by conventional sputtering, for example, using a raw material capable of forming the functional layer as a target 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, so that the amount of the functional layer formed can be minimized and the effect of improving adhesion can be obtained.

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.

There are no particular restrictions on the combination of the material of the functional layer and the material of the metal layer A, and they can be selected appropriately depending on the purpose. For example, it is possible to use Ti as the functional layer and form a Cr mixed phase film with α-Cr (alpha chromium) as the main component as the metal layer A.

In this case, for example, the metal layer A can be formed by magnetron sputtering using a raw material capable of forming a Cr mixed phase film as a target and introducing Ar gas into a chamber. Alternatively, the metal layer A can be formed by reactive sputtering using pure Cr as a target and introducing an appropriate amount of nitrogen gas together with Ar gas into a chamber. In this case, the ratio of CrN and Cr ₂ N contained in the Cr mixed phase film and the ratio of Cr ₂ N in CrN and Cr ₂ N can be adjusted by changing the amount and pressure (nitrogen partial pressure) of the nitrogen gas introduced or by adjusting the heating temperature by providing a heating process.

In these methods, the growth surface of the Cr mixed-phase film is determined by the functional layer made of Ti, and a Cr mixed-phase film can be formed that is mainly composed of α-Cr, which has a stable crystal structure. In addition, the Ti that constitutes 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 gauge factor temperature coefficient TCS and the resistance temperature coefficient TCR can be set within the range of -1000 ppm/°C to +1000 ppm/°C. Note that when the functional layer is formed from Ti, the Cr mixed-phase film may contain Ti and TiN (titanium nitride).

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

In this way, by providing a functional layer below the metal layer A, it is possible to promote crystal growth in the metal layer A, and to produce a metal layer A consisting of a stable crystalline phase. As a result, the stability of the gauge characteristics of the strain gauge 1 can be improved. In addition, the material constituting the functional layer diffuses into the metal layer A, thereby improving the gauge characteristics of the strain gauge 1.

Next, the second metal layer 42 and the second metal layer 52 are formed on the upper surface of the metal layer A. The second metal layer 42 and the second metal layer 52 can be formed, for example, by a photolithography method.

Specifically, first, a seed layer is formed by, for example, sputtering or electroless plating so as to cover the upper surface of metal layer A. Next, a photosensitive resist is formed over the entire upper surface of the seed layer, and is exposed and developed to form openings that expose the areas in which second metal layer 42 and second metal layer 52 are to be formed. At this time, the pattern of second metal layer 42 can be formed into any shape by adjusting the shape of the openings in the resist. For example, a dry film resist can be used as the resist.

Next, for example, the second metal layer 42 and the second metal layer 52 are formed on the seed layer exposed in the opening by electrolytic plating using the seed layer as a power supply path. The electrolytic plating method is advantageous in that it has high tact and can form low-stress electrolytic plating layers as the second metal layer 42 and the second metal layer 52. By making the thick electrolytic plating layer low-stress, it is possible to prevent warping of the strain gauge 1. The second metal layer 42 and the second metal layer 52 may also be formed by electroless plating.

Next, the resist is removed. For example, the resist can be removed by immersing it in a solution that can dissolve the resist material.

Next, a photosensitive resist is formed on the entire upper surface of the seed layer, and is exposed and developed to be patterned into a planar shape similar to the resistor 30, wiring 40, and electrode 50 in FIG. 1. For example, a dry film resist can be used as the resist. Then, the resist is used as an etching mask to remove the metal layer A and the seed layer exposed from the resist, forming the resistor 30, wiring 40, and electrode 50 in the planar shape of FIG. 1.

For example, unnecessary portions of the metal layer A and the seed layer can be removed by wet etching. If a functional layer is formed below the metal layer A, the functional layer is patterned by etching into the planar shape shown in FIG. 1, similar to the resistor 30, the wiring 40, and the electrode 50. At this point, a seed layer is formed on the resistor 30, the first metal layer 41, and the first metal layer 51.

Next, the second metal layer 42 and the second metal layer 52 are used as an etching mask to remove the unnecessary seed layer exposed from the second metal layer 42 and the second metal layer 52, thereby forming the second metal layer 42 and the second metal layer 52. Note that the seed layer directly below the second metal layer 42 and the second metal layer 52 remains. For example, the unnecessary seed layer can be removed by wet etching using an etching solution that etches the seed layer but does not etch the functional layer, resistor 30, wiring 40, and electrode 50.

After that, a cover layer 60 is provided on the upper surface 10a of the substrate 10 to cover the resistor 30 and the wiring 40 and expose the electrodes 50. Specifically, first, a semi-cured thermosetting insulating resin film is prepared with a plurality of recesses 60x formed on the upper surface by a mold or the like. Then, for example, the semi-cured thermosetting insulating resin film is laminated on the upper surface 10a of the substrate 10 so as to cover the resistor 30 and the wiring 40 and expose the electrodes 50. The semi-cured thermosetting insulating resin film is then heated and pressurized to harden it, forming the cover layer 60. Lamination is preferably performed in a vacuum atmosphere. The above steps complete the strain gauge 1.

It is also possible to prepare a thermosetting insulating resin film without the recesses 60x on its upper surface, laminate it to the upper surface 10a of the substrate 10 so as to cover the resistor 30 and the wiring 40 and expose the electrode 50, and then form the recesses 60x using a mold or the like. However, in this case, there is a risk that stress will be applied to the resistor 30, etc., when the recesses 60x are formed using a mold or the like, which may lead to breakage. For this reason, it is preferable to laminate a semi-cured thermosetting insulating resin film with multiple recesses 60x formed on its upper surface onto the substrate 10.

When a functional layer is provided on the upper surface 10a of the substrate 10 as an underlayer for the resistor 30, the first metal layer 41, and the first metal layer 51, the strain gauge 1 has a cross-sectional shape as shown in FIG. 5. The layer indicated by the reference numeral 20 is the functional layer. When the functional layer 20 is provided, the planar shape of the strain gauge 1 is, for example, the same as that shown in FIG. 1. However, as described above, the functional layer 20 may be formed in a solid shape on part or all of the upper surface of the substrate 10.

Although the preferred embodiments have been described above in detail, 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.

For example, the present invention can be applied to a strain gauge in which multiple resistors are formed on a substrate. The present invention can be applied to a strain gauge in which two resistors are formed on a substrate to form a half-bridge circuit. Alternatively, the present invention can be applied to a strain gauge in which four resistors are formed on a substrate to form a full-bridge circuit.

REFERENCE SIGNS LIST 1 strain gauge, 10 substrate, 10a upper surface, 20 functional layer, 30 resistor, 30e ₁ , 30e ₂ termination, 40 wiring, 50 electrode, 41, 51 first metal layer, 42, 52 second metal layer, 60 cover layer, 60x recess

Claims (11)

A flexible substrate;
A resistor formed on the substrate;
A wiring formed on the base material and electrically connected to the resistor;
an insulating resin layer formed on the base material and covering the resistor and the wiring;
the insulating resin layer has a plurality of recesses recessed from an upper surface toward the base material ,
The recess has a bottom surface and four inner sides connected to the bottom surface . The strain gauge according to claim 1, wherein the recesses are regularly arranged on the upper surface of the insulating resin layer when viewed from the normal direction of the upper surface of the insulating resin layer. The strain gauge according to claim 1 or 2 , wherein a roughness of the upper surface of the insulating resin layer is greater than a roughness of a side surface of the insulating resin layer. 4. The strain gauge according to claim 1, wherein, when an area of the upper surface of the insulating resin layer when the recess is not formed is taken as 1 , an area of the upper surface of the insulating resin layer including a surface area of the recess when the recess is formed is 1.36 or more. 5. The strain gauge according to claim 1, wherein the upper surface of the insulating resin layer has a contact angle with water of 90 degrees or more. 6. The strain gauge according to claim 1, wherein the wiring includes a first metal layer formed of the same material as the resistor, and a second metal layer formed on an upper surface of the first metal layer. The strain gauge of claim 6 , wherein the second metal layer comprises copper. The strain gauge according to claim 1 , wherein the resistor is formed from a film containing Cr, CrN, and Cr ₂ N. 9. The strain gauge according to claim 8, having a gauge factor of 10 or more. 10. The strain gauge according to claim 8 , wherein the resistor contains 20% by weight or less of CrN and _Cr2N . The strain gauge according to claim 10 , wherein a ratio of said _Cr2N in said CrN and said _Cr2N is 80% by weight or more and less than 90% by weight.

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