Strain gauge

Abstract

To provide a strain gauge with a higher gauge factor and a lower lateral sensitivity ratio than a conventional one.SOLUTION: A strain gauge has a base material and a resistor formed on the base material. The resistor is formed of a material essentially comprising Cr. A line width W and a film thickness T of the resistor satisfy the following formula (1) so that a lateral sensitivity ratio is equal to or less than 70% and a gauge factor is equal to or more than 5. (1): w≤0.0177 t2+0.1521 t+2.9541, wherein w=logW and t=logT.SELECTED DRAWING: Figure 9

Description

[Technical field]

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

[0002] Conventionally, strain gauges are known that are attached to an object to be measured. The gauge factor of a strain gauge is usually about 2, but some strain gauges equipped with a resistor whose main component is Cr have a gauge factor of about 10 (see, for example, Patent Document 1).

[0003] In addition, it is known that strain gauges equipped with resistors whose main component is Cr not only have high longitudinal sensitivity, which is the sensitivity in the principal axis direction, but also have lateral sensitivity, which is the sensitivity in the direction perpendicular to the principal axis direction, that is as high as the longitudinal sensitivity, and this property is said to be independent of the shape of the resistor (see Non-Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6084393 [Non-patent literature]

[0005] [Non-Patent Document 1] Isotropy of Gauge Factor in Cr-N Thin Films for Strain Sensors, Proceedings of the 2013 Annual National Conference of the Institute of Electrical Engineers of Japan 3(2013):240-241 Summary of the Invention [Problem to be solved by the invention]

[0006] However, when detecting strain with a strain gauge, a large gauge factor is preferable, but if the transverse sensitivity ratio, which is the ratio of the transverse sensitivity to the longitudinal sensitivity, is large, detection errors will occur. Therefore, there is a demand for strain gauges with a higher gauge factor than the conventional gauge factor (about 2) and a lower transverse sensitivity ratio.

[0007] The present invention has been made in view of the above-mentioned points, and has an object to provide a strain gauge having a higher gauge factor and a lower transverse sensitivity ratio than conventional ones. [Means for solving the problem]

[0008] A strain gauge according to an embodiment of the present disclosure includes a substrate and a resistor formed on the substrate, the resistor being made of a material containing Cr as a main component, and the line width W and film thickness T of the resistor satisfy the following formula (1) so that the transverse sensitivity ratio is 70% or less and the gauge factor is 5 or more. 2 +0.1521t+2.9541···(1), where w = logW and t = logT in equation (1). Effect of the Invention

[0009] According to the disclosed technique, it is possible to provide a strain gauge having a higher gauge factor and a lower transverse sensitivity ratio than conventional ones. [Brief description of the drawings]

[0010] [Figure 1] FIG. 2 is a plan view illustrating the strain gauge according to the first embodiment. [Diagram 2] 1 is a cross-sectional view (part 1) illustrating a strain gauge according to a first embodiment. FIG. [Diagram 3] 4 is a cross-sectional view (part 2) illustrating the strain gauge according to the first embodiment. FIG. [Figure 4] FIG. 1 is a diagram illustrating a lateral sensitivity ratio. [Diagram 5] FIG. 1 is a diagram (part 1) showing the results of the study of Example 1. [Figure 6] FIG. 2 is a second diagram showing the results of the study of Example 1. [Figure 7] FIG. 3 is a diagram (part 3) showing the results of the study of Example 1. [Figure 8] FIG. 4 shows the results of the study of Example 1. [Figure 9]FIG. 5 shows the study results of Example 1. [Figure 10] FIG. 13 is a diagram showing the results of the study of Example 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In the drawings, the same components may be denoted by the same reference numerals.

[0012] First embodiment Fig. 1 is a plan view illustrating the strain gauge according to the first embodiment. Fig. 2 is a cross-sectional view (part 1) illustrating the strain gauge according to the first embodiment, showing a cross section along line AA in Fig. 1.

[0013] 1 and 2, the strain gauge 1 has a substrate 10, a resistor 30, wiring 40, electrodes 50, and a cover layer 60. The cover layer 60 can be provided as necessary. For convenience, only the outer edge of the cover layer 60 is shown by a dashed line in Figs. 1 and 2. First, each part constituting the strain gauge 1 will be described in detail.

[0014] 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", and the side on which the resistor 30 is not provided is referred to as the "lower side". The surface located on the upper side of each part is referred to as the "upper surface", and the surface located on the lower side of each part is referred to as the "lower surface". However, the strain gauge 1 can also be used upside down. The strain gauge 1 can also be arranged at any angle. The planar view refers to viewing the object in the normal direction from the upper side to the lower side of the upper surface 10a of the substrate 10. The planar shape refers to the shape of the object when the object is viewed in the normal direction.

[0015] The substrate 10 is a member that serves as a base layer for forming the resistor 30 and the like. The substrate 10 has flexibility. The thickness of the substrate 10 is not particularly limited and may be appropriately determined depending on the intended use of the strain gauge 1 and the like. For example, the thickness of the substrate 10 may be about 5 μm to 500 μm. A strain generator may be bonded to the lower surface side of the strain gauge 1 via an adhesive layer or the like. From the viewpoint of the transferability of strain from the surface of the strain generator to the sensing part and dimensional stability against environmental changes, the thickness of the substrate 10 is preferably within the range of 5 μm to 200 μm. From the viewpoint of insulation, the thickness of the substrate 10 is preferably 10 μm or more.

[0016] The substrate 10 is 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. The film refers to a member having a thickness of about 500 μm or less and having flexibility.

[0017] When the substrate 10 is formed from an insulating resin film, the insulating resin film may contain a filler, impurities, etc. For example, the substrate 10 may be formed from an insulating resin film containing a filler such as silica or alumina.

[0018] 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, and perovskite ceramics (CaTiO3, BaTiO3). In addition to the above-mentioned crystalline materials, amorphous glass or the like may be used as the material for the substrate 10. Metals such as aluminum, aluminum alloys (duralumin), stainless steel, and titanium may also be used as the material for the substrate 10. When a metal is used, an insulating film is provided so as to cover the upper and lower surfaces of the metallic substrate 10.

[0019] The resistor 30 is a thin film formed in a predetermined pattern on the upper side of the substrate 10. In the strain gauge 1, the resistor 30 is a sensing part that receives strain and generates a resistance change. The resistor 30 may be formed directly on the upper surface 10a of the substrate 10, or may be formed on the upper surface 10a of the substrate 10 via another layer. For convenience, the resistor 30 is shown in FIG. 1 as having a dense matte pattern.

[0020] The resistor 30 includes a plurality of elongated portions and a plurality of folded portions. In the resistor 30, the elongated portions are arranged in parallel with their longitudinal directions facing in the same direction. The folded portions alternately connect the ends of adjacent elongated portions among the plurality of elongated portions to connect the elongated portions in series. This gives the resistor 30 a structure in which it is folded back in a zigzag pattern as a whole. The longitudinal direction of the elongated portions corresponds to the grid direction, and the direction perpendicular to the grid direction corresponds to the grid width direction.

[0021] 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 terminal ends 30e1 and 30e2 in the grid width direction of the resistor 30. The terminal 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 terminal ends 30e1 and 30e2 in the grid width direction of the resistor 30 to the electrodes 50.

[0022] In the strain gauge 1, the gauge factor is preferably 5 or more. The gauge factor of the strain gauge 1 does not depend on the line width W of the resistor 30. Here, the gauge factor not depending on the line width W of the resistor 30 refers to the case where the difference between the maximum and minimum values ​​of the gauge factor in the line width W of the resistor 30 in the range of 0.1 μm to 70 μm is 2 or less. In addition, the transverse sensitivity ratio of the strain gauge 1 has a positive correlation with the logarithm of the line width W of the resistor 30, and becomes lower as the line width W of the resistor is narrower.

[0023] Moreover, the gauge factor of the strain gauge 1 does not depend on the film thickness T of the resistor 30. Here, the gauge factor not depending on the film thickness T of the resistor 30 refers to a case where the difference between the maximum and minimum values ​​of the gauge factor in the film thickness T of the resistor 30 ranges from 30 nm to 800 nm is 2 or less. Moreover, the transverse sensitivity ratio of the strain gauge 1 has a negative correlation with the film thickness T of the resistor 30, and decreases as the film thickness T of the resistor 30 increases.

[0024] The value of the transverse sensitivity ratio of the strain gauge 1 changes depending on both the line width W and film thickness T of the resistor 30. The definition of the transverse sensitivity ratio and the relationship between the line width W and film thickness T of the resistor 30 and the gauge factor and the transverse sensitivity ratio will be described in detail later.

[0025] The strain limit has a negative correlation with the film thickness T of the resistor 30, and the thicker the film thickness T of the resistor 30, the lower the strain limit becomes. Here, the strain limit is the value of mechanical strain at which cracks or breaks start to occur when strain is applied to the strain gauge. The relationship between the film thickness T of the resistor 30 and the strain limit will be described in detail later.

[0026] The resistor 30 can be made of, for example, a material containing Cr (chromium) as a main component. Here, the term "main component" refers to a component that accounts for 50% by weight or more of the total material constituting the resistor 30. An example of a material containing Cr as a main component is a Cr mixed-phase film. The Cr mixed-phase film is a film in which Cr, CrN, Cr2N, and the like are mixed. The Cr mixed-phase film may contain inevitable impurities such as chromium oxide.

[0027] For example, when the resistor 30 is a Cr mixed-phase film, the stability of the gauge characteristics can be improved by using α-Cr (alpha chromium) which is a stable crystal phase as the main component. Also, for example, when the resistor 30 is a Cr mixed-phase film, the resistor 30 uses α-Cr as the main component, so that the gauge factor of the strain gauge 1 can be 10 or more, and the gauge factor temperature coefficient TCS and the resistance temperature coefficient TCR can be in the range of -1000 ppm / °C to +1000 ppm / °C. From the viewpoint of improving the gauge characteristics, the resistor 30 preferably contains α-Cr in an amount of 80% by weight or more. Furthermore, from the same viewpoint, the resistor 30 more preferably contains α-Cr in an amount of 90% by weight or more. Note that α-Cr is Cr with a bcc structure (body-centered cubic lattice structure).

[0028] In addition, 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 Cr2N. By containing 20% ​​by weight or less of CrN and Cr2N in the Cr mixed-phase film, a decrease in the gauge factor of the strain gauge 1 can be suppressed.

[0029] In addition, the ratio of CrN and Cr2N in the Cr mixed phase film is preferably such that the ratio of Cr2N is 80% by weight or more and less than 90% by weight with respect to the total weight of CrN and Cr2N. More specifically, the ratio is more preferably such that the ratio of Cr2N is 90% by weight or more and less than 95% by weight with respect to the total weight of CrN and Cr2N. Cr2N has semiconductor properties. Therefore, by setting the ratio of Cr2N to 90% by weight or more and less than 95% by weight, the decrease in TCR (negative TCR) becomes more significant. Furthermore, by setting the ratio of Cr2N to 90% by weight or more and less than 95% by weight, the resistor 30 is less likely to become ceramic, and the resistor 30 is less likely to be brittle fractured.

[0030] On the other hand, CrN has the advantage of being chemically stable. By including more CrN in the Cr mixed-phase film, the possibility of unstable N being generated can be reduced, resulting in a stable strain gauge. Here, "unstable N" refers to trace amounts of N2 or atomic N that may be present in the Cr mixed-phase film. This unstable N may escape to the outside of the film depending on the external environment (e.g., high-temperature environment). When unstable N escapes to the outside of the film, the film stress of the Cr mixed-phase film may change.

[0031] In the strain gauge 1, when a Cr mixed-phase film is used as the material of the resistor 30, high sensitivity and miniaturization can be achieved. For example, the output change of a conventional strain gauge was about 0.04 mV / 2 V, whereas an output change of 0.3 mV / 2 V or more can be obtained when a Cr mixed-phase film is used as the material of the resistor 30. In addition, while the size (gauge length x gauge width) of a conventional strain gauge was about 3 mm x 3 mm, the size (gauge length x gauge width) of the resistor 30 can be reduced to about 0.3 mm x 0.3 mm when a Cr mixed-phase film is used as the material of the resistor 30.

[0032] The wiring 40 is provided on the substrate 10. One end of the wiring 40 is electrically connected to both ends of the resistor 30, and the other end is electrically connected to the electrodes 50. The wiring 40 is not limited to being linear, and may have any pattern. The wiring 40 may have any width and any length. For convenience, the wiring 40 is shown in FIG. 1 as having a matte pattern with a lower density than the resistor 30.

[0033] The electrodes 50 are provided on the substrate 10. The electrodes 50 are electrically connected to the resistor 30 via the wiring 40. The electrodes 50 are formed in a substantially rectangular shape wider than the wiring 40 in a plan view. 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. For example, a lead wire for external connection is joined to the electrodes 50. A metal layer having low resistance such as copper or a metal layer having good solderability such as gold may be laminated on the upper surface of the electrode 50. Although the resistor 30, the wiring 40, and the electrode 50 are denoted by different reference numerals for convenience, they can be integrally formed from the same material in the same process. In FIG. 1, the electrodes 50 are shown with a matte pattern having the same density as the wiring 40 for convenience.

[0034] The cover layer 60 is provided on the upper surface 10a of the base material 10 as necessary so as to cover the resistor 30 and the wiring 40 and expose the electrodes 50. Examples of materials for the cover layer 60 include insulating resins such as PI resin, epoxy resin, PEEK resin, PEN resin, PET resin, PPS resin, and composite resins (e.g., silicone resin, polyolefin resin). The cover layer 60 may contain a filler or a pigment. The thickness of the cover layer 60 is not particularly limited and can be appropriately selected depending on the purpose. For example, the thickness of the cover layer 60 can be about 2 μm to 30 μm. By providing the cover layer 60, it is possible to suppress mechanical damage and the like from occurring to the resistor 30. In addition, by providing the cover layer 60, it is possible to protect the resistor 30 from moisture and the like.

[0035] [Strain gauge manufacturing method] In the strain gauge 1 according to this embodiment, a resistor 30, wiring 40, electrodes 50, and a cover layer 60 are formed on a substrate 10. Note that another layer (such as a functional layer described below) may be formed between the substrate 10 and these layers of components.

[0036] A method for manufacturing the strain gauge 1 will be described below. To manufacture the strain gauge 1, first, a base material 10 is prepared, and a metal layer (for convenience, referred to as metal layer A) is formed on the upper surface 10a of the base material 10. Metal layer A is a layer that is ultimately patterned to become resistor 30, wiring 40, and electrode 50. Therefore, the material and thickness of metal layer A are the same as those of resistor 30 and the like described above.

[0037] The metal layer A can be formed, for example, by magnetron sputtering using a target made of a raw material capable of forming the metal layer A. Instead of magnetron sputtering, the metal layer A may be formed by reactive sputtering, vapor deposition, arc ion plating, pulsed laser deposition, or the like. During formation of the metal layer A, the thickness of the metal layer A can be adjusted to a desired value.

[0038] After forming a metal layer A on the upper surface 10a of the substrate 10, the metal layer A is patterned by a well-known photolithography method into a planar shape similar to that of the resistor 30, the wiring 40, and the electrode 50 in Fig. 1. During patterning by the photolithography method, the line width of the resistor 30 can be adjusted to a desired value. The resistor 30 includes a plurality of juxtaposed elongated portions and folded portions that alternately connect the ends of adjacent elongated portions among the plurality of elongated portions to connect the elongated portions in series.

[0039] Alternatively, a base layer may be formed on the upper surface 10a of the substrate 10, and then the metal layer A may be formed. For example, a functional layer of a predetermined thickness may be vacuum-formed by conventional sputtering on the upper surface 10a of the substrate 10. By providing a base layer in this manner, the gauge characteristics of the strain gauge 1 can be stabilized.

[0040] In the present application, the functional layer refers to a layer having a function of promoting the crystal growth of at least the upper layer, metal layer A (resistor 30). The functional layer preferably further has a function of preventing oxidation of metal layer A due to oxygen or moisture contained in the substrate 10, and / or a function of improving adhesion between the substrate 10 and metal layer A. The functional layer may further have other functions.

[0041] The insulating resin film constituting the substrate 10 may contain oxygen and moisture, and Cr may form a self-oxidized film. Therefore, particularly when the metal layer A contains Cr, it is preferable to form a functional layer having a function of preventing the oxidation of the metal layer A.

[0042] 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 crystal phase. As a result, the stability of the gauge characteristics of the strain gauge 1 is 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.

[0043] Examples of materials for the functional layer include 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.

[0044] 3 is a cross-sectional view (part 2) illustrating the strain gauge according to the first embodiment. FIG. 3 shows the cross-sectional shape of the strain gauge 1 when a functional layer 20 is provided as an underlayer for the resistor 30, the wiring 40, and the electrodes 50.

[0045] The planar shape of the functional layer 20 may be patterned to be substantially the same as the planar shapes of the resistor 30, the wiring 40, and the electrodes 50, for example. However, the planar shapes of the functional layer 20, the resistor 30, the wiring 40, and the electrodes 50 may not be substantially the same. For example, when the functional layer 20 is formed from an insulating material, the functional layer 20 may be patterned to be different from the planar shapes of the resistor 30, the wiring 40, and the electrodes 50. In this case, the functional layer 20 may be formed in a solid shape in the region where the resistor 30, the wiring 40, and the electrodes 50 are formed, for example. Alternatively, the functional layer 20 may be formed in a solid shape on the entire upper surface of the substrate 10.

[0046] After forming the resistor 30, the wiring 40, and the electrodes 50, a cover layer 60 is formed on the upper surface 10a of the base material 10 as necessary. The cover layer 60 covers the resistor 30 and the wiring 40, but the electrodes 50 may be exposed from the cover layer 60. For example, the cover layer 60 can be formed by laminating a semi-cured thermosetting insulating resin film on the upper surface 10a of the base material 10 so as to cover the resistor 30 and the wiring 40 and expose the electrodes 50, and then heating and curing the insulating resin film. Through the above steps, the strain gauge 1 is completed.

[0047] [Reduction in transverse sensitivity ratio] First, the transverse sensitivity ratio will be described. FIG. 4 is a diagram for explaining the transverse sensitivity ratio. In FIG. 4, axis M indicates the direction in which strain is to be measured. In FIG. 4, strain gauge 1A, whose grid direction faces the direction of axis M, and strain gauge 1B, whose grid direction faces a direction perpendicular to axis M, are attached to strain gauge 100. Note that the x-axis, y-axis, and z-axis in FIG. 4 indicate axes that are orthogonal to each other. Axis M is parallel to the y-axis, and the direction perpendicular to axis M is parallel to the x-axis.

[0048] In Figure 4, when a tensile strain occurs in the direction of axis M, the resistor of strain gauge 1A expands in the direction of axis M, and the resistor of strain gauge 1B contracts due to Poisson's ratio in a direction perpendicular to axis M. Arrow N in Figure 4 indicates the direction of contraction due to Poisson's ratio.

[0049] Here, the sensitivity to strain occurring in the direction of axis M is called the longitudinal sensitivity F. a The sensitivity to strain occurring in a direction perpendicular to the axis M is called the transverse sensitivity F. t Then, equations (1) and (2) hold. Also, the lateral sensitivity F t and vertical sensitivity F a The transverse sensitivity ratio K, defined as the ratio of

[0050] In addition, in formulas (1) and (2), ε y is the strain generated in the direction of axis M, and ε x is the strain generated in the direction perpendicular to the axis M. Also, R A is the resistance value of the resistor of the strain gauge 1A when no strain is generated. Also, R B is the resistance value of the resistor of the strain gauge 1B when no strain is generated. Also, ΔR A is the strain ε x and ε y This is the change in resistance that occurs in the resistor of the strain gauge 1A when a strain B is the strain ε x and ε y This is the change in resistance value that occurs in the resistor of the strain gauge 1B when

[0051]

number

[0052]

number

[0053]

number

[0054] The transverse sensitivity ratio is more preferably 50% or less, further preferably 40% or less, and particularly preferably 30% or less. The gauge factor is more preferably 8 or more, more preferably 11 or more, and particularly preferably 14 or more. The smaller the transverse sensitivity ratio and the larger the gauge factor, the larger the output change amount can be when a bridge circuit is formed with strain gauges.

[0055] Examples will be described below, but the present invention is not limited to the following examples in any way.

[0056] [Example 1] (Relationship between resistor line width, transverse sensitivity ratio and gauge factor) The inventors prepared samples of strain gauges and investigated the relationship between the line width of the resistor and the transverse sensitivity ratio and the gauge factor.

[0057] First, a 3 nm thick Ti film was vacuum-formed as the functional layer 20 on the upper surface 10a of the substrate 10 made of a 25 μm thick polyimide resin by conventional sputtering. Next, a 200 nm thick Cr mixed phase film was formed as the resistor 30, wiring 40, and electrode 50 on the entire upper surface of the functional layer 20 by magnetron sputtering. Then, the functional layer 20, resistor 30, wiring 40, and electrode 50 were patterned by photolithography as shown in FIG. 1 to prepare a strain gauge sample. In this sample, the resistor 30 was patterned so that the line width W was 500 μm.

[0058] Next, in a similar manner, strain gauge samples were fabricated in which the line widths W of the resistor 30 were patterned to be 200 μm, 100 μm, 50 μm, 40 μm, 20 μm, 10 μm, and 5 μm, respectively. The transverse sensitivity ratio and gauge factor of each strain gauge sample were then measured in an environment of 20° C., and the results are summarized in FIG.

[0059] As shown in FIG. 5, the gauge factor of the prepared sample does not depend on the line width W of the resistor 30. In FIG. 5, the difference between the maximum and minimum values ​​of the gauge factor when the line width W of the resistor 30 is changed is about 1.5. In contrast, the transverse sensitivity ratio has a positive correlation with the logarithm of the line width W of the resistor 30, and the narrower the line width of the resistor 30, the lower it becomes. FIG. 6 shows the relationship between the line width of the resistor and the transverse sensitivity ratio and the gauge factor when a sample with a gauge factor of about 9 is prepared by the same method. In this case, too, the gauge factor does not depend on the line width W of the resistor 30, whereas the transverse sensitivity ratio has a positive correlation with the logarithm of the line width W of the resistor 30, and the narrower the line width of the resistor 30, the lower it becomes. Although not shown, other samples with a gauge factor of 5 or more also exhibit similar characteristics.

[0060] From these results, it was found that by forming resistor 30 from a material mainly composed of Cr and narrowing the line width W of resistor 30, it is possible to reduce the transverse sensitivity ratio while maintaining a high gauge factor. In other words, it was found that by forming resistor 30 from a material mainly composed of Cr and controlling the line width W of the resistor, it is possible to realize a strain gauge with a higher gauge factor and a lower transverse sensitivity ratio than conventional ones. In this way, by reducing the transverse sensitivity ratio while maintaining a high gauge factor, it is possible to improve the accuracy of strain detection.

[0061] Fig. 7 is a diagram showing the relationship between the line width of a resistor, the transverse sensitivity ratio, and the output change of a strain gauge. Fig. 7 shows the relationship between the line width, the transverse sensitivity ratio, and the output change of a strain gauge, which was determined by the inventors through experiments for a strain gauge with a gauge factor of 5 and a strain gauge with a gauge factor of 15. Fig. 7 also shows the output change of a commercially available strain gauge with a gauge factor of 2 as a comparative example. The output change shown in Fig. 7 is the output change when a bridge circuit is formed with strain gauges.

[0062] As shown in FIG. 7, if the gauge factor is 5 and the transverse sensitivity ratio is 70% or more, it is possible to obtain an output change amount equal to or greater than that of commercially available strain gauges. By setting the line width W of resistor 30 to 70 μm or less, the transverse sensitivity ratio can be set to 70% or less. Therefore, by setting the line width W of resistor 30 to 70 μm or less so that the transverse sensitivity ratio is 70% or less and the gauge factor is 5 or more, a strain gauge with an output change amount equal to or greater than that of commercially available strain gauges can be realized. For example, by setting the line width W of resistor 30 to 70 μm so that the transverse sensitivity ratio is 70% or less and the gauge factor is 15, a strain gauge with an output change amount about three times that of commercially available strain gauges can be realized.

[0063] As described above, the transverse sensitivity ratio is more preferably 50% or less, further preferably 40% or less, and particularly preferably 30% or less. As shown in Fig. 5, the transverse sensitivity ratio can be reduced to 50% or less by setting the line width W of the resistor 30 to 20 μm or less. As shown in Fig. 5, the transverse sensitivity ratio can be reduced to 40% or less by setting the line width W of the resistor 30 to 10 μm or less. As shown in Fig. 5, the transverse sensitivity ratio can be reduced to 30% or less by setting the line width W of the resistor 30 to 5 μm or less.

[0064] Furthermore, to obtain an output change amount equal to or greater than that of commercially available strain gauges, a gauge factor of 5 or more is sufficient, but the gauge factor of each sample using a Cr mixed-phase film for the resistor 30 was 14 or more, which is a sufficiently large value. Even when a Cr mixed-phase film is used for the resistor 30, the gauge factor can be adjusted to various values ​​of 5 or more by changing the content of α-Cr in the resistor 30. Furthermore, by using a Cr mixed-phase film for the resistor 30 and increasing the crystallinity of α-Cr, the gauge factor becomes approximately 5 or more and 10 or less.

[0065] In this way, a strain gauge can be realized in which the resistor 30 is formed from a material containing Cr as a main component, the gauge factor is 5 or more, is independent of the line width of the resistor 30, and the transverse sensitivity ratio has a positive correlation with the logarithm of the line width of the resistor 30 and decreases as the line width of the resistor 30 becomes narrower. Such characteristics can be realized at least when the line width of the resistor 30 is in the range of 0.1 μm to 70 μm.

[0066] (Relationship between resistor film thickness, transverse sensitivity ratio and gauge factor) Next, the inventors prepared samples of strain gauges and investigated the relationship between the film thickness of the resistor and the transverse sensitivity ratio and the gauge factor.

[0067] First, a 3 nm thick Ti film was vacuum-formed as the functional layer 20 on the upper surface 10a of the substrate 10 made of a 25 μm thick polyimide resin by conventional sputtering. Next, a 800 nm thick Cr mixed phase film was formed as the resistor 30, wiring 40, and electrode 50 on the entire upper surface of the functional layer 20 by magnetron sputtering. After that, the functional layer 20, resistor 30, wiring 40, and electrode 50 were patterned by photolithography as shown in FIG. 1 to prepare a strain gauge sample. The resistor 30 was patterned so that its line width W was 50 μm.

[0068] Next, strain gauge samples were fabricated in the same manner, with the resistor 30 having a film thickness T of 500 nm, 200 nm, 100 nm, 50 nm, and 30 nm, respectively. The transverse sensitivity ratio and gauge factor of each strain gauge sample were then measured in an environment of 20° C., and the results are summarized in FIG.

[0069] As shown in Fig. 8, the gauge factor of the fabricated sample does not depend on the film thickness T of resistor 30. In Fig. 8, the difference between the maximum and minimum values ​​of the gauge factor when the film thickness T of resistor 30 is changed is about 1.5. In contrast, the transverse sensitivity ratio has a negative correlation with the film thickness T of resistor 30, and it is observed that the value of the transverse sensitivity ratio tends to decrease as the film thickness T of resistor 30 increases.

[0070] In FIG. 8, the gauge factor is about 15. However, it has been confirmed that samples with various gauge factors of 5 or more exhibit characteristics similar to those in FIG. 8, although not shown in the figures.

[0071] From these results, it was found that by forming resistor 30 from a material mainly composed of Cr and increasing the film thickness T of resistor 30, it is possible to reduce the transverse sensitivity ratio while maintaining a high gauge factor. In other words, it was found that by forming resistor 30 from a material mainly composed of Cr and controlling the film thickness T of the resistor, it is possible to realize a strain gauge with a higher gauge factor and a lower transverse sensitivity ratio than conventional ones. In this way, by reducing the transverse sensitivity ratio while maintaining a high gauge factor, it is possible to improve the accuracy of strain detection.

[0072] As mentioned above, if the transverse sensitivity ratio is 70% or less, an output change amount equal to or greater than that of commercially available strain gauges can be obtained, but as shown in Fig. 8, the transverse sensitivity ratio can be made 70% or less by setting the film thickness T of resistor 30 to 200 nm or more. In other words, a strain gauge can be realized in which the film thickness of resistor 30 is 200 nm or more so that the transverse sensitivity ratio is 70% or less and the gauge factor is 5 or more.

[0073] Furthermore, to obtain an output change amount equal to or greater than that of commercially available strain gauges, a gauge factor of 5 or more is sufficient, but the gauge factor of each sample using a Cr mixed-phase film for the resistor 30 was 14 or more, which is a sufficiently large value. Even when a Cr mixed-phase film is used for the resistor 30, the gauge factor can be adjusted to various values ​​of 5 or more by changing the content of α-Cr in the resistor 30. Furthermore, by using a Cr mixed-phase film for the resistor 30 and increasing the crystallinity of α-Cr, the gauge factor becomes approximately 5 or more and 10 or less.

[0074] In this way, a strain gauge can be realized in which resistor 30 is formed from a material containing Cr as a main component, the gauge factor is 5 or more, is independent of the film thickness of resistor 30, and the transverse sensitivity ratio is negatively correlated with the film thickness of resistor 30 and becomes lower as the film thickness of resistor 30 becomes thicker.

[0075] (Relationship between resistor line width and film thickness and transverse sensitivity ratio and gauge factor) As a result of further investigations, the inventors have found that the value of the transverse sensitivity ratio of the strain gauge 1 varies depending on the influence of both the line width W and the film thickness T of the resistor 30 .

[0076] That is, the above experiment showed that when the film thickness T of resistor 30 is 200 nm, the line width W of resistor 30 should be 70 μm or less to keep the transverse sensitivity ratio at 70% or less. Also, the above experiment showed that when the line width W of resistor 30 is 50 μm, the film thickness T of resistor 30 should be 200 nm or more to keep the transverse sensitivity ratio at 70% or less.

[0077] However, since the transverse sensitivity ratio of the strain gauge 1 changes depending on both the line width W and film thickness T of the resistor 30, it is possible to make the transverse sensitivity ratio 70% or less in a wider range than the line width W and film thickness T obtained in the above experiment. That is, even if the line width W of the resistor 30 is thicker than 70 μm, the transverse sensitivity ratio can be made 70% or less by adjusting the film thickness T to an appropriate value. Similarly, even if the film thickness T of the resistor 30 is thinner than 200 nm, the transverse sensitivity ratio can be made 70% or less by adjusting the line width W to an appropriate value.

[0078] Fig. 9 is a contour diagram showing the transverse sensitivity ratio with respect to line width and film thickness, and summarizes the results of experiments conducted by the inventors. In Fig. 9, in the first region shown by the high-density dot pattern, 0% ≦ transverse sensitivity ratio ≦ 70%. In the second region shown by the low-density dot pattern, 70% < transverse sensitivity ratio ≦ 100%.

[0079] The boundary between the first and second regions is w=0.0177t 2 It can be approximated as +0.1521t+2.9541. Here, if the resistor line width is W and the film thickness is T, then w=logW and t=logT. In other words, w=logW and t=logT are w≦0.0177t 2+0.1521t+2.9541 (1) is satisfied, the transverse sensitivity ratio can be set to 70% or less. In other words, a strain gauge can be realized in which the line width W and film thickness T of resistor 30 have a relationship that satisfies the above formula (1) so that the transverse sensitivity ratio is 70% or less and the gauge factor is 5 or more.

[0080] Furthermore, as can be seen from FIG. 9, by setting the line width W of the resistor 30 to 40 μm or less, the lateral sensitivity ratio can be set to 70% or less, almost independently of the film thickness T.

[0081] In addition, although polyimide resin was used for the substrate 10 in Example 1, the characteristics shown in Figures 5 to 9 do not depend on the material of the substrate 10. For example, even when a material other than resin is used for the substrate 10, characteristics similar to those shown in Figures 5 to 9 can be obtained.

[0082] Also, instead of the Cr mixed phase film, other materials can be used to obtain a gauge factor of 5 or more. Examples of materials that can obtain a gauge factor of 5 or more include Ge, Pt, Si, Ni, Cu, C, Ti, Cr, and materials containing these. Specifically, these materials include Cr-N, Ge, Ge-In, Ge-Ga, Ge-P, Pt, Pt-In, Cu, Ni-C, Si, Ti-CO, etc. Also, by adjusting the line width W and / or film thickness T of the resistor 30 using these materials, a desired transverse sensitivity ratio can be obtained.

[0083] That is, even if these materials are used, it is possible to realize a strain gauge in which the line width W and film thickness T of the resistor 30 satisfy the above formula (1) so that the transverse sensitivity ratio is 70% or less and the gauge factor is 5 or more. It is also possible to realize a strain gauge in which the gauge factor is 5 or more and does not depend on the line width of the resistor 30, and the transverse sensitivity ratio has a positive correlation with the logarithm of the line width of the resistor 30, and becomes lower as the line width of the resistor 30 becomes narrower. Alternatively, it is possible to realize a strain gauge in which the resistor 30 is formed from these materials, and the gauge factor is 5 or more and does not depend on the film thickness of the resistor 30, and the transverse sensitivity ratio has a negative correlation with the film thickness of the resistor 30, and becomes lower as the film thickness of the resistor 30 becomes thicker.

[0084] [Example 2] The inventors prepared multiple strain gauge samples with the same specifications as those used in "Relationship between resistor film thickness and transverse sensitivity ratio and gauge factor" in Example 1, for each film thickness, and investigated the relationship between resistor film thickness and strain limit. The types of resistor film thickness were 1100 nm, 800 nm, 500 nm, 200 nm, 100 nm, 50 nm, and 30 nm. The resistor line width was fixed at 50 μm, but the strain limit is almost independent of the resistor line width.

[0085] Fig. 10 is a diagram showing the results of the study of Example 2, plotting the minimum value of the strain limit for each film thickness for multiple strain gauge samples. As shown in Fig. 10, the strain limit has a negative correlation with the film thickness T of the resistor 30, and the value of the strain limit tends to decrease as the film thickness T of the resistor 30 increases. In particular, when the film thickness T of the resistor 30 is in the range of 30 nm to 200 nm, the strain limit value decreases significantly. In contrast, when the film thickness T of the resistor 30 exceeds 200 nm, the strain limit value decreases gradually.

[0086] When actually using strain gauge 1, 6000 x 10 -6 In some cases, a strain limit of about 6000×10 is required. Therefore, from the results of FIG. 10, the film thickness T of the resistor 30 is set to a value that satisfies the strain limit of about 6000×10 -6 It can be said that 800 nm or less is preferable.

[0087] The strain gauge 1 is attached to a strain body and detects the amount of strain in the strain body by expanding and contracting in accordance with the movement of the strain body. Therefore, in order to detect a larger amount of strain, the strain gauge 1 itself must not be damaged (disconnected, etc.) during the expansion and contraction process, and a higher strain limit is required. In the strain gauge 1, by setting the film thickness T of the resistor 30 to 800 nm or less, it is possible to improve the strain limit.

[0088] Also, from the viewpoint of the strain limit, it is more preferable to set the film thickness T to 600 nm or less, further preferably to set it to 400 nm or less, and particularly preferably to set it to 200 nm or less. As can be seen from FIG. 10, by setting the film thickness T to 200 nm or less, the strain limit can be significantly improved, and by setting the film thickness T to 100 nm or less, the strain limit can be further improved. Also, from FIG. 9, it can be seen that even if the film thickness T is 100 nm or 200 nm, the transverse sensitivity ratio can be set to 70% or less by adjusting the line width W. For example, by setting the line width W of the resistor 30 to 40 μm or less and the film thickness T to 200 nm or less, the transverse sensitivity ratio can be set to 70% or less, and the strain limit can be significantly improved.

[0089] In Example 2, polyimide resin was used for the substrate 10, but the characteristics shown in Fig. 10 do not depend on the material of the substrate 10. For example, even when a material other than resin is used for the substrate 10, characteristics similar to those shown in Fig. 10 can be obtained.

[0090] Moreover, even if other materials are used instead of the Cr mixed phase film, the same characteristics as those in Fig. 10 can be obtained. Examples of other materials include Ge, Pt, Si, Ni, Cu, C, Ti, Cr, and materials containing these. Specifically, these materials include Cr-N, Ge, Ge-In, Ge-Ga, Ge-P, Pt, Pt-In, Cu, Ni-C, Si, Ti-CO, etc.

[0091] [Preferable range of film thickness T of resistor 30] From the results of Example 2, focusing on the strain limit, the strain limit is 6000 × 10 -6 To satisfy the above, it is preferable that the film thickness T of resistor 30 is 800 nm or less. Therefore, taking into account the results of Example 2 in addition to the results of Example 1, it can be said that it is preferable that the line width W and film thickness T of resistor 30 satisfy the above formula (1) so that the transverse sensitivity ratio is 70% or less and the gauge factor is 5 or more when the film thickness T is in the range of 800 nm or less.

[0092] This allows the transverse sensitivity ratio to be 70% or less, the gauge factor to be 5 or more, and the strain limit to be 6000×10-6 As described above, a strain gauge can be realized in which the line width W and film thickness T of the resistor 30 are adjusted. This strain gauge can improve the accuracy of strain detection and can detect a larger amount of strain.

[0093] Depending on the required specifications, the strain limit may be as high as 6000×10 -6 Since a thickness T of the resistor 30 of less than 800 nm may be acceptable in some cases, it is not essential that the thickness T of the resistor 30 be 800 nm or less.

[0094] The preferred embodiments and the like have been described above in detail. However, the strain gauge according to the present disclosure is not limited to the above-described embodiments and modifications. For example, various modifications and substitutions can be made to the strain gauge according to the above-described embodiments and the like without departing from the scope of the claims. [Explanation of symbols]

[0095] 1, 1A, 1B strain gauge, 10 substrate, 10a upper surface, 20 functional layer, 30 resistor, 30e1, 30e2 termination, 40 wiring, 50 electrode, 60 cover layer, 100 strain body

Claims

1. a base material, and a resistor formed on the base material, wherein the resistor is formed from a material having Cr as a main component, a strain gauge in which the line width W and the film thickness T of the resistor satisfy the following formula (1) so that the transverse sensitivity ratio is 70% or less and the gauge factor is 5 or more. w ≤ 0.0177t ２ + 0.1521t + 2.9541 ··· (1) However, in formula (1), w = log W and t = log T.

2. The transverse sensitivity ratio is 70% or less, the gauge factor is 5 or more, and the strain limit is 6000×10 －６ The strain gauge according to claim 1, wherein the film thickness T of the resistor is 800 nm or less so as to be 6000×10 or more.

3. The strain gauge according to claim 2, wherein the film thickness T of the resistor is 200 nm or less.

4. The strain gauge according to claim 2 or 3, wherein the strain limit has a negative correlation with the film thickness T of the resistor, and the thicker the film thickness T of the resistor, the lower the strain limit.

5. The strain gauge according to any one of claims 1 to 3, wherein the gauge factor does not depend on the line width W of the resistor, and the transverse sensitivity ratio has a positive correlation with the logarithm of the line width W of the resistor, and the narrower the line width W of the resistor, the lower the transverse sensitivity ratio.

6. The strain gauge according to any one of claims 1 to 3, wherein the gauge factor does not depend on the film thickness T of the resistor, and the transverse sensitivity ratio has a negative correlation with the film thickness T of the resistor, and the thicker the film thickness T of the resistor, the lower the transverse sensitivity ratio.

7. The strain gauge according to any one of claims 1 to 3, wherein the line width W of the resistor is 40 μm or less.

8. The resistor is formed of a film containing Cr, CrN, and Cr ２ The strain gauge according to any one of claims 1 to 3, wherein the resistor is formed of a film containing N.

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