strain gauge

Abstract

To provide a strain gauge with a resistor formed on a flexible substrate, which features reduced warping.SOLUTION: A strain gauge provided herein comprises a flexible resin substrate, a functional layer formed of a metal, alloy, or metal compound directly on one surface of the substrate, and a resistor principally made up of α-Cr and formed of a film containing Cr, CrN, and Cr2N directly on one surface of the functional layer, where the functional layer features a function of promoting α-Cr crystal growth to form a film principally made up of α-Cr. The resistor has a thickness of 0.05 to 2 μm, inclusive, and the functional layer has a thickness of 1 to 100 nm, inclusive. The substrate has an expansion coefficient in a range of 7 to 20 ppm / K.SELECTED DRAWING: Figure 3

Description

[Technical Field]

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

[0002] There is known a strain gauge that is attached to 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, unlike when a substrate made of a material with high mechanical strength such as ceramic is used, when a flexible substrate is used, warping of the strain gauge becomes a problem. Warping of the strain gauge can cause cracks in the resistor, deteriorating the gauge characteristics and causing the strain gauge to cease functioning.

[0005] The present invention has been made in view of the above points, and has an object to reduce warpage 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, and an α-Cr-based resistor formed directly on one side of the functional layer from a film containing Cr, CrN, and CrN, wherein the functional layer has the function of promoting crystal growth of the α-Cr and forming the film mainly composed of α-Cr, the resistor has a thickness of 0.05 μm or more and 2 μm or less, the functional layer has a thickness of 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, warping can be reduced 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] FIG. 10 is a diagram showing the relationship between the expansion coefficient of the substrate and the internal stress of the 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, taken along line AA in Fig. 1. Referring to Figs. 1 and 2, the strain gauge 1 has a substrate 10, a resistor 30, and a terminal portion 41.

[0011] In this embodiment, for 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 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 strain is applied. 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 matte finish.

[0016] The resistor 30 can be formed from, for example, a material containing Cr (chromium), a material containing Ni (nickel), or a material containing both Cr and Ni. That is, the resistor 30 can be formed from a material containing at least one of Cr and Ni. An example of a material containing Cr is a Cr mixed phase film. An example of a material containing Ni is Ni-Cu (nickel copper). An example of a material containing both Cr and Ni is Ni-Cr (nickel chromium).

[0017] Here, the Cr mixed phase film is a film containing a mixture of Cr, CrN, Cr2N, etc. The Cr mixed phase film may contain inevitable impurities such as chromium oxide.

[0018] 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.

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

[0020] 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.

[0021] 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.

[0022] A cover layer 60 (insulating resin layer) may be 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. The cover layer 60 may be provided so as to cover the entire portion except for the terminal portions 41.

[0023] 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.

[0024] To manufacture the strain gauge 1, first, a substrate 10 is prepared, and then the resistor 30 and terminal portion 41 having the planar shape shown in Fig. 1 are formed on the upper surface 10a of the substrate 10. The material and thickness of the resistor 30 and terminal portion 41 are as described above. The resistor 30 and terminal portion 41 can be integrally formed from the same material.

[0025] The resistor 30 and the terminal portion 41 can be formed, for example, by depositing a film by magnetron sputtering using a target made of a material capable of forming the resistor 30 and the terminal portion 41, and then patterning the film by photolithography. Instead of magnetron sputtering, the resistor 30 and the terminal portion 41 may also be deposited by reactive sputtering, vapor deposition, arc ion plating, pulsed laser deposition, or the like.

[0026] From the viewpoint of stabilizing the gauge characteristics, it is preferable to vacuum-form a functional layer having a film thickness of about 1 nm to 100 nm by, for example, conventional sputtering as a base layer on the upper surface 10a of the substrate 10 before forming the resistor 30 and the terminal portion 41. After the resistor 30 and the terminal portion 41 are formed on the entire upper surface of the functional layer, the functional layer is patterned together with the resistor 30 and the terminal portion 41 into the planar shape shown in FIG.

[0027] In this 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 preferably also has a function of preventing oxidation of the resistor 30 due to oxygen or moisture contained in the substrate 10, and a function of improving adhesion between the substrate 10 and the resistor 30. The functional layer may also have other functions.

[0028] The insulating resin film that constitutes the substrate 10 contains oxygen and moisture, and since Cr forms a self-oxidized film, it is effective for the functional layer to have the function of preventing oxidation of the resistor 30, especially when the resistor 30 contains Cr.

[0029] The material of the functional layer is not particularly limited as long as it has the function of promoting the crystal growth of at least the upper layer, the resistor 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 suitable metals include one or more metals selected from the group consisting of Fe (iron), Mo (molybdenum), W (tungsten), Ru (ruthenium), Rh (rhodium), Re (rhenium), Os (osmium), Ir (iridium), Pt (platinum), Pd (palladium), Ag (silver), Au (gold), Co (cobalt), Mn (manganese), and Al (aluminum), alloys of any of the metals in this group, and compounds of any of the metals in this group.

[0030] Examples of the alloys include FeCr, TiAl, FeNi, NiCr, CrCu, etc. Examples of the compounds include TiN, TaN, Si3N4, TiO2, Ta2O5, SiO2, etc.

[0031] The functional layer can be formed in vacuum by conventional sputtering, for example, using a target made of a material capable of forming the functional layer and introducing Ar (argon) gas into a chamber. By using conventional sputtering, the functional layer is formed while etching the upper surface 10a of the substrate 10 with Ar, thereby minimizing the amount of the functional layer formed and achieving an improvement in adhesion.

[0032] 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.

[0033] There are no particular restrictions on the combination of the material of the functional layer with the material of the resistor 30 and the terminal portion 41, and it can be selected appropriately depending on the purpose. For example, it is possible to use Ti for the functional layer 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.

[0034] 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.

[0035] In these methods, the Ti functional layer defines the growth plane of the Cr mixed-phase film, allowing the formation of a Cr mixed-phase film primarily composed of α-Cr, which has a stable crystal structure. Furthermore, the Ti constituting the functional layer diffuses into the Cr mixed-phase film, improving the gauge characteristics. For example, the gauge factor of the strain gauge 1 can be set to 10 or more, and the temperature coefficient of gauge factor (TCS) and temperature coefficient of resistance (TCR) can be set within the ranges of -1000 ppm / °C to +1000 ppm / °C. When the functional layer is made of Ti, the Cr mixed-phase film may contain Ti or TiN (titanium nitride).

[0036] When the resistor 30 is a Cr mixed phase film, the functional layer 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 instead of Ti.

[0037] In this way, by providing a functional layer 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 constituting the functional layer into the resistor 30, it is possible to improve the gauge characteristics of the strain gauge 1.

[0038] After forming the resistor 30 and the terminal portions 41, a cover layer 60 that covers the resistor 30 and exposes the terminal portions 41 is provided on the upper surface 10a of the substrate 10 as needed, thereby completing the strain gauge 1. 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.

[0039] [Example 1] In Example 1, 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 3 were obtained.

[0040] 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 Figure 3 is a value per unit thickness and does not depend on the thickness of resistor 30.

[0041]

number

[0042] 3, 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] [Example 2] In Example 2, a plurality of strain gauges 1 were fabricated using a substrate 10 whose expansion coefficient was set within the range of 7 ppm / K to 20 ppm / K.

[0047] First, a 3 nm thick Ti film was vacuum-formed as a functional layer by conventional sputtering on the upper surface 10a of the substrate 10 made of polyimide resin and having a thickness of 25 μm.

[0048] Next, a Cr mixed phase film was formed as resistor 30 and terminal portion 41 on the entire upper surface of the functional layer by magnetron sputtering, and then the functional layer, resistor 30, and terminal portion 41 were patterned by photolithography as shown in FIG.

[0049] Next, the gauge characteristics were measured for each sample of Example 2. As a result, the gauge factor for each sample of Example 2 was 14 to 16. Furthermore, the gauge factor temperature coefficient TCS and the temperature coefficient of resistance TCR for each sample of Example 2 were within the range of -1000 ppm / °C to +1000 ppm / °C.

[0050] In this way, it was confirmed that warpage is reduced and a strain gauge 1 with good gauge characteristics can be produced by using a substrate 10 with an expansion coefficient in the range of 7 ppm / K to 20 ppm / K. It was also confirmed that the presence of a functional layer does not lead to worsening warpage of the strain gauge 1.

[0051] 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]

[0052] 1 strain gauge, 10 substrate, 10a upper surface, 30 resistor, 41 terminal portion, 60 cover layer

Claims

1. a flexible resin substrate; a functional layer formed directly on one surface of the substrate from a metal, 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; 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 strain gauge has an expansion coefficient of the substrate within the range of 7 ppm / K to 20 ppm / K.

2. 2. The strain gauge according to claim 1, wherein the substrate is made of a polyimide resin.

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