Strain gauge and method for manufacturing the same

Abstract

To improve the stability of gauge characteristics in strain gauges. [Solution] This strain gauge comprises a flexible resin substrate, a functional layer formed directly on one surface of the substrate from a metal, alloy, or metal compound, and a resistor mainly composed of α-Cr formed directly on one surface of the functional layer from a film containing Cr, CrN, and Cr2N. The functional layer has the function of promoting the crystal growth of α-Cr and forming a film mainly composed of α-Cr. The thickness of the resistor is 0.05 μm to 2 μm, and the thickness of the functional layer is 1 nm to 100 nm. The resistor includes a first layer which is the bottom layer and a second layer which is a surface layer laminated on the first layer, and the second layer is a higher density layer than the first layer.

Description

Technical Field

[0001] The present invention relates to a strain gauge and a method for manufacturing the same.

Background Art

[0002] A strain gauge that is attached to a measurement object to detect the strain of the measurement object is known. The strain gauge includes a resistor that detects strain, and as the material of the resistor, for example, a material containing Cr (chromium) or Ni (nickel) is used. Further, the resistor is formed in a predetermined pattern, for example, by etching a metal foil (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, due to reasons such as the formation of an oxide layer on the surface of the material used as the resistor, the etching property may decrease. If the etching property decreases and the resistor cannot be etched into a desired pattern, for example, there is a problem that the resistance value of the resistor varies and the stability of the gauge characteristics is lacking.

[0005] The present invention has been made in view of the above points, and an object thereof is to improve the stability of gauge characteristics in a strain gauge.

Means for Solving the Problems

[0006] This strain gauge comprises a flexible resin substrate, a functional layer formed directly on one surface of the substrate from a metal, alloy, or metal compound, and a resistor mainly composed of α-Cr, formed directly on one surface of the functional layer from a film containing Cr, CrN, and Cr2N. The functional layer has the function of promoting the crystal growth of α-Cr and forming a film mainly composed of α-Cr. The thickness of the resistor is 0.05 μm to 2 μm, and the thickness of the functional layer is 1 nm to 100 nm. The resistor includes a first layer which is the bottom layer and a second layer which is a surface layer laminated on the first layer, and the second layer is a higher density layer than the first layer. [Effects of the Invention]

[0007] According to the disclosed technology, the stability of the gauge characteristics in strain gauges can be improved. [Brief explanation of the drawing]

[0008] [Figure 1] This is a plan view illustrating a strain gauge according to the first embodiment. [Figure 2] This is a cross-sectional view illustrating a strain gauge according to the first embodiment. [Figure 3] This is a diagram illustrating the layered structure of a resistor. [Figure 4] This diagram illustrates the oxide layer formed on the resistor. [Modes for carrying out the invention]

[0009] The embodiments for carrying out the invention will be described below with reference to the drawings. In each drawing, the same reference numerals are used for identical components, and redundant explanations may be omitted.

[0010] <First Embodiment> Figure 1 is a plan view illustrating a strain gauge according to the first embodiment. Figure 2 is a cross-sectional view illustrating a strain gauge according to the first embodiment, showing a cross-section along line AA in Figure 1. Referring to Figures 1 and 2, the strain gauge 1 has a base material 10, a resistor 30, and a terminal portion 41.

[0011] In this embodiment, for convenience, the side of the base material 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. Furthermore, the surface on which the resistor 30 is provided at each part is referred to as one surface or the upper surface, and the surface on which the resistor 30 is not provided is referred to as the other surface or the lower surface. However, the strain gauge 1 can be used upside down or positioned at any angle. Moreover, "plan view" refers to viewing the object from the direction normal to the upper surface 10a of the base material 10, and "planar shape" refers to the shape of the object when viewed from the direction normal to the upper surface 10a of the base material 10.

[0012] The base material 10 is a member that serves as a base layer for forming the resistor 30, etc., and is flexible. The thickness of the base material 10 is not particularly limited and can be appropriately selected depending on the purpose, but for example it can be about 5 μm to 500 μm. In particular, a thickness of 5 μm to 200 μm of the base material 10 is preferable in terms of the transmission of strain from the surface of the strain-generating body joined to the lower surface of the base material 10 via an adhesive layer, etc., and dimensional stability against the environment, and a thickness of 10 μm or more is even preferable in terms of insulation.

[0013] The base material 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, or polyolefin resin. The term "film" refers to a flexible material with a thickness of approximately 500 μm or less.

[0014] Here, "formed from an insulating resin film" does not prevent the base material 10 from containing fillers or impurities in the insulating resin film. For example, the base material 10 may be formed from an insulating resin film containing fillers such as silica or alumina.

[0015] The resistor 30 is a thin film formed on the substrate 10 in a predetermined pattern, and is a sensitive part that undergoes a change in resistance when strained. The resistor 30 may be formed directly on the upper surface 10a of the substrate 10, or it may be formed on the upper surface 10a of the substrate 10 via another layer. For convenience, in Figure 1, the resistor 30 is shown with a textured surface.

[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 multiphase 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).

[0017] Here, a Cr multiphase film is a film in which Cr, CrN, Cr2N, etc., are mixed. The Cr multiphase film may contain unavoidable impurities such as chromium oxide.

[0018] The thickness of the resistor 30 is not particularly limited and can be appropriately selected depending on the purpose, but for example, it can be about 0.05 μm to 2 μm. In particular, a thickness of 0.1 μm or more is preferable because it improves the crystallinity of the crystals constituting the resistor 30 (for example, the crystallinity of α-Cr), and a thickness of 1 μm or less is even preferable because it can reduce cracks in the film and warping from the substrate 10 caused by internal stress in the film constituting the resistor 30.

[0019] For example, when the resistor 30 is a Cr mixed-phase film, by using α-Cr (alpha chromium), which is a stable crystal phase, as the main component, the stability of the gauge characteristics can be improved. Also, when the resistor 30 has α-Cr as the main component, 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 within the range of -1000 ppm / °C to +1000 ppm / °C. Here, the main component means that the target substance occupies 50 mass% or more of all the substances constituting the resistor. From the viewpoint of improving the gauge characteristics, it is preferable that the resistor 30 contains 80 wt% or more of α-Cr. Note that α-Cr is Cr with a bcc structure (body-centered cubic lattice structure).

[0020] As shown in Fig. 3(a), the resistor 30 preferably has a laminated structure in which a second layer 32 is laminated on a first layer 31. The first layer 31 is a layer having a relatively low-density columnar structure. The second layer 32 is a layer having a homogeneous structure formed at a higher density than the first layer 31. The first layer 31 and the second layer 32 can be formed of, for example, α-Cr. An oxide layer 31x (self-oxidized film) is formed on the surface of the second layer 32.

[0021] A layer having a columnar structure such as the first layer 31 has excellent etching properties and a high etching rate when etching for pattern formation is performed, and an accurate pattern shape can be formed.

[0022] However, a Cr thin film easily forms an oxide layer. Therefore, for example, as shown in Fig. 3(b), when the resistor is formed only from the first layer 31 having a columnar structure, not only an oxide layer 31x that is like a passive state is formed only on the surface of the Cr thin film, but also the oxide layer 31x penetrates into the film. This is considered to be because oxygen encloses the columns constituting the Cr thin film.

[0023] Figure 4(a) is a graph showing the degree of oxidation in the columnar structure, measured by X-ray photoelectron spectroscopy (XPS). In Figure 4(a), the horizontal axis represents etching time (min). Here, the degree of oxidation is measured while etching the Cr thin film, so an etching time of 0 indicates the surface of the Cr thin film, and a longer etching time indicates a deeper location in the Cr thin film. From Figure 4(a), it can be seen that the Cr oxide layer is formed not only on the surface of the Cr thin film but also at deeper locations.

[0024] Cr thin films with an oxide layer formed on the surface and within the film lose the advantages of the columnar structure described above, becoming less etchable. If etching to the desired pattern is not possible, gauge characteristics such as resistance will become unstable. Furthermore, the presence of the oxide layer itself may adversely affect gauge characteristics.

[0025] On the other hand, a Cr thin film having a homogeneous structure formed at a higher density than the first layer 31, such as the second layer 32, has high corrosion resistance, and although oxidation occurs near the surface, oxidation within the film can be reduced. Therefore, for example, as shown in Figure 3(c), if the resistor is formed only from the second layer 32 having a homogeneous structure formed at a high density, the area near the surface of the Cr thin film is oxidized and an oxide layer 31x is formed, but since the oxide layer 31x does not penetrate into the film, oxidation within the film can be reduced.

[0026] Figure 4(b) is a graph showing the degree of oxidation in a homogeneous Cr thin film, measured using the same method as in Figure 4(a). From Figure 4(b), it can be seen that the Cr oxide layer is formed only near the surface of the Cr thin film and not at deeper levels.

[0027] However, because the homogeneous Cr thin film has high hardness, forming the resistor from only the second layer 32 results in a thick film thickness and poor etchability. As a result, if etching to the desired pattern is not possible, the gauge characteristics such as resistance will become unstable.

[0028] Therefore, in the resistor 30, as shown in Figure 3(a), the bottommost layer, the first layer 31, is a layer with a columnar structure, and the surface layer (uppermost layer), the second layer 32, is a layer with a homogeneous structure formed at a higher density than the first layer 31. By forming such a structure in a vacuum, the first layer 31 can be made into a columnar structure layer without an oxide layer, and the area where the oxide layer 31x is formed can be limited to the vicinity of the surface of the second layer 32. Furthermore, by making the resistor 30 a laminated structure, the film thickness of the second layer with a homogeneous structure can be made thinner.

[0029] With this structure, although an oxide layer 31x, which is a self-oxidizing film, is formed on the surface of the second layer 32, no oxide layer 31x is formed within the film, and the lower layer can be made into a columnar structure with excellent etching properties, thus improving the etching properties of the resistor 30. In order to improve the etching properties of the resistor 30, it is preferable to make the film thickness of the second layer 32 half or less of the film thickness of the first layer 31.

[0030] Furthermore, this structure makes it possible to reduce the film stress of the resistor 30, which is a cause of warping of the strain gauge 1.

[0031] The above explanation used a Cr thin film as an example, but the same effect can be obtained with Cu-Ni thin films and Ni-Cr thin films by making the resistor 30 into the layered structure described above.

[0032] Returning to the explanation of Figures 1 and 2, the terminal portion 41 extends from both ends of the resistor 30 and, in a plan view, is wider than the resistor 30 and formed in a roughly rectangular shape. The terminal portion 41 is a pair of electrodes for outputting the change in the resistance value of the resistor 30 caused by strain to the outside, and for example, lead wires for external connection are joined to it. The resistor 30 extends from one terminal portion 41 in a zigzag pattern and is connected to the other terminal portion 41. The upper surface of the terminal portion 41 may be covered with a metal that has better solderability than the terminal portion 41. Although the resistor 30 and the terminal portion 41 are given different reference numerals for convenience, both can be formed integrally from the same material in the same process.

[0033] A cover layer 60 (insulating resin layer) may be provided on the upper surface 10a of the base material 10 so as to cover the resistor 30 and expose the terminal portion 41. Providing the cover layer 60 prevents mechanical damage to the resistor 30. In addition, providing the cover layer 60 protects the resistor 30 from moisture and other elements. The cover layer 60 may be provided so as to cover the entire portion excluding the terminal portion 41.

[0034] 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 fillers or pigments. There are no particular restrictions on the thickness of the cover layer 60, and it can be appropriately selected depending on the purpose, but for example, it can be about 2 μm to 30 μm.

[0035] To manufacture the strain gauge 1, first, a base material 10 is prepared, and a resistor 30 and terminal portion 41 in the planar shape shown in Figure 1 are formed on the upper surface 10a of the base material 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 formed integrally from the same material.

[0036] The resistor 30 and terminal portion 41 can be formed, for example, by depositing a film using a magnetron sputtering method targeting a raw material capable of forming the resistor 30 and terminal portion 41, and then patterning it by photolithography. The resistor 30 and terminal portion 41 may also be deposited using reactive sputtering, evaporation, arc ion plating, pulsed laser deposition, or other methods instead of magnetron sputtering.

[0037] From the viewpoint of stabilizing the gauge characteristics, it is preferable to vacuum-deposit a functional layer with a thickness of approximately 1 nm to 100 nm on the upper surface 10a of the substrate 10 as an underlayer, for example by conventional sputtering, before depositing the resistor 30 and terminal portion 41. After forming the resistor 30 and terminal portion 41 on the entire upper surface of the functional layer, the functional layer is patterned together with the resistor 30 and terminal portion 41 into the planar shape shown in Figure 1 by photolithography.

[0038] In this application, the functional layer refers to a layer that has the function of promoting crystal growth of the resistor 30, which is at least the upper layer. Preferably, the functional layer also has the function of preventing oxidation of the resistor 30 by oxygen and moisture contained in the substrate 10, and the function of improving the adhesion between the substrate 10 and the resistor 30. The functional layer may also have other functions.

[0039] Since the insulating resin film that makes up the base material 10 contains oxygen and moisture, and especially when the resistor 30 contains Cr, the Cr forms an oxidized film, it is effective for the functional layer to have a function that prevents oxidation of the resistor 30.

[0040] The material of the functional layer is not particularly limited as long as it is a material that has the function of promoting crystal growth of the upper layer resistor 30, and can be appropriately selected according to the purpose, but 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 include one or more metals selected from the group consisting of Fe (iron), Mo (molybdenum), W (tungsten), Ru (ruthenium), Rh (rhodium), Re (rhenium), Os (osmium), Ir (iridium), Pt (platinum), Pd (palladium), Ag (silver), Au (gold), Co (cobalt), Mn (manganese), and Al (aluminum), an alloy of any of these metals, or a compound of any of these metals.

[0041] Examples of the alloys mentioned above include FeCr, TiAl, FeNi, NiCr, and CrCu. Examples of the compounds mentioned above include TiN, TaN, Si3N4, TiO2, Ta2O5, and SiO2.

[0042] The functional layer can be deposited using a conventional sputtering method, for example, by targeting a raw material capable of forming a functional layer and introducing Ar (argon) gas into a chamber. By using the conventional sputtering method, the functional layer is deposited while etching the upper surface 10a of the substrate 10 with Ar, thus minimizing the amount of functional layer deposited and achieving improved adhesion.

[0043] However, this is just one example of a method for forming a functional layer, and the functional layer may be formed by other methods. For example, the upper surface 10a of the substrate 10 may be activated by plasma treatment using Ar or the like before forming the functional layer to improve adhesion, and then the functional layer may be formed in a vacuum by magnetron sputtering.

[0044] There are no particular restrictions on the combination of materials for the functional layer and the resistor 30 and terminal portion 41, and they can be appropriately selected according to the purpose. For example, it is possible to use Ti as the functional layer and to deposit a Cr multiphase film mainly composed of α-Cr (alpha-chromium) as the resistor 30 and terminal portion 41.

[0045] In this case, for example, the resistor 30 and terminal portion 41 can be formed by magnetron sputtering with Ar gas introduced into the chamber, using a raw material capable of forming a Cr multiphase film as the target. Alternatively, the resistor 30 and terminal portion 41 may be formed by reactive sputtering with pure Cr as the target, using an appropriate amount of nitrogen gas introduced into the chamber along with Ar gas.

[0046] In these methods, a functional layer made of Ti dictates the growth surface of the Cr multiphase film, enabling the formation of a Cr multiphase film primarily composed of α-Cr, which has a stable crystalline structure. Furthermore, the diffusion of Ti constituting the functional layer into the Cr multiphase film improves the gauge characteristics. For example, the gauge factor of strain gauge 1 can be set to 10 or higher, and the gauge factor temperature coefficient TCS and 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 multiphase film may contain Ti or TiN (titanium nitride).

[0047] Furthermore, when the resistor 30 is a Cr multiphase 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 by oxygen and moisture contained in the substrate 10, and improving the adhesion between the substrate 10 and the resistor 30. The same applies when Ta, Si, Al, or Fe are used instead of Ti as the functional layer.

[0048] In this way, by providing a functional layer beneath the resistor 30, it becomes possible to promote crystal growth in the resistor 30, and a resistor 30 consisting of a stable crystalline phase can be fabricated. As a result, the stability of the gauge characteristics in the strain gauge 1 can be improved. Furthermore, the diffusion of the material constituting the functional layer into the resistor 30 can improve the gauge characteristics in the strain gauge 1.

[0049] To form a laminated structure in which the resistor 30 and terminal portion 41 are laminated with a second layer 32 on a first layer 31, the first layer 31 and the second layer 32 can be formed by changing the film deposition conditions of the sputtering method. For example, the pressure of the Ar gas introduced into the chamber can be controlled. Although the film produced by the sputtering method contains Ar (peening effect), the Ar content in the film can be changed by controlling the pressure of the Ar gas, thereby changing the film density.

[0050] Specifically, when depositing the first layer 31 of the columnar structure, the pressure of the Ar gas is controlled to a relatively high first value. This reduces the plasma energy due to collisions between the sputtered particles and the Ar gas, causing the sputtered particles to randomly inject onto the substrate and form a relatively low-density columnar structure.

[0051] On the other hand, when forming the second layer 32, which has a higher density and more homogeneous structure than the first layer 31, the pressure of the Ar gas is controlled to a second value, which is lower than the first value. As a result, the plasma energy increases, the Ar content in the film increases (the peening effect increases), and the second layer 32, which has a higher density and more homogeneous structure than the first layer 31, is formed.

[0052] By performing the deposition of the first layer 31 and the second layer 32 as a series of steps in a vacuum chamber, the first layer 31 and the second layer 32 are deposited without the formation of an oxide layer. In other words, the second layer 32 can be deposited on the non-oxidized surface that forms the upper surface of the first layer 31. When the process is moved from the vacuum chamber to the atmosphere, an oxide layer (self-oxidized film) will form near the surface of the second layer 32, which is the surface layer, but no oxide layer will form on the first layer 31 that is covered by the second layer 32, and the upper surface of the first layer 31 will remain a non-oxidized surface.

[0053] Using a gas containing one or more of the noble gases Ne (neon), Kr (krypton), and Xe (xenon) instead of Ar will produce the same effect as using Ar. Furthermore, controlling the sputtering power instead of controlling the Ar pressure will produce the same effect as controlling the Ar pressure.

[0054] In other words, when depositing the first layer 31, the sputtering power is controlled to a relatively high third value, and when depositing the second layer 32, the sputtering power is controlled to a fourth value lower than the third value, thereby achieving the same effect as when controlling the Ar pressure.

[0055] The resistor 30 may have a laminated structure in which at least a second layer 32 is laminated on a first layer 31, and may have a laminated structure of three or more layers. That is, the resistor 30 may include one or more other layers between the bottommost first layer 31 and the surface second layer 32. In this case, only the surface second layer 32 may be a layer with a homogeneous structure formed at high density, while the layers below the second layer 32 may be layers with a columnar structure that is less dense than the second layer 32.

[0056] When the resistor 30 has a laminated structure of three or more layers, in order to improve the etching properties of the resistor 30, it is preferable that the film thickness of the second layer 32 be half or less of the total thickness of the layers having a columnar structure.

[0057] Furthermore, if the resistor 30 has a laminated structure of three or more layers, the density may be increased in stages from the first layer 31, which is the bottom layer, to the second layer 32, which is the surface layer. For example, the density of each layer having a columnar structure can be adjusted by controlling the pressure of the Ar gas. In this way, by gradually increasing the density of the layers closer to the surface layer among the multiple layers having a columnar structure, the etching properties can be further improved.

[0058] After forming the resistor 30 and terminal portion 41, the strain gauge 1 is completed by providing a cover layer 60 on the upper surface 10a of the base material 10, if necessary, to cover the resistor 30 and expose the terminal portion 41. The cover layer 60 can be made, for example, by laminating a semi-cured thermosetting insulating resin film onto the upper surface 10a of the base material 10 so as to cover the resistor 30 and expose the terminal portion 41, and then heating and curing it. Alternatively, the cover layer 60 may be made by applying a liquid or paste-like thermosetting insulating resin to the upper surface 10a of the base material 10 so as to cover the resistor 30 and expose the terminal portion 41, and then heating and curing it.

[0059] Although preferred embodiments have been described in detail above, the invention is not limited to the embodiments described above, and various modifications and substitutions can be made to the embodiments described above without departing from the scope of the claims. [Explanation of symbols]

[0060] 1 Strain gauge, 10 Base material, 10a Top surface, 30 Resistor, 31 First layer, 32 Second layer, 41 Terminal section, 60 Cover layer

Claims

1. A flexible resin base material, A functional layer formed directly from a metal, alloy, or metal compound is provided on one side of the aforementioned substrate. On one side of the functional layer, Cr, CrN, and Cr ２ It comprises a resistor mainly composed of α-Cr, formed from a film containing N, The functional layer has the function of promoting the crystal growth of α-Cr and forming a film mainly composed of α-Cr. The thickness of the resistor is 0.05 μm or more and 2 μm or less. The thickness of the functional layer is 1 nm or more and 100 nm or less. The resistor includes a first layer which is the bottom layer, and a second layer which is a surface layer laminated on the first layer. The second layer is a strain gauge, which is a layer with a higher density than the first layer.

2. The strain gauge according to claim 1, comprising one or more other layers between the first layer and the second layer, wherein the density is progressively increased from the first layer to the second layer.

3. The strain gauge according to claim 1 or 2, wherein the layer below the second layer has a columnar structure.

4. The strain gauge according to claim 3, wherein the thickness of the second layer is less than or equal to half the total thickness of the layer having the columnar structure.

5. A strain gauge according to any one of claims 1 to 4, comprising an insulating resin layer covering the resistor.

6. A step of forming a functional layer directly on one side of a flexible resin substrate, which is made of metal, an alloy, or a metal compound. On one side of the functional layer, Cr, CrN, and Cr ２ The process includes a step of forming a resistor mainly composed of α-Cr, which is formed from a film containing N, by sputtering, The functional layer has the function of promoting the crystal growth of α-Cr and forming a film mainly composed of α-Cr. The thickness of the resistor is 0.05 μm or more and 2 μm or less. The thickness of the functional layer is 1 nm or more and 100 nm or less. The resistor includes a first layer which is the bottom layer, and a second layer which is a surface layer laminated on the first layer. The aforementioned second layer is a denser layer than the aforementioned first layer. A method for manufacturing a strain gauge, wherein in the step of forming the resistor, the density of the first and second layers is changed by controlling the pressure of the rare gas introduced into the chamber or the sputtering power.

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