JP7659746B2 - Strain gauges - Google Patents

Description

The present invention relates to a strain gauge.

There is known a strain gauge that is attached to an object to be measured to detect the strain of the object. The strain gauge has a resistor that detects the strain, and the resistor is made of a material that contains, for example, Cr (chromium) or Ni (nickel). The resistor is formed on a base material made of insulating resin, for example (see Patent Document 1).

JP 2016-74934 A

However, when a flexible substrate is used, it is difficult to form a stable resistor on the substrate, and there is a problem in that the gauge characteristics (gauge factor, temperature coefficient of gauge factor TCS, and temperature coefficient of resistance TCR) are lacking in stability.

The present invention has been made in consideration of the above points, and aims to improve the stability of the gauge characteristics in a strain gauge having a resistor formed on a flexible substrate.

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 a resistor mainly composed of α-Cr, formed directly on one side of the functional layer from a film containing Cr, CrN, and Cr ₂ N , the functional layer having a function of promoting crystal growth of the α-Cr and forming a film mainly composed of the α-Cr, the resistor having a thickness of 0.05 μm or more and 2 μm or less, and the functional layer having a thickness of 1 nm or more and 100 nm or less .

The disclosed technology can improve the stability of the gauge characteristics in a strain gauge having a resistor formed on a flexible substrate.

FIG. 1 is a plan view illustrating a strain gauge according to a first embodiment. 1 is a cross-sectional view illustrating a strain gauge according to a first embodiment. 3A to 3C are diagrams illustrating a manufacturing process of the strain gauge according to the first embodiment; FIG. 13 is a diagram showing the results of fluorescent X-ray analysis of the functional layer. FIG. 13 is a diagram showing the results of X-ray diffraction of a resistor.

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

First Embodiment
Fig. 1 is a plan view illustrating a strain gauge according to a first embodiment. Fig. 2 is a cross-sectional view illustrating the strain gauge according to the first embodiment, showing a cross section along line A-A in Fig. 1. Referring to Figs. 1 and 2, the strain gauge 1 has a substrate 10, a functional layer 20, a resistor 30, and a terminal portion 41.

In this embodiment, for convenience, in the strain gauge 1, the side of the substrate 10 on which the resistor 30 is provided is referred to as the upper side or one side, and the side on which the resistor 30 is not provided is referred to as the lower side or the other side. Also, the surface on which the resistor 30 is provided in each portion is referred to as the one side or upper surface, and the surface on which the resistor 30 is not provided is referred to as the other side or lower surface. However, the strain gauge 1 can be used upside down or placed at any angle. Also, a planar view refers to viewing an object from the normal direction of the upper surface 10a of the substrate 10, and a planar shape refers to the shape of the object viewed from the normal direction of the upper surface 10a of the substrate 10.

The substrate 10 is a flexible member that serves as a base layer for forming the resistor 30 and the like. There are no particular limitations on the thickness of the substrate 10, and it can be selected appropriately depending on the purpose, but it can be, for example, about 5 μm to 500 μm. In particular, a thickness of 5 μm to 200 μm is preferable in terms of the transmission of strain from the surface of the strain generator that is joined to the underside of the substrate 10 via an adhesive layer or the like, and dimensional stability against the environment, and a thickness of 10 μm or more is even more preferable in terms of insulation.

The substrate 10 can be formed from an insulating resin film such as PI (polyimide) resin, epoxy resin, PEEK (polyether ether ketone) resin, PEN (polyethylene naphthalate) resin, PET (polyethylene terephthalate) resin, PPS (polyphenylene sulfide) resin, polyolefin resin, etc. Note that a film refers to a flexible material with a thickness of about 500 μm or less.

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

The functional layer 20 is formed on the upper surface 10a of the substrate 10 as a lower layer of the resistor 30. That is, the planar shape of the functional layer 20 is substantially the same as the planar shape of the resistor 30 shown in FIG. 1. The thickness of the functional layer 20 can be, for example, about 1 nm to 100 nm.

In this application, the functional layer refers to a layer that has the function of promoting the crystal growth of at least the upper layer, the resistor 30. The functional layer 20 preferably also has the function of preventing oxidation of the resistor 30 due to 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 20 may also have other functions.

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

The material of the functional layer 20 is not particularly limited as long as it has the function of promoting the crystal growth of at least the upper layer, the resistor 30, and can be appropriately selected according to the purpose. For example, one or more metals selected from the group consisting of Cr (chromium), Ti (titanium), V (vanadium), Nb (niobium), Ta (tantalum), Ni (nickel), Y (yttrium), Zr (zirconium), Hf (hafnium), Si (silicon), C (carbon), Zn (zinc), Cu (copper), Bi (bismuth), Fe (iron), Mo (molybdenum), W (tungsten), Ru (ruthenium), Rh (rhodium), Re (rhenium), Os (osmium), Ir (iridium), Pt (platinum), Pd (palladium), Ag (silver), Au (gold), Co (cobalt), Mn (manganese), and Al (aluminum), an alloy of any of the metals in this group, or a compound of any of the metals in this group can be mentioned.

Examples of the alloy include FeCr, TiAl, FeNi, NiCr, CrCu, etc. Examples of the compound include TiN, TaN, _Si3N4 , _TiO2 , _Ta2O5 , _{SiO2 , etc.}

The resistor 30 is a thin film formed in a predetermined pattern on the upper surface of the functional layer 20, and is a sensing part that generates a resistance change when strained. For convenience, the resistor 30 is shown in FIG. 1 with a matte finish.

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

Here, the Cr mixed phase film is a film in which Cr, CrN, Cr ₂ N, etc. are mixed. The Cr mixed phase film may contain inevitable impurities such as chromium oxide. In addition, a part of the material constituting the functional layer 20 may be diffused into the Cr mixed phase film. In this case, the material constituting the functional layer 20 and nitrogen may form a compound. For example, when the functional layer 20 is formed of Ti, the Cr mixed phase film may contain Ti or TiN (titanium nitride).

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

By forming the resistor 30 on the functional layer 20, the resistor 30 can be formed with a stable crystal phase, improving the stability of the gauge characteristics (gauge factor, temperature coefficient of gauge factor TCS, and temperature coefficient of resistance TCR).

For example, if the resistor 30 is a Cr mixed phase film, providing the functional layer 20 makes it possible to form a resistor 30 whose main component is α-Cr (alpha chromium). α-Cr is a stable crystal phase, which can improve the stability of the gauge characteristics.

Here, the term "main component" means that the target substance accounts for 50% by mass or more of the total substances that make up the resistor. When resistor 30 is a Cr mixed phase film, it is preferable that resistor 30 contains 80% by weight or more of α-Cr from the viewpoint of improving the gauge characteristics. α-Cr is Cr with a bcc structure (body-centered cubic lattice structure).

In addition, the metal (e.g., Ti) constituting the functional layer 20 diffuses into the Cr mixed phase film, thereby improving the gauge characteristics. Specifically, the gauge factor of the strain gauge 1 can be set to 10 or more, and the gauge factor temperature coefficient TCS and the resistance temperature coefficient TCR can be set within the range of -1000 ppm/°C to +1000 ppm/°C.

The terminals 41 extend from both ends of the resistor 30 and are formed in a generally rectangular shape in plan view, wider than the resistor 30. The terminals 41 are a pair of electrodes for outputting the change in resistance value of the resistor 30 caused by distortion to the outside, and are connected to, for example, lead wires for external connection. The resistor 30 extends from one of the terminals 41 while folding back in a zigzag pattern and is connected to the other terminal 41. The upper surface of the terminal 41 may be covered with a metal that has better solderability than the terminal 41. Although the resistor 30 and the terminals 41 are given different reference numerals for convenience, they can be integrally formed from the same material in the same process.

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 portion 41. By providing the cover layer 60, mechanical damage to the resistor 30 can be prevented. Furthermore, by providing the cover layer 60, the resistor 30 can be protected from moisture and the like. The cover layer 60 may be provided so as to cover the entire portion except for the terminal portion 41.

The cover layer 60 can be formed from an insulating resin such as PI resin, epoxy resin, PEEK resin, PEN resin, PET resin, PPS resin, or composite resin (e.g., silicone resin, polyolefin resin). The cover layer 60 may contain a filler or pigment. There is no particular limit to the thickness of the cover layer 60 and it can be appropriately selected depending on the purpose, but it can be, for example, about 2 μm to 30 μm.

Figure 3 illustrates the manufacturing process of the strain gauge according to the first embodiment, showing a cross section corresponding to Figure 2. To manufacture the strain gauge 1, first, in the process shown in Figure 3(a), a substrate 10 is prepared, and a functional layer 20 is formed on the upper surface 10a of the substrate 10. The materials and thicknesses of the substrate 10 and the functional layer 20 are as described above.

The functional layer 20 can be formed in a vacuum by conventional sputtering, for example, using a raw material capable of forming the functional layer 20 as a target and introducing Ar (argon) gas into a chamber. By using conventional sputtering, the functional layer 20 is formed while etching the upper surface 10a of the substrate 10 with Ar, so that the amount of the functional layer 20 formed can be minimized and the effect of improving adhesion can be obtained.

However, this is just one example of a method for forming the functional layer 20, and the functional layer 20 may be formed by other methods. For example, a method may be used in which the upper surface 10a of the substrate 10 is activated by a plasma treatment using Ar or the like before forming the functional layer 20, thereby improving adhesion, and then the functional layer 20 is vacuum-formed by magnetron sputtering.

3B, the resistor 30 and the terminal portion 41 are formed on the entire upper surface of the functional layer 20, and then the functional layer 20, the resistor 30, and the terminal portion 41 are patterned by photolithography into the planar shape shown in FIG. 1. The material and thickness of the resistor 30 and the terminal portion 41 are as described above. The resistor 30 and the terminal portion 41 can be integrally formed from the same material. The resistor 30 and the terminal portion 41 can be formed by, for example, a magnetron sputtering method using a raw material capable of forming the resistor 30 and the terminal portion 41 as a target. The resistor 30 and the terminal portion 41 may be formed by a reactive sputtering method, a vapor deposition method, an arc ion plating method, a pulsed laser deposition method, or the like instead of the magnetron sputtering method.

The combination of the material of the functional layer 20 and the material of the resistor 30 and terminal portion 41 is not particularly limited and can be selected appropriately depending on the purpose, but for example, it is possible to use Ti for the functional layer 20 and form a Cr mixed phase film with α-Cr (alpha chromium) as the main component for the resistor 30 and terminal portion 41.

In this case, for example, the resistor 30 and the terminal portion 41 can be formed by magnetron sputtering using a raw material capable of forming a Cr mixed phase film as a target and introducing Ar gas into the 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 together with Ar gas into the chamber.

In these methods, the growth surface of the Cr mixed phase film is defined by the functional layer 20 made of Ti, and a Cr mixed phase film can be formed that is mainly composed of α-Cr, which has a stable crystal structure. In addition, the Ti that constitutes the functional layer 20 diffuses into the Cr mixed phase film, improving the gauge characteristics. For example, the gauge factor of the strain gauge 1 can be set to 10 or more, and the temperature coefficient of gauge factor TCS and temperature coefficient of resistance TCR can be set within the range of -1000 ppm/°C to +1000 ppm/°C.

When the resistor 30 is a Cr mixed phase film, the functional layer 20 made of Ti has all of the following functions: promoting crystal growth of the resistor 30, preventing oxidation of the resistor 30 due to oxygen and moisture contained in the substrate 10, and improving adhesion between the substrate 10 and the resistor 30. The same applies when Ta, Si, Al, or Fe is used instead of Ti as the functional layer 20.

3(b), the strain gauge 1 is completed by providing a cover layer 60 on the upper surface 10a of the substrate 10 as necessary, which covers the resistor 30 and exposes the terminal portion 41. 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 portion 41, and then heating and hardening 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 portion 41, and then heating and hardening the film.

In this way, by providing the functional layer 20 under the resistor 30, it is possible to promote crystal growth of the resistor 30, and a resistor 30 consisting of a stable crystalline phase can be produced. As a result, the stability of the gauge characteristics of the strain gauge 1 can be improved. In addition, the material constituting the functional layer 20 diffuses into the resistor 30, thereby improving the gauge characteristics of the strain gauge 1.

[Example 1]
First, as a preliminary experiment, a Ti film was vacuum-formed as the functional layer 20 by conventional sputtering on the upper surface 10a of a substrate 10 made of a polyimide resin having a thickness of 25 μm. At this time, five samples were fabricated by forming the Ti film with different thicknesses.

Next, X-ray fluorescence (XRF) analysis was performed on the five samples produced, obtaining the results shown in Figure 4. The presence of Ti was confirmed from the X-ray peaks in Figure 4, and the X-ray intensity of each sample at the X-ray peak confirmed that the thickness of the Ti film could be controlled in the range of 1 nm to 100 nm.

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

Next, a Cr mixed phase film was formed as the resistor 30 and the terminal portion 41 on the entire upper surface of the functional layer 20 by magnetron sputtering, and then the functional layer 20, the resistor 30, and the terminal portion 41 were patterned by photolithography as shown in FIG. 1.

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

Next, X-ray diffraction (XRD) evaluation was performed on the sample of Example 1 and the sample of Comparative Example 1, and the results shown in Figure 5 were obtained. Figure 5 shows X-ray diffraction patterns in the 2θ diffraction angle range of 36 to 48 degrees, with the diffraction peak of Example 1 shifted to the right compared to the diffraction peak of Comparative Example 1. In addition, the diffraction peak of Example 1 is higher than the diffraction peak of Comparative Example 1.

The diffraction peak in Example 1 is located near the diffraction line of α-Cr (110), and it is believed that the provision of the functional layer 20 made of Ti promoted the crystal growth of α-Cr, forming a Cr mixed phase film mainly composed of α-Cr.

Next, multiple samples of Example 1 and Comparative Example 1 were prepared and the gauge characteristics were measured. As a result, the gauge factor of each sample of Example 1 was 14 to 16, whereas the gauge factor of each sample of Comparative Example 1 was less than 10.

In addition, while the gauge factor temperature coefficient TCS and the temperature coefficient of resistance TCR of each sample in Example 1 were within the range of -1000 ppm/°C to +1000 ppm/°C, the gauge factor temperature coefficient TCS and the temperature coefficient of resistance TCR of each sample in Comparative Example 1 were not within the range of -1000 ppm/°C to +1000 ppm/°C.

In this way, by providing a functional layer 20 made of Ti, the crystal growth of α-Cr is promoted to form a Cr mixed-phase film mainly composed of α-Cr, and a strain gauge is produced with a gauge factor of 10 or more, and a gauge factor temperature coefficient TCS and a resistance temperature coefficient TCR in the range of -1000 ppm/°C to +1000 ppm/°C. It is believed that the diffusion effect of Ti into the Cr mixed-phase film contributes to the improvement of the gauge characteristics.

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

1 strain gauge, 10 substrate, 10a upper surface, 20 functional layer, 30 resistor, 41 terminal portion, 60 cover layer

Claims (1)

A flexible resin base material;
A functional layer formed directly on one surface of the substrate from a metal, an alloy, or a metal compound;
a resistor mainly composed of α-Cr, formed of a film containing Cr, CrN, and Cr ₂ N, directly on one surface of the functional layer ;
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 thickness of the resistor is 0.05 μm or more and 2 μm or less,
A strain gauge , wherein the functional layer has a thickness of 1 nm or more and 100 nm or less .

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