JP7846293B2 - Strain gauge - Google Patents

Description

This invention relates to strain gauges.

A strain gauge is known that is attached to an object to be measured to detect its strain. A strain gauge is equipped with a resistor for detecting strain, and the material of the resistor is, for example, a material containing Cr (chromium) or Ni (nickel). Alternatively, for example, one resistor may be formed on one surface of a substrate made of insulating resin (see, for example, Patent Document 1).

Japanese Patent Publication No. 2016-74934

Incidentally, when a strain gauge is attached to a strain-generating body to detect changes in the resistance of a resistor, external noise such as electromagnetic waves or power supply noise may be superimposed on the output of the strain gauge. In this case, the signal-to-noise ratio (S/N) of the strain gauge decreases, and the sensor sensitivity decreases.

This invention has been made in view of the above points, and aims to provide a strain gauge capable of improving the signal-to-noise ratio (S/N).

This strain gauge comprises a flexible resin base material, a first functional layer formed directly on one surface of the base material from a metal, alloy, or metal compound, a second functional layer formed directly on the other surface of the base material from a metal, alloy, or metal compound, and a resistor. The resistor is formed directly on one surface of the first functional layer from a film containing Cr, CrN, and Cr2N, and has a first sensitive part mainly composed of α-Cr that changes resistance when strained, and has a first sensitive part formed directly on the other surface of the second functional layer from a film containing Cr, CrN, and _{Cr2N .} The resistor comprises a second sensitive portion formed from a film containing N and mainly composed of α-Cr, which undergoes a change in resistance upon strain, wherein the first functional layer and the second functional layer have 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 first functional layer and the second functional layer is 1 nm or more and 100 nm or less, the first sensitive portion and the second sensitive portion have substantially the same pattern and are arranged at opposing positions with the substrate in between.

According to the disclosed technology, a strain gauge capable of improving the signal-to-noise ratio (S/N) can be provided.

This is a plan view illustrating a strain gauge according to the first embodiment. This is a bottom view illustrating a strain gauge according to the first embodiment. This is a cross-sectional view illustrating a strain gauge according to the first embodiment.

The following describes embodiments for carrying out the invention with reference to the drawings. In each drawing, identical components are denoted by the same reference numerals, and redundant explanations may be omitted.

<First Embodiment>
Figure 1 is a plan view illustrating a strain gauge according to the first embodiment, showing the strain gauge as viewed from the top side of the base material. Figure 2 is a bottom view illustrating a strain gauge according to the first embodiment, showing the strain gauge as viewed from the bottom side of the base material. Figure 3 is a cross-sectional view illustrating a strain gauge according to the first embodiment, showing a cross-section along the line A-A in Figure 1. Referring to Figures 1 to 3, the strain gauge 1 includes a base material 10, a resistor 30 (resistance parts 31 and 32), and terminal parts 41 and 42.

In this embodiment, for convenience, the side of the strain gauge 1 on which the resistance portion 31 is located is referred to as the upper side or one side, and the side on which the resistance portion 32 is located is referred to as the lower side or the other side. Furthermore, the surface on which the resistance portion 31 is located at each part is referred to as one surface or the upper surface, and the surface on which the resistance portion 32 is located 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. Furthermore, "planar 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 viewed from the direction normal to the upper surface 10a of the base material 10.

The substrate 10 is a base layer component for forming the resistor 30 and the like, and is flexible. The thickness of the substrate 10 is not particularly limited and can be appropriately selected depending on the purpose, but for example, it can be approximately 5 μm to 500 μm. In particular, a substrate thickness of 5 μm to 200 μm is preferable because it can reduce the strain sensitivity error of the resistors 31 and 32.

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

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.

The resistor 30 is formed on the substrate 10. The resistor 30 includes resistive sections 31 and 32 laminated via the substrate 10. That is, the resistor 30 is a collective term for the resistive sections 31 and 32, and is referred to as the resistor 30 when there is no need to distinguish between them. For convenience, in Figures 1 and 2, the resistive sections 31 and 32 are shown with a textured surface.

The resistive portion 31 is a thin film formed in a predetermined pattern on the upper surface 10a of the substrate 10. The resistive portion 31 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. The resistive portion 31 includes a sensing portion 31S that changes resistance when subjected to strain. The portion of the resistive portion 31 other than the sensing portion 31S functions as a wiring pattern connected to the terminal portion 41.

The resistive portion 32 is a thin film formed in a predetermined pattern on the lower surface 10b of the substrate 10. The resistive portion 32 may be formed directly on the lower surface 10b of the substrate 10, or it may be formed on the lower surface 10b of the substrate 10 via another layer. The resistive portion 32 includes a sensing portion 32S that changes resistance when subjected to strain. The portion of the resistive portion 32 other than the sensing portion 32S functions as a wiring pattern connected to the terminal portion 42.

The sensing parts 31S and 32S have substantially the same pattern and are positioned opposite each other on the substrate 10. In other words, the sensing parts 31S and 32S have substantially the same pattern and are positioned in overlapping locations in a plan view.

Here, "the sensing parts 31S and 32S have substantially the same pattern" indicates that, as a result of being manufactured based on the same design, their patterns are almost identical, meaning that a certain degree of manufacturing error is acceptable.

Furthermore, the wiring pattern portion of resistor 31 and the wiring pattern portion of resistor 32 may be positioned opposite each other across the base material 10, or they may not be positioned opposite each other across the base material 10. Also, terminal portions 41 and 42 may be positioned opposite each other across the base material 10, or they may not be positioned opposite each other across the base material 10.

The resistor 30 (resistor portions 31 and 32) 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 Cr-containing material is a Cr multiphase film. An example of a Ni-containing material is Cu-Ni (copper-nickel). An example of a material containing both Cr and Ni is Ni-Cr (nickel-chromium).

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.

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 reduces cracks in the film and warping from the substrate 10 caused by internal stress in the film constituting the resistor 30.

For example, if the resistor 30 is a Cr multiphase film, the stability of the gauge characteristics can be improved by using α-Cr (alpha-chromium), 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 set to 10 or higher, and the temperature coefficient of gauge factor TCS and the temperature coefficient of resistance TCR can be set within the range of -1000 ppm/°C to +1000 ppm/°C. Here, "main component" means that the substance in question accounts for 50% or more by mass of the total substances constituting the resistor. From the viewpoint of improving gauge characteristics, it is preferable that the resistor 30 contains 80% by weight or more of α-Cr. Note that α-Cr is Cr with a bcc structure (body-centered cubic lattice structure).

The terminal portion 41 extends from both ends of the resistive portion 31 on the upper surface 10a of the base material 10, and in a plan view, it is wider than the resistive portion 31 and formed in a substantially rectangular shape. The terminal portion 41 is a pair of electrodes for outputting the change in the resistance value of the resistive portion 31 caused by strain to the outside, and for example, an external connection lead wire is joined to it. The resistive portion 31 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 resistive portion 31 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.

The terminal portion 42 extends from both ends of the resistive portion 32 on the lower surface 10b of the base material 10, and in a plan view, it is wider than the resistive portion 32 and formed in a substantially rectangular shape. The terminal portion 42 is a pair of electrodes for outputting the change in the resistance value of the resistive portion 32 caused by strain to the outside, and for example, an external connection lead wire is joined to it. The resistive portion 32 extends from one terminal portion 42 in a zigzag pattern and is connected to the other terminal portion 42. The upper surface of the terminal portion 42 may be covered with a metal that has better solderability than the terminal portion 42. Although the resistive portion 32 and the terminal portion 42 are given different reference numerals for convenience, both can be formed integrally from the same material in the same process.

Furthermore, through-holes (through-wiring) may be provided to penetrate the base material 10, and terminal sections 41 and 42 may be concentrated on either the upper surface 10a or the lower surface 10b of the base material 10.

A cover layer 61 (insulating resin layer) may be provided on the upper surface 10a of the base material 10 so as to cover the resistor portion 31 and expose the terminal portion 41. Alternatively, a cover layer 62 (insulating resin layer) may be provided on the lower surface 10b of the base material 10 so as to cover the resistor portion 32 and expose the terminal portion 42. Providing cover layers 61 and 62 prevents mechanical damage to the resistor portions 31 and 32. Furthermore, providing cover layers 61 and 62 protects the resistor portions 31 and 32 from moisture and other elements. Note that cover layers 61 and 62 may be provided to cover the entire portion excluding the terminal portions 41 and 42.

The cover layers 61 and 62 can be formed from insulating resins such as PI resin, epoxy resin, PEEK resin, PEN resin, PET resin, PPS resin, or composite resins (e.g., silicone resin, polyolefin resin). The cover layers 61 and 62 may also contain fillers or pigments.

The thickness of the cover layers 61 and 62 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 for the cover layer 62 is preferable in terms of strain transmission from the surface of the strain-generating body joined to the lower surface of the cover layer 62 via an adhesive layer, etc., and dimensional stability against the environment. A thickness of 10 μm or more is even preferable in terms of insulation. Note that the cover layers 61 and 62 may be formed from different materials, or they may be formed with different thicknesses.

To manufacture the strain gauge 1, first, a base material 10 is prepared, and a planar resistance portion 31 and a terminal portion 41, as shown in Figure 1, are formed on the upper surface 10a of the base material 10. The material and thickness of the resistance portion 31 and the terminal portion 41 are as described above. The resistance portion 31 and the terminal portion 41 can be formed integrally from the same material.

The resistor portion 31 and terminal portion 41 can be formed, for example, by depositing a film using a magnetron sputtering method targeting a material capable of forming the resistor portion 31 and terminal portion 41, and then patterning it using photolithography. Alternatively, the resistor portion 31 and terminal portion 41 may be deposited using reactive sputtering, vapor deposition, arc ion plating, pulsed laser deposition, or other methods instead of magnetron sputtering.

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 onto the upper surface 10a of the substrate 10 as a base layer, for example, by conventional sputtering, before forming the resistive portion 31 and the terminal portion 41. The functional layer is then patterned with the resistive portion 31 and the terminal portion 41 together with the functional layer into the planar shape shown in Figure 1 by photolithography after the resistive portion 31 and the terminal portion 41 have been formed across the entire upper surface of the functional layer.

In this application, the functional layer refers to a layer having the function of promoting crystal growth of at least the upper layer, the resistive portion 31. Preferably, the functional layer further has the function of preventing oxidation of the resistive portion 31 by oxygen and moisture contained in the substrate 10, and the function of improving the adhesion between the substrate 10 and the resistive portion 31. The functional layer may further have other functions.

Since the insulating resin film constituting the base material 10 contains oxygen and moisture, and especially when the resistive portion 31 contains Cr, the Cr forms a self-oxidizing film, it is effective for the functional layer to have a function that prevents oxidation of the resistive portion 31.

The material of the functional layer is not particularly limited as long as it has the function of promoting crystal growth in at least the upper layer, the resistive portion 31. It can be appropriately selected according to the purpose. Examples 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 these metals, or a compound of any of these metals.

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} .

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 argon (Ar) 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.

However, this is merely one example of a method for forming a 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 to improve adhesion, and then the functional layer is formed using a magnetron sputtering method under vacuum.

The combination of materials for the functional layer and the resistive portion 31 and terminal portion 41 is not particularly limited and can be appropriately selected according to the purpose. For example, it is possible to use Ti as the functional layer and deposit a Cr multiphase film mainly composed of α-Cr (alpha-chromium) as the resistive portion 31 and terminal portion 41.

In this case, for example, the resistive portion 31 and terminal portion 41 can be formed by magnetron sputtering using a raw material capable of forming a Cr multiphase film as the target, with Ar gas introduced into the chamber. Alternatively, the resistive portion 31 and terminal portion 41 may be formed by reactive sputtering using pure Cr as the target, with an appropriate amount of nitrogen gas introduced into the chamber along with Ar gas.

In these methods, a functional layer made of Ti (Ti) initiates 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 from 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).

Furthermore, when the resistive portion 31 is a Cr multiphase film, the functional layer made of Ti has all of the following functions: promoting crystal growth of the resistive portion 31, preventing oxidation of the resistive portion 31 by oxygen and moisture contained in the substrate 10, and improving the adhesion between the substrate 10 and the resistive portion 31. The same applies when Ta, Si, Al, or Fe is used instead of Ti as the functional layer.

Thus, by providing a functional layer beneath the resistive portion 31, crystal growth in the resistive portion 31 can be promoted, enabling the fabrication of a resistive portion 31 composed of a stable crystalline phase. 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 resistive portion 31 further improves the gauge characteristics of the strain gauge 1.

Next, the resistive portion 32 and terminal portion 42, which have a planar shape as shown in Figure 2, are formed on the lower surface 10b of the substrate 10. The resistive portion 32 and terminal portion 42 can be formed in the same manner as the resistive portion 31 and terminal portion 41. Similarly, it is preferable to form a functional layer as an underlayer on the lower surface 10b of the substrate 10 before forming the resistive portion 32 and terminal portion 42.

After forming the resistance portion 31 and terminal portion 41, and the resistance portion 32 and terminal portion 42, a cover layer 61 covering the resistance portion 31 and exposing the terminal portion 41 may be provided on the upper surface 10a of the base material 10, and a cover layer 62 covering the resistance portion 32 and exposing the terminal portion 42 may be provided on the lower surface 10b of the base material 10, if necessary. This completes the strain gauge 1.

The cover layer 61 can be manufactured, for example, by laminating a semi-cured thermosetting insulating resin film onto the upper surface 10a of the base material 10 so that the resistor portion 31 is covered and the terminal portion 41 is exposed, and then heating and curing it. The cover layer 62 can be manufactured, for example, by laminating a semi-cured thermosetting insulating resin film onto the lower surface 10b of the base material 10 so that the resistor portion 32 is covered and the terminal portion 42 is exposed, and then heating and curing it. Instead of laminating with insulating resin film, the cover layers 61 and 62 may also be manufactured by applying a liquid or paste-like thermosetting insulating resin and heating and curing it.

Thus, in the strain gauge 1, the sensing parts 31S and 32S have substantially identical patterns and are positioned opposite each other across the base material 10. Therefore, for example, when bending stress occurs where sensing part 31S is on the tensile side and sensing part 32S is on the compression side, the output of terminal 41 connected to sensing part 31S and the output of terminal 42 connected to sensing part 32S have substantially equal absolute values but opposite signs. In contrast, external noise such as electromagnetic waves and power supply noise are applied substantially equally to both sensing parts 31S and 32S. Therefore, by taking the difference signal between the output of terminal 41 and the output of terminal 42, the output is approximately doubled, and the noise is reduced. As a result, it is possible to improve the signal-to-noise ratio (S/N) in the strain gauge 1, thereby improving sensor sensitivity.

While it is possible to separately manufacture two strain gauges, each with a nearly identical pattern of sensing element, and attach them to opposing positions on either side of the strain-generating body, this process is inefficient, and ensuring attachment accuracy is difficult. This problem can be solved by using a single strain gauge 1, where two nearly identical sensing elements are positioned opposite each other on the base material 10.

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.

1. Strain gauge, 10. Base material, 10a. Top surface, 10b. Bottom surface, 30. Resistor, 31, 32. Resistive section, 31S, 32S. Sensitive section, 41, 42. Terminal section, 61, 62. Cover layer

Claims (2)

A flexible resin base material,
A first functional layer formed directly on one surface of the substrate from a metal, alloy, or metal compound,
A second functional layer formed directly on the other surface of the aforementioned substrate from a metal, alloy, or metal compound,
It has a resistor,
The resistor is
A first sensitive portion is formed directly on one surface of the first functional layer from a film containing Cr, CrN, and _Cr₂N , with α-Cr as the main component, which undergoes a change in resistance when strained,
The other surface of the second functional layer includes a second sensitive portion formed directly from a film containing Cr, CrN, and _Cr₂N , with α-Cr as the main component, which undergoes a change in resistance upon strain,
The first functional layer and the second functional layer have 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 first functional layer and the second functional layer is 1 nm or more and 100 nm or less.
The first sensing portion and the second sensing portion have substantially the same pattern and are arranged at opposing positions with the substrate in between. The strain gauge according to claim 1, further comprising an insulating resin layer covering at least one of the first sensing portion and the second sensing portion.