Patent Description:
Conventionally, a strain gauge is attached to an object to be measured, and the strain of the measurement object is detected. The strain gauge has a resistor for detecting strain, and the resistor is formed, for example, on insulating resin. The resistor is connected to an electrode via a conductive trace, for example (see Patent Document <NUM>, for example).

With a strain gauge such as the one described above, stability becomes a problem due to leakage current when used underwater or in a water-splashing environment. In particular, when the strain gauge has a high gauge factor, leakage current has a significant impact, and the output voltage varies. Other examples of strain gauges are disclosed in <CIT>.

The present disclosure has been prepared in view of the above, and it is therefore an object of the present disclosure to provide a strain gauge that can reduce the variation of output voltage due to the impact of leakage current.

The strain gauge according to the present invention is defined in appended claim <NUM>. Other advantageous embodiments are defined in the corresponding dependent claims.

According to the technique disclosed herein, it is possible to provide a strain gauge that can reduce the variation of output voltage due to the impact of leakage current.

An embodiment of the present disclosure will be described below with reference to the accompanying drawings. In each drawing, the same components will be assigned the same reference signs, and redundant description will be omitted.

<FIG> is a plan view that illustrates a strain gauge according to a first embodiment. <FIG> is a cross-sectional view that illustrates the strain gauge according to the first embodiment, showing a cross-section along line A-A in <FIG>. Referring to <FIG> and <FIG>, a strain gauge <NUM> has a substrate <NUM>, a resistor <NUM>, conductive traces <NUM>, electrodes <NUM>, and a barrier layer <NUM>.

Note that, with the present embodiment, for ease of explanation, the side of the substrate <NUM> in the strain gauge <NUM> on which the resistor <NUM> is provided will be referred to as "the upper side" or "one side," and the side of the substrate <NUM> on which the resistor <NUM> is not provided will be referred to as "the lower side" or "the other side. " Furthermore, in each component or part, the surface on the side where the resistor <NUM> is provided will be referred to as "one surface" or "the upper surface," and the surface on the side where the resistor <NUM> is not provided will be referred to as "the other surface" or "the lower surface. " However, the strain gauge <NUM> can be used upside down, or can be disposed at any angle. Furthermore, plan view herein means that an object is viewed in a direction normal to an upper surface 10a of the substrate <NUM>, and plan shape herein refers to the shape of an object as viewed in a direction normal to the upper surface 10a of the substrate <NUM>.

The substrate <NUM> is a member that serves as a base layer for forming the resistor <NUM> and the like, and is flexible. The thickness of the substrate <NUM> is not particularly limited and can be selected in accordance with the purpose of use. For example, the substrate <NUM> may be approximately <NUM> to <NUM> thick. In particular, it is preferable if the substrate <NUM> is <NUM> to <NUM> thick, because the transmission of strain from the surface of the strain generating body joined to the lower surface of the substrate <NUM> via a bonding layer or the like, the dimensional stability in the environment, and so forth are excellent. It is even more preferable if the substrate is <NUM> thick or more because the insulation is excellent.

The substrate <NUM> can be formed from an insulating resin film made of, for example, a polyimide (PI) resin, an epoxy resin, a polyether ether ketone (PEEK) resin, a polyethylene naphthalate (PEN) resin, a polyethylene terephthalate (PET) resin, a polyphenylene sulfide (PPS) resin, a liquid crystal polymer (LCP) resin, a polyolefin resin, and so forth. Note that a film herein refers to a flexible member that is approximately <NUM> thick or less.

Here, "the substrate <NUM> can be formed from an insulating resin film" by no means precludes the substrate <NUM> from containing fillers, impurities, and so forth, in the insulating resin film. The substrate <NUM> may be formed from, for example, an insulating resin film that contains fillers such as silica or alumina.

Examples of materials of the substrate <NUM> other than resin include crystalline materials such as SiO<NUM>, ZrO<NUM> (including YSZ), Si, Si<NUM>N<NUM>, Al<NUM>O<NUM> (including sapphire), ZnO, perovskite ceramics (CaTiO<NUM>, BaTiO<NUM>, etc.) and so forth. Further, additional examples include amorphous glass and the like. Furthermore, as materials of the substrate <NUM>, metals such as aluminum, an aluminum alloy (duralumin), titanium, and others may be used. In this case, for example, an insulating film is formed on the substrate <NUM> of metal.

The resistor <NUM> is a thin film formed on the substrate <NUM> in a predetermined pattern, and is a sensitive part configured such that that resistance changes in response to strain. The resistor <NUM> may be formed directly on the upper surface 10a of the substrate <NUM>, or may be formed on the upper surface 10a of the substrate <NUM> via other layers. Note that, in <FIG>, the resistor <NUM> is shown with a dark satin pattern for ease of explanation.

The resistor <NUM> has a plurality of elongated portions, arranged at predetermined intervals with their longitudinal directions oriented in the same direction (the direction of line A-A in <FIG>), and the ends of adjacent elongated portions are alternately connected to form a zigzag structure on the whole. The longitudinal direction of the elongated portions is the grid direction, and the direction perpendicular to the grid direction is the grid width direction (the direction of line B-B in <FIG>).

Ends of the two elongated parts in the longitudinal direction, located on the outermost side in the grid width direction, are bent in the grid width direction, and form respective ends 30e<NUM>, and 30e<NUM> of the resistor <NUM> in the grid width direction. The ends 30e<NUM> and 30e<NUM> of the resistor <NUM> in the grid width direction are electrically connected with the electrodes <NUM> via the conductive traces <NUM>. In other words, the conductive traces <NUM> electrically connect the ends 30e<NUM> and 30e<NUM> of the resistor <NUM> in the grid width direction with respective electrodes <NUM>.

The resistor <NUM> can be made of, for example, a material containing Cr (chromium), a material containing Ni (nickel), or a material containing both Cr and Ni. That is, the resistor <NUM> can be made of a material containing at least one of Cr and Ni. Materials containing Cr include, for example, a Cr composite film. Materials containing Ni include, for example, Cu-Ni (copper nickel). Materials containing both Cr and Ni include, for example, Ni-Cr (nickel chromium).

Here, a Cr composite film refers to a composite film of Cr, CrN, Cr<NUM>N, and the like. A Cr composite film may contain incidental impurities such as chromium oxide.

The thickness of the resistor <NUM> is not particularly limited and can be selected in accordance with the purpose of use. The resistor <NUM> can be, for example, approximately <NUM> to <NUM> thick. In particular, it is preferable if the resistor <NUM> is <NUM> thick or more, because the crystallinity of crystals (for example, the crystallinity of α-Cr) constituting the resistor <NUM> improves. It is even more preferable if the resistor <NUM> is <NUM> thick or less, because the cracks in the film constituting the resistor <NUM> due to the film's internal stress, and the warping of the film from the substrate <NUM> can be reduced. The width of the resistor <NUM> may preferably be optimized in accordance with the required specifications such as the resistance value and the lateral sensitivity, and be set to, for example, approximately <NUM> or more and <NUM> or less, as a countermeasure against disconnections.

For example, if the resistor <NUM> is a Cr composite film, the stability of gauge characteristics can be improved by using α-Cr (alpha-chromium), which has a stable crystalline phase, as the main component. Additionally, when the resistor <NUM> contains α-Cr as its main component, the strain gauge <NUM> can have a gauge factor of <NUM> or higher, as well as a gauge factor temperature coefficient TCS and a resistance temperature coefficient TCR in the range of -<NUM>,<NUM> ppm/degrees Celsius to +<NUM>,<NUM> ppm/degrees Celsius. Here, "the main component" means that the substance of interest takes up <NUM>% or more, by weight, of all substances constituting the resistor. It is preferable if the resistor <NUM> contains <NUM>% or more of α-Cr by weight, and more preferable if the resistor <NUM> contains <NUM>% or more of α-Cr by weight, from the perspective of improving the gauge characteristics. Note that α-Cr is a Cr having a body-centered cubic structure (bcc structure).

Also, when the resistor <NUM> is a Cr composite film, the CrN and Cr<NUM>N contained in the Cr composite film are preferably <NUM>% or less by weight. When the CrN and Cr<NUM>N contained in the Cr composite film are <NUM>% or less by weight, the decrease in the gauge ratio can be reduced.

Furthermore, the proportion of Cr<NUM>N in CrN and Cr<NUM>N is preferably <NUM>% or more by weight and less than <NUM>% by weight, and, more preferably, <NUM>% or more by weight and less than <NUM>% by weight. When the proportion of Cr<NUM>N in CrN and Cr<NUM>N is <NUM>% or more by weight and less than <NUM>% by weight, Cr<NUM>N, which has semiconductor-like properties, makes the decrease of TCR (negative TCR) more noticeable. Furthermore, reduced use of ceramic materials enables the reduction of brittle fracture as well.

Meanwhile, when a small amount of N<NUM> or atomic N gets mixed or is present in the film, the external environment (for example, high temperature environment) might make them escape from the film, which might result in a change in the film stress. By creating a chemically stable CrN, a stable strain gauge can be obtained without generating the above unstable N.

The conductive traces <NUM> are formed on the substrate <NUM>. The electrodes <NUM> are formed on the substrate <NUM>, and electrically connected to the resistor <NUM> via the conductive traces <NUM>. For example, the electrodes <NUM> are wider than the conductive traces <NUM> and formed in a substantially rectangular shape. The electrodes <NUM> are a pair of electrodes for outputting the strain-induced changes in the resistance value of the resistor <NUM> to the outside, and are connected to lead wires for external connection, for example. In <FIG>, for ease of explanation, the conductive traces <NUM> and the electrodes <NUM> are shown with a satin pattern that is sparser than that of the resistor <NUM>.

Note that the resistor <NUM>, the conductive traces <NUM>, and the electrodes <NUM> are assigned different reference signs for ease of explanation, but they can be formed integrally from the same material through the same process. Therefore, the resistor <NUM>, the conductive traces <NUM>, and the electrodes <NUM> have substantially the same thickness.

The upper surfaces of the conductive traces <NUM> and the electrodes <NUM> may be covered by a metal that is made of a material having a lower resistance than the conductive traces <NUM> and the electrodes <NUM>. For example, when the resistor <NUM>, the conductive traces <NUM>, and the electrodes <NUM> are a Cr composite film, the metallic material having a lower resistance than the Cr composite film may be Cu, Ni, Al, Ag, Au, or Pt, an alloy of any of these metals, a compound of any of these metals, or a laminated film obtained by appropriately laminating any of these metals, alloys, or compounds.

The barrier layer <NUM> is provided to cover the resistor <NUM>. The barrier layer <NUM> is a resin layer that is formed to reduce the leakage current that flows in the resistor <NUM>, and is made from an insulating material. Note that, in the present disclosure, an insulating material refers to a material having a volume resistivity of <NUM> MΩ·m or more, preferably <NUM> MΩ·m or more.

With this barrier layer <NUM>, the ratio of the moisture permeability (g/m<NUM>/<NUM>) to the thickness (mm) is <NUM>:<NUM> or greater. For example, if the moisture permeability of the barrier layer <NUM> is <NUM>/m<NUM>/<NUM>, the thickness of the barrier layer <NUM> is <NUM> or more. If the moisture permeability of the barrier layer <NUM> is <NUM>/m<NUM>/<NUM>, the thickness of the barrier layer <NUM> is <NUM> (<NUM>) or more. Furthermore, if the moisture permeability of the barrier layer <NUM> is <NUM>/m<NUM>/<NUM>, the thickness of the barrier layer <NUM> is <NUM> (<NUM>) or more. The minimum thickness x (mm) of the barrier layer <NUM> is given by: the moisture permeability A (g/m<NUM>/<NUM>) of the barrier layer material × the thickness B (mm) upon measurement of the moisture permeability A / a coefficient C. The coefficient C is a coefficient value for calculating the minimum thickness (mm) and is <NUM> (g/m<NUM>/<NUM>).

In the present disclosure, the moisture permeability is the mass (g) of water vapor that permeates through a unit area (<NUM><NUM>) of the barrier layer <NUM> in <NUM> hours. Also, in the present disclosure, the moisture permeability is measured in an environment of <NUM> degrees Celsius and <NUM>% RH, in accordance with the provisions of JIS Z0208.

Also, the thickness of the barrier layer <NUM>, when determining its ratio to the moisture permeability, refers to the thickness of the portion of the barrier layer <NUM> that is in contact with the upper surface of the resistor <NUM>. That is, it is the thickness of the barrier layer <NUM> measured from the upper surface of the resistor <NUM> in a direction normal to the upper surface of the resistor <NUM>.

The material of the barrier layer <NUM> can be selected as appropriate, and the barrier layer <NUM> can be made from, for example, a fluoropolymer, an epoxy resin, or an acrylic resin, insofar as the ratio of the moisture permeability (g/m<NUM>/<NUM>) to thickness (mm) is <NUM>:<NUM> or greater. The barrier layer <NUM> may have a laminated structure in which a plurality of layers of different materials are laminated. Alternatively, for example, a hybrid resin combining an epoxy resin and an acrylic resin may be used.

For example, if there is a risk of contact with chemicals, a fluoropolymer, which has high chemical resistance, is a suitable material for the barrier layer <NUM>. An epoxy resin, which is excellent in heat resistance, is a suitable material for the barrier layer <NUM> when the strain gauge <NUM> used at a relatively high temperature. Furthermore, in the event the strain gauge <NUM> is used at a relatively low temperature, an acrylic resin, which can ensure flexibility even at low temperatures, is a suitable material for the barrier layer <NUM>.

As described above, with the barrier layer <NUM>, the ratio of the moisture permeability (g/m<NUM>/<NUM>) to the thickness (mm) has only to be <NUM>:<NUM> or greater. However, even if there is a resin having a high moisture permeability, it does not necessarily mean that the resin can be used by simply making it thicker. If the barrier layer <NUM> becomes thicker, the cure-shrinkage stress and the like becomes a problem, and so it is necessary to keep the barrier layer <NUM> somewhat thin. Considering the cure-shrinkage stress and the like, the thickness of the barrier layer <NUM> is preferably <NUM> or less. Meantime, due to manufacturing limitations, the thickness of the barrier layer <NUM> is preferably <NUM> or more.

Thus, in the strain gauge <NUM>, the resin barrier layer <NUM> is formed so as to cover the resistor <NUM>, and, in the barrier layer <NUM>, the ratio of the moisture permeability (g/m<NUM>/<NUM>) and the thickness (mm) is <NUM>:<NUM> or greater, and its thickness is <NUM> or more and <NUM> or less.

By this means, even when the strain gauge <NUM> is used underwater or in a water-splashing environment, the barrier layer <NUM> suppresses the permeation of water vapor, so that the leakage current flowing in the resistor <NUM> can be reduced. As a result of this, in the strain gauge <NUM>, the variations of output voltage due to the impact of leakage current can be reduced.

In particular, a highly sensitive strain gauge that uses a Cr composite film for the resistor <NUM> and that has a gauge factor of <NUM> or more is more sensitive than a conventional strain gauge having a gauge factor of less than <NUM>, thus being more sensitive to the impact of moisture, and measurement errors and variations in the output voltage occur more often. Therefore, when the strain gauge uses a Cr composite film for the resistor <NUM> and has a gauge factor of <NUM> or more, it is particularly effective to form a barrier layer <NUM>, in which the ratio of the moisture permeability (g/m<NUM>/<NUM>), the thickness (mm) is <NUM>:<NUM> or greater, and the thickness of which is <NUM> or more and <NUM> or less, and which covers the resistor <NUM>.

The barrier layer <NUM> is provided so as to cover the upper surface and side surfaces of the resistor <NUM> in a continuous fashion. As a result of this, the effect of reducing the impact of moisture becomes greater than when the barrier layer <NUM> covers only the upper surface of the resistor <NUM>. Nevertheless, insofar as the barrier layer <NUM> covers at least part of the resistor <NUM>, a certain effect of reducing the impact of moisture can be achieved.

<FIG> shows an outcome of an experiment for confirming the leakage current reduction effect of the barrier layer. In <FIG>, in test sample A, a strain gauge having the structure shown in <FIG> and <FIG> without the barrier layer <NUM> is used, while, in test sample B, the barrier layer <NUM> is provided as in the strain gauge <NUM> having the structure shown in <FIG> and <FIG>.

In test samples A and B, a Cr composite film having a film thickness of <NUM> is used as the resistor <NUM>. Also, in test sample B, an acrylic resin having a film thickness of <NUM> is used as the barrier layer <NUM>. In this case, the ratio of the moisture permeability (g/m<NUM>/<NUM>) and the thickness (mm) is <NUM>:<NUM>.

In the experiment, after a certain period of time elapsed (the one-dot chain line in <FIG>) and the measurement became stable, test samples A and B were sprayed with water by using a commerciallyavailable pressurized spray, and were exposed to a water-splashing environment, and thereupon the changes in their output voltage were measured over time.

<FIG> shows that, with test sample A that does not have a barrier layer, when test sample A is exposed to moisture, the resistance value of the resistor drops due to the impact of leakage current, and a sharp rise in the output voltage can be seen. On the other hand, with test sample B having a barrier layer <NUM>, an increase in the output voltage due to leakage current cannot be seen. Note that, with test sample B, the output voltage rises slowly, but this simply shows the same behavior as temperature changes and depends on the thermal characteristics (TCR) of the strain gauge, and the impact of moisture is unrelated.

The present inventors repeated the same experiment as shown in <FIG>, and found out that, insofar as the ratio of the moisture permeability (g/m<NUM>/<NUM>) and the thickness (mm) of the barrier layer <NUM> was <NUM>:<NUM> or greater, the output voltage did not rise sharply as in the case with test sample A, and exhibited the same behavior as temperature changes as in the case with test sample B. Furthermore, according to further studies by the present inventors, even when an epoxy resin and a fluoropolymer were used for the barrier layer <NUM>, the same result as with test sample B in <FIG>, which used an acrylic resin, was obtained.

To make the strain gauge <NUM>, first, the substrate <NUM> is prepared, and a metal layer (referred to as a "metal layer" A for ease of explanation) is formed on the upper surface 10a of the substrate <NUM>. The metal layer A is a layer that is ultimately patterned so as to serve as the resistor <NUM>, the conductive traces <NUM>, and the electrodes <NUM>. Therefore, the material and the thickness of the metal layer A are the same as those of the resistor <NUM>, the conductive traces <NUM>, and the electrodes <NUM> described above.

The metal layer A can be formed, for example, by magnetron sputtering which uses, as a target, a raw material that can form the metal layer A. The metal layer A may be formed by using reactive sputtering, vapor deposition, arc ion plating, pulse laser deposition, and so forth, instead of magnetron sputtering.

From the perspective of achieving stable gauge characteristics, before forming the metal layer A, it is preferable to vacuum-form a functional layer of a predetermined thickness as an underlying layer, over the upper surface 10a of the substrate <NUM> by, for example, conventional sputtering.

In the present disclosure, a functional layer refers to a layer having a function of promoting the crystal growth of at least the upper metal layer A (resistor <NUM>). The functional layer preferably further has a function of preventing oxidation of the metal layer A due to the oxygen and moisture contained in the substrate <NUM>, a function of improving the adhesion between the substrate <NUM> and the metal layer A, and so forth. The functional layer may also have other functions as well.

The insulating resin film that constitutes the substrate <NUM> contains oxygen and moisture. In particular, when the metal layer A contains Cr, Cr forms a self-autoxidized film, and it is therefore effective if the functional layer has a function of preventing oxidation of the metal layer A.

The material of the functional layer is not particularly limited as long as it at least has a function of facilitating the crystal growth of the metal layer A (resistor <NUM>), which is an upper layer, and can be selected in accordance with the purpose of use. The material may be, for example, one or more types of 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 some of the metals in this group, or a compound of some of the metals in this group.

Examples of the above alloy include FeCr, TiAl, FeNi, NiCr, CrCu, and the like. Examples of the above compound include TiN, TaN, Si<NUM>N<NUM>, TiO<NUM>, Ta<NUM>O<NUM>, SiO<NUM>, and the like.

When the functional layer is formed from a conductive material such as a metal or an alloy, the film thickness of the functional layer is preferably <NUM>/<NUM> or less of the film thickness of the resistor. When the film thickness of the functional layer is in this range, it is possible to facilitate the crystal growth of α-Cr, and prevent a situation where part of the current flowing in the resistor flows into the functional layer and lowers the sensitivity of strain detection.

When the functional layer is formed from a conductive material such as a metal or an alloy, the film thickness of the functional layer is more preferably <NUM>/<NUM> or less of the film thickness of the resistor. When the film thickness of the functional layer is in this range, it is possible to facilitate the crystal growth of α-Cr, and prevent, more effectively, a situation where part of the current flowing in the resistor flows into the functional layer and lowers the sensitivity of strain detection.

When the functional layer is formed from a conductive material such as a metal or an alloy, the film thickness of the functional layer is even more preferably <NUM>/<NUM> or less of the film thickness of the resistor. When the film thickness of the functional layer is in this range, it is possible to prevent, even more effectively, a situation where part of the current flowing in the resistor flows into the functional layer and lowers the sensitivity of strain detection.

When the functional layer is formed from an insulating material such as an oxide or a nitride, the film thickness of the functional layer is preferably <NUM> to <NUM>. When the film thickness of the functional layer is in this range, it is possible to facilitate the crystal growth of α-Cr, and form a film with ease without forming cracks in the functional layer.

When the functional layer is formed from an insulating material such as an oxide or a nitride, the film thickness of the functional layer is more preferably <NUM> to <NUM>. When the film thickness of the functional layer is in this range, it is possible to facilitate the crystal growth of α-Cr, and form a film even more easily without forming cracks in the functional layer.

When the functional layer is formed from an insulating material such as an oxide or a nitride, the film thickness of the functional layer is even more preferably <NUM> to <NUM>. When the film thickness of the functional layer is in this range, it is possible to facilitate the crystal growth of α-Cr, and form a film even more easily without forming cracks in the functional layer.

Note that the plan shape of the functional layer is patterned substantially the same as the plan shape of the resistor illustrated in <FIG>, for example. However, the plan shape of the functional layer does not necessarily have to be substantially the same as the plan shape of the resistor. When the functional layer is formed from an insulating material, it does not have to be patterned in the same shape as the plan shape of the resistor. In this case, the functional layer may be formed solidly at least in the region where the resistor is formed. Alternatively, the functional layer may be formed solidly over the entire upper surface of the substrate <NUM>.

Also, when the functional layer is formed from an insulating material, the functional layer may be made relatively thick, such as <NUM> thick or more and <NUM> thick or less, and may be formed in a solid shape, so that the thickness and the surface area of the functional layer increase, and the heat that is generated when the resistor warms up can be readily dissipated to the substrate <NUM> side. As a result of this, with the strain gauge <NUM>, it is possible to reduce the decrease of the accuracy of measurement due to the resistor's self-heating.

The functional layer can be vacuum-formed by, for example, conventional sputtering, in which a raw material that can form the functional layer is used as a target and an Ar (argon) gas is introduced into a chamber. By using conventional sputtering, the functional layer is formed while the upper surface 10a of the substrate <NUM> is being etched with Ar, and therefore it is possible to form the functional layer only in a minimal amount, and achieve an effect of improved adhesion.

However, this is simply one example of the method of forming the functional layer, and the functional layer may be formed by using other methods as well. For example, a method may be used here in which: before the functional layer is formed, the upper surface 10a of the substrate <NUM> is activated by plasma treatment using Ar or the like, so as to gain an adhesion improving effect; and subsequently, the functional layer is vacuum-formed by magnetron sputtering.

The combination of the material of the functional layer and the material of the metal layer A is not particularly limited and can be selected in accordance with the purpose of use. For example, it is possible to form a Cr composite film by using Ti as the main component of the functional layer and α-Cr (alpha-chromium) as the main component of the first metal layer.

In this case, the first metal layer can be formed by magnetron sputtering, in which a raw material that can form a Cr composite film is used as a target, and an Ar gas is introduced into a chamber. Alternatively, reactive sputtering, which targets pure Cr and introduces an appropriate amount of nitrogen gas into a chamber with an Ar gas, may be used to form the metal layer A. In this case, by changing the amount and pressure (nitrogen partial pressure) of nitrogen gas to be introduced, adjusting the heating temperature by providing a heating step, and so forth, it is possible to adjust the proportions of CrN and Cr<NUM>N contained in the Cr composite film, as well as the proportion of Cr<NUM>N in CrN and Cr<NUM>N.

According to these methods, the growth surface of the Cr composite film is defined based on the functional layer consisting of Ti, and a Cr composite film, in which α-Cr having a stable crystalline structure is the main component, can be formed. Also, Ti that constitutes the functional layer is diffused in the Cr composite film, so that improved gauge characteristics can be gained. For example, the gauge factor of the strain gauge <NUM> can be made <NUM> or more, and the gauge factor temperature coefficient TCS and the resistance temperature coefficient TCR can be kept in the range of -<NUM>,<NUM> ppm/degrees Celsius to +<NUM>,<NUM> ppm/degrees Celsius. Note that, when the functional layer is formed from Ti, the Cr composite film might contain Ti or TiN (titanium nitride).

Note that, when the metal layer A is a Cr composite film, the functional layer consisting of Ti has all of: the function of facilitating the crystal growth of the metal layer A; the function of preventing oxidation of the metal layer A due to the oxygen or moisture contained in the substrate <NUM>; and the function of improving the adhesion between the substrate <NUM> and the metal layer A. The same applies when Ta, Si, Al, or Fe is used for the functional layer, instead of Ti.

In this way, by providing a functional layer in a lower layer of the metal layer A, it becomes possible to facilitate the crystal growth of the metal layer A, and fabricate a first metal layer consisting of a stable crystalline phase. As a result of this, the stability of gauge characteristics in the strain gauge <NUM> can be improved. Furthermore, since the material to constitute the functional layer is diffused in the metal layer A, the strain gauge <NUM> can have improved gauge characteristics.

Next, the metal layer A is patterned by photolithography, and the flat resistor <NUM>, conductive traces <NUM>, and electrodes <NUM> shown in <FIG> are formed.

Note that, when a functional layer is provided on the upper surface 10a of the substrate <NUM> as a base layer for the resistor <NUM>, the conductive traces <NUM>, and the electrodes <NUM>, the strain gauge <NUM> has the cross-sectional shape shown in <FIG>. The layer denoted by the reference sign <NUM> is the functional layer. The plan shape of the strain gauge <NUM> when the functional layer <NUM> is provided is, for example, the same as that shown in <FIG>. However, as described above, the functional layer <NUM> may be formed solidly in part or all of the upper surface of the substrate <NUM>.

Next, a barrier layer <NUM> is formed on the upper surface 10a of the substrate <NUM> so as to cover the resistor <NUM> and expose the electrodes <NUM>. As long as the barrier layer <NUM> covers at least the resistor <NUM>, the barrier layer <NUM> may additionally cover part or all of the conductive traces <NUM>. The material and thickness of the barrier layer <NUM> are as described above. Note that, in <FIG>, the barrier layer <NUM> is formed so as to cover part of the conductive traces <NUM>.

The method of forming the barrier layer <NUM> is not particularly limited and can be selected as appropriate depending on the purpose of use. For example, the barrier layer <NUM> may be produced by laminating a semi-hardened thermosetting insulating resin film over the upper surface 10a of the substrate <NUM>, so as to cover the resistor <NUM> and expose the electrodes <NUM>, and then by heating and hardening the laminate. Alternatively, the barrier layer <NUM> may be produced by applying a liquid or paste thermosetting insulating resin to the upper surface 10a of the substrate <NUM>, so as to cover the resistor <NUM> and expose the electrodes <NUM>, and then by heating and hardening the resin. Through the above steps, the strain gauge <NUM> is completed.

A modification <NUM> of the first embodiment shows an example in which a stress relaxing layer is provided in the strain gauge. Note that, in modification <NUM> of the first embodiment, the description of the same components as in the above-described embodiment may be omitted.

<FIG> is a diagram that illustrates an example of a strain gauge according to modification <NUM> of the first embodiment. <FIG> is a cross-sectional view that illustrates the strain gauge according to modification <NUM> of the first embodiment, showing a cross-section along line B-B in <FIG>. Referring to <FIG> and <FIG>, the strain gauge 1A differs from the strain gauge <NUM> (see <FIG>, <FIG>, etc.) in that it has an insulating stress relaxing layer <NUM>.

In the strain gauge 1A, the stress relaxing layer <NUM> is placed between the resistor <NUM> and the barrier layer <NUM>. That is, the stress relaxing layer <NUM> is formed to cover the resistor <NUM>, and the barrier layer <NUM> is formed to cover the stress relaxing layer <NUM>. Alternatively, the stress relaxing layer <NUM> may be formed only on the upper surface of the resistor <NUM>, and the side surfaces of the resistor <NUM> may be directly covered with the barrier layer <NUM>.

The stress relaxing layer <NUM> is made from an organic film having a lower Young's modulus than the barrier layer <NUM>. If the barrier layer <NUM> has a high Young's modulus or cure-shrinkage stress, the gauge factor of the strain gauge 1A might decrease. By placing an insulating stress relaxing layer <NUM>, made from an organic film having a lower Young's modulus than the barrier layer <NUM>, between the resistor <NUM> and the barrier layer <NUM>, the stress generated in the barrier layer <NUM> is relieved by the stress relaxing layer <NUM>, and transmitted less to the resistor <NUM>.

As a result of this, it is possible to minimize the decrease of the gauge factor of the strain gauge 1A. Considering this together with the effect of the barrier layer <NUM>, it is possible to reduce the variation of output voltage due to the impact of leakage current, without sacrificing the gauge factor of the strain gauge 1A.

It is preferable if the coefficient of linear expansion of the stress relaxing layer <NUM> is close to the coefficient of linear expansion of the substrate <NUM>. When the difference between the coefficient of linear expansion of the stress relaxing layer <NUM> and the coefficient of linear expansion of the substrate <NUM> increases, greater strain arises due to the difference in expansion in response to temperature changes, making the resistance value of the resistor <NUM> vary more significantly. In the meantime, making the linear expansion coefficient of the stress relaxing layer <NUM> close to the linear expansion coefficient of the substrate <NUM> brings about an effect of reducing the TCR.

Examples of materials for the stress relaxing layer <NUM> include organic materials such as resin and rubber. For example, the material of the stress relaxing layer <NUM> may be the same as the material of the substrate <NUM>. The thickness of the stress relaxing layer <NUM> may be the same as the thickness of the substrate <NUM>. The thickness of the stress relaxing layer <NUM> can be, for example, <NUM> or more and <NUM> or less. The stress relaxing layer <NUM> can be formed, for example, by the same method as that of the barrier layer <NUM>.

Although a preferred embodiment and other forms of the present disclosure have been described above in detail as examples, the present disclosure is by no means limited to these examples, and a variety of modifications and replacements can be introduced to the above examples without departing from the scope set forth in the claims.

Claim 1:
A strain gauge comprising:
a flexible substrate (<NUM>);
a resistor (<NUM>) made from a film containing Cr, CrN, and Cr<NUM>N, on one surface of the substrate (<NUM>); and
a resin barrier layer (<NUM>) covering an upper surface and side surfaces of the resistor (<NUM>) in a continuous manner,
wherein, in the barrier layer (<NUM>), a ratio of moisture permeability (g/m<NUM>/<NUM>) to thickness (mm) is <NUM>:<NUM> or less than <NUM>:<NUM>, and the thickness of the barrier layer (<NUM>) is <NUM> or more and <NUM> or less, wherein the moisture permeability is measured in an environment of <NUM> and <NUM>% relative humidity in accordance with the provision of JIS Z0208.