Strain gauge

1. A strain gauge comprising:

a flexible resin substrate;

resistors each formed of a film that includes Cr, CrN, and Cr 2 N; and

a functional layer formed of a metal, an alloy, or a metal compound, the functional layer being in a lower surface of the resistors,

wherein an element included in the functional layer is diffused into the film,

wherein the resistors include a first resistor and a second resistor that are formed on one side of the substrate; and a third resistor and a fourth resistor that are formed on another side of the substrate,

wherein the first resistor, the second resistor, the third resistor, and the fourth resistor constitute a Wheatstone bridge circuit, and

wherein a gauge factor of the strain gauge is 10 or more.

2. The strain gauge according to claim 1 , further comprising two electrodes for applying an input voltage to the Wheatstone bridge circuit; and two other electrodes for obtaining an output voltage of the Wheatstone bridge circuit,

wherein the two electrodes and the two other electrodes are formed on the one side of the substrate.

3. The strain gauge according to claim 2 , wherein the two electrodes and the two other electrodes are electrically each connected, through a given via hole provided in the substrate, to a corresponding pad from among pads extending from both end portions of each of the third resistor and the fourth resistor, and

wherein each of the two electrodes and the two other electrodes is continuously formed, from the one side of the substrate, on a sidewall of the via hole and on a surface of the pad exposed in the via hole, so that a recessed portion is formed in the via hole.

4. The strain gauge according to claim 1 , further comprising a first insulating resin layer with which the first resistor and the second resistor are coated and that is formed on the one side of the substrate; and

a second insulating resin layer with which the third resistor and the fourth resistor are coated and that is formed on the another side of the substrate.

5. The strain gauge according to claim 1 , wherein a main component of each resistor is alpha-chromium.

6. The strain gauge according to claim 5 , wherein each resistor includes alpha-chromium at 80% by weight or more.

7. The strain gauge according to claim 1 , wherein the functional layer includes a function of promoting crystal growth of the resistor.

8. A strain gauge comprising:

a flexible resin substrate:

resistors each formed of a film that includes Cr, CrN, and Cr 2 N; and

a functional layer formed of a metal, an alloy, or a metal compound, the functional layer being in a lower surface of the resistors,

wherein an element included in the functional layer is diffused into the film,

wherein the resistors include a first resistor and a second resistor that are formed on one side of the substrate; and a third resistor and a fourth resistor that are formed on another side of the substrate,

wherein the first resistor, the second resistor, the third resistor, and the fourth resistor constitute a Wheatstone bridge circuit, and

wherein a temperature coefficient of resistance of the strain gauge is in a range of from −1000 ppm/° C. to +1000 ppm/° C.

9. A strain gauge comprising:

a flexible resin substrate;

resistors each formed of a film that includes Cr, CrN, and Cr 2 N; and

a functional layer formed of a metal, an alloy, or a metal compound, the functional layer being in a lower surface of the resistors,

wherein the resistors include a first resistor and a second resistor that are formed on one side of the substrate; and a third resistor and a fourth resistor that are formed on another side of the substrate, and

wherein the first resistor, the second resistor, the third resistor, and the fourth resistor constitute a Wheatstone bridge circuit.