JP2013217763A - Material for thin film strain sensor and thin film strain sensor using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide material for a thin film strain sensor having high strain sensitivity, low lateral sensitivity, and a low resistance temperature coefficient, and to provide a thin film strain sensor using the material.SOLUTION: The Cr content of material for a thin film strain sensor of the present invention is 14-25 mass%, and the rest is composed of Pd and inevitable impurities. A thin film strain sensor 1 according to the present invention is obtained by forming, on an insulating film (not shown): a sensing part 3 for sensing strain; and lead parts 4 formed at both ends of the sensing part 3. At least the sensing part 3, among the sensing part 3 and the lead parts 4, is formed using the material for a thin film strain sensor.

Description

  The present invention relates to a material for a thin film strain sensor and a thin film strain sensor using the same.

A thin film strain sensor (also referred to as a thin film strain gauge) is known as a sensor for measuring a strain amount when an external force is applied to a mechanical component.
The thin film strain sensor is formed on a base material with a sensing portion for sensing strain and a lead portion formed at both ends of the sensing portion and connected to a bridge circuit such as a Wheatstone bridge. The lead portion is formed using an alloy having a specific composition.

  When an external force is applied to the thin-film strain sensor, the sensing unit expands and contracts, and a minute output voltage proportional to the change in resistance value caused by the expansion and contraction can be obtained. taking measurement.

  The performance and characteristics of the thin film strain sensor greatly depend on the alloy forming the sensing part and the lead part and the shape of the sensor. Therefore, various alloys for forming the sensing part and the lead part have been developed. In general, a Cu—Ni-based alloy or a Ni—Cr-based alloy is used, but there are, for example, those described in Non-Patent Document 1 and Patent Document 1.

  Non-Patent Document 1 discloses a thin film manufactured using a Pd—Cr alloy (88.5Pd-11.5Cr) having a Pd content of 88.5 mass% and a Cr content of 11.5 mass%. The electrical characteristics of the strain sensor are described.

  Japanese Patent Application Laid-Open No. 2004-228561 describes that the sensing portion is made to have a specific shape, and the alloy forming the sensing portion and the lead portion is preferably a Ni—Cr—Al alloy. In addition, the composition of the Ni—Cr—Al alloy is such that the Cr content is 18.5% by mass, the Al content is 11.0% by mass, and the balance is Ni (18.5Cr-11.0Al—Ni rest). It is described that.

P. Kayser, J.C.Godefroy and L. Leca, “High-temperature thin-film strain gauges”, Sensors and Actuators A, 37-38 (1993) P.328-332

Patent No. 4482250

  However, as shown in FIG. 6, Non-Patent Document 1 has a low thermal stability, so the temperature coefficient of resistance (TCR) with respect to temperature is as large as 210 ppm / ° C. or higher, and the strain sensitivity increases at higher temperatures. There was a problem of getting worse. The strain sensitivity is also called a gauge factor, and can be expressed as a ratio of a resistance change rate ΔR / R caused by uniaxial stress applied in the gauge axis direction of the strain gauge and a strain ε in the gauge axis direction. it can. The strain sensitivity can be obtained by K = (ΔR / R) / ε, where K is the strain sensitivity.

  Further, the Ni—Cr—Al alloy described in Patent Document 1 has low strain sensitivity (vertical direction 1.40 to 1.50) and large lateral sensitivity that causes noise (lateral direction −0.42). There is a problem that the thermal stability is low. The lateral sensitivity refers to sensitivity to excitation in an arbitrary direction perpendicular to the gauge axis direction (sensitive axis).

  For this reason, neither NPL 1 nor PTL 1 could be used in an internal combustion engine having a high temperature of 400 ° C. or higher.

  This invention is made | formed in view of the said problem, and makes it a subject to provide the material for thin film strain sensors with a high strain sensitivity, a low lateral sensitivity, and a low resistance temperature coefficient, and a thin film strain sensor using the same. .

(1) The material for a thin film strain sensor according to the present invention that solves the above-mentioned problems is characterized in that the Cr content is 14 to 25% by mass, and the balance is composed of Pd and inevitable impurities.

  Since the thin film strain sensor material according to the present invention is a Pd—Cr alloy having a specific composition range, the strain sensitivity can be increased, the lateral sensitivity can be decreased, and the resistance temperature coefficient can be decreased.

(2) Moreover, the thin film strain sensor according to the present invention is a thin film strain sensor in which a sensing portion for sensing strain and lead portions formed at both ends of the sensing portion are formed on an insulating film, At least the sensing part of the sensing part and the lead part is formed using the material for a thin film strain sensor described in (1).

  Since the thin film strain sensor according to the present invention forms the sensing portion using the material for the thin film strain sensor described in (1), the sensor has high strain sensitivity, low lateral sensitivity, and low resistance temperature coefficient. can do.

  Since the thin film strain sensor material according to the present invention is a Pd—Cr alloy having a specific composition range, the strain sensitivity is high, the lateral sensitivity is low, and the resistance temperature coefficient is low. That is, since the material for a thin film strain sensor according to the present invention has high thermal stability, it can be used at a high temperature of, for example, 400 ° C. or higher.

  In the thin film strain sensor according to the present invention, at least the sensing portion of the sensing portion and the lead portion is formed using the thin film strain sensor material according to the present invention, so that the strain sensitivity is high, the lateral sensitivity is low, the resistance temperature The coefficient is low. That is, since the thin film strain sensor according to the present invention has high thermal stability, it can be used even at a high temperature of 400 ° C. or higher, for example.

It is a graph explaining the relationship between content of Cr with respect to Pd, strain sensitivity, and resistance temperature coefficient (TCR). The horizontal axis represents Cr content [% by mass] relative to Pd, the first vertical axis represents strain sensitivity K, and the second vertical axis represents TCR [ppm / ° C.]. The broken line represents the longitudinal strain sensitivity, the solid line represents the lateral sensitivity, and the alternate long and short dash line represents the TCR. It is a top view which shows one Embodiment of the thin film strain sensor which concerns on this invention. It is a top view which shows other embodiment of the thin film strain sensor which concerns on this invention. It is a figure explaining the strain sensitivity tester used for the measurement of a strain sensitivity, Comprising: The right figure is a top view and the left figure is a side view. It is a graph which shows the strain sensitivity with respect to the presence or absence of a thermal history of 400 degreeC and 1 hour about the thin film strain sensor produced with the Pd-Cr alloy, and the thin film strain sensor produced with the Ni-Cr-Al alloy. It is a graph which shows the temperature coefficient of resistance (TCR) with respect to the temperature of the conventional thin film strain sensor using 88.5Pd-11.5Cr. The horizontal axis represents temperature [° C.], and the vertical axis represents TCR [ppm / ° C.].

  The gist of the present invention is that the material for the thin film strain sensor is a Pd—Cr alloy having a specific composition range, so that the strain at a high temperature is higher than that of a conventionally known Pd—Cr alloy or Ni—Cr—Al alloy. The purpose is to suppress a decrease in sensitivity.

Hereinafter, an embodiment for implementing a thin film strain sensor material according to the present invention and a thin film strain sensor using the same will be described with reference to the drawings as appropriate.
The thin film strain sensor material according to the present invention has a Cr content of 14 to 25% by mass, and the balance is made of Pd and inevitable impurities.

  When the Cr content is in the range of 14 to 25% by mass, a high strain sensitivity K (longitudinal direction) of 1.75 or more can be obtained as shown in FIG.

  Further, when the Cr content is in this range, as shown in FIG. 1, the lateral sensitivity is in the range of -0.3 to 0.3, so that noise can be reduced. That is, the main strain value can be detected with high accuracy.

  Furthermore, when the Cr content is in this range, as shown in FIG. 1, the temperature coefficient of resistance (TCR) changes little, so that the amount of temperature drift can be reduced. That is, the thermal stability is high, and the strain sensitivity is unlikely to deteriorate at high temperatures. Therefore, the strain can be measured stably even at high temperatures.

  In other words, when the Cr content is within this range, the thin film strain sensor has a high strain sensitivity and a region where the change in strain sensitivity is small, a region where the lateral sensitivity can be used, and a region where the change in TCR is small. Can be a material.

On the other hand, when the Cr content is less than 14% by mass, the lateral sensitivity becomes lower than −0.3, so that noise increases, and the TCR increases, so that the amount of temperature drift increases. Accordingly, the thermal stability is low, and the strain sensitivity is likely to deteriorate at high temperatures.
On the other hand, if the Cr content exceeds 25% by mass, the lateral sensitivity increases beyond 0.3 and noise increases. That is, the main strain value cannot be detected with high accuracy.
In addition, content of Cr can also be 14.5-20.3 mass%. Even in such a range, the above-described effects can be reliably achieved.

  The balance is Pd and inevitable impurities as described above. Inevitable impurities in the present invention refer to elements that are inevitably contained in the production process. It is preferable not to contain such inevitable impurities as much as possible. However, in most cases, the content is extremely small, and does not affect the effect of the thin film strain sensor material of the present invention.

  Such a material for a thin film strain sensor is formed by, for example, sputtering so that the Cr content is 14 to 25% by mass, and the remaining portion is made of Pd and inevitable impurities, and the sensing portion 3 and lead portion 4 ( 2) can be formed.

  Next, the thin film strain sensor according to the present invention will be described. As shown in FIG. 2, a thin film strain sensor 1 according to the present invention includes a sensing unit 3 for sensing strain on an insulating film (not shown) formed on the surface of a substrate 2, and both ends of the sensing unit 3. And at least the sensing part 3 of the sensing part 3 and the lead part 4 is formed using the thin film strain sensor material according to the present invention described above. Is.

  Examples of the base material 2 include an internal combustion engine and various devices that have a high temperature of 400 ° C. or higher. Taking an internal combustion engine as an example, specific examples include main bearings, connecting rod shafts, piston pin bosses, and the like.

The insulating film is provided in order to prevent the energization of the substrate 2 and the sensing unit 3 and the lead unit 4 formed thereon and to accurately measure the strain.
The insulating film can be formed with an arbitrary film thickness using, for example, Al ₂ O ₃ or AlN. The thickness of the insulating film can be set to 1 to 20 μm, for example. If the thickness of the insulating film is less than the lower limit described above, the strength of the insulating film is lowered, and the product life may be shortened. Further, if the thickness of the insulating film exceeds the above upper limit, the merit as a thin film sensor may be lost.

  As shown in FIG. 2, the sensing unit 3 does not have a folded part (that is, in a straight line) or, as shown in FIG. 3, between the two lead parts 4, one or more folded parts. It is formed with. The sensing unit 3 can be formed with a width of 0.03 mm and a thickness of 0.2 μm, for example. Here, the length of the sensing unit 3 differs depending on the presence or absence of the folded portion, and can be arbitrarily set. In addition, when the sensing unit 3 is formed between the two lead units 4 without a folded part (in a straight line), for example, the sensing unit 3 can be 3 mm wide, for example. Needless to say, the width, thickness, and lateral dimensions of the sensing unit 3 are not limited to the above.

In addition to being connected to the sensing unit 3, the lead unit 4 is connected to a bridge circuit (not shown) such as a Wheatstone bridge. The lead portion 4 is preferably formed of the material for a thin film strain sensor according to the present invention as in the case of the sensing portion 3 in terms of the manufacturing process, but may be formed of a different material.
The lead portion 4 can be, for example, 3 mm wide × 2 mm wide, but is not limited to this.

Such a thin film strain sensor 1 is formed by forming an insulating film on the surface of a substrate 2 by a known technique, forming a Pd—Cr alloy layer thereon by sputtering, and performing photolithography on the formed Pd—Cr alloy layer. It can be manufactured by making the sensing part 3 and the lead part 4 into a predetermined shape by a method or the like. In addition, after forming the sensing part 3 and the lead part 4, a protective film (not shown) may be formed thereon using Al ₂ O ₃ or AlN. The protective film can have a thickness of 1.5 μm, for example.

  A thin film strain sensor material and a thin film strain sensor using the same according to the present invention will be specifically described with reference to an example that satisfies the requirements of the present invention and a comparative example that does not.

  As materials for the thin film strain sensor, Pd—Cr alloys according to Examples 1 to 3 and Comparative Examples 1 and 2 were prepared so as to have the compositions shown in Table 1 below.

Sputtering was performed using the Pd—Cr alloys according to Examples 1 to 3 and Comparative Examples 1 and 2, and a 0.2 μm thick Pd—Cr alloy layer was formed on the strain sensitivity tester having the shape and dimensions shown in FIG. Formed. The dimension (millimeter) is shown in the same figure. Note that an insulating film having a thickness of 1.5 μm is formed using Al ₂ O ₃ on the surface on which the Pd—Cr alloy layer is formed.

  The Pd-Cr alloy layer consists of a thin plate portion (thickness 2 mm) of a strain sensitivity tester, a sensing portion (refer to FIG. 2) of width 0.03 mm × width 3 mm, and a lead portion of width 3 mm × width 2 mm at both ends of the sensing portion. Two sensors having (see FIG. 2) were formed.

Each of the two sensors has a direction in which the length direction of the sensing unit is parallel to the short direction of the strain sensitivity tester (this is called a vertical direction) and a direction perpendicular to the short side direction (this is a horizontal direction). It was formed to become. That is, the former sensor is a strain sensitivity K sensor, and the latter sensor is a lateral sensitivity sensor. A Wheatstone bridge circuit is formed in advance at the location where the lead portion is formed.
Hereinafter, the strain sensitivity testers equipped with the thin film strain sensors thus produced are referred to as Examples 1 to 3 and Comparative Examples 1 and 2, respectively.

Examples 1 to 3 and Comparative Examples 1 and 2 are affixed with a monitor gauge, and a strain sensitivity K sensor and a lateral sensitivity sensor when 1000 με is generated in the maximum principal strain direction and the maximum lateral strain is 5 με or less. Strain sensitivity was obtained from the output. Note that the monitor gauge has one axis of a laminated biaxial 90 degree gauge (gauge length 3 mm) coaxially with the sensing part of the lateral sensitivity sensor of Examples 1 to 3 and Comparative Examples 1 and 2. In this way, it was pasted (see FIG. 4).
The strain sensitivity (vertical direction) and lateral sensitivity (horizontal direction) of Examples 1 to 3 and Comparative Examples 1 and 2 are also shown in Table 1. The strain sensitivity (longitudinal direction) passed 1.75 or more and passed less than 1.75, and the lateral sensitivity passed -0.3 to 0.3, lower than -0.3 and A case of exceeding 0.3 was regarded as a failure.

Moreover, about Examples 1-3 and Comparative Examples 1 and 2, the temperature coefficient of resistance (Temperature Coefficient of Resistance; TCR) with respect to temperature was calculated | required. A TCR of less than 70 was accepted and 70 or more was rejected.
Table 1 shows TCRs of Examples 1 to 3 and Comparative Examples 1 and 2.

As shown in Table 1, in Examples 1 to 3, since the composition of Pd—Cr is appropriate, the strain sensitivity (longitudinal direction) is a sufficiently high value of 1.75 or more, and the lateral sensitivity that causes noise is also present. It became -0.3-0.3. Furthermore, the TCR was less than 70, and the amount of temperature drift was low.
Therefore, it was confirmed that Examples 1 to 3 have high strain sensitivity, are less affected by noise and the like, and can be used at high temperatures.

On the other hand, in Comparative Examples 1 and 2, since the composition of Pd—Cr was not appropriate, the strain sensitivity (longitudinal direction) was high, but the lateral sensitivity was lower than −0.3. Moreover, TCR became large.
Therefore, it was confirmed that Comparative Examples 1 and 2 could not be used at high temperatures because the strain sensitivity was high but the noise was large and the thermal stability was sometimes low.

  Next, heating under predetermined conditions is performed using a 79.6Pd-20.3Cr alloy (Example 2) and an 18.5Cr-11.0Al-Ni rest alloy (Comparative Example 3 (see Patent Document 1)). Changes in strain sensitivity during the test were examined.

A thin film strain sensor was fabricated under the same conditions as in Example 1 and Example 2 and Comparative Example 3.
About the produced Example 2 and the comparative example 3, when the strain sensitivity (longitudinal direction) when the heat history was not carried out and when heating at 400 ° C. for 1 hour was measured, it was as shown in FIG.

  As shown in FIG. 5, in Example 2, the strain sensitivity without thermal history was 1.75, while in Comparative Example 3, the strain sensitivity without thermal history was 1.28. That is, Example 2 was 37% more sensitive than Comparative Example 3.

Further, in Example 2, the strain sensitivity did not change even after heating at 400 ° C. for 1 hour, whereas in Comparative Example 3, the strain sensitivity was as follows when heated at 400 ° C. for 1 hour. It was 1.18, a decrease of 8%.
Therefore, it was confirmed that Example 2 in which the composition of Pd—Cr is appropriate has higher thermal stability than the case of using a Ni—Cr—Al alloy.

  As described above, from the results of Examples 1 to 3, the material for a thin film strain sensor that satisfies the requirements of the present invention and the thin film strain sensor using the material have high strain sensitivity, low lateral sensitivity, and low resistance temperature coefficient. It was confirmed that it could be used even in an internal combustion engine having a high temperature of 400 ° C. or higher.

DESCRIPTION OF SYMBOLS 1 Thin film strain sensor 2 Base material 3 Sensing part 4 Lead part

Claims (2)

  A thin film strain sensor material, wherein the Cr content is 14 to 25% by mass, and the balance is Pd and inevitable impurities. On the insulating film,
A sensor for detecting strain;
Lead portions formed at both ends of the sensing portion;
A thin film strain sensor formed of
The thin film strain sensor, wherein at least the sensing portion of the sensing portion and the lead portion is formed using the thin film strain sensor material according to claim 1.

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