JPS5814751B2 - Diaphragm strain gauge - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a diaphragm strain gauge, and particularly to a diaphragm strain gauge in which the strain gauge is disposed on the surface of a diaphragm made of a semiconductor single crystal.

Semiconductor strain gauges using silicon diaphragms are
Silicon itself is used as a diaphragm having a fixed part and a strain-generating part, and a plurality of strain gauges are formed on the diaphragm by diffusion of impurities, and these plurality of strain gauges are wired in a Wheatstone bridge. It is possible to effectively convert the strain occurring within the body into an electric signal, and convert physical quantities that are converted into strain, such as mechanical force or liquid pressure caused by various causes, into an electric signal.

The Kono type diaphragm type strain gauge includes a radial strain gauge arranged symmetrically to an arbitrary or specific axis passing through the center on the surface of a silicon diaphragm having a specific surface orientation, and parallel to this axis. A tangential strain gauge arranged parallel to an axis orthogonal to is formed by diffusion of impurities, and these strain gauges are combined to form a full bridge.

Figure 1 shows an example of a diaphragm-type strain gauge prototyped by the inventors, in which figure a shows the arrangement of the strain gauge formed on the diaphragm, and figure b shows a longitudinal section. It is something.

As shown in Figure b, impurities are diffused by a diffusion method into the strain-generating portion 11 of the silicon element, which has a diaphragm 10 formed with a concave hole on the back surface and whose diaphragm surface has a (110) plane orientation. Strain gauges 1, 2, 3, and 4 are formed, and the fixing part 12 of the diaphragm is attached to the housing 1 with an adhesive 13.
4 is fixed with adhesive.

The radial strain gauges 1 and 2 pass through the center of the diaphragm on the diaphragm surface with a surface orientation of (110) <111>
The radial strain gauges 1, 2, tangential strain gauges are formed parallel to the axial direction, and the tangential strain gauges 3, 4 are formed in the <112> axis direction passing through the center of the diaphragm, perpendicular to this axis. 3 and 4 are both arranged equidistant from the center of the diaphragm.

However, the diaphragm strain gauge in which the strain gauge is arranged in this manner is inherently sensitive to any direction of the crystal axis, and has the drawback of large variations in characteristics from manufacture to manufacture.

Also, instead of making the diaphragm surface (110), (11
1) Although it is possible to form using a surface, there is a drawback that a large output voltage cannot be obtained due to low output sensitivity.

The present invention has high output sensitivity, small characteristic fluctuations,
The purpose of this strain gauge is to provide a diaphragm strain gauge with excellent linearity.It is formed by impurity diffusion on a diaphragm made of a semiconductor single crystal substrate, and the radial stress acting on the diaphragm surface and its current direction diaphragm type strain gauge having at least one radial strain gauge whose radial strain gauges are parallel to each other, and at least one tangential strain gauge of H stiffness whose tangential stress acting on the diaphragm surface is parallel to its current direction; has a (110) plane orientation, the radial strain gauges are arranged in the <111> axis direction, and the tangential strain gauges are arranged in the axial directions making an angle of 45° from the <110> axis and the <100> axis, respectively. It is characterized by the fact that it is arranged.

That is, as a result of theoretically analyzing the resistance changes experienced by strain gauges arranged on a diaphragm made of a semiconductor single crystal, the present invention has developed a method to most effectively analyze the changes in resistance based on the dependence of the piezoresistance coefficient on the crystal axis (angle). This was done based on the confirmation that there was an array of strain gauges that could be used.

In other words, the strain gauge placed on the diaphragm of the peripheral fixed model is subjected to surface stress as shown in Figure 2a, and its sensitivity coefficient, the Viezor resistance coefficient, is
It will look like the figure.

In Figure 2a, the horizontal axis shows the ratio of the distance r from the diaphragm center to the diaphragm radius a, O indicates the center 1 of the diaphragm, and the vertical axis shows the stress. There is.

σr and σt in the figure respectively indicate stress in the radial direction and stress in the tangential direction.

The bending stresses σx and σt are respectively expressed by the following equations.

In addition, the horizontal and vertical axes in FIG. 3 respectively indicate the <110> and <100> axes passing through the center of the silicon tire phragm having the (110) plane orientation, and πt is the piezoresistance coefficient in the longitudinal direction, πt indicates the piezoresistance coefficient in the lateral direction.

The rate of change in resistance of a strain gauge whose longitudinal direction is arranged in the radial direction of the diaphragm, that is, a two-radius gauge, is roughly expressed by the following equation.

Similarly, the tangent gauges arranged in the tangential direction are as follows:

The magnitudes of the first and second terms in equations (1) and (2) vary depending on the arrangement of the strain gauges, so there is an arrangement where the resistance change is the largest.

As is clear from Figure 3, for the radius gauge, the crystal axis passing through the center of the diaphragm is the <111> axis, and for the tangential gauge, the crystal axis passing through the center of the diaphragm is the <110> axis.
It can be seen that the piezoresistance coefficient is maximum at an angle of 45° from the > axis and the <100> axis, and since it is at the inflection point of the curve, there is little variation due to angular deviation.

Furthermore, if the distances of the radius gauge 1 and the tangent gauge 3 from the center of the diaphragm are changed, and the tangent gauge 3 is placed closer to the center of the diaphragm than the radius gauge 1, as shown in FIG. 1. Since stress in the longitudinal direction occupies most of the stress in both tangent gauges 3, the first term in equations (3) and (4) becomes dominant.

That is, in Figures 2 and 3, points A and B of the radius gauge correspond to terms 1 and 2 of equations (3) and (4), respectively, and points a and b of the tangent gauge correspond to (1) and (1), respectively. 2) Corresponds to the 1st and 2nd term of equation.

As a result of the above, the tangent gauge is arranged at an angle of 45° from the radius gauge <111> axis, the <110> axis and the 100> axis, and the radius gauge is placed near the tangential stress σt=0, The tangent gauge is inside, and the radial stress σr
It is understood that it is sufficient to arrange it near where =0.

These gauges are each used as a bare bridge in the well-known bridge configuration as shown in Figure 4, and the pressure at this time ~
The nonlinearity between outputs is the sum of the nonlinearity of each gauge, but
Regarding this point as well, qualitatively speaking, the orthogonal stress component to the gauge has greater nonlinearity, so we have confirmed that if the gauge is arranged as shown in Figure 2, the nonlinear error will be small. The same applies to the temperature effect of nonlinearity.

However, in a diaslam that detects low pressure,
The amount of displacement relative to the plate thickness becomes large, and so-called non-linearity between pressure and strain due to membrane stress becomes a problem.
From the viewpoint of nonlinearity between pressure and output when each gauge is assembled into a bridge, such a gauge arrangement is not necessarily optimal.

Examples will be described below.

FIG. 5 shows an embodiment of the silicon diaphragm type strain gauge of the present invention.

Figure a shows the arrangement of strain gauges formed on the diaphragm, and figure b shows a longitudinal section, where the same reference numerals as in Figure 1 indicate the same parts.

The strain gauges are made of n-conductivity type silicon single crystal and include a pair of radius gauges 1 and 2 arranged radially along the <111> axis on a silicon diaphragm with a plane orientation of (110).
;21,22,31,32;41,42 are formed by selective diffusion of impurities such as boron, and <100> and <1
00> A pair of tangent (circumferential) gauges 3, 4; 23, each arranged perpendicular to the axial direction forming an angle of 45° from the axis;
24, 33, 34; 43, 44 are similarly formed by selective diffusion at positions closer to the center of the diaphragm than the radius gauge.

Although only a portion of the lead wire 15 is shown, a lead wire 16 is bonded onto an aluminum vapor-deposited electrode for extracting an output signal from the strain gauge.

In this embodiment, the radius gauge and tangent gauge placed in each quadrant are placed at the same position from the center of the diaphragm, but the position of the radius gauge or tangent gauge in each quadrant from the center of the diaphragm is By slightly changing the angle, the problem of non-linearity due to the difference in the pressure measurement range mentioned above can be overcome, and even with diaphragms that measure the same pressure, it is possible to overcome the fluctuation of the product.

When a pressure p is applied to the upper surface of the diaphragm 10 having such a configuration, the diaphragm deforms, each strain gauge receives strain, and correspondingly a resistance change occurs based on the viezoresistance effect.

That is, a positive resistance change occurs in the radius gauges, for example 1 and 2, and a negative resistance change occurs in the tangential gauges, for example 3 and 4. Since these are configured as a bridge as shown in Fig. 4, their output terminals An electrical signal proportional to the pressure p is obtained.

The effects of using such a diaphragm strain gauge will be explained using specific numerical values.

For example, the radius gauge and tangential cage on the strain-generating part of a silicon diaphtem with a thickness of 0.175 mm and a diameter of 8.4 mm are each placed 3.61 mm from the center of the diaphragm (r/a=0.8
When 62.69mm (r/a=0.64), the bridge output voltage sensitivity is 222mVFS/3.5V, FS=
5Kg/cm2, non-linear error is -0.13%FS
, the temperature effect on nonlinear error (-40°C to 120°C) is 0.
1% FS to -0.2% FS was obtained.

Therefore, the diaphragm strain gauge described in the example is
It has the following effects.

(1) Since both the radius gauge and the tangential gauge are arranged on the crystal axis that has the maximum sensitivity of the p-type strain gauge, a large resistance change can be obtained and the output sensitivity is high.

(2) Since both the radius gauge and the tangent gauge are arranged at the inflection point of the sensitivity curve, variations in sensitivity due to crystal axis deviation are small, and by applying this example, products with uniform characteristics can be obtained. .

In other words, the variation in gauge resistance due to angular misalignment is improved compared to the case of the embodiment shown in FIG. is 1/2 of the conventional value
Improved to 1/6.

Figure 6 shows the rate of change in resistance of the tangent gauge of this embodiment in comparison with that of the conventional example, and the vertical axis shows (ΔR/R).
t (%) is taken, where A is the case in this embodiment and B is the case in FIG.

Figure 7 shows the linearity of the bridge constructed using the strain gauge of this embodiment in comparison with that of the conventional example, with linearity (%) plotted on the vertical axis; In this embodiment, D is as shown in FIG.

In all characteristics, variations between samples are extremely small.

(3) On the silicon diaphragm, a radius gauge,
Since the tangential gauges are arranged so that the stress in the longitudinal direction is dominant, they are under the action of so-called uniaxial stress, and the non-linearity of resistance change with respect to pressure (strain) and temperature effects are small.

In other words, when assembled into a constant voltage excitation bridge, if the resistance change ratio between the tangential gauge and the radius gauge is not 1, nonlinearity will appear based on the impedance change of the bridge, but in this example, the resistance change will fluctuate due to angular deviation. is extremely small, so any changes in nonlinearity caused by this can be ignored.

(4) Nonlinear errors caused by so-called membrane stress, which are a problem when manufacturing silicon diaphragms for low pressure, can be avoided by slightly changing the position of the gauge to reduce the nonlinear errors caused by orthogonal stress acting on the gauge. It can be reduced by canceling each other out.

Note that with this diaphragm type strain gauge, the radius gauge and tangent gauge are arranged at different distances from the center of the diaphragm, but if the radius gauge and tangent gauge are extremely far apart, good linearity cannot be obtained, and the resistance As a result of the variation in values, the unbalanced voltage and temperature characteristics of the bridge output will be impaired.

Also, the angle at which the radius gauge and tangent gauge are arranged is approximately 10
°, they must be placed very close to each other, and to obtain a high resistance of 5 kΩ in a single strip requires a longer gauge, resulting in a larger diaphragm.

Therefore, the strain gauge is constructed from, for example, four strips, and the diaphragm is made smaller while ensuring high resistance.

Furthermore, although radius gauges and tangent gauges require each pair of strain gauges to constitute a full bridge, as in the embodiment, radius gauges and tangent gauges are provided in each quadrant or some of all quadrants, and appropriate strain gauges are used. Using the combination is manufacturing efficient.

As described above, the diaphragm strain gauge of the present invention provides a diaphragm strain gauge with high sensitivity, little variation in characteristics, and excellent linearity, and is an industrially effective dog.

[Brief explanation of the drawing]

Figure 1a is a layout diagram of a conventional silicon diaphragm type strain gauge, Figure 1b is also a vertical cross-sectional view,
Figure 2a is a distribution diagram of the stress applied to the strain gauge on the diaphragm, Figure 2b is an explanatory diagram showing the gauge position of the silicon diaphragm type strain gauge of the present invention, and Figure 3 is (11
0) Distribution diagram of the piezoresistance coefficient in the silicon single crystal plane, Figure 4 is a diagram of the bridge constructed by strain gauges, Figure 5a is the arrangement of strain gauges in one embodiment of the silicon diaphragm type strain gauge of the present invention. FIG. 5B is a longitudinal sectional view, and FIGS. 6 and 7 are characteristic diagrams showing the effect. 10...Diaphragm, 1, 2, 21, 22, 31,
32, 41, 42...radius gauge, 3, 4, 23, 2
4, 33, 34, 43, 44.・．． tangent gauge.

Claims (1)

[Scope of Claims] 1. at least one radial strain gauge formed by impurity diffusion on a diaphragm made of a semiconductor single crystal substrate, the radial stress acting on the diaphragm surface being parallel to the current direction; In a diaphragm type strain gauge having a tangential stress acting on a diaphragm surface and at least one tangential strain gauge whose current direction is parallel to the diaphragm surface, the diaphragm surface is
10) The radial strain gauge has a plane orientation of (1
11) axially arranged, the tangential strain gauge element being 45 mm from the (110) axis and the (100) axis, respectively;
A diaphragm strain gauge characterized by being arranged in the axial direction forming an angle of °. 2. The radial strain gauge is installed at a position where tangential stress is approximately zero, and the tangential cracking cage is located closer to the center of the diaphragm than the radial strain gauge is at a position where radial stress is approximately zero. A diaphragm strain gauge according to claim 1. 3. The diaphragm strain gauge according to claim 1 or 2, wherein the strain gauge comprises a plurality of filaments parallel to each other, and the filaments are connected in series through a low resistance region. 4. The diaphragm shape according to any one of claims 1 to 3, wherein the radial strain gauge and the tangential strain gauge are formed in each quadrant of the (110) plane of the tie ram. strain gauge. 5. The diaphragm strain gauge according to any one of claims 1 to 4, wherein the semiconductor is silicon.