JPH059746B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a semiconductor acceleration sensor that outputs a change in resistance value of a semiconductor strain gauge that occurs during elastic change as an electrical signal.

[Conventional technology]

Conventionally, the structure commonly used to detect vibrations, acceleration, etc. is to form a thin diaphragm part on a semiconductor substrate, on which a semiconductor strain gauge is formed, and a part of a thick semiconductor substrate to form a thin diaphragm part on a semiconductor substrate. This is a so-called cantilever type, which is fixed to a pedestal as a supporting part, and the other part of the thick semiconductor substrate is used as a free end to be displaced according to the force to be measured, and the force to be measured is detected by the change in the resistance value of the semiconductor strain gauge. A semiconductor type acceleration sensor is known.

[Problem that the invention seeks to solve]

The characteristics of such a cantilever-type semiconductor acceleration sensor are almost determined by the shape of the cantilever, and the resonant frequency range of the cantilever itself (usually around 350 to 1500 Hz) is several tens of times lower than the output in other frequency ranges. It will produce twice the output. Therefore, when trying to detect the acceleration of a car, for example, only a low frequency range (usually around 10Hz or less) is required, so silicone oil is used to cut out relatively high frequency ranges, including the resonance frequency range. By placing a cantilever in a damping liquid, the damping liquid is used as a mechanical high-cut filter by utilizing the damping effect caused by viscous resistance.

Furthermore, compared to metal wire strain gauges, such semiconductor acceleration sensors have the advantage of having a large strain sensitivity output, but have the disadvantage of having low breaking strength and being susceptible to impact. In order to compensate for such drawbacks, a semiconductor acceleration sensor having a configuration in which a recess of a predetermined depth is provided in the pedestal to mechanically limit the displacement of the free end is proposed as a patent application previously filed by the present inventors. Gansho 61-
It has been proposed in Publication No. 225437, etc.

However, in order to improve the impact resistance, the depth of the recess and the distance between the free end and the bottom of the recess should be set short, but if this is done, the frequency f and output as shown in Figure 5. As shown in the diagram showing the relationship between the cutoff frequency and the _size of
f ₀ decreases and frequency response deteriorates. In other words, the effective usable frequency range R that can be used in a state where the output is large and the responsiveness is good becomes small. The problem arises that the output of the sensor becomes unstable.
Incidentally, in the above explanation, the cutoff frequency is defined as the frequency at which the magnitude of the output decreases by 3 dB. Further, in FIG. 5, the horizontal axis is on a logarithmic scale.

Here, the above problem arises because conventional means for limiting the displacement of the free end and its weight part (in the case of the above example, corresponds to the bottom of the recess of the pedestal)
This is based on the fact that the relationship between the distance from Therefore, it is possible to optimally control the distance between the free end and the means for mechanically limiting its displacement according to the user's requests such as the frequency range of the force to be measured, and improve the frequency response of the sensor. The purpose of this research is to provide a semiconductor-type acceleration sensor that enables the following.

[Means for Solving the Problems] In order to achieve the above object, the semiconductor acceleration sensor of the present invention includes a thin-walled diaphragm portion having a semiconductor strain gauge, and a cantilever having a thick-walled free end. a pedestal for fixing a portion of the cantilever; a protective wall that sets a predetermined distance between the free end and allows the free end to be displaced within that distance; and a damping liquid in the package. A semiconductor acceleration sensor comprising: a distance between the free end and the protective wall, h;
When the displacement distance of the free end per 1 G is dχ, the cutoff frequency f _cf in the configuration without the protective wall, and the desired cutoff frequency of the semiconductor acceleration sensor is f _c or more, the value of h is expressed by the formula f It is characterized by being greater than the value set by _c = f _cf 1/1 + Kdχ/h (where K is a proportionality constant).

〔Example〕

Hereinafter, the present invention will be explained using embodiments shown in the drawings.

FIG. 1 is an overall perspective view of an embodiment of the present invention, showing the package before it is hermetically sealed. In the figure, 100 is a stem made of metal such as Kovar, and except for the welded part 101 which is the outer periphery, a convex part on which a pedestal 700, a stopper 200, etc., which will be described later, is mounted, is formed by pressing etc. For example, there are four through holes for electrical connection with the outside, and a lead terminal 400 is fixed to the through holes by welding hard glass 500 therebetween. The lead terminal 400 and the stem 100 are electrically insulated by a hard glass 500, and the hard glass 500 is interposed with good airtightness.

A sensor element consisting of a cantilever 800 and a pedestal 700 is mounted on the stem 100, and the cantilever 800 is, for example, N
It consists of a type silicon single crystal substrate, A- in Figure 1.
As shown in FIG. 2, which is a cross-sectional view taken along line A, a thin diaphragm portion 8 is formed by etching the substrate.
01, and the thick portion has a free end 80 that can be partially displaced according to the acceleration to be measured.
2, and the other parts are fixed to the pedestal 700. or,
A solder load 803 is bonded to one main surface of the free end 802 at multiple locations, and the diaphragm portion 80
1 or on the diaphragm part 801.
As shown in the top view of the cantilever 800 in the figure,
Four semiconductor strain gauges 804 are formed by known semiconductor processing techniques, such as thermal diffusion or ion implantation of P-type impurities such as boron, and each semiconductor strain gauge 804 is electrically connected to each other by a wiring member (not shown). and constitutes a full bridge. The pad portion of the wiring member and the lead terminal 400 are electrically connected by wire bonding the wire wire 300.

The pedestal 700 is made of silicon in order to match the thermal expansion coefficient with that of the cantilever 800, and as shown in the perspective view of FIG. , the recess 701 is formed from one end surface 703 to the other end surface 704. Note that the direction of the acceleration to be measured is the cantilever 80.
0, the protective wall referred to in the present invention corresponds to the bottom of the recess 701, and the distance h between the bottom and the load 803 of the cantilever 800 corresponds to the protection wall of the free end 802. Displacement is allowed.

And the above sensor element has a free end 8
When the acceleration to be measured acts on the diaphragm 801, the resistance value of the semiconductor strain gauge 804 changes depending on the magnitude of the acceleration. It generates an unbalanced voltage and detects acceleration according to that voltage.

In FIG. 1, 200 is a stopper, and its shape is as shown in the perspective view of FIG.
Two plates 201 and 202 perpendicular to 00 and parallel to the stem 100, in other words, the cantilever 80
The plate members 203 parallel to 0 are combined perpendicularly to each other.
a and the upper surface of the cantilever 800 are adjusted to have a predetermined distance. The material thereof is, for example, Kovar, and it is bonded to the stem 100 with solder. Furthermore, when the direction of the acceleration to be measured is on the stopper 200 side with respect to the cantilever 800, the protective film referred to in the present invention corresponds to the lower surface 203a of the stopper 200, and the distance between the lower surface 203a and the cantilever 800 is Displacement of the free end 802 is allowed at .

Next, 600 is a shell made of metal such as Kovar, which also has a welded part 601 at a position opposite to the welded part 101 of the stem 100, and the other parts are box-shaped with recesses formed by press working. There is. Further, the shell 600 is formed with a hole 602 for injecting a damping liquid after hermetically sealing and a hole 603 for letting air escape at that time. The hermetic seal is made between the welded portion 601 and the welded portion 101.
This is done by welding the shell 600 and the stem 100 by applying electricity to the shell 600 and the stem 100 while applying mechanical pressure. After that, for example, a syringe with a needle-shaped discharge tip is inserted into the hole 602, and a certain amount of damping liquid 900, such as silicone oil, is injected into the hole 602, for example, about 70 to 80% of the total volume.
2 and 603 are sealed with solder.

Therefore, according to the semiconductor acceleration sensor of this embodiment, when a strong impact (acceleration to be measured) is applied to the sensor, the direction of the impact is directed toward the cantilever 800.
If the direction is on the pedestal 700 side, the stop will be forcibly stopped when it comes into contact with the bottom of the recess 701 of the pedestal 700, and if the direction is on the stopper 200 side with respect to the cantilever 800, the stopper 200 will be forcibly stopped.
Since it is forcibly stopped when it comes into contact with the lower surface 203a, destruction due to excessive change can be prevented. Distance h between load 803 of 800, or lower surface 203a of free end 802 and stopper 200
The distance from is the desired cutoff frequency for the sensor.
To set f _c , it can be determined as shown below.

To simplify the explanation, the measured acceleration is the pedestal 7.
Only the case where it joins the 00 side will be explained. Now, the cutoff frequency in the free area where there is no means to limit the displacement of the free end 802 is f _cf , and the area of the free end 802 facing the pedestal 700 is S (fourth
It has been found that the desired cutoff frequency f _c is expressed by the following equation, where dχ is the tip displacement distance of the free end 802 per 1 G, and ρ is the viscosity of the damping fluid.

f _c =k ₁ /ρSdχ・1/1+K ₂ dχ/h…(1) =f _cf・1/1+K ₂ dχ/h…(2) (Here, K ₁ and K ₂ are constants) Here, constant Regarding K ₁ and K ₂ , either calculate back using equation (1) by making two or more prototype sensors and measuring each parameter, or calculate the cutoff frequency f in the free area using one prototype sensor. It can be obtained by measuring _cf and each parameter and calculating backwards using equation (2), but in a normal small semiconductor acceleration sensor, each parameter is S
= ² ~20mm2, dχ=0.02~10μm, ρ=10~500cp
If it is within the range, K ₂ ≒ 200, and calculations can be made using this value.

Figure 7 shows the results of measurements made by the inventors on a prototype sensor and the theoretical values obtained from equations (1) and (2) above.
It is a characteristic diagram showing the relationship between dχ/h and f _c /f _cf , and it can be seen that the measured value and the theoretical value show good consistency. In addition, the circle plot in the figure indicates that each parameter is f _cf
These are the actual measured values for a prototype product with f cf = 30Hz, S = 9.0mm ² , dχ = 2×10 ^-3 mm, and the triangular plot shows that each parameter is f _cf = 400Hz, S = 3.6mm ² , dχ = 0.3×10 ^{- 3}
This is an actual measurement value for a prototype product.

In this way, it is clear that the cut-off frequency f _c can be controlled using the above equations (1) and (2) using dχ/h, so the above (2) ) is modified from the following equation: dχ/h=1/K ₂ [f _cf /f _c −1]=C …(3) From the equation, the value of dχ/h is determined to be less than or equal to the constant C set by the right-hand side of equation (3). That is, if dχ/h≦C, it becomes possible to set the frequency range of the acceleration to be measured within the effective frequency range R determined by the cutoff frequency f _c , and the output of the sensor becomes stable. Here, the value of dχ is determined by the shape, size, etc. of the cantilever 800, so the distance h between the bottom of the recess 701 and the load on the cantilever 800 should be set to h≧dχ/C (4). become.

The lower value of distance h is determined by the above, and by controlling that value, the frequency response of the sensor can be improved. However, in order to improve the shock resistance of the sensor, the range of the value of distance h must be further increased. It has also been found that it is necessary to limit it, and this is shown below.

When each parameter is set as shown in FIG. 2 and FIG. 4 which is a plan view of the cantilever 800, that is, the length of the long side of the free end 802 is A,
The length of the short side is B, the length of the diaphragm part 801 is L, the thickness of the diaphragm part 801 is t, and the free end 8
02 and the load 803 weight is M, and the silicon breaking stress is σ (=4×10 ⁹ dmn/cm ² ), the maximum acceleration that the cantilever 800 can withstand without being destroyed (the maximum allowable acceleration G _nax is G _nax = Bt ² σ/3M (L + A) ...(5) When this G _nax is applied to the cantilever 800, the displacement h _nax of the same end of the cantilever 800 is expressed as h _nax = MG _nax gA {(1 -A) L ² +15/4AL +A ² }/(6EI) ...(6) Here, g is the gravitational acceleration, E is Young's modulus, and I is the second moment of area (=Bt ³ /12).

It is expressed as Therefore, if the value of the distance h is made smaller than this h _nax , the cantilever 800 can be saved from destruction, and the upper limit value of the distance h is set by this h _nax . In addition, when actually designing, take into account the possibility of an error of several tens of percent in the thickness t of the diaphragm part 801 during the manufacturing process, and take into account this error in advance.
It is preferable to set h _nax .

Although the present invention has been described above using the above embodiments, the present invention is not limited thereto and can be modified in various ways without departing from the spirit thereof. For example,
It is also possible to adopt a configuration without the load 803 bonded to the cantilever 800, a configuration without the stopper 200, etc.

〔Effect of the invention〕

As described above, according to the semiconductor acceleration sensor of the present invention, the distance h between the free end and the protective wall is optimally controlled, so the desired cutoff frequency f _c can be arbitrarily set based on the distance h. is possible,
This has the effect of improving the frequency response of the sensor.

[Brief explanation of the drawing]

Fig. 1 is an overall perspective view of an embodiment of the present invention before hermetic sealing, Fig. 2 is a sectional view taken along line A-A in Fig. 1, Fig. 3 is a perspective view of the pedestal, and Fig. 4 is a cantilever. Fig. 5 is a diagram showing the relationship between frequency and output size, Fig. 6 is a perspective view of the stopper, and Fig. 7 is a diagram showing the relationship between the measured value and the theoretical value dχ/h and f _c /f _cf. FIG. 100... Stem, 200... Stopper, 6
00... Shell, 700... Pedestal, 701... Recessed portion, 800... Cantilever, 801... Diaphragm portion, 802... Free end, 804... Semiconductor strain gauge, 900... Damping liquid.

Claims (1)

[Scope of Claims] 1. A cantilever having a thin-walled diaphragm portion having a semiconductor strain gauge and a thick-walled free end, a pedestal for fixing a portion of the cantilever, and a predetermined distance between the free end and the cantilever. A semiconductor acceleration sensor comprising, in a package, a protective wall that allows displacement of the free end over a set distance, and a damping liquid, between the free end and the protective wall. The distance is h,
When the displacement distance of the free end per 1G is dχ, the cut-off frequency in the configuration without the protective wall is f _cf , and the desired cut-off frequency of the semiconductor acceleration sensor is f _c or more, the value of h is expressed by the formula A semiconductor type acceleration sensor characterized in that the value is greater than or equal to the value set by f _c =f _cf・1/1+Kdχ/h (where K is a proportionality constant). 2. The damping liquid has a viscosity of 10 to 500 cp, the free end has an area of 2 to 20 ^mm2 , the displacement distance dχ is 0.02 to 10 μm, and the proportionality constant K is 200. A semiconductor acceleration sensor according to claim 1.