JPH0829446A - Semiconductor acceleration sensor - Google Patents

Abstract

PURPOSE:To enhance the S/N of a semiconductor acceleration sensor by reducing the stress concentration at the shape-changing part between a deflection part and a supporting frame. CONSTITUTION:The thickness of a deflection part 12 is wholly uniformized, and the deflection part 12 is made up of a mass 121 and a narrow part 12i whose width is narrower than that of the mass 121. The width of the narrower part 122 is increased on the side of a supporting frame 11 for reducing the stress concentration at the shape-changing part or root part 123. Further, a detection element 19 is dislocated from the shape-changing part 123 to the mass 121 side by a distance OS. By this constitution, the stress at the shape- changing part 123 can be significantly reduced, to be less than the maximum nominal stress being detected by the detection element 19. Thus, even if the stress concentration is taken into consideration, the stress at the shape-changing part is less than the maximum nominal stress, and the deflection part 122 is prevented from being fractured at the root part 123. As a result, the maximum nominal stress can be set to a larger value, so that the S/N of a semiconductor acceleration sensor can be enhanced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor acceleration sensor, and more particularly, it detects stress generated by a physical external force such as pressure, acceleration and mechanical vibration and converts the stress into an electric signal to represent the external force. The present invention relates to a semiconductor acceleration sensor that outputs a signal.

[0002]

2. Description of the Related Art Recently, as a sensor for detecting acceleration and deceleration (hereinafter collectively referred to as "acceleration"),
A semiconductor acceleration sensor that can directly output acceleration information as an electric signal is widely used. This semiconductor acceleration sensor is small and lightweight, and is used as, for example, a collision detection sensor in an automobile air bag system.

FIG. 3 and FIG. 4 show an example of a semiconductor acceleration sensor. In FIG. 3, the acceleration sensor 1 is a GaAs
This substrate is manufactured by etching into a desired shape. The acceleration sensor 1 includes a frame 11 and a flexure 12 that extends inward from the frame 11 in a cantilever manner. FIG.
As shown in the sectional view of (b), a weight portion (hereinafter, referred to as a “mass”) 14 having a thickness larger than that of the root portion 13 is formed at the tip of the flexible portion 12, and the amount of deflection due to acceleration is formed. Is designed to be large. A stress detecting element 13a such as an FET formed by stacking semiconductor layers is provided on the surface of the root portion 13, that is, a stress generating portion.

As described above, the mass 14 is provided, and the stress detecting element is provided on the surface of the root portion 13 where the generated stress is maximum, so that the generated stress can be made large for the same acceleration. This is to increase the sensitivity or the output signal / noise ratio (S / N ratio).

When acceleration is applied to the acceleration sensor 1 in the direction of arrow A, the base portion 13 is bent. Due to this deflection, the FET1
3a of the drain (source) current I _DS changes. Current I
The amount of change in _DS is converted into a voltage value and extracted as an electric signal representing the magnitude of acceleration.

FIG. 4 shows an acceleration sensor having a structure in which the flexible portion 12 is supported by the frame 11 in the form of both ends supporting beams. In this acceleration sensor 1, when acceleration is applied in the direction of arrow A, the flexure of the flexure 12 causes the two roots 1 of the frame 11 to move.
In addition to Nos. 5 and 16, large stresses are generated at the roots 17 and 18 of the mass 14, so that the number of acceleration samples is large. In other words, FET as a stress sensing element
13a can be provided at the four roots or in the vicinity thereof. However, in the root portions 15, 16 and 17, 18, there is a difference in the detected stress between tensile stress and compressive stress.

As described above, in the conventional acceleration sensor,
The S / N ratio can be increased, but
On the other hand, when the generated stress is larger than the breaking stress of the flexible portion 12 for the magnitude of the acceleration, the flexible portion 12 may be broken from the root portion 13. Therefore, the shape, dimensions, etc. of the flexible portion 12 are designed in consideration of safety so that the generated stress does not exceed the breaking stress.

However, in the conventional structure, the root portion 1
Since the cross-sectional shape is changed and the thickness is changed at 3, 15 to 18, stress concentrates on the thickness changing portion, and there is a possibility that the breaking stress is easily exceeded. The stress concentration has a high shape dependency such as an opening angle and a curvature of a corner.

In order to eliminate such a problem due to stress concentration, an acceleration sensor has been proposed in which a rounded portion, that is, an arc portion is formed in the portion where the thickness changes to avoid the concentration of stress. KAISHO 64-18063,
JP-A-1-302167). In the acceleration sensors described in these publications, isotropic etching is performed to drop corners to form smooth arc portions.

[0010]

However, in the etching process, there is a large variation in processing accuracy, and it is not always easy to accurately obtain a desired arc shape. For this reason, it is necessary to consider not only the safety factor and the strength variation of the material but also the magnitude of the concentrated stress due to the accuracy variation of the etching process. As a result, it becomes necessary to reduce the stress by reducing the mass 14, and the S /
There is a problem that the N ratio decreases.

The object of the present invention is to solve the above problems,
An object of the present invention is to provide a semiconductor acceleration sensor that can reduce the influence of variations in the accuracy of etching processing on stress concentration and can have a structure with a large S / N ratio.

[0012]

SUMMARY OF THE INVENTION The present invention for solving the above problems and achieving the object comprises a flexible portion having a uniform thickness and a flexible portion supporting frame thicker than the flexible portion.
The flexible portion is provided with a narrow portion for integrating the flexible portion with the support frame, and the narrow portion is formed wide at a boundary portion with the support frame, and the narrow portion deviated from the boundary portion. It is characterized in that a stress detector is provided at the planned position of the section.

[0013]

According to the above characteristics, since the flexure has a uniform thickness, the flexure itself has no stress concentration portion. Further, the stress detecting portion is provided in the narrow portion of the bending portion, and the boundary between the bending portion and the support frame, that is, the thickness changing portion which is the only stress concentration portion is wide. Therefore, the maximum stress is generated in the narrow portion where the stress detecting portion is provided. As a result, if a nominal maximum stress in design is set in the stress detection section, the stress generated at the boundary of the flexure and the support frame will change to a nominal value even if the concentration of stress is taken into consideration. Less likely to exceed maximum stress.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the drawings. 1A is a plan view of a semiconductor acceleration sensor according to an embodiment of the present invention, and FIG. 1B is a sectional view taken along line BB in FIG. 1A. In addition, FIGS. 1C and 1D are distribution diagrams of stress generated in the flexure portion of the semiconductor acceleration sensor. In FIG. 1, a frame 11 of a semiconductor acceleration sensor 1 obtained by processing a substrate made of GaAs by etching has a flexible portion 12 projecting in a cantilever form inside thereof. In order to clarify the range of the flexible portion 12, the range is shown by reference numeral x0 in FIG. The flexible portion 12
Consists of a mass 121 and a narrow portion 122 that supports the mass 121 on the frame 11. In the figure, the range of the narrow portion 122 is indicated by reference numeral x1. As can be seen from the cross-sectional view shown in FIG. 1B, there is no thickness change portion between the mass 121 and the narrow portion 122, and the narrow portion 122 is smaller than the mass 121 to increase the S / N ratio. Except that the width of the
The overall thickness of 2 is uniform. On the other hand, the narrow portion 122 has a frame 1
The width is gradually enlarged toward the 1 side, and the root portion 123
The width is maximum. In this way, the mass 1
Since 21 and the narrow portion 122 have no change in thickness, it is possible to avoid concentration of stress due to variation in etching.

On the other hand, it is unavoidable that the cross-sectional shape change due to the thickness change occurs at the boundary between the flexible portion 12 and the frame 11, that is, at the root portion 123. In this embodiment, since the width of the narrow portion 122 is gradually increased toward the frame 11 to increase the width of the cross-sectional shape change portion where stress is concentrated, the resistance force to stress concentration is increased. Will increase.

On the other hand, the stress detecting element 19 is deviated from the root portion 123 of the bending portion by the distance OS, and the narrow portion 122 is formed.
It was arranged in the minimum width part of. Since this arrangement position is the minimum width and thickness of the flexible portion 12 and is close to the root portion 123, the maximum tensile stress σmax is generated as shown in FIG. 1 (c). On the other hand, in the stress concentration portion, that is, the root portion 123, the narrow portion 1
Since the width of 22 is wide, the stress σx generated in this portion is relatively smaller than the stress σmax generated in the arrangement portion of the stress sensing element 19 even if the stress concentration is taken into consideration. Therefore,
If the stress σ max is the maximum nominal stress in design, only a stress σx smaller than the maximum nominal stress of the semiconductor acceleration sensor 1 is generated in the root portion 123, so that the stress is broken at an acceleration that generates a stress equal to or less than the maximum nominal stress. There is nothing to do.

It should be noted that the position shown in the figure is the optimum position for arranging the stress detecting element 19, but the position is not limited to this, and the boundary portion of the narrow portion 122 between the flexible portion 12 and the frame 11, that is, the root portion, is not limited to this. Any position may be used as long as it has a certain amount of offset from 123.

Next, for comparison with the semiconductor acceleration sensor of this embodiment, the stress distribution of the conventional semiconductor acceleration sensor is shown in FIG. In the figure, the same reference numerals as those in FIG. 3 denote the same or equivalent parts. As shown in FIG. 2, in particular, when the stress concentration on the back surface root portion 13 of the flexible portion 12 and the root portion 20 of the mass 14 is large and the maximum nominal stress σmax occurs at the stress on the surface side of the root portion 20, The generated stress σz on the back surface side of the portion 20 and on the back surface side of the root portion 13 of the mass 14 exceeds the maximum nominal stress σmax. When this maximum nominal stress σ max is a fracture stress, the result of stress concentration on the back surface of the root portion 13 and the back surface of the root portion 20 of the mass 14,
Even if the stress on the surface portion is less than or equal to the maximum nominal stress, the flexure portion 12 may be broken by the small stress. From such a stress distribution, in the conventional acceleration sensor, the flexure 12
It is necessary to set the maximum nominal stress so that the concentrated stress on the back surface of the
This means that the stress detector on the surface of No. 2 can measure only a small stress, that is, a small acceleration.

In this embodiment, the cantilevered narrow portion is described as an example, but the present invention is not limited to this embodiment, and a plurality of narrow portions 122 are radially formed toward the frame 11 at equal angular intervals. The same can be applied to the type, that is, the both-end support beam type or the four-direction support beam type in which the flexure is supported from four directions. The point is that the thickness of the flexible portion is uniform, a narrow portion is provided near the support frame in the flexible portion, and the width of the narrow portion is increased on the side of the support frame to form the narrow portion. It suffices that the stress detection unit is provided at a predetermined position of the narrowed portion offset from the boundary between the flexible portion and the support frame. In this case as well, it goes without saying that the stress and offset amount, etc., are multiplied by a safety factor in consideration of material variations and the like.

[0020]

As is apparent from the above description, according to the present invention, the boundary between the flexible portion and the support frame, that is, the shape change portion and the stress detecting portion are displaced from each other, and the width of the shape change portion is set to the stress. It was made wider than the width of the detection part. Therefore, the stress concentration in the shape change part can be relaxed, so that the stress in the shape change part can be limited to the maximum stress detected by the stress detection part or less even if the processing accuracy due to etching varies. As a result, it is possible to prevent the bending portion from being broken by the stress equal to or less than the maximum nominal stress.

Further, since the stress concentration at the shape change portion can be relaxed, the maximum nominal stress can be increased, resulting in S / N.
The ratio can be improved.

Furthermore, since the flexure has a flat shape with a uniform thickness, the bending moment due to the acceleration applied laterally to the flexure is greater than in the case where the mass is formed by the thickness of the flexure. Does not work. Therefore, the acceleration sensor having high directivity can be obtained.

[Brief description of drawings]

FIG. 1 is a plan view, a sectional view, and a stress distribution diagram of an acceleration sensor showing an embodiment of the present invention.

FIG. 2 is a plan view, a sectional view, and a stress distribution diagram of a conventional acceleration sensor.

FIG. 3 is a plan view and a cross-sectional view of a conventional cantilever narrow section type acceleration sensor.

4A and 4B are a plan view and a cross-sectional view of a conventional acceleration sensor of both ends supporting narrow portion type.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Acceleration sensor, 11 ... Support frame, 12 ... Deflection part, 19 ... Detection element, 121 ... Mass, 122 ... Narrow part, 123 ... Root part

Claims (3)

[Claims] 1. A flexure having a uniform thickness, and a flexure support frame surrounding the flexure and thicker than the flexure, wherein the flexure is provided in the flexure and the support frame. A narrow portion for integration is provided, and the narrow portion is formed wide at a boundary portion with the support frame, and a stress detecting portion is provided at a predetermined position of the narrow portion deviated from the boundary portion. Characteristic semiconductor acceleration sensor. 2. The semiconductor acceleration sensor according to claim 1, wherein a plurality of the narrow portions are radially formed toward the support frame at equal angular intervals. 3. The semiconductor acceleration sensor according to claim 1, wherein processing in at least the thickness direction is performed by etching.