JPH09166618A - Semiconductor acceleration sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a highly reliable semiconductor acceleration sensor. SOLUTION: The sensing element 1 of a semiconductor acceleration sensor is provided with a weight section 2, a pair of thin flexible sections 3 one ends of which are integrally connected to the weight section 2, and a supporting section 4 to which the other ends of the sections 3 are integrally connected and which supports the weight section 2 in a swingable state by means of the sections 3. An upper cap section 10 and a lower cap section 20 which respectively seal at least the weight section 2 and flexible sections 3 are anode-connected to the top and bottom faces of the element 1. In the caps 10 and 20, recessed sections 10a and 20a having very small recessing and projecting sections are formed on the entire inner peripheral surface at parts facing the weight section 2.

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 formed by processing a semiconductor substrate.

[0002]

2. Description of the Related Art A semiconductor acceleration sensor has been conventionally manufactured by utilizing a semiconductor etching technique, and has features of small size, high sensitivity and high reliability. FIG. 6 shows an example of a conventional semiconductor acceleration sensor, Lynn Michell R
oylance "A Batch-Fabricated Silicon Accekerometer"
(IEEE Transaction on Electron Devices, Vol1.ED-26,
No. 12, December 1979).

This semiconductor acceleration sensor has a weight portion 2,
A thin flexible portion 3 having one end integrally connected to the weight portion 2 and a support portion 4 integrally connected to the other end of the flexible portion 3 for supporting the weight portion 2 swingably by etching the silicon substrate. It has a sensing element 1 integrally formed by processing, and an upper cap 10 and a lower cap 20 fixed by anodic bonding above and below a supporting portion 4, and the surface of the bending portion 3 has a resistance value according to strain. Piezoresistance R that changes
_{1 to} R ₄ (see FIG. 7) are formed by a diffusion technique into a silicon substrate. It should be noted that recesses 10a and 20a are formed by etching on the surfaces of the upper cap 10 and the lower cap 20 facing the sensing element 1 to secure a swing space for the weight portion 2 and to obtain an air damping effect. is there. The upper cap 10 and the lower cap 20 are for sealing the sensing element 1, and are formed of glass (heat-resistant glass) having a thermal expansion coefficient substantially equal to that of the silicon substrate.

When, for example, an acceleration G in the direction of the arrow in FIG. 6 (b) is applied to the semiconductor acceleration sensor, the weight portion 2 swings in accordance with the magnitude of the acceleration G, so that the semiconductor acceleration sensor bends. The portion 3 bends, and as a result, stress due to strain is generated in the bending portion 3, and the resistance value of the piezo resistance R on the surface of the bending portion 3 changes due to the piezo effect. Then, the applied acceleration can be detected by extracting the change in the resistance value as a voltage output.

Further, in the above semiconductor acceleration sensor, as shown in FIG. 7, a pair of bending portions 3 are provided, and a pair of piezoresistors (R ₁ and R ₄₎ are diffused near the respective surfaces of the pair of bending portions 3 by diffusion. And R ₂ and R ₃ ) to form a bridge circuit BC as shown in FIG. 8 to increase the detection sensitivity. In such a semiconductor acceleration sensor, to the terminal T _14, T ₂₃ of the bridge circuit BC, 9
Connect a constant current source I or a constant voltage source E as shown in
Than is detected as a voltage change between the piezoresistive R ₁ to R terminal T ₁₂ changes in the resistance value of _4, T ₃₄ by acceleration.

[0006]

In the above semiconductor acceleration sensor, the upper cap 10 and the lower cap 20 are fixed to the sensing element 1 by anodic bonding. At the time of anodic bonding, the space between the sensing element 1 and the upper cap 10 is increased. Also, a voltage of several hundreds of volts is applied between the sensing element 1 and the lower cap 20. Therefore, the weight portion 2 and the upper cap 1
0 and the lower cap 20 generate an electrostatic attraction force.

Here, the distance between the weight portion 2 and the upper cap 10 or the lower cap 20 is d, the applied voltage is V, the area of the electrode to which the voltage is applied is S, and the vacuum dielectric constant is ε ₀ . The electrostatic attraction force F is given by F = (1/2) × ε ₀ × S × (V / d) ² .

That is, the magnitude of the electrostatic attraction force F is the voltage V
Is proportional to the area S of the applied electrode and inversely proportional to the square of the distance d. In the semiconductor acceleration sensor described above, an electrostatic attraction force is generated both between the weight portion 2 and the upper cap 10 and the lower cap 20, so that the electrostatic attraction force generated in the weight portion 2 is canceled to some extent. Since the electric attraction force F is inversely proportional to the square of the distance d, once the upper cap 10
Alternatively, when attracted to one of the lower caps 20, no electrostatic attraction force is generated on the other. Therefore, during anodic bonding,
The weight 2 is pulled toward the upper cap 10 side,
A part of the weight portion 2 may hit the bottom surface of the recess 10a of the upper cap 10.

The recesses 10a and 20a are formed by wet etching using an etching solution such that the inner peripheral surface becomes a smooth surface. Therefore, the recesses 10a, 2
The inner peripheral surface of 0a is extremely smooth and is in a state of being easily anodically bonded. That is, a part of the weight portion 2 is a recess 10
When it hits a, there is a problem that the contacted portion is anodically bonded and the weight portion 2 remains held on the bottom surface of the recess 10a.

In order to reduce the electrostatic attraction force generated during anodic bonding, the depths of the recesses 10a and 20a are increased to increase the distance between the weight portion 2 and the bottoms of the recesses 10a and 20a. It is possible to prevent a part of the weight portion 2 from touching the bottom surfaces of the recesses 10a and 20a, but in this case, the air damping effect cannot be obtained, and the weight portion 2 largely shakes at the resonance point. Since the bending portion 3 is generally formed to have a thickness of several microns or several tens of microns, it is more likely to be damaged than other portions, and there is a problem that the bending portion is damaged if the vibration at the resonance point is large. is there. Therefore, the distance between the weight portion 2 and the bottom surfaces of the recesses 10a and 20a is limited.

By the way, the semiconductor acceleration sensor is required to have high reliability because it is used for acceleration detection for an air bag of an automobile or the like. For this reason, a failure diagnosis function that can clarify the failure mode of the sensor is required. However, in the semiconductor acceleration sensor, for example, the bending portion 3
Is broken at the end connected to the weight portion 2, that is, at the position shown in FIG. 8A, between the piezoresistors R _{1 to} R ₄ and the piezoresistors R _{1 to} R ₄ forming the bridge circuit BC. Since the wiring connecting the lines remains normal, the bridge circuit BC itself operates normally. That is, since the output of the bridge circuit BC is substantially zero in a state where no acceleration is applied,
There is a problem that the failure mode cannot be easily detected and the failure mode is unclear regardless of the normal state.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a highly reliable semiconductor acceleration sensor.

[0013]

In order to achieve the above object, the invention of claim 1 is such that a weight portion, a bending portion whose one end is integrally connected to the weight portion, and the other end of the bending portion are integrally connected. A supporting portion for swingably supporting the weight portion by the bending portion is formed by processing a semiconductor substrate, and a sensing element having a piezoresistor formed in the vicinity of the surface of the bending portion, and an anode above and below the sensing element, respectively. In a semiconductor acceleration sensor, which is fixed by bonding and includes an upper cap and a lower cap that seal at least the weight portion and the flexible portion, the upper cap and the lower cap are each made of glass and at least the weight. Characterized in that a concave portion having minute irregularities is formed over the entire inner peripheral surface in a portion facing the portion, The presence of the fine irregularities over the entire surface of the peripheral surface, since the weight portion during anodic bonding the weight portion also hit the inner circumferential surface is not joined to the inner peripheral surface, it is possible to operate normally.

According to a second aspect of the present invention, the weight portion, the bending portion whose one end is integrally connected to the weight portion, the other end of the bending portion which is integrally connected, and the support for swingably supporting the weight portion by the bending portion. A portion formed by processing a semiconductor substrate, a sensing element having a piezoresistor formed in the vicinity of the surface of the bending portion, and at least the weight portion and the bending portion fixed by anodic bonding above and below the sensing element, respectively. In a semiconductor acceleration sensor including an upper cap and a lower cap to be sealed, each of the upper cap and the lower cap is made of glass, and a recess is formed at least in a portion facing the weight portion,
A metal film, which is electrically connected to the semiconductor substrate forming the sensing element, is deposited between an inner peripheral surface of the recess and a surface in contact with the sensing element. It is one of the
Since the sensing element and the metal film have the same potential, and the metal film is also adhered to the inner peripheral surface of the recess, electrostatic attraction between the weight portion and the inner peripheral surface. No force is generated, and as a result, the weight portion can be normally operated without being joined to the inner peripheral surface.

According to the invention of claim 3, in the invention of claim 2, since the thickness of the metal film is set to several hundred angstroms or less, the joining / sealing is hindered at the joining surface of the sensing element and the upper cap and the lower cap. It is possible to prevent the weight portion from being anodically bonded to the inner peripheral surface of the recess without causing such a step, and as a result, it is possible to operate normally.

The invention of claim 4 is claim 1 or claim 2.
In the invention, there is provided a pair of bending portions and a bridge circuit formed of a pair of piezoresistors in each of the bending portions, and at least extended from each pair of piezoresistors formed in each of the bending portions. Since one wire is routed so as to straddle the supporting portion and the weight portion, the wiring is cut if the bending portion is damaged in a state where acceleration is applied, so that the bridge You can see that the circuit has been disconnected and has failed.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to embodiments. (First Embodiment) A semiconductor acceleration sensor according to a first embodiment of the present invention will be described with reference to FIGS.

As shown in FIG. 1, the acceleration sensor of the present embodiment has a weight portion 2, a pair of thin-walled bending portions 3 whose one end is integrally connected to the weight portion 2, and the other ends of both the bending portions 3. And a supporting portion 4 which is integrally connected to each other and supports the weight portion 2 swingably by the bending portion 3 by anisotropically etching a rectangular plate-like silicon substrate with an alkaline etching solution containing potassium hydroxide (KOH) or the like. An upper cap 10 and a lower cap 20 that have the sensing element 1 formed by the above-mentioned method and are anodically bonded to the upper and lower sides of the sensing element 1 and seal at least the weight portion 2 and the bending portion 3.
have.

The upper cap 10 and the lower cap 20 are made of heat-resistant glass, so-called Pyrex glass, in the shape of a rectangular plate, and are anodically bonded to the upper and lower surfaces of the support portion 4 of the sensing element 1 at their peripheral portions. The sensing element 1 is sealed. Here, each of the upper cap 10 and the lower cap 20 has recesses 10a and 20a formed by, for example, sandblasting on the surface facing the weight portion 2. The weight 2 swings in the recesses 10a and 20a.

In this semiconductor acceleration sensor, the recess 10a,
Since 20a is formed by sandblasting, minute irregularities are formed on the entire inner peripheral surfaces of the recesses 10a and 20a.
Even if it hits the inner peripheral surface of 0a, 20a, the weight portion 2 will have a recess 1
Therefore, the inner peripheral surfaces of 0a and 20a are not anodically bonded.

As described above, in the configuration of this embodiment,
Even when heat-resistant glass is used for the upper cap 10 and the lower cap 20, it is possible to prevent the weight portion 2 from being bonded to the upper cap 10 and the lower cap 20 during anodic bonding. By the way, a pair of piezoresistors (R ₁ and R ₄ and R ₂ and R ₃ ) are formed on each surface of the pair of flexible portions 3 by a diffusion technique. These piezoresistors R _{1 to} R ₄ are formed in a strip shape, and as shown in FIG. 2A, the piezoresistors (R ₁
And R ₄ and R ₂ and R ₃ ) so that their longitudinal direction coincides with the longitudinal direction of the bending portion 3, and the other piezoresistor makes its longitudinal direction substantially orthogonal to the longitudinal direction of the bending portion 3. Has been formed. In addition, these piezoresistors R _{1 to} R ₄
The bridge circuit BC (see FIG. 2 (b)) is constituted by the wiring, and the wiring for detecting the resistance value changes of the piezoresistors R _{1 to} R ₄ as voltage is extended as shown in FIG. 2 (a). That is, in the present semiconductor acceleration sensor, the wiring extending from the connection points N ₁₄ and N _{23 of} each pair of piezoresistors R ₁ and R ₄ and R ₂ and R ₃ formed in each bending portion 3 is connected to the support portion 4. Weight part 2
Therefore, in the state where the acceleration is applied, the flexible portion 3 is, for example, as shown in FIG.
As shown in (c), the wiring is disconnected no matter which end is damaged, so that it can be seen that there is no input to the bridge circuit BC and there is a failure.

(Second Embodiment) A semiconductor acceleration sensor according to a second embodiment of the present invention will be described with reference to FIGS. The basic configuration of the semiconductor acceleration sensor of the present embodiment is substantially the same as that of the conventional example, and in this semiconductor acceleration, as shown in FIG. 4, the upper cap 10 and the lower cap 20 made of glass face the weight portion 2. Recesses 10a, 20a
And a metal film 30 made of, for example, aluminum, which is electrically connected to the silicon substrate forming the sensing element 1, is deposited between the recesses 10a and 20a facing the weight portion 2 and the surface contacting the sensing element 1. It is characterized in that it is anodically bonded to the sensing element 1.

In this semiconductor acceleration sensor, as shown in FIG. 5, the upper cap 10 and the lower cap 20 are set to be minus, and the sensing element 1 is set to be positive, between the sensing element 1 and the upper cap 10 and between the sensing element 1 and the lower cap 20. The metal film 30 and the upper cap 10 are anodically bonded and fixed to the sensing element 1 by applying a voltage of several hundred volts. At this time, since the silicon substrate forming the sensing element 1 and the metal film 30 have the same potential, the weight portion 2 and the metal film 3 are formed.
The electrostatic attraction does not work between 0. Therefore, the weight portion 2 is not anodically bonded to the bottom surfaces of the recesses 10a and 20a.

By the way, a normal semiconductor acceleration sensor has two
The size is about 4 mm square, and the thickness of the sensing element 1, the upper cap, and the lower cap at the portions to be anodically bonded is several hundred μm. Therefore, the metal film 30
If the thickness is less than several hundred angstroms, the sensing element 1, the upper cap 10 and the lower cap 2
At the joint surface of 0, the step caused by the metal film 30 is
It does not interfere with the sealing.

The acceleration sensor according to the first embodiment is also used.
Since the bridge circuit BC of the same wiring as is formed,
As in the first embodiment, when the bending portion 3 is damaged, it can be seen that the sensor has failed.

[0026]

According to the first aspect of the present invention, the anode having the minute concave and convex portions is formed over the entire inner peripheral surface at least in the portions of the upper cap and the lower cap facing the weight portion. Even if the weight portion contacts the inner peripheral surface at the time of joining, the weight portion is not joined to the inner peripheral surface, so that there is an effect that a normal operation can be performed.

According to a second aspect of the present invention, a recess is formed in at least a portion of the upper cap and the lower cap facing the weight portion, and the recess is formed between the inner peripheral surface of the recess and the surface contacting the sensing element. Since the metal film that is conductive to the semiconductor substrate forming the sensing element is deposited, the sensing element and the metal film have the same potential at the time of anodic bonding, and the metal film is the inner portion of the recess. Since it is also adhered to the peripheral surface, no electrostatic attraction force is generated between the weight portion and the inner peripheral surface, and as a result, the weight portion does not bond to the inner peripheral surface and operates normally. There is an effect that can be done.

According to the invention of claim 3, in the invention of claim 2, since the thickness of the metal film is several hundred angstroms or less,
It is possible to prevent the weight part from being anodically bonded to the inner peripheral surface of the recess without forming a step that hinders bonding and sealing at the bonding surface of the sensing element and the upper cap and the lower cap, and as a result, normal It has the effect of being able to operate.

The invention according to claim 4 is claim 1 or claim 2.
In the invention, there is provided a pair of bending portions and a bridge circuit formed of a pair of piezoresistors in each of the bending portions, and at least extended from each pair of piezoresistors formed in each of the bending portions. Since one wire is routed so as to straddle the supporting portion and the weight portion, the wiring is cut if the bending portion is damaged in a state where acceleration is applied, so that the bridge The effect is that there is no input to the circuit and it can be seen that there is a failure.

[Brief description of the drawings]

FIG. 1 is a side sectional view showing a first embodiment.

FIG. 2A is a plan view showing a main part of the first embodiment. (B) is an equivalent circuit diagram of the above.

FIG. 3 is a plan view for explaining a damage mode of a main part of the first embodiment.

FIG. 4 is a side sectional view showing a second embodiment.

FIG. 5 is an explanatory diagram of a main manufacturing process of the above.

FIG. 6A is a plan view showing a conventional example. (B) is AA 'sectional drawing same as the above.

FIG. 7A is a plan view showing a main part of another conventional example. (B) is a BB 'sectional view same as the above.

8A is an enlarged view of a region C of FIG. 7A.
(B) is an equivalent circuit diagram of the above.

FIG. 9 is an operation explanatory view of the above.

[Explanation of symbols]

 1 Sensing Element 2 Weight Part 3 Bending Part 4 Support Part R Piezoresistor 10 Upper Cap 10a, 20a Recess 20 Lower Cap

Claims (4)

[Claims] 1. A semiconductor substrate comprising: a weight portion; a bending portion whose one end is integrally connected to the weight portion; and a support portion which is integrally connected to the other end of the bending portion and supports the weight portion swingably by the bending portion. A sensing element having a piezoresistor formed in the vicinity of the surface of the bending portion by processing, and an upper cap fixed to the upper and lower sides of the sensing element by anodic bonding and sealing at least the weight portion and the bending portion. And a lower cap, the upper cap and the lower cap are each made of glass and have minute irregularities over at least a portion facing the weight portion over the entire inner peripheral surface. A semiconductor acceleration sensor having a recess formed therein. 2. A semiconductor substrate comprising: a weight portion; a bending portion whose one end is integrally connected to the weight portion; and a support portion which is integrally connected to the other end of the bending portion and swingably supports the weight portion by a semiconductor substrate. A sensing element having a piezoresistor formed in the vicinity of the surface of the bending portion by processing, and an upper cap fixed to the upper and lower sides of the sensing element by anodic bonding and sealing at least the weight portion and the bending portion. And a lower cap, the upper cap and the lower cap are made of glass, and a recess is formed in at least a portion facing the weight portion, and an inner peripheral surface of the recess is formed. And a surface abutting the sensing element are electrically connected to the semiconductor substrate forming the sensing element. A semiconductor acceleration sensor, which is formed by depositing a metal film. 3. The semiconductor acceleration sensor according to claim 2, wherein the metal film has a thickness of several hundred angstroms or less. 4. A pair of flexures and a bridge circuit composed of a pair of piezoresistors formed in each flexure, and extending from each pair of piezoresistors formed in each flexure. The at least one wire is laid out so as to extend over the support portion and the weight portion.
Alternatively, the semiconductor acceleration sensor according to claim 2.