JP2010185781A - Acceleration sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a piezoresistive element type acceleration sensor which is stable electrically and mechanically and suitable for being made compact. <P>SOLUTION: The piezoresistive element type triaxial acceleration sensor including as components: a frame part; a flexible part; a spindle part held by the frame part through the flexible part; metal wiring for connecting a piezoresistive element disposed at the flexible part to a chip terminal disposed at the frame part; and a heavily doped diffusion layer, has acceleration sensors each having a bridge circuit composed of the piezoresistive element, the metal wiring and the high-concentration diffusion layer for respective three axes perpendicularly intersecting with another, wherein shield wiring is arranged on the piezoresistive element through a second insulating film, such that the shield wiring makes up a portion of the bridge circuit as a part of the metal wiring. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

The present invention relates to a piezoresistive element type acceleration sensor, in particular, an automobile, an aircraft, a portable terminal device,
The present invention relates to an acceleration sensor used for toys and the like.

As a semiconductor sensor using a semiconductor piezoresistive element, a pressure sensor, an acceleration sensor, a strain sensor, and the like have been studied. In recent years, in particular, attention has been focused on acceleration sensors used in automobiles, aircraft, portable terminal devices, toys and the like. Hereinafter, the semiconductor piezoresistive element is simply referred to as a piezoresistive element. A piezoresistive element type semiconductor sensor is a sensor that measures various physical quantities with an electric circuit using a piezoresistive element formed by implanting impurities into silicon. Each physical quantity is detected by causing a resistance change in the piezoresistive element in response to the physical quantity to be measured.

For example, a piezoresistive element type acceleration sensor is composed of a physical structure such as a flexible portion formed on a silicon substrate using a photolithography technique and an electric circuit such as a piezoresistor formed by implanting impurities into silicon using a semiconductor technique. It is an acceleration sensor. When acceleration acts on the physical structure, distortion occurs in the piezoresistive element, and this distortion changes the resistance value of the piezoresistive element. As a result, the output value of the electric circuit changes, so that acceleration can be detected.

2. Description of the Related Art Conventionally, an acceleration sensor that measures a collision acceleration in a plane direction, that is, a uniaxial or biaxial direction, has been widely used for operating an air bag of an automobile. Recently, it is also frequently used for mobile terminal devices and robots, and measures spatial movement, that is, acceleration of X, Y, and Z axes.
An axial acceleration sensor has been put into practical use. Furthermore, high stability is required for detecting minute accelerations, and small size is required for mounting on various devices.

Japanese Patent Laid-Open No. 08-86671 JP 2003-92413 A

20 and 21 illustrate a piezoresistive element type acceleration sensor (hereinafter referred to as an acceleration sensor). The acceleration sensor has a structure in which the weight portion 11 is supported by the beam portion 33. Piezoresistive elements 13, 14, and 15 are formed on the beam to form a bridge circuit. The resistance value of the piezoresistive element varies with strain. Therefore, the film stress of the wiring forming the bridge circuit affects the stabilization of the output of the bridge circuit.
More details are as follows. Since the wiring has a coefficient of thermal expansion different from that of silicon as a base material, a film stress is generated in the process of heating and cooling during manufacturing. Furthermore, this film stress may be relaxed by creep. Since the generation and relaxation of such film stress acts on the piezoresistive element, the output of the bridge circuit tends to fluctuate if the ratio of the wiring occupying the beam is high. Especially the acceleration sensor 20
If the size of the beam is reduced, the beam is reduced in size, so that the influence of wiring is relatively increased.

It is known that the presence of surface ions and moisture tends to affect the resistance value of the piezoresistive element. This means that the output of the acceleration sensor becomes unstable. This is called electrical destabilization. On the other hand, in Patent Document 1 (Japanese Patent Laid-Open No. 08-86671), a shield electrode fixed at a high potential side or a low potential side potential of a sensor circuit is disposed on a piezoresistive element via an insulating film. Thus, a method for suppressing fluctuations is disclosed.

However, when providing the acceleration sensor with the method of arranging the shield electrode with a fixed electric potential disclosed in Patent Document 1, it is necessary to newly arrange the shield electrode wiring in addition to the original detection circuit wiring. Since the acceleration sensor has the piezoresistive element arranged on the beam, it is necessary to arrange the circuit wiring, the shield electrode, and the shield electrode wiring on the beam.

Such an increase in the number of wirings causes an increase in film stress on the beam due to the wirings, that is, destabilization of the output of the acceleration sensor due to the film stress. This is called mechanical destabilization. Mechanical destabilization has a great influence especially on downsizing. In addition, when the size is further reduced, it becomes difficult to secure a wide area on the beam to which the shield electrode and the shield electrode wiring are added.

Patent Document 2 discloses that a part of the wiring protrudes above the piezoresistive element to serve as a shield. However, there are five wires around the piezoresistive element and five wires near the center of the beam. There is a problem that the number of wires is large and the film stress on the beam due to the wires increases.

In view of the above problems, an object of the present invention is to realize an electrically and mechanically stable piezoresistive element type acceleration sensor suitable for downsizing.

The acceleration sensor of the present invention has a frame part, a flexible part, and a weight part held by the frame part through the flexible part,
It has metal wiring and high-concentration diffusion layers used for connecting the piezoresistive element provided in the flexible part and the chip terminal provided in the frame part,
An acceleration sensor having a bridge circuit composed of the piezoresistive element, metal wiring, and high-concentration diffusion layer for each of three orthogonal acceleration detection axes,
A shield wiring is disposed on the piezoresistive element via an insulating film, and the shield wiring constitutes a part of a bridge circuit as a part of the metal wiring. Separately
Since no shield is added, it is effective for suppressing an increase in wiring stress, and also contributes to suppressing electrical destabilization.

In the bridge circuit, the potential of the shield wiring is the highest potential in the bridge circuit (=
Input voltage), minimum potential (= ground voltage), or an intermediate potential between them. This is effective for suppressing stress by suppressing an increase in the number of wires.

The bridge circuit is composed of four sets of piezoresistive elements,
The combination of the potentials of the shield wiring corresponding to the two sets of high-potential side piezoresistive elements is the same between the high-potential side piezoresistive elements,
The combination of the potentials of the shield wiring corresponding to the two sets of piezoresistive elements on the low potential side is the same between the low potential piezoresistive elements. Here, “corresponding” means that the layer is formed so as to be opposed to the piezoresistive element through the insulating film. By adopting this configuration, the effect of the shield wiring potential on the piezoresistive element is balanced, so that it is electrically stable.

The number of metal wirings passing through the beam portion is the same for all the beam portions. Stress balances and stabilizes. The number is particularly preferably 3. Stress is balanced and stable with a small number.

Except for local through holes, the metal wiring is protected by an insulating film, and the insulating film is a silicon nitride film. The insulating film contributes to prevention of corrosion during processing. The silicon nitride film has high chemical resistance.

The bridge circuit is characterized in that at least one of a high potential side chip terminal and a low potential side chip terminal is shared by different bridge circuits. .

According to the present invention, an electrically and mechanically stable piezoresistive element type acceleration sensor suitable for downsizing can be realized.

Hereinafter, the present invention will be described in detail based on examples with reference to the drawings. In order to make the explanation easy to understand, the same reference numerals are used for the same parts and parts.

Example 1
The acceleration sensor according to the first embodiment of the present invention will be described below. 1 and 2 show the element structure of the acceleration sensor of the embodiment. 1 is a plan view, and FIG. 2 is a sectional view taken along the line kk ′ of FIG. The acceleration sensor element 9 of FIG. 1 can be applied to an acceleration sensor assembled as shown in FIGS. 19 and 20, an acceleration sensor assembled in a resin package after being hermetically sealed with caps shown in FIGS. Since the present invention is characterized by the structure of the acceleration sensor element, particularly the wiring structure thereof, the acceleration sensor element will be described in detail below based on examples.

The acceleration sensor element 9 according to the first embodiment includes a first beam portion 33a, a second beam portion 33b, and a third beam of a beam portion 33 having four beam-shaped flexible portions 11 in a support frame portion 10. Part 33c, fourth beam part 33d
Is supported by. In FIG. 1, the first beam portion 33a and the second beam portion 33b are to detect X-axis acceleration, and the third beam portion 33c and the fourth beam portion 33d are to detect Y-axis and Z-axis acceleration. did. At the longitudinal end of each beam portion, acceleration detecting piezoresistive elements 13 (eight from 13-1a to 13-4b), 14 (eight from 14-1a to 14-4b), 15 (from 15-1a) 15-4b).

The first beam portion 33a and the second beam portion 33b include a piezoresistive element 1 that detects acceleration in the X-axis direction.
3 was formed. A third beam portion 33c and a fourth beam portion arranged at right angles to the first beam portion 33a and the second beam portion 33b;
A piezoresistive element 14 for detecting acceleration in the Y-axis direction and a piezoresistive element 15 for detecting acceleration in the Z-axis direction are formed on the beam portion 33d. The piezoresistive element 15 for detecting the Z-axis acceleration may be disposed on either the X-axis or Y-axis beam, but in this embodiment, it is formed on the Y-axis beam. Eight piezoresistive elements for detecting the acceleration in each axis direction are one set (two sets in total), and four sets are connected by metal wiring 31 and high-concentration diffusion layer wiring 38, and bridged for each axis. A circuit was constructed. For example, as shown in FIG. 10, the high concentration diffusion layer wiring is formed as a connection wiring other than the metal wiring. One bridge circuit has two paths, and each path includes a high-potential-side piezoresistive element and a low-potential-side piezoresistive element. The potential between the high-potential side piezoresistive element and the low-potential side piezoresistive element in one path is the midpoint A, and the other path between the high-potential side piezoresistive element and the low-potential side piezoresistive element Was set to the midpoint B. In FIG. 1, a piezoresistive element is represented by a hatched rectangular pattern, a portion where the rectangular pattern and the wiring are electrically connected is represented by a small round dot, and a branch of the wiring is represented by a large round dot. . The wiring passing over the piezoresistive element without a small round dot is insulated from the piezoresistive element through an insulating film, and is a shielded wiring.

When the weight portion 11 is displaced by applying acceleration from the outside, the flexible beam portion 33 is deformed, so that the electric resistance of the piezoresistive element changes. The acceleration can be detected by extracting the potential difference caused by the resistance change amount of the four sets of piezoresistive elements constituting the bridge circuit on each axis by the bridge circuit.

A method of manufacturing the acceleration sensor element 9 will be briefly described with reference to FIG. For manufacturing the acceleration sensor element 9, an SOI wafer having a silicon layer of about 1 μm and a silicon layer of about 5 μm on a silicon layer of about 400 μm thickness was used. The silicon oxide film layer was used as an etching stop layer for dry etching, and the structure was formed on two silicon layers. Less than,
The thinner silicon layer is called the first layer 41, the thicker silicon layer is called the second layer 42, the surface of the first layer not joined to the silicon oxide film layer is the first surface 43, and the surface of the second layer is The connection surface through the second surface 44 and the silicon oxide film layer is referred to as a third surface 45.

The shape of the piezoresistive element was patterned with a photoresist, and boron was implanted into the first surface 43 to form a piezoresistive element. A silicon oxide film was formed on the first surface 43 to protect the piezoresistive element.

Piezoresistive elements were connected by metal wiring 31 and high-concentration diffusion layer wiring 38 to form a bridge circuit for detecting acceleration. The metal wiring 31 was formed by sputtering an aluminum-based metal on the first insulating film (silicon oxide film) formed on the first surface 43. High concentration diffusion layer wiring 3
A diffusion layer 8 is formed by implanting boron into the first surface 43 at a high concentration. The first insulating film (silicon oxide film) 36 formed on the first surface 43 also functions as an insulating film between the first layer silicon 41 and the metal wiring 31. The first insulating film (silicon oxide film) 36 formed on the first surface 43 partially formed a through hole, and connected between the piezoresistive element and the high concentration diffusion layer wiring 38 and the metal wiring 31. Further, a second insulating film (protective layer) 37 for protecting the metal wiring 31 was formed. The insulating film and the metal wiring were processed into desired shapes by photolithography.

Here, a first insulating film (silicon oxide film) 36 is formed on the piezoresistive elements 13, 14, 15.
The metal wiring 31 was arranged via This portion of the metal wiring is referred to as shield wiring 32. The shield wiring 32 arranged on the piezoresistive element gives a shielding effect to the piezoresistive element. Here, the metal wiring takes different potentials (high potential, low potential, or an intermediate potential between them) depending on the position in the bridge circuit. The potential of the metal wiring affects the resistance value of the piezoresistive element. That is, different potentials have different effects on the piezoresistive element. As a result, in the bridge circuit, the resistance balance may be lost, that is, electrical instability may occur.
Details of a configuration that does not cause electrical instability with shields of different potentials will be described later.

Next, after forming a photoresist pattern on the first surface 43, the shapes of the beam portion 33, the weight portion 11, and the support frame portion 10 were processed by dry etching. Further, after forming a photoresist pattern on the second surface 44, the shapes of the weight portion 11 and the support frame portion 10 were processed by dry etching. The silicon oxide film layer remaining between the first layer and the second layer was removed by wet etching.

A large number of acceleration sensor elements 9 formed on one wafer were separated into chips (chips) by dry etching or dicing. The acceleration sensor element 9 divided into pieces (chips) can be used for assembling the acceleration sensor 20 as shown in FIGS. Alternatively, a plurality of acceleration sensor elements 9 formed on one wafer are bonded to the upper cap chip 22 and the lower cap chip 23 in the wafer state, and then separated into chips (chips) by dicing or the like to obtain the MEMS assembly 21. Acceleration sensor 20 as shown in FIGS.
Can be used for assembling.

As described above, a part of the metal wiring 31 is a shield wiring 32 for the piezoresistive elements 13 to 15.
To function as. A configuration example is shown in FIG. In FIG. 3, the central metal wiring 31 is divided and branched into a total of four metal wirings, which are arranged on the piezoresistive element.
FIG. 3c) is a cross section of the field of view from A in FIG. 3b). The piezoresistive element 13 (or 14, 15) and the high-concentration diffusion layer wiring 38 are formed on the first layer 41, and then the first insulation. The film 36 and the metal wiring 31 are laminated, and further a second insulating film (protective layer) 37 is covered.

4, 5 and 6 show the bridge circuit in the triaxial acceleration sensor according to the first embodiment extracted for each axis. In an actual acceleration sensor, these three circuits are arranged in substantially the same plane so as to intersect with each other as appropriate so as not to cause a short circuit. 4, 5, and 6 indicate metal wirings that are located on the beam but are not directly related to the respective bridge circuits shown in the drawings.
The broken lines in FIGS. 14, 16, 17, and 18 are also displayed with the same meaning.

FIG. 4 shows a bridge circuit related to X-axis detection. FIG. 5 shows a bridge circuit related to Y-axis detection. FIG. 6 shows a bridge circuit related to Z-axis detection.
In these bridge circuits, one piezoresistive element forms one bridge circuit. Two eight piezoresistive elements located near the same beam root are connected in series. That is, one bridge circuit (Wheatstone bridge) is formed by four sets of piezoresistive elements. The two piezoresistive elements connected in series are arranged symmetrically with respect to the rotation axis of the beam, thereby canceling a change in the resistance value of the piezoresistive element when a torsion element is generated in the beam. The resistance value of the piezoresistive element gave a dominant resistance value compared to other resistances, for example, the wiring resistance of metal wiring.
Here, the bridge circuit for X-axis detection is an electrode pad (13), in addition to the main circuit formed by the metal wiring and the high-concentration diffusion layer wiring (38) connecting the piezoresistive elements.
24-V, 24-G, 24-x) and a circuit including a metal wiring from the main circuit to the electrode pad. Similarly, the bridge circuit for Y-axis detection is in addition to the circuit formed by the metal wiring connecting the piezoresistive element (14) and the piezoresistive element and the high-concentration diffusion layer wiring (38),
It means a circuit including electrode pads (24-V, 24-G, 24-y) and metal wiring from the main circuit to the electrode pads. The bridge circuit for Z-axis detection is a piezoresistive element (14).
In addition to the circuit formed by the metal wiring and the high-concentration diffusion layer wiring (38) for connecting between the piezoresistive elements and the electrode pads (24-V, 24-G, 24-z) and the main circuit to the electrode pads It means a circuit containing metal wiring.

As described above, a part of the metal wiring 31 constituting the bridge circuit is arranged on the eight piezoresistive elements via the first insulating film (silicon oxide film) 36, and the piezoresistive elements 13, 14 are arranged.
, 15 was given an electrical shielding effect.
The potential of the metal wiring 31 takes one of a maximum potential, a minimum potential, and an intermediate potential thereof depending on the position in the bridge circuit. The potential of the shield wiring 32 on the piezoresistive element has a property of changing the resistance value of the piezoresistive element. However, in the acceleration sensor of the present invention, a metal wiring having a potential that gives a change in resistance that is located near the piezoresistive element and does not disturb the output balance of the bridge circuit is used as the shield wiring 32. With this configuration, it is necessary to use metal wiring located in the vicinity of the piezoresistive element simultaneously with electrical stabilization due to the shielding effect, that is, to arrange an electrode having a function of shielding wiring separately from the bridge circuit. In the absence of this, mechanical stabilization can be achieved.

FIGS. 7 to 9 (a) show circuit diagrams of a bridge circuit composed of four sets (eight) of piezoresistive elements for detecting the acceleration of each axis. 7 to 9 (b) shows a weight 1 for detecting the acceleration of each axis.
1 operation | movement and the deformation | transformation of the beam part 33 were shown. These (FIGS. 7-9) and FIGS.
In relation to Example 1, Comparative Example 1 and Comparative Example 2 shown in FIG. 8, the potentials of the eight piezoresistive elements constituting the bridge circuit, the metal wiring arranged on each piezoresistive element, and the output change and output thereby The presence or absence of change was indicated.

First, an ideal state for detecting the X-axis acceleration will be described.
In the bridge circuit for detecting the X-axis acceleration, the acceleration is the high potential side piezoresistive element 13.
-1 and the potential of the intermediate point between the low potential side piezoresistive element 13-4 (midpoint A) and the potential of the intermediate point between the high potential side piezoresistive element 13-2 and the low potential side piezoresistive element 13-3 (midpoint) Detection is based on the difference from B). Since the resistance values of the piezoresistive elements are common, ideally or in the initial design, the potential at the midpoint between the piezoresistive elements 13-1 and 13-4 is the electrode pad 24.
The value becomes half of the input voltage at −V, and the piezoresistive element 13-2 and the piezoresistive element 13
Since the potential at the intermediate point of −3 is also half the value of the input voltage at the electrode pad 24-V, the difference between them, that is, the output is zero. The electrode pad corresponds to a chip terminal.

Next, regarding the detection of the X-axis acceleration in the first embodiment, the electrical influence due to the arrangement of the shield wiring 32 will be described with reference to Table 1.
As shown in Table 1, in the bridge circuit that detects X-axis acceleration, the potential of the shield wiring corresponding to the piezoresistive element 13-1 and the piezoresistive element 13-2 on the high potential side is high. Therefore, the resistance values of the piezoresistive element 13-1 and the piezoresistive element 13-2 change (increase) greatly (compared to when the potential of the shield wiring is zero potential). On the other hand, the potential of the shield wiring 32 corresponding to the piezoresistive element 13-3 and the piezoresistive element 13-4 on the low potential side is low. Therefore, the resistance values of the piezoresistive element 13-3 and the piezoresistive element 13-4 are (
The change (increase) is small (compared to when the shield potential is high).

As described above, in the bridge circuit that detects the X-axis acceleration, the acceleration is the potential between the high potential side piezoresistive element 13-1 and the low potential side piezoresistive element 13-4 (middle point A) and the high potential side. Potential at the midpoint between the piezoresistive element 13-2 and the low potential side piezoresistive element 13-3 (middle point B)
It is detected by the difference between Potential at the midpoint between the piezoresistive element 13-1 and the piezoresistive element 13-4 (
The middle point A) is lower than half of the input voltage at the electrode pad 24-V because the resistance of the piezoresistive element 13-1 is increased as compared with the resistance of the piezoresistive element 13-4. At the same time, the potential of the intermediate point between the piezoresistive element 13-2 and the piezoresistive element 13-3 (middle point B) is such that the resistance of the piezoresistive element 13-2 increases compared to the resistance of the piezoresistive element 13-3. Pad 24
-V drops below half of the input voltage. However, since these rates of change are common,
The difference between the potential at the midpoint between the piezoresistive element 13-1 and the piezoresistive element 13-4 (midpoint A) and the potential at the midpoint between the piezoresistive element 13-2 and the piezoresistive element 13-3 (midpoint B) That is, the output can be kept at zero (middle point A = middle point B). In Table 1, X of 1, 2, 3, 4 is 13
-1, 13-2, 13-3, 13-4, Z 1, 2, 3, 4 are 15-1, 15-
2, 15-3, 15-4.

As shown in FIGS. 8A and 8B and Table 1, the detection of the Y-axis acceleration has the same effects as the X-axis acceleration detection, the initial circuit configuration, and the shield wiring 32. Therefore, the description for detecting the Y-axis acceleration is the same as the description for detecting the X-axis acceleration described above, and a detailed description thereof will be omitted.

Next, an ideal state for detection of Z-axis acceleration will be described.
In the bridge circuit that detects the Z-axis acceleration, the acceleration is the high potential side piezoresistive element 15.
-1 and the low potential side piezoresistive element 15-3 (midpoint A) and the intermediate potential between the high potential piezoresistive element 15-2 and low potential piezoresistive element 15-4 (midpoint) Detection is based on the difference from B). Since the resistance values of the piezoresistive elements are common, ideally or in the initial design, the potential (middle point A) between the piezoresistive elements 15-1 and 15-3 is the electrode. The value is half the input voltage at the pad 24-V, and the potential (middle point B) between the piezoresistive element 15-2 and the piezoresistive element 15-4 is also half the value of the input voltage at the electrode pad 24-V. Therefore, the difference between them, that is, the output is zero.

Next, regarding the detection of the Z-axis acceleration in the first embodiment, the electrical influence due to the arrangement of the shield wiring 32 will be described based on Table 1.
As shown in Table 1, the bridge circuit for detecting the Z-axis acceleration corresponds to each of the two piezoresistive elements constituting the high potential side piezoresistive element 15-1 and the low potential side piezoresistive element 15-2. The potential of the shield wiring is a high potential for the piezoresistive elements 15-1a and 15-2a, and is an intermediate potential of the input voltage for the piezoresistive elements 15-1b and 15-2b. Accordingly, the resistance values of the piezoresistive element 15-1 and the piezoresistive element 15-2 change (increase) somewhat greatly (compared to when the shield potential is zero).

On the other hand, the potential of the shield wiring corresponding to the piezoresistive element 15-3 and the low potential side piezoresistive element 15-4 is an intermediate potential between the input voltages for the piezoresistive elements 15-3a and 15-4a. The resistance elements 15-3b and 15-4b are at a low potential.
Accordingly, the change (increase) in the resistance values of the piezoresistive elements 15-3 and 15-4 is slightly smaller (compared to when the shield potential is high).

As described above, in the bridge circuit that detects the Z-axis acceleration, the acceleration is the potential at the midpoint between the high potential side piezoresistive element 15-1 and the low potential side piezoresistive element 15-3 (middle point A) and the high potential side. Potential at the midpoint between the piezoresistive element 15-2 and the low potential side piezoresistive element 15-4 (middle point B)
It is detected by the difference between Potential at the midpoint between the piezoresistive element 15-1 and the piezoresistive element 15-3 (
The middle point A) is lower than half of the input voltage because the resistance of the piezoresistive element 15-1 is increased compared to the resistance of the piezoresistive element 15-3. At the same time, the potential of the intermediate point between the piezoresistive element 15-2 and the piezoresistive element 15-4 is the same as that of the piezoresistive element 15-2.
Since the resistance is increased compared to −4, the voltage is lower than half of the input voltage at the electrode pad 24-V. However, since these rates of change are common, the potential (middle point A) between the piezoresistive element 15-1 and the piezoresistive element 15-3, the piezoresistive element 15-2, and the piezoresistive element 15-4. The difference between the potentials of the intermediate points (midpoint B), that is, the output can be kept at zero (midpoint A = midpoint B).

From the above, even if the shield wiring is arranged from the position where the arrangement is easy, even if the potential of the shield wiring 32 is different in the bridge circuit (by using the wiring configuration of the present invention).
, Fluctuations in output can be avoided. As a result, since electrical instability can be avoided without increasing the number of wires, an acceleration sensor that is mechanically and electrically stable and suitable for downsizing can be realized.

Note that the potential arrangement of the shield wiring shown in Table 1 is an example, and the potential of the shield wiring with respect to the piezoresistive element on the high potential side in the two paths in the bridge circuit is the same as long as the bridge circuit balance is not lost. Further, other combinations may be employed as long as the potential of the shield wiring with respect to the piezoresistive element on the low potential side is the same. In this embodiment, the piezoresistive element and the shield wiring disposed on the piezoresistive element are the piezoresistive element and the metal wiring in the same bridge circuit. These may be configured to extend over another bridge circuit. For example, the X-axis detection piezoresistive element may be used as the Y-axis detection metal wiring as the shield wiring. However, in order to ensure the degree of freedom of design change, it is desirable that the configuration be closed in the same circuit. For example, when scaling down from a 3-axis acceleration sensor to a 2-axis acceleration sensor, if the piezoresistive element and the shield wiring for the piezoresistive element are in different circuits,
It may be necessary to leave unnecessary shield wiring. This can cause mechanical instability.

The difference between Example 1 of the present invention and Comparative Example 1 will be described with reference to FIGS. In the wirings shown in FIGS. 10 to 12, a solid line indicates a high potential (input voltage potential) wiring, and a broken line indicates a non-high potential (low potential, intermediate potential) wiring.

FIG. 10 is a wiring diagram and a cross-sectional view in the vicinity of the piezoresistive element in Example 1 of the present invention.
In the first embodiment, the number of wires is four at each piezoresistive element position, and the number of wires is three at the center of the beam.

(Comparative Example 1)
11 and 12 are a wiring diagram and a cross-sectional view in the vicinity of the piezoresistive element in the first comparative example. Comparative Example 1 is an example in which the shield potential is fixed at a high potential. Since all shield potentials are common, the resistance change of each piezoresistive element is also common, and the output change due to electrical factors does not occur.
This is shown in Table 1 (b). However, as shown in FIGS. 11 and 12, in the comparative example 1, the number of wirings is 5 (A1-A1 ′, A2-A2 ′) at the position of the piezoresistive element.
) Or 7 (A3-A3 ′, A4-A4 ′). In the center of the beam,
There were 3 (between A1-A1 ′ and A2-A2 ′) or 4 (between A3-A3 ′ and A4-A4 ′). When the shield potential is fixed in this way, a piezoresistive element (A3-A3 ′ or A4) in which a low-potential wiring is arranged in the comparative example 1 near a specific piezoresistive element.
It is inevitable that the number of wires increases in the vicinity of the piezoresistive element in the vicinity of -A4 '.
This becomes a factor of mechanical instability, and it becomes difficult to secure a space for wiring, which becomes an obstacle to miniaturization.

If the input voltage at the electrode pad 24-V is constant, the potential difference between the midpoint A and the midpoint B can be corrected as the initial offset value. That is, the shield wiring potential is not balanced. However, it is desirable that the input voltage at the electrode pad 24-V can be arbitrarily set within a certain voltage range, for example, a range of 2.5 to 5V in accordance with the convenience of the user. In this case, since the potential difference between the midpoint A and the midpoint B changes depending on the input voltage, it cannot be corrected with a constant value. Therefore, as disclosed in the embodiments, it is necessary to balance the shield wiring potential.

In FIG. 1, the bridge circuit for detecting the acceleration of each axis of X, Y, and Z shares the electrode pad 24-V on the high potential side (input voltage side) and the electrode pad 24-G on the low potential side. The common use contributes to miniaturization of the acceleration sensor element.

A silicon nitride film (Si ₃ N ₄ ) was used as the protective layer 37. As a result, a structure with excellent resistance to chemicals such as potassium hydroxide (KOH) used for wet etching of Si was obtained. In general, it is said that a silicon nitride film (Si ₃ N ₄ ) is easily charged,
Although there is a concern that it may become an electrical noise source for the bridge circuit, the influence of charging can be avoided by using the shield wiring of the present invention. Further, it is desirable to adopt a bridge circuit configuration in consideration of the resistance of the metal wiring 31.

In the embodiment, the beam is a straight beam. However, for example, a non-linear beam shape (a stress buffering portion 4 on the beam portion) is used.
The present invention can also be applied to an acceleration sensor element having a beam shape having zero. FIG. 26 illustrates metal wiring.

(Example 2)
FIG. 13 shows a plan view of the acceleration sensor element 9 of the second embodiment. The acceleration sensor element 9
Is different from the acceleration sensor element of the first embodiment only in the wiring structure. Therefore, an acceleration sensor using the acceleration sensor element can have the shape shown in FIGS. 19 and 20 or FIGS.
14 to 16 show the triaxial acceleration sensor internal bridge circuit according to the second embodiment divided for each axis. In an actual acceleration sensor, these three circuits are arranged in the same plane so as to intersect with each other as appropriate so as not to cause a short circuit.
Since the electrical characteristics of this example are the same as those of Example 1, the description in Table 1 is omitted.
The description is also omitted.
In the second embodiment, the number of wires is four at each piezoresistive element position, which is the same as in the first embodiment. However, the number of wires is two at the center of the beam, and the stability of the mechanical characteristics is as follows. It is higher than Example 1.

(Comparative Example 2)
17 and 18 show Comparative Example 2. Comparative Example 2 is common to the acceleration sensor element 9 of Example 2 except for some wiring. Therefore, the mechanical stability is equivalent to that of Example 2.
However, on the piezoresistive elements 14-1b, 14-2a, 14-4a, and 14-4b, the portion protruding from the metal wiring is extended as a shield. It does not constitute a part of the bridge circuit.
Table 1 (c) shows the electrical characteristics of Comparative Example 2. As shown in Table 1 (c), in the comparative example, since the balance of the shield potential is not ensured in the bridge circuit for Y-axis acceleration detection, a potential difference between the midpoint A and the midpoint B occurs. That is, in the wiring configuration of Comparative Example 2, electrical stability cannot be ensured.

The configuration of the example and the comparative example is that the piezoresistive element position is confirmed by observation means such as composition analysis and infrared transmission, the positional relation between the piezoresistive element and the wiring is confirmed by observing the wafer cross section, and the piezoresistor is confirmed by checking the circuit pattern. This can be easily confirmed by determining the positioning of the shield located on the element in the bridge circuit.

It is a top view which shows the structure of the acceleration sensor element of Example 1. FIG. FIG. 2 is a cross-sectional view (a) showing a h-h ′ cross-section of FIG. 1 according to the first embodiment and a plan view (b) of an acceleration sensor element. 1 is a plan view showing the periphery of a piezoresistive element of the present invention, a partially enlarged plan view, and a cross-sectional perspective view. It is the figure which extracted the bridge circuit of the X-axis acceleration detection of the acceleration sensor element of Example 1. FIG. It is the figure which extracted the bridge circuit of the Y-axis acceleration detection of the acceleration sensor element of Example 1. FIG. It is the figure which extracted the bridge circuit of the Z-axis acceleration detection of the acceleration sensor element of Example 1. FIG. It is a partial cross section figure which shows the circuit diagram and operation mode of the bridge circuit of the X-axis acceleration detection of the acceleration sensor element of this invention. It is a partial cross section figure which shows the circuit diagram and operation mode of the bridge circuit of the Y-axis acceleration detection of the acceleration sensor element of this invention. It is a partial cross section figure which shows the circuit diagram and operation mode of the bridge circuit of the Z-axis acceleration detection of the acceleration sensor element of this invention. FIG. 2A is a diagram illustrating a vicinity of a piezoresistive element of the acceleration sensor element according to the first embodiment, and FIG. FIG. 5A is a diagram illustrating a vicinity of a piezoresistive element of the acceleration sensor element of Comparative Example 1, and FIG. FIG. 5A is a diagram illustrating a vicinity of a piezoresistive element of the acceleration sensor element of Comparative Example 1, and FIG. It is a top view which shows the structure of the acceleration sensor element of Example 2. FIG. It is the figure which extracted the bridge circuit of the X-axis acceleration detection of the acceleration sensor element of Example 2. FIG. It is the figure which extracted the bridge circuit of the Y-axis acceleration detection of the acceleration sensor element of Example 2. FIG. It is the figure which extracted the bridge circuit of the Z-axis acceleration detection of the acceleration sensor element of Example 2. FIG. 10 is a plan view showing a structure of an acceleration sensor element of Comparative Example 2. FIG. It is the figure which extracted the bridge circuit of the Y-axis acceleration detection of the acceleration sensor element of the comparative example 2. It is a disassembled perspective view of a 3-axis acceleration sensor. FIG. 20 is a cross-sectional view taken along the line h-h ′ in FIG. 19 and a plan view of the acceleration sensor element. It is a perspective view which shows the structure of the acceleration sensor element of a 3-axis acceleration sensor. It is a disassembled perspective view (b) which shows the structure (a) of the MEMS assembly of a 3-axis acceleration sensor. It is a transmission perspective view of a 3-axis acceleration sensor. FIG. 24 is a cross-sectional view of the three-axis acceleration sensor A-A ′ of FIG. 23. FIG. 24 is a cross-sectional view of the B-B ′ triaxial acceleration sensor of FIG. 23. It is sectional drawing. It is a top view which shows the periphery of the other piezoresistive element of this invention.

1 case,
3 Regulatory plate,
4 Chip terminal,
5 wires,
6 Case terminal,
7 External terminal
8 Case lid,
9 Acceleration sensor element,
10 Support frame,
11 weight,
13 (13-1a to 13-4b) X-axis acceleration detecting piezoresistive element,
14 (14-1a to 14-4b) Y-axis acceleration detecting piezoresistive element,
15 (15-1a to 15-4b) Z-axis acceleration detecting piezoresistive element,
16 Adhesive,
17 Adhesive,
20 Accelerometer,
21 MEMS assembly,
22 Upper cap tip,
23 Lower cap tip,
24, 24 '(24-V, 24-G,) electrode pad,
24-x, 24-y, 24-z electrode pads,
25 IC for detection,
26a, 26b, 26c, 26d lead frame,
27 wires,
29 Mold resin,
31 metal wiring,
32 Shield wiring,
33, 33a, 33b, 33c, 33d Beam part,
35 joints,
36 1st insulating film 37 2nd insulating film (protective layer),
38 High concentration diffusion layer wiring,
40 stress buffer,
41 1st layer,
42 second layer,
43 First side,
44 Second side,
45 Third side,
50 space part,
61a Upper die attach film,
61b Lower die attach film.

Claims (7)

A frame portion, a flexible portion, and a weight portion held by the frame portion via the flexible portion;
It has metal wiring and high-concentration diffusion layers used for connecting the piezoresistive element provided in the flexible part and the chip terminal provided in the frame part,
An acceleration sensor having a bridge circuit composed of the piezoresistive element, metal wiring, and high-concentration diffusion layer for each of three orthogonal acceleration detection axes,
An acceleration sensor, wherein a shield wiring is disposed on a piezoresistive element via an insulating film, and the shield wiring forms a part of a bridge circuit as a part of the metal wiring. 2. The acceleration according to claim 1, wherein the potential of the shield wiring in the bridge circuit is any two or more of a maximum potential, a minimum potential, or an intermediate potential between the two in the bridge circuit. sensor. The bridge circuit is composed of four sets of piezoresistive elements,
The combination of the potentials of the shield wiring corresponding to the two sets of high-potential side piezoresistive elements is the same between the high-potential side piezoresistive elements,
3. The acceleration sensor according to claim 1, wherein the combination of the potentials of the shield wiring corresponding to the two sets of piezoresistive elements on the low potential side is the same between the low potential side piezoresistive elements. The acceleration sensor according to any one of claims 1 to 3, wherein the number of metal wirings passing through the beam portion is the same in all the beam portions. The acceleration sensor according to any one of claims 1 to 3, wherein the number of metal wirings passing through the beam portion is three in all the beam portions. The acceleration sensor according to any one of claims 1 to 3, wherein the metal wiring is protected by an insulating film except for a local through hole, and the insulating film is a silicon nitride film. 4. The acceleration sensor according to claim 1, wherein the bridge circuit shares at least one of a high potential side chip terminal and a low potential side chip terminal between different bridge circuits.

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