JP3066211B2 - Semiconductor acceleration sensor - Google Patents

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 having a self-diagnosis function.

[0002]

2. Description of the Related Art Conventionally, a semiconductor acceleration sensor uses a Newton's equation of motion F = ma (F is a force, m is a mass, and a is an acceleration), and detects acceleration a from the amount of displacement of the sensor based on the force F. Such a two-dimensional or three-dimensional semiconductor acceleration sensor is extremely important, for example, as a sensor for controlling the operation of an airbag, which is one of safety devices for automobiles.

In this case, in order to improve reliability, Japanese Patent Laid-Open No.
As described in -200038 (GO / L 25/00) and the like, the sensor has a self-diagnosis function utilizing electrostatic attraction. Next, a conventional diaphragm type three-dimensional semiconductor acceleration sensor having a self-diagnosis function will be described with reference to FIGS. 1, 3, and 4 corresponding to one embodiment of the present invention.

As shown in FIG. 1, lower surfaces of a plurality of glass fixing legs 2 having a height (thickness) of, for example, about 1.5 mm are electrostatically pressed on a silicon pedestal 1, and a sensor is mounted on the upper surfaces of these fixing legs 2. A silicon sensor chip 3, which is a main body, is mounted on the frame portion 4 by electrostatic pressure bonding.

In the sensor chip 3, the space between the projection 5 and the frame 4 at substantially the center is etched from the lower surface to form a diaphragm 6 as a thin portion. A rectangular parallelepiped weight body 7 of a semiconductor such as silicon is attached by electrostatic pressure bonding so as not to contact the pedestal 1, and the weight section 8 supported by the diaphragm section 6 by the projection section 5 and the weight body 7 is formed. Have been. In the actual manufacturing process, the fixing leg 2 and the weight 7 are electrostatically pressed to the frame portion 4 and the projection 5, and then the pedestal 1 is electrostatically pressed and attached.

When the weight 7 is made of glass, the fixing legs 2 and the weight 7 are formed, for example, as described below in order to simplify the manufacturing process. That is, as shown in FIG. 3, a glass plate 9 having the same area as the sensor chip 3 and a thickness of about 1.5 mm is prepared.
Is adhered to the frame portion 4 and the projection portion 5 by electrostatic crimping, and then the vertical and horizontal separation grooves 10 of the glass plate 9 are diced to fix the fixing legs 2.
And the weight body 7 are separately formed, and the lower surface of the weight body 7 is etched by about 10 μm, whereby each fixed leg 2 and the weight body 7 are formed.

Since the weight 8 is supported by the diaphragm 6 so as to be movable up, down, left and right, when acceleration is applied to the sensor, the weight 8 fluctuates up, down, left, and right in accordance with the direction and magnitude of the acceleration. Due to this change, the diaphragm 6 is displaced by the stress strain.

The sensor chip 3 has a crystal plane orientation (001) in consideration of the crystal orientation dependence of the piezoresistance effect described later.
As shown in FIG. 4, each of the four X electrodes is formed on the upper surface of the diaphragm portion 6.
Piezoresistors 11, 12, and 13 for detecting acceleration in the axial direction, the Y-axis direction, and the Z-axis direction are radially formed by a thermal diffusion method, an ion implantation method, or the like.

At this time, four piezo resistors 11 to 11 of each axis are used.
13 are actually linearly arranged along a total of three directions, that is, two directions perpendicular to the <110> direction and the substantially <111> direction that exhibit a large piezoresistance effect on the (110) plane.

Then, four piezo resistors 11 to 1 of each axis are provided.
Reference numeral 3 denotes a DC connection which is bridge-connected by aluminum wiring or the like for each axis, and when the diaphragm 6 is displaced by acceleration in the Z-axis direction (vertical direction), the balance of the bridge of the piezoresistor 13 is broken and acceleration is applied to the bridge. When a corresponding output (voltage) is generated and the piezoresistors 11 and 12 are displaced so as to be twisted by the acceleration in the X-axis direction and the Y-axis direction, an output corresponding to the acceleration is generated in each bridge of the piezoresistors 11 and 12. Therefore, three-dimensional acceleration is independently detected for each axis by the bridge of the piezoresistors 11 to 13.

If the displacement of the weight 8 becomes too large, the diaphragm 6 is damaged and the sensor breaks down. Therefore, the cap 14 of a semiconductor substrate such as a glass substrate or a silicon substrate is provided on the sensor chip 3. Are attached by electrostatic crimping or the like. The lower surface of the cap portion 14 substantially facing the weight portion 8 and the diaphragm portion 6 is etched and shaved, and the displacement of the weight portion 8 is regulated by the cap portion 14 and the pedestal 1.

Further, in order to provide a self-diagnosis function, the piezoresistors 11 are provided on the upper surfaces of the projection 5 and the diaphragm 6.
Chip-side self-test electrode 1 so as not to overlap
5 is formed by diffusion, and a cap-side self-test electrode 16 made of a metal thin film such as a gold thin film or an aluminum thin film is formed on the lower surface of the cap portion 14 at a position facing the chip-side self-test electrode 15. The interval between the electrodes 15 and 16 is several μm, which is almost equal to the gap between the pedestal 1 and the weight 7.
When the sensor chip 3 is formed of an N-type silicon substrate, each of the piezoresistors 11 to 13 and the chip-side self-test electrode 15 are formed in a P-type by diffusion of P ⁺ impurities.

When the diagnosis is performed only in the Z-axis direction, a DC test voltage is applied between the self-test electrodes 15 and 16 using the chip-side self-test electrode 15 as a single electrode.
F∝V ² / d based on the surface potential of both electrodes 15 and 16
^{2 A} predetermined acceleration is applied in the Z-axis direction (vertical direction) by an electrostatic attraction F (V is an applied voltage, d is a distance between the electrodes), and a Z-axis piezoresistor 13 bridge output obtained at this time is applied. Pass / fail is detected.

When the diagnosis is made in each of the three axes of X, Y, and Z, a torsional displacement is generated in the diaphragm portion 6 at the time of diagnosis in the X-axis direction and the Y-axis direction, and the piezoresistors 12, 1 are generated.
In order to obtain the bridge output of No. 3, the chip-side self-test electrode 15 is, for example, patterned electrodes 15a to 15d shown in FIG.
Is formed by dividing into four.

The pattern electrodes 15a to 15a
15d, and one or more of the selected pattern electrodes 15a to 15d and the cap-side self-test electrode 16 are selected.
And a test voltage is applied between them, and the quality of the test is detected from the bridge outputs of the piezo resistors 11 to 13 of the diagnostic axis obtained at this time.

[0016]

In the case of this type of conventional semiconductor acceleration sensor having a self-diagnosis function, a silicon dioxide (SiO ₂ ) film and a silicon nitride film (SiN) are formed on the upper surface of the sensor chip 3 as passivation films for surface protection. 5), at least the protrusion 5 and the diaphragm 6 are formed by the insulating film 17 as shown in FIG.
The whole is covered.

At this time, the insulating film 17 is
Since it is formed so as to cover 5a to 15d, at the time of self-diagnosis, for example, any one of the pattern electrodes 15a to 15d
Even if a test voltage is applied between the cap-side self-test electrode 16 and the cap-side self-test electrode 16, the entire insulating film 17 is charged and all the pattern electrodes 15a to 15d and the cap-side self-test electrode 1
6 is equivalent to applying a test voltage.

Therefore, the chip-side self-test electrode 1
5 is divided into the pattern electrodes 15a to 15d, the chip-side self-test electrode 15 cannot be divided and driven by selectively applying a voltage to the pattern electrodes 15a to 15d. There is a problem that self-diagnosis cannot be performed.

Note that, even when two sets of piezoresistive bridges are provided to detect accelerations in, for example, two axes, the X-axis direction and the Y-axis direction, the chip-side self-test electrode is divided into a plurality of pattern electrodes. When the self-diagnosis is performed for each axis, the same problem as described above occurs.

In this type of semiconductor acceleration sensor, the voltage applied to the cap-side self-test electrode 16 is applied to the electrode provided on the sensor chip 3. Therefore, as shown in FIG. On the upper surface of the end portion away from the upper surface 6, a cap extraction electrode 18 of a metal thin film such as an aluminum thin film is laminated and formed via an insulating film 17, and this electrode 18 is brought into contact with the cap-side self-test electrode 16. There is.

In this case, a test voltage can be applied to the cap-side self-test electrode 16 through the extraction electrode 18, but the insulating film 17 below the extraction electrode 18 is damaged and a pinhole is formed, and the extraction electrode 18 is damaged. When the sensor contacts the sensor chip 3, at the time of self-diagnosis, the extraction electrode 18 and each piezoresistor 11 to 1 are connected.
There is a problem that a leak current flows between the third bridge and the output of each bridge, causing fluctuations in the offset voltage of the outputs of the respective bridges.

This problem also occurs in a one-dimensional semiconductor acceleration sensor in which the chip-side self-test electrode 15 is not divided into pattern electrodes.

An object of the present invention is to form an insulating film for surface protection on the upper surface of a silicon sensor chip so that a chip-side self-test electrode can be divided and driven, so that self-diagnosis can be performed for each axis. It is another object of the present invention to prevent the occurrence of a leak current between a cap lead-out electrode provided on a sensor chip and a piezoresistor bridge, thereby enabling accurate self-diagnosis.

[0024]

In order to achieve the above object, in the semiconductor acceleration sensor according to the present invention, in the case of claim 1, each of the divided pattern electrodes is formed by covering each piezoresistor on the upper surface of the sensor chip. An insulating film for protecting the surface is formed so as not to cover. In the case of the second aspect, a diffusion resistance layer for insulation wider than the extraction electrode of the same conductivity type as the piezo resistor is formed directly below the extraction electrode for cap on the upper surface of the sensor chip.

[0025]

In the case of the semiconductor acceleration sensor of the present invention constructed as described above, in the configuration of the first aspect, the insulating film on the upper surface of the sensor chip does not cover each pattern electrode as the chip-side self-test electrode. At this time, the insulating film is not charged by the voltage applied to each pattern electrode, the test voltage can be selectively applied to each pattern electrode, the chip-side self-test electrode can be divided and driven, and diagnosis can be performed for each axis. .

According to the second aspect of the present invention, even if a flaw or a pinhole is formed in the insulating film below the cap lead-out electrode, if the conductivity type of the sensor chip is N, the connection between the lead-out electrode and the piezoresistor is made. A P-type diffusion resistance layer, an N-type sensor chip, and a P-type piezoresistive PNP diode are formed therebetween.
This diode prevents the occurrence of a leak current, and enables accurate self-diagnosis.

[0027]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment will be described with reference to FIGS. In these drawings, reference numeral 19 denotes an insulating film for protecting the surface such as an oxide film and a nitride film corresponding to the insulating film 17 of FIG.
3 is formed so as not to cover the pattern electrodes 15a to 15d.

Reference numerals 20a to 20d denote each of the pattern electrodes 15a to 15d.
15d and the boundary between the insulating film 19 and the sensor chip 3
Is exposed. Reference numeral 21 denotes an insulating diffusion resistance layer formed on the upper surface of the sensor chip 3 directly below the cap lead-out electrode 18 and wider than the electrode 18.
Is formed on an N-type silicon substrate, it is formed in the same P-type as the piezoresistors 11 to 13 by diffusion of P ⁺ impurities or the like.

The insulating film 19 is uniformly formed on the upper surface of the sensor chip 3 through, for example, a drive-in process and a thermal oxidation process in the same manner as in the related art, and then the portions of the pattern electrodes 15a to 15d are removed by etching. Formed. The diffusion resistance layer 21 is simultaneously formed on the sensor chip 3 by using, for example, a silicon dioxide film as a mask when forming the piezoresistors 11 to 13.

Then, the insulating film 19 is formed on each of the pattern electrodes 15.
Since a test voltage is not applied to any one or more of the pattern electrodes 15a to 15d and a cap-side self-test electrode 16 during self-diagnosis, the insulating film 19 is not charged and the chip is not charged. The split driving of the side self-test electrode 15 can be performed.

Then, as a result of actual measurement by detecting 2 G acceleration at full scale (FS), application of a test voltage of 30 V causes a vertical displacement corresponding to the full scale on the Z axis, resulting in a sufficient displacement of 4 mV. A large bridge output was obtained, and a displacement in the torsional direction occurred in the X-axis and the Y-axis, resulting in a bridge output of about 1 mV that can be confirmed for operation. The displacement time of 95% of the full scale is about 0.3.
As short as one second, the bridge output occurs instantaneously for any axis.

On the other hand, when the insulating film 17 covers each of the pattern electrodes 15a to 15d as shown in FIG. 5, a bridge output of 4 mV is obtained for the Z axis by applying a test voltage of 30 V, but the X axis and the Y axis are obtained. No bridge output is obtained for the axis, and diagnosis cannot be performed. Then, since the chip-side self-test electrode 15 is divided and driven, the surface of the sensor chip 3 is protected by the insulating film 19 and self-diagnosis can be performed for each axis.

Next, since the diffusion resistance layer 21 is provided, a scratch or a pinhole is generated in the insulating film 19 immediately below the extraction electrode 18, and even if the extraction electrode 18 contacts the sensor chip 3, A diffusion resistance layer 21 of P ⁺ , an N-type sensor chip 3, and a PNP diode of each of the piezo resistors 11 to 13 of P ⁺ are formed between the piezo resistors 11 to 13, and this diode prevents generation of a leak current. Is done.

Therefore, there is no fluctuation in the offset voltage of the bridge output of each of the piezoresistors 11 to 13 due to the leak current, and the operation is stable and accurate self-diagnosis can be performed. And
The shape and the like of each of the pattern electrodes 15a to 15d are not limited to the embodiment. In addition, the weight 8 is formed only by
And a sensor provided with a thin portion having a cantilever configuration.

Further, in the above-described embodiment, the present invention is applied to a three-dimensional acceleration sensor. However, it is needless to say that the present invention can be applied to a two-dimensional acceleration sensor having two sets of piezoresistive bridges. The provision of the diffusion resistance layer 21 has the same effect even when applied to a one-dimensional acceleration sensor in which the chip-side self-test electrode is a single electrode.

[0036]

Since the present invention is configured as described above, the following effects can be obtained. First, in the case of the configuration of claim 1, the insulating film 19 formed on the upper surface of the silicon sensor chip 3 is
In the self-diagnosis, the insulating film 19 is not charged by applying a voltage to each of the pattern electrodes 15a to 15d.
A test voltage can be selectively applied to 15d to drive the chip-side self-test electrode 15 in a divided manner. A self-diagnosis can be performed for each axis of a two-dimensional or three-dimensional acceleration sensor, and a highly reliable self-diagnosis function is provided. 2D or 3D semiconductor acceleration sensor can be provided.

In the case of the second aspect, since the diffusion resistance layer 21 is formed on the upper surface of the silicon sensor chip 3 immediately below the extraction electrode 18 for the cap, the insulating film 19 below the extraction electrode 18 has scratches or pins. Even if a hole is formed, the extraction electrode 1
8 and the piezoresistors 11 to 13, a diffusion resistance layer 21, a sensor chip 3, and diodes of the piezoresistors 11 to 13 are formed. These diodes prevent the occurrence of a leak current and stabilize the operation at the time of self-diagnosis. , Accurate self-diagnosis can be performed.

[Brief description of the drawings]

FIG. 1 is a cutaway front view of one embodiment of a semiconductor acceleration sensor of the present invention.

FIG. 2 is a plan view of the silicon sensor chip of FIG.

FIG. 3 is an explanatory view showing a part of FIG.

FIG. 4 is an explanatory view of the arrangement of the piezoresistors in FIG. 1;

FIG. 5 is a plan view of a silicon sensor chip of a conventional sensor.

[Explanation of symbols]

 Reference Signs List 3 silicon sensor chip 6 diaphragm portion as thin portion 8 weight portion 11 to 13 piezoresistor 14 cap portion 15 chip-side self-test electrode 15a to 15d pattern electrode 16 cap-side self-test electrode 18 cap extraction electrode 19 insulating film 21 diffusion resistance layer

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Tadashi Kobayashi 5-4-1, Higashi 5-chome, Hanyu-shi, Saitama Prefecture Inside Akebono Brake Research Institute (56) Reference JP-A-3-200038 (JP, A) JP-A-5-209894 (JP, A) JP-A-4-274766 (JP, A) JP-A-3-112170 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01P 15 / 12 G01P 21/00 H01L 29/84

Claims (2)

(57) [Claims] 1. A silicon sensor chip in which a weight located substantially at the center is supported by a thin portion, a cap located above the sensor chip for restricting displacement of the weight, and a radially extending upper surface of the thin portion. A piezoresistor for detecting acceleration in a plurality of axes formed on the chip portion; and a chip-side self formed by diffusion into a plurality of pattern electrodes on the upper surface of the weight portion and the thin portion so as not to overlap the piezoresistors. In a semiconductor acceleration sensor including a test electrode and a cap-side self-test electrode formed on the lower surface of the cap portion so as to face the chip-side self-test electrode, the upper surface of the sensor chip covers each of the piezoresistors. A semiconductor acceleration sensor, wherein an insulating film is formed so as not to cover the pattern electrodes. 2. A silicon sensor chip in which a weight located substantially at the center is supported by a thin portion, a cap located above the sensor chip for restricting displacement of the weight, and a cap formed on an upper surface of the thin portion. A piezoresistor for acceleration detection, a chip-side self-test electrode diffused and formed on the upper surface of the weight portion and the thin portion so as not to overlap the piezoresistor, and a chip-side self-test on a lower surface of the cap portion. A cap-side self-test electrode formed to generate an electrostatic attraction between the electrode and an upper surface of the sensor chip separated from the weight portion and the thin portion via an insulating film; A semiconductor accelerometer having a cap lead-out electrode in contact with a self-test electrode, wherein the piezo resistor is provided on the upper surface of the sensor chip immediately below the lead-out electrode. A semiconductor acceleration sensor, wherein a diffusion resistance layer for insulation wider than the extraction electrode of the same conductivity type as the resistance is formed.