DE9320965U1 - Semiconductor accelerometer - Google Patents

Description

5 May 1995 Hitachi, Ltd. N 17294 Gbm A/La/sz

Documents required for registration of the utility model
are to be based on

Semiconductor accelerometer

is The present invention relates to a semiconductor accelerometer and, in particular, to a semiconductor accelerometer suitable, for example, for actuating an airbag in a motor vehicle and for controlling a motor vehicle.

Fig. 17 illustrates an example of a capacitance type semiconductor accelerometer as illustrated in U.S. Patent No. 5,095,752, wherein a weight 5 for sensing acceleration is formed in a silicon plate 1 by etching while being supported by a cantilever portion 10 connected to the outer frame portion of the silicon plate 1, the silicon plate 1 is sandwiched between glass plates 4 and 9 with a bonding material, stationary electrodes 13 are formed on the respective surfaces opposite to the two main surfaces of the weight 5, which serves as a movable electrode in response to acceleration acting thereon, and a capacitance change caused thereby is transmitted to a signal processing unit (not shown) via bonding pads 8.

In the above capacitance type semiconductor accelerometer, the center of gravity, indicated by a star, of the weight 5 is designed to be located on a parallel plane containing a central line in the direction of the thickness of the cantilever portion 10 s.

JP-3-214064(1991) also discloses a semiconductor accelerometer that uses piezoresistors to detect acceleration and has a similar weight configuration as the above.

In the above conventional art, the mass portion, which is the weight for sensing acceleration, was formed by processing the silicon plate itself, and microscopic processing using multiple times of two-side adjustment was required. Therefore, it was difficult to manufacture such semiconductor accelerometers with sufficient processing accuracy. For this reason, mass production of such semiconductor accelerometers was difficult.

An object of the present invention is to provide a semiconductor accelerometer that allows mass production with sufficient processing accuracy and is designed to detect accelerations ranging from a small acceleration necessary for motor vehicle body control to an acceleration of about 100 G.

The above object of the present invention is achieved by a semiconductor accelerometer having a frame portion to be mounted on an object for detection, a mass portion which converts an acceleration of the object for detection into an inertial

force and a membrane portion bridging between the frame portion and the mass portion, wherein a metal film for providing additional mass is formed on at least one surface of the mass portion.

Furthermore, the above object of the present invention is achieved by a semiconductor accelerometer having a frame portion to be mounted on an object for detection, a mass portion that converts an acceleration of the object for detection into an inertial force, and a diaphragm portion bridging between the frame portion and the mass portion, wherein a metal film for providing additional mass is formed on both the upper and lower surfaces of the mass portion.

is Still further, the above object of the present invention is achieved by a semiconductor accelerometer having a frame portion to be mounted on an object for detection, a mass portion that converts an acceleration of the object for detection into an inertial force, and a diaphragm portion bridging between the frame portion and the mass portion, wherein an additional mass having a higher density than that of the mass portion is provided on at least one surface of the mass portion.

The above object of the present invention is achieved by a semiconductor accelerometer having a frame portion to be mounted on an object for detection, a mass portion that converts an acceleration of the object for detection into an inertial force, and a diaphragm portion bridging between the frame portion and the mass portion, wherein an additional

Mass is arranged on at least one surface of the mass portion in such a way that the center of gravity of the weight formed from the mass portion and the additional mass is located on a parallel plane containing a central line in the direction of the thickness of the membrane portion.

In the present invention, the weight for sensing acceleration is composed of a combination of the mass portion formed by deep etching of the silicon plate and an additional mass,

γ formed on the mass portion, eliminating the conventional microscopic processing requiring multiple, two-sided alignment, thereby obtaining a semiconductor accelerometer having high processing accuracy. Further, the density and configuration of the additional mass formed on the

is mass portion is selected so that the center of gravity of the weight formed by the mass portion and the additional mass is located on a parallel plane containing a central line in the direction of the thickness of the diaphragm portion, thereby facilitating zeroing of output signals from the resulting semiconductor accelerometer according to the present invention.

Further advantages, properties and possible applications of the present invention emerge from the following description of embodiments in conjunction with the drawing. In the drawing:

Fig. 1 is a cross-sectional view showing a first embodiment of piezo-resistive type semiconductor accelerometers according to the present invention;

Fig. 2 is a plan view of the semiconductor accelerometer shown in Fig. 1;

Fig. 3 is a cross-sectional view showing a second embodiment of piezo-resistive type semiconductor accelerometers according to the present invention;

Fig. 4 is a cross-sectional view showing a third embodiment of piezo-resistive type semiconductor accelerometers according to the present invention;

Fig. 5 is a circuit diagram applied to the semiconductor accelerometers shown in Fig. 1, Fig. 3 and Fig. 4 for processing signals from a bridge circuit of piezo-resistive elements formed on the semiconductor accelerometer;

Fig. 6 (a) is a plan view schematically illustrating the layout of the circuit elements shown in Fig. 5 on a semiconductor substrate;

Fig. 6 (b) is a cross-sectional view of the semiconductor substrate shown in Fig. 6 (a).

Fig. 7 is an enlarged cross-sectional view of the circled portion in Fig. 6(b);

Fig. 8 is a circuit diagram for processing signals from three sets of semiconductor accelerometers shown in any of Figs. 1, 3 and 4 to determine three-dimensional acceleration;

Fig. 9 is a cross-sectional view showing a first embodiment of capacitance type semiconductor accelerometers according to the present invention;

Fig. 10 is a cross-sectional view showing a second embodiment of capacitance type semiconductor accelerometers according to the present invention;

Fig. 11 is a perspective view showing a third embodiment of capacitance type semiconductor accelerometers for detecting three-dimensional acceleration and which is integrally formed according to the present invention;

Fig. 12 is a circuit diagram for processing signals from the is capacitance type semiconductor accelerometer shown in Fig. 11, for determining three-dimensional acceleration;

Fig. 13 is a plan view showing a fourth embodiment of a piezo-resistive type semiconductor accelerometer according to the present invention;

Fig. 14 is a plan view showing a fifth embodiment of a piezo-resistive type semiconductor accelerometer according to the present invention;

Fig. 15 shows a nonlinearity error of a semiconductor accelerometer without a center of mass adjusting measure with respect to the variation of an acceleration component along the detection axis;

Fig. 16 shows a non-linearity error of a semiconductor accelerometer with a center of mass adjusting measure according to the present invention with respect to the variation of an acceleration component along the detection axis; and

Fig. 17 is a cross-sectional view of a conventional capacitance type semiconductor accelerometer.

A first embodiment of piezo-resistive type semiconductor accelerometers according to the present invention will be explained with reference to Figs. 1 and 2.

A mass section 5, a frame section 3 and a membrane section 6, which is arranged between the mass section 5 and the frame section 3

is bridged, for a piezo-resistive type semiconductor accelerometer is formed from a single crystalline silicon plate 1 by deep etching it. The movement of the mass portion 5 in response to acceleration is balanced in combination with the length and thickness of the diaphragm portion 6. Further, the balancing is controlled via a combination of the mass portion 5 and an additional metal film 7 formed on the mass portion 5 in the present embodiment. Piezo-resistive elements 21, 22, 23 and 24 are formed at predetermined positions on a main surface of the diaphragm portion 6.

In a conventional capacitance type semiconductor accelerometer, as illustrated in Fig. 17, the mass portion is shaped in such a manner that the center of gravity of the mass portion, as shown by a star, is positioned on a parallel plane containing the center in the direction of the thickness of the cantilever 10, in

In other words, the center line of the mass portion 5 in the direction of thickness thereof is aligned with that of the cantilever 10 in its direction of thickness to keep a linearity or hysteresis and a sensitivity of outputs of the semiconductor accelerometer at predetermined levels.

However, the configuration of the mass portion 5 as shown in Fig. 17 required four to five times of dual side alignment for performing a photolithographic process on a semiconductor wafer, which is a very complex microscopic processing, and made it very difficult to maintain a predetermined dimensional accuracy on both sides of the wafer.

Furthermore, although single-side stepping type trimmers are generally used for manufacturing semiconductors such as ultra-large scale integration (ULSI), there are no two-side stepping type trimmers, so a contact type two-side trimmer, which is unsuitable for mass production, e.g., forming hundreds of chips on single wafers, had to be used for the production of semiconductor accelerometers having the above configuration, which requires highly accurate pattern trimming several times on both the front and back sides of the wafer.

In contrast to power semiconductors such as gate-turn thyristors (GTO thyristors) which are configured via double-side exposure on a wafer representing a single chip by using a contact-type two-side adjuster, several hundred semiconductor sensors such as semiconductor accelerometers which have a three-dimensional configuration must be configured in a form of chips on

a single wafer, therefore, the double-side adjuster and the process used for producing the power semiconductors can hardly be used for producing semiconductor sensors, furthermore, incident angles and aberration of an exposure light on a chip at the center of the wafer and on a chip at the edge portion of the wafer vary significantly, whereby accuracy of the exposed pattern on respective chips is exponentially deteriorated by frequent double-side adjustment operations, which prevents mass production of semiconductor accelerometers having a &iacgr;&ogr; configuration requiring a plurality of double-side adjustment operations.

However, the molding of the silicon plate 1 as shown in Fig. 1 is carried out with one-time two-side adjustment like conventional semiconductor pressure sensors mass-produced nowadays, further a deep etching process from one side of the wafer is also required only once to complete the configuration. Further, in order to maintain the predetermined sensor characteristics such as linearity of output and sensitivity, the additional metal film 7 made of such as Au and having a corresponding weight to that of the mass portion 5 formed by deep etching is formed on the mass portion 5 in such a manner that the center of gravity of the weight formed in combination of the mass portion 5 and the additional metal film 7 is located on a parallel plane containing a center line of the diaphragm portion 6 in the direction of thickness.

The silicon plate 1 thus configured is machined onto a lower glass plate 4 by eye, such as by anodic bonding, to facilitate mounting onto the semiconductor accelerometer.

When a chip size of the semiconductor accelerometer as shown in Fig. 2 is reduced to a square of several mm such as 3 mm ² , in order to have a limited weight of the mass portion, the thickness of, e.g., the diaphragm portion 6 must be reduced so that the limited weight can respond to acceleration, but the thickness of the diaphragm portion 6 cannot be reduced below a predetermined thickness for maintaining the mechanical strength.

Fig. 3 shows a second embodiment of piezo-resistive type semiconductor accelerometers according to the present invention which eliminates the need for reducing the thickness of the diaphragm portion 6 as explained above, wherein the same or equivalent elements as in the embodiment of Fig. 1 and Fig. 2 are designated by the same reference numerals. In the embodiment of Fig. 3, a second additional metal film 11 is formed on the back of the mass portion 5 and the thickness of the first additional metal film 7 is determined to compensate for the combined weight of the mass portion 5 and the second additional metal film 11.

Fig. 4 shows a third embodiment of piezo-resistive type semiconductor accelerometers according to the present invention, wherein the same or equivalent elements as in the embodiment of Fig. 1 and Fig. 2 are designated by the same reference numerals. In the embodiment of Fig. 4, a recess 16 is provided on the back surface of the mass portion 5 for adjusting the weight and balance formed in combination of the mass portion 5 and the first additional metal film 7.

Fig. 5 is a circuit diagram comprising a bridge circuit 2 constructed by piezo-resistive or diffusion type strain gauges 21, 22, 23 and 24 formed on the diaphragm portion 6 of the semiconductor accelerometer such as shown in Fig. 1 to Fig. 4, and a signal processing circuit 40 for signals from the bridge circuit.

Fig. 6 (a) is a plan view illustrating a layout of the circuit elements shown in Fig. 5 formed on a common semiconductor substrate 1 for the semiconductor accelerometer as shown in Figs. 1 to 4 and the signal processing circuit 40 for processing the signals from the bridge circuit 2 formed on the semiconductor accelerometer portion.

Fig. 6(b) is a cross-sectional view of the integrated semiconductor body shown in Fig. 6(a).

Fig. 7 is an enlarged and detailed cross-sectional view of the circled portion of Fig. 6 (b), wherein in addition to the piezo resistors 21 to 24, a parallel resistor for an amplifier and an npn transistor for the amplifier in the signal processing circuit 40 are illustrated.

Fig. 8 is a circuit diagram for determining three-dimensional accelerations V ₁ , V ₂ and V ₃ in x, y and z directions by using three sets of bridge circuits 2, 2' and 2", which constitute a detection unit 30. The signal processing circuit 40 is a conventional one which includes an amplifying unit 41, a PWM unit 42, a processing and output unit 43, an oscillator 44 and a clock generator 45 which generates clock signals ^ ₁ to §5 for phase shifting

the detection signals generated by the respective bridge circuits 2, 2' and 2".

Fig. 9 is a first embodiment of capacitance type semiconductor accelerometers according to the present invention, wherein the same or equivalent elements as those shown in Fig. 17 are designated by the same reference numerals. In the embodiment of Fig. 9, the mass portion 5 is formed by deep etching the silicon plate 1 from one side and the additional metal film 7 is formed on the other side of the mass portion 5 in the same manner as in the embodiment shown in Figs. 1 and 2, thereby facilitating the microscopic processing for configuring the silicon plate 1.

Fig. 10 is a second embodiment of capacitance type semiconductor accelerometers according to the present invention, wherein similar or equivalent elements to those shown in Fig. 17 are designated by the same reference numerals. In the embodiment of Fig. 10, the second additional metal film 11 is provided on the back surface of the mass portion 5 in addition to the first additional metal film 7 in the same manner as in the embodiment shown in Fig. 3.

Fig. 11 is a perspective view, partially broken away, of a third embodiment of capacitance type semiconductor accelerometers comprising four semiconductor accelerometer units integrally formed for detecting three-dimensional acceleration, wherein the same or equivalent elements as in Fig. 9 are designated by the same reference numerals, and

the upper glass plate 9 is removed for a clear illustration of the structure.

Fig. 12 is a signal processing circuit 50 of the signals φ ₁ to φ ₄ from the capacitance type semiconductor accelerometer shown in Fig. 11 and having four semiconductor accelerometer units C ₁ , C ₂ , C ₃ and C ₄ for determining three-dimensional accelerations V ₁ , V ₂ and V ₃ in x, y, z directions. The signal processing circuit 50 is a usual one which includes an AC detection unit 51, an amplifying unit 52, a PWM unit 53, a processing and output unit 54, an oscillator 55 and a clock generator which generates clock signals φ ₁ to φ ₅ for phase shifting the detection signals _φ 1 to φ ₄ from the respective semiconductor accelerometers C ₁ to C ₄ .

Fig. 13 is a plan view of a fourth embodiment of piezo-resistive type semiconductor accelerometers according to the present invention, wherein the same or equivalent elements as in Fig. 1 and Fig. 2 are designated by the same reference numerals. In the embodiment of Fig. 13, a pair of communicating holes 14 are provided on the diaphragm portion 6 on both sides of the diffusion type strain gauges 21 and 23 for allowing communication between the upper and lower spaces separated by the diaphragm 6 and the mass portion 5 to enhance movement of the weight formed in combination of the mass portion 5 and the additional metal film 7 in response to accelerations acting thereon.

The location and configuration of the communication holes 14 are not limited to those illustrated in Fig. 13, but any locations

and configurations can be used if this does not disturb a balanced movement of the weight in response to accelerations acting on it.

Fig. 14 is a plan view of a fifth embodiment of piezo-resistive type semiconductor accelerometers according to the present invention, wherein the same or equivalent elements as in Figs. 1 and 2 are designated by the same reference numerals. In the embodiment of Fig. 14, reinforcing portions 15 having an increased thickness are provided across the diaphragm portion 6 at positions where the diffusion type strain gauges 21 to 24 are formed, thereby reinforcing the mechanical strength of the diaphragm portion 6 in case the thickness of the diaphragm portion 6 is reduced.

Fig. 15 and Fig. 16 are graphs for explaining an improvement of non-linearity errors by the present invention.

Fig. 15 shows a non-linearity error of a semiconductor accelerometer without additional metal film as schematically illustrated in the graph when the schematically illustrated semiconductor accelerometer is rotated in a clockwise direction, then rotated back in a counterclockwise direction as indicated by arrows, to vary the acceleration components Fp(G) in the sensing axis. As can be seen from the graph, a comparatively large non-linearity error is observed.

Fig. 16 shows a non-linearity error of a semiconductor accelerometer with an additional metal film 7 according to the present invention, as schematically illustrated in the graph when the

schematically illustrated semiconductor accelerometer is rotated clockwise, then rotated back counterclockwise as indicated by arrows to vary the acceleration components Fp(G) in the sensing axis. As can be seen from the graph, the non-linearity error is essentially eliminated.

In the embodiments shown in Fig. 9, Fig. 10 and Fig. 11, the upper and lower glass plates 4 and 9 are used, but the glass plates 4 and 9 may be replaced by silicon plates.

According to the semiconductor accelerometer explained here, since the center of gravity of the weight formed by combining the mass portion and the additional metal film can be designed to be located in a region of the mass portion defined by an extension of the diaphragm portion or the cantilever portion and having a depth corresponding to the thickness of the diaphragm portion or the cantilever portion, zero-point adjustment of the detection signals outputted therefrom can be easily performed and acceleration detection accuracy is improved.

Since the semiconductor accelerometers according to the present invention can detect acceleration in the range from a low acceleration used for a motor vehicle body control to an acceleration of about 100G, the semiconductor accelerometers according to the present invention can be combined with an air bag control system as well as with a vehicle body control system in a motor vehicle with high accuracy.

According to the present invention, semiconductor accelerometers having a small nonlinearity output and high sensitivity, suitable for mass production, are obtained.

Claims (1)

5 May 1995 Hitachi, Ltd. N 17294 Gbm A/La/sz Protection claims 1. A semiconductor accelerometer comprising: a mass portion formed at a center of a silicon plate; a frame portion formed around the periphery of the silicon plate to surround the mass portion; a diaphragm portion formed in the silicon plate between the mass portion and the frame portion to bridge the mass portion to the frame portion, wherein one of the major surfaces of the silicon plate serves as a common continuous major surface for the mass portion, the frame portion and the diaphragm portion; Piezo-resistive elements formed on the common continuous main surface on the diaphragm portion; and an additional metal film having a higher density than that of the silicon plate formed on the common continuous main surface at the mass portion, the additional metal film in combination with the mass portion of the silicon plate representing a weight which acts on a load acting thereon. · ·· acceleration, and wherein the mass of the additional metal film is selected in such a way that the center of gravity of the weight formed in combination of the mass portion and the additional metal film is located in a region in the mass portion having a depth corresponding to the thickness of the membrane portion and which is defined by an extension of the membrane portion. 2. A semiconductor accelerometer according to claim 1, wherein the additional metal film is made of Au. 3. A semiconductor accelerometer according to claim 1, further comprising: is Another additional metal film formed on the other main surface of the mass portion for adjusting the mass of the weight. 4. A semiconductor accelerometer according to claim 1, further comprising: A recess formed on the other major surface of the mass portion for adjusting the mass of the weight. 5. A semiconductor accelerometer according to claim 1, further comprising: A plurality of communicating holes formed on the membrane portion to facilitate communication between the spaces separated by the membrane section and the mass section. 6. A semiconductor accelerometer according to claim 1, wherein the portions of the diaphragm portion where the piezo-resistance elements are formed are reinforced by increasing the thickness of the diaphragm portion. 7. A semiconductor accelerometer comprising: a mass portion formed at the center of a silicon plate; a frame portion formed around the periphery of the silicon plate to surround the mass portion; a cantilever portion formed in the silicon plate between the mass portion and the frame portion to bridge the mass portion with the frame portion, wherein one of the major surfaces of the silicon plate serves as a common continuous major surface for the mass portion, the frame portion and the cantilever portion; an additional metal film having a higher density than that of the silicon plate formed on the common continuous major surface at the mass portion, the additional metal film in combination with the mass portion of the silicon plate constituting a weight responsive to an acceleration acting thereon, and the mass of the additional metal film being selected in such a way that the center of gravity the weight formed in combination of the mass portion and the additional metal film is located in a region in the mass portion having a depth corresponding to the thickness of the cantilever portion and which is defined by an extension of the cantilever portion; another additional metal film formed on the other main surface of the mass portion for adjusting the mass of the weight; a first glass plate provided with an electrode thereon, the first glass plate being connected to the silicon plate through the frame portion to oppose the electrode thereof to the additional metal film formed on the mass portion with a predetermined opening; and a second glass plate provided with an electrode thereon, the second glass plate being connected to the silicon plate through the frame portion to face the electrode thereon to the other main surface of the mass portion with a predetermined opening. 8. A semiconductor accelerometer according to claim 7, wherein the additional metal film is made of Au.