Microaccelerometer employing resonant circuit detection of seismic mass displacement

A first embodiment of an improved microaccelerometer includes a seismic mass, a support wafer, a cover wafer and a beam (or beams) for flexibly mounting a seismic mass between the support and cover wafers. A first oscillator includes a resonant circuit whose capacitance comprises conductive plates on one surface of the seismic mass and a conductive coating on an opposed surface of the support wafer. A second oscillator includes a resonant circuit whose capacitance is comprised of conductive coatings on another surface of the seismic mass and on an opposed surface of the cover wafer. A difference circuit provides an acceleration output that is dependent on a difference in oscillation frequencies between the first and second oscillators, when the accelerometer is subjected to an acceleration event. A second embodiment includes a structure similar to the aforedescribed, however, the second oscillator is replaced by an ac levitation circuit that exerts a single direction restoring force on the seismic mass during an acceleration event. A third embodiment provides ac levitational restoring forces when the seismic mass is subject to acceleration in either of two opposed directions. In the latter embodiment, ac levitating circuits are disposed on opposed surfaces of the support and cover wafers.

FIELD OF THE INVENTION 
This invention relates to microaccelerometer structures and, more 
particularly, to a microaccelerometer that employs resonant oscillating 
circuits to determine a physical displacement of a seismic mass within the 
microaccelerometer structure. 
BACKGROUND OF THE INVENTION 
Prior art microaccelerometers often employ a seismic mass that is 
cantilevered on one or more support beams. The seismic mass is displaced 
generally normal to the direction of beam extension by an accelerating 
force. To measure the amount of accelerating force, strain gauges are 
often embedded in the beam or beams which support the seismic mass and are 
connected to a bridge circuit to obtain an output proportional to an 
amount of seismic mass displacement. U.S. Pat. No. 5,060,504 to White et 
al., discloses such a microaccelerometer. White et al. further employ a 
conductive coating on one surface of the seismic mass and a similar 
coating on an adjacent fixed surface to enable application of a dc voltage 
therebetween. A resulting electrostatic field is established between the 
seismic mass and the adjacent fixed surface which tends to restore the 
mass to a null position. Those skilled in the art will realize that 
positioning through use of applied dc potentials to create electrostatic 
forces is inherently unstable (i.e., as a result of drift, voltage 
variations, etc.). 
U.S. Pat. No. 4,805,456 to Howe et al. discloses a resonant accelerometer 
which employs drive electrodes positioned adjacent a beam (or beams) 
supporting a seismic mass. The beams are mechanically resonant and are 
electrostatically excited to resonate by pulses applied to the drive 
electrodes. As the beams are stressed, their resonant frequencies change, 
which changes are sensed to determine the amount of seismic mass 
displacement. Howe et al. employ a counter or a phase locked loop to 
detect the frequency of oscillation of each beam and then provide a 
frequency difference (as measured between beams along a common axis) as an 
indication of acceleration. The frequency difference is achieved by 
converting each frequency signal into a voltage which is then applied to 
control a measuring circuit. Howe et al. further describe (FIG. 10) the 
use of dc electrostatic potentials to enable a repositioning of their 
seismic mass. More recently, Cho et al. (U.S. Pat. No. 5,015,906) and Carr 
et al. (U.S. Pat. No. 5,187,399) have disclosed an ac stabilization 
procedure for a seismic mass which avoids instabilities inherent in dc 
levitated systems. In an ac levitation system, a planar rotor having a 
conductive surface is displaced above (or below) a pair of planar 
conductive levitation plates that are disposed on an adjacent fixed 
surface. A high frequency voltage source is connected across the 
conductive plates so as to create a continuous ac circuit from the one 
side of an ac voltage source through an inductor to one of the levitation 
plates, to the conductive surface on the rotor, back to the other 
levitation plate and the other side of the ac voltage source. The 
resulting ac circuit exhibits a natural frequency of resonance. The ac 
voltage source is chosen to have a frequency of excitation that is in 
excess of the natural frequency of resonance of the ac circuit. 
In operation, when the ac voltage source is energized, the rotor structure 
is held in vertical equilibrium in relation to the fixed surface that 
supports the levitation plates. Stability of levitation comes about 
because the net forces acting on the rotor, when it is displaced from its 
equilibrium position, are restoring. More specifically, an upward 
displacement of the rotor produces a net downward force and vice versa. 
That is, the oscillating force field acting on the rotor acts to oppose 
displacement movements. The rotor therefore experiences a net restoring 
force that is essentially a null at some levitation position. 
This phenomenon is further described by Kumar et al. in "Experimental Study 
of Electric Suspension for Microbearings", JOURNAL OF 
MICROELECTROMECHANICAL SYSTEMS, Vol. 1, No. 1, March 1992, pages 23-30. 
Accordingly, it is an object of this invention to provide an improved 
microaccelerometer that exhibits improved sensing of the deflection of a 
seismic mass. 
It is another object of this invention, to provide an improved 
microaccelerometer that employs ac levitation to achieve an adjustability 
of acceleration outputs. 
It is yet another object of this invention to provide an improved 
microaccelerometer wherein ac levitation is employed in a feedback circuit 
to restore a positionally displaced seismic mass to a null position. 
SUMMARY OF THE INVENTION 
A first embodiment of an improved microaccelerometer includes a seismic 
mass, a support wafer, a cover wafer and a beam (or beams) for flexibly 
mounting a seismic mass between the support and cover wafers. A first 
oscillator includes a resonant circuit whose capacitance comprises 
conductive plates on one surface of the seismic mass and a conductive 
coating on an opposed surface of the support wafer. A second oscillator 
includes a resonant circuit whose capacitance is comprised of conductive 
coatings on another surface of the seismic mass and on an opposed surface 
of the cover wafer. A difference circuit provides an acceleration output 
that is dependent on a difference in oscillation frequencies between the 
first and second oscillators, when the accelerometer is subjected to an 
acceleration event. A second embodiment includes a structure similar to 
the aforedescribed, however, the second oscillator is replaced by an ac 
levitation circuit that exerts a single direction restoring force on the 
seismic mass during an acceleration event. A third embodiment provides ac 
levitational restoring forces when the seismic mass is subject to 
acceleration in either of two opposed directions. In the latter 
embodiment, ac levitating circuits are disposed on opposed surfaces of the 
support and cover wafers.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, a microaccelerometer 10 includes a cover wafer 12, a 
support wafer 14, and a frame 16. A seismic mass 18 is supported by a beam 
20 that extends from frame 16. Cover wafer 12 is bonded to frame 16 by 
glass seals 22 and frame 16 is, in turn, bonded to support wafer 14 by 
glass seals 24. Each of cover wafer 12, support wafer 14, and frame 16 are 
comprised of silicon (or gallium arsenide and compounds thereof) and are 
constructed using known photolithographic procedures. 
A conductive layer 26 is present on the uppermost surface of seismic mass 
18 and a conductive layer 28 is present on the lowermost surface thereof. 
An insulating layer (not shown) separates conductive layers 26 and 28 from 
the silicon body of seismic means 18. 
A semiconductor chip 30 is mounted on the lowermost surface of cover wafer 
12 and an identical semiconductor chip 32 is located on the uppermost 
surface of support wafer 14. An inductor is schematically shown with each 
of chips 30 and 32 to indicate that each of those chips contains an 
inductor. Conductors 34 are shown schematically and enable connections to 
be made between semiconductor chip 30 and bonding pads 36. Similarly, 
schematically shown conductors 38 enable electrical connections to be made 
between semiconductor chip 32 and bonding pads 40. The circuits on 
semiconductor chips 30 and 32 can also be processed, in their entirety or 
partially, using standard integrated circuit and micromachining directly 
on cover and support wafers 12 and 14. 
Each of semiconductor chips 30 and 32 includes a Hartley oscillator circuit 
42 as shown in FIG. 2. Hartley oscillator circuit 42 comprises an FET 
transistor 44 that is connected to a resonant circuit which comprises 
inductor 46 and capacitor plates 48 and 50. The opposed plate of the 
capacitors is conductive layer 26 on seismic mass 18 (see FIG. 1). When 
seismic mass 18 is positionally displaced by an acceleration event, the 
frequency across the resonant circuit of oscillator 42 will change. That 
frequency is fed through a buffer amplifier 52 to a downcounter 54. An 
identical oscillator 42 is included on chip 32 and provides its frequency 
output to downcounter 56. 
Each of downcounters 54 and 56 count down by a given number of counts and 
then recommence their countdown operation. The difference between each set 
of counts is detected by a subtractor 58 which provides a difference 
output on line 60. That output is indicative of an amount of deflection of 
seismic mass 18. For example, in the event an acceleration moves 
microaccelerometer 10 in an upward direction, conductive layer 26 moves 
away from capacitor plates 48 and 50 in oscillator 42 on chip 30. By 
contrast, conductive layer 28 moves closer to capacitor plates 48 and 50 
on chip 32. As a result, the oscillation frequencies of oscillators 42 on 
chips 30 and 32 are oppositely affected, causing the down count values fed 
to subtractor 58 to exhibit a differential value that is directly related 
to the frequency difference between the two oscillator frequencies. 
Subtractor 58 provides an output indicative of that differential value as 
a measurement of acceleration. 
FIG. 3 is a plot of acceleration versus deflection and FIG. 4 is a plot of 
acceleration versus frequency output, both for a model of the structure 
shown in FIG. 1. The model of the structure of FIG. 1 was derived 
employing the following device parameters: 
width of beam 20=800.times.10.sup.-6 m 
thickness of beam 20=30.times.10.sup.-6 m 
length of beam 20=2000.times.10.sup.-6 m 
width of seismic mass 18=3000.times.10.sup.-6 m 
thickness of seismic mass 18=200.times.10.sup.-6 m 
length of seismic mass 18=400.times.10.sup.-6 m 
nominal gap=1.5.times.10.sup.- 6m. 
The physical/material constants used in the modeling were as follows: 
.mu.=1.8.times.10.sup.-5 Ns/m.sup.2 =viscosity 
.rho.=2.3.times.10.sup.3 Kg/m.sup.3 =density 
e=180.times.10.sup.9 N/m.sup.2 =modulus of elasticity 
g=9.81 m/sec.sup.2 =unit gravity 
.epsilon.=8.85.times.10.sup.-12 F/m=permittivity 
FIG. 5 illustrates the microaccelerometer structure of FIG. 1 (like 
portions are numbered identically) that has been modified to include ac 
levitation. The structure of FIG. 1 has been changed by the removing 
oscillator chip 30 from the lowermost surface of cover wafer 12 and 
placing thereon of a pair of conductive levitation plates 70 and 72 that 
are insulated from the silicon (or gallium arsenide) body of cover wafer 
12. Levitation plates 70 and 72 are connected via conductors through 
bonding pads 74 to a levitating ac source 76 and an inductor 78. 
As taught in U.S. Pat. No. 5,015,906, the entire circuit connected to ac 
source 76 has a natural resonant frequency. The frequency of ac source 76 
is selected to exceed that natural resonant frequency so as to achieve a 
stable levitation of seismic mass 18. A control input 80 enables 
alteration of the signals emanating from ac source 76 so as to achieve a 
repositioning of seismic mass 18 during an acceleration event. 
Levitating plates 70 and 72, in combination with conductive layer 26, 
provide a levitating force that is attractive to seismic mass 18. As a 
result, the microaccelerometer of FIG. 5 is constructed for use wherein 
the acceleration force is as shown by arrow 82. Such acceleration causes 
an downward movement of seismic mass 18 (as constrained by beam 20), with 
the ac levitating circuit tending to return seismic mass 18 to a null 
position as a result of a compensating feedback control signal applied to 
control line 80. 
Control circuits which enable alteration of the ac levitation force are 
shown in FIGS. 6 and 7. Input to the control circuit of FIG. 6 is derived 
from the Hartley oscillator 42 present on semiconductor chip 32. The 
output signal frequency f.sub.s of oscillator 42 is fed from chip 32 to 
bonding pads 84. Signal frequency f.sub.s is then fed to analog/digital 
convertor 86 which provides digital voltage values to microprocessor 88 
(at the frequency f.sub.s). Microprocessor 88 detects any change in 
f.sub.s from a nominal frequency that is expected from Hartley oscillator 
42 on chip 32, and employ the frequency difference to provide an 
acceleration output value on line 90. Furthermore, that frequency 
difference value is used to derive a frequency control signal that is fed, 
via control line 80, to ac source 76. The frequency of ac source 76 is 
decreased by the fed back control signal (causing a reduction in the 
induced attractive force) and enabling beam 20 (and seismic mass 18) to 
return towards a null position. Eventually frequency f.sub.s becomes equal 
to the nominal frequency of Hartley oscillator 42 when seismic mass 18 is 
at the null position. 
This operation can be more fully understood by realizing that levitating 
plates 70 and 72, in combination with conductive layer 26, exhibit an 
attractive force on seismic mass 18 that is opposed by the stiffness of 
beam 20. When the structure shown in FIG. 5 accelerates in a downward 
direction, the distance between levitating plates 70 and 72 and conductive 
layer 26 decreases. By decreasing the frequency applied to levitating 
plates 70 and 72 from ac source 76, the resulting attractive force is 
decreased and enables beam 20 to return seismic mass 18 to an approximate 
null position. The frequency of oscillation of oscillator 42 on chip 32 
provides the signal feedback that enables the restoration of seismic mass 
18 to a null position. 
The speed of operation of the restorative action is dependent upon the time 
constant of the feedback circuit, which time constant may be adjusted to 
achieve a desired speed of restoration. Such a null mode of operation of 
the microaccelerometer provides a highly accurate measure of acceleration 
in the direction shown by arrow 82, while preventing anomalies that occur 
in non-null mode accelerometers. 
In FIG. 7, a similar feedback circuit is shown, however, there 
microprocessor 88 provides an amplitude control signal on line 80 that is 
proportional to the difference between feedback frequency f.sub.s and the 
nominal frequency of oscillator 42 on chip 32. As a result, the amplitude 
of oscillations emanating from ac source 76 are reduced thereby reducing 
the attractive force exerted on seismic mass 18 (much the same as above 
described). 
The microaccelerometer as shown in FIG. 5 has been modeled using the same 
parameters as for the microaccelerometer shown in FIG. 1 except for 
physical dimensions, which were altered for minimal mechanical support as 
follows: 
width of beam 20=50.times.10.sup.-6 m 
thickness of beam 20=10.times.10-.sup.-6 m 
length of beam 20=500.times.10.sup.-6 m 
width of seismic mass 18=1000.times.10.sup.-6 m 
thickness of seismic mass 18=200.times.10.sup.-6 m 
length of seismic mass 18=2000.times.10.sup.-6 m. 
FIG. 8 is a plot of acceleration versus deflection of seismic mass 18 of 
FIG. 5, employing a 5 volt, 200 MHz ac source 76. In FIG. 9, acceleration 
is plotted versus frequency for a 5 volt, 200 MHz ac source 76. FIGS. 10 
and 11 are identical plots to those shown in FIGS. 8 and 9, however, the 
ac drive voltage has been reduced from 5 volts to 1 volt. This action 
reduces the levitation "stiffness", increases the sensitivity of the 
microaccelerometer by a factor of 5 and provides finer resolution. 
Turning to FIG. 12, a microaccelerometer structure is shown that is 
substantially identical to that shown in FIG. 5, except that an additional 
pair of levitating plates 100 and 102 have been emplaced on the upper 
surface of support wafer 14 and are connected to a second source of ac 
levitating signal 104. Other components of the microaccelerometer of FIG. 
12 are numbered identically to those shown in FIG. 5. The 
microaccelerometer of FIG. 12 is particularly adapted to sensing 
accelerations in the directions indicated by arrows 106. 
As indicated with respect to FIG. 5, levitating plates 70 and 72, in 
combination with conductive layer 26, provide an attractive levitating 
force for seismic mass 18 when ac source 76 is energized. In a similar 
manner, levitating plates 100 and 102, in combination with conductive 
layer 28, provide an attractive force for seismic mass 18 when ac source 
104 is energized. Thus, in the event of no acceleration, seismic mass 18 
is positionally stable between cover wafer 12 and support wafer 14. 
In the event of an acceleration, seismic mass 18 is displaced either 
towards or away from chip 32, thus causing a change in the oscillation 
signal frequency f.sub.s fed from chip 32. Signal f.sub.s is fed via 
output pads 84 to A/D converter 86 in FIG. 13. In a similar manner to that 
described for FIG. 6, a frequency difference between f.sub.s and the 
nominal oscillator frequency causes an acceleration output value to be 
impressed on output line 90 and a frequency control signal to be applied 
via control line 80 to ac source 76. In this case, microprocessor 88 
includes an additional output frequency control line 106 that manifests an 
equal and opposite sense control signal on line 80. As a result, the 
frequency output of ac source 104 is altered in an opposite manner from 
that of ac source 76, thereby causing an equal and opposite sense changes 
in levitation forces applied to seismic mass 18. Seismic mass 18 is thus 
returned to a null position by the combined actions of levitating plates 
70, 72 and 100, 102. 
In FIG. 14, a substantially identical control circuit is shown which, in 
lieu of modifying the frequency outputs of ac sources 76 and 104, modifies 
the amplitudes of their signal output in an equal and opposite direction 
so as to achieve a restoration of seismic mass 18 to a null position. 
The microaccelerometer of FIG. 12 provides a sensitivity that is less 
dependent upon the stiffness of beam 20. Furthermore, by variation of the 
amplitude of control feedback signals to ac sources 76 and 104, the 
sensitivity of the microaccelerometer structure can be altered so as to 
accommodate different levels of acceleration. 
It should be understood that the foregoing description is only illustrative 
of the invention. Various alternatives and modifications can be devised by 
those skilled in the art without departing from the invention. 
Accordingly, the present invention is intended to embrace all such 
alternatives, modifications and variances which fall within the scope of 
the appended claims.