A microaccelerometer is provided for use in on-board automotive safety control and navigational systems. The microaccelerometer includes a central support body which is supported upon a backing chip, a peripheral proof mass which circumscribes the central support body, and at least one pair of microbridges, each of which are attached to both the central support body and the peripheral proof mass. The pair of microbridges extend outwardly in opposite directions from the central support body such that a longitudinal axis through each of the microbridges forms a common axis through the central support body. The microbridges are attached to the peripheral proof mass at the end opposite the central support body so as to suspend the peripheral proof mass circumferentially about the central support body and above the backing chip. Piezoelectric drivers and sensors are provided, respectively, for exciting the microbridges at their resonant frequencies, and for detecting changes in the resonant frequencies of the microbridges which occur as a function of acceleration of the peripheral proof mass. The sensors are connected to a feedback circuit which amplifies and buffers their output and provides feedback to the drivers to properly maintain the microbridges at their resonant frequencies.

This invention generally relates to sensors of the accelerometer type. More 
specifically, this invention relates to a two-axis silicon 
microaccelerometer having heavily-doped single-crystal silicon resonant 
microbridges which are driven by a piezoelectric driver. The construction 
of the microaccelerometer is characterized by low off-axis response while 
having reduced vacuum packaging requirements and improved structural 
ruggedness. 
BACKGROUND OF THE INVENTION 
An accelerometer is one of the primary sensors used in on-board automotive 
safety control systems and navigational systems, particularly inertial 
navigational systems. Examples of such automotive applications include 
anti-lock braking systems, active suspension systems, and seat belt 
lock-up systems. 
More specifically, an accelerometer is a device which measures 
acceleration, or more accurately, accelerometers measure the force that is 
exerted by a body as the result of a change in the velocity of the body. A 
moving body possesses inertia which tends to resist the change in 
velocity. It is this resistance to any change in velocity that is the 
source of the force which is exerted by the moving body. This force is 
directly proportional to the acceleration component in the direction of 
movement when the moving body is accelerated. 
In a conventional accelerometer, a mass is suspended between two spring 
members which are coaxially attached on opposite sides of the mass The 
mass is maintained in a neutral position so long as the system is at rest 
or is in motion at a constant velocity. When the mass-spring system 
undergoes a change in velocity in the direction of the springs' axis, such 
as an acceleration or deceleration in that direction, the spring mounted 
mass will resist the movement because of its inertia. This resistance to 
the change in velocity will force one of the springs to be in tension 
while the other spring is compressed. Accordingly, the force acting on 
each spring is equal but opposite in magnitude. The well-known 
mathematical interrelationship of the three variables--force, weight and 
acceleration--provides that the force generated is equal to the product of 
the weight of the mass and the acceleration of the mass, divided by the 
gravitational constant. 
Silicon-based microaccelerometers having resonant-type microbridges are 
also known. An example of this type of accelerometer is disclosed in U.S. 
Pat. No. 4,901,570 to Chang et al, assigned to the assignee of the present 
invention. Chang et al disclose a microaccelerometer having a central 
square-shaped proof mass which is suspended by at least one pair of 
resonant microbridges. The resonant bridges are attached to a supporting 
substrate which circumscribes the proof mass with a gap provided 
therebetween. As such, the central proof mass is supported within and has 
free movement relative to the supporting substrate. The individual 
microbridges within each pair of microbridges are positioned at opposing 
edges of the proof mass such that the pair's longitudinal axes constitute 
a common axis across the surface of the proof mass. 
In a microaccelerometer employing resonant microbridges, acceleration in 
the plane of the substrate causes differential axial loads on oppositely 
disposed resonant microbridges, i.e., causes one supporting resonant 
bridge to be in compression and the other in tension. It is the inertial 
force of the proof mass which generates the axial load on the resonant 
microbridges. In turn, it is the compressive and tensile loads which 
produce a shift in the resonant frequencies associated with each resonant 
microbridge. The resulting difference between the resonant frequencies of 
the compressive and tensile members can then be measured and used to 
determine the magnitude of the acceleration component in the direction of 
the common axis shared by the pair of resonant microbridges. 
The microbridges taught by Chang et al are driven electrostatically at 
their respective resonant frequencies by a separate drive electrode. The 
maximum amplitude of the vibration of each microbridge occurs when the 
microbridge is at resonance, whereby the frequency of the drive voltage 
which is supplied to the drive electrode coincides with the natural 
frequency of the microbridge. To sustain the microbridge in resonance, any 
shift in its resonant frequency due to the externally imposed stress of 
the proof mass must be compensated for by a corresponding change in the 
frequency of the drive electrode's drive voltage. 
The frequency of vibration of each microbridge is detected by monitoring 
the change in a voltage-induced capacitance between the microbridge and a 
sensing electrode. The capacitance varies with time according to the 
frequency of vibration of the microbridge. By placing the sensing 
electrode in close proximity to the microbridge, the shift in the 
microbridge's frequency of vibration can be detected. 
Since the resulting capacitance is small and stray capacitances are usually 
much larger than the sensed capacitance, the signal derived from the 
time-varying capacitance change must be amplified and buffered by an 
on-chip circuitry, such as a clamping diode in conjunction with a 
depletion mode n-channel metal-oxide-semiconductor field-effect 
transistor, or MOSFET. To sustain the microbridge in resonance, this 
enhanced signal is provided through feedback circuitry to the drive 
electrode, causing the frequency of the drive voltage to change so that it 
again coincides with the shifted resonant frequency of the microbridge. 
This type of resonant microaccelerometer is attractive for precision 
measurements because the frequency of a micromechanical resonant structure 
exhibits good linearity with high sensitivity, resolution, and bandwidth. 
However, a shortcoming of such structures as that taught by Chang et al is 
that the gap between the sensing electrode and the corresponding 
microbridge must be sufficiently small so as to maximize the capacitance 
being detected. This requirement necessitates the evacuation of the 
microaccelerometer package so as to reduce the damping effects of the air 
squeezed between the components, typically referred to as a squeeze film 
effect. 
For purposes of assessing the quality of a vibrating structure as a 
harmonic oscillator, the art has derived a dimensionless number which is 
referred to as the quality factor (Q). The quality factor of a given 
structure is inversely related to the damping factor associated with the 
structure and generally relates to the sharpness or width of the response 
curve in the vicinity of the resonant frequency of the vibrating 
structure. The concepts of quality factor, damping, and resonant 
frequencies are primary factors when considering the vibrational 
characteristics of a vibrating structure, and will therefore be referred 
to and further discussed in relation to the present invention. 
With regards to the microaccelerometer taught by Chang et al, the 
evacuation of the microaccelerometer package is necessary to reduce the 
damping effects of the air in order to achieve a high quality factor. As 
an example, testing has indicated that vacuum packaging of approximately 
100 mTorr is necessary to achieve a quality factor of 600 with the 
structure taught by Chang et al. Microbridge resonance cannot be initiated 
in the structure taught by Chang et al when operated at one atmosphere as 
a consequence of the high damping effect of the air, and hence a low 
quality factor. 
Another significant shortcoming of the teachings of Chang et al is that the 
microaccelerometer structure exhibits a relatively high off-axis (which is 
the axis orthogonal to the plane in which acceleration is being detected) 
response on the order of 10% as compared to the on-axis response. The 
off-axis response is attributed to a geometrical mismatch between the 
paired microbridges. The mismatch itself is created in part by non-ideal 
microfabrication techniques which produce less than ideal symmetry of the 
proof mass and bridge dimensions. As a result, the bridges are 
asymmetrically stressed and therefore have different resonant frequencies. 
Another factor is the package-induced stresses within the bridges which 
result from such conditions as less than perfect positioning of the proof 
mass relative to the supporting substrate and inherent stresses associated 
with the bonding and packaging techniques employed. As a result, the 
bridges are further asymmetrically stressed. 
The detrimental effects of this geometric mismatch are exacerbated by the 
resonant frequency measuring technique adopted by Chang et al. For an 
electrostatic drive as taught by Chang et al, the electrically measured 
resonant frequency is a strong function of the gap obtained between the 
bridge and drive electrode, and thus, this effect on the measured resonant 
frequency is amplified. This sensitivity produces an erroneous shift in 
the measured resonant frequency of each microbridge as detected by the 
sensing electrode. As a result, the unequal built-in stresses associated 
with a pair of microbridges directly contribute to a resonant frequency 
error which does not cancel out during computation. The result is an 
erroneous acceleration amplitude reading. 
Therefore, it would be advantageous to provide a microaccelerometer which 
employs silicon integrated circuit technology while minimizing the effects 
of any geometrical mismatch of the microaccelerometer's construction. In 
addition, it would be advantageous to provide a microaccelerometer which 
does not employ an electrostatic driver for driving the microbridges at 
their resonant frequencies, so as to alleviate the shortcomings associated 
with the prior art. Finally, it would be desireable to reduce the 
microaccelerometer package's vacuum requirements while maintaining or 
improving the microaccelerometer's quality factor. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a single-crystal 
silicon microaccelerometer which is suitable for use in automotive 
applications. 
It is a further object of this invention that such a microaccelerometer 
employ a support body, proof mass and at least one pair of microbridges 
which are constructed and arranged so as to minimize the adverse effects 
of any geometrical mismatch therebetween. 
It is still a further object of this invention that the microaccelerometer 
employ a microbridge driver and sensor which operate piezoelectrically, 
thereby avoiding exacerbation of the adverse influence that geometrical 
mismatch has on the measured resonant frequency of each microbridge. 
It is yet another object of this invention that the support body be 
attached to the microaccelerometer package so as to avoid propagation of 
any packaging-induced stresses to the microbridges. 
It is a further object of this invention that the microaccelerometer employ 
on-chip circuitry to amplify and buffer the sensor signal derived from 
detecting the acceleration-induced stresses in the microbridges. 
Lastly, it is still a further object of the present invention that the 
package vacuum requirement for the microaccelerometer be capable of being 
relaxed while still achieving satisfactory vibrational response from the 
microbridges and at the same time attaining critical damping for the proof 
mass. 
In accordance with a preferred embodiment of this invention, these and 
other objects and advantages are accomplished as follows. 
A microaccelerometer is provided which is suitably rugged for use in 
on-board automotive safety control and navigational systems. The 
microaccelerometer includes a backing chip, a central support body 
supported upon the backing chip, a peripheral proof mass which 
circumscribes the central support body, and at least one pair of 
microbridges, each microbridge being attached to both the central support 
body and the peripheral proof mass. A gap is provided between the central 
support body's perimeter and the peripheral proof mass. The gap is 
sufficient to prevent physical interference between the peripheral proof 
mass and the central support body when the peripheral proof mass is 
displaced relative to the central support body. 
The pair of microbridges extend outwardly in opposite directions from the 
central support body such that a longitudinal axis through each of the 
microbridges forms a common axis through the central support body. The 
microbridges are attached to the peripheral proof mass at their ends 
opposite to the central support body so as to suspend the peripheral proof 
mass circumferentially about the central support body and above the 
backing chip. 
A driving device is provided on each microbridge for exciting the 
microbridge at its resonant frequencies. The driving devices operate 
piezoelectrically to induce stresses parallel to the plane of the 
microbridges at a frequency corresponding to each microbridge's resonant 
frequency. In so doing, the driving devices are able to drive their 
corresponding microbridges at their resonant frequencies. However, the 
resonant frequencies of the microbridges shift when under tension or 
compression as a consequence of the peripheral proof mass's acceleration 
in the direction of the common axis of the microbridges. Therefore, 
piezoelectric sensors are provided for detecting the shift in the resonant 
frequency of each microbridge during acceleration of the proof mass. The 
piezoelectric sensors respond to the change in stress within each 
microbridge, which correspond to a change in the frequency of vibration of 
the microbridge. 
As such, by comparing the shifted resonant frequencies of opposing 
microbridges, the piezoelectric sensors provide an output which indicates 
the acceleration (or deceleration) of the peripheral proof mass in the 
direction of the opposing microbridges, common axis. Circuitry connected 
to the individual piezoelectric sensors provides amplification and 
buffering of the output signal to enhance the signal. 
Together the piezoelectric driver and sensors form a frequency-measuring 
circuit for each microbridge which is preferably formed so as to be 
integral with the supporting substrate of both the peripheral proof mass 
and the central support body. This frequency-measuring circuitry is 
connected to a feedback circuit which produces an output signal 
corresponding to the change in difference between frequencies for the pair 
of microbridges. The feedback circuitry provides feedback to the driving 
devices to allow each driving device to compensate for the change in 
resonant frequency of each microbridge. As a result, each driving device 
is able to properly maintain its corresponding microbridge at its resonant 
frequency. 
In a preferred embodiment, a second pair of microbridges are similarly 
attached to opposite sides of the central support body and form a second 
common axis which is perpendicular to the common axis formed by the first 
pair of microbridges. Again, each microbridge is excited to vibrate at its 
respective resonant frequency, and the magnitude of the difference in 
resonant frequency between the members of each pair due to the 
acceleration is then measured to provide an indication of acceleration 
along the common axis formed by the respective pair of microbridges. 
A particularly advantageous feature of this invention is that the proof 
mass of the microaccelerometer is provided as a peripheral proof mass 
which eliminates the effects of packaging-induced stress. Another distinct 
advantage of the present invention is that the microbridges are driven and 
sensed piezoelectrically, which eliminates the presence of the narrow gap 
underneath the microbridge and thus avoids exacerbation of the detrimental 
effects due to any deviation from four-fold symmetry of the proof mass and 
the presence of any geometrical mismatch between each pair of 
microbridges. A further advantage is that due to the elimination of the 
narrow gap underneath the microbridge, the squeeze film effect is avoided 
and hence, packaging vacuum requirements may be relaxed without 
detrimental results to the operation of the microaccelerometer. 
Other objects and advantages of this invention will be better appreciated 
from the detailed description thereof, which follows.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A resonant bridge microaccelerometer is disclosed which measures 
acceleration in the plane of a silicon substrate forming the primary 
components of the microaccelerometer. In a preferred embodiment 
illustrated in FIG. 1, this invention comprehends a two-axis single 
crystal silicon microaccelerometer 10 having a peripheral proof mass 12 
which movably circumscribes a central support body 14. The peripheral 
proof mass 12 and the central support body 14 lie in a plane designated as 
the x-y plane of the microaccelerometer 10. As seen in FIG. 2, the central 
support body 14 is supported above a backing chip 16 by a plurality of 
support members 18 which extend upwardly (in the z-direction) from the 
upper surface of the backing chip 16. The central support body 14 is 
secured to the backing chip 16 with a suitable epoxy 42. A gap 20 is 
provided between the central support body's perimeter and the peripheral 
proof mass 12 such that no physical interference exists between the 
peripheral proof mass 12 and the central support body 14 when the 
peripheral proof mass 12 is displaced in the z-direction. 
To limit displacement of the peripheral proof mass 12 in the z-direction 
(i.e. perpendicular to the x-y plane), the backing chip 16 is provided 
with mechanical stoppers 38 near its perimeter. In addition, constraining 
bridges 40 are provided on the lower surface of the central support body 
14 to prevent rotational movement of the peripheral proof mass 12 and to 
inhibit its z-directional displacement. Accordingly, the movement of the 
peripheral proof mass 12 is predominantly limited to the x-y plane. 
Although the backing chip is not absolutely necessary for the operation of 
the microaccelerometer, it is preferred that the backing chip be used for 
the above purposes. 
With reference again to FIG. 1, two pairs of microbridges 22, 24 and 26, 28 
are preferably provided for suspending the peripheral proof mass 12 from 
the central support body 14 and above the backing chip 16. The 
microbridges 22 and 24 of the first pair are oriented such that their 
longitudinal axes are parallel to the x-axis, while the microbridges 26 
and 28 of the second pair are oriented such that their longitudinal axes 
are parallel to the y-axis. As shown in FIG. 2, the microbridges 22, 24 
and 26, 28 are formed to be substantially coplanar with the upper surface 
44 of the central support body 14. Each pair is orthogonally attached to 
opposite sides of the central support body 14 and span the gap 20 between 
the central support body 14 and the peripheral proof mass 12. The 
microbridges 22, 24 and 26, 28 are also formed to be coplanar with and 
orthogonally attached to an adjacent edge of the peripheral proof mass 12. 
Each pair of microbridges is oriented such that each member of a 
microbridge pair is longitudinally aligned with the other to form a common 
axis. As illustrated, the first microbridge pair 22 and 24 form a common 
axis 30 parallel to the x-axis while the second microbridge pair 26 and 28 
form a common axis 32 parallel to the y-axis. The common axes 30 and 32 
are perpendicular to each other such that excessive movement of the 
peripheral proof mass 12 is inhibited in both the x-direction and the 
y-direction. 
The microbridges 22, 24 and 26, 28 are formed from single-crystal silicon 
heavily doped with boron, and therefore are under tensile stress. Internal 
stresses within the microbridges 22, 24 and 26, 28 are dependant in part 
upon the dopant species, dopant concentration, and the post-doping heat 
treatment. The microbridges 22, 24 and 26, 28 can be formed such that the 
level of internal stress within the microbridge member is minimal, 
preferably stress-free or under minimal tensile stresses. 
In addition, the microbridge pairs are geometrically matched as nearly as 
possible such that a given microbridge pair will experience differential 
axial stresses of nearly equal (though opposite) magnitude during 
acceleration of the peripheral proof mass 12. Ideally, geometrical 
matching also provides a common mode rejection capability such that 
stresses induced by temperature and material effects and z-directional 
forces, are cancelled. However, this capability is severely reduced as 
microbridge pairs diverge from an ideal geometrical match. As will become 
apparent, the effect of geometrical mismatch is greatly reduced by the 
advantageous features of the present invention. 
The acceleration component aligned with each microbridge pair 22, 24 and 
26, 28 is measured through the use of a fibration drive means 34 and a 
fibration sense means 36 assigned to each microbridge 22, 24 and 26, 28. 
Both the drive means 34 and sense means 36 are piezoelectric elements 
composed of a zinc oxide (ZnO) film. The desired properties of the ZnO 
film include a high electromechanical coupling constant, high electrical 
resistance, and low intrinsic stress--i.e. stress-free or slightly under 
tensile stress. The thickness of the ZnO film used was in the range of 
about 0.5 to about 1.0 microns, because this was a sufficient thickness 
for piezoelectric operation, yet not excessively thick so as to impair the 
movement of the microbridge 22,24 and 26, 28. Foreseeably, other suitable 
piezoelectric material such as lead zirconate titanate (PZT) or aluminum 
nitride (AlN) could also be used. 
In plan view, portions 34 and 36 of the zinc oxide film appear as if they 
were electrodes on the ends of the microbridges. Hence, they are 
hereinafter referred to as electrodes. 
Each drive electrode 34 is positioned on one end of microbridge 22, 24 and 
26, 28. When a voltage is applied to the drive electrode 34, the 
piezoelectric effect induces a stress in the microbridge parallel to the 
common axis 30 or 32 of the pair of microbridges. By alternating the 
voltage to the drive electrode 34, the microbridge can be driven at its 
mechanical resonant frequency--that is, the frequency at which the 
microbridge's maximum amplitude of vibration occurs. To sustain the 
microbridge at resonance, the frequency of the drive electrode's voltage, 
and thus the induced stress, is tuned to coincide with the resonant 
frequency of the microbridge. 
In a preferred embodiment, the sense electrodes 36 are located on the end 
of the microbridges 22, 24 and 26, 28 opposite their corresponding drive 
electrodes 34, as best seen in FIG. 4. The vibration of each microbridge 
22, 24 and 26, 28 generates a time-varying strain within the microbridge 
which is sensed piezoelectrically by the sense electrode 36. As a result, 
the sense electrode 36 is able to sense any shift in the resonant 
frequency of the microbridge which occurs when the corresponding pair of 
microbridges are under the tensile/compressive stresses induced by the 
acceleration of the peripheral proof mass 12. 
The sense electrodes 36 convert the resulting strain into a voltage. 
Because the resulting voltage is small, the sense electrodes 36 are each 
accompanied by an on-chip circuitry consisting of a clamping diode 54 and 
a depletion-mode n-channel MOSFET 56. The on-chip circuitry acts to buffer 
and amplify the sense electrode's output to provide a more readily 
detectible output signal. To sustain the microbridge in resonance, this 
enhanced signal is provided through a feedback circuitry (not shown) to 
the drive electrodes 34, causing the frequency of the drive electrode 34 
to change so that it again coincides with the shifted resonant frequencies 
of the microbridges 22, 24 and 26, 28. 
In order to induce the necessary stress within the corresponding 
microbridge, it is preferable to mount the drive electrode 34 on the end 
of the microbridge attached to the peripheral proof mass 12, as 
illustrated in FIG. 4. Alternative embodiments which are not illustrated 
are also suitable for purposes of the present invention. A first 
alternative embodiment has the positions of the drive electrode 34 and the 
sense electrode 36 reversed, with the drive electrode 34 placed on the end 
of the microbridge attached to the central support body 14 and the sense 
electrode 36 placed on the end of the microbridge attached to the 
peripheral proof mass 12. Another embodiment involves placing both the 
drive electrode 34 and the sense electrode 36 in parallel lengthwise on 
the microbridge. Preferably, the drive electrode 34 is located on the 
corresponding axis 30 or 32 while the sense electrode 36 is provided as a 
pair of electrodes positioned on both sides of the drive electrode 34. Yet 
another embodiment involves locating the drive electrode 34 at both ends 
of each microbridge while the sense electrode 36 is located between the 
drive electrodes 34 approximately in the middle of the microbridge. 
When considering to each of the above alternative embodiments, of 
significant importance is that both the drive electrodes 34 and the sense 
electrodes 36 are positioned such that they do not cross the points of 
inflection of the fundamental flexure mode of the microbridge. The points 
of inflection for a beam with both ends clamped are located approximately 
one fourth of the beam length from the clamping ends. Consequently, the 
length of the ZnO films which comprise the drive and sense electrodes 34 
and 36 should extend less than about one fourth of the length of the 
microbridges 22, 24 and 26, 28. 
The inertial force on the peripheral proof mass 12, due to acceleration in 
the x-y plane of the microaccelerometer, generates the differential axial 
loads on the opposing microbridges 22, 24 and 26, 28. The resulting 
compressive and tensional loads on the microbridges cause a shift in their 
corresponding resonant frequencies. This effect produces a change in the 
difference between the respective resonant frequencies of each member of a 
pair of microbridges. The magnitude of this change in difference between 
resonant frequencies in a bridge pair corresponds to the acceleration of 
the peripheral proof mass 12 along one of the common axes 30 and 32 formed 
by the respective bridge pair. 
For purposes of illustration, an x-axis acceleration component, a.sub.x, 
results in an inertial force, defined by the equation F.sub.i =-ma.sub.x, 
on the peripheral proof mass 12. The inertial force F.sub.i is 
predominantly shared by the microbridges 22, 24 aligned on the common axis 
30 in the x-axis direction, as illustrated in FIG. 1. The acceleration in 
the x-axis direction causes one of the pair, microbridge 22 as 
illustrated, to be in compression while the other microbridge 24 is in 
tension. The states of compression and tension cause a shift in the 
resonant frequencies f.sub.x1 and f.sub.x2 in the microbridges 22 and 24, 
respectively. 
An analysis based upon Rayleigh's Energy Method, assuming the fundamental 
vibrational mode, leads to the following expression for the resonant 
frequency of the microbridge 24 in tension. Therefore: 
EQU f.sub.x2 =f.sub.o [1+0.293(1.sup.2 /EWt.sup.3) (0.5ma.sub.x)].sup.0.5 
where f.sub.o is the unperturbed resonant frequency, E is the Young's 
modulus of elasticity for the material and 1, W and t are the length, 
width and thickness of the microbridge, respectively. For simplicity, this 
result neglects the very minimal load-sharing provided by the microbridges 
26 and 28 provided in the orthogonal direction. 
By subtracting the corresponding expression for the member in compression, 
i.e. microbridge 22, and considering small perturbations, the difference 
in frequencies becomes: 
EQU .DELTA.f.sub.x =f.sub.x2 -f.sub.x1 =(0.146f.sub.o ml.sup.2 
/EWt.sup.3)a.sub.x =s.sub.x a.sub.x, 
where S.sub.x is defined as the sensitivity to x-axis acceleration 
components. From this analytical result, the x-axis acceleration component 
may be determined. In addition, the above relationships may be used to 
determine the y-axis component of acceleration also. 
A z-axis component of acceleration, which is the axis orthogonal to the x-y 
plane of the peripheral proof mass 12, causes vertical displacement of the 
peripheral proof mass 12. For small vertical displacements, the resulting 
perturbations in f.sub.x1 and f.sub.x2, for the x-axis component, are 
common to both bridges 22 and 24 aligned with the x-axis, and are 
therefore cancelled by common mode rejection in f.sub.x. Additional 
restraint on the z-axis displacement of the peripheral proof mass 12 is 
provided by the mechanical stoppers 38 so as to ensure only small vertical 
displacements over a practical range of acceleration in the z-axis. As 
noted before, geometrical matching of the pair of microbridges 22 and 24 
will also ameliorate the effect of the z-axis component. However, 
historically the ability to minimize geometrical mismatching has been 
limited by the capabilities of the fabrication process and technologies. 
However, inventive features of the present invention are able to minimize 
the influence of geometrical mismatching. Firstly, the drive electrodes 34 
operate piezoelectrically. Therefore, their operation does not rely upon a 
polarization voltage which, as noted with the electrostatic drivers of the 
prior art, is adversely influenced by geometrical mismatch between the 
microbridge pairs 22, 24 and 26, 28. Secondly, the placing of the sense 
electrodes 36 directly on the microbridges eliminates the gap previously 
necessary therebetween with the electrostatic microaccelerometer of the 
prior art. Accordingly, the influence of the gap distance--a product of 
processing variations--on the sensed capacitance, and hence the detected 
frequency, is eliminated. 
In addition, the structure of the microaccelerometer allows for better 
control of the z-directional spacing between the microbridges 22, 24 and 
26, 28 and the backing chip 16, and the z-directional spacing between the 
bottom surface of the peripheral proof mass 12 and the backing chip 16. In 
the preferred embodiment, the spacing of the microbridges 22, 24 and 26, 
28 with the backing chip 16 is the thickness of a 3 inch 
wafer--approximately 400 microns, while the spacing between the bottom 
surface of the peripheral proof mass 12 and the mechanical stops 38 of the 
backing chip 16 can be controlled to be approximately 10 microns. Another 
advantage is that the central support body 14 is attached to the backing 
chip 16 at a point remote from the peripheral proof mass 12 and the 
microbridges 22, 24 and 26, 28. As a result, package-induced stresses are 
completely isolated from the microbridges 22, 24 and 26, 28. 
The preferred method for forming the microaccelerometer 10 of the present 
invention uses bulk and surface micromachining techniques particularly 
suitable for single-crystal silicon. In addition, the preferred method 
employs orientation-dependent silicon etching to form the peripheral proof 
mass 12 and other components. Considerations for the fabrication of the 
microaccelerometer pertain to the process integration of the active 
devices of the sense electrodes 36 with the micro-mechanical structures of 
the device. For example, the thermal budget for high temperature processes 
and the overall fabrication sequence must be coordinated such that both 
the active devices and the micro-mechanical structures function as 
intended. The process sequence described below is capable of integrating 
the required devices on the same chip without compromising their 
performance. 
Using conventional and unconventional semiconductor fabrication techniques, 
the preferred fabrication process for the microaccelerometer requires 
employing three different technologies to form a micromachined mechanical 
structure (the peripheral proof mass 12 and the microbridges 22, 24 and 
26, 28); an electrically transductive material (the ZnO thin film for the 
drive and sense electrodes 34 and 36); and a conventional silicon active 
device (the MOSFET 56). Starting with a standard single-crystal silicon 
substrate oriented along the [100] crystallographic plane 62, two deep 
boron diffusions are carried out by high temperature diffusion using a 
boron oxide source. The boron concentration must be greater than about 
5.times.10.sup.19 cm.sup.-3 to ensure a hard etch-stop in ethylene diamine 
pyrocatechol (EDP) or potassium hydroxide (KOH), which are the more 
commonly used etchants for the micromachining of silicon structures. 
A series of first heavily-doped boron layers 58 are selectively formed to 
be approximately ten microns thick on both sides of the substrate 62, as 
illustrated in FIG. 5. The first layer 58 defines the length of the 
microbridges 22, 24 and 26, 28 and provides the etch mask for the EDP etch 
of the substrate 62 which forms the peripheral proof mass 12 and the 
microbridges 22, 24 and 26, 28. The constraining bridges 40 will also be 
formed from the first boron-doped layer 58 on the lower surface of the 
substrate 62. The ten micron thickness of the constraining bridges 40 
helps restrain z-directional displacement of the peripheral proof mass 12 
to approximately 0.2 microns when submitted to a 1 g z-directional 
acceleration. A second heavily-doped boron layer 60 is formed to be 
approximately 4 microns thick. The second layer 60 defines the thickness 
and width of the microbridges 22, 24 and 26, 28, which in the preferred 
embodiment were approximately 4 microns and 100 microns, respectively. 
The MOSFETs and diodes associated with the sense electrodes 36 are 
fabricated prior to the ZnO thin film processes and the formation of both 
the resonant microbridges 22, 24 and 26, 28 and the peripheral proof mass 
12. Such a process arrangement gives better control of the MOSFET 
fabrication temperatures of approximately 900.degree. C. which might 
otherwise cause the ZnO films to interact with the adjacent layers, thus 
leading to contamination. 
To prevent excessive dopant diffusion, arsenic implantations of 
approximately 5.times.10.sup.11 cm.sup.-2 at 150 keV were used for 
adjustment of the threshold voltage. Phosphorus implantations of 
approximately 5.times.10.sup.15 cm.sup.-2 at 150 keV were used to form the 
source and drain regions of the MOSFET. Low pressure chemical vapor 
deposition (LPCVD) polysilicon was used for the gate electrode of the 
MOSFET. In the preferred embodiment, the length and width of the MOSFET 
channel were approximately 10 microns and 100 microns, respectively. 
Insulating field silicon dioxide (SiO.sub.2) regions 64 having a thickness 
of 1.0 micron were selectively grown into the first boron-doped layers 58 
on the upper surface of the substrate 62, as shown in FIG. 6. The field 
SiO.sub.2 regions 64 reduced the stray capacitance associated with the 
electrical interconnects of the frequency-measuring circuit. A 
phosphosilicate glass (PSG) layer 66 was then deposited upon the field 
SiO.sub.2 regions 64. A second insulating SiO.sub.2 layer 67 was grown 
upon the second boron-doped layer 60 to a thickness of about 0.1 microns. 
The ZnO films for the drive and sense electrodes 34 and 36 were prepared by 
using an rf magnetron sputter-deposition technique. For providing good 
crystallinity and low internal stresses, the substrate 62 was heated to 
approximately 275.degree. C. during deposition of the ZnO film to the 
substrate 62. The thickness of the ZnO films were in the range of 
approximately 0.5 to 1.0 microns while their widths and lengths were 
delineated by using a wet chemical etching of a 5% solution of ammonium 
chloride (NH.sub.4 Cl) at about 55.degree. C. Other materials used in 
fabricating the microaccelerometer--aluminum, silicon and silicon 
oxide--appeared to withstand attack by the ZnO etchant. 
Next, a layer 68 of silicon nitride (Si.sub.3 N.sub.4) was deposited to a 
thickness of approximately 0.1 microns to encapsulate the ZnO films, as 
shown in FIG. 7. During the deposition of the silicon nitride layer 68 the 
substrate 62 wash held at approximately 275.degree. C. The silicon nitride 
layer 68 was provided to prevent the ZnO films from being attacked by the 
EDP subsequently used in the micromachining of the peripheral proof mass 
12. In addition, the sandwiching of the drive and sense ZnO elements 34 
and 36 between the SiO2 layer 67 and the Si.sub.3 N.sub.4 layer 68 blocked 
the leakage paths of the piezoelectrically induced charges, and the 
formation of depletion layers in the ZnO films were ensured. Consequently, 
the piezoelectrically-induced charges have long decay time constants, thus 
ensuring device operation in the low frequency spectrum. 
The electrical interconnects were next formed as a 0.2 micron thick 
aluminum layer 70 as indicated in FIG. 7. Since the EDP used in the 
micromachining process would attack both the aluminum 70 and the PSG 66, a 
protective 4 micron layer of polyimide (not shown) was applied next. The 
polyimide layer also served to support the microbridges 22, 24 and 26, 28 
so as to prevent damage during subsequent processing. 
The microbridges 22, 24 and 26, 28 were then formed during the 
micromachining of the peripheral proof mass 12 and the constraining 
bridges 40. In a preferred embodiment, the thickness, width and length of 
the microbridges 22, 24 and 26, 28 were 4, 100 and 500 microns, 
respectively. The peripheral proof mass 12 was micromachined using an 
orientation-dependent EDP etch. Preferably, the same etching step yields 
eight boron-doped silicon constraining bridges 40 from the first 
boron-doped layers 58 on the lower surface of the substrate 62. 
As noted before, the microaccelerometer of the present invention is 
generally intended to operate under a vacuum in order to maximize 
vibrational amplitude. The packaging process encompasses mounting the 
central support body 14 to the backing chip 16 using vacuum epoxy 42 which 
has a low vapor pressure and is stable up to approximately 250.degree. C. 
As noted above, the gap between the mechanical stoppers 38 and the lower 
surface of the peripheral proof mass 12 is provided to be approximately 10 
microns. The assembled chip is then attached to a standard integrated 
circuit (IC) package such as a 24 pin DIP 48 again using a vacuum epoxy 
46. The DIP is then hermetically sealed with a metal cover 52 under vacuum 
using a low melting tin-gold alloy 50. 
A significant advantage of the present invention as described above is that 
the proof mass of the microaccelerometer is provided as a peripheral proof 
mass 12 which is less affected by off-axis disturbances. Another distinct 
advantage of the present invention is that the microbridges 22, 24 and 26, 
28 are driven piezoelectrically, which alleviates the exacerbation of the 
detrimental effects due to any deviation from four-fold symmetry of the 
proof mass 12 and the presence of any geometrical mismatch between each 
opposing pair of microbridges 22, 24 and 26, 28, and between each 
microbridge and its corresponding sense electrode 36. 
A further advantage is that there is no gap provided between the sense 
electrodes 36 and their corresponding microbridges 22, 24 and 26, 28. 
Hence, no squeeze film effect occurs to interfere with the vibration of 
the microbridges 22, 24 and 26, 28. Therefore, packaging vacuum 
requirements may be relaxed while still achieving a high quality factor. 
For example, quality factors of 300 and 3,000 have been attained in tests 
performed at one atmosphere and under a vacuum of 100 mTorr, respectively. 
This large increase in the quality factor realized at 100 mTorr over the 
previously noted value of 600 for the electrostatically driven 
microaccelerometer of the prior art is mainly attributed to the 
elimination of the narrow squeeze film gap between the microbridges 22, 24 
and 26, 28 and their sense electrodes 36. As a result, in certain 
applications which do not require high sensing resolution, vacuum 
packaging may not be necessary at all. Moreover, the ability to achieve a 
high quality factor without the requirement for packaging vacuum enhances 
the feasibility of mass production. 
The above structural characteristics, in conjunction with a predetermined 
package vacuum of as little as approximately 1 Torr, will cause 
underdamping in the microbridges 22, 24 and 26, 28 while producing 
critical damping of the peripheral proof mass 12 so that the z-directional 
displacement of the peripheral proof mass 12 is effectively suppressed, 
further minimizing the effect of z-directional perturbations. 
Finally, the use of the peripheral proof mass 12 yields a large mass for a 
given chip area, hence the silicon real estate is more efficiently used. 
Therefore, while our invention has been described in terms of a preferred 
embodiments, it is apparent that other forms of the device could be 
adopted by one skilled in the art. By example, the intent of the present 
invention could also be met through by employing thermal drive techniques 
which also avoid the previous problems noted with electrostatic drives. In 
addition, an alternative microbridge pattern could be used with the same 
piezoelectric technique disclosed herein. Also, it is clear that these 
teachings could be used with modified or alternative materials, or with 
modified processing parameters. It is therefore readily observable by 
those skilled in the art that there are other parameters which can affect 
geometrical matching, including the specific processes used, the 
thicknesses of the different materials used, and the specific geometry of 
the proof mass 12 and its supporting substrate. Accordingly, the scope of 
our invention is to be limited only by the following claims.