Silicon micromachined motion sensor and method of making

A method for making and vacuum packaging a silicon micromachined motion sensor, such as a gyroscope, at the chip level. The method involves micromachining a trench-isolated sensing element in a sensing chip, and then attaching a circuit chip to enclose the sensing element. Solder bumps serve to attach the circuit chip to the sensing chip, form a hermetic seal to enable vacuum-packaging of the sensor, and electrically interconnect the sensing chip with the circuit chip. Conductive runners formed on the enclosed surface of the circuit chip serve to electrically interconnect the sensing element with its associated sensing structures.

FIELD OF THE INVENTION 
The present invention generally relates to micromachined semiconductor 
sensing devices. More particularly this invention relates to a method for 
making and packaging a motion sensor that includes a micromachined sensing 
chip and a circuit chip, in which electrical connections between the 
sensing and circuit chips is achieved by solder bumps formed on the 
circuit chip, which is assembled with the sensing chip as a capping chip. 
BACKGROUND OF THE INVENTION 
Motion sensors, which include gyroscopes and their components (e.g., 
angular rate sensors, yaw rate sensors and accelerometers), are widely 
used in VCR cameras and aerospace and automotive safety control systems 
and navigational systems. Examples of automotive applications include 
anti-lock braking systems, active suspension systems, supplemental 
inflatable restraint systems such as air bags and seat belt lock-up 
systems, and crash sensing systems. Automotive yaw rate sensors sense 
movement of an automobile about a vertical axis through its center of 
gravity, while accelerometers measure acceleration, or more accurately, 
the force that is exerted by a body as the result of a change in the 
velocity of the body. Both types of sensors operate on the basis of a 
moving body possessing inertia which tends to resist a change in velocity. 
In the past, electromechanical and electronic motion sensors have been 
widely used in the automotive industry to detect an automobile's 
deceleration. More recently, sensors that employ an 
electrically-conductive, micromachined silicon sensing element have been 
developed which can be integrated with signal conditioning circuitry on 
silicon chips. As is known by those skilled in the art, micromachined 
silicon sensing elements are formed by etching a "sensing" chip formed of 
a single crystal silicon wafer or a polysilicon film on a silicon or glass 
handle wafer. In one configuration, the sensing element is formed by a 
resonating ring supported by arcuate springs extending radially from a 
post or hub. Circumscribing the ring is an electrode pattern composed of a 
number of individual electrodes. Finally, a capping wafer is often used to 
enclose the ring structure within an evacuated cavity defined by and 
between the sensing and capping wafers. Conductive runners formed on the 
sensing chip run beneath the edge of the capping chip to enable the 
electrodes to be electrically interconnected with appropriate signal 
conditioning circuitry and to provide a biasing voltage to the sensing 
element. 
With the above construction, a sensor is able to detect rotary movement 
about the axis of the hub and, therefore, rotary movement about any chosen 
axis of an automobile. In operation, some of the electrodes are typically 
energized to drive the ring into resonance, others are energized to 
balance the resonant peaks of the rotary movement by inducing stiffness in 
the ring and springs, while other electrodes are used to capacitively 
sense rotary motion of the ring. 
Sensors of the type described above are capable of extremely precise 
measurements, and are therefore desirable for use in automotive 
applications. However, the intricate sensing element required for such 
sensors must be precisely formed in order to ensure the proper operation 
of the sensor. For example, a sufficient gap must exist between the 
electrodes and the sensing element ring to prevent shorting, yet 
sufficiently close to maximize the capacitive output signal of the sensor. 
In addition, the electrodes must be spaced sufficiently apart along the 
perimeter of the ring in order to minimize crosstalk. In addition to these 
operational considerations, there is a continuing emphasis for motion 
sensors that are lower in cost and smaller in size, yet maintain high 
reliability and a high performance capability. 
Therefore, it would be highly desirable if further advancements could be 
made toward reducing manufacturing complexity and costs without any loss 
in sensor performance, and preferably with improvements in performance. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a motion sensor and a method 
for fabricating such a motion sensor. 
It is another object of this invention that such a method yields a 
monolithic motion sensor structure, in which the motion sensor includes a 
sensing element on a sensing chip and conditioning circuitry on a capping 
wafer that encloses the sensing element. 
It is yet another object of this invention that such a method entails 
making electrical connections between the conditioning circuitry and the 
sensing chip through electrical connections at the interface between the 
sensing and capping chips. 
It is still another object that the motion sensor utilizes trench isolation 
in order to dielectrically isolate the various electrical structures of 
the motion sensor. 
In accordance with a preferred embodiment of this invention, these and 
other objects and advantages are accomplished as follows. 
According to the present invention, there is provided a motion sensor 
having a micromachined sensing element and sensing electrodes formed in 
the near-surface region of a silicon sensing chip, and a circuit chip on 
which is formed signal conditioning circuitry for the sensor. The sensing 
and circuit chips are configured such that, once bonded together, 
electrical connections are made between the conditioning circuitry and the 
sensing element and sensing electrodes without resorting to exterior 
electrical connections. 
The sensing chip of this invention generally includes the sensing element 
and an array of electrodes that define an electrode pattern circumscribing 
the sensing element. A trench circumscribes the sensing element to 
physically isolate the sensing element from the electrodes. A contact is 
present on the sensing chip and is electrically connected to the sensing 
element. The circuit chip is attached to the sensing chip so as to enclose 
the sensing element between the circuit chip and the sensing chip. A 
second contact is present on the circuit chip and registered with the bond 
pad on the sensing chip in order to electrically connect the sensing 
element to the circuit chip. Additional contacts are also present on the 
circuit chip and are electrically connected to the electrodes on the 
sensing chip. Finally, conductive runners are present on the circuit chip 
to electrically connect the contacts on the circuit chip to the integrated 
circuit and, if necessary, to each other. 
In a preferred embodiment, the motion sensor further includes a seal that 
circumscribes the electrode pattern so as to hermetically enclose the 
sensing element between the circuit chip and the sensing chip. The seal is 
preferably formed by a continuous bead of solder. The solder for the seal 
can be simultaneously deposited with solder used to form the electrical 
connections between the contacts on the sensing and circuit chips. 
Also in a preferred embodiment, the sensing element comprises a hub 
supporting the sensing element on the sensing chip, a plurality of spring 
members extending radially from the hub, and a ring circumscribing the 
spring members and supported by the spring members. The trench 
circumscribing the sensing element separates the ring from the electrodes. 
To reduce crosstalk between the electrodes, the trench can be formed to 
surround each of the electrodes, such that each of the electrodes is a 
salient feature on the sensing chip, and adjacent electrodes are 
dielectrically isolated by a portion of the trench. 
The method for fabricating the sensor of this invention generally entails 
the steps of forming the sensing element, electrodes, trench and contacts 
for the sensing chip, and forming the circuit chip and its signal 
conditioning circuitry, contacts and conductive runners. Importantly, the 
contacts on the circuit chip are located so as to register with the 
appropriate contacts on the sensing chip. Preferably, the solder used to 
form the seal and make the electrical connections between the contacts is 
then deposited on one of the sensing and circuit chips. Thereafter, the 
circuit chip is attached to the sensing chip so as to enclose the sensing 
element between the circuit chip and the sensing chip, and simultaneously 
register and electrically connect the contacts on the circuit chip with 
the contacts on the sensing chip. 
From the above, it can be appreciated that the present invention provides a 
motion sensor characterized by having its inter-chip electrical 
connections formed between the chips, instead of exterior wiring or 
runners routed from chip to chip. Furthermore, the method of this 
invention yields a micromachined motion sensor capable of achieving high 
reliability and high performance as a result of trench isolation between 
the sensing element and its sensing electrodes, and also between adjacent 
electrodes. Another advantage of this invention is that the resulting 
motion sensor can be fabricated to yield a relatively large sensing 
element within a relatively small sensor package. 
Other objects and advantages of this invention will be better appreciated 
from the following detailed description.

DETAILED DESCRIPTION OF THE INVENTION 
FIGS. 1 and 2 represent an all-silicon monolithic sensing wafer 12 for a 
motion sensor 10 shown in FIGS. 3 and 5, all of which are in accordance 
with the present invention. As illustrated, the sensor 10 includes a 
micromachined sensing element 14 formed in the sensing wafer 12, and a 
circuit wafer 16 bonded to the sensing wafer 12 so as to enclose the 
sensing element 14. In accordance with this invention, conditioning 
circuitry 18 for the sensing element 14 is formed on the circuit wafer 16. 
Contacts, such as the bond pad 20 shown, are present on the sensing and 
circuit wafers 12 and 16 and enable the conditioning circuitry 18 and the 
sensing element 14 to be electrically interconnected with solder bumps 22. 
In addition, bond pads 24 formed on the circuit wafer 16 enable electrodes 
26 formed on the sensing wafer 12 to be electrically interconnected to the 
conditioning circuitry 18 with solder bumps 22. Finally, according to a 
preferred aspect of this invention, a seal 28 is formed by a continuous 
bead of solder on a bond pad 25 that completely surrounds the sensing 
element 14 and electrodes 26. The seal 28 forms a hermetic seal between 
the sensing and circuit wafers 12 and 16, thus enabling the sensing 
element 14 to be enclosed in an evacuated cavity 30 defined between the 
wafers 12 and 16. 
As shown in FIG. 1, the sensing element 14 is formed by a ring 32 that is 
supported by a number of arcuate springs 34 extending from a center post 
36. As best seen in FIG. 1, the electrodes 26 form an electrode pattern 
that circumscribes the ring 32 without physically contacting the ring 32. 
The ring 32 and the electrodes 26 are formed to be electrically 
conductive, as will be explained below, enabling the ring 32 to form a 
capacitor with each of the electrodes 26 when a voltage potential is 
present. In a preferred embodiment, some of the electrodes 26 are 
energized to drive the ring 32 into resonance, others are energized to 
balance the resonant peaks of the rotary movement by inducing stiffness in 
the ring 32 and springs 34, while the remaining electrodes 26 are used to 
capacitively sense a signal generated from the ring 32 as a result of 
being subjected to rotary motion. With this construction, the sensor 10 is 
able to detect rotary movement about the vertical axis through the center 
post 16 and, therefore, rotary movement about an axis of a body, such as 
an automobile, to which the sensor 10 is mounted. Those skilled in the art 
will appreciate that, though described in terms of a motion sensor, the 
structure of FIGS. 1-3 and its method of manufacture can also be adapted 
for use as an accelerometer. 
As shown in FIGS. 1-3 and 5, the sensing element 14 and each of the 
electrodes 26 are defined in a near-surface region of the sensing wafer 12 
by a trench 38. As such, the sensing element 14 and the electrodes 26 are 
freestanding conductive structures of the sensing wafer 12. To facilitate 
their fabrication and maximize spacing therebetween, the electrodes 26 are 
preferably formed to have the shape shown in FIG. 1. The shape of the 
electrodes 26 is useful to avoid shorting between the ring 32 and the 
solder 22 forming the connections between the bond pads 24 of the sensing 
and circuit wafers 12 and 16. The larger solder bump bond pad 24 is 
located away from the ring 32, while a thinner radial extension extends 
toward the ring 32. The radial extension maximizes the width of the trench 
38 between electrodes 26 in the region of the ring 32 as compared to the 
gap between each electrode 26 and the ring 32, with the result that 
crosstalk and parasitic capacitance between the electrodes 26 is 
significantly reduced. The inner radial end of each electrode 26 is 
preferably T-shaped as shown to increase the capacitive plate area, or 
otherwise to increase the capacitive signal produced between the ring 32 
and the electrodes 26. A conductive strip (not shown) can be formed along 
the radial extension of each electrode 26 to reduce parallel resistance. 
As more readily seen in FIGS. 2, 3 and 5, the sensing element 14 and the 
electrodes 26 are also dielectrically isolated by the trench 38, as a 
result of the trench 38 extending down to a dielectric sublayer 44, e.g., 
a layer of silicon oxide or silicon nitride within the sensing wafer 12. 
As such, the sensing element 14, electrodes 26 and the seal 28 surrounding 
the electrodes 26 are both physically and electrically isolated from each 
other. The electrical connections provided by the solder bumps 22 between 
the bond pads 20 and 24 on the sensing and circuit wafers 12 and 16 
eliminate the conventional requirement for metal runners that provide 
conductive paths from the interior of the motion sensor 10 to an exterior 
region of the sensing wafer 12, where wire bond connections would 
typically be formed. 
As is apparent from FIG. 1, unetched surface regions of the sensing wafer 
12 define finger-like projections 40 having their terminal ends adjacent 
the ring 32. When the surface of the sensing wafer 12 is appropriately 
processed, and with the ends of the projections 40 closer to the ring 32 
than the electrodes 26, the projections 40 can serve as g-stops that 
prevent the ring 32 from sticking to the electrodes 26 due to excessive 
lateral motion. The projections 40 are shown as being electrically 
interconnected to contacts 42, enabling the projections 40 to be held at 
approximately the same potential as the ring 32 so as to prevent the ring 
32 from becoming electrostatically held by, and therefore irreparably 
shorted to, one of the electrodes 26 following fabrication of the sensor 
10 or in the event the sensor 10 is subjected to an acceleration 
sufficiently high to deflect the ring 32 toward one of the electrodes 26. 
The above structure yields a motion sensor 10 characterized by a monolithic 
construction. The relative physical dimensions indicated in the Figures 
are primarily for illustrated purposes and should not be construed as a 
restriction to the teachings of the present invention. Essentially, the 
size of the sensing element 14, sensing and circuit wafers 12 and 16, and 
the sensor 10 as a whole can vary considerably, with the process 
encompassed by this invention being most efficient if multiple sensors are 
simultaneously formed from a wafer stack that is subsequently sawed to 
separate individual sensors. 
Processing steps for the fabrication and assembly of the sensing wafer 12 
of FIGS. 1 and 2 are represented in FIGS. 4A and 4B, which represent the 
same cross-section of the sensing wafer 12 shown in FIG. 2. With reference 
to FIG. 2, the sensing wafer 12 is shown as being comprised of a 
heavily-doped silicon region 12a over a substrate 12b. The sensing element 
14, electrodes 26 and projections 40 are formed from the silicon region 
12a in order to be electrically conductive. The silicon region 12a may be 
single-crystal or polycrystalline, and doped either P+ or N+ depending on 
the type of silicon etching used to form the trench 38. The substrate 12b 
may be a silicon or glass. If the substrate 12b is silicon, the dielectric 
sublayer 44 is silicon oxide to provide a bond interface and an etch stop 
between the silicon region 12a and the substrate 12b, as will be explained 
below. It is also foreseeable that the sublayer 44 could be silicon 
nitride. If the substrate is glass, such as PYREX 7740, the sublayer 44 is 
unnecessary as an etch stop or to form the bond between the silicon region 
12a and substrate 12b. 
With reference to FIG. 4A, a suitable process for fabricating the sensing 
wafer 12 involves bonding together two silicon wafer slices 46a and 46b. 
The upper wafer slice 46a includes the heavily-doped silicon region 12a 
and a lightly-doped region 12c that will be subsequently removed by 
etching. The silicon region 12a may be a diffused layer or an 
epitaxially-grown layer, or a combination of both. Alternatively, it is 
foreseeable that the entire wafer 46a could be heavily doped to yield the 
silicon region 12a required by the sensing wafer 12. The second wafer 46b 
includes a lightly-doped region that forms the substrate 12b. In a 
preferred embodiment, the second wafer 46b also has a thin heavily-doped 
silicon layer 46 that can later serve as a ground plane for the motion 
sensor 10, and an oxide layer 48 that protects the silicon layer 46 during 
etching. Finally, each of the wafers 46a and 46b further includes an oxide 
layer 44a and 44b that, once the wafers 46a and 46b are bonded together, 
form the dielectric sublayer 44 of the sensing wafer 12. For the 
embodiment in which the heavily-doped silicon region 12a is a diffused 
layer using boron as the dopant, oxidation of the surface of the silicon 
region 12a to form the oxide layer 44a causes the doping profile in the 
wafer 46a to be more uniform, which reduces the tendency for warpage of 
the micromachined sensing element 14 formed from the silicon region 12a. 
FIG. 4B shows the lightly-doped region 12c removed from the wafer 46a, the 
bond pads 20, 24 and 25 formed on the exposed surface of the silicon 
region 12a, and an oxide mask 50 overlying the silicon region 12a and bond 
pads 20, 24 and 25. The oxide mask 50 is then patterned to allow the 
trench 38 to be formed by a high aspect ratio anisotropic etch in 
accordance with known etching techniques. FIG. 2 represents the result of 
performing the etching step and removing the oxide mask 50. The etching 
process is carried out such that the trench 38 forms the capacitive 
structures that include the ring 32, the adjacent wall portions of the 
electrodes 26, and the gaps therebetween. As can be seen from FIG. 1, the 
etching process also serves to form the springs 34. 
As illustrated in FIG. 3, the etching process can also be used to remove 
peripheral regions of the sensing wafer 12 to enable access to a portion 
16a of the circuit wafer 16 on which one or more bond pads 27 are formed. 
These bond pads 27 are electrically connected to the integrated circuit 
18, and serve as connection sites with which the motion sensor 10 can be 
wire bonded to its exterior packaging (not shown). 
Thereafter, the oxide sublayer 44 exposed at the bottom of the trench 38 
can be selectively etched, such that the sensing element 14 is undercut as 
seen from FIG. 2. The result of this etching process is the creation of 
the post 36, which enables the sensing element 14 to resonate. A suitable 
etching technique for the oxide sublayer 44 is to employ a buffered 
solution of dilute hydrofluoric acid (HF). 
As an alternative to the above, the sensing element 14 can be formed 
entirely from a heavily-doped polysilicon layer grown on top of the oxide 
sublayer 44 if the substrate 12b is silicon, or directly on the substrate 
12b if the substrate 12b is glass. As those skilled in the art will 
appreciate, use of heavily-doped polysilicon as the sensing element 14 
would require annealing at temperatures of about 1100.degree. C. for 
several hours to reduce stresses induced in the polysilicon during doping. 
As noted previously, an alternative would be to grow amorphous silicon or 
a combination epitaxial-polysilicon layer using known deposition methods. 
Processing of the circuit wafer 16 can proceed in accordance with 
conventional techniques to form suitable signal conditioning circuitry 18, 
as well as the bond pads 20, 24 and 25 for the sensing element 14, 
electrodes 26 and seal 28, respectively. The bond pads 20 and 24 are 
electrically interconnected by appropriate means to the conditioning 
circuitry 18. As represented in FIG. 5, metal runners 52 are formed on the 
lower surface of the circuit wafer 16, so as to be enclosed within the 
cavity 30 formed between the wafers 12 and 16. As shown, the runners 52 
can serve to electrically interconnect bond pads 24 for certain paired 
electrodes 26 that serve as balance electrodes for the sensing element 14 
if used as a gyroscope or yaw sensor. Any number of runners 52 can be 
formed to appropriately interconnect the other electrical structures of 
the sensing wafer 12, e.g., the sensing element 14, other electrodes 26 
and g-stop projections 40, as may be necessary for the desired operation 
of the sensor 10. As an alternative to metal runners 52, polysilicon or 
diffused conductors could be used in accordance with known practices in 
the art. 
As also shown in FIG. 5, the circuit wafer 16 may be formed to include a 
ground plane 54 of metal or polysilicon that is insulated from the bond 
pads 20, 24 and 25 by a dielectric layer 56, such as an oxide. The 
presence of the ground plane 54 greatly reduces crosstalk and parasitic 
signals between the bond pads 20, 24 and 25 and the runners 52. 
Finally, to obtain the motion sensor 10 shown in FIGS. 3 and 5, the sensing 
and circuit wafers 12 and 16 are bonded together. Bonding is preferably 
done under vacuum with a fluxless solder, eutectic bonding or through the 
compression of a conductive material between the bond pads 20, 24 and 25 
on both wafers 12 and 16. Compression bonding can be performed at room or 
elevated temperatures, and be assisted by forming the bond pads 20, 24 and 
25 over bumps of silicon nitride or oxide, which assist in concentrating 
the compression force at the center of the bond pads 20, 24 and 25. As is 
evident from FIGS. 3 and 5, the advantageous effect of forming inter-chip 
connections with the bond pads 20 and 24 and solder 22 is the elimination 
of metal runners that are conventionally required to provide conductive 
paths from the interior of the motion sensor 10 to an exterior region of 
the sensing wafer 12, where wire bond connections are required to make the 
inter-chip connections between the sensing wafer 12 and the circuit wafer 
16. 
FIG. 6 is an alternative embodiment of this invention, in which a shallow 
annular-shaped cavity 58 is etched into the surface of the substrate 12b 
prior to bonding the wafers 46a and 46b (FIGS. 4A and 4B) together. During 
the subsequent trench etch into the silicon region 12a, the trench 38 
extends to the cavity 58, with the result that a larger vertical gap is 
formed between the sensing element 14 and the underlying substrate 12b, 
thereby reducing the likelihood of stiction between the sensing element 14 
and substrate 12b. 
FIG. 6 shows another optional feature of this invention, in which the 
heavily-doped silicon region 12a is thicker than that represented for 
FIGS. 2, 3 and 5. As shown, an annular-shaped cavity 60 is formed in the 
surface of the silicon region 12a prior to the wafers 46a and 46b being 
bonded together. The cavity 60 serves to reduce the stiffness of the 
springs 34 of the sensing element 14, as may be desired if the silicon 
region 12a is relatively thick as shown. The effect is that the sensing 
ring 32 has a tubular shape, as can be appreciated from FIG. 6. The 
greater surface area of the ring allows the bias voltage for the ring 32 
to be greatly reduced, since the capacitive signal produced between the 
ring 32 and the electrodes 26 is proportional to voltage and the 
capacitive plate area. Alternatively, the trench 38 could be patterned to 
yield comb-like features on the sensing element 14 for sensing 
acceleration. Such a sensing element is disclosed in Funk et al., 
Surface-Micromachining of Resonant Silicon Structures, Transducers 
'95--Eurosensors IX, The 8th International Conference on Solid State 
Sensors and Actuators, and Eurosensors IX (Jun. 25-29, 1995) pp. 50-52. 
FIG. 7 is a plan view of another embodiment for the sensing wafer 12 of 
this invention, in which the surface area of the sensing wafer 12 
subjected to the trench etch is significantly increased, such that the 
only portions of the silicon region 12a remaining include the sensing 
element 14, the electrodes 26 and raised regions 62 for the bond pads 42. 
According to this aspect of the invention, removal of the heavily-doped 
silicon region 12a to the extent shown will have the greatest impact on 
reducing parasitic lateral capacitance within the sensing wafer 12. 
FIG. 7 also illustrates the situation in which bond pads 20 for the sensing 
element 14 are formed on electrodes 24a between adjacent pairs of springs 
34. In this embodiment, the bond pads 20 can be electrically connected to 
the sensing element 14 through a heavily-doped region in the substrate 
12b. By offsetting the bond pads 20 in this manner, complications caused 
by a thermal mismatch between the solder 22 and sensing element 14 are 
avoided. 
As also shown in FIG. 7, multiple inner electrodes 64 can be formed beneath 
the ring 32 and between adjacent springs 34. For this purpose, the shape 
of the springs 34 is preferably modified to the S-shape shown in FIG. 7. 
Each of the inner electrodes 64 is preferably defined by the trench etch, 
and has a bond pad 66 similar to the bond pads 20 for the sensing element 
14. The use of inner electrodes 64 in combination with the outer 
electrodes 26 is advantageous to obtain differential signals compensate 
for changes in capacitance due to thermal expansion of the ring 32, and 
for increasing the sensing area, and hence the capacitive signal, obtained 
from the sensing element 14. 
While conventional silicon processing materials can be employed to form the 
sensor 10 of this invention, other materials can be used, including 
elemental and compound semiconductor materials, or layers of conducting 
and insulating materials. As those skilled in the art will appreciate, the 
choice of materials will determine the appropriate processes for forming 
selective regions of conductivity and resistance in the wafers. In 
addition, while the polarity of p-type and n-type regions in the wafers 
are generally interchangeable, each conductivity type incurs tradeoffs 
that would be appreciated and accommodated by one skilled in the art. 
Consequently, it is foreseeable that the present invention can be utilized 
to encompass a multitude of applications through the addition or 
substitution of other processing or sensing technologies. 
While our invention has been described in terms of a preferred embodiment, 
other forms could be adopted by one skilled in the art. Accordingly, the 
scope of our invention is to be limited only by the following claims.