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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates generally to the field of devices that are
responsive to acceleration, particularly to devices which provide
switching action in response to multiple threshold levels of acceleration
and specifically to miniature mechanical switching devices fabricated from
silicon.
As performance requirements have advanced there has been an increased
demand for acceleration switches which are smaller in size and lower in
cost but offer improved accuracy and reliability. Micromechanical devices
of silicon can be batch-fabricated using silicon technology and as such
can be made to high accuracy and high reliability at relatively low cost.
SUMMARY OF THE INVENTION
The present invention utilizes multiple cantilever beams integral to an
accelerometer die which is sandwiched between a recessed glass contact
plate having fixed contacts and a glass support plate. Each cantilever
beam carries an integral silicon end mass and a movable contact.
Deflection of a cantilever beam due to acceleration causes the switch
contacts to close. The threshold acceleration levels at which the contacts
close is determined by the contact gap distance, beam length, beam width,
beam thickness, and end mass value. Thus, application specific
acceleration switches may be designed to offer the needed acceleration
switching levels and to fit into standard packaging configurations. The
present invention utilizes a contact arrangement which permits operation
even when an off-normal input acceleration causes some rotation of the end
mass and twisting of the beam. Further, mechanical biasing is provided
which reduces the sensitivity of the accelerometer to noise and increases
its resistance to mechanical shock.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention may be best understood from a reading of the following
detailed description with the drawings in which:
FIG. 1 is a perspective partially cross-sectioned view of an accelerometer
switch in accordance with the principles of the invention;
FIG. 2 is an enlarged fragmentary cross-sectional view of an accelerometer
switch in accordance with the principles of the invention; and
FIG. 3 is a cross-sectional view of an accelerometer switch in accordance
with the principles of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 illustrates a partially cross-sectioned pictorial view of an
embodiment of the invention in which reference numeral 10 generally
identifies a microminiature accelerometer switch. The accelerometer switch
includes an accelerometer die 12, a contact plate 14, and a backplate 16,
each of which may be fabricated by using the batch-fabrication techniques
used in microelectronics. The use of such techniques to make miniature
mechanical devices is well known and described in various publications,
for example "Silicon Micromechanical Devices," Scientific American, pp.
44-55, April 1983.
In device 10, die 12 is planar and in the shape of a frame surrounding an
opening or void 18. Accelerometer beams 20, 22, and 24 extend into void
18. The general structure of the accelerometer beams is similar, and
accelerometer beam 20 will be described as typical. Accelerometer beam 20
is of the cantilever type having a fixed end 26, an intermediate section
28, and a free end 30. An integral end mass 31 is carried by free end 30.
The starting material for silicon die 12 may be a boron-doped silicon wafer
with an epitaxially grown layer of phosphorous doped silicon. This layer
will be the thickness of beam 20 after the processing described. The front
side of the wafer will be oxidized to provide an insulating substrate
which will serve as the upper surface 33 of die 12 and on which a contact
material may be deposited.
A standard tri-metal system may be used to deposit conductor material on
upper surface 33 of die 12. The metal used may be a sandwich of three
metals such as titanium-tungsten, palladium, and gold. Gold forms the
actual electrical contact surface.
Using standard etching practices, the metal may then be etched to leave the
necessary conductive runners. Specifically, runners 32, 34, and 36 will be
needed on first contact faces 38, 40, and 42, respectively, of
accelerometer beams 20, 22, and 24, respectively. Runners 32, 34, and 36
may extend from near the free end of their respective beams to wire bond
pads 44, 46, and 48 located on the periphery of surface 33. The width of
metal runners 32, 34, and 36 on first contact faces 38, 40, and 42,
respectively, may be chosen to give adequate tolerance for alignment, yet
not significantly alter the mechanical behavior of the accelerometer
beams. In addition, contact pad 50, runner 52, and wire bond pad 54 will
be needed to provide, electrical input to device 10. In the assembled
accelerometer switch, feed-through conductors such as 56 are associated
with the four wire bond pads 44, 46, 48, and 54. The feed-through
conductors are electrically connected to their associated wire bond pad by
a wire as shown in the figures.
After completion of the front side wafer processing, the wafer will be
lapped down from the backside to the desired thickness of the die 1 and
then polished to optical quality. Silicon nitride may then be deposited on
the backside and patterned to open the areas in which the silicon will be
etched. A silicon anisotropic etchant such as a solution of potassium
hydroxide, alcohol, and water may be used to thin the silicon chip in the
appropriate region where beams are to be formed. For example, the etching
will determine the thickness of beam 20, and the end mass 31 may be formed
by masking during the etching so that silicon is left at the end of the
beam.
Anisotropic etching of (100) oriented silicon results in the sloping side
surfaces of the beam 20, the end mass 31, and void 18 as shown in the
figures.
Finally the beams will be defined by laser milling through the remaining
silicon web. A computer-controlled, YAG, thin film laser trimming system
with optical positioning may be used.
Contact plate 14 may be formed from glass such as glass wafers using
similar microelectronic techniques. Plate 14 is of smaller size than die
12 so that when plate 14 is placed over die 12, it covers void 18 but does
not cover wire bond pads 44, 46, 48, and 54. The inner side of contact
plate 14 has a peripheral mating surface 58 for mating with surface 33 of
die 12 and a second contact face 60. Contact face 60 is generally
recessed, and according to an important feature of the present invention,
the recessed area is in a two-level configuration. The first level 62 is a
raised portion of the contact face 60 having a contact 63. Second level 64
is the contact face surrounding the raised portion 62.
The starting material for contact plate 14 will be glass wafers polished to
optical flatness. The glass may be batch-processed in a way similar to the
silicon. The glass will first be masked and etched to form an etch to the
level of the raised portion 62 of the contact face 60. The glass will then
be patterned and etched to the depth of the surrounding second level 64.
Contact plate 14 will then be metallized using the same metal system as
used on accelerometer die 12. The metal will then be etched to form the
opposing fixed contact wire runner 66 and contact pad 68.
Accelerometer backplate 16 is fabricated from a glass wafer. The backplate
increases the overall strength of the accelerometer backplate 16 includes
peripheral mating surface 72 and recessed area 74. In accordance with an
important feature of the present invention, the same metal system used
with the die and contact plate may be used to leave a small pedestal or
bump 76 directly below the end mass. Pedestal 76 provides for mechanical
biasing of the accelerometer beam toward closure, making it less
responsive to vibration and noise and more resistant to mechanical shock.
Accelerometer die 12, contact plate 14, and backplate 16 may be aligned and
bonded together using the known technique of anodic bonding. This
technique produces a hermetic and irreversible seal between silicon and
glass. The result is a bond without an interface material such as epoxy or
solder so that the gap 65 between first contact 39 and second contact 63
is defined solely by the depth of the glass etch forming raised portion
62, the thickness of the contact metal on raised portion 62, and the
thickness of the metal on first contact face 38.
The operation of the accelerometer switch may be described with reference
to typical beam 20. In operation, an acceleration in the direction of
arrow 1 in FIG. 2 will cause a deflection from the relaxed or no
acceleration condition. As the acceleration increases, accelerometer beam
20 will move to an increasingly strained condition with free end 30 moving
toward contact face 60. If the acceleration is sufficient, first contact
39 will strike second contact 63.
Frequently the acceleration is not normal to the surface 33 of die 12. For
example, the acceleration may contain a component transverse to beam 20,
as indicated by arrow 2 in FIG. 2. As further illustrated in FIG. 2, a
transverse acceleration will cause twisting of beam 20 and some rotation
of end mass 31 about an axis parallel to the longitudinal axis of beam 20.
It may be appreciated that the two-level configuration of contact face 60
will allow end mass 31 to tilt or have some rotation but still allow
closure of first contact 39 and second contact 63. Specifically, second
level 64 provides a relief to allow for the tilting or rotation of end
mass 31.
The invention disclosed herein may be embodied in other specific forms
without departing from the spirit of the invention. Thus, the scope of the
invention is to be indicated by the appended claims.
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