Silicon Designs, Inc. (SDI) developed a miniature
technology which combines additive micro machining and integrated
circuit technology to produce a highly reliable, exceptionally rugged,
capacitive acceleration sensor. Since initial
development, ongoing research has resulted in improvements
have increased the reliability, sensitivity, and survivability of our
accelerometers to a point that they are being used in entirely new
areas such as inertial navigation and high temperature environments.
The SDI approach is also one of the first commercial
non-silicon MEMS (Micro-ElectroMechanical Systems) sensors.
SDI's accelerometers use capacitance change due to acceleration force
as the sensed parameter. A capacitive approach allows several
benefits when compared to the piezoresistive sensors used in many other
accelerometers. In general, gaseous dielectric capacitors are
relatively insensitive to temperature. Although spacing
with temperature due to thermal expansion, the low thermal coefficient
of expansion of many materials can produce a thermal coefficient of
capacitance about two orders of magnitude less than the thermal
coefficient of resistivity of doped silicon.
Capacitance sensing therefore has the potential to provide a wider
temperature range of operation, without compensation, than
piezoresistive sensing. As compared with piezoelectric type
accelerometers, which require a dynamic input of some minimum frequency
to generate a response, SDI capacitive sensing allows for
to DC accelerations as well as dynamic vibration. This allows
capacitive accelerometer to be used in a wider range of applications.
Silicon Designs' basic accelerometer unit is a 20 pin LCC package
containing two parts: the Sense Element or sensor chip and
integrated electronics or ASIC chip (see figure below). The
are attached using standard die attach and gold wire bonding techniques
and the package is solder sealed to provide a simple, strong, fully
hermetic device. Built with one of two ASIC chips to provide
either an Analog
output, this basic accelerometer can be easily surface mounted to a
circuit board and is used to build all of SDI's single and three axis
MICRO-MACHINED SENSE ELEMENT
The basic structure of the SDI sense element is shown below.
sense element wing is a flat plate of nickel supported above the
substrate surface by two torsion bars attached to a central
pedestal. The structure is asymmetrically shaped so that one
is heavier than the other, resulting in a center of mass that is offset
from the axis of the torsion bars. When an acceleration force
produces a moment around the torsion bar axis, the plate or wing is
free to rotate, constrained only by the spring constant of the torsion
On the substrate surface, beneath the sense
wing, two conductive capacitor plates are symmetrically
each side of the torsion bar axis. The upper wing and the two lower
capacitor plates on the substrate form two air-gap variable capacitors
with a common connection. This creates a fully active
bridge. When the wing rotates about the torsion bar axis, the
average distance between the wing and one surface plate decreases,
increasing the capacitance for that plate, while the distance to the
other plate increases, decreasing its capacitance.
The sense element wings are approximately 1000 microns long by 600
microns wide and 5 to 10 microns thick. The wing to substrate
spacing of about 5 microns results in a capacitance from the wing to
each lower plate of about 0.15 pF. The sensitivity
sense elements (the ratio of deflection to acceleration) is determined
by the mass of the sense element, the distance from the center of mass
to the torsion bar axis, and the torsion bar stiffness. Each
complete sense element chip contains two wings for a total of four
Fabrication of surface structures using selective electroforming is
different from traditional methods for building MEMS devices in bulk
silicon. In this technique, a metal is electroplated onto a
conductive substrate through a patterned photo resist layer.
After the photo resist has been stripped, the metal remains on the
surface in a pattern determined by open areas of the photo
resist. To produce suspended sense elements, the structure is
fabricated partially on the top of a previously deposited sacrificial
After the sense elements have been formed, the spacer material is
removed, leaving the sense element supported only where it was formed
directly on the surface. The use of such additive techniques,
opposed to the selective etching limitations of silicon processing,
allows for more complex structures to be produced with a potential for
partially enclosed voids or complex multi-layered structures.
addition, with recent photoresist technology advances, high aspect
ratio, straight wall features can be created with ease.
Silicon Designs' sensor production is carried out with four inch wafer
substrates utilizing standard photoresist processing techniques, UV
contact aligners, and custom built electroplating equipment.
single wafer contains approximately 1600 individual sense elements, and
is tested and diced in-house to complete processing.
The second key component in this design is the ASIC (application
specific integrated circuit) which is needed to convert the small
capacitance changes of the sense element into a useful electrical
signal. These electronics must be closely coupled to the
element to accurately measure the miniscule acceleration-caused changes
in capacitance that occur in the presence of much larger stray
capacitances. Silicon Designs has developed two versions of
ASIC; one provides a digital output and the other provides an analog
output. The digital ASIC generates a pulse stream whose
(or, more precisely, pulse density) is proportional to
acceleration. The analog ASIC generates a differential
output proportional to acceleration.
Having both analog
accelerometers increases the number of applications that can take
advantage of our unique sense element technology.
equipment and older accelerometer systems are based on analog
sensors. This makes it simple to switch to a higher
SDI accelerometer (analog 1210 or 1221) without major redesign and
allows for the use of familiar analog signal processing
techniques. The availability of a digital accelerometer
1010) allows for easy integration with modern microprocessor based
systems without the trouble of additional A/D
simple microprocessor, such as one from Microchip's PIC series, is all
that is needed to read the accelerometer output.