SDI's Capacitive Accelerometer Technology
Silicon Designs, Inc. (SDI) has been the leader in capacitive MEMS accelerometer technology for over 25 years. Our latest technology combines bulk silicon micro machined capacitive MEMS and integrated circuit technology to produce a highly reliable, exceptionally rugged, capacitive acceleration sensor. Since initial development, ongoing research has resulted in improvements that 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.
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 changes 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. Moreover, the differential design of the SDI MEMS accelerometer ensures that errors due to thermal expansion are cancelled.
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 response to DC accelerations as well as dynamic vibration. This allows the 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 the integrated electronics or ASIC chip (see figure below). The chips 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 or Digital 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 modules.
MICRO-MACHINED SENSE ELEMENT (MEMS)
The basic structure of the SDI sense element is shown below. The sense element consists of a plate of single crystal silicon that is free to rotate about a torsional axis. Each side of the plate form capacitors with electrodes on the fixed substrate. The structure is asymmetrically shaped so that one side is heavier than the other, resulting in a center of mass that is offset from the axis of rotation.
On the substrate surface, beneath the sense element wing, two conductive capacitor plates are symmetrically located on 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 capacitance 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 sensitivity of the 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 sensing capacitors.
The SDI accelerometer design is unique in that it contains to moving sense elements in a mirror image configuration. This performs two functions. First, common mode asymmetries are eliminated by the electronics. Second, the acceleration dependent cross axis sensitivity of pendulous accelerometers is eliminated. In a standard pendulous accelerometer, as the proof mass rotates out of plane, it becomes sensitive to accelerations in the cross axis. These unwanted inputs are cancelled by the two mirrored moving plates in the SDI accelerometer.
SENSE ELEMENT FABRICATION
Sense element fabrication is carried out in SDI's own state of the art MEMS clean room fabrication facility at the Silicon Designs Headquarters. Unlike other companies that outsource production of sense elements, SDI has been building MEMS accelerometers in its own on site clean room for over 25 years. As such, they are a world leader in MEMS fabrication technology.
Over time as market entrants into the high precision MEMS accelerometer segment have come and gone, SDI has continuously developed and improved its technology. With packaging, fabrication, and calibration all located in the same building, along with top floor sales, marketing, and product development, SDI has leveraged the synergies that emerge from the close collaboration of these business units. Silicon Designs invests heavily in internal research and development of its sensor fabrication and packaging. These efforts continuously and directly drive improvement in the quality, reliability and performance of our accelerometers.
ELECTRONICS CHIP (ASIC)
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 sense 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 this ASIC; one provides a digital output and the other provides an analog output. The digital ASIC generates a pulse stream whose frequency (or, more precisely, pulse density) is proportional to acceleration. The analog ASIC generates a differential voltage output proportional to acceleration.
Having both analog and digital accelerometers increases the number of applications that can take advantage of our unique sense element technology. Most test equipment and older accelerometer systems are based on analog sensors. This makes it simple to switch to a higher performance SDI accelerometer without major redesign and allows for the use of familiar analog signal processing techniques. The availability of a digital accelerometer allows for easy integration with modern microprocessor based systems without the trouble of additional A/D conversion. A simple microprocessor, such as one from Microchip's PIC series, is all that is needed to read the accelerometer output.