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OVERVIEW Silicon Designs, Inc. (SDI) developed a miniature accelerometer 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 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. The SDI approach is also one of the first commercial successes of non-silicon MEMS (Micro-ElectroMechanical Systems) sensors. MEMS ACCELEROMETER 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. 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. PACKAGING 
 MICRO-MACHINED SENSE ELEMENT The basic structure of the SDI sense element is shown below. The 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 side 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 bars.  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 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 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. SENSE ELEMENT FABRICATION 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 spacer material. 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, as 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. In 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. A single wafer contains approximately 1600 individual sense elements, and is tested and diced in-house to complete processing. ELECTRONICS CHIP 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 (analog 1210 or 1221) without major redesign and allows for the use of familiar analog signal processing techniques. The availability of a digital accelerometer (digital 1010) 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. |