A highly sensitive micromachined accelerometer that is intended to be exposed to very small accelerations during operation incorporates features for protection against much larger accelerations during handling and transport. More specifically, the accelerometer is designed to measure acceleration as small as 10 -8× that of normal Earth gravitation (g), yet can withstand accelerations as large as 200 g when the protective function is activated.
The accelerometer is made from four bulk micromachined silicon dice. Two of the dice are made into a square proof mass 1 cm² in area and 0.8 mm thick supported by eight folded cantilever springs, each spring being 1 cm long, 100 µm wide, and 25 µm thick. Another die contains four quadrature electrodes, plus a tip electrode that is placed near an electrode on the proof mass. The displacement of the proof mass is inferred from measurement of the quantum-mechanical-tunneling electron current between the tip electrode and an electrode on the proof mass; this tunneling-tip aspect of the design is similar to that of a scanning tunneling microscope and offers the advantage of high sensitivity (displacements as small as 10 -3Å can be measured). The remaining die, called the "force plate," contains a single electrode that is used to control the position of the proof mass.
The spring-and-mass and tip-electrode structures are delicate; this is an unavoidable aspect of the design necessary for achieving the required sensitivity, but it makes these structures vulnerable to damage in the presence of large accelerations. Accordingly, during handling and transport, the single control electrode on the force plate is also used to clamp the proof mass away from the tip electrode and against the force plate. This clamping is effected by applying sufficient electrostatic potential (see figure) between the control electrode and an electrode on the proof mass to pull the proof mass into contact with the force plate. An oxide layer on the surface of the force plate prevents undesired electrical contact between the control and proof-mass electrodes.
This work was done by Frank T. Hartley of Caltech and Paul M. Zavracky of Northeastern University for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Physical Sciences category, or circle no. 188 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).