Micromachined Accelerometers With Optical Interferometric Read-Out and Integrated Electrostatic Actuation

Abstract

A micromachined accelerometer device structure with diffraction-based optical detection and integrated electrostatic actuation is introduced. The sensor consists of a bulk silicon proof mass electrode that moves vertically with respect to a rigid diffraction grating backplate electrode to provide interferometric detection resolution of the proof-mass displacement when illuminated with coherent light. The sensor architecture includes a monolithically integrated electrostatic actuation port that enables the application of precisely controlled broadband forces to the proof mass while the displacement is simultaneously and independently measured optically. This enables several useful features such as dynamic self-characterization and a variety of force-feedback modalities, including alteration of device dynamics in situ. These features are experimentally demonstrated with sensors that have been optoelectronically integrated into sub-cubic-millimeter volumes using an entirely surface-normal, rigid, and robust embodiment incorporating vertical cavity surface emitting lasers and integrated photodetector arrays. In addition to small form factor and high acceleration resolution, the ability to self-characterize and alter device dynamics in situ may be advantageous. This allows periodic calibration and in situ matching of sensor dynamics among an array of accelerometers or seismometers configured in a network.

Towards a sub 15-dBA optical micromachined microphone

“…This system has been modeled and experimentally studied in detail in several prior works, with application to force microscopy, 42 ultrasonic sensors, 43 accelerometers, 44,45 and microphones. 37 The system has been used to create surface-mountable optical MEMS microphones 23 as presented schematically in Fig.…”

Low Voltage Electrostatic Actuation and Displacement Measurement Through Resonant Drive Circuit

“…Van Spengen et al [29] also use the capacitance measurement to detect a displacement of a comb drive. Hall et al [30] use diffraction-based optical detection to measure the displacement of an accelerometer. Although this measurement method is immune to the parasitic capacitance and provides high sensitivity to the displacement, its fabrication is more complicated and expensive to be implemented.…”

“…In turn, this increase in a capacitance results in less voltage gain and more phase delay in an actuation voltage and a current that flows through the resonant drive circuit. Through the measurement of this change in the amplitude or the phase of the voltage and the current with respect to the input voltage, we are able to measure a displacement of an electrostatic actuator without any sensing electrodes [28] or any optical diffraction measurement [30]. An envelope detector is implemented to measure a change in a voltage gain of the resonant drive circuits, and a phase detector is used to measure a phase change of an actuation voltage or a current through the resonant drive circuits with reference to an input AC voltage.…”

Micro-system inertial sensing technology overview.

“…The physical mechanism for a vibratory gyroscope is the transfer of energy from one resonator axis to another via the Coriolis acceleration coupling. The suspension for this device can have a unique natural frequency,  x ,  y and a unique damping ratio for  x ,  y each axis, equation 10. The relative positioning of the suspension natural frequencies is a gyroscope design decision.…”