William C. Tang

Laterally Driven Polysilicon Resonant Microstructures

Abstract Interdigitated finger (comb) structures are demonstrated to be effective for exciting electrostatically the resonance of polysilicon microstructures parallel to the plane of the substrate. Linear plates suspended by a folded-cantilever truss and torsional plates suspended by spiral and serpentine springs are fabricated from a 2 μm-thick phosphorus-doped low-pressure chemical-vapor-deposited (LPCVD) polysilicon film. Resonance is observed visually, with frequencies ranging from 18 kHz to 80 kHz and quality factors from 20 to 130. Simple beam theory is adequate for calculating the resonance frequencies, using a Youngs modulus of 140 GPa and neglecting residual strain in the released structures.

Electrostatic-comb drive of lateral polysilicon resonators

This paper mvestlgates the elcctrostatlc dnve and sense of polynhcon resonators parallel to the substrate, using an mterdlgtated capacitor (electrostatic comb) Three expenmental methods are used nucroscoplc observation mth contmuous or stroboscopic iummatlon, capacltlve sensing using an amphtude-modulation technique and SEM observation The mtnnslc quality factor of the phosphorus-doped low-pressure chemlcal-vapordeposited (LPCVD) polyslhcon resonators is 49 000 + 2000, whereas at atmosphenc pressure, Q < 100 The finger gap IS found to have a more pronounced effect on comb charactenstlcs than finger width or length, as expected from simple theory

Electrostatic Comb Drive Levitation And Control Method

This paper presents the theory, simulation results, and experimental study of the levitating force (normal to the substrate) associated with Interdigitated capacitor (electro static comb) lateral actuators. For compliant suspensions, normal displacements of over 2 /spl mu/m for a comb bias of 30 V are observed. This phenomenon is due to electrostatic attraction induced on top of the suspended structure. By electrically lsolating alternating drive-comb fingers and applying voltages of equal magnitude and opposite sign, levitation force can be reduced by an order of magnitude, while reducing the lateral drive force by less than a factor of 2. The levitation theory incorporates electrostatic simulation results, and agrees well with experimental data.

Laterally driven polysilicon resonant microstructures

Interdigitated finger (comb) structures are demonstrated to be effective for electrostatically exciting the resonance of polysilicon microstructures parallel to the plane of the substrate. Linear plates suspended by a folded-cantilever truss and torsional plates suspended by spiral and serpentine springs are fabricated from a 2- mu m-thick phosphorus-doped low-pressure chemical vapor deposited polysilicon film. Resonance is observed visually, with frequencies ranging from 18 kHz to 80 kHz and quality factors from 20 to 130. Simple beam theory is adequate for calculating the resonant frequencies, using a Youngs modulus of 140 GPa and neglecting residual strain in the released structures.<<ETX>>

Viscous air damping in laterally driven microresonators

A systematic experimental study of viscous air damping in laterally moving planar microstructures is reported. Previous studies indicated that Couette and Stokes flow models underestimated microstructural damping. To investigate this discrepancy, a series of lateral resonant microstructures with different damp plates and combs was fabricated with polysilicon surface micromachining. The resonant frequencies and quality factors of the structures were measured electrically. By examining these data, the damping due to different geometries were isolated and compared to theory. The results indicated that if edge and finite-size effects are included in the model, reasonably accurate prediction on the quality factors can be obtained even for small geometries and comb drives. An empirical formula was developed that predicts quality factor for a range of plate size and comb designs. The damping effects as a function of structural thickness and structure-to-substrate separation are also reported.

MEMS micro-valve for space applications

We report on the development of a micro-electro-mechanical systems (MEMS) valve that is designed to meet the rigorous performance requirements for a variety of space applications, such as micro-propulsion, in situ chemical analysis of other planets, or microbiology. These systems often require very small yet reliable silicon valves with extremely low leak rates and long shelf lives. Also, they must survive the perils of space travel, which include unstoppable radiation, monumental shock and vibration forces, as well as extreme variations in temperature. Currently, no commercial MEMS valve meets these requirements. At JPL, we are developing a piezoelectric MEMS valve that attempts to address the unique problem of space. We begin with proven configurations that may seem familiar. However, we have implemented some major design innovations that should produce a superior valve. The JPL micro-valve is expected to have an extremely low leak rate, limited susceptibility to particulates, vibration or radiation, as well as a wide operational temperature range.

KNUDSEN COMPRESSOR AS A MICRO- AND MACROSCALE VACUUM PUMP WITHOUT MOVING PARTS OR FLUIDS

Applications of Knudsen compressors as both microscale and macroscale vacuum pumps have been investigated. The study is based on a cascade analysis incorporating available transitional thermal transpiration and Poiseuille flow results for slender channels. It was found that the Knudsen compressor is an attractive possibility for microscale pumps down to a pressure of about 1 mTorr and for macroscale pumps to about 0.1 mTorr. A microscale pump for a micromass spectrometer providing a molecule flow rate of 5×1014 molecules/s results in the following pump characteristics: energy use of 2.4 W, pump volume of 13.9 ml at an inlet pressure of 1 mTorr and an energy use of 28.5 mW, and pump volume of 0.16 ml at an inlet pressure of 10 mTorr. A macroscale pump providing a pumping speed of 103 l/s results in a pump with an energy use of 1786 W, and pump volume of 1695 l at an inlet pressure of 0.1 mTorr. Several Knudsen compressor characteristics such as pressure rise, pumping speed, volume, energy use and mass flow...

Surface-micromachined capacitive differential pressure sensor with lithographically defined silicon diaphragm

A capacitive surface-micromachined sensor suitable for the measurement of liquid and gas pressures was fabricated. The structure consists of a polysilicon stationary electrode suspended 0.7 /spl mu/m above a 20-/spl mu/m-thick lightly doped silicon diaphragm formed by a patterned etch stop. The a priori patterning of the buried etch stop yields diaphragm widths independent of wafer thickness variations with excellent alignment. The design described here has a pressure range of 100 PSI, a nominal capacitance of 3.5 pF with a full scale span of 0.8 pF, and a temperature coefficient of 100 ppm/spl deg/C/sup -1/. Each device, including a matched reference capacitor, occupies 2.9 mm/sup 2/, yielding approximately 2000 devices per 100-mm wafer.

Electrostatically balanced comb drive for controlled levitation

An experimental study of the levitation force, normal to the substrate, associated with interdigital capacitor electrostatic comb lateral actuators is reported. For compliant suspensions, normal displacements of over 2 mu m for a comb bias of 30 V were observed. This phenomenon is due to electrostatic repulsion by image charges mirrored in the ground plane beneath the suspended structure. By electrically isolating alternating drive-comb fingers and applying voltages of equal magnitude and opposite sign, levitation can be reduced by an order of magnitude while reducing the lateral drive force by less than a factor of two.<<ETX>>

Total dose effects on Microelectromechanical Systems (MEMS): accelerometers

Microelectromechanical sensors, ADXL50 and XMMAS40G accelerometers which are fabricated with surface micromachining techniques are characterized for their total dose radiation response. Different failure mechanisms were observed when the sensor element or the whole device was irradiated.