Surface micromachined mechanisms and micromotors

Electric micromotors are sub-millimeter sized actuators capable of unrestrained motion in at least one degree of freedom. Polysilicon surface micromachining using heavily phosphorus-doped LPCVD polysilicon for the structural material, LPCVD silicon nitride for the electrical isolation and deposited silicon dioxide for the sacrificial material has formed the fabrication technology base for the development of these micromotors. Two polysilicon surface micromachining processes, referred to here as the center-pin and flange, have been demonstrated for the fabrication of passive mechanisms and micromotors. Passive mechanisms such as gear trains, cranks and manipulators have been implemented on silicon. Reported operational micromotors have been of the rotary variable-capacitance salient-pole and harmonic (or wobble) side-drive designs. These micromotors are capable of motive torques in the 10 pN m order of magnitude range. Preliminary progress has been made in studying the operational, friction and wear characteristics of these micromechanical devices. Typical operational voltages have been as low as 37 V and 26 V across 1.5 mu m air gap salient-pole and harmonic micromotors. These excitations correspond to electric field intensities above 10(8) Vm-1 in the micromotor air gaps. Salient-pole and wobble micromotors have been reported to operate at speeds as high as 15000 rpm and 700 rpm, respectively. Micromotor lifetimes of at least many millions of cycles over a period of several days have been reported

Ultrasensitive mode-localized micromechanical electrometer

We report a highly sensitive prototype micromechanical electrometer that employs the phenomena of mode-localization and curve veering for monitoring minute charge fluctuations across an input capacitor. The device consists of a pair of weakly coupled, nearly identical single crystal silicon, double-ended tuning fork (DETF) resonators. An addition of charge across an input capacitor on one of the coupled resonators induces a differential axial strain on that resonator relative to the other consequently perturbing the structural symmetry of the nearly periodic system. The resulting shifts in the eigenstates for the same magnitudes of charge input are theoretically and experimentally demonstrated to be nearly three orders of magnitude greater than corresponding resonant frequency variations. The topology chosen may also be adapted for force or strain monitoring thereby widening the relevance of the results reported here to precision inertial sensing as well

CMOS systems and circuits for sub-degree per hour MEMS gyroscopes

The objective of our research is to develop system architectures and CMOS circuits that interface with high-Q silicon microgyroscopes to implement navigation-grade angular rate sensors. The MEMS sensor used in this work is an in-plane bulk-micromachined mode-matched tuning fork gyroscope (M² – TFG ), fabricated on silicon-on-insulator substrate. The use of CMOS transimpedance amplifiers (TIA) as front-ends in high-Q MEMS resonant sensors is explored. A T-network TIA is proposed as the front-end for resonant capacitive detection. The T-TIA provides on-chip transimpedance gains of 25MΩ, has a measured capacitive resolution of 0.02aF /√Hz at 15kHz, a dynamic range of 104dB in a bandwidth of 10Hz and consumes 400μW of power. A second contribution is the development of an automated scheme to adaptively bias the mechanical structure, such that the sensor is operated in the mode-matched condition. Mode-matching leverages the inherently high quality factors of the microgyroscope, resulting in significant improvement in the Brownian noise floor, electronic noise, sensitivity and bias drift of the microsensor. We developed a novel architecture that utilizes the often ignored residual quadrature error in a gyroscope to achieve and maintain perfect mode-matching (i.e.0Hz split between the drive and sense mode frequencies), as well as electronically control the sensor bandwidth. A CMOS implementation is developed that allows mode-matching of the drive and sense frequencies of a gyroscope at a fraction of the time taken by current state of-the-art techniques. Further, this mode-matching technique allows for maintaining a controlled separation between the drive and sense resonant frequencies, providing a means of increasing sensor bandwidth and dynamic range. The mode-matching CMOS IC, implemented in a 0.5μm 2P3M process, and control algorithm have been interfaced with a 60μm thick M2−TFG to implement an angular rate sensor with bias drift as low as 0.1°/hr ℃ the lowest recorded to date for a silicon MEMS gyro.Ph.D.Committee Chair: Farrokh Ayazi; Committee Member: Jennifer Michaels; Committee Member: Levent Degertekin; Committee Member: Paul Hasler; Committee Member: W. Marshall Leac

Silicon micromachined resonant accelerometer with CMOS interface circuits

Ph.DDOCTOR OF PHILOSOPH

The tapered-width micro-cantilevers: design and characterization

Tapered-width cantilevers are proposed to give designers more design space in obtaining lower pull-in voltage and higher capacitance change for a cantilever with relatively short length and moderate area. Several types of the width taper functions have been investigated in this research. Based on the analytical form of the pull-in voltage for a simple cantilever, we have obtained empirical forms of the pull-in voltages for tapered-width cantilevers. These formulae can be the basis of the pull-in voltage synthesis by geometry control of a cantilever and might be helpful for integrated cantilever sensor systems in which only low actuating voltage is available. The measured pull-in voltages of cantilevers together with these formulae were used to extract the Young\u27s modulus of the material (polysilicon), which is 175 GPa with a standard deviation 40 GPa.;For applications of micro-mechanical structures, on-chip circuitry is preferred to do the signal conditioning to avoid parasitics of bonding pads/wires, which is usually comparable to or larger than the motional induced capacitance change if interest. Unfortunately, not every micro-machining process is compatible with the fabrication processes for integrated circuits; instead they are usually very customized for their particular applications. In this project, we have investigated RF measurement techniques for the characterization of microstructures and a direct-conversion circuit based on the electromechanical amplitude modulation/demodulation scheme has been built for possible off-chip readout circuitry applications. A direct measurement of input voltage and through-current was used to obtain the input admittances of devices under test, from which the equivalent circuits can be extracted

Interface Circuit for a Multiple-Beam Tuning-Fork Gyroscope with High Quality Factors

This research work presents the design, theoretical analysis, fabrication, interface electronics, and experimental results of a Silicon-On-Insulator (SOI) based Multiple-Beam Tuning-Fork Gyroscope (MB-TFG). Based on a numerical model of Thermo-Elastic Damping (TED), a Multiple-Beam Tuning-Fork Structure (MB-TFS) is designed with high Quality factors (Qs) in its two operation modes. A comprehensive theoretical analysis of the MB-TFG design is conducted to relate the design parameters to its operation parameters and further performance parameters. In conjunction with a mask that defines the device through trenches to alleviate severe fabrication effect on anchor loss, a simple one-mask fabrication process is employed to implement this MB-TFG design on SOI wafers. The fabricated MB-TFGs are tested with PCB-level interface electronics and a thorough comparison between the experimental results and a theoretical analysis is conducted to verify the MB-TFG design and accurately interpret the measured performance. The highest measured Qs of the fabricated MB-TFGs in vacuum are 255,000 in the drive-mode and 103,000 in the sense-mode, at a frequency of 15.7kHz. Under a frequency difference of 4Hz between the two modes (operation frequency is 16.8kHz) and a drive-mode vibration amplitude of 3.0μm, the measured rate sensitivity is 80μVpp/°/s with an equivalent impedance of 6MΩ. The calculated overall rate resolution of this device is 0.37/°hr/√Hz, while the measured Angle Random Walk (ARW) and bias instability are 6.67°/\u27√hr and 95°/hr, respectively

Suspended microchannel resonators for biomolecular detection

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2005.Includes bibliographical references (leaves 115-124).Microfabricated transducers enable the label-free detection of biological molecules in nanoliter sized samples. Integrating microfluidic detection and sample-preparation can greatly leverage experimental efforts in systems biology and pharmaceutical research by increasing analysis throughput while dramatically reducing reagent cost. Microfabricated resonant mass sensors are among the most sensitive devices for chemical detection, but degradation of the sensitivity in liquid has so far hindered their successful application in biology. This thesis introduces a type of resonant transducer that overcomes this limitation by a new device design: Adsorption of molecules to the inside walls of a suspended microfluidic channel is detected by measuring the change in mechanical resonance frequency of the channel. In contrast to resonant mass sensors submersed in water, the sensitivity and frequency resolution of the suspended microchannel resonator is not degraded by the presence of the fluid. Our device differs from a vibrating tube densitometer in that the channel is very thin, and only molecules that bind to the walls can build up enough mass to be detected; this provides a path to specificity via molecular recognition by immobilized receptors.(cont.) Suspended silicon nitride channels have been fabricated through a sacrificial polysilicon process and bulk micromachining, and the packaging and microfluidic interfacing of the resonant sensors has been addressed. Device characterization at 30 mTorr ambient pressure reveals a quality factor of more than 10,000 for water filled resonators; this is two orders of magnitude higher than previously demonstrated Q-values of resonant mass sensors for biological measurements. Calculation of the noise and the sensitivity of suspended microchannel resonators indicate a physical limit for mass resolution of approximately 0.01 ng/cm2 (1 Hz bandwidth). A resolution of -0.1 ng/cm2 has been experimentally demonstrated in this work. This resolution constitutes a tenfold improvement over commercial quartz crystal microbalance based instruments. The ability to detect adsorbing biomolecules by resonance frequency has been validated through binding experiments with avidin and various biotinylated proteins.by Thomas P. Burg.Ph.D