Farrokh Ayazi

Micromachined inertial sensors

This paper presents a review of silicon micromachined accelerometers and gyroscopes. Following a brief introduction to their operating principles and specifications, various device structures, fabrication, technologies, device designs, packaging, and interface electronics issues, along with the present status in the commercialization of micromachined inertial sensors, are discussed. Inertial sensors have seen a steady improvement in their performance, and today, microaccelerometers can resolve accelerations in the micro-g range, while the performance of gyroscopes has improved by a factor of 10/spl times/ every two years during the past eight years. This impressive drive to higher performance, lower cost, greater functionality, higher levels of integration, and higher volume will continue as new fabrication, circuit, and packaging techniques are developed to meet the ever increasing demand for inertial sensors.

A HARPSS polysilicon vibrating ring gyroscope

This paper presents the design, fabrication, and testing of an 80-/spl mu/m-thick, 1.1 mm in diameter high aspect-ratio (20:1) polysilicon ring gyroscope (PRG). The vibrating ring gyroscope was fabricated through the high aspect-ratio combined poly and single-crystal silicon MEMS technology (HARPSS). This all-silicon single-wafer technology is capable of producing electrically isolated vertical electrodes as tall as the main body structure (50 to 100s (/spl mu/m tall)) with various size air-gaps ranging from submicron to tens of microns. A detailed analysis has been performed to determine the overall sensitivity of the vibrating ring gyroscope and identify its scaling limits. An open-loop sensitivity of 200 /spl mu/V/deg/s in a dynamic range of /spl plusmn/250 deg/s was measured under low vacuum level for a prototype device tested in hybrid format. The resolution for a PRG with a quality factor (Q) of 1200, drive amplitude of 0.15 /spl mu/m, and sense node parasitic capacitances of 2 pF was measured to be less than 1 deg/s in 1 Hz bandwidth, limited by the noise from the circuitry. Elimination of the parasitic capacitances and improvement in the quality factor of the ring structure are expected to reduce the resolution to 0.01 deg/s/(Hz)/sup 0.5/.

Process and temperature compensation in a 7-MHz CMOS clock oscillator

This paper reports on the design and characterization of a process, temperature and supply compensation technique for a 7-MHz clock oscillator in a 0.25-/spl mu/m, two-poly five-metal (2P5M) CMOS process. Measurements made across a temperature range of -40/spl deg/C to 125/spl deg/C and 94 samples collected over four fabrication runs indicate a worst case combined variation of /spl plusmn/2.6% (with process, temperature and supply). No trimming was performed on any of these samples. The oscillation frequencies of 95% of the samples were found to fall within /spl plusmn/0.5% of the mean frequency and the standard deviation was 9.3 kHz. The variation of frequency with power supply was /spl plusmn/0.31% for a supply voltage range of 2.4-2.75 V. The clock generator is based on a three-stage differential ring oscillator. The variation of the frequency of the oscillator with temperature and process has been discussed and an adaptive biasing scheme incorporating a unique combination of a process corner sensing scheme and a temperature compensating network is developed. The biasing circuit changes the control voltage of the differential ring oscillator to maintain a constant frequency. A comparator included at the output stage ensures rail-to-rail swing. The oscillator is intended to serve as a start-up clock for micro-controller applications.

High aspect-ratio combined poly and single-crystal silicon (HARPSS) MEMS technology

This paper presents a single-wafer high aspect-ratio micromachining technology capable of simultaneously producing tens to hundreds of micrometers thick electrically isolated poly and single-crystal silicon microstructures. High aspect-ratio polysilicon structures are created by refilling hundreds of micrometers deep trenches with polysilicon deposited over a sacrificial oxide layer. Thick single-crystal silicon structures are released from the substrate through the front side of the wafer by means of a combined directional and isotropic silicon dry etch and are protected on the sides by refilled trenches. This process is capable of producing electrically isolated polysilicon and silicon electrodes as tall as the main body structure with various size capacitive air gaps ranging from submicrometer to tens of micrometers. Using bent-beam strain sensors, residual stress in 80-/spl mu/m-thick 4-/spl mu/m-wide trench-refilled vertical polysilicon beams fabricated in this technology has been measured to be virtually zero. 300-/spl mu/m-long 80-/spl mu/m-thick polysilicon clamped-clamped beam micromechanical resonators have shown quality factors as high as 85 000 in vacuum. The all-silicon feature of this technology improves long-term stability and temperature sensitivity, while fabrication of large-area vertical pickoff electrodes with submicrometer gap spacing will increase the sensitivity of micro-electromechanical devices by orders of magnitude.

High-Q single crystal silicon HARPSS capacitive beam resonators with self-aligned sub-100-nm transduction gaps

This paper reports on the fabrication and characterization of high-quality factor (Q) single crystal silicon (SCS) in-plane capacitive beam resonators with sub-100 nm to submicron transduction gaps using the HARPSS process. The resonating element is made of single crystal silicon while the drive and sense electrodes are made of trench-refilled polysilicon, yielding an all-silicon capacitive microresonator. The fabricated SCS resonators are 20-40 /spl mu/m thick and have self-aligned capacitive gaps. Vertical gaps as small as 80 nm in between 20 /spl mu/m thick silicon structures have been demonstrated in this work. A large number of clamped-free and clamped-clamped beam resonators were fabricated. Quality factors as high as 177000 for a 19 kHz clamped-free beam and 74000 for an 80 kHz clamped-clamped beam were measured under 1 mtorr vacuum. Clamped-clamped beam resonators were operated at their higher resonance modes (up to the fifth mode); a resonance frequency of 12 MHz was observed for the fifth mode of a clamped-clamped beam with the fundamental mode frequency of 0.91 MHz. Electrostatic tuning characteristics of the resonators have been measured and compared to the theoretical values. The measured Q values of the clamped-clamped beam resonators are within 20% of the fundamental thermoelastic damping limits (Q/sub TED/) obtained from finite element analysis.

Thin-film piezoelectric-on-silicon resonators for high-frequency reference oscillator applications

This paper studies the application of lateral bulk acoustic thin-film piezoelectric-on-substrate (TPoS) resonators in high-frequency reference oscillators. Low-motional impedance TPoS resonators are designed and fabricated in 2 classes--high-order and coupled-array. Devices of each class are used to assemble reference oscillators and the performance characteristics of the oscillators are measured and discussed. Since the motional impedance of these devices is small, the transimpedance amplifier (TIA) in the oscillator loop can be reduced to a single transistor and 3 resistors, a format that is very power-efficient. The lowest reported power consumption is ~350 muW for an oscillator operating at ~106 MHz. A passive temperature compensation method is also utilized bThis paper studies the application of lateral bulk acoustic thin-film piezoelectric-on-substrate (TPoS) resonators in high-frequency reference oscillators. Low-motionalimpedance TPoS resonators are designed and fabricated in 2 classes--high-order and coupled-array. Devices of each class are used to assemble reference oscillators and the performance characteristics of the oscillators are measured and discussed. Since the motional impedance of these devices is small, the transimpedance amplifier (TIA) in the oscillator loop can be reduced to a single transistor and 3 resistors, a format that is very power-efficient. The lowest reported power consumption is ~350 muW for an oscillator operating at ~106 MHz. A passive temperature compensation method is also utilized by including the buried oxide layer of the silicon-on-insulator (SOI) substrate in the structural resonant body of the device, and a very small (-2.4 ppm/degC) temperature coefficient of frequency is obtained for an 82-MHz oscillator.y including the buried oxide layer of the silicon-on-insulator (SOI) substrate in the structural resonant body of the device, and a very small (-2.4 ppm/degC) temperature coefficient of frequency is obtained for an 82-MHz oscillator.

Piezoelectric-on-Silicon Lateral Bulk Acoustic Wave Micromechanical Resonators

This paper reports on the design, fabrication, and characterization of piezoelectrically-transduced micromechanical single-crystal-silicon resonators operating in their lateral bulk acoustic modes to address the need for high-Q microelectronic-integrable frequency-selective components. A simple electromechanical model for optimizing performance is presented. For verification, resonators were fabricated on 5-mum-thick silicon-on- insulator substrates and use a 0.3-mum zinc oxide film for transduction. A bulk acoustic mode was observed from a 240 mum times 40 mum resonator with a 600-Omega impedance (Q=3400 at P=1 atm) at 90 MHz. A linear resonator absorbed power of -0.5 dBm and an output current of 1.3 mA rms were measured. The same device also exhibited a Q of 12 000 in its fundamental extensional mode at a pressure of 5 torr.

Electronically Temperature Compensated Silicon Bulk Acoustic Resonator Reference Oscillators

The paper describes the design and implementation of an electronically temperature compensated reference oscillator based on capacitive silicon micromechanical resonators. The design of a 5.5-MHz silicon bulk acoustic resonator has been optimized to offer high quality factor (> 100 000) while maintaining tunability in excess of 3000 ppm for fine-tuning and temperature compensation. Oscillations are sustained with a CMOS amplifier. When interfaced with the temperature compensating bias circuit, the oscillator exhibits a frequency drift of 39 ppm over 100degC as compared to an uncompensated frequency drift of 2830 ppm over the same range. The sustaining amplifier and compensation circuitry were fabricated in a 2P3M 0.6-mum CMOS process.

A Mode-Matched Silicon-Yaw Tuning-Fork Gyroscope With Subdegree-Per-Hour Allan Deviation Bias Instability

In this paper, we report on the design, fabrication, and characterization of an in-plane mode-matched tuning-fork gyroscope (M2-TFG). The M2-TFG uses two high-quality-factor (Q) resonant flexural modes of a single crystalline silicon mi- crostructure to detect angular rate about the normal axis. Operating the device under mode-matched condition, i.e., zero-hertz frequency split between drive and sense modes, enables a Q -factor mechanical amplification in the rate sensitivity and also improves the overall noise floor and bias stability of the device. The M2 -TFG is fabricated on a silicon-on-insulator substrate using a combination of device and handle-layer silicon etching that precludes the need for any release openings on the proof-mass, thereby maximizing the mass per unit area. Experimental data indicate subdegree-per-hour Brownian noise floor with a measured Allan deviation bias instability of 0.15deg /hr for a 60-mum-thick 1.5 mm X 1.7 mm footprint M2-TFG prototype. The gyroscope exhibits an open-loop rate sensitivity of approximately 83 mV/deg/s in vacuum. [2007-0100].

Low-Impedance VHF and UHF Capacitive Silicon Bulk Acoustic Wave Resonators—Part I: Concept and Fabrication

This paper presents high-performance high-frequency single-crystal silicon (SCS) capacitive resonators. Long and thick bulk-micromachined resonating block structures, which are referred to as ldquosilicon bulk acoustic wave resonatorrdquo (SiBAR), are fabricated using the high-aspect-ratio poly and single crystalline siliconrdquo (HARPSS) fabrication process on silicon-on-insulator (SOI) substrates. Such resonators operate in their horizontal width extensional modes with quality factors in the range of 10000-100000. With their comparatively large electrode area and deep-submicrometer capacitive transduction gaps, such resonators have demonstrated comparatively low impedances for capacitive resonators that are well within the desired range for high-frequency electronic applications. Sub-kilo-Ohm total electrical resistances and extracted motional resistance as low as 200 are demonstrated for the fundamental width extensional modes of SiBARs in the very-high-frequency range. Resonant frequencies up to 1.55 GHz are demonstrated for the higher resonance modes of the capacitive SiBARs with comparatively low impedances. Part I of this paper presents the basic operation concepts and fabrication methodology for the HARPSS-on-SOI SiBARs. Extensive resonator measurement data, including temperature characteristics, are presented in Part II of this paper, and different frequency tuning approaches for temperature compensation of such resonators are discussed and investigated.