Gavin K. Ho

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.

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.

High frequency micromechanical piezo-on-silicon block resonators

This paper reports on the design, implementation and characterization of high-frequency single crystal silicon (SCS) block resonators with piezoelectric electromechanical transducers. The resonators are fabricated on 4/spl mu/m thick SOI substrates and use sputtered ZnO as the piezo material. The centrally-supported blocks can operate in their first and higher order length extensional bulk modes with high quality factor (Q). The highest measured frequency is currently at 210 MHz with a Q of 4100 under vacuum, and the highest Q measured is 11,600 at 17 MHz. The uncompensated temperature coefficient of frequency (TCF) was measured to be -40ppm//spl deg/C and linear over the temperature range of 20-100/spl deg/C.

Vertical capacitive SiBARs

This work introduces high frequency, vertical silicon bulk acoustic resonators (SiBAR). A combination of the new resonator structures with much larger transduction area and the HARPSS fabrication process is used to demonstrate high frequency capacitive resonators with significantly lower impedances compared to the previous capacitive resonators. Impedances as low as a few kilo-Ohms and quality factors in the range of 20,000 to 50,000 in the VHF range have been achieved for the first thickness mode of the fabricated resonators. Resonant frequencies as high as 983MHz are demonstrated for the third thickness modes of the capacitive SiBARs.

Quality factor in trench-refilled polysilicon beam resonators

In this paper, thermoelastic damping (TED) in trench-refilled (TR) polysilicon microelectromechanical beam resonators is studied as a mechanism for limiting quality factor (Q) at low frequencies. An approximate model based on Zeners theory is developed and verified by numerical simulations in FEMLAB. According to the proposed model a double-dip characteristic is expected for the quality factor versus frequency curve of TR beam resonators. To verify the model experimentally, equal-width TR micro-resonators are fabricated in different length to cover a broad range of frequencies. Frequency response of these devices agrees well with our model. By using the theoretical and numerical models developed in this paper, an upper bound for the quality factor in TR beam resonators or any similar structure such as TR polysilicon gyros can be predicted.

Temperature Compensated IBAR Reference Oscillators

This work presents a two-chip automatically temperature-compensated micromechanical IBAR reference oscillator with a temperature drift of 39ppm over 100 ° C. Temperature compensation is provided by a parabolic VPcorrection scheme and provides 10X improvement over previously reported results. Tunable 6MHz, 10MHz, and 20MHz resonators were characterized with 2000– 4500ppm tuning and Q up to 119000. Motional impedances as low as 218Ω were extracted from measurement data with VP= 18V. The interface IC for temperature compensation and oscillation consumes only 1.9mW. Measurements also show that temperature compensation of a 10MHz resonator with 65nm gaps is possible with less than 5V.

High-order composite bulk acoustic resonators

In this article, we present lateral and thickness mode low-impedance UHF resonators to obtain dispersed- frequency devices simultaneously on a single substrate. The low-impedance is enabled by using high-order modes of resonators consisting of a piezoelectric transduction film on an underlying silicon layer. The impedance of these devices reduces as mode number increases. This is attributed to the increase in transduction area. The lowest measured impedance is 55Omega at 373MHz. Resonators with 373MHz and 640MHz lateral modes and 2.5GHz thickness modes from the same substrate are presented.