Siavash Pourkamali

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.

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.

VHF single crystal silicon capacitive elliptic bulk-mode disk resonators-part II: implementation and characterization

This work, the second of two parts, reports on the implementation and characterization of high-quality factor (Q) side-supported single crystal silicon (SCS) disk resonators. The resonators are fabricated on SOI substrates using a HARPSS-based fabrication process and are 3 to 18 /spl mu/m thick. They consist of a single crystal silicon resonant disk structure and trench-refilled polysilicon drive and sense electrodes. The fabricated resonators have self-aligned, ultra-narrow capacitive gaps in the order of 100 nm. Quality factors of up to 46 000 in 100 mTorr vacuum and 26000 at atmospheric pressure are exhibited by 18 /spl mu/m thick SCS disk resonators of 30 /spl mu/m in diameter, operating in their elliptical bulk-mode at /spl sim/150 MHz. Motional resistance as low as 43.3 k/spl Omega/ was measured for an 18-/spl mu/m-thick resonator with 160 nm capacitive gaps at 149.3 MHz. The measured electrostatic frequency tuning of a 3-/spl mu/m-thick device with 120 nm capacitive gaps shows a tuning slope of -2.6 ppm/V. The temperature coefficient of frequency for this resonator is also measured to be -26 ppm//spl deg/C in the temperature range from 20 to 150/spl deg/C. The measurement results coincide with the electromechanical modeling presented in Part I.

VHF single-crystal silicon elliptic bulk-mode capacitive disk resonators-part I: design and modeling

This work, the first of two parts, presents the design and modeling of VHF single-crystal silicon (SCS) capacitive disk resonators operating in their elliptical bulk resonant mode. The disk resonators are modeled as circular thin-plates with free edge. A comprehensive derivation of the mode shapes and resonant frequencies of the in-plane vibrations of the disk structures is described using the two-dimensional (2-D) elastic theory. An equivalent mechanical model is extracted from the elliptic bulk-mode shape to predict the dynamic behavior of the disk resonators. Based on the mechanical model, the electromechanical coupling and equivalent electrical circuit parameters of the disk resonators are derived. Several considerations regarding the operation, performance, and temperature coefficient of frequency of these devices are further discussed. This model is verified in part II of this paper, which describes the implementation and characterization of the SCS capacitive disk resonators.

High-Frequency Thermally Actuated Electromechanical Resonators With Piezoresistive Readout

This paper presents fabrication, characterization, and modeling of micro/nanoelectromechanical high-frequency resonators actuated using thermal forces with piezoresistive readout. Thermally actuated single-crystalline silicon resonators with frequencies (up to 61 MHz) have been successfully demonstrated. It is shown both theoretically and experimentally that, as opposed to the general perception, thermal actuation can be a viable actuation mechanism for high-frequency resonators, and using appropriate design guidelines, this actuation mechanism could even be more suitable for higher frequency rather than lower frequency applications. It has been shown through comprehensive thermoelectro-mechanical modeling that thermal-piezoresistive nanomechanical resonators with frequencies in the gigahertz range can exhibit motional conductance values as high as 1 mA/V while consuming static power as low as a few microwatts.

A 2.5-V 14-bit /spl Sigma//spl Delta/ CMOS SOI capacitive accelerometer

This paper presents a 2.5-V 14-bit fully differential /spl Sigma//spl Delta/ interface circuit in 0.25-/spl mu/m CMOS technology for a high-resolution silicon-on-insulator capacitive accelerometer fabricated using a simple CMOS-compatible stictionless process. The integrated circuit is based on programmable front-end back-end first-order /spl Sigma//spl Delta/ architecture and provides a 1-bit pulse-width modulated digital output. Using correlated double sampling, the low-frequency noise is suppressed by 10 dB. Capacitive resolution is 22 aF at 75 Hz (resolution bandwidth = 1 Hz), equivalent to 110 /spl mu/g with a dynamic range of 85 dB (14-bit resolution) and a sensitivity of 500 mV/g. The chip occupies 2 mm/sup 2/ and consumes 6 mW.

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.

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.

Individual Air-Borne Particle Mass Measurement Using High-Frequency Micromechanical Resonators

This work demonstrates mass measurement of individual submicron air-borne particles using resonant micromechanical nano-balances. Thermally actuated high-frequency single crystalline silicon resonators fabricated using a single mask process have been used as mass sensors. Mass sensitivity of the resonators have been characterized using artificially generated airborne particles of known size and composition. Mass sensitivities as high as 1.6 kHz/pg have been demonstrated for devices with resonant frequencies in the tens of MHz range. The measured mass sensitivities are in good agreement with the calculated values based on the resonator physical dimensions. Due to the high mass sensitivities, the shift in the resonator frequencies caused by individual particles as small as ~200 nm in diameter is distinguishable. Counting and individual mass measurement of single arbitrary particles in air samples from a cleanroom have also been demonstrated. The results in this work present the possibility of implementation of low-cost and small-size instruments for airborne particle mass and size distribution analysis in highly controlled environments (e.g., for cleanroom classification) or for environmental applications.