Wanling Pan

Thin-film piezoelectric-on-substrate resonators with Q enhancement and TCF reduction

In this paper we report on lateral mode thin-film piezoelectric-on-substrate (TPoS) resonators with techniques to enhance the quality factor (Q) and reduce the temperature coefficient of frequency (TCF). Such techniques utilize a highly or degenerately doped Si substrate layer as the ground electrode, and reduce the thickness and volume ratio between the AlN piezoelectric layer and the Si substrate. By patterning the AlN and eliminating the bottom metal electrode, a record quality factor of over 30,000 is observed in air at 27MHz (≫66,000 in vacuum). The highly-doped Si brings the resonator average TCF to around −12ppm/°C.

A 76 dB

This paper reports on the design and characterization of a high-gain tunable transimpedance amplifier (TIA) suitable for gigahertz oscillators that use high-Q lateral micromechanical resonators with large motional resistance and large shunt parasitic capacitance. The TIA consists of a low-power broadband current pre-amplifler combined with a current-to-voltage conversion stage to boost the input current before delivering it to feedback voltage amplifiers. Using this approach, the TIA achieves a constant gain of 76 dB-Ohm up to 1.7 GHz when connected to a 2 pF load at the input and output with an input-referred noise below 10 pA/√(Hz) in the 100 MHz to 1 GHz range. The TIA is fabricated in a 1P6M 0.18 μm CMOS process and consumes 7.2 mW. To demonstrate its performance in high frequency lateral micromechanical oscillator applications, the TIA is wirebonded to a 724 MHz high-motional resistance (Qunloaded ≈ 2000, Rm ≈ 750 Ω, CP ≈ 2 pF) and a 1.006 GHz high-parasitic (Qunloaded ≈ 7100, Rm ≈ 150 Ω, CP ≈ 3.2 pF) AIN-on-Silicon resonator. The 724 MHz and 1.006 GHz oscillators achieve phase-noise better than -87 dBc/Hz and -94 dBc/Hz @ 1 kHz offset, respectively, with a floor around -154 dBc/Hz. The 1.006 GHz oscillator achieves the highest reported figure of merit (FoM) among lateral piezoelectric micromechanical oscillators and meets the phase-noise requirements for most 2G and 3G cellular standards including GSM 900 MHz, GSM 1800 MHz, and HSDPA.

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This paper reports on the demonstration of series tuning for lateral micromechanical oscillators and its application for electronic temperature compensation of piezoelectric lateral bulk acoustic resonator (LBAR) micromechanical oscillators. Two aluminum nitride-on-silicon (AlN-on-Si) piezoelectric LBARs, one operating at 427 MHz (Rm ≈180 Ω, Qunloaded ≈ 1400) and the other operating at 541 MHz (Rm ≈ 55 Ω, Qunloaded ≈ 3000) are interfaced with a 13 mW three-stage tunable TIA implemented in 0.18 μm 1P6M CMOS process to sustain the oscillation. Recognizing the impact on the frequency tuning range due to the body capacitances appearing in parallel with the ports of the resonator, the TIA uses parasitic cancellation techniques to neutralize this effect and boost the tuning range of 427 MHz and 541 MHz oscillators, by as much as 12× to 810 ppm and 1,530 ppm, respectively, with negligible impact on the phase noise performance. The shunt parasitic capacitor is either resonated out with an active inductor or is cancelled out by using a single-terminal negative capacitor of equal value. However, the oscillator that uses negative capacitance parasitic cancellation yields larger tuning. This extended tuning range is used for temperature compensation. A 2 mW bandgap-based temperature compensation circuit which uses second-order parabolic approximation is fabricated on the same chip. Using this temperature compensation circuit has lowered the overall frequency drift of a 427 MHz tunable oscillator using negative capacitance cancellation from ±390 ppm to ±35 ppm in the -10°C to 70°C temperature range. The phase noise of this oscillator reaches -82 dBc/Hz at 1 kHz offset. The total phase noise variation for offset frequencies below 10 kHz is under 5 dB within the specified tuning range, and the best phase noise floor is under -147 dBc/Hz . Due to the higher Q and lower insertion loss of the resonating tank, the 541 MHz oscillator achieves -86 dBc/Hz at 1 kHz offset, and lower phase noise floor of -158 dBc/Hz.

1.7 GHz 0.18

A 1.8 GHz thin-film piezoelectric-on-substrate (TPoS) filter with insertion loss of less than 4 dB and bandwidth of 18 MHz (50Omega termination) is presented in this paper. Such filters are alternatives to conventional ladder-type filters working in the frequency range of up to a few GHz. A monolithic TPoS filter is a coupled mode system based on a single composite resonant structure, with a thin piezoelectric layer deposited on a high acoustic velocity, low loss substrate layer. An interdigitated top electrode structure is used that eliminates the spurious modes and provides a mechanism for adjusting the bandwidth.

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In this, the second of two papers, we present numerical simulations and comprehensive analysis of acoustically coupled thickness-mode AlN-on-Si filters. We simulate the scattering parameters of such acoustically coupled filters using commercially available finite element analysis software and compare the simulation results with a set of measurements. The simulations are in good agreement with the measurements, allowing the optimization of filter characteristics. We analyze the filter response under varying geometric parameters and demonstrate that variations in the top electrode geometry allow the design of low-loss filters (insertion loss <;5 dB) with percentage bandwidth up to about 1% and ripple less than 1 dB.

m CMOS Tunable TIA Using Broadband Current Pre-Amplifier for High Frequency Lateral MEMS Oscillators

This paper shows improved phase-noise performance of MEMS oscillators when the sustaining amplifier operates a lateral bulk acoustic wave AlN-on-Si resonator in the nonlinear regime. An empirical exponential-series-based model that closely describes the phase noise in nonlinearity is presented, reflecting the increased resonator filter order and the reduced amplifier flicker-noise contribution. An oscillator using a 23MHz in-plane shear mode resonator with a quality factor of 4,000 exhibits a phase noise of −130dBc/Hz at 1KHz offset-frequency, corresponding to an improvement of about 20dB with respect to linear operation.

Electronic Temperature Compensation of Lateral Bulk Acoustic Resonator Reference Oscillators Using Enhanced Series Tuning Technique

In this, the first of two papers, we present the working principle and the implementation of laterally acoustically coupled thickness-mode thin-film piezoelectric-on-substrate (TPoS) filters. This type of filter offers low insertion loss and small bandwidth in a broad frequency range- from a few hundred megahertz up to a few gigahertz-and occupy a small chip area. In this paper, we discuss several design concerns, including the choice of materials for TPoS filters. We demonstrate a design for an air-suspended AlN-on-Si filter, which offers a low insertion loss of 2.4 dB at 2.877 GHz. The bandwidth of this filter is 12 MHz with a return loss of better than 30 dB. In Part II of this paper, we present a comprehensive analysis of the effect of physical layout parameters on the frequency response of TPoS filters.

A low-loss 1.8GHz monolithic thin-film piezoelectric-on-substrate filter

This paper reports on the first demonstration of series tuning for lateral micromechanical oscillators and its application in a temperature-compensated 427MHz AlN-on-Si reference oscillator. The sustaining amplifier is a 13mW tunable TIA implemented in 0.18µm CMOS that uses shunt-parasitic cancellation to increase the tuning by 12× to 810ppm. The tunable oscillator along with a 2mW on-chip temperature compensation circuit has reduced the overall frequency drift to 70ppm in −10°C to 70°C. The phase-noise of the oscillator reaches −82dBc/Hz at 1kHz offset with floor below −147dBc/Hz.

Acoustically coupled thickness-mode AIN-on-Si band-pass filters-Part II: simulation and analysis

An approach to build thin-film piezoelectric-on-substrate (TPoS) filters working at different frequencies on the same wafer is proposed. In this approach, filters with different substrate layer thicknesses working at different orders of resonance are used. The insertion loss for each thickness and mode is predicted by introducing damping elements in the material constants used in Masonpsilas model simulations. In a first demonstration, ZnO-on-Si filters with different Si layer thicknesses are fabricated and tested. Measurement results are in good agreement with simulations. Several filters with low insertion loss at different frequencies are fabricated on the same wafer.

Phase noise shaping via forced nonlinearity in piezoelectrically actuated silicon micromechanical oscillators

We present in this paper IDTs efforts and progress in the development and commercialization of worlds first piezoelectric MEMS resonators for high performance timing applications. IDTs pMEMS™ resonators consist of a piezoelectric material on a single-crystal silicon layer substrate. Based on pMEMS™ technology, IDT is now shipping worlds lowest jitter MEMS Oscillators with less than 0.3ps RMS jitter (12kHz to 20MHz). In addition, pMEMS™ oscillators have demonstrated great long term frequency stability and immunity to shock / vibration testing.