Electrostatically tunable piezoelectric-on- silicon micromechanical resonator for real-time clock

This paper reports on the design, fabrication, and characterization of a small form factor, piezoelectrically transduced, tunable micromechanical resonator for real-time clock (RTC) applications (32.768 kHz). The device was designed to resonate in an out-of-plane flexural mode to simultaneously achieve low-frequency operation and reduced motional resistance in a small die area. Finite element simulations were extensively used to optimize the structure in terms of size, insertion loss, spurious-mode rejection, and frequency tuning. Microresonators with an overall die area of only 350 × 350 μm were implemented on a thin-film AlN on silicon-on-insulator (SOI) substrate with AlN thickness of 0.5 μm, device layer of 1.5 μm, and an electrostatic tuning gap size of 1 μm. A frequency tuning range of 3100 ppm was measured using dc voltages of less than 4 V. This range is sufficient to compensate for frequency variations of the microresonator across temperature from -20°C to 100°C. The device exhibits low motional impedance that is completely independent of the frequency tuning potential. Discrete electronics were used in conjunction with the resonator to implement an oscillator, verifying its functionality as a timing reference.

[1]  Farrokh Ayazi,et al.  A Low-Power Oven-Controlled Vacuum Package Technology for High-Performance MEMS , 2009, 2009 IEEE 22nd International Conference on Micro Electro Mechanical Systems.

[2]  G. K. Ho,et al.  High-Q single crystal silicon HARPSS capacitive beam resonators with self-aligned sub-100-nm transduction gaps , 2003 .

[3]  Wan-Thai Hsu,et al.  Stiffness-compensated temperature-insensitive micromechanical resonators , 2002, Technical Digest. MEMS 2002 IEEE International Conference. Fifteenth IEEE International Conference on Micro Electro Mechanical Systems (Cat. No.02CH37266).

[4]  C. Nguyen,et al.  MEMS technology for timing and frequency control , 2005, Proceedings of the 2005 IEEE International Frequency Control Symposium and Exposition, 2005..

[5]  Farrokh Ayazi,et al.  Electronically Temperature Compensated Silicon Bulk Acoustic Resonator Reference Oscillators , 2007, IEEE Journal of Solid-State Circuits.

[6]  Farrokh Ayazi,et al.  Thin-film piezoelectric-on-substrate resonators with Q enhancement and TCF reduction , 2010, 2010 IEEE 23rd International Conference on Micro Electro Mechanical Systems (MEMS).

[7]  T. Kenny,et al.  Temperature-Insensitive Composite Micromechanical Resonators , 2009, Journal of Microelectromechanical Systems.

[8]  Reza Abdolvand,et al.  Piezoelectric-on-Silicon Lateral Bulk Acoustic Wave Micromechanical Resonators , 2008, Journal of Microelectromechanical Systems.

[9]  Farrokh Ayazi,et al.  Tunable piezoelectric MEMS resonators for real-time clock , 2011, 2011 Joint Conference of the IEEE International Frequency Control and the European Frequency and Time Forum (FCS) Proceedings.

[10]  F. Ayazi,et al.  Combined capacitive and piezoelectric transduction for high performance silicon microresonators , 2011, 2011 IEEE 24th International Conference on Micro Electro Mechanical Systems.

[11]  C. S. Lam,et al.  A review of the recent development of MEMS and crystal oscillators and their impacts on the frequency control products industry , 2008, 2008 IEEE Ultrasonics Symposium.

[12]  Reza Abdolvand,et al.  Voltage-tunable piezoelectrically-transduced single-crystal silicon micromechanical resonators , 2004 .

[13]  Don L. DeVoe,et al.  Piezoelectric thin film micromechanical beam resonators , 2001 .

[14]  G. Casinovi,et al.  Passive TCF compensation in high Q silicon micromechanical resonators , 2010, 2010 IEEE 23rd International Conference on Micro Electro Mechanical Systems (MEMS).

[15]  E. Momosaki,et al.  A brief review of progress in quartz tuning fork resonators , 1997, Proceedings of International Frequency Control Symposium.

[16]  Farrokh Ayazi,et al.  MEMS for integrated timing and spectral processing , 2009, 2009 IEEE Custom Integrated Circuits Conference.

[17]  Wan-Thai Hsu,et al.  32KHz MEMS-based oscillator for low-power applications , 2005, Proceedings of the 2005 IEEE International Frequency Control Symposium and Exposition, 2005..

[18]  François Krummenacher,et al.  Silicon Resonator Based 3.2 $\mu$W Real Time Clock With $\pm$10 ppm Frequency Accuracy , 2010, IEEE Journal of Solid-State Circuits.

[19]  F. Ayazi,et al.  Temperature-Stable High-Q AlN-on-Silicon Resonators with Embedded Array of Oxide Pillars , 2010 .