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Switchable Piezoelectric Transduction in AlGaN/GaN MEMS Resonators

• Authors: L. C. Popa, D. Weinstein
• Sponsorship: Raytheon

High-Q MEMS resonators, with small footprint and monolithic integration, are excellent building blocks for configurable RF systems. While these resonators provide narrow bandwidth selectivity, broad-band operation typically requires a large bank of switchable devices. This bank introduces a large capacitive load at the input due to the drive transducers. Typically, piezoelectric resonators have strong electromechanical coupling coefficients enabling low loss filters. However, they must be switched in line of the RF signal, resulting in insertion loss and reduced power handling.

This work presents a new implementation of piezoelectric transduction in an AlGaN/GaN heterostructure that enables on/off switching of transduction with DC voltage applied out-of-line of the RF signal and reduces the capacitive load of the resonator by >10× when in the off state. This transducer is formed in the AlGaN, between a top Schottky electrode and a 2D electron gas (2DEG) as a second electrode^[1],^[2] (Figure 1).  When a negative bias of -7 volts is applied to the Schottky electrode, the 2DEG is depleted. The removal of this bottom electrode suppresses electromechanical transduction and serves to reduce the drive capacitance by >10×.

Mechanical resonances can be detected with a passive transducer equivalent to the drive, or with a high electron mobility transistor (HEMT) embedded in the resonator, which has been previously shown to enable sensing at higher frequencies^[3],^[4]. The HEMT-sensed device is illustrated in Figure 1c. The DC behavior of the embedded HEMT is shown in Figure 2a, while the measured frequency response of the resonator is illustrated in Figure 2b. Applying a negative bias to the drive transducer depletes the 2DEG and suppresses the resonance signal while reducing the drive capacitance by 13×. The resonance at 2.67 GHz has Q of 650 in air with f·Q of 1.7×10¹², the highest in GaN resonators to date.



Figure 1: (a) 2DEG confined in conduction band potential well at the interface between AlGaN and GaN. (b) Principle of switchable piezoelectric transduction. (c) SEM of piezoelectric-drive, HEMT-sensed resonator.



Figure 2: (a) Measured IV curves of the HEMT embedded in resonator (b) Measured frequency response of resonator, in air. The transconductance, g_m = Y_21-Y_12 , is suppressed when the 2DEG under the drive electrode is depleted.




1. K. Brueckner, et al., “Two-dimensional electron gas based actuation of piezoelectric AlGaN/GaN microelectromechanical resonators,” Applied Physics Letters, vol. 93, pp. 173504:1-3, 2008. [↩]
2. K. Tonisch, et al., “Piezoelectric actuation of (GaN)/AlGaN/GaN heterostructures,” Journal of Applied Physics, vol. 104, pp. 084516:1-8, 2008. [↩]
3. D. Weinstein and S. Bhave, “The Resonant Body Transistor” Nano Letters, vol. 10(4), pp. 1234-37, 2010. [↩]
4. M. Faucher, et al., “Amplified piezoelectric transduction of nanoscale motion in gallium nitride electromechanical resonators,” Applied Physics Letters, vol. 94, pp. 233506:1-3, 2009. [↩]

Deep Trench Capacitor Drive of Unreleased Si MEMS Resonator

• Authors: W. Wang, D. Weinstein
• Sponsorship: SRC/FCRP MSD

With frequency-quality factor products (f•Q) often exceeding 10¹³, MEMS resonators offer a high-Q, small footprint alternative to conventional LC tanks and off-chip crystals for clocking and wireless communication. Over the past three decades, much progress has been made in the key figures of merit of MEMS resonators including small footprint, high Q, low motional impedance, and efficient energy coupling k_T². In parallel, efforts have focused on system-level metrics including high yield, low cost, robustness, easy packaging, and integration with circuits. A key challenge in MEMS resonator design is to achieve high performance yet manufacturable devices. The unreleased deep trench (DT) resonators in this work address this challenge.

Beyond the performance goals of high Q and low loss, these devices target two key features desired for monolithically integrated MEMS resonators. First, lithographic definition of resonance frequency enables a broad range of frequencies to be fabricated on a single chip. Second, unreleased bulk-acoustic resonators do not require any low-yield, complex steps to freely suspend the moving structure and are robust in harsh environments without packaging. Unreleased resonators such as the HBAR^[1] have been demonstrated but have thickness-defined frequency. Lateral bulk acoustic resonators with lithographically defined frequency such as the LoBAR^[2] have achieved high Q but require low d₃₁ coupling to drive and sense resonance. Meanwhile, sidewall AlN resonators^[3] excite lateral resonance with d₃₃ coupling but still require a release step. This work provides the benefits of all of these devices with high Q, efficient dielectric transduction, lateral resonance, and no release step. The DT resonator implements deep trench capacitors as both electrostatic transducers and Acoustic Bragg Reflectors (ABRs), defined in a single mask and self-aligned (Figure 1). While ABRs provide acoustic isolation in a solid medium, the DT capacitors function as internal dielectric transducers^[4], which have achieved the highest frequencies in Si to date^[5]. A 3.3-GHz unreleased Si resonator is demonstrated with Q of 2057 and motional impedance R_X of 1.2 kΩ (Figure 2). This realization of high-Q unreleased resonators in a bulk Si process provides a high yield, low cost, no packaging solution for on-chip clocking, wireless communication, and electromechanical signal processing.



Figure 1: Scanning electron micrographs (top) and schematic (bottom) of unreleased deep trench (DT) MEMS resonator. Acoustic Bragg reflectors (ABRs) formed from periodically spaced DTs define a high-Q resonant cavity in the center of the device. DT capacitors inside the resonant cavity form electrostatic drive and sense transducers.



Figure 2: Measured frequency response of DT resonators with 7.2-µm-long resonant cavity, DT length of 950 nm, and 1.7-µm DT pitch in the ABRs. Device with 50 ABRs is shown. Wide frequency sweeps show no spurious modes in a multi-GHz range.




1. J. T. Haynes, M. S. Buchalter, R. A. Moore, H. L. Salvo, S. G. Shepherd, and B. R. McAvoy, “Stable microwave source using high overtone bulk resonators,” IEEE MTT-S 1985, pp. 243-246. [↩]
2. M. Ziaei-Moayyed, S. D. Habermehl, D. W. Branch, P. J. Clews, and R. H. Olsson III, “Silicon carbide lateral overtone Bulk Acoustic Resonator with ultrahigh quality factor,” IEEE MEMS, 2011, pp. 788-792. [↩]
3. R. Tabrizian, and F. Ayazi, “Laterally excited silicon Bulk Acoustic Resonator with sidewall AlN,” Transducers 2011, pp. 1520-1523. [↩]
4. D. Weinstein, and S. A. Bhave, “Internal dielectric transduction of a 4.5 GHz silicon bar resonator,” IEDM 2007, pp. 415-418. [↩]
5. W. Wang, L.C. Popa, R. Marathe, and D. Weinstein, “An unreleased mm-wave resonant body transistor,” IEEE MEMS, 2011, pp. 1341-1344. [↩]