Two of the greatest challenges in MEMS are those of packaging and integration with CMOS technology. Development of unreleased MEMS resonators at the transistor level of the CMOS stack will enable direct integration into front-end-of-line (FEOL) processing and minimal or no packaging, making these devices an attractive choice for on-chip signal generation.

Toward this goal, the authors have previously demonstrated the first fully unreleased MEMS resonator operating at 39 GHz with a quality factor (Q) of 129 ^[1] . The Si bulk acoustic resonator, surrounded on all sides by SiO₂, demonstrates the feasibility of unreleased resonators, providing a Q that is only 4x lower than its released counterpart ^[2] . At mm-wave frequency in the Landau-Rumer regime, resonator Q is limited primarily by anchor loss ^[3] . In the case of fully-clad resonators, the quality factor can be significantly improved by localization of acoustic energy using acoustic Bragg reflectors ^[4] .

The HybridMEMS lab has performed a study of fully unreleased resonator surrounded by lithographically defined ABRs, embedded in a homogeneous cladding layer (Figure 1). This one-mask design enables resonator banks of various frequencies on the same chip, providing multiple degrees of freedom in ABR design. With the goal of direct integration into FEOL CMOS processing, resonator performance is investigated for materials commonly found in the CMOS stack. The characteristics of these unreleased structures are compared with freely suspended resonators, released resonators isolated with lithographically defined ABRs ^[5] , and phononic crystal ^[6] based unreleased resonators (Figure 2).

1. W. Wang, L. C. Popa, R. Marathe, and D. Weinstein, “An unreleased mm-wave resonant body transistor,” IEEE MEMS Conference, 2011, pp. 1341-1344.
2. D. Weinstein and S. A. Bhave, “Acoustic resonance in an independent-gate FinFET,” Hilton Head Workshop, 2010, pp. 459-462.
3. R. Tabrizian, M. Rais-Zadeh, and F. Ayazi, “Effect of phonon interactions on limiting the f.Q product of micromechanical resonators,” IEEE Transducers Conference, 2009, pp. 2131-2134.
4. K. M. Lakin, “Thin film resonators and filters,” IEEE Ultrasonics Symposium, 1999, pp. 895-906.
5. R. H. Olsson, J. G. Fleming, and M. R. Tuck, “Contour mode resonators with acoustic reflectors,” US Patent 7385334 B1, 2008.
6. S. Mohammadi, A. A. Eftekhar, W. D. Hunt, and A. Adibi, “High-Q micromechanical resonators in a two-dimensional phononic crystal slab,” Applied Physics Letters, vol. 94, pp. 051906:1-3, Feb. 2009.