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.

Collaborators

• A. Chandrakasan, MIT
• T. Palacios, MIT
• D. Boning, MIT
• G. Fedder, CMU
• T. Kazior, Raytheon
• G. Bourianoff, Intel

Graduate Students

• R. Marathe, Research Assistant, EECS
• W. Wang, Research Assistant, ME
• L. Popa, Research Assistant, Physics

Support Staff

• V. Dinardo, Admin. Asst.

Publications

W. Wang, D. Weinstein, “Acoustic Bragg reflectors for Q-enhancement of unreleased MEMS resonators,” IEEE Frequency Control Symposium (FCS 2011), San Francisco, CA, May 1-5, 2011.

W. Wang, L.C. Popa, R. Marathe, D. Weinstein, “An unreleased mm-wave resonant body transistor,” IEEE International Conference on Micro Electro Mechanical Systems (MEMS 2011), 1341-44.

D. Weinstein, S.A. Bhave, “Acoustic resonance in an Independent-Gate FinFET,” Solid State Sensor, Actuator and Microsystems Workshop (Hilton Head 2010), Hilton Head Island, South Carolina, June 6-10, 2010.

D. Weinstein, S.A. Bhave, “The resonant body transistor,” Nano Letters 10(4) 1234-37 (2010).

D. Weinstein, S.A. Bhave, “Internal dielectric transduction in bulk-mode resonators,” IEEE Journal of Microelectromechanical Systems (JMEMS), 18(6), 1401-1408 (2009).

L. Sekaric, O. Gunawan, A. Majumdar, X.H. Liu, D. Weinstein, J.W. Sleight, “Size-dependent modulation of carrier mobility in top-down fabricated silicon nanowires,” Applied Physics Letters, 95(2), 023113 (2009).

D. Weinstein, S.A. Bhave, “Frequency scaling and transducer efficiency in internal dielectrically transduced silicon bar resonators,” International Conference on Solid-State Sensors, Actuators and Microsystems (Transducers 2009), pp. 708-711.

D. Weinstein, S.A. Bhave, “Piezoresistive sensing of a dielectrically actuated silicon bar resonator,” Solid State Sensor, Actuator and Microsystems Workshop (Hilton Head 2008), pp. 368-371 (2008).

D. Weinstein, S.A. Bhave, “Internal dielectric transduction of a 4.5 GHz silicon bar resonator,” IEEE International Electron Devices Meeting (IEDM 2007), pp. 415-418.

D. Weinstein, S.A. Bhave, “Internal dielectric transduction: optimal position and frequency scaling,” IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 54(12), 2696-98 (2007).

D. Weinstein, S.A. Bhave, M. Tada, S. Mitarai, S. Morita, K. Ikeda, “Mechanical coupling of 2D resonator arrays for MEMS filter applications,” IEEE Frequency Control Symposium (FCS 2007), pp. 1362-1365.

Electromechanical resonators such as quartz crystals, surface acoustic wave (SAW) resonators, and ceramic resonators have become essential components in electronic systems. However, due to their large footprint and difficulty in integrating with CMOS processes, there has been much interest in developing microelectromechanical systems (MEMS) resonators that achieve comparable performance yet have smaller footprint and are compatible with CMOS. Recently, MEMS resonators have been proposed that overcome physical limitations in traditional resonators to reach frequencies in the GHz range. In addition, they have the potential for compatibility with CMOS, opening up possibilities for new circuits and systems ^[1] . As with other semiconductor devices, with increasing frequency and with decreasing device size into the submicron scale, variability has started to become a critical issue in MEMS resonators. Thus vigorous characterization of important device parameters such as resonant frequencies, quality factors, and variations associated with them has become necessary. However, one of the critical challenges is the lack of a characterization method that is accurate but efficient enough to be used for testing of the large number of devices necessary to acquire accurate statistical distribution of the parameters of interest. This project proposes an on-chip test circuit that can accurately characterize a large number of resonators for variation analysis. The desired test circuit is general enough that it can be used with a wide range of resonators, not limited to specific frequencies or other properties. Previous works have attempted to achieve similar goals, but most of them were restricted to characterization of a single device or a narrow range of properties. The proposed test circuit is based on a transient step response method using a voltage step that can accurately measure the resonant frequencies and the quality factor of devices ^[2] . The circuit employs a sub-sampling method to capture the high-frequency decay signal ^[3] and a simple analog-to-digital converter (ADC) ^[4] allowing complete digital interface, an important feature for test automation.

1. D. Weinstein and S. A. Bhave, “The resonant body transistor,” Nano Letters, vol.10, no. 4, pp. 1234-37, 2010.
2. M. Zhang, N. Llaser, H. Mathias, and F. Rodes, “CMOS offset-free circuit for resonator quality factor measurement,” IEEE Electronic Letters, vol. 46, no. 10, p. 706, May 2010.
3. R. Ho et al., “Applications of on-chip samplers for test and measurement of integrated circuits,” in Proc. 1998 IEEE Symposium on VLSI Circuits, June, pp. 138-139.
4. E. Alon, V. Stojanović, and M. A. Horowitz, “Circuits and techniques for high-resolution measurement of on-chip power supply noise,” IEEE Journal of Solid-State Circuits, vol. 40, no. 4, pp. 820-828, Apr. 2005.