December 7, 2010 – At this week’s IEDM 2010 in San Francisco, IMEC and Panasonic are revealing a SiGe thin-film packaged SOI-based MEMS resonator with the industry’s highest-recorded quality factor ("Q factor") — i.e., the ratio of energy stored vs. energy dissipated during a cycle.

MEMS resonators’ desirability vs. traditional resonators (quartz crystals, piezoelectrics) include miniaturization, better frequency stability, integration with CMOS, use of standard IC packaging, and ideally lower costs — but they also tend to have a lower Q factor and high bias voltage. (A higher Q factor means lower energy dissipation, thus oscillations persist longer.) Panasonic and IMEC addressed this in two ways:

— Applying a torsional vibration mode (Figure 1), enabling low anchor losses and lower squeeze film damping (vs. flexural mode resonators).

— Vacuum-encapsulating it in a thin-film package (Figure 2): 4μm thick SiGe film, realized with monolithic fabrication process with the MEMS.

Figure 1: Illustration of the torsional vibration mode. (Source: IMEC)

The result: a record-high Q factor: 220,000 at 20MHz resonant frequency (f • Q product of 4.3 × 10¹²Hz).

A narrow 130nm gap between the beam/drive and sense electrodes enables a low bias voltage (1.8V_dc), eliminating the need for a charge pump in the oscillator circuit. Sacrificial layer etching through a microcrystalline SiGe layer minimizes deposition of sealing material inside the cavity, so etching holes can be better lined up with the beam surface.

IMEC and Panasonic built the device through IMEC’s CMORE service.

Figure 2: SEM of a cross-sectional structure of the developed packaged MEMS resonator. (Source: IMEC)