1. Field
The invention relates to microelectromechanical structures (MEMS).
2. Background
Communication systems generally require partitioning of the electromagnetic frequency spectrum. Communication transceiver devices therefore must be capable of high frequency selectivity, i.e., capable of selecting a given frequency band while rejecting all others. Frequency-selective devices, such as filters, oscillators and mixers are therefore some of the most important components within a transceiver and the quality of the devices generally dictates the overall architecture of a given transceiver.
In many communication systems (e.g., cordless and cellular phones), off-chip, passive components are used as part of the frequency-selective devices. Such passive components are typically implemented at the board level and therefore impede the ultimate miniaturization of portable transceivers.
Micromachining technologies have been applied to the miniaturization and integration of frequency-selective devices to bring such devices to the chip level. Polycrystalline silicon-based device structures represent one specific micromachining technology. High frequency (HF) and very high frequency (VHF) vibrating micromechanical resonators, for example, for use in bandpass filters and reference oscillators have been formed through a sequence of integrated circuit-compatible film deposition and patterning. To form small gaps, such as, for example, when fabricating a vibratable resonator, traditional integrated circuit lithography and etch processes may be employed. Such processes typically include depositing and patterning polycrystalline silicon, structural material, the patterning defining gap between device structures through photolithographic means. Potential problems with using traditional lithographic and etch patterning include the gap width miniaturization brought about by the limit associated with lithography and etch. In addition, certain design specifications require near exact symmetric gaps for balance. Such symmetry cannot be guaranteed by lithography and etch processes because the critical dimension (CD) variation is large when approaching the lithography limit. Still further, etching profiles for high aspect ratio gaps can create non-uniform gaps (e.g., vertical gaps) which can give rise to non-uniform charge distribution.
What is needed are methods and structures that are not constrained by the limitations of lithography and etch.