A novel gap narrowing process for creating high aspect ratio transduction gaps for MEM HF Resonators
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1139-GG01-05
A novel gap narrowing process for creating high aspect ratio transduction gaps for MEM HF Resonators S. Stoffels1,2, G. Bryce1, R. Van Hoof1, B. Du Bois1, R.P. Mertens1,2, R.Puers2, H.A.C. Tilmans1 and A. Witvrouw1 1 2
IMEC , Kapeldreef 75, 3001 Leuven, Belgium KULeuven, Kasteelpark Arenberg 10, 3001 Leuven
ABSTRACT In this work a novel technique to create nanometer sized air gaps for high frequency (HF) mechanical resonators will be presented. The technique is based on the narrowing of initially wide gaps with a conformal ‘narrowing’ layer. The novelty of this technique is that it enables the creation of narrow high-aspect ratio gaps (e.g. 100nm gaps in 10µm thick layers) without the need for complex lithography or high aspect ratio etching. Furthermore, the electrodes and the resonator itself can be patterned in a single processing step. The process methodology will be explained and validation experiments in a silicon-germanium (SiGe) based technology will be shown. This technology uses low temperature (~450ºC) poly silicon-germanium (SiGe) as the structural layer, which can be processed above CMOS, and therefore allows the fabrication of MEM devices above CMOS. INTRODUCTION In the field of wireless communication, micro electro-mechanical (MEM) HF resonators are promising replacement components for use in filters and reference oscillators due to their tiny size, high quality factors (5000-100,000) and achievable frequencies ranging from tens of MHz till several GHz [1]. Electrical signals can be filtered with these mechanical resonators, however the signals need to be transduced between the electrical and mechanical domain. This transduction can be achieved by e.g. electrostatic, electrodynamical, piezoelectric, etc… means [2]. Electrostatic transduction with air gaps is the prevalent method used to drive MEM resonators due to the simplicity of the process. However, for these electrostatically transduced air gap MEM resonators, a small air gap in the range of 50-100nm is needed to achieve an efficient conversion between electrical and mechanical energy. In this work we will present a novel process to create nanometer sized air gaps for high frequency (HF) mechanical resonators and evaluate the process in a poly-SiGe based technology. In prior work, narrow gaps were created by the use of a spacer layer in between the separately deposited resonator and electrode layers [3]. In other research, the narrow gaps were directly defined using expensive advanced lithography in combination with high aspect ratio etching [4]. Our technology differs from these methods as the electrode and resonator are patterned in a single processing step with relatively wide air gaps. The desired width of the gaps is achieved afterwards by narrowing the wide air gaps in a controllable manner. The novelty of this technique is that it enables the creation of narrow high-aspect ratio air gaps (e.g. 100nm gaps in 10µm thick layers) without the need for advanced lithography combined with high aspect ratio etching as reported in li
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