CMOS-compatible micromachining techniques for fabricating high-performance edge-suspended RF/microwave passive components on silicon substrates

Abstract

This paper reports the micromachining techniques for fabricating edge-suspended RF/microwave passive components, which are proposed to deliver enhanced performance without sacrificing their mechanical strength and reliability. The fabrication incorporates ICP-DRIE dry etching and TMAH anisotropic etching techniques, which are both CMOS compatible. The edge-suspended structures were realized by a TMAH solution consisting of 5 wt% TMAH, 1.6 wt% Si and 0.5 wt% (NH4)2S2O8. This solution offers effective etching of silicon along the {100} and {110} planes, while having negligible etching on aluminum and {111} planes. The layout requirement for achieving edge-suspended passive components is also outlined on the basis of the analysis of the anisotropic etching along different crystal orientations. Using the techniques described here, high performance spiral inductors and coplanar waveguides (CPW) with significantly reduced loss are demonstrated. For a three-turn 4.5 nH inductor, a 70% increase (from 6.8 to 11.7) in maximum Q-factor and a 57% increase (from 9.1 GHz to 14.3 GHz) in self-resonance frequency are obtained with an 11 µm suspended edge in 25 µm wide lines compared to the conventional inductors without being micromachined. A 50 Ω edge-suspended CPW exhibits a reduction in insertion loss, from 2.4 dB mm−1 in a conventional CPW to 0.4 dB mm−1 at 39 GHz.