CMOS-compatible micromachined edge-suspended spiral inductors with high Q-factors and self-resonance frequencies

This paper reports a new category of high-Q edge-suspended inductors (ESI) that are fabricated using CMOS-compatible micromachining techniques. This structure was designed based on the concept that the current was crowded at the edges of the conducting metal wires at high frequencies due to the proximity effect. The substrate coupling and loss can be effectively suppressed by removing the silicon around and underneath the edges of the signal lines. Different from the conventional air-suspended inductors that have the inductors built on membranes or totally suspended in the air, the edge-suspended structures have the silicon underneath the center of the metal lines as the strong mechanical supports. The ESIs are fabricated using a combination of deep dry etching and anisotropic wet etching techniques that are compatible with CMOS process. 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 to 14.3 GHz) in self-resonance frequency were obtained with a 11-μm suspended edge in 25-μm-wide lines.