High-Q CMOS-compatible micromachined edge-suspended spiral inductors

This paper reports a new category of high-Q edge-suspended inductors (ESI) that are realized 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. Since the coupling to the low resistivity silicon substrate is dominated by the current carrying parts (the edges), 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 membrane 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 edge-suspension structures 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, a 57% increase (from 9.1 GHz to 14.3 GHz) in self-resonance frequency are obtained with a 11 μm suspended edge in 25 μm wide lines.