On-chip spiral inductors suspended over deep copper-lined cavities

A silicon micromachining method has been developed to fabricate on-chip high-performance suspended spiral inductors. The spiral structure of an inductor was formed with polysilicon and was suspended over a 30-μm-deep cavity in the silicon substrate beneath. Copper (Cu) was electrolessly plated onto the polysilicon spiral to achieve low resistance. The Cu plating process also metallized the inner surfaces of the cavity, forming both a good radio-frequency (RF) ground and an electromagnetic shield. High quality factors (Qs) over 30 and self-resonant frequencies higher than 10 GHz have been achieved. A study of the mechanical properties of the suspended inductors indicates that they can withstand large shock and vibration. Simulation predicts a reduction of an order of magnitude in the mutual inductance of two adjacent inductors with the 30-μm-deep Cu-lined cavity from that with silicon as the substrate. This indicates very small crosstalk between the inductors due to the shielding effect of the cavities