High-Q passives for mm-wave SiGe applications

Deep-silicon etching technique was used for achieving high-Q inductors in a 0.25 mum SiGe:C BiCMOS process. Low-resistive silicon regions under passive structures were removed using deep-silicon plasma etch technique. The lithography and etch of the silicon were performed from the backside of the wafer. Both thick (750 mum) and thin (370 mum) 8-inch wafers were processed without any handling and reliability problems. Inductors with different number of turns and values were evaluated. RF measurements were performed up to 110 GHz. Performance increase of multi-turn, high value inductors was mainly limited by the inter-winding capacitance. For low value inductances, significant increase of the quality factor and self-resonance frequency was observed. The results demonstrate that the deep-silicon etching technique is an extremely suitable method for fabricating passives which exhibit low losses even at mm-wave frequencies.