Investigation of Q enhancement for inductors processed in BiCMOS technology

This paper reports the investigation of the quality factor (Q) enhancement approaches for on-chip spiral inductors which can be implemented without modifying standard CMOS/BiCMOS processes. These approaches include multilevel metal cascade, multilevel metal shunt, patterned ground shield, and tapered trace width. To make the comparison consistent, the effective inductance of all inductors is designed to be 5 nH at 1 GHz. This work shows that multilevel metal shunt is the most effective method to increase the Q below 2.3 GHz. The multilevel metal cascade can also improve the Q up to 10% below 3.4 GHz and reduce the inductor area by half. The tapered width inductor has the highest Q above 2.1 GHz, but its equivalent inductance is reduced by 30%. All of these approaches improve the Q by sacrificing the self-resonant frequency, so the single metal spiral inductor is more suitable for the applications above 3.4 GHz.