The effect of a metal PGS on the Q-factor of spiral inductors for RF and mm-wave applications in a 28-nm CMOS technology

In this paper, the effect of a metal patterned ground shield (PGS) on the performance of monolithic inductors is investigated. To this aim, three spiral inductors integrated in a 28-nm fully depleted (FD) silicon-on-insulator (SOI) CMOS technology are analyzed by means of a 3-D FEM-based commercial software. The inductors have been designed at different operating frequencies in the RF and mm-wave ranges to better explore the effect of the PGS. Extensive analysis revealed that the shield is able to improve the quality factor (Q-factor) only of the inductor operated at the lowest frequency (ie, K-band). On the contrary, it has a detrimental effect on the Q-factor of the inductors working at higher frequencies. This is mainly due to induced losses in the PGS itself, which are so high to frustrate the substrate loss reduction. This result gives a different perspective to the adoption of the PGS for CMOS integrated inductors, which is largely recommended to improve inductor performance in the current state of the art.