On-chip Spiral Inductors With Patterned Ground Shields For Si-based RF IC's

Abstract

This paper presents a patterned ground shield in- serted between an on-chip spiral inductor and silicon substrate. The patterned ground shield can be realized in standard silicon technologies without additional processing steps. The impacts of shield resistance and pattern on inductance, parasitic resistances and capacitances, and quality factor are studied extensively. Experimental results show that a polysilicon patterned ground shield achieves the most improvement. At 1-2 GHz, the addition of the shield increases the inductor quality factor up to 33% and reduces the substrate coupling between two adjacent inductors by as much as 25 dB. We also demonstrate that the quality factor of a 2-GHz tank can be nearly doubled with a shielded inductor. In this paper, we present a patterned ground shield, which is compatible with standard silicon technologies, to reduce the unwanted substrate effects. To provide some background, Section II presents a discussion on the fundamental definitions of an inductor and an tank . Next, a physical model for spiral inductors on silicon is described. The magnetic energy storage and loss mechanisms in an on-chip inductor are discussed. Based on this insight, it is shown that energy loss can be reduced by shielding the electric field of the inductor from the silicon substrate. Then, the drawbacks of a solid ground shield are analyzed. This leads to the design of a patterned ground shield. Design guidelines for parameters such as shield pattern and resistance are given. In Section III, experiment design, on-wafer testing technique, and parasitic extraction procedure are presented. Experimental results are then reported to study the effects of shield resistance and pattern on inductance, parasitic resistances and capacitances, and inductor . Next, the improvement in of a 2-GHz tank using a shielded inductor is illustrated. A study of the noise coupling between two adjacent inductors and the efficiency of the ground shield for isolation are also presented. Lastly, Section IV gives some conclusions.