Noise coupling in heavily and lightly doped substrate from planar spiral inductor

Recently, much studies have been done to include on-chip inductors for wireless communication IC design. The large area metal plane of the spiral inductor can form large capacitor to the silicon substrate. At high enough frequency, excitation of the inductor will induce noise to the substrate. In this paper, we will present the simulation results of the coupling effect of a planar spiral inductor on heavily and lightly doped substrate by using a compact model of the substrate. Results show that the insertion of a P+ diffusion guard ring can reduce about 15 dB of noise for lightly doped substrate and 7 dB of noise for heavily doped substrate at 3 GHz. Variation in the size of the P+ diffusion guard ring affects the efficiency of noise reduction and also the inductance. Results show that in order to optimize for inductance, the side length of the guard ring needs to be twice the side length of the inductor and the guard ring can only reduce about 6 dB of noise for lightly doped substrate and 1.5 dB of noise for heavily doped substrate at 3 GHz