Frequency-Response-Oriented Design and Optimization of N+ Diffusion Guard Ring in Lightly Doped CMOS Substrate

This paper presents a new design strategy of n+ diffusion guard ring for CMOS integrated circuit design, namely the frequency-response-oriented codesign. The frequency response of the n+ diffusion guard ring is investigated, based on which a new design is proposed to achieve frequency-dependent isolation characteristics. In this structure, the p-n junction capacitance in the guard ring resonates with the inductive interconnect to the ground, forming a very low impedance path for the substrate noise current, which results in better isolation than the conventional guard rings. Implemented in GSMC generic 0.13-$\mu$m radio frequency CMOS technology, the design concept is verified by full-wave simulation and measurement. Measurement results show that the designed n+ diffusion guard rings have the much higher isolation at most about 30 dB at the resonant frequency than conventional structures with a broad frequency band about 10 GHz. This suggests that the proposed technique is able to enhance the isolation of the desired frequency range.