Isolation strategy against substrate coupling in CMOS mixed-signal/RF circuits

A deep n-well guard-ring, DNW-GR, provides effective isolation from substrate-coupled high frequency noises in combination with high-resistive p-type substrates. AC-measurements of S21 with port geometry and guard-ring structure dependencies are reported in a 0.25-μm CMOS standard mixed-signal technology with p-type substrates having the bulk resistivity from 10 Ωcm to 3 kΩcm. It is shown that DNW-GR on a 1-kΩcm substrate gains 20dB improvement in isolation against 1-GHz coupling, compared with a p⁺ guard-ring on a standard 10-Ωcm substrate. S21 simulations with substrate equivalent circuit models derived by way of F-matrix computation on test structures are in good agreement with measurements for a frequency range over 5 GHz. The substrate models can compactly as well as precisely include layout-level isolation effects in circuit simulations for a mixed-signal/RF design.