RF Performance of Devices Processed in Low-Temperature Sequential Integration

RF performance and intertier coupling of CMOS processed in 3-D sequential integration are investigated. pMOS transistor fabricated with a 500 °C thermal budget features good RF figures of merit with ${f}_{t} =105$ GHz and ${f}_{\text {max}} =175$ GHz for a gate length of 45 nm and ${V}_{\text {DD}} = -1$ V. Moreover, we demonstrate that the low- ${k}$ SiCO oxide spacer and low polysilicon gate resistance obtained with the low temperature process contribute to ${f}_{\text {max}}$ results that are better than high temperature process (above 1000°). Finally, we illustrate the crosstalk effects and the influence of the low-tier transistor gate voltage ${V}_{\text {G}}$ on the top-tier threshold voltage ${V}_{T}$ . We also show that a polysilicon ground shield integrated under the top-tier transistor substantially attenuates the intertier RF field coupling effects.