An analysis of perfect-magnetic-coupling ultra-low-loss micromachined SMIS RF transformers for RFIC applications

Selective removal of the silicon underneath a set of single-turn multi-layer interlaced stacked (SMIS) RF transformers with nearly perfect magnetic-coupling factor (k_IM ∼ 1) and high resistive-coupling factor (k_Re) is demonstrated. This process is based on the inductively-coupled-plasma (ICP) deep trench technology. A 102% (from 4.963 to 10.03) and a 23.4% (from 2.236 to 2.76) increase in Q-factor, a 11.8 % (from 0.756 to 0.845) and a 4.4 % (from 0.881 to 0.92) increase in G_Amax, and a 0.484 dB (from 1.215 dB to 0.731 dB) and a 0.188 dB (from 0.549 dB to 0.361 dB) decrease in NF_min were achieved at 5.2 and 8 GHz, respectively, for a SMIS transformer with an overall dimension of 170×240 μm² after the backside ICP dry etching. The G_Amax of 0.845 and 0.92 are both state-of-the-art results among all reported on-chip transformers. Furthermore, the reasons why the SMIS transformer exhibits better performances than the traditional bifilar and stacked transformer are explained.