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Selective removal of the silicon underneath a set of single-turn multilayer interlaced stacked (SMIS) radio-frequency (RF) transformers with nearly perfect magnetic-coupling factor ([[kappa].sub.IM] ~ 1) and high resistive-coupling factor ([[kappa].sub.Re]) is demonstrated. This process is based on the inductively coupled-plasma (ICP) deep trench technology. Improvement of 20.6 and 15.7 dB in isolation ([S.sub.21]) were achieved at 5.2 and 8 GHz, respectively, for a dummy open device after the backside ICP etching. Q-factor increases of 102% (from 4.96 to 10.03) and 23.2% (from 2.24 to 2.76), [G.sub.Amax] increases of 11.8% (from 0.76 to 0.85) and 4.5% (from 0.88 to 0.92), and N[F.sub.min] decreases of 0.49 dB (from 1.22 to 0.73 dB) and 0.19 dB (from 0.55 to 0.36 dB) were achieved at 5.2 and 8 GHz, respectively, for an SMIS transformer with an overall dimension of 170 x 240 [micro][m.sub.2] after the backside ICP etching. The [G.sub.Amax] of 0.85 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 the traditional stacked transformer are explained. These results show that the micromachined SMIS transformers are very promising for RF integrated circuit applications. Index Terms--Inductively coupled plasma (ICP), magnetic coupling, quality factor (Q-factor), radio-frequency integrated circuit (RFIC), single-turn multilayer interlaced stacked (SMIS), transformer, ultra-low-loss.