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Through-the-wafer porous Si (PS) trenches have been used to provide radio frequency (RF) isolation in Si because of their semi-insulating property. Reduction of crosstalk by 70 dB at 2 GHz and 45 dB at 8 GRz is demonstrated between Al pads with 800 /spl mu/m separation on p/sup +/Si. Crosstalk suppression increases linearly with increasing PS width to beyond 320 /spl mu/m. This suppression is degraded by one order of magnitude when the Si underneath the PS trenches remains and serves as a residual path for crosstalk. These results show that PS is an excellent candidate for RF isolation in modern VLSI technology.