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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 μm separation on p/sup +/Si. Crosstalk suppression increases linearly with increasing PS width to beyond 320 μm. 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.