We are interested in etching very deep anisotropic trenches (similar to 100 mu m)with high aspect ratios (depth/width) (similar to 20-50) and high etch rates (similar to 5 mu m/min), A high density plasma helicon reactor using SF6/O-2 chemistry and a cryogenic chuck has been used for etching very narrow trenches from 1.2 to 10 mu m wide on n-type Si wafers with a SiO2 mask. The first results show significant features that demonstrate the feasibility of this method. Two-micron-wide trenches have been etched to a depth of 80 mu m at an average etch rate of 2.7 mu m/min. The resulting profiles are highly anisotropic and selectivity of Si/SiO2 is remarkably high (>500).