An electron cyclotron resonance source has been used to generate a Cl-2 plasma and etch Si trenches >100 mu m deep. High microwave power was used to achieve Si etch rates of 1.09 mu m/min for trench openings >500 mu m With pressure ranging from 2 to 30 mTorr, etching at higher pressures, up to 20 mTorr, increased the anisotropy of the etch, due to sidewall passivation. In addition, Si etch rates were faster at high pressures, increasing from 0.47 to 0.78 mu m/min as the pressure was increased from 2 to 30 mTorr. Features with aspect ratios >30 have been fabricated using 1200 W microwave power and 50 W radio-frequency power at 20 mTorr with 20 seem of Cl-2 flow. Under these conditions,the self-induced de bias at the stage was 50 V; and a selectivity >250 over an evaporated Ni mask was achieved. This allowed the use of an evaporated Ni mask for trench depths >50 mu m. Electroplated Ni could be used as an etch mask, and the selectivity of Si over Ni was 75. Also, the addition of SF6 increased the etch rate to as high as 1.89 mu m/min for trench openings >500 mu m, while keeping the vertical profile and smooth surface. The Si etch rate was found to depend on the aspect ratio and an etch rate reduction of 18% was seen in the Cl-2, plasma for 1.5 mu m wide trench openings relative to the etch rate of a 50 mu m wide trench opening. The etch rate also increased as the etch time increased. For a 30 min etch in the Cl-2, plasma, an etch rate of 0.79 mu m/min was measured; however, for a 2 h etch, an etch rate of 1.00 mu m/min was measured.