Dry etching of deep Si trenches for released resonators in a Cl 2 plasma

An electron cyclotron resonance source has been used to generate a Cl ₂ plasma and etch Si trenches >100 μm deep. High microwave power was used to achieve Si etch rates of 1.09 μm/min for trench openings >500 μm. With pressure ranging from 2 to 30 mTorr, etching at higher pressures, up to 20 mTorr, increased the anisotropy of the etch, due to side-wall passivation. In addition, Si etch rates were faster at high pressures, increasing from 0.47 to 0.78 μm/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 dc 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 μm. Electroplated Ni could be used as an etch mask, and the selectivity of Si over Ni was 75. Also, the addition of SF ₆ increased the etch rate to as high as 1.89 μm/min for trench openings >500 μm, 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 μm wide trench openings relative to the etch rate of a 50 μm 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 μm/min was measured; however, for a 2 h etch, an etch rate of 1.00 μm/min was measured.