A plasma etch chemistry which allows a near perfectly vertical etch of silicon is disclosed. A Cl-containing compound such as BCl.sub.3 has Br.sub.2 added to it, readily allowing anisotropic etching of silicon. This is due to the low volatility of SiBr.sub.4. The silicon surface facing the discharge is subjected to ion bombardment, allowing the volatilization (etching) of silicon as a Si-Cl-Br compound. The Br which adsorbs on the sidewalls of the etched silicon protects them from the etching. This new plasma etch chemistry yields a very smooth etched surface, and the etch rate is relatively insensitive to the electrical conductivity of the silicon.