A method and system for selectively and uniformly etching a dielectric layer with respect to silicon and polysilicon in a dry plasma etching system are described. The etch chemistry comprises the use of fluorohydrocarbons, such as CH.sub.2F.sub.2 and CHF.sub.3. High etch selectivity and acceptable uniformity can be achieved by selecting a process condition, including the flow rate of CH.sub.2F.sub.2 and the power coupled to the dry plasma etching system, such that a proper balance of active etching radicals and polymer forming radicals are formed within the etching plasma.