The problem of polymer contamination in W trench etching is studied tor fluorocarbon plasmas. Polymer contamination of narrow trenches has been observed on samples where isolated positive-relief structures (mesas) are clean. This effect is linked to sidewall sputtering by ions backscattered at low angles from the substrate : the sidewalls of a mesa are bombarded by reflected ions originating over a larger area of the substrate than the sidewalls of a trench. The presence of a blast of backscattered ions is confirmed (in an SF6/Br2 plasma) by images where sidewalls adjacent to large substrate areas etch isotropically while those in the shadow of an adjacent feature etch anisotropically. Finally, it is shown that ultrathin, durable polymer coatings which protect mesa sidewalls without contaminating nanometer-scale trenches can be formed in a magnetically enhanced, triode etching system using a CF4/O2 source gas : 40 nm wide, 300 nm deep trenches are shown.