Mask charging and profile evolution during chlorine plasma etching of silicon

Nonideal feature profile anomalies such as under-cut, tapered, or bowed sidewalls and microtrenches at the base of bench sidewalls are often observed after etching masked silicon (Si) in chlorine (Cl-2) plasmas. Off-normal impact with subsequent scattering and/or focusing of ions is believed to be the primary cause of these anomalies. Localized buildup of negative charge on the insulating mask sidewalls is one possible source of the ion deflection. Here we show that nearly identically shaped sidewalls and microtrenches were formed when Si features were etched in several Cl-2 plasmas (the rf source and substrate bias powers and pressure were varied) regardless of whether the mask was insulating (SiO2) or conducting (Si or tungsten). Therefore, differential charging of the insulating mask material is not the fundamental cause of these profile characteristics. Ion angular distributions for each experimental plasma condition were estimated and the angle through which an ion would be deflected by a charged mask sidewall was calculated using simulated charged-mask field strengths reported in the literature. (C) 1999 American Vacuum Society. [S0734-211X(99)00406-0].