Mask Charging and Feature Profile Evolution During Chlorine Plasma Etching of Silicon

Non-ideal feature profile anomalies such as undercut, tapered, or bowed sidewalls and microtrenches at the base of the sidewall are often observed after etching SiO sub 2-masked silicon (Si) in chlorine (Cl sub 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 sub 2 plasmas (rf source and substrate bias powers and pressure were varied) regardless of whether the mask was insulating (SiO sub 2) 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 was calculated using simulated charged mask field strengths reported in the literature.