Reactive sputter etching of silicon with very low mask-material etch rates

Directional etching of deep structures in silicon is often made difficult by a high mask erosion rate. Recent results have given a Si/SiO2etch rate ratio of up to 8 without the undercut problems associated with other selective etches. In this paper a new selectivity mechanism is described which can reproducibly give Si/SiO2etch rate ratios of more than 100 with a nonloading target, and more than about 50 with a loading target. Similar etch ratios are also obtained with masks of MgF2, Al2O3, Al, and Cr. The inherently high Si/SiO2etch rate ratio obtained in Ar/Cl2discharges is here enhanced by causing selective deposition of SiO2onto slowly etched materials. The silicon may be obtained from the target, or, for easier control, from input gases such as SiCl4. The deposition rate is controlled by the oxygen concentration. The results of etching deep grooves in Si are presented. Etch-mask faceting and Si surface decoration appear to limit the attainable etch rate ratios with fine structures; however, 18-µm-deep gratings of 4.5-µm period have been etched in Si.