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/SiO
2
etch 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/SiO
2
etch 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 MgF
2
, Al
2
O
3
, Al, and Cr. The inherently high Si/SiO
2
etch rate ratio obtained in Ar/Cl
2
discharges is here enhanced by causing selective deposition of SiO
2
onto slowly etched materials. The silicon may be obtained from the target, or, for easier control, from input gases such as SiCl
4
. 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.