High contrast marks for e-beam direct write made by reactive ion etching

Abstract

Square wells, etched in silicon (111) substrates by reactive ion etching, are characterized as alignment marks for direct write electron beam lithography. In addition to compatibility with subsequent silicon bipolar processing steps, the contrast of the marks is comparable to etched V-shaped holes in silicon (100). The mark properties are investigated systematically by varying the size, the depth and the slope of the edges. Marks with a size smaller than 10 μm and a depth larger than about 1 μm have a sufficient contrast (>0.1) for automatic detection by a Philips EBPG-3 after deposition of 0.4 μm SiO 2 and 0.25 μm Ti/ 0.2 μm Pt/0.6 μm Au. Although the slope of the mark edges degrades after deposition of the layers, the marks allow an overlay accuracy between 50 to 100 nm. The application of the marks is not restricted to silicon (111) since the fabrication of the marks only depends on the anisotropic nature of reactive ion etching and this technique is applicable to many other substrates.