Two-layer resist process

Depth of field depends on substrate reflectivity, the degree of partial coherence and the minimum feature size (5). In practice, however, the classical depth of field for the incoherent case (X-f2(N.A.)2) gives a reasonable approximation. Two layer resist processes in which the image is formed in a thin, flat, resist layer on top of a much thicker planarizing layer, alleviate the need for a large depth of field and make it easier to form high resolution, high aspect ratio, resist patterns (6,7). Satisfactory results can be obtained at contrast levels as low as 40%. 

With the two layer resist process, no effect of the surface structure is observed in the resulting resist pattern. A 0.35 wide lines with an aspect ratio of 6 have almost vertical side-walls and show a linewidth loss less than 0.1 iim by the etching of 2.0 thick AZ layer. In spite of the thin film of SNR, pin-holes were hardly observed in the dry-etched AZ patterns, presumably due to good wettability of silicone resin on organic film surface. 

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