The reasons that imaging is tone-dependent come from two fundamental concepts: the aerial images of complimentary mask patterns for partially coherent projection systems are not complimentary, and the exposure reaction is highly non-linear in the concentration of the soluble species. Complimentary mask patterns are simply patterns of opposite tone. If mp(x) describes a positive mask pattern, then its complimentary mask pattern, mn(x), is given by mn(x) = 1 - ntp(x). For incoherent imaging systems, complimentary mask patterns result in complimentary images; however, partially coherent imaging systems do not produce complimentary images. For a first order exposure reaction, the concentration of the photosensitive species is exponentially related to the exposure energy. However, the dependence of the concentration of developer- soluble species on exposure is different for positive and negative resist systems, resulting in different exposure properties. The net result is lithographic behavior which can vary significantly with resist tone.

Date Published: 1 June 1992
PDF: 11 pages
Proc. SPIE 1674, Optical/Laser Microlithography V, (1 June 1992); doi: 10.1117/12.130332

Published in SPIE Proceedings Vol. 1674:
Optical/Laser Microlithography V
John D. Cuthbert, Editor(s)