Relationship between the slope of the HD curve and the fundamental resist process contrast

Abstract
One of the fundamental tests for a resist process is to determine the characteristic curve of photoresist thickness versus exposure dose, sometimes called the HD plot. Standard analysis of characteristic curve data in an HD plot yields a slope which we term γHD. γHD is generally regarded as an experimental determination of the process contrast or process nonlinearity, and high values of it are considered desirable. It is possible to define a more fundamental measure of the process contrast, which we term γtrue. The present work describes the relationship between γtrue and γHD using a simplified theoretical treatment that includes the effects of resist absorption and substrate reflection. This relationship shows that increasing resist absorption tends to decrease γHD for positive resist processes. On the other hand, for negative resist processes, increasing resist absorption tends to increase |γHD|. Without accounting for the effects of absorption, |γHD| is a poor indicator of true process nonlinearity.

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