= Photoresist contrast
{title2=$\gamma$}
<Photoresist contrast> measures how sharply a <photoresist> changes its developed thickness as the <exposure dose> changes. For a <positive photoresist>, let $h(D)$ be the remaining thickness after a specified development process and $h_0$ the initial thickness. A contrast curve plots $h(D)/h_0$ against $\log_{10}D$. The measured <photoresist contrast> is the magnitude of the slope near clearing; it need not describe the entire curve.<Glossary of Lithography Terms by Chris A. Mack>[Entry Photoresist Contrast: measured H-D-curve slope versus theoretical development-rate contrast]{ref}
In an idealized model, the thickness decreases linearly with $\log_{10}D$ between a retention <exposure dose> $D_0$ and a clearing <exposure dose> $D_c$, where $0<D_0<D_c$. Then
$$
\frac{h(D)}{h_0}=1-\gamma\log_{10}\frac{D}{D_0},\qquad \gamma=\frac{1}{\log_{10}(D_c/D_0)}.
$$
Outside that interval the idealized thickness is clipped to $1$ or $0$. A large $\gamma$ means a smaller <exposure dose> ratio suffices to change from retention to clearing. This is why <photoresist contrast> influences the <critical modulation for a photoresist>; the inverse-logarithm formula assumes this particular contrast-curve model.
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