Photosolubility of diazoquinone resists

The photochemical properties of a diazoquinone/novolak resist system were studied over a wide range of material and processing parameters in order to determine the optimum values for a given application. These resists are representative of the commercial positive-working photoresists being used for high-resolution lithography. The solubility of the resist in an alkaline developer depends on exposure E as
\Delta g^{-1} = [g_{0}(E/E_{e})^{m}]^{-1} + [\Delta g_{\infin}]^{-1}
where the net development rate (
\Delta g
) is the difference between solubility rates of the exposed (
g
) and unexposed (g
0
) sample. Three of these parameters characterize the lithographic response of the resist. They depend mainly on the resist composition but not on the processing conditions. E
e
is a measure of the sensitivity and ranges from 10 to 20 mJ/cm
²
of 405-nm light for useful resist formulations. The contrast parameter (
m
) increases slowly with sensitizer concentration while the saturated solubility ratio (
g_{\infin}/g_{0}
) inereases very rapidly. The fourth parameter (g
0
) depends strongly on processing parameters. It can readily be set to provide the desired development time, e.g., by adjusting the developer strength. On a more fundamental level, it is found that the dependence of the solubility on exposure can be expressed in a unified manner for all the resist formulations studied as
g \approx g_{0} \exp (2 \times 10^{-20} n_{e})
where n
e
is the number of exposed sensitizer molecules per cubic centimeter.