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 (g0) 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. Eeis a measure of the sensitivity and ranges from 10 to 20 mJ/cm2of 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 (g0) 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 asg \approx g_{0} \exp (2 \times 10^{-20} n_{e})where neis the number of exposed sensitizer molecules per cubic centimeter.