Advanced Functional Materials, Vol.21, No.9, 1593-1601, 2011
The Control of Shrinkage and Thermal Instability in SU-8 Photoresists for Holographic Lithography
The negative-tone epoxy photoresist, SU-8, expands approximate to 1% by volume after postexposure baking. However, if the maximum optical fluence is comparable to that at the insolubility threshold, as in a holographic exposure, the developed resist shrinks (approximate to 35% by volume) due to the removal of light oligomers not incorporated into the polymeric network. IR spectroscopy shows that, at this level of exposure, only 15% of the epoxy groups in the insoluble polymer have reacted; consequently microstructural elements soften and collapse at > 100 degrees C. When the light oligomers are removed, the sensitivity of the resist is unchanged, provided that 5% (w/w) of a high-molecular-weight reactive plasticizer (glycidoxy-terminated polyethylene glycol) is added, but it shrinks less on development and, when used as a photonic crystal template, shows improved uniformity with less cracking and buckling. Reinforcing the polymer network by reaction with the polyfunctional amine (bis-N,N'-(3-aminopropyl) ethylenediamine) increases the extent of cross-linking and the thermal stability, allowing inverse replicas of photonic crystal templates to be fabricated from both Al:ZnO and Zr3N4 using atomic layer deposition at temperatures up to 200 degrees C.