The Control of Shrinkage and Thermal Instability in SU-8 Photoresists for Holographic Lithography

Journal article

The negative-tone epoxy photoresist, SU-8, expands ≈1% by volume after postexposure baking. However, if the maximum optical fl uence is comparable to that at the insolubility threshold, as in a holographic exposure, the developed resist shrinks ( ≈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 °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 °C. © 2011 Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim.

Authors: Denning, R; Blanford, C; Urban, H; Bharaj, H; Sharp, D

• Publication status: Published
• Journal: ADVANCED FUNCTIONAL MATERIALS
• Volume: 21
• Issue: 9
• Pages: 1593-1601
• Publication date: 2011-05-10
• DOI: 10.1002/adfm.201002653
• EISSN: 1616-3028
• ISSN: 1616-301X
• Language: English