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The Control of Shrinkage and Thermal Instability in SU‐8 Photoresists for Holographic Lithography
Author(s) -
Denning Robert G.,
Blanford Christopher F.,
Urban Henning,
Bharaj Harpal,
Sharp David N.,
Turberfield Andrew J.
Publication year - 2011
Publication title -
advanced functional materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 6.069
H-Index - 322
eISSN - 1616-3028
pISSN - 1616-301X
DOI - 10.1002/adfm.201002653
Subject(s) - materials science , photoresist , resist , epoxy , polymer , thermal stability , lithography , photopolymer , composite material , polymerization , chemical engineering , prepolymer , polycarbonate , holography , polymer chemistry , layer (electronics) , optoelectronics , optics , polyurethane , physics , engineering
The negative‐tone epoxy photoresist, SU‐8, expands ≈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 (≈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 Zr 3 N 4 using atomic layer deposition at temperatures up to 200 °C.