Network properties and acid degradability of epoxy-based SU-8 resists containing reactive gamma-butyrolactone

SU-8 is an octafunctional epoxy-based negative resist supplied with a reactive diluent, gamma-butyrolactone (GBL). This paper characterizes the network properties and acid degradability of cured SU-8 resists with varying GBL monomer content and ultraviolet (UV) irradiation time. The SU-8/GBL network structure was characterized by Fourier transform infrared (FTIR) spectroscopy, 13C nuclear magnetic resonance (NMR) spectroscopy and gas chromatography–mass spectrometry (GC/MS). GBL was found to copolymerize with epoxy to bridge two neighbouring epoxy groups and does not homopolymerize. The maximum GBL:SU-8 molar ratio whereby all GBL fully reacted with available epoxy functionalities in the network was found to be 8:1. Excess GBL beyond the maximum GBL:SU-8 ratio remains in the network as a plasticizer. GBL content and UV irradiation time affect glass transition temperature (Tg), epoxy conversion and molecular weight between cross-links (Mc) which were measured by dynamic mechanical analysis (DMA) and FTIR. The mechanism of cross-linked network acid degradation was found to be surface erosion. Lower epoxy conversion, higher Mc and higher GBL content resulted in a higher dissolution rate, which can be exploited in applications requiring SU-8 removal. A patterned SU-8 grating with relatively high-GBL content (10%) was successfully used as a template for Cu electroforming.