Processing of thin SU-8 films

Abstract

This paper summarizes the results of the process optimization for SU-8 films with thicknesses 5 μm. The influence of soft-bake conditions, exposure dose and post-exposure-bake parameters on residual film stress, structural stability and lithographic resolution was investigated. Conventionally, the SU-8 is soft-baked after spin coating to remove the solvent. After the exposure, a post-exposure bake at a high temperature T PEB 90 • C is required to cross-link the resist. However, for thin SU-8 films this often results in cracking or delamination due to residual film stress. The approach of the process optimization is to keep a considerable amount of the solvent in the SU-8 before exposure to facilitate photo-acid diffusion and to increase the mobility of the monomers. The experiments demonstrate that a replacement of the soft-bake by a short solvent evaporation time at ambient temperature allows cross-linking of the thin SU-8 films even at a low T PEB = 50 • C. Fourier-transform infrared spectroscopy is used to confirm the increased cross-linking density. The low thermal stress due to the reduced T PEB and the improved structural stability result in crack-free structures and solve the issue of delamination. The knowledge of the influence of different processing parameters on the responses allows the design of optimized processes for thin SU-8 films depending on the specific application.

Cracking-assisted photolithography for mixed-scale patterning and nanofluidic applications

“…8). This is because an increased exposure energy causes an increase in the material failure strength, thanks to stronger cross-linking 19 . We note that the maximum crack length may not be limited but the ultraviolet exposure time needs to increase to reduce the crack width and depth, resulting in the increase of development time.…”

Characterisation of the fabrication process of freestanding SU-8 microstructures integrated in printing circuit board in microelectromechanical systems

“…This stress appears due to the different values of the coefficient of thermal expansion (CTE) between SU-8 and the used substrate. The expression to determine this value is [15]:…”

Micro-particle filter made in SU-8 for biomedical applications

“…After solvent evaporation at room temperature for 3 h, the SU-8 layer was structured by a standard UV exposure (SUSS Mask Aligner MA6/BA6) and baked for 1 h at 50 °C [6]. The development was carried out in PGMEA for 2 x 2 min.…”