Gabriela Blagoi

Processing of thin SU-8 films

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 TPEB ≥ 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 TPEB = 50 °C. Fourier-transform infrared spectroscopy is used to confirm the increased cross-linking density. The low thermal stress due to the reduced TPEB 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.

Polymeric cantilever-based biosensors with integrated readout

The authors present an SU-8 cantilever chip with integrated piezoresistors for detection of surface stress changes due to adsorption of biomolecules on the cantilever surface. Mercaptohexanol is used as a model biomolecule to study molecular interactions with Au-coated SU-8 cantilevers and surface stress changes of about 0.1N∕m are observed. The measurements indicate that the SU-8 surface is inert to mercaptohexanol molecules, which enables the use of a bare SU-8 cantilever as a reference. Moreover, the sensitivity of the cantilevers to pH changes is presented. The results demonstrate that SU-8 cantilevers are a promising alternative to Si-based cantilevers.

Photochemical modification and patterning of SU-8 using anthraquinone photolinkers.

Bioactive protein patterns and microarrays achieved by selective localization of biomolecules find various applications in biosensors, bio-microelectromechanical systems (bio-MEMS), and in basic protein studies. In this paper we describe simple photochemical methods to fabricate two-dimensional patterns on a Novolac A derivative polymer (SU-8) and, subsequently, their functionalization with biomolecules. Anthraquinone (AQ) derivatives are used to chemically modify and pattern SU-8 surfaces. Features as small as 20 mum are obtained when using uncollimated light. The X-Y spatial resolution of micropatterned AQ molecules is improved to 1.5 mum when a collimated light source is used. This micropatterning process will be important for the functionalization of MEMS-based biosensors. The method saves several processing steps and can be integrated in cleanroom fabrication thus avoiding contamination of the sensor surfaces.