Design of experiments optimization of PMMA for LIGA applications

The most common resist used for LIGA applications with deep X-ray lithography is PMMA. It achieves very good resolution of the mask structures with a typical resist thickness from 200 mum to 2 mm. The resist system employed here is crosslinked on the substrate after preparation, and this can lead to problems with internal stresses and resist cracking. To optimize the resist performance, a design of experiments procedure was implemented. Design of experiments assists the determination of optimised process parameters through the use of statistical procedures. The principal components of the PMMA resist employed in the optimization procedure were PMMA, adhesion promoter, crosslinker, initiator and accelerator. The PMMA type employed was kept constant and the concentrations of adhesion promoter, crosslinker, initiator and accelerator were altered following a design of experiments methodology. At first 18 resists compositions were prepared and irradiated to determine preliminary exposure behaviour. A selection was made from this composition on the basis of the performance, and then these resist composition were irradiated with standard test structures. This determined the performance of the resist to stress, its inherent stability and the inherent resolution that can be achieved. A smaller parameter set was then implemented in the final optimization of the resist. A design of experiments procedure has assisted the optimization of the PMMA resist for LIGA application with deep X-ray lithography. The results obtained demonstrate the effectiveness of the procedure in quickly attaining resist compositions suitable for deep X-ray lithography.