Study on diffraction effect and microstructure profile fabricated by one-step backside lithography

Abstract

Backside exposure lithography has been proven to be able to generate needle-like microstructures. The structure profile can be controlled by varying the aperture diameter on the photomask and the distance between the photomask and the photoresist. This distance is usually defined by the glass thickness of the glass in backside exposure lithography. However, in our experience, needle-like structures can be generated easily in some cases but not in others. In order to accurately predict the microstructure profile fabricated by backside exposure lithography, in this study, we built a complete three-dimensional Fresnel–Kirchhoff diffraction model and used a binary approach to simulate the curing threshold. We found that the microstructure profile is influenced by diffraction in both the near-field (Fresnel) and far-field regions (Fraunhofer). Diffraction depends on the design pattern on the photomask and the glass thickness. In many cases, it changes gradually from the near-field to the far-field. This is exactly the reason that our approach generates needle-like structures. Structures ranging from 50 to 450 µm in height were simulated by our model and had a high degree of consistency with the fabricated results. This research may provide potential guidelines for the prediction and the fabrication of needle-like structures for future applications.