Ion beam etching of multilevel masking layers written by two-photon lithography

Abstract

Ion beam etching (IBE) provides high surface quality. Finding suitable masking layers is one of the key issues for process optimization. In case of high-intensity and long-term IBE conventional photoresists are not appropriate as masking layers. As an alternative in terms of thermal durability, photoresist masking layers polymerized with two-photon lithography were investigated here and their IBE etch rates were measured. A hard bake (200 °C) lowered them due to higher crosslinking without an alteration of the structure shapes.Two-photon lithography enables the fabrication of multilevel structures which can be etched in one process step. Two types of 3D masking layers were transferred into fused silica to demonstrate this approach. Diffractive structures were chosen because their diffraction efficiency benefits from the high surface quality provided by IBE and it is influenced by fabrication induced deviations of the geometry: 3D line gratings with overlapping photoresist areas are a new approach to avoid delamination problems without the necessity of the integration of unwanted gaps into the resist patterns. Measurements proved good agreement of the diffraction efficiency with simulated results, differ only by 1.14%. The transfer of blazed grating structures illustrated the effect of the angle dependence of the etch rate. The transferred structures showed good agreement with the step heights forecast on the basis of process selectivity.