Ultra-thick positive photoresist layers for maskless grayscale lithography

Abstract

Maskless grayscale lithography is a key technology to create structured surfaces in photoresist, especially for micro-optic applications. It uses spatially modulated light intensity to expose a layer of low-contrast positive resist. A digital design that contains gray values or height information, corresponding to a certain target depth in the photoresist, can conveniently be exposed with a DWL 66⁺ from Heidelberg Instruments. Processes are well known for thicknesses up to 60 μm. The possibility to fabricate taller structures, are of great interest in the micro-fabrication world. Two experimental and one commercial resists have been compared to reach and exceed the 100 μm symbolic height. After a validating experiment in a single coated layer of an experimental DNQ-based photoresist, we doubled and tripled the coating cycles at relatively low velocity and quickly obtained promising results. The triple-coated film while enabling the possibility to fabricate structures 100 μm high showed some sort of delamination in the deepest region of the layer, close to the substrate's surface. The delamination indicates the formation of N₂ bubbles, a disadvantage of DNQ-based photoresists that release nitrogen when exposed to light. Experiments with a commercially available resist seems to show similar behavior for thicknesses above 80 μm. Recent experiments using a second experimental resist, from a different supplier, showed some promising results: structures slightly higher than 100 μm without visible defect caused by nitrogen have been fabricated. Another advantage is that overall dose required to reach this depth was significantly lower than in the previous test.