Greyscale lithography is applied to manufacture complex 2.5D and freeform microstructures in photoresists which serve as master for the pattern transfer into materials for permanent applications, often used in micro-optics. We present the results and the challenges in reproducible generation of deep greyscale patterns in a highly sensitive greyscale positive photoresist, mr-P 22G_XP, when using photomask-based mask aligner greyscale lithography in contrast to laser direct writing on which resist development had been focused. Furthermore, we show the influence of resist aging on the resist response, and ways to correct it by process adaption, as well as we conclude requirements to greyscale photomasks suitable to make use of the full potential of the mr-P 22G_XP resist dedicated for >100μm deep greyscale patterns.
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.
Greyscale lithography for the manufacture of complex 2.5D and freeform microstructures in photoresists receives increasing attention from industry for the fabrication of advanced micro-optical elements. The thus obtained structures serve as master or template for different methods of pattern transfer into materials for final, permanent applications, such as refractive and diffractive lenses, blazed gratings, beam-shapers etc. However, many such applications require large structure heights beyond 100 μm which was not easily accessible until now. We present a novel photoresist, mr-P 22G_XP, enabling greyscale lithography of very deep patterns. Issues limiting the pattern depth caused by the photoresist chemistry were addressed. Greyscale pattern depths of 120 μm were possible with an easily accessible set-up with this prototype, with a well-considered choice of photoresist ingredients, and lithography process adjustments focusing on laser direct writing, with the prospect of even deeper patterns up to 140–150 μm.
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