Diazo-type positive photoresists are commonly used for pattern replication by the integrated circuit industry in a thickness range ≈ O.3-3μm. We are using these same resists at thicknesses as great as 40 μm to form electroplating molds for the fabrication of micro-Fresnel zone plates. Difficulties are encountered when films thicker than l5-18μm are used for pattern replication. Most significant of these difficulties are: i) the occurance of bulk microfractures throughout the resist volume, ii) loss of UV sensitivity, and iii) sidewall taper in high aspect ratio structures. These difficulties, with the exception of the sidewall taper, can be overcome with appropriate resist processing schedules.
D . R . Ciarlo,
N . M. Ceglio,
"Ultrathick Photoresist Processing", Proc. SPIE 0221, Developments in Semiconductor Microlithography V, (5 September 1980); doi: 10.1117/12.958626; https://doi.org/10.1117/12.958626