Characterization of "Bulk Lithography" Process for Fabrication of Three-Dimensional Microstructures

Various ways of fabricating a three-dimensional (3D) component in a single-layer exposure using spatial variation of exposure dose have been presented in the literature. While some of them are based on dynamic mask process, more recently, a process based on varying intensity of a scanning Gaussian laser beam termed as "bulk lithography" has been proposed. In bulk lithography, the entire varying depth 3D microstructure gets fabricated because of spatial variation of intensity of laser imposed at every point in single layer scan. For the bulk lithography process, this paper first presents experimental characterization of unconstrained depth photopolymerization of resin upon exposure to Gaussian laser beam. Experimental characterization carried out for two resins systems: namely 1,6 hexane diol-diacrylate (HDDA) and trimethylolpropane triacrylate (TMPTA), over relatively wider range of Ar+ laser exposure dose and time, show behavior well beyond Beer-Lambert law. A unified empirical model is proposed to represent the nondimensional depth variation with respect to the time and energy of exposure for both resins. Finally, using these models, successful fabrication of several microstructures including micro-Fresnel lens, textured curved surface, otherwise difficult or impossible to fabricate, is demonstrated. Several advantages of the bulk lithography as compared to other similar processes in the literature are highlighted.