Experimental and Numerical Study of 3D Nanolithography Using Photoinitiator Depletion

Fabricating complex submicron 3D structures can be achieved by multi-photon lithography, especially two-photon lithography is commonly used to obtain precision and flexibility in printing sophisticated sub-micron 3D structures. Several disadvantages stemmed from a two-photon lithography experiment setup, including cost, the necessity of a large laboratory space to use a femtosecond laser and a high-order process. A two-step absorption is chosen instead of two-photon lithography as a primary excitation process achieving the same degree of quadratic optical non- linearity as two-photon lithography at a lower cost with a relatively compact laboratory size. The working mechanism of Two-step absorption is the following. Quadratic nonlinearity comes from radicals from excited triplet states photoinitiators. Ground states of photoinitiators get excited by the incident laser. Those excited singlet photoinitiators go through the intersystem crossing, becoming the ground triplet state of photoinitiators. There are two branches after the ground triplet states, especially for photoinitiator benzil molecules with the incident laser on. Either it becomes a radical without photons received from the incident laser or gets excited again to an excited triplet state by the incident laser. Those excited triplet-state photoinitiator molecules become radicals that occur in polymerization. However, those from the ground triplet states add linearity to polymerization. When it comes to multiple exposures, the linearity becomes problematic, especially outside the region and tails of the voxel. For example, suppose the intensity at two tails of the voxel is 1% relative to the maximum intensity at the focal point. In that case, the absorbed dose will be added up to the maximum intensity at the focal point when it comes to 100 exposures. Quadratic nonlinearity and linearity are jumbled together in the current two-step absorption process. In this work, optimization of photoinitiator concentration was conducted to reduce the linearity. Confined and high throughput 3D structure fabrications are achieved by controlling initiator depletion. Simulations are also developed with multi-physics models to compare with the empirical results.