Optimizing two-photon initiators and exposure conditions for three-dimensional lithographic microfabrication

The radical photoinitiation characteristics of a range of D-π-D chromophores (where D is an electron-donor and π is a biphenyl, stilbene, or bis(styryl)benzene conjugated bridge) under two-photon excitation are reported. Photo-crosslinkable resins were formulated with these initiators and were used to fabricate a variety of complex three-dimensional structures by two-photon induced polymerization (TPIP). These structures illustrate that TPIP can be a highly versatile technique for the rapid single-step fabrication of complex microstructures and devices. The new two-photon resins were found to be as much as 50 times more sensitive than resins containing conventional UV initiators. The increased two-photon sensitivity is attributed to the large two-photon absorption cross-sections and the efficient electron -transfer mediated initiation of the polymerization by the D-π-D chromophores. A three-dimensional test-structure was used to evaluate how varying the intensity and the exposure times affects the shape and dimensions of the polymerization volume element (voxel). Conditions were found for which the voxel could be as small as ∼200 nm in width and ∼700 nm in length, even though the wavelength of the two-photon excitation radiation was 775 nm.