overall performance by the addition of small amounts of base to reduce proton diffusion into unexposed regions. We can estimate the width of the etched voxel to be~1.0 lm. As the width of the voxel is much larger than that of the focal spot, the experimental conditions used to process the resist may not be optimized for achieving the smallest voxel. We are currently exploring other exposure and development conditions and modified resin systems to further improve the resolution in this type of 3D microlithography.In conclusion, we have developed a chemically-amplified positive resist material and processing conditions that enable the microfabrication of buried 3D structures using twophoton lithography. The ability to selectively remove material in exposed regions allows for efficient creation of small hollow features within a larger solid body. By tailoring the polymer structure, it should be possible to fabricate the 3D structures with selected surface properties. For example, in our material, the pendent chemical groups at the patterned surface are methacrylic acid groups that provide a variety of options for surface functionalization. This feature may be useful for microfluidic devices for biological applications, and makes possible the study of both chemical and geometrical factors for such applications. These results suggest that positive tone two-photon 3D microfabrication may be a valuable route to precision microfluidic and micro-optical devices.
ExperimentalThe polymers used in this study were synthesized by copolymerization of THPMA, MMA, and tBMA [18,19]. The number average molecular weight (M n ) and the polydispersity of the copolymers were determined by gel permeation chromatography (THF, solvent) using polystyrene as the standard. The copolymer compositions were verified by 1 H NMR measurements that were carried out in CDCl 3 at room temperature with a Varian INOVA-400 spectrometer operating at 400 MHz for protons (Table 1). A two-photon 3D microfabrication resin system was created by blending 4 wt.-% of the two-photon acid generator [17] into the above-synthesized copolymers. 3D microfabrication was carried out using thick polymeric films (~50 lm) prepared by blade casting from c-butyrolactone (GBL). The films were baked at 90 C for 1 h under a nitrogen atmosphere. The system for two-photon 3D microfabrication has been described elsewhere [17]. The PAG was excited with~100 fs pulses at a wavelength of 745 nm. The samples were mounted on a computer-controlled 3D translation stage. The incident beam was focused through an oil-immersion objective with a numerical aperture of 1.4. By scanning the focal point of the laser beam through the film, 2D and 3D patterns were defined. The total exposure dose is determined by the laser power and linear scan speed (dwell time in a voxel) of the stage. After exposure, the film was baked and developed in 0.26 N tetramethyl ammonium hydroxide (TMAH) aqueous solution and monitored by optical microscopy. The 3D microstructures shown in Figures 3 and 4 were fabricated ...
