Three-dimensional laser nano-structuring: contrast in three-photon and two-photon polymerization of SU-8 Ladan E. Abolghasemi, Shane Eaton, Abbas Hosseini and Peter R. Herman Dept. of Electrical and Computer Engineering, Institute for Optical Science, University of Toronto, Toronto, ON M5S 3G4, Canada ladan.abolghasemi@utoronto.ca Abstract: A femtosecond fiber laser with 100-kHz repetition rate was optimized for 3-D nanostructuring of photoresist. Contrasts in three-photon (1045nm) and two-photon (522nm) resolution are presented together with prospects for creating photonic crystal templates and optical phasemasks. OCIS codes: (220.4000) Microstructure fabrication; (320.2250) Femtosecond phenomena Short-pulse lasers enable three-dimensional (3-D) nano-patterning of negative-tone photoresist SU-8 by driving multi–photon absorption (MPA) within a tightly focused interaction volume. Rapid, flexible fabrication of arbitrary 3-D patterns have been demonstrated with high spatial resolution [1,2], most often driven by two-photon polymerization (2PP) using amplified Ti:Sapphire laser systems with 1-kHz repetition rate, 120-fs pulse width, and 800-nm central wavelength [3-5]. Three-photon- polymerization (3PP) has also been studied in other materials. For example, Farsari et al. [6] reported 3PP of UV photocurable organic-inorganic liquid (ORMOCER) with a diode-pumped laser oscillator providing 200-fs pulses, 1028-nm wavelength, and high 50-MHz repetition rate. However, solid-phase photoresists like SU-8 are better developed and more widely applied. This epoxy-based resist offers high mechanical strength that supports formation of high aspect-ratio nanostructures, while high chemical resistance is attractive for pattern transfer in etching chambers. For these reasons, SU-8 photoresist has been selected in our program for high-speed laser direct writing of multi-level 2-D optical phasemasks and 3-D photonic crystal templates with tailored optical defects. In this paper, we present laser exposure and resist development recipes for high-speed 2-D and 3-D nanostructuring of SU-8 photoresist. Both fundamental (1045 nm) and second-harmonic (522 nm) output from a fiber-amplified femtosecond laser was applied to drive respective 3PP and 2PP in this resist, for the first time at these wavelengths to our best knowledge. SU-8 absorbs at λ < 420 nm, and is highly transparent at both laser wavelengths. One objective is a 35% improvement in optical resolution possible for 2PP at λ = 522 nm in contrast with the many 2PP studies based on the 800-nm Ti:Sapphire lasers [4,7]. Contrasts in resolution limits for 3PP and 2PP are presented together with examples of high aspect ratio 2-D and 3-D structures having sub- diffraction limited feature sizes. A commercial femtosecond Yb-fiber laser system (IMRA America, FCPA μJewel) provided 1045 and 522 nm wavelength light and variable repetition rate from 0.1 to 5.0 MHz. Results will show that polymerization is possible with the long 488-fs pulse duration of the laser in contrast to the <130 fs typically applied in prior studies [1-5, 7]. The samples consisted of 1 to 50 μm thick SU-8 layers, spincoated onto a glass substrate. The laser pulses were focused into the SU-8 by an aspheric lens with moderate numerical aperture (NA) of 0.55 that provided estimated spot diameters of 0.60 μm for λ = 1045 nm and 0.30 μm for λ = 522 nm. Smaller spot sizes available [7] from higher (NA) lenses and oil immersion are also presently under investigation. The samples were translated with scan speeds of 1 to 25 mm/s along pre-programmed paths using precise air-bearing motion stages (Aerotech, ABL1000). The pulse energy was varied between 2 to 31 nJ/pulse (average power: 0.2 - 3.1 mW), while a low repetition rate of 100 kHz was selected to improve the positioning accuracy and to avoid heat-accumulation effects in the resist. An optimum development procedure applied to all laser exposed resist was postbaking at 65 o C for 1 min and at 95 o C for 5 min immediately after exposure followed by emersion in SU-8 developer and gentle rinsing with isoproponal (IPA). Results of a systematic study of resolution limits in 1045-nm exposure of 1μm-thick SU-8 resist are shown in Fig. 1a. The laser was focused at the SU-8/glass interface and scanned laterally in a grid pattern with variable combinations of laser pulse energy and scan speed. The width of SU-8 ribs was measured with an atomic force microscope (AFM) from images such as shown in Fig. 1b. Here, a 2-μm grid of ribs with average waist diameter of 341 nm ± 25 nm and 600-nm height was generated. The observed linewidths, d, in Fig. 1a are well represented by an expected logarithmic dependence on pulse energy, E, of [7] 0 2 ln( ) th d w EE N = (1) where E th is the threshold of polymerization for the laser focal volume, w 0 is the Gaussian beam waist, and N=3 is the order of multi-photon absorption. Overly high laser pulse energy (>31 nJ) and/or low scan speed (<10 μm/s) resulted in boiling of the SU- 8, as shown inset of the Fig. 1, features also observed in other work [3]. Axial spatial resolution was not examined here because the laser depth of focus exceeded the film thickness. Equation 1 predicts a 38% decrease in feature size for 522-nm 2PP in comparison with 1045-nm 3PP. However, the smallest SU-8 voxel diameter observed to date with λ = 522 nm was ~410 nm. The optimum resist development procedure for 522 nm exposure is possibly different than the 1045-nm procedure that was adopted here. Nevertheless, small periodic nanostructures were successfully fabricated in 50-μm thick SU-8 resist as shown by the SEM images of high-aspect ratio walls in Fig. 2 (a-c) and a2748_1.pdf CThN6.pdf