Abstract
We report on the challenges in a hybrid sub-micrometer fabrication process while using three dimensional femtosecond direct laser writing and electroplating. With this hybrid subtractive and additive fabrication process, it is possible to generate metallic polarization elements with sub-wavelength dimensions of less than 400 nm in the cladding area. We show approaches for improving the adhesion of freestanding photoresist pillars as well as of the metallic cladding area, and we also demonstrate the avoidance of an inhibition layer and sticking of the freestanding pillars. Three-dimensional direct laser writing in a positive tone photoresist is used as a subtractive process to fabricate free-standing non-metallic photoresist pillars with an area of about 850 nm × 1400 nm, a height of 3000 nm, and a distance between the pillars of less than 400 nm. In a subsequent additive fabrication process, these channels are filled with gold by electrochemical deposition up to a final height of 2200 nm. Finally, the polarization elements are characterized by measuring the degree of polarization in order to show their behavior as quarter- and half-wave plates.
Highlights
Laser material processing technologies comprise innumerable subtractive and additive manufacturing approaches.Typical subtractive, material-removing, processes are, e.g., laser modification and removal [1,2], laser ablation [3,4], laser micro-drilling [5,6], laser cutting [7,8], and laser structuring [9,10]
In order to evaluate the quality of the indium tin oxide (ITO) layer in this respect, the current underneath the pillar structure, as described above dissolved by the stripper, as well as in the channels between the original pillar structure, i.e., areas that were expose to the reactive ion etching (RIE) process, are measured
The difference between these currents ∆I is regarded as a qualitative measure of the integrity and quality of the ITO layer with respect to the forthcoming electroplating process step and it is plotted in Figure 6c as a function of the (RIE) etching time
Summary
Laser material processing technologies comprise innumerable subtractive and additive manufacturing approaches. The blue area in the figure depicts the substrate, the light gray area the unexposed photoresist, the dark gray areas the exposed photoresist, the red lines approximate the envelope of the laser beam used for exposure, which is strongly focused by an objective with a high numerical aperture and the red area illustrates the focal volume (voxel) in which the polymerization occurs. Deep mask-based lithography is a suitable manufacturing process, the achievable resolution is correspondingly high due to the short wavelength of the used source, and, due to the parallel exposure of all structures, is significantly faster than 3D DLW Disadvantages of this method, are the complex mask fabrication for the deep UV exposure and the limited access to a synchrotron radiation source [32]. A quarterand a half-waveplate are manufactured, both being based on the concept of hollow waveguides, as theoretically described by Helfert et al and recently experimentally demonstrated by the authors [33,34,35]
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