On the Limits of Precision Glass Molding for Diffractive Optical Elements

Diffractive optical elements (DOEs) consist of surface reliefs with dimensions in the micrometer range and nanometer precision. Two technologies dominate: Elements replicated in plastic and directly microfabricated elements in fused silica. Plastic DOEs are mostly used in mass production because they can be fabricated very cost efficient by replication technologies such as plastic injection molding and hot embossing. Glass DOEs are only used when its superior characteristics e.g. higher temperature stability, higher form accuracy due to a low reaction to humidity and stress are necessary for the specific application. Today, glass DOEs are fabricated by cleanroom technology based on direct structuring of fused silica. In this thesis we investigate the possibility to use precision glass molding to fabricate glass DOEs. Up to now, precision glass molding is used only for continuous surfaces like in aspherical lenses or freeform elements. Diffractive optical elements with more complex structures including surfacediscontinuities (steps) are not found. One reason is the lack of a suitable mold material that can withstand the high molding temperatures and can be microstructured with the necessary accuracy. One potential candidate to close this technology gap is glassy carbon. Glassy carbon is a fullerene like carbon with extreme temperature resistance and unmatched chemical inertness. The key factor of applying glassy carbon is the possibility to structure its surface with the suitable dimensions and conformity. We tested and developed microstructuring processes to overcome limitations such as process compatibility, etch selectivity, structural integrity and surface roughness. Our major objective of this work aimed to investigate the limits of precision glass molding for DOEs. Of special interest are the minimal feature size and maximal aspect ratio that can be obtained by keeping the optical quality of surfaces. We showed that precision glass can replicate features down to 800 nm and the process is stable. With this newly established processes the whole fabrication chain could be tested for the first time including lifetime tests of the stamp. For each step extensive characterization was done. Measuring the optical performance was the last step that allowed us to develop a complete guideline for the fabrication of diffractive optics with precision glass molding. Beamsplitting elements were chosen as test designs, because they allow a rigorous evaluation of the optical performance by measuring the diffraction efficiencies and uniformity distribution. As the main results we could show that precision glass molding of DOEs can reach a comparable optical performance as directly etched fused silica DOEs, which is the state-of-the-art technology. We could confirm that glassy carbon is an excellent mold material for precision glass molding of complex optical surfaces.

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