Multilevel Micro-Structuring of Glassy Carbon for Precision Glass Molding of Diffractive Optical Elements

A consumer market for diffractive optical elements in glass can only be created if high efficient elements are available at affordable prices. In diffractive optics the efficiency and optical properties increases with the number of levels used, but in the same way the costs are multiplied by the number if fabrication steps. Replication of multilevel diffractive optical elements in glass would allow cost efficient fabrication but a suitable mold material is needed. Glassy carbon shows a high mechanical strength, thermal stability and non-sticking adhesion properties, which makes it an excellent candidate as mold material for precision compression molding of low and high glass-transition temperature materials. We introduce an 8 level micro structuring process for glassy carbon molds with standard photolithography and a Ti layer as hard mask for reactive ion etching. The molds were applied to thermal imprinting onto low and high transition temperature glass. Optical performance was tested for the molded samples with different designs for laser beamsplitters. The results show a good agreement to the design specification. Our result allow us to show limitations of our fabrication technique and we discussed the suitability of precision glass molding for cost efficient mass production with a high quality.

Surface micro-structuring of glassy carbon for precision glass molding of diffractive optical elements

Hans Peter Herzig, Karin Prater, Toralf Scharf

Glassy carbon is used nowadays for a variety of applications because of its mechanical strength, thermal stability and non-sticking adhesion properties. This makes it also a suitable candidate as mold material for precision compression molding of low and high glass-transition temperature materials. To fabricate molds for diffractive optics a highresolution structuring technique is needed. We introduce a process that allows the micro-structuring of glassy carbon by reactive ion etching. Key parameters such as uniformity, surface roughness, edge definition and lateral resolution are discussed. They are the most relevant parameters for a stamp in optical applications. The use of titanium as a hard mask makes it possible to achieve a reasonable selectivity of 4:1, which has so far been one of the main problems in microstructuring of glassy carbon. We investigate the titanium surface structure with its 5-10 nm thick layer of TiO2 grains and its influence on the shape of the hard mask. In our fabrication procedure we were able to realize optically flat diffractive structures with slope angles of more than 80° at typical feature sizes of 5 μm and at 700 nm depth. The fabricated glassy carbon molds were applied to thermal imprinting onto different glasses. Glassy carbon molds with 1 mm thickness were tested with binary optical structures. Our experiments show the suitability of glassy carbon as molds for cost efficient mass production with a high quality. © (2014) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Spie-Int Soc Optical Engineering2014

Micro-structuring of glassy carbon for precision glass molding of binary diffractive optical elements

Hans Peter Herzig, Karin Prater, Toralf Scharf

Precision glass molding is a more cost efficient process for the large volume manufacturing of highly complex optical surfaces than direct manufacturing. Glassy carbon (GC) molds are used for precision glass molding, because they can be operated at temperatures up to 2000°C. Used today mainly for manufacturing aspheric lenses, we consider here material technology for diffractive optical element (DOE). For diffractive optics the surface structuring is in the micrometer range and a surface roughness Ra lower than 20 nm is required. We introduce a reactive ion etching process with a titanium hard mask. Fused silica (FS) molds with identical optical functionality were fabricated for comparison. All molds were used for precision glass molding of a low Tg glass L-BAL42. We will compare GC and FS as mold materials in terms of quality and robustness. Optical performance measurements of the molded glass DOEs are shown and are in good agreement with the theoretical predictions. The results confirm that precision glass molding based on GC molds is a very promising technology to economically fabricate small structures in glass for DOEs.

Optical Society of America2016

On the Limits of Precision Glass Molding for Diffractive Optical Elements

Karin Prater

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.

EPFL2017

On the Limits of Precision Glass Molding for Diffractive Optical Elements

Karin Prater

EPFL2017