Glassy carbon

Glass-like carbon, often called glassy carbon or vitreous carbon, is a non-graphitizing, or nongraphitizable, carbon which combines glassy and ceramic properties with those of graphite. The most important properties are high temperature resistance, hardness (7 Mohs), low density, low electrical resistance, low friction, low thermal resistance, extreme resistance to chemical attack, and impermeability to gases and liquids. Glassy carbon is widely used as an electrode material in electrochemistry, for high-temperature crucibles, and as a component of some prosthetic devices. It can be fabricated in different shapes, sizes and sections. The names glassy carbon and vitreous carbon have been registered as trademarks, and IUPAC does not recommend their use as technical terms. A historical review of glassy carbon was published in 2021. History Glassy carbon was first observed in the laboratories of The Carborundum Company, Manchester, UK, in the mid-195

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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

Wafer-level micro-structuring of glassy carbon

Loïc Etienne Hans, Hans Peter Herzig, Cédric Kilchoer, Karin Prater, Toralf Scharf

Glassy carbon is used nowadays for a variety of applicationsbecause of its mechanical strength, thermal stability and non-sticking adhesion properties. One application is glass molding that allows to realize high resolution diffractive optical elements on large areas and at affordable price appropriate for mass production. We study glassy carbon micro-structuring for future precision compression molding of low and high glass-transition temperature. For applications in optics the uniformity, surface roughness, edge definition and lateral resolution are very important parameters for a stamp and the final product. We study different methods of microstructuring of glassy carbon by etching and milling. Reactive ion etching with different protection layers such as photoresists, aluminium and titanium hard masks have been performed and will be compare with Ion beam etching. We comment on the quality of the structure definition and give process details as well as drawbacks for the different methods. Inour fabrications we were able to realize optically flat diffractive structures with slope angles of 80° at typical feature sizes of 5 micron and 700 nm depth qualified for high precision glass molding.

Spie-Int Soc Optical Engineering2014

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

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