One of the key questions concerning the concept of a system for hot embossing lithography is whether or not it should provide for imprinting under vacuum. We have performed experiments comparing the embossing in vacuum and in atmospheric pressure in a semi-automated imprint system. The stamps used were fully patterned, 10 cm diameter with pattern sizes ranging from 400 nm to 100 μm. It turned out that vacuum enhances the large area uniformity of the imprint by avoiding an air cushion remaining between stamp and sample during automated contact after a non-contact assembly and alignment step. Lower molecular weight polymers turned out to be more sensitive to uniformity deviation than higher molecular weight materials. Detailed analysis showed that defects typically found for relatively high processing temperatures, caused by overheated compressed air, remaining solvent in the polymer layer or even beginning polymer decomposition could be reduced substantially under vacuum embossing conditions, where the excess volume of the polymer is evacuated and free to accommodate gaseous constituents. The best result with complete cavity filling and negligible defects was obtained for imprint of a 99 kg/mol polymer at 200°C and 50 bar under vacuum. Residual layers measured across the diameter of the sample were 44.5 nm ± 9.8 nm. The non-uniformity of the residual layer is a result of the locally different pattern sizes and pattern densities of the stamp, typical for all mechanical patterning processes.
The combination of nanoimprint and UV-lithography has been demonstrated. For this purpose a UV-sensitive epoxy based resin with a low glass temperature was prepared by adding low molecular weight components, in particular by increasing the monomer content. The suitability of our approach to minimize process temperatures was tested by embossing and VU-lithography. Mix and match of both techniques was used to demonstrate that the embossing step did not degrade the UV sensitivity of the material. UV processing provided in addition a simple means for stabilization of this low Tg material. Resists like mr-L 6000-1 xp may close the gap between 'hot embossing' 'UV-molding' and UV-lithography.
In order to reduce the cost for stamps featuring nanometer structures in a hot embossing lithography (HEL) process the production and performance of working stamps made of thermoset polymer are of interest. Fabrication of stamps made of thermosetting material no silicon substrates by hot embossing with 2 X 2 cm2 templates and their replication by HEL has already been demonstrate. In this paper the enlargement of this principle to 4 inch wafer- scale is presented. Two procedures to obtain working stamps by hot embossing are compared, one solely based on hot embossing, the other enhanced by additional UV-exposure. The produced working stamps are tested for performance under standard embossing conditions are the topic of anti-sticking layers, a key issue in all large area imprint applications is addressed. Two methods of tailoring adhesion properties of thermosets are proposed, plasma-depositing a fluorinated film and coating with a self-assembled monolayer of fluoroalkyltrichlorosilane, only the former of which was employed successfully. The achieved fidelity of pattern replication with working stamps and imprints thereof is assessed by cross-sectional SEM investigation, showing only the UV-enhanced method to be well suited for the task of obtaining low-cost replications of silicon stamps.
In order to examine the suitability of nanoimprinting for wafer scale pattern definition, a commercially available hot embossing system, the EV520HE of EVGroup, Austria, has been used to imprint 4 inch substrates. The EV520HE is based on a production-proven wafer bonding system which guarantees compatibility with semiconductor fabrication conditions. A 4 inch silicon wafer fully patterned with structures from 400 nm to 100 micrometers size was used as a stamp. The patterns, having a nominal height of 260 nm were defined in poly-Si over SiO2 by reactive ion etching. Different anti- sticking layers were applied to the stamps by monolayer self-assembling, among them (1,1,2,2 H perfluoroctyl)- trichlorosilane. Two different polymers, polymethylmethacrylate (PMMA) and a commercially available nanoimprint resist were used to spin-coat the substrates. Imprints were performed with temperatures of up to 225 degree(s)C, forces between 10 bar and 55 bar and holding times of 5 and 15 minutes. After separation of stamp and sample the imprints were characterized by a surface profiler and inspected by an optical microscope as well as a scanning electron microscope. Different qualities of pattern transfer according to the used process parameters were achieved, but patterning of the whole sample surface was always observed. In contrast to radiation-based lithography, the difficulties are based in imprinting of larger features whereas structures of 400 nm size were reproduced with high quality. Therefore the largest patterns of the stamp, 100 micrometers square bond pads, were used for imprint quality assessment, judged by the degree of stamp cavity filling around the pads. High quality was achieved by embossing at 225 degree(s)C with a hold time of 5 minutes at a pressure of 55 bar. For full wafer imprint only a small degradation of imprint quality from the center towards the periphery was observed. Further optimization of the process is required to minimize residual layer thickness for the hot embossing lithography step, taking into account the visco-elastic properties of the polymer material.
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