Impact of vacuum environment on the hot embossing process

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 mum. 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 degreesC 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.