Nanoimprint Lithography with a commercial 4 inch bond system for hot embossing

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 mum 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 perfluorooctyl)-trichlorosilane. Two different polymers, polymethyhnethacrylate (PMMA) and a commercially available nanoimprint resist were used to spin-coat the substrates. Imprints were performed with temperatures of up to 225 degreesC, 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 gm, 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 degreesC with a hold timeof 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.