Tham khảo tài liệu 'recent optical and photonic technologies part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 286 Recent Optical and Photonic Technologies charges. The generated electric field is several hundreds of volts meter at one centimeter distance from the stamp measured with a fieldmill-based static electric field meter. On the nanoscopic scale the field is nonuniform and causes forces strong enough to bend patterns together. Rather interestingly as the patterns move the electric field changes and the neighboring patterns can under the right circumstances change their pair. This can be observed through the optical microscope as a dynamically and chaotically changing pairing. Unfortunately as the surface charge discharges over time the patterns do not return to the non-paired situation because PDMS surfaces kept in contact react chemically and are glued together. Pairing can be reduced by reduction of the aspect ratio increase of the pattern spacing and by using stiffer stamp materials . h-PDMS and Ormostamp . Long grating lines and tightly spaced pillars on PDMS are most prone to pairing with their neighbors. Larger patterns pair less likely than small ones and softness of the stamp causes less deformations with microstructures than with nanostructures. For this reason Sylgard 184 is a rather popular material for micro-structuring of UV-polymers on the nanoscale more rigid materials are required. Fig. 5. Scanning electron microscope picture of pairing effect observed on soft stamp. h-PDMS aka hard-PDMS was developed at IBM as early as 2000 Schmid and Michel 2000 . They tried to formulate a better imprint material by trying different combinations of vinyl and hydrosilane end-linked polymers and vinyl and hydrosilane copolymers with varying mass between cross-links and junction functionality. A nanoimprint resolution record of 2 nm Hua et al. at 2004 was demonstrated using soft stamps based on h-PDMS. Based on Schmid s work and our studies we started to use a formulation according to table 3. Toluene was added to h-PMDS since it has very low viscosity mPa-s
