Surface patterning by means of soft lithography for molecular and bio-electronics (2007)

The aim of this thesis was establishing Soft Lithography, mainly Microcontact Printing, as a powerful patterning technique for Molecular and Bio-Electronics. Especially patterns having sub 100 nm dimensions were subject of this thesis. Therefore a main issue addressed was the adoption of stamp materials with a Young´s modulus larger than 100MPa that allowed the transfer of patterns with low critical dimensions. The Young´s modulus of these materials was more than an order of magnitude larger than that of materials commonly used. From a functional point of view emphasis was laid on the transfer of proteins. A process was to be developed that allowed the transfer of fully functional protein patterns. In addition the influence of Contact Inking on the functionality of transferred proteins was investigated. Beside that, protein patterns were used to demonstrate the influence of sub 1 μm patterns on the cell culture of neurons. Another major object of this thesis was the establishment of a technique to transfer metal patterns to arbitrary surfaces. Commonly used methods only work with a specific choice of stamp material, metal and target surface. The concept proposed and demonstrated in this thesis was more universal. Crossbar junctions having molecular interlayers were demonstrated as one application for this process. A special design was developed based on lines and spaces with variable widths and gaps, which allowed the detailed investigation of various scaling issues. This pattern was used as template for the fabrication of masters. A process compromising Electron-Beam Lithography of PMMA resist and megasonic development followed by a Reactive Ion Etching process using a hydrogen-bromide plasma to etch into polysilicon provided best results. Ideal passivation of masters was obtained by vapor deposition of a perfluoro-octyl-trichlorosilane. It formed a smooth monolayer on the master´s surface prohibiting sticking of the stamp. PDMS stamps were fabricated by casting the liquid pre-polymer against the master. Since PDMS has got a rather low Young´s modulus defects such as Pairing were observed for the pattern design used. Thermoplastic material Affinity VP polyolefin was used as stamp material patterned by Hot Embossing. Although its Young´s modulus is fifty times higher than that of PDMS Pairing was observed. Therefore thermoplastic materials with an even higher Young´s modulus were used, namely polyolefins Zeonor, Zeonex, Topas, ionomere Surlyn and methacrylate Plexiglas. All these materials could be used to obtain defect free replications of the master. For Surlyn the edges were slightly rounded, while for the other materials the shape of the stamp pattern were rectangular. Alkanethiols served as model molecules to investigate the printing process since they are commonly used in various applications and widely studied. For printing octadecanethiol Contact Inking was performed. Diffusion of the molecules for printing with PDMS stamps was found in agreement with previous publications. For Affinity VP stamps however, the diffusion was found to be less pronounced due to the different composition of the stamp material. For printing with Surlyn, which is rather [...]