Ordered porous Materials
Block copolymer lithographie
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M. Steinhart
Block copolymer lithography (M. Park et al., Science 1997, 276, 1401; T. P. Russell et al., Science 1997, 275, 1458) is an established approach to laterally extended, continuous porous films with uniform pore size distribution, regular arrangement of the pores, and lattice constants of few tens of nanometers. Based on the self-assembly of block copolymers, this high-throughput method allows patterning laterally large areas. Block copolymer lithography is considerably less costly than electron beam lithography and allows realizing feature sizes not easily accessible by optical lithography. We use block copolymer films along with advanced dry and wet etching techniques to generate ordered pore arrays in various substrates, such as silicon wafers covered with silicon nitride (Fig. 1), and to position nanoparticles that may catalyze the growth of nanowires in such a way that the arrays of the nanoparticles conserve their order upon heating (Fig. 2).
Figure 1. Transmission electron microscopy image of a cross-sectional specimen containing a silicon wafer covered with a silicon nitride layer. Ordered arrays of nanopores in the wafer were obtained by block copolymer lithography and plasma etching.
Figure 2. Scanning electron microscopy image of a silicon wafer after block copolymer lithography, dry plasma etching, decoration with Au and annealing at 450°C for 30 min. At the top, the block copolymer template, a porous polystyrene film, is coated with large gold islands. At the bottom, an ordered array of Au nanoparticles uniform in size within the pores of a nanoporous Si wafer can be seen. It is obvious that the fixation of the gold in the nanopores of the wafer is essential for the conservation of the order initially imposed by the block copolymer template.