A lithographic approach to generate clean patterns of multiple types of nanoparticles on one 4-inch silicon wafer is demonstrated in this paper. Each type of nanoparticle is precisely directed to the desired location. The process is mainly based on conventional microelectronic techniques with extremely high reproducibility. This enables the possibility of industrial applications to fabricate devices made of nanocrystals. A thin film of polystyrene spheres, 150 nm in diameter, was first coated on the silicon wafer with layer-by-layer self-assembly, followed by a layer of aluminum deposited on the thin film. A layer of positive photoresist was spun on the surface of aluminum and then patterned by lithographic technique. The unprotected aluminum was removed by wet etching until the polystyrene thin film underneath was exposed to the air. Oxygen plasma was employed to etch the polystyrene thin film all the way to the silicon surface. Subsequently, a thin film of another type of nanoparticle, silica particle 78 nm in diameter, was adsorbed onto the surface with layer-by-layer self-assembly. Eventually, aluminum and photoresist were removed and each type of nanoparticle was located next to each other as the pattern was designed. A scanning electron microscope was used to produce the image of the pattern.
Introduction.Nanoparticles have been the focus of many material researchers due to their unique properties in the microelectronics, optoelectronics, and chemical fields. 1,2 A great deal of attention has been attracted to the various potential applications of nanoparticles to complex nanoelectronic devices, photonic crystals, and biochemical sensors. [3][4][5][6][7] Among numerous nanoparticle deposition techniques, layer-by-layer (LbL) self-assembly, since its introduction by Decher et al., 8,9 has become, due to its simplicity and versatility, an increasingly popular technique, which enables adsorption of colloidal nanoparticles onto almost any material. The alternate immersion of substrate in oppositely charged solutions allows thin films of nanoparticles, enzymes, or protein in nanometer scale to be coated by electrostatic interaction. [10][11][12][13] However, before the LbL self-assembled nanoscale colloidal structure is applied to real devices, an approach must be developed to easily generate complex and distinct patterns on the multilayer films. Recently, some results have been reported for the creation of spatially resolved nanoparticle films. These works are mostly based on the microprinting technique in which a template is first fabricated by stamping two functional chemicals on the flat substrate, then directing nanoparticles onto adhesion-promoting regions while they
