One of the frontiers in the basic science in the advanced electronic device engineering is fabrication of nanometer-scale structure on the surface. Mass production of structures on silicon wafers on such scale will open new fields of basic research and technology, such as quantum electronics, high-density memory devices, minute magnetic devices and functionalized/mechanized surfaces .
Drawing desired patterns on wafer surfaces with ultimately thin material, i.e., monolayer adsorbate, is one of the strategic solutions to achieve the spatial resolution. Our strategy to realize this whole process is as follows:
Deposition of a monolayer of organic molecules covalently bonded on surface Si atoms, to supply ultimately thin materials to construct surface structures.
Alteration of the monolayer by fine-beam electron bombardment, as the method that has practically the best resolution and mass-productivity for minute scale patterning , .
Deposition of metal atoms selectively over the bombarded portions, to enhance the utility of patterns formed for practical purposes.
By employing a fine beam (diameter <20 nm) electron gun, the ultimate goal of our strategy for nanometer-scale monolayer fabrication will be reached. The work to be done next will be to realize the ultimate resolution of fabrication of surface structures with the process presented with using a fine electron beam.
The substrate silicon wafers used were n-type Si(111) (resistivity 3–8 cm) supplied by Shinetsu Handotai. Prior to all experiments, Si(111) pieces were sonicated in organic solvents and then treated in hot SPM solution (four parts of concentrated H2SO4+one part of 30% H2O2 mixture, heated at 120°C) for 10 min and stored in Milli-Q-purified water. Just before use, they were immersed in 40% NH4F for 10 min. This is the procedure to prepare the hydrogen-terminated Si(111) (H:Si(111)) .
Results and discussion
Preparation of p-substituted phenyl moieties were carried out by cyclic voltammetry according to Allongue et al. , . For electrolysis we used a clean Pt wire as the quasi-reference electrode, a carbon rod for the counter electrode, and the Si wafer pressed at a bore made on the container. The aqueous solutions for electrolysis were 0.1 M H2SO4+2% w/w HF added with 2–0.2 mM of diazonium salts. We used BF4− salts of p-Br–C6H4–N2+, p-NO2–C6H4–N2+, p-CH3–C6H4–N2+, p-CF3–C6H4–N2+, p-CH3)2CH–C6H
The electrolysis of para-substituted benzenediazonium salts on H:Si(111) can deposit a monolayer of organic moieties and a precipitate layer simultaneously, in aqueous and dehydrated acetonitrile electrolytes. Rinsing in a proper solvent can remove the precipitate. The reaction of Grignard reagents and H:Si(111) can deposit a homogeneous robust monolayer of alkyl groups corresponding to the delivered Grignard reagents. The alkyl moieties formed are arranged randomly within the monolayer. The
This work was supported by the Research for the Future Project “Wafer-Scale Formation Process of Nano Dots”, the Japan Society for the Promotion of Science, Iketani Science and Technology Foundation, the Murata Science Foundation and Yazaki Memorial Foundation for Science and Technology. The authors are thankful to the generous donation by Matsushita Research Institute, Tokyo, Inc.
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