Delta layer formation of silver nanoparticles in thin silicon dioxide film by negative ion implantation

Two formation methods of nanoparticles distributed at a certain depth in a delta layer by negative ion implantation were investigated. Such delta layer formation of isolated metal nanoparticles is desired in a very thin gate oxide film, with thickness of less than 10 nm for the development of a single electron device using nanoparticles as very small capacitance for Coulomb blockade. We have tried two methods by metal negative ion implantation in a thermally grown SiO2 film on Si substrate and postannealing for (1) a delta layer formation of gold nanoparticles in the surface region of SiO2 film, and for (2) a delta layer formation of silver nanoparticles in the bottom region of SiO2 film. In the former with gold negative ion implantation with a low energy, the depth distribution of nanoparticles formed at annealing at less than 900 °C depended on depth deviation of implanted atoms. The almost delta layer of Au nanoparticles with diameters of 4–8 nm was obtained just under the surface by Au negative ion implantation at 1 keV and annealing at 900 °C. In the latter with silver negative ion implantation at 10 keV, 1×1015 ions/cm2, 25-nm-thick SiO2/Si, and annealing at 700 °C, Ag nanoparticles with a diameter of 6–8 nm were aligned at the same depth in a delta layer in the bottom region of the SiO2 at a distance of 2 nm from the boundary.