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.