An investigation of cluster formation in an ionized cluster beam deposition source

We have investigated the concentration of Ag_n clusters (25<n<1600) in a vapor of Ag expanding from the nozzle of a crucible heated to temperature as high as 1950 K (which corresponds to a silver vapor pressure of about 30 Torr). The goal of the experiment was to determine whether there were enough of these clusters present to have a significant effect on the ion‐enhanced deposition of Ag films, as postulated for ion cluster beam (ICB) deposition. Time‐of‐flight mass spectra were obtained by using an excimer laser at 248 nm (KrF) or 193 nm (ArF) to photoionize a fraction of the vapor. The ions were extracted normal to the direction of average velocity of the neutral species and detected with a Faraday plate after a 20 cm field‐free flight path. At the level of experimental uncertainty, no convincing evidence for the presence of large clusters was found for Ag vapor pressures in the 2–30 Torr range (typical for ICB). We conclude that under the conditions of our experiment, not more than one atom in 5000 is contained in a cluster with between 25 and 1600 atoms.