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.