MAGNESIUM CLUSTER-BEAM DEPOSITION ON GLASS AND SI(111)

Magnesium cluster beams have been obtained from supersonic nozzle expansions. The low boiling point of Mg (1363 K) allows conditions for clustering to be reached with and without argon as a carrier gas. This article reports on cluster size and intensity as a function of expansion conditions, and discusses the characteristics of magnesium films obtained by cluster-beam deposition. The source parameters were temperature T0 less-than-or-equal-to 1600 K, total pressure p0 less-than-or-equal-to 5000 hPa, magnesium partial pressure p(Mg) less-than-or-equal-to 2600 hPa, supersonic conical nozzle: diameter d = 0.25 mm, cone angle 2alpha = 10-degrees, and cone length l = 27 mm. For the Ar/Mg mixture the cluster-beam intensity corresponded to deposition rates of up to 83 nm/s at a 0.3 m distance from the nozzle. This exceeds the ''ideal'' intensity from a sonic nozzle operated with the same magnesium mass flow by about a factor of 3.5. For the neat Mg vapor the deposition rates extended up to 190 nm/s, but at a higher Mg mass flow compared to the Ar/Mg mixture. The cluster beams were deposited on room-temperature glass and Si(111) substrates. Films about 1000 nm thick were examined by x-ray diffraction. Compared to films produced by atomic beam deposition with a typical polycrystalline structure the cluster films are distinguished by a preferential orientation of (002) planes parallel to the substrate. This feature was observed for both types of cluster beams, with and without argon as a carrier gas, and for both glass. and Si substrates. The highly specular film surfaces turned usually into a golden-yellow color after, exposure to air, with the notable exception of the film formed with the highest intensity (510 nm/s). The surface structure studied with an atomic force microscope showed an increase in grain size with increasing deposition rate.