Measured velocity distribution of sputtered Al atoms perpendicular and parallel to the target

An external cavity diode laser tuned on the 396.15 nm Al line is used to probe the velocity distribution function (vdf) of Al atoms sputtered from a radio-frequency biased aluminium target in low pressure high density Ar plasma (0.06 Pa). Measurements are performed in an industrial inductively coupled plasma etching reactor. Absorption spectroscopy provides the vdf of atoms parallel to, and 1 mm above, the target; while an original laser-induced fluorescence setup gives the vdf perpendicular to the target. Comparison between these recorded Doppler profiles and the theory of sputtering indicates that at low bombardment energy (similar to 200 eV), the angular distribution of ejected particles is heart-shaped.