Velocity distribution function of sputtered gallium atoms during inductively coupled argon plasma treatment of a GaAs surface

Abstract

A GaN laser diode at 403.3nm is used to measure the velocity distribution function (vdf) of Ga atoms sputtered from a radio-frequency biased GaAs substrate in a low pressure inductively coupled plasma (ICP) argon discharge. To investigate both perpendicular (Vz normal to wafer) and longitudinal (Vx parallel to wafer) velocity components, laser induced fluorescence (LIF) measurements are performed in the z direction and atomic absorption spectroscopy (AAS) in the x direction. The longitudinal vdf of Ga sputtered atoms is very close to a Lorentzian function with Vx comprised between 0 and 7500ms−1, while the perpendicular velocities Vz can reach 10000ms−1. Experimental results are compared to molecular dynamics (MD) simulations of Ar+ ion sputtering of GaAs under 200eV bombardment. MD predictions and experiments are in fairly good agreement, which confirms the existence of products sputtered from the surface with kinetic energies larger than 10eV. In etching processes dominated by physical bombardment, these energetic atoms could alter passivation layers on sidewalls and be responsible for defects observed in nanodevices. The best fit of the Doppler-broadened LIF and AAS profiles with the vdfs predicted by sputtering theory allows one to estimate the surface binding energy of Ga atoms in GaAs, Eb, to be around 3eV.