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

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 $3eV$⁠.