ArF laser etching of n-GaAs (100) in Cl2 gas ambient has been investigated as a function of laser fluency and the main thermodynamical parameters of the process (pressure, temperature and gas phase composition). Experimental results indicate that GaAs laser etching in the presence of Cl2 occurs by a combination of thermally and photochemically initiated reaction steps. The etch rate was found to be kinetically controlled by the surface reaction between photogenerated CI adatoms and GaAs, with an activation energy of 6.3 Kcal/mol. Vertical etch profiles with smooth bottom surfaces were obtained at substrate temperatures above 150°C, low Cl2 partial pressures (≤ 1.5 m Torr), and low laser fluences (≤ 25 mJ/cm2) for etch masks parallel to the | 01 1 | direction.