The potential of excimer laser micro-processing for surface modification of aluminum nitride (AIN) thin films was studied. Thin films of AIN were deposited by plasma-source molecular beam epitaxy (PSMBE) on silicon and sapphire substrates. These films were then exposed to different fluence levels of KrF (lambda = 248 nm) excimer laser radiation in an ambient air environment, and the changes in the film surface were studied by X-ray photoelectron spectroscopy, atomic force microscopy and optical spectrophotometry. The results show that there is a narrow range of laser fluences, just above 1.0 J/cm(2), within which mostly photochemical transformations of the film surface take place. These transformations consist of both oxidation and decomposition to metallic Al of the original film within a very thin sub-surface layer with thickness of several tens of nanometers. No changes were observed at fluences below 1.0 J/cm(2). Above a fluence of 1.0 J/cm(2), severe photomechanical damage consisting of film cracking and detachment was found to accompany the photochemical and photothermal changes in the film. (c) 2004 Elsevier B.V. All rights reserved.