We present an atomic force microscopy study on the morphology and roughness of sexithienyl (T6) thin films evaporated on mica in high vacuum. The effects of two thermal processes are investigated : (i) temperature of the substrate during evaporation, and (ii) annealing temperature in high vacuum. The former yields a good control on the extension of grain boundaries and the aggregate ordering, but does not affect surface roughness. The latter is characterized by a threshold temperature (similar to 175 degrees C) above which a smooth continuous surface is formed, The roughness scaling behaviour is analysed by the power spectrum of the topographical profiles. T6 surface is self-affine over 1-2 orders of magnitude of the spatial frequencies. However, films prepared at substrate temperatures above 200 degrees C or annealed above the critical temperature exhibit an extended self-affine behaviour. The roughness scaling factor suggests a growth process of the Kardar-Parisi-Zhang universality class.