Surface chemistry and morphology of polysilicon thin films etched in a high-density fluorocarbon plasma under various conditions are studied by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and spectroscopic ellipsometry. XPS reveals the presence of a fluorocarbon layer, which composition and thickness depend on the plasma conditions. Ellipsometry measurements show the need to consider the superficial roughness. Surface roughness and morphology obtained by AFM are used to define geometric models suitable to represent the top layer when processing the ellipsometry data. Results are discussed and compared to that given by the Bruggeman effective medium approximation (BEMA). The BEMA model always agrees with the geometric model, which is the closest to the observed surface morphology. However, if a good agreement is obtained between surface roughness and top layer thickness for unetched or weakly damaged samples, a discrepancy is observed for the etched samples. Formation of a non-transparent fluorocarbon layer on these sample is put forward to explain this behaviour.