We measured the ac magnetic permeability of an oxygen-deficient YBa2Cu3O7-δ thin film epitaxially grown by Pulsed Laser Deposition on a (100) SrTiO3 substrate, before and after fluorination under diluted NF3, as a function of temperature T and amplitude of the applied field. Fluorine apparently occupies empty sites on a and b axes of the perovskite structure, with very different reaction rates. For each fluorine content we determined the width of the transition, the field H1(T) of first flux penetration in the sample and the peak field Hp(T) giving maximum dissipation at temperature T. The transition width appears to be related to the difference between fluorine populations in the two axes. Hp(T) obeys the same power law as in YBa2Cu3 O7-δAx (A = F, Cl) ceramics, but this is not the case of H1(T). Our results are compatible with a model of chemical disorder generating a random distribution of superconducting critical temperatures Ts in the sample.