1. A method of determining one or more profile parameters of a photomask covered with a pellicle, the method comprising: developing an optical metrology model of a pellicle covering a photomask, the pellicle having a first and second surface; developing an optical metrology model of the photomask, the photomask separated from the pellicle by a medium and having a structure, the structure having profile parameters, the optical metrology model of the photomask taking into account the optical effects on the illumination beam transmitted through the pellicle and diffracted by the photomask structure, the diffraction generating a detection beam; integrating the optical metrology model of the pellicle and the optical metrology model of the photomask, generating an integrated optical metrology model; optimizing the integrated optical metrology model; and determining one or more profile parameters of the mask structure using the optimized integrated optical metrology model.