a thin flexible steel mask is provided for use in vacuum depositing high resolution back metal electrodes on the surface of a glass substrate already having deposited thereon front transparent electrodes and a thin - film structure including an electroluminescent layer sandwiched between layers of a dielectric . the spaced filaments provided on the mask for defining the openings through which the metal electrodes are deposited are joined together along the lengths thereof by reinforcing portions which provide for rigidly holding the filaments in the plane of the mask and enable a permanent magnet to retain the filaments of the mask in position flush against the surface of the substrate . after the metal has been deposited a first time through the openings in the mask , the mask is repositioned on the substrate such that the reinforcing portions now lie over the areas already deposited on and uncover the portions of the substrate that have yet to be deposited on . the metal is then deposited a second time through the openings in the mask to complete the forming of the electrodes on the substrate .