Near-angle transmission scattering in excimer laser components

Results are presented for preliminary investigations into the way optical elements that are exposed to corrosive environments exhibit an increase in transmissive scatter the longer that such exposure exists. Near angle (i.e. < 1 degree(s)) scatter data is presented for fused silica substrates exposed to fluorine atmospheres of differing concentrations. The fluorine reacts with any water vapor present to form hydrofluoric acid which etches materials such as silica. The scatter probe wavelength used was 633 nm and scattering angles from 4 to 15 mR studied. Results are presented in bidirectional transmission distribution function (BTDF) format as well as photographs of the resulting diffraction patterns arising from passing a HeNe laser beam through the bulk of the sample which seem to show a periodic roughness profile. It is suggested that surface cleaning prior to exposure provides seeding sites, or micro scratches, for preferential attack by hydrofluoric acid.