<i>E</i>-beam deposition characteristics of reactively evaporated Ta2O5 for optical interference coatings

Abstract

Experiments with Ta2O5 as a starting material were undertaken to produce nonabsorbing Ta2O5 as a thin film material for multilayer dielectric coatings. The dependence of refractive index on oxygen pressure, on substrate temperature, and on deposition rate was measured. An interesting relationship was discovered between the evaporation rate and the rate of outgassing of the chemically bound oxygen from the Ta2O5 melt. A condition for nonabsorbing Ta2O5 films, regulated O2 being held at a constant pressure, is indicated when the outgassing rate of oxygen from the melt follows the deposition rate in a linear fashion increasing with increasing deposition rate and remaining constant with constant deposition rate. When this condition is met the maximum rate for nonabsorbing films can be realized, i.e., when the deposition rate is increased beyond a critical limit the outgassing ceases to follow linearly but degenerates and the film becomes absorbing. Automatic rise, soak, and deposit cycles enabled 40%, 35 g, of the starting material to be deposited as nonabsorbing films from the same 15 cc crucible. Ta2O5/SiO2 multilayer structures were fabricated with theoretical stop and pass bands being obtained.