Mechanical properties of optical films

Abstract Tensile stresses are retained in silver, aluminium and chromium films made by fast deposition in a high vacuum with a non-reactive residual atmosphere. Many investigations have been performed for the thickness range between 10 and 100 nm. Silver and aluminium films of 50 nm thickness have low intrinsic stress values of σ Ag ≈ 3 × 10 8 dyn cm −2 and σ Al ≈ 2 × 10 8 dyn cm −2 . Well-prepared chromium films of the same thickness, however, show much higher values of σ cr ≈ 16 × 10 9 dyn cm −2 . The dependence of the stress on the thickness of the growing films reaches a maximum between 5 and 15 nm at a stage when the last holes in the island microstructure of the films are filling up. The density of thicker films is near that of the bulk. The adhesion of silver films is poor, that of thin chromium films excellent. With oxygen-sensitive metals, adhesion may increase with time (diffusion) by the formation of an oxide film at the interface. The stress of aluminum and chromium films is very sensitive to reactive gas incorporation. In oxygen-doped aluminium films the stress changes sign, becoming compressive. An increase in oxygen incorporation leads to a maximum in the compressive stress. A further supply of oxygen decreases the compressive stress and through reactive deposition favours the formation of Al 2 O 3 films which show tensile stresses σ Al 2 O 3 of about 3.5 × 10 9 dyn cm −2 . In chromium films the compressive component associated with the incorporation of small quantities of oxygen decreases the high tensile stress of the pure films. The doped chromium films remain in the tensile region, however. With dielectric materials (alkaline earth fluorides) films 100 nm thick have tensile stresses. The stresses decreases in the sequence MgF 2 (with with σ MgF 2 ≈ 6.9. × 19 9 dyn cm −2 ), CaF 2 (with σ CaF 2 ≈ 4.7 × 10 8 dyn cm −2 ) and BaF 2 (with σ CaF 2 ≈ 4.7 × 10 8 dyn cm −2 ). T e adsorption of H 2 O molecules at the inner surfaces of a still growing or freshly deposited film decreases the stress values by more than 50%. Film deposition on heated glass substrates also reduces the intrinsic tensile stress but, depending on the thermal expansion of the glass and the film material, a thermal stress component comes into action which may be disadvantageous. Deposition onto heated substrates, however, increases the film density ( e.g. for MgF 2 the packing density p 25° C = 0.72 and p 300° C = 0.92), hardness and adherence. Zinc chalcogenide films exhibit compressive stresses. ZnS (with σ ZnS ≈ −2 × 10 8 dyn cm −2 ) has a higher stress than ZnSe. With structure, microstructure and elastic data from previous investigations the tensile stresses in crystalline films can be explained in a reasonable semiquantitative manner by grain boundary interactions. Using such model considerations to find a way to influence the stress permanently it was possible to develop specially doped MgF 2 films which have markedly reduced intrinsic tensile stresses compared with the pure films.