Mechanical, optical and structural properties of TiO2 and MgF2 thin films deposited by plasma ion assisted deposition

Abstract

A study of the properties of TiO2 and MgF2 thin films deposited both by plasma ion assisted deposition (PIAD) and conventional electron beam evaporation in a Leybold APS 904 vacuum system is carried out. Experimental results for stress measurements of single films performed on a KLA-Tencor FLX-2320 system as well as for the film surface structure performed by AFM are reported. The presented experimental results for the studied films are discussed in terms of film columnar structure, surface free energy and sorption processes at the grain boundaries and columns. Both TiO2 and MgF2 deposited by PIAD have higher refractive indices and packing densities than by conventional processes. MgF2 films deposited by PIAD have higher optical absorption, which can substantially be reduced by adding oxygen during deposition. The AFM study corroborates that TiO2 films deposited by PIAD and conventionally have different structures. In both cases, however, the crystallization is still not fully completed. Annealing to 350°C leads to further crystallization in both films, though the crystallization achieved after heating differs significantly and is much more advanced in the PIAD case. All TiO2 films have tensile stress. The stress of TiO2 films deposited conventionally is influenced more rigorously by the air exposure than the PIAD films. Both film relaxation with time and film stress–temperature behaviour are explained following the grain boundary approach. All MgF2 films have tensile stress immediately after deposition. The stress decreases gradually with time and after 2 days the stress of conventionally deposited MgF2 thin films turns to compressive, while the stress of all PIAD MgF2 films remains tensile. The stress transformation is explained by the chemical absorption of water onto the film. MgF2 thin films also crystallize at temperatures above 250°C. After crystallization their internal stress does not change and the stress–temperature behaviour follows closely the linear stress–temperature model.