Development of mechanical stress in fluoride multilayers for UV applications

Abstract

Since excimer laser applications extend to UV wavelengths at 193 nm and 157 nm, renewed research interest has recently arisen on fluoride thin films due to their unrivaled position as wide- band-gap material for the vacuum UV (VUV). In order to evaluate the critical development of mechanical stress in all dielectric UV mirrors a systematic study was performed on LaF 3/MgF2 and LaF3/AlF3 stacks on silica and silicon substrates. The samples, were investigated by means of complex ex - situ mechanical stress analysis including temperature dependence of stress, optical measurements, evaluation of structural and morphological parameters by AFM and XRD. When deposited at high substrate temperature of about 300°C, LaF3/MgF2 tends to show high tensile stress due to the large thermal expansion coefficient difference between the substrate and the film materials. It results in micro crack formation already starting after deposition of about 10 layer pairs. LaF3/AlF3 appears to have a larger crack resistance due to lower stress which can be correlated to the higher water content in these kind of stacks. By adjusting the deposition temperature, mirror stacks with high reflectance at 193nm can be grown

Narrowband mirrors based on fluorides tuned at vacuum ultraviolet wavelengths

“…(a) Cross-sectional back-scattered electrons SEM images of AlF3/LaF3 and MgF2/LaF3 MLs (b) derived RMS roughness as a function of accumulated thickness/interface from the substrate (#0) to the outermost layer; the plot includes AFM measurements reported in [ 12 ] for similar MLs. Empty (filled) symbols correspond to the low-index (highindex) material for each combination.AlF3/LaF3 MLs show remarkably lower stress than equivalent MgF2/LaF3 MLs, which is easily explained by the smaller thermal expansion coefficient of AlF3 compared to MgF2, as reported in[ 12 ]. The lower stress of AlF3/LaF3 MLs allows…”

“…In this context, thermally evaporated, hot-deposited AlF3/LaF3 MLs have proven to be the most successful combination to obtain better overall performance compared to more traditional MgF2/LaF3 MLs. The first combination gives higher reflectance and bandwidth [ 8,11 ], lower total stress [ 12 ], and smaller roughness [ 9 ] than the second combination, as can also be seen in Figures 1 and 2. 13 and (AlF3/LaF3) 13 MLs tuned at ~121.6 nm (b) Total stress of hot-deposited (~250ºC), thermally evaporated (MgF2/LaF3) 13 and (AlF3/LaF3) 13 MLs tuned at ~160 nm both on fused silica and CaF2 substrates.…”

Influence of substrate temperatures on the properties of GdF_3 thin films with quarter-wave thickness in the ultraviolet region

“…The absolute stress measurements were carried out immediately after the deposition was finished, and the mechanical stress of the GdF 3 thin films was derived from the deformation of substrates due to the deposited films before and after the coating. Figure 7 shows the stress of GdF 3 films deposited at different temperatures by using Stoney's equation [12]; independent of the substrate temperature all the GdF 3 films exhibited tensile stress values between 62 and 125 MPa. The GdF 3 thin films deposited at a higher substrate temperature show slightly higher tensile stress due to the larger thermal expansion coefficient difference between the GdF 3 thin films and the substrates.…”