Structure characteristics of stratified and single fluoride optical coatings

The structure evolution characteristics of MgF 2 and NdF 3 optical thin films on CaF 2 (111) substrates have been investigated by cross sectional transmission electron microscopy and atomic force microscopy and were related to their extinction coefficient. The films were deposited by physical vapor deposition under ultra high vacuum conditions. A series of experiments have been carried out at various substrate temperatures at 300 nm thickness and by the stratification of the two fluoride materials. The structure evolution of single and stratified films was investigated and discussed in relation to the structure zone models. The effect of stratification on the structure and extinction coefficients is discussed in comparison to the effect of segregating impurities. The development of morphology of stratified films is analyzed in relation to the thickness of the interlayers and temperature. The optimum conditions for preparing films of low extinction coefficients are found at parameters corresponding both to the transition between zone T and zone II growth and to the formation of nanocrystalline structures.