ORIGIN OF MECHANICAL-STRESS IN VACUUM-DEPOSITED MGF2 AND ZNS FILMS

In this paper an attempt is made to find an explanation for the observed intrinsic stresses of clean and contaminated vapour-deposited MgF2 and ZnS films. The tensile stress of a stoichiometric and pure MgF2 film 1000 Å thick can be calculated assuming a grain boundary model. Small quantities of impurities acting in the grain boundary region change the stress behaviour according to their space requirements. In MgF2 films low concentrations of CaF2 and ZnF2 decrease the tensile stress appreciably. The compressive stress of ZnS films 1000 Å thick can be explained by an impurity grain boundary model. Calculation and measurement with both types of films are in a reasonable agreement. In the case of films deposited at low rates it is assumed that the introduction of oxygen atoms into the ZnS lattice, ogether with a resulting lower packing density, is responsible for the observed tensile stress. The tensile stresses of isolated crystalline aggregates may be produced by strain caused by the many point defects observed and by the partial disorder of the lattice. The stress behaviour of thicker films containing high concentrations of impurities seems to be determined by strains caused by distance variations in the lattice of the film of columnar crystals as well as by grain boundary effects. For stress calculations in this complex case the required mathematical formulations are still not available. © 1979.