The Growth and Structure of Thin Metallic Films

The structure of thin metallic films was studied by means of electron diffraction and electron microscopy. Microcrystal size can be correlated with the melting point of the metals, those with high melting points producing continuous films of small unoriented microcrystals while those with low melting points produce large microcrystals oriented preferentially with respect to the substrate. The effects of varying experimental conditions on the film structure of selected metals are reported. The electron beam of the electron microscope affects metals with low melting points most directly, producing melting, sublimation or crystallization. Variations in the velocity of the impinging atom, obtained by passing thermal atoms through a mechanical velocity selector, produce no effect for metals whose vapor is monatomic. Antimony whose vapor is found to consist of polyatomic molecules as well as atoms produces films whose grain size seems to vary with the size of the molecules forming the antimony film. The rate of evaporation merely determines whether zinc or cadmium films may or may not be formed while for antimony films it also determines the type of the films formed. Those formed by rapid evaporation consist of small crystallites while those formed by slow evaporation consist of large amorphous patches. This effect is explained on basis of the mobility of the atoms on the substrate, the mobility time being determined to be less than 2×10−5 second. The mobility time and therefore the structure depends upon the type of substrate. The degree of vacuum is critical when the residual vapor reacts with the film being formed, but if no reaction takes place between residual vapor and film, pronounced variation for films formed in a vacuum are produced only when the pressure is above 10−2 mm of mercury.