Zone Descriptions of Film Structure-A Rationale

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ABSTRACT The microstructure of films is a result of the interplay of nucleation and growth processes which occur during deposition. A columnar structure is characteristic of films irrespective of the material or the deposition process. The length scale of the microstructure depends systematically on the atomic mobility during growth.

INTRODUCTION The structure of deposited films has been studied for many years by a variety of techniques beginning with optical microscopy and subsequently exploiting the increased capabilities of scanning electron microscopy, x-ray line broadening and ultimately transmission electron microscopy, field-ion microscopy and scanning probe microscopy [1-6]. Optical and scanning electron microscopy of cross sections prepared by fracture reveal morphology rather than microstructure and x-ray line broadening is insensitive to large grains in a population of smaller grains. These techniques therefore suffer from intrinsic limitations which can lead to misleading results. The microstructure of thin films is the outcome of the nucleation, growth and coarsening processes which occur during deposition. Each process is affected systematically by substrate temperature, film composition and the ambient. Consequently, irrespective of the material or the specific deposition process, general trends in the dependence of film microstructure on the processing variables are observed. In particular a columnar morphology is ubiquitous. Consideration of atomic mobility, grain coarsening and deposition rate provides a basis for understanding the origins of the microstructural features in films deposited by physical vapor deposition, (PVD), sputtering, chemical vapor deposition (CVD) and wet chemical methods. The substrate, for the purposes of this paper, acts mainly as a support and a heat-sink.

NUCLEATION

At the high supersaturations which exist in vapor deposition almost all the atoms or molecules which are incident on a substrate are captured [reviewed in 7]. This process involves thermal accommodation followed by surface diffusion. As more material arrives the adatoms may form a monolayer or develop an island morphology. The actual morphology depends on the interplay of

401 Mat. Res. Soc. Symp. Proc. Vol. 317. ©1994 Materials Research Society

thermodynamic factors, principally interfacial energies and kinetic factors especially surface diffusion and nucleation rates. Since nuclei can coalesce and coarsen during film growth, the resulting grain size is only exceptionally a measure of the number of nuclei. Some precursors to the formation of the first monolayer of deposit are illustrated in Fig. 1 which shows (a) a field-ion micrograph of single atoms of nickel deposited onto a tungsten substrate at room temperature (b) a field-ion micrograph of close packed rows of rhenium deposited onto a tungsten substrate at room temperature and (c) an electron micrograph of islands of gold embedded in amorphous carbon after deposition onto sodium chloride.

Fig. 1: (a) a field-ion micrograph of single atoms of nick