COLUMNAR MICROSTRUCTURES IN MAGNETRON-SPUTTERED REFRACTORY-METAL THIN-FILMS OF TUNGSTEN, MOLYBDENUM AND W-TI-(N)

Refractory metal thin films based on metals such as tungsten, molybdenum or titanium, having thicknesses of the order of 100 nm, are well known for their applicability in advanced silicon technology, These high melting point metals are commonly deposited by magnetron sputtering onto substrates at room temperature. In spite of the energetic particle bombardment of the growing films the surface mobility of the adatoms is rather limited. This, in combination with the occurrence of geometric shadowing, usually results in films exhibiting a columnar microstructure. In this paper we describe the experimental results as obtained with transmission electron microscopy using the cross-sectional technique. The successive deposition of material on the growing columns may be traced by the observation of multilayer structures, e.g. W/Ti composites. An important parameter which influences the microstructural details of the columnar structure is the internal stress state. The relationship between stresses and microstructure will therefore be illustrated, as well as the influence of the sputter gas pressure on both microstructure and stress state for the case of molybdenum thin films. Another interesting parameter which influences the columnar morphology is the chemical composition of the films. A comparison of the microstructure of pure tungsten and tungsten-rich films alloyed with titanium has shown that, apart from a one-phase to multiphase transition, the mutual alignments of the columns are different: the grains formed by similarly oriented columns are substantially larger in the binary W-Ti alloy films. Additional alloying with nitrogen gives rise to morphological changes: the incorporation of nitrogen forces the column boundaries to fade away and induces a change into a mixture of small crystallites.