Effects of elastic strain and diffusion-limited aggregation on morphological instabilities in sputtered nitride thin films

The nature of morphological instabilities in sputtered titanium and niobium nitride thin films grown on amorphous borosilicate glass and single-crystal Si (311) substrates is investigated. All the films were grown by RF magnetron sputtering at constant power and pressure but with thickness varying from 40 to 400 nm and substrate temperatures of 250-300°C. The surfaces of the thin films can be divided into two areas: one in which the morphology is smooth with densely packed grains and the other in which there are morphological instabilities. A closer observation of the morphological instabilities reveals the coexistence of elastic strain-induced Asaro-Tiller-Grinfeld (ATG) type of instability and dendritic and snowflake structures due to diffusion-limited aggregation (DLA). The ATG instabilities extend over lengths of several tens of micrometers, whereas the DLA structures are confined to lengths of less than 10 μm in the same film. At low thickness (40-100 nm) only the elastic strain-induced instabilities emerge. High growth rates and a thickness of 150 nm are required to cause DLA and coexistence of the two kinds of instabilities. It has also been found that crystallization is not a prerequisite for the formation of dendritic structures.