Effect of substrate temperature on the microstructure of thin niobium films

Niobium films with constant thickness have been deposited on sapphire (1120) by electron-beam evaporation at different substrate temperatures (150 °C ⩽TS ⩽ 750 °C). The samples were characterized by X-ray diffraction and resistivity measurements. X-ray reflectivity shows that all films are covered with an oxide layer of about 20 Å in ambient atmosphere. The (110) texture at high \{TS\} decreases towards lower TS, accompanied by an increasing surface roughness. Below \{TS\} = 350 °C the grain size in the growth direction becomes smaller than the film thickness and a relaxation of intrinsic stress is observed. A transition from a columnar growth structure to a fine grained microstructure is inferred. The change of the microstructure with \{TS\} is probably due to the temperature dependent grain boundary mobility during the deposition process.