A study of adhesion of tantalum films at elevated temperatures via advanced synchrotron techniques.

Abstract

Tantalum films are subjected to elevated temperatures in many of their diverse applications due to use or post-deposition processing. Thus, response to elevated temperature must be avoided or incorporated into the selection and design of a coating for a particular application. The evolution of stress, morphology, composition, and microstructure were studied in dc magnetron sputtered Ta coatings on Si (100) during one hour thermal tests at 600&deg;C in air, with the goal of relating as-deposited film characteristics to their elevated-temperature stability. Morphology, microstructure, and impurity content of the as-deposited films were controlled by adjusting the Ar deposition pressure over a range of 2--18 mTorr. Films' microstructure, phase content, morphology, and composition were assessed with x-ray diffraction, atomic force microscopy, scanning electron microscopy, and Rutherford backscattering spectrometry. An analytical method was developed which allowed the instantaneous curvature of the substrates, and therefore the stress in the overlaying coatings, to be determined using transmission Laue diffraction topography. This technique was used for <italic>in situ</italic>, real-time observations of film delamination and stress evolution during thermal testing. Coatings deposited at low (2--5 mTorr), intermediate (6--7 mTorr), and high (8--18 mTorr) pressures exhibited severe, moderate, and no delamination, respectively. These results were explained by the attendant stress data, which indicated the development of high (&le;3.8 GPa), moderate (&le;2 GPa), and low (&le;1.4 GPa) maximum compressive stress in the low, intermediate, and high-pressure coatings, respectively. Stress originated mainly from formation of less-dense oxide phases in the films, both in grain boundaries and at the film surface. All films converted to orthorhombic Ta₂O₅ during the thermal test, with a systematic increase in amorphous content with increasing deposition pressure. Disparities in stress evolution and attendant adhesive performance between high and low-pressure coatings were explained by large variations in as-deposited morphology and impurity content. Films deposited at high pressures possessed a voided zone 1 morphology (&sim;67% dense) and up to 40 at.% O, which allowed them to better accommodate oxide formation during thermal testing. Low-pressure films exhibited dense zone T morphology (up to 80% dense) and fewer impurities, such that oxidation created high delamination-inducing compressive stresses.