Correlation of Stress and Phase Evolution in Thin Ta Films On Si (100) During Thermal Testing
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CORRELATION OF STRESS AND PHASE EVOLUTION IN THIN Ta FILMS ON Si (100) DURING THERMAL TESTING B. L. French and J. C. Bilello Center for Nanomaterials Science, Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2136. ABSTRACT In this study the stress and morphology of Ta coatings sputtered on Si (100) substrates were monitored in situ and in real-time during thermal testing to allow interruptions at critical events in the coatings’ stress evolution. These observations were carried out using white beam Laue transmission diffraction topography/radiography and a high-temperature sample stage at the Stanford Synchrotron Radiation Laboratory (SSRL). The structure of specimens from interrupted thermal tests was then analyzed using x-ray diffraction θ-2θ scans. The structure and phases present in the film at different stages of the thermal test were correlated with specific mechanical responses in the coating such as stress generation. This information was employed to elucidate the role of phase evolution in the respective stress responses of films deposited in high and lowpressure regimes. INTRODUCTION While much information exists about the oxidation and high-temperature stability of bulk bcc α-Ta, little is know about the impurity-stabilized tetragonal β-Ta phase, which results most frequently from sputter deposition [1]. In thin coatings, oxidation processes are affected by higher concentrations of impurities and numerous diffusion “short-circuits” such as grain boundaries and voids [2]. Thin β-Ta films are of interest for a number of applications, including protection against corrosion and wear at elevated temperatures, thin film resistors, and thin film diffusion barriers [1,3]. It is therefore imperative to obtain information concerning the mechanical behavior and structure of these films at elevated temperatures in an oxidizing ambient environment. Previously, thin Ta films sputtered at Ar pressures ≥ 8 mTorr were observed to adhere completely to their substrates when subjected to a 60 minute hold at 585oC, while films deposited at lower pressures delaminated [4]. This behavior was explained by the respective stress responses of low and high-pressures films: films grown at higher pressures simply did not attain sufficient stress to cause delamination, while lower-pressure films did [5]. The present study examines the possibility of a correlation between the previously determined mechanical response of thin Ta coatings and their phase development during thermal testing.
EXPERIMENT Thin Ta films were deposited onto the native oxide of Si (100) substrates with dc magnetron sputtering in a chamber with a base pressure less than 10-7 Torr. The films were deposited at working gas pressures of 2, 15, and 18 mTorr using ultrahigh-purity Ar. The curvature of the substrates’ lattice planes was measured before and after deposition using double
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crystal diffraction topography [6], from which the residual growth stress in the films was calculated using the modified Ston
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