Thermomechanical behavior and microstructural evolution in tantalum thin films
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Thermomechanical behavior and microstructural evolution in tantalum thin films Robert Knepper, Katherine Jackson, Blake Stevens, and Shefford P. Baker Cornell University, Department of Materials Science and Engineering Ithaca, NY 14853 U.S.A. ABSTRACT Ta films were prepared in the metastable β phase using an ultra-high vacuum sputter deposition system. The stresses that arose during thermal cycles to 750ºC were measured using an in situ substrate curvature measurement system, allowing oxygen content in the films to be minimized. A phase transformation from β to the stable α phase takes place in conjunction with distinct “jumps” in stress in the tensile direction during heating at approximately 400ºC and 650ºC. Xray and electron backscatter diffraction (EBSD) analyses were used to determine grain sizes, along with crystal phase and orientation information. These results indicate a significant amount of grain growth accompanying the phase transformation. It is found that the measured total stress change is in reasonable agreement with that predicted by the combination of grain growth, crystal densification associated with the phase transformation, and stress relaxation. INTRODUCTION Tantalum thin films have been studied due to their applications in microelectronic devices and in x-ray optics. Depending on the deposition conditions and substrate [1-5], tantalum films can be formed with two different crystal structures: either the stable bcc α phase, or the metastable tetragonal β phase. The β phase is found only in thin films and has a high resistivity (~180 µΩ cm), making it suitable for thin film resistors and heaters. β tantalum is also easier to pattern using reactive ion etching than the α phase [6] and is therefore used for masks in x-ray optics. The α phase has a much lower resistivity (~13 µΩ cm), is much more ductile, and is commonly used for thin film interconnections, capacitors, and as a diffusion barrier between copper and silicon in microelectronic devices. Stresses that build up in these films during thermal cycling can cause reliability problems in devices, as they may lead to cracking or delamination [7]. In addition to differential thermal expansion, two different processes having to do with irreversible microstructural changes can act to produce very large changes in stress in Ta films during thermal cycling: the β−α phase transformation and the incorporation of oxygen into the crystal lattice. The thermomechanical behavior of tantalum films has been previously studied by Clevenger et al. [8], who showed a large change in stress in the tensile direction at about 750ºC. X-ray measurements showed that films were β phase before and α phase after thermal cycling, and these authors attributed the stress change to the phase transformation. Other groups [7,9,10] have reported large changes in stress in the compressive direction in films cycled to temperatures below that of the phase transformation, which they have attributed to the incorporation of oxygen into the crystal lattice. While the crystal
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