Effect of Deposition Conditions on Intrinsic Stress, Phase Transformation and Stress Relaxation in Tantalum Thin Films
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EFFECT OF DEPOSITION CONDITIONS ON INTRINSIC STRESS, PHASE TRANSFORMATION AND STRESS RELAXATION IN TANTALUM THIN FILMS A. Mutscheller, L.A. Clevenger, J.M.E. Harper, C. Cabral Jr. and K. Barmakt IBM T.J. Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598 t IBM General Technology Division, Hopewell Junction, NY 12533 Abstract We demonstrate that the high temperature polymorphic tantalum phase transition from the tetragonal beta phase to the cubic alpha phase causes complete stress relaxation and a large decrease in the resistance of tantalum thin films. 100 nm beta tantalum thin films were deposited onto thermally oxidized < 100> silicon wafers by dc magnetron sputtering with argon. In situ stress and resistance at temperature were measured during temperature-ramped annealing in purified He. Upon heating, films that were initially compressively stressed showed increasing compressive stress due to thermo-elastic deformation from 25 to 550°C, slight stress relief due to plastic deformation from 550 to 700 0 C and complete stress relief due to the beta to alpha phase transformation at approximately 700-800°C. Incomplete compressive stress relaxation was observed at high temperatures if the film was initially deposited in the alpha phase or if the beta phase did not completely transform into alpha by 800°C. This incomplete beta to alpha phase transition was most commonly observed on samples that had radio frequency substrate bias greater than -100 V. We conclude that the main stress relief mechanism for tantalum thin films is the beta to alpha phase transformation that occurs at 700 to 800 0 C. Introduction Interest in the electrical and structural properties in tantalum thin films over the past twenty years has been stimulated by its potential applications in electronic devices and x-ray optics.", 2 The crystal structure of these as-deposited thin films is usually the metastable tetragonal beta phase.1' 3 However, depending on the deposition techniques used, the as-deposited crystal structure can also be bcc alpha phase or a mixture of beta and alpha phase tantalum.".3 The beta tantalum has an as-deposited resistivity of 170 to 210 un-cm and is used for thin film resistors. 3 The resistivity of alpha Ta is 15 to 50 ,A2-cm which makes it a good choice for thin film interconnections. 3 Previous research has mostly focused on determining the experimental deposition conditions for which beta or alpha phase Ta form in thin films.'. 3 In this work, we chose to examine the stress and resistance versus temperature behavior of tantalum films deposited in both the beta and alpha phases using substrate bias from 0 to -400 V and with a variety of asdeposited stresses. In particular, we demonstrate that the main stress relief and resistance decrease mechanism in these films is the tetragonal beta to bcc alpha phase transition which occurs at approximately 750'C.
Mat. Res. Soc. Symp. Proc. Vol. 239. ©1992 Materials Research Society
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Experimental 100 nm tantalum thin films were deposited at 2.5 nm/sec onto thermal
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