Compressive Stress Increase with Repeated Thermal Cycling in Tantalum(Oxygen) Thin Films
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COMPRESSIVE STRESS INCREASE WITH REPEATED THERMAL CYCLING IN TANTALUM(OXYGEN) THIN FILMS C. Cabral, Jr., L.A. Clevenger, and R.G. Schad IBM T.J. Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598
Abstract Stresses which build up in thin films, such as tantalum, during thermal processing, can cause major reliability problems in x-ray optics and electronic applications. We have demonstrated that 50 nm to 200 nm thick sputtered beta tantalum thin films undergo repeated compressive stress increases when thermally cycled from room temperature to 400 0C (at 10°C/min) and back in a purified He ambient because of low levels of oxygen gettered by the tantalum. The oxygen contamination is a result of the poor quality of the quartz annealing chamber atmospheric seal. As-deposited stress in the sputter deposited tantalum films ranges from -1 to -4 GPa. The compressive stress build up was monitored in situ and was shown to increase -0.5 GPa on average after each thermal cycle for a final value of -6 to -7 GPa after seven cycles. After being cycled thermally seven times any perturbation of the film such as a four point probe resistivity measurement can cause the film to instantaneously crack in a serpentine pattern relieving the large compressive stress. Auger electron spectroscopy depth profiling analysis indicated that the as-deposited films contained one atomic percent oxygen which increased to eight to twelve percent after seven thermal cycles accompanied by an approximate doubling in resistivity. In conclusion, the increase in oxygen concentration in tantalum thin films which occurs upon thermal cycling leads to a repetitive increase in compressive stress which could be detrimental when the films are used in x-ray or electronic applications.
Introduction Tantalum thin films have been used in the microelectronics industry in passive components for over twenty five years. 1 Recently they have also been used as diffusion barriers2 ,3 in microelectronic applications, as alignment marks in photolithography applications and in x-ray optics applications. 4 We have demonstrated that thermally cycling beta Ta thin films to 400°C in the presence of minimal amounts of oxygen, which may occur during thermal processing in electronic applications, causes the compressive stress in the film to increase during each thermal cycle. Eventually the compressive stress builds up to values of -6 to -7 GPa where any perturbation of the film can cause cracking and peeling thus relieving the high compressive stresses. This of course leads to a major reliability problem. To prevent this problem the Ta film must be annealed in an extremely clean (02 free) environment or capped to stop oxygen diffusion.
Mat. Res. Soc. Symp. Proc. Vol. 308. ©1993 Materials Research Society
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Experimental An MRC 643 magnetron sputtering system was used to deposit 50 nm to 200 nm
beta Ta thin films at a deposition rate of 2.0 nm/s on 200 nm thermally oxidized < 100 > Si wafers. The base pressure of the system was between 0.8 and 1 x l0-7 Torr, with an
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