Plastic Relaxation in Thin Copper Films
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Plastic Relaxation in Thin Copper Films Jonathan B. Shu and Shefford P. Baker Department of Materials Science and Engineering, Cornell University Bard Hall, Ithaca, NY 14853-1501 ABSTRACT We have studied the isothermal relaxation behavior of 500 nm Cu films with SiNx passivation and barrier layers on Si substrates. Oxygen content in the Cu films was varied by deposition in various oxygen partial pressures in the range 10-10 to 10-4 Torr. The substrate curvature method was used to investigate the thermomechanical behavior of the Cu films. Isothermal relaxation experiments were performed in the temperature range 50-175˚C. Comparison with constitutive creep deformation equations shows that the relaxation data in this temperature range are well described by power law behavior. Under certain isothermal conditions related to temperature, stress, and thermal history, anelastic recovery was observed— i.e. while in a tensile stress state and with the temperature held constant, the overall film stress was seen to increase over a relatively short time scale. INTRODUCTION The study of thin film behavior and mechanical properties becomes increasingly important as microelectronic and microelectromechanical (MEMS) features are continually made smaller. In order to improve device reliability and performance, knowledge and prediction of film stresses are of great importance. Previous experiments show that small impurities of oxygen can dramatically affect the thermomechanical behavior of passivated Cu films [1-3]. Several unusual behaviors, collectively referred to as the “oxygen effect” [3], have been observed. Following that work, we have developed an ultra high vacuum (UHV) system in which we are able to sputter films and measure stress in-situ using the substrate curvature technique, thus allowing excellent control of film composition as well as a very clean testing environment. In the present work we have reproduced the oxygen effect by depositing Cu films in certain partial pressures of oxygen, and studied deformation mechanisms of these films through isothermal relaxation measurements. There are several characteristics of films exhibiting the oxygen effect that are unusual compared to films deposited without oxygen. Figure 1 shows the thermomechanical behavior of films prepared with and without oxygen. Both films show very high tensile stresses at room temperature and follow thermoelastic lines with similar slopes on heating. Both films deviate from thermoelastic behavior, indicating the onset of yielding, while still in tension. The deviation is to higher stresses (above the thermoelastic line) indicating that the plastic strains are compressive. Thus, this effect has been dubbed early or negative yielding. The film displaying the oxygen effect shows more pronounced early yielding and continues to yield plastically at much lower stresses until exhibiting a “tail”, generated by a higher rate of stress increase with increasing temperature at high temperatures. It has been shown that this tail can be shifted along the temperatu
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