(a) Thermomechanical behavior of a Ta film cycled repeatedly to in an atmosphere containing a small but undetermined amount of oxygen, after Cabral et al. (Ref. 17). (b) Thermomechanical behavior of a Ta film thermally cycled to , after Clevenger et al. (Ref. 21). X-ray analysis showed the film to be phase as deposited and phase after cycling.
Thermomechanical behavior of Ta films cycled to 335, 355, 450, and deposited and cycled in a deposition system with base pressure. The stress jump is associated with the phase transformation and occurs at much lower temperatures than in previous reports based on oxygen-containing films [e.g., Fig. 1(b)].
x-ray scans of Ta films (a) as deposited and after thermal cycling to (b) , (c) , (d) , and (e) in ultrahigh vacuum. The as-deposited film shows a strong (002) phase texture. The films show a gradual transformation from to with increasing cycling temperature. The film cycled to has completely phase transformed.
Plot of the x-ray peak volume ratio of the (002) peak after thermally cycling to various temperatures. This ratio gives a quantitative estimate of the volume fraction of phase remaining. The temperature range over which the volume fraction of decreases coincides with the large increase in stress shown in Fig. 2.
Stress-temperature behavior of Ta films deposited in partial pressures of oxygen ranging from . The shift in the stress jump to higher temperatures with increasing oxygen pressure arises from solute drag on phase boundaries. At the highest oxygen content, no jump occurs when cycling the film between room temperature and .
Change in activation energy for phase boundary motion normalized to that of an oxygen-free film as a function of calculated as-deposited oxygen content. The curve is a line to guide the eye and should not be interpreted as a functional form.
Stress-temperature behavior of Ta films exposed to various amounts of oxygen between deposition and thermal cycling to form an oxide layer. The phase transformation is arrested when the oxygen from the oxide layer diffuses into the film. While the total stress change remains essentially constant, higher temperatures are needed to complete the transformation as the film is exposed to greater amounts of oxygen.
Stress-temperature behavior of an Ta film thermally cycled in an atmosphere containing oxygen. As temperature increases, there is a competition between oxygen incorporation (leads to higher activation energies for boundary motion, inhibiting the phase transformation and causing increased compressive stresses) and the increase in thermal energy (allows the phase transformation to progress, causing increased tensile stresses). Note that the shape of the curve is similar to that reported by Clevenger et al. (Ref. 21) [Fig. 1(b)].
Summary of data for films with oxygen incorporated during deposition: oxygen partial pressure during deposition, estimated oxygen concentration in the film, , estimated lower bound of activation energy for phase boundary motion, , and additional tensile stress due to oxygen leaving the film during the phase transformation.
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