Kinetics and Mechanism of the C49 to C54 Titanium Disilicide Polymorphic Transformation
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consumed and converted into the polycrystalline C49-TiSi 2 . An in situ resistance measurement was used to monitor this process to avoid any C49-to-C54 pretransformation at this stage. This was also double checked using transmission electron microscopy (TEM) and x-ray diffraction. Kinetics measurements were carried out under isothermal annealing condition over a temperature range from 660 to 720 *C in a high vacuum furnace with a base pressure of better than 5 X 10-8 Torr. With the aid of the resistance probe, a set of samples annealed to various stages of the transformation were also obtained for establishing the microstructural information. Microstructural studies of the phase transformation were performed using plan-view and crosssectional TEM, selected area electron diffraction, and electron microdiffraction. 111. RESULTS AND DISCUSSION A. Kinetics Study Because of the distinct change in resistance associated with the C49 to C54 polymnorphic transformation, this feature was used to study the kinetics of the transformation. When the C49TiSi 2 film is isothermally annealed in a suitable temperature range, it gradually transforms to the stable C54 disilicide and the film resistance continuously drops down with annealing time, i.e., R =f(t), due to the increase in volume fraction of the low resistivity C54 phase formed by consuming the high resistivity C49 phase. Figure 1 shows normalized resistance plots as a function of annealing time at several temperatures. For the sake of clarity, only three temperatures are shown in the figure, while the actual measurement was made at seven different temperatures. The change in resistance can be related to the progress of the phase transformation using a resistance model [6]: X7(t)
=
[R(O) - R(r)]/[R(O) - R( f)]
(1)
where XT(1t) is the volume fraction of the transformed C49-TiSi2, R(O) is the initial film resistance of the C49 phase, R(f) is the final film resistance of the C54 phase, and R(t) is the time-dependent film resistance. Using this relation, a set of transformation curves are obtained and presented in Fig. 2. It is observed that the time needed to complete the transformation decreases with increasing annealing temperature, indicating that the transformation is thermally activated. Also, the sigmoidal shape of the curves suggests that the transformation proceeds by a nucleation and growth mechanism. A John son-Mehl-Avrami type of kinetics analysis [10] was used to deduce an effective activation energy and other kinetic information, i.e., X74t)
=
1 - exp(-btfl)
(2)
where b is a kinetic parameter, depending on the nucleation and growth rate of the C54 disilicide, n is a mode parameter, generally implying the mode of nucleation or the dimensionality of growth, and t is the reaction time. By defining X7,)=0.5
(3)
and ,r= roexp(Ealff)
(4)
the effective activation energy Ea for the transformation can be obtained by plotting In (T)versus 1/kT (Figure 3). In the case of polymorphic transformation, the Avrami exponent n is related to the nucleation mode of the tran
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