Phase Transformation Kinetics of TiSi 2
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PHASE TRANSFORMATION KINETICS OF TiSi 2
R.W.MANN*, L.A.CLEVENGER** and Q.Z.HONG** *IBM Technology Products, Essex Junction, VT 05452 **IBM T.J.Watson Research Center, Yorktown Heights, NY 10598 ABSTRACT The microstructure and kinetics of the polymorphic C49 to C54-TiSi2 phase transformation have been studied using samples prepared as in self-aligned silicide applications. For C49-TiSi2 thin films formed at temperatures of 600°C and 625°C on (100) single-crystal silicon substrates, the effective activation energy was 5.6 ± 0.3 and 5.7 + 0.08 eV, respectively, for this phase transformation carried out in the temperature range of 600'C to 700'C. The transformation process was observed to occur by nucleation and growth of the orthorhombic face-centered (C54) phase from the as-formed orthorhombic base-centered (C49) phase. INTRODUCTION The C49 to C54 phase transformation in titanium silicide is of significant practical importance in the VLSIC industry. Titanium silicide has become the most common silicide in the industry for self-aligned silicide (SAlicide) applications because of its combined characteristics of low resistivity, ability to be self-aligned and relatively good thermal stability. Although TiSi2 possesses certain advantages and similarities when compared to other silicides, it is unique in that it is a polymorphic material. TiSi2 may exist as orthorhombic base-centered (C49) phase having 12 atoms per unit cell, or as the thermodynamically favored orthorhombic face-centered (C54) phase which has 24 atoms per unit cell,. It has been found experimentally that the high-resistivity (60-90 UQf-cm) metastable C49 phase forms first 2 -3. It is generally accepted that the C49 phase forms first because of a lower surface energy and, hence, barrier to nucleation of this phase4 -5. The thermodynamic driving force to convert from C49 to C54 is a bulk free-energy difference. The subsequent transformation to the lower resistivity (12-20 p.Q-cm) C54 phase requires additional thermal energy to overcome the nucleation barrier associated with forming the new surface and the energy required to grow the newly formed crystal. Surface energy, film thickness and microstructure are three fundamental factors that influence the phase transformation kinetics. The surface energy can be varied by adding impurities6 or by varying the substrate material,5 . The silicide fdim thickness, which typically decreases with each new technology generation can be varied by changing the formation conditions or the as-deposited thickness 7 . The C49 microstructure can be modulated by varying the formation temperatures. In a previous evaluation using an evaporated film of near stoichiometric (Ti = 1, Si= 2) composition, the effective activation energy for the C49 to C54 transformation was determined to be 4.45 eV on an SiO2 substrate7 . In a subsequent study using similar preparation techniques and substrate type, it was shown that the activation energy for this phase transformation could be reduced by adding small quantities (0.3-2.5 atomic %) of an
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