Controlling CoSi 2 nucleation: the effect of entropy of mixing
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Controlling CoSi2 nucleation : the effect of entropy of mixing C. Detavernier*, R.L. Van Meirhaeghe*, K. Maex+0, F.Cardon* *
Laboratorium voor Kristallografie en Studie van de Vaste Stof, Universiteit Gent, Krijgslaan 281/S1, B-9000 Gent, Belgium. + IMEC, Kapeldreef 75, B-3001 Leuven, Belgium. 0 also at E.E. Dept, K.U. Leuven, B-3001 Leuven, Belgium.
ABSTRACT It is generally known that nucleation effects strongly influence the CoSi to CoSi2 phase transition. According to classical nucleation theory, the small difference in Gibbs free energy between the CoSi and CoSi2 phase is responsible for the nucleation barrier. Adding elements that are soluble in CoSi and insoluble in CoSi2 will influence the entropy of mixing, and thus change ∆G. In this way, the height of the nucleation barrier may be controlled. By depositing Fe or Ge (respectively replacing Co and Si in the CoSi lattice) in between the Co and the Si substrate, we were able to increase the nucleation barrier. In the presence of Ni, the nucleation barrier is lowered, and low-resistive disilicide is formed at lower temperatures. INTRODUCTION It is known from literature that when a film of Co is deposited onto a Si substrate, annealing results in sequential growth of Co2Si, CoSi and CoSi2. It has been reported that the formation of CoSi2 is nucleation controlled [1,2]. The activation energy for nucleation is given by [3] (∆σ )3 ( ∆σ ) 3 (1) ∆G * ≈ = (∆G ) 2 ( ∆H − T ∆S )2 with ∆σ the difference in interfacial energy caused by the formation of the nucleus and ∆G the change in free energy. From eqn. 1, it is clear that nucleation phenomena will only be important if ∆G is small. In the case of CoSi2, the small difference in free energy ∆H between the CoSi and CoSi2 is responsible for the nucleation barrier. However, ∆G also contains an entropy term ∆S, which is usually neglected for solid state reactions. In this work, it will be shown that one can control the nucleation temperature of CoSi2 by changing this ∆S term. This can be accomplished by adding alloying elements that are soluble in CoSi or CoSi2. We were able to control the nucleation temperature in the range 400-800°C. EXPERIMENT The substrates (p-type Si(100), Na = 1013-1014 cm-3) were cleaned using standard RCA cleaning, followed by a HF dip. Layers of Fe, Ge, Ni and Co were deposited by e-beam evaporation in a vacuum of 10-6 mbar. To study the silicidation reaction, isochronal annealing (30 seconds) was done at various temperatures. Annealing was carried out in a Rapid Thermal Processing (RTP) system, in N2 ambient. The samples were analyzed using a four point probe for sheet resistance
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measurements, X-Ray Diffraction (XRD, CuKα radiation) for phase identification and X-ray Photoelectron Spectroscopy (XPS) to determine elemental depth distribution. Because of the large difference in resistivity between CoSi and CoSi2, sheet resistance measurements provide a fast means of obtaining information about phase formation. In this paper, because of the limited amount of space, we will only present th
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