Silicidation Mechanism of Co/(Refractory-Metal) Bilayer and Epitaxial Growth of CoSi 2 on Si(100) Substrate
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Co
RM
Si ............ :...-....
S4 2 ..-. '...-...-.-.-......
Fig. 1 Metallurgical phenomena involved in the silicidation of Co/(refractory- metal) bilayer: mass transfer; intermediate phase formation(D) and solid solution(@)), surface reaction; silicide formation()) and reduction of native oxide()). 379
Mat. Res. Soc. Symp. Proc. Vol. 320.
01994
Materials Research Society
reduction of a native oxide by the refractory layer is involved in the surface reaction.[2] Thus, if the silicidation temperature of the refractory layer is higher than that of Co, a preferential reaction between Si and Co, namely, the formation of CoSi 2, takes place. And if the oxidation potential of the refractory metal is negatively higher than that of Si, the native oxide formed on Si substrate is reduced by the chemical potential of the refractory film at the early stage of thermal annealing.[8] In addition, if the refractory layer effectively limits the flux of Co atoms toward the substrate and consequentially delays the Co-Si interaction, the effective heat of formation of CoSi 2 phase comes to have the most negative value.[9] Since the coherent interface is the most stable, in that case, the epitaxial CoSiz layer which has the coherent interface with Si substrate might be formed. Cr, V, Ta, and Zr were chosen as the representative refractory metals after the following considerations.[7-91 (1) Cr has the highest equilibrium solid solubility in Co, while the intermediate phase of Co-Cr is the most unstable among all Co-refractory couples. And also the silicidation temperature of Cr is rather lower than that of Co. (2) The properties of the Co-V couple are nearly the same as those of Co-Cr couple except that the silicidation temperature is higher than that of Co. (3) The properties of the Co-Ta couple can be summerized as follow: it has lower solubility than any other couples; the silicidation temperature of Ta is higher than any other refractory metals; moreover, the oxidation potential of Ta is negatively higher than that of Si. Finally, (4) the thermodynamic properties of Co/Zr bilayer ia nearly the same as those of the Co/Ta bilayer. However, it has several distinctive features: the silicidation temperature and the negative value of the oxidation potential of Zr are the highest among all refractory metals. And the equilibrium solid solubility between Co and Zr is the lowest among other Co-refractory couples, which means that the mass transport of Co is extremely limited irrespective of annealing temperature and annealing ambients. EXPERIMENTAL PROCEDURES N-type (100)Si wafers, of 10-20 Q.cm resistivity and 10 cm diameter, were cleansed in HF solution just prior to loading in an electron gun evaporator which was evacuated below 5X 10-7 torr during the deposition. Refractory metal layer, such as Cr(25nm), V(25nm), Ta(3nm), or Zr(25nm), and Co layer of 25 nm thick were sequentially deposited on Si substrate without a vacuum break. All samples were annealed in a halogen lamp annealer at the temperature of 9001C for 20 sec in a N2 o
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