Liquid Phase Growth of Epitaxial Ni and Co Silicides by Pulsed Laser Irradiation
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LIQUID PHASE GROWTH OF EPITAXIAL Ni AND Co SILICIDES BY PULSED LASER IRRADIATION R. T. TUNG, J. M. GIBSON, D. C. JACOBSON, AND J. M. POATE Bell Laboratories, Murray Hill, NJ 07974 ABSTRACT Epitaxial Ni and Co silicides have been fabricated using pulsed laser melting. Interfacial instabilities and cell formation are suppressed during the liquid-phase epitaxy by melting mono or disilicide layers. Single crystal NiSi 2 and CoSi 2 films have been grown on (100) and (111) Si following a post-anneal. This method does not require UHV deposition or reaction techniques.
Thin film metal silicides are usually formed by deposition of the metal film on Si followed by heating until the requisite silicide phase is attained. Reaction usually starts by formation of the metal rich phase until all the metal is consumed; further heating will then result in conversion to the silicon rich silicides. These silicide layers are typically polycrystalline with grain sizes of a 1000 A or so. CoSi 2 and NiSi2 have the cubic CaF 2 structure and lattice parameters within 1.2% of Si and are ideal candidates for epitaxy. However, conventional furnace annealing [1] does not result in planar epitaxial silicides but rather heavily facetted films on Si(100), and textured films on Si(111) with grains of normal orientation (type A) and grains rotated 180' about the surface normal (type B); see Fig. 1. We have recently demonstrated [1-31, by the use of ultra high vacuum (UHV) deposition and reaction techniques that it is possible to form single-crystal, epitaxial CoSi2 and NiSi 2 films on both Si(ll) and (100). The techniques are complicated but are necessary to control the interfacial and grain boundary impurity concentrations so that the reaction kinetics and mass transport can be controlled in the solid phase to produce the epitaxial material. Here we show that these epitaxial silicides can be produced by liquid-phase epitaxial growth using pulsed laser melting. Poate et al [41 first demonstrated that thin silicide layers could be produced by pulsed laser melting of films of Ni, Pd or Pt on Si. Those studies and measurements by Stacy et al [51 showed that for laser heating with pulse lengths of -50 nsec, the films were not of uniform composition of equilibrium phases but consisted of cells of Si surrounded by walls of silicides. When the metal film is irradiated the melt front will penetrate very rapidly through the metal and Si. The melt depth will depend upon the deposited laser energy. Solidification will then ensue with the solid-liquid interface moving rapidly to the surface. During melting the metal film and Si will interdiffuse in the liquid phase. The solidifying interface will therefore encounter a liquid of continuously varying composition. Segregation at the moving interface can produce constitutional supercooling of the melt. This supercooling can lead to interfacial instabilities and the formation of cellular structures. Of course this phenomenon will not occur if the melt has the same composition as the equilibrium solid state phase. Th
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