Formation of a ternary silicide for Ni/Ti/Si (100) and Ni/TiSi 2 structures

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INTRODUCTION The reactions of polycrystalline alloys have been the object of several studies because of their potential use in shallow silicide contacts. The separation of the alloy and the movement of the various species are reviewed in the study by Thompson et al.' However, the situation for bilayers is different and a definite model to determine the first growing phase, the reaction sequence, or the final equilibrium state is still lacking. These reactions are not limited to two metals in contact with Si but concern also the behavior of a metal on top of a silicide submitted to thermal processing. This last point is extremely important since it defines the stability of a contact and might impose the addition of a diffusion barrier in the manufacturing process. Of course, because of the extensive use of Al as the final layer, most of the studies included this metal as one of the three components, but little is known about the interaction of two transition metals with Si. The effect of interposing a thin layer can have several effects. It could favor nucleation; because of the increased number of sites, a smoother, more uniform surface could result. It could also promote heteroepitaxy. For example, a layer of Ti improves the epitaxial growth of Pd2Si on both (111) and (100) Si substrates.2 The changes of the various reaction temperatures (TI for the system Mi-Si, T2 for M2-Si, and Tm for the metals Ml and M2) are also of interest as one moves from binary to ternary systems. One should keep in mind that the addia)Also

with Laboratoire des Materiaux et du Genie Physique, Ecole Nationale Superieure de Physique de Grenoble, UA CNRS 1109, Saint Martin d'Heres, France. b)Also with the Department of Electrical Engineering, The University of Pennsylvania, Philadelphia, Pennsylvania 19104. 1218

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tion of a new component could result in the occurrence of a eutectic at lower temperatures than any of the ones present in the binary phase diagrams. Examples from the different categories of reactions for metal-metal-Si systems such as the growth of a ternary ordered compound, a pseudo-binary solid solution, or phase separation have been presented in a previous publication.3 The objective of this study of the Ti—Ni—Si system is to examine the formation of a ternary silicide and the stability of a Ni overlayer in contact with TiSi2. II. EXPERIMENTAL PROCEDURE Si (p type) wafers with (100) orientation and resistivities of 6 to 9 fi • cm are cleaned using a standard procedure, etched in 5% HF, rinsed in DI water, dried, and loaded in an MRC 8667 RF sputtering chamber. The base pressure before deposition is below 2.10~7 Torr. After a 5-min cleaning of the targets at a power of 1 kW, the substrates are Ar+ sputter etched for 3 min to remove the native oxide. For all depositions, parameters such as deposition rates and argon pressure are kept unchanged in order to minimize variations in resulting film stresses and morphology. Deposition rates are typically 11 nm/min for Ti (0.4 kW, PM = 8 fim) and 12 nm/mi

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