Crystallography and Structural Studies of CoSi₂ Growth on Si(001) by Reactive Deposition Epitaxy:

Effects of high-flux low-energy ion irradiation during magnetically-unbalanced magnetron sputtering

C. W. Lim, C.-S. Shin, D. Gall, M. Sardela, R. D. Twesten, J. M. Zuo, I. Petrov, and J. E. Greene

Department of Materials Science, the Frederick Seitz Materials Research Laboratory

University of Illinois, 104 South Goodwin Avenue, Urbana, Illinois 61801

Metal-silicides have evolved as the contact and interconnect materials of choice for deep sub-micron scale devices. Among the large family metal-silicides, CoSi₂ is favorable for microelectronic applications owing to its process compatibility and ease of implementation in a variety of silicidation schemes. In addition, recent results demonstrating the successful use of CoSi₂ as a catalyst for the selective formation of carbon nanotubes have generated more interest in CoSi₂ for future device applications. The necessity for epitaxial metal-silicides has become apparent for next generation ultra-shallow junctions devices in order to reduce the effect of drain-induced barrier lowering and to prevent junction punch-through and leakage. However, the growth of phase-pure single-crystal CoSi₂ films directly on Si(001), without the use of interlayers, remains a challenge due to the competitive growth of several epitaxial CoSi₂ variants on Si(001).

In this work, CaF₂-structure CoSi₂ layers were formed on Si(001) by reactive deposition epitaxy (RDE) and compared with CoSi₂ layers obtained by conventional solid phase growth (SPG). CoSi₂ formation via RDE occurred during Co deposition while SPG silicidation, for which the Co deposition temperature was 25 ^°C, took place during subsequent annealing. The Co deposition rate as well as the CoSi₂ formation temperature, 700 ^°C, were identical in the two cases. X-ray diffraction pole figures and transmission electron microscopy results demonstrate that RDE CoSi₂ layers are epitaxial, with a cube-on-cube relationship || and ||, and containing fourfold symmetric {111} twins with|| and ||. In contrast, SPG films are polycrystalline with an average grain size of • 100 nm and a mixed 111/002/022/112 texture. We attribute the striking difference to rapid Co diffusion into Si during RDE; the high Co/Si reactivity gives rise to a flux-limited reaction resulting in the direct formation of the disilicide phase. In contrast, sequential nucleation and transformation among increasingly Si-rich phases -- from orthorhombic Co₂Si to cubic CoSi to CoSi₂ -- during SPG results in polycrystalline layers with a complex texture. In order to improve the crystalline quality of RDE CoSi₂, a variable axial magnetic field B_ext was used to control the Ar⁺ ion flux incident at the growing film without affecting the Co deposition flux J_Co. increases by more than a factor of ten, with the ion energy remaining constant at 9.6±0.9 eV, as B_ext is raised from 0 to 180 G. We show that the volume fraction f₁₁₁ of twinned regions is sharply reduced, from 81% with = 1.0 to 42% with = 13.3, through the use of high-flux low-energy ion irradiation during RDE CoSi₂(001) growth. We attribute the increased crystalline quality to an ion-irradiation-induced enhancement in the effective Co surface mobility which, in turn, decreases the probability of metal atoms becoming trapped at higher energy sites and nucleating {111} twin.