Oxide Mediated Epitaxial Growth of CoSi 2 in a Single Deposition Step
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(SADS) scheme make epitaxial Co silicide
ideally suited for the formation of ultra-shallow junctions [3,4]. Not all processing conditions lead to epitaxial CoSi 2 growth, however. Presently, there are three processing techniques for epitaxial CoSi 2 which are compatible with routine fab line environments [2]. High temperature sputtering (HTS) leads to partially epitaxial growth on heavily doped Si [5], but is not effective on lightly doped Si. Ti-interlayer mediated epitaxy (TIME) generates single crystal CoSi 2 layers on lightly doped and p+ Si [6], but is plagued by void formation at the edges of oxide patterns [7,8]. OME allows the growth of single crystal CoSi 2 layers and successfully avoids the void formation problem [9], but this technique is complicated as it involves repeated deposition and annealing procedures for silicide layers more than 10nm thick. In this work, the OME technique has been modified to allow thick (-20-40nm) epitaxial CoSi2 layers to be grown in a single deposition sequence. The techniques involved are (a) thin Ti cap, (b) thin Ti blocking layer, (c) co-deposition of metal-rich CoSix, and (d) co-deposition of Col-xTix. Uniform and high quality growth of epitaxial silicide has been demonstrated. The implications of these results on the epitaxial mechanism of the OME process are also discussed. EXPERIMENTAL PROCEDURES
Lightly doped Si(100) wafers were used in these studies.
Substrates were cleaned
repeatedly by oxide growth and removal, ending with the growth of a thin protective oxide layer on the surface in a peroxide bath. They were transferred to an ultrahigh vacuum evaporation 117
Mat. Res. Soc. Symp. Proc. Vol. 564 ©1999 Materials Research Society
system and properly degassed before metal deposition(s). Cobalt, silicon and titanium were ebeam evaporated from individually controlled sources, while the pressure in the growth chamber remained below Ix10 9 torr. Throughout this paper, a lnm* thick CoSix or CoTi5 layer is defined as a mixed layer, with the said composition, which contains the equivalent of 1nm of pure Co (9x10' 5 atoms cm2 ). Anneals were carried out in UHV or in a side chamber backfilled with ultra-high purity gas, without breaking vacuum. Rutherford backscattering (RBS) spectra were recorded with a 2MeV He+ beam, using a glancing exit geometry. RESULTS AND DISCUSSIONS With pure Co, formation of epitaxial CoSi 2 is limited to -1-3nm Co by OME [9]. When a Ti cap is added to the OME process, thicker epitaxial CoSi 2 layers were found to be grown within a single deposition step. A Ti or TiN cap has previously been shown to eliminate bridging problems in a cobalt salicide process [10], presumably through the reduction of surface diffusion. The Ti cap in particular, getters oxygen and improves Co silicide formation near the edges of the oxide pattern [11,12]. It is also known, however, that the intermixing of Co and Ti reduces the amount of Co taking part in the Co silicide reaction. The present study shows that a relatively thick Ti cap is important for the epitaxial
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