Key Issues Related to the Self-Aligned Formation of CoSi 2 in a Salicide Process
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KEY ISSUES RELATED TO THE SELF-ALIGNED FORMATION OF CoSi 2 IN A SALICIDE PROCESS L. VAN DEN HOVE, R. WOLTERS*, M. GEYSELAERS*, R. DE KEERSMAECKER and G. DECLERCK Interuniversity Microelectronics Center (IMEC), Kapeldreef 75, B-3030 LeuvenBelgium *Philips Research Laboratories, 5600 JA Eindhoven, The Netherlands
ABSTRACT The self-alignment of the Co-Si interaction at the edge of a Si-Si0 2 transitionhas previously been ascribedto the fact that Co is the main moving species during silicideformation. However, the reactions in the Co-Si system are more complex. The phenomena of lateral silicidationand encroachmentare therefore investigated at various stages of the Co/Si reactionusing samples starting eitherfrom sputtered Co layers or from cosputtered Co-Si multilayers with varying average stoichiometries. The mechanisms resulting in the absence of lateralsilicidationare clarified.
INTRODUCTION Self-aligned silicidation technologies in which a silicide is formed selectively on silicon regions have received considerable attention during recent years [1-3]. Although TiSi 2 has become the most popular silicide for this application [3], CoSi 2 has been presented as an attractive alternative for TiSi 2 [4-7]. Since the silicon regions in a real MOS device structure may be separated by a distance of only 0.3 to 0.5 g.m in some areas, a perfect self-alignment is required to achieve an acceptable yield in the silicidation process. Figure 1 shows a cross-sectional TEM micrograph of CoSi 2, formed near an SiO 2 edge. The silicidation reaction was performed using rapid thermal processing (RTP) at 700'C for 30 s in N2 ambient. A perfect self-alignment can be noticed. Unlike for the case of TiSi 2, no difference has been observed when using N 2 or Ar ambients. The key parameter influencing the lateral silicidation at oxide edges is the identity of the moving species during silicide formation. The absence of any lateral silicidation in the Co-Si system is often loosely ascribed to the fact that Co is the moving species during silicide formation. This, however, should be considered as an oversimplification. For the Co-Si system, three silicide phases are formed in sequence: Co 2Si, CoSi and CoSi 2[4-7]. It has been shown that Co is indeed the moving species for Co 2Si and CoSi 2 formation [8,9], but Si is diffusing during CoSi formation [8].
Fig. 1. Cross-sectional TEM micrograph of CoSi2 formed selectively on Si (with
respect to SiC 2) using the standardprocess (38nm Co was used, silicidationat 700 'C for 30 s in N2).The unreacted Co was etched off. Moreover, Co as the moving species causes concern for silicide encroachment under a Si-Si02 step. Co could diffuse through the formed CoSi 2 layer and react with Si under the SiO 2 edges. Surprisingly, this was never observed on any of the available cross-sectional TEM micrographs
Mat. Res. Soc. Symp. Proc. Vol. 100. ý1988 Materials Research Society
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of which Fig. 1 is a typical illustration. Severe encroachment would lead to junction leakage of shallow diodes under the silicid
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