Facilitated C54-TiSi 2 Formation With Elevated Deposition Temperature: A Study of CO-Deposited Layers
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(PAM) Si substrates [5].
During the HTS process, a thicker
amorphous (cc) TiSi, layer is formed at the interface between the sputtered Ti film and PAM-Si substrates, which eventually leads to a reduction in the C54-TiSi 2 formation temperature [5, 6]. A possible reason for the beneficial effect of a thick ct-TiSi5 layer was suggested by high resolution transmission electron microscopy (HRTEM) to be the formation of more defective C49-TiSi2 which led to an easier transformation into the C54-TiSi 2 phase [5]. It was also speculated that since the initial silicide formation rate was actually slower for samples with thicker ct-TiSi5 interlayer, the nucleation of the TiSi2 phase(s) took place at a later time during a rapid thermal anneal (RTA) ramp-up and, hence, a higher temperature [6]. Recently, HRTEM images of a-TiSi, layers at the interfaces of HTS prepared samples were analyzed by autocorrelation function method and shown to contain a higher density of silicide crystallites [7]. It was thus concluded that a thicker ct-TiSix layer led to a smaller grain size for the C49-TiSi2 phase that led to the enhanced formation of C54-TiSi 2 . In this paper, the C49- to C54-TiSi2 phase transformation is further investigated using TiSi5 co-deposited with a precise composition, and by varying the deposition temperatures. The effect of thin MoSi 2.0 interlayer in the codeposited TiSi, layer is also investigated.
47 Mat. Res. Soc. Symp. Proc. Vol. 564 © 1999 Materials Research Society
EXPERIMENTS Lightly doped (10- 15 C2acm) Si(l 00) wafers were used as substrates in this experiment. Most wafers were implanted with BF 2 (3x 1015 cm 2 , 35 keV) and given a 900'C, 20 min anneal. The surfaces of some substrates were then pre-amorphized by an As implantation at 50 keV using a dose of 3x 1014 cm 2 . Substrates were cleaned chemically, etched in a dilute HF solution, and loaded immediately in an ultrahigh vacuum (UHV) system, with a 7x1011 Torr base pressure. Ti, Si and Mo were deposited by electron-beam evaporation sources at typical rates of 0.02-0.1 nm/s for the metals. Co-deposition ratio (x) of TiSix layers was varied from x=0 to 2.5. The temperature of the substrate was maintained at either room temperature (RT) or 400'C during the co-deposition. Throughout this paper, a "1 nm*" thick TiSi5 (MoSi 2) layer is defined as a co-deposited layer which contains the equivalent of I nm Ti (Mo) and the necessary amount of Si to account for the specified composition. The thickness of TiSi, layer was 15 nm*. The in-situ RTA processes for silicidation were done typically at 650TC for 60s. Sheet resistance was measured using a 4-point probe. Planview transmission electron microscopy (TEM) and diffraction (TED) patterns were performed for structural characterizations. RESULTS AND DISCUSSION Both the co-deposition ratio and the deposition temperature were found to have a significant effect on the formation of the C54-TiSi 2 phase. Figure 1 shows the sheet resistance of co-deposited TiSi5 (x=0-2.5) on p-type PAM (p-PAM) substrates which were
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