Selective Rapid Thermal Chemical Vapor Deposition of Titanium Disilicide on Silicon and Polysilicon
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In these experiments, TiCLj and Sil-4 (10% in He) were used as the Ti and Si source gases respectively. All silicide layers characterized in this study were selectively deposited on crystalline silicon or polycrystalline silicon through windows defined in 100 nm SiO 2 layers. TiCl4 flow was kept constant at 0.5 sccm corresponding to a TiC14 partial pressure of - 10-4 Torr. Different SiH 4 :TiCLt gas flow ratios were used to characterize the process. Ex-situ surface preparation consisted of a standard RCA clean followed by a dip in 5% HF solution and rinse in deionized water immediately before loading into the main chamber. No insitu clean (e.g. in hydrogen) or any other in-situ surface pre-treatment was employed in our process. The deposited silicide films were characterized by current-voltage measurements, profilometry, scanning electron microscopy (SEM) and atomic force microscopy (AFM). RESULTS Figure 1 shows scanning electron micrographs obtained from samples with TiSi 2 layers deposited selectively on Si and polysilicon. Excellent selectivity of the process is evident in both cases. The films were deposited at 775°C using a SiH4TiCI4 flow ratio of 75:1. With this ratio, Si consumption on crystalline Si is completely eliminated. An interesting feature of these two micrographs is that TiSi 2 grains on crystalline silicon are noticeably larger than those on polysilicon. Based on this result, we can further speculate that TiSi 2 grain structure is influenced by the grain structure of the underlying polysilicon.
Figure 1. SEM micrographs of TiSi 2 layers deposited on single crystal silicon and polysilicon. TiSi 2 was deposited selectively in windows defined in - 100 nm SiO2 . To investigate the effects of temperature on the TiSi2 microstructure, silicide layers were deposited at different temperatures and analyzed by AFM. Shown in Figure 2 is a typical AFM micrograph obtained from a TiSi 2 film deposited on polysilicon at 750TC. Figure 3 shows the effect of temperature on grain density and the average grain size extracted from AFM micrographs. We observe that the grain density increases while the average grain size decreases as the deposition temperature is increased. The dependence on temperature is linear for both parameters. A potential explanation for this behavior is the effect of temperature on TiSi 2 nucleation. Our previous results from very short (six second) deposition cycles show an increase in initial nuclei density with temperature. We speculate that once the initial nuclei form, they provide sites more attractive for Si (Ti) chemisorption and film growth than the Si surface. Therefore, we propose that a lower initial nuclei density should result in a larger grain size or equivalently, a smaller grain density. At high temperatures, initial nuclei form readily leaving little room for subsequent grain growth. The same observation can be made for TiSi 2 films deposited on crystalline silicon, however, the dependence is a lot weaker.
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Shown in Figure 4.a is TiSi 2 thickness obtained on crystalline
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