Influence of SiGe Thickness on the Co/SiGe/Si Solid State Reaction
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lo%% resistivity and possibility of self-aligned formation at relatively low temperatures. The reaction between SiGe and metals such as Pt [3], Pd [3]. Ti [4] and Co [5-9] have been reported. Recently. the effect of the Co thickness on the reaction of Co with 260 nm thick SiGe layers was investigated by Boyanov el al. [9]. They observed that the reaction products after annealing at 800 'C depend on the thickness of the Co film and attributed this thickness effect to preferential Co-Si bonding in the reaction zone and the energy change due to Ge segregation. In the present work, we investigate the effect of the SiGe layer thickness on the Co-SiGe/Si solid state reaction: phase formation sequence and temperature, crystalline orientation of the phases formed and the tendency of the silicide film to agglomerate. EXPERIMENT The epitaxial Sil.,Ge, layers used in the experiments were grown on Si (100) substrates by a rapid thermal chemical vapor deposition (RTCVD) process. The Ge content in these layers was determined from Rutherford backscattering spectroscopy (RBS) and showed a value of 20 at.%. Sif 5Geo 2 layers with thickness of 17, 35, and 80 nm were used in the experiments, n-type Si (100) wafers were used as control samples. All the wafers received a RCA clean followed by an IIF 2% for 30 s to remove any native oxide on the silicon surface. Co films were deposited in DC magnetron sputtering systems. The silicidation was carried out in a rapid thermal processor in N2 ambient for 30 s. Annealing temperatures varied between 500 and 1050 'C. 151 Mat. Res. Soc. Symp. Proc. Vol. 564
1999 Materials Research Society
10000
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"3"
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Structural characterization of the layers formed was done by transmission electron microscopy (TEM), x-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS). Sheet
SIGe
- 35 nm SlGe
1000
2
80 nm
--A.17 nrnSlGe
*I .. /
-
100
resistance was measured using a standard
four-point probe method.
10o
•X-X-X-A-XA 500
600
A ...... A---The 800
700
900
1000
RESULTS sheet resistance (R,) of the layers formed as a function of the annealing temperature is shown in Fig. I for 15/17,
17/35
RTP temperature (C)
and
15/80 (nm Co / nm SiGe)
samples. The Rs values between 60 and 100 Q/sq indicate that the predominant phase in the film is the high resistivity monosilicide phase, and the drop in the Rs values shows that the disilicide phase is formed and represents a larger fraction in the film. For the sample with the thinnest SiGe layer (17 nm), the disilicide phase is formed at 775 'C, whereas the sample prepared on 35 nm thick SiGe layer showed the disilicide formation at 850 'C. The significant increase of the Rs for the thicker SiGe layer (80 nm) for temperatures above 750 'C results from the agglomeration of the silicide film. Comparing these results with those of the reaction of Co with pure Si substrates, it is clear that the disilicide formation is retarded in the case of the reaction with SiGe alloys and that the increase of the SiGe thickness results in a furth
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