Antimony Clustering due to High-dose Implantation
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Antimony Clustering due to High-dose Implantation
Kentaro Shibahara and Dai Onimatsu Research Center for Nanodevices and Systems Hiroshima University 1-4-2 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8527, Japan ABSTRACT Antimony implantation is a promising technique for fabricating ultra-shallow n+/p junctions for extensions of sub-100-nm n-MOSFETs. By increasing the Sb+ implantation dose to 6x1014 cm-2, sheet resistance (Rs) of an implanted layer was reduced to 260 Ω/sq. for rapid thermal annealing (RTA) at 800°C. The obtained junction depth of 19 nm is suitable for sub-100-nm MOSFETs. However, the reduction in the sheet resistance showed a tendency to saturate. No pileup at the Si-SiO2 interface, which was the major origin of dopant loss in lower dose cases was, observed in Sb depth profiles in this case. However, in the case of 900°C RTA, Sb depth profiles indicated that Sb was nearly immobile in the region where Sb concentration exceeded 1x1020 cm-3. These results imply that the major limiting factor of Rs reduction under the highdose condition is Sb precipitation, which is different from the lower dose cases. INTRODUCTION Antimony is suitable for the fabrication of ultra-shallow n+/p junctions by ion implantation for sub-100-nm MOSFETs because it is heavier than As or P and its diffusion coefficients are smaller than theirs. Recently, not only junction depth (Xj) but also sheet resistance (Rs) has become an increasingly important factor. This is because the parasitic resistance sometimes limits the MOSFETs performance as their gate length is scaled and channel resistance is lowered. We have demonstrated ultra-shallow (19 nm) and low-resistive (1.4 kΩ/sq.) n+/p junction formation by 10 keV 1x1014 cm-2 Sb implantation and applying it to source and drain extensions of 150-nm gate length MOSFETs [1]. Annealing was performed with a furnace at 850°C for 30 min in that case. The Rs should be reduced to meet the demand for sub-100-nm generations. Increasing implantation dose is a conventional way to achieve this reduction, but it is not effective because of dopant loss due to pileup at the SiO2-Si interface. About half of the implanted Sb was lost due to the pileup [2]. The pileup was suppressed by changing the annealing method from furnace annealing to rapid thermal annealing (RTA) [3,4]. Figure 1 shows Sb depth profiles after RTA. Pileup was observed only for 1000°C RTA. The Rs was reduced to 450Ω/sq. for 800°C 10 s RTA by suppressing pileup and increasing implantation dose
B8.5.1
800˚C 900˚C
20
10
1000˚C 19
10
18
10
Dose 3x1014 cm-2 Annealing Time 10 s
SiO2 Si
Sb Concentration [ cm -3 ]
21
10
17
10
0
5 10 15 20 25 30 35 40
Depth [ nm ] Figure 1. Sb depth profiles after RTA. The pileup is observed only for 1000°C RTA. to 3x1014 cm-2. However, Sb concentration exceeds thermal equilibrium solid solubility (1x1019 cm-3 at 850°C and 2x1019 cm-3 at 1000°C [5]) around a peak region. In the case of furnace annealing for a long time, Sb clusters or precipitates [6]; on the other hand, in the case of RTA
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