Cu gettering in Si cavities observed by Positron Annihilation Doppler Broadening
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Cu gettering in Si cavities observed by Positron Annihilation Doppler Broadening H. Schut, A. van Veen and S.W.H. Eijt Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, NL-2629 JB Delft, The Netherlands.
ABSTRACT In this study we present the results of Positron Beam Annihilation (PBA) experiments on the gettering of copper in Cz-Si implanted with 33 keV He+ ions with doses ranging from 0.5 to 3× 1016 cm-2 followed by an anneal treatment at 1100 K under N2 ambient. For the higher doses this yields a sub-surface layer containing nanometer sized cavities. Copper is introduced into these cavities by diffusing from the backside of the wafer at 1000 K, again under N2 ambient. Mapping of the S and W Doppler broadening parameters, with the implantation dose as running parameter, shows the formation of cavities. After the Cu in-diffusion the change in the values of the characteristic S-W cluster points clearly demonstrates the arrival of Cu at the internal surface of the cavities.
INTRODUCTION The reduction in device dimensions and the increasing complexity of IC technology puts limits to the levels of unwanted (metallic) impurities, such as Fe, Ni and Cu, in e.g. Si. Impurity gettering at cavities is one method to reduce the detoriating effects of the impurities on the performance of the devices. From work by Griffioen et al. [1] , Myers et al. [2] and Raineri et al. [3] it is known that in Si nano-cavities can be generated by keV implantation of light ions (H and He) at doses beyond approximately 1016 cm-2 followed by an annealing at typically 1100 K. Due to the stabilizing effect of the He and H, the irradiation induced defects cluster to nanometer sized cavities. At the final temperatures the implanted atoms dissolve and permeate to the surface. A method to detect the formation of such cavities and the modification of their internal surfaces is the Positron Beam Annihilation Doppler broadening (PBA) technique. A general overview of the applications of this technique is given in reference [4]. Gettering of Cu at ion implantation induced defects in the Rp/2 region (half of the projected ion range) studied by PBA and SIMS is addressed by Kögler et al. [5]. Trapping and subsequent annihilation of positrons in the cavities is observed by a significant change of the so-called S and W annihilation parameters. In bulk materials these relate to the annihilation of positrons with valence and core electrons, respectively. The trapping at Sicavities opens a channel for the formation of Positronium (Ps) with annihilation characteristics clearly different from that of the bulk material. Furthermore, the trapping of positrons in a surface state and the annihilation of the ortho-Ps (a bound state of an electron and a positron with spins oriented parallel) by pick-off reactions with electrons at the internal cavity surface enables the detection of impurities arriving at these cavities.
O6.5.1
EXPERIMENTAL Sample preparation 0.016 0.014
E =2 keV
0.012
P(z,E)
Polished Cz-Si (1-5 Ωcm) wafers were used to
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