What Does Self-Diffusion Tell Us about Ultra Shallow Junctions?
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What Does Self-Diffusion Tell Us about Ultra Shallow Junctions?
Ant Ural, Serene Koh, P. B. Griffin, and J. D. Plummer Department of Electrical Engineering, Stanford University, Stanford, CA 94305, U.S.A. Electronic address: [email protected]
ABSTRACT
Understanding the coupling between native point defects and dopants at high concentrations in silicon will be key to ultra shallow junction formation in silicon technology. Other effects, such as transient enhanced diffusion (TED) will become less important. In this paper, we first describe how thermodynamic properties of the two native point defects in silicon, namely vacancies and self-interstitials, have been obtained by studying self-diffusion in isotopically enriched structures. We then discuss what this tells us about dopant diffusion. In particular, we show that the diffusion of high concentration shallow dopant profiles is determined by the competition between the flux of mobile dopants and those of the native point defects. These fluxes are proportional to the interstitial or vacancy components of dopant and self-diffusion, respectively. This is why understanding the microscopic mechanisms of silicon self-diffusion is important in predicting and modeling the diffusion of ultra shallow dopant profiles. As an example, we show experimental data and simulation fits of how these coupling effects play a role in the annealing of shallow BF2 ion implantation profiles. We conclude that relatively low temperature furnace cycles following high temperature rapid thermal anneals (RTA) have a significant effect on the minimum junction depth that can be achieved.
INTRODUCTION
Recently, we have studied self-diffusion in silicon using isotopically enriched structures under nonequilibrium point defect conditions [1-4]. From these results, we have extracted the thermodynamic properties of the native point defects in silicon, namely vacancies (V) and selfinterstitials (I). The goal of this paper is to show that these defect properties play a key role in determining the diffusion profiles of technologically important dopants in silicon. In this section, we review the results of the self-diffusion experiments. Then, in the theory section, we explain the physics behind the coupling between the diffusion of dopants and point defects (see Refs. [5-9] for previous work on coupled diffusion). We conclude with the experiment and simulation section, where we present experimental results of low temperature anneals of activated shallow BF2 implants, and corresponding simulation fits obtained using the properties of self-interstitials extracted from self-diffusion data. The self-diffusion experiments were carried out using Si isotope structures grown by chemical vapor deposition (CVD) having a surface layer containing the three stable isotopes of Si in
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their natural relative abundances, and a buried layer heavily depleted in 29Si and 30Si. The experimental details are given in Refs. [1-4]. Using thermal oxidation and nitridation to perturb the point defect concentrations f
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