Investigation of Fluorine Effect on the Boron Diffusion by Mean of Boron Redistribution in Shallow Delta-doped Layers
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Investigation of Fluorine Effect on the Boron Diffusion by Mean of Boron Redistribution in Shallow Delta-doped Layers. A. Halimaoui1, J. M. Hartmann3, C. Laviron3, R. El-Farhane2, and F. Laugier3 1 STMicroelectronics, 2 Philips, 850 rue Jean Monnet 38926 Crolles Cedex France 3 CEA-LETI, 17 rue des Martyrs 38054 Grenoble Cedex France ABSTRACT Previously published articles have shown that co-implanted fluorine reduces transient enhanced diffusion of boron. However, it is not yet elucidated whether this effect is due to interaction of fluorine with point-defects or boron atoms. In this work, we have used boron redistribution in a shallow Delta-doped Si structures in order to get some insights into the role of fluorine in the boron diffusion. The structures consisted of 3 boron-doped layers separated by 40nm-thick undoped silicon. The samples were given to Ge preamorphization and F co-implant. SIMS depth profiling was used to analyse boron redistribution after annealing. The results we obtained strongly suggest that fluorine is not interacting with point-defects. The reduction in boron TED is most probably due to boron-fluorine interaction. INTRODUCTION In microelectronics industry, the doping of the silicon substrate is achieved by ion-implantation followed by activation anneal. The ion implantation process results in a supersaturation of selfinterstitials in the implanted silicon crystal. These interstitials have been identified to be at the origin of the well known Transient Enhanced Diffusion (TED) that takes place during the activation anneal, even at temperature as low as 500°C [1-3]. The scaling-down of CMOS devices is requiring Ultra Shallow Junctions (USJ) that are becoming more and more difficult to achieve, in particular when using boron. A possible approach to achieve this challenging task is to reduce or even suppress TED. It has been experimentally demonstrated [4-9] that fluorine co-implant resulted in a significant reduction in boron diffusion yielding shallower junction. The mechanisms at the origin of this diffusion reduction have not yet been unambiguously elucidated. Downey et al [5] have used preamorphization of Si to separate the chemical and damage effects of fluorine on TED of boron. They conclude that the reduction in boron TED is a chemical species effect affecting the interstitials. This hypothesis is supported in other reports [4, 6] mentioning that fluorine interacts with interstitials after their release from End Of Range defects (EOR). Dielbel et al [8] have also suggested interaction of fluorine with point defects. They developed a model from ab-initio calculation to get some insights into the effects of coimplanted fluorine on boron and phosphorous diffusion. This model is based on the formation and dissolution of Fluorine-Vacancy clusters (FnVm). The authors conclude that in the nonamorphized regions boron and phosphorous diffusion get enhanced due to excess interstitials, while in the amorphized region diffusion is retarded due to grown in F3V. Robertson et al [7] have demonst
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