Post-Oxidation Enhanced Diffusion of Low-Energy Implanted Boron in Ultra-Shallow P + /N Junctions Formation
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POST-OXIDATION ENHANCED DIFFUSION OF LOW-ENERGY IMPLANTED BORON IN ULTRA-SHALLOW P+/N JUNCTIONS FORMATION D. LENOBLE, A. HALIMAOUI, A. GROUILLET France-Telecom R&D – CNET, 28 chemin du vieux chêne – 38243 Meylan – France ABSTRACT In this paper, we report for the first time the effect of sacrificial oxide (sacox) on the boron diffusion in ultra-shallow P+/N junctions. It is shown that the boron diffusivity is enhanced when low energy implantations are performed through sacrificial oxide. The various experimental data lead to conclude that the Post-Oxidation Enhanced Diffusion (POED) is due to a « mirror effect » seen by the Si interstitials incoming into the sacox layer. POED occurs even for sacox as thin as 1.5 nm. From a simple model, the reflection coefficient is estimated to be about 100 % for a 2.5 nm-thick sacox. INTRODUCTION The formation of ultra-shallow junctions is a critical issue for the future CMOS device technology. The recent international roadmap for semiconductors [1] has pointed out that junctions shallower than 33 nm is required for the 0.1-µm technology node. This represents a very challenging task in particular for the formation of the ultra-shallow P+/N junctions due to the B features. From one side, its low atomic mass imposes an ultra-low implantation energy (ULE) which is not trivial to achieve with sufficient uniformity, reproducibility, throughput, etc.. Moreover, the important channeling of B during ULE implantation and the energetic contamination [2] drive the junction depth deeper into the Si bulk. From the other side, the Transient Enhanced Diffusion (TED) of B during the postimplantation annealing is one of the critical physical issue to handle for the ultra-shallow junction formation. It is well known that the excess of crystalline point-defects is at the origin of the TED [3]. Mathiot and Pfister have first exposed the basic general diffusion equation of the TED [4]. Today, it is well accepted that B diffuses via a kick-out reaction with Si self-interstitials (SiI) [5]. The enhanced diffusion coefficient for boron as defined by equation 1 represents a general consensus of the scientific community. Identically to the TED C phenomenon, any process which would increase the supersaturation *i of SiI would enhance Ci the anomalous assisted point-defect diffusion. The well known Oxidation Enhanced Diffusion [3] (OED) is a perfect illustration. C DED = Db* *i (1) Ci with : D ED : enhanced diffusion coefficient Db* : intrinsec B diffusion coefficient [6] C i : mobile Si interstitials concentration C i* : equilibrium Si interstitials concentration A sacrificial oxide (sacox) is currently used in implantation technology in order to prevent any contamination of the silicon surface. Such sacox can also be integrated to a Si front-end process flow to minimize the ion channeling particularly important at low implantation energy [7]. In this study, we have investigated the sacox influence on the
B3.9.1
diffusion during the subsequent rapid thermal annealing of the implanted dopants. We evidenced
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