Ultra-Shallow Junction Formation by Excimer Laser Annealing of Ultra-Low Energy B Implanted in Si
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Ultra-shallow junction formation by excimer laser annealing of ultra-low energy B implanted in Si G. Fortunato1, L. Mariucci1, V. Privitera2, A. La Magna2, S. Whelan2, G. Mannino2. 1 CNR-IFN, Via Cineto Romano 42, 00156 Rome, Italy 2 CNR-IMM, Stradale Primosole 50, 95121 Catania, Italy ABSTRACT Formation of ultra-shallow junctions by excimer laser annealing (ELA) of ultra-low energy (1keV –250 eV) B implanted in Si has been investigated. High resolution TEM has been used to assess the as-implanted damage and the crystal recovery following ELA. The electrical activation and redistribution of B in Si during ELA has been studied as a function of the laser energy density (melt depth), the implant dose and the number of laser pulses (melt duration). Under appropriate ELA conditions, ultra-shallow profiles, extending to a depth as low as 35 nm with an abrupt profile (2.5 nm/dec), have been achieved. A significant amount of the implanted dopant was lost from the sample following ELA. However, the dopant that was retained in crystal material was fully activated following rapid re-solidification. We developed a theoretical model, that considers the dopant redistribution during melting and regrowth, showing that the fraction of the implanted dopant not activated during ELA was lost from the sample through out diffusion. The lateral distribution of the implanted B following laser annealing has been studied with 2-D measurements, using selective etching and cross-section TEM on samples where the implanted dopant was confined by using test structures including windows opened in silicon dioxide masks and patterned gate stack structures. INTRODUCTION Scaling gate length below 0.13 µm for Complementary Metal-Oxide-Semiconductor (CMOS) devices, will require formation of source/drain (S/D) junction depths shallower than 50 nm [1]. Ultra-shallow, electrically activated layers can be formed in Si through a combined process of ultra low energy (ULE) ion implantation and high ramp rate (400°C/s), short time (1000°C), "spike" annealing [2]. Although the ions are implanted at low energy and their range in the solid is very shallow, high temperature annealing is required in order to activate a sufficient fraction of the implanted dopant. On the other hand, non-equilibrium diffusion can lead to increased diffusivity during post-implantation annealing (transient enhanced diffusion, TED) [3], seriously limiting the minimum junction depth. Excimer laser annealing (ELA) of ion implanted Si, thoroughly investigated in the early ‘80s [4, 5] has received renewed interest within the semiconductor community recently for its possible application to the formation of ultra-shallow junctions in Si [6-9]. The technique offers many advantages compared to conventional rapid thermal anneal (RTA) procedures, such as control over the junction depth and a higher dopant activation efficiency [6-9]. When irradiating Si with laser light at a sufficient energy density, a well defined melted zone, with a sharp transition phase from liquid to crystal, is form
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