Rapid Thermal Process Requirements for The Annealing of Ultra-Shallow Junctions
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ABSTRACT 2.0 keV "B+, 2.2 keV 49 BF 2+ ion implanted and 1.0 kV Plasma Doped (PLAD) wafers of a dose of 1E 15/cm2 were annealed at various times and temperatures in a variety of ambients: 600 to 50,000 ppm 02 in N 2; 5% NH 3 in N2; N20; N2 or Ar, in order to investigate the effects of the annealing ambient on the formation of ultra-shallow junctions. RGA data was collected during some of the anneals to assist in identifying the complex surface chemistry responsible for boron outdiffusion. Subsequent to the anneals, ellipsometric, XPS, four-point probe sheet resistance and SIMS measurements were performed to further elucidate the effects of the different ambients on the retained boron dose, the sheet resistance value, the RTP grown oxide layer and the junction depth. In the cases where oxygen was present, e.g. N20 and 02 in N2, an oxidation enhanced diffusion of the boron was observed. This was most dramatic for the N20 anneals, which at 1050°C 10 s diffused the boron an additional 283 to 427 A, depending on the particular doping condition and species. For the case of BF2 implants and PLAD, anneals in 5% NH 3 in N 2 reduced the junction depth by a nitridation reduced diffusion mechanism. RGA data indicated that the out-diffusion mechanisms for B and BF 2 implanted wafers are different, with the BF 2 exhibiting dopant loss mechanisms during the 950°C anneals, producing F containing compounds. B implants did not show doping loss mechanisms, as observed by the RGA, until the 1050'C anneals and these signals did not contain F containing compounds. Equivalent effective energy boron implants of 8.9 keV BF 2 vs. 2.0 keV B, however,
indicated that the overall effect of the F in the BF 2 implants is very beneficial in the creation of ultrashallow junctions ( compared to B implants): reducing the junction depth by 428 A, and increasing the electrical activation ( determined by SRP) by 11.7%, even though the retained dose (resulting from an increased out-diffusion of B), was decreased by 5.4%. INTRODUCTION
The scaling down of the source/drain junction depth is one of the key requirements in the successful shrinking of device geometries. Current roadmaps foresee the p-type source/drain extension regions being scaled down to post-anneal depths of around 400 A, with a sheet resistance of 500 Ohms/sq. for devices with 0.12 gm gate lengths [1]. While the achievement of these shallow junctions is contingent on innovations in ion implantation technology, the annealing of these low energy implanted profiles presents several challenges. Control of the boron diffusion both out of the wafer and within the substrate is essential to maximize the retained post-anneal dopant with the shallowest junction profile. Previous research [2,3] conducted in our laboratories identified boron dopant loss during the anneal stage, either consumed into an oxide layer grown during the anneal or simply out-diffused into the ambient, as one of the most challenging problems related to the creation of ultra-shallow junctions with desirable sheet resistance.
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