AN APPROACH USING A SUBAMORPHIZING THRESHOLD DOSE SILICON IMPLANT OF OPTIMAL ENERGY TO ACHIEVE SHALLOWER JUNCTIONS

Significant channeling during the B+/BF2+ implant and enhanced diffusi on during the subsequent anneal step limit the ability of forming ultr ashallow p(+)-type junctions. The problem is becoming even more seriou s as device dimensions are shrinking and shallower junctions are neede d to prevent short channel effects. An approach using a subamorphous t hreshold dose Si+ preimplant of optimal energy is proposed and demonst rated, and it reduces both channeling during the subsequent dopant imp lant and enhanced B diffusion during the anneal step. In this approach , the Si+ implant is performed prior to the dopant implant (B+/BF2+); thus it is referred to as a Si+ preimplant. The energy of the Si+ prei mplant determines the spatial distribution of the vacancies and inters titials formed by it. The vacancy excess created by the Si+ preimplant creates strain in the lattice by disrupting the periodic potential of the lattice, which in turn helps in reducing channeling during the su bsequent dopant implant. The vacancy distribution created by the Si+ p reimplant also recombines with the interstitials produced by the dopan t implant resulting in fewer interstitials that can contribute to the enhanced diffusion of B, thereby reducing the enhanced B diffusion and thus resulting in shallower junctions. The energy of the Si+ preimpla nt is chosen such that the excess interstitials produced by it lie bey ond the depth to which the dopants would diffuse in the absence of a S i+ preimplant, thus the interstitials produced by the Si+ preimplant d o not contribute to enhanced B diffusion. Junction depth reduction was studied using unpatterned n-type wafers by secondary ion mass spectro metry analysis. A Si+ preimplant of the desired energy (ranging from 5 0 to 100 keV) was then performed through a thin screen oxide. The cont rol samples were not implanted with Si+. Subsequent B+ or BF2+ implant s were carried out at 5 keV. The samples were annealed at 1000 degrees C. The junction depth was reduced by 200 Angstrom for 5 keV, 6 x 10(1 4) cm(-2) BF2+ implants and 450 Angstrom for 5 keV, 2 x 10(14) cm(-2) B+ implants using this approach of preimplanting with a neutral specie s (Si+ was used in this study) of optimal energy. Low leakage devices have been fabricated using this approach to demonstrate that the Si+ p reimplant does not create any unwanted defects. (C) 1998 American Inst iture of Physics.