Interface-Mediated Photostimulation Effects on Diffusion and Activation of Boron Implanted into Silicon

Abstract

Recent work has demonstrated the existence of nonthermal illumination effects on the diffusion of boron, arsenic, and isotopic silicon ion-implanted into silicon for ultra-shallow junction applications. In some cases, the degree of electrical activation is affected as well. The effects arise from super-bandgap light's direct action on bulk point defects via changes in their charge state. By contrast, the present work demonstrates the existence of a distinct mechanism whereby light acts indirectly on bulk defects via changes at a nearby interface. The changes occur in interfacial annihilation probability and/or electric potential, and affect the efficiency with which the interface absorbs point defects emitted by extended end-of-range defects during annealing.Formation of shallow pn junctions for integrated circuits requires ion implantation of dopants followed by annealing to reduce implantation damage and to promote electrical activation of the dopants. Annealing technology relies upon rapid heating by incandescent lamps, flash lamps or lasers. 1 The optical stimulation was long thought to supply heating alone, although scattered experimental evidence had hinted at additional nonthermal influences on diffusion and activation of phosphorous, 2 arsenic, 3-5 and boron. 4,6-8 However, the experiments were difficult to interpret reliably because lamps supplied the heating, thereby complicating the decoupling of heating and photostimulation effects. Some progress was achieved in quantifying photoenhanced diffusion in II-VI semiconductors (using computational methods) 9 and a-Si:H (experimentally), 10 yet the understanding of the photostimulated dopant diffusion in c-Si remained inadequate. This laboratory has employed a novel experimental configuration to eliminate the ambiguity, and has thereby demonstrated the influences of low-intensity (<1 W/cm 2 ) super-bandgap illumination − first on the surface diffusion of various adsorbates on Si, 11,12 and subsequently on the bulk self-diffusion of Si 13,15 as well as the bulk diffusion and electrical activation of As and B implanted into silicon. 14 As first suggested for surface diffusion 11,12,15,16 and later modeled quantitatively for bulk Si self-diffusion, 13 photostimulation can influence diffusional flux rates by changing the average electrical charge state of point defects involved with diffusion, although the exact mechanism depends upon the specific case. For surface diffusion of Group III, IV and V elements on Si, the mass flux is carried by adatoms that can be rendered immobile by substitution into the top layer of the substrate. Escape from this substitutional state leaves behind a surface vacancy that can take on a variety of charge states depending upon the local concentrations of electrons and holes. Photostimulation exerts its influence by changing these carrier concentrations, which propagates into the average charge state of the vacancies, then into the effective formation energy for creating an adatom and vacancy, and finally into the concentration of ad...

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