6 - Challenges in ultra-shallow junction technology

Abstract

The aim of this chapter is to review the issues associated with shallow junction formation in silicon and germanium CMOS. In a first part, some basic quantities will be defined with respect to dopants in semiconductors and the basic diffusion mechanism of dopant atoms will be reviewed. The important role played by the intrinsic point defects, namely, vacancies (Vs) and interstitials (Is), will be highlighted. In first order, one can state that dopant diffusion in Ge is dominated by vacancies, while mixed behavior can be expected for silicon. In a second part, different doping methods will be summarized, whereby the dominant one is still relying on an ion implantation and annealing scheme. At the same time, the issue of displacement damage as a source of point defects and enhanced diffusion will be outlined. This indicates the important role played by defect engineering in order to control diffusion and dopant activation. This is illustrated in more detail in two case studies: the diffusion of B in silicon and of n-type dopants in Ge. Boron diffuses through the interstitialcy mechanism, i.e., by B-I pairs in silicon (and Ge). The presence of excess interstitials in the end-of-range damage region acts as a source of the so-called transient-enhanced diffusion (TED) and of the formation of boron-interstitial clusters (BICs). It will be shown that, for example, co-implantation with C, which is an efficient interstitial trap in silicon, contributes to a suppression of TED. BICs, on the other hand, consume part of the excess interstitials, so that they also suppress TED at short annealing times, but at the same time they form an immobile B fraction which is inactive. At longer annealing, BICs dissolve and release interstitials, causing delayed TED. In the case of n-type dopants in Ge, it is shown that diffusion occurs through X-V pairs. This is characterized by a diffusion coefficient which increases with increasing active electron concentration. As will be shown, this results in concentration-enhanced diffusion, characterized by a box-like dopant profile and a limitation of the active level to a value well below the maximum solid solubility. Countermeasures for the successful activation of ultra-shallow n+ junctions in Ge will be discussed next. They mainly rely on co-implantation with other (non)-dopant impurities or on the use of self-interstitials to control the excess vacancies. The role of ultrashort annealing schemes and of high- or low-temperature implantations will also be pointed out. The chapter is wrapped up by a summary.