Stress relaxation and dopant activation in nsec laser annealed SiGe

Abstract

Defect evolution and dopant activation are intimately related to the use of ion implantation and annealing, traditionally used to dope semiconductors during device fabrication. However, during the last decade, the increased difficulties to maintain the transistor miniaturization pace have led to a diversification of both the basic device architecture and processes. In this context, the recent advances in nanosecond laser annealing have opened the way to solve a wide spectrum of difficult challenges in semiconductor technology, well beyond the traditional source/drain fabrication issues. Indeed, thanks to its low thermal budget, laser annealing is very attractive not only for the achievement of abrupt and highly doped junctions, but more generally for all semiconductor technologies in which dopants need to be activated while preserving the integrity of the surrounding areas, as in the case of CMOS backside imagers or 3D integration technologies.After giving an overview of some relevant published studies in this domain [1], we will present our recent investigations focused on damage and strain evolution in SiGe and their impact on dopant distribution and electrical activation during nanosecond laser thermal annealing. These will concern compressively strained SiGe layers (with Ge fraction, x, ranging from 0 to 0.4), which were submitted to Ultraviolet Nanosecond Laser Anneal (UV-NLA) at different energies. The impact of the process parameters on the annealing regimes and on the relaxation of the initial compressive strain are investigated in detail. Optimised conditions to achieve pseudomorphic, defect-free structures are identified, which depend on the elastic energy stored in the recrystallized structure (Fig. 1). The impact of the structural modifications induced by UN-NLA on the efficiency of dopant activation is also discussed. Finally, Ge segregation towards the surface occurs during melt laser annealing, resulting in the formation of a Ge-rich surface layer (Fig. 2). Such pseudomorphic SiGe layers with a graded composition and a Ge-rich surface may find promising applications such as contact resistance lowering in doped layers.In the last part, some recent investigations of phosphorus-doped ultra-thin SOI layers submitted to UV-NLA will be presented. Their recrystallization is investigated as a function of the implanted dose as well as the laser energy density, while the best conditions to achieve high dopant activation (up to ~2 % at.) are identified.