Source-drain Engineering for Channel-limited PMOS Device Performance: Advances in Understanding of Amorphization-Based I
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Source-drain Engineering for Channel-limited PMOS Device Performance: Advances in Understanding of Amorphization-Based Implant Techniques Nick E. Cowern School of Electrical, Electronic and Computer Engineering, University of Newcastle upon Tyne, Merz Court, Newcastle upon Tyne, NE1 7RU, United Kingdom ABSTRACT This paper discusses the role of amorphisation and residual end-of-range defects in p-channel source/drain engineering. A comparison between preamorphisation and molecular implant approaches shows up some important common features of electrical activation, diffusion, and junction leakage, related to the formation and location of boron-interstitial and self-interstitial clusters. The success of these techniques depends on confining ‘end-of-range’ defects – whether TEM-visible defects or sub-microscopic clusters – within the narrow region between the boron implant peak and the source-drain/halo depletion region. This observation points to significant improvements that can still be made in implantation processing for ultrashallow junctions.
INTRODUCTION Formation of highly-doped, ultrashallow junctions with low junction leakage is essential for future CMOS technology generations. As channel engineering techniques continue to advance, performance scaling requires comparable improvements in source/drain series resistance, while maintaining very shallow and low leakage junctions. Here we focus on understanding the similarities and differences in two key techniques for achieving this goal. P-channel MOS devices present particular difficulties owing to the tendency of boron to form electrically inactive interstitial clusters at high concentrations after implantation and annealing. To minimize this problem a variety of solutions have been attempted, including flash or laser annealing, with or without the use of preamorphization or molecular dopant ion implantation (which form amorphous layers prior to annealing). This paper looks at the physical processes of defect formation, dopant activation and diffusion that play a part in ultrashallow junction formation using these approaches. Traditionally, p-channel source-drain extensions have been created by ion implantation of Bcontaining ion species, either direct implantation of the doping ion into crystalline silicon, or pre-amorphization with heavy ions such as Ge to form an amorphous layer to limit boron channelling. Recently increasing emphasis has been placed on implantation of molecular species, because of the relatively high implantation energy that can be used even for ultrashallow junction formation, and because, empirically, reduced junction leakage is observed under appropriate annealing conditions [1] Since the benefits of pre-amorphisation arise from the fact that dopant is implanted into a ‘pre-fabricated’ amorphous layer in
crystalline silicon, it is interesting to consider the similarities and differences between (a) preamorphization and dopant implantation, (b) direct amorphization during implantation of a heavy molecular dopant species. By
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