Abstract
As the CMOS transistor scaling is approaching its physical limits, the semiconductor industry is forced to seek an alternative approach to conventional methods of device manufacturing. One of the key requirements, which sets the performance characteristics of p-channel MOSFET device is the fabrication of p-type source and drain extension regions. These regions are required to be as shallow as possible whilst containing the maximum number of electrically active dopant atoms. The current methods of achieving this include pre-amorphisation implantation (PAI) which is used to disorder the crystal structure in the near surface region, typically with germanium ions, followed by low energy boron implant. The amorphous region is then transformed into a near perfect crystal, via solid phase epitaxial re-growth process (SPER). As a result, boron becomes substitutionally incorporated into the lattice. The problem associated with this approach is the fact that, after SPER, excess silicon interstitials, generated as a result of amorphisation remain in material just below the former amorphous/crystalline interface and condense into clusters. These clusters transform into End of Range (EOR) defects. The EOR defect dissolve during subsequent annealing releasing silicon interstitials which migrate into the bulk of the material and towards the surface, interacting with boron atoms which leads to Transient Enhanced Diffusion (TED) and formation of Boron-Interstitial Clusters (BICs). These interactions result in increased junction depth and reduction in electrically active dopant respectively, where any remaining defects can act as conduction paths through the junction which retards transistor performance. In order to overcome these problems novel approaches to materials and doping species are currently being researched. This dissertation examines the use of alternatives to monomer boron implants into bulk silicon. It is demonstrated that detrimental effects of using PAI can be avoided by using self amorphising BF2 implant, where the presence of fluorine prevents the silicon interstitials from reaching the boron profile. The self-amorphising molecular B18H22 implant, which allows eliminating the PAI step, shows high electrical activation and thermal stability. Implants into Silicon-Germanium (SiGe) compound substrate show improvements in terms of diffusion. The compressive stress present in SiGe promotes higher electrical activation. Optimisation of the use of SiGe substrate in conjunction with B18H22 implants can prove to be a viable choice for fabrication of future devices.