Silicides and germanides for nano-CMOS applications

An overview of silicides and germanides for nano-CMOS applications is presented. The historical evolution describing the migration from the use of Ti silicide to Co silicide to Ni silicide as contacting material is first discussed. These changes in silicide material were mainly motivated by the inability to form the target low resistivity silicide phase in small structures due to low nucleation density. This issue was found first for the low resistivity C54 TiSi2 at linewidths below 200 nm and later for the low resistivity COSi2, at linewidths below 40 nm. A detailed description of scalability and thermal stability issues for NiSi is then presented. No nucleation issues were found in small structures for NiSi, which grows by diffusion or interface limited kinetics with Ni as main moving species. However, silicidation can be excessive in small structures due to Ni diffusion from surrounding areas, resulting in thicker films than targeted in small devices. This can be controlled by using a silicidation process with two rapid thermal processing steps, the first one to control the amount of Ni reacted and the second one to convert the silicide to the target low resistivity monosilicide phase. One of the main issues for applications of NiSi is its low thermal stability: thin NiSi films agglomerate at relatively low temperatures. The process window and thermal stability of Ni and Pt-based films reacted with Si. Si:Ge and Si:C substrates is reviewed. Addition of Ge is shown to degrade thermal stability while addition of C or Pt improves it. Contact resistivity considerations and implementation of dual band-edge silicides are discussed, as well as promising results for the extension of Ni-based silicides to future nodes. Finally a brief overview of germanides is presented discussing NiGe and PdGe as main candidates. (C) 2008 Elsevier B.V. All rights reserved.