Strained Si NMOSFET on relaxed Si 1-x Ge x formed by ion implantation of Ge

Abstract

Strained-Si films show considerably higher electron mobility than conventional silicon films which results in increased transconductance and drive current of strained-Si channel NMOSFETs. However, in order to form tensile strained silicon it is necessary to use relaxed Si<SUB>1-x</SUB>Ge<SUB>x</SUB> 'substrates,' typically requiring the growth of several microns of a graded Si<SUB>1-x</SUB>Ge<SUB>x</SUB> layer, followed by a buffer layer. In this work, we have used ion implantation of Ge followed by high- temperature annealing to form a relaxed substrate, eliminating the growth of graded, relaxed layers, and simplifying the fabrication process. Upon this film, a 1000 angstrom buffer layer of Si<SUB>0.85</SUB>Ge<SUB>0.15</SUB> was grown. X-ray analysis indicates that the films formed by this method are 75% relaxed. This was followed by a 200 angstrom thick strained-Si layer. For comparison, unstrained Si epitaxial films and a 2000 angstrom thick film of Si<SUB>0.85</SUB>Ge<SUB>0.15</SUB> (on unimplanted Si) followed by 200 angstrom of Si were used. A typical self-aligned MOS process with modifications to achieve low-thermal budget was used to fabricate NMOSFETs with gate lengths ranging from 10 micrometer to 0.8 micrometer. Strained-Si devices show a 17.5% higher peak linear g<SUB>m</SUB> than control devices as a result of higher electron mobility in the strained-Si channel.