Strained silicon is a layer ofsilicon whose atoms interatomic distance is stretched beyond their originaldistance and used in channel.
This is achieved by superimposing the siliconlayer on Silicon Germanium (SiGe) substrate. Fig 2 shows the normal silicon vsstrained silicon over SiGe. The silicon atoms get aligned with the underlyingSiGe atoms, which are place little farther apart when compared to Si. Thisstretched strained silicon atoms interatomic space is increased and hence theinteratomic force reduces thereby increasing the easy movement of electrons.This increases the mobility of electrons resulting in better chip performanceand lower power consumption 2. These electrons can move 70% faster allowing strainedsilicon to switch 35% faster 24.Thistechnique has become an integral part of the International Technology Roadmapfor Semiconductors (ITRS), starting at 90nm technology node 24. Strainedsilicon was implemented in Intel’s 90nm technology in 2003 11.
There are twobasic approaches to introduce strain into transistor channel: global and localapproach.Globalstrain is the type where the stress is induced in the channel of the MOSFET.This is done by epitaxially growing a SiGe buffer layer above the siliconsubstrate as shown in Fig. 3. Now, silicon is grown above the SiGe layer whosecrystalline lattice is slightly larger than the crystalline lattice structureof silicon. Silicon grown above the SiGe layer is stretched which inducesbiaxial strain to provide increased spacing between Si atoms 4% more thannormal Si atoms.
The strain produced causes change in the energy bands for bothelectrons and holes. This results in increased mobility of the carriers and increasesthe drive current to provide improved performance 25. Localstrain is introduced by replacing the conventional silicon source and drainwith SiGe source and drain as shown in Fig.
4. This is achieved by etching thesilicon from the source and drain regions selectively and growing SiGeepitaxially over it. As said earlier than SiGe cystal lattice is slightlylarger than Si, the channel region between SiGe source and drain experiencesuniaxial compressive stress. This uniaxial compressive stress enhances themobility of holes and thus results in improved performance of 65% drain currentin PMOS 25. This type of strain is a real boost for PMOS since theirperformance is always on the lower side when compared to NMOS. Therefore, thislocal strain very advantageous for PMOS.