2.4 Recent Developments

The strained Si technologies discussed so far assumed the standard (100) Si substrate for improving the carrier mobilities. Since the carrier mobilities are dependent on the crystal orientation of the substrate as well as the direction of the channel [Momose02], the mobilities can be further enhanced by adopting different substrate orientations and channel directions. Maximum benefit in CMOS performance can be drawn when the nMOS and pMOS transistors are grown on (100) and (110) substrate orientations, respectively, with [110] as the channel direction. Combining the benefits of this hybrid orientation technology (HOT) on SOI with the stress induced from processing steps, significant improvements in pMOS mobility has been reported [Yang06].

Another promising strained Si technology suggested recently [Issacson06] is strained Si on Si (SSOS). In this technique, a strained Si layer is formed directly on relaxed Si without any intermediate SiGe or oxide layers. Due to the removal of the oxide layer from the process, the thermal conductivity near the active regions of the device is significantly improved. The strain here is created purely from the homochemical heterojunction between strained Si and unstrained Si and not due to any compositional changes. back