Global Strain

Global strain on wafer level is mostly induced by the epitaxial growth of Si1-x Gex and Si layers. Because the lattice parameter of Si1-x Gex (0 ≤ x ≤ 1) alloys varies between 0.5431 nm for silicon (x = 0) and 0.5657 nm for germanium (x = 1) tensile strain is induced in a silicon layer epitaxially grown on top of the SiGe. The strain is generally biaxial. Furthermore, uniaxial strained layers are also obtained by mechanical straining.

Biaxially strained layers. Figure 1 illustrates various heterostructure substrates that have been applied to biaxial strain and high-mobility channel materials. Epitaxially grown Si1-x Gex layers on Si bulk wafers are generally applied acting as substrate for a strained silicon layer grown on top (bulk materials). In order to reduce the defect density in the strained silicon a relaxed Si1-x Gex buffer is required grown on a graded Si1-x Gex layer (Fig. 2a). Because the Ge concentration x increases continuously by about 10 % per µm, the thickness of the graded buffer is several micrometers [1, 2]. An alternative is the relaxation of a thin pseudomorphic SiGe layer (< 500nm) induced by hydrogen or helium implantation and subsequent annealing [3, 4]. Thinner SiGe buffer makes the process costs effective. Variations of the basic structure (Fig. 2a) have been also published including dual channel structures incorporating an additional strained Si1-y Gey layer with y > x (Fig. 1b) and heterostructures on bulk using a second strained silicon layer (Fig. 2c) [9].

The realization of SSOI wafers from bulk materials is a complex process combining wafer bonding, layer transfer, and etch-back methods. The SSOI technologies provide a pathway to implementing mobility enhancement in partially or fully depleted devices, in ultrathin-body MOSFETs, or nonplanar (double-gate) MOSFETs. Mobility enhancement in SSOI was reported in [5] and [6] for the different SSOI configurations. Furthermore, long channel devices (Lg ≥ 1µm) show clearly improvements of the device characteristics. For instance, drive current (IDSAT) improvements of 80% at the same source-to-drain leakage (IOFF) has been measured. The combination of biaxially strained SSOI and optimized uniaxial stressors (dual-stress nitride capping layer and embedded SiGe) was already demonstrated resulting in IDSAT improvements of 27% and 36% for n-channel MOSFETs and p-channel MOSFETs, respectively, in sub- 40 nm devices [7]. In addition, the gate leakage current was also reduced by 30%. All investigations suggest that the combination of biaxially strained SSOI and uniaxial strain by process-induced stressors is the optimum way for future requirements.


Fig. 1: Schematic illustration of various heterostructure substrates produced by epitaxial growth on bulk substrates (bulk materials) and by transfer of the strained layers to oxidized substrates (strained silicon on insulator (SSOI), strained Si/SiGe on insulator (SGOI)) [5,6].


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