Brief Review of Strained Si Technology

Rapid growth in the study of implementing strained silicon to the channel has been

witnessed in the past several years. Historically, improvements on MOSFET’s

performance have been attained by shrinking device dimensions. However, the practical

benefit of scaling is compromised as physical and economic limits are being approached,

and novel solutions are being sought. The 2003 ITRS roadmap started to schedule the

mobility enhancement factor by stress controls. Strain improves MOSFET drive current

by altering the band structure of the channel and can therefore enhance performance

even at aggressively scaled channel lengths. Bi-axial and uni-axial strained silicon

technologies are promising for enhancement of CMOS performance [25~30]. For the

case of a silicon layer under bi-axial tensile strain, it is mainly implemented by the

lattice mismatch with an underling relaxed SiGe layer. Note that, to avoid the generation

of high amount of dislocations, thickness of the top strained Si layer must be thinner

than the critical thickness that depends on the Ge content of the underlying relaxed SiGe

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layer. In contrast, uni-axial strain can be engineered by modifying capping layer

deposition [9][31], shallow trench isolation [32][33], source/drain material [6],

silicidation [34], packing process [35], and so on. Furthermore, the behaviors of carrier

mobility under uni-axial strain depend on the strength of the strain and the orientation.

Electron and hole mobilities respond to the complex three-dimensional mechanical

stress in different, even opposite ways, as shown in Fig. 3.1.

Several different behaviors caused by bi-axial and uni-axial stress were reported,

such as the drop of mobility enhancement at high electric field and threshold voltage

shift. Bi-axial strain improves electron transport more than hole transport, and vice

versa for the perpendicular uni-axial strain. Uni-axial strain can be applied arbitrarily in

any direction relative to the carrier transport direction. Enhancements of carrier mobility

under bi-axial and uni-axial strain were induced by different factors and mechanisms.

For NMOSFETs, recent reports and theoretical calculations indicate that

strained-Si under bi-axial or uni-axial tension should exhibit a higher mobility than bulk

Si. The differences in electronic conduction due to bi-axial and uni-axial strain can be

explained by examining the splitting of the degeneracy at the conduction band edges.

The biaxial tensile strain induces splitting of degeneracy in the triangular potential well

of the MOS inversion layer. This splits the six-fold degenerate Si conduction band into a

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two-fold (Δ2) and a four-fold (Δ4) branches. The energy of sub-band in four-fold

valley is lower than that in two-fold valley under uni-axial tensile stress, opposing to

what occurs in the case of bi-axial tensile stress. The energy difference (ΔE) between

Δ2 and Δ4 sub-bands determines the total population of the bands. The enhancement

of uni-axial and bi-axial strained-Si caused by the splitting of conduction band can

suppress inter-valley phonon scattering [36]. At high electric field, even in bulk Si

NMOSFETs, the six-fold degeneracy is broken near the Si/SiO2 interface by the

confinement of carrier at surface (as shown in Fig. 3.2(a)). The two-fold sub-band is

also preferentially occupied at high gate bias. Most inversion electrons are expected to

reside in two-fold sub-band even for devices fabricated on bulk Si. In contrast, bi-axial

tension causes nearly 100% of inversion electrons to occupy the two-fold sub-band at

all gate biases. Note that the enhancement of bi-axial tension at high electrical field,

albeit not as high as that at low field, is still very significant [37].

To quantify the mobility enhancement of holes, changes in the scattering and

effective mass depend on the altered valence band caused by the strain. For both

bi-axial tensile and longitudinal uni-axial compressive stresses, the effective mass is

nearly constant over the surface energy range of a few kT below the valence band in

contrast to un-strained case. The constant effective mass results since strain removes the

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degeneracy and reduces the band-to-band coupling. From full-band Monte Carlo

simulation [38], uni-axial compressive strained MOSFETs may have lighter in-plane

effective mass thus improve hole mobility. For bi-axial tensile stress, the effective mass

is heavier than un-strained case. The hole mobility enhancement is only possible

through the reduction of inter-valley scattering [39]. This effect becomes significant

only when the strain level is high enough (e.g., Ge > 20 %). Reducing the intra-band

acoustic scattering by altering the light- and heavy-hole band density-of-states is

negligible for uni-axial strain in Si, even at several hundreds of mega-pascal. On the

other hand, the energy difference ΔEs (as shown in Fig. 3.2) between light-hole band

and heavy-hole band was split by uni-axial stress at gamma-point (k=0) and reduces the

optical phonon scattering (as shown in Fig. 3.2(b)). Significant scattering reduction

requires ΔEs > 60meV (optical phonon energy in Si) [13].

Hole mobility at high vertical field with uni-axial compressive and biaxial tensile

stresses would have different behaviors. Splitting of light- to heavy-hold band caused by

uni-axial and biaxial stresses has no significant difference without considering surface

quantization confinement. However, the splitting of light- and heavy-hole bands caused

by bi-axial tensile stress would be nullified at high electric field due to surface

confinement [40]. In contrast, hole mobility enhancement under uni-axial compressive

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strain is not nullified by surface confinement, which represents a major advantage for

MOSFETs operating at high electric fields. The splitting magnitude of the surface

confinement depends on the relative magnitude of the stress altered light and heavy hole

out-of-plane effective masses. Recent reports [41] showed the interesting result that the

out-of-plane effective mass of light hole is heavier than heavy hole for uni-axial stress

and causes the light to heavy hole band splitting to increase. On the contrary, for

bi-axial stress the previously-reported out-of-plane effective mass of light hole is lighter

than heavy hole and causes the band splitting to be reduced. This is why the bi-axial

stress degrades hole mobility enhancement at high vertical electric fields (as shown in

Fig. 3.2(c)).

The threshold voltage shift caused by bi-axial tensile stress is larger than the case

with uni-axial tensile strain has been reported for NMOSFETs [42]. For PMOSFETs,

larger shift of light-hole band edge under bi-axial tensile strain leads to larger shift in

Vth than the case with uni-axial compressive strain [41].