Strain Technology

With the scaling of the device size, performance improvement of CMOS devices

faces a number of obstacles. It is becoming more and more difficult to maintain high

transistor performance because of mobility degradation caused by the increase in

substrate doping. To address this issue, mobility enhancement technology is essential. In

order to realize high-speed performance, it is necessary to increase the carrier mobility

for devices with the gate length down to the sub-100-nm and below. Strain improves

MOSFET drive current by altering the band structure of the channel and can therefore

enhance performance even at aggressively scaled channel lengths [11-13].

MOSFETs with biaxial tensile channel stress by growing a Si channel layer on a

relaxed SiGe substrate has been demonstrated [14]. Drive current of both NMOSFET

and PMOSFET was enhanced by the biaxial tensile stress when Ge is incorporated by

more than 20% in the relaxed SiGe layer. It is noted that the thickness of the top

strained-Si layer must be thinner than the critical thickness which depends on the Ge

content of the underlying relaxed SiGe layer to avoid the generation of a large amount

of dislocations. However, the yield issue associated with high threading dislocation

density (typically > 10⁴ cm^-2) of the virtual SiGe substrates still represents a major

obstacle for practical applications. In addition, other concerns such as high Ge content

and up-diffusion, fast diffusion of n-type dopants, and expensive wafer cost further

blight the situation.

In contrast, uniaxial channel strain is free from the aforementioned concerns.

Uniaxial strain can be engineered by modifying contact-etch-stop-layer (CESL)

deposition [15], shallow trench isolation (STI) [16], source/drain (S/D) material [17],

silicidation [18], packing process [19], and so on. Furthermore, the behaviors of carrier

mobility under uniaxial strain depend on the strength of the strain and the orientation

[20]. Electron and hole mobilities respond to the complex three-dimensional mechanical

stress in different and even opposite ways. The channel tensile and compressive stress

can be applied separately to NMOS and PMOS devices to enhance performance,

respectively (as shown in Fig. 1.2). Depending on the CESL deposition conditions, the

SiN layer can generate either tensile or compressive stress [21]. The channel tensile and

compressive stress can be applied on NMOS and PMOS devices to enhance

performance, respectively [20].

The carrier mobility is given by _* m qτ

µ = , where1/τ is the scattering rate and m^* is

the conductivity effective mass. Strain enhances the mobility by reducing the

conductivity effective mass and/or the scattering rate. Both effective mass and scattering

rate changes are important for mobility enhancement in electrons [22]. However, only

effective mass change due to band warping and repopulation [23] plays a significant

role in holes. For electron transports in bulk Si, the conduction band is composed of six degenerate valleys (∆6) of the same energy. Strain removes the degeneracy between the

four in-plane valleys (∆4) and the two out-of-plane valleys (∆2) by splitting them in

energy. The energy difference (∆E) between ∆2 and ∆4 sub-bands determines the total

population of the bands. The enhancement caused by the splitting of conduction band

can suppress inter-valley phonon scattering [24]. The lower energy of the ∆2 valleys

indicates that they are preferentially occupied by electrons. The electron mobility is

improved partly by reducing in-plane and increasing out-of-plane effective mass due to the favorable mass of the ∆2 valleys, which results in more electrons with an in-plane

transverse effective mass and out-of-plane longitudinal mass.

For holes, the valence-band structure of Si is more complex than the

conduction-band structure. The complex band structure as well as valence-band warping

under strain results in a much larger mobility enhancement of holes than electrons.

These two factors also explain why strained-channel PMOSFETs is a key focus in

advanced logic technologies. Holes occupy the top two (the heavy- and light- hole)

bands for unstrained Si. With the application of strain, the hole effective mass becomes

highly anisotropic due to band warping, and the energy levels become mixtures of the

pure heavy, light, and split-off bands. Thus, the light and heavy hole bands lose their

meaning, and holes increasingly occupy the top band at higher strain due to the energy

splitting. To quantify the mobility enhancement of holes, changes of the scattering and

effective mass depend on the altered valence band caused by the strain. From full-band

Monte Carlo simulation [25], uniaxially compressive-strained PMOSFETs may have

lighter in-plane effective mass thus improve hole mobility. But, for biaxial tensile stress,

the effective mass is heavier than that in the unstrained case. Thus the hole mobility

enhancement is only possible through the reduction of inter-valley scattering [26]. 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 density-of-states of the light-

and heavy-hole bands is negligible for uniaxial strain in Si, even at several hundreds of

mega-pascal.

Hole mobility at high vertical field with uniaxial compressive and biaxial tensile

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

uniaxial and biaxial stresses has no significant difference without considering surface

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

by biaxial tensile stress would be nullified at high electric field due to surface

confinement [27]. In contrast, hole mobility enhancement under uniaxial compressive

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

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

depends on the relative magnitude of the stress-altered out-of-plane masses of the light

and heavy holes. Recent reports [23] showed the interesting result that the out-of-plane

effective mass of light hole is heavier than that of heavy hole for uniaxial stress, and

causes the increase in the splitting of light- to heavy-hole bands. On the contrary, for

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

than that of the heavy hole, leading to a reduced band splitting. This is why the biaxial

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

Fig. 1.3).

For NMOSFETs, it has been report that the threshold voltage shift caused by

biaxial tensile stress is larger than the case with uniaxial tensile strain [28]. For

PMOSFETs, larger shifts of light-hole band edge under biaxial tensile strain leads to

larger shift in Vth, compared with the case with uniaxial compressive strain [23].