Abstract

Starting with the introduction of the 90 nm CMOS technology node many important microprocessor manufacturers improve the transport characteristics of silicon using techniques that induce strain in the transistor channel. The most common techniques that produce strain globally or locally on the wafer are being reviewed in this work.

While most theoretical work has been performed for biaxially strained Si with {001} substrate orientation, a thorough theoretical analysis of electron mobility enhancement in arbitrarily strained Si is missing. In this work the effect of a general homogeneous strain - described by the strain tensor $\hat{\varepsilon}$ - on the transport in bulk Si and Si inversion layers is analyzed. The band structure of strained Si is calculated numerically using the empirical pseudopotential method with nonlocal and spin-orbit corrections. The results of band structure calculations reveal that shear strain changes the effective electron masses in addition to the splitting of the six $\Delta$-valleys. The effective mass change can be attributed to the lifting of the degeneracy of the two lowest conduction bands at the X-point due to shear strain. Using the kp method analytical expressions for the effective mass change are derived.

The transport properties of strained Si are investigated by solving the semiclassical Boltzmann equation using the Monte Carlo (MC) method employing fullband and analytical band models. The low-field electron mobility resulting from MC simulations using an analytical description of the electron bands and the fullband description coincide only for not too high shear strain (<0.5%). At larger shear strain the band deformation is so pronounced, that fullband modeling is required.

The hybrid-orientation technology combines different silicon substrate orientations and channel directions on the same wafer and can be used in conjunction with strain techniques. Since strain yields an anisotropic mobility, the proper channel direction and substrate orientation have to be chosen to obtain the maximum mobility enhancement. The effect of strain on the inversion layer mobility of electrons is investigated by calculating the subband structure using a self-consistent Schrödinger-Poisson solver. In the two-dimensional electron gas shear strain results in a shift of the subband ladders and a change of the effective electron masses. From simulations of the effective electron mobility in ultra-thin-body MOSFETs it can be concluded that the change of the effective masses is the dominant effect leading to mobility enhancement, since the strong geometrical confinement yields a large intrinsic splitting of the subband ladders, such that an additional splitting induced by strain has only a little effect.

The effect of degeneracy both on the phonon-limited mobility and the effective mobility including surface-roughness scattering is studied using a new MC algorithm developed in this work. By comparison with results from MC simulations where the Pauli exclusion principle is neglected, it is shown that a correct treatment of degenerate carrier statistics of the 2DEG of Si inversion layers is important.
E. Ungersboeck: Advanced Modelling Aspects of Modern Strained CMOS Technology