The Strain Characteristics Of Strained Si MOS Devices

Recently, Strained Si technology has attracted wide attention because of improving significantly the MOS devices performances. For example, the hole mobility in PMOS and the electron mobility in NMOS can be significantly enhanced by introducing appropriate compressive and tensile channel stresses, respectively. Thus, through the optimization design of the process,material and structural parameters, investigating the influence of stress and strain in semiconductor nano-devices has important scientific significance and practical value.The accurate measurements of local micro-stress and strain in ultra deep sub-micron semiconductor structures usually resort to complicated microstructure analysis, measurement methods. Therefore, this paper explored to use the finite element analysis tool ANSYS to research stress and strain distributions and influencing factors for a uniaxial strained Si MOS device with typical SiGe source-drain structure and a biaxial strained Si MOS device with strained Si/SiGe heterostructure, respectively. Firstly there introduced the relationship between stress and strain, and found them only related to the material Young's modulus and Poisson's ratio, verified the feasibility by using ANSYS. By compared the simulation results in the uniaxial strained Si MOS device obtained by ANSYS and the experimental data measured by the convergent beam electron diffraction (CBED), there found them fit very well, also verified the reliability by using ANSYS.According to the differences of Young's modulus and the virtual thermal expansion coefficient of SiGe structure, this paper established a two-dimensional model of uniaxial SiGe source-drain MOS devices, and then increased a uniform temperature 1000℃, to simulate stress and strain caused by the lattice mismatch. The strain in the simulation results charts hierarchically distributed, can well explained the distribution of strain in the device channels. And then this paper respectively simulated Ge composition, source and drain spacing, source and drain etch depth and the elevated height on the impact of the device, mapping out their strain curve trends, found that higher Ge composition, smaller source and drain spacing, deeper etch depth and higher elevation height can effectively raised the channel strain.Similarly, using finite element method, this paper analyzed the distributions of strain in a biaxial strained MOS device with the Si/SiGe structure, established a two-dimensional model, used the same principle of the uniaxial device, analyzed Ge composition, thickness of strained Si layer, thickness of relaxed SiGe layer and device width on the impact of the device, mapping out their strain curve trends, found that higher Ge composition, smaller thickness of strained Si layer, larger thickness of relaxed SiGe layer, smaller device width can effectively improved the strain in strained Si layer.