Nanobeam electron diffraction strain mapping in monocrystalline silicon of modern trench power MOSFETs

Despite a decisive influence of residual strains in Si on its electronic properties and mechanical stability, strain distributions in vertical power transistors are still not fully investigated and understood. In this work, transmission electron microscopy (TEM) nanobeam electron diffraction (NBED) and finite element (FE) modeling were applied to reveal cross-sectional residual strain distributions in monocrystalline Si of a modern trench power MOSFET. Scanning NBED was realized in a region of interest with lateral and vertical dimensions of 0.69 μm and 1.38 μm, respectively, with a spatial resolution of 5.4 nm and a high precision better than 0.05%. The acquired results are interpreted in terms of device structure as well as process flow and are compared with the FE simulation. Complex strain distributions for the vertical, lateral and shear strain components are shown, which are not only dependent on structure and geometry but also influenced by the doping profile in Si. The comparison with the FE strain simulation shows good agreement especially for the vertical and shear strain components. However, it also reveals a limitation of the FE strain simulation with regard to the influence of doping on the Si lattice strain.