High power density switching solutions such as Si split-gate trench power MOSFETs require continuous performance improvement to follow the current trend towards digitalization and electrification. The device performance, however, is limited by intrinsic material properties since performance improvement solely due to conventional scaling is approaching a physical and economical limit. Yet the functional device properties strongly depend on the residual strain distributions in monocrystalline Si as a result of a strain-dependent charge carrier mobility. Strain engineering, therefore, offers an effective way to further improve the performance by reducing the on-state resistance RON without the need for additional device scaling. Nevertheless, stresses and strains in a trench power MOSFET as well as their influence on the functional properties of a split-gate trench power MOSFET are not fully investigated and understood.

This work reveals the interaction of mechanical strain distributions in a Si split-gate trench power MOSFET and its functional device properties. Therefore, procedures to accurately analyze residual stresses and strains on a nanoscale within the complex power MOSFET structure consisting of insulator, semiconductor and conductor multilayers with amorphous, single and polycrystalline structures are presented. The determination of residual stresses in polysilicon electrodes of a state-of-the-art split-gate trench power MOSFET was conducted by cross-sectional X-ray nanodiffraction. The results show geometry, grain size and doping dependent residual stresses, which are in the range of -65 to -95 MPa and -185 to -225 MPa in the horizontal and vertical direction, respectively, in the lower source electrode as well as 70 to 150 MPa for both directions in the upper gate electrode. The shear stresses are generally on a very low level, ranging from -40 to 20 MPa. Residual strain distributions in monocrystalline Si of the investigated power MOSFET were evaluated using transmission electron microscopy (TEM) nanobeam electron diffraction (NBED). Strain maps with a size of 1 µm2 were obtained with a resolution of 5.4 nm/pixel and a precision better than 0.05 %, revealing complex strain distributions that are dependent on structure and geometry of the transistor as well as the doping in Si. Furthermore, the residual stresses and strains acquired by physical measurements are compared to finite element (FE) simulation showing a good agreement.
In addition, a novel, fully integrated strain engineering concept is applied to a state-of-the-art split-gate trench power MOSFET which is able to withstand a blocking voltage of 25 V. The concept includes thermally grown SiO2 functional strain layers that are introduced into source and gate electrode by partial oxidation. The influence of the functional strain layers on the strain distributions in the adjacent monocrystalline Si is described by FE simulation and the resulting device properties are assessed by means of detailed electrical characterization. In the channel and drift region, the FE simulation shows stronger tensile strains in the direction of current flow and stronger compressive strains perpendicular to it, which positively affect the electron mobility. The RON values of the strain-modified devices are improved by up to 16.8 and 13.8 % at a gate voltage VG of 4.5 and 10 V, respectively, while the breakdown voltage VBD does not change. The threshold voltage VTH is reduced in the presence of a functional strain layer in the gate electrode, also contributing to the RON improvement. It is shown that the RON improvement increases with higher oxide share in the trenches and is generally at a higher level when the functional strain layer is introduced into the source electrode, mainly influencing the strain distribution in the drift region. Furthermore, the transfer and breakdown characteristics show low leakage currents and no parasitic behavior, indicating that the introduction of functional strain layers by partial oxidation of the poly-Si electrodes does not harm the device functionality.
In summary, this work shows that strain engineering enables a significant performance improvement of split-gate trench power MOSFETs as a result of an increased charge carrier mobility in monocrystalline Si. The combination of both the successful implementation of a novel strain engineering concept and the ability to map residual stresses and strains on a nanoscale allows for a deeper understanding of the interaction between electrical and mechanical properties of Si split-gate trench power MOSFETs, which will facilitate performance improvement of future device generations.