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Residual stresses in poly-Si electrodes of a state-of-the-art Si trench power MOSFET were characterised using cross- sectional X-ray nanodiffraction (CS-nanoXRD) at beamline ID13 (Figure 139). An incident X-ray beam with a photon energy of 13.0 keV and a diameter of 200 nm was used to scan 42 transistors with a step size of 100 nm. The data were subsequently averaged and showed residual stresses in the range of -65 to -95 MPa for the vertical stress component sx and -185 to -225 MPa for the lateral stress component sy in the lower electrode, while the upper electrode exhibited tensile stresses between 90 and 160 MPa for both directions.
The stress distributions correlate well with the electrodes dimensions, which effectively predetermine the maximum grain size reached during crystallisation after their deposition in the amorphous state and, additionally, indicate a stress dependency on silicon doping. In summary, CS-nanoXRD was shown to be a powerful tool for nanoscale characterisation of the mechanical state in poly-Si electrodes of modern power MOSFETs.
Fig. 139: a) Cross-section of the investigated Si trench power MOSFET. b) CS-nanoXRD phase map of averaged diffraction intensities (green = poly-Si, blue = W, red = Ti/TiN). c) CS-nanoXRD residual stress distributions compared to (d) finite element simulation.
PRINCIPAL PUBLICATION AND AUTHORS
X-ray nanodiffraction analysis of residual stresses in polysilicon electrodes of vertical power transistors, S. Karner (a,c), O. Blank (a), M. Rösch (a), M. Burghammer (b), J. Zalesak (c), J. Keckes (c), J. Todt (c), Materialia 24, 101484 (2022); https:/doi.org/10.1016/j.mtla.2022.101484 (a) Infineon Technologies Austria AG, Villach (Austria) (b) ESRF (c) Department of Materials Science, Montanuniversität Leoben, Leoben (Austria)
 P. Chidambaram et al., IEEE Trans. Electron Devices 53, 5, 1010-1020 (2006).  H. Miura et al., Appl. Phys. Lett. 60, 22, 2746-2748 (1992).
X-ray nanodiffraction analysis of residual stresses in polysilicon electrodes of vertical power transistors
The influence of residual stress concentrations on the mechanical stability and functional properties of vertical power transistors is not fully understood. X-ray nanodiffraction and finite element (FE) modelling were used to analyse residual stresses in two polycrystalline Si electrodes.
The strain distributions in modern silicon (Si) trench power MOSFETs decisively influence the mechanical stability and the functional transistor properties . During the fabrication of polycrystalline Si (poly-Si) electrodes inside the trench structure, a high amount of residual stress is built up due to the crystallisation of amorphous Si . Since the magnitude and multiaxiality of the stresses significantly differs from those in planar layers, it is of high interest to quantify nanoscopic residual stress distributions across the poly-Si electrodes and understand their correlation with electric properties.