We have investigated the thermal stability of TiN/HfSiON gate stack structures using synchrotron-radiation photoemission spectroscopy. Spectral intensities of the Si-oxide components in Si 2p core-level spectra systematically increase with annealing temperature, which strongly depends on the thickness of the TiN metal gate layer. Changes brought by annealing procedures in depth profiles of atomic concentration indicate segregation of Si-atoms at the TiN surface. Furthermore, chemical-state-resolved depth analyses by angle-resolved photoemission spectroscopy suggest formation of TiSi x and HfNy components due to chemical bond breaking in the HfSiON layer during TiN film growth. This can be related to the degradation of thermal stability.