Mechanism of Improved Thermal Stability of B in Poly-SiGe Gate on SiON

Post-annealing characteristics of in situ B-doped poly-SiGe films have been investigated. Thermal stability of substitutional B atoms at a supersaturated concentration was significantly improved by Ge doping, for example, the stability at 800°C for poly-Si0.6Ge0.4 films was nine times as high as that for poly-Si films. Both fast and slow processes exist for the deactivation of B. The fast process was due to movement of B atoms from substitutional to interstitial sites, enhanced by a local strain induced by the difference in atomic radii between Si and B atoms, and the slow process was due to trapping of B at grain boundaries during grain growth. The mechanism of the improved thermal stability of B atoms is discussed on the basis of local strain compensation by Ge atoms.