The use of numerical simulation to predict the unlocking stress of dislocations in Cz-silicon wafers

Under certain conditions, interstitial oxygen atoms in Czochralski-grown silicon (Cz-Si) are known to hinder or completely stop dislocation motion. As a result, oxygen impurities can remarkably improve the mechanical strength of silicon wafers as they are transported and bound to dislocations. The amount of oxygen bound to dislocations—and with it the wafer’s resistance to plastic deformation—is oxygen concentration, time, temperature and, importantly, thermal history dependent. It is also reversible. A numerical model has been developed to predict the shear stress necessary to move glide dislocations in Cz-Si wafers during the course (time evolution) of different heat treatments and sequences of heat treatments typical of integrated circuit fabrication. This model accurately accounts for the experimentally observed behaviour of isolated straight dislocations over a wide range of controlled conditions. Modifications to heat treatments can be predicted by using this numerical simulation so that wafer warpage can be minimised during device processing.