Nickel monosilicide (NiSi) has been well recognized as a promising silicide for future ULSI devices. It can have low resistivity of 15 mu Omega cm on deep submicron lines and after high temperature (> 600 degrees C) annealing. This work studies the resistivity and thermal stability of thin NiSi layer on B, As, P-doped and in situ boron doped deep submicron polycrystalline silicon (poly-Si) lines after 500-800 degrees C/1 h of annealing. The stability of NiSi on crystalline silicon (c-Si), poly-Si and amorphous silicon (a-Si) film will also be studied.