Effects of base layer thickness on reliability of CVD Si₃N₄ stack gate dielectrics

Effects of the base layer in Si₃N₄/SiON stack gate dielectrics, in particular, the physical thickness of the base layer, on the dielectric reliability, MOSFET performance and process controllability are investigated. It is found that the electrical characteristics such as TDDB lifetime as well as the Si₃N₄ film property in Si₃N₄/SiON stack dielectrics with the same capacitance oxide equivalent thickness strongly depend on the SiON-base layer thickness. From the TDDB measurements for both stress polarities and from the Si₃N₄ stoichiometry by the X-ray photoelectron spectroscopy analysis, the optimum SiON-base layer thickness is determined to be approximately 1 nm, in order to obtain longer TDDB lifetime and surperior n-ch MOSFET performance. The obtained results are considered to attribute to the nitrogen profile in the Si₃N₄/SiON stack dielectrics and the strained layer thickness near SiON/Si interface.