Optimization of multilayer thin film passivation processes for improving cache memory device performance

A multilayer thin film passivation structure based on alternating plasma-en hanced chemical vapor deposited (PECVD) SiOx-SiNx layers are prepared and c haracterized, in order to improve and optimize the electrical performance a nd hot carrier reliability of polyload resistors in 4-transistor (4-T) cach e static random access memory devices. SiH4-N2O gas mixtures are utilized a s precursors for oxide CVD process. Adopting a higher SiH4/N2O flow rate ra tio during deposition renders the resulting oxide films more silicon rich, as manifested by their higher refractive index (RI) and wet etch rates. The se modifications in film characteristics are also accompanied by enhanced r esistance of polyload resistor and lower percentage hot-carrier linear drai n current (I-dlin) degradation. An increase in RI from 1.46 to 1.67 transla tes to a rise in resistance of polyload resistor from 98 to 225 G Omega and a fall in I-dlin from 5.8 to 4.5%. Further improvement in device performan ce can be realized by modifying the stoichiometry of the overlying nitride passivation layer. This is achieved by increasing bias power while reducing the SiH4/NH3 gas flow rate ratio during the PECVD nitride deposition proce ss. The nitride thus thus deposited contain lower Si-H bond density, and ex hibit lower buffered oxide etch rates and compressive stress. Passivation s tructures based on the combination of a high RI oxide and a low Si-H conten t nitride layers yield the most promising device performance and reliabilit y. Defect species in the oxide passivation layer are identified and their c harge trapping mechanisms clarified. Impact of moisture and hydrogen from t he passivation on polygate and load resistor are both held responsible for device degradation. Interfacial defect reactions involving both hydrogen an d moisture are proposed to account for the carrier trapping mechanisms resp onsible for device failure. (C) 1999 The Electrochemical Society. S0013-465 1(98)02-078-3. All rights reserved.