28nm Key Process Defect Inspection And Yield Enhancement

The tolerance of the defects becomes very small as the continuous shrinkage of the geometric size of the device for various applications involving new processes,new materials and new equipments.Defects from those different process sources could result in end-of-line yield failure.Traditionally,optical inspection system including bright-field and dark-field inspection is the prior choice for defect detection and monitoring,while the electron-beam inspection(EBI)system plays as the assistant scheme.However,the scenario goes in opposite way while semiconductor technology entered 28 nanometer scale.E-beam inspection changed to be the indispensable approach during the development and research of 28 nm product,because of optical inspection's limitation.Three types of defects in 28 nm key process were discovered and investigated in this paper including contact(CT)bottom tungsten missing(W missing),Nickel Silicide(NiSi)poor formation and extreme tiny oxide(OX)residue on shallow trench isolation(STI).Mechanisms of those detection methods,with proper electron charging and discharging rate to increase resistance voltage contrast(RVC)signal of W missing,material voltage contrast(MVC)signal of NiSi poor formation and topography voltage contrast(TVC)signal of OX residue,were also studied.With these defect scan methods,inline indexes were setup which made it fast to perform defect failure model analysis and reduction.Accordingly,a series of defect reduction actions,including the optimization of post etch treatment(PET),enlargement of CT CD and improving the micro-environment of wafer surface for W missing,shrinking poly line CD from 49 nm to 44 nm and reduce SPACER thickness from 310 to 270 angstrom for NiSi poor formation,and the optimization of STI chemical mechanical polish(CMP)process with a new slurry for OX residue,were carried out.Furthermore,a new approach of E-beam inspection with bare Silicon wafer post NiSi formation was developed to enhance the sensitivity and shorten the cycle time of defect reduction experiments.Subsequently,as the reduction of these key process defects,the yield of 28 nm product increased rapidly.