Radiation-Induced Defects in SiO2 as Determined with XPS

Device quality gate oxides (~ 850 Å) grown on Si (100) substrates are irradiated with 0 - 20 eV electrons during in situ XPS measurements. These structures have been thinned stepwise to 25 to 60 Å using a relatively benign wet-chemical depth-profiling procedure. An analytical method based on oxide/substrate intensity ratios is used to deduce the product of the atomic number density (D) and the electron mean free path (λ) as a function of depth for these structures. Samples showing a wide variety of hole trapping efficiencies were examined. Si+3 species are formed in the region of the Si/SiO2 interface and are observed during the course of their relaxation and annihilation. These formation results are correlated with the presence of strained Si-O-Si bonds at the interfaces. Radiation hard and soft structures show different strained bond distributions in the interfacial region. The direct observation of bond cleavage and bond strain gradients in these samples is used to extend silica devitrification models to explain the generation of fixed oxide charge and interface states. This bond strain gradient (BSG) model is shown to be consistent with a variety of experimental EPR and electrical observations of hole- and electron-trap generation by ionizing radiation.