Origin of MeV ion irradiation-induced stress changes in SiO2

The 4 MeV Xe ion irradiation of a thin thermally grown SiO2 film on a Si su bstrate leads to four different effects in which each manifests itself by a characteristic change in the mechanical stress state of the film: densific ation, ascribed to a beam-induced structural change in the silica network; stress relaxation by radiation-enhanced plastic flow; anisotropic expansion and stress generation; and transient stress relaxation ascribed to the ann ealing of point defects. Using sensitive wafer-curvature measurements, in s itu measurements of the in-plane mechanical stress were made during and aft er ion irradiation at various temperatures in the range from 95 to 575 K, i n order to study the magnitude of these effects, the mechanism behind them, as well as their interplay. It is found that the structural transformation leads to a state with an equilibrium density that is 1.7%-3.2% higher than the initial state, depending on the irradiation temperature. Due to the co nstraint imposed by the substrate, this transformation causes a tensile in- plane stress in the oxide film. This stress is relaxed by plastic flow, lea ding to densification of the film. The anisotropic strain-generation rate d ecreases linearly with temperature from (2.5 +/- 0.4)x10(-17) cm(2)/ion at 95 K to (-0.9 +/- 0.7)x10(-17) cm(2)/ion at 575 K. The spectrum of irradiat ion-induced point defects, measured from the stress change after the ion be am was switched off, peaks below 0.23 eV and extends up to 0.80 eV. All fou r irradiation-induced effects can be described using a thermal spike model. (C) 2000 American Institute of Physics. [S0021-8979(00)00513-2].