Low-energy ion bombardment of silicon dioxide films on silicon

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Abstract
Exposure of SiO2 layers on silicon to a plasma environment during sputtering or backsputtering leads to degradation of the electrical properties of the interface. In this paper the effects of the ion bombardment occurring during such exposure are detailed. A low-energy ion-bombardment apparatus, capable of giving ion beams ranging from 100 nA at 10 eV to 5 μA at 3.0 keV of mass-analyzed gas ions, constructed for this study, is described. Bombardment of SiO2 layers by Ar+, N+, or N2+ ions of any energy from 10 to 2800 eV leads to increase in interface state density and oxide interface charge. The oxide charge after bombardment is located both in the region of the Si/SiO2 interface and in the outside 50 Å of the SiO2. Both induced oxide interface charge and interface state density increase linearly with dose to a saturation value, typically to 8×1012 charges/cm2 and 1013 interface states/cm2 eV at midgap after a dose of 8×1013 ions/cm2 at 600 eV, on 1000Å of SiO2. At constant dose the induced charge and interface state density increase linearly with energy to approximately 500 eV, then are independent of energy to the maximum energy studied. After ion bombardment to a dose of ions of 5×1013/cm2 or greater, SiO2 films exhibit a significant reduction in dielectric strength. This reduction is not completely recoverable by annealing to temperatures up to 600°C. Annealing to 600°C does, however, remove all interface charge, interface states, and room-temperature trapping instabilities produced by the bombardment, as measured at room temperature after annealing. These experiments show that ion bombardment is capable of producing MOS degradation similar to some of the MOS degradation observed after sputter deposition of metals over oxide films. The increase in charge and interface states is a function of dose, so adjustment of plasma parameters during sputtering to give minimum ion bombardment should minimize this buildup of charge.