Plasma-parameter dependence of thin-oxide damage from wafer charging during electron-cyclotron-resonance plasma processing

In this work, the effects of plasma-parameter variations on the charging damage of polysilicon-gate MOS capacitor test structures exposed to O₂ electron-cyclotron-resonance (ECR) plasmas are investigated. The results show that charging damage is generated when large potential differences exist across the gate-oxide layers of the MOS capacitor test structures and that these potential differences can only occur in the presence of plasma nonuniformities. These results demonstrate the critical need for plasma uniformity during processing, in particular as device dimensions shrink and gate-oxide thicknesses decrease. The plasma parameters were varied by adjusting the neutral gas pressure and by independently biasing a circular grid and a ring electrode located above the wafer. The damage induced in the test wafers during the plasma exposure was characterized with ramp-voltage breakdown measurements. Radial profiles of the floating potential measured with a Langmuir probe were found to vary nonuniformly when the grid electrode was positively biased due to preferential depletion of electrons relative to ions beneath the grid electrode. An equivalent-circuit model of the test wafer and the wafer-stage electrode predicts that the silicon substrate acquires a potential equal to the average of the wafer surface potential. Comparisons of the calculated profiles of the potential difference across the gate-oxide layers of the test structures and whole-wafer maps of the breakdown-voltage measurements show that the majority of the damage occurs where the oxide potential difference is largest and that the damage only occurs in the presence of plasma nonuniformities