Investigation of electron-shading effects during high-current ion implants

Charging characteristics of As⁺, BF₂⁺, and B⁺ high-current ion implants, performed at different energies and different plasma flood system settings, were measured using bare and resist-covered CHARM®-2 wafers patterned with a six-field mask containing holes ranging from 2 μm to 0.5 μm (clear and resist-covered fields were also used). The results show significant differences in the charging characteristics of high-current ion implanters compared to contemporary plasma-based process tools. The differences appear to be independent of ion energy, but depend on the set-up conditions of the plasma flood system used to limit positive charging caused by the ion beam. In contrast to plasma tools, the implants typically exhibited positive and negative potentials independent of hole size. The positive and negative current densities measured in the resist holes were also independent of hole size (and significantly higher than in the clear field). However, a 500 eV B⁺ implant with modern plasma flood control produced positive and negative potentials that scaled with hole size, as expected for electron shading, but with current densities below CHARM®-2 detection levels. This establishes an existence proof that optimal plasma flood can achieve near perfect current balance between the positive charging from the ion beam and the negative charging from the flood plasma. Altogether, these results suggest that charging damage in high-current ion implanters should be controllable when implant mask and device features are scaled down.