Ultralow Electron Temperature Plasma for Damage-Reduced Processing of High-k/Metal Gate Films toward Vertically Stacked Nanosheet Devices

In vertically stacked nanosheet devices, repeated plasma-processing steps are required to realize multiple threshold voltages (Vth), but such repeated exposure can accumulate plasma-induced damage (PID) in ultrathin high-k/metal gate (HKMG) stacks. Here, we demonstrate that ultralow electron temperature (ULET, Te < 1 eV) plasma generated using a DC-biased grid suppresses PID by controlling the electron temperature and ion energy distribution function (IEDF). Compared with conventional inductively coupled plasma processing at the same substrate bias (−100 V), ULET plasma reduces the mean ion energy from 131 to 101 eV and reduces the IEDF standard deviation by 44% (from 37.4 to 20.9 eV). Consequently, surface roughness decreases by up to 58%, fluorine incorporation decreases by up to 58%, and HfO2 Raman-band intensities are 1.32–1.81 times those under conventional ICP processing. These results establish electron-temperature control as a key parameter for minimizing PID in HKMG films relevant to vertically stacked nanosheet devices.