The role of mask charging in profile evolution and gate oxide degradation

Through detailed numerical simulations we investigate the role of insulator mask thickness in altering the fidelity of pattern transfer and causing damage to buried gate oxides during plasma etching. While a certain scaling with the mask aspect ratio is known to exist, we find that the mask thickness changes the contact time of ions with the local electric fields, which can perturb the ion trajectories leading to sidewall bowing and microtrenching. For very thick masks, the simulations reveal an ion focusing effect due to significant positive charging of the mask sidewalls which could lead to rounded profiles. The ion flux to the trench bottom is reduced with a concomitant decrease in charging damage, as suggested by the drop in net current to a buried gate electrically connected to the etched structure.