Plasma Etching Charge-up Damage to Thin Oxides

Abstract

Plasma processes can cause dc current to flow through thin oxides, resulting in plasma-induced damage, which can be simulated as the response to a constant current electrical stress. Monitoring the density of interface traps through capacitance-voltage (CV) analysis is a more sensitive index of plasma-induced damage than testing for oxide breakdown. Oxide-charging current distributions for reactive ion etching (RIE) and magnetically enhanced RIE (MERIE) have been compared and explained. The charge is collected through the conductor surfaces, which are not covered by photoresist during plasma processing. The process sensitivity of charge-up in a MERIE system is reviewed. Little degradation occurs when extremely large capacitors are plasma etched with the photoresist mask in place, but the process sensitivity is magnified when the same capacitors are exposed to the plasma without a photoresist mask. For very thin oxides, the plasma behaves more like an imperfect current source than a voltage source. This is fortunate in terms of future devices because it means that the thinner oxides anticipated with such devices will be exposed to about the same current densities and not to increasing electric fields. Although a forming gas anneal can passivate many of the traps generated during plasma etching, latent damage remains, and subsequent electrical stressing causes roughly half of the passivated traps to be reactivated.