Analysis of injection current through thin gate oxide during metal etch

A charging damage mode due to “latent antenna” formation for high aspect ratio structures in a metal etch process was investigated by a new surface charging simulation technique. The latent antenna is formed at the end of the metal etch process because of the microloading effect, and is connected to a transistor gate with 35 Å thickness. By using a new simulation algorithm, the plasma stress current current–voltage (I–V) characteristic caused by topography dependent charging was calculated for different ranges of the injection current to the antenna structure. Subsequent gate injection current through the thin gate oxide was estimated by the plasma I–V with a measured holding current I–V characteristic especially optimized for high current injection. The plasma I–V was calculated for the latent antenna and a mask-defined structure (overetch structure), which are formed during an overetch period. The steady state gate injection current is largest during the latent antenna formation. Because of a device degradation characteristic that is mostly controlled by an initial gate injection charge, the high injection current for the latent antenna defines the degree of damage, and the observed degradation independence of the overetch time is well explained by the results. Since the initial gate injection charge depends on the period of latent antenna formation, damage increases as the etching rate decreases at the narrow space region. The injection current gets smaller with a smaller antenna ratio, and antenna structures without high aspect from the gate injection current. For the analysis of thin gate oxide degradation, the injection current is more important than the charging voltage.