OBSERVATION OF MICROSCOPIC NONUNIFORMITY DURING OVERETCH IN POLYSILICON GATE ETCHING

Microscopic uniformity during overetch has been investigated for polys ilicon gate etching in electron cyclotron resonance Cl-2/O-2 plasmas, using atomic force microscopy as well as scanning electron microscopy. As the O-2 percentage was increased in Cl-2/O-2 plasmas, the aspect-r atio dependence of the poly-Si etch rate changed from a weak reactive- ion-etching (RIE) lag to a slightly inverse RIE lag; on the other hand , the aspect-ratio dependence of the SiO2 etch rate exhibited a strong ly inverse RIE lag at high O-2 percentages. As a result, the microscop ic etch selectivity of poly-Si over SiO2 was considerably degraded wit h increasing aspect ratio, giving rise to gate oxides broken in large aspect-ratio features during overetch. These results are indicative of the transport of incoming oxygen atoms in microstructures limited by neutral shadowing.