Characterization of highly selective SiO2/Si3N4 etching of high-aspect-ratio holes

The pattern size dependence of SiO2 and Si3N4 etch rates of contact holes (RIE-lag) in C4F8 + CO plasma was studied. It was found that these etch rates can be characterized by the aspect ratio, regardless of the pattern size. SiO2 etch rate decreased with increasing aspect ratio and became 0 at an aspect ratio of 6. Si3N4 etch rate also decreased; however, etching still occurred at an aspect ratio of 30. From ion current measurements through capillary plates (CPs), it was deduced that etch rates decreased because of decreasing ion current. XPS analyses revealed that fluorocarbon film deposited on the Si3N4 surface at the bottom of a hole was more F-rich than that deposited on a flat Si3N4 surface. This explained why Si3N4 is etched even in high-aspect-ratio holes. A small amount of O-2 addition to the C4F8 + CO plasma resolved the RIE-lag. It was found that the ion current density at high aspect ratio increased with O-2 addition, which would enhance SiO2 etching and contribute to suppressing RIE-lag.