Observation of surface reaction layers formed in highly selective SiO2 etching

Abstract

We characterized the surface reaction layers formed by a fluorocarbon plasma for SiO2 selective etching over Si and Si3N4, in order to understand the etch mechanism and develop a process and tool for future ultralarge-scale integrated circuit processing. Specimens were etched using C4F8/Ar/O2 plasma in a dual-frequency (27/0.8 MHz) parallel-plate reactive ion etching system. The relationship between ion energy (assumed to be equal to the peak-to-peak voltage Vpp of the rf bias) and the thickness of the surface reaction layers was quantitatively analyzed using x-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The fluorocarbon polymer layer and the SiFxOy layer on the substrates were observed. We found that the etch rate was strongly affected by the ion energy and the thickness of the fluorocarbon film on etched materials. In a highly selective etch process, the thickness of the fluorocarbon layer on the SiO2 surface was below 1 nm, while that on the Si3N4 and Si substrates were about 5–6 nm. It is considered that the difference in the fluorocarbon layer thickness on each material is the cause of the selectivity. Both TEM and XPS observations revealed that reaction layers (1–5 nm) were formed at the interface between the fluorocarbon layer and Si, Si3N4. The XPS analysis showed the composition of the reaction layer was SiFxOy. These SiFxOy layers were thicker when the ion energy was high and the fluorocarbon film was thin, i.e., a high etch rate condition for Si and Si3N4. SiFxOy is thought to be an intermediary product when the Si3N4 and Si are etched. In a highly selective etch process, the fluorocarbon film on SiO2 was so thin that ion energy was not reduced when ions passed through the film. However, at the surface of Si3N4 and Si, thicker fluorocarbon films were formed and reduced the etch rate, resulting in thin SiFxOy layers being formed.