Aspect ratio effects in submicron contact hole plasma etching investigated by quantitative x-ray photoelectron spectroscopy

Abstract

The submicron contact hole etching process using CHF3–CF4–Ar plasma was performed in a commercial parallel plate reactive ion etching reactor (LAM 4520). Response surface methodology was employed to correlate input parameters with etching results. For the aspect ratio investigations, a special test pattern consisting of line/space and contact/space arrays of various dimensions (1 and 0.6 μm) was used. Aspect ratio dependent etching effects were observed. The bottom surface chemistry of SiO2/Si patterned structures after plasma etching have been investigated by using quantitative x-ray photoelectron spectroscopy (XPS). After the SiO2 etching treatments, the silicon surface of the contact (or trench) bottom presents modifications similar to that observed on unpatterned SiO2/Si samples. These modifications are described by a two-layer model involving a fluorocarbon overlayer and an interfacial oxide layer. The fluorocarbon layer thickness strongly increases when the surface area of silicon substrate exposed to the plasma decreases whereas, as unexpected, the fluorine density of the fluorocarbon film decreases. A refluorination mechanism depending on the diffusion of the fluorine atoms from the plasma through the fluorocarbon overlayer is proposed. This work points out the interest of XPS surface analysis in improving the contact hole etching process.