Characterization and removal of silicon surface residue resulting from CHF3/C2F6 reactive ion etching

Abstract

The surface properties of an underlying Si substrate after reactive ion etching of SiO2 in CHF3/C2F6 gas plasmas have been studied using x-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry, Rutherford backscattering spectroscopy, and high resolution transmission electron microscopy (HRTEM). A 50-nm-thick silicon layer that contains carbon and fluorine and a 4-nm-thick uniform residue layer composed entirely of carbon, fluorine, oxygen, and hydrogen on the silicon surface have been observed. The residue film has nine different kinds of chemical bonds. At the surface, O-F bond is found on C-F polymer that contains C-CF(x) (x less-than-or-equal-to 3), C-F1, C-F2, and C-F3 bonds. Between the C-F polymer layer and the silicon substrate, C-C/H, Si-C, Si-O, and Si-F bonds exist. Neither point defect clusters nor distinct planar defects are found in cross-sectional HRTEM images of the silicon substrate. The changes of peak shapes for C, Si, O, and F in the residual film have been analyzed through an in situ resistive anneal under ultrahigh vacuum condition. C-F1, C-F2, and C-F3 bonds decompose and from C-CF(x) bonds above 200-degrees-C. Above 400-degrees-C, C-CF(x) bonds also decompose to C-C/H bonds. For removal of the silicon surface residue, reactive ion etched specimens have been exposed to O2, NF3, Cl2, and SF6 plasmas. By XPS analysis, NF3 treatment has been revealed to be the most effective. With 10 s exposure to NF3 plasma, the fluorocarbon residue film decomposes and the remaining fluorine is mostly bound to silicon. The fluorine completely disappears after wet cleaning.