Study on modified silicon surface after CHF ₃ /C ₂ F ₆ reactive ion etching

The effects of reactive ion etching (RIE) of SiO ₂ layer in CHF ₃ / C ₂ F ₆ on the underlying Si surface have been studied by X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometer, Rutherford backscattering spectroscopy, and high resolution transmission electron microscopy. We found that two distinguishable modified layers are formed by RIE: (i) a uniform residue surface layer of 4 nm thickness composed entirely of carbon, fluorine, oxygen, and hydrogen with 9 different kinds of chemical bonds and (ii) a contaminated silicon layer of about 50 nm thickness with carbon and fluorine atoms without any observable crystalline defects. To search the removal condition of the silicon surface residue, we monitored the changes of surface compositions for the etched silicon after various post treatments as rapid thermal anneal, O ₂ , NF ₃ , SF ₆ , and Cl ₂ plasma treatments. XPS analysis revealed that NF ₃ treatment is most effective. With 10 seconds exposure to NF ₃ plasma, the fluorocarbon residue film decomposes. The remained fluorine completely disappears after the following wet cleaning.