X-RAY PHOTOELECTRON-SPECTROSCOPY STUDIES OF CONTAMINATION AND CLEANING OF SURFACES EXPOSED TO A FLUOROCARBON PLASMA

A reactive ion etching (RIE) with a mixture of fluorocarbon gases is used to etch selectively SiO2 patterns on Si wafer. X-ray photoelectron spectroscopy (XPS) measurements were carried out to determine the walls contamination of the reactor with different cathode coatings like Al, Si, and photoresist. The contamination of different wafers (SiO2, Si with and without resist) following RIE processes was also studied in order to understand the mechanism of the selective etching. Then different cleaning processes, in particularly NF3 and O2 plasmas followed or not by a hydrophilic treatment, were also qualified by XPS. For every sample, the Al(2p) or Si(2p), C(1s), O(1s), and F(1s) photoelectron lines were monitored and concentrations determined as usual by integration of these lines. F(1s) and C(1s) were decomposed into several components corresponding to different chemical bonds. The results show that the contamination of the reactor walls by fluoropolymers is due to the presence of Si or of the photoresist on the cathode; there is no polymer formation with an Al cathode coating. The polymer contamination on the reactor walls is cleaned off by an O2 plasma and the residual fluorine forms a passivative layer with the Al of the reactor walls. During the plasma etching of a SiO2 film, there is formation of C-Si, C-C, O-Si-F, F-Si bonds but it is only when there is no oxygen left that CFx bonds are formed. These polymer layers stop the etching process on silicon. They are cleaned by O2 or NF3 plasma, but the latter removes the polymers without increasing the oxygen, fluorine or nitrogen contamination and appears to be most appropriate for improving the surface quality of silicon after fluorocarbon plasma etching.