In the shallow trench isolation process, a stack of thin films on a Si wafer must be etched using different gas discharges. During plasma etching of a thin-film stack consisting of an organic antireflection coating, Si3N4 and Si with Cl-2, CHF3/CF4 and Cl-2/O-2 discharges, respectively, halogenated silicon oxide and fluorocarbon films form on the chamber walls. The chemical nature of these films and the methods for removing them were studied using multiple plasma and surface diagnostics. We find that the film which deposits on the chamber walls is a. composite consisting primarily of two components, a halogenated silicon oxide film-created during Cl-2 /O-2 etching of Si-and a fluorocarbon film formed during CF4 /CHF3 etching of Si3N4. The halogenated silicon oxide film can be removed by etching with F formed by dissociation in SF6 plasma, while the fluorocarbon film is etched by O atoms created in O-2 plasma. A sequential SF6 and O-2 plasma process can be used to etch the composite film, but even an O-2 plasma maintained under proper conditions was found to remove this film. During the removal of the fluorocarbon film by O-2 discharge, the etching products released into the gas phase dissociate to produce F, which can etch the oxide film. Thus, the fluorocarbon film serves as a solid source of F allowing the removal of all of the composite film including the oxide component with O-2 plasma alone. However, O-2 plasma cleaning conditions must be optimized to allow high residence time for F atoms to react with the oxide film.