Plasma etching of organic materials. I. Polyimide in O2–CF4

Gas phase and surface phenomena responsible for etching polyimide in O2–CF4 rf plasmas have been investigated. The dependence of the etch rate of polyimide on the relative concentrations of these two gases was compared with data from optical emission spectra, mass spectra, and x-ray photoelectron spectroscopy (XPS). The increase in O atom number density in the plasma with addition of CF4 to O2 certainly increases etch rates over those achieved in pure oxygen plasmas. However, etch rates do not track identically with atomic oxygen emission intensities. Etch rates are enhanced through the reaction of fluorine in the polyimide surface layer while fluorine present in excess inhibits etching through competition with oxygen atoms, resulting in the formation of CF2 type bonding at the surface. Removal of the resulting passivation is most efficiently accomplished by exposure of the polyimide to a plasma with higher oxygen concentration, probably due to the presence of ions. The position of the maximum in etch rate distribution with respect to CF4 concentration depends on an optimum ratio of O to F atoms in the plasma.