The effects of HBr or Cl-2 addition to CF4/O-2 plasmas on resist trimming were examined. Because the reactivity of halogens on trimming decreases from F to Br and then to Cl, the addition of HBr and Cl-2 to CF4/O-2 decreases the trim rate. In contrast, the etch selectivity of the resist to the underlying polysilicon and the trim-rate uniformity are generally improved with the addition of enough HBr to CF4/O-2, whereas addition Of Cl-2 has the opposite effect in that it degrades the selectivity and uniformity. The constituents of the resist sidewall in different plasma etchants with HBr or Cl-2 gas added to CF4/O-2 were examined using angle-resolved XPS to study how the trim rate is influenced by the sidewall films formed during resist trimming. The XPS results showed that the films formed on the resist sidewall had different constituents and concentrations according to the plasma chemistry. COxBryFz and COxClyFz films were formed on the resist sidewall during trimming with HBr/CF4/O-2 and Cl-2/CF4/O-2, respectively. The presence of halogen in the plasma affects the availability of the oxygen atom to the resist surface. A greater atomic concentration ratio of oxygen to carbon and a higher degree of halogenation of the films coincide with a higher trim rate.