Negative ions in etching plasma were extracted from a plasma chamber and mass spectra were measured. For the extraction of the negative ions, the plasma sheath was broken by a cone-shaped extractor electrode which had an orifice on the tip. To avoid disturbing the plasma, the tip of the extractor electrode was slightly stuck out of an electrically earthed wall. To remove electrons from the extracted negative charged flux, a magnetic field was applied to the flux. By using the extractor electrode and the magnetic field for the removal of the electrons, the negative ions could be extracted efficiently and the mass spectra could be measured with low noise. ©1998 American Institute of Physics.