ELECTRON-ATTACHMENT MASS-SPECTROSCOPY AS A DIAGNOSTICS FOR ELECTRONEGATIVE GASES AND PLASMAS

Electron attachment mass spectrometry (EAMS) has been developed to stu dy mixtures of electronegative,eases and plasmas. A quadrupole mass sp ectrometer (QMS) has been used to detect negative ions, formed from sa mpled species by attachment of low energy electrons. Varying the elect ron energy allows to collect the attachment cross section of the consi dered species. EAMS appears to be a very powerful technique to study t he chemistry of electronegative,eases. Unlike ionization mass spectrom etry, where cross sections are low at the threshold and rather flat ov er a broad range of electron energies, attachment resonances are sharp and distinct. Also very limited fragmentation of the parent negative ion occurs, so a given molecule yields only a few different negative i ons. This facilitates identification of components in a gas mixture. I t is particularly advantageous for detection of large, fragile molecul es, which break up after ionization, but can be easily transformed int o large negative ions. Moreover, sensitive detection of active species is possible due to their relatively high attachment cross sections. A particularly important application of EAMS is the determination of an effective attachment cross section in a plasma. Recording this cross section allows to decide on the actual negative ion formation mechanis m in the plasma environment, where active products of plasma conversio n can significantly alter the negative ion production channels and con sequently the whole balance of charged particles. Examples of EAMS app lications to fluorocarbon gases and low-pressure radio-frequency plasm as are discussed. In a CF4 discharge conversion of the parent gas into species like C2F6 and C3F8 is easily visualized. The dominant mechani sm of negative ion formation in the plasma is electron attachment to t hese minority species and not to the parent gas. Also larger polymers are readily formed in fluorocarbon plasmas. In a C2F6 discharge molecu les with up to ten carbon atoms (the mass limit of our apparatus) have been detected using EAMS. (C) 1998 American Institute of Physics.