POSTIONIZATION OF SPUTTERED NEUTRALS BY A FOCUSED ELECTRON-BEAM

We have explored a novel approach crf electron-impact postionization involving the use of a focused low-energy electron beam directed at the flux of sputtered particles in very close proximity to the sample surface [''matched'' e-beam sputtered neutral mass spectrometry (SNMS)]. The first version of an electron gun developed for this purpose delivers a 300 eV beam of approximately 100 muA into a 100 mum spot. The postionization efficiency was investigated in a quadrupole based ion microprobe using samples of germanium and gallium arsenide bombarded with 10 keV Ar+. Singly and multiply charged species M(n+) (with n up to 5 for As) were produced by electron impact. The position and size of the electron beam with respect to the ion beam could be controlled and optimized by recording raster scanning ion images of the postionized species. Inspection of the energy spectra showed that the ionization probability is larger the lower the velocity of the sputtered neutrals. At low energies ( < 5 eV) and with the maximum electron current density presently achievable, the intensity ratio of singly charged postionized atoms to secondary ions was almost-equal-to 1 for Ge and almost-equal-to 5 for As. Fractional ion yields of 2 X 10(-8) and 3 X 10(-8), respectively, are estimated for these species. These numbers are higher by more than one order of magnitude than the yields obtained previously by ''remote'' e-beam SNMS. Interference with the secondary ion flux was small or negligible in the case of doubly and multiply charged ions. The fractional ion yields for doubly charged ions ranged from 3 to 5 X 10(-9). The results are compared with available cross sections for electron impact ionization. The measured SNMS ion yields are briefly compared with yields that can be achieved by charge transfer ionization using Ar+ beams at maximum current density. Advantages and limitations of the matched e-beam SNMS technique are discussed together with conceivable improvements using a more advanced electron gun and an improved ionization and ion extraction geometry.