Abstract: Mass microanalyzer using electron-beam guiding ion source

Abstract

A new apparatus which performs microanalysis by analyzing ions evaporated from a specimen by the impact of a focused, scanned electron beam from an electron-optical column which includes a ring cathode and electrostatic lenses is described. A model is developed that involves four steps: (1) Material is locally evaporated from a specimen by bombardment with electrons of energy about 10 keV. (2) The evaporated atoms are ionized by inelastic collisions with both primary and secondary electrons near the point of impact of the electron beam. (3) The ions are trapped in the potential well (∠0.1–1 eV) formed by the electron beam. (4) The ions are accelerated through the electron-optical column toward the entrance slit of the mass spectrometer and are mass analyzed. The model predicts that for a primary electron current Ie, the collected ion current Ii is equal to (2m/M) Ie or 4×10−8 A for N2 ions and Ie=1 mA. For high beam currents (1 mA), the spatial resolution of the system was demonstrated to be less than 1 mm. The ultimate spatial resolution of the system is estimated to be 1 μm with the removal of 44 atom layers from the specimen; this estimate is based on a minimum detectable ion current of 10−16 A and a collection angle of 10−2 rad. Experimental results were obtained from a specimen of evaporated NaCl on stainless steel. The preliminary experiments demonstrate the possible use of this instrument for surface analysis and the theory indicates the technique could provide micron spatial resolution.