New wide angle, high transmission energy analyzer for secondary ion mass spectrometry

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Abstract
A conically tapered resistive disk with a single conducting hemisphere is used to effect the same energy selection as a pair of concentric hemispheres. The combination of this energy filter with a quadrupole mass spectrometer produces a secondary ion mass spectrometry (SIMS) instrument with a high total transmission attributable to its open geometry and lack of restricting apertures. An approximate form for the energy filter-mass spectrometer combined transmission function is constructed for operating conditions pertaining experimentally, and the theory and experiment are shown to be in good agreement. For a primary beam spot large compared with the electrical entrance aperture of the quadrupole, the energy bandwidth increases linearly with the spot diameter, and the transmission increases quadratically with the mass bandwidth and inverse quadratically with the spot diameter. The overall transmission (absolute sensitivity) is typically on the order of 5×10−4 in the static mode, and would be much higher (∼12% for typical applications) in the dynamic mode with small spot rastering. Static mode ion transmissions are measured for Cu+, Al+, Si+, and Ni+ at the peak of the kinetic energy distribution with an energy bandwidth of less than 3 eV. The instrument is used to study the copper surface in the static mode at current densities as low as 1.5×10−9A/cm2, using 90° incidence for the primary ions, with energies as low as 500 eV. The kinetic distribution of Cu+ ions is only weakly dependent upon primary ion energy over the range 500 eV to 3 keV. The Cu+ distributions show a 13-eV halfwidth and a maximum at 10 eV, while the Cu2+ ion kinetic energy distribution has a 10-eV halfwidth and a maximum at 7 eV. Molecular ions ejected from a copper surface after a plasma oxidation are CuOH+,Cu2O+ Cu2OH+, and Cu2(C2H2)+. These ions have kinetic energy distributions with ∼7 eV half-widths and peaks at ∼5 eV. The aforementioned measurements are made with an energy resolution of less than 3 eV, which is compatible with a mass resolution equal to the mass number of at least 250 amu.