Calibration of the Atom Probe FIM

Abstract

Accurate identification of atomic species in the atom probe FIM requires a highly shielded tip assembly, as well as a revision of the equation formerly used to compute the m/n ratios. Voltage reflections on the unterminated pulse transmission line cause the actual evaporation pulse at the tip to be greater than the applied pulse by a ``pulse factor'' α. In addition, the recorded flight time of the ion under investigation differs from the actual flight time by a constant electronic time delay δ. Using either of two single isotope calibration techniques, both α and δ can be determined. For a shielded tip assembly either technique gives a value of α=2.00±0.05 and a value of δ=0.06±0.02 μsec. Typical absolute mass determination using these values of α and δ is within ±0.6 amu at m/n=20, and ±1.6 amu at m/n=100. When masses near a known species, like hydrogen- or helium-metal molecular ions, are to be detected, the practical resolution reaches ±0.2 amu at the middle of the mass range.