In a time-of-flight atom probe instrument using voltage pulses, the acceleration of the field-evaporated ions through a time-varying field leads to an uncertainty in their kinetic energy, and thus, degradation in the mass resolution [1]. Atom probes, have tended to include some energy compensation, using either a Poschenrieder lens [2] or a reflection [3] to compensate for this variation and improve mass resolution. In the scanning atom probe (SAP), originally proposed by Nishikawa et al. [4], and the local electrode atom probe described by Kelly et al.[5], the distance from specimen to counter electrode is <100μm. Mass resolution in these configurations can be improved using different methods than previously. Kelly et al. [6] have used post acceleration to improve mass resolution. This relies on a very small separation between tip and electrode which allows field evaporation at lower applied voltages - around 5 kV as opposed to the usual more 10-20 kV. By post accelerating the ions, the energy variation is reduced relative to the total energy, thus improving in the mass resolution. Cerezo et al. [8] adopted a different approach, using a design for the counter electrode in which the ions evaporated on the pulse are decelerated to the applied dc potential. The voltage used in deceleration is therefore only a small part of the total applied voltage since the pulse fraction is usually 15%-20%, and the lensing effects generated are small. The total ion energy is also kept small, such that the time resolution is not compromised.
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