Abstract
The mass resolution of a time-of-flight mass spectrometer (TOFMS) with a two-stage electrostatic reflector is calculated for SIMS and SNMS (Secondary lon, and Secondary Neutral Mass Spectrometry) applications. The parameters in reflector operation are optimized to satisfy the time focusing conditions, where the flight time spread in an accerelation region is also taken into account. The calculation method used is a numerical one, as well as an analytical one for a second order time focusing.
The main factors which limit the resolution in SIMS are (i) the time spread ΔT due to the energy spread ΔE of secondary ions and (ii) the uncertainty in the time-of-flight Δτ caused by the primary ion beam and the signal detecting system. Although the time spread ΔT can be reduced by increasing the acceleration voltage Ua, excessively high Ua reduced the total flight time and therefore the resolution. The resolution attains a maximum value of nearly 10,000 at the accerelation voltage of 100V for ΔE=10eV, Δτ=5ns and a 2m total drift length.
In the case of SNMS, not only ΔE but also the spatial width Δs of an ionizing laser beam affects the time spread ΔT. However, the effect of Δs can be partially compensated by applying a mild accerelation voltage to the ionizing region. The maximum value of resolution is about 1,000 at the total accerelation voltage of 1,000V for ΔE=10eV, Δs=1.0mm and Δτ=10ns.
