Digital Mass Control for an Ion Microprobe Mass Analyzer

Abstract

With a double-focussing mass spectrometer as employed in Applied Research Laboratories Ion Microprobe Mass Analyzer (IMMA) and other SIMS instruments various atomic mass numbers are scanned by varying the magnetic field of the mass spectrometer. The relationship between the mass number M and the magnetic field B is given by $${\rm{M = C}}{{\rm{B}}^{\rm{2}}}$$ where C is a constant. Also for constant size of the final slit, the mass resolution is given by: $${\rm{M/\Delta M = R}}$$ where R is typically 300-500. The magnetic field is varied by changing the current to the coils driving the magnet and the field is controlled in the usual case by comparing the output of a Hall Sensor with a reference voltage which is varied according to mass number. This system has two complications which are apparent from the foregoing equation, namely: 1) a non-linear function must be used to program the current drive to the magnet for various mass numbers, and, 2) the control system must provide for greater resolution at low mass numbers.