Some basic considerations concerning the sensitivity of sputter ion mass spectrometers

Abstract

The importance of proper phase space matching of source emittance and analyzer acceptance to achieve maximum sensitivity in sputter ion mass spectrometers is emphasized. Quantitative relations between transmitted current and instrument parameters are given. In an optimized instrument ion currents of the order of microamperes can be expected from large samples. Using low background detection systems a dynamic current range of 14 orders of magnitude is achievable. Under ideal conditions this corresponds to the detectability of bulk impurities with concentrations between 10 −3 and 10 −6 p.p.b., depending on the element. Depth gradients of p.p.b.-impurities can be measured with a material consumption corresponding to a single monolayer of sample. Physical processes and not sensitivity problems ultimately limit the depth resolution. The statistical nature of sputtering seems to set this limit at 10–15 atomic layers at the surface and even higher than that as surface erosion progresses. For small samples the trace sensitivity is proportional to the projected area. For a 1 μm diameter particle, sensitivity limits are characteristically of the order of 1 p.p.m. In the limit an impurity (continuously distributed over a few atomic layers or in the form of an inclusion) can be identified with 10% accuracy if it contains at least 10 4 atoms of one kind.