NanoSIMS

The Instrument

Technical overview

The CAMECA NanoSIMS 50 has been optimised for high lateral resolution SIMS analysis. Although conventional dynamic SIMS is a powerful tool, the NanoSIMS 50 is the only dynamic SIMS instrument which allows simultaneously high lateral resolution (50 nm), high mass resolution (M/ΔM = 5,000) and high sensitivity (ppb for some elements). Most  conventional SIMS instruments, for example most ToF-SIMS instruments, have to compromise on one of these factors, although this is still the case to some extent for the NanoSIMS, the compromises are not as severe. For instance a mass resolution of 3500 is achievable while still maintaining 100% transmission.

Modes of operation

The NanoSIMS has several unique features such as coaxial optics and a normal incidence primary ion beam. Working in the ion microprobe mode, the primary ion beam is rastered across the surface and secondary ions are detected as a function of the beam position on the sample, resulting in high lateral resolution and collection efficiency. The NanoSIMS is capable of various modes of operation including chemical maps, depth profiles, mass spectra, isotopic ratios and line scans.

Primary ion sources

The use of reactive primary like caesium in positive mode (only negatively charged secondary ions can be detected) or oxygen in negative mode (only positively charged secondary ions can be detected) is required to increase the secondary ion yield of negative and positive ions respectively. Other non-reactive primary ions like Ga or Bi, both of which are widely used in liquid-metal ion sources (LMIG) to achieve lateral resolutions below 100nm in most other SIMS instruments, yield 100-1000x lower signal intensities for elements. This would be problematic as very low beam currents are employed in the NanoSIMS to allow focusing to fine incident beams and this would make trace element analysis almost impossible.

Ion optical setup

The main conflict in the design of conventional ion optics in other secondary ion mass spectrometers is that the objective lens of the primary ion column must be as close as possible to the sample to produce a small and intense beam but the extraction optics must also be placed as close to the sample as possible to collect as many secondary ions as possible. The primary and secondary optics both have a physical size leading to a large working distance and the optics are not in their optimum positions. The co-linear optics of the NanoSIMS solves this problem by simultaneously focusing the primary ion beam and collecting most of the secondary ions. The co-axial optics has several advantages including a shorter working distance, smaller probe size, higher collection efficiency (improved transmission) and minimisation of shadowing effects (normal incidence to the sample). The main disadvantages with this technique are that the primary and secondary ions must be of opposite polarity and equal energy. This constraint means that minimisation of matrix effects using the MCs⁺ technique is not possible and oxygen flooding cannot be used. In a coaxial optic system, the secondary ions are extracted back through the primary ion beam diaphragm which controls lateral resolution but limits transmission and thereby sensitivity at the same time.

The mass spectrometer

The NanoSIMS uses a mass analyser in a Mauttach-Herzog configuration with a 90° spherical electrostatic sector and an asymmetric magnet. This type of mass spectrometer has reasonable transmission, unrivalled mass resolution (M/ΔM> 10⁴) high energy acceptance and preserves the lateral resolution which allows high mass resolution with a small spot size. After mass separation in the magnetic sector field the secondary ions are detected using electron multipliers which can accurately measure low intensity ion signals. Higher intensity signals can be measured using a Faraday cup. The NanoSIMS 50 is equipped with 4 moveable and one fixed detector, making it possible to detect 5 secondary ions simultaneously. This multicollection capability allows a more accurate reconstruction of the sample chemistry as each ion image is coming from exactly the same volume. This also ensures reliable isotopic measurements and image registration.