Kore SurfaceSeer - Model S TOF-SIMS -High Sensitivity Surface Analysis

The instrument uses a 5keV Cs+ ion beam (optionally inert gas Ar+) that is pulsed to minimise the surface damage that would otherwise be caused by a continuous ion beam. The gun pulses on for only 60ns in every 100µs TOF cycle, and thus the primary beam delivers more than 1,000 times less current than if it were on continuously. A typical experiment time of 10s delivers a dose equivalent to only a few milliseconds of continuous beam current. Several minutes of analysis in a single spot would be required to reach the so called “Static SIMS” limit, the point at which surface damage becomes clearly visible in the data (focus and beam-current dependent). Despite the low ion doses, the TOF analyser is very efficient, and this accounts for the high secondary ion data rates (typically 5000 c/s even on relatively low ion yield polymers).

Where required, the primary ion current can be increased and the ion gun turned on continuously and rastered for sputter cleaning of suitable samples.

The instrument also employs a technique known as ‘delayed extraction’ for the secondary ions produced. In this technique the primary ions bombard the surface and produce the analytically important secondary ions. A short time after the primary beam pulse has finished bombarding the sample, the ion extraction field is pulsed on. This results not only in secondary ion extraction, but also secondary ion compression as the ions travel through the analyser to the detector. In some TOF-SIMS instruments the primary beam is compressed or ‘bunched’, but in this instrument it is the secondary ions that are bunched. This delayed extraction is set so that the secondary ions of the same m/z are temporally focused to produce better mass resolution than would otherwise be obtained with the long primary pulse (60ns) on its own.

CHARGE NEUTRALISATION

One of the advantages of using a pulsing ion beam/delayed extraction combination is that there are relatively long periods in each TOF cycle when there is no ion extraction field applied. In that period a pulse of low energy electrons (30eV) is directed at the analytical area. By doing this it is possible to neutralise the effect of positive charge that would otherwise build up on the surface as the primary ion beam bombards an insulating sample.

TOF ANALYSER

The instrument has a 150mm diameter reflectron analyser, with a total effective flight-length (including the flight tube) of 2 metres. It is a dual-slope reflectron with in-vacuo high precision resistors, and has an adjustable ‘retard’ potential within the reflectron that has been set for optimum spectral performance.

VACUUM PUMPING

The instrument has a single rack-mounted vacuum controller that controls the vacuum system. Turbomolecular pumps are used to maintain vacuum in the analytical chamber and the sample load lock, each of which is backed by a 2-stage rotary pump. Load lock venting and pumping is achieved with a single manual button on the vacuum controller. A high vacuum gauge (inverted magnetron) monitors the pressure in the analytical chamber at all times, and is used to provide vacuum interlock protection, shutting down high voltages if the pressure rises beyond a set point.