The use of cluster beams for sputtering organic and polymer materials is now well established, with the high sputter rates and low damage levels achieved making their use as sputtering systems widespread in SIMS and XPS.
For use as a primary ion source for SIMS analysis, these cluster beams were limited, as they had large spot sizes, and poor ionization levels. The GCIB 40, operating at 40 keV beam energy, has addressed these issues, and has two clear advantages over working with lower energy beams: higher ionization yields, and improved spatial resolution.
Higher Ionization Yields
In organic analysis, the material being bombarded is often damaged by the ion beam, resulting in loss of molecular signal. With a GCIB, the induced damage is reduced so that the material can be analysed and depth profiled. But at lower beam energies the secondary ion yield with the GCIB is not very high, so signals can be low. Operating at higher energies results in a much higher yield of secondary ions, while still maintaining the low damage characteristic.
Using a sample of Irganox 1010, with equal primary ion dose, figure 1(a) shows results for signal intensity with four different cluster beam energies. It shows a very strong trend for signal to increase with energy.
The figure below shows that fragmentation by the higher energy beam is mostly on a par, or slightly better, than the lower energy beams.
Signal Enhancement with no increase in fragmentation. (a) normalised signal intensity for molecular and significant fragment signals from Irganox 1010, showing much higher ion yields for higher beam energy. (b) normalised signal ratios, comparing levels of fragmentation for four different beam energies. Data courtesy of Dr J. Fletcher, University of Gothenburg.
Greater Resolving Power
The figure below illustrates the enhanced spot size performance with the 40 keV beam. The first image, 2(a), shows an overlay of four masses found in a section of human hair ends supported in gelatine. The image size is 256 × 256 μm2, 128 x 128 pixels, positive ion mode. the primary ion dose density was approximately 1e13 ions/cm2. Figure 2(b) has image size 90 x 90 μm2 with 128 x 128 pixels and (c) shows a line scan plot taken from image (b). It indicates a spatial resolution of less than 3 microns.
ToF-SIMS imaging of human hair. (a) overlay, red: m/z 318.16, m/z 333.19; green: m/z 284.27, m/z 324.28; blue: m/z 331.17, m/z 845.52 (d18:1/23:0 di-hydro-sphingomyelin); white: m/z 114.88 (In). 2(b) total ion image and (c) a line scan across an edge of the hair in (b). Data courtesy of Dr J. Fletcher, University of Gothenburg.
The GCIB 40 system comprises:
Ion source, comprising cluster generation and ionization chambers.
Ion optical column, including a mass filter.
Power supplies to drive the ion source and column.
Source pumping system.
Gas clusters are formed by adiabatic expansion and then ionised by electron bombardment.
The cluster ions are accelerated into an ion column which contains a Wien filter, 5 selectable apertures (for selection of current and spot size ranges), a gate valve (for isolation of source from instrument during maintenance), a pulsing unit, a bend to remove neutrals, scan plates and a final focussing lens.
The Wien filter selects single cluster sizes for the small clusters; for the larger clusters (~Ar100 upwards) the beam consists of a mass distribution around the
nominal cluster size.
The GCIB 40, with its high-energy gas cluster analysis beam, offers better signal with low damage in organic SIMS analysis, with < 3 microns spatial resolution.