Greg Gillen

Objectives: To evaluate new approaches for organic surface analysis using energetic cluster bombardment (Cluster SIMS) and to develop a fundamental understanding of the mechanisms of cluster-organic surface sputtering in order to optimize the Cluster SIMS technique.

Problem: Secondary Ion Mass Spectrometry (SIMS) is a well-established analytical tool for organic surface characterization. High sensitivity, monolayer sampling depth and high spatial resolution make it the technique of choice for industrial applications in polymer surface characterization, biomolecule analysis and evaluation of organic surface contamination. The basis of the technique, as well as its fundamental limitation, is the requirement for bombardment of the organic surface, in vacuum, with an energetic (keV) primary ion beam. This primary ion bombardment, typically with a species such as Ga+, produces characteristic sputtered molecular secondary ion signals that give information on the surface composition of the sample. Unfortunately, the bombardment process also results in extensive alteration (damage) of the near-surface region of the sample preventing the acquisition of in-depth molecular compositional information and greatly reducing sensitivity. Another complication is that only a small fraction of the sputtered molecules will be desorbed as secondary ions and thus be amenable to detection. In an effort to expand the capabilities of the SIMS technique for organic materials, the Surface and Microanalysis Science Division has been actively pursuing the development of energetic cluster ion bombardment for organic surface analysis. Compared to the conventional SIMS experiment, the use of small cluster primary ion beams, such as C8-, can enhance molecular secondary ion signals by several orders of magnitude. In addition, accumulation of beam-induced damage can be substantially reduced in some materials. While initial results have been encouraging, we have not yet conducted a systematic evaluation of the experimental parameters required to optimize the cluster SIMS technique. Furthermore, there currently exists no definitive explanation for the observed signal enhancement or damage resistance. Understanding the fundamental processes that give rise to these effects is a key requirement for further development of the Cluster SIMS approach as a viable surface analysis tool.

Approach: We have used a CSTL prototype cluster ion source to bombard organic and polymer surfaces with Cx- (x=1-10) projectiles under various experimental conditions. Parameters evaluated included the influence of cluster size and bombardment energy on measured secondary ion yields (number of molecular ions produced/primary ion impact), the sputter yield (total number of molecules desorbed/impact) and the rate of sample degradation as a function of increasing primary ion dose. Computer modeling has also been used to study the subsurface penetration of cluster ion projectiles and the build-up of subsurface damage under a variety of experimental conditions.

Results and Future Plans: We have found that optimal secondary ion yields and minimal accumulation of damage are obtained by using cluster ions in the range from C6--C8-. Smaller cluster ions demonstrate a rapid fall-off in signal enhancement and more pronounced degradation of the sample. Use of larger clusters (above C8-) does not provide additional benefit. Computer modeling of the cluster-surface interaction indicates that the reduction in damage accumulation is primarily related to the dissociation of a cluster ion after impact with the surface leading to a significant reduction in the depth of the altered layer below the sputtered surface. Recent experiments suggest that the enhancement in secondary ion signal under cluster bombardment may result not from a higher sputter yield (which is the prevailing theory), but from a much higher fraction of intact molecules that survive the cluster impact event. These studies are continuing and we anticipate they will provide further insight into defining optimal experimental condition for analysis. It is hoped that these fundamental studies will eventually lead to a robust new method for organic surface analysis that could be transferred to US industry.

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