J.G. Gillen and B.D. Freibaum
Objective: To develop new approaches for secondary ion mass spectrometry (SIMS) using energetic cluster ion bombardment.
Problem: Secondary ion mass spectrometry (SIMS) is a surface analysis technique widely used for the chemical characterization of organic and semiconductor surfaces. The basis of the technique, as well as its fundamental limitation, is the requirement for bombardment of the material to be analyzed, in vacuum, with an energetic (keV) primary ion beam. This primary ion bombardment, typically with a species such as Ar+, O2+, Ga+ or Cs+, produces the characteristic sputtered secondary ion signal that gives information on the surface and in-depth composition of the sample. Unfortunately, the bombardment process also results in extensive alteration of the near-surface region of the sample. The depth of this damaged or "altered layer" is directly related to the penetration depth of the primary ion and is largely responsible for determining the depth resolution of a SIMS depth profile. For organic surfaces, the creation of a subsurface damage layer results in the rapid degradation of the molecular structure of the surface, preventing the acquisition of molecular depth profiles and greatly reducing sensitivity.
Approach: The Surface and Microanalysis Science Division has become actively involved in the development and utilization of polyatomic and cluster primary ion beams for SIMS analysis. Because a cluster ion dissociates upon impact with a surface, the penetration depth of the constituents of the cluster are greatly reduced as compared to monoatomic primary ion bombardment under the same conditions. Furthermore, the simultaneous and temporally correlated impacts of multiple atoms from the cluster produce very large, non-linear enhancements in the number of atoms or molecules sputtered per cluster impact. We have demonstrated that the combination of these two effects can increase the yield of characteristic molecular secondary ions, more efficiently desorb higher molecular weight species, and reduce the accumulation of primary-beam-induced damage. For depth profiling of semiconductors, a cluster beam may offer substantial improvements in depth resolution. We are working with a small U.S. company (Peabody Scientific, Peabody, MA) to develop and explore the use of a negative cesium sputter ion source for generating carbon cluster ion beams for practical SIMS analysis.
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Figure 1. Sputter Ion Source |
Results and Future Plans: Figure 1 shows a computer- generated model of the carbon cluster source currently being used at NIST. The cluster ions are produced by energetic Cs+ bombardment of a graphite target in the source region. Primary ion beam currents of 1 µA of C2- have been produced. Cluster ions ranging from C2- to C10- are routinely used for analysis and depth profiling. For organic SIMS applications, the use of carbon cluster ions greatly increases the yield of molecular secondary ions. Under low primary ion dose bombardment conditions, the molecular ion yield from amino acid targets was found to increase by as much as a factor of ~800 when comparing C1- to C8- primary ions. Under high dose bombardment, the yield enhancement can be greater than 10,000. The larger carbon cluster ions also reduce the accumulation of beam-induced damage, allowing for sustained molecular ion emission at high primary ion doses. Semiconductor characterization has focused on examination of low energy As implants in silicon. Preliminary results suggest that the depth resolution obtained with a CsC6- cluster ion is improved by a factor of four as compared to conventional Cs+ depth profiling. The source can also be operated in a microfocused mode, allowing micrometer spatial resolution images to be obtained. Current work includes further optimization of the source design and studies of the fundamental interactions of cluster ions with surfaces. |
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Last Updated
March 5, 2002
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