Ultra High Brightness Plasma Ion Source for SIMS Imaging of Actinides at the Theoretical Resolution Limit (Final Report)
This project was aimed at providing a radical improvement in ion beam technology that could be applied to the mass spectrometric analysis of actinides. The state-of-the-art secondary ion mass spectrometers (SIMS) have largely employed the duoplasmatron ion source for many decades to create the primary ion beam for actinide analysis. The duoplasmatron only has sufficient optical brightness to create oxygen focused ion beams as small as 200nm in diameter, with an ion beam current of ~0.5pA. However, to achieve this small beam diameter, the duoplasmatron has also had to operate with an ion optical column that has been designed to operate on a very specific type of instrument (ie the Cameca NanoSIMS50). The ion optical column employed by the Cameca NanoSIMS50 instrument, can only operate with a negatively charged oxygen beam, a beam impact energy that is fixed at 13keV, and with a primary beam working distance of <<1mm. Having to operate with only negative ions and at 13keV is restrictive in itself, but the very short working distance results in a very short depth of focus, limited scanned imaging field and it also restricts the samples being analyzed to ones that are very flat. In order to analyze sub-micron actinide particles, focused oxygen beam diameters as small as 10nm are required to provide the highest possible imaging resolution using the SIMS technique, but with the duoplasmatron this has been far from possible. Oregon Physics developed its first high density plasma source (HyperionTM) prior to this project, for pilot studies and as a stepping stone to the work being carried out here. Hyperion has realized a factor of 10 increase in current density over the duoplasmatron and extended source life significantly. This work has been leveraged in the 1st phase of this project. Phase I incorporated a series of detailed analytic and numeric simulations of heat transfer, electromagnetic field calculations and ion extraction optics employed with the new ion source. A proof-of-concept (PoC) source has been engineered, built and has undergoing detailed experimental studies. Results have shown a significant gain in RF power efficiency, resulting in a factor of 20 increase in source brightness, over and above that demonstrated with Hyperion, to give a energy normalized brightness for xenon of ~2x105 Am-2sr-1V-1. This project also targeted a reduction in the energy spread of the extracted beam (targeting 2.5eV, rather than Hyperion’s 5eV), but this was not demonstrated in conjunction with the enhanced brightness. Although the improved energy spread was not realized, this author still remains confident that 2.5eV is attainable, for which evidence is provided in this report. An ion optical column was designed and built, that is suitable for ultra-high resolution SIMS imaging. The ion column is capable of operating with both negative ions or positive ion species and beam energies ranging from 0 to 30keV. With the significantly higher optical brightness of both Hyperion and the ion source being developed in this project, the ion column can operate with a more typical working distance of 10-20mm to accommodate larger scanned imaging fields, a greater depth of focus and the ability to work with non-planar samples. With Hyperion attached to this new ion column, we have been able to achieve beam diameters as small as 25nm. At the end of this project, the new ion source concept had been built for ‘proof-of-concept’ (PoC) experiments, however, the final prototype design faced a number of manufacturing challenges that have yet to be overcome. The PoC source performance suggests that the new source (named ‘Aurora’) would in principle achieve <15nm oxygen beam diameters with the new optical column and <10nm xenon beam diameters. There is strong evidence to suggest that with further work, that the goal of 10nm oxygen beams and 7nm xenon beams could be achieved with improved energy spread of the Aurora ion source. The Aurora ion source exceeded the brightness target of this project, and leaves us with the confidence that this ion source technology is indeed capable of achieving the required plasma density, coupled with a low enough thermal ion energy to achieve an ion source brightness that can further reduce the ion beam diameter of an oxygen beam, and suitable for SIMS analysis of actinide particles The primary unresolved engineering challenge is the ability to create hermetically sealed braze joints between aluminum nitride and molybdenum. Aluminum nitride is a ceramic material that has a similar thermal conductivity as molybdenum (~140 W/m/K) and with very similar thermal expansion coefficients. However, creating hermetic braze joints between these materials is not commonly carried out. In fact, only one company in the US was able to make the brazed assemblies for the PoC ion source, but unfortunately the prototype Aurora ion source brazements were not successful and suffered from significant vacuum leaks and insufficiently precise braze joints, despite the best efforts of the fabricators. Despite not having met the goal of demonstrating a 10nm diameter, 30keV oxygen beam, the new ion column has demonstrated a beam diameter as small as 30nm, which is almost a factor of 2 smaller than Hyperion could achieve on a NanoSIMS50. While it looks quite plausible that a 10nm oxygen beam diameter could be attainable with the completion of Aurora, the combination of the new ion column and Hyperion is still providing the highest resolution oxygen beam for SIMS analysis, thus far achieved. Whereas the NanoSIMS50 ion optics can produce a 0.5pA, 50nm focused beam when using the Hyperion ion source, the ion optical column developed under this project can provide 5nA, 50nm beam diameters by virtue of being able to operate at up to 30keV and a positive ion mode, while still having a relatively large working distance of 10mm.
- Research Organization:
- Oregon Physics
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- SC0008201
- OSTI ID:
- 1771932
- Type / Phase:
- SBIR (Phase II)
- Report Number(s):
- DOE - Oregon Physics - 08201
- Country of Publication:
- United States
- Language:
- English
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