This Small Business Innovation Research (SBIR) Phase I project will develop a high brightness C60 ion source that can be used in TOF SIMS instruments for 2D and 3D chemical analysis and image mapping in nanobiology applications. Energetic cluster ions such as C60+ yield large molecular secondary ion species when impacting a sample, reducing subsequent surface damage for depth profiling. However, C60+ ions produced from state-of-the-art electron impact sources suffer from low beam current and relatively broad ion beams resulting in limited spatial resolution and throughput. The proposed inductively coupled plasma (ICP) source would provide a 10x increase in brightness and a 50% reduction in energy spread. The resulting C60 ion beams with higher probe current density would vastly improve spatial resolution, and allow for larger volumes of material to be analyzed at increased analysis speed. This would be extremely useful for characterizing the spatial distribution of molecules within cell components without the need for labeling. The ability to analyze and determine cell process mechanisms could have far reaching implications in health and disease control.
The broader impact/commercial potential of this project is the improved C60 ion source would enable numerous researchers, faculty, students, and industrial users, to site specifically and spatially identify molecular species within cell and nanobiology structures at superior resolution and speed compared to the current state-of-the-art. More accurate mapping and imaging of cell functions in 3D could give rise to the discovery of new cell mechanisms, impacting society through improved health and disease control. The improved source would allow the unique sputtering properties of C60 to be applied to FIB applications in fields where existing FIB technology is limited by beam charging or material issues. These fields include semiconductor packaging, polymer science, fiber science, and paper science. The improvements proposed in the description of this new source could immediately supplant the current state-of-the-art C60+ ion sources used in commercial SIMS instruments, allowing for successful technology transfer to the marketplace through a direct supply of sources to SIMS vendors. In addition, upgrades to the current installed base of SIMS instruments would be possible, providing existing C60+ source users with the improved performance benefits. This source would also be utilized on commercial dual-beam microscopes used for cross-sectional metrology.
This Small Business Innovation Research Phase I project has focused on the development of a high brightness C60 ion source that could be used in TOF SIMS instruments for 2D and 3D chemical analysis and imaging of biological structures at the nanoscale. This phase I study has clearly shown that it is possible to create a source of C60+ ions from an inductively coupled plasma source, with properties suitable for creating a focused ion beam. The evidence suggests that the C60+ ion beam created in this project, was approximately a factor of 10-20 more intense than other commercial C60 ion sources. However, further measurements would need to be made to determine the precise gain in performance that has been achieved here. In this work, a precise measurement of the C60+ current density extracted from the source has been made (16uA/cm2), however, we can only estimate that the mean thermal ion energy is somewhere between 0.05 and 0.1eV, which would result in an energy normalized brightness of 1.5 to 3 Am-2sr-1V-1 (ie estimated to be a factor of 10 to 20 higher than the state of the art electron impact C60 ion sources). For this project to move forward towards a commercialization phase, we would need to see yet another factor of 10 gain in performance (ie a factor of 100 gain over the state-of-the-art commercial products). While we have not reached our target of 100x gain in C60 source brightness, there are no indications that this goal could not be achieved with further work. The primary limitation to the brightness of our proof-of-concept design, has been the ability to deliver a high enough flux of C60 vapor into the plasma chamber. WIth further investigation and development, the vapor deliver rate could certainly be increased. The mass spectrum seen here, is of ions extracted from the source, when operated with xenon as a carrier gas (~10mTorr) and a C60 partial pressure that is estimated to be <0.05mTorr. The overall beam current extracted from the ion source was 22uA, with 5nA being due to C60+ ions. If we assume that the mean thermal ion energy is elevated to 0.1eV due to oven heating and electron impact heating (N.B. 0.05eV of electron impact heating is typically observed for xenon ions, with this ion source), then we surmise that the energy normalized brightness will be 1.5 Am-2sr-1V-1.
