NEW DEVELOPMENTS FOR CLUSTER ION BEAMS IN SECONDARY ION MASS SPECTROMETRY IMAGING EXPERIMENTS
Open Access
- Author:
- Kozole, Joseph
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 19, 2007
- Committee Members:
- Nicholas Winograd, Committee Chair/Co-Chair
Barbara Jane Garrison, Committee Member
Blake R Peterson, Committee Member
John H Golbeck, Committee Member - Keywords:
- Molecular Depth Profiling
Chemical Imaging
Buckminsterfullerene
ToF-SIMS
3-D Imaging - Abstract:
- The analysis of the desorption of atomic and molecular ions from surfaces bombarded using energetic projectiles constitutes the technique of secondary ion mass spectrometry (SIMS). In principle, SIMS is a powerful tool in the characterization of organic and biological material. However, when the projectiles used to irradiate the sample are atomic in nature (i.e. Cs+, Ga+, In+, Au+, Bi+, etc.), several fundamental flaws exist, such as low yields, extensive fragmentation, and the incidence of significant sample damage after erosion beyond 1% of the surface molecules. Polyatomic (cluster) ions (i.e. Au3+, Bi3+, C60+) have recently been introduced as possible replacements for atomic ions. Since each constituent atom in a cluster ion has less kinetic energy then an atomic counterpart, the incident energy remains nearer the solid surface during the impact event. Hence, the energy is deposited into the solid at a depth that is efficient for desorption and inefficient for sample damage. The new physics associated with cluster ion bombardment has led to the observation of several special properties relevant to SIMS measurements, including increased yields, reduced physical and chemical sample damage, and the feasibility of molecular depth profiling. The aim of this research is to maximize the performance of these properties in the SIMS imaging modality. Specifically, two approaches are emphasized – optimization of the type of cluster ion and optimization of the SIMS instrumental/experimental strategy. Since a large number of cluster ion types are available for use, a series of experiments are presented in which the quartz microbalance (QCM), atomic force microscope (AFM), and SIMS are used to measure surface sensitivity, sputter yield, and physical and chemical damage during ion bombardment of water-ice, phospholipid, cholesterol, and silicon. The studies compare projectile type, i.e. Au+, Au2+, Au3+, and C60+, projectile incident energy, i.e. 10 keV – 120 keV, and projectile incident angle, i.e. 0° – 75°. The results show that the total sputtering yield relative to the degree of sample damage is largest when C60+ projectiles are employed at large kinetic energies and glancing incident angles. Therefore, these conditions are most practical for increasing the overall sensitivity of SIMS and improving the prospect for three-dimensional (3-D) characterization. Since the current SIMS instrumental/experimental strategy does not fully exploit the optimized cluster ion beam type identified here, the instrument/experiment is modified. Specifically, the conventional strategy, which employs a pulsed ion beam and axial time-of-flight (ToF) MS detection, is restricted by its duty cycle, secondary ionization efficiency, and sample preparation. Accordingly, we introduce a novel instrument in which a C60+ ion beam is operated in direct current (dc) mode and the desorbed ions are detected using an orthogonal ToF MS. In addition to rapid sampling, we show the scheme is capable of tandem MS-MS when a sequence of quadropoles is placed between the sampling region and the ToF region. For all of the situations presented here, the secondary ionization efficiency is quite low. Hence, strategies for enhancing the secondary ion yield are needed. Here, we modify the instrumental/experimental strategy by using tunable vacuum ultraviolet (VUV) radiation obtained from the Lawrence Berkeley National Laboratory (LBNL) to photoionize C60+ desorbed neutral particles. The results show that although a C60+/VUV postionization spectrum can be attained, the ion yield is not currently large enough to be considered an improvement upon secondary ionization efficiency. However, it is suggested that by increasing the photon flux of the VUV radiation, an acceptable ion yield may potentially be obtained. Finally, cluster ion beams are shown to change the nature of sample preparation for biological material. Preliminary data is presented which demonstrates the utility of a C60+ ion beam in tandem with cryogenic freeze-fracture hardware for the interrogation of single biological cells. The results suggest that the experimental strategy is practical for maintaining the 3-D structure of the single cell while exposing an intact, meaningful surface for SIMS imaging analysis. In summary, the work presented here identifies the ion beam type and the instrumental/experimental strategy best-suited for utilizing the special properties of cluster ion bombardment in the SIMS imaging modality. First, we find that by depositing the largest amount of energy as possible in the near-surface region of the solid using a single projectile, the total sputtering yield relative to sample damage becomes optimal for increased sensitivity and 3-D characterization in imaging analysis. Second, we find that by modifying the instrumental/experimental strategy to include a dc cluster ion beam with orthogonal ToF detection, a high fluence VUV light source, and freeze-fracture hardware, the door is opened to rapid sampling, tandem MS-MS, improved secondary ionization efficiency, and well-preserved sample preparation during image analysis. Overall, the developments presented in this thesis improve the prospect for SIMS imaging experiments using cluster ion beams.