Developing molecular depth profiling and dynamic imaging with TOF-SIMS and cluster ion beams.

摘要

This thesis work is aimed toward developing molecular depth profiling with time-of-flight secondary ion mass spectrometry (TOF-SIMS) and the emerging cluster ion sources, mainly C60+. This unique tool for molecular surface characterization exhibits surface sensitivity and chemical imaging capability, both highly desirable features in materials characterization. However, the approach has not yet found full application to the study of organic and biological samples since the traditional atomic ion sources induce crucial fragmentation of molecules during desorption, and the sensitivity and formation of molecular ions generally drops significantly. This situation has been dramatically changed recently with the development of cluster ion sources. New physics is believed to be involved with the cluster ion bombardment that is quite different from atomic bombardment, and a number of exciting new properties have been observed. The most interesting property is the potential of molecular depth profiling.;A fundamental knowledge of these properties is highly desirable to speed future application development. Hence, the basic concepts of the issues discussed in this thesis are introduced in Chapter 1. Next, a defined, reproducible platform for examining the details of cluster/solid interactions has been constructed as presented in Chapter 2. Successful molecular depth profiling, as well as sputtering parameters were obtained from this platform. With these parameters, an erosion dynamics analytical model is also developed in Chapter 3. Two parameters have been found to be critical for successful molecular depth profiling, a high sputtering yield and a minimized damage factor of the bombardment. Although this model is semi-quantitative, it facilitates the elucidation of the essential physics of cluster/solid interactions. This physical understanding is key to fully applying new properties of cluster ion bombardment. Multiple examples based on this platform are studied in Chapters 3 and 5 to confirm this model and to examine the basic sputtering properties of C60+ on more complex materials. This approach has also proven valuable in comparing different ion sources quantitatively. Results in Chapters 3 and 4 have demonstrated that the C60 + ion source exhibits superior sputtering properties when compared to Aun+ (n=1-3). The results from this study are opening new research avenues for cluster ion sources. An important goal is to be able to perform three-dimensional molecular dynamic SIMS imaging. Preliminary studies aimed toward identifying the critical issues for 3-D imaging will be presented in Chapter 6 with future directions summarized in Chapter 7. Overall, the results presented here establish the foundation for the characterization of molecular solids using cluster ion beams and mass spectrometry.