AVS 57th International Symposium & Exhibition
    Applied Surface Science Tuesday Sessions
       Session AS-TuP

Paper AS-TuP28
An Investigation of Primary Ion Choices in Depth Profiling Using Time-of-Flight Secondary Ion Mass Spectrometry

Tuesday, October 19, 2010, 6:00 pm, Room Southwest Exhibit Hall

Session: Applied Surface Science Poster Session
Presenter: Z.H. Zhu, Pacific Northwest National Laboratory
Authors: Z.H. Zhu, Pacific Northwest National Laboratory
V. Shutthanandan, Pacific Northwest National Laboratory
Correspondent: Click to Email

Depth profiling is one of the important applications of time-of-flight secondary ion mass spectrometry (ToF-SIMS). Dual beam depth profiling strategy is commonly used because the current of the primary ion beam is normally very weak (~10-12 A), and the second beam with high current (10-8-10-6 A) is introduced for sputtering. Recent years, a major development in ToF-SIMS field is application of cluster primary ions. It has been found that cluster primary ions can dramatically enhance signal intensity of molecular ions with a factor of 10-1000. So far, cluster primary ions have been introduced into commercial ToF-SIMS instruments for over five years. However, in presently available commercial ToF-SIMS instruments, the usable currents of cluster primary ion beams are considerably smaller than that of monatomic primary ion beams. More importantly, enhancements using cluster primary ions are not only material-dependent but also ion species-dependent. Therefore, large amounts of experimental data are needed to develop an understanding of how to choose an optimal primary ion for ToF-SIMS depth profiling.

In this work, we studied depth profiling of several representative systems: (1) Hydrogen depth profiling in silicon wafer, zinc oxide crystal and normal glass; (2) molecular depth profiling of a thin sucrose film on silicon substrate; (3) molecular depth profiling of Irganox “delta” layers prepared by NPL, United Kingdom . Six primary ion beams (25 keV Bi+, Bi3+, Bi5+, 50 keV Bi3++, 10 keV C60+ and 20 keV C60++) were compared. Our data show that enhancement of cluster primary ions for elemental species is limited (~1.5-4.0 times) if compared to that for molecular species (>>10 times). Considering the highest usable currents of difference primary ion species on presently available commercial ToF-SIMS instruments, we suggest that monatomic primary ion beams may be the best choice for elemental depth profiling, and cluster primary ion beams are better choices for molecular depth profiling.