Fundamental Studies of Molecular Depth Profiling with ToF-SIMS and Cluster Ions

Open Access
Author:
Lu, Caiyan
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 06, 2010
Committee Members:
  • Nicholas Winograd, Dissertation Advisor
  • Nicholas Winograd, Committee Chair
  • Barbara Jane Garrison, Committee Member
  • Christine Dolan Keating, Committee Member
  • Qiming Zhang, Committee Member
Keywords:
  • ToF-SIMS
  • molecular depth profiling
  • cluster ions
Abstract:
Cluster ion beams opened up new capabilities for molecular depth profiling with time-of-flight secondary ion mass spectrometry (ToF-SIMS). The thesis work presented here is aimed at understanding the fundamentals of molecular depth profiling and improving the performance of ToF-SIMS with C60+ projectiles on 3-dimensional (3-D) characterization of organic and biological materials. Ionization effects in depth profiling of trehalose films are investigated to elucidate the influence of water and salt contents on the formation of molecular ions under C60+ bombardment. An Irganox standard obtained from NPL with delta layers is used to evaluate the reliability and reproducibility of depth profiling through organic samples. A new delta layer system constructed with lipids and fatty acids is formed via the Langmuir-Blodgett (LB) technique. This sample is used to optimize depth profiling parameters, such as, sample temperature, primary ion beam energy and incident angles, as well as, determine the depth resolution of biological molecules. The results indicate that the depth resolution is predominately influenced by ion beam induced mixing at the organic-organic interfaces and not an artifact of topography. Overall, the optimal experimental parameters for biological depth profiling and 3-D imaging are at cryogenic temperature with a low kinetic energy C60+ beam at a glancing angle. A different experimental strategy is tested on a ~400 nm LB multilayer thin film using both Au+ and C60+ beams. Bombardment by Au+ projectiles results in significant damage and extremely low sputter efficiency on the LB film. The following sputtering by a C60+ beam is able to remove the damage and recover the molecular ion signals.