Paper AS-WeA1
Exploring the Surface Sensitivity of ToF-SIMS: Measuring the Implantation Depths and Sampling Depths of Bi_n and C₆₀ Ion Sources in Organic Films

Wednesday, October 20, 2010, 2:00 pm, Room Cochiti

When a pulsed Bi_n⁺ analysis beam and a direct current C₆₀ etching beam are used in ToF-SIMS dual-beam depth profiling, we observed that increasing the ion dose of the analysis beam decreased the molecular signal as a function of depth. This decay was seen to occur much earlier in the depth profile for less clustered ions, suggesting that damage is related to the implantation depth of the primary ion. Recent experimental and simulation results in this field indicate that implantation depth is inversely related to the clustering of the primary ion, and are observed to affect outcomes such as sputter yield and escape depth (sampling depth). There is a need to characterize these parameters on organic systems, since a better understanding of these parameters will help us choose experimental parameters for the optimal analysis of samples.

In this study, the above mentioned quantities for Bi_n^q+ (n = 1, 3, 5, q = 1, 2) and C₆₀⁺⁺ ions were characterized on molecular and polymeric organic films; a spin-cast trehalose and a plasma polymerized tetraglyme, both about 100 nm thick. The implantation depths of Bi ions were determined using C₆₀ single-beam depth profiling of the same area bombarded by bismuth. The depth scale was calibrated using AFM. The sputter yield was obtained by AFM imaging of the sputter crater. The molecular escape depth was determined by capping a protein-adsorbed mica with tetraglyme overlayers of varying thicknesses and monitoring the protein fragment intensities as a function of thickness. Results show that an ion’s penetration depth is directly proportional to the energy-per-constituent of the ion. For example, increasing the number of Bi atoms from 1 to 3 (Bi₁⁺ to Bi₃⁺) for 25 keV single charged ions decreased the average penetration depth from 18 to 10 nm. When the energy of Bi₃ was increased from 25 to 50 keV, the implantation depth increased from 10 to 14 nm. When implanting bismuth ions, we observed etching of the sample surface, with the crater depths proportional to the cluster size of the bismuth ion. These crater depths were also proportional to the escape depth of the protein fragments. Bi₁⁺ at 25 keV was found to be the most surface sensitive; its sampling depth was only 1.8 nm despite its implantation of 18 nm. The second most surface sensitive was C₆₀⁺⁺ at 20 keV with a molecular escape depth of 2.4 nm. It turned out that the single most important factor that determined the surface sensitivity of the ion source was its sputter depth; both Bi₁⁺ and C₆₀⁺⁺ eroded 0.3 and 1.0 nm, respectively, whereas Bi₅⁺⁺ with a sputter depth of 1.8 nm was the least surface sensitive with a molecular escape depth of 4.7 nm.