Direct comparison of Au(3)(+) and C(60)(+) cluster projectiles in SIMS molecular depth profiling

Abstract

The sputtering properties of two representative cluster ion beams in secondary ion mass spectrometry (SIMS), C(60)(+) and Au(3)(+), have been directly compared. Organic thin films consisting of trehalose and dipalmitoylphosphatidylcholine (DPPC) are employed as prototypical targets. The strategy is to make direct comparison of the response of a molecular solid to each type of the bombarding cluster by overlapping the two ion beams onto the same area of the sample surface. The ion beams alternately erode the sample while keeping the same projectile for spectral acquisition. The results from these experiments are important to further optimize the use of cluster projectiles for SIMS molecular depth profiling experiments. For example, Au(3)(+) bombardment is found to induce more chemical damage as well as Au implantation when compared with C(60)(+). Moreover, C(60)(+) is found to be able to remove the damage and the implanted Au effectively. Discussions are also presented on strategies of enhancing sensitivity for imaging applications with cluster SIMS.

Figure 1

Secondary ion signal intensities are plotted against the primary ion dose during the alternative bombardment by ${\text{Au}}_3^{+}$ and ${\text{C}}_{60}^{+}$. The order of the projectile that is used to sputter for each turn is indicated on the figure. The inset is an optical image of the film after the overlap depth profile experiment. The smaller crater is created by ${\text{C}}_{60}^{+}$ sputtering while the bigger crater on the outside is created by ${\text{Au}}_3^{+}$ sputtering. All of the spectra are taken from the overlapped center area. The field of view is 1360 μm × 1020 μm.

Figure 2

(a) Plot of the ratio between m/z 437 to m/z 452 (M + H of GGYR) versus accumulated total ion dose. (b) XPS image of Au (Au 4f in red) and C (C1s in green) from part of the crater after extensive bombardment of ${\text{Au}}_3^{+}$. The field of view is 800 μm × 800 μm.

Figure 3

Plot of the ratio between m/z 19 to m/z 18 versus accumulated total ion dose.

Figure 4

Depth profile plot of the DPPC molecular ion M + H⁺ (m/z = 735) and principle DPPC headgroup fragments phosphocholine ${\text{C}}_5{\text{H}}_{15}{\text{NPO}}_4^{+}(m/z=184)$ and choline C₅H₁₂N⁺ (m/z = 86)secondary ion signal intensities versus the primary ion dose accumulated during alternative bombardment with ${\text{Au}}_3^{+}$ and ${\text{C}}_{60}^{+}$. The sputter projectile used over a specified primary ion dose is illustrated in the figure.

Figure 5

Plots of the ratio between the fragmented choline ion C₅H₁₂N⁺ (m/z = 86) and the phosphocholine ion ${\text{C}}_5{\text{H}}_{15}{\text{NPO}}_4^{+}(m/z=184)$ and the ratio between H₃O⁺ (m/z = 19) and H₂O⁺ (m/z = 18) (as a measure of surface protons accumulated during the depth profile) versus primary ion dose accumulated during alternative bombardment with ${\text{Au}}_3^{+}$ and ${\text{C}}_{60}^{+}$.

Figure 6

Depth profile plot of DPPC molecular fragments phosphocholine ${\text{C}}_5{\text{H}}_{15}{\text{NPO}}_4^{+}(m/z=184)$ and choline C₅H₁₂N⁺ (m/z = 86) secondary ion intensities versus primary ion dose. Solid symbols represent secondary ion intensities acquired during a depth profile using ${\text{C}}_{60}^{+}$ only (as both the sputter source and the analysis source) and open symbols represent secondary ion intensities acquired during a depth profile using ${\text{Au}}_3^{+}$ only. The inset is an AFM image of a dry, porous DPPC film bombarded with ${\text{C}}_{60}^{+}$ at 200 × 200 μm² field of view. Image is taken at a 400 × 400 μm² field of view and indicates the film is 1000 nm thick with a 100 nm surface roughness.