Molecular dynamic‐secondary ion mass spectrometry (D‐SIMS) ionized by co‐sputtering with C₆₀⁺ and Ar⁺

Abstract

Dynamic secondary ion mass spectrometry (D-SIMS) analysis of poly(ethylene terephthalate) (PET) and poly(methyl methacrylate) (PMMA) was conducted using a quadrupole mass analyzer with various combinations of continuous C(60)(+) and Ar(+) ion sputtering. Individually, the Ar(+) beam failed to generate fragments above m/z 200, and the C(60)(+) beam generated molecular fragments of m/z ~1000. By combining the two beams, the auxiliary Ar(+) beam, which is proposed to suppress carbon deposition due to C(60)(+) bombardment and/or remove graphitized polymer, the sputtering range of the C(60)(+) beam is extended. Another advantage of this technique is that the high sputtering rate and associated high molecular ion intensity of the C(60)(+) beam generate adequate high-mass fragments that mask the damage from the Ar(+) beam. As a result, fragments at m/z ~900 can be clearly observed. As a depth-profiling tool, the single C(60)(+) beam cannot reach a steady state for either PET or PMMA at high ion fluence, and the intensity of the molecular fragments produced by the beam decreases with increasing C(60)(+) fluence. As a result, the single C(60)(+) beam is suitable for profiling surface layers with limited thickness. With C(60)(+)-Ar(+) co-sputtering, although the initial drop in intensity is more significant than with single C(60)(+) ionization because of the damage introduced by the auxiliary Ar(+), the intensity levels indicate that a more steady-state process can be achieved. In addition, the secondary ion intensity at high fluence is higher with co-sputtering. As a result, the sputtered depth is enhanced with co-sputtering and the technique is suitable for profiling thick layers. Furthermore, co-sputtering yields a smoother surface than single C(60)(+) sputtering.

Modeling dynamic cluster SIMS experiments

“…Computer simulations show that by using sample rotation, these elongated ridges and valleys do not form, the surface gets smoother, and the sputtering yield increases by approximately 10%. This value is actually comparable with the yield increase with co-sputtering reported by You et al [4] for organic, polymeric samples. Thus, we propose that the Ar beam, which is oriented 33 with respect to the C 60 beam, is breaking up the ridges formed by the C 60 bombardment, effectively smoothing the surface.…”

“…There is another possibility that may contribute to the enhancement of the depth profiles [4,5] of organic solids with co-bombardment by C 60 and 200 eV Ar when compared with C 60 bombardment. As discussed earlier, high-fluence simulations of C 60 impacts show that C 60 bombardment at 70 incidence creates a roughened surface with elongated ridges and valleys parallel to the beam direction.…”

“…First, as predicted by an erosion dynamics model, the total sputtering yield relative to the damage created should be large for effective depth profiling. [18] The experimental sputtering yield reported by You et al [4] ranges from 1 to 3 nm 3 per incident ion. This yield is, however, very small compared with, for example, the experimental yield for 40 keV C 60 bombardment of cholesterol at 73 incident angle, for which the yield is two orders of magnitude larger.…”

“…In the second example, we examine the effect of low energy Ar co-bombardment as a means of improving C 60 depth profiles. [4,5] In this case, the interpretation is more intricate, weaving together a logical explanation based on the simulations and the experimental data.…”

“…On the basis of calculations for a single angle of incidence, we predict that sample rotation at 70 will also help experiments using Au 3 and large Ar cluster beams. [6] C 60 and low energy Ar co-bombardment An intriguing set of experiments from Shyue et al [4,5,[9][10][11][12][13][14][15] shows promise of assisting depth profiles by co-bombardment of the system with C 60 and low energy Ar ions. This strategy for eroding samples has, as yet, only been implemented by one research group because most SIMS instruments with C 60 ion beams do not also have a low energy Ar ion beam.…”