Secondary ion mass spectrometry depth profiling of ultrathick films using an argon gas cluster source: Crater shape implications on the analysis area as a function of depth

Abstract

Argon cluster ions have enabled molecular depth profiling to unprecedented depths, with minimal loss of chemical information or changes in sputter rate. However, depth profiling of ultrathick films (>100 μm) using a commercial ion source oriented at 45°to the surface causes the crater bottom to shrink in size because of a combination of the crater wall angle, sputter rate differences along the trailing-edge crater wall, and undercutting on the leading-edge. The shrinking of the crater bottom has 2 immediate effects on dualbeam depth profiling: first is that the centering of the analysis beam inside the sputter crater will no longer ensure the best quality depth profile because the location of the flat crater bottom progressively shifts toward the leading-edge and second, the shifting of the crater bottom enforces a maximum thickness of the film that could be depth profiled. Experiments demonstrate that a time-of-flight secondary ion mass spectrometry instrument equipped with a 20 keV argon cluster source is limited to depth profiling a 180 μm-thick film when a 500 μm sputter raster is used and a 100 μm square crater bottom is to be left for analysis. In addition, depth profiling of a multilayer film revealed that the depth resolution degrades on trailing-edge side of the crater bottom presumably because of the redeposition of the sputtered flux from the crater wall onto the crater bottom.

ToF‐SIMS depth profiling of oral drug delivery films for 3D visualization of active pharmaceutical particles

“…The size of the particles was also not uniform across the volume, and this was reflected in the ToF‐SIMS depth profiles as well as the quantitated results of each run in Table . Another possible confounding issue was the change in sputtering yield caused by topography generation at the crater bottom or redeposition of sputtered neutrals, both of which have been observed to occur at these depths . As topography is generated, the slowing sputter rate would be expected to yield changes in the measured intensities.…”

“…Since depth profiling of ultrathick organic films (>100 μm) is just now being explored, parameters such as sputter rate, damage accumulation, and topography generation in these materials have not yet been carefully studied. For example, redeposition of the sputtered flux along the crater bottom occurs due to the low sputter take‐off angles of the cluster source, but its effect on quantification or the quality of the image remains unclear. As such, studying the effects of these parameters is an important step in developing the SIMS technique for biomedical device validation and quality control.…”

Deep depth profiling using gas cluster secondary ion mass spectrometry: Micrometer topography development and effects on depth resolution

“…Ar cluster bombardment allows sputtering of samples such as conjugated polymers that were previously not feasible using SF 5 + or C 60 + sources 1–3 and the large cluster size with low incidence energy per atom combine to minimize damage accumulation 4–6 and topography development 7–9 . These benefits allow for molecular compositional depth profiling of organic films with depth resolutions approaching a few nanometers, while preserving the sputter rate and chemical information even at sputter depths greater than 10s of micrometers 10–12 . Although the strength of ToF‐SIMS depth profiling lies in its ability to characterize nanometer features, the cluster source gives it the flexibility to provide mass‐specific 3D images of chemical components inside very thick films such as biological samples, 13 biomaterials, 14 and drug delivery systems 12 where 10s and even hundreds of micrometers of material need to be removed.…”

“…The result of this is that a micrometer‐sized sphere embedded in a film would appear as a diagonally stretched spheroid in the 3D SIMS image. Second, the sputter beam is also fixed at 45° which leads to a lateral shift in the sputter crater with depth, forming a wedge‐shaped crater 11 . Because the two ion beams are orthogonal to each other, this results in the reduction of the analysis area with depth which places a physical limit on the maximum analyzable depth.…”

“…In addition, the gas cluster source creates a pressure wave that results in a very low angular distribution for the sputtered material of less than 30° from the surface 20,21 . Inside a wedge‐shaped crater, the sputtered flux will redeposit along the crater wall and affect the depth resolution through resputtering 11 . Lastly, the crater bottom roughness encountered in deep depth profiling is on the order of a few micrometers, 11,12 which is thought to induce nonuniform sputter yields and degrade depth resolution, but no studies have yet fully addressed the impact of this effect on the quality of the 3D SIMS image or explored approaches for its mitigation.…”