Combining Anisotropic Etching and PDMS Casting for Three-Dimensional Analysis of Laser Ablation Processes

Abstract

State-of-the-art laser ablation (LA) depth-profiling techniques (e.g. LA-ICP-MS, LIBS, and LIMS) allow for chemical composition analysis of solid materials with high spatial resolution at micro- and nanometer levels. Accurate determination of LA-volume is essential to correlate the recorded chemical information to the specific location inside the sample. In this contribution, we demonstrate two novel approaches towards a better quantitative analysis of LA craters with dimensions at micrometer level formed by femtosecond-LA processes on single-crystalline Si(100) and polycrystalline Cu model substrates. For our parametric crater evolution studies, both the number of applied laser shots and the pulse energy were systematically varied, thus yielding 2D matrices of LA craters which vary in depth, diameter, and crater volume. To access the 3D structure of LA craters formed on Si(100), we applied a combination of standard lithographic and deep reactive-ion etching (DRIE) techniques followed by a HR-SEM inspection of the previously formed crater cross sections. As DRIE is not applicable for other material classes such as metals, an alternative and more versatile preparation technique was developed and applied to the LA craters formed on the Cu substrate. After the initial LA treatment, the Cu surface was subjected to a polydimethylsiloxane (PDMS) casting process yielding a mold being a full 3D replica of the LA craters, which was then analyzed by HR-SEM. Both approaches revealed cone-like shaped craters with depths ranging between 1 and 70 μm and showed a larger ablation depth of Cu that exceed the one of Si by a factor of about 3.

A method for improvement of mass resolution and isotope accuracy for laser ablation time‐of‐flight mass spectrometers

“…Many degraded spectra were recorded close to the surface (left side in Figure , upper panel). We attribute this effect to the initial crater formation process, occurring typically during the first 5000 to 10 000 laser shots . Afterwards, the mass peak intensities and mass resolution stabilise, and only a few files have to be discarded in a random occurrence pattern.…”

“…In particular, when Q‐switched lasers (eg, Nd:YAG laser operating in the ns pulse range) are used, the shot‐to‐shot fluctuations of the pulse are commonly in the range of a few percent, but values high as 40% were measured in our previous measurements . If working close to the ablation threshold, such fluctuations can indeed have a strong impact on the ion production rates …”

“…We can therefore exclude excessive fluctuations in the laser shot‐to‐shot stability as a source for the observed distortions and rather attribute their origin to non‐linear behaviour of the ion yield during crater formation processes and on grain boundaries on heterogeneous or porous sample material . A high ion yield then causes charging of the sample surface or space charge effects inside the ablated ion plume .…”

Review—Laser Ablation Ionization Mass Spectrometry (LIMS) for Analysis of Electrodeposited Cu Interconnects

“…In fact, it is well known that uniform laser erosion of semiconductors requires harsher ablation conditions than metals. 69,70 Therefore, optimal laser conditions had to be initially determined to promote a nonselective removal of the irradiated material above the Si ablation threshold that would at least reach the bottom of single TSV structures (50 μm deep). Thus dedicated LIMS ablation investigations were performed on the TSVs to determine optimal laser irradiances and number of applied laser pulses.…”

Novel 2D binning approach for advanced LIMS depth profiling analysis

“…Recent studies have demonstrated an exponential decrease of the ablation rate for Si and Cu substrates using the same instrument setup. 27 This results in decreased recorded signal intensity and therefore in a lower signal-to-noise-ratio of the detected mass peaks. The lateral averaging of the material, in comparison, keeps the ablation plane at constant depth and therefore also the amount of ablated material.…”