New insights into laser ablation processes of heterogeneous samples: Towards analysis of Through-Silicon-Vias

Moreno-García, Pavel; Grimaudo, Valentine; Riedo, Andreas; Cedeño López, Alena; Wiesendanger, Reto; Tulej, Marek; Gruber, Cynthia; Lörtscher, Emanuel; Wurz, Peter; Broekmann, Peter (2018). New insights into laser ablation processes of heterogeneous samples: Towards analysis of Through-Silicon-Vias. Analytical chemistry, 90(11), pp. 6666-6674. American Chemical Society 10.1021/acs.analchem.8b00492

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State-of-the-art three-dimensional very large-scale integration (3D-VLSI) relies, among other factors, on the purity of
high-aspect-ratio Cu interconnects such as through-silicon-vias (TSVs). Accurate spatial chemical analysis of electroplated TSV
structures has been proven to be challenging due to their large aspect ratios and their multi-material composition (Cu and Si) with
distinct physical properties. Here, we demonstrate that these structures can be accurately analyzed by femto-second (fs) laser beam
ablation techniques in combination with ionization mass spectrometry (LIMS). We specifically report on novel preparation
approaches for the post-ablation analysis of craters formed upon TSV depth profiling. The novel TSV sample preparation is based on
deep and material-selective reactive-ion etching of the Si matrix surrounding the Cu interconnects thus facilitating systematic focusedion-
beam (FIB) investigations of the high-aspect-ratio TSV structures upon ablation. The particular structure of the TSV analyte
combined with the ∅beam > ∅Cu-TSV condition allowed for an in-depth investigation of fundamental laser ablation processes,
particularly focusing on the re-deposition of ablated material at the inner side-walls of the LIMS craters formed. This phenomenon is
of imminent importance for the ultimate quantification in any laser ablation-based depth profiling. In addition, we have developed a
new method which allows the unambiguous determination of the crossing-point of the Si/Cu║bare Si interface upon Cu-TSV depth
profiling which is based on pronounced, depth-dependent changes in the mass-spectrometric detection of those Six
y+species formed
upon the LIMS depth erosion.

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