Spatial Chemical Analysis of Electrodeposited Metal Films By Femtosecond Laser Ablation Ionization Mass Spectrometry
Authors: Moreno-García Pavel , Grimaudo Valentine , Riedo Andreas , Cedeño López Alena , Wiesendanger Reto , Tulej Marek , Gruber Cynthia , Lörtscher Emanuel , Wurz Peter , Broekmann Peter 
Published: February 2020

In recent years, the development of three-dimensional large-scale integration (3D-LSI) has been accelerated to overcome the limitations of the classical 2D integration approach, which has begun to deviate from the ideal scaling trends1. Ever increasing complexity of these novel structures that assemble a great variety of materials with distinct physical properties pose new challenges for fabrication and subsequent composition analysis of microchips. In this context, metal electrodeposition plays a crucial role in the manufacturing of the interconnect networks inside the devices. Since these metallic interconnects are manufactured by an additive-assisted metal electrodeposition process, incorporation of the employed additives in the metallic matrices in trace amounts might heavily influence their performance and durability2. Herein we present case studies that analyze two key components of state-of-the-art integrated circuits (ICs), namely through-silicon-vias (TSV)2 and lead-free solder interconnects3 (Sn-Cu). In the former case, accurate targeting and chemical composition analysis of TSVs was addressed by means of three complementary techniques: femtosecond laser ablation ionization mass spectrometry (LIMS technique; τ ~ 190 fs, λ = 775 nm, laser crater diameter Ø ~ 15 µm)4, Auger electron spectroscopy and focused ion beam. Top-down laser ablation analysis of the TSVs allowed qualitative depth profiling that showed a decreasing trend of the C content with depth. This indicates preferential incorporation of organic impurities from the employed plating additives on the top level of the TSV feature. Quantitative C content analysis inside the copper lines was enabled by an alternative experimental approach, in which the chemical composition of the copper structures was analyzed over the feature cross-sections. In agreement with top-down profiling this approach revealed a 1.5-fold increased embedment of C in the upper part of the TSV with respect to the bottom section.
Regarding the second case study, we focused on enabling quantitative depth profiling investigations of Sn-Cu bilayers as representative model system of the target Sn solder bumps deposited on Cu pillars. We report a dedicated study on the quantification of side-walls contributions in the course of fs-laser ablation mass spectrometry depth profiling experiments using the LIMS technique. We identified the prerequisites for quantitative depth profiling of multi-layer systems similar to these Sn/Cu solder bumps and compared different general approaches. Preliminary LIMS results on Sn/Cu solder bumps will be discussed.
Overall, these investigations shed light on the spatial chemical distribution of contaminants in metal films that might enable further improvements of additive-assisted electrodeposition processes.
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