Spatial Chemical Analysis of Electrodeposited Metal Films By Femtosecond Laser Ablation Ionization Mass Spectrometry

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 trends 1 . 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 durability 2 . 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 interconnects 3 (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 thes