(Invited) Towards Spatially Resolved Chemical Analysis of Sn/Ag Solder Bumps By Means of Laser Ablation Ionization Mass Spectrometry (LIMS)
Authors: Cedeño López Alena , Moreno-García Pavel , Grimaudo Valentine , Riedo Andreas , Tulej Marek , Wiesendanger Reto , Lukmanov Rustam , Wurz Peter , Broekmann Peter 
Published: February 2020

Following the ban of Sn/Pb solders from the markets eutectic Sn/Ag alloy has become the industrially most relevant option in the field of microchip fabrication1. Sn/Ag solder bumps are mostly prepared by means of through-mask electrodeposition. Due to the inherently different electrochemical properties of Sn and Ag, their efficient co-deposition requires optimization of plating parameters as well as the use of numerous organic plating additives2-5, rendering the development of high-performance plating bath compositions a very demanding task. Major criteria for assessing the quality of a plating formulation are the fraction of Ag and the homogeneity of Ag distribution within the as-deposited solder alloy, as these characteristics are crucially relevant for homogeneous melting and defect-free recrystallization during the soldering process1. It is of urgent interest to microchip industry to minimize embedment of the plating additives into the solder material, as these might cause voids during soldering and may lead to a reduced lifetime of the devices under working conditions.
Investigation of these aspects requires sophisticated and highly spatially resolved depth profiling methods providing both lateral and vertical information on the solder composition and possible organic residues. To date, well-established composition analysis techniques for solid materials are only partly able to provide such data. In this contribution, we present depth profiling by means of femtosecond laser ablation ionization mass spectrometry (fs-LIMS) using the LMS instrument7, 8 as an approach to achieve these goals.
For this purpose, we study fundamentals of fs-laser-matter interaction on Sn and Sn/Ag alloys with particular focus on the influence of different laser characteristics (IR and UV light, laser intensity) on ablation and reflow processes taking place on these materials. Furthermore, we compare different depth profiling approaches, i.e., spatially resolved one-dimensional depth profiling and two-dimensional raster matrices with different geometries. Moreover, we aim towards three-dimensionality with a sample-tailored raster approach that allows to investigate the electroplated deposit layer by layer or to even select specific regions of a given layer only. We demonstrate how this approach yields improved results especially in the analysis of laterally highly confined solder bump arrays.
The thus obtained data allow for extraction of depth profiles of the major species Sn and Ag, as well as C, O, and S from the embedded additives within the alloy matrix. These are required to assess the homogeneity and the purity of Sn/Ag electroplated depositions from different bath formulations or under differential plating parameters. Correlation of different elemental profile data to each other might contribute to the evaluation of whether additive or electrolyte embedment takes place and thereby may crucially help to bring forward plating formulation development.

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