Chip Scale Review - March April 2020

28 Chip Scale Review March • April • 2020 [ChipScaleReview.com] c onduc t i ng , t he r oug h ne s s of t he polymer-facing side of the adhesion promoter does not lead to elongation of the high-frequency signal path on account of the skin effect. Other than with a roughening adhesion promoter, w i t h AP 2 t h e c o n d u c t i ve c o p p e r surface retains the f latness of the as- deposited surface, ensuring the same suitabilit y for 5G, radar, and other high-frequency applications. T h e e f f e c t s o f t h e c o p p e r deposit t h ick ness on duct il it y, t he corresponding scaling issue, and the approach to overcome this issue upon composite formation are summarized in Figure 7 . A comparison of copper deposits without dielectric once more shows lower ductility values of medium purity ( Figure 7a ) in comparison to high purity ( Figure 7b ). The decrease of ductility upon decreasing deposit thickness was refer red to earlier as a scaling issue. In general, a strong stabilization of fine copper structures by proper composite formation with the dielectric was observed. This could be deduced from the strong increase of the ductilit y of the composite in comparison to the individual copper mat e r ia l s. The r ei n for cement wa s much more pronounced for high-purity copper deposits. While the benefit in case of medium-purity copper started smaller at low deposit thickness and diminished upon increasing thickness, t h e d u c t i l i t y b e c a m e v i r t u a l l y independent of the thickness in the case of high-purity copper. Summary Chemist r y solut ions can help t o opt imi ze t he r el iabi l it y of coppe r RDL traces, especially at decreasing L/S dimensions. Optimization of the i ndividual copper mater ial may be a ch ieved by i nc r ea si ng t he pu r it y of the deposit via proper design of organic additives. Further significant r e i n f o r c eme n t of t h e me c h a n i c a l properties may be obtained by using a holistic approach, i.e., using synergies with the surrounding materials within the package. Proper adhesion between copper and dielectric allows composite for mat ion . The r efor e, t he va r iou s materials of the package may no longer be considered as individual, but as a whole. This approach allows significant r e i n f o r c eme n t of t h e me c h a n i c a l properties upon a combination of high- purity copper material with a suitable adhesion promoter. References 1. C. Melvin, R. Massey, Chip Scale Review 2017 , 21, 40-44. 2. R. Schmidt, T. Beck, R. Rooney, A. A. Gewirth, “Optimization of electrodeposited copper for sub- 5µm L/S redistribution layer lines by plating additives,” IEEE 68th Electronic Components and Tech. Conf. (ECTC), 1220-1225 (2018). Biographies Ralf Schmidt is Manager R&D Semiconductor at Atotech Group, Berlin, Germany. He holds a PhD in Organic Chemistry and joined the company in 2011. Before his current position, he was team manager in the central R&D department at Atotech. Email ralf.schmidt@atotech.com Thomas Beck is Regional VP, South East Asia at Atotech Group, Berlin, Germany. Prior to his current position, he was Global Product Director Semiconductor—responsible for the definition of R&D projects, integration/introduction and marketing-related activities for ECD (pillar/RDL) and electroless (ENEPIG) processes, respectively, and lead frame and connector products. Figure 7: Normalized ductility of a) medium; and b) high-purity copper as a function of the deposit thickness for copper only (•), as well as a composite of copper and dielectric ( • ).

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