Chip Scale Review - March April 2020

28 Chip Scale Review March • April • 2020 [] 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 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 ( • ).