Minimum Thickness of Continuous Cu Layers on Mo from Fluoride Free Electrolytes

Abstract

Mo as a refractory metal has physical properties which are of interest for applications in CIGS photovoltaic cells fabrication and for next generation of liners for Cu interconnects[1]. Therefore it is of fundamental and practical interest to develop a process for Cu metallization of Mo surfaces where contiguity and high coverage is achieved at minimum thickness. However, one of the obstacles that prevent readily use of Mo as substrate for Cu electrodeposition is tendency of its surface to form a high quality oxide[2] ,[3].The surface energy of Mo-oxide is much lower than Cu which results in sporadic 3D nucleation and low adhesion of Cu deposit[2]. The presented work addresses this fundamental problem by combining the knowledge/breakthroughs demonstrated in the field of electro-polishing of refractory metals such as Nb[4] with common pulse electrodeposition practice[1] ,[5]. A design of deposition pulse reverse potential/current function for Cu is presented which converts a Mo-oxide/Cu to a Mo/Cu interface. Elaborate measurements of the thin film impedance during the Cu growth complemented with Pb UPD decoration studies and AFM surface morphology analysis are presented. These data indicate that Cu layers reach full contiguity and coverage of the Mo substrate at the thickness level of 3-5 nm. Considering that common Cu electrodeposition bath chemistry for interconnects contains additives whose incorporation makes Cu deposit somewhat incompatible with photovoltaic layers[1], this work presents the Cu solution chemistry which is additive free and can serve as a base for further improvement of the specific Cu deposition processes depending on its application.[1] . J. Bi et al, Journal of Power Sources, 326, (2011) 211.[2] . D. Mercier et al, Journal of the Electrochemical Society, 160 (2013) 3103.[3] . M. Pourbaix, Atlas of electrochemical equilibria in aqueous solutions. 2d English ed. 1974, Houston, Tex.: National Association of Corrosion Engineers.[4] . Journal of The Electrochemical Society, 160 (9) E94-E98 (2013).[5] . J.C. Puippe and F. Leaman, Theory and Practice of Pulse Plating, AESF, (1986).