Citation: Santos, R.F.; Oliveira, B.M.C.; Ferreira, P.J.; Vieira, M.F. The Effect of Ultrasonic Agitation on the Seedless Growth of Cu on Ru-W Thin Films. Materials 2023, 16, 167. https://doi.org/10.3390/ ma16010167 Academic Editors: Geoffrey R. Mitchell and Nuno Alves Received: 14 November 2022 Revised: 16 December 2022 Accepted: 19 December 2022 Published: 24 December 2022 Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article The Effect of Ultrasonic Agitation on the Seedless Growth of Cu on Ru-W Thin Films Rúben F. Santos 1,2, * ,† , Bruno M. C. Oliveira 1,2,† , Paulo J. Ferreira 3,4,5 and Manuel F. Vieira 1,2 1 Department of Metallurgical and Materials Engineering, University of Porto, 4200-465 Porto, Portugal 2 LAETA/INEGI—Institute of Science and Innovation in Mechanical and Industrial Engineering, 4200-465 Porto, Portugal 3 International Iberian Nanotechnology Laboratory, 4715-330 Braga, Portugal 4 Materials Science and Engineering Program, University of Texas at Austin, Austin, TX 78712, USA 5 Mechanical Engineering Department and IDMEC, IST, University of Lisbon, 1749-016 Lisbon, Portugal * Correspondence: rbns@fe.up.pt † These authors contributed equally to this work. Abstract: Ru attracted considerable attention as a candidate to replace TaN as a diffusion barrier layer for Cu interconnect metallisation. The addition of W improves the diffusion barrier properties of Ru but appears to weaken the adhesion strength between the barrier and Cu and the direct (seedless) electroplatability behaviour. Although Cu can be directly electroplated on near equimolar Ru-W thin films, no complete substrate coverage is obtained. The understanding of Cu electrocrystallisation on Ru–W is essential to develop methods of fabricating thin, continuous, and well adherent films for advanced interconnect metallisation, where Ru–W thin films could be used as diffusion barriers. This work studies the effect of ultrasonic agitation on the growth of Cu films electroplated on Ru–W, namely on the impact on substrate coverage. Film structure, morphology and chemical composition were evaluated by digital and scanning and transmission electron microscopies, and X-ray diffraction. The results show that Cu particles decrease with increasing current density, but when no electrolyte agitation is applied, substrate coverage is incomplete in the central region, with openings around larger Cu particles, regardless of current density. Under ultrasonic agitation, substrate coverage is remarkably improved. An active particle detachment mechanism is proposed as responsible for attaining improved substrate coverage, only possible at intermediate current density. Lower current densities promote growth over nucleation, whereas higher currents result in extensive hydrogen reduction/formation. Ultrasonic agitation also enhances a preferential Cu growth along <111> direction. Keywords: Cu; Ru; W; diffusion barrier; seedless; electroplating; acidic; ultrasound; agitation 1. Introduction Powerful and efficient integrated circuits (IC) such as central processing units (CPU) and graphic processing units (GPU) are possible due to a continuous miniaturisation of transistors, down to the nanoscale. Such evolution implies a continuous miniaturisation of all the structures inside an IC, including the interconnects. This translates into new challenges related to the materials being used for interconnect lining and metallisation, among others [1]. Interconnect metallisation with Cu has been the industry standard for more than 20 years now, since its introduction by IBM in the late 1990s. Cu suitably replaced Al inter- connects because of its lower resistivity (ρ Al = 2.65 × 10 -8 Ω·m; ρ Cu = 1.68 × 10 -8 Ω·m) and higher electromigration resistance. The fabrication of Cu interconnects, however, re- quires different fabrication processes, different equipment, and the use of a lining to prevent Cu diffusion into the surrounding dielectric materials. TaN has been the industry standard material for that lining purpose, combined with a Ta adhesion layer sitting between TaN and Cu [2]. Although effective as a diffusion barrier, TaN has a relatively high resistivity Materials 2023, 16, 167. https://doi.org/10.3390/ma16010167 https://www.mdpi.com/journal/materials