Electrochimica Acta 56 (2011) 9448–9452 Contents lists available at ScienceDirect Electrochimica Acta j ourna l ho me pag e: www.elsevier.com/locate/electacta Bitrex: A new levelling agent for copper J.F.K. Cooper ∗ , C.H.W. Barnes Department of Physics, Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom a r t i c l e i n f o Article history: Received 16 May 2011 Received in revised form 10 August 2011 Accepted 10 August 2011 Available online 18 August 2011 Keywords: Levelling Copper Bitrex EQCM Corrosion a b s t r a c t We investigate the effects of denatonium benzoate (Bitrex) on the electrodeposition of copper ﬁlms from a boric acid bath using scanning electron microscopy, cyclic voltammetry, electrochemical quartz microbalance (EQCM) measurements and corrosion studies. In the absence of Bitrex, pure copper ﬁlms grown by this method are optically black owing to the appearance of complex surface nanostructures. The addition of Bitrex acts as a levelling agent preventing the formation of these nanostructures even for concentrations as low as 0.02 mM producing a lustrous ﬁlm with low surface roughness. Bitrex is also found to improve the corrosion resistance by up to a factor of 20 and increase the current efﬁciency by over a factor of two. Bitrex is hypothesised to act directly on the cathode, partially inhibiting the growth or lowering the deposition current. © 2011 Elsevier Ltd. All rights reserved. 1. Introduction Levelling and brightening agents are widely used in electrode- position as an economical way to modify the morphology of the resulting deposit [1], usually creating a smoother, shinier sur- face. The agents are incredibly varied [2–4], ranging from artiﬁcial sweeteners (saccharin) [5] to foaming agents used in toothpaste (sodium laurel sulphate) and animal glue [6]. Their microscopic actions are still poorly understood [1,7], though they are able to act to produce geometric levelling, orien- tate and reﬁne grains, recent phenomenological models can also now predict their behaviour well [8]. It has also been found that in most cases it is organic molecules which provide levelling [9] and that some are able to complex the metals in solution [10,11]. Herein we present a new levelling agent for copper, denato- nium benzoate (Bitrex) [12]; a commonly used food denaturant and among the most bitter substances known to man. It is non toxic, non cyanide containing and chemically inert. Thus Bitrex is an excel- lent candidate for an environmentally friendly levelling agent for copper. Bitrex was chosen owing to its interesting structure, with an aniline derivative and ionic salt composition. The exact func- tionality of these components is unknown, though the increased corrosion resistance of the Bitrex ﬁlms implies that they are impor- tant (Fig. 1). Copper electrodeposition is a large ﬁeld and much work has been done on levelling agents for copper, particularly from sul- ∗ Corresponding author. E-mail addresses: J.F.K.Cooper@gmail.com, jfkc2@cam.ac.uk (J.F.K. Cooper). phate baths, e.g. [13] and even on Pt [14]. In this study we use boric acid as a buffer solution as it is found to reduce the evolution of hydrogen at the cathode [15]. 2. Experimental The electrolyte was pH 3.9 and was composed of high purity water from Millipore TM with 500 mM boric acid (99.5% purity), 50 mM CuSO 4 (99.995% purity) and a varied amount of denatonium benzoate (Bitrex), all obtained from Sigma–Aldrich. The electro- chemical cell used a saturated calomel reference electrode (with respect to which all potentials are now referred) and a platinum mesh as the counter electrode, placed 4 cm from the working elec- trode. The cell was controlled using a PAR versastat II potentiostat, used in potentiostatic mode. The substrates used were highly pol- ished glass wafers (surface roughness 15–20 ˚ A) with a conducting layer of platinum 10 nm thick, grown by UHV magnetron sputter- ing. They were kept under vacuum until use, and then washed with DI water before being cleaned by oxygen plasma. A set of continu- ous ﬁlms were grown, as well as a set of ﬁlms grown on an array of 5 m × 25 m ellipses lithographically deﬁned in AZ9260 pho- toresist. These were grown in order to easily compare the surface morphology of the ﬁlm to that of the conducting layer as well as to witness the effects of constrained growth. All the ﬁlms were grown under potentiostatic control at −0.55 V (apart from the corrosion study ﬁlms which were grown at −0.65 V) and the thicknesses were controlled using a cut-off charge, aiming for a total thickness of 1 m, based on the pure copper solution current efﬁciency. A range of Bitrex concentrations was used from 0.02 mM to 2 mM. At each concentration cyclic voltammetry was performed 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.08.035