Copper electrodeposition from a pH 3 sulfate electrolyte A. VICENZO and P.L. CAVALLOTTI* Dipartimento di Chimica, Materiali e Ingegneria Chimica ‘‘Giulio Natta’’, Politecnico di Milano, via Mancinelli 7, 20131 Milan, Italy (*author for correspondence, fax: þ39 02 23993180, e-mail: pietro.cavallotti@polimi.it) Received 17 December 2001; accepted in revised form 10 April 2002 Key words: copper electrodeposition, electrokinetics, interconnects, plating additives Abstract Kinetics and growth modes of copper electrodeposited from copper sulfate 0.8 M, pH 3 electrolytes are studied and compared to those for standard acidic sulfate baths. The influence of chlorides and three different additives (polyethylene glycol, a thiocompound and a quaternary ammonium salt) on steady state and transient electrokinetic behaviour, structure and morphology is investigated. Chlorides behave as a surface stabilizing agent, promoting epitaxy and contributing to the electrolyte microlevelling power. Each organic additive shows a specific effect which, however, may change substantially in the presence of other additives. Polyethylene glycol is a suppresser with the strongest action on copper electrodeposition, as shown by its effects on discharge kinetics and growth behaviour. The main role of the brightener (thiocompound) and the leveller (quaternary ammonium salt) consists in mitigating or tuning the surfactant blocking action respectively, realising the best balance between catalytic and suppressing factors. The pH 3 electrolyte is shown to be a valuable alternative to the standard acid bath, allowing higher deposition rates and promising improvements with regard to microthrowing power and growth control of thin films. 1. Introduction Essential requirements for copper plating of ultrascale integrated interconnects are voids-free filling of trenches and vias and thickness distribution uniformity at wafer scale. The achievement of both results depends on different and related factors: in particular, overpotential distribution and concentration gradients at the surface. Phenomena controlling uniformity at micro and macro scale should be mastered through a careful selection of plating bath composition, including additives, and deposition rate. In this respect, a suitable choice of the base electrolyte composition (i.e., CuSO 4 and H 2 SO 4 concentration) should be considered the starting point for improving plating performance. The acid sulfate bath has become the standard electrolyte for copper plating in microelectronics for its long-lasting use in printed circuits manufacture. A ‘low- acid’ sulfate bath with high copper concentration was proposed by Landau [1], stressing its potential benefit with respect to standard electrolytes, thanks to conduc- tivity decrease and copper transport rate enhancement. As discussed by Landau, lowering the electrolyte con- ductivity minimises the terminal effect due to the resistive seed layer, thus improving deposit uniformity. With regard to the ability to fill trenches and vias, the relative value of activation and concentration resistance is important [2], since both high metal cations concen- tration and low activation resistance at the bottom of the vias are desirable. Thus, high copper concentration is expected to be beneficial to via filling, as also stated in other recent reports [3, 4] and easily explained when the factors affecting deposit microdistribution are taken into consideration [5]. Actually, it was ascertained that increasing pH and copper sulfate concentration micro- roughness decreases [6], as a consequence of increasing copper transport number, that is, lowering concentra- tion polarization has a beneficial effect on deposit microdistribution. The standard copper plating bath for wafer metalli- zation contains, besides H 2 SO 4 to provide a high throwing power, a multicomponent additive system, especially formulated to achieve super-filling [7]. Addi- tives selection must be optimized, since only their synergetic action produces the desired balance of inhi- bition and acceleration [8], particularly with regard to the topographic features of the patterned substrate. In the present study, the specific role of a set of additives, representative of the kind of addition agents employed in copper plating for interconnects, is investigated. Kinetic behaviour during deposition, structure and morphology of copper layers on blanket wafers are characterised and discussed with regard to additive influence. Journal of Applied Electrochemistry 32: 743–753, 2002. 743 Ó 2002 Kluwer Academic Publishers. Printed in the Netherlands.