JOURNAL OF APPLIED ELECTROCHEMISTRY 25 (1995) 603-608 Effect of surface active agents on the initial formation of electrodeposited copper layers R. RASHKOV, C. NANEV Institute of Physical Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria Received 15 August 1994; revised 20 December 1994 The kinetics of formation and the structure of copper layers are investigated in the presence of a com- plex additive for bright acid copper plating. It is shown that, in the presence of additive, two maxima are observed on the voltammetric curve. The first may be explained in terms of the formation of a complex between the components of the additive and the cuprous ion. The structure of the initial copper deposits is studied using the double pulse technique. It is shown that the size of separate crystallites strongly decreases in the presence of the additive which also blocks vertical growth and favours lateral growth. 1. Introduction In recent decades, numerous detailed investigations on the effects exerted by additives with different com- position upon sulfuric acid copper plating electrolytes have been carried out. Many papers dealing with the behaviour of surface active agents during the electro- deposition of copper coatings with good physical- mechanical properties have been published [1-11], but a complete picture of the mechanisms by which the respective additives influence the formation of the layers is still lacking. Compact polycrystalline metallic layers are highly complex structures and, according to Baraboshkin [12], when deposited on substrates, three stages can generally be observed as a function of time. These are: (i) the nucleation of separate crystallites on the substrate, (ii) the growth of separate crystallites until they merge into a compact layer, and (iii) the collec- tive growth of crystallites in the compact layer. These stages depend on the conditions and regime of electrodeposition. For example, the second stage terminates with the merger of crystallites in a com- pact layer only in cases when their size, up to the moment they merge, has not reached dimensions at which the growth becomes unstable, that is, it is trans- formed into a dendrite. After the merger into a compact layer, the crystal- lites continue their growth. At this stage the aniso- tropy of growth rate begins to exert a strong effect. The so-called geometrical selection becomes active: crystallites with favourable orientations with respect to the electrical field lines start growing faster and, as a result, a predominant orientation of crystalline grains appears. Thus, compact metallic layers with a columnar structure are formed. When morphological stability is disturbed, that is, when the size of crystallites reaches a certain critical value [13] skeletal growth starts and the crystal layer is transformed into a dendritic polycrystalline 0021-891X © 1995 Chapman & Hall structure. This suggests that single crystals, compact layers and dendritic layers are cognate structures, since they are developed from crystallites initially originating on the substrate. The present paper is aimed at the in situ investi- gation of the effect of surface active agent upon the initial stages of copper layer formation. The complex additive (CA) for bright acid copper plating [14] was investigated. 2. Experimental details The investigations were carried out by applying a method [15] which combines both microscopic- interferometry and electrochemical (potentio- dynamic) techniques, shown in Fig. 1. It offers the possibility of studying both the initial stages of metal deposition in situ and to modelling the levelling processes caused by surface active agents, dendritic formation, etc. The basic element of the method is the 'two-dimensional' glass cell, provided with a Luggin capillary (d = 100#m) for fixing the SSE reference electrode (Hg-sulfate, potential + 678 mV). This approach offers the possibility of monitoring changes in the concentration during the process by interferometry. Due to the small distance between the plane-parallel glass plates (the gap is less than 600#m) and the uniform temperature of 20°C, natural convective flows are thought to be negligible for the case of vertically placed electrodes [16, 17] while in other geometries [18] the convection may play a crucial role. The lack of convection permits us to use low scan rates (1-2mVsq). The cathodes were made of platinum or nickel 300 #m thick foil fixed in the cell at a distance 30-50#m from the Luggin capillary. The anodes were copper plates. The nickel electrodes were coated in advance with an amorphous Ni-P layer to eliminate any epitaxial effect of the substrate (area about 10 mm2). The effect of oxygen was eliminated by purging the electrolytes 603