A Co-based surface activator for electroless copper deposition

Abstract

The activation of the dielectric surface by the colloidal solution of cobalt compounds was examined as a palladium-free activation process. The principle of the process is based on the deposition of a Co-based precursor film on the substrate surface to be metalized and on the reduction of the adsorbed cobalt particles. The reduction of the adsorbed cobalt oxy/hydroxy compounds was carried out by dipping the treated surface in an alkaline solution of borohydride both at room and elevated temperatures. It has been determined that the presence of a small amount of Cu^2 + ions in the colloidal solution of cobalt compounds catalyzes the reduction of adsorbed Co-based precursor on the dielectric surface. Furthermore, the formed Co(0) seeds initiate electroless copper deposition. A continuous copper film was deposited on a glass sheet after its activation in a colloidal solution of cobalt compounds containing Cu^2 + ions in contrast to that activated in the same solution without Cu^2 + ions.

Introduction

Electroless copper deposition is an essential process in manufacture of printed circuit boards (PCB). This process is a heterogeneous autocatalytic electron transfer reaction, in which electrons are transferred from a reducing agent to Cu(II) ions at the interface. The formed metallic copper layer acts as a catalyst for further reduction of copper ions. However, for deposition of Cu on noncatalytic surfaces the activation of these surfaces is needed to produce catalytic nuclei centers, which initiate the catalytic deposition of Cu. The precious metals such as silver [1], [2], [3], gold [4], [5] and palladium can be used as activators for nonconductive surfaces, with palladium being the best of them. Therefore palladium is widely used in practice.

Despite a very small amount of catalyst used for activation of metal deposition, a great quantity of printed schemes requires considerable amounts of palladium to be spent. A steady decrease in raw palladium recourses and increase in price enforces to find ways to replace palladium by non-precious metals. The metals of the iron group (Fe, Co, Ni) and copper are the best ones for initiation of electroless copper and nickel deposition.

As seen from the survey of references [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], the creation of new palladium-free processes for electroless metallization of non-catalytic surfaces continue to be relevant [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. Diverse methods and their combinations were proposed to seed the surface both with Pd(0) [11], [14], [15] and non-precious metal species: immobilizing metal nanoparticles as catalyst site on the plastic surface [6], [7], [8], grafting of functional groups onto surface capable to bind metal ions [13], [14], [15], the creation of activation solutions containing non-precious metals [9], [10]. The latter method is simple, easily arranged to the existing equipment in practice and cost effective, while the other mentioned ones require the Ar plasma, UV or VUV treatment, high temperature and pressure [6], [7], [13], [14], [15]. The Pd-free activation of dielectrics prior to electroless copper metallization using Ni(0) or Cu(0) seeds for the initiation of electroless deposition is described in [7], [10], [13], [14], [15].

In our previous studies a stable colloidal solution of cobalt complexes [16], which is an improved version of so-called PLATO technologies [17], was proposed for deposition of cobalt sulfide films on various surfaces. These films are used for galvanic metallization of insulating surfaces, and the coatings have a good adhesion to them. The idea to use this colloidal solution for initiation of electroless copper deposition was the beginning of present investigation. The colloidal nanoparticles of cobalt compounds adsorbed on the solid surface were reduced to the metal state chemically. In the present study we investigated the autocatalytic property of Co. The first step deals with the deposition of a thin layer which consists of cobalt compounds (denoted as a Co-based precursor) and with its chemisorption on the substrate surface. For this purpose a colloidal solution of cobalt compounds was used. The second step is the reduction of adsorbed cobalt compounds on the substrate surface from the Co-based precursor to Co(0). The cobalt compounds adsorbed on the substrate surface were reduced chemically by dipping the latter in an alkaline borohydride solution. The third step concerns the copper metallization itself. Fig. 1 shows the process of dielectrics metallization using the Co-based precursor. The main step of dielectrics metallization is the seeding of the substrate surface to be metalized with Co(0) particles which should be able to initiate the electroless copper plating. In the present work additional studies on the activator (Co-based precursor) have been carried out using electrochemical quartz crystal microbalance (EQCM). Due to its sensitivity this method allows determining in-situ small changes in the electrode mass which are proportional to the changes in the quartz crystal resonant frequency by Sauerbrey's equation [18]:

$$\Delta f=-2\frac{f_0^2\Delta m}{S\sqrt{{\mu }_q{\rho }_q}}=–K\Delta m$$

where f₀ — is the resonant frequency of the quartz crystal, S is the piezoelectrically active area (cm²), μ_q is the shear modulus of the quartz (2.947 10¹¹ g cm^− 1 s^− 2) and ρ_q its density (2.648 g cm^− 3) [19]. K includes all the constants of Eq. (1) and for fundamental resonance frequency of 6 MHz is equal to 128.152 × 10⁶ Hz g^− 1 corresponding to a sensitivity of 12.26 ng Hz^− 1 cm^− 2. This corresponds to the sensitivity of the QCM used 0.081 Hz ng^− 1 cm². This method allows obtaining more comprehensive data on the processes occurring during the chemical reduction of the adsorbed Co-based precursor to Co(0) in the borohydride solution. The surface morphology and composition of the samples were examined using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS).