AFM investigations for development of printed circuit boards

Abstract

The deposition and dissolution of copper are key processes for the manufacturing of printed circuit boards (PCBs). Prior to copper deposition the insulating substrate surfaces are often activated by palladium particles. One important quality factor of this activation layer is the surface structure, especially the layer thickness and the particle size. AFM investigations can offer access to both of these parameters. The structures of different palladium layers, which were deposited electrochemically, electrolessly, by immersion or by adsorption of colloids, are compared. In the second part the electrochemical dissolution of copper under various electrochemical conditions is studied. This process has only rarely been investigated by SXM, in contrast with electrochemical deposition. Our AFM measurements demonstrate the influence of anions, temperature and overpotential on the dissolution.The electrochemical deposition of metals is a common method for manufacturing printed circuit boards and for coating surfaces with metal films. In recent years many scanning probe microscopy (SXM) investigations of electrochemical metal deposition have been performed [1-3]. These measurements have improved our understanding of these processes significantly. In contrast with model electrolytes, technical systems often contain organic compounds which influence the properties of the coatings. The first efforts have been undertaken to close the gap between technical and model systems [4-6]. Thus it has been possible to demonstrate how organic additives influence the growth mode on an atomic level. For some systems it has even been possible to image the adsorption structure of the organic molecules on the surface [7]. Such measurements will help in understanding how some organic molecules can change the layer properties so drastically.Electrochemical deposition is only one way to metallize surfaces. In many cases, i.e. when the surface is nonconductive, other processes, such as electroless deposition, the deposition of colloids and immersion processes, are used in the plating industry. We have measured the surface structure of palladium surfaces deposited in four different ways that are commonly used in technical processes. Palladium plays an important role in plating processes because it can be used as a catalytic active layer for the chemical deposition of further metals and as a surface finish that can be used for the bonding of microelectronic devices. Figure 1 compares typical images of the surface structure of such metal layers. Metal particles, which are surrounded by a ligand shell, are adsorbed by colloidal deposition onto the surfaces. The protecting ligands are dissolved after the deposition step. The particle density on the surface can be tuned by several parameters of the adsorption step. In electroless metal deposition, each colloid is a starting site for the metallization process. The particle density influences the structure of the metal coating and its adhesion strength. If the particle density is sufficiently high the s...