Direct metallization methods for microelectronics and advanced manufacturing

Abstract

This thesis examines various direct metallization methods for microelectronics and advanced manufacturing. The reason for this lies in that these methods do not require vacuum-based metal evaporation, complex equipment or multi-lithographic steps. They also eliminate copper waste, as they require no etching, and the need of toxic materials and inert gases. Novel direct metallization methods have been developed for insulating substrates such as glass, silicon, polydimethylsiloxane, thermoplastic polyurethanes and liquid photosensitive acrylic resins. The thesis starts with the investigation of a direct metallization method of the modified photosensitive 2-diazo-2H-naphthalen-1-one (DNQ)-novolac polymer. This was achieved through mixing a positive photoresist with silver salt. Silver ions were thermally reduced to silver nanoparticles inside the polymeric matrix, which allowed a deposition of the electroless copper onto the nanocomposite. This method resulted in the electroless copper films of 0.44 ± 0.05 μm in thickness and (1.6 ± 0.4) ✕ 107 S/m in conductivity. Due to the growing interest in using 3D printing for fabricating electronics, another aim of this thesis is to examine the metallization of newly developed 3D printable thermoplastic polyurethanes. This was carried out by loading polyurethanes with silver ions through an ion exchange mechanism. Surface-modified polyurethanes were exposed to a blue light in order to photo-reduce silver chloride to silver nanoparticles. The photo-reduced silver nanoparticles served as catalysts for the deposition of electroless copper, which resulted in a sheet resistance of (139.4 ± 7.2) mΩ/☐. Being able to modify the liquid photosensitive resin that is used for digital light processing (DLP) 3D printing made it more favorable for use in direct metallization. It was modified with AgClO4 before printing. Once the 3D printed structure was immersed into electroless copper solution, silver ions acted as catalysts for copper deposition. The resultant electroless copper reached a conductivity of (0.29 ± 0.05) ✕ 107 S/m. The interchanged printing with the modified and unmodified resin allowed a selective metallization in which the former was coated with the electroless copper, while the latter was left unaffected.