Abstract
A highly selective and self-activated Co-based deposition with dimethylamine borane (DMAB) as reductant for Cu-lines capping process is presented. SEM and TEM images show higher selectivity of Co-based alloy on Cu surface as compared with conventional Pd-activation as a pretreatment step in conventional electroless deposition. Furthermore, an 8.6 % Rs increase via Pd-activation process over the proposed self-activated process indicates that Pd may diffuse into Cu line and induce Rs increase. AFM analysis indicates less roughness for CoPB than CoB film especially for thin film with less than ~17.5 nm thick. Results from GIXRD analysis on as-deposited Co-based films reveal that it has nano-crystalline structure. Such structure changes very little after annealing over 400 °C for 30 min. AES depth profiles also reveal uniform distribution of the elemental components and extremely low B content. Additionally, Cu was not detected on Co cap film, indicating such films could serve as diffusion barrier layer to inhibit Cu diffusion.The oxidation process of electroless CoBP was characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The oxidation peak of DMAB was greatly suppressed with addition of hypophosphite. At low [OH-], obvious oxidation peak of hypophosphite was attained due to insufficient formation of BH3OH-, which showed slow charge transfer on Cu surface; at high [OH-], distinct competition between hypophosphite and OH- and sufficient BH3OH- formed, which enhanced the oxidation of DMAB and indicated faster charge transfer. The selectivity of electroless Co deposition was found to be strongly influenced by [DMAB], [hypophosphite], and [OH-]. Extraneous Co deposition was observed with increasing [DMAB] and [OH-], however, selective Co deposition was obtained with increasing [hypophosphite]. In addition, a one-step, room-temperature route to form carbon-metal bonds via spontaneous reduction of diazonium ion in an acid solution has been developed. This property was later employed to design a new improved process for self-activated electroless Co deposition on Cu without Pd activation. XPS and IR analysis show the evidence of direct organic molecules attachments on Cu. From SEM, electrical resistance (Rs) and line-to-line leakage current observations, the selectivity of Co deposition on the modified Cu surface has been greatly improved. Impedance measurements indicated that the charge transfer of electroless reaction was partially blocked by highly covalent bond to Cu surface.