Electrostatic Coating with Charge-Compensated Ligandless Copper Nanoparticles

Abstract

A nonaqueous electroless deposition (ELD) coating process that uses a charge compensator in lieu of a ligand or complexing agent is presented. Si(100) coupons were hydroxylated using a sulfuric acid-hydrogen peroxide mixture (SPM or piranha). The surface was terminated with an amine group by immersion in a 5 mM solution of (3-aminopropyl)-trimethoxysilane (APTMS) in methanol followed by a 150°C anneal. Metal films were deposited by suspending samples in a bath made by dissolving Cu(II) chloride in ethylene glycol, which also served as the reducing agent, and adding 1-butyl-3-methylimidazolium tetrafluoroborate as a charge compensator. Annealing the coupons at 200°C in nitrogen promoted the formation of an electrically conductive thin film. Four-point probe measurements of the films yielded electrical conductivities in the range 10⁶-10⁷ S/m (10-80% of bulk conductivity). Electron microscopy images of the coated substrates showed a layer structure consisting of nanoparticles. The Cu particle core-ion shell complex is attracted to the positively charged amine groups at high pH depositing a thin metal particle film that is both continuous and conformal. With increasing ionic liquid concentration, film morphology changes from conformal films to discrete islands. In the ionic liquid concentration range from 2.0-2.5 mM, the metal films exhibit increased optical absorbance, luminescence and electrical conductivity. The film properties are correlated to interparticle interactions with electron imagery and spectroscopic ellipsometry. Lastly, a thin metal film was deposited that is both continuous and cohesive on the walls and floor of 5-10X aspect ratio trenches and vias.