TEM investigation on the metal-oxide nanoparticle formation by the reaction of polyimide with thin metal films

Abstract

Transmission electron microscopy (TEM) has been used to characterize the metal-oxide nanoparticles dispersed in a polyimide and their particle size, distribution, and morphology were investigated. 10 nm-thick Cu, Zn, or Sn films were spin-coated with polyamic acid (PAA) in N-ethyl-2-pyrrdione. The samples were baked at 135℃ for 30 min to remove the solvent and then fully cured in N2 atmosphere at 400℃ for 1 hour. TEM specimens were prepared by using a NaCl or an oxidized-wafer. TEM observation revealed oxide nanoparticles formed in a polyimide. The electron diffraction analysis proved that Cu oxide is cubic Cu2O, Zn oxide is hexagonal ZnO, and Sn oxide is tetragonal SnO2. The Cu2O or ZnO particles were randomly dispersed within the polyimide. Particle size of Cu2O was ~ 10 nm and that of ZnO was ~ 9 nm, having a relatively uniform and narrow distribution. On the other hand, the distribution of Sn oxide particle in the polyimide is different from that of Cu or Zn oxides. The SnO2 particles existed only at the interface between PI and substrate. The particles showed a bimodal particle size distribution. Small particles were dispersed among relatively large particles. The radius of the small particles was ~ 10 nm and that of large particles was ~ 30 nm. The difference in the particle distributions was ascribed to the degree of reactivity between thin metal film and PAA. Cu or Zn reacted with PAA and was completely dissolved in the PAA. During curing, polyamic acid will dehydrates and imidizes, forming polyimide, accompanied by precipitation of Cu2O or ZnO particles. In case of SnO2 particles, Sn has relatively low reactivity compared to Cu or Zn. The coated polyamic acid attacks the Sn surfaces through the grain boundaries. This reaction drives the continuous etching of the Sn surface and the polyamic acid reacts through the grain boundaries of Sn and laterally separates the continuous Sn layer