The Chemical Deformation of a Thermally Cured Polyimide Film Surface into Neutral 1,2,4,5-Benzentetracarbonyliron and 4,4'-Oxydianiline to Remarkably Enhance the Chemical-Mechanical Planarization Polishing Rate.

Abstract

The rapid advancement of 3D packaging technology has emerged as a key solution to overcome the scaling-down limitation of advanced memory and logic devices. Redistribution layer (RDL) fabrication, a critical process in 3D packaging, requires the use of polyimide (PI) films with thicknesses in the micrometer range. However, these polyimide films present surface topography variations in the range of hundreds of nanometers, necessitating chemical-mechanical planarization (CMP) to achieve nanometer-level surface flatness. Polyimide films, composed of copolymers of pyromellitimide and diphenyl ether, possess strong covalent bonds such as C-C, C-O, C=O, and C-N, leading to inherently low polishing rates during CMP. To address this challenge, the introduction of Fe(NO₃)₃ into CMP slurries has been proposed as a polishing rate accelerator. During CMP, this Fe(NO₃)₃ deformed the surface of a polyimide film into strongly positively charged 1,2,4,5-benzenetetracarbonyliron and weakly negatively charged 4,4'-oxydianiline (ODA). The chemically dominant polishing rate enhanced with the concentration of the Fe(NO₃)₃ due to accelerated surface interactions. However, higher Fe(NO₃)₃ concentrations reduce the attractive electrostatic force between the positively charged wet ceria abrasives and the negatively charged deformed surface of the polyimide film, thereby decreasing the mechanically dominant polishing rate. A comprehensive investigation of the chemical and mechanical polishing rate dynamics revealed that the optimal Fe(NO₃)₃ concentration to achieve the maximum polyimide film removal rate was 0.05 wt%.