A Study of Catalyst and Scavenger for Tunable Fenton Reaction in Tungsten Chemical Mechanical Planarization

Abstract

In tungsten (W) film chemical-mechanical-planarization (CMP), the chemical and mechanical reaction behaviors of the W film surface play a critical role in the CMP performance, as determined by oxidation (i.e.,WO3), corrosion (i.e., WO42-), and the electrostatic force at the interface between abrasives and the surface. The Fenton reaction is used to oxidize the W film surface to WO3. The Fenton reaction is a reaction in which radicals and oxygen are generated by the chemical reaction of iron ions and hydrogen peroxide. The radicals and oxygen formed by the Fenton reaction oxidize the surface of W to enhance the polishing rate of W film, but deteriorate the abrasive stability of the slurry. Therefore, in this thesis, the chemical and mechanical properties of the zirconia (ZrO2)-abrasive based slurry using potassium ferric oxalate [K3Fe(C2O4)3] as catalyst and using trilithium tetrahydrate (TCT-Li) as scavenger were studied, respectively. The concentration of K3Fe(C2O4)3 determines the mechanism by which oxidation- or corrosion-dominant of W surface. It was confirmed that the oxidation reaction dominates in the low concentration region (i.e., K3Fe(C2O4)3 concentration 0.001 ~ 0.03 wt%) and the corrosion reaction dominates in the high concentration region (i.e., K3Fe(C2O4)3 concentration 0.03 ~ 1 wt%). To analyze the polishing mechanism, chemical properties such as static etch rate and potentiodynamic polarization curves were investigated. As a result, the ZrO2-based slurry using K3Fe(C2O4)3 as a catalyst showed a maximum W-film polishing rate at a specific concentration (i.e., 0.03 wt%), and corrosion did not occur on the W-film surface after CMP. In addition, the electrostatic force between the abrasives and films affects the polishing rate and the number of remaining particle on the surface after CMP. By measuring the zeta potential of the particles according to the concentration, the electrostatic force between the abrasives and the films is calculated using Coulomb’s law and the number of remaining particle on the surface after CMP was confirmed through surface analysis. In particular, compared to the colloidal-silica based slurry, it showed excellent dishing characteristics of less than ± 4 Å at all concentrations. By selecting the appropriate catalyst concentration, it is possible to achieve sufficient polishing rate, improve abrasive stability, and achieve dishing-less characteristics. In addition, a ZrO2-based W slurry with improved abrasive stability was designed by adding TCT-Li, a scavenger. The formation of Fe(III)-citrate complex improved the abrasive stability by inhibiting the generation of the radical ions that promote the degradation of the dispersant. The characteristics of the W polishing mechanism was studied according to the concentration. Resultantly, the W-film polishing rate and abrasive stability are a trade-off relation. It was found that the tendency for the SiO2 polishing rate to be divided into a region in which the polishing rate increases and a region in which the polishing rate decreases according to the concentration of TCT-Li. The polishing mechanism was understood by analyzing the binding energy of the SiO2 surface and the electrostatic force between the abrasives and the film. I conducted a basic mechanism analysis of a new catalyst and scavenger for controlloing a Fenton reaction based on crystalline-ZrO2 abrasives in W CMP slurry.