Barrier and Packaging Level CMP Optimization

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The yield of chip packaging operations is a function of the surface finish that is solderable and wire-bondable. It has been shown that the use of palladium (Pd) plated lead-frame for packaging has improved the processing cost and reliability by simplifying the process integration [1]. Pd can also be used as a sacrificial layer to protect the copper (Cu) substrate material from oxidation and interdiffusion before the SnPb solder application. In packaging applications, Pd is advantageous in forming a Ni(Pd)Si allowing a lower sheet resistance at higher temperatures as compared to the NiSi in addition to much better surface morphology [2].The new integration schemes of Pd into the circuitry at the packaging level also require a CMP application where the Pd is deposited on the dielectric layer with bond-pad recess and polished to form a Pd based pad surrounded by Ni [3]. In combination with the applications of the 2.5 and 3-D microelectronics packaging technologies, the CMP electivity becomes critical where a competitive removal rate is desired between the Cu, Cu-barrier (TaN) and Ni filmsIt has been demonstrated on W-CMP in an earlier study that the 1:1:1 removal rate selectivity of the W/Ti/TiN layers can be achieved through adjusting CMP formulations in terms of the slurry chemistry and the particle concentrations. The systematic study has demonstrated a methodology for developing optimal CMP configurations for the newly introduced films to CMP applications based on electrochemical evaluations [5].In this paper, we study the removal rate selectivity for Pd integrated packaging level CMP applications through electrochemical and surface energy evaluations in commercial bulk and barrier CMP slurries. It can be seen that the thin film dissolution rates relate to the surface passivation as well as the surface energy measured by the contact angle evaluations on the Cu, Pd, TaN and Ni wafer coupons. Pd being a novel metal shows strong passivation that results in minimal dissolution in the slurry environment which requires alternative formulations for CMP slurries to promote the removal rate performance.