Inline Refractive Index Replaces Auto-titration in Qualifying H2O2 Concentration in CMP of Tungsten

Abstract:
Refractive index measurements have established themselves as the technique of choice for qualifying peroxide content in slurries for CMP of tungsten. Many emerging process flows use CMP as a critical tool for building circuit structures, dramatically increasing the number of CMP steps — and thus the number of opportunities for yield loss if slurry composition deviates from the specification. While auto-titration measurements can give extremely accurate results, they impose large capital equipment and ongoing maintenance costs and offer only discrete sampling at specified intervals. Refractive index, a continuous, non-slurry-consuming measurement, helps fabs identify slurry composition faults quickly, reducing the number of wafers at risk. Once calibrated to a specific slurry’s temperature/refractive index characteristics, refractive index measurements can determine the concentration of hydrogen peroxide in tungsten slurry to within ±0.03% by weight. Moreover, unlike conductivity probe tests, refractive index measurements can monitor slurry density, an indicator of settling and degradation of the slurry over time. Therefore, refractive index is used to qualify not only the final product for supply, but also to monitor batch to batch variation of incoming raw slurry and validate the blend addition steps. In long-term studies at a leading-edge analog device fab, refractive index measurements have replaced auto-titration in qualifying H2O2 concentration in slurry delivery systems. The measurements have remained stable for four years with no instrument maintenance beyond routine flushing of the slurry blender. An attractive feature in some slurry delivery systems has been the ability to use an automated chemical spiking function in the day tank. Keywords: Slurry delivery system with H2O2 spike function in day tank, inline refractive index concentration measurements, monitoring of incoming raw material