In-line refractive index monitoring for CMP slurry fault detection

Inline refractive index measurements have established themselves as the technique of choice for detecting faults in the CMP slurry blending and dispense systems of leading fabs. Refractive index, a continuous, non-sampling measurement, helps fabs identify slurry composition changes quickly. Once calibrated to a specific slurry´s temperature/refractive index characteristics, refractive index measurements can determine the concentration of hydrogen peroxide in slurry with a precision to within ±0.02% by weight, for both copper and tungsten slurries. In long-term studies at this leading edge fab, measurements for low node technology CMP processes detected slurry compositions reliably for three years, with no instrument maintenance beyond routine flushing of the slurry blender tank. In data extracted over a 40-day period, repeatable measurements within the process specification were achieved for both 3% hydrogen peroxide in tungsten slurry and 1% peroxide in copper slurry. Measurements on slurries with 0.1% peroxide concentration were repeatable, but with measurement noise potentially exceeding the process limits. Regardless of the slurry concentration level, refractive index measurements can both monitor intentional slurry dilutions and detect unexpected slurry composition changes in the process. Unique with refractive index is the ability to monitor both the raw slurry and DI water dilution as well as the subsequent peroxide addition.