Chemical mechanical polishing (CMP) is used to smooth the surface of semiconductor wafers and remove excess material. The slurry used in the CMP process has both mechanical (abrasive) and chemical properties. If the slurry composition isn’t consistent, wafer quality is affected. Vaisala’s inline process refractometers are uniquely suited to slurry monitoring, controlling batch-to-batch variation, dilution, and blending and ensuring optimal wafer quality and yield.
 

Slurry consistency is essential

Slurry density and composition is a critical parameter for CMP process quality, but it’s not as simple as just always using the same slurry mix. Slurry is delivered from the manufacturer in concentrated form and can vary from batch to batch. The semiconductor fabrication plant (fab) dilutes the concentrated slurry with deionized water and adds   an oxidizing agent, usually hydrogen peroxide (H2O2). This needs to be done just before the CMP process because H2O2 degrades into water and oxygen over time.

This degradation introduces a limited time window where maintaining a consistent surface planarization quality and minimizing wafer defects and scrappage is possible. To ensure wafer quality, fabs need to be able to continuously monitor slurry composition throughout the day. Inline process refractometers offer a simple and cost-effective way to perform this monitoring. Refractometers can be installed in the following locations:  

• On the supply lines to monitor incoming slurry and H2O2 and to define the slurry dilution target concentration level.
• In the slurry blender tank for real-time control of H2O2 blending to ensure the correct concentration and blending time. 
• At the CMP tool to ensure slurry composition is within set limits before it comes into contact with the wafers.

Real-time results with inline refractive index (RI) measurement

Process refractometers work by measuring the RI of the slurry. Every blend has a unique composition and therefore a unique RI, which can be validated and characterized using the refractometer. Real-time measurement using an inline refractometer provides a more accurate picture of variations in slurry composition than any discrete sampling method. Furthermore, measurement is virtually drift-free, does not depend on the flow rate, and accuracy not affected by bubbles in the slurry.

Once calibrated to a specific slurry’s refractive index and temperature characteristics, RI measurements can achieve excellent repeatability when used to measure, for example, copper and tungsten slurries. Inline measurements are superior to lab-based methods because they remove any uncertainty introduced in sampling and sample handling such as H2O2 deterioration or temperature changes.

The Vaisala PR-33-S Semicon Refractometer

The Vaisala PR-33-S Semicon Refractometer is specifically designed for in-line, real-time measurement in semiconductor manufacturing environments. It has a small footprint and metal-free Teflon and sapphire-wetted parts and surfaces, making it ideal for measuring slurries and harsh chemicals. The robust design promises long-term stability and a long lifespan, and the built-in diagnostics give an instant overview of measurement performance. 

The PR-33-S offers direct slurry density measurement and the integrated temperature sensor ensures highly accurate RI measurement, tolerating process vibrations with no measurement errors. Thanks to the unique optical measurement principle and robust design, there is no measurement drift and the device is virtually maintenance-free. The device requires no consumables and does not generate chemical waste, providing a cost-efficient, reliable, and sustainable alternative to laboratory sampling and titration. Studies have proven Vaisala refractometers to have excellent agreement with reference titration – but with the advantage of real-time results, direct and repeatable from the process. 

Fast, accurate, and reliable

Inline RI measurement has become the industry standard for fast, accurate, real-time slurry monitoring. Refractometers are a safe, cost-effective, and low-maintenance method for monitoring slurry density and composition. They are also the tool of choice for leading fabs to detect faults in slurry blending and dispense systems. By verifying that slurries consistently meet requirements, refractometers allow fabs to ensure optimal wafer quality and yield while reducing waste.