Improving Wafer Quality and Yield with UPW Resistivity and TOC Measurements

An earlier SemiWiki post discussed water sustainability in semiconductor manufacturing, related challenges and solutions. Whether first time use or recycled use, water purity needs to meet certain stringent criteria for the processing task on hand. This article will look at it from a wafer quality and yield perspective and is based on a recently published whitepaper by Mettler Toledo.

The Significance of Water Purity in Semiconductor Manufacturing

Water serves as a cleaning agent, a heat transfer medium, and a crucial ingredient in the chemical processes used to etch and deposit materials onto wafers. Any impurities in the water can lead to defects on the wafer, reducing yield and affecting the overall product quality. Minute amounts of ionic impurities and organic contamination even at the sub-parts per billion (ppb) level, can negatively impact the yield and quality of wafers. Therefore, it is essential to monitor and control the purity of the water used in semiconductor manufacturing.

Ensuring Water Purity

Two key parameters that must be monitored in real-time to ensure ultrapure water (UPW) are Total Organic Carbon (TOC) and resistivity. TOC refers to the measurement of organic carbon compounds present in the water, which can be indicative of contamination. Resistivity, on the other hand, measures the water’s ability to conduct electrical current and can highlight any ionic impurities. As such, real-time, continuous monitoring of resistivity and Total Organic Carbon (TOC) has long been a standard practice in the industry. Strict control of these parameters is crucial for semiconductor manufacturers to enhance wafer quality and maximize yield in this highly competitive industry.

Challenges Faced

UPW is produced through a complex and costly multistage purification process. This process involves various techniques, such as reverse osmosis, micro-filtration, electrodeionization, ion exchange, adsorption, and UV photo-oxidation. However, one challenge associated with UPW is that any water used for Total Organic Carbon (TOC) measurement, a key indicator of water purity, cannot be returned to the process water stream and is instead directed to drain. UPW has a very high resistivity of 18.18 Megohm-cm, making it crucial for resistivity instruments used in semiconductor manufacturing to accurately detect even the smallest resistivity changes on a non-zero background.

While UPW is extremely important, being overly tight in measuring the purity level could lead to potentially throwing away a significant quantity of water as wastewater. On the other hand, a more relaxed approach could impact product quality and yield. To meet the industry’s stringent expectations for water purity, resistivity instrumentation must provide stable, precise measurements with effective noise reduction techniques in place.

Consequently, semiconductor manufacturing facilities seek solutions that are reliable, easy to integrate into existing systems, and operator-friendly.

UPW Monitoring For Stable and Precise Measurements

Advanced sensor technology such as Mettler Toledo’s UniCond sensors can simplify monitoring processes in several ways. These sensors are equipped with onboard memory that stores their unique identity and calibration data, and this information is automatically transmitted to the connected transmitter. Mettler Toledo’s M800, is a multi-parameter transmitter that offers installation flexibility and simplified process control measurements. It can simultaneously monitor one, two, or four in-line sensors, making it a versatile and cost-effective solution for UPW monitoring. This “plug and measure” approach simplifies installation and ensures the sensor’s performance integrity, even if it’s relocated to another process location.

The 6000TOCi sensor from Mettler Toledo provides additional features to streamline routine system maintenance. Users benefit from local storage of calibration data, allowing them to access historical calibration records, which ensures compliance with water system requirements.

Mettler Toledo’s Intelligent Sensor Management (ISM®) technology not only facilitates the communication of calibration data but also offers sensor diagnostics. These diagnostics can identify out-of-range resistivity measurements and temperature variations, contributing to improved process control. ISM also supports calibration planning and provides advance warnings of potential sensor failures through Dynamic Lifetime Indicators (DLI) for components like the UV lamp, filter, and ballast. This proactive approach helps reduce downtime and increase yield in industrial processes.

Summary

Repeatable and precise measurements are essential to maintain a consistent supply of UPW to wafer tools and wet benches. Mettler Toledo offers the most advanced tools for on-line continuous measurement and process control required for UPW systems. Its suite of plug-and-measure sensors, including UniCond and 6000TOCi sensors, can be easily integrated into existing water systems via a user-friendly M800 transmitter interface. These solutions ensure that TOC and resistivity are monitored at sub-ppb levels, reducing risks and improving yield.

For more details, download Mettler Toledo’s whitepaper here.

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