The Cost of Inaccuracy at Semiconductor Fabricators

Resistivity Measurement in Ultrapure Water

Semiconductor fabricators are pushing the limits of ultrapure water. As node tolerances shrink, water purity must increase to protect wafers. Therefore, water analytics have to detect minute changes in quality that can affect final yield.

Resistivity is measured throughout the water purification process and downstream systems. Being one of the key analytics, resistivity is a go or no-go measurement for releasing water to production. If resistivity decreases, final wafer yield can decline. So to protect yield, facilities often shut down water to production rather than send poor‑quality water downstream. Tolerances on resistivity are tighter than ever, where 18.00 MΩ-cm water is considered the limit of ionic purity. But now the industry target is trending toward 18.18 MΩ-cm.

Accurate and stable resistivity sensors are essential for well‑controlled systems and reliable, data‑driven decisions. The UPW UniCond™ sensor is a state-of-the-art resistivity sensor with unmatched stability and accuracy. In this article, we explore a few scenarios that are common in semiconductor facilities, along with how the UPW UniCond can help remedy them.

In these first two scenarios, you are a water system owner who wants to make sure you are not sending poor-quality water downstream. Your resistivity alarm is set at the industry standard, 18.00 MΩ-cm. If the reading is above 18.00 MΩ-cm, the system is producing water of sufficient ionic purity to protect product yield.

If the reading goes below 18.00 MΩ-cm, you need to shut down the water system to prevent yield loss. The resistivity sensor you are using has a 1% accuracy specification, which is very common in the marketplace but double that of a UPW UniCond. Let’s see how the UPW UniCond’s much tighter specification of 0.5% offers greater protection in terms of preventing expensive shutdowns and even more expensive impacts to yield.

Scenario 1: False Sense of Security

Imagine the water quality resistivity falls to 17.90 MΩ-cm, which is out of tolerance. With a 1% accuracy specification, your resistivity sensor could be reporting up to 18.08 MΩ-cm, tricking you into thinking your water system is behaving normally (see Figure 1).

Unfortunately, in this scenario, poor‑quality water would be sent downstream without detection. However, if you were using a UPW UniCond, you would never report 17.90 MΩ-cm water as 18.00 MΩ-cm water. A tighter accuracy specification reduces system risk and increases confidence that the water meets quality requirements.

Scenario 2: Prevent Costly Shutdowns

Suppose again that you have a resistivity sensor with a 1% accuracy specification. The sensor suddenly begins reading at 17.96 MΩ-cm. You decide to shut down the water system to protect yield. The shutdown lasts 24 hours because a full investigation is required to analyze the water system and understand the root cause. Each hour that the system does not produce water costs your company $50,000.

After one full day, the investigation concludes that there was no problem with the water system. The water quality was 18.15 MΩ-cm, but the resistivity reading at 17.96 MΩ-cm—within specification of the resistivity sensor—was false (see Figure 2). That false reading and your resulting shutdown cost your company $1.2 million.

If you had used a UPW UniCond sensor, this scenario would have been avoided. With a 0.5% accuracy specification, the UPW UniCond would not display 18.15 MΩ-cm water as out of tolerance. In short, UPW UniCond helps to protect your yield and the water system from unexpected, costly shutdowns.

In this scenario, again imagine you are the water system owner. You are looking at trending for resistivity and trying to determine if there was a small release of an ionic contaminant that passed through the purification steps.

With standard resistivity sensors, it’s challenging to determine the difference between noise and minute changes in resistivity. As the industry pushes to detect ever-smaller changes in water quality, analytics need to improve accordingly.

We tested a competitive resistivity sensor side‑by‑side with a UPW UniCond in the same water. The results showed that UPW UniCond was ten times more stable than the competitor sensor.

We then injected electrical noise directly into the UPW UniCond's measuring circuits to simulate harsher conditions. This produced only slightly more instability than the UPW UniCond’s baseline without noise injection.

As a water system owner, how should you interpret the resistivity readings? Should you read it as 18.12 MΩ-cm or 18.14 MΩ-cm? The UPW UniCond reads 18.19 MΩ-cm consistently in this test, leaving no room for error.

As specifications for semiconductor fabrication are becoming increasingly strict, analytics must keep pace. Because analytics monitor product worth millions of dollars, only the most reliable analytics should be considered.

For more information on the UPW UniCond, please visit the link below.

www.mt.com/UPWUniCond