A New Ultra-Stable Resistivity Monitor for Ultra-Pure Water

I am straying from my normal range of topics here, but confess I am developing an interest in semiconductor metrology since it takes me back to my physics and math roots. Even better I get to learn about the state of the art in ultra-pure water for semiconductor applications, an area where resistivity monitors play a role since resistivity is how purity is estimated these days.

The role of ultra-pure water, and challenges

Ultra-pure water is used throughout semiconductor design for cleaning, rinsing, conditioning and particularly in CMP which requires high volumes for slurry production and rinsing. Unsurprisingly the water must be incredibly pure because even small impurities at current process scales can completely kill yield. Also not a surprise, flow rates have been increasing driven by wafer sizes.

You can’t buy water at this purity (that would be impossibly expensive). It’s all created in-house in fabs, through a complex and expensive series of steps to progressively remove impurities, through filtration, reverse osmosis, de-gasification, UV irradiation, ion exchange, more filtration, more UV irradiation, more ion exchange, etc, etc.

These steps aim to filter out to the greatest extent possible: particles, bacteria, organic carbons, silica, and anything else which isn’t water. Further, setting up or making a change to such a system can incur months of delay due to need to flush the system thanks to outgassing, particle shedding from equipment and pipes, bacteria/biofilm removal and so on.

But perfect purity isn’t possible, just like a perfect vacuum isn’t possible. The goal is to stay within upper limits on impurities, subject to one or more guidelines. One very important way to determine inline that water falls within these guidelines is through resistivity measurements.

Resistivity as a measure of purity

Resistivity is the standard way to check for ionic impurities, measured in MΩ/cm. Seawater (lots of salts) comes in at 20-30 Ω/cm, tap water around 1-5 kΩ/cm and distilled water at 500 kΩ/cm. The expectation for semiconductor grade ultra-pure water is 18.18 MΩ/cm. I have seen slightly different values for absolutely pure water around 18.24 MΩ/cm, an obviously theoretical value since the real thing is not attainable.

In other words, semiconductor grade ultra-pure water is very close to theoretically pure, but real-life metrology requires very high accuracy in measurements.

The Mettler Toledo (MT) UPW UniCond sensor

This sensor is pictured above. MT quote a fab customer calibrating this sensor to 18.2 MΩ/cm once installed, thoroughly rinsed and stabilized. At first, I thought this was just marketing rounding generosity on 18.18 MΩ/cm, but they also show a graph of standard deviation (𝛔) in measurements versus other production sensors. They claim an order of magnitude better 𝛔 at notably 0.0003 MΩ/cm. Even marketing can’t fudge 18.18 to 18.20 with that 𝛔.

Note this performance reflects not only on accuracy but also on stability. MT attribute this performance to significantly better temperature compensation, signal stability, and environmental isolation than have been previously available. They report that the customer that ran these tests has now mandated this sensor for all new fabs and fab upgrades, the ultimate endorsement.

You can learn more about the MT UPW UnicCond sensor HERE.