High Purity Water

Depending on the required water quality, UPW treatment plants often also feature degasification, microfiltration, ultrafiltration, ultraviolet irradiation, disinfection/sanitation processes. Therefore, instruments are needed that measure contaminant levels, such as: resistivity/conductivity, (dissolved) oxygen, turbidity, redox (ORP), pH, total organic carbon [TOC] and specialty measurements for specific ions.

Chlorine, Ozone, Chlorine dioxide

Chlorination, ozonation and chlorine dioxide (ClO2) are commonly used and effective techniques for disinfecting and sanitizing ultrapure water and treatment systems. Measurements of dissolved chlorine, ozone and ClO2 concentrations are an essential part of high purity treatment processes to assure adequate treatment levels.

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In pure water systems, electrolytic conductivity or resistivity measurement is the most common indicator of ionic contamination.

Ultrapure water is easily contaminated by traces of carbon dioxide from the atmosphere passing through tiny leaks or diffusing through thin wall polymer tubing when sample lines are used for measurement. Carbon dioxide forms conductive carbonic acid in water. Therefore, conductivity probes are generally permanently inserted directly into the main ultrapure water system piping to provide real-time continuous monitoring of contamination. These probes contain both conductivity and temperature sensors to enable accurate compensation for the very large temperature influence on the conductivity of pure waters.

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Oxygen (dissolved)

Advanced microelectronics manufacturing processes require low single digit to 10 ppb dissolved oxygen (DO) concentrations in the ultrapure rinse water to prevent oxidation of wafer films and layers. DO in power plant water and steam must be controlled to ppb levels to minimize corrosion.

Impurities in the rinse water such as high dissolved oxygen content, organic contaminants and gas bubbles can cause defects. Controlling oxide formation on the wafer surface is one of the reasons for reducing residual dissolved oxygen levels. As chip line widths continue to decrease, the quality of the water and the dissolved oxygen content becomes more critical.

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High-purity water is frequently used in many water treatment applications to avoid possible contaminations caused by water impurities. Measuring the pH of this water is challenging and necessitates specialized equipment.

pH measurement in high purity water is challenging for various reasons. The low conductivity of pure water in combination with the vulnerability to contamination, the variability of reference electrodes and additional temperature effects converge to make this a particularly challenging measurement. Overcoming these obstacles requires a thorough understanding of the potential problems and their systematic solution.

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Redox (ORP)

Redox or ORP (Oxidation-Reduction Potential) measurement is used to indicate the presence of oxidizing or reducing conditions in water. In pure water treatment, ORP is generally used to assure the removal of chlorine or other oxidizing agent ahead of membranes or deionization resins that could be damaged by oxidation.

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In the high purity water processing systems, silica levels are usually the first to increase if there are any glitches during the production.

High purity water systems often measure trace silica as it is a reliable primary indicator of contaminants. Silica is one of the most abundant minerals on the planet and is found in all water supplies. In the chip manufacturing industry, trace silica has the potential to remain on the wafer of microchips as the UPW dries, which can lead to a significant loss in yield.

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Water quality is critical to the efficiency and performance of water treatment applications and to the longevity of its associated equipment. Sodium measurement is one of the most common techniques that achieves accurate, continuous and rapid response times to detect possible salt contaminations that are known to be contributory factors in the stress and pitting of corrosion resistant steel.

Sodium concentration monitoring has therefore become one of the most essential water quality determinants throughout various water treatment applications.

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Total Organic Carbon

Total organic carbon in ultra-pure water can contribute to bacterial proliferation by providing nutrients, can react in unwanted ways in sensitive thermal processes and, in severe cases, leave unwanted residues on production parts. TOC can come from the feed water used to produce UPW, from the components used to convey the, from subsequent manufacturing and cleaning operations of piping systems or from dirty pipes, fittings and valves.

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Many methods exist for the determination of water contaminants, yet turbidity measurement is still considered significant because it is a simple and undeniable indicator of water quality change. A sudden change in turbidity may indicate an additional pollution source (biological, organic or inorganic) or may signal a problem in the water treatment process. 

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