Ultrapure water
[2] Bacteria, particles, organic, and inorganic sources of contamination vary depending on a number of factors, including the feed water to make UPW, as well as the selection of the piping materials used to convey it.
[3] Techniques that help to minimize bacterial colony growth within UPW streams include occasional chemical or steam sanitization (which is common in the pharmaceutical industry), ultrafiltration (found in some pharmaceutical, but mostly semiconductor industries), ozonation, and optimization of piping system designs that promote the use of Reynolds Number criteria for minimum flow,[4] along with minimization of dead legs.
Sources can include bacterial fragments, the sloughing of the component walls within the conduit's wetted stream, and the cleanliness of the jointing processes used to build the piping system.
Metallic and anionic contamination in UPW systems can shut down enzymatic processes in bioprocessing, corrode equipment in the electrical power generation industry, and result in either short or long-term failure of electronic components in semiconductor chips and photovoltaic cells.
Depending on the level of purity needed, detection of these contaminants can range from simple conductivity (electrolytic) readings to sophisticated instrumentation such as ion chromatography (IC), atomic absorption spectroscopy (AA) and inductively coupled plasma mass spectrometry (ICP-MS).
While each industry uses what it calls "ultrapure water", the quality standards vary, meaning that the UPW used by a pharmaceutical plant is different from that used in a semiconductor fab or a power station.
For instance, semiconductor plants use UPW as a cleaning agent, so it is important that the water not contain dissolved contaminants that can precipitate or particles that may lodge on circuits and cause microchip failures.
Today, ion exchange (IX) and electrodeionization (EDI) are the primary deionization technologies associated with UPW production, in most cases following reverse osmosis (RO).
[7]: 118 Water quality standards for use in the semiconductor industry It is used in other types of electronics manufacturing in a similar fashion, such as flat-panel displays, discrete components (such as LEDs), hard disk drive platters (HDD) and solid-state drives NAND flash (SSDs), image sensors and image processors/ wafer-level optics (WLO), and crystalline silicon photovoltaics; the cleanliness requirements in the semiconductor industry, however, are currently the most stringent.
Currently, the World Health Organization (WHO) [22] as well as the Pharmaceutical Inspection Co-operation Scheme (PIC/S) [23] developed technical documents which outline validation requirements and strategies for water systems.
[24][25] 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.
For this reason, conductivity probes are most often permanently inserted directly into the main ultrapure water system piping to provide real-time continuous monitoring of contamination.
Sodium is also measured in power plant water and steam samples because it is a common corrosive contaminant and can be detected at very low concentrations in the presence of higher amounts of ammonia and/or amine treatment which have a relatively high background conductivity.
[26] 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.
The measurement adds reagents including a molybdate compound and a reducing agent to produce a blue silico-molybdate complex color which is detected optically and is related to concentration according to the Beer–Lambert law.
Colloidal silica is considered more critical than dissolved in the electronic industry due to the bigger impact of nano-particles in water on the semiconductor manufacturing process.
When speciation of the organics is required for troubleshooting or design purposes, liquid chromatography-organic carbon detection (LC-OCD) provides an effective analysis.
This method allows for identification of biopolymers, humics, low molecular weight acids and neutrals, and more, while characterizing nearly 100% of the organic composition in UPW with sub-ppb level of TOC.
[34][35] Similar to TOC, SEM particle analysis represents a lower cost alternative to the expensive online measurements and therefore it is commonly a method of choice in less critical applications.
[37][38] In addition, the degree of filtration upstream of these processes will be dictated by the level of suspended solids, turbidity and organics present in the source water.
The figure on the right illustrates an interesting coincidence that the largest diameter of the main supply line of UPW is equal to the size of the wafer in production (this relation is known as Klaiber's law).
In the former case the pore distribution is uniform through the filter, while in the latter the finer surface provides the particle removal, with the coarser structure giving physical support as well reducing the overall differential pressure.
The UF is a highly efficient filtration product for UPW, and the sweeping of the particles into the retentate stream yield extremely long life with only occasional cleaning needed.
Point of use treatment is often applied in critical tool applications such as Immersion lithography and Mask preparation in order to maintain consistent ultrapure water quality.
In the case when urea, THM, isopropyl alcohol (IPA) or other difficult to remove (low molecular weight neutral compounds) TOC species may be present, additional treatment is required thru advanced oxidation process (AOP) using systems.
[39][40] The semiconductor industry uses a large amount of ultrapure water to rinse contaminants from the surface of the silicon wafers that are later turned into computer chips.
These are fairly common practices in many semiconductor facilities worldwide, however there is a limitation to how much water can be reclaimed and recycled if not considering reuse in the manufacturing process.
Some companies have incorporated an approach that uses complex large scale treatment designed for worst case conditions of the combined waste water discharge.
A water management plan will also require a significant amount of sample data and analysis to determine proper drain segregation, application of online analytical measurement, diversions control, and final treatment technology.
Due to its metallic contribution, most steel was removed from microelectronics UPW systems in the 1980s and replaced with high performance polymers of polyvinylidene fluoride (PVDF),[1] perfluoroalkoxy (PFA), ethylene chlorotrifluoroethylene (ECTFE) and polytetrafluoroethylene (PTFE) in the US and Europe.
