||This article is written like a personal reflection or opinion essay rather than an encyclopedic description of the subject. (August 2011)|
Ultrapure water (UPW) refers to water that has been purified to uncommonly stringent specifications. The primary use of ultrapure water is as the prime cleansing agent in semiconductor manufacturing.
Until the advent of SEMI F63, ultrapure water used for semiconductor manufacturing was loosely defined. The SEMI F63 template has been followed recently with a UPW version for photovoltaic cell manufacturing. For years SEMATECH has referred to UPW as “de-ionized and filtered water.” Additionally, the foremost publication devoted to the subject of UPW, The Journal of Ultrapure Water, reaches across a wide readership audience ranging from water used in electrical power generation to pharmaceuticals to wafer fabrication in semiconductor houses. Likewise, ASTM D5127 recognizes a host of UPW water qualities.
It is this diversity of individual end-user high quality water needs that makes UPW difficult to define. Therefore, instead of a unique definition, it is often relegated to characterization by those in need of its attributes. Both intra- and inter-industry water needs sometimes differ so greatly that one individual's UPW may not work for the other. Nevertheless, there are several characteristic requirements that are important to every end-user of UPW. These include detection in the UPW stream and control of:
- Bacterial proliferation
- Organic contaminants, typically measured as total organic or total oxidizable carbon (TOC)
- Metallic contaminants
- Anionic contaminants
Sources and control 
Bacteria, particles, organic and inorganic sources of contamination vary depending on a number of items including the feed water to make UPW as well as the selection of the piping materials to convey it. Bacteria are typically reported out in colony-forming units (CFU) per volume of UPW. Particles use number per volume of UPW. TOC, metallic and anionic contaminates are measured in dimensionless terms of parts per notation, such as ppm, ppb, ppt and ppq.
Bacteria are one of the most obstinate in the list to control. Techniques that help in minimizing 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 flow along with minimization of dead legs.
Particles in UPW are the bane of the semiconductor industry, causing defects in sensitive photolithographic processes that define nanometer sized features. For other industries their presence can range from being a nuisance to life threatening. Particles can be controlled by use of filtration and ultrafiltration. Sources can include bacterial fragments, the sloughing of the component walls within the conduit’s wetted stream and also the cleanliness of the jointing processes used to build the piping system; both installation technique and installation environment.
TOC in UPW can contribute to bacterial proliferation as a foodstuff, substitute as a carbide for another chemical species in a sensitive thermal process, react in unwanted ways with biochemical reactions in bioprocessing and, in severe cases, leave unwanted residues on production parts. TOC can come from the feed water to make UPW, from the components used to convey the UPW (additives in the manufacturing piping products or extrusion aides and mold release agents), from subsequent manufacturing and cleaning operations of piping systems or overall dirty pipe, fittings and valves.
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. Sources are similar to those mentioned for TOC. 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).
Purification process 
Typically city feed water (containing all the unwanted contaminates previously mentioned) is taken through a series of purification steps that, depending on the quality of UPW wanted, includes gross filtration for large particulates, carbon filtration, water softening, reverse osmosis, exposure to ultraviolet (UV) light for TOC and/or bacterial static control, polishing using either ion exchange resins or electrodeionization (EDI) and finally filtration or ultrafiltration.
Some systems use direct return, reverse return or serpentine loops that return the water to a storage area, providing continuous recirculation, while others use dead end systems that run from point of UPW production to point of use. The constant recirculation action in the former continuously polishes the water with every pass. The latter can be prone to contamination build up if it is left stagnant with no use.
Stainless steel remains a piping material of choice for the pharmaceutical industry. 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), perfluoroalkoxy (PFA), ethylene chlorotrifluoroethylene (ECTFE) and polytetrafluoroethylene (PTFE) in the US and Europe. In Asia, polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC) and polypropylene (PP) are popular, along with the high performance polymers.
Methods of joining thermoplastics used for UPW transport 
Thermoplastics can be joined by different thermofusion techniques.
- Socket fusion (SF) is a process where the outside diameter of the pipe uses a “close fit” match to the inner diameter of a fitting. Both pipe and fitting are heated on a bushing (outer and inner, respectively) for a prescribed period of time. Then the pipe is pressed into the fitting. Upon cooling the welded parts are removed from the clamp.
- Conventional butt fusion (CBF) is a process where the two components to be joined have the same inner and outer diameters. The ends are heated by pressing them against the opposite sides of a heater plate for a prescribed period of time. Then the two components are brought together. Upon cooling the welded parts are removed from the clamp.
- Bead and crevice free (BCF), uses a process of placing two thermoplastic components having the same inner and outer diameters together. Next an inflatable bladder is introduced in the inner bore of the components and placed equidistance within the two components. A heater head clamps the components together and the bladder is inflated. After a prescribed period of time the heater head begins to cool and the bladder deflates. Once completely cooled the bladder is removed and the joined components are taken out of the clamping station. The benefit of the BCF system is that there is no weld bead, meaning that the surface of the weld zone is routinely as smooth as the inner wall of the pipe.
- Infrared fusion (IR) is a process similar to CBF except that the component ends never touch the heater head. Instead, the energy to melt the thermoplastic is transferred by radiant heat. IR comes in two variations; one uses overlap distance when bringing the two components together while the other uses pressure. The use of overlap in the former reduces the variation seen in bead size, meaning that precise dimensional tolerances needed for industrial installations can be maintained better.
- SEMI F63 "Guide for ultrapure water used in semiconductor processing"
- SEMI PV3 " GUIDE FOR HIGH PURITY WATER USED IN PHOTOVOLTAIC CELL PROCESSING"
- SEMATECH Official Dictionary, Revision 5.0, Technology Transfer 91010441C-STD
- ASTM D5127 Standard Guide for Ultra-Pure Water Used in the Electronics and Semiconductor Industries
- Burkhart M, Cassie S, Hawelka K and Klaiber F, "Injection Molding of High-Purity Water Components," UltraPure Water, December 1997
- Mittlemann MW and Geesey GC,"Biofouling of Industrial Water Systems: A Problem Solving Approach", Water Micro Associates, 1987
- Libman S, "Use of Reynolds Number as a Criteria for Design of High-Purity Water Systems", Ultrapure Water, October 2006
- Buesser DS,”Part 1: Flow requirements, pressure differential and pressure control of distribution systems”,Ultrapure Water, November 2002, pp 41-48
- ASTM D5127 Standard Guide for Ultra-Pure Water Used in the Electronics and Semiconductor Industries, page 4
- Sixsmith T, Wermelinger J, Williamson C and Burkhart M, "Advantages of Infra-Red Welding of Polyethylene Pipes for Industrial Applications”, presented at the Plastic Pipes Conference XV, Vancouver, Canada, September 20–22, 2010