Cover story22Filtration+Separation June 2006

Ultrapure water:

systems formicroelectronics

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Electronics are the heartbeat of the productsand services of many industries – includingaerospace, communications, entertainment,defence, and health – and, as a consequence,the global electronics market is valued at US$1 trillion worldwide.

The electronics market is a stage for intensecompetition, primarily among US and Asianfirms. This in turn spurs rapid productobsolescence and demands high levels ofinvestment in all areas of semiconductor, board,and assembly technologies, and at all stages ofresearch and development (R&D).

Electronic products are steadily becomingsmaller, thinner, lighter, faster, and lessexpensive. Advances now occur so rapidly that,to cite just one example, portablecommunication products are conceived,designed, and produced in 12 to 15 months,compared to 21 to 27 months a few years ago.

These trends are expected to continue andaccelerate, thus challenging the foundation oftoday’s electronics technology. However, rapidchange is possible only with an agile, responsivesupply and manufacturing infrastructure.

This article will look at only one aspect of thistechnological arena – high-purity watersystems for the microelectronics industry –although the same general design principlesapply in all areas of this high-tech market.

The manufacture of all electronic productsinvolves the use of high purity water, frequentlyin large quantities. In the microelectronicsindustry, manufacturing processes arecumulative, meaning that each process in afacility is affected by the output of the previous

processes. When particles and other impuritiesare present in the chemicals, water and gasesused in the process, product yield is reduced.

Of these three potential sources ofcontamination, water is arguably the mostsignificant one, since water comes into contactwith the product many times, and at every stageof the manufacturing process.

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Microelectronics facilities demand large volumesof water – as much as two thousand gallons perminute – to manufacture a product. And whileproducts increase in complexity and decrease insize, quality requirements for water increaseproportionately.

As the industry continues to expand anddevelop even more advanced technologies, anincreasing burden is being placed on thepurification technologies used in watergeneration systems. And the increased demand

on limited water resources and local watersupport infrastructures goes hand in hand withincreasingly tougher environmental regulations.As a result, the cost of water usage, treatmentand discharge is rising – and will continue torise.

Some years ago, it was not particularly commonfor a microelectronics manufacturing facility toreuse water. The perceived wisdom was that therisks involved in reclaiming and recycling waterwithin the facility were greater than the risksinvolved in processing ‘clean’ potable water. Thismeant that very large quantities of relativelyhigh purity water were being sent to waste.

There now appears to be a trend reversing thisand modern facilities are now more frequentlyreclaiming and recycling water, although therewill still be some waste streams that must bedisposed of.

Carefully planned filtration and purification canoptimise the output of each process, resulting in

O f all the potential sources of contamination in microelectronicsmanufacturing, water is the most significant – being necessaryat different stages of the production process. John Hutchesonof Hutch2O looks at how this potential problem can beavoided.

Figure 1: high-purity water production.

improved yields and manufacturing costreductions. Thus, by effectively filteringcontaminants out of the effluent, waste disposalcosts can be significantly reduced, and cleanpermeate can be reused, resulting in additionalsavings.

To efficiently control the full cycle of waterwithin a facility, and thereby control costs andincrease yield, knowledgeable filtration andwater experts should manage all the fluids in amanufacturing process.

A variety of different technologies are used inthe water purification system and thereclaim/wastewater treatment plant – many ofwhich are membrane-based technologies (seebox, ‘water purification technologies’).

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Semiconductor companies have expandedglobally over the past few years. According toThomas Flude in Ultrapure Water, November2004, semiconductor device fabricators havebenefitted from many significant technologyadvances developed over the last decade. Forexample, the production of 200-mm wafers isbecoming more common for all new fabinvestments.

Achieving a line-width geometry of less than0.13 µm is a normal development today, andmany major companies have successfullyventured into the 0.09 µm line-widthtechnology. Just a few years ago, a definition of‘high-purity’ water referred to impurities in theparts per billion (ppb) range – while now it iscommon to set limits in the parts per trillion(ppt) range.

However, it is difficult to grasp what these verylow limits actually mean. It used to be said(rather glibly) that if it was possible to measureany impurity, then there was too much of itthere. In other words, it is developments inchemical analysis that drive the movetowards the lower and lower limits ofimpurities. It is certainly true that there isno such thing as ‘zero’, only ‘too low tomeasure’.

To give some idea of what we mean, here isa comparison. The chances of winning thejackpot in the UK National Lottery are onein 13,983,816, i.e. 71.5 in a billion or71,511 in a trillion. The level of impuritiesthat we are seeking to achieve is less thanone in a billion for all the importantelements.

Put another way, 1 ppb is equivalent to anaccuracy of 1 mm in 1,000 km or 1 secondin 11,574 days (over 31 years). A humanhair is around 60 – 100 µm; bacteria used totest sterilising grade filters are around 0.2 –0.3 µm; the largest viruses are just under 0.1µm, and chips are being manufactured witha line width of 0.09 µm or less.

High-purity water is a very aggressivesolvent, and it has an excellent ability todissolve the impurities out of every materialit comes into contact with. It is thereforenot only very difficult to achieve therequired water quality standards but tomaintain them; all the while, transportingthe high-purity water from the locationwhere it is purified to the location where it isused.

It is clear that great care must be taken withany chemical used in the generation system orregeneration steps, as well as the materials usedto build the purification system and the storageand distribution system. It is also important toensure accuracy while using the analysers thatmeasure down to these very low ppt levels, andhave to cope with the high-purity waterenvironment.

While most ‘impurities’ are relatively simple toremove and do not pose much of a challenge,others are much tougher and are typicallythe focus of every system. These includetotal organic carbon (TOC), boron, silica,metals, oxygen and particles (includingbacteria).

So what limits are set for these impurities?Flude, quoted above, included a table in hisarticle, part of which is reproduced below.At this level of purity, conductivity becomesa rather meaningless measurement and theresistivity will always be expected to be 18.2MegOhm at 25oC.

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Interestingly, there are no revolutionary newtechnologies used in achieving theseexceptionally high levels of purity. The systemsuse combinations of technologies that have beenavailable for some years, but the performance ofthese technologies has been refined consistentlyover the years. (Figure 1 on page 22 illustratesone possible example of a microelectronics watersystem).

This diagram is from the website of KochMembrane Systems (www.kochmembrane.com)and is generally similar to the system illustratedin the Fulde article.

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The key membrane separation technologies usedare RO, UF and microporous membranefiltration. Degassing membranes and ion-

Cover story 23Filtration+Separation June 2006

Memory Size 256M 1G 1G 4G 16G 64G

Geometry rules (µm) 0.25 0.18 0.15 0.13 0.10 0.07

Manufacturing date 1997 1999 2001 2003 2006? 2009?

TOC (ppb) < 1 < 1 - < < 1 - < < 1 - < < 1 - < < 0.5 ?

0.5 0.5 0.5 0.5

Particle counts

/ L </= 0.03 µm - < 500 ? < 300 >

Particle counts < 500 < 300 < 300 < 300 < 100 < 100 ?

/ L </= 0.05 µm

Boron (ppt) < 100 < 50 < 50 < 50 10 - 50 10 - 50 ?

Na+ (ppt) < 7 < 5 < 5 < 2 < 2 < 1 ?

K+ (ppt) 10 < 5 < 5 < 2 < 2 < 1 ?

F- (ppt) 30 30 30 < 10 < 10 ? < 5 ?

Cl- (ppt) < 20 < 20 < 20 < 10 < 5 ? < 5 ?

Chromium (Cr) (ppt) 4 2 2 2 2 < 1 ?

Table 1: Impurity removal.

Water purification technologies

Reclamation:

• CMP effluents;

• Low TDS/TOC rinses;

• Backside grind/effluent;

• HF rinses;

• Copper plating rinses;

• Ultrapure water system effluents;

Wastewater treatment:

• pH neutralisation;

• Fluoride removal;

• Ammonia reduction;

• TOC/BOD/LOD reduction;

• Arsenic removal;

• CMP effluent treatment (with or withoutcopper);

• Phosphate reduction;

• Suspended solids removal;

Ultrapure water:

• Reverse Osmosis (RO);

• Continuous electrodeionisation;

• Degasification;

• Deionisation;

• Advanced oxidation;

• Ultrafiltration (UF);

• Microfiltration (MF).

selective membranes in CEDI systems also playan increasingly key role in these types of systems.

While it is beyond the scope of this article todiscuss the system designs in detail, theremainder of this article will consider the use ofeach membrane technology in the system andhow it contributes to the final water quality.

The overall design of microelectronics watersystems is similar to that of all other systems:

• Pretreatment – designed for the localincoming water supply. Its purpose is tominimise the fouling or damage of membranesand ion exchange resins in the purificationstage;

• Purification – designed to remove thecontaminants present in the water to achievethe required final level of purity;

• Storage and distribution – designed totransport the water to the points of use at therequired flow and pressure with minimaldegradation of the quality. However, inmicroelectronics water systems, furtherpurification steps are included in thedistribution system and at point of use becauseof the difficulties in obtaining, thenmaintaining, the very high purity levelsrequired.

Membrane-based technologies are used in allthree sections of a microelectronics water system.

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UF membranes are typically considered to havean effective molecular weight filtration range of10,000 – over 100,000 Daltons, which equates toa size of around 0.005 – over 0.1 µm.

Despite this very ‘tight’ filtration capability, UFmembrane systems are now typically used in boththe pre-treatment stage of systems as well as inthe high purity stage.

UF membranes are an attractive alternative toconventional chemical and mechanical filtration,because they can provide a more effective and

reliable pretreatment process for RO and ionexchange.

UF membranes are available as spiral woundmodules or as hollow fibre modules. Theseshollow fibre membranes are typically ‘doubleskinned’ i.e. they have a UF membrane on theinside of the hollow fibre, as well as the outside.

The design of the UF system will typically use across flow configuration where the feedwater isrecirculated around an internal recirculation loopwith permeate being taken off through themembranes, and a portion of concentrate beingbled off to drain with the concentratedimpurities.

The use of UF units in pre-treatment systems isincreasing, particularly in locations where thefeed water is variable in quality and can havehigh organic levels. UF membranes give a goodbalance of physical strength, resistance to foulingand ‘cleanability’ with economically viable fluxrates (flux is a measure of flow per unit area ofmembrane).

There are various methods of operating suchsystems, from a high cross flow and lower percentage conversions, through to minimal orno cross flow with periodicbackpulsing/backflushing and cleaning cycles.

Some systems – from the likes of ChristWaterman for example – are designed to operatewith a cross flow, but include the capability ofbackwashing the UF modules with permeateperiodically, as well as carrying out chemicalcleaning cycles.

In this type of pretreatment application, the UFmembranes are being used to removecontaminants across a wide range of particle sizes– and certainly much larger than 0.1 µm. Theyare a particularly effective method of reducingthe organic contaminant problem, which cancause considerable fouling to first-stage ROmembranes and ion exchange units, and whichtraditional technologies (such as organicscavengers, coagulation/flocculation and

filtration) sometimes have difficulty in removingconsistently.

The UF membranes used for such applicationswill typically have a molecular weight cut-offrating of 30,000 – 100,000 Daltons.

As an alternative to UF, continuous membranefiltration (CMF) can be used in this pre-treatment application. One example is theMemcor 0.2 µm membrane system, which isperiodically backflushed with a compressed gaspurge.

UF membranes can also be used very effectivelywhen installed in the distribution pipeworksystem in the high-purity section. The double-skinned design can minimise particle sheddingwith the permeate exiting the membranethrough a UF membrane rather than through asupporting matrix, as is the case withconventional spiral wound modules.

The UF membranes used in this criticalapplication will typically have a molecular weightcut-off rating of 6,000 – 10,000 Daltons (< 0.01µm). At this level of filtration, the membranesremove not only particles but bacteria,breakdown products from bacterial cells andpolymeric organic molecules. By operating UFunits with a small reject flow, the rejectedcontaminants are removed from the system, notretained within the filter where they can build upand cause an increasing challenge.

Finally, smaller UF membranes may be used aspoint-of-use filters, when the user-take-off pointsremove any particles that might have beengenerated within the distribution piping system.

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RO has been the ‘work horse’ of high puritywater systems for many years now. Althougheven the most modern RO membranes onlyachieve a percentage removal of contaminants,this percentage removal is both very high andcomprehensive across the full range ofcontaminants present. RO is therefore anexcellent first step in removing contaminants inthe feed water. Remaining contaminants can be

Cover story24Filtration+Separation June 2006

Techniques and equipment used in the production of ultrapure water (courtesy of Christ Water Technologies).

removed with other technologies such as UF,CEDI and Ozone.

In today’s systems it is now standard for double-pass RO to be used. In this technology, thepermeate from the first bank of RO membranesbecomes the feed to the second bank, so that theproduct water passes through two sets of ROmembranes.

RO membrane manufacturers, such as Filmtec,have developed new membranes that have ahigher overall rejection, and specifically a higherrejection of lower molecular weight organiccompounds, silica and boron. They also have anaccelerated TOC rinse down profile. Theproducts also have higher surface areas, whichallow fewer elements to be used along with loweroperating pressure. Developments to the endcapsand interconnectors have also reduced the risk ofleakage.

In order to improve RO performance, further pHadjustment using caustic soda injection issometimes carried out, either prior to the firstpass or between the two RO passes (interstage).

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Arguably one of the most significant changes intechnology usage in microelectronics watersystems, is the growth in use of CEDI systems.CEDI technology is now commonly used as apost-RO makeup demineralisation process in avariety of applications, includingmicroelectronics.

Some efforts have been made in the past todevelop CEDI technology as a final polishingstep, and as a replacement for mixed beds formicroelectronics applications. Although theseefforts have met with limited success, and havenot been cost effective, the latest innovations inthe CEDI process, combined with low-cost CEDImodules and system designs – such as theIonpure VNX – have resulted in a cost effectivealternative to mixed bed DI.

CEDI devices are comprised of cation- andanion-permeable membranes alternating in amodule with spaces in between, and configuredto create liquid flow compartments with inletsand outlets. The diluting compartments arebound by an anion exchange membrane (AEM)facing the positively charged anode, and a cationexchange membrane (CEM) facing thenegatively charged cathode. The concentratingcompartments are bound by an AEM facing thecathode and a CEM facing the anode. MostCEDI systems use a ‘plate and frame’ typeconfiguration to build up a complete module,although a spiral configuration is also available,an example being the Septron systemmanufactured by Christ Water Technology.

To facilitate ion transfer in low ionic strengthsolutions, the dilute compartments are filled withion exchange resins, and in some systems theconcentrate compartments are also filled withion exchange resin.

A transverse DC electrical field is applied by anexternal power source using electrodes at the

bounds of the membranes and compartments.When the electric field is applied, ions in theliquid are attracted to their respective counter-electrodes. The result is that the dilutingcompartments are depleted of ions and theconcentrating compartments are concentratedwith ions.

Although mixed bed polishing systems havecontinued to be used there has been anincreasing use of CEDI systems as a polishingtechnique – for quality and economic reasons.Ongoing developments in CEDI technologycontinue; for example, Ionpure UP technologyhas been developed to remove weakly-ionisedspecies such as boron and silica from the feedwater, which is typically RO permeate, and attainvery high purity (18 M.-cm) in the productwater.

Development of layered bed CEDI devices havethe advantage of being able to configure the ionexchange layers within the device to optimisethe removal of specific contaminants. Over thelast few years there has been a shift towardsthicker diluting cells and modular systemapproaches, both of which have lowered costs.

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The removal of oxygen has become a veryimportant item with typical levels now beingspecified at <1 ppb. Vacuum towers werepreviously the standard but now, in almost allnew plants, membrane degasification technologyis used.

Typically two-membrane degasification systemsare used, the primary one removing oxygen toless than 100 ppb and the polishing system thenreducing oxygen to less than 1 ppb. By using twosystems, removal of oxygen and THM can begreatly improved and it also allows ozonation ofthe polishing system without oxygen excursionsoccurring.

As with CEDI technology, where contaminantions pass through the membrane material andwater does not, in degassing membrane systemsthe gas being removed passes through themembrane while water is retained.

An example of a degassing membrane system isthe Liqui-Cel product supplied by Membrana.This is a hollow fibre, microporous, naturallyhydrophobic, polypropylene filter which has goodgas transfer capabilities. The membrane also hasrelatively high burst and tensile strengths.

For ease in device manufacturing, the hollowfibre is knitted into a fabric array so that thefibres are evenly spaced. Unlike a filter, whichmembrane contactors are not, the liquid flowsover the outside of the hollow fibres while thegas/vacuum phase is on the inside of the hollowfibres. There is much more surface area for gastransfer to occur when the liquid flows shell sideor on the outside of the hollow fibres. Theoxygen is drawn through the membrane andremoved from the system. (Degassing membranesare also increasingly being used to remove carbon

dioxide from RO permeate to reduce thechallenge onto CEDI modules).

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Space does not permit any in-depth review of thisarea, however, one example given on the Kochwebsite is of a microchip manufacturer whosecustom water treatment chemistry enabledreclaimed wastewater to replace municipal wateras make-up for cooling towers. The companysaved a total of US$300,000 per annum in citywater expenses and reduced chemical usage fromincreased cycles of concentration and lowerblowdown volume.

The use of recycling systems today is not only astandard but a must-have in a system. Therecycling rate typically lies between 60-80%, anddepends on the location (country and end user)where the system is installed. The recycled wateris usually used again in the UPW treatment plantso recycled water is returned from the recyclingplant to the raw water or the filtered water tank.

A typical recycling system consists of thefollowing equipment:

• Collecting tanks;

• Circulating pumps;

• Booster pumps;

• Activated carbon filters;

• RO units.

The use of membrane systems in thereclaim/recycle/reuse of previously dischargedwaste streams will increase as water resourcesbecome scarcer and more expensive. •AAcckknnoowwlleeddggmmeennttssSpecial thanks to John Yen at IIoonnppuurreeTTeecchhnnoollooggiieess for the material supplied and to ElkeTures at CChhrriisstt WWaatteerr TTeecchhnnoollooggyy.Thank you to the following companies for thematerial made available on their websites: CChhrriisstt WWaatteerr TTeecchhnnoollooggyy;; FFiillmmTTeecc;; KKoocchh MMeemmbbrraannee SSyysstteemmss;; MMeemmbbrraannaa;; PPaallll;; UUSSFFiilltteerr.

Cover story 25Filtration+Separation June 2006

Microelectronics facilities demand large volumes ofwater - as much as two thousand gallons per minute- to manufacture a product. And while productsincrease in complexity and decrease in size, qualityrequirements for water increase proportionately.