Ultrapure Water Production
John DeGenova
Department of Chemical and Environmental Engineering
University of Arizona
 1999 Arizona Board of Regents for The University of Arizona
INTRODUCTION
Throughout the wafer manufacturing process, there are several steps that involve the use of
ultrapure water (UPW), both in chemical bath mixtures, and in the rinse process following these
chemical processes. These include standard cleans, wet etch, and Chemical Mechanical
Planarization (CMP). Furthermore, the wafer is often redundantly cleaned to remove
contaminants and prepare the surface between processes. These wet standard cleaning
operations can account for as much as one third of the total processing steps, depending on the
type of product.
Wet cleans and other wet processes, such as some etch processes, solvent processes and CMP,
utilize only UPW for the process chemistry and subsequent rinses. In fact, more than 3000
gallons of UPW can be used to process each eight inch wafer from start to finish. Standard wet
cleans consume about 60% of the total UPW used in wafer processing, while etch processes,
which typically use acids, consume 20%. Solvent processes use 10%, and process tool cleaning
steps consume approximately 10%.
UPW GENERATION PROCESS
The UPW generation process is a series of chemical engineering unit operations designed to
remove contaminants from water to achieve an ultrapure level. These contaminants include
particles, organic compounds, dissolved ions, and dissolved gases. The final UPW quality that
must be achieved is often the highest of any industry. This is due to the fact that any
contaminants that remain on the wafer surface can render a device non-functional; the
dimensions of the device structure are so small that virtually any molecule of contaminant can be
detrimental.
A comparison of typical water qualities is presented in Table 1. A simplified diagram of a
typical UPW system is presented in Figure 1.
Water Quality
Parameter
Units
Typical Municipal
Water Supply
Typical Ultrapure
Water Product
Resistivity
pH
TOC
Ammonium
Calcium
Magnesium
Potassium
Silica
Sodium
Chloride
Fluoride
Sulfate
M Ohms-cm
units
ppb
ppb
ppb
ppb
ppb
ppb
ppb
ppb
ppb
ppb
0.004
8
3500
300
22000
4000
4500
4780
29000
15000
740
42000
>18
6
<10
<1
<1
<1
<1
<10
<1
<1
<1
<1
Table 1 – Water Quality Comparison
Figure 1: Typical UPW Schematic
F e e d F lo w
(g p m )
744
120
79
133
Feed
Tank
1776
A c t.
R e v e rse
R e v e rse
UV
C a rb o n
O sm o si s
O sm o si s
254 n m
1199
U P W S u p p ly
1000
1776
1332
1199
444
IW W
1079
120
P ri m a ry
D EG A S
P ri m a ry
(R O )
M ix e d
UV
Tank
1199
254 n m
Be d
1199
1199
IX
1199
U P W R e tu rn
500
1079
500
P o l i sh
UP W
M ix e d
UV
P o l i sh
Tank
1579
185 n m
Be d
1579
IX
U .F .
1579
1500
79
Makeup Water
Feed water to a UPW facility is typically pumped from either a local well or from a municipal
water treatment plant (sometimes both) to a raw water storage tank. Pumps from this storage
tank re-pressurize the stream and provide a steady pressure flow through the pretreatment
process. This pretreatment usually includes filtration steps such as multimedia filters (a
combination of gravel, sand and anthracite) and/or activated carbon filtration. In the
multi-media filtration process, water flows downward through the layers of media, leaving
suspended solids trapped in the interstitial spaces. Multimedia filters require periodic
backwashing, which fluidizes the media and releases the trapped particles. In an activated
carbon process, water usually flows downward, adsorbing some of the organic compounds and
typically all of any oxidizing compounds.
Some manufacturers heat the water at this point to ensure better processing downstream.
Heating the water improves hydraulic performance of the reverse osmosis (RO) units. It also,
however, increases the species flux for most contaminants through the membrane, thus rendering
the process less efficient at removing those contaminants.
Chemical pretreatment to the RO process typically includes pH adjustment, such as an acid
injection, and possibly a chlorine reducing agent, such as sodium bisulfite. If activated carbon
filtration is present, addition of a reducing agent is not necessary. In some cases, an antiscalant
is also added to help prevent precipitation and fouling on the RO membrane surface. Some
UPW makeup systems incorporate water softeners to help remove ionic impurities that might
otherwise foul RO membranes. These usually consist of filter housings that contain cartridge
filters with a nominal pore rating of 0.5 to 1.0 m located immediately upstream of the RO
pumps. These filters serve as a mixing chamber for the pretreatment chemicals as well as a
vehicle for removing relatively large particles before the water enters the RO high pressure
pumps.
Reverse Osmosis
The RO process is considered to be the workhorse of the UPW system. This process typically
removes >98% of all dissolved solids, TOC, silica and particles from the feedwater stream.
Most RO systems are labor intensive. They also consume a significant amount of the energy and
chemicals required to maintain a dependable UPW system. Because the RO process is a
membrane process rather than a simple cutoff filter, separation occurs based on the osmotic
pressure difference across the membrane. Small molecular compounds that can pass through the
membrane will not permeate if the osmotic pressure difference is favorable. Due to this
behavior, many UPW systems contain dual pass RO systems, in which two RO systems are
arranged in series. Product water from the first pass is fed into the second. In this way, an even
better quality RO permeate can be produced.
To maintain flow velocity across the membrane surface as the volume of remaining feed and
concentrate is reduced by removal of permeate, all industrial RO systems are arranged in tapered
arrays or stages. In each train, the concentrate from the first set of vessels is the feed to the
second set of vessels. Typically, 100% of the feedwater fed to the first array is separated into
two streams; a 50% permeate solution, and a 50% concentrate solution. The vessel
configuration typically uses the concentrate produced from the first array to feed the second
array, where this is again split into two streams; a 50% permeate solution, and a 50% concentrate
solution. The overall permeate conversion is typically 75%. Different array designs are possible
depending on feedwater characteristics and flow requirements.
Vacuum Degasification
Depending on the amount of chemical pretreatment, RO permeate water usually contains
undesirable dissolved gasses such as carbon dioxide and oxygen. Typically, these gases are
mostly removed with a vacuum degasifier. This system consists of a degasification tower that is
under negative pressure by means of a vacuum pump. Water enters the degasifier at the top of
the tower through an inlet header containing spray nozzles. As the sprayed water falls through
the packing it is broken into fine droplets to create a larger surface area. Carbon dioxide,
oxygen, and other volatile compounds are liberated and removed in the vapor flow. There is no
waste stream since all water is recovered for further treatment.
Storage Tanks
Storage tanks utilized for UPW systems are typically lined with an inert polymer coating such as
polyvinylidene fluoride (PVDF). Tanks vary in size but all are designed to store water for use in
the high purity water re-circulation loops. High purity water storage tanks also receive return
flow from the distribution piping network supplying the fab. These storage tanks are also
blanketed with high purity nitrogen to prevent contamination from the air and maintain high
levels of purity. When the tanks are full, water from the make-up system is diverted back to the
raw water storage tank. This allows the RO plant to run continuously using water already
metered, and thus reduce the intake of City Water.
Mixed Bed Ion Exchange
Ion exchange columns or beds, are a series of rubber-lined steel pressure vessels containing
either cation exchange resin, anion exchange resin, or in the case of Mixed Bed Ion Exchange, a
mixture of two synthetic ion exchange resins. These are commonly referred to as "mixed beds".
The two different resins are utilized to perform specific ion exchange functions. As water
passes downward through the mixed bed, the cation resins chemically exchange undesirable
aqueous cations (e.g., Na+, Ca2+, Mg2+, etc.) with a hydrogen ion (H+) from the resins functional
group. Undesirable aqueous anions (e.g., Cl-, SO42-, etc.) are exchanged with a hydroxyl
molecule (OH-) attached to the anion resins. The hydrogen and hydroxyl ions then chemically
combine to form H-O-H, or H2O, at the vessel outlet.
The chemical reaction is fully reversible so that when the resins are completely loaded with the
undesirable ions they become exhausted. At this point, dilute acid and caustic solutions are
employed to flush away the "contaminant" ions and replenish the supply of hydrogen and
hydroxyl ions. This cleansing step is also known as regeneration. All water is recovered while
the mixed bed is in service. Regeneration procedures create various waste streams that are
recoverable. Depending on the quality they are typically directed to either a reclaim storage tank
for reuse in other facilities systems, diverted back to the regeneration water supply tank, or routed
to an industrial wastewater neutralization system.
Electrolytic Deionization (EDI) is a rather new technique that essentially regenerates the ion
exchange resin continuously using electricity rather than regeneration chemicals. This process
requires a membrane to separate channels into a permeate and a reject stream. Ions, once
removed from the water onto a resin, are pulled from the resin via a charge across electrodes.
These ions are pulled toward the electrode through the membrane and then into the reject stream,
where they are washed away. EDI usually requires pretreatment with RO to obtain a pure
enough feed stream to prevent fouling from salt formation. This process typically recovers 90 to
95% of the feedwater as permeate water.
Ultraviolet Oxidizers/Sterilizers
Ultraviolet oxidizers/sterilizers (UV) are cylindrical stainless steel flow chambers containing
quartz sleeves with mercury lamps inside. Compression fittings connect the quartz sleeves to
the flow chamber providing a waterproof seal. A perforated baffle surrounds the quartz sleeves
and forces the inlet water to take a tortuous path before exiting. The turbulence created
increases exposure time and surface area to the UV light. Ultraviolet processes serve different
functions in a UPW system depending on their location and the wavelength of light emitted.
Primary functions include:
 185 nm UV light is typically used for the reduction of organic compounds.
 254 nm UV light is typically used for destruction of bacteria.
 254 nm UV light is typically used for the conversion of ozone and hydrogen peroxide
to dissolved oxygen and water.
All UV sterilization units are intended to provide in excess of 99.5% reduction of biological
organisms.
Ultrafilter Skid
Spiral-wound ultrafilter (UF) membranes are similar to RO membranes. Ultrafilter membranes
have a higher molecular weight cutoff than RO membranes but require less energy to produce
equal volumes of permeate. Ultrafilter membranes are well suited for the removal of colloidal
silica, endotoxins or pyrogens that may be created by other system components located upstream.
They are employed towards the backend of a UPW system for these reasons.
Typical UF membranes are spiral wound and maintain either a 6,000 or 10,000 molecular weight
cut-off (MWCO) range, as opposed to the 100 MWCO for a standard RO membrane. Low
pressure pumps supply the driving force necessary to divide the feedwater stream into two exit
streams. Ninety-five percent of the feedwater is recovered as permeate. The remaining 5% is
collected in the reject manifold but is also of very high quality. This UF reject can be recovered
and sent to the feed water storage tank, or it can be used as feedwater to boilers, fab humidity
control systems, the ion exchange regeneration loop, or other systems requiring high purity water.