Ultrapure Water Recycle in the Semiconductor Industry
John DeGenova
Department of Chemical and Environmental Engineering
University of Arizona
 1999 Arizona Board of Regents for The University of Arizona
This paper summarizes the benefits and risks associated with water recycling in the
semiconductor industry. The specific items that will be covered in this paper include
characterization of the spent rinsewater types, the composition range of key impurities,
and different recycling strategies. The discussion will include the development of new
purification methods for the removal of organic impurities, and the development of a
computer model for simulating the effects of recycle.
INTRODUCTION
The semiconductor industry has recently experienced rapid growth at an unprecedented
rate. This expansion is causing concern in some communities due to the large quantities
of water presently required for semiconductor wafer manufacturing. Along with this
expansion comes the construction and installation of new wafer fabrication facilities
(fabs). Each new fab will use 1 to 3 million gallons per day for just wafer processing.
The water use in some locations may approach twice that range. The only cost effective,
long term solution is proper segregation and collection of waste rinsewaters as part of the
implementation of a true recycling strategy.
Recycling of water in the semiconductor industry is the reuse of water, that had been
previously purified to an extremely high quality, used in wafer processing, collected,
retreated, and used again to process wafers. The practical range of recoveries for recycled
water is from a low of 10-20% to over 95%. Fab rinses can be relatively easily recycled
because they consist of water, already extremely contamination-free, with low levels of
easily removed ions from wafer processing chemicals. Much of the collected rinsewater is
already cleaner than the municipal supply water it replaces. This water can be directly
re-used as a replacement for some of the municipal water supply, without additional
treatment. Various amounts of recovered water can be recycled. Some of the water may
require treatment prior to reintroduction to the UPW system.
Figure I indicates the wide range of water usage rate reported by semiconductor
manufacturing facilities. This chart is the result of a survey performed by Sematech,
along with data obtained from its member companies. The amount of UPW used per
wafer produced varies with wafer size and from site to site between the different
manufacturers. Each wafer fabrication facility typically uses 1-3 MGPD (million gallons
per day). Some companies have 3-4 fabs per site, resulting in consumption of UPW
exceeding 10 MGPD. This amount does not refer to the municipal water demand but
rather only the demand for UPW. The actual demand on the municipal water supply is
approximately 25%-40% greater than the quantity of UPW due to losses in the
purification processes. These demands can be quite significant on the municipal water
supply and have been a problem for some communities, especially those located in arid
regions.
Most of the UPW consumed is used for wafer rinsing purposes. Figure II indicates some
typical semiconductor process tool setups and a general indication of the various types of
wastewater generated1.
The first and primary strategy to conserve UPW is simply the reduction of water used in
the wafer rinsing process. Some of the techniques, indicated in Table I are quite simple
but can make significant impacts to the overall water consumption. Spray type rinsing
has been shown to use much less water than the typical overflow or dump-rinse methods.
It can reduce water consumption by 75-95%, and dramatically lower processing times.







Spray rinsing vs. overflow/quick dump
Rinse tank geometry improvements
Hot UPW vs. cold UPW
Megasonic rinsing
Idle flow rate reduction
Analytical monitoring of rinse water
Computer modeling, convective/diffusive
Table I: Rinse Water Reduction Techniques
Megasonic excitation during rinsing is a typical method of indirectly improving the rinse
process. By adding megasonic action, the cleaning process in some cases can be
optimized, and possibly allowing for a reduction of necessary rinse water.
Until recently, rinse tanks were typically designed with relatively large volumes,
significantly larger than the wafer carriers (boats) which hold the wafers to be rinsed. As
the wafers are typically loaded into the boats tightly spaced relative to one another, the
path of least resistance for the water flow is actually around the wafer boat, rather than in
between the wafer product spacing.
As can be seen in Figure III, in an overflow rinse process, water flows at a constant rate
from the bottom of the tank overflowing out of the top of the tank, for usually 5-10
minutes. The initial turbulent mixing mechanism while wafers enter the tank changes
quickly to one of non-turbulent, laminar flow. In this case, most of the water flows to the
outside of the wafer boat, bypassing the wafers entirely. It has been shown that nearly
80% of the rinse water actually bypasses the product4. A smaller rinse tank, or one that
confines the flow to the wafer volume, actually provides for a more water-efficient rinse.
New semiconductor process tools are being designed with smaller rinse tanks and with
directional flow patterns to force the water in between the wafer spaces, producing faster
and more effective rinsing. Less process time is required for the rinse, and better process
control is achieved.
Studies are under way to evaluate sensors for monitoring the quality of water in each rinse
tank to determine when adequate rinsing is achieved. Other investigations have focused
on the development of rinse models using both diffusion and convection equations to help
optimize the rinsing process5. These models incorporate the processes that occur in
between the tightly spaced wafers, including the desorption of the chemicals from the
wafer surface and the diffusion of the chemicals through the boundary layer into the bulk
fluid, where impurities are carried away by convection. Further work needs to be done in
the area of turbulent mixing, dump rinse efficiencies, and elucidating mechanisms for
spray rinsing.
Reduction of UPW flow rates during idle periods can also make significant differences in
water reduction. A minimum flow is required to prevent bacteria colonies from forming
colonies on to the pipe walls.
A second water reduction strategy that is currently underway is in the re-use of water in
other areas, in which the quality of water is not a primary concern. Some of the more
common applications of water re-use are listed in Table II. The reject water from the
reverse osmosis process is a good candidate for this type of re-use. Spent rinse water can
also be used for some of these applications. However, since it may contain corrosive
ions, this water may be quite aggressive and could corrode equipment components.
Additional treatment may be necessary prior to re-use of this water, such as alkalinity
adjustments.




RO Reject; C/T, Air Scrubbers, Irrigation
Ultrafilter Reject; UPW system rinsing
Analytical instrument discharge; various use
Spent rinse water; C/T Makeup
Table II: Water Reclamation /Reuse
Another water use optimization and reduction strategy is the recycling of spent rinse
water back into the UPW treatment system. Because of the relatively high purity level of
the rinse water, it can be added back into the UPW purification process at various points
within the system. It can be combined with the feed water at the treatment input or be
combined at other points such as with the reverse osmosis purified water. Depending on
the possible contaminants in the spent rinse water, it can also be added back into the
UPW process at a purer stage, such as in the ultrapure water storage tank where it will be
re-polished prior to use. Table III lists these options, which are also illustrated in Figure
IV. Figure IV indicates a generic UPW treatment process, replete with primary treatment,
ion exchange or secondary treatment, and a final polishing treatment step with holding
tanks. The spent rinse water can be brought back to the holding tanks at any of these
three major stages.
 Feed Water storage tank
 Semi-Pure storage tank
 Ultrapure Water storage tank
Table III: Recycle Flow Options
There are significant benefits, related to both cost and processing, associated with
recycling. For example, one is the improvement in final water quality that is achieved.
With certain ions and most organic compounds, recycling is able to lower contamination
levels proportional to the amount recycled; an 80% recycle amount cuts contamination by
a similar amount.
Some of the key benefits to recycling are included in Table IV. Note that the
improvements will vary with the amount recycled and the technologies used to treat
recovered water. This is not an exhaustive list.
1. Improved final UPW quality
2. Rinse recipes can be optimized for cleanliness and throughput, instead of
minimized water consumption
3. Improved reliability of UPW facility, less downtime:
Reduced frequency of RO membrane cleaning processes
Reduced frequency of ion exchange regenerations
Reduced frequency of filter backwashes/rinses
Improved efficiencies in UPW treatment processing
Improved control of feed water problems
4. Reduced chemical usage for ion exchange regenerations and waste treatment
5. Significant cost savings:
Less feed and discharge water costs
Less regeneration chemical and backwash water costs
Less industrial waste treatment cost
Less maintenance and cleaning chemical cost
6. Possibly Improved RO reject quality for other reclamation purposes
7. Less demand on the municipal water supply and waste water treatment systems
Table IV: Benefits Associated with Recycling Water
There are also risks, however, associated with recycling spent process rinsewater. These
risks include the items listed in Table V. Notice, though, that some of the benefits of
recycling directly answer risks associated with municipal feed water supply and quality;
and that some of the risks are similar for either water source. The key is to manage the
risks with the best engineering practices wherever possible. In some cases, this can only
be done with great difficulty, cost, and/or complexity on municipal feed streams, but can
easily be achieved with a comprehensive recycle strategy.
Recycled water
1. Introduction of impurity spikes into the
system.
2. The potential buildup of recalcitrant
compounds.
3. Qualification of the present purification
methods in removing process generated
contaminants
4. Risk of new chemical interactions caused
by recycle
5. Contamination due to biofouling
Municipal Supply Water
Same
none
none
none
Same
Table V: Risks Associated with UPW Treatment
The risks of recycling, listed in Table V, include the possible introduction of new and
unknown process contaminants into the UPW purification system. Compounds that are
not properly removed could remain and possibly even build up in the system, and the
product yield could be negatively impacted. There is very little data available on the
removal of organic compounds typically used in wafer processing. The presence of any
of these compounds in the UPW product water at the point of use could have devastating
consequences. Hence, the proper treatment or segregation of the spent rinse water is
imperative in order to keep these compounds out of the polish loop. The characterization
of the behavior of specific compounds subjected to standard units processes, such as
activated carbon, Reverse Osmosis, and UV oxidation in a UPW system is an ongoing
task.
Spent ultrapure rinse water is contaminated with residual chemicals carried over from the
process flow. Rinses that typically follow acid/base chemistries can usually be readily
recycled; the contaminants can be removed from the water using standard separation
techniques, such as ion exchange. One must only segregate these rinse water drains
away from the industrial waste drain to collect the water for recycle purposes.
Typical compounds found in spent rinsewaters are listed in Table VI. These compounds
are typically quite easy to detect with available analytical instruments, and are quite easy
to remove with standard unit operations found in most UPW treatment facilities.
Through proper segregation, detection, and diversion techniques, most of the spent
rinsewaters generated in wafer fab facilities is readily recyclable.
Due to the requirements of some of the semiconductor processing chemistries, some rinse
water may contain compounds that are not readily removed with standard separation
techniques. For example, organic surfactants and co-solvents may be present. The rinse
water after photoresist strip chemistry may contain organics if the resist oxidation process
is somehow only partially complete. Certainly, the rinsewater that follows organic
chemical baths will contain compounds that may not be readily removable.
Characterization and correct treatment, or even segregation of these waste streams, are
key to achieve high levels of water recovery concomitant with guaranteed quality.
The contaminants in rinse water that generate most of the concern for recycling in the
industry today are organics. The method of choice for the removal of these organic
compounds in a recycle system is diversion. Metrology is available to rapidly detect and
divert organic-containing water.
As recycle systems become more robust and take on the role as the primary source of
water, organic oxidation will be the mainstay. The concept of photocatalytic oxidation
reactors as applied to UPW recycle systems has been developed at the University of
Arizona in Tucson 2. The photocatalyzed oxidation of process-generated impurities like
organic solvents, surfactants, and trace-chlorinated hydrocarbons in ultrapure water is
promising.
Table VI indicates some typical water purity levels at different stages of processing. It
depicts a typical municipal supply water purity, a typical Ultrapure Water quality, and
expected levels from collecting approximately 50% of the spent rinse water, specifically
segregated according to recycle readiness.
Water
Quality
Parameter
Resistivity
pH
TOC
Ammonium
Calcium
Magnesium
Potassium
Silica
Sodium
Chloride
Fluoride
Sulfate
Units
Typical
Municipal
Water
Supply
Typical
Ultrapure
Water
Product
Segregated
(50%)
Spent
Rinse water
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
0.8
3-7
20
300
0
0
0
338
0
100
100
500
Table VI: Typical Water Quality Comparison
This data proves that the spent rinse water is still in a semi-pure state and is of better
quality than water supplied by the municipality. Thus, one approach of recycling is to
accept this water as is, and return it to the UPW make-up (feed) stream. The recycled
water can even deteriorate in quality to the level of the municipality's water, and it would
still be usable. As there is no pretreatment required, this is a low-labor, low-equipment
alternative. However, although the RO unit affords a high level of protection, it will
waste 20-40% of the water as reject. It is a compromise in favor of risk-avoidance at the
cost of a large fraction of the water recycled. Much greater benefits can be realized when
the risk associated with water recycling is lowered to below that of not recycling. Some
companies have done this with excellent success and cost savings.
The benefits of recycling, listed in Table IV, can be quite substantial. Consider the
configuration in which recovered water is brought back to the front-end. It will
essentially replace an equal volume of outside feed water. With a better quality of feed
water at the source, the unit processes in the ultrapure water facility can operate with
improved efficiency. There will be less required maintenance and downtime, resulting in
a more reliable and safer facility. A reduction of chemical usage can also be realized, as
less membrane cleaning and less ion exchange regeneration would be required.
Recycling also leads to a reduction in the amount of industrial wastewater requiring
treatment. In fact, with a better quality feed source, the reject from a reverse osmosis
system also improves in quality, rendering this water much more amenable for other
reclamation purposes. Figure V illustrates a schematic of a typical UPW system with
recycle back to the primary feed water storage tank. Indicated here are quality levels
through various points in the UPW facility, with and without the recycle of water. Based
on quality levels listed in Table VI, the estimated water quality with recycle feeding the
final polishing loop is superior.
Another configuration that shows the potential for even greater benefits includes a modest
level of pretreatment, and return of collected water to the RO water storage (product)
tank. The system includes the standard hardware from Figure IV, plus:
1. An activated carbon process for the destruction of hydrogen peroxide,
2. An Electrodeionization unit for removal of ions, and optionally,
3. An ultraviolet unit for the destruction of organics.
This system has shown that it's possible to collect virtually all rinse water from a fab, with
treatment through a very low maintenance treatment process, and return over 95% of the
rinse water back to be used again. This level of recycling has shown extremely favorable
returns on the investment, great reductions in key contaminants in the product water,
elimination of dozens of system shutdowns due to feed water quality excursions, and a
variety of other benefits. Development efforts are targeting TOC reduction methods and
sensors for rapid response.
The greatest benefit would be gained by introducing the recovered and treated water to
the UPW tank just prior to the polish steps. This approach maximizes the amount of
water recovered. It assumes that the water being returned is at a quality consistent with
the water it replaces (output from the primary loop), or it will exhaust the polisher resins
sooner, or degrade the output water quality. However, this option also carries the highest
risk.
Process simulation is another important tool in the design and analysis of recycle systems.
The University of Arizona3 is presently developing a process simulator for this
application. This simulation consists of a solution to the governing equations for
transport and removal of impurities. The equations for the simulator modules
representing reactions and transport of impurities are combined to determine the
processes that take place in a typical ultrapure water treatment facility. By solving these
equations, a dynamic view of contaminant distribution in the primary supply, the
polishing loop, the recycle system, and at the point of use can be obtained.
In addition to the process simulation, improved metrology and sensing techniques are
required to minimize the risks of recycling. Of utmost importance is the incorporation of
on-line instrumentation with fast response that can be used to monitor recycle water
quality. Quick response is necessary so that important decisions can be made on whether
to direct the recycle water back into the UPW process or divert the flow away from the
UPW system. This action must be fast enough to avoid any UPW quality upsets. Special
valve switching arrangements can be installed, based on the monitoring devices, in order
to minimize the risk or system upsets. Eventually, there will be a need to integrate
metrology and simulator technologies for both predictive and control purposes.
A Sample of 1996 Ultrapure Water Use Among SEMATECH
Member Company Fabrication Facilities
60.0
50.0
40.0
UPW Usage
(gallons per
square inch)
30.0
20.0
10.0
0.0
10 11 12
13 14 15
16 17
18 19
Approximate Mask Levels (+/-2)
20 21
22 23
24 4
8"
6" Wafer
5"
Diameter
(Inches)
This chart represents ultrapure water (UPW) used in wafer processing (not local water
resources) at 16 U.S. chip manufacturing facilities. Total demand on local water
resources has been reduced by many companies through implementation of recycle and
reclaim strategies. SOURCE: SEMATECH
Figure I: Ultrapure Water Use In Semiconductor Wafer Processing
DRAIN
TYPE
PREFURNACE HOOD
SC-1
UPW
Rinse
1OO:1
HF
UPW
Rinse
SC-2
UPW
Rinse
IPA
DRYER
UPW
Rinse
SOLVENT
IWW
RECYCLE
RESIST STRIP HOOD
SC-1
SC-1
UPW
Rinse
UPW
Rinse
SC-2
UPW
Rinse
IPA
DRYER
UPW
Rinse
SOLVENT
IWW
SOLVENT HOOD
Organic Organic Organic
Solvent Solvent Solvent
IPA
IPA
UPW
Rinse
UPW
Rinse
SPIN
DRYER
SOLVENT
IWW
Figure II: Typical Process Tool Setup
Most of the UPW bypasses the wafers.
Desorption
Convection
Diffusion
Figure III: Typical Overflow Rinsing
Recycle
Recycle
IW
Recycle
IW
IW
Raw
Water
Storage
Tank
Primary
Treatment
R.O.
Water
Storage
Tank
Ion Exchange
Treatment
Ultrapure
Water
Storage
Polishing
Treatment
Point
of Use
D.I. Return
IW
Other
Reclaim
Figure IV: Water Recycling Strategies
TYPICAL UPW SYSTEM w/ RECYCLE
M unicipal
Water
Supply
Recycle
Recycle
M onitoring/
Treatment
60 gpm
73 gpm
IW
Raw
Water
Storage
Tank
100
133
Primary
Treatment
R.O.
Water
Storage
Tank
I on Exchange
Treatment
Ultrapure
Water
Storage
I on
Na+
Ca+2
ClSO4-2
Polishing
Treatment
D.I . Return
33
(ppb)
100
60
Point
of Use
40
Water Quality
City w/ Rec
29,000/16,573
22,000/12,120
15,000/ 8,278
42,000/23,278
City w/ Rec
2,300/1,314
500/ 275
1,000/ 552
1,300/ 720
City w/ Rec
0.9
0.5
0.1
0.05
0.4 0.2
0.4 0.2
c:\ftab\eshjan.ppt
Figure V: Recycle Effects on Water Quality Levels
IW
Other
Reclaim