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SEMI F75-1102 © SEMI 2002 3 reduced organic matter will limit the n utrients available for bacteria growt h and thus biofil m developm ent. Increasing TOC values at the FF can indicate degradation a nd/or fouling of syst…
SEMI F75-1102 © SEMI 2002 2
batch samples can prove time-consuming and be prone
to sampling error.
4 Referenced Standards
4.1 SEMI Standards
SEMI F61 — Guide for Ultrapure Water System Used
in Semiconductor Processing
SEMI F63 — Guidelines for Ultrapure Water Used in
Semiconductor Processing
NOTE 2: As listed or revised, all documents cited shall be the
latest publications of adopted standards.
5 Terminology
5.1 Abbreviations and Acronyms
NOTE 3: All other abbreviations and acronyms are defined in
the SEMI Compilation of Abbreviations and Acronyms
available at web site, http://www.semi.org/.
5.1.1 CFU — Colony-Forming Units
5.1.2 DO — Dissolved Oxygen
5.1.3 EDI — Electrodionization
5.1.4 EDX — Energy Dispersive X-Ray Microanalysis
5.1.5 FF — Final Filter
5.1.6 IC— Ion Chromatography
5.1.7 NDIR — Non-Dispersive Infrared Analysis
5.1.8 OPC — Optical Particle Counters
5.1.9 POD — Point Of Distribution
5.1.10 RO — Reverse Osmosis
5.1.11 TDS — Total Dissolved Solid
5.1.12 THM — Trihalomethanes
6 Units
6.1 Parts per million (ppm) is equivalent to µg/mL or
mg/L.
6.2 Parts per billion (ppb) is equivalent to ng/mL or
µg/L.
6.3 Parts per trillion (ppt) is equivalent to pg/mL or
ng/L.
7 Use of the Guides
7.1 Monitoring programs should reflect the age and
complexity of the UPW equipment and the needs of the
manufacturing process.
7.2 The quality of UPW at the POU may be negatively
impacted by 1.) contamination sources within a tool, 2.)
the design of the tool, 3.) the materials of construction
of the tool and piping distribution, and 4.)
contamination loading in the bath from build-up after
multiple rinse cycles.
7.3 Sampling methods and contamination control are
of paramount importance when attempting to measure
the listed parameters at very low levels of sensitivity.
7.4 The quality of the data measured may depend on
which testing methods and calibration techniques are
used. Consequently, trends observed in the values may
be more meaningful than absolute values, especially for
certain on-line monitors calibrated for ultrasensitive
detection. In addition, online and offline measurements
may not correlate depending on the measurement
technique and level of sensitivity of the measurement.
8 Tests For Monitoring UPW Production and
Distribution
NOTE 4: Since SEMI Guides do not require analytical data or
methods to support them, the recommendation of specific
analytical methods are only for informational purposes.
Alternative methods may also be applicable.
NOTE 5: See Table 1 at the end of this document for
summary of recommended testing frequency and sampling
points.
8.1 Resistivity (megohm-centimeters)
8.1.1 Resistivity (the inverse of conductivity) is a
general measure of ionic activity and is measured by an
on-line meter. The resistivity of UPW should be
approximately 18.2 mΩcm depending on the resolution
of the instrument.
NOTE 6: 18.25 mΩcm is the theoretical upper limit for pure
water at 25°C.
8.2 Total Oxidizable (Organic) Carbon (TOC) (ppb)
8.2.1 Carbon Sources in Water Supplies
8.2.1.1 Incoming feed water contains both inorganic
and organic carbon. Inorganic carbon as dissolved
carbon dioxide (CO
2
), bicarbonate, and carbonate can
be present at high ppm levels according to the geology
of the water supply. Organic carbon reflects biological
input and man-made contaminants such as oils,
pesticides, and fertilizers. Ground and well waters
normally have significantly lower organic content than
surface waters. To remove the majority of organics,
most UPW systems employ reverse osmosis; anion
exchange resin and ultraviolet (UV) destruction are also
employed to remove organics. Some volatile organics,
such as trihalomethanes (THM), may be controlled by
the use of vacuum degasification. The control of
organics is essential for preventing organic fouling and
maintaining resin beds at high efficiency. In addition,
SEMI F75-1102 © SEMI 2002 3
reduced organic matter will limit the nutrients available
for bacteria growth and thus biofilm development.
Increasing TOC values at the FF can indicate
degradation and/or fouling of system components,
contamination from routine operational maintenance, or
return contamination from the factory. Changes in the
incoming feed water due to seasonal or extreme
weather conditions can also cause increases in TOC
readings.
8.2.2 Method of TOC Measurement for Source Water
8.2.2.1 There are various methods for measuring TOC,
and several TOC analyzers are available. TOC
measurement generally involves the oxidation of
organic materials by means of temperature, UV
radiation, and/or chemicals. The CO
2
produced by these
reactions can then be measured by non-dispersive
infrared analysis (NDIR) or conductivity (resistivity)
differential. The exact method utilized will vary
depending on the TOC instrumentation employed and
the method may also affect the TOC readings as
different methods have different recoveries of various
organics. Some instruments will also require the use of
a carrier gas such as air or nitrogen, while others
measure TOC directly.
8.2.3 Monitoring TOC in the UPW System
8.2.3.1 TOC is a useful test to measure the organic
removal effectiveness of the UPW system components
including Carbon, RO, Degasification, and Ion
Exchange. Suggested measurement points are city
feed, pre and post RO membrane, and final filter, with a
minimum frequency that reflects possible changes in
organics from the feed source or UPW components.
After the RO, TOC drops from low ppm levels in the
source water to mid ppb range, and to single digit ppb
levels after the mixed resin beds and UV TOC
reduction units.
8.2.4 Monitoring TOC at the POU
8.2.4.1 TOC may be measured at the point-of-use to
determine quality changes from the distribution system
and the manufacturing tool. Short wavelength UV (185
nm) is capable of breaking up residual organics into
charged organic molecules. TOC which survives to the
point of use in a UPW system are typically either
“light” molecules or small fragments of larger
molecules such as acetate and formate. While low TOC
means that the UPW system is working effectively to
eliminate the source water organic load, this test is not
an accurate measure of sterility of a UPW system. In
addition, TOC levels at POU can also reflect carryover
from chemical baths and contaminants in cleanroom air.
8.3 Dissolved Oxygen (ppb)
8.3.1 Dissolved oxygen (DO) content can be
controlled, if desired. Rinsing hydrogen passivated
silicon wafer surfaces with high DO UPW can result in
etching of the silicon by the oxygenated UPW and loss
of control of gate oxide thickness.
8.4 Particulate Matter (Particles/L)
8.4.1 Sources of Particles in Ultrapure Water Supplies
8.4.1.1 Particles that adhere to wafer surfaces at each
step of the integrated circuit device manufacture may
impair the application of thin-films and
photolithographic substances, and ultimately cause
discrete and integrated transistors to fail because of
resultant physical imperfections. Particles originate in
the incoming city water and may also be generated
from degradation of the UPW system components or
operational activities. Incoming source water contains a
high level of particles and is initially filtered by
prefilters and mixed media in order to remove gross
physical turbidity in the micron range.
8.4.1.2 Following reverse osmosis, source water
particles have been greatly reduced, as the RO
membranes also reject particles. It is essential to
minimize the particle load to the RO to prevent
membrane fouling and premature or frequent RO
cleanings. From the RO outlet forward in the system,
the particle load in the UPW originates from system
components. Particle sources can be RO membrane
modules, piping components, valves and other similar
control devices, tank linings, resin fines and other
sources where materials are undergoing wear or
degradation and are shedding particles.
8.4.1.3 After the ion-exchange resin beds, increasingly
smaller dimension filters are used in the range 0.2 to 1.0
microns to remove residual particles and resin fines. In
most ultrahigh purity applications, ultra filtration at
10,000 molecular weight is utilized. In most UPW
systems, the presence of significant quantities of sub-
micron particles at the final filter may indicate the
breakdown of upstream UPW system components. In
addition, POU filters on tools can be a source of
contamination if not maintained.
8.4.2 Optical Particle Counters (OPC)
8.4.2.1 On-line methods using laser optical particle
counting technology are recommended for trend
analysis and specification compliance. These
instruments can monitor particles in the 0.03 to > 0.5
micron range depending on the system component
being tested. Particle counting using an OPC requires a
good sample port connection to eliminate false counts.
Good counter maintenance, including annual calibration
and replacement of tubing and fittings is critical
especially if monitoring particles ≤ 0.05 microns. In
order to monitor trend analysis with good statistical
SEMI F75-1102 © SEMI 2002 4
process control (SPC), the number of particles per
sample interval should be sufficient for good statistics
(e.g. > 20 particles per sample interval). For example,
to obtain a coefficient of variation (standard
deviation/mean) of 10%, it is necessary that at least 100
particles greater than the control be counted.
8.4.3 Scanning Electron Microscopy (SEM) Analysis
or Direct Count Analysis of Particles and Bacteria
8.4.3.1 SEM is used to analyze particles and bacteria
within the UPW system. Three major aspects of the
SEM method are the enumeration of particles,
enumeration of bacteria and the determination of the
elemental composition of the captured particles.
Depending on the particulate level in the UPW, a large
volume of UPW is filtered on-line, typically at the final
filter. Due to particles created by the act of sampling
and the preexisting particles found normally on a new
filter prior to use, a volume of over 1000 liters may be
required to get a statistically valid result. In the
laboratory, the filter is removed from the filter holder
and placed in the vacuum chamber of a SEM
instrument. A focused beam of electrons is scanned
across the filter, systematically allowing the size, shape
and distribution of the particles to be measured. The
electron microscope is capable of extremely high power
detection (10,000× magnification), and the counting
process may be automated. Bacteria and particles in the
0.05 to > 0.5 micron range are detected and counted.
8.4.3.2 When SEM particle capture filters are installed
at critical areas in the UPW system on a continuous
basis, they may be utilized to determine the source and
nature of intermittent particle excursions.
8.4.4 Identification of Particles
8.4.4.1 As part of the SEM analysis, an Energy
Dispersive X-ray Beam (EDX) may be applied to a
particle, which produces a spectral output of the
composition of the particle. EDX results allow the
basic classification of organic and inorganic
components. Elemental compositions are provided that
can be used to trace the source of particulates. For
example, a particle having Cr and Fe peaks is probably
from a stainless steel source, while one having C and F
peaks is most probably composed of fluoropolymer
materials.
8.5 Bacteria
8.5.1 Sources of Bacteria in Water Supplies
8.5.1.1 Incoming city water supplies and UPW system
components are all potential sources of biological
contaminants if not monitored and controlled. Viable
(live) counts are usually non-detectable or very low in
the city water due to chlorination. Chlorine removal and
low flow areas in the components are the main catalysts
for rapid biological growth in an UPW system. Fouling
of RO membranes and ion-exchange resins may occur,
requiring costly remediation. Seasonal variations may
greatly change the bacterial count of source water.
8.5.1.2 The oligotrophic nature of the UPW causes the
bacteria to become hydrophobic which creates a
thermodynamic adhesion to the wall. The bacteria,
once adhered to the wall have a higher degree of
probability of having the nutrients required for growth
brought by them than the planktonic bacteria in the
system. This process initiates the creation of a biofilm.
While some system owners employ sanitization
programs in hopes of safeguarding against microbial
activity, biofilm can be prove resistant and may
permanently coat the inaccessible surfaces of valves
and dead-legs. Despite the fact that UPW systems are
designed to be hostile to most bacterial species, the
formation of biofilm on filters, in membranes, and in
ion-exchange resins is widespread and periodic
flourishing of bacterial colonies can be a costly long-
term problem. Once in the tool, live bacteria can
multiply further particularly in low flow area and pose
an ever greater threat.
8.5.2 Viable Bacteria by Culture
8.5.2.1 Replicate samples are collected at each
sampling point in sterile containers, passed through
special sterile filters, and then dosed with a growth
medium. In an incubator, bacteria are cultured and
grow to form colonies. The bacteria colonies are
counted under low power magnification after a
specified interval and at a specified temperature.
Different methods will enumerate different results.
Results may vary depending upon factors such as
sample volume, growth media, incubation time and
temperature, and enumeration methods. It may be
necessary to evaluate different methods to determine
the best recovery for a specific UPW system. Bacteria
results are reported as colony-forming units (CFU) per
unit volume (e.g. 100 mL or 1000 mL). For very low
bacteria UPW systems, larger sized samples may be
collected to provide lower detection limits. Larger
volumes can be filtered using this method, making the
bacterial counts more statistically significant.
8.5.3 Total Bacteria by Epifluorescence
8.5.3.1 After collection, samples are filtered onto
polycarbonate membranes, which are then stained with
dyes that cause biological materials to fluoresce under
ultraviolet light. A high power microscope is employed
to visually identify bacteria, both live and dead, and a
counting method is used to give statistical accuracy for
the sample size. This test provides accurate
information about actual bacterial content (both viable
and non-viable) of the UPW system, rather than an