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SEMI F75-1102 © SEMI 2002 4 process contr o l (SPC), t he number of particl es per sample interval should b e sufficient for good statistics (e.g. > 20 partic les per sample interval). For e x ample, to obtain a coeff…
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
SEMI F75-1102 © SEMI 2002 5
estimate of bacteria that is capable of being cultured
during a growth opportunity (the culture method). On
line samples may also be obtained utilizing a method
similar to that of the SEM particle capture method.
Larger volumes can be filtered using this method
making the bacterial counts more statistically
significant. The culture method may also be performed
concurrently to determine if an increase in levels is due
to live or dead bacteria.
8.5.4 Total Viable Organisms by Scan RDI
8.5.4.1 Scan RDI is a semi-automated method of
measuring total viability counts. The method is able to
detect a single cell based on direct measurements of cell
activity and includes bacteria and other live organisms
that may be present in biofilm.
8.5.4.2 The sample is filtered through a membrane and
a counterstain is added to minimize background
fluorescence. Viable organisms are labeled using a
nonfluorescent substrate that diffuses across the cell
membrane. The labeling differentiates between viable
and dead cells based on the presence or absence of
esterase activity and intact cell membranes. Only viable
cell with membrane activity have the ability to perform
this cleavage and retain the fluorescent label. No cell
growth is required. The membrane is then scanned by a
solid-phase laser-scanning cytometer. The cells are
detected, counted and verified within minutes.
8.6 Silica
8.6.1 Forms of Silica in UPW Systems
8.6.1.1 Silica exists primarily in two forms as silicates
and as polymeric silica. Silicates are referred to as
dissolved silica and are reactive to molybdate using the
heteropoly method. The polymeric forms are called
colloidal silica or particulate silica and are not
measured directly by on-line analyzers. As pH in a
UPW system is lowered, silicates polymerize to form
colloidal silica, which range in size from a few
nanometers to several tenths of a micron.
8.6.1.2 As with TOC, RO membranes are responsible
for the gross rejection of silica in incoming waters.
Within the UPW system, dissolved silica is further
removed in the anionic ion exchange resin beads.
However, dissolved silica is only weakly attracted to
the anionic resins, and is one of the first species to be
liberated when the resin beds approach exhaustion.
After anion resins are exhausted, increasingly high
levels of silica are released into the UPW stream. Since
dissolved silica is only weakly negatively charged, the
on-line resistivity meter is not sensitive enough to
detect silica leakage prior to total resin bed exhaustion,
i.e. breakthrough. It is very important to detect this on-
set of silica breakthrough before a drop in resistivity. If
this does occur, then Cl
-
and other monovalent ions may
pass from the resin to the point of use. This would also
be accompanied by a decrease in the resistivity from the
resin beds. This occurs because ions that are stored on
the resin beds over a period of time become liberated as
their exchange sites are taken over by other higher-
affinity ions (e.g. nitrate, sulfate).
8.6.2 Dissolved Silica Techniques
8.6.2.1 For the measurement of dissolved silica, a
colorimetry heteropoly blue method is used. Dissolved
silica reacts with the molybdate reagent at acidic pH,
and forms a yellow complex in direct proportion to
concentration. A reducing agent is added to the
prepared sample, causing a blue color to develop. This
color change can be quantified by a sensitive
spectrophotometer for ppb to sub-ppb levels of
dissolved silica. On-line instruments provide
continuous monitoring of UPW systems. Ion
Chromatography may also be used to measure
dissolved silica.
8.6.3 Total Silica Technique
8.6.3.1 Since all forms of silica are not reactive in the
heteropoly method, the quantity of total silica in UPW
must be verified independently. Techniques such as
ICP-MS are capable of quantifying the silicon present
to sub ppb levels after pre-concentration. The method
detection limit of total silica obtained by this technique
is in the range of 0.5 ppb.
8.6.4 Measurement Methods For Total Silica
8.6.4.1 Total Silica may be measured by Graphite
Furnace Atomic Absorption Spectroscopy (GFAAS),
Inductively Coupled Plasma Atomic-Emission
Spectroscopy (ICP-AES) or Inductively Coupled
Plasma-Mass Spectrometry (ICP-MS) at ppb levels.
8.6.5 Colloidal Silica
8.6.5.1 The difference between total silica and
dissolved silica is assumed to be represented by the
colloidal form of silica. Colloidal silica is partially
removed by the RO membrane, however, some hot
UPW feed streams may carry a higher concentration of
colloidal silica. Colloidal silica cannot be detected by
on-line resistivity or dissolved silica testing, but may be
detected as a component of non-volatile residue.
8.7 Non-Volatile Residue (NVR)
8.7.1 Non-volatile residue in UPW primarily consists
of dissolved inorganic material. In the final polished
UPW the most likely dissolved inorganic material is
silica in both the dissolved and colloidal form. The
primary source of this breakthrough silica is the anion
resin bed as it approaches exhaustion. Sub-ppb levels
of dissolved silica can be detected as non-volatile