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SEMI F75-1102 © SEMI 2002 5 estimate of bacteria that is cap able of being cultured during a growth opportunity (the culture method). On line samples may also b e obtained utilizing a method similar to that of t he SEM p…
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
SEMI F75-1102 © SEMI 2002 6
residue. There is also evidence that as an ion-exchange
bed approaches exhaustion, previously removed
dissolved silica can be subsequently released as
colloidal silica if the pH in the ion-exchange bed is
favorable. There is currently no specific measurement
of on-line colloidal silica, but detecting it as a
component of non-volatile residue can be beneficial.
8.7.2 Method of Measurement for NVR
8.7.2.1 The on-line measurement of nonvolatile residue
is based on the principle of determining “residue after
evaporation” of atomized ultrapure water droplets.
Droplets evaporate in a fraction of a second leaving an
ultrafine nonvolatile residue particle agglomerate a few
nanometers in diameter; the size of which is related to
the amount of residue impurity originally present in the
ultrapure water. A condensation particle counter then
monitors the concentration of ultrafine nonvolatile
residue particles. The measurement range of this
technique is 0.01 parts per billion to 20 parts per
million.
8.8 Ions
8.8.1 Mass Balance and Ionic Removal in UPW
Systems
8.8.1.1 The ionic composition of natural waters is in a
dynamic equilibrium (mass balance) with the geological
source. The total dissolved solid (TDS) content is
composed of negatively charged (anionic) and
positively charged (cationic) species that can be
analyzed by Ion Chromatography (IC). RO membranes,
ion-exchange resins, and electronic deionization (EDI)
are used to remove ionic species from water. The UPW
resin beds need to be monitored because each unit has a
fixed capacity to remove ions over time. Weakly bound
singly charged (monovalent) ions such as Na
+
and Cl
-
are continually displaced in favor of more highly
charged doubly charged (divalent) ions such as SO
4
2-
and Ca
2+
throughout the life of the resin. When ionic
breakthrough occurs, the weakly bound ions are
liberated in excess, and resin beds must be regenerated.
8.8.2 Ion Chromatography Techniques
8.8.2.1 Ionic (anions and cations) removal in UPW
systems can be quantified at very sensitive levels using
the technique of ion chromatography. During IC
analysis, a UPW sample is injected into an analytical
column packed with a special ion-exchange resin. The
ions from the samples are eluted (washed) down the
column by a suitable eluent. Depending on their
binding energy and sizes, the ions move down the
column at different speeds. In time, all of the ions are
separated and measured by a conductivity detector.
External calibration of a sensitive conductivity detector
across a wide range of concentrations allows accurate
quantification of anion and cations. The anions and
cations determined by IC are fluoride (F
-
), chloride
(Cl), nitrite (NO
2
-
), bromide (Br
-
), nitrate (NO
3
-
), sulfate
(SO
4
2-
). phosphate (PO
4
3-
) and lithium (Li
+
), sodium
(Na
+
), ammonium (NH
4
+
), potassium (K
+
), magnesium
(Mg
2+
), calcium (Ca
2+
) respectively.
8.8.3 Ions Critical in Semiconductor Processing
8.8.3.1 Certain types of haze formation on wafer
surfaces could be caused by high concentrations of
anions and cations on the silicon surface as residue.
There could be other causes for haze formations,
therefore, testing of anions and cations at the POU in
the rinse bath may be important to identify and prevent
this source of contamination.
8.9 Metals
8.9.1 Measurement Methods
8.9.1.1 Up to 68 metals may be determined by
instruments such as Graphite Furnace Atomic
Absorption Spectroscopy (GFAAS), Inductively
Coupled Plasma Atomic-Emission Spectroscopy (ICP-
AES) or Inductively Coupled Plasma-Mass
Spectrometry (ICP-MS).
8.9.2 Sources of Trace Metals
8.9.2.1 Trace metal concentrations in feed waters vary
over orders of magnitude according to the geology of
source rocks and the residence time of water in aquifers
and surface systems. Surface water supplies are often
mixed by the local municipal services, and seasonal
variations are also introduced as ground water and well
waters are brought in and out of service. Consequently,
periodic analysis of metals in incoming feedwater and
at key points of the UPW system including after the
final filter is essential so that significant changes in
performance may be corrected early in the process of
deionization.
8.9.3 Boron
8.9.3.1 Boron is a trace element that, like silica, is
weakly attracted to anion resins. Both elements serve
as a flag for the onset of ionic breakthrough through the
anion exchange resin. A rapid and systematic rise in
the boron content immediately after the resin bed is
typical when ion-exchange resins approach exhaustion
in the UPW system. Silica and boron breakthough rates
and each element’s selectivity in the ion exchange
process is a matter of ongoing study. The safest route is
to measure both, but at least to focus on the one most
prevalent in the city water feed stream.
8.9.4 Contaminants from the Distribution System
8.9.4.1 Certain heavy elements such as Zn, Mn, Ni, Cu,
Cr, and Pb may be present as trace impurities in plastic