Characterization of Organic Contaminants Outgassed from
Materials Used in Semiconductor Fabs/Processing
Peng Sun, Caroline Ayre, and Matthew Wallacea
California Materials Technology Department, Intel Corporation, Santa Clara, CA 95052
a
current address: 3306 South 256th Street, Kent, WA 98032
Abstract. As ULSI technology continues to advance, semiconductor manufacturers are facing new contamination
control and monitoring challenges, including airborne molecular contamination (AMC). AMC is being recognized as
one of the yield limiting factors in newer generation microelectronics fabrication processes. A major AMC source,
materials' outgassing can introduce a variety of organic contaminants into semiconductor fabs, impacting many
processes. This paper provides a brief overview of typical organic outgassing contaminants, their sources, process
impacts and analytical techniques used to detect these species. In addition, outgassing study results for
polycyclodimethylsiloxanes and several other contaminants using thermal desorption-gas chromatography-mass
spectrometry (TD-GC-MS) analysis are employed to demonstrate the relationships among (1) outgassing level and
outgassing time (linear), (2) outgassing quantity and the inverse of outgassing temperature (logarithmic), and (3)
outgassing quantity and material surface area (linear). A new method, based on gas diffusion conductivity detection, for
ammonia and volatile amines' outgassing analysis is also presented.
INTRODUCTION
Understanding the effects of Airborne Molecular
Contamination (AMC) on microelectronic fabrication
processes is of increasing concern as ULSI technology
continues to develop. SEMI F2-951 standard
"Classification of Airborne Molecular Contaminant
Levels in Clean Environments" specified AMC in four
classes - molecular acids (MA), molecular bases
(MB), molecular condensables (MC), and molecular
dopants (MD) [1]. Molecular condensables are organic
contaminants that may adversely impact many
semiconductor processes. The 2002 International
Technology Roadmap for Semiconductor (ITRS-02)
indicates organic contamination on silicon wafers after
critical cleans should be below 2.6E13 carbon
atoms/cm2 for 130 nm technologies. The value drops
to 1.5E13 for 90 nm technologies [2]. As one of the
major sources of molecular condensables, materials'
outgassing can contribute to organic contamination
from a variety of cleanroom materials including filters,
sealants, walls, adhesives, floor tiles, paints, wafer
carrier and packaging materials, as well as
consumables such as garments, gloves, tapes and
cleaners.
Material's outgassing is also a source of airborne
base contamination in semiconductor cleanrooms. As
lithography continues to progress to deeper DUV
wavelengths, the resists employed in semiconductor
processing are increasingly sensitive to airborne bases.
The current ITRS specification for total bases in
lithography is 750 pptM (parts per trillion Molar) [2].
Therefore, better understanding of materials'
outgassing behaviors is necessary for cleanroom
material selection and the control of both molecular
base and molecular condensable concentrations in
microelectronic Fab environments.
EFFECTS OF MOLECULAR
CONDENSABLES AND ANALYTICAL
METHODS FOR MATERIALS'
OUTGASSING
Effects of Outgassed Organic
Contaminants
Organic contaminants can affect semiconductor
processing in a variety of ways. Tamaoki, et al.,
CP683, Characterization and Metrology for ULSI Technology: 2003 International Conference,
edited by D. G. Seiler, A. C. Diebold, T. J. Shaffner, R. McDonald, S. Zollner, R. P. Khosla, and E. M. Secula
© 2003 American Institute of Physics 0-7354-0152-7/03/$20.00
245
reported that organic contamination on the initial SiO2
surface caused degradation in the polysilicon layer
resulting in breakdown field strength reduction [3].
Kasi, et al., found that significant organic
contamination (1014-1015 C atoms/cm2) could cause
serious degradation in MOS devices grown on
hydrogen-passivated silicon (Si) [4]. Both oxide and
nitride film growth and quality can be affected by
organic contamination as well. Licciardello, et al.,
reported that plasticizers such as dioctyl phthalate
(DOP) could react with HF-etched surfaces to generate
carbon-rich hydrophobic surfaces that retarded silicon
oxide growth [5]. Saga et al, found that wafers
contaminated with butylated hydroxytoluene (BHT)
and dibutyl phthalate (DBF) experienced gate oxide
integrity (GOI) degradation and low-pressure CVD
nitride growth retardation [6,7]. Organophosphorus
contaminants such as triethyl phosphate (TEP) and
tris(choloropropyl) phosphate (TCPP) flame retardants
can cause serious unintentional doping of Si device
wafers resulting in yield losses [8,9]. FTIR and GC
studies have shown that polycyclodimethylsiloxanes
outgassed from sealants of silicone polymers form
particles [10]. Organic contaminants and photo resist
outgassing can present contamination and haze
problems in DUV lithography [11].
benefits of indirect outgassing measurements is that
they provide a means to study the adsorption behavior
of outgassed organic species on Si wafer surfaces
[12,13]. A drawback of the indirect measurement is
that it does not provide a complete picture of the total
outgassed organic contaminants.
Related industry standard methods include: ASTM
F1227-89 "Standard Test Method for Total Mass Loss
of Materials and Condensation of Outgassing Volatiles
on Microelectronics Related Substrates" [14]; ASTM
F1982-99: "Standard Test Methods for Analyzing
Organic Contaminants on Silicon Wafer Surfaces by
Thermal Desorption Gas Chromatography" [15];
SEMI E108-0301 "Test Method for the Assessment of
Outgassing
Organic
Contamination
from
Minienvironments Using Gas Chromatography/Mass
Spectroscopy" [16]; EEST Working Group CC031
ongoing activity: Recommended Practice (RP) for
method for characterizing outgassed organic
compounds from cleanroom materials and components
[17].
CHARACTERIZATION OF
OUTGASSED ORGANIC
CONTAMINANTS USING TD-GC-MS
Analytical Methods for Materials'
Outgassing Measurements
Detection of Organic Contaminants
Outgassed from Cleanroom Materials
Both direct and indirect techniques may be used for
materials' outgassing measurements. Direct methods
commonly used in the microelectronic industry
include material weight loss analysis and thermal
desorption gas chromatography-mass spectrometry
(TD-GC-MS, also referred as dynamic headspace GCMS). Weight loss measurements are an excellent
method to determine total mass loss due to outgassing
of volatiles and semi-volatiles. They provide useful
information for assessing physical property changes
such as shrinkage during the curing process of a
sealant material. TD-GC-MS combines the superior
separation ability of GC and the powerful unknown
identification and quantification ability of mass
spectrometry. The TD-GC-MS technique allows for
qualitative and quantitative analysis of volatile and
semi-volatile compounds outgassed from different
materials used in semiconductor fabs and processes.
Because of its good qualitative and quantitative
capabilities, TD-GC-MS was used in all of the organic
outgassing characterization experiments and wafer
surface organics' analyses described here. Fig. 1
shows a block diagram of a TD-GC-MS system. For
materials' outgassing analysis, a piece of selected
material was placed in a outgassing thermal desorption
tube and analyzed on a TD-GC-MS system (Fig.lA).
Method B in ASTM F1982-99 standard [15] was used
for wafer surface organic analysis (Fig. IB).
outgassing
A.
B.
Indirect measurements of outgassed volatiles and
semi-volatiles can be made by exposing witness Si
wafers to materials of interest and subsequently
analyzing the outgassed or adsorbed species by TDGC-MS or Time-of-Flight Secondary Ion Mass
Spectrometry (TOF-SIMS), respectively. One of the
tube
tube
organic outgassing
* result: ug/g
surface organic
* result: ng/cmA2
wafer TD unit
FIGURE 1. Block diagram of TD-GC-MS system.
246
2 o
o
A
4
^1=|1
V.
1
»
Total outgassing: 680 ug/g
Siloxanes: 290 ug/g
I
L
8
03
6
I
5.00
T3
C
3
^
10.00
15.00
B
20.00
25.00
30.00
Total organics: 7.6 ng/cm2
Siloxanes: 6 ng/cm2
3
6
2
i ,
1 „
7
1
.
s no_____in no_____i.s.no
Retention Time (min)
C
unknown spectrum
8
(0
T5
^5
-«.„
=3000
_[
-t ooo
~--~~—'
seL«s««o<,
Dodecamethyl-cyclohexasiloxane
standard spectrum
^».ooo
T 000
-»o
oo
«o -, oo-, io-, Ao-, Ao-, sosoo^o^o^o^o^oo^o^
T
m/z
FIGURE 2. A. TD-GC-MS outgassing chromatogram for a cleanroom sealant, (1): acetic acid, (2)-(6):
polycyclodimethylsiloxanes; B. Witness wafer surface organic analysis result, (2)-(6): polycyclodimethylsiloxane, (7) organic
acid ester; C. Identification of dodecamethyl- cyclohexasiloxane by MS detector. Retention time = the time when a specific
compound elutes out from the GC column.
detected on the witness wafer surface with
dodecamethyl cyclohexasiloxane as the most abundant
contaminant (peak 4 in Fig. 2B). The total polycyclodimethylsiloxane level of 6 ng/cm2 (~ 2.5E14 carbon
atoms/cm2) was significantly higher (> 10 times) than
the ITRS-02 requirement for 130 nm technologies.
Another major outgassing compound, acetic acid was
not detected on the wafer surface, suggesting that
acetic acid did not deposit there. Fig. 2C demonstrates
the powerful contaminant identification capability of
TD-GC-MS technique. The chemical structure of the
contaminant eluted from the GC column at 10.9 min
Fig. 2A shows a TD-GC-MS outgassing
chromatogram of a silicone-based cleanroom sealant.
A series of polycyclodimethylsiloxanes, known
problematic contaminants, outgassed from this
material at 50 °C.
The total amount of
polycyclodimethylsiloxane outgassing was 290
microgram/gram (ug/g). Another major outgassing
compound was acetic acid, 380 ug/g. Fig. 2B shows
the TD-GC-MS chromatogram for a witness wafer that
was exposed to one gram of the sealant shown in Fig.
2A. After three days of exposure at room temperature,
the same types of polycyclodiemethylsiloxanes were
247
Triethyl phosphate: 49
ug/g
0)
o
CO
•o
kJJ
c
3 XXfc> «_i rt cita r t ot»
.Q
TIC: 1216O2O2-D
B
Triethyl phosphate: 5.8
ng/cm2
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•7OOOOOO
©oooooo
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•4OOOOOO
3OOOOOO
2OOOOOO
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>.OO
2O.OO
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3O.OO
35.OO
Retention Time (min)
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T929 (I 1 -5©3 r-nir-i>: 12-1SO2O2.O
>i-S
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C
Unknown spectrum
©000
o
(0
Alil . jU . .in . ,
2000
•o
c
**'o
20
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, 1.
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•i e,-v "' s-?©0
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-=.ov
200
->-=>«3
220
2*3-0
"o-^
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:
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s-s Triethyl phosphate
standard spectrum
SOOO
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2000
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20
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'
-MS
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ill
12
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m/z
FIGURE 3. A. TD-GC-MS outgassing chromatogram for a cleanroom sealant; B. Witness wafer surface organic analysis
result; C. Identification of TEP by MS.
cleanroom pop-out sealant material at 50 °C (Fig. 3A).
A witness wafer exposed to the pop-out sealant at
room temperature showed a surface TEP level of 5.8
ng/cm2 (-2E13 P atoms/ cm2), well above the critical
level that can cause unintentional phosphorus doping
of Si wafers and yield loss [17]. Once again, TEP was
positively identified by the MS detector (Fig. 3C).
was identified to be dodecamethyl-cyclohexasiloxane
by matching the unknown spectrum (top panel in Fig.
2C)
with
the
standard
dodecamethylcyclohexasiloxane spectrum (bottom panel in Fig. 2C)
stored in the MS spectral library.
Fig. 3 shows the detection of triethyl phosphate
(TEP), a commonly used flame retardant.
Approximately 49 ug/g of TEP outgassed from a
248
The influence of outgassing temperature on
outgassing concentration is shown in Fig. 5. A linear
relationship was observed between the logarithm of
outgassing quantity and the inverse of outgassing
temperature (1/T) for polycyclodimethylsiloxanes,
DBF and BHT. These results are in agreement with
the findings reported by Takeda, et al., in their paper,
which demonstrated a method of estimating outgassing
rates of contaminants of interest based on their vapor
pressure [18].
Outgassing Characterization
Several polymer materials known to outgas organic
contaminants commonly seen in semiconductor Fab
cleanroom environments were selected as part of a
study designed to understand outgassing behavior as a
function of time, temperature, material surface area
and weight. Organic contaminants tested included
polycyclodimethylsiloxanes, dibutyl phthalate (DBF),
butylated hydroxytoluene (BHT), 2-ethyl-l-hexanol,
methylstyrene and naphthalene.
Fig. 6 and Fig. 7 illustrate the relationship among
surface area, sample weight and outgassing quantity.
As shown in Fig. 6, outgassing levels of
polycyclodimethylsiloxanes are proportional to the
sample surface area when the sample weight remains
constant. However, outgassing is independent of
sample weight when the surface area remains constant
(Fig. 7). Similar behavior was observed for methylstyrene, 2-ethyl-hexanol and naphthalene.
As shown in Fig. 4, a linear relationship was
observed between the concentration of various
polycyclodimethylsiloxanes outgassed at 50 °C and
outgassing time. Outgassing rates for different
compounds were estimated from the slopes of
outgassing concentration vs. outgassing time plots.
Similar results were also obtained for DBF and BHT.
Data for tests conducted at 75 °C also showed a linear
relationship with higher slope values, indicative of
higher outgassing rates at 75 °C verses 50 °C.
Outgassing vs. Temperature
The influence of outgassing temperature on
outgassing concentration is shown in Fig. 5. A linear
relationship was observed between the logarithm of
outgassing quantity and the inverse of outgassing
temperature (1/T) for polycyclodimethylsiloxanes,
DBF and BHT. These results are in agreement with
the findings reported by Takeda, et al, in their paper,
which demonstrated a method of estimating outgassing
rates of contaminants of interest based on their vapor
pressure [18].
2.50
2.00 O
o
y = -3.9015x + 11.825
R2 = 0.9726
1.50 -
y = -1.782x+5.4216
R? = 0.9977
o
0)
J?
y=-1.5764x+4.6101
R2 = 0.9841
0.50
0.00
2.D
Outgassing Time Dependence (at 50°C)
1.20 -i
1.00 ^
-0.50
0.0358x + 0.2154
R 2 ^ 0.9816
L0394X + 0.0609
FP = 0.9872
D
°-80 ]
2.70
2.90
3.80
y =-1.724x + 5.301
R2 = 0.9945
-1.00
1/Tx10 3 (1/Kx10 3 )
2 £ 0.60
X
o E
C
FIGURE 5.
Outgassing quantity vs. outgassing
temperature.
(O): Octylmethyl-cyclotetrasiloxane; (n):
Decamethyl-cyclopentasiloxane;
(x):
Dodecamethylcyclohexasiloxane; (A): Dibutyl phthalate (DBF).
Q.
0.40 ]
= 0.0236X + 0.0744
R2 = 0.9993
0.20
0.00
10
20
30
40
Outgassing Time (min)
FIGURE 4. Outgassing quantity vs. outgassing time.
(O): Octylmethyl-cyclotetrasiloxane; (n): Decamethylcyclopentasiloxane; (x): Dodecamethyl-cyclohexasiloxane.
249
(1/T); and 4) outgassing quantity is proportional to the
material surface area and independent of material
weight when surface area remains constant,
suggesting that outgassing is a surface phenomenon.
However, due to the large number of variables in the
physical properties and chemical composition of
materials used in cleanroom construction and
operation, the authors believe that materials'
outgassing can be far more complicated than the
limited findings reported in this paper.
Siloxane Outgassing Dependence on
Sample Surface Area
(outgassing condition: 50°C, 15 min)
700 O)
600 -
0)
500 400
-
300 200
TOO
OctcmefrhylcydotetrcBiloxcne
DeocrrefhylcydopentcBiloxcne
AMMONIA/AMINES' OUTGASSING
T
cydoheptcsiloxcne
D 1x surface area, 1x sample weight
& 2x surface area, 1x sample weight
Ammonia/Amines' Impact
The impact of airborne molecular base (MB)
contamination on the performance of chemically
amplified (CA) resist has been a long-standing
problem in semiconductor lithography [20-21]. MB
can neutralize the photo-generated acid during the time
delay between the exposure and post-exposure bake
(PEB), generating insoluble products that cannot be
dissolved by the developer solvent. A lip, known as
"T-topping", forms at the top of the developed resist
profile. Extreme cases of "T-topping" cause bridging
between adjacent patterns. Among numerous possible
MB contamination sources, materials' outgassing is a
common one. Volatile molecular bases can outgas
from cleanroom materials such as ceiling tiles, sealant,
paints, adhesives, cleaning solutions and process
chemicals, among others. Screening new cleanroom
materials for NH3/amines outgassing before using
them in the cleanroom and/or DUV bay may
substantially reduce the likelihood of MB
contamination.
FIGURE 6. Outgassing quantity vs. material surface.
Outgassing Dependence on Sample
Weight (50°C, 15m in)
600
O)
500 400 -
300 O)
3
O
75
200 100 -
OctcmeThylcydotetrcBiloxcne
Deocmethylcydopentcsiloxcne
T efrcdeccnrerh/lo/doheptailoxcne
n 1x sample weight, 1x surface area
0 2x sample weight, 1x surface area
Gas Diffusion-Conductivity
Ammonia/Amines' Outgassing Analysis
FIGURE 7. Outgassing quantity vs. material weight.
The typical TD-GC-MS technique used for
condensable organics outgassing is generally not
suited for detecting ammonia (NH3) and very volatile
amines, which have poor recovery in TD-GC-MS due
to their high volatility. Therefore, a new materials'
NH3/amines' outgassing method based on gas
diffusion conductivity detection has been developed.
Fig. 8 shows a schematic diagram of this NH3/amines'
outgassing technique. Detection limits of less than 1
part per billion (ppb or ng/g) can be achieved using
this method.
Based on the results of these material outgassing
characterization
experiments,
the
following
conclusions may be drawn for the contaminants
studied under the conditions outlined above: 1) there is
a linear relationship between outgassing concentration
and outgassing time; 2) outgassing rate can be
estimated from the slope of the outgassing
concentration vs. outgassing plot; 3) there is a linear
relationship between the logarithm of the outgassing
quantity and the inverse of outgassing temperature
250
level of sealant B was < 5 ng/g and this sealant had no
impact on the photo resist develop process.
sample chamber
carrier gas
liquid conductivity
detector
buffer
solution
T diffusion module
In time-dependent outgassing studies, three (3)
days of curing were required for the NH3/amines'
outgassing level from sealant A to drop to the single
ppb range (Fig. 10A). The initial quantity of
NH3/amines outgassed was in excess of 700 ppb. The
outgassing level dropped to 30 ppb after an 8-hour
cure.
gas outlet
NH3/Amine Outgassing vs. Cure Time
B
onn
(ng/g)
JH3/Amine Outgassinc
_
FIGURE 8. Schematic diagram of gas diffusion
conductivity based NH3/amines' outgassing technique.
V
0
10
20
30
40
50
60
70
80
Cure Time (hour)
Wafer Surface NH4+ vs. Cure Time
C
(1 hour exposure at room temperature)
0
O
O
O
O
0
L_
4
i*
O
O
O
"————————
D
go
(E10NH4+/cm2)
Wafer Surface NH4+
NH3/amines signal
_k -L ro ro c
01 o oi c
C
onnn
0
10
20
30
40
50
60
70
80
Cure Time (hour)
Wafer Exposure Test
(1 hour at room temperature)
onnn
+
S«
2500 -
Z
c0
onnn
00g^2
•t x 1500 -
^+
^^^
C
^^
3 Z
*2O 2
1000 - +^"^———————————————————
UJ
<5 ^
;>
cr\n
0 .———,———,——————————,—— _,___,___,
FIGURE 9. NH3/amines' outgassing of two cleanroom
sealants. A. Sealant A; B. Sealant B; C. SEM image of
"T-topping" caused by NH3/amines' outgassing of sealant
A.
0
100 200 300 400 500
600 700 800
NH3 Outgassing
(ng/g)
Fig. 9 shows the NH3/amines' outgassing of two
FIGURE 10. NH3/amines' outgassing of and deposition
on witness wafer surfaces.
A.
Time dependent
NH3/amines' outgassing of sealant; B. Surface NH4+ on
witness wafers exposed to sealant A; C. Correlation of
witness wafer surface NH4+ and sealant outgassing levels.
cleanroom sealants. Sealant A outgassed 723 ng/g
NH3/amines at room temperature (Fig. 8 A) and caused
significant "T-topping" defects in EO delay photo
resist tests (Fig. 8C). The NH3/amines' outgassing
251
In order to understand the adsorption behavior of
outgassed NH3/amines on Si wafers as a function of
curing time, clean Si wafers were exposed to samples
of sealant A that had been cured for different lengths
of time. Each wafer was subsequently extracted with
deionized (DI) water and the extract analyzed by ion
chromatography (1C) [22] in order to determine the
ammonium (NH4+) content. The wafer exposed to
fresh sealant A for one (1) hour at room temperature
showed a very high surface NH4+ concentration of
2600E10 NH4+ ions/cm2 (Fig. 10B). By contrast, the
wafer exposed to a sealant A sample that has been
cured for 69 hours exhibited a surface NH4+
concentration close to that of the control wafer, which
had no sealant exposure. The linear relationship
between witness wafer surface NH4+ concentration
and NH3/amines' outgassing measurements for sealant
A appears in Fig. 10C.
define specifications applicable to the new ULSI
technology.
ACKNOWLEDGMENTS
The authors thank Yaacov Maoz of Intel Fab 18
for providing the "T-topping" defect SEM image, Zari
Pourmotamed of Intel California
Materials
Technology Department for collecting the timedependent NH3/amines' outgassing results, and Joseph
O'Sullivan
of
Intel
Facilities
Technology
Development for providing valuable technical inputs.
REFERENCES
The results from the time-dependent outgassing
studies and wafer exposure tests have demonstrated
that the gas diffusion-conductivity based technique
provides a sensitive method for materials'
NH3/amines'
outgassing
measurements.
The
combination of this technique with the widely used
TD-GC-MS method allows for a more complete
screening of cleanroom materials for potential airborne
base and condensable contamination, respectively.
1.
2.
3.
4.
SUMMARY
5.
Microelectronic
fabrication
processes
with
decreasing device geometries are increasingly
susceptible to AMC. Many AMC contaminants found
in semiconductor Fabs come from outgassing of
cleanroom materials. Outgassed contaminants can
adversely affect many processes, resulting in yield
loss, shortened tool life and reduced long-term device
reliability. The large variety of cleanroom materials
and numerous outgassing contaminants combined with
the complexity of process steps makes understanding
detrimental levels of particular contaminants in
particular processes very challenging. Screening
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outgassing prior to bringing them into Fabs can be
used as a first-line-of-defense against molecular
contaminants such as organo-phosphorus, siloxanes,
plasticizers and ammonia/amines. As processes and
chemistries change, requirements for monitoring,
control and analysis of materials' outgassing will
continue to evolve. Cooperative efforts among
manufacturers of integrated circuits, materials and
analytical tools are needed to better understand the
impact of molecular contaminants and to properly
6.
7.
8.
9.
10.
11.
12.
13.
14.
252
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Surfaces
by
Thermal
Desorption
Gas
Chromatography", West Conshohocken, ASTM
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Assessment of Outgassing Organic Contamination
from
Minienvironments
Using
Gas
Chromatography/Mass Spectroscopy" (2001).
17. IEST
WG
CC031
Ongoing
Activity:
Recommended Practice
for Characterizing
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