Introduction
Metal atoms and conductive particle contaminants are undesirable and
potentially damaging in semiconductor manufacturing processes. Oxidative
cleaning baths and large volumes of ultrapure rinse water are used to remove
metallic contamination from wafer surfaces.
1
For optimal cleaning efficiency,
the concentrations of iron and other metals in cleaning solutions should be
minimized.
2
To monitor metals in cleaning baths and rinse water, improved
analytical methods are needed.
This Application Note describes a method for determining low ng/L
amounts of transition metals in high purity water and semiconductor bath
solutions. The Thermo Scientific
™
Dionex
™
IonPac
™
CS5A column is used
for the separation of transition metals. This column has a unique bilayer
latex structure consisting of both anion- and cation-exchange retention
mechanisms. Transition metals can be separated using either anion or cation
exchange chromatography, depending on the choice of the complexing agent
used in the eluent. Pyridine-2,6-dicarboxylic acid (PDCA) is a very strong
complexing agent that forms stable, anionic metal complexes. This results in
very efficient chromatography. Another commonly used complexing agent is
oxalic acid, a moderate chelating agent. Because the complexation is weaker,
metals are separated either as free metal cations (Pb, Mn, Cd), as anionic
Authors
Mark Laikhtman and Jeff Rohrer
Thermo Fisher Scientific,
Sunnyvale, CA
Keywords
Ion chromatography, IC, copper,
zinc, absorbance, IonPac
CS5A, iron, nickel, PAR, PDCA,
post column, semiconductor,
semiconductor cleaning bath,
t-metals, transition metals, visible
absorbance
Determination of transition metals at ppt levels in
high-purity water and SC2 (D-clean) baths
APPLICATION NOTE 131
2
complexes (Cu), or as a combination of the two (Co, Zn,
Ni). Transition metals are detected using postcolumn
derivitization with 4-(2-pyridilazo)resorcinol (PAR)
with absorbance detection at 520–530 nm. The PAR
displaces the PDCA and forms highly absorbing metal
complexes. PAR exhibits broad selectivity for transition
metals and provides a very sensitive detection method
with low background.
Experimental
Equipment
• Thermo Scientific
™
Dionex
™
DX-500 Ion
Chromatography system* consisting of:
– GP40 Gradient Pump (microbore configuration)
– AD20 UV/Vis detector with 10 mm path length cell
– LC30 Chromatography Enclosure with rear-loading
Rheodyne
®
injection valve
– Concentrator Pump, DQP
– RP-1 Postcolumn reagent pump with pulse damper
(2’ of 0.020” i.d. tubing after the RP-1)
– Postcolumn reagent bottle (P/N 044411 with its
O-ring replaced by a Teflon
®
encapsulated O-ring
(P/N 043523))
– Knitted Reaction Coil (P/N 053640)
– Pressurizable Reservoir Chamber
* Equivalent or improved results can be achieved using
the Thermo Scientific
™
Dionex
™
ICS-5000
+
HPIC
™
system.
• Thermo Scientific
™
Dionex
™
PeakNet
™
Chromatography
Workstation
Reagents and standards
• Deionized water (DI H
2
O), Type 1 reagent grade,
18 MΩ∙cm resistance or better
• PDCA Eluent: 7.0 mM Pyridine-2,6-dicarboxylic acid
(PDCA), 66 mM KOH, 5.6 mM K
2
SO
4
, 74 mM Formic
acid (Thermo Scientific
™
Dionex
™
MetPac
™
PDCA
Eluent Concentrate P/N 046088).
• Postcolumn reagent: 0.06 g of PAR (P/N 039672)
in 1 L Dionex MetPac PAR postcolumn reagent
diluent (P/N 046094). The formulation of the diluent is
1.0 M 2-dimethylaminoethanol + 0.50 M ammonium
hydroxide + 0.30 M Sodium bicarbonate.
Conditions
Columns: Dionex IonPac CS5A Analytical,
2 × 250 mm (P/N 052576)
Dionex IonPac CG5A Guard,
2 × 50 mm (P/N 052836)
Thermo Scientific
™
Dionex
™
IonPac
™
TCC-2 Concentrator,
3 × 35 mm (P/N 043103)
Eluent: PDCA
Eluent Flow Rate: 0.3 mL/min
LC-30
Temperature: 30.0 °C
Postcolumn
Reagent: 0.06 g of PAR in 1 L Dionex MetPac
PAR Postcolumn Diluent
Postcolumn
Flow Rate: 0.15 mL/min
Concentrator Pump
Flow Rate: 2.0 mL/min
Run Time: 15 min
Detection: Visible, High setting, 530 nm
System
Backpressure: 1700–2000 psi
Preparation of solution and reagents
Transition metals standards
Appropriate concentrations of standards are prepared
from 1 g/L stock standards solutions. All standards
were prepared in 2 mM HCl to ensure their stability and
prevent the formation of insoluble oxides and hydroxides.
Eluent solution
PDCA eluent
Add 200 mL of Dionex MetPac PDCA Eluent Concentrate
to 800.0 mL of degassed water for a total volume of
1000.0 mL or 204.0 g of Dionex MetPac PDCA Eluent
Concentrate to 800.0 g of degassed water for a total
weight of 1004.0 g.
• Hydrochloric acid, ultrapure reagent. ULTREX
®
II (J.T.
Baker
®
or equivalent)
• Hydrogen peroxide, semiconductor grade (Aldrich,
99.999% solution in water)
• 1 g/L individual transition metal standards (VWR, atomic
absorption grade)
3
Postcolumn reagent
Dissolve 0.06 g of PAR in 1.0 L Dionex MetPac PAR
postcolumn diluent.
Stock solution for sample and pH adjustment
1 M Hydrochloric acid
Weigh 909.70 g of deionized water (Type I reagent grade,
18 MΩ∙cm resistance or better) into an eluent bottle. Tare
the bottle and carefully add 90.3 mL of ultrapure reagent
grade hydrochloric acid directly to the bottle.
Standard and sample preparation
Add 1.0 mL of 1 M hydrochloric acid to 499 g of
sample or standard solution. The final concentration of
hydrochloric acid is 2 mM.
Glassware cleanings
Prior to use, high density polyethylene (HDPE) containers
used for samples and standards preparation were rinsed
with DI water and an aliquot of the sample to reduce the
amount of leachable transition metals from the bottle. To
avoid contamination and pH errors when formulating the
eluent and the PAR reagent, use the high purity reagents
offered by Thermo Fisher Scientific.
System operation
System configuration and operation parameters for
this application are outlined in a previously published
document.
3
To ensure efficient 2 mm column operation, 0.125 mm
(0.005 in.) tubing must be used. Lengths of connecting
tubing should be kept as short as possible to minimize
system void volume. Carefully use a razor blade or
plastic tubing cutter so that the ends of the tubing cuts
are straight and smooth. Irregularity on the surface of a
tubing end can result in unwanted dead volume.
Sample preconcentration is used to improve
sensitivity and lower the detection limits. Samples with
transition metal concentrations below 2 µg/L must be
preconcentrated for accurate quantification. The sample
is loaded onto the Dionex IonPac TCC-2 (Trace Cation
Concentrator) with a pressurized reservoir or Dionex DQP
concentrator pump. The Dionex IonPac TCC-2 column
stationary phase is surface-functionalized sulfonated
resin. We used a flow rate of 2 mL/min and times of 5
and 15 minutes to concentrate 10 and 30 mL of sample.
A Dionex RP-1 pump was used to deliver postcolumn
reagent (PAR). Pneumatic delivery is also acceptable and
either of these techniques can be used successfully in
this method. Figure 1 shows the system configuration.
Results and discussion
Trace level analysis of transition metals is limited by the
purity of water and the reagents. PEEK
™
, metal-free flow
paths are a very important factor in the integrity of the
analytical system. Precautions must be taken at every
step of sample and standard preparation to minimize
contamination. All plastic containers and pipettes
must be cleaned with highest purity reagents (soak in
10 mM HCl overnight and rinse thoroughly with water).
Information about the content of leachable transition
metals in these containers should be obtained from
the supplier. The analytical system flow path, including
tubing, pumps, postcolumn reagent, and sample must be
thoroughly cleaned with 50% IPA/H
2
O at start-up.
To perform analysis of trace levels less than 2 µg/L,
samples must be preconcentrated rather than directly
injected. Figure 2 shows the analysis of 30 mL of a
1 µg/L transition metals standard. All peaks are well
separated from the void volume and from each other and
are therefore easily quantified. Figures 3 and 4 show the
analyses of 10 and 30 mL of high quality deionized water.
These samples were concentrated at 2 mL/min for
5 and 15 min respectively. Iron, copper, and zinc are
major contaminants. Trace analysis of real samples
containing these analytes will depend on the levels of
transition metals present in the water blank. The iron
concentration in 30 mL of water is estimated to be
45 ng/L (ppt) based on the iron area count in the
standard (30 mL of 1 µg/L (ppb) of each transition metal).
Figure 1. System configuration for detection of transition metals.
Waste
Loading
Pump
Concentrator
Column
Guard
Column
Pulse
Damper
Reagen
t
Pump
Analytical
Column
3-way
Manifold
Waste
10 mm Vis Cell
Knitted Reaction Coil
Gradient
Pump
Valve
4
Figure 3. Water blank (10 mL concentrated).
Figure 2. 1 µg/L (ppb) transition metals standard (30 mL
concentrated).
Table 1. Analysis of iron in 30 mL DI H
2
O.
Day #
Concentration
ng/L (ppt)
Area
count
Retention
Time
1 38.1 71,123 6.13
2 37.4 69,786 6.17
3 38.7 72,075 6.10
RSD 1.71 1.62 0.57
Figure 4. Water blank (30 mL concentrated).
The concentration of Fe
3+
in 10 mL of water was also
approximately 45 ng/L (ppt). Therefore, the concentration
of the Fe
3+
is due to the Fe
3+
in the water and reagents
and not the chromatography system. Because it is
possible to quantify the amount of Fe in the 10 mL
sample, if that amount was found in a 30 mL sample
(less contaminated water), the minimum detection
limit would be 15 ppt. Table 1 shows the result of the
analysis of 30 mL of high quality deionized water on three
consecutive days. The peak area and retention time
RSDs were less than 2%.
Analytical Columns:
CG5A (2 mm)
Eluent: PDCA
Eluent Flow Rate: 0.3 mL/min
Post Column Reagent: PAR
Post Column Reagent:0.15 mL/min
Concentrator Column:
Concentrator Pump: DQP
Concentrator Pump Flow:2 mL/min
Concentration Time: 15 min
Sample Volume: 30 mL
Total Run Time: 30 min
Detection:VIS, 530 nm
Peaks: 1. Iron
2. Copper
3. Nickel
4. Zinc
0
2
4
1
2
3
4
6
8
10
12
14
Minutes
0.04
AU
-
0.04
Dionex IonPac CS5A,
Dionex IonPac TCC-2
Analytical Columns:
CG5A (2 mm)
Eluent: PDCA
Eluent Flow Rate: 0.3 mL/min
Post Column Reagent: PAR
Post Column Reagent:0.15 mL/min
Concentrator Column:
Concentrator Pump: DQP
Concentrator Pump Flow:2 mL/min
Concentration Time:5 min
Sample Volume: 10 mL
Total Run Time: 20 min
Detection: VIS, 530 nm
Peaks: 1. Iron
2. Copper
3. Nickel
4. Zinc
0.02
AU
-0.01
1
14
121086
Minutes
420
2
3
4
Dionex IonPac CS5A,
Dionex IonPac TCC-2
Concentration
Time: 15 min
Sample Volume:30 mL
Total Run Time:30 min
Detection: Vis, 530 nm
Peaks: 1. Iron
1 µg/L
2. Coppe
r1
3. Nickel 1
4. Zin
c1
5. Cobalt 1
6. Cadmium1
7. Manganese1
0.2
AU
0
0
26 2101
44
18
Minutes
2
1
3
4
5
6
7
Analytical Columns:
CG5A (2 mm)
Eluent:
PDCA
Eluent Flow Rate:
0.3 mL/min
Post Column Reagent:
PAR
Post Column Reagent
:0.15 mL/min
Concentrator Column:
Concentrator Pump:
DQP
Concentrator Pump Flow
:2 mL/min
Dionex IonPac CS5A,
Dionex IonPac TCC-2
©2015, 2017 Thermo Fisher Scientific Inc. All rights reserved. All trademarks are the property of Thermo Fisher Scientific and
its subsidiaries unless otherwise specified. Rheodyne is a registered trademark of IDEX Health & Science LLC Corp. JT Baker and
ULTREX are registered trademarks of Avantor Performance Materials, LLC. Teflon is a registered trademark of The Chemours
Company FC LLC. This information is presented as an example of the capabilities of Thermo Fisher Scientific products. It is
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Find out more at thermofisher.com/IC
Figure 5 shows the chromatogram of an SC2 bath.
Concentrations of Fe
+3
, Cu
+2
, and Zn
+2
were 80, 75, and
106 ng/L (ppt), respectively. Recovery of iron from the
bath is higher than from water, indicating that chemicals
used in bath preparation contain iron.
Summary
The method outlined in this Application Note describes
the chromatographic analysis of trace levels of transition
metals using a preconcentration technique.
References
1. Wayne M. Moreau, Semiconductor Lithography Principles, Practices, and Materials,
1988 Plenum Press, New York,1988, 270–280.
2. Suggested Guidelines for Pure Water used in Semi-conductor Processing. Doc 2796.
SEMI, 1998, 1–3.
3. Dionex 2-mm Transition Metal System with Postcolumn Delivery Installation and
Troubleshooting Manual, P/N 031355
Figure 5. SC2 batch (30 mL concentrated) containing
1 mL HCl / 5 mL H
2
O
2
/ 494 mL H
2
O.
CG5A (2 mm)
Eluent: PDCA
Eluent Flow Rate: 0.3 mL/min
Post Column Reagent: PAR
Post Column Reagent: 0.15 mL/min
Concentrator Column:
Concentrator Pump: DQP
Concentrator Pump Flow: 2 mL/min
Time: 15 min
Sample Volume: 30 mL
Total Run Time:30 min
Detection: VIS, 530 nm
Peaks: 1. Iron8
2. Copper 75
3. Nickel -
4. Zinc106
0.02
AU
0
2
4
1
2
3
4
6
8
10
12
14
Minutes
Dionex IonPac TCC-2