Determination of Impurities in Electronic-Grade Hydrochloric Acid with the NexION 350S ICP-MS

APPLICATION NOTE
ICP - Mass Spectrometry
Author
Kenneth Ong
PerkinElmer, Inc.
Singapore
Determination of
Impurities in ElectronicGrade Hydrochloric Acid
with the NexION
300S/350S ICP-MS
Introduction
In the production of semiconductor
devices, the wafers are subjected to
a so-called “Standard Clean 2” step,
commonly referred to as an “SC2” step.
The SC2 step is thought to desorb atomic
and ionic contaminants from the wafers.
In particular, the SC2 step is intended
to remove metals deposited on the
wafer surface. In a typical SC2 step, the wafers are submerged in a solution
of H2O:HCl:H2O2. Thus, it is important to analyze for the presence of metal
contaminants in electronic-grade hydrochloric acid (HCl). SEMI Standard C270708 specifies the maximum concentration of metal contaminants by element
and tier for hydrochloric acid.
Inductively coupled plasma mass spectrometry
(ICP-MS) has been used for determination of
ultra-trace impurity levels in various process
chemicals. Nevertheless, under conventional
plasma conditions, argon ions combine with
matrix components to generate polyatomic
interferences. Examples of chloride-based
interferences observed during the analysis of
HCl are listed in Table 1.
Table 1. Chloride interferences
observed during HCl analysis.
InterferenceAnalyte
37
Cl1H2
39
K
35
Cl O
51
V
35
C16O1H
52
Cr
37
Cl O
53
Cr
37
Cl O O
69
Ga
40
35
Ar Cl
75
As
40
Ar Cl
77
Se
16
16
16
37
16
Traditionally, two primary techniques have been used to
overcome the interferences: double-focusing high resolution
ICP-MS (HR-ICP-MS) and cool plasma. With HR-ICP-MS,
the resolution of the mass analyzer is increased so that
the analytes and interfering ions are separated. While cool
plasma has been shown to be effective in reducing argonbased interferences, it is more prone to matrix suppression
than hot plasma, especially for refractory elements or
elements with high ionization potentials. In addition, some
analyses cannot be accomplished successfully with HR-ICPMS or cool plasma, such as the determination of the major
vanadium (V) isotope and the only isotope of arsenic (As)
in HCl because of the formation of ClO+ and ArCl+ at m/z
51 and 75, respectively. The theoretical resolutions required
to separate V+ from ClO+ and As+ from ArCl+ will mean a
drop in ion transmission to 18% and 2%, respectively. This
makes low part per trillion (ppt) determination of V and As
extremely difficult. The cool plasma not only cannot reduce
the formation of ClO+ and ArCl+, it results in significant drop
of As+ and V+ sensitivity: As+ ion formation is reduced in
cool plasma due to its high ionization potential, and V-oxide
(formed in the plasma) cannot be effectively decomposed to
form V+.
Dynamic Reaction Cell™ (DRC) is an alternative technique to
eliminate these kinds of interferences utilizing hot plasma.
The DRC uses a quadrupole mass filter to create Dynamic
Bandpass Tuning (DBT), where only ions of a specific mass
range pass through the cell, thus allowing only controlled
reactions to take place where undesirable by-product ions
do not form within the cell, even when very reactive gases,
such as NH3 or O2, are used.
This application note demonstrates the ability of the NexION
300 ICP-MS to remove interferences so that trace levels of
impurities in HCl can be measured under a single set of hot
plasma conditions for all analytes in one analysis. This is best
accomplished using both Standard and Reaction modes in a
single method.
Experimental conditions
Normally, the concentration of HCl is around 35%. In this
experiment, a direct analysis is carried out on 20% ultra
pure HCl (Tamapure-AA 10, Tama Chemicals, Tokyo, Japan).
Standard solutions were made from a 10 mg/L multielement standard (PerkinElmer Pure, PerkinElmer, Shelton,
CT USA). The instrument used for this experiment was a
NexION 300S ICP-MS (PerkinElmer, Shelton, CT USA). The
operating conditions were determined using a 100 ppt
standard solution in 1% HNO3. Since a robust, hightemperature plasma was always used, optimization with
matrix-matched standards was not required. Instrumental
parameters and sample introduction components are shown
in Table 2.
Results
HCl samples were quantitatively analyzed using additions
calibrations; the calibration curves for As, Se, Cr, Fe, V and
K are shown in Figures 1–6 and demonstrate good linearity,
which is possible with all the polyatomic interferences
removed by the reactive NH3 gas in combination with the
bandpass. In the case of As and Se, reactive O2 is used to
convert them into AsO+ and SeO+, moving them away from
interferences at mass 75 and 80 arising from 40Ar35Cl and 40Ar2.
The PerkinElmer NexION® 300 ICP-MS incorporates Universal
Cell Technology™, which allows the use of Collision mode
(with kinetic energy discrimination), Reaction mode (incorporating
DBT), and Standard mode (where a cell gas is not used).
The user can select whichever mode(s) are most appropriate
for the application.
Table 2. Instrumental parameters and sample introduction components for the NexION 300S ICP-MS.
2
Spray Chamber: Quartz Cyclonic
Plasma Gas: 18 L/min
Torch:
Auxiliary Gas:
1.1 L/min
Torch Injector: 2-mm Quartz
Nebulizer Flow:
1.01 L/min
Sampler Cone: Platinum
RF Power:
1500 W
Standard Quartz
Skimmer Cone: Platinum
Integration Time: 1 sec/mass
Nebulizer:Meinhard® Type A Concentric Quartz
Replicates:
3
The detection limits (DLs) and background equivalent
concentrations (BECs) were both determined in 20% HCl,
while accounting for the sensitivities in 20% HCl. DLs were
calculated by multiplying the standard deviation of the blank
by three, and BECs were determined by measuring the
signal intensities. A spike recovery test was carried out based
on SEMI guidelines. According to SEMI, the spike level must
be 50% of the proposed specification level, and the recovery
should be between 75% and 125% of the expected value.
The specification level for Tier C grade is 100 ppt in 37%
HCl; as such, 25 ppt spikes in 20% HCl were used in this
experiment. Recoveries were determined from 25 ng/L
spikes. The results are summarized in Table 3.
Table 3. Detection limits (DLs) and background equivalent
concentrations (BECs) for all analytes in 20% HCl, spike
recoveries at 25 ng/L level.
Cell Gas Flow*
DL BEC
25 ppt
AnalyteMass (mL/min) RPq (ppt) (ppt) Recovery
Li
7
0
0.250.020.04 101%
Be
9
0
0.250.020.01 103%
B
11
0
0.252 10 92%
Na
23
0
0.25
Mg
24
0
0.250.2 0.7 102%
Al
27
0.6
0.50.2 2 102%
K
39
1
0.651 30 105%
Ca
40
1
0.50.32.1 100%
0.3
< DL
101%
Ti480.30.75
0.5
1.3
97%
V
Figure 1. As calibration, as AsO91, with O2 cell gas flow of 0.5 mL/min.
Figure 2.
80
Se+ calibration, as 96SeO+, with O2 cell gas flow of 0.5 mL/min.
51
1
0.750.1 0.1 93%
Cr 52
0.6 0.71 7 96%
Mn 55
0.6
Fe
56
0.6 0.51 10 90%
Co
59
0.3
Ni
60
0.3 0.71 4 91%
Cu
63
0.3
0.70.34.7 105%
Zn
66
0.3
0.650.4 2
Ga
71
0.6
0.650.1 0.4 101%
Ge
74
0.3
0.650.6 3
As
91
Sr
88
Se
96
Zr
90
0
0.250.7 3
Nb
93
0
0.250.1 0.5 100%
Mo
98
0
0.250.3 0.6 98%
Ru
102
0
0.25
0.1
< DL
104%
Rh
103
0
0.25
0.1
< DL
101%
Pd
106
0
0.25
0.3
< DL
102%
Ag
107
0
0.250.1 0.2 99%
Cd
114
0
0.250.2 0.4 104%
In
115
0
0.250.1 0.5 102%
Sn
120
0
0.250.9 5.2 95%
Sb
121
0
0.250.8 2.7 103%
Ba
138
0
0.250.020.04 102%
Ta
181
0
0.250.020.01 101%
W
184
0
0.25
0.2
< DL
100%
Pt
195
0
0.25
0.3
< DL
99%
Au
197
0
0.250.1 0.3 102%
Tl
205
0
0.25
Pb
208
0
0.250.060.15 101%
Bi
209
0
0.25
0.07 < DL
102%
U
238
0
0.25 < 0.02 < DL
102%
0.70.20.6 101%
0.5
0.4
< DL
103%
100%
94%
0.5 (O2)0.5 5 50 94%
0
0.250.090.75 101%
0.5 (O2)0.5 5 7 99%
0.02 < DL
101%
101%
*Cell gas used is NH3, unless otherwise noted
3
Figure 3. Cr calibration, with NH3 cell gas flow of 0.6 mL/min.
Figure 6. K calibration, with NH3 cell gas flow of 1 mL/min.
Figures 7 and 8 show excellent stability, with RSDs of < 3%
over 2 hours. The stability results, combined with the spike
recovery data, highlight the ability of the NexION 300S
ICP-MS for the determination of all SEMI-required elements
in the HCl matrix.
Conclusion
Figure 4. Fe calibration, with NH3 cell gas flow of 0.6 mL/min.
Figure 7. Two-hour stability (normalized intensity) for a 100 ng/L spike for
first group of analytes.
Figure 5. V calibration, with NH3 cell gas flow of 1 mL/min.
Figure 8. Two-hour stability (normalized intensity) for a 100 ng/L spike for
second group of analytes.
4
The NexION 300S ICP-MS is shown to be robust and suitable
for the routine quantification of ultratrace impurities at the
ng/L level in HCl. The data demonstrates the effectiveness of
Reaction mode of the UCT for the elimination of chloridederived interferences. By applying Reaction mode, 20% HCl
can be analyzed directly for contaminants at SEMI Tier C levels,
without the need for cumbersome sample pre-treatment. By
means of computer-controlled switching between Standard
mode and Reaction mode in the Universal Cell, interferencefree analysis using hot plasma conditions for all analytes is
possible during a single sample run.
References
1.SEMI Standard C27-0708, SEMI Standards, http://www.
semi.org/en/index.htm
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