Determination of Impurities in Semiconductor-Grade Sulfuric Acid with the NexION 350S ICP-MS

APPLICATION NOTE
ICP - Mass Spectrometry
Author
Kenneth Ong
PerkinElmer, Inc.
Singapore
Determination of
Impurities in
Semiconductor-Grade
Sulfuric Acid with
the NexION 300S/
350S ICP-MS
Introduction
The making of a semiconductor device comprises
of forming a sacrificial layer on a substrate. Usually,
a patterned resist layer forms the sacrificial layer
so that ion implantation to the substrate can be
performed, after which a wet etching solution is
used to remove the patterned photoresist layer.
Typically, an etching solution comprises of sulfuric
acid (H2SO4) and peroxide (H2O2), known as piranha
or ozonated sulfuric acid. As with other chemicals used, any metal impurities present
would have detrimental effect on the reliability of an IC device and thus need to be of high
purity and quality. SEMI Standard C44-0708 specifies the maximum concentration of metal
contaminants by element and tier for sulfuric acid.
Inductively coupled plasma mass spectrometry (ICP-MS) is an indispensable analytical tool for
quality control because of its superior capability to detect at the ultratrace (ng/L or parts-pertrillion) level. Nevertheless, under the conventional plasma conditions, argon ions combine
with matrix components to generate polyatomic interferences. Some of the interferences in
sulfuric acid are 32S15N+ on 47Ti+, 32S16O2+ on 64Zn+, ArS+ on 70-74Ge+, 38Ar1H+ on 39K+, 40Ar+ on
40
Ca+, 40Ar16O+ on 56Fe+.
The Dynamic Reaction Cell (DRC™), which uses a quadrupole mass filter to create Dynamic
Bandpass Tuning (DBT), is a powerful correction technique to remove interferences on
analytes of interest. Collision cells, using nonreactive gases, have proven to be another
simple method in reducing specific polyatomic interferences. Both of these techniques
are available in PerkinElmer’s NexION® 300 ICP-MS through its unique Universal Cell
Technology™, which allows the use of all three modes (Standard, Collision and Reaction)
within one analytical method.
This application note demonstrates the ability of the NexION
300 ICP-MS to remove interferences so that trace levels of
impurities in H2SO4 can be measured under a single set of
hot plasma conditions for all analytes in one analysis.
instrumentation used for this experiment was the NexION
300S ICP-MS (PerkinElmer, Shelton, CT USA). Instrumental
parameters and sample introduction components are shown
in Table 1.
Experimental conditions
Results
Normally, the concentration of H2SO4 is around 98%. In
this experiment, a ten-fold dilution is carried out on 98%
semiconductor-grade H2SO4 from our customer. Standard
solutions were made from a 10 mg/L multi-element standard
(PerkinElmer Pure, PerkinElmer, Shelton, CT USA). The
H2SO4 samples were quantitatively analyzed using additions
calibrations; the calibration curves for Zn, K, Ca, Fe and Ni
are shown in Figures 1–3, indicating good linearity. This is
possible with all the polyatomic interferences removed by
the reactive NH3 gas in combination with the bandpass.
Table 1. Instrumental parameters and sample introduction components for the NexION 300S ICP-MS.
Spray Chamber:
PFA-Scott
Plasma Gas: 18 L/min
Torch:
High Efficiency Quartz
Auxiliary Gas:
1 L/min
Torch Injector:
PFA-Platinum
Nebulizer Flow:
0.96 L/min
Sampler Cone:
Platinum
RF Power:
1500 W
Skimmer Cone:
Platinum
Integration Time: 1 sec/mass
Nebulizer:
PFA-100 (100 µL/min)
Replicates:
Figure 1. Zn calibration, with He cell gas flow of 3 mL/min.
3
Figure 2. Fe calibration, with NH3 cell gas flow of 0.6 mL/min.
Figure 3. Ni calibration, with NH3 cell gas flow of 0.3 mL/min.
2
The detection limits (DLs) and background equivalent
concentrations (BECs) were both determined in 10%
H2SO4, while accounting for the sensitivities in 10%
H2SO4. DLs were calculated by multiplying the standard
deviation by three, and BECs were determined by
measuring the signal intensities. Recoveries were
determined from 20 ng/L spikes. The results are
summarized in Table 2.
Stability was determined by continuous introduction
into the NexION 300S of 10 ng/L spikes in 10% H2SO4
(without rinse) for 10 hours. Figures 4 and 5 show
excellent stability, with RSDs of < 3% over 10 hours.
The stability results, combined with the spike recovery
data, highlight the ability of the NexION 300S ICP-MS
to determine all SEMI-required elements in the H2SO4
matrix.
Figure 4. Ten-hour long-term stability results at 10 ng/L level for first
group of analytes.
Figure 5. Ten-hour long-term stability results at 10 ng/L level for
second group of analytes.
Table 2. Detection limits (DLs), background equivalent
concentrations (BECs), and 20 ng/L spike recoveries for all
analytes in 10% H2SO4.
Cell Gas Flow*
DL BEC 20 ppt
Analyte Mass (mL/min) RPq (ppt) (ppt) Recovery % RSD
Li
7
0
0.25 0.040.04 102% 2.7
Be
9
0
0.25 0.20.03103% 3.3
B
11
0
0.250.9 11 100% 3.2
Na
23
0
0.250.7 3.3 103% 2.1
Mg
24
0
0.250.2 0.4 102% 1.3
Al
27
0.6
0.5 0.71.0 96% 1.5
K
39
0.6 0.72 7113%1.7
Ca
40
1
0.5 13.297% 1.1
V
51
0.6
0.5 0.9ND103% 1.3
Cr
52
0.3
0.5 5 155102% 1.8
Mn
55
0.6
0.7 0.42.2 98% 1.3
Fe
56
0.6
0.7 2 22113% 1.5
Co
59
0.3
0.650.1 0.3 106% 1.4
Ni
60
0.3 0.71 2 99% 1.8
Cu
63
0.3 0.751 2 103% 1.9
Zn
68
3(He) 0.2510 40 103% 2.9
Ga
69
0.6
Ge
74
0.3 0.651 2 105% 1.8
As
75
0
0.250.4 0.9 100% 2.1
Sr
88
0
0.250.1ND106% 1.4
Zr
90
0
0.250.5 1.9 102% 1.4
Nb
93
0
0.250.1ND100% 1.2
Mo
98
0.3
0.5 0.9ND 98% 1.8
Ru
102
0
0.25 0.30.19 96% 1.5
Rh
103
0
0.250.03 0.4 104% 1.2
Pd
106
0
0.250.4ND102% 1.8
Ag
107
0
0.250.4 0.5 102% 1.7
Cd
114
0
0.250.3 0.6 101% 1.5
In
115
0
0.250.07 0.1 102% 1.6
Sn
120
0
0.251 8 99% 1.7
Sb
121
0
0.250.2ND103% 1.6
Ba
138
0
0.250.07ND 101% 1.3
Ta
181
0
0.250.2ND103% 2.0
W
184
0
0.250.3ND100% 1.8
Pt
195
0
0.250.5ND105% 1.9
Au
197
0
0.251ND93% -
Tl
205
0.6
0.5 0.02ND 104% 1.2
Pb
208
0.6
0.5 0.1ND102% 1.5
Bi
209
0.6
0.5 0.02ND 103% 1.8
U
238
0
0.250.06ND 102% 1.9
0.7 0.20.3103% 1.1
*Cell gas used is NH3.
3
Conclusion
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 H2SO4. By means of computer-controlled
switching between Standard mode and Reaction mode
in the Universal Cell, interference-free analysis using hot
plasma conditions for all analytes is possible during a single
sample run.
References
1.SEMI Standard C44-0708, SEMI Standards, http://www.
semi.org/en/index.htm
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