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

APPLICATION NOTE
ICP - Mass Spectrometry
Author
Kenneth Ong
PerkinElmer, Inc.
Singapore
Determination of
Impurities in
Semiconductor-Grade
Nitric Acid with the
NexION 300S/350S
ICP-MS
Introduction
Semiconductor devices are
currently being designed with
smaller line widths and are more
susceptible to low-level impurities.
Nitric acid (HNO3) is widely used
as a mixture with hydrofluoric
acid (HF) to alter between diffusion-limited or rate-limited etching in the
semiconductor industry. The mixture is commonly used to etch and expose
the critical layer in the front-end processing. In this stage, the actual devices,
including transistors and resistors, are created. A typical front-end process
includes the following: preparation of the wafer surface, growth of silicon
dioxide (SiO2), patterning and subsequent implantation or diffusion of dopants
to obtain the desired electrical properties, growth or deposition of a gate
dielectric and via etching. Any metal impurities present would have detrimental
effects on the reliability of an IC device. Nitric acid is also commonly used
in semiconductor laboratories to carry out analysis of other semiconductor
materials, and thus needs to be of high purity and quality. SEMI Standard
C35-0708 specifies the maximum concentration of metal contaminants by
element and tier for nitric acid.
Inductively coupled plasma mass spectrometry (ICP-MS) is
an indispensable analytical tool for quality control because
of its superior capability to detect at ultratrace (ng/L or
parts-per-trillion) levels and ability to rapidly determine
analytes in various process chemicals. Nevertheless, under
conventional plasma conditions, argon ions combine with
matrix components to generate polyatomic interferences.
Some of the common interferences are 38Ar1H+ on 39K+,
40
Ar+ on 40Ca+, and 40Ar16O+ on 56Fe+. While cool plasma
has been shown to be effective in reducing argon-based
interferences, it is more prone to matrix suppression than
hot plasma, especially for refractory elements or elements
with high ionization potentials. In addition, other polyatomic
interferences may be preferentially formed under the low
plasma energy, which are not seen under hot plasma
conditions.
impurities in HNO3 can be measured under a single set of
hot plasma conditions for all analytes in one analysis. This
was best accomplished using both Standard and Reaction
modes in a single method.
Collision cells using multipoles and un-reactive gases
have proven useful in reducing polyatomic interferences.
This approach necessitates the use of kinetic energy
discrimination (KED) to remove the unwanted by-products
which could result in loss of sensitivity. The Dynamic
Reaction Cell™ (DRC™) is another correction technique which
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. As a result of this capability, undesirable
by-product ions do not form within the cell, even when
very reactive gases are used, such as NH3 or O2.
Results
Experimental conditions
Normally, the concentration of HNO3 is around 70%. In
this experiment, a five-fold dilution is carried out on 55%
ultra pure HNO3 (Tamapure-AA 10, Tama Chemicals, Tokyo,
Japan). Standard solutions were made from a 10 mg/L
multi-element standard (PerkinElmer Pure, PerkinElmer,
Shelton, CT USA). The instrument used for this experiment
was a NexION 300S ICP-MS (PerkinElmer, Shelton, CT
USA). Instrumental parameters and sample introduction
components are shown in Table 1.
HNO3 samples were quantitatively analyzed using additions
calibrations; the calibration curves for K, Ca, Fe and Ni
are shown in Figures 1–4, indicating good linearity. This is
possible with all the polyatomic interferences removed by
the reactive NH3 gas in combination with the bandpass.
The PerkinElmer NexION® 300 ICP-MS incorporates Universal
Cell Technology™, which allows the use of Collision mode
(with KED), Reaction mode (incorporating DBT), and
Standard mode (where a cell gas is not used), and the
user has the ability to select whichever mode(s) are most
appropriate for the application, and switch between modes
within the same analytical method.
This application note demonstrates the ability of the NexION
300 ICP-MS to remove interferences so that trace levels of
Figure 1. K calibration, with NH3 cell gas flow of 0.6 mL/min.
Table 1. Instrumental parameters and sample introduction components for the NexION 300S ICP-MS.
Spray Chamber:
Quartz Cyclonic
Plasma Gas: 18 L/min
Torch:
Standard Quartz
Auxiliary Gas:
1.1 L/min
Torch Injector:
2-mm Quartz
Nebulizer Flow:
1.01 L/min
Sampler Cone:
Platinum
RF Power:
1500 W
Skimmer Cone:
Platinum
Integration Time: 1 sec/mass
Nebulizer:Meinhard® Type A Concentric Quartz
2
Replicates:
3
The detection limits (DLs) and background equivalent
concentrations (BECs) were both determined in 10% HNO3,
while accounting for the sensitivities in 10% HNO3. DLs
were calculated by multiplying the standard deviation by
three, and BECs were determined by measuring the signal
intensities. Recoveries were determined from 10 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 (without rinse) for 10 hours.
Figures 5 and 6 show excellent stability, with RSDs of < 3%
over 10 hours (Table 2, last column). 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 HNO3 matrix.
Figure 2. Ca calibration, with NH3 cell gas flow of 1 mL/min.
Figure 5. Ten-hour long-term stability (normalized intensity) for a
10 ng/L spike for first group of analytes.
Figure 3. Fe calibration, with NH3 cell gas flow of 0.6 mL/min.
Figure 6. Ten-hour long-term stability (normalized intensity) for a
10 ng/L spike for second group of analytes.
Figure 4. Ni calibration, with NH3 cell gas flow of 0.3 mL/min.
3
Table 2. Detection limits (DLs) and background equivalent concentrations (BECs)
for all analytes in 10% HNO3, and 10 ng/L spike recoveries.
Analyte Mass
Cell Gas Flow*
(mL/min)
RPq
BEC
(ppt)
10 ppt
Recovery % RSD
Li
7
0
0.250.03 0.04 102% 2.7
Be
9
0
0.250.1 0.03 103% 3.3
B
11
0
0.251 11 100%3.2
Na
23
0
0.250.2 3.3 103% 2.1
Mg
24
0
0.250.1 0.4 102% 1.3
Al
27
0.6
0.50.4 1.0 96% 1.5
K
39
0.6
0.50.8 7 113% 1.7
Ca
40
1
0.50.5 3.2 97% 1.1
Ti
48
0.3
0.50.3 1.2 103% 1.8
V
51
0.6
0.5
Cr
52
0.3
0.50.5 2.0 102% 1.8
Mn
55
0.6
0.70.06 0.44 98% 1.3
Fe
56
0.6
0.50.9 7 113% 1.5
Co
59
0.3
0.50.03 0.18 106% 1.4
Ni
60
0.3
0.70.6 1.0 99% 1.8
Cu
63
0.3
0.50.2 1.3 103% 1.9
Zn
64
0.3
0.650.5 0.8 103% 2.9
Ga
69
0
0.250.07 0.24 103% 1.1
Ge
74
0.3
0.650.4 0.6 105% 1.8
As
75
0
0.250.2 0.7 100% 2.1
Sr
88
0
0.50.02 0.03 106% 1.4
Zr
90
0
0.25
Nb
93
0
0.250.02 0.03 100% 1.2
Mo
98
0
0.25
0.1
< DL
98%
1.8
Ru
102
0
0.25
0.1
< DL
96%
1.5
Rh
103
0
0.250.03 0.07 104% 1.2
Pd
106
0
0.250.3 0.4 102% 1.8
Ag
107
0
0.250.2 0.4 102% 1.7
Cd
114
0
0.25
0.1
< DL
101%
1.5
In
115
0
0.25
0.01
< DL
102%
1.6
Sn
120
0
0.250.2 0.7 99% 1.7
Sb
121
0
0.250.03 0.10 103% 1.6
Ba
138
0
0.250.03 0.04 101% 1.3
Ta
181
0
0.25
0.01
< DL
103%
2.0
W
184
0
0.25
0.05
< DL
100%
1.8
Pt
195
0
0.250.1 0.3 105% 1.9
Au
197
0
0.250.3 0.5 93%
Tl
205
0
0.25
Pb
208
0
0.250.04 0.09 102% 1.5
Bi
209
0
0.25
0.02
< DL
103%
1.8
U
238
0
0.25
0.005
< DL
102%
1.9
*Cell gas used is NH3.
4
DL
(ppt)
0.04
0.05
0.01
< DL
< DL
< DL
103%
102%
104%
1.3
1.4
–
1.2
Conclusion
The NexION 300S ICP-MS has shown to be robust and
suitable for the routine quantification of ultratrace impurities
at the ng/L level in HNO3. 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 C35-0708, SEMI Standards, http://www.
semi.org/en/index.htm
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