On-line Analysis of Process Chemicals by Inductively
Coupled Plasma Mass Spectrometry (ICP-MS)
Yoko Kishi*, Katsu Kawabata*, David Palsulich** and Dan Wiederin***
*PerkinElmer Life and Analytical Sciences, Concord, Ontario, L4K4V8, Canada
**Micron Technology, Inc., Boise, ID 83707-0006, USA
*** Elemental Scientific Inc., Omaha, NE 68131, USA
Abstract. An initial study of on-line remote system for monitoring of trace metals using the inductively coupled plasma
mass spectrometer equipped with dynamic reaction cell system is described. The three types of cleaning solutions (HF,
SC-1 and SC-2) used in the traditional RCA clean procedure are analyzed periodically with one set of simple calibration
curves to simulate the on-line monitoring. A remote sampling system for on-line monitoring with the combination of the
ICP-MS is also demonstrated.
reduction of the interferences, and several technical
papers on semiconductor application have been
published [5-9]. The DRC technique is independent
from the plasma temperature and can eliminate variety
of interferences with normal or hot plasma condition.
Since the DRC does not require the cool plasma
condition for the determination of various chemicals
used in the semiconductor industry, matrix suppression
can be minimized. Another practical feature of the
DRC technique is elimination of Cl related
interferences. Since the cool plasma technique cannot
avoid Cl related interferences, elimination of Cl matrix
by evaporation has been required prior to the ICP-MS
analysis. However, some of volatile elements such as
B, Ge and As are lost during the evaporation, therefore
it has been troublesome for the analysis of one of the
most commonly used cleaning solution, standard
solution-2 (SC-2), which is consisted of H2O2 and HC1.
INTRODUCTION
As integration of semiconductor devices continues
to increase and junctions become shallower,
contamination control of chemicals and materials used
in the semiconductor industry is becoming more
critical and important. The International Technology
Roadmap for Semiconductors [1] suggested that
critical metallic impurity levels in the ultrapure water
(UPW) and the process chemicals should be lower
than 20 and 10 ppt, respectively, and that required
impurity level of critical surface metals on Si wafers
should be lower than IE 10 atoms/cm2.
In order to determine such a low concentration of
impurities in chemicals, inductively coupled plasma
spectrometry (ICP-MS) has been commonly used.
However, the ICP-MS has not been applied for on-line
monitoring of chemicals used in FABs due to some
critical issues such as interference and matrix
suppression. Although a cool plasma technique [2]
has been widely used to overcome some of
interference problems, it cannot eliminate some of
critical interferences such as C1H2, CIO, C1OH, C12,
ArCl, PO2, SO2 and PO2H on K, V, Cr, Ge, As, Cu and
Zn, respectively in HC1, H3PO4 and H2SO4 matrices.
In addition, the cool plasma suffers from severe matrix
suppression, which requires cumbersome matrix
elimination or matrix matched calibration solutions.
Using the benefit of hot plasma and the DRC
technology, a possibility of on-line monitoring for
three cleaning chemicals commonly used in the
traditional RCA cleaning procedure was investigated
using an autosampler and a lwt% nitric acid
calibration curve. Another critical issue of on-line
monitoring was how to take samples from chemical
baths to the ICP-MS instrument. A sampling length
between a bath to the instrument might be up to 30
meters or so, which might cause an adsorption
problem of elements in a transfer tube and a long
sample uptake time. A feasibility study of remote
Recently, ICP-MS with a dynamic reaction cell
(DRC) technique has been developed [3, 4] for
CP683, Characterization and Metrology for VLSI 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
606
EXPERIMENTAL
Perfluoroalkoxy (PFA) transfer tube by Ar gas flow.
The sample flow was switched from one to another by
the PFA switching valves located near the ICP-MS.
This system enables to transport even limited amount
of samples very quickly minimizing the sample uptake
time.
Instrumentation
Standards and Reagents
The experiment was carried out on an ELAN DRC
II ICP-MS (PerkinElmer Sciex, ON, Canada),
equipped with several types of introduction system
(Elemental Scientific, Inc., NE, USA). The operating
conditions were determined using a 1 jig/L (ppb)
standard solution in lwt% HNO3, which are listed in
Table 1.
Three types of cleaning solutions were prepared by
simply diluting high purity grade chemicals (Tama
Chemicals, Inc., Tokyo, Japan): lwt% HF, SC-1
(2.5wt% NH4OH + 3.5wt% H2O2) and SC-2 (3.5wt%
HC1 + 3.5wt% H2O2) with high purity water obtained
from a Milli-Q Element system (Millipore Corp., MA,
USA).
A beta version of remote sampling device
(Elemental Scientific, Inc., NE, USA) which was
equipped with nebulizers, spray chambers, transfer
lines and switching valves as shown in Figure 1 was
evaluated in this experiment. One set of a nebulizer
and a spray chamber was placed close to each sample
(it could be a rinse bath in the FABs). In this
experiment, three sets were placed close to HNO3, SC1 and SC-2 solutions in the bottles. The sample
solution was self-aspirated to the nebulizer where
aerosol is generated, and this aerosol was transported
to the ICP-MS through a 30 m x 8 mm I.D.
TABLE 1. Operating Conditions.__________
Parameter/System
Setting/Type
Nebulizer
PFA concentric type
Spray chamber
PFA Scott type
/Quartz cyclonic type
Torch injector
Pt
Sampling/Skimmer cones
Pt
RF power
1550 W
18L/min
Plasma gas flow
Aux. Gas flow
1.8L/min
Nebulizer gas flow
1.0- UL/min
NH3
Cell gas for DRC
sampling system in order to minimize these issues was
investigated.
1 sec/mass
Integration time
mm
i
Oralri
FIGURE 1. Schematic Diagram of Remote Sampling System.
607
used to monitor the signals, and re-calibration and the
QC sample check were performed every 30 samples.
Since the self-aspiration was used to introduce the
samples into the nebulizer, it was considered the
sample uptake rate would be varied due to the variance
of each chemical viscosity. Besides, there would be
unevenness on the nebulizers used for remote
sampling system. To compensate these issues, internal
standard correction by 5 |o,g/L (ppb) Be, and 1 |0,g/L
(ppb) Sr, In and Tl was used in this experiment.
Standard solutions were made from three types of 10
mg/L (ppm) multi-element standard solutions
(PerkinElmer Life and Analytical Sciences, CT, USA)
by serial dilution with the high purity water.
The spike recovery obtained from the unspiked
sample and the 100 ng/L (ppt) spiked sample for each
sample type showed 90-110% and its stability over 15
-48 hours was less than 1% for most of the elements.
The detection limits calculated by dividing the
standard deviation of blank solution with the
sensitivity were below 10 ng/L (ppt). The stability of
internal standard elements in the SC-2 sample, which
indicates the real instrument stability before the
internal standard correction, is shown in Figure 2. All
the signals were very stable over 48 hours, 2.1 -4.3%.
RESULTS AND DISCUSSION
-*— Be (4.3%)
-A—Sr(2.1%)
Recovery and Stability
-+--- In (2.6%)
*
As a first step, the ICP-MS performance for the
analysis of three types of rinse solutions was verified
without the remote sampling device. The standard
solutions, 0 and 1 |0,g/L (ppb), in lwt% HNO3 were
used to establish one set of calibration curves in the
beginning, followed by a QC sample, 500 ng/L (ppt) in
lwt% HNO3. Then, HF, HF + 100 ng/L (ppt) spike,
SC-1, SC-1 + 100 ng/L (ppt) spike, SC-2 and SC-2 +
100 ng/L (ppt) spike samples were alternately
analyzed for 15-48 hours by the DRC-ICP-MS with an
Autosampler, AS-93 Plus (PerkinElmer Life and
Analytical Sciences, CT, USA). More than 20
elements including critical elements in the
semiconductor field such as Na, Al, K, Ca, Fe, Cu and
Zn were analyzed in one run within 5 minutes. The
QC software equipped with the ELAN system was
200
11
Tl (3.6%)
31
21
Measurement number
FIGURE 2. 48 Hours Stability for Internal Standard
Elements in SC-2.
800
500
561
700
Fe
400
600
100
500
300
400
200
300
200
100
10
20
30
ppt
40
50
60
10
20
30
ppt
40
50
60
FIGURE 3. Calibration Curves for Na, K and Fe.
608
10
20
30
ppt
40
50
60
Na(3.2%)
1.20
Mg(3.0%)
1.00
15 0.80
c
0)
"55
| 0.60
0.40
0.20
0.00
Mo (1.8%)
0
100
200
300
400
500
600
700
800
Time (min)
Pb(1.4%)
FIGURE 4. Stability of Analytes with 30 m Transfer Tubing for 13 Hours.
sampling system. The samples were nebulized with
the PFA concentric nebulizer and transported to the
ICP-MS through the 30 m transfer tubing which was
connected to the torch of the ICP-MS system. One
series of standard solutions, from 0 to 50 ng/L (ppt),
were analyzed sequentially to check the capability to
analyze ultra trace level. Calibration curves obtained
for some critical elements in the semiconductor
industry are shown in Figure 3. As can be seen, very
good linearity was obtained even at ultra trace level.
0
Using the same system, the long-term stability test
was performed for 39 elements with a 1 (ig/L (ppb)
multi-element standard solution for 13 hours.
Excellent stability was obtained even without the
internal standard correction: the RSDs of the signals
over that period were around 1 -2% for most of the
elements. The results of some key elements are shown
in Figure 4.
100 200 300 400 500 600 700
Time (min)
FIGURE 5.
System.
Sample Switching with Remote Sampling
The performance of switching valve was studied
with a 10 jig/L (ppb) Y in SC-1, and 1 (ig/L (ppb) In in
SC-2 and Rh in HNO3. Transient signals were
monitored to check the effect of switching on the
signals. Each sample was transported with the remote
sampling device shown in Figure 1.
Remote Sampling Device
In the beginning, standard solutions in lwt% HNO3
was used to evaluate the performance of remote
609
The valve was switched every 100 sec, and three
types of samples were introduced into the ICP-MS
alternatively. As can be seen in Figure 5, the signal
response was very quick: the signals were stabilized in
20 sec after switching the valve, and three orders of
magnitude signal reduction could be achieved in 50
sec when the valve was switched to another samples.
9. Kawabata K., Kishi Y. and Thomas R., Spectroscopy, 18
(1), 2-9 (2003)
CONCLUSIONS
The data presented in this work demonstrate that
the ICP-MS equipped with the DRC system can
analyze typical cleaning solutions used in the FABs
just after simple dilution. It is also shown that the
DRC-ICP-MS with the combination of innovative
remote sampling system would be applicable to real
time on-line monitoring of chemicals used in the
semiconductor FABs.
ACKNOWLEDGMENTS
The authors would like to thank Matt ew Knapp
(PerkinElmer Life and Analytical Sciences) for his
technical support.
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