Download Elemental Analysis of Semiconductor Gases Using a Gas Exchange

Elemental Analysis of Semiconductor Gases using a Gas Exchange Device
Coupled to High Sensitivity ICP-MS
Tomoko VINCENT1, Kohei NISHIGUCHI2, Keisuke UTANI2, Julian D. WILLS1, Lothar ROTTMANN1
1Thermo Fisher Scientific, Bremen, Germany
2J-SCIENCE LAB Co. LTD, Kyoto, Japan
FIGURE 3. 74Ge calibration.
Instrumentation
Overview
Purpose: To demonstrate the direct analysis of trace
amounts of semiconductor gases
Methods: The Thermo Scientific™ iCAP Q™ ICP-MS and
J-SCIENCE LAB Gas Exchange Device were used to
measure trace amounts of semiconductor gases.
Results: AsH3 and GeH4 gases were measured by the
iCAP Qs connected to the Gas Exchange Device as
described. Linear calibration curves were obtained for
AsH3 and GeH4 from 1 vol.ppt to 0.5 vol.ppb and detection
limits of less than 1 vol.ppt were achieved.
Introduction
In the semiconductor industry, ultrapure gases such as
AsH3, GeH4, PH3, SiH4 and SO2 are used in the
manufacturing process. For example, AsH3 is used to
produce GaAs by CVD (Chemical Vapor Deposition) while
PH3 is used in the deposition of dopants on
semiconductors materials by MOVPE (Metal Organic
Vapor Phase Epitaxy).
The sample out gas line from the GED was simply
connected to the iCAP Qs injector using a ball joint
connector. Due its high transmission interface and
proprietary 90 degree ion optics for the removal of neutral
species, the iCAP Qs provides the high elemental
sensitivity and low back grounds required for trace
elemental determinations in semiconductor applications.
The iCAP Q was operated in a single collision cell mode,
with a 3 voltage kinetic energy discrimination (KED) barrier,
using pure He as the collision gas.
A high purity 2.0 mm ID sapphire injector and platinum
tipped sampler and skimmer cones were necessary
because of the ultra trace element target analyte levels.
The operating parameters used for the GED and iCAP Qs
used in this work are shown in Table 1.
FIGURE 2. Direct gas analysis system used: GED and
RC with the iCAP Qs
FIGURE 4. 75As calibration.
The use of these gases is not without risk due to their
toxicity and flammability and therefore careful monitoring
of ambient air in semiconductor manufacturing facilities is
required.
This paper describes the direct measurement of trace
amounts of AsH3 and GeH4 in ambient air using a new
high performance quadrupole ICP-MS system, the Thermo
Scientific iCAP Qs (Figure 2), coupled with a Gas
Exchange Device (J-SCIENCE LAB, Kyoto, Japan). The
Gas Exchange Device (GED) consists of two concentric
glass tubes with pores of 0.1 µm in diameter that act as a
membrane. Upon introducing semiconductor gases into
the device, the gas molecules in the sample (N2, O2, CO2
and H2O etc.) are exchanged with Ar across the
membrane, thus ensuring a gas stream flowing to the
plasma which contains principally Ar. The analytes in the
sample gas are maintained in the central channel of the
GED thanks to a process in a Reaction Cell (RC) which
prevents them from travelling through the porous
membrane.
Reaction Cell and Gas Exchange Device Processes
In the RC, O3 converts the metal hydrides to their
respective oxides:
2AsH3 (g)＋ 2O3 → As2O3 ＋ 3H2O
GeH4 (g)＋ O3 → GeO2 ＋ H2O + H2
A 2% ammonia solution (prepared from 20% Thermo
Scientific Optima™ ammonium solution) is used to
generate ammonium nitrate via the following reaction:
2NH3 (g) ＋
4O3
→
NH4NO3
TABLE 1. Instrumental Configuration.
iCAP Qs
Parameter
Plasma power
Nebulizer gas flow
Sample depth
Injector
Interface
Dwell time
QCell He gas flow
QCell KED voltage
GED and RC
Oxygen gas flow
2% ammonia solution gas flow
Nitrogen gas flow
Argon gas flow
＋ H2O ＋ 4O2
Value
1550 W
0.46 L/min
3.5 mm
2.0 mm I.D., sapphire
Platinum sampler and high
sensitivity platinum skimmer
100 ms per peak
4.5 mL/min
3V
25 mL/min
25 mL/min
6 mL/min
100 mL/min
BEC (Background equivalent concentrations) and LoD (Limits
of Detection) for GeH4 and AsH3 were calculated from three
times the standard deviation of10 analyses of the gas blank.
Sub ppt.vol values were obtained for both gases.
TABLE 2. BEC and LoD obtained for the direct analysis of
AsH3 and GeH4.
Analyte
74
Data Analysis
The Thermo Scientific Qtegra™ software was used to
control the iCAP Qs and perform all data acquisition and
calculations.
The AsO3 and GeO2 agglomerate with the NH4NO3 to
form larger particles that cannot pass through the porous
membrane of the GED and are swept in an Ar stream into
the iCAP Qs. The NH4NO3 particles are quickly
dissociated in the ICP ion source into water, N2 & O2.
FIGURE 1. Schematic of the GED.
Sample Gas
Particle
Particle
Sample Gas /
Methods
Sample Preparation
Standards at concentrations of 1, 2, 5, 100, 200 and 500
vol. ppt for As and 5, 10, 25, 500, 1000 and 2500 vol.ppt
for Ge were prepared in bags from the standard stock
1 ppm gas (GASTEC™, Kanagawa, Japan) by appropriate
dilution with 99.9995% pure nitrogen gas. The blank gas
was repeated after the highest concentration gas (0.5 and
2.5 vol.ppb) to check for memory and wash out effects.
Results
Calibrations
Linear calibrations for Ge (as GeO2) and As (as As2O3) are
shown in Figures 3 and 4 respectively. Calibration points
for As at 1, 2, 5, 100, 200 and 500 vol.ppt and 5, 10, 25,
500, 1000 and 2500 vol.ppt for Ge are shown. Each point
is made from 3 separate measurements. The zoomed
section of each calibration shows excellent linearity at <50
vol.ppt concentrations. The fourth calibration point (100
vol.ppt for As and 500 vol.ppt for Ge) was measured 3
times as an unknown and precisions of 2.6% As and 3.3%
Ge (RSD) were obtained.
In order to assess any memory effect from the GED and
the RC sampling system, a pure nitrogen gas blank was
introduced immediately after the highest concentration
standard and the Ge and As intensities were monitored on
the iCAP Qs. After only 6 minutes the Ge and As count
rates dropped below the gas blank showing that GED and
RC didn’t cause any significant memory effect.
Ge
75
As
BEC (ppt)
LOD (ppt)
0.4
0.8
0.1
0.1
Conclusion
 The combination of the J-SCIENCE Gas Exchange Device
with the Thermo Scientific iCAP Qs ICP-MS has been
shown to provide a highly sensitive and specific method
for the direct analysis of semiconductor process gases in
ambient air.
 The detection limits obtained indicate the suitability of the
described technique for the routine environmental
protection of semiconductor manufacturing facilities.
 Applications of gas sampling with high sensitivity ICP-MS
iCAP Q are not limited to just the semiconductor field.
Additional applications include the monitoring of a range of
gas samples (e.g. nuclear, engine exhaust and tobacco
smoke etc) and these will be explored in subsequent
studies.
Acknowledgements
We would like to thank J-SCIENCE Lab for providing the gas
exchange device and reaction system used in this study.
A total measurement times of 2.5 minutes was achieved for
routine analyses including uptake and washout.
At such trace levels Ge and As can be affected by
polyatomic interferences coming from the air, but these
were shown to be effectively suppressed through the use
of He KED mode on the iCAP Qs.
GASTEC is a trademark of the Gastec CORPORATION. All trademarks are the property of Thermo Fisher Scientific
and its subsidiaries.
This information is not intended to encourage use of these products in any manners that might infringe the intellectual
property rights of others.
Presented at the Winter Conference on Plasma Spectrochemistry, Poland, 2/2013.