Analytical Methodologies for Semiconductor Process
Characterization—Novel Mass Spectrometric Methods
Victor H. Vartanian and Brian Goolsby
Motorola, Semiconductor Products Sector
Advanced Products Research and Development Laboratory
Physical Analysis Laboratory
3501 EdBluestein Boulevard
Austin, TX 78721
Abstract. New analytical techniques and applications are needed to address the challenges facing the
semiconductor industry as transistor feature sizes continue to decrease beyond the 100 nm technology node.
Several new applications of quadrupole ion trap (QIT) and Fourier transform ion cyclotron resonance
(FTICR) mass spectrometry are presented, specifically applied to process tool effluent characterization. A
QIT with atmospheric pressure transfer line and pneumatically driven valves is used to characterize a
dielectric etch process, with response times comparable to extractive Fourier transform infrared (FTIR)
spectroscopy. The QIT allows application of collision-induced dissociation (CID) for structure elucidation,
and is useful when high-molecular weight metal organic precursors are used in processes that evolve
byproducts that are ambiguous by gas-phase infrared analysis. Similarly, a transportable FTICR with a fixed
magnet and pulsed-valve sample introduction allows matrix ion ejection to improve the sensitivity to analyte
ions. The FTICR has also been evaluated for a low-pressure chemical vapor deposition (LPCVD) process
using a high-molecular weight precursor. Byproducts are ambiguous in FTIR spectra, but the FTICR
provided structural confirmation of the effluent species, and is a useful complement to FTIR. The FTICR
was a Iso u sed with F TIR t o characterize p rocess t ool e ffluent e missions i n a study using S F6 a nd Ar t o
plasma etch a candidate metal oxide gate electrode material, RuO2. The results confirmed that no significant
amount of RuO4, a toxic byproduct, were produced in this process.
Confirmation of ambiguous spectral signatures is
important when toxic byproducts may be emitted.
INTRODUCTION
The accelerated introduction of new semiconductor
technology generations requires new and innovative
applications of existing analytical methodologies for
process tool exhaust effluent characterization. While
Fourier transform infrared spectroscopy (FTIR) and
mass spectrometry (MS) remain the mainstays of gasphase analysis, new applications and extensions of
existing techniques must be developed for challenging
problems requiring gas phase analysis. New materials,
such as low-vapor pressure metal organic precursors
used for high-k dielectric deposition, can result in
increased
byproduct
complexity, convoluting
traditional methods of species identification.
ANALYTICAL METHODOLOGY
MASS SPECTROMETRY
Mass spectrometry is one of the most widely used
analytical techniques for semiconductor process and
emissions monitoring. Specialized inlets for highpressure sampling have enabled mass spectrometry to
be used for tool process effluent characterization,
complementing FTIR. Ion storage mass spectrometers
applied to semiconductor processes offer further
CP683, Characterization and Metrology for ULSI 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
205
flexibility in their ability to isolate and manipulate
ions, providing increased sensitivity and structure
elucidation in processes where high-molecular weight
precursors are used.
Quadrupole Ion Trap Mass Spectrometry
Quadrupole ion trap mass spectrometry differs
from linear quadrupole mass spectrometry mainly due
to the electrode geometry, which is modified to create
a three-dimensional trapping volume. The advantage
of an ion trap mass spectrometer is the ability to store
and manipulate ions by application of various
waveforms to the ring or endcap electrodes, resulting
in increased experimental flexibility and utility. A
background h elium b uffer g as a t a p ressure o f a few
mTorr is used to collisionally damp the ion motion to
the center of the trap prior to ejection and subsequent
detection, improving spectral resolution and
sensitivity. The buffer gas can also be used to perform
collisionally-induced dissociation (CID) for molecular
structure information [1] in combination with rf fields
applied to the endcap electrodes. This results in
increased amplitude of the ion motion; the increase in
ion translational velocity increases the collision
frequency with the buffer gas, and the internal energy
of the precursor increases until bond fragmentation
occurs (Figure 2).
Two commercial instruments, a quadrupole ion trap
(QIT) mass spectrometer, and a Fourier transform ion
cyclotron (FTICR) mass spectrometer, are used for
process monitoring and emissions characterization.
Each has been adapted for high-pressure sampling by
inclusion of specialized inlets for fast response and
low limits of detection.
A typical sampling
arrangement showing application of the two
instruments is illustrated in Figure 1, in which the
instruments are arranged in parallel. The process tool
effluent is introduced to each instrument via a heated
extraction line at approximately 600 Torr using a
simple diaphragm pump, allowing continuous
monitoring of the process effluent by pulsing the
sample introduction valves periodically. The two ion
storage devices operate similarly by allowing greater
experimental flexibility.
1,
*
*
FIGURE 2. Ion i solation and fragmentation s equence
for structure elucidation is illustrated in an ion trap. Ions are
formed externally and injected into the trap (1). By ramping
the rf amplitude on the ring electrode, or by applying
selective waveforms across the endcaps, ions of interest are
isolated (2). A supplementary waveform is applied to the
endcap electrodes in resonance with the ion motion, inducing
fragmentation (3). The ion fragment is ejected from the trap
and detected by the electron multiplier (4).
in
An atmospheric sampling transfer loop injector
(ASTLI) interface is used to introduce the effluent gas
stream near atmospheric pressure to the ion trap mass
spectrometer [2]. The ASTLI injector has a 100 uL
loop volume and programmable pneumatic valve (PV)
sequencing that may be used for pressures between 10
mTorr (ultimately dependent on the vacuum attained
by the turbomolecular pump used in the device) and
FIGURE 1. QIT/MS and FTICR/MS in parallel are
shown downstream of a process chamber. Tool effluent near
atmospheric pressure (post mechanical pump) is drawn past
the instrument sample inlets by a diaphragm pump.
206
The QIT process response is similar to an
extractive FTIR as shown in the time-resolved spectra
of a C4F8 etch process in Figure 5. The QIT m/z 131
ion (C3F5*) response is virtually the same as the FTIR
C4F8 response, indicating there is no significant lag
caused by the injection loop interface.
1000 Torr. Tool process exhaust effluent is introduced
to the interface through a heated extraction line.
The injection loop sequence (Figure 3) begins by
specifying the pump down time or pump down
pressure, as well as the injection time. The loop is
pressurized with helium at the time it is injected into
the GC interface of the ion trap mass spectrometer,
and helium is also used to purge the loop injector
between injections. Either the mass spectrometer
vacuum or an auxiliary turbomolecular pump is used
to evacuate the loop. An interlock prevents both the
loading and the injecting valve or the loading and the
helium valve being open simultaneously to avoid
contamination to the process chamber. A typical
sequence can be performed in 5-6 s, with mass
analysis performed in 100-250 ms.
FTIR Response Curve
-
o
250
Q1TMS Response Curve
ASTLI Injection Scheme
Sanjipl
A.
-m/z 131
250
Evacuate Loop
Load Sample
FIGURE 3. ASTLI injector sequence.
FIGURE 5. Comparable response is shown by FTIR
and QITMS during a 120 s CF4 etch process.
Inject Sample
Fourier Transform Ion Cyclotron
Mass Spectrometry
Matrix or contaminant ions can be selectively
ejected from the trap, removing chemical interference
and increasing ion abundance. An example of an ionselective experiment is shown in Figure 4. Isolation
of a t-butanol ligand fragment ion (top right) is
followed by dehydrolyis (bottom right) for a byproduct
from a TiN deposition process using a metal organic
precursor TDMAT (tetrakis-dimethyl amino titanium.
ITMS used to identify
unknown FTIR features
Fourier transform ion cyclotron mass Spectrometry
is based on magnetic and electrostatic ion confinement
as opposed to radio frequency confinement in the ion
trap [3]. Inherent advantages to FTICR include the
ability to achieve high-mass resolving power, as well
as the capability of performing ion-molecule reactions.
FTICR, like ITMS, is a mass storage device, and ions
can be selectively isolated by ejecting unwanted ions
or collisionally-excited to perform ion-molecule
reactions. This is somewhat more difficult to achieve
in the FTICR because the getter ion pump does not
allow operation at pressures usually associated with
CID. Even so, the initial pressure burst that occurs
with sample introduction allows a few hundred
milliseconds of elevated pressure, sufficient time for
CID to occur.
Isolation of
ligand
fragment
Dehydrolysis
Typical of FTICR trapped-ion cells are three pairs
of electrodes that comprise a volume in which the
experiment sequence occurs. As indicated in Figure 6,
two sets of parallel electrodes are employed for ion
excitation and detection.
Orthogonal to these
FIGURE 4. Ion trap isolation of a t-butanol
fragment ion followed by dehydrolyis (loss of m/z 18).
207
electrodes are two trapping electrodes, to which an
electrostatic potential of like charge with the ions is
applied, creating a one-dimensional potential well.
Because the trap resides in a homogeneous magnetic
field, the Lorentz force confines ions in the other two
dimensions.
trace species. In this case, several single-frequency
excitation waveforms were applied at the cyclotron
frequencies of the unwanted ions, causing their
cyclotron radii to exceed the dimensions of the cell.
An example is shown in Figure 8 for an SF6 test
application.
Normally the background N2
predominates in the top spectrum, and only by
selectively ejecting matrix ions during the ion
formation event does the SF6 fragment ions become
observable in the bottom spectrum. This technique is
routinely used to improve the analytical results.
FTICR Spectral Trace of SF6 Without N2 Matrix Ejection
•8
(0
FIGURE 6. FTICR schematic indicating ion trapping,
excitation, and detection aspects.
Mass (m/z)
FTICR Spectral Trace of SF 6 With N2 Matrix Ejection
The specific FTICR used in this evaluation has a
permanent magnet of approximately 1 Tesla, and a
getter ion pump (Figure 7) allowing transport under
vacuum. Sample is introduced to the chamber via a
pulsed v alve; a mechanical p ump d raws t ool e xhaust
gases past the FTICR's sample port pulsed valve via a
1/16 in. s.s. capillary. This sampling arrangement
eliminates dead volume and permits rapid response.
I
3
4
CO
NVI*VlAmJwJLv»A««*\^-U
Mass (m/z)
FIGURE 8. Spectrum obtained without ion ejection
results in minimal numbers of SF6 fragment ions trapped
(top). N2 ions are ejected from the trap by applying a radio
frequency waveform at 508,616 Hz during the ionization
event (bottom), resulting in increased analyte signal-to-noise.
Ejection of background matrix ions is shown during an
SF6/Ar high-density plasma etch process on a RuO2 substrate
in which volatilized Ru-containing byproducts were of
interest, especially toxic RuO4, which is a potent respiratory
irritant. FTIR data were ambiguous, showing several
unidentified spectral features. The predominant background
ions of N2+ and Ar+ were ejected, allowing only the
analytically important process-derived ions to be observed.
No Ru-containing species were detected in the mass
spectrum using SF6/Ar, indicating that at least to the
detection limits of the mass spectrometer, there were no
detectable ruthenium compounds.
FIGURE 7. FTICR vacuum chamber and sample
schematic, illustrating introduction of process effluent.
The ability to eject undesired ions greatly increases
sensitivity by enriching the fraction of ions in the cell
that have analytical utility. By selectively applying an
excitation waveform at the cyclotron frequency of
undesired ions, ion ejection occurs, eliminating
chemical interference and improving ion abundance of
208
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CONCLUSION
27(SF5+)
The accelerated introduction of new semiconductor
processes and chemical precursors requires innovative
applications of analytical methodologies. Process
precursors for advanced dielectric deposition and
metalization are becoming more exotic, expensive and
difficult to deliver and require more diagnostic
assistance to increase process learning, performance,
and utilization. With these new precursors it is
necessary to characterize unreacted precursors and
process byproducts to determine whether persistent or
toxic byproducts are produced.
3000
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FIGURE 9. FTICR spectra during a high-density
plasma etch of a RuO2 substrate using SFe/Ar, with
background N2/Ar ions ejected.
The a pplication o f i on t rap a nd F ourier t ransform
ion cyclotron mass spectrometry to process tool
effluent studies allows more definitive identification of
chemical species when FTIR data is ambiguous. This
may occur when characterizing processes using new or
high-molecular weight precursors. The ability to
selectively trap and eject unwanted ions decreases
chemical interference and increases the dynamic range
of the instrument.
Application of excitation
waveforms
to
perform
collisionally-induced
dissociation is also useful when structural information
can assist in identification of complex species. The
sensitivity and response times of novel sample
introduction
techniques
also
allows
mass
spectrometers to achieve performance comparable to
traditional FTIR.
A CVD deposition process using the liquid
precursor bis(t-butylamino)silane (BTBAS) was
studied using the FTICR. This starting material can be
used in the deposition of various Si-based films and
exhibits numerous properties desired in nextgeneration LPCVD, including high conformality,
lower temperature deposition, and elimination of
halogens from the process, which can form nucleation
sites for particle agglomeration. One goal in the
analysis was to determine what byproducts are formed
and attempt to correlate them to reactions at the wafer
surface. Process utilization was also of interest. An
experiment flowing unreacted BTBAS was performed
to observe the molecules response to electron
ionization (El) (Figure 10). Shown below is an FTICR
spectrum of the unreacted BTBAS and the result of
electron ionization induced fragmentation. Numerous
fragment ions are shown. FTIR spectra suggested that
BTBAS utilization was low, but the spectra were
ambiguous. FTICR data confirmed that process
emissions were almost entirely unreacted BTBAS.
ACKNOWLEDGMENTS
ThermoQuest/Finnigan—Scott Quarmby
SC Beu Consulting—Steve Beu
Siemens/Applied Automation—Dean Davis, Kenneth
Gallaher, Alan Kania, Wayne Rimkus
REFERENCES
Spectrum of Unreacted BTBAS
1) Quadrupole Mass Spectrometry and Its Applications,
edited by P. Dawson, American Institute of Physics
Press, Woodbury, NY (1995).
2) V. Vartanian, S. Quarmby, M. Platt, High Pressure
Sampling Mass Spectrometry of Semiconductor Tool
Process Emissions, The Electrochemical SocietyEnvironmental Issues in the Electronics and
Semiconductor Industries (2000).
3) FT-ICR/MS: Analytical Applications of Fourier
Transform Ion Cyclotron Resonance Mass Spectrometry,
edited by B. Asamoto, VCH Publishers, NY (1991).
Experiment Parameters
mass-to-charge
20 eV ionization potential chirp excitation
500 nA ion current
32k data points
100 ms beam time
5 co-added scans
matrix ejection
FIGURE 10.
FTICR spectrum of a CVD liquid
precursor bis(t-butylamino)silane.
209