an XPS-based metrology system to determine film thickness and composition
Thomas Larson, Emir Gurer, and J. Kelly Truman, ReVera
IC industry is experiencing a major transition. Mobility and
integration have become the drivers of semiconductor products, replacing
computational speed as the primary growth factor. New features determining
design are the number of integrated functions, portability, and battery-life
hours. Devices such as cell phones integrated with cameras, wireless cable
setup boxes, and plasma TVs are propelling chip scaling as much as new
network servers and workstations.
is well understood that new materials, processes, and tools must be developed
to satisfy the growing demand for powerful, portable chips. In the process-tool
area, the metrology segment is growing faster than any other group. It
is becoming increasingly true for devices at the 90-, 65-, and 45-nm nodes
and below that process latitude is shrinking as new materials are being
integrated. Established processes are being pushed until the last bit
of margin can be squeezed out of them. At the nanometer level, process
tools are being forced to operate at the atomic scale.
must be increasingly deployed in fabs to enable faster process development,
help put inoperable process tools back on-line quickly, provide a mechanism
to ensure that duplicate process tools are well matched, and—most
importantly—detect process excursions. In the past, metrology has
been focused solely on the dimensions of a target—its width, depth,
or thickness. That is changing, however. Metrology must adapt as final
device performance in a growing number of process areas is being determined
not by the size of a feature but by how many atoms of a specific type
are in it—in other words, by its composition.
emergence of atomic-scale metrology is predictable. In the 1970s, labs
used reflectometers to measure film thicknesses, while fabs used visual
color charts based on process margins. In the 1990s, labs used x-ray technologies
to measure very thin films and determine film composition, while fabs,
still relying on wide production margins, only controlled thicknesses.
In the nanometer era, however, tiny atomic-scale process margins require
that x-ray techniques, which used to be confined to the laboratory, must
be automated and integrated into the fab.
metrology began to appear with the advent of 90-nm logic technology. Starting
with transistor processes, leading-edge (65- and 45-nm) logic processes
require atomic-composition process control steps in addition to traditional
thickness measurements. Likewise, DRAM and nonvolatile memory applications
rely heavily on complex films to achieve ever-increasing memory densities.
DRAM products face the toughest challenge because of changing transistor
and capacitor materials. In copper processes, the likely switch to atomic-layer
barrier deposition at 45 nm will require chipmakers to control metal nitrides
6–9 nm thick and films composed of various materials.
metrology is most crucial in the transistor gate dielectric process. While
silicon dioxide has been used traditionally in this process since the
advent of CMOS, the intrinsic inability of SiO2 to maintain
necessary electrical properties as the technology is scaled has led to
the introduction of a new atom into the SiO2 matrix—nitrogen.
Between 1 and 2 nm thick, a state-of-the-art SiON film contains between
5 and 25% nitrogen, depending on the device design rules. As the transistor
gate dielectric process has evolved from a thermal diffusion process to
include various plasma-enhanced deposition processes, metrology needs
have shifted as well.
2-nm-thick film is about eight atoms high. At that scale, thickness is
difficult to define, let alone to measure. With nitrogen comprising one
to three of those atoms, it too must be controlled. While this film has
traditionally been measured using optical film-thickness metrology, the
introduction of nitrogen has changed the metrology requirements.
1: Correlation between the change in nitrogen concentration of the
SiON film and the dielectric constant and EOT.
1–3 demonstrate the difficulty of controlling compositionally complex
processes. Figure 1 shows the dependence of the SiON dielectric constant
on nitrogen concentration. In this case, the dielectric constant is represented
as the ratio of the physical film thickness to the electrically equivalent
oxide thickness (EOT). Ultimately, EOT is the variable that is changed
by adding nitrogen. The gate dielectric process, using nitrogen, increases
the dielectric capacity of the gate oxide while maintaining a physically
thicker film. However, the film behaves electrically as though it were
thinner. Figure 1 illustrates that any change in the nitrogen concentration
of the film causes a proportional change in the dielectric constant and
EOT. Therefore, if the nitridation process drifts, it causes a drift in
the electrical performance of the device.
2: Schematic diagram showing the formation of SiON gate dielectric
with the addition of nitrogen.
form a SiON gate dielectric, as shown in Figure 2, a starting film of
SiO2 is created on a silicon substrate. After the SiO2
is formed, nitrogen is added. Traditionally, in order to keep the gate
dielectric process in control, SiO2 film thickness is monitored.
However, because the critical electrical properties of the film strongly
depend on nitrogen concentration, it too must be measured.
Figure 3 illustrates the impact of controlling only film thickness. In
this case, a <2-nm SiON film was created. If film thickness alone had
been measured, the film would have presented no obvious yield problems,
as indicated in Figure 3a, in which thickness uniformity was found to
be <1%. But at the first electrical test, a significant within-wafer
electrical nonuniformity was revealed, as shown in the nitrogen map of
the film in Figure 3b, in which nitrogen concentration uniformity across
the wafer was >8%.
3: Wafer maps illustrating (a) that final film thickness with a uniformity
of <1% at 15 Å is in control, and (b) that the final nitrogen
distribution with a uniformity >8% at 1.0 X 1015
is out of control.
measurements presented in Figures 1–3 were made using an RVX 1000
in-line compositional metrology system from ReVera (Sunnyvale, CA). The
tool is based on x-ray photoelectron spectroscopy (XPS), which has been
used in the laboratory for more than 50 years and is emerging as a reliable
method for measuring the thickness and composition of semiconductor ultrathin
films. Two main features distinguish the semiconductor metrology tool
from its laboratory cousin: its production-worthiness and precision. Precision
at a useful throughput is needed to ensure adequate process control, and
round-the-clock operation is required in today's fabs.
4a presents a schematic diagram of the XPS concept. Low-energy x-rays
illuminate a surface and penetrate to a few micrometers in depth. When
an x-ray is absorbed by the surface atoms, the atoms must conserve energy.
In many cases, the atom ejects an electron. The electron is ejected with
energy equal to the initial kinetic energy of the illuminating x-ray plus
the energy that bound the electron to the atom. Ejected electrons are
4: Schematic diagram showing (a) emission of photoelectrons from a
film surface, and (b) typical compositional spectrum from an SiON
surface with several photoelectrons' binding energies.
photoelectrons are collected and analyzed according to their energy. Because
of the low energy of the illuminating x-rays, only photoelectrons near
the surface are collected. Photoelectrons from deep within the material
are reabsorbed. The XPS method can generally detect photoelectrons approximately
10 nm from the top surface of the film.
4b shows a typical compositional spectrum from an SiON surface with several
important binding energies. The first is the binding energy for silicon
at approximately 100 eV. The inset indicates that the silicon photoelectron
energy actually has different components. The spectroscopic information
is composed of the outer-shell electrons of the measured material, and
XPS is sensitive enough to differentiate among the material's bonding
states. In this case, the lower- binding-energy peak corresponds to photoelectrons
emitted from the the silicon substrate (Si-Si) bond, while the higher-
binding-energy portion of the spectrum corresponds to photoelectrons emitted
from silicon atoms inside the SiON film itself, whose binding energy increased
because of its chemical structure (Si-O and Si-N bonds). Most of the silicon
atoms are bound to oxygen, but some are bound to nitrogen.
figure also shows the relative amounts of important elements in the film.
Nitrogen, oxygen, and—interestingly—carbon are of interest.
The carbon may come from the process or an adsorbed layer on the film
surface. Such adsorbed organic layers are typically referred to as molecular
atmospheric contamination (MAC) and are caused by amines and other airborne
contaminants. In evaluating the composition and thickness of the film,
the MAC layer can be disregarded, since its effect is in the spectral
region of carbon and not in the energy regions of interest.
5: Schematic diagram showing (a) that when x-rays pass through a surface,
Si-O and Si-Si photoelectrons are emitted, and (b) Si-O and Si-Si
peaks. Thickness is determined by measuring the attenuation of one
type of photoelectron as it passes through a film above it.
5 illustrates another feature of the x-ray technique. XPS thickness measurements
are made by comparing peak ratios. The general theory governing this method
is called attenuation theory. Thickness is determined unambiguously
by measuring the attenuation of one type of photoelectron as it passes
through a film above it. For example, in the case of SiON on silicon,
photoelectrons from the silicon substrate are measured by evaluating the
peak of the Si-Si bond. When an SiON film is on top of the silicon, Si-Si
photoelectrons must pass through that film to be detected. As SiON film
thickness increases, fewer Si-Si photoelectrons pass through the film.
Hence, by calculating the ratio of the Si-O peak to the Si-Si peak, the
thickness of the film can be determined. The attenuation theory enables
investigators to determine the thickness of multiple film stacks, provided
their total thickness is in the range of 10 nm. The literature provides
a good overview of this theory and its relationship to thickness measurement
is conceptually simple, nondestructive, and analytically powerful. All
composition and thickness information is contained in the spectral output
of the photoelectron energy analyzer system. The number of photoelectrons
detected at a given energy is proportional to the number of atoms of that
type in the surface. In the lab, a surface scientist is typically required
to operate the tool and interpret the results. However, in order for the
XPS technique to qualify as production worthy, fab personnel not intimately
familiar with the underlying technology must be able to operate it. Just
like its laboratory predecessors, the reflectometer and the ellipsometer,
XPS has emerged from the lab enhanced and optimized for use as an in-line
metrology system to control process variations associated with atomic-scale
thickness and composition.
analytical techniques that provide compositional information can generally
probe only the inner, core electrons of atoms. Chemical bonding information
cannot be obtained from most electrons because of the electrons' very
high binding energies.
Using In-Line XPS
example of how in-line ultrathin film compositional metrology can be applied
is shown in Figure 6 and Table I. The figure presents typical wafer maps
from an SiON process that was altered to improve process tools' particle
generation performance and to accommodate a new device design. While the
process tool modifications resulted in reduced particle levels, they unexpectedly
caused compositional variations in the nitrogen concentration.
6: Typical wafer maps from an SiON process showing (a) and (b) film
thickness and nitrogen-dose maps before a process tool modification,
and (c) and (d) film thickness and nitrogen-dose maps after the modification.
While the process tool modifications resulted in reduced particle
levels, they caused compositional variations in the nitrogen concentration.
engineers would have made the tool modifications and verified the results
using thickness metrology only. In this case, however, they were saved
from having to perform weeks of troubleshooting work on a degraded process.
Using the in-line XPS tool, further adjustments and modifications enabled
the device maker to satisfy both film uniformity and particle requirements.
I: Thickness and nitrogen dose uniformity before and after process
processes are not the only compositionally critical ones in semiconductor
manufacturing. The new DRAM and flash devices use dielectrics that are
composed of hafnium oxide alloys. Much work is being conducted on the
integration of some form of aluminum and hafnium oxide. Evidently, aluminum
improves leakage performance and stabilizes HfO2 thermally.
However, the use of aluminum in terms of device performance reduces the
overall capacitance and, therefore, potential memory density. Hence, a
careful balance must be struck to manufacture such devices optimally.
developing typical ALD films composed of hafnium and aluminum, engineers
must be able to measure process uniformity across the wafer and achieve
compositional process repeatability, since stoichiometry is critical to
determining capacitor performance. Figure
7 shows thickness uniformity for three different high-k capacitor
films (HfO2, Al2O3, and SiO2)
and compositional uniformity (in %) for hafnium, aluminum, and oxygen.
In a production setting, the ability to monitor the uniformity of the
relative ratios of hafnium to aluminum or oxygen is crucial for achieving
8: Compositional metrology results showing the correlation between
residual chlorine and surface resistivity.
addition to its application in the capacitor process, titanium nitride
is beginning to appear in the electrodes used in some DRAM architectures.
Sometimes this film is created from a CVD process based on chlorine precursor
chemistry. While generally a well-understood process, the use of this
chemistry leaves residual chlorine on the surface, which can cause the
electrode to exhibit poor resistivity. As illustrated in Figure 8, compositional
metrology was used to demonstrate the dependence of surface resistivity
on residual chlorine. These data show that even a 1% change in chlorine
on the surface doubles resistivity. Hence, proper control of chlorine
on the wafer surface is critical.
high-k films that have been developed for memory and other hafnium systems,
which are typically ALD films composed of nitrogen-doped HfSiO or HfO2,
are also being contemplated for future generations of the gate-oxide process.
The XPS metrology tool has been qualified and is being used to develop
and control these next-generation films.
ALD process generally requires a specific surface condition to enable
the deposition reaction cycle to saturate. The first few ALD cycles are
key. ALD works by exposing a surface to a sequential and alternating set
of precursors; at each cycle, it lets the surface reaction saturate, after
which no further film deposition occurs. If complex films are required,
alternating exposures to several different precursors are performed.
the HfO2 gate dielectric case, the initial surface state of
the silicon is the key to success. Because of the reactions used, there
is always an interfacial layer of SiOx (suboxide). As a result, a two-stack
film with a high-k film (HfO2) in series with a low-k film
(SiO2) is formed. If the optimal suboxide is not formed, the
resulting electrical characteristics are degraded. Figure 7 shows how
compositional metrology can be used to determine the thickness and composition
of the HfO2 and suboxide layer simultaneously.
9: Compositional maps illustrating the concentration ratio of silicon
to germanium in the SiGe film.
other critical front-end of-line processes require compositional control.
For example, silicon germanium depositions are used increasingly to form
the strained-silicon channels for high-performance logic devices. While
these films are typically at least 100 nm thick, the top surface is where
the action is. For example, Figure 9 indicates that by measuring the silicon-to-germanium
concentration ratio at the surface, users of the XPS system can control
the processes typically used to deposit the film. Temperature and gas-flow
stability are crucial in these processes, and any process drift manifests
itself as a variable ratio of silicon to germanium. That variation affects
the strain capacity of the film.
very important process under investigation is shown in Figure 10. The
addition of a specific concentration of fluorine to the transistor process
may help offset poor neutral bias thermal instability (NBTI) performance.
NBTI manifests itself as a long-term drift in the threshold voltage. Occurring
more frequently with heavily nitrided gate oxides and high-dose ultrashallow
junctions, it is not a conveniently reversible process. The addition of
fluorine tends to stabilize the phenomenon.
10: Compositional map of fluorine distribution.
future will see many new composition-critical films. According to the
International Technology Roadmap for Semiconductors, future ultrashallow
junctions will be formed at depths of 10 nm or less, with dopant concentrations
surpassing the solid solubility limit of silicon. This establishes a clear
need for new metrology methods to control very critical implant process
steps. Traditional in-line control metrologies will fail to detect implanter
dose excursions, because they cannot measure the presence of dopants that
are not activated. True physical compositional metrology is required.
At high doses and low implant depths, XPS-based technology provides an
excellent mechanism for monitoring implant dose.
the back end of the line, where the copper dual-damascene process has
been used since the 0.18-Ám technology node, most yield-limiting process
problems have been physical failures resulting from integration defects
such as voids and delamination. In the coming device generations, surface
conditioning will enable more-robust integration, so that the chemical
state of a surface will determine its yield. For example, a low-k surface
will be prepared for subsequent barrier deposition and an ultrathin TaN
barrier will be prepared for optimal electroless copper deposition. These
process steps will require new methods of process control. Because of
their strong surface sensitivity, XPS-based technologies will be a strong
the same time, process control of film deposition will continue to use
traditional metrologies at an increasing rate. Last year, VLSI Research
projected that the overall film metrology market will reach approximately
$900 million in 2009. Although not explicitly tracked by the major market
agencies, the segment devoted to compositional metrology is projected
to occupy a significant part of that market. Many XPS-based compositional
metrology tools are used in advanced fabs around the world. With an increasing
installed base, the technology has arrived as a standard tool for process
authors wish to thank Kathy Barla, Jerome Bienacel, and Nicolas Emonet
from the Crolles2 Alliance in Crolles, France, for the data appearing
in Figures 1 and 8. They would also like to thank Zoe Osborne, Don Wayne,
and Mike Kwan from ReVera's applications engineering team.
D Briggs and MP Seah, Practical Surface Analysis, vol.
1, 2nd ed. (New York: Wiley, 1990).
Thomas Larson is vice president of product marketing at ReVera
(Sunnyvale, CA), which emerged out of the metrology division of Physical
Electronics in 2003. He came to Physical Electronics from Applied Materials,
where he was director of marketing for the advanced process control and
integrated metrology programs. He was also involved in strategic and field
marketing at Schlumberger, where he developed and supported worldwide
field operations for E-beam inspection, CD SEM, and overlay registration
tools. Larson began his career in capital equipment at KLA-Tencor, where
he held applications and product-management positions in the Surfscan
and other strategic units. He received a BS in physics from Arizona State
University in Tempe and performed postgraduate work in physics at San
Francisco State University. (Larson can be reached at 408/205-9235 or
Gurer, PhD, is director of applications engineering at ReVera.
He came to the metrology division of Physical Electronics from ASML's
track division, where he was director of process technology. His responsibilities
at ASML included advanced process technology and applications development,
field applications, and new product development. Before joining ASML,
Gurer was a postdoctoral research associate at the Zettlemoyer Center
for Surface Studies at Lehigh University in Bethlehem, PA. He holds eight
patents and has presented and published more than 25 papers in national
and international conferences. He received a BS and MS in physics from
METU in Ankara, Turkey, and an MS and PhD in physics from Lehigh University.
(Gurer can be reached at 408/530-3828 or email@example.com.)
Kelly Truman, PhD, is vp of strategic marketing and technology
at ReVera. Before joining the company, he served as general manager of
the wet-clean division at Applied Materials, where he had previously been
responsible for product and strategic marketing for the start-up of the
rapid thermal processing group. Also before joining ReVera, he spent a
year on the materials science faculty at the University of Florida in
Gainesville, where he investigated nano/biosensor materials while consulting
for major semiconductor capital equipment companies. He has also held
technical positions at Conductus, CVC, Tektronix, and IBM. He received
a BS in metallurgical engineering from the University of Notre Dame in
South Bend, IN, and MS and PhD degrees in materials science and engineering
from the University of Florida. (Truman can be reached at 408/530-3858
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