Advanced Characterization of High-K Materials Interfaces by
High-Resolution Photoemission using Synchrotron Radiation
Olivier Renault*, Nicholas T. Barrett\ David Samour*, Jean-Fransois Damlencourt*
Delphine Blin*1^ and Sybil Quiais-Marthon*
* CEA-DRT-IETI/DTS, CEA-Grenoble, 17, av. des Martyrs, 38054 Grenoble Cedex 9, France
^CEA-DSM-DRECAM/SPCSI, CEA-Saday, 91191 Gif-sur-Yvette, France
ASM France, 1025 rue Henri Becquerel, 34046 Montpellier Cedex 1, France
Abstract. We present in this paper the results of advanced characterization, by photoemission using the soft x-rays of a synchrotron
source, of the interface between chemically oxidized Si and 0.6 nm HfO2. The benefits of such a source enables us to determine the
chemical nature and estimate the spatial structure of the intermediate layer between the high-K material and the Si substrate. It is
shown that this layer consists of two sub-layers, the first being pure SiO2 extending over 0.64 nm and the second being Hf-silicate
(HfO2)x(SiO2)i-x with x=0.44 and a thickness estimated to be 0.22 nm. The present approach should apply to the study of other
Si/high-K materials interfaces.
INTRODUCTION
Integration of new high-permittivity (high-K) dielectric
materials as gate oxides such as HfO2 or HfSiO:N for
the sub-0.1 jim generation of CMOS transistors will
involve unprecedented control of the interface with the
substrate. As the thickness of this oxide gate reaches 23 nm, this control requires an accurate understanding
of the interface properties at the atomic scale, during
deposition of the material and upon subsequent thermal
treatments required by the CMOS manufacturing
technology.
Photoemission is a well known characterization
technique providing informations about both chemical
(bonding states) and electronical properties of surfaces
and interfaces. It has been successfully used for
investigating many issues related to silicon technology
[1]. Laboratory photoemission though appears to be
limited for some specific diagnostic related to interface
chemistry of high-K/Si or high- K/SiOi systems [2]
because of its poor energy resolution, low x-ray source
brilliance and fixed, high x-ray energy that yields low
surface sensitivity due to relatively large photoelectron
mean-free path.
The use of a tunable, soft x-ray synchrotron source
usually overcomes all these drawbacks. In this paper,
we shall highlight the benefits of such a source through
recent results of high-resolution photoemission
experiments obtained on HfO2/SiO2 oxide gate stacks
[3]. The high energy resolution coupled with the high
brightness of the source enables us to unambiguously
separate, on both Hf 4/ and Si 2p core-level spectra,
interfacial from bulk contributions. The binding energy
of the Si 2p fingerprint related to interfacial species
can then be used to infer chemical composition of the
HfO2-SiO2 interface, in agreement with conclusions
gained from both electrical and transmission
microscopy experiments in the laboratory. Some
insights are given concerning quantitative data about
the spatial extension of the mixed interface.
EXPERIMENTAL
Analysis conditions
Photoemission measurements were performed on
beamline SA73 using the tunable synchrotron X-ray
source from the SuperACO storage ring (positrons 800
MeV) of LURE in Orsay. The base pressure in the
analysis chamber was 2x10"10 Torr. Photoelectrons
were detected at a take-off angle of 90° with respect to
the sample surface (i.e. normal emission) by an angleresolved hemispherical analyzer, with an angular
acceptance of 1°, and a pass energy of 16 eV (energy
resolution of 100 meV). The overall energy resolution
(monochromator band-pass and electron analyser
combined) was less than 150 meV. The energy of
incident photons was fixed at 160 eV, yielding
photoionization cross-section values for both Si 2p and
Hf 4/ core-levels close to their maxima. The zero
binding energy (BE) is the Fermi level, measured at the
leading edge of a clean metallic copper surface in
electrical contact with the samples. Gaussian-
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
139
Lorentzian line shapes were used for decomposition of
the spectra after standard Shirley background
subtraction. For the decomposition of the Si2p corelevel, the following lineshapes were found to provide
the best fit: 100% gaussian for the oxidized
components and 85% gaussian for the substrate
component.
Samples
HfO2 films were deposited at 350°C in a Pulsar™
2000 reactor from ASM Microchemistry Ltd on low
doped (2.1015 cm'3), p-type, 8" oxidized Si(lOO)
substrates. The oxidation was performed using
ozonated deionized water (concentration: 6 ppm) after
an HF-last step and yielded 0.7 nm-thick SiO2 layers,
as estimated using ellipsometry; then, a 0.6 nm-thick
HfO2 film was grown for study of the SiO2/HfO2
interface, while a 2.5 nm-thick film was made for
comparison. Before all XPS measurements, except in
the case of the oxidized Si sample, the samples surface
was cleaned in-situ by gentle (200 eV) Ar+ sputtering.
This allowed a complete surface contamination
removal without inducing any modification on the
samples, as evidenced by the disappearence of the C Is
signal and the invariance of the Hf 4/spectra.
106
104
102
100
98
96
Binding Energy (eV)
FIGURE 1. High-resolution Si 2p spectra recorded at 160
eV photon energy of (a) 0.7 nm SiO2/Si, before HfO2
deposition, and (b) 0.6 nm HfO2/0.7 nmSiO2/Si.
Hf4f
hv=160 eV
(a)
£
(0
RESULTS AND DISCUSSION
Assignment of the spectral
interfacial features
The Si 2p spectrum of the chemically oxidized Si
surface obtained before HfO2 deposition is shown in
Fig.
la after background subtraction and
decomposition (each component including both 2/?3/2
and 2pi/2 sub-components with a spin-orbit splitting
value of 0.6 eV). It exhibits the typical spectral features
of an ultra-thin SiO2 layer on Si, related to fully
oxidized Si (Si4+) at the surface and to sub-oxide
species (Si3+, Si2+, Si1+) located at the Si/SiO2 interface.
The energy shift (table 1) of the oxidized Si spectrum
components are in agreement with those reported for
an in-situ grown, thermal SiO2 film [4]. After HfO2
deposition (fig. Ib), an additional oxidation state of Si
clearly emerges between Si4+ and Si3+, shifted by 0.7
eV to lower energies relative to Si4+ (table 1). We point
out at this stage that the peak parameters (energy and
full-width at half maximum) that were used for
spectrum (a) were also used for the fit of spectrum (b).
We assign this extra-component to the spectral
fingerprint of Hf-silicate bonds that form at the SiO2HfO2 interface from interfacial Si4+, and leaving
(b)
Hf-O-Si
22
20
19
18
17
16
14
Binding Energy (eV)
FIGURE 2. High-resolution Hf 4f spectra at 160 eV of (a)
2.5 nm HfO2/0.7 nm SiO2/Si; (b) 0.6 nm HfO2/0.7 nm
SiO2/Si.
burried Si4+basically unchanged. The Hf 4/core-level
spectrum of the 0.6 nm HfO2 is presented in fig. 2b
after background subtraction and decomposition (each
component including both 4/7/2 and 4/5/2 subcomponents with a spin-orbit splitting value of 1.66
eV) The spectrum also shows up an extra-component
to higher energy than the main contribution at 17.6 eV
assignable to Hf-O bonds in HfO2.
140
that is, the silicate layer at the HfO2/SiO2 interface is
Si-enriched. This is in fairly good agreement with other
XPS results on a series of Hf- and Zr-silicate
compounds [6], showing that a shift <1 eV for the Si2p
silicate feature relative to Si4+is expected when x<0.5.
This composition of the interfacial silicate layer is also
consistent with apparent differences found in the
electrical thickness of the intermediate SiO2 layer of 5
nm-thick HfO2 samples, according to the technique
used: as derived from the Equivalent Oxide Thickness
curves this thickness is found to be 0.7 nm, whereas
the physical thickness determined by HRTEM is
around 1.2 nm [7]. This result suggests that the
interfacial layer is not pure SiO2/SiOx, and has a
dielectric constant estimated at around 7; this
independent estimate is in agreement with an
intermediate layer consisting off, according to our
model, both Si-rich silicate (dielectric constant around
10 [6]) and pure SiO2/SiOx sub-layers (dielectric
constant 3.9).
TABLE 1. Energy shift of the Si oxidation states (eV) and
estimated SiO2 reduced thickness
Ref [4]
Si1"
Si2+
Si3+
Before
After
HfO2 deposition
0.95
1.75
2.48
0.93
1,73
2.56
Si-O-Hf
Si4+
3.90
3.89
0.94
1.83
2.56
3.16
3.86
1.36
1.01
Oxidation
state
This additional component is assigned to interfacial
Hf-silicate bonds for two reasons : first, its energy shift
relative to the main Hf-O contribution is of similar
magnitude (0.6-0.7 eV) and opposite direction
compared to that observed in the Si 2p spectrum.
Moreover, it does not appear in the spectrum of the
thicker sample, for which the interface with SiO2
cannot be detected (fig. 2a).
In an attempt to understand the origin of the shift
to lower binding energy of the Si 2p silicate component
relative to SiO2, we had assumed [3] that this shift was
mainly due to the initial-state, charge-transfer term
(fingerprint of covalence/ionicity of a bond) arising
from the change in the nature of some of the secondnearest neighbors of Si (Si atoms in SiO2 and Hf- or Si
atoms in the interfacial silicate). Because Hf has a
lower electronegativity (1.4) than Si (1.8), the lower
energy shift of the silicate feature relative to the Si4+
one is understood to arise from enhanced charge
transfer from the Hf atom to the silicate Si-O bond,
compared to the Si-O-Si situation. One of the
consequences of this view is that the shift should
depend upon the number of Hf-second neighbors. This
has been confirmed by the recent modeling of the Si 2p
core-level shift at Si-(ZrO2)x(SiO2)i-x interfaces [5]
from first-principles calculations that show a linear
dependence of the Si4+-silicate Si2p component
spacing on both the number of second-neighbor Zr
atoms and the O coordination of these Zr atoms.
Because Hf and Zr atoms have very similar electronic
structures, it is reasonable to combine these results
with our data. It is then possible to infer the chemical
composition of the interfacial (HfO2)x(SiO2)i_x silicate
layer simply by assessing the magnitude of the silicate
energy shift in the Si2p spectrum, provided the relation
between the number of Hf atoms in the Si secondneighbor shell and the Hf content x is known.
According to Ref. 5, and assuming the validity of the
calculation when Hf is substituted with Zr, a shift of
0.7 eV of the silicate feature with respect to Si4+ (table
1) would correspond to an average number of secondnearest Hf neighbor of 3.2: this would yield x=0.44,
Semi-quantitative model
of the HfO2/SiO2 interface
In this section, we make an attempt to estimate
indirectly the spatial extension of the mixed Hf-silicate
sub-layer that forms upon HfO2 deposition, between
SiO2 and HfO2. Quantitative treatments of the Si2p
oxide-component intensity at low photon energies is
notoriously delicate, if not impossible, because of the
requirement of known cross sections of each of the
oxidation states in order to calculate the oxide and suboxide thicknesses. It seems difficult to include directly
in the classical quantitative treatments of Himpsel et al.
concerning the Si/SiO2 interface [4] the silicate
component as an oxidation state of Si, because this
would require the knowledge of too many unkown
physical parameters (e.g the photoionisation cross
section at 160 eV) that would yield to excessive
uncertainties. Rather, we estimate the Hf-silicate
thickness by the difference in the SiO2 (Si4+) thickness
before and after HfO2 deposition, following our
assumption that the Hf-silicate sub-layer forms from
interfacial Si4+ bonding states.
The thickness of the SiO2 sub-layer (not taking into
account the sub-oxides at the Si/SiO2 interface) can be
estimated using the following equation :
,
(1)
where ?tSi02 is the inelastic mean-free path of Si2p
photoelectrons in SiO2, 0 is the photoelectron take-off
141
angle (here, 90°), I4+/I° and IJ^/L0 being the intensity
ratios for the sample and for bulk Si and SiO2,
respectively. Lo4+/Ioo0 is the product of the ratios of the
inelastic mean-free-paths in SiO2 and in Si, of the Si
atomic densities in SiO2 and in Si, and of the
photoionisation cross sections at 160 eV. We can
estimate the value of L^/L0 at 160 eV photon energy
by extrapolating the data of Himpsel et al [4] given for
120, 130 145 and 200 eV photon energies. This yields
L.^/LM.2. The results of the calculation of dsi02
before and after HfO2 deposition is given in Table 1.
The thickness of fully oxidized Si decreases from
1.36A,Si02 before deposition to LOlXSi02 after
deposition of 0.6 nm HfO2. This decrease is due to the
conversion of the Si4+ oxidation states at the surface of
the initial SiO2 layer into Si atoms in the
(HfO2)o.44(SiO2)o.56 silicate sub-layer. Consequently,
this silicate sub-layer is believed to extend over a
thickness of 0.35XSi02- The value at 160 eV of ^Sio2 is
taken to be 0.64 nm, according to HimpseFs values of
0.63 nm at 145 eV and 0.65 nm at 200 eV [4]. Taking
into account the A,SjO2 value derived from Himpsel's
data, this would yield 0.87 nm for the thickness before
HfO2 deposition and 0.22 nm for the thickness of the
Hf-silicate sub-layer. We prefer to keep in mind a
relative value expressed using ^-5102 as a umt (table 1)
simply because the uncertainty on the value of this
parameter is generally high. Nevertheless, this estimate
shows that the mixed Hf silicate sub-layer extends over
less than 2 monolayers.
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