# X-Ray Photoemission Study of the Oxidation of Hafnium

Authors: A. R. Chourasia; J. L. Hickman; R. L. Miller; G. A. Nixon; M. A. Seabolt
Journal: International Journal of Spectroscopy
 (2009)

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## Abstract

About 20Å   of hafnium were deposited on silicon substrates using the electron beam evaporation technique. Two types of samples were investigated. In one type, the substrate was kept at the ambient temperature. After the deposition, the substrate temperature was increased to 100, 200, and 300∘C. In the other type, the substrate temperature was held fixed at some value during the deposition. For this type, the substrate temperatures used were 100, 200, 300, 400, 500, 550, and 600∘C. The samples were characterized in situ by the technique of X-ray photoelectron spectroscopy. No trace of elemental hafnium is observed in the deposited overlayer. Also, there is no evidence of any chemical reactivity between the overlayer and the silicon substrate over the temperature range used. The hafnium overlayer shows a mixture of the dioxide and the suboxide. The ratio of the suboxide to dioxide is observed to be more in the first type of samples. The spectral data indicate that hafnium has a strong affinity for oxygen. The overlayer gets completely oxidized to form HfO2 at substrate temperature around 300∘C for the first type of samples and at substrate temperature greater than 550∘C for the second type.


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## Body

## 1. Introduction

In the semiconductor industry, the success of metal-oxide-semiconductor structures
depends largely on the “near perfect” SiO2/Si interface.
With the continued downscaling of complementary MOS (CMOS) devices, SiO2 seems to have reached its limits. As the MOS devices are scaled down to the 65 nm technology mode (physical gate length equal to 45 nm), the thickness of the
gate dielectric needs to be less than 2 nm [1, 2]. In this regime, the major
concerns are related to dramatic increase in direct tunneling leakage current
and reliability issues associated with the SiO2 films [3]. The only
alternative is to increase the dielectric constant of the gate dielectric.
Hafnium dioxide (HfO2) is emerging as a high κ dielectric substitute for SiO2 because of its large band gap, high κ and good
thermodynamic stability on Si [4], compatibility with complimentary metal oxide
semiconductor processing, and good reliability [5].Investigation of HfO2-Si systems has been focused on the direct deposition of HfO2 thin films on silicon substrate using different deposition techniques: atomic
layer deposition [2, 6], chemical deposition [7], reactive dc magnetron
sputtering [4, 8], chemical vapor deposition [9], and e-beam technique [10].
Very few studies have been performed involving evaporation of elemental
hafnium. Xing et al. [11] have deposited elemental hafnium on silicon and
investigated the Hf/Si interface. Suzer et al. [12] have studied the oxidation
of elemental hafnium using soft X-ray photoemission spectroscopy (XPS). Morant
et al. [13] have studied the initial oxidation of hafnium foil by XPS. It has
been observed that hafnium suboxide is formed in addition to the hafnium
dioxide [9, 12, 13]. The formation of the suboxide depends on the substrate
temperature and the depositing conditions. To our knowledge, no systematic
investigation on the formation of hafnium suboxide and hafnium dioxide as a function of depositing
conditions has been carried out.In this investigation, we have studied the formation of hafnium dioxide and
hafnium suboxide as a function of the substrate temperature. The technique of
XPS has been employed for in situ
characterization of the samples. The formation of the suboxide and the dioxide
has been compared in two types of samples. One type consists of depositing the
hafnium film at room temperature followed by annealing. In the other type, the
substrate was brought to the desired temperature before the deposition. The
results show a systematic variation in the content of the suboxide and the
dioxide in these samples. The formation of the dioxide and the suboxide indicates that hafnium
has a strong affinity for oxygen.


## 2. Experiment

Hafnium wires (2 mm diameter) of 96.7% (Alfa Aesar, Mass, USA) were used in this work. The sample
contained nominal 3% Zr as impurity according to the supplier. The silicon
substrates (100;p-type) used in the present investigation were cleaned with HF
to get rid of any native oxide that might be present on the surface. Hafnium
films with estimated thickness of 20 Å  were deposited on silicon substrates.
The thickness of the deposited films was calibrated using a quartz crystal
oscillator. The deposition was carried out by the electron beam technique. The
EGN4 electron gun (Oxford Applied Research, Oxfordshire, UK) was used for the deposition. Four samples in the form of wires can be mounted
onto this gun. The sample to be deposited can be selected by selecting the
appropriate filament. Two types of samples were investigated. For one type
(labeled as type A), the substrate was kept at the ambient temperature during
the deposition. After the deposition, the temperature of the sample+overlayer was raised to the desired value and held
at that temperature for 30 minutes. For this type, the substrate temperatures
used were room temperature (as-deposited), 100, 200, and 300°C. In
the other type (labeled as type B), the substrate was brought to the desired
temperature prior to the deposition. After the deposition, the sample was held
at that temperature for 30 minutes. The substrate temperatures used for this
type of samples were 100, 200, 300, 400, 500, 550, and 600°C.
The base pressure before deposition was high 10−9 torr and rose to
low 10−8 torr during the deposition. The deposition chamber is
connected to the analysis chamber. The overlayers were characterized in situ by XPS.Physical Electronics PHI 5100 ESCA system was used in the present investigation. The
magnesium anode (energy = 1253.6 eV) was used as the source of excitation. For
calibration purposes, pure silver, gold, and copper samples were used. The Cu2p3/2 and Au 4f7/2 lines were set to give a binding energy (BE) difference
of 848.6 eV. This established the linearity of the BE scale. The Ag 3d5/2 line was set at 368.2 eV, the Cu 2p3/2 line at 932.6 eV, and the Au
4f7/2 line at 84.0 eV. The full width at half maximum (FWHM) of the
Ag 3d5/2 peak was determined to be 1.8 eV which gives a measure of
the resolution. The base pressure during
analysis was <10−9 torr. The survey spectra were taken with pass
energy of 89.45 eV, and the multiplex spectra were taken with 35.75 eV pass
energy.


## 3. Results and Discussion

The multiplex spectra in the hafnium 4f and oxygen 1s regions were recorded in the
samples. The hafnium 4s peak has a BE of~537 eV and thus overlaps (see, e.g.,
[14]) with the oxygen 1s region (around 535 eV). The analyses were, therefore,
confined to the Hf 4f region. In the spectra recorded for all the samples, a
shoulder is observed on the low BE side of the main peak in the Hf 4f region.
Investigation of the silicon 2p region did not show any significant changes in
shape and binding energy. The shoulder, therefore, does not correspond to any
chemical interaction between the deposited hafnium overlayer and the silicon
substrate under the deposition conditions used in the present investigation.Figure1 shows the multiplex spectra in the Hf 4f region for type A samples for the
various temperatures. The multiplex spectra recorded in oxygen 1s region in
these samples are shown in Figure 2. The spectra represent the average of
thirty runs taken to reduce the statistical noise and thus to improve the
signal-to-noise ratio. The Hf 4f spectra have been normalized to have equal
intensity at 12.0 eV, and the oxygen 1s spectra have been normalized to have
equal intensity at 527.0 eV. The major peak in spectra has been aligned for
comparison purposes. In Figure 2, a shoulder is observed on the high BE side of
the main peak in the oxygen 1s region. This is due to the Hf 4s core peak. This
confirmed by the fact that this shoulder exists even when the hafnium overlayer
is completely oxidized (spectrum corresponding to 300°C in Figure 2).Figure 1
Normalized Hf 4f spectra in type A samples as a function of substrate
temperature. The major peaks have been aligned.Figure 2
Normalized oxygen 1s spectra in type A samples
as a function of substrate temperature. The major peaks have been aligned.The BE difference between the Hf4f7/2 levels in elemental Hf and in the
dioxide has been determined to be 3.6 eV [12, 15]. Since BE difference between
the main peak and the shoulder is less than this value (~1.7 eV), the spectra
indicate no presence of metallic hafnium. The main peak is due to the formation
of the hafnium dioxide as the elemental film gets deposited on the substrate.
The presence of the shoulder is, therefore, interpreted as the formation of Hf suboxide.
A slight increase in the intensity of the shoulder is observed as the
temperature is increased to 200°C. At temperatures higher than 200°C, the
intensity decreases and the shoulder is seen to disappear at a temperature of
300°C. The spectrum of the sample at this temperature shows the presence of two
peaks corresponding to the 4f7/2 and 4f5/2 levels of
hafnium in hafnium dioxide. This is further corroborated by the BE difference
explained later. Our data, therefore, show that in this type of samples, the
presence of suboxide is detected in addition to the dioxide. The suboxide
exists till 200°C.
At temperatures around 300°C, the overlayer is completely oxidized to form HfO2.To determine the amount of the dioxide and the suboxide present in the samples,
the spectra in the 4f region were fitted with four curves corresponding to the4f7/2 and 4f5/2 levels in the dioxide and in the
suboxide. During the curve fitting, the ratio of the areas of the peaks was
kept fixed at the statistical ratio 1.4, the FWHM for the dioxide at 2.1 eV,
and that for the suboxide of
4f7/2 level at 1.3 eV. The separation between the 4f7/2 and 4f5/2 peaks was locked at the value of 1.7 eV. A combination of Gaussian-Lorentzian was used
with about 80% Gaussian. These parameters have been taken from our earlier
study [15] and from [12, 13]. An example of curve fitting is shown in Figure 3
for the sample with substrate temperature 100°C. The areas under the 4f7/2 peak for both the dioxide and the suboxide were determined from such a curve
fitting. The areas were normalized against the total area and are plotted in
Figure 4. The suboxide peak clearly shows an initial increase as compared to
the as-deposited sample. It is found to remain nearly constant for the 100 and
200°C samples and is found to disappear for the 300°C sample. The dioxide peak
shows an initial decrease as compared to the as-deposited sample and remains
approximately constant for the 100 and 200°C samples. The data, therefore, show
the presence of hafnium suboxide in this type of sample till the substrate
temperature of 200°C. For temperatures around 300°C, the suboxide disappears
and the hafnium overlayer undergoes complete oxidation to form HfO2.Figure 3
Curve fitting of Hf 4f region in type A sample with substrate temperature
of 100°C. The peaks correspond to 4f7/2 and 4f5/2 levels
in the dioxide and in the suboxide. The dashed curve shows the fit.Figure 4
Normalized intensity (in percentage) of the4f7/2 peak in the dioxide
and the suboxide for type A samples as a function of the
substrate temperature. The areas were determined from curve fitting.
The error bars are shown. The dashed curve is the total area under
the 4f7/2 peaks.Figure5 shows the multiplex spectra in the Hf 4f region for type B samples as a
function of temperature. The corresponding oxygen spectra in these samples are
shown in Figure 6. Again, these spectra are normalized to have equal intensity
at 12.0 eV in the Hf 4f region and at 527.0 eV in the oxygen region. The major
peaks in these figures have been aligned. As seen in Figure 5, the shoulder on
the right-hand side of the Hf 4f main peak is seen to increase for the 200°C
sample as compared to the 100°C sample. Some slight variation is observed in
the intensity of this
region as a function of the substrate temperature. The shoulder is seen to
disappear completely for the sample at temperature 600°C. To determine the
temperature at which the suboxide disappears, we have taken data for the sample
with substrate temperature of 550°C. It is observed that the suboxide persists
even at 550°C. The data, therefore, indicate that the suboxide in the annealed
samples disappears
at temperatures greater than 550°C. The normalized areas under the 4f7/2 peak for the oxide and the suboxide have been determined from curve fitting the
4f region. The parameters used are the same as those used for type A samples.
These normalized areas are plotted in Figure 7. This figure shows an initial
increase in the suboxide as the temperature is increased to 200°C. No
significant variation in the content of the suboxide is observed between the
temperature range from 200 to 400°C. The
concentration of the suboxide is seen to decrease at higher temperatures going
to zero at 600°C. The dioxide shows an initial decrease and is found to remain
somewhat constant in the temperature range from 200 to 400°C. At temperatures
greater than 400°C, the dioxide increases. The dashed lines in Figures 4 and 7
represent the total hafnium content as determined from the normalized Hf 4f7/2 data in both the dioxide and the suboxide. A comparison of the areas in these
figures demonstrates that the total amount of hafnium deposited in both types
of the samples remains the same. A redistribution of hafnium is observed
between the dioxide and the suboxide. The content of the suboxide is seen to be
more in type B samples than in type A samples. However, the dioxide content is
observed to be more in type A samples than in type B samples.Figure 5
Normalized Hf 4f spectra in type B samples as a function of substrate
temperature. The major peaks have been aligned.Figure 6
Normalized oxygen 1s spectra in type B samples as a function of
substrate temperature. The major peaks have been aligned.Figure 7
Normalized intensity (in percentage) of the4f7/2 peak in the dioxide
and the suboxide for type B samples as a function of the
substrate temperature. The areas were determined from curve fitting.
The error bars are shown. The dashed curve is the total area under
the 4f7/2 peaks.Xing et al. [11] deposited about 1000 Å  film of elemental hafnium on silicon
substrates at a base pressure of high 10−7 torr using the focused
electron beam evaporation technique. The Hf/Si interface was subjected to rapid
thermal oxidation process at temperatures ranging from 300 to 700°C. They have
characterized the interface using X-ray diffraction, Auger electron
spectroscopy, XPS, and electrical properties. Their results indicate that at
oxidation temperatures lower than 500°C, the film is partially oxidized. The
hafnium film is found to be completely transformed to HfO2 at
temperatures above 500°C. They have also observed an interfacial layer of
hafnium silicate between the HfO2 and silicon substrate. No further
investigation on the presence of the suboxide has been performed. In contrast
to their study, our data show no chemical interaction between hafnium and
silicon up to the temperature (600°C) investigated in this study. This may be
due to the difference in the annealing conditions performed on the samples.Morant et al. [13] have used XPS to investigate the oxidation of Hf foil by exposing
it to oxygen keeping the foil at room temperature. Prior to exposure, they had
cleaned the sample surface by ion etching and high-temperature annealing.
Despite the cleaning process adopted by them and with a base pressure of <10−10 torr in the chamber, their analysis still showed the presence
of some oxygen on the foil. This indicates that hafnium has a strong affinity
for oxygen; consistent with our observation that hafnium gets deposited as
hafnium oxide(s) at higher pressures. Upon oxygen exposures ≤5 L,
they observed the formation of hafnium suboxide. Hafnium dioxide was found to
be formed at oxygen exposures ≥10 L.
In their study, no mention has been made about the charge correction to the BEs
in the presence of the oxides. Their study shows that the thickness of the
suboxide increases to 4 monolayers as the oxygen exposure is increased to ~25 L. This thickness then decreases to a saturation value of ~4Å upon further
exposure to oxygen. Their study also shows that the suboxide is present in the
sample even at higher exposures of oxygen. Our data demonstrates the presence
of the suboxide as a function of the substrate temperature and the complete
oxidation of the overlayer at higher temperatures.Suzer et al. [12] have utilized soft X-ray photoemission spectroscopy to determine
the BE shifts and the valence band offsets of HfO2 grown on Hf
metal. In their study, the polycrystalline hafnium foil was kept at a
temperature of 200°C and was exposed to oxygen for various durations. They have
observed the formation of hafnium suboxide in addition to the dioxide. The BE
difference between the 4f7/2 peaks in the dioxide and the suboxide
has been determined to be 1.7 eV. In our investigation, this difference is seen
to be in the range 1.6–2.8 eV and is
found to depend upon the amount of the suboxide present in the sample.In order to get more information on the formation of the oxides, we have
determined the BE difference between the main peak in the oxygen 1s region and
the4f7/2 peak in the hafnium dioxide as well as in the
suboxide. These differences are plotted
in Figure 8. The dashed horizontal line in this figure corresponds to the
corresponding BE difference value of 513.2 eV in the pure HfO2 sample, as determined in the previous study [15]. In case of HfO2, the difference is
observed to start from a slightly higher value and shows a decreasing trend as
the temperature is increased in type A samples. The expected difference is seen
to be reached at temperature above 200°C. Type B samples show a different
behavior. For these samples, the BE difference starts from a higher value and
is observed to increase until about 400°C. For temperatures greater than 400°C,
it decreases sharply reaching the expected value at about 600°C.  The data indicate that the presence of the
suboxide produces different surface charging effects in the samples. The
charging effect is more pronounced in type B samples (which contain more
suboxide) than in type A samples. The presence of oxygen may be attributed to
the chemisorbed oxygen on the hafnium wire and/or residual water vapor in the
chamber.Figure 8
The difference in the BE between the oxygen 1s
peak and the Hf4f7/2 peak as a function of substrate temperature. The dashed curves
correspond to type A samples and the solid curves to type
B samples. The dashed-dotted horizontal line at 513.2 eV is
the BE difference in the pure HfO2 sample.In conclusion, we have compared the
formation of the hafnium dioxide and the hafnium suboxide in two types of samples.
Elemental hafnium gets deposited as a mixture of the dioxide and the suboxide.
The content of the suboxide in type A samples is observed to be more than that
in type B samples. The dioxide is found to be more in type B samples than in
type A samples. The hafnium overlayer is observed to be completely oxidized to
HfO2 at a substrate temperature of 300°C for type A sample and at
600°C for type B sample. The spectral data also show no chemical interaction
between the hafnium overlayer and the underlying silicon substrate under the
depositing conditions used in this investigation.



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