Characterization of Hafnium Oxide Thin Films Prepared
By MOCVD
!
Siew Fong Choy, Vanissa Sei Wei Lim, *R. Gopalakrishan, ^lastair Trigg,
^akshmi Kanta Bera, ^hajan Matthew, *N. Balasubramanian, 2Moon-Sig Joo,
2
Byung-Jin Cho, 2Chia Ching Yeo
2
Institute of Microelectronics, 11 Science Park Road, Singapore Science Park II, Singapore 117685
Silicon Nano Device Laboratory, Department of Electrical and Computer Engineering, National University of
Singapore, 10 Kent Ridge Crescent, Singapore 119260
Abstract. Hafnium oxide thin films deposited by MOCVD were annealed in nitrogen at various temperatures. The asdeposited films and annealed films were characterized using Auger electron spectroscopy (AES), atomic force
microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The films
were found to be slightly oxygen deficient. Angle-resolved XPS revealed oxygen to be residing in two different
chemical states, that of oxygen in hafnium oxide, and possibly, a hafnium silicate. Auger depth profiling revealed
nitrogen enrichment in an interfacial layer at the film-substrate interface, which could be the result of an ammonia pretreatment prior to deposition. The thickness of this interfacial layer was determined to be ~ 15 A fom TEM.
Progressively larger grains were found from AFM measurements with increasing annealing temperature.
INTRODUCTION
Until
recently,
improvements
in
device
performance and functionality have been achieved
chiefly by scaling down channel lengths and gate
dielectric thickness. However, this reduction in gate
dielectric thickness cannot continue indefinitely. While
the
International
Technology
Roadmap for
Semiconductors (ITRS) predicts that an equivalent
oxide thickness of < 1.5 nm is necessary for 0.1 |im
technology, the gate leakage current for SiQz becomes
unacceptably high as its thickness is scaled below 2
nm [1]. In addition, there are concerns with boron
penetration and reliability in ultrathin SiQ [2].
Many high-K materials such as Ta2O3, Y2O3,
and HfC^ have been studied as alternative gate
dielectrics [3]. Their higher dielectric constant
compared to SiO2 allows a greater physical thickness
while retaining an equivalent gate capacitance [4], Of
these, HfCb appears most promising due to its high K
value (~25), relatively large bandgap (~5.8 eV) and
high heat of formation (~ 271 kcal/mol) [1, 5].
Hafnium oxide films have been prepared by many
methods, including sputtering [6-8], atomic layer
deposition (ALD) [9-11], chemical vapour deposition
(CVD) [12] and ion-beam assisted deposition (IBAD)
techniques
[13,14].
However, post-deposition
annealing is often required to improve film quality and
reduce leakage current.
Several authors have found the interfacial layer
(between HfC>2 and silicon) to be composed of
hafnium silicate [3,6,15], but the actual thickness of
this layer has not been determined. Although several
aspects of hafnium oxide films have been investigated,
there are limited studies on the structural and surface
characteristics of the films after post-deposition
annealing in nitrogen.
In this work, hafnium oxide films prepared by
MOCVD were annealed in nitrogen at 700°C, 800°C
and 900°C. Auger electron spectroscopy (AES) and
angle-resolved X-ray Photoelectron Spectroscopy
(ARXPS) were used to study the surface composition
and their variation with depth. Atomic Force
Microscopy (AFM) and Transmission Electron
Microscopy (TEM) were employed to study the
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
176
surface roughness and
respectively.
structure of the films,
representative of the other samples, are presented in
Figure 1 below.
EXPERIMENTAL
Hafnium oxide films were deposited onto p-type Si
(100) substrates by MOCVD. Prior to deposition, the
substrates were subjected to HF-last cleaning process
followed by a NH3 pre-gate treatment at 700° C for one
minute. HfO2 film deposition was then carried out
using Hf[OC(CH3)3]4 precursor (20 mg/s) and argon
(450 seem) as carrier gas. The chamber temperature
and pressure was maintained at 400° C and 400 mTorr
respectively during deposition. After deposition, the
HfO2 films were annealed in nitrogen at 700°C, 800°C
and 900°C for one minute and their thicknesses
measured using spectroscopic ellipsometry (Jusung
Eureka-2000).
FIGURE 1. Auger spectra of hafnium oxide film annealed
at 700°C in nitrogen (SI). Top: as-received, Bottom: after ~1
min ion etching.
Auger spectra and depth profiles were acquired
using a JEOL JAMP 7800F Scanning Auger
Microprobe equipped with a 0.5 keV argon ion gun.
An accelerating energy of 10 keV and a probe current
of 10 nA were employed for all spectra. For XPS, a
VG Escalab 220IXL system with a monochromatized
Al Koc (1486.6 eV) source was employed. A constant
analyzer energy (CAE) of 100 eV was used and the C
Is peak (assigned a binding energy of 284.6 eV), was
used as an internal reference for charge shift
corrections. Both survey and narrow scan spectra were
acquired using take-off angles of 90° and 30°.
Due to peak overlap between Si KLL and Hf MNN
Auger peaks, the low energy Si LVV and Hf NVV
peaks were monitored in the depth profiles. Relative
sensitivity factors (RSFs)1 as provided by JEOL, were
used to quantify the spectra.
Figure 2 shows a typical depth profile obtained. A
sharp drop in oxygen intensity can be seen after ~1
minute of ion etching. A nitrogen peak can also be
clearly seen at the hafnium oxide-silicon interface; this
peak could have resulted from the ammonia pre-gate
treatment. Carbon contamination on the film was
minimal. Table 1 shows the relative concentration of
the elements found. The corresponding RSFs are given
in parentheses. All films have a Hf/O ratio of ~0.56
after ion etching and are slightly oxygen deficient.
The structure and surface roughness of the films
were investigated using a Philips CM200 FEG
HRTEM and a DI Dimension 3000 Atomic Force
Microscope, respectively. For the latter, the images
(three or more scans) were acquired using silicon
nitride probes in tapping mode over 1 x 1 (im2 area.
TABLE 1. Relative Concentration of Elements (at.%)
Thick Anneal
Hf
C
O
Temp (0.128) (0.371)
NW
Sample
ness
(0.104)
/°C
(A)
RESULTS AND DISCUSSION
Auger Analysis
Auger spectra were acquired from the hafnium
oxide films in the as-received condition and after ~ 1
minute of ion etching. All four films consisted of
carbon, oxygen and hafnium in the as-received state,
with partial or total removal of adventitious carbon
after ion etching. The spectra for sample SI, which is
1
177
S4
SI
S2
S3
57.6
50.0
56.0
59.3
S4
SI
S2
S3
57.6
50.0
56.0
59.3
As-received
9
4
700
8
800
6
900
After Ion Etching
700
800
900
-
61
62
61
63
30
34
31
31
64
63
65
64
36
37
35
36
Use of RSFs is only semiquantitative and for comparison pusposes.
800.00
600.00
400.00
200.00
0
0.00
Binding Energy (eV)
FIGURE 3. A wide scan survey spectrum of Sample SI at
90° take-off angle.
OlsK^i^S:..............
0.0
2,5
7,5
1O.O
Tim*
fminl
FIGURE 2. Depth profile of sample S4 (as-deposited).
As seen from Table I, all four hafnium oxide films
have similar surface compositions before and after ion
etching and have carbon contents from 4 to 9 at.%.
XPS Analysis
The chemical bonding of hafnium oxide was
evaluated qualitatively by XPS. A typical survey
spectrum of sample SI annealed at 700°C at normal
incidence is shown in Figure 3. The spectrum reveals
the presence of carbon, oxygen, hafnium and silicon.
Carbon probably arose from adventitious surface
contaminants or from residues of the carboncontaining precursors used [11], while the presence of
a silicon signal from the substrate suggests a film
thickness of around 5 nm. This is consistent with the
observation that the Si 2p intensity varies directly with
the film thickness (see Figure 4).
Figure 4 shows the narrow scan spectra of O Is, Hf
4f and Si 2p for SI at 90° take-off angle. While the
binding energies (BE) of the Hf 4f peaks remain
relatively constant with annealing temperature, the
BEs of O Is varied with annealing temperature.
Samples S2 and S3 have relatively lower BE compared
to Sample SI and S4 (indicated by diamonds and
squares respectively). A small hump, suggestive of a
second component, is also visible at X. Similarly, the
Si 2p spectra revealed the presence of at least two peak
components, as indicated by A and B. Table 2 lists the
BEs of Si, SiO2, Hf, HfQ, and HfSixOy with those of
the hafnium oxide films obtained here.
It can be seen from Table 2 that the BE of oxygen
in these films are fairly close to that of HfO2. XPS
spectra were also acquired at 30° take-off angle to
probe the topmost surface of the films. An example of
the peak-fitted O Is spectrum for this take-off angle is
given in Figure 5. Silicon peaks were not observed.
TABLE 2. Comparison of the binding energies of our films (at 9(f take-off angle), Hf, Si, SiO2 and HfSixOy.
Sample /Material
Ols
Hf4f7/
Si2p
Bond
Reference
530.2
SI
98.2
this work
16.2
98.2
this work
S2
529.7
16.3
S3
98.0
529.6
16.3
this work
S4
529.9
16.2
98.0
this work
HfO2
530.4
Hf-0
16.7
[16]
Hf
14.4
Hf-Hf
[16]
533.0
103.6
Si-O
SiO2
[16]
99.3, 99.5
Si- Si
Si
[16], [17]
HfSixOy
-532
Hf, Si-O
[17]
-
178
iOls
eV) and HfO2 (~530 eV), and is closer to that of HfO2.
It is thus reasonable to attribute it to a chemical state
intermediate between that of HfC>2 and SiC^, i.e a nonstoichiometric silicate HfSixOy. The formation of a
compound between hafnium oxide and SiO2 is
supported by the small Si feature at B (Figure 4),
which is distinct from that of SiO2 and Si.
X
TEM Results
Figure 6 gives the TEM cross-section of the four
hafnium oxide films. The hafnium oxide layer was
polycrystalline for all samples and an amorphous
interfacial layer of ~13 - 17 A was found.
Unfortunately, it was not possible to determine the
composition of the interfacial layer by electron energy
loss spectroscopy (EELS) due to mechanical and
thermal shifts. However, given the clear indication of
nitrogen enrichment at the film-substrate interface
from AES depth profiles, it is likely that this layer is a
form of nitride or oxynitride.
100
Binding Energy (eV)
FIGURE 4. O Is, Hf 4f and Si 2p XPS spectra of sample SI
at 90° take-off angle, as-deposited and for various annealing
temperatures.
FIGURE 6. TEM cross-sections of SI to S4, with thickness
of hafnium oxide and interfacial layer (IL) indicated.
531.0
528.0
525.0
AFM Results
Binding Energy («V)
FIGURE 5. Peak-fitted O Is XPS spectrum for SI at 30°
take-off angle.
The AFM images for the as-deposited film and
films annealed at different temperature are shown in
Figure 7. Films annealed at 700°C and 800°C exhibit
smaller grains and pin holes. Both films appeared to be
smoother (rms <3 A) as compared to as -deposited film
(rms~3.6A). Annealing at 900°C led to significant
change in surface morphology of the film. Grain
growth was observed (image rms~3.3 A), and the
The O Is spectrum consisted of two different
chemical states. The main component at 530.3 eV and
a smaller component at 531.7 eV were assigned to
hafnium oxide (530.2 eV) [18] and non-stoichiometric
hafnium silicate (-532 eV) respectively [19]. The
feature at 531.7 eV lies between that of SiQj (533.0
179
2. Wilk, G. D., Wallace, R.M. and Anthony, J.M., J. Appl
Phys. 89 (10), 5243 - 5275 (2001).
maximum grain diameter was measured to be
approximately 50 nm.
3. Zhan, N., Ng, K.L., Poon, M.C., Kok, C.W., Chan, M.
and Wong, C., "Characteristics of High Quality Hafnium
Oxide Gate Dielectric", IEEE Hong Kong Electron
Devices Meeting Proceedings, 2002, pp. 43-46.
4. Lee, B.H., Kang, L., Nieh, R., Qi, W.J. and Lee, J.C.,
Appl Phys. Letters 76, 1926 - 1928 (2000).
5. Zhu, W.J., Ma, T.P., Tamagawa, T., Kirn, J. and Di, Y.,
IEEE Electron Device Lett 23, 97-99 (2002).
6. Lee, B.H., Kang, L., Qi, W.J., Nieh, R., Jeon, Y., Onishi,
K. and Lee, J.C., "Ultrathin Hafnium Oxide with Low
Leakage and Excellent Reliability for Alternative Gate
Dielectric Application" in Technical Digest, International
Electron Devices Meeting, 1999, pp. 133 - 136.
1, Callegari, A., Carrier, E., Gribelyuk, M., Okorn-Schmidt,
H.F., and Zabel, T., J. Appl Phys. Letters 90, 6466 6475 (2001).
8. Kang, L., Lee, B. H., Qi, W.J., Jeon, Y., Nieh, R.,
Gopalan, S., Onishi, K. and Lee, J.C., IEEE Electron
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FIGURE 7. AFM 3D images of hafnium oxide films.
9. Lin, Y.S., Puthenkovilakam, R. and Chang, J.P., Appl
Phys. Letters 81, 2041-2043 (2002).
CONCLUSION
10. Lysaght, P.S., Chen, P.J., Bergmann, R., Messina, T.,
Murto, R.W. and Huff, H.R., J. Non-cryst. Solids 303,
54-63 (2002).
Hafnium oxide films prepared by MOCVD and
annealed in nitrogen were found to be oxygen
deficient. Oxygen was found to exists in two chemical
environments, that of hafnium oxide and possibly,
hafnium silicate. A nitrogen-rich amorphous interfacial
layer (~13-17 A thick) was formed in all samples,
which was not possible to confirm with TEM. More
work needs to be done to develop electron microscopy
techniques for such analyses. Films annealed at 700°C
and 800° C were smoother than the as-deposited film
and that annealed at 900° C, where significant grain
11.Kuki, K., Ritala, M., Aarik, J., Lu, J., Sajavaara, T.,
Leskala, M. and Harsta, A., J. ApplPhys. 92, 5698-5703
(2002).
12. Lee, S.J., Jeon, T.S., Kwong, D.L. and Clark, R.,.7. Appl
Phys. 92, 2807-2809 (2002).
13. Manory, R.R., Mori, T., Shimizu, L, Miyake, S. and
Kimmel, G., J. Vac. Scl Technol A 20, 549-554 (2002).
14. Miyake, S., Shimizu, I, Manory, R.R., Mori, T. and
Kimmel, G., Surf. And Coatings. Technol. 146-147, 237242(2001).
growth was observed. Work is currently on-going to
confirm the grain size of the latter using grazing angle
XRD.
15. Cosnier, V., Olivier, M., Theret, G. and Andre, B., J.
Vac. Sci Technol A 19, 2267-2271 (2001).
ACKNOWLEDGMENTS
16. Moulder, J.F., Stickle, W.F., Sobol, P.E. and Bomben,
K.D.., Handbook of X-ray Phototelectron Spectroscopy,
Minnesota: Perkin-Elmer Corporation, 1992.
The authors would like to thank Mr. Mo Zhiqiang
of PSB for performing the XPS measurements and Dr.
Du An Yan (IME) for his TEM cross-sections.
17. Wilk, G.D., Wallace, R.M. and Anthony, J.M., JAppl
Phys. 87, 484-492 (2000).
18. Nefedov, V.I., Gati, D., Dzhvrinskii, B.F., Sergushin,
N.D. and Salyn, Ya. V., Russian J. Inorg. Chem. 20,
2307-2314(1975).
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180