Atomic layer deposition of ZrO2 on W for metal-insulator-metal capacitor application

Article abstract of DOI:10.1063/1.1569985

Atomic layer deposition of ZrO₂ on W was investigated for metal-insulator-metal (MIM) capacitor application. A 13～14-A-thick interfacial layer of WO_x was observed between ZrO₂ and W through angle-resolved x-ray photoelectron spectroscopy. Results showed an effective dielectric constant of 22～25 was obtained for MIM capacitors with Pt top electrodes.

Full text of DOI:10.1063/1.1569985

Atomic layer deposition of ZrO 2 on W for metal–insulator–metal capacitor application
Sang-Yun Lee, Hyoungsub Kim, Paul C. McIntyre, Krishna C. Saraswat, and Jeong-Soo Byun
Citation: Applied Physics Letters 82, 2874 (2003); doi: 10.1063/1.1569985
View online: http://dx.doi.org/10.1063/1.1569985
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APPLIED PHYSICS LETTERS
VOLUME 82, NUMBER 17
28 APRIL 2003
Atomic layer deposition of ZrO on W for metal–insulator–metal
2
capacitor application
Sang-Yun Lee, Hyoungsub Kim,a) and Paul C. McIntyre
Department of Materials Science and Engineering, Stanford University, Stanford, California 94305
Krishna C. Saraswat
Department of Electrical Engineering, Stanford University, Stanford, California 94305
Jeong-Soo Byun
Applied Materials, Inc., 3330 Scott Boulevard, Santa Clara, California 95054
͑
Received 6 January 2003; accepted 1 March 2003͒
A metal–insulator–metal ͑MIM͒ capacitor using ZrO on tungsten ͑W͒ metal bottom electrode was
2
demonstrated and characterized in this letter. Both ZrO and W metal were synthesized by an atomic
2
layer deposition ͑ALD͒ method. High-quality 110ϳ115 Å ZrO films were grown uniformly on
2
ALD W using ZrCl and H O precursors at 300 °C, and polycrystalline ZrO in the ALD regime
4
2
2
could be obtained. A 13ϳ14-Å-thick interfacial layer between ZrO and W was observed after
2
fabrication, and it was identified as WO through angle-resolved x-ray photoelectron spectroscopy
x
analysis with wet chemical etching. The apparent equivalent oxide thickness was 20ϳ21 Å. An
effective dielectric constant of 22ϳ25 including an interfacial WOx layer was obtained by
measuring capacitance and thickness of MIM capacitors with Pt top electrodes. High capacitance
2
Ϫ7
2
per area (16ϳ17 fF/␮m ) and low leakage current (10 A/cm at Ϯ1 V͒ were achieved. © 2003
American Institute of Physics. ͓DOI: 10.1063/1.1569985͔
With the continuing down-scaling of integrated circuit
dimensions, high-density dynamic random access memory
strong candidates for capacitor dielectric materials in high
density DRAMs requiring complicated capacitor geometries.
In terms of the bottom electrode, highly doped poly-Si is
conventionally used in DRAM capacitors. However, it easily
reacts with high-k dielectric materials and generates thick
͑
DRAM͒ requires increasing capacitance per unit area to im-
1
prove sensing and noise immunity. Several possible high-k
materials, such as Al O (␧ ϭ10), Ta O (␧ ϭ26), and
2
3
2
3
r
2
5
r
4
8
(
BaSr)TiO ͑BST͒ (␧ ϭ200ϳ350) have been investigated
interfacial oxides having a low dielectric constant. There-
3
r
to meet the required trend of relentless device miniaturiza-
tion. However, Ta O and BST are usually thermodynami-
fore, MIM capacitors using transition metals, such as Pt, Ru,
Ta, and W, as bottom electrodes, have been considered for
future capacitor structures. Recently, ALD deposition of a
2
5
9
cally unstable with the bottom electrode used in such capaci-
tors ͑in particular, with doped poly-Si͒, and, as a result, it is
difficult to control the stoichiometry of films during the sub-
sequent thermal treatment.3 Al O is stable in contact with
single-element metal like W using WF and Si H was dem-
6
2
6
onstrated for filling contacts and vias in microelectronic
,4
10
circuits.
2
3
many bottom electrodes; however, it has a rather low dielec-
tric constant which limits its effectiveness as an enabler for
continued on-chip capacitor dimensional scaling. Recently,
ultrathin films of other high-k dielectric materials, such as
ZrO and HfO have been are widely studied, especially for
gate dielectric applications, because of their relatively high5
dielectric constant ͑ϳ25͒ and thermodynamic stability.
However, little is known regarding the behavior of these
metal-oxide films in metal–insulator–metal ͑MIM͒ struc-
tures for charge storage capacitor applications in DRAM de-
vices.
In this letter, results are reported on the preparation and
properties of a high-quality thin ALD-ZrO dielectric layer
2
which was deposited on an ALD-W metal electrode to form
a MIM capacitor structure with a Pt top electrode. The com-
positional and structural properties were investigated using
transmission electron microscopy ͑TEM͒ and angle-resolved
x-ray photoelectron spectroscopy ͑ARXPS͒. Electrical prop-
erties, such as capacitance–voltage and leakage-current–
voltage characteristics, were measured using MIM capaci-
tors.
2
2
An ALD-W film of 500-Å thickness was deposited at
Among the possible deposition techniques for preparing
nanoscale high-k dielectric films, atomic layer deposition
300 °C on a TiN/SiO /Si structure using tungsten hexafluo-
2
ride (WF ) and diborane (B H ) as the precursors. Without
6
2
6
͑ALD͒ is very promising, because it can produce high qual-
any surface pre-cleaning of the W, 110ϳ115 Å of ZrO was
2
ity films with precise thickness control and near-perfect con-
formality, which are crucial for nonplanar structures, such as
three-dimensional high-geometry capacitors. The conformal-
ity of ALD film growth results from its adsorption-controlled
deposited at 300 °C using alternating surface-saturating reac-
tions of ZrCl and H O in a cold wall-type, high-vacuum
4
2
laboratory-scale ALD system. Each precursor was pulsed for
2 s and N purging followed for 30 or 60 s after H O or
2
2
6
deposition mechanism. As a consequence, high-k metal-
ZrCl pulsing, respectively. The base pressure of the system
4
7
Ϫ8
oxide films, such as ZrO synthesized by ALD, could be
was in the mid-10 Torr range and the process pressure was
2
maintained at 0.5 Torr. Various process results, such as a
deposited thickness that depends linearly on the number of
H O/ZrCl cycles, the independence of growth rate on pre-
a͒Author to whom correspondence should be addressed; electronic mail:
hsubkim@stanford.edu
2
4
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003-6951/2003/82(17)/2874/3/$20.00 2874 © 2003 American Institute of Physics
30.18.123.11 On: Sat, 20 Dec 2014 03:10:27
0
1

Appl. Phys. Lett., Vol. 82, No. 17, 28 April 2003
Lee et al.
2875
FIG. 1. Cross-sectional HR-TEM micrograph of ALD-ZrO /ALD-W.
2
FIG. 2. X-ray photoelectron spectrum of W 4f on ALD-ZrO /ALD-W
2
cursor pulse and purging duration, and the near-perfect step
coverage on a high-aspect-ratio structure, confirmed that the
specimen after HF etching of the ZrO layer as a function of the detection
2
take-off angle.
ZrO process used in this experiment was in the ALD re-
2
gime. No annealing experiments to densify the ZrO film
were performed after capacitor fabrication.
2
tungsten oxide (WO ) or compound layer formed by reac-
x
tion with the overlying ZrO , ARXPS was performed. To
2
In order to measure the electrical properties of the MIM
capacitors, various sizes of circular Pt electrodes were depos-
ited by e-beam evaporation through a shadow mask. The
compositional characterization and the depth profiling of a
avoid unwanted interfacial mixing and sputtering artifacts
commonly observed during XPS depth profiling of ultrathin
high-k dielectrics by ion sputtering,11 wet chemical etching
was incorporated with the ARXPS analysis. The ZrO layer
2
ZrO /W structure were carried out by ARXPS using a Sur-
2
was partially thinned down by exposure to aqueous HF so-
face Science Instruments S-Probe (Al K x-ray source͒. The
␣
lution (HF:H Oϭ1:10) until a significant W 4f signal com-
2
thickness and the microstructures of various samples were
analyzed using both cross-sectional and plan-view transmis-
sion electron microscopy ͑TEM, a Philips CM20 FEG-TEM
operating at 200 kV with a point-to-point resolution of 2.4
Å͒.
ing from the bottom substrate was observed. Sequentially,
the take-off angle of generated photoelectrons in relation to
the film surface was varied from 35° to 85°, in order to
collect depth-profiling information. Figure 2 shows the re-
sulting W 4f signals as a function of take-off angle, and
signals from both the W substrate and the interfacial layer
were observed. Clearly, W 4f signals coming from the inter-
facial layer were located at higher binding energies and the
Zr 4p signal decreased as the take-off angle increased. To
identify the binding status in the interfacial layer, a bare W
film with native tungsten oxide was also analyzed using
ARXPS. The binding energies of photoelectrons coming
from native tungsten oxide were ϳ37.5 and ϳ35.5 eV for W
(4f7/2) and W (4f5/2), respectively. These binding energy
values agree well with the binding energies from the inter-
face layer shown in Fig. 2. Although it is not clear whether
some Zr diffused and mixed with interfacial tungsten oxide
during the deposition, the major constituent of the interfacial
layer can be identified as tungsten oxide, and not a com-
Figure 1 shows a cross-sectional high-resolution TEM
͑
HR-TEM͒ image of ALD-ZrO on ALD-W before the Pt
2
top electrode deposition. The ZrO film had a uniform thick-
2
ness of 110ϳ115 Å. The deposition rate of ZrO in the ALD
2
system used in this experiment was 0.5ϳ0.6 Å per cycle on
an SiO surface, and this matches well with results obtained
2
on the W substrate based on measurement of ZrO thickness
2
using HR-TEM after 200 alternating cycles. Typically, ALD
growth based on H O and chloride precursors occurs readily
2
on a uniform hydroxyl-terminated surface formed prior to or
during the first H O pulse. Otherwise, irregular island-type
2
7
growth is commonly observed at a reduced film growth rate.
Therefore, the ALD-W bottom electrode that was covered
with a native tungsten oxide during air exposure that may
provide a good hydrophilic surface condition for the follow-
ing ALD-ZrO2 deposition. This resulted in uniform film
pound formed by reaction with the adjacent ZrO film. Ac-
2
cording to the ZrO –WO binary phase diagram reported by
2
3
growth and the same growth rate obtained as on a SiO sur-
Chang et al.,12 there is no compound formation and a ZrO2
solid solution ͑Ͻ5 at. % WO ) can coexist with WO up to
2
face. As shown in the cross-sectional HR-TEM image, the
as-deposited microstructure of the ALD-ZrO2 film on
ALD-W was almost completely polycrystalline, and a tetrag-
3
3
1100 °C. Our ARXPS data are, therefore, consistent with the
reported phase equilibria in this materials system.
onal crystalline ZrO phase was identified through electron
Figure
3
shows the C–V characteristics of
2
diffraction analysis ͑not presented here͒. A thin interfacial
amorphous layer of thickness of 13ϳ14 Å was observed be-
ALD-W/ZrO /Pt capacitors measured at different frequen-
cies. The 18 500 ␮m circular capacitor was swept from
2
2
tween the ALD-ZrO and ALD-W, as shown in Fig. 1.
negative to positive bias on top electrode and back to check
the amount of hysteresis. Although a modest dispersion was
2
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In order to study whether this interfacial layer was native
130.18.123.11 On: Sat, 20 Dec 2014 03:10:27

2876
Appl. Phys. Lett., Vol. 82, No. 17, 28 April 2003
Lee et al.
FIG. 3. Capacitance vs applied voltage characteristics of a MIM capacitor
FIG. 4. Leakage current characteristics of a MIM capacitor structure
structure (Pt/ALD-ZrO /ALD-W) for different measurement frequencies.
2
2
(
Pt/ALD-ZrO /ALD-W). The inset of the figure shows a J/E vs 1/E plot
The applied voltage is defined as positive when the top Pt electrode is
positively biased.
2
for the positive biasing condition, consistent with Fowler–Nordheim tunnel-
ing behavior.
observed with different measuring frequencies and its origin
is not clear at this point, the resulting equivalent oxide thick-
ness varied only by Ϯ1 Å. Based on the measured physical
thickness and the capacitance, the apparent equivalent oxide
thickness was 20ϳ21 Å and the calculated effective dielec-
tric constant of a capacitor was 22ϳ25, including the inter-
facial tungsten oxide. Because the reported dielectric con-
WO surface. Using ARXPS analysis with wet chemical
etching, the amorphous interfacial layer between the dielec-
x
tric and W bottom electrode was identified as WO , possibly
x
containing some Zr. Through MIM capacitor fabrication with
a Pt top electrode, a 20ϳ21 Å equivalent oxide thickness
was obtained and the effective dielectric constant of the ca-
pacitor dielectric, including the contribution of the interfacial
layer, was 22ϳ25. The leakage current magnitude obtained
for carrier injection from the W electrode into the ALD-ZrO2
dielectric is suitable for application in gigabit DRAM.
5
stant of a ZrO film on SiO /Si is around 25, this measured
2
2
value is quite reasonable and was not degraded significantly
by the presence of the interfacial tungsten oxide. The re-
3
ported dielectric constant of WO2.9 is ϳ42. A high capaci-
2
tance per unit area of 16ϳ17 fF/␮m was obtained for these
This work was supported in part by the NSF/SRC Center
for Environmentally Benign Semiconductor Manufacturing,
award No. Q423740 and by a Mayfield Stanford Graduate
Fellowship. One of the authors would like to thank Dr. C.
Knepfler for her useful comments.
ALD-ZrO dielectrics of 110ϳ115 Å thickness, and this is
2
comparable to or even higher than that of typical Ta O5
2
MIM capacitors. However, because deposition of much thin-
ner ZrO films by ALD is relatively straightforward, there is
2
significant opportunity to scale the areal capacitance to much
larger values than those obtained in this work.
1
International Roadmap for Semiconductors ͑Semiconductor Industry As-
Figure 4 shows the leakage current behavior of
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2
Pt/ALD-ZrO /ALD-W capacitors. Asymmetric J–V charac-
2
teristics can be seen due to the asymmetric MIM capacitor
structure having different Schottky barrier heights at the
electrode interfaces. In the case of electron injection from the
bottom ALD-W electrode ͑when the top electrode was posi-
tively biased͒ current-voltage behavior consistent with
Fowler–Nordheim tunneling was observed above ϳ 3 V in
keeping with the J/E2 versus 1/E graph ͑inset of Fig. 4͒.
Because the interfacial tungsten oxide is relatively thin, we
assume that its effect on the barrier height between W and
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2
1
3
9
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