Contact properties of Pt/RuO2Ru electrode structure integrated on polycrystalline silicon

Article abstract of DOI:10.1149/1.1393532

The buffer layers of RuO₂/Ru in Pt/RuO₂/Ru/poly-Si (polycrystalline silicon) structure were prepared by metallorganic chemical vapor deposition (MOCVD) and dc magnetron sputtering. The barrier layers of RuO₂/Ru deposited by MOCVD showed a stable interface, and did not affect the surface morphology of the platinum bottom electrode even at a high annealing temperature of 800°C. The barrier layers effectively prevented the interdiffusion of Pt, O, and Si at annealing temperatures above 700°C in O₂ ambient. On the other hand, the barrier layers of RuO₂/Ru formed by dc sputtering showed severe intermixing and strongly influenced the platinum morphology at high temperature annealing. Contacts in Pt/MOCVD(RuO₂/Ru)/-poly-Si and Pt/dc sputtered (RuO₂/Ru)/poly-Si structures showed the specific contact resistance of 5.0 × 10^-5 and 2.0 × 10^-3 Ω cm², respectively. The barrier layers of RuO₂/Ru formed by MOCVD in Pt/RuO₂/Ru/poly-Si structure were attractive for integration of high dielectric constant (Ba,Sr)TiO₃.

Full text of DOI:10.1149/1.1393532

2340
Journal of The Electrochemical Society, 147 (6) 2340-2342 (2000)
S0013-4651(99)11-088-7 CCC: $7.00 © The Electrochemical Society, Inc.
Contact Properties of Pt/RuO2/Ru Electrode Structure Integrated on
Polycrystalline Silicon
Eun-Suck Choi, Jun-Sik Hwang, and Soon-Gil Yoon*,z
Department of Materials Engineering, Chungnam National University, Taejon 305-764, Korea
The buffer layers of RuO2/Ru in Pt/RuO2/Ru/poly-Si (polycrystalline silicon) structure were prepared by metallorganic chemical
vapor deposition (MOCVD) and dc magnetron sputtering. The barrier layers of RuO2/Ru deposited by MOCVD showed a stable
interface, and did not affect the surface morphology of the platinum bottom electrode even at a high annealing temperature of
800ЊC. The barrier layers effectively prevented the interdiffusion of Pt, O, and Si at annealing temperatures above 700ЊC in O2
ambient. On the other hand, the barrier layers of RuO2/Ru formed by dc sputtering showed severe intermixing and strongly influ-
enced the platinum morphology at high temperature annealing. Contacts in Pt/MOCVD(RuO2/Ru)/poly-Si and Pt/dc sputtered
(RuO2/Ru)/poly-Si structures showed the specific contact resistance of 5.0 ϫ 10Ϫ5 and 2.0 ϫ 10Ϫ3 ⍀ cm2, respectively. The bar-
rier layers of RuO2/Ru formed by MOCVD in Pt/RuO2/Ru/poly-Si structure were attractive for integration of high dielectric con-
stant (Ba,Sr)TiO3.
© 2000 The Electrochemical Society. S0013-4651(99)11-088-7. All rights reserved.
Manuscript submitted November 24, 1999; revised manuscript received March 3, 2000.
Barium strontium titanate, (Ba,Sr)TiO3 (BST), thin films have
rier layers were also expected to block the interdiffusion of Pt and Si
elements. The structural and the electrical properties of Pt/dc sput-
tered (RuO2/Ru)/poly-Si and Pt/MOCVD (RuO2/Ru)/poly-Si elec-
trode structures were evaluated under more severe annealing condi-
tions than under the deposition of BST by MOCVD.
received a great deal of attention as a likely candidate for dynamic
random access memory (DRAM) applications.1-3 The integration of
BST films into a DRAM capacitor structure has many problems.
One of the primary problems is the need to deposit the BST films
under oxidizing conditions and to minimize postdeposition thermal
treatments under low oxygen partial pressures, as the perovskites are
susceptible to reduction. This also limits the choice of electrodes for
the capacitor stack to either Pt, Ru, or conducting oxides, since
forming an insulating oxide at the BST-electrode interface will lower
the stack capacitance. The choice of electrode materials is important
for both processing and integration. The major developments in the
bottom electrode contacts were performed by NEC,4 Mitsubishi,5
and earlier work by NEC.6 The electrode stack structures suggested
by NEC, Mitsubishi, and earlier work by NEC were Al/TiN/
BST/RuO2/TiN/thin Ti/poly-Si plug, Ru/BST/Ru/poly-Si, and top
electrode/BST/Pt/TiN/Ti/poly-Si, respectively. However, thus far,
the suggested electrode structures have many problems for DRAM
devices applications.
Generally, electrode structures for integration of the high dielec-
tric materials onto polysilicon were fabricated by sputtering. How-
ever, the electrodes and the barrier layers formed by sputtering have
large stress. When they are exposed to high temperature for deposi-
tion of high dielectric materials, hillocks are formed by the interdif-
fusion of elements to relieve the high stress, resulting in electrical
failure to capacitors. However, because metallorganic chemical
vapor deposition (MOCVD) was performed by the diffusion and the
surface reaction of sources under the equilibrium state, MOCVD
layers have a dense structure and low stress. Therefore, the electrode
structures formed by MOCVD were expected to be able to alleviate
the formation of hillocks and the intermixing of elements when they
are exposed to high temperature in order to deposit the high dielec-
tric materials. The evaluation of the electrode structures formed by
MOCVD for integration of high dielectric thin films has not been
reported elsewhere.
The barrier layers of RuO2/Ru were deposited onto poly-Si
(200 nm)/SiO2 (100 nm)/Si (001) substrates by MOCVD and dc
sputtering, respectively. The native oxides on poly-Si were eliminat-
ed with the following schedule. Wafers were etched for 10 s using a
2.5% HF solution and then rinsed with deionized water for 5 min in
ultrasonic cleaner. After rinsing, wafers were etched again for 5 s in
a 6:1 HF solution buffered using C2H5OH. Finally, they were dried
with nitrogen (99.9999% purity). The bottom electrodes of Pt were
also prepared on RuO2/Ru/poly-Si by MOCVD. The growth system
of MOCVD used for the Ru, RuO2, and Pt deposition consists of a
vertical cold-wall reactor. The detailed deposition conditions by
MOCVD and dc sputtering are summarized in Table I. Ruthenium
dioxide by MOCVD was sequentially deposited on ruthenium in O2
ambient after deposition of Ru on poly-Si without oxygen. The plat-
inum electrodes and RuO2/Ru barrier layers were deposited on poly-
Si as a thickness of about 150 and 50 nm, respectively. The barrier
layers are composed of 30 nm thick RuO2 and 20 nm thick Ru. The
residual stress existing at the films was measured by X-ray diffrac-
tion (XRD) using sin2 ⌽.7,8 The film thickness and the surface mor-
phologies were determined from cross-sectional and surface images
by scanning electron microscopy (SEM, AKASHI DS-130C). The
depth-profile of electrode structures was determined by Auger elec-
tron spectroscopy (AES, VG Scientific Microlab 310-D).The resis-
Table I. Deposition conditions of Ru/RuO2 by MOCVD and dc
sputtering, and Pt films by MOCVD.
Deposition
parameter
dc-sputtered
(RuO2/Ru)
MOCVD
(RuO2/Ru)
MOCVD-Pt
In this paper, we report the bottom electrode stack structures of
Pt/RuO2/Ru/poly-Si. The barrier layers of RuO2/Ru on poly-Si were
deposited by MOCVD and dc sputtering. Ruthenium barrier layer
plays an important role in increasing the adhesion between Pt and
poly-Si and in preventing the oxidation of poly-Si during the depo-
sition of high dielectric materials. Ruthenium dioxide also plays role
in preventing the diffusion of Ru through Pt grain boundary and then
the formation of RuOx phase on the platinum surface. RuO2/Ru bar-
a
Source material
Deposition
temperature
Substrate
Ar gas flow rate
Bubbling
Ru metal target Ru3(CO)12 MeCpPtMe3
300Њ/400ЊC
250ЊC
400ЊC
Poly-Si
10 sccm
—
Poly-Si RuO2/Ru/poly-Si
200 sccm
110ЊC
50 sccm
10ЊC
temperature
Deposition pressure 1 ϫ 10Ϫ2 Torr
2 Torr
50/0 sccm
0.5 Torr
50 sccm
Oxygen flow rate
10/0 sccm
* Electrochemical Society Active Member.
E-mail: syoon2@unity.ncsr.edu
z
a (Trimethyl)methylcyclopentadienylplatinum {(CH3)3(CH3C5H4)Pt)}.
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Journal of The Electrochemical Society, 147 (6) 2340-2342 (2000)
2341
S0013-4651(99)11-088-7 CCC: $7.00 © The Electrochemical Society, Inc.
Figure 1. SEM surface and cross-sectional images of Pt/MOCVD (RuO2/Ru)/poly-Si (a) as-deposited, and annealed at (b) 600, (c) 700, and (d) 800ЊC for 1 h
in O2 (760 Torr).
tivity of electrode structures was measured by electrometer (CMT-
SR 1000) using a four-point probe. Current-voltage (I-V) character-
istics were then measured using an HP 34401A multimeter and
Keithley 224 programmable current source. The specific contact
resistance of electrode structures was measured with the experimen-
tal configuration known as the transfer length method (TLM).9 The
contact patterns were prepared by Ar-ion milling and photolithogra-
phy. Ion milling is conducted at a tilt angle of 15Њ from the ion beam
direction at 8 ϫ 10Ϫ4 Torr with a 350 V argon ion beam. The pho-
tolithography is done with AZ5214E photoresist (PR).
Figure 1 shows SEM surface and cross-sectional images of
Pt/RuO2/Ru/poly-Si electrode structures as-deposited by MOCVD
and annealed at different temperatures in oxygen ambient (760 Torr).
The barrier layers of RuO2/Ru formed by MOCVD showed clear
interfaces after annealing at sufficiently high temperature of 800ЊC.
The platinum bottom electrodes also show the smooth and dense
morphologies, and slight grain growth with increasing annealing
temperature. These results suggested that the RuO2/Ru layer formed
by MOCVD did not affect the microstructure of Pt bottom electrode.
Figure 2 shows SEM surface and cross-sectional images of Pt/RuO2/
Ru/poly-Si electrode structures as-deposited by dc sputtering and
annealed at different temperatures in oxygen ambient (760 Torr). The
interface between platinum and RuO2/Ru was not clear due to severe
interdiffusion with increasing annealing temperature. Platinum sur-
face showed extremely rough morphology in samples annealed at
700ЊC. This result might be considered because ruthenium diffused
through the Pt grain boundary was formed RuOx second phase on the
platinum surface during annealing in oxygen ambient. This behavior
of ruthenium in electrode structure can be explained by residual
stress of ruthenium. The residual stress of 20 nm thick ruthenium
deposited on polysilicon was measured at room temperature using
XRD peak of (101) existing at 2␪ ϭ 42Њ. The dc-sputtered and
MOCVD-Ru films have a compressive stress of about 2.1 and a ten-
sile stress of about 0.05 GPA, respectively. When dc-sputtered ruthe-
nium having a large compressive stress was annealed at high tem-
perature of 700ЊC, ruthenium diffused to platinum surface through
RuO2 and platinum grain boundary to relieve the large compressive
stress. Then ruthenium diffused to platinum surface forms the second
phase on platinum surface during oxygen ambient annealing. Be-
cause RuO2 layer formed by dc-sputtering is oxygen-poor compared
with that by MOCVD, dc-sputtered RuO2 layer is less effective than
that of MOCVD-RuO2 as a diffusion barrier of Ru. The platinum sur-
face annealed at high temperature in oxygen ambient shows a rough
morphology due to the formation of second phase. On the other hand,
because MOCVD was performed by diffusion and the surface reac-
tion of sources under the equilibrium state, deposition layers have a
dense structure and a low tensile stress compared with sputtering
method. Therefore, the electrode structures formed by MOCVD
Figure 2. SEM surface and cross-sectional images of Pt/dc sputtered (RuO2/Ru)/poly-Si (a) as-deposited, and annealed at (b) 500, (c) 600, and (d) 700ЊC for 1
h in O2 (760 Torr).
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2342
Journal of The Electrochemical Society, 147 (6) 2340-2342 (2000)
S0013-4651(99)11-088-7 CCC: $7.00 © The Electrochemical Society, Inc.
Figure 4. Specific contact resistivities of electrode structures as a function of
annealing temperature.
a function of annealing temperature is shown in Fig. 4. The contact
resistivity of Pt/MOCVD (RuO2/Ru)/poly-Si structure with increasing
annealing temperature almost did not vary, compared with that of as-
deposited electrode structure. On the other hand, the contact resistivi-
ties of Pt/dc-sputtered (RuO2/Ru)/poly-Si structure slightly increase
with increasing annealing temperature. The contact resistivity of as-
deposited electrode structure could not measure because of adhesion
problem between layers. The contact resistivities of Pt/MOCVD
(RuO2/Ru)/Poly-Si and Pt/dc-sputtered (RuO2/Ru)/poly-Si structure
were about 5.0 ϫ 10Ϫ5 and 2.0 ϫ 10Ϫ3 ⍀ cm2, respectively.
Figure 3. AES depth-profile of (a) Pt/MOCVD (RuO2/Ru)/poly-Si and (b)
Pt/dc sputtered (RuO2/Ru)/poly-Si structures annealed at 700ЊC in O2
ambient.
Conclusions
were expected to prevent the formation of hillocks and the intermix-
ing of elements when they are experienced under high temperature.
Figures 3a and b showed AES depth-profiles of Pt/MOCVD(RuO2/
Ru)/poly-Si and Pt/dc sputtered (RuO2/Ru)/poly-Si structures
annealed at 700ЊC in oxygen ambient(760 Torr), respectively. In
Pt/MOCVD(RuO2/Ru)/poly-Si structure, the dense RuO2 deposited
on Ru prevents the diffusion of Ru into the RuO2 layer, and then an
intermixing with platinum during the high temperature annealing in
oxygen ambient. The diffusion of Ru into the platinum layer was not
observed, and the RuO2/Ru barrier layer seems to maintain a stable
interface after annealing at 700ЊC. The barrier layers of RuO2/Ru
formed by MOCVD effectively prevented the interdiffusion of Pt, O,
and Si. The RuO2/Ru barrier layers also prevent the diffusion of oxy-
gen through the Pt bottom electrode during annealing in oxygen
ambient and the oxidation of poly-Si plug. On the other hand, in
Pt/dc sputtered (RuO2/ Ru)/poly-Si structure, RuO2/Ru barrier layer
did not prevent the diffusion of platinum into silicon and showed a
unstable interface structure. The ruthenium and oxygen exist on the
platinum surface, compared with electrode structure formed by
MOCVD. From this result, the second phase observed from SEM
surface image might be RuOx formed on platinum surface by the
reaction of ruthenium and oxygen.
The I-V curves of as-deposited and annealed contacts at various
temperatures for 1 h in oxygen ambient for Pt/MOCVD (RuO2/
Ru)/poly-Si structures become linear, which is indicative of Ohmic
characteristics independent of annealing temperature after annealing
at various temperatures. However, the I-V characteristics for Pt/dc-
sputtered (RuO2/Ru)/poly-Si structure cannot be measured because of
high contact resistance. While the I-V characteristics are useful for ex-
amining changes in contact behavior with annealing temperature,
measuring the specific contact resistance (␳c) gives more quantitative
information. The specific contact resistivity of electrode structures as
The barrier layers of RuO2/Ru in Pt/RuO2/Ru stack structures
formed on polycrystalline silicon by MOCVD and dc sputtering were
suggested as electrode structures for integration of high dielectric
materials. The barrier layers of RuO2/Ru deposited by MOCVD
showed a stable interface and did not affect the surface morphology
of the platinum bottom electrode at high annealing temperature, com-
pared with those of dc sputtering. The barrier layers formed by
MOCVD effectively prevented the interdiffusion of Pt, O, and Si even
at sufficiently high annealing temperature in oxygen ambient. Con-
tacts in Pt/MOCVD (RuO2/Ru)/poly-Si and Pt/dc sputtered (RuO2/
Ru)/poly-Si structure have a specific contact resistance of 5.0 ϫ 10Ϫ5
and 2.0 ϫ 10Ϫ3 ⍀ cm2, respectively. The electrode structures of
Pt/RuO2/Ru prepared on polycrystalline silicon by MOCVD were
attractive for the integration of high dielectric capacitors.
Dr. Yoon assisted in meeting the publication costs of this article.
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