Journal of The Electrochemical Society, 153 共8兲 F180-F187 共2006兲
F180
0013-4651/2006/153共8兲/F180/8/$20.00 © The Electrochemical Society
Scaling to Sub-1 nm Equivalent Oxide Thickness with Hafnium
Oxide Deposited by Atomic Layer Deposition
Annelies Delabie,*,z Matty Caymax, Bert Brijs, David P. Brunco, Thierry Conard,
Erik Sleeckx, Sven Van Elshocht, Lars-Åke Ragnarsson, Stefan De Gendt,*,a
and Marc M. Heyns
IMEC, B-3001 Leuven, Belgium
The implementation of HfO2 gate dielectrics in sub-45 nm devices requires optimization of nanometer-thin HfO2 layers, deposited,
e.g., by atomic layer deposition 共ALD兲. In this work, we optimize ALD conditions such as precursor pulse time and deposition
temperature for HfO2 layers with physical thicknesses below 2 nm. Additionally, we investigate intermediate treatments in the
ALD reaction cycle, such as exposure to gas-phase moisture or remote plasma at low temperature and thermal anneals. Such
intermediate treatments affect both growth-per-cycle 共GPC兲 and Cl-impurity content of the HfO2 layers. The analysis of the
process modifications allows a better understanding of the reaction mechanisms. H2O pulse times of 10 s must be applied to
achieve saturation in GPC and Cl content. Using saturated H2O pulses decreases the gate leakage current in the sub-1 nm
equivalent oxide thickness 共EOT兲 range. The GPC is enhanced from ⬃1.8 Hf/nm2 for conventional ALD to 4 Hf/nm2 for
intermediate plasma treatments at low temperature. Intermediate anneals reduce the Cl content by about two orders of magnitude.
Sufficient hydroxylation of the HfO2 surface is one important factor controlling electrical properties in the sub-1 nm EOT range.
The reduction of the Cl content does not systematically improve the electrical properties.
© 2006 The Electrochemical Society. 关DOI: 10.1149/1.2209568兴 All rights reserved.
Manuscript submitted January 16, 2006; revised manuscript received April 13, 2006. Available electronically June 15, 2006.
The continuous downscaling in complementary metal-oxidesemiconductor 共CMOS兲 devices requires the introduction of gate
dielectric layers with high permittivity. The low equivalent oxide
thickness 共EOT兲 requirement for the sub-45 nm nodes necessitates
deposition of nanomter-thin high-k layers. Scaling to the sub-1 nm
EOT range has been demonstrated with HfO2 deposited by atomic
layer deposition 共ALD兲 using the HfCl4 and H2O precursors.1,2 EOT
values as low as 0.8 nm were reached with ALD HfO2 deposited on
a 0.4 nm chemical oxide interface.
ALD is a chemical vapor deposition 共CVD兲 technique that is
based on the sequential use of self-terminating gas-substrate
reactions.3 The basic unit in the deposition is the reaction cycle, in
which the substrate is exposed to well-separated pulses of at least
two precursors 共steps 1–4 in Fig. 1兲. The precursors chemisorb on
the substrate in a self-terminating reaction 共steps 1 and 3兲. In between the precursor pulses, nonreacted precursors and gaseous reaction by-products are purged away by inert gas 共steps 2 and 4兲. The
alternating supply avoids gas-phase reactions and the film thus
grows only by means of surface reactions. Two factors can cause
saturation of the surface reactions: steric hindrance of the precursor
ligands adsorbed on the surface and a limited amount of reactive
sites at the surface. The growth-per-cycle 共GPC兲 is the amount of
material deposited in one reaction cycle. The GPC can be affected
by the substrate on which the ALD is performed. Steady GPC is
obtained when all substrate effects have vanished and the deposition
occurs on the ALD-grown material itself.
Several studies have investigated the process optimization of
HfCl4 /H2O ALD.4-11 Both deposition temperature and surface
preparation control the material and electrical properties. Films deposited at temperatures between 300 and 370°C have the highest
density and contain the least amount of impurities. Good-quality
continuous films are deposited on ozone-based wet-oxide surfaces,12
while HfO2 deposited on hydrofluoric acid 共HF兲-cleaned Si surfaces
shows island-like morphology and poor electrical properties due to
poor nucleation on H-terminated Si. However, the results mainly
focused on films with thicknesses larger than 2.5 nm.
In this work, we focus on thinner films to improve electrical
performance in the technologically relevant ⬃1 nm EOT range. The
optimization of the quality of nanometer-thin HfO2 films could al-
* Electrochemical Society Active Member.
a
z
Present address: Katholieke Universiteit Leuven, B-3001 Leuven, Belgium.
E-mail: [email protected]
low further downscaling in EOT and/or improve the EOT leakage
current performance in general. Two characteristics of HfCl4 /H2O
ALD may limit the downscaling of the HfO2 thickness. First, the
steady GPC of the conventional HfCl4 /H2O process at 300°C is
only 14–17% of a monolayer.4-6 This is much lower than the theoretical maximum in ALD of one monolayer per cycle. Theoretical
studies based on the random deposition model indicate that a low
GPC can limit the scalability of the HfO2 thickness.13 A low GPC
can result in a less two-dimensional atomic stacking of the deposited
material, resulting in atomic-scale roughness and lower density.
Most studies indicate that the GPC is controlled by the number of
reactive sites at the surface in the HfCl4 half reaction, the hydroxyl
groups10,11,14,15
x−OH + HfCl4共g兲 → −Ox −HfCl共4−x兲 + xHCl共g兲
关1兴
surface species are denoted by គគ, gas phase species by 共g兲, and x is
the number of hydroxyl groups reacting per HfCl4. Substrate inhibition occurs typically on surfaces with too low OH density.16 Furthermore, the decreased GPC as a function of temperature is attributed to decreased hydroxylation of the HfO2 surface.10 This implies
that HfCl4 is not very reactive toward oxygen-bridging sites and
Si–H bonds. A second characteristic of HfCl4 /H2O ALD is the presence of Cl impurities in the HfO2 films, originating from the HfCl4
precursor. Cl is mainly located at the HfO2 bottom interface.7-9,17
Figure 1. Schematic representation of a conventional 共steps 1–4兲 and extended ALD 共steps 1–5兲 reaction cycle with intermediate treatment applied
after the H2O reaction. The intermediate treatment is performed in the EPSILON reactor or transport module of the polygon cluster 共Fig. 2兲.
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Journal of The Electrochemical Society, 153 共8兲 F180-F187 共2006兲
Figure 2. Polygon 8200 platform at IMEC.
Therefore, its effect on electrical performance is expected to be especially pronounced for thin HfO2 layers.
We have optimized the conventional ALD process conditions of
precursor pulse and purge times and reactor temperature to improve
electrical performance in the sub-1 nm EOT range. Next, the ALD
reaction cycle was extended with different intermediate treatments
共step 5 in Fig. 1兲, such as thermal anneals, cooling in gas phase
moisture, or remote microwave plasma at low temperature. A first
goal of the intermediate treatments is to increase the OH density at
the HfO2 surface in order to enhance the GPC. A second goal is to
reduce the Cl content of the HfO2 layers. The electrical properties of
thin ALD HfO2 layers are improved by better surface hydroxylation
during steps 3–5 共Fig. 1兲. Furthermore, the analysis of the process
modifications allows a better understanding of the reaction mechanisms in the HfCl4 /H2O ALD.18
Experimental
Sample preparation.— Prior to ALD, the native oxide of the
200 mm Si substrates was stripped in HF and the bare Si surface
was reoxidized in an O3 /H2O solution to a nominal thickness of 0.4
or 1 nm.12 HfO2 was deposited in an ALCVD PULSAR 2000 reactor, attached to a Polygon 8200 platform 共Fig. 2兲.19,b The PULSAR
2000 is a hot-wall cross-flow-type reactor. The pressure in the reactor was 1 Torr. All depositions were performed at temperatures between 225 and 370°C with HfCl4 and H2O precursors. HfCl4 is a
solid at room temperature. It was heated to approximately 185°C to
achieve sufficient vapor pressure for the HfCl4 pulses. The H2O
bubbler was kept at 18°C. The pulse times were varied from the
standard pulse length 共0.3 s兲 to several minutes. For the long H2O
pulse times, the post-H2O purge times were also elongated to several minutes to ensure that the precursor pulses remained wellseparated.
Intermediate or postdeposition treatments were performed in an
ASM EPSILON nitride CVD reactor attached to the same Polygon
platform 共Fig. 2兲.19 Intermediate anneals were performed in O2 or
N2 at temperatures between 420 and 500°C. Remote N2O plasma
treatments at room temperature were also studied. The plasma was
generated in a microwave discharge in a side arm of the reactor with
the sample in downstream arrangement. The base pressure and leak
rate in the EPSILON Nitride reactor are 6 mTorr and
0.2 mTorr/min, respectively.
During the transport between ALD and EPSILON reactors,
samples stay about 2 min in the transport module. During this time,
the sample cools and is subjected to the H2O background of the
transport module, with a typical partial pressure of 10–20 mTorr.
The total pressure in the transport module can vary between 1 and
760 Torr.
b
ALCVD, PULSAR, EPSILON, and Polygon are trademarks of ASM International.
F181
Physical and electrical analysis.— The GPC was evaluated
from both spectroscopic ellipsometry 共SE兲 and Rutherford backscattering 共RBS兲 for samples where 10–200 reaction cycles of HfO2
were deposited on a 1 nm chemical oxide. RBS gives quantitative
results, whereas SE results should be interpreted only qualitatively.20
SE was performed on a KLA-Tencor ASET F5. RBS was performed
in a RBS400 Endstation 共Charles Evans and Associates兲 which is
installed around a 6SDH-1 2MV tandem accelerator 共National Electrostatics Corporation兲. The measurements were performed with a
1 MeV He+ beam in a rotating random mode. The scatter angle was
168 degrees. The accumulation dose was 20 ␮C. Beam current was
limited to 5 nA to avoid pileup in the electronics. A beamchopper
was used for normalization. The RUMP simulation code was applied
to calculate the areal density of Hf 共number of atoms/cm2兲.
Time-of-flight secondary ion mass spectroscopy 共TOF-SIMS兲
surface measurements and depth profiles were performed with an
ION-TOF-IV instrument using a dual-beam setup with a 500 eV Ar+
ion beam.
Atomic force microscopy 共AFM兲 was performed on a Veeco
Nanoscope IV dimension 3100 operating in tapping mode. The
root–mean–square 共rms兲 roughness was extracted from the AFM
images with a scan range of 500 nm 共or 1–2 ␮m兲.
Cross-sectional transmission electron microscopy 共TEM兲 specimens were prepared by conventional ion milling and observed in
JEOL 200CX and Philips CM30 TEMs at 200 and 300 kV, respectively.
Electrical measurements were made on n-MOS capacitors with a
physical vapor deposition 共PVD兲 TaN gate. The highest thermal
budget in the capacitor fabrication was a 1 min anneal at 650°C
after metal-gate deposition. This low-thermal budget limits the regrowth of interfacial oxide. As such, the electrical characterization
focuses as much as possible on the effect of the deposition parameters on the ultrathin HfO2 dielectric layers. Capacitance–voltage
共C–V兲 characteristics were measured at 10 and 100 kHz on 30
⫻ 30 and 100 ⫻ 100 ␮m capacitors. The EOT was estimated by
fitting of the C–V curve with the chemical vapor cleaning 共CVC兲
tool.21 The leakage current was taken at a gate voltage of −1 V or
VFB − 0.6 V on 3 ⫻ 3, 10 ⫻ 10, 30 ⫻ 30, and 50 ⫻ 50 ␮m capacitors. 60 devices per wafer were analyzed. The trends in leakage
current when taken at a gate voltage of −1 V or VFB − 0.6 Vwere
very similar.
n-MOS transistors with PVD TaN gates were manufactured with
a process flow as described in detail in Ref. 2. The highest thermal
budget in the transistor fabrication was a spike activation anneal at
1030°C. The capacitance equivalent thickness 共CET兲 and effective
mobility were estimated from long channel 共L⫽10 µm兲 devices. The
effective mobility was extracted with the split C–V technique.22
Results and Discussion
Optimization of the conventional HfCl4 /H2O ALD process parameters.— H2O pulse length.— The H2O reaction is not saturated
for a pulse time of ⬃0.3 s, typically used in ALD 共Fig. 3a and b兲. If
the H2O reaction is not fully saturated, not all Hf–Cl bonds are
converted to Hf–OH
Hf–Cl + H2O共g兲 → Hf–OH + HCl共g兲
关2兴
Unsaturated H2O reactions therefore result in a higher amount of Cl
impurities in the HfO2 films than in the case of full saturation. At the
same time, unsaturated H2O reactions reduce the GPC because of
lower OH density.
At 300°C, H2O pulses of at least 10 s must be applied to obtain
saturation of the GPC and the Cl content in the HfO2 layer 共Fig. 3a
and b兲. Much longer H2O pulses, e.g., 90 s, result in minor changes
in both GPC and Cl content. The GPC saturates to 1.8 Hf/nm2 or
20% of a monolayer. With more standard H2O pulse times, the GPC
is only 1.4 Hf/nm2 or 15% of a monolayer, in agreement with previous studies.4,5,8 At higher deposition temperatures 共370°C兲, H2O
pulses of at least 10 s are necessary to saturate the hydroxylation.
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Journal of The Electrochemical Society, 153 共8兲 F180-F187 共2006兲
F182
Figure 4. Leakage current as a function of EOT for conventional ALD with
standard vs saturated H2O pulses at 300°C and HfO2 deposited at 370°C.
Trend lines are drawn as a guide to the eye.
Figure 3. Effect of the H2O pulse length on 共a兲 the GPC as determined by
RBS at deposition temperature of 300 and 370°C and 共b兲 the Cl content as
determined by TOF-SIMS for ⬃4 nm thick HfO2 layers deposited at 300°C.
Too short 共0.3 s兲 H2O pulses result in a 10 times higher Cl content
than in the saturation case. The Cl profile is similar to that reported
previously in literature.7-9,17
Saturating the H2O pulse time in the HfO2 deposition improves
the electrical properties in the sub-1 nm EOT range as compared to
standard H2O pulse times. H2O pulses of 10 s systematically reduce
the leakage current by about half a decade at the same EOT 共Fig. 4兲.
Using H2O pulse times longer than 10 s gives no further reduction
of the leakage current, in agreement with the saturation behavior in
GPC and Cl content. C–V and leakage current also indicate better
uniformity over the wafer 共not shown兲. VFB, hysteresis, and Dit do
not vary significantly with the H2O pulse time 共Table I兲. The extracted k-value for HfO2 is 20 and does not depend on the H2O
pulse time 共Fig. 5兲. Also, the thickness of the interfacial silicon
oxide 共0.6 nm兲 does not depend on the H2O pulse time. With this
process it is possible to scale the EOT down to about 0.8 nm when
depositing on the 0.4 nm chemical oxide.
The effect of longer H2O pulses was also evaluated for transistors, in which the gate stack is subjected to a 1030°C activation
anneal. We compared 35 cycles HfO2 with long H2O pulses with
40 cycles HfO2 with standard H2O pulses to compensate for the
higher GPC with longer H2O pulses 共Table II兲. Gate stacks with the
same EOT show some increase in mobility when 10 s H2O pulses
are used.
Deposition temperature.— It has been reported previously that both
GPC and Cl content decrease by increasing the deposition
temperature.10,8 The decreased GPC at high temperatures is attributed to decreased hydroxylation of the HfO2 top surface.10,8 Fourier
transform infrared spectroscopy 共FTIR兲 has shown a decrease of
surface hydroxyl density between 200 and 400°C and elimination of
OH at temperatures as high as 600°C.23 We also observe a decrease
in GPC in the temperature range between 225 and 300°C, but the
GPC stabilizes at temperatures between 300 and 370°C 共Fig. 6a兲.
The Cl content keeps on decreasing between 300 and 370°C 共Fig.
6b兲.
Thus, reduced Cl levels are not necessarily accompanied by better hydroxylation and higher GPC. This implies that besides the
normal ligand exchange reaction 共Eq. 2兲 additional reaction共s兲 occur
Table I. Electrical characteristics of TaN gate capacitors: EOT, JG (leakage current taken at VFB − 1 V), VFB (flatband voltage), ⌬V (hysteresis),
and Dit (interface state density).
H 2O
pulse
共s兲
0.3
0.3
0.3
10
10
10
10
0.3
0.3
0.3
0.3
0.3
Deposition
temperature
共°C兲
Intermediate
treatment
Number
of
cycles
EOT
共Å兲
300
300
300
300
300
370
370
300
300
300
300
300
—
—
—
—
—
—
—
Cooling
Cooling
Plasma
Plasma
Plasma
25
30
40
25
30
25
30
10
12
8
10
12
8.3
8.8
9.7
8.8
9.2
9.6
9.5
7.7
8.1
9.6
1.0
1.1
JG
共A/cm2兲
VFB
共V兲
⌬V
共mV兲
⫻
⫻
⫻
⫻
⫻
⫻
⫻
⫻
⫻
⫻
⫻
⫻
−0.442
−0.432
−0.422
−0.460
−0.461
−0.447
−0.457
−0.428
−0.456
−0.403
−0.404
−0.402
3.1
4.2
3.8
6.1
6.9
7.1
6.6
5.6
5.8
2.3
1.8
1.4
2.17
3.38
2.20
1.81
4.26
4.37
3.20
2.09
1.72
9.24
1.09
2.24
10−1
10−2
10−3
10−4
10−3
10−1
10−3
10
10−1
10−2
10−2
10−3
Dit
共cm−2 eV−1兲
4.8 ⫻ 1011
4.5 ⫻ 1011
5.1 ⫻ 1011
4.6 ⫻ 1011
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Journal of The Electrochemical Society, 153 共8兲 F180-F187 共2006兲
Table II. Peak mobility and CET for TaN-gated transistors with
different substrate doping „NA… for HfO2 gate dielectric deposited
with 10 and 0.3 s H2O pulse times.
Peak mobility 共cm2 /Vs兲
CET 共nm兲
H2O pulse length 共s兲
10
0.3
10
0.3
NA = 1 ⫻ 10 /cm
NA = 3 ⫻ 1017 /cm3
NA = 6 ⫻ 1017 /cm3
270
253
228
262
241
202
1.67
1.67
1.76
1.66
1.66
1.75
17
3
during the H2O pulse. One likely reaction is the condensation of
neighboring Hf–Cl and Hf–OH at the HfO2 surface, forming a Hf–
O–Hf bridge and releasing HCl
Hf–Cl + Hf–OH → Hf–O–Hf + HCl共g兲
关3兴
This reaction can account for simultaneous dehydroxylation and Cl
elimination.
HfO2 deposited at 370°C has less EOT downscaling potential
than HfO2 deposited at 300°C, despite the lower Cl content in the
layer 共Fig. 4兲. Moreover, at 370°C the electrical properties of different capacitors on the same wafer show a much larger spread than
at 300°C 共Fig. 7兲. The median leakage current increases in the sub1 nm EOT range, even with saturated H2O pulses. For our standard
H2O pulse times of 0.3 s, the spread in leakage current over the
wafer was so large that no results are reported in Fig. 4 and Table I.
The degradation for HfO2 deposition at 370°C is particularly apparent for thin HfO2 films 共Fig. 4兲. Good electrical characteristics are
observed at higher EOT, in agreement with Triyoso et al.8 This
indicates that the reduced EOT downscaling potential of HfO2 deposited at 370°C is related to poor initial growth on the scaled
chemical oxide substrate24 used for low-EOT applications.2 The dehydroxylation of Si–OH groups is favored at higher temperatures
2Si–OH → Si–O–Si + H2O共g兲
F183
density at the HfO2 surface and as such enhance the GPC. Especially treatments at low temperatures are expected to increase the
hydroxyl density. We have therefore investigated cooling in gasphase moisture and remote microwave plasma at room temperature.
Growth enhancement is defined as deposition with a GPC higher
than the steady value of 1.4–1.8 Hf/nm2 of the conventional ALD at
300°C. Growth enhancement occurring on the ALD-grown material
itself, in the region where normally the steady HfO2 GPC is
achieved, is referred to as steady HfO2 growth enhancement, as
opposed to substrate growth enhancement occurring in the first reaction cycles of the ALD process.25 In the latter case, it is induced
by the substrate, for example, due to a higher density of reaction
sites at the substrate than on the ALD-grown material itself. The
observation of steady growth enhancement is interesting because a
higher GPC could, according to theoretical studies, result in faster
layer closure and smoother HfO2 films6 and as such a better scalability toward low EOT values. Intermediate thermal treatments after the H2O reaction are expected to decrease the hydroxyl density
of the HfO2 substrate but could reduce residual Cl impurities in the
HfO2 layers. Intermediate treatments after the HfCl4 reaction are
also considered for comparison.
Steady GPC enhancement by intermediate treatments.— Low-temperature intermediate treatments after the H2O reaction in the ALD
reaction cycle enhance the GPC 共Table III兲. Intermediate plasma
treatments at room temperature give the largest enhancement of the
GPC 共from 1.4 to 4.1 Hf/nm2兲. Intermediate cooling of the HfO2
substrate in a moisture background enhances the steady GPC to
关4兴
Too low OH density on the starting surface can result in growth
inhibition and poor HfO2 film quality.4 This is currently under further investigation.
ALD process optimization by intermediate treatments in the reaction cycle.— In the next two sections, we investigate different
treatments after the H2O reaction in the ALD reaction cycle 共Fig. 1兲.
The goal of the intermediate treatments is to increase the hydroxyl
Figure 5. EOT as a function of physical thickness 共as measured with ellipsometry, including both HfO2 and interfacial SiO2 thickness兲 for conventional ALD with standard and saturated H2O pulses and for intermediate
cooling and plasma after every H2O reaction. Trend lines are drawn as a
guide to the eye.
Figure 6. Dependence of the ALD deposition temperature on 共a兲 the GPC as
determined by RBS and 共b兲 the Cl content as measured by TOF-SIMS. The
trend line in 共a兲 is drawn as a guide to the eye.
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F184
Journal of The Electrochemical Society, 153 共8兲 F180-F187 共2006兲
Figure 8. Effect of H2O pulse time and intermediate treatments on the Cl
content as determined by TOF-SIMS for ⬃4 nm thick HfO4 layers deposited
at 300°C. Intermediate cooling, remote plasma, or annealing were performed
every 10 reaction cycles after the H2O reaction.
Figure 7. Spread in leakage current for 60 capacitors on a 200 mm wafer for
HfO2 deposited at 300 and 370°C with saturated H2O pulses.
3.0 Hf/nm2. Both 90 s and 5 min cooling give a similar growth
enhancement. Intermediate annealing followed by cooling enhances
the GPC to a level similar to that of intermediate cooling only.
Annealing without cooling was not possible with our current hardware.
Intermediate cooling or annealing applied after the HfCl4 pulse
does not affect the GPC. However, the plasma treatments enhance
the GPC to 2.4 Hf/nm2.
Properties of ALD HfO2 deposited with enhanced GPC.— TOFSIMS can qualitatively compare the Cl content of HfO2 deposited
with intermediate treatments after the H2O reaction. In order to deposit the 4 nm HfO2 layers for TOF-SIMS within a realistic time
frame, the intermediate treatments were applied every 10 reaction
cycles 共corresponding to slightly more than one HfO2 monolayer兲
instead of every reaction cycle. The effect of the intermediate treatment is still apparent in the Cl profiles 共Fig. 8兲.
The enhanced GPC results in a larger amount of Hf–Cl bonds
that need to be hydrolyzed in each H2O reaction. Therefore, intermediate plasma or cooling in combination with standard 0.3 s H2O
pulses increases the Cl content of HfO2 by a factor of 2 共Fig. 8兲.
10 s H2O pulses reduce the Cl content to a level comparable to that
of the standard GPC processes with saturated H2O pulse times.
Intermediate anneals reduce the Cl content of HfO2 by about 2
orders of magnitude, even with the use of standard H2O pulses 共Fig.
8兲. The temperature and frequency of the intermediate anneal determine the efficiency of the Cl reduction. The Cl content decreases by
2 orders of magnitude with intermediate anneals at 500°C every
10 cycles, while essentially no Cl is removed during a postdeposition anneal 共PDA兲 at 500°C. This indicates that the mechanism for
Cl reduction is a substrate reaction occurring at the HfO2 top surface. The ambient of the intermediate anneal 共O2 or N2兲 did not
influence the Cl content of the layers. The condensation of OH and
Cl groups at the top surface 共Eq. 2兲 is a possible mechanism for the
reduction of Cl during intermediate annealing in O2 or N2. Probably,
annealing after every single reaction cycle instead of after every
deposited monolayer will not further reduce the Cl content. Intermediate anneals at 420°C give a smaller reduction in the Cl content
than 500°C 共not shown兲. Thus, Eq. 2 becomes more efficient at
higher temperature.
Intermediate remote plasma treatments do not physically damage
the HfO2 layer. Plasma treatments in general can induce surface
diffusion which can affect the surface morphology, roughness, and
islanding of the treated film.26 The roughness of HfO2 deposited
with intermediate treatments is similar to conventionally deposited
HfO2, as demonstrated by AFM 共rms = 0.2 nm兲. Note that AFM
cannot detect all the atomic-scale roughness because the AFM tip
cannot penetrate openings smaller than the tip diameter. Crosssectional TEM images also show smooth top and bottom interfaces
共Fig. 9兲. In a remote plasma process the substrate is far from the
glow region of the plasma. Electrically charged species and high
energy radicals will not leave the cavity of the radical generator due
to their short lifetime. Only ground-state low-energy radicals reach
the process chamber. Thus, physical effects such as ion bombardment are minimized in our current setup.
Origin of the steady GPC enhancement.— The HfO2 surface is
very sensitive to H2O adsorption due to the high polarity of the
Hf–O bond. Its sensitivity to moisture increases at low
temperatures.27 For HfO2 at room temperature, moisture concentrations as low as 10 ppm are sufficient for monolayer coverage with
H2O.27 Indeed, a large contribution of the growth enhancement
comes from cooling the sample in a moisture background, increasing the OH density of the substrate: intermediate cooling in the
moisture background of 10–20 mTorr enhances the GPC from
1.4 to 3.0 Hf/nm2 共Table III兲. The HfO2 surface hydroxyl density
obtained by cooling in a moisture background is stable at 300°C for
at least 5 min, as the GPC enhancement is also observed when the
cooled HfO2 surface is prestabilized at 300°C for 5 min before introducing the HfCl4 pulse 共Table III兲. Therefore, we suggest that the
H2O adsorbed at low temperature is decomposed on the HfO2 surface, resulting in increased OH density
Hf–O–Hf + H2O共g兲 → 2Hf–OH
关5兴
Physisorbed H2O would mostly be desorbed by 5 min degassing at
300°C. Further support for the H2O adsorption at low temperature
comes from the observation that HfO2 can be deposited without
H2O pulses in the ALD reactor at 300°C but with intermediate
cooling in the transport module instead 共Table III兲. Probably, the Cl
content of this HfO2 layer is very high, as the Cl reduction becomes
more difficult at lower temperatures.
Thus, increased hydroxylation of the HfO2 surface by adsorbed
water is the origin of the GPC enhancement. We indicate several
possible adsorption reactions of H2O with the HfO2 top surface.
However, a detailed study of the reaction pathway is beyond the
scope of this study. One possibility is H-bonding of H2O with the
HfO2 top surface. Either 1 or 2 H bonds could be formed
Hf–OH + H2O共g兲 → Hf–OH . . . H2O
关6兴
2Hf–OH + H2O共g兲 → Hf–OH . . . H2O . . . OH–Hf
关7兴
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Journal of The Electrochemical Society, 153 共8兲 F180-F187 共2006兲
F185
Table III. GPC [Hf/nm2 or % ML (monolayer)] for different extended ALD reaction cycles.
GPC RBS
共Hf/nm2兲
GPC RBS
共% ML兲
GPC SE
共% ML兲
1.40–1.50
15–16
15–20
Intermediate treatment after the H2O reaction
Remote N2O plasma at room temperature
5 min cooling in moisture background
90 s cooling in moisture background
Annealing at 420°C in N2 + cooling in moisture background
Cooling in moisture background + 5 min pre-stabilization at 300°C
4.10
3.21
2.89
2.95
2.77
42
34
32
32
30
49
40
35
35
37
Intermediate treatments after the HfCl4 reaction
Room temperature plasma treatment
5 min cooling
2 min 420°C + 2 min cooling
2.36
—
—
30
—
—
29
17
15
Special types of reaction cycles
HfCl4 pulse, purge, 4 min cooling, purge 共noH2O pulses in ALD reactor兲
0.85
9
12
Standard process
A second possibility is Lewis electron donation of H2O to coordinate unsaturated Hf cations at the surface23
Hf共␦+兲 + H2O共g兲 → Hf␦+ − OH2
关8兴
These H-bonded 共Eq. 6 and 7兲 or Lewis-bonded 共Eq. 8兲 intermediates at low temperature could assist the decomposition of H2O on
the HfO2 surface, 共Eq. 5兲 resulting in additional –OH terminations
and enhancing the GPC.
The GPC is also enhanced for intermediate anneals at 420 and
500°C, followed by intermediate cooling 共Table III兲. Probably, the
OH density on HfO2 decreases by thermal treatment. However, the
HfO2 surface easily readsorbs moisture during cooling after annealing. Intermediate annealing without cooling afterward is not possible with our current hardware because the wafer cools during the
transport from anneal 共EPSILON兲 to ALD reactor, but annealing
without cooling might show growth inhibition due to lower OH
density.
Intermediate plasma treatments at room temperature cause a
larger enhancement of the GPC as compared to cooling alone. The
GPC increases from 1.4 to 4.1 Hf/nm2 for plasma treatments after
the H2O reaction. Plasma treatments are the only treatments that
also enhance the GPC when applied after the HfCl4 reaction. We
propose that the plasma makes the HfO2 top surface more susceptible to H2O adsorption. The N2O plasma is a rich source of atomic
oxygen28,29 that could eliminate residual impurities at the HfO2
surface.30 It has been shown previously that the HfO2 growth behavior was improved by plasma-surface preparation.31 The plasma
treatments improved the GPC in the first reaction cycles as compared to untreated HF-cleaned Si substrates.
Analysis of the OH density.— The growth enhancement is further
analyzed by a model for GPC considering steric hindrance of metal
ligands.32 The maximum number of Cl ligands remaining on a flat
surface when steric hindrance prevails 共nCl共max兲兲 can be estimated
from the radius of Cl and assuming a hexagonal close packing.32
From the ionic radius of Cl 共in contrast to Ref. 32, we have used the
ionic radius of Cl instead of the van der Waals radius, because of the
ionic nature of the Hf–Cl bond兲, nCl共max兲 is calculated as
8.8 Cl/nm2. The OH density on the substrate is extracted from the
mass-balance equation of Reaction 1
nOH = 4 ⫻ GPC − nCl共max兲
关9兴
2
In this equation, the GPC is expressed in Hf/nm . The OH density of
the substrate is equal to the number of Cl that have reacted with OH,
and this is estimated as the difference between the total number of
Cl associated with Hf 共4 ⫻ GPC兲 and the maximum Cl content
according to steric hindrance. The GPC model gives an underestimate of the OH density, as the maximum amount of Cl is most likely
slightly lower than that estimated from a hexagonal packing because
of the specific Hf–Cl bond arrangements at the surface.
First, the model illustrates that the observed values for enhanced
GPC do not exceed the maximum allowed by steric hindrance, characteristic for ALD. Second, the model roughly estimates the OH
density in case of growth enhancement with intermediate treatments,
where the steric hindrance limit is reached 共Table IV兲. For intermediate plasma treatments, the calculated OH density is 7 nm2, while
intermediate cooling results in a lower OH density of 3–4 nm2. For
conventional ALD, the GPC model cannot predict the OH density of
HfO2 as the limit of steric hindrance is not reached.
EOT downscaling potential of ALD HfO2 deposited with enhanced GPC.— The scaling potential with conventional and enhanced GPC is analyzed by two methods. First, the layer closure is
analyzed by TOF-SIMS surface measurements. Next, the EOT
downscaling potential is studied by means of electrical analysis of
capacitors.
TOF-SIMS.— Static TOF-SIMS uses very low primary-ion doses,
which limits the analysis depth to the first two or three outermost
atomic layers. Therefore, static TOF-SIMS can be used to monitor
the composition of the top surface of the HfO2 sample. In order to
Table IV. OH density on HfO2 for different intermediate treatments after the H2O ALD half reaction obtained using Eq. 9.
Intermediate treatment after the H2O reaction
Figure 9. Cross-sectional TEM of 30 cycles HfO2 deposited with intermediate N2O plasma after the H2O reaction every 5 cycles on 1 nm chemical
oxide.
Remote N2O plasma at room temperature
5 min cooling in moisture background
90 s cooling in moisture background
Annealing at 420°C in N2 + cooling in
moisture background
nOH 共/nm2兲
7
3
4
3
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F186
Journal of The Electrochemical Society, 153 共8兲 F180-F187 共2006兲
Figure 10. TOF-SIMS Si intensity as a function of the RBS Hf-coverage for
HfO2 deposited with intermediate plasma treatments after every H2O pulse
and conventional ALD on chemical oxide substrates. The Si intensity decays
faster if intermediate plasma treatments are used as compared to conventional ALD, illustrating slightly faster layer closure.
analyze the closure of the HfO2 layer on the Si substrate, we observe
how fast the TOF-SIMS Si substrate intensity decreases as a function of the amount of HfO2 deposited, as measured by RBS 共Fig.
10兲. For a similar amount of HfO2 deposited, less Si is visible with
TOF-SIMS for HfO2 layers that are covering the Si substrate
better.4,25 We systematically observe a lower Si intensity if intermediate plasma treatments are used 共Fig. 10兲, as compared to conventional HfO2 growth. This indicates that the enhanced GPC indeed
results in a slightly faster HfO2 closure and a better HfO2 quality.
Capacitor results.— Intermediate cooling with a growth enhancement to 3.0 Hf/nm2 is most promising for EOT downscaling. The
lowest EOT 共0.77 nm兲 is observed with intermediate cooling 共Fig.
11兲, indicating the good quality of HfO2. Intermediate cooling does
not affect the interfacial oxide thickness 共Fig. 5兲. The k-value of
HfO2 is not significantly affected by the growth enhancement.
Within the measurement error, the leakage current is comparable to
that of conventional ALD. VFB and hysteresis are not much affected
by intermediate cooling 共Table I兲.
Although a larger growth enhancement 共to 4.1 Hf/nm2兲 was observed with intermediate plasma treatments, this process is less
promising for sub-nm EOT applications. The reason is regrowth of
the interfacial oxide 共SiO2兲 during the plasma treatments. The thickness increase of the scaled chemical oxide is about 1.5 Å 共Fig. 5兲.
Interfacial oxide growth is also observed in the EOT-leakage curve,
where films with the same leakage current as standard HfO2 are
obtained at higher EOT 共Fig. 11兲. Despite the thicker interfacial
layer, an EOT as low as 0.9 nm is obtained, illustrating the good
quality of HfO2 deposited with enhanced GPC, in agreement with
TOF-SIMS.
In spite of the very low Cl content, scaling below 1 nm EOT
could not be demonstrated with intermediate anneals. The leakage
current was too high. Thus, applying a thermal budget higher than
300°C during HfO2 deposition severely degrades its scaling potential.
Conclusions
The HfO2 ALD was optimized to improve the electrical performance in the sub-1 nm EOT range. Sufficient hydroxylation of the
HfO2 surface is one important factor controlling both growth behavior 共HfCl4 nucleation兲 and electrical properties. The H2O reaction is
not saturated for pulse times of a few tenths of a second, as currently
applied in ALD. H2O pulse times of at least 10 s must be applied to
achieve saturation in GPC and Cl content. Using saturated H2O
pulses decreases the gate leakage current in the sub-1 nm EOT
Figure 11. Leakage current as a function of EOT for HfO2 deposited with
intermediate cooling and plasma after every H2O reaction, as compared to
conventional ALD with standard H2O pulses at 300°C. Trend lines are drawn
as a guide to the eye.
range. The present study did not explore the cause of the slow saturation of the H2O reaction. If the slow saturation is limited by the
H2O supply, more manufacturable options such as higher H2O vapor
pressure sources may be envisaged.
Increased hydroxylation of the HfO2 surface is achieved by intermediate treatments at low temperature after the H2O reaction.
With intermediate cooling, the GPC is enhanced by a factor of 2,
and good electrical properties are observed in the sub-1 nm EOT
range. Intermediate remote-plasma treatments give the largest
growth enhancement, but interfacial oxide regrowth during the
plasma treatments makes it unsuitable for aggressive 共sub-1 nm兲
EOT scaling.
Applying a thermal budget higher than 300°C during HfO2
deposition by either increased deposition temperature or intermediate anneals severely degrades the HfO2 scaling potential. For 370°C
depositions, the higher leakage in the sub-1 nm EOT range could be
related to poor initial growth of HfO2 on scaled chemical oxide
substrates. In an extended reaction cycle, we combine the efficient
reduction of Cl impurities at high temperature with GPC enhancement at low temperature. However, scaling to 1 nm EOT could not
be achieved with this process, indicating that factors other than GPC
and Cl content are also important to achieve sub-1 nm EOT scaling.
Acknowledgments
Riikka L. Puurunen 共VTT Finland兲, Jan-Willem Maes, Hilde De
Witte, Johan Swerts, and Yanina Fedorenko 共ASM-Belgium兲 are
kindly acknowledged for many scientific discussions. We acknowledge Olivier Richard and Danielle Vanhaeren 共IMEC兲 for crosssectional TEM and AFM analyses, respectively.
IMEC assisted in meeting the publication costs of this article.
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