Electrical property improvements of high- k gate oxide by in situ nitrogen incorporation during atomic layer deposition

Article abstract of DOI:10.1063/1.2472189

Atomic layer deposition (ALD) process for oxynitrides of high- k gate dielectrics employing N H4 OH as a single source for reactants, water and N H3, was studied. By this method, nitrogen was incorporated up to 1-3 at. % for ALD Al2 O3 and Ta2 O5 films from metal organic precursors. A comparative study with water based ALD showed that the electrical properties were improved. The leakage current of oxide films from N H4 OH based ALD had been reduced and, more importantly, the dielectric strength was found to be enhanced by more than two orders of magnitude from a time dependent dielectric breakdown measurement.

Full text of DOI:10.1063/1.2472189

Electrical property improvements of high- k gate oxide by in situ nitrogen incorporation
during atomic layer deposition
W. J. Maeng, S. J. Lim, Soon-Ju Kwon, and H. Kim
Citation: Applied Physics Letters 90, 062909 (2007); doi: 10.1063/1.2472189
View online: http://dx.doi.org/10.1063/1.2472189
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APPLIED PHYSICS LETTERS 90, 062909 ͑2007͒
Electrical property improvements of high-k gate oxide by in situ nitrogen
incorporation during atomic layer deposition
W. J. Maeng, S. J. Lim, Soon-Ju Kwon, and H. Kima͒
Department of Materials Science and Engineering, Pohang University of Science and Technology
(POSTECH), San 31, Hyoja-Dong, Nam-Gu, Pohang 790-784, Republic of Korea
͑Received 1 November 2006; accepted 10 January 2007; published online 9 February 2007͒
Atomic layer deposition ͑ALD͒ process for oxynitrides of high-k gate dielectrics employing
NH4OH as a single source for reactants, water and NH3, was studied. By this method, nitrogen was
incorporated up to 1–3 at. % for ALD Al2O3 and Ta2O5 films from metal organic precursors. A
comparative study with water based ALD showed that the electrical properties were improved. The
leakage current of oxide films from NH4OH based ALD had been reduced and, more importantly,
the dielectric strength was found to be enhanced by more than two orders of magnitude from a time
dependent dielectric breakdown measurement. © 2007 American Institute of Physics.
͓DOI: 10.1063/1.2472189͔
For nanoscale high performance and low power devices,
small leakage current and good reliability of gate oxides are
essential. Regarding these, the incorporation of nitrogen has
been considered as a way to eliminate a number of concerns,
since the incorporated nitrogen atoms reduce the generation
of defects, prohibit boron penetration,1 and passivate oxygen
vacancy states.2 Additionally, nitrogen incorporation can im-
prove structural stability and can increase dielectric constant
of gate oxide.1 While direct thermal oxidation using nitrogen
containing oxidants such as N2O or NO gas has been widely
used for the formation of Si oxynitride, thermal or plasma
annealing using NH3 or N2O gas before or after gate oxide
film deposition has been employed for high-k gate oxides.3,4
Previous reports have shown that about 1%–2% of nitrogen
composition in ZrO2 and Ta2O5 is reported to improve de-
vice properties with high mobility and good reliability.5,6
While high nitrogen concentration at a poly-Si gate-oxide
interface is beneficial by preventing dopant diffusion, too
much incorporation of nitrogen at the interface between the
channel region and the gate oxide can induce a large flatband
voltage shift caused by an interface positive charge and can
degrade channel mobility. Thus, the ability to control the
nitrogen profile with atomic layer accuracy can take a sig-
nificant impact on the improvements of the gate oxide prop-
erty for future nanoscale complementary metal-oxide semi-
conductor ͑CMOS͒ devices.1
Atomic layer deposition ͑ALD͒ is one of the most prom-
ising techniques for high-k gate oxides. Since the thickness
and composition of the thin films can be controlled with
atomic accuracy by ALD, the in situ nitrogen incorporation
during ALD can be a promising technique that can lead to an
atomically modulated nitrogen profile. Although in situ dop-
ing of nitrogen into high-k gate oxides has been reported for
chemical vapor deposition ͑CVD͒ using nitrogen containing
reactants such as N2O ͑Ref. 7͒ and NO ͑Ref. 8͒ plasmas, in
situ nitrogen incorporation during ALD has rarely been stud-
ied. Only a brief report on nitrogen addition to oxygen
plasma for plasma enhanced ALD of ZrO2 and Al2O3 is
found, without a detailed study on electrical properties.9
In this report, we investigated the in situ nitrogen incor-
poration during ALD of high-k gate oxides. For nitrogen in-
corporation during ALD, we developed a simple ALD pro-
cess using NH4OH as a source for reactants, which produces
a mixture of vapors of water and NH3.10 Considering their
practical importance as capacitor dielectric materials in dy-
namic random access memory and as gate dielectrics for
nanoscale CMOS devices, the film properties of the common
water based ALD Ta2O5 and NH4OH based ALD TaOxNy
were comparatively studied focusing on the electrical prop-
erties of gate stack including interface trap density, leakage
current, and dielectric breakdown. To demonstrate the uni-
versality of the process, another representative ALD high-k
oxide, Al2O3, was also investigated using the same process.
The results have shown that in situ nitridation is feasible
during ALD by using NH4OH as a single source for reactants
and the electrical properties of high-k oxides are improved
significantly without seperate nitridation.
A homemade ALD system was used using pentakis͑dim-
ethylamino͒tantalum ͑PDMAT͒ and trimethyl aluminum
͑TMA͒ as Ta and Al precursors, respectively. The water and
NH4OH ͑ammonium hydroxide, 29% diluted in water͒ were
contained in silicone oil to maintain a constant temperature,
and the flow rate of vapor was controlled by a leak valve. Ar
was used as a carrier and purging gas, and the flow was
controlled by a mass flow controller upstream to the main
chamber. The PDMAT ͑powder͒ was contained in a bubbler
and its temperature was kept at 65 °C to generate high
enough vapor pressure, while TMA was used in room tem-
perature without carrier gas due to its high vapor pressure. A
typical Ta2O5 or TaOxNy ALD sequence was composed of
PDMAT with a reactant exposure time of 1.5 s and a purging
time of 4 s between those. The growth temperature was
250 °C. Similar process parameters were employed for
Al2O3 ALD. The stoichiometry and impurity content in the
films were analyzed by x-ray photoemission spectroscopy
͑XPS͒ and secondary ion mass spectroscopy ͑SIMS͒. The
microstructures of the films were analyzed by x-ray diffrac-
tion ͑XRD͒ and transmission electron microscopy ͑TEM͒.
For electrical measurements, Ta2O5 thin films were depos-
ited on p-type ͑boron, 1018/cm3͒ Si͑001͒ substrates, which
were cleaned by the RCA method following dipping in a
buffered oxide etch solution for 1 min to remove native ox-
a͒
Electronic mail: hyungjun@postech.ac.kr
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0003-6951/2007/90͑6͒/062909/3/$23.00 90, 062909-1 © 2007 American Institute of Physics
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062909-2
Maeng et al.
Appl. Phys. Lett. 90, 062909 ͑2007͒
FIG. 2. TEM image of as-deposited ͑a͒ ALD Ta2O5 and ͑b͒ ALD TaOxNy
films using water and NH4OH as a single source for reactants, respectively.
nomenon is SiNx formation at the interface.13 Although these
results show the nitrogen incorporation from NH4OH, the
experimental results of nitrogen incorporation might be am-
biguous since TDMAT contains N in its ligands. To clarify
this point, we have carried out the Al2O3 ALD process from
TMA, which does not have nitrogen atoms in it, and NH4OH
at Ts=150 °C. By XPS, the nitrogen incorporation up to
3.5% was observed, indicating that the nitrogen incorpora-
tion by using NH4OH is applicable to other high-k ALD.
Previously, it has been reported that NH4OH produces va-
pors composed of water and NH3 molecules, whose ratio is
dependent on NH3 concentration and temperature.10 Thus,
we can infer that the nitrogen incorporation in the current
study is from the NH3 vapor produced by the NH4OH solu-
tion. For quantification of ammonia verus water flow from
the NH4OH solution, separate experiments were carried out
based on NH3 concentration measurements with exposure
time, and the generation rate of NH3 vapors was determined
to be about 1.8 times larger than that of water vapors for our
experimental conditions.
Cross sectional high resolution TEM images of as-
deposited ALD Ta2O5 and ALD TaOxNy thin films are
shown in Figs. 2͑a͒ and 2͑b͒, respectively. The TEM images
show a clean and smooth interface with an amorphous struc-
ture for both cases. Although both of the films show the
interface layer formation, the thickness of the interlayer for
the ALD TaOxNy film ͑1.3 nm͒ is thinner than that of ALD
Ta2O5 ͑1.5 nm͒. These results indicate that the nitrogen in-
corporation can reduce the formation of an interfacial layer.
In a previous report on CVD HfO2 gate oxide, the activation
energy of interlayer formation during air exposure was
higher for a film deposited by NO than O2, attributed to the
reduced oxygen diffusion by higher nitrogen content.8 The
thinner interlayer by nitrogen incorporation can lead to a
smaller equivalent oxide thickness, which is beneficial to de-
vice scaling.
FIG. 1. SIMS profiles of as-deposited ͑a͒ ALD Ta2O5 and ͑b͒ ALD TaOxNy
films using water and NH4OH as a single source for reactants, respectively.
ide. After deposition, a metal-oxide-semiconductor ͑MOS͒
capacitor structure was fabricated by patterning a 50 nm
thick evaporated Al film through photolithography and dry
etching using BCl3. Capacitance-voltage and current-voltage
characteristics were determined using a Keithley 4200 semi-
conductor analyzer with a HP4284 LCR meter.
Growth characteristics of thermal Ta2O5 using PDMAT
were discussed in detail in our previous report.11 The growth
characteristics of TaOxNy using PDMAT and NH4OH were
similar to those of thermal ALD using water. The growth rate
was 0.8 Å/cycle at saturation conditions of exposure times
longer than 1.5 s, which is almost the same as that of Ta2O5
ALD using water. Also, the microstructure of the as-
deposited film was determined to be amorphous by XRD,
which is same as ALD Ta2O5 using water. The film compo-
sition was initially analyzed by XPS. A small nitrogen peak
was observed for ALD TaOxNy, and the concentration was
determined to be about 1.2% while no nitrogen peak was
observed for ALD Ta2O5. But since the exact comparison of
nitrogen concentration in each sample was difficult by XPS
due to the similar locations of N 1s and Ta 4p, a SIMS
measurement was performed for better sensitivity of the ni-
trogen content. SIMS depth profiles of the as-deposited water
and ALD TaOxNy film are shown in Figs. 1͑a͒ and 1͑b͒,
respectively. Figure 1͑a͒ shows a small nitrogen composition
even for ALD Ta2O5 in spite of the use of water as a reac-
tant, which is probably caused by nitrogen incorporation
from an alkylamide precursor used in this study. However,
for ALD TaOxNy, at least five times more nitrogen incorpo-
ration was observed. Meanwhile, the contents of other pos-
sible contaminants, such as hydrogen and oxygen, were simi-
lar. After postdeposition annealing ͑PDA͒ at 700 °C for
1 min in O2 ambient, the nitrogen contents in the bulk of
both films were slightly decreased to about 60%–70% level,
with relatively small change at the Si-dielectric interface
͑data not shown͒. Higher nitrogen content at the TaOxNy –Si
interface is a common observation for nitrided high k made
Capacitance-voltage measurements were carried out for
a postdeposition annealed p-type Si/ALD Ta2O5 ͑10 nm͒/Al
MOS capacitor ͑Fig. 3͒. Also, additional forming gas anneal-
ing was performed at 400 °C for 30 min in 5% H2–95% N2
to reduce the interface trap charge. While both ALD TaOxNy
and Ta2O5 films have low hysteresis ͑Ͻ0.05 V͒, the latter
was found to have a relatively thinner equivalent oxide
thickness ͑4 nm͒ than the former ͑4.3 nm͒. The increase of
dielectric constant with nitrogen incorporation is a general
observation, which can be explained by reduced interface
layer formation and higher dielectric constant of the
film itself.13 The interface state density ͑Dit͒ was measured
by conductance method at various frequencies,
from 1 kHz to 1 MHz. The midgap Dit values were mea-
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by other methods.12 One possible explanation of this phe-
sured to be 1.8ϫ1012 cm−2 eV−1 for ALD Ta2O5 and
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062909-3
Maeng et al.
Appl. Phys. Lett. 90, 062909 ͑2007͒
FIG. 5. Time dependent dielectric breakdown data for MOS capacitors with
10 nm ALD Ta2O5 ͑solid line͒ and TaOxNy ͑dotted line͒ as insulators. The
voltage stress measurement was done at an applied voltage of 6.5 MV/cm.
FIG. 3. Capacitance-voltage curves of MOS capacitors with 10 nm ALD
Ta2O5 and TaOxNy as insulators.
1.4ϫ1011 cm−2 eV−1 for ALD TaOxNy. Thus, Dit near the
midgap is almost an order of magnitude lower for ALD
TaOxNy, indicating the significant improvement by in situ
nitridation.
almost comparable to other previous reports by employing a
separate nitridation process. For example, tbd of 4 nm ZrO2
has been increased from 8 to 1000 s ͑Ref. 5͒ and 1000 times
improvement was reported for 16 nm thick CVD Ta2O5 by
NH3 PDA.5,16
Thus, by in situ nitridation using NH4OH as a single
source for reactants, considerable improvements in electrical
properties can be made without a separate nitridation pro-
cess. Moreover, by using this process, the nitrogen profile
control is feasible, since ALD itself is beneficiated by atomic
level control in thickness and composition. This would give
additional degrees of freedom in reliablity control of gate
oxide in future nanoscale device fabrication.
Figure 4 shows current-density–voltage measurement re-
sults of the same MOS capacitors. The leakage current den-
sity at 1 MV/cm was 7.6ϫ10−6 A/cm2 for ALD Ta2O5 and
4ϫ10−7 A/cm2 for ALD TaOxNy. Thus, by using NH4OH
as a single source for reactants, the leakage current is signifi-
cantly reduced. The leakage current reduction by nitrogen
incorporation has been commonly observed in previous re-
ports. For example, two orders of magnitude reduction in
leakage current density was reported by high density plasma
N2O annealing.14 One of the plausible explanations for this
phenomenon is that nitrogen passivates oxygen vacancies,
which serve as an electron leakage path.15 More noticeable
improvement in electrical properties by using NH4OH is the
improvement in dielectric breakdown strength. Figure 4
shows that the dielectric breakdown occurs at 20 MV/cm for
the ALD TaOxNy case, which is significantly higher than at
10 MV/cm for ALD Ta2O5.
This work was supported by Korea Research Foundation
grants funded by the Korean government ͑MOEHRD, Grant
No. KRF-2005-003-D00144; Basic Research Promotion
Fund, Grant No. KRF-2006-311-D00114͒.
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