Fabrication and characterization of ALD-grown ZrO2:Ge thin films on Si(100) using CpZr(NMe2)3 and (NMe2)2Ge(ipr2en) precursors with ozone

Full text of DOI:10.1002/bkcs.10406

Note
DOI: 10.1002/bkcs.10406
BULLETIN OF THE
J.-S. Jung et al.
KOREAN CHEMICAL SOCIETY
Fabrication and Characterization of ALD-grown ZrO :Ge Thin Films on
2
i
Si(1 0 0) using CpZr(NMe ) and (NMe ) Ge( pr en) Precursors
2
3
2 2
2
with Ozone
Jae-Sun Jung,† Dae-Hyun Kim, Jin-Ho Shin, Jung-Soo Kang, Joseph P. Thomas,
,‡
‡
‡
§
§
§
†,
*
K. T. Leung, and Jun-Gill Kang
†Department of Chemistry, Chungnam National University, Daejeon 305-764, Korea.
*
E-mail: jgkang@cnu.ac.kr
‡
Soulbrain Sigma-Aldrich Co., Ltd, Gongju-si 314-240, Korea
§
WATLab and Department of Chemistry, University of Waterloo, Waterloo, Canada
Received April 1, 2015, Accepted April 25, 2015, Published online July 28, 2015
Keywords: Atomic layer deposition, Ge-doped ZrO thin film, Growth rate
2
The continuous scaling of complementary metal-oxide semi-
conductor (CMOS) device dimensions to a few nanometers
thick has led to the use of high-dielectric materials. The tradi-
independent plateau in its growth rate, remaining at 0.40 Å/
cycle in the range 200–320 C. Here we describe an ALD
ꢀ
growth process for Ge(IV)-doped ZrO ultra-thin films using
2
i
tional SiO layer in the application to the nanosized configura-
CpZr(NMe ) and (NMe ) Ge( pr en) with ozone. We inves-
2
2 3
2 2
2
tions faces a problem, such as the leak currents from electron
tunneling through the dielectrics due to its low dielectric con-
tigated the relationships between film thickness, stoichiome-
try of the precursor mixture, and chemical composition,
with the aim of controlling Ge doping and achieving high-
1–3
stant with k = 3.9.
material because of its large dielectric constant (k > 20),
ZrO2 is a particularly attractive
4,5
1,6
quality ZrO :Ge thin films on Si(1 0 0).
2
7
wide band gap (5.8–7.8 eV), and high thermal stability with
Three types of ZrO :Ge/ZrO multilayer were fabricated on
2
2
8
Si. ZrO has already been used in dynamic random access
Si(1 0 0) substrates by controlling precursor ratios. Both ALD
conditions and the method of injection of the different precur-
sors significantly affected the quality of the resulting films.
2
memory (DRAM); however, incorporation of ZrO2 into
next-generation DRAM devices remains a challenge. Zirconia
exists in three polymorphs: monoclinic, tetragonal, and cubic,
as well as the amorphous state. The dielectric constants of the
monoclinic, tetragonal, and cubic phases are 20, 47, and
Table 1 lists the Ge-doped ZrO ALD growth conditions with
2
i
the CpZr(NMe ) and [(NMe ) Ge ( pr en)] precursors. Two
2
3
2 2
2
basic subcycles were designed for separately feeding pure Zr
precursor (A) and a mixture of Zr and Ge precursors (B), as
shown in Figure 1. Each subcycle corresponded to a pulse
sequence, as listed in Table 1; hereafter, pulses for feeding
A and B are referred to as cycles A and B, respectively. First,
we fabricated the multilayered thin films on Si wafers via a
sequence of five consecutive A-cycles, followed by one B-
cycle (creating film F1), and the total number of cycles was
198. Two further sets of multilayered thin films were fabri-
cated using the following sequences: three consecutive A-
cycles followed by one B-cycle (F2) and one A-cycle, fol-
lowed by one B-cycle (F3); for all of these, the total number
of cycles was 200. The molar ratios of Ge to Zr were 0.09,
0.14, and 0.33 for the F1, F2, and F3 films, respectively.
The thickness of each film was determined using ellipsome-
try (MG-1000; Nano-View, Seoul, Korea). The film F1 was
213.8 Å thick, and the growth rate was 1.08 Å/cycle. The
observed growth rate of the Ge-doped thin films was equal
9–11
37, respectively.
The monoclinic phase is the most stable,
however, and is in equilibrium at room temperature, whereas
the tetragonal and cubic phases can be stabilized at tempera-
tures greater than 1400 and 2570 K, respectively. For ultra-
thin films, however, the amorphous state is stable up to
ꢀ
50 C. Recently, doping ZrO with Ge has been shown
2
12
7
13–16
to increase k,
which is attributed to favorable formation
of the metastable phase. Atomic layer deposition (ALD) has
been shown to be an excellent method for deposition of
high-quality ultra-thin films for semiconductor device appli-
17
cations; however, few studies have reported the properties
of ALD-grown GeO , and to date, Ge-doped ZrO thin
2
2
films have been formed using molecular beam epitaxy at
high temperatures. Germanium ALD growth precursors
include 1,2-bis[(2,6-diisopropylphenyl)imino] acenaphthene
germanium(II), germanium(IV) ethoxide, and tetrakis-
(dimethylamino) germanium(IV), which are expensive and
limited in supply. We have previously reported an ALD
growth process for ZrO and GeO ultra-thin films using
to that of un-doped ZrO thin films grown under identical
2
ALD conditions. As the fraction of B-cycles increased, the
thickness of the films decreased (the total number of cycles
remained approximately constant at ~200). The F2 film was
208 Å thick and grew at a rate of 1.04 Å/cycle, and the F3 film
was 187 Å thick with a growth rate of 0.935 Å/cycle. Given
2
2
cyclopentadienyl-type Zr(IV) and ethylenediamine-type
0
Ge(IV) precursors, respectively. Bis(dimethylamino)(N,N -
di-isopropyl-ethylenediamine) germanium [(NMe ) Ge
2
2
i
(
pr en)] with ozone exhibits a distinct temperature-
2
Bull. Korean Chem. Soc. 2015, Vol. 36, 2162–2165
© 2015 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Wiley Online Library
2162

BULLETIN OF THE
Note
KOREAN CHEMICAL SOCIETY
that the growth rate of a pure GeO thin film grown underiden-
signal. These results indicated that the Ge dopants were homo-
geneously distributed throughout the ALD-grown thin film.
Figure 4 shows the relative Ge content in the films, which
was consistent with the molar ratios of the precursors listed
in Table 2. These results indicated that the concentration of
the Ge dopant was proportional to the molar ratio of the
precursor.
2
tical conditions was 0.4 Å/cycle, these results indicated that
Ge concentration increased as the fraction of B-cycles
increased.
The surface roughness and film morphology were analyzed
using atomic force microscopy (AFM) using a NanoScope V
Scanning Probe Microscope (Bruker, Santa Barbara, CA,
USA). As shown in Figure 2, the F1 film was relatively
X-ray photoelectron spectroscopy (XPS) data for the three
+
smooth, with a root-mean-square (rms) roughness of R =
filmswereobtainedfollowing Ar ionsputteringfor10 susing
q
0
.59 nm. Increasing Ge content resulted in an increase in the
a MultiLab 2000 Thermo Scientific (Waltham, MA, USA)
with a monochromatic Al Kα source (1486.6 eV). As shown
in Figure 5, peaks corresponding to Zr 3d (170–188 eV) and
O 1 s (~530 eV) can be seen, as well as weaker peaks corre-
sponding to Ge 3d (~30 eV), 3p (129–122 eV), and 2p
(~1220 eV). Characteristic Si 2s (152 eV) and 2p (100 eV)
peaks, as well as C 1s (285 eV) and N 1s (402 eV) peaks were
not observed, or appeared only as trace features. The highly
resolved spectrum of Ge 3p, scanned with a resolution of
0.05 eV, displayed two bands with peaks at 124.1 and
surface roughness: R of 0.65 and 1.10 nm for F2 and F3,
q
respectively. The greater surface roughness of the F3 film
may be due to the greater mismatch in the growth rates of
the Zr and Ge precursors. Even with the F3 film, however,
Rq was less than 6% of the film thickness; all films may thus
be considered to have a smooth morphology.
3/2
18
The spatially resolved chemical composition of the ZrO2:
Ge/ZrO multilayer structure was characterized using time-
2
of-flight secondary ion mass spectroscopy (TOF-SIMS,
ION-TOF GmbH, Münster, Germany). Note that the intensi-
ties of carbon and nitrogen were lower than the detection limit.
Figure3showsdepthprofilesofSi, Zr, andGeinthethinfilms.
The Sisignal did notexceedthe background until 90–120 min,
followed by a rapid increase. The region where the intensities
of Zr and Ge ions were maintained at a near-constant level can
be clearly distinguished prior to the appearance of the Si
(a)
Table 1. ALD growth conditions for a subcycle injection.
Growth conditions
ꢀ
Evaporation temperature
Carrier gas (sccm)
105 C
Ar (80)
(b)
Reactant gas (sccm)
Substrate temperature
Working pressure
O
300 C
1 Torr
3
(100)
ꢀ
Pulse sequence (precursor/purge/O
Number of cycle
3
/purge)
5/10/5/10 s
198–200
(c)
Figure 1. Schematic diagram for injection of two different precursor-
compositions—A: a canister containing a pure Zr precursor; B: a can-
ister containing an equivalent mixture of Zr and Ge precusors, S; Si
substrate.
Figure 2. AFM images of Ge-doped ZrO
via the ALD process: (a) F1, (b) F2, and (c) F3.
2
thin film surfaces grown
Bull. Korean Chem. Soc. 2015, Vol. 36, 2162–2165
© 2015 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.bkcs.wiley-vch.de 2163

BULLETIN OF THE
Note
KOREAN CHEMICAL SOCIETY
(a)
(b)
Figure 4. Depth profiles of Geof the preparedfilms (the baseline was
subtracted).
Table 2. Relative Ge concentrations of the F1, F2, and F3 thin films.
Film
Injection (mole ratio)
TOF-SIMS
XPS (3p)
F1
F2
F3
1 (0.09)
1.6 (0.14)
3.7 (0.33)
1
1.7
3.3
1
1.6
3.4
(
a)
(c)
(
b)
(c)
Figure 3. Depth profile performed on (a) F1,(b) F2, and (c) F3.
128.9 eV. The 124.1 eV peak appears as a well-defined line,
and can be attributed to Ge 3p3/2 (Figure 5(b)). We assign
the 128.9 eV peak (which appeared with moderate intensity)
to the 3p1/2 line of the ionized Ge dopant, which has not pre-
viouslybeenreported. Theapproximatecomposition oftheGe
atom was determined from the 3p peak area, since the uncer-
tainty in the peak area from 2p3/2 was too high arisen from its
low resolution (1 eV) scanning (Figure 5(c)). As listed in
Table 2, the Ge 3p peaks were well-matched with the experi-
mental relative concentration, although the strongest 2p3/2
peak did not exhibit the expected proportional behavior.
Figure5. XPSspectraofthe F1(black), F2 (red), andF3 (green)films
treated by Ar ion sputtering (a) for 10s, and bands of (b) Ge 3p
and (c) Ge 2p.
+
In conclusion, we have demonstrated high-quality Ge-
doped ZrO2 thin films via ALD using the precursors
i
CpZr(NMe ) and (NMe ) Ge( pr en). A dual-subcycle sys-
2 3
2 2
2
tem was used to separately feed the Zr precursor and a mixture
Bull. Korean Chem. Soc. 2015, Vol. 36, 2162–2165
© 2015 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.bkcs.wiley-vch.de 2164

BULLETIN OF THE
Note
KOREAN CHEMICAL SOCIETY
of the Zr and Ge precursors, which was crucial in growing
films without forming precipitates. The growth rate and dop-
ant concentration were characterized as a function of the num-
ber of subcycles of the mixed precursors. The growth rates
with Ge mole fractions of 0.09, 0.14, and 0.33 were 1.08,
reactant gases, respectively. The p-type Si substrates
(LG Siltron Inc., 2 × 2 cm) were washed using a mixture of
H SO and H O (at a volume ratio of 3:1) for 10 min, then
rinsed in deionized water. The substrate was then washed in
HF solution (1 vol%) for 1 min and then rinsed in deionized
water before loading into the reactor.
2
4
2
1.04, and 0.935 Å/cycle, respectively. These thin films had
a smooth surface morphology, and the carbon and nitrogen
impurity concentrations were smaller than the detection limit
of the TOF-SIMS system.
References
1
2
. G. D. Wilk, R. M. Wallace, J. M. Anthony, J. Appl. Phys. 2001,
89, 5243.
. J. L. Padilla, F. Gámiz, A. Godoy, Appl. Phys. Lett. 2013, 103,
Experimental
1
12105.
Synthesis. To synthesize CpZr(NMe ) , 4.21 mol of n-BuLi
dissolved in hexane was added to a 10-L flask containing 3.3 L
2
3
3
4
. D. Panda, T.-Y. Tseng, Thin Solid Films 2013, 531, 1.
. W. Zhang, Y. Cui, Z. G. Hu, W. L. Yu, J. Sun, N. Xu, Z. F. Ying,
J. D. Wu, Thin Solid Films 2012, 520, 6361.
. K. Kato, T. Saito, S. Shibayama, M. Sakashita, W. Takeuchi,
N. Taoka, O. Nakatsuka, S. Zaima, Thin Solid Films 2014,
of toluene, then cooled to a temperature in the range −20 to
ꢀ
−
15 C. HNMe (5.46 mol) was slowly bubbled and white
2
5
precipitate of LiNMe formed immediately. The mixture
2
ꢀ
was warmed to 60 C for 5 h. Following removal of excess
5
57, 192.
HNMe , the mixture was cooled to room temperature, and
2
6
7
. S. H. Jeong, I. S. Bae, Y. S. Shin, S.-B. Lee, H.-T. Kwak,
J.-H. Boo, Thin Solid Films 2005, 475, 354.
. J. Robertson, J. Vac. Sci. Technol. B 1785, 2000, 18.
ZrCl (1.0 mol) was slowly added to the stirred solution.
4
ꢀ
The reaction mixture was refluxed at 80 C for 4 h to form
the crude Zr(NMe ) . The addition of cyclopentadiene mono-
8. T. S. Jeon, J. M. White, D. L. Kwong, Appl. Phys. Lett. 2001,
78, 368.
2
4
ꢀ
mer to the crude Zr(NMe ) cooled at −15 to −10 C produced
2
4
CpZr(NMe ) . The solidwasremoved byfiltration and all vol-
9. X. Zhao, D. Vanderbilt, Phys. Rev. B 2002, 65, 075105.
0. S. K. Kim, C. D. Hwang, Electrochem. Solid-State Lett. 2008,
11, G9.
1. S. Sayan, N. V. Nguyen, J. Ehrstein, T. Emge,
E. Garfunkel, M. Croft, X. Zhao, D. Vanderbilt, I. Levin,
E. P. Gusev, H. Kim, P. J. McIntyre, Appl. Phys. Lett. 2005,
2
3
1
1
atile species were removed by evaporation. The product (61%
1
yield) was obtained by fractional vacuum distillation. H
NMR (C D ): δ 6.06 (s, 4H), 2.92 (s, 18H). The (NMe ) Ge
6
6
2 2
i
(
pr en) was synthesized in a 5-L three-neck flask filled with
2
2
.5 L of toluene, and 0.28 mol of GeCl was added and then
4
ꢀ
8
6, 152902.
ꢀ
cooled to a temperature in the range −20 C to −15 C. Follow-
ing the gradual addition of 0.25 mol of N,N -di(isopropyl)eth-
1
1
2. J. P. Chang, Y.-S. Lin, Appl. Phys. Lett. 2001, 79, 3666.
3. D. Tsoutsou, G. Apostolopoulos, S. F. Galata, P. Tsipas,
A. Sotiropoulos, G. Mavrou, Y. Panayiotatos, A. Dimoulas,
A. Lagoyannis, A. G. Karydas, V. Kantarelou,
S. Harissopoulos, J. Appl. Phys. 2009, 106, 024107.
4. D. Tsoutsou, G. Apostolopoulos, S. F. Galata, P. Tsipas,
A. Sotiropoulos, G. Mavrou, Y. Panayiotatos, Microelectron.
Eng. 2009, 86, 1626.
0
ylenediamine and 0.84 mol of N(C H ) , the mixture was
2
5 3
warmed to room temperature and stirred for 5 h to form
i
(
pr en)GeCl . The mixture was then cooled to between −20
2 2
ꢀ
1
and −10 C, 0.88 mol of Li(NMe ) was added, and the mixture
2
ꢀ
was refluxed at 110 C for 12 h to form the crude (NMe ) Ge
2
2
i
(
pr en). The solid was removed by filtration and all volatile
2
1
1
5. Y.-H. Wu, C.-C. Lin, L.-L. Chen, Y.-C. Hu, J. R. Wu, M.-L. Wu,
Appl. Phys. Lett. 2011, 98, 013506.
6. F. Boscherini, F. D'Acapito, S. F. Galata, D. Tsoutsou,
A. Dimoulas, Appl. Phys. Lett. 2011, 99, 121909.
species were removed by evaporation. The product (47.5%
yield) was obtained by fractional vacuum distillation.
Atomic LayerDeposition. Ge-doped ZrO films weredepos-
2
ited on Si(1 0 0) substrates using a showerhead-type hot-wall
ALD reactor (ASM Genitech, Inc., Seungnam, Korea). The
working pressure during deposition was 1 Torr; Table 1 lists
a summary of the ALD growth conditions. Argon
1
1
7. A. M. George, Chem. Rev. 2010, 110, 111.
8. A. V. Naumkin, A. Kraut-Vass, S. W. Gaarenstroom,
C. J. Powell, NIST X-ray Photoelectron Spectroscopy Data
V. 4.1, the Measurement Services Division of the National
Institute of Standards and Technology, 2012.
(>99.999%) and ozone (99.999%) were used as carrier and
Bull. Korean Chem. Soc. 2015, Vol. 36, 2162–2165
© 2015 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.bkcs.wiley-vch.de 2165