STUDY OF RF-SPUTTERED Ba(ZrTi1 -)O 3 THIN FILMS FOR ULSI DRAM
APPLICATION
JIAN-HUNG LEE, TUNG-SHENG CHEN, VENKATASUBRAMANI BALU, JEONG HAN,
RAZAK MOHAMMEDALI, SUNDAR GOPALAN, CHUN-HUI WONG, AND JACK C. LEE
Microelectronics Research Center, Mail Code R9950, The University of Texas at Austin, Austin,
TX 78712
ABSTRACT
Barium zirconate-titanate (Ba(ZrTi 1 •,)03, BZT) films with thickness around 60 nm were
deposited on Ir substrates using RF magnetron sputtering. The effect of zirconium atomic
fraction (x = 0.14 to 0.7), substrate temperature (380 'C to 550 'C) and oxygen partial pressure
(0 to 5 mTorr with total pressure 30 mTorr) on leakage current, dielectric constant and dielectric
dispersion (capacitance reduction with increasing frequency) was studied. We found that the
Zr/Ti ratio played a crucial role in determining the dielectric constant and dispersion. The
dielectric constant varies from 26 to 168 while dispersion ranges from 0.80 to 2.58 % loss in
capacitance (dielectric constant) per decade of frequency. Low leakage currents ( <lx 10-7
A/cm 2) were observed.
INTRODUCTION
High-permittivity materials, such as barium strontium titanate (BST) have been
extensively studied for future memory application [1-3]. One of the major challenges with BST
is high dielectric dispersion, which leads to loss in stored charge under DRAM operating
conditions.
Ba(ZrTilx)0 3 has been investigated as an alternative material for DRAM applications
with x = 0 to 0.4. The introduction of Zr" is expected to suppress electron hopping between Ti"
and Ti" since Zr" ions are more stable than Ti". This may improve the electrical characteristics
and device reliability. BZT films of thickness greater than 160 nm have been studied [4, 5]. High
dielectric constant (T. B. Wu et al reported a dielectric constant of 300 at x = 0.12 and film
thickness -180 nm) and low leakage currents (lxl0"9 A/cm 2 @1 V) were reported. In this work,
we scaled down the thickness of BZT films to 60 nm to study the feasibility of using BZT for
ULSI DRAM storage capacitors.
EXPERIMENT
The Ba(Zr.Ti1 ,)O3 thin films studied in this paper were prepared by RF-sputtering 3-inch
ceramic targets with three Zr/Ti ratios. The stoichiometry of the films, determined using
Rutherford Backscattering Spectroscopy (RBS) were: x = 0.14, 0.6, and 0.7. Test capacitors
with a metal-insulator-metal structure were fabricated on SiO 2 (400 nm)/Si substrates. DCsputtering was adopted to deposit Ir bottom electrodes (200 nm). RF-sputtered Ba(ZrTi1 x,)03
thin films (60 nm) of the three compositions mentioned above were then deposited at various
temperatures (380 'C, 460 'C, and 550 'C) and 0 2/Ar pressure ratios (0, 1/9 and 1/5), with a total
pressure of 30 mTorr. The Pt top electrodes (200 nm) were deposited through a shadow mask
using DC-sputtering. The area of the devices was 1.17x10 3 cm 2 .
The thicknesses of the BZT films were determined by cross-section scanning electron
microscopy (SEM). The crystallographic orientations were identified by X-ray diffraction
(XRD). The current-voltage characteristics were measured with a HP 4156A semiconductor
parameter analyzer, and the capacitance-frequency behavior in the frequency range of 100 Hz to
1 MHz was determined using a HP 4194A impedance analyzer.
47
Mat. Res. Soc. Symp. Proc. Vol. 541 0 1999 Materials Research Society
RESULTS
Zr/Ti ratio effect
Figure 1 shows the variation of the dielectric constant at 100 KHz with Zr/Ti ratios at
different substrate temperatures. The dielectric constant decreases with increasing Zr/Ti ratio
within this range. It is also observed that higher substrate temperature results in higher
dielectric constant, except for Ba(Zr0 7Ti0 3)0 3 thin films for which the dielectric constant is fairly
independent of the substrate temperature. The dependence of the dielectric dispersion and
leakage current on Zr/Ti ratio is shown in Figures 2 and 3, respectively. Increasing zirconium
fraction in BZT appears to reduce dielectric dispersion and leakage current.
Among the BZT thin films investigated in this study, Ba(Zr0 .14Ti 086)0 3 thin films have the
highest dielectric constant. However, the high leakage current and dielectric dispersion of these
films make them unsuitable for DRAM storage capacitors. On the other hand, the dielectric
constant of Ba(ZroTTi 0 .3)03 thin films is too low. Hence, Ba(Zr 0 6Ti04)0 3 was selected for further
studies on the effect of processing parameters such as the substrate temperature and oxygen
partial pressure on electrical characteristics.
180
160
140
.. . . i.
. ..
,.
. . .
.
. a.
.
.
. a.
I
3
.
.Ba(ZrTiI)O3
2.5
120
0
2
100
80
60
40
20
Ba(ZrxTi1 ,x)03
"7:3
.£4 1.5
-o--380CT
-460'C
- -.-- 55ý0'ýC
-
0.5
0.1 0.2 0.3 0.4 0.5 0.6 0.7
x
0.8
0.1 0.2 0.3
Figure 1. Dielectric constant as a function of
Zr atomic fraction at various temperatures.
0.4 0.5 0.6 0.7 0.8
x
Figure 2. Dielectric dispersion as a function of
Zr atomic fraction at various temperatures.
10-5
q
.- 380 'C
- -460------.&--550 T
1
Ba(Zr Ti 1-)O3
10-6
10-7
10-8
S10-9
--.. 0-380
C
-40-0 C
-
10-10
*-550 'C
...
........
0.1 0.2 0.3
S.
.
.
.
.
.
0.4
.
.
.
.
0.5
.
.
.
|
l
0.6
l l |
i l
0.7
|
|
i
0.8
x
Figure 3. Leakage current at 1 V as a function of Zr atomic fraction at various temperatures.
48
Effect of substrate temperature
Figures 4 and 5 show the dielectric constant-voltage (e-V) and dielectric constantfrequency (r -F) characteristics for Ba(Zr 0 6Ti04 )O3 at various substrate temperatures. The loss
tangent for all the films is less than 0.02 at I KHz. We observe that films deposited at 550 'C
have the best combination of high capacitance (137 at 100 KHz), low dispersion (1.37 % per
decade) and fairly low leakage current (2.2x 108 A/cm 2 at 1 V, Figure 6). From the C-V curves,
we see a slight voltage dependence of capacitance (AC(0 to 1V)/C(peak) < 3% @ 550 'C). The
voltage dependence is greater for films with higher capacitance. Also, the c-V curves do not
depend on the direction of voltage sweep, indicating that the BZT films are paraelectric.
140
140
120
460 'C
100
Bak/_ro0
611I04)"3
550 C, D=l.371%/decade
120
a
1
100
60
380 C
AV= 3V M I
..
460 °C. D=1.6037%/deca•de .
C
80
•0
14
80
40
,80 °C,D=1.126%/decade.
660 6O
20
0
Ba(Zro 6Tio 4)O, Ar = 30 mT
0
-2
0
-1
102
2
1
104
10 3
Voltage (V)
Frequency (Hz)
Figure 4. Fairly low voltage dependence of
the dielectric constant is observed for
Ba(Zro 6Tio 4 )O3 films.
Figure 5. Dielectric constant as a function
of frequency for Ba(Zro6Tio.4 )O3 films.
Films deposited at 550 'C have highest
dielectric constant.
10-2
10-3
10-4
10-5
10-6
10-8
10-9
10-10
-10
-5
0
v
top
5
10
(V)
Figure 6. Leakage current density as a function of voltage at 380 'C, 460 'C and 550 'C.
49
20
25
30
35
40
45
50
55
60
20 (degree)
Figure 7. XRD patterns for Ba(Zr0 .6Ti0.4)O3 thin films at various temperatures.
The XRD patterns (Figure 7) suggest a mixed <100>/<110> orientation for all
Ba(Zr 0.6Ti0.4)0 3 thin films. However, for those films deposited at 550 'C, the intensity of the
<100> peak is stronger than that for <110>. Similar results were observed by Kamehara et al [6]
for BZT films deposited on Pt/SiO 2 /Si. Films deposited at 380 'C and 460 'C appear to be more
<110> oriented. This, in addition to better crystallinity, may explain the higher dielectric
constant for films deposited at 550 'C.
10-6
150
Ba(Zre6Ti0 )03 deposited @ 550 C
140
10-7
0
c;)
W
0
10-8
130
120
110
Ba(ZroTio.4)O3deposited @550 C
10-9
I.I
0
.
i ..
.
0.04 0.08
02 pressure
100
. I,.,
0.12
0.16
I
0
fraction
,
, ,
I
.
I
. . .
I
_, .
0.04 0.08 0.12
02 pressure fraction
. I
0.16
(b)
(a)
Figure 8. The oxygen partial pressure appears to have little influence on both (a) leakage current
at 1 V and (b) dielectric constant at 100 KHz.
Effect of % oxygen
The effect of oxygen percentage in the reactive gas mixture on the leakage currents at 1 V
and dielectric constant at 100 KHz is shown in Figure 8. Both seem to be fairly independent of
50
oxygen partial pressure. However, we observe lower dielectric dispersion for films deposited
with oxygen in the reactive gas (Figure 9).
2.5
Ba(Zro-6T.i )O-3deposited @556
-C
7.)
C.)
2
1.5
0.5
0
0.04
0.08
02 pressure
0.12
0.16
fraction
Figure 9. Significant reduction in the dielectric dispersion is found using oxygen reactive
sputtering.
RF-sputtering with gas mixtures of oxygen/argon may reduce the amount of oxygen
vacancies in the perovskite thin films and thus improve the overall quality of the films. The
dielectric dispersion could be reduced as a result.
CONCLUSIONS
The zirconium atomic fraction in the BZT films has major influence on their electrical
characteristics. Increasing zirconium atomic fraction generally results in reduction of leakage
current, dielectric constant and dispersion. From the films used in this study, we observe a
dielectric constant as high as 168 and leakage currents ranging from IxIl -' to lxl0-6 A/cm 2 (at
1V) when the film thickness is scaled down to 60 nm. Further reduction of the film thickness
may lead to even higher leakage current. This suggests that more work is needed in order to
successfully apply BZT thin films as dielectric material of storage capacitors in the future ULSI
DRAM technology.
ACKNOWLEDGMENTS
The authors would like to acknowledge the support by Motorola and the advice and
cooperation from the MRST group.
REFERENCES
1. R. Khamankar, B. Jiang, R. Tsu, W. Y. Hsu, J. Nulman, S. Summerfelt, M. Anthony, and J.
Lee, VLSI Technical Digest, p. 127 (1995).
2. .A. I. Kingon, S. K. Streiffer, C. Basceri, and S. R. Summerfelt, MRS Bulletin, Vol. 21, No. 7,
p. 46 (1996).
51
3. S. 0. Park, C. S. Hwang, C. S. Kang, H. J. Cho, B. T. Lee, W. J. Yoo, Y. S. Park, S. I. Lee,
and M. Y. Lee, VLSI Technical Digest, p. 24 (1996).
4. T. B. Wu, C. M. Wu, and M. L. Chen, Appl. Phy. Lett. 69, p. 2659 (1996).
5. S. Hoffmann and R. M. Waser, , Integrated Ferroelectrics, Vol. 17 p. 141-152 (1997).
6. N. Kamehara, M. Tsukada, J. S. Cross, and K. Kurihara, AIChE Journal Vol. 43, No. 1 A, p.
2844-2848 (1997).
52