Highly insulative barium zirconate-titanate thin films prepared by rf
magnetron sputtering for dynamic random access memory applications
T. B. Wu, C. M. Wu, and M. L. Chen
Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan,
Republic of China
͑Received 1 July 1996; accepted for publication 22 August 1996͒
Polycrystalline thin films of Ba͑Zr
x
Ti
1Ϫx
)O
3
with xϭ0–0.4 and a thickness of ϳ180 nm were
deposited on platinum-coated silicon substrates by rf magnetron sputtering at 500 °C. Ba-rich
targets were used to prepare films of stoichiometric composition. The film having xϭ0.12 exhibited
a satisfactory dielectric property of dielectric constant, kϷ300, and dissipation factor, tan
␦
Ͻ0.02,
at frequencies from 10
3
to 10
6
Hz under ambient temperatures ranging from 20 to 180 °C. More
importantly, the film showed a very stable and highly insulative characteristic against applied
voltage. The leakage current density J increases only smoothly to a value less than 10
Ϫ7
A/cm
2
followed by an Ohmic relation of J ϭ
E with
ϭ1.4ϫ10
Ϫ14
(⍀ cm)
Ϫ1
up to an extremely high
electric field E of 5.6 MV/cm without any sign of abrupt increase of leakage current or electrical
breakdown. Also, no time-dependent electric degradation was observed for the film subjected to an
electric field as high as 5 MV/cm at room temperature up to 3600 s of measurement. © 1996
American Institute of Physics. ͓S0003-6951͑96͒04444-0͔
Preparation of ferroelectric thin films of high dielectric
constant with reliably low leakage current and high dielectric
breakdown strength is important in the development of
Gbit DRAM ͑dynamic random access memory͒ tech-
niques.
1–3
Among various materials, the perovskite-type
(Ba
1Ϫx
Sr
x
)TiO
3
͑BST͒ is a popular composition of study for
the above application.
4–6
The addition of strontium into
BaTiO
3
is to shift the Curie temperature below room tem-
perature and make the BST a paraelectric material at room
temperature offering high dielectric constant, low leakage
current, and low dielectric dispersion against frequency.
However, although the BST thin films have a low leakage
current at low electric field, the leakage often abruptly turns
up in orders of magnitude as the applied voltage exceeds a
critical field of several hundreds kV/cm,
7–9
and breakdown
occurs at a field around 2 MV/cm.
10,11
It limits the minimum
thickness of BST films still having a satisfactory electrical
property for applications. Searching for an alternative mate-
rial thus attracts attention.
It is known that there are several isovalent substituents
in BaTiO
3
other than Sr which are also able to shift the Curie
temperature below room temperature.
12
Among them, Zr is
of interest because a different character of dielectric response
with respect to the paraelectric-to-ferroelectric phase transi-
tion can be achieved by the substitution of Zr from that of Sr
in BaTiO
3
, due to the disparate distortions of oxygen octa-
hedra in the perovskite lattice.
13
Moreover, the Zr
4ϩ
ion is
chemically more stable than the Ti
4ϩ
and has a larger ionic
size to expand the perovskite lattice. Therefore, the conduc-
tion by electron hopping between Ti
4ϩ
and Ti
3ϩ
, if any,
would be depressed by the substitution of Ti with Zr. In this
work, a preliminary study concerning the dielectric and con-
duction characteristics of Ba(Zr
x
Ti
1Ϫx
)O
3
͑abbreviated as
BZT, hereafter͒ films prepared by rf magnetron sputtering is
reported.
Ba-rich targets ͑2 in. in diameter͒ made by sintering of
pressed pellets composed of mixed powders of BaZrO
3
and
Ba
2
TiO
4
of different ratios were used. Platinum-coated sili-
con substrates, i.e., Pt͑150 nm͒/Ti͑50 nm͒/SiO
2
͑150 nm͒/Si,
were placed parallel to the target at a distance of 4.5 cm. The
Pt and Ti films were deposited by electron beam evaporation
without substrate heating. The sputtering chamber was first
pumped down to a base pressure of 1 ϫ10
Ϫ5
Torr, and then
argon and oxygen gas (Ar/O
2
ϭ90/10) were introduced into
the chamber and controlled at a working pressure of 5
ϫ10
Ϫ3
Torr with mass flow controllers. An rf power density
of 2.5 W/cm
2
was employed for the sputtering deposition.
The substrates were heated to a temperature of 500 °C during
deposition for 2 h, which gave a film thickness around 180
nm. For electrical measurement, upper electrodes of 100-nm-
thick Pt film ͑0.3 mm in diameter͒ were fabricated by rf
magnetron sputtering and subsequent photolithography, sub-
sequently. The BZT thin films having a stoichiometric com-
position ͑examined by EPMA͒ of Ba(Zr
x
Ti
1Ϫx
)O
3
with x
ϭ0, 0.12, 0.22, and 0.40, and Ba/͑TiϩZr͒ϭ1Ϯ0.01, were
obtained.
Figure 1͑a͒ shows the x-ray diffraction patterns ͑XRD͒
of the BZT films. Perovskite peaks were observed for all the
films of different composition. A gradual shift of 2
angles
to the low angle side with increasing the Zr content in the
BZT films reveals the expansion of perovskite lattice by the
addition of Zr in BaTiO
3
. The lattice constants of the BZT
films, as well as those of bulk specimens for reference pre-
pared in this work, were determined from the XRD patterns,
as shown in Fig. 1͑b͒. The films were found to have a larger
lattice constant than that of bulk material of the same com-
position, i.e., a phenomenon also often observed from sput-
tered BST thin films, indicating nonequilibrium and highly
distorted states within the films.
14
The BZT film of x ϭ0
showed a pseudocubic structure ͑i.e., no peak split was ob-
served͒ rather than a tetragonal structure as that of bulk
specimen, which was also observed from the sputtered BST
films and was attributed to the highly distorted states within
the films.
14
2659Appl. Phys. Lett. 69 (18), 28 October 1996 0003-6951/96/69(18)/2659/3/$10.00 © 1996 American Institute of Physics