APPLIED PHYSICS LETTERS
VOLUME 73, NUMBER 11
14 SEPTEMBER 1998
Self-patterning nano-electrodes on ferroelectric thin films for gigabit
memory applications
M. Alexe,a) J. F. Scott,b) C. Curran, N. D. Zakharov,c)
D. Hesse, and A. Pignoletd)
Max Planck Institute of Microstructure Physics, Weinberg 2, D-06120 Halle (Saale), Germany
~Received 2 March 1998; accepted for publication 13 July 1998!
In the present work, we report self-assembling bismuth-containing nano-electroded cells of layered
perovskite ferroelectric thin films that are about 200 nm in size, that is 50 times smaller than the
smallest cell reported to date. Heteroepitaxial Bi-rich Bi4Ti3O12 films were grown by pulsed laser
deposition ~PLD! on top of epitaxial conductive La0.5Sr0.5CoO3 ~LSCO! layers equally deposited by
PLD. The epitaxial LSCO has been grown on top of an epitaxial CeO2 yttrium-stabilized zirconia
~YSZ! stack, itself deposited by PLD on Si~100! substrates. As a consequence of the high substrate
temperature during the epitaxial deposition of the Bi4Ti3O12 layer, the excess Bi segregates,
migrates to the surface where it forms a self-organized array of epitaxial mesas which possess
metallic-like electrical characteristics. © 1998 American Institute of Physics.
@S0003-6951~98!00237-X#
Immense efforts have been undertaken in microelectronics to provide ultrahigh density ultralarge scale integration
~ULSI! memory chips. The use of submicron lithography
together with complicated geometries involving stacking and
trenching of silicon devices in order to obtain more capacitance per unit area has been emphasized, the resistors and
transistors in the integrated circuits taking up negligible
space. In the effort to increase capacitance, the silicon-oxide
capacitors will potentially be replaced with ferroelectric materials, whose dielectric constants are 100 times larger, permitting the same capacitance in a 100 times smaller area.
Most of the problems with ferroelectric thin films have been
solved recently by using either unusual bismuth compounds1
or high-T c superconducting electrodes.2 However, the main
limitation on the recent development of ferroelectric thinfilm memory3 in 64 Mbit–4 Gbit densities has been the ability to scale capacitor cell sizes below 1 mm2. Until now the
smallest cells reported were preliminary studies on 0.730.7
mm cells from NEC produced by conventional submicron
lithography,4 and 1.031.0 mm cells from Mitsubishi produced by the chemical solution deposition ~CSD! method,
adding a photoresist to the ferroelectric precursors and optically patterning.5 In the present work we report selfassembling nano-electroded cells of ferroelectric thin films
that are 50 times smaller than the previous NEC record, although these cells are not yet registered. This shows that
scaling of ferroelectric memories to 1 Gbit densities is indeed possible, provided registered formation will prove to be
possible. For practical devices the valleys surrounding the
a!
Permanent address: National Institute of Physics of Materials, P.O. Box
MG7, R-76900, Bucharest, Romania.
b!
Humboldt Award visitor. Permanent address: School of Physics, University of New South Wales, Sydney 2052, Australia.
c!
Permanent address: Institute of Crystallography, Academy of Science,
Leninsky Prospect, Moscow, Russia.
d!
Electronic mail: [email protected]
Bi-containing crystalline mesas can be filled ~e.g., with a
spin-on glass! to produce a mechanically robust memory
chip.
Here we report on rectangular planar arrays of Bicontaining crystalline electrodes showing metallic conductivity on the surface of epitaxial films of bismuth titanate prepared by pulsed later deposition. The bismuth titanate layer
has been deposited on top of an epitaxial layer of
La0.5Sr0.5CoO3 ~LSCO! serving both as electrode and as template for the ferroelectric layer. An epitaxial layer of CeO2
and an underlying epitaxial yttrium-stabilized zirconia ~YSZ!
layer between the LSCO and the Si substrate prevent Bi diffusion. The nano electrodes are rather uniform in size and
well separated from each other with no overlap or terraces
@Fig. 1~a!# yielding ;1 Gbit of accessible ferroelectric cells
per chip ~0.530.5 cm2). Depending upon processing conditions the maximum sizes of the nano electrodes are typically
125 nm and can exceed 200 nm.
Under sol-gel, metalorganic decomposition ~MOD!, or
pulsed laser deposition ~PLD! bismuth titanate ~BiT! and
strontium bismuth tantalate ~SBT! films segregate out metallic, elemental bismuth at the surface.6 The reported x-ray
photoelectron spectroscopy ~XPS! data showed that about
30% of the Bi at the surface of SBT is elemental Bi0 and
70% is Bi31, and the angular dependence revealed that the
metallic Bi was confined to the surface. The XPS study was,
however, not space resolved and therefore no lateral information was available. Our new scanning electron microscopy ~SEM!, transmission electron microscopy ~TEM!, and
atomic force microscopy ~AFM! studies of PLD-processed
Bi4Ti3O12 show that an array of crystalline, mesa-like Bicontaining islands, 40 nm high, grows on the surface. The
heights are uniform and are shown in Fig. 1~b! and, via TEM
cross sections, in Fig. 2. The Bi-containing mesas are likely
to grow during the Bi4Ti3O12 film deposition at temperatures
of 1098640 K in an oxygen atmosphere. It has to be noted
that Bi melts at 544.5 K and that Bi2O3 reduces at 643.2 K,
so that formation of metallic Bi island is possible in vacuum
0003-6951/98/73(11)/1592/3/$15.00
1592
© 1998 American Institute of Physics
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Appl. Phys. Lett., Vol. 73, No. 11, 14 September 1998
Alexe et al.
1593
FIG. 3. Pair correlation function for Bi-containing mesas on bismuth titanate, showing a well-defined ~nonrandom! nearest-neighbor distance of 500
nm.
FIG. 1. ~a! Plan view SEM micrograph of a Bi-containing nano-electrode
array, ~b! Atomic force microscopy vertical contours of the crystalline Bicontaining mesas on bismuth titanate.
or in an oxygen-poor ambient.7 In our case, however, the
pure 13.3 Pa oxygen atmosphere prevents the reduction of
the bismuth oxide, and the islands are mainly composed of
crystalline cubic d -Bi2O3 as determined from the interatomic
distances of, and the angles between, $111% planes in highresolution electron microscopy ~HREM!. The nanoscale islands have a short-range glassy correlation function ~Fig. 3!,
i.e., a well-defined nearest neighbor separation of ;150–500
nm, which depends on processing conditions, and a strong
rectangular angular correlation peaking at 90° intervals.
Their size and density permit a self-patterning 256 Mbit or 1
Gbit ferroelectric electroded array, provided the mesas can
be induced to grow in a registered array.
Ferroelectric switching of these small capacitors has
been obtained using Hg-ball contacts. Two Hg balls were put
on the top surface and observed via SEM to wet only the Bi
mesas, the LSCO conducting bottom layer serving as a
middle floating electrode. Figure 4 shows the ferroelectric
properties of a nanoscale capacitor array consisting of a 180
nm thick Bi4Ti3O12 film epitaxially deposited on a stack of
YSZ/CeO2 /LSCO. The curve shown is neither square nor
saturated and typically characterizes a lossy ferroelectric, but
the quality of the hysteresis is quite comparable to that in
large-area cells of the same material.8 The switching ~coercive! voltage is 3 V, corresponding to fields of ;55 kV/cm
and the remanent polarization P r >4 mC/cm2, comparable to
the known value for large-area cells. It is an assumption in
our analysis that the lines of force from top to bottom electrodes are normal, so that only the ferroelectric directly under
the Bi-containing nano electrode switches. Although fringing
fields for these aspect ratios may be large, such fields will
not be in a direction to induce significant switching of nearby
regions of the dielectric film. Above 0.5 V, current–voltage
data show almost perfectly quadratic dependence typical of
space-charge limited currents. The leakage current is about
FIG. 2. Cross-section TEM micrograph showing two Bi-containing mesas
on a 40 nm thick bismuth titanate (Bi4T3O12) film; the lower layers are
FIG. 4. Dielectric hysteresis for a 180 nm thick Bi4Ti3O12 film demonstratLSCO, CeO2, yttrium-stabilized zirconia ~YSZ!, and finally silicon.
ing the ferroelectric properties of the bismuth titanate film.
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1594
Alexe et al.
Appl. Phys. Lett., Vol. 73, No. 11, 14 September 1998
10 pA through 83105 /cm2, i.e., 12 mA/cm2, for 3 V across
180 nm of Bi4Ti3O12.
Similar outgrowths or hillocks forming at the surface of
SrBi2Ta2O9 films annealed in an hydrogen ambient have recently been reported.9 These hillocks have been identified to
be metallic bismuth arising from a chemical reaction of the
SrBi2Ta2O9 surface with hydrogen, reduction of Bi, and subsequent outdiffusion of the metallic bismuth to the surface.
However, since the reported films are polycrystalline, no
definite orientation of these outgrowths can be expected or
has been observed. In contrast, our film stack is epitaxial and
it is clearly seen in Fig. 1~a! that the Bi-containing nano
electrodes have a definite orientation correlated with the
crystallographic axes of the underlying layers and with the
@100# axes of the Si substrate. A seeded growth of the nano
electrodes should thus permit a regular patterning, that is a
registration of the nano-electrode pattern and therefore an
addressing of the nanoscale ferroelectric memory cells. A
discussion of techniques for registering and addressing these
arrays by seeding growth will be given elsewhere. It has to
be noted that cubic d -Bi2O3 composing the nanoscale mesas
has a defect fluorite structure and is the best ion conductor
known with a resistivity of ;1 V cm at 1023 K, therefore
perfectly suitable to serve as nano electrodes.10
In summary, self-assembling nano-electroded cells of
ferroelectric thin films that are 50 times smaller than the
previous record have been achieved. This nearly two orders
of magnitude reduction in cell size shows that scaling of
ferroelectric memories to 1 Gbit densities is indeed possible,
provided registered arrays may be grown by appropriate
seeding techniques. Although the reported phenomenon has
been observed for an epitaxial Bi4Ti3O12 film, it should also
occur on a variety of ferroelectric thin-film capacitors of
bismuth-containing layer-structure perovskites, such as
strontium bismuth tantalate ~SBT!.
The authors wish to thank Professor U. Gösele for his
continued support during this project.
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