Appl. Phys. A (2000) / Digital Object Identifier (DOI) 10.1007/s003390000418
Applied Physics A
Materials
Science & Processing
100-nm lateral size ferroelectric memory cells fabricated by
electron-beam direct writing
M. Alexe, C. Harnagea, W. Erfurth, D. Hesse, U. Gösele
Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, 06120 Halle/Saale, Germany
(Fax: +49-345/5511-223, E-mail: [email protected])
Received: 15 September 1999/Accepted: 22 October 1999/Published online: 23 February 2000 –  Springer-Verlag 2000
Abstract. A fundamental limitation in the recent development of non-volatile ferroelectric memories of 64-Mbit to
4-Gbit densities was found to be the problem to scale ferroelectric capacitor cell sizes down below 1 µm2 . In this paper
we report on the preparation of ferroelectric memory cells
with lateral sizes down to 100 nm using electron-beam direct writing. Switching of single 100-nm cells was achieved
and piezoelectric hysteresis loops were recorded using a scanning force microscope working in piezoresponse mode. The
piezoelectricity and its hysteresis acquired for 100-nm PZT
cells demonstrate that a further decrease in lateral size under
100 nm appears to be possible and that the size effects do not
fundamentally limit the increase of the density of non-volatile
ferroelectric memories in the Gbit range.
ferroelectric properties. So far, the smallest switching single cells fabricated by a lithography process have been reported by Mitsubishi-Symetrix [6] (1.0 × 1.0 µm) and NEC
(0.7 × 0.7 µm) [7].
The aim of this work is to prove the feasibility of highdensity ferroelectric memories by fabricating ferroelectric
structures with lateral dimensions down to 100 nm and by
demonstrating polarization switching of such small cells.
PACS: 85.50.+k; 85.40.Ux; 07.79.-v
Although the thickness of thin films can be very well controlled down to the monolayer range using techniques such
as Langmuir–Blodgett or molecular beam epitaxy (MBE) [8],
the control of lateral dimensions (patterning) of individual
elements in the nm range, i.e. below 100 nm, is very difficult. Generally, the features are patterned by photolithography, which is extensively used in the integrated circuit (IC)
technology. Due to the intrinsic wavelength limitation, features of lateral dimensions of 100 nm or less can be patterned
only using extreme UV, X-ray, ion or electron beam lithography (EBL) [9, 10]. EBL is a low-cost and simple method to
produce nano-size features having lateral dimensions in the
sub-hundred-nm region, but it is known as a low-throughput
process and is considered too slow for IC manufacturing [11].
For ferroelectric thin films a conventional patterning process based on usual resist lithography followed by etching of
the oxide film involves severe problems. Side-wall redeposition and contamination, both altering the switching process
even for micrometer range features, are unsolved issues in
the patterning of complex ferroelectric thin films [12, 13].
Therefore, an approach that circumvents the etching problems
and makes use of the advantages of lithography (nanometric
resolution and maskless method) is electron-beam direct writing (EBDW) largely applied for writing metallic nanostructures using metalorganic precursors or metal colloids [14, 15].
This method was already applied by Okamura et al. [16] to
pattern ferroelectric structures having lateral dimensions of
about 150 nm.
The current high interest in ferroelectric thin films is due to
the wide range of their potential applications in microelectronics, especially in non-volatile memories [1]. In a prospective 1-Gbit non-volatile memory the lateral area of the whole
memory cell should not exceed 150 × 150 nm and this implies
ferroelectric capacitors having lateral dimensions of 100 nm
or less. At these nano-size dimensions, finite size effects such
as depolarizing fields and surface states will induce anomalies of the ferroelectric behavior, the most critical being the
possible inhibiting of polarization switching [2]. Most theoretical studies on size effects in ferroelectrics were focused
on the transition from three-dimensional (3D) systems to
two-dimensional (2D) systems by decreasing the thickness
of ferroelectric thin films, with the main result that below
a certain critical thickness the bulk ferroelectricity is suppressed by depolarization energies [3, 4]. Recently, Bune et
al. [5] experimentally proved the existence of ferroelectricity
in a ferroelectric polymer film as thin as 10 Å and thus the
absence of a critical thickness below which ferroelectricity
would vanish.
Accordingly, the fundamental question remaining in the
fabrication of high-density ferroelectric memories is how
small a ferroelectric capacitor can be, still exhibiting polarization switching, and how the capacitor size will affect the
1 Experimental
1.1 Patterning of nano-size ferroelectric cells
Fig. 1. Process flow chart of electron-beam direct writing of ferroelectric
cells
In the present work electron-beam direct writing (EBDW)
of metalorganic precursors, outlined in Fig. 1, was used to
pattern regular structures of strontium bismuth tantalate,
SrBi2 Ta2 O9 (SBT), and lead zirconate titanate,
Pb(Zr0.70 Ti0.30 )O3 (PZT). Two-dimensionally periodic arrays
of squares with lateral sizes of 1, 0.5, 0.3, and 0.2 µm were
exposed by irradiating a 1-µm-thick metalorganic thin film
obtained by spinning a corresponding precursor solution of
stoichiometric SBT and PZT onto the substrates. Raw materials used for the preparation of the solution were Sr-, Bi-,
Pb-ethylhexanoates, Ti-, Zr-isopropylene, and Ta-methoxide
as metal precursors, xylene and 2-methoxyethanol as solvents. EBDW was performed using a commercial electron
beam lithography system (ELPHY Plus, Raith Co.) adapted
to a JEOL JSM 6400 scanning electron microscope (SEM)
with LaB6 cathode and working at 40 kV acceleration voltage. The structures are patterned by scanning an electron
beam of 3 nm diameter over selected areas of the metalorganic film. The electron doses used to expose the metalorganic film were from 1500 µC/cm2 to 6000 µC/cm2 for PZT,
and 600 µC/cm2 to 1200 µC/cm2 for SBT. After the e-beam
exposure, the structures were developed by immersing the
exposed sample 1 minute in toluene and were then dried by
blowing with dry nitrogen. The metalorganic mesas obtained
after the developing were subsequently transformed into an
oxide by a low temperature annealing in air for 5 min at
300 ◦ C and crystallized by conventional thermal annealing
performed also in air for 1 h at temperatures ranging from
600 ◦ C to 850 ◦ C.
1.2 Measuring the ferroelectric properties of nano-size cells
Ferroelectric properties, viz. remanent polarization, Pr , saturation polarization, Ps , and coercive field, E c , are generally
determined from the dielectric hysteresis loop recorded using
either a Sawyer–Tower setup or a virtual-ground pulse measurement system. The total charge released by a 0.1 × 0.1 µm
cell having Pr = 2–4 µC/cm2 is about 2000 electrons. The
extremely low capacitance and high impedance of the sample prevents measuring this amount of charge without “onchip” integrated amplifiers. These technical limitations made
the efforts in acquiring a dielectric hysteresis loop of a single nanocell unsuccessful [17]. Therefore, switching of single
nano-cells was demonstrated using an indirect method that
measures the local piezoelectric behavior by scanning force
microscope (SFM) working in the piezoresponse mode.
SFM working in the piezoresponse mode has been proven
to be a powerful tool to characterize ferroelectric films at the
nm scale [18, 19]. It can be used for both visualizing the ferroelectric domain structure of a ferroelectric thin film and
to locally record the piezoelectric hysteresis loops [20]. In
the present work a scanning force microscope (Dimension
5000, Digital Instruments) working in contact mode was upgraded to work in the piezoresponse mode. A conductive tip
is scanned over the sample surface while maintaining a constant deflection of the cantilever (constant force mode). Locally, underneath the contact point between the tip and the
sample, piezoelectric oscillations are induced in the sample
by applying a probing ac signal with a frequency of about
15 kHz and an amplitude A = 2.8 V between the tip and the
bottom electrode of the sample. The mechanical oscillations
are converted into an electrical signal by the optical detector
of the SFM and are subsequently extracted from the global
deflection signal using a lock-in amplifier (EG&G Instruments, Model 7260). The image of the ferroelectric domain
structure is obtained by simultaneously monitoring the topography of the sample and the first harmonic of the signal
(further referred to as piezoresponse signal).
Ferroelectric properties of the sample are locally measured by probing the sample with the conductive tip and measuring the piezoelectric constant versus a dc voltage applied
between the tip and the bottom electrode. In order to avoid the
electrostatic interactions between tip/cantilever and the bottom electrode and to measure only the remnant piezoelectric
coefficient as a function of the dc voltage applied, each point
of the piezoelectric hysteresis loop was measured as follows:
A 100-ms dc polarizing voltage pulse of a corresponding
voltage is applied to the tip, and after 2 s from the suppression of the dc polarizing voltage the piezoresponse signal is
recorded and stored [21]. The piezoelectric hysteresis loops
are acquired by sweeping the polarizing pulse between minimum and maximum values. A hysteresis in this piezoelectric
loop is always associated with ferroelectric properties of the
sample [22].
2 Results and discussions
As already mentioned, the structures are patterned into the
metalorganic layer by scanning a 4-nm-diameter electron
beam over selected areas. The electrons cause a chemical
change and alter the solubility of the metalorganic film, similar to the normal exposure process for an electron-beam resist. Figure 2a shows the morphology of a PZT metalorganic
film after electron-beam direct writing of a regular pattern.
The periodic pattern of rectangularly shaped holes shown in
this figure is made by the e-beam exposure. It can also be
seen that the overall average film thickness of the exposed
area is reduced. This is most probably caused by a proximity effect. Scattered, backscattered, and secondary electrons
are exposing the volume adjacent to the written area, but
this exposure is below the threshold which induces solubility modifications. After the exposure the unexposed and underexposed areas are removed in toluene. The result of this
developing process is a pattern of metalorganic mesas shown
in Fig. 2b. These mesas are converted into oxide mesas by
Fig. 3a,b. Scanning electron micrographs of a an array of high-density PZT
ferroelectric cells, and b one PZT cell of 100 nm lateral size
Fig. 2a,b. AFM topography images showing a the morphology of a PZT
metalorganic layer after EBDW exposure of a regular pattern, and
b metalorganic mesas after developing the exposed layer
an annealing in air for 5 min at 300 ◦ C. The ferroelectric
phase is then obtained by a crystallization thermal annealing in air for 1 h at 650 ◦ C in the PZT case and at 850 ◦ C
in the SBT case. During the entire thermal processing the
cells are shrinking to about half of the initially patterned size.
The cells were in all cases polycrystalline consisting of grains
having diameters of 10–20 nm for PZT and 30–40 nm for
SBT.
Using this patterning process, high-density ferroelectric
structures made of PZT and SBT were obtained. As an example, Fig. 3a shows a periodic pattern of ferroelectric PZT
cells obtained on a conductive single-crystalline substrate of
niobium-doped SrTiO3 . The equivalent memory density is
roughly 1.5 Gbit/cm2 considering each cell as one bit. The
smallest cell size ever obtained was 100 nm and was achieved
for both ferroelectric materials PZT and SBT. Figure 3b
shows one PZT cell of 100 nm lateral size after the crystallization annealing.
Ferroelectric properties of single cells having lateral sizes
down to 100 nm were determined recording the piezoelectric
hysteresis loops by piezoresponse SFM. Figure 4 shows hysteresis loops acquired for a 1-µm and a 100-nm PZT cell, respectively. The piezoelectric hysteresis of 100-nm PZT cells
demonstrates unambiguously the ferroelectricity of the nmsize cells. Note that the coercive voltage measured does not
depend on the cell dimension. This is a first indication that
size effects are not affecting the coercive field of PZT for
structures that have lateral sizes down to 100 nm, although the
aspect ratio is now 1:1 and the fringing fields are important.
The structure is not a two-dimensional thin film but actually
a three-dimensional nano-size structure, the lateral surface
being four times larger than the electrode surface. Besides the
problem of fringing fields which is important at aspect ratios
lower than 5:1, surface states on the side walls can drastically
affect the ferroelectric behavior [23]. Surface states can pin
domains, create a non-switching region or, more generally, affect the switching properties of the capacitor and/or can lead
to an excessive leakage current. Using piezoresponse SFM
the ferroelectric domains can be visualized and in this way
it may be determined whether the switching process is affected by surface states on the lateral surface. Figure 5 shows
a switched domain within a 1-µm PZT cell [24]. The cell was
first positively polarized by scanning the surface and simultaneously applying a bias of +30 V. Subsequently, the middle
Fig. 4. Piezoelectric hysteresis loops of a 1-µm PZT cell ( ) and of
a 100-nm PZT cell (◦) recorded by SFM
Fig. 6. Polarization hysteresis loops of a 1-µm PZT cell: ( ) calculated
from the piezoresponse signal, and (◦) macroscopically measured for
a 0.3-mm-diameter PZT capacitor
nent polarization measured macroscopically. This is a proof
for relationship (1) being also valid for microscopic measurements. Notably, the coercive voltage of the cell determined by
SFM is considerably larger than the value for the thin film.
As the interaction between the tip and the sample is extremely
complex [27] and at the present time poorly known, the true
coercive field can not be extracted from these piezoresponse
measurements.
Fig. 5a,b. Topographic a and piezoresponse b images of a 1-µm cell. The
cell was polarized applying a poling pulse of 10 ms at −10 V
of the cell was probed by the tip and a negative pulse of
−30 V applied. As can be seen, the switched domain extends
to the cell edge thus proving that there are no side-wall effects
for the EBDW-patterned cells. However the size and surface
effects on fatigue remain to be studied for these nano-size
cells.
For ferroelectric materials with a cubic paraelectric phase
the piezoelectric coefficient, d33 , is related to the polarization,
P, by: [25]
d33 = 2Qε33 P ,
(1)
where Q is the electrostrictive coefficient, and ε33 the dielectric permittivity of the ferroelectric material. Using this
relation (1) it is possible to estimate the remanent polarization of the cell from the SFM hysteresis measurements,
provided that the values of Q and ε33 are either known or
measured [26]. The results in case of a 1-µm PZT cell are
shown in Fig. 6. The polarization estimated based on relation (1) is plotted together with a dielectric hysteresis loop
measured macroscopically on a PZT thin film grown under
the same conditions as the e-beam patterned structures. The
polarization values determined from the SFM measurements
have the same order of magnitude as the polarization measured macroscopically. Moreover, the negative polarization at
zero field has exactly the same value as the relaxed rema-
3 Conclusions
Three-dimensional nano-size ferroelectric structures having
lateral sizes down to 100 nm were patterned by electronbeam direct writing. Ferroelectric switching of these cells was
achieved by scanning force microscopy working in piezoresponse mode. It was shown that PZT cells with lateral sizes of
100 nm exhibit piezoelectric hysteresis loops. This shows that
the problems of fringing fields and surface states on the side
walls are not preventing the switching of nano-size ferroelectric cells.
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