Improved etch profiles and selectivity of CaxBa1-xNb2O6 using a hard mask
1
2
S. Vigne1, J. Muñoz2, S. Delprat1, J. Margot2, M. Chaker1
Institut National de la Recherche Scientifique (INRS), Varennes, J3X1S2 Canada
Département de Physique, Université de Montréal, Montréal, H3C 3J7, Canada
Abstract: It is shown that even though the new electro-optic material CaxBa1-xNb2O6
(CBN) is chemically inert when etched in Cl2 plasma, it is still possible to pattern it with
good selectivity and sidewall angle taking advantage of a chemical reaction taking place on
a nickel hard mask. It is observed that, at high temperature, a net growth of NiCl2 occurs on
the mask that improves significantly the selectivity and etching profiles of CBN as
compared to room temperature conditions.
Keywords: Plasma, Etching, Electro-optic, Ferroelectric
1. Introduction
Research in the microelectronic processes field is
becoming more and more challenging as the size of
electronic devices is reduced further. Beside this race for
miniaturization, another way to obtain more performing
devices is emerging with the use of multifunctional
materials with less demanding critical dimensions. For
example, a lot of research focus on active optical devices,
that are thought to potentially replace the current CMOS
technology. A variety of new materials have been
successfully synthesized towards this goal among which
the ferroelectrics occupy a large place.
Ferroelectrics exhibit interesting properties such as
dielectric tunability or electro-optic effect [1][2] but are
generally complex oxides very difficult to pattern. The
most common electro-optic ferroelectric is lithium niobate
(LiNbO3) that has an electro-optic coefficient r33 of 30 pm
/ V in its bulk form [3]. New electro-optic materials such
as bulk Sr0.75Ba0.25Nb2O6 (SBN-0.75) have much higher
electro-optic coefficient of 1300 pm / V [4]. However,
these electro-optic properties drop significantly when the
materials are synthesized as thin films, with a r33 of 844
pm / V for SBN and only 10.7 pm / V for LiNbO 3 [5][6].
More recently, several groups have successfully
synthesized a new type of ferroelectric material with high
electro-optic properties: calcium barium niobate (CBN-x)
of chemical formula CaxBa1-xNb2O6 [7]. For x=0.28 this
material exhibits an electro-optic coefficient r33 of 130 pm
/ V in its thin film form [8]. Its advantage over SBN is a
higher Curie temperature (around 280°C [ 9] instead of
80°C [10] for SBN) that makes it more suitable for very
high speed optical applications where high density optical
data packets generate a lot of heat.
In order to fabricate active electro-optic devices such as
phase or amplitude modulators based on this material, it is
first necessary to pattern epitaxial CBN thin films with
very good precision. Plasma etching is a common
technology used for CMOS fabrication of very small
transistors because it is possible to control very precisely
the etching conditions, i.e. chemistry, ion energy, substrate
temperature, etc.
In this paper, we present an etching study of CBN-0.28
using a high density Cl2 plasma. The use of nickel as a
hard mask to pattern CBN thin film is investigated. The
optimized processing conditions are determined and used
to fabricate an optical waveguide made of epitaxial CBN.
2. Experimental
CBN-0.28 thin films were deposited on MgO and Si
substrates using Pulsed Laser Deposition (PLD) from a
stoechiometric target. Deposition conditions are described
in ref. [ 11 ]. X-Ray Diffraction (XRD) and X-Ray
Photoelectron Spectroscopy (XPS) characterization of the
thin films show that the films deposited on Si are
polycristalline while those deposited on MgO are epitaxial
with a composition close to bulk CBN-0.28. Etching
experiments were conducted in an Inductively Coupled
Plasma (ICP) reactor from Oxford Instruments
(PlasmaLab 100). The plasma is generated by applying a 2
MHz RF signal at 1000 W on a cyclindrical inductive coil
placed around an alumina tube. The samples are placed on
a chuck table at the bottom of the plasma chamber and the
energy of the ions impinging the surface is controlled by a
13.56 MHz RF signal applied on the chuck. The
temperature of the substrate is controlled by either a heater
or a liquid nitrogen flow depending on the temperature
required. To ensure a good thermal control on the samples,
a backside helium flow is used between the chuck table
and the clamped Si carrier wafer. Small CBN samples are
set on the table using perfluoroether oil or high
temperature vacuum grease bonding depending on the
temperature range used (from -75 °C to +125 °C in the
first case and above +125 °C in the second one). The etch
rate (ER) was measured by in-situ laser interferometry
from the recorded amplitude signal reflected from the
surface of the samples. The sidewall angles were measured
from cross-section Scanning Electron Microscopy (SEM)
images of the etched samples. The total ion density in the
plasma was determined by Langmuir probe measurements
and the positive ion composition was determined with a
plasma sampling mass spectrometer (model HAL 511S/2)
positioned between the source and the chuck.
3.
Results and discussion
The ion fluxes were determined from the saturation current
of a Langmuir probe assuming a collisionless probe theory
that is valid for the low pressure conditions used [12].
First, the ion density is measured according to eq. 1
(1)
where ni+ is the positive ion density, Ii+ the positive ion
current measured by the probe, Mi+ the (effective) ion
mass, Te the electron temperature and Ap is the probe area
exposed to the plasma.
From this measurement and using the Bohm velocity, it is
possible to calculate the positive ion flux in the plasma
using eq. 2
(2)
where Ji+ is the positive ion flux and vBohm is the Bohm
velocity determined by
.
As it can be observed on Fig. 1, the ion flux is nearly
constant below 5 mTorr and decreases above this value.
Figure 1 also shows the dependence of the Cl+ and Cl2+
ions on the pressure. Clearly the atomic ions are dominant
over the range of pressure investigated even though their
population decreases linearly while Cl2+ ions increases
proportionally. The decrease in the ion flux as a function
of pressure is consistent with a previous study in an ICP
reactor where it was observed that from 2 to 10 mTorr, the
flux decreases and stays almost stable up to 50 mTorr [13].
Fig. 1 Positive ion composition and total ion flux in a
Cl2 plasma at 25 °C, 1000 W ICP power with pressure
varying from 1 mTorr to 10 mTorr.
Modeling of a Cl2 plasma by Thorsteinsson et al. [14] also
shows that the Cl+ fraction decreases with increasing
pressure between 1 and 100 mTorr whereas Cl2+
population increases. This decrease of atomic ions
population is mainly due to the decrease of the molecular
dissociation with increasing pressure. Stafford et al. [15]
also confirm that the dominant positive ion is Cl+ at low
pressure in a Cl2 plasma and that its percentage decreases
with increasing pressure.
It has been previously shown that the etching of CBN is
purely physical [16]. Therefore, in order to achieve the
highest sputtering efficiency, we have chosen in this study
to operate at low pressure (1 mTorr), which provides both
high ion flux and low redeposition rate. Indeed,
redeposition was shown to increase dramatically with
pressure in the case of sputter etching [17].
In order to etch CBN with good anisotropy and selectivity,
it is necessary to choose a mask very resistant to etching.
Nickel is known to be chemically inert and very resistant
to physical sputtering. It has been successfully used as a
hard mask to etch lithium niobate. Fig. 2 presents the etch
rate of both CBN and Ni materials in a Cl2 plasma at 1 kW
ICP power, 200 V bias and 1 mTorr pressure as a function
of substrate temperature (ranging from -75 °C to +375 °C).
The figure shows that the CBN etch rate does not depend
on the substrate temperature and remains almost constant
at about 50 nm / min over the whole temperature range.
The slight slope observed (activation energy lower than 5
meV) can be attributed to the variation in the surface
energy of the material with temperature. It also confirms
that plasma etching of CBN in a Cl2 plasma is purely
physical. On the other hand, the measured etch rate of Ni
in the same conditions is clearly abnormal. Indeed, from -
Fig. 2 Dependence of the etch rate of CBN-0.28 and Ni in a
Cl2 plasma on the substrate temperature. Plasma conditions
are 1000 W ICP power, 200 V bias voltage and 1mTorr
background pressure for 2 min.
Fig. 3 Effect of the substrate temperature on the measured
sidewall angle using a nickel mask for CBN-0.28 etched
in a 1mTorr Cl2 plasma generated at 1000 W ICP power
and 200 V bias voltage
75 °C to +375 °C, the Ni etch rate decreases linearly from
30 nm / min to a net growth of the nickel mask for
temperatures higher than 150 °C. At room temperature, the
occupied by chlorinated nickel is three times larger, thus
explaining the net growth on Ni.
selectivity of the Ni over CBN (defined as
) is about
2.3, which is acceptable. However, it considerably
improves as the temperature increases. At 150°C, the etch
rate of Ni is null and the selectivity is then infinite, while
above 150°C, material is observed to grow on the top of
the mask and the underlying materials (here CBN) can be
etched without any limitation.
The chemical composition of the compound growing on
the nickel layer during etching was analyzed by XPS. It
was found to be nickel chloride of formula NiClx.
Compounds like NiCl, CuCl, NiCl2 and CuCl2 were
observed by other groups who were interested to etch Ni
and Cu with chlorine plasmas [18][19]. The presence of
such compounds were considered as a nuisance. In our
case, we can take advantage of this phenomenon to pattern
materials known to be difficult to etch. The formation of
NiCl2 is believed to result from the diffusion of
chlorinated compounds coming from the plasma, thus
forming covalent bonds with the metal. This assumption
was confirmed by depth-profile XPS data (not shown here)
that exhibit an increase of the Cl penetration depth into Ni
as temperature increases. It is worthwhile to notice that, in
contrast to CuClx, NiClx seems to display a boiling point at
a temperature higher than the highest one investigated
(375°C) so that the resulting growth of NiClx over Ni
protects increasing its life duration forever. As NiCl2 has a
density three times lower than that of Ni, the volume
The use of Ni mask at high temperature in chlorine
plasmas has also a beneficial impact on the CBN sidewall
angle. Fig. 3 shows the CBN sidewall angle achieved after
5 min etching in Cl2 (approx. 250 nm of CBN etched) for
different temperatures. The 200 nm thick Ni mask was
patterned by the lift-off technique. The CBN sidewall
angles were measured from SEM images. Figure 3 shows
that the sidewall angle varies almost linearly from about
55° at room temperature to 75° at the highest temperature
investigated (90° corresponding to perfectly vertical
sidewalls). This improvement is most probably due to the
increase of the selectivity of Ni over CBN when the
temperature increases. Indeed, as the Ni mask edge does
not suffer from significant erosion, the only limitation to
the sidewall angle achievable is due to the decrease of the
sputtering cross-section with increasing incident ion angle
When impinging the substrate close to vertical, the
incident ions tend to bounce on the sidewalls rather than to
sputter the material, thus limiting the attainable angle to
75°- 80°. Finally, let us mention that using a Ni mask
could also be used to etch a large number of materials
known to be difficult to etch.
These significant improvements of the CBN etch
selectivity and sidewall angle were used to pattern an
optical waveguide made of a 1 µm thick CBN-0.28 film
epitaxially grown on MgO. Etching was performed using
the optimal conditions found for CBN etching, i.e. Cl2
generated at 1 kW ICP power with 200 V bias voltage,
1mTorr pressure and substrate temperature of 375 °C.
5.
Acknowledgements
The authors are grateful to the technical staff of the
Laboratory of Micro and Nanofabrication (LMN) from
INRS. The authors also acknowledge NSERC and the
Canada Research Chair program for financial support.
6.
Fig. 4 Cross-section SEM image of an epitaxial CBN optical
waveguide fabricated by Cl2 plasma etching at 1000 W ICP
power, 1mTorr pressure, 200 V bias and 375 °C substrate
temperature using a 500 nm thick nickel mask.
Patterning of the Ni mask was performed using
conventional UV lithography with a S1813 photoresist and
subsequent electroplating deposition of 500 nm thick Ni
using a Cr-W seed layer. The resist and seed layers were
then removed chemically to form the hard mask on top of
the CBN waveguide to be etched.
The etched CBN waveguide is shown on the cross-section
SEM image of Fig. 4 (the nickel mask and remaining seed
layers were removed chemically prior to observation). As
can be seen, the CBN epitaxial thin film was entirely
etched with sidewall angles around 75°, demonstrating the
high potential of the process developed. Moreover, no
residues were found on the MgO layer after etching and
the resulting sidewalls were very smooth, which also
demonstrates the compatibility of this etching process with
optical applications, where surface roughness has a major
effect on the optical losses.
4.
Conclusion
This study shows that ferroelectric materials such as CBN,
that are known to be very hard to etch, can be successfully
patterned using a nickel hard mask and optimized plasma
etching conditions. These optimal conditions in a Cl2
plasma are achieved at high substrate temperatures due to
the hardening of the Ni mask used. At high temperature (>
200 °C) and under ion bombardment (200 V DC bias), the
chlorinated species induce the growth of a protective NiCl2
layer on the top of the mask. This results in a significant
improvement of selectivity and sidewall angles. Using this
process, smooth and near-vertical sidewalls were obtained
over a 1 µm thick CBN layer, demonstrating the feasibility
of fabricating optical waveguides based on CBN, and
opening the door to the design of active photonic devices.
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