In situ synthesis of metallic nanowire arrays for ionization gauge electron sources
Farough Roustaie, Sebastian Quednau, Florian Dassinger, and Helmut F. SchlaakMarcel LotzStefan Wilfert
Citation: Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics: Materials,
Processing, Measurement, and Phenomena 34, 02G103 (2016); doi: 10.1116/1.4939756
View online: http://dx.doi.org/10.1116/1.4939756
View Table of Contents: http://avs.scitation.org/toc/jvb/34/2
Published by the American Vacuum Society
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In situ synthesis of metallic nanowire arrays for ionization gauge electron
sources
Farough Roustaie,a) Sebastian Quednau, Florian Dassinger, and Helmut F. Schlaak
Institute of Electromechanical Design (EMK), Department of Electrical Engineering and Information
Technology, Technische Universit€
at Darmstadt, Merckstr. 25, D-64283 Darmstadt, Germany
Marcel Lotz
GSI Helmholtz Centre for Heavy Ion Research, Common Systems - Vacuum Systems, Post Box 110552,
D-64220 Darmstadt, Germany and IAP, Goethe University Frankfurt, Max-von-Laue-Str. 1,
D-60438 Frankfurt am Main, Germany
Stefan Wilfert
GSI Helmholtz Centre for Heavy Ion Research, Common Systems - Vacuum Systems, Post Box 110552,
D-64220 Darmstadt, Germany
(Received 13 July 2015; accepted 28 December 2015; published 12 January 2016)
An in situ synthesis process for nanowire arrays was used to fabricate a customized field emitter
array for use as a nonthermal electron source in an ionization gauge. The wire arrays fabricated
with this process had a density of 1.6 106 cm2 using optimized deposition of the wires in
template foils on predefined electrodes. The diameter of the wires varied from 100 to 400 nm and
their length ranged from 8 to 100 lm. This method can enclose with nanowires a wide area of
predefined electrodes, and is possible for areas larger than 3 cm2. Further, the cylindrical shape of
the nanowires was modified into a conical geometry to achieve improved thermomechanical
C 2016 American Vacuum Society. [http://dx.doi.org/10.1116/1.4939756]
stability. V
I. INTRODUCTION
Cryogenic vacuum systems, such as those found in particle accelerators using superconducting beam guiding components, are typically operated at liquid helium (LHe)
temperatures. In these low-temperature systems, pressures in
the lower ultrahigh vacuum range and even in the upper
extreme-high vacuum range can be routinely generated via
cryopumping.1 However, pressure measurement, which is
more precisely defined as the measurement of the residual
gas density in these systems, is a metrological challenge
because conventional pressure measuring techniques are not
possible without substantial problems and restrictions. For
example, crossed-field ion gauges (commonly known as
“cold-cathode gauges”) do not seem to work reliably at LHe
temperatures.2 Though hot-filament ion gauges have been
proven to work well under cryogenic conditions,2–4 because
of their directly heated thermionic cathode, they represent an
unwanted and unacceptable heat source within the cryogenic
environment.
With the rapidly progressing development of novel highperformance field emitter cathodes in the past two decades,
the research on pressure measuring techniques has focused
on the development of field emitter-based vacuum sensors.5
Especially in ionization gauges, the application of cold (i.e.,
nonthermal) electron sources has the potential to eliminate
some of the disadvantages of the thermionic cathodes.
Meanwhile, numerous researchers6–8 have experimentally
shown that ion gauges equipped with field emitter cathodes
work adequately and deliver a reliable pressure reading.
Owing to the almost negligible heat generation of those
a)
Electronic mail: f.roustaie@emk.tu-darmstadt.de
02G103-1 J. Vac. Sci. Technol. B 34(2), Mar/Apr 2016
electron sources, it is hoped that field emitter-based ion
gauges can successfully be used in cryogenically cooled vacuum systems for pressure measurements without noticeable
restrictions.
One of the most thoroughly studied field emitter cathodes
in ionization gauges are carbon nanotube (CNT) cathodes,
which are characterized by a low turn-on field and outstanding field emission properties in terms of current density and
short-term stability. In most cases, however, CNT films do
not emit uniformly across their emitting surface area (i.e.,
they possess emission hot spots) because film homogeneity
and growth behavior are difficult to control during preparation.9 Furthermore, the poor electric contact between the
substrate and the CNT layer can be problematic because it
restrains the electron transport and reduces the field emission
yield. As a consequence, when extraction voltages get too
high, excessive amounts of ohmic power is dissipated in the
resistive substrate so that it can even melt.10 In addition, the
rapidly progressing emission degradation of the CNT cathodes when operated at higher pressures indicates that CNT
cathodes cannot be used unreservedly as an alternative cathode in ion gauges.
A promising alternative to CNT cathodes are metallic
nanowire arrays. Owing to their specific mechanical, electrical, and chemical properties, they offer the opportunity to
develop highly effective field emission sources with field
emission properties similar to CNT cathodes but with the
likelihood of fewer drawbacks. In contrast to CNT cathodes,
metallic nanowire array preparation can be well-controlled
in terms of number density, wire diameter, wire length, and
tip radius, and, additionally, electrical contact problems
between the substrate and emission layer are less significant.
Moreover, metallic nanowires do not have the “hairy”
2166-2746/2016/34(2)/02G103/6/$30.00
C 2016 American Vacuum Society
V
02G103-1
02G103-2 Roustaie et al.: In situ synthesis of metallic nanowire arrays
02G103-2
surface structure of CNT cathodes, where huge amounts of
gases can be adsorbed and accumulated for a long time.
Recently, the field emission properties of gold nanowires
have been investigated intensively by Dangwal et al.,11 who
showed that the ratio of emitting wires to deposited wires in
gold nanowire cathodes is substantially higher than that in
typical CNT cathodes.
Pursuing this approach, in this paper, we report on the
fabrication process of a gold nanowire-based electron field
emitter cathode by using the integration technique of metallic nanowires.12 The goal of this project is to substitute the
thermionic cathode in an ionization gauge of the extractor
type with a gold nanowire-based field emitter cathode, which
can be used as a reliable residual gas density sensor in cryogenic vacuum systems.
II. FABRICATION PROCESS
The combination of the well-established ion-track etching
process13 with conventional MEMS processes12 led to a cost
effective and flexible in situ developing process for structuring metallic nanowires and integrating them with microstructured electron extraction grids in 3D microsystems. In
this technique, the metallic nanowires were grown by electrodeposition in the pores of a template foil. The template
foil was fabricated by irradiating a polymer (e.g., polycarbonate) with swift heavy ions possessing mega-electronvolt–giga-electronvolt kinetic energy. Etching the iontracks in the polymer freed up the pores, and, depending
upon the etching time and solution concentration, the diameter of the pores varied from tens to 400 nm. The thickness of
the template foils ranged from 8 to 100 lm and determined
the maximum length of the wires. The number of pores per
unit area (pore density) could be controlled by changing the
irradiation parameters.
A. Fabrication process of metallic nanowires
The fabrication process began with a glass wafer, upon
which was deposited a 25 nm-thick layer of chrome and a
150 nm-thick layer of nickel. Subsequently, the base electrode, which was used for nanowire deposition, was structured through lithographic and etching processes. Later, the
base electrodes were gilded by electroless nickel immersion
gold surface plating, where the deposited gold layer reached
a thickness of about 80 nm and was used as starting layer for
electroplating. Next, the template foil was placed and fixed
on the structured gold electrode. Laminating the template
foil on a bare metallic or glass surface is not possible, so we
used a layer of 7 lm-thick AZ-9260 or 1.5 lm-thick map1215 photoresist to build 100 lm-wide supporting walls as
an adhesive surface [see Fig. 1(b)].
The lamination took place using a heated roll laminator at
a temperature of 140 C. An alternative method was to soften
the surface of the resist with the vapor of a proper solvent
and laminate the foil at room temperature.
To enable initiation of the electrodeposition process, the
electrolyte should penetrate the pores and wet the cavity
underneath. For pores over 200 nm in diameter or for foils
J. Vac. Sci. Technol. B, Vol. 34, No. 2, Mar/Apr 2016
FIG. 1. (Color online) (a) Structured gold electrode. (b) Structuring the supporting walls by a photolithography process. (c) Lamination of the porous
polymer template foil on the photoresist. (d) Electrodeposition of the nanowires in the pores of the template foil. (e) Removing the foil and
photoresist.
with a pore density greater than 1 108 cm2, the structure
could be immersed for some hours in deionized water. For
pores smaller than 200 nm in diameter or low pore densities
of about 1 106 cm2, especially in broad area nanowire
arrays larger than 1 cm2, the cavities were formed as communicating vessels and the electrolyte was injected under the
foil through a channel. After wetting the cavities, the wafer
was clamped in a specially designed galvanic cell for a
potentiostatic-controlled electrodeposition using a gold-SF
electrolyte, an Ag/AgCl reference electrode (Meinsberg SE
11), and a 99.9% gold anode (see Fig. 2).
To obtain a more homogeneous wire growth, a 20 Hz
deposition pulse potential with 450 mV amplitude and
450 mV level offset at 60 C was applied [see Fig. 3(a)].
FIG. 2. (Color online) Cross-section of the galvanic cell used for electrodeposition of nanowires. The cell is made of an aluminum case and a TeflonTM
container. The gilded wafer is clamped under the cell and used as the working electrode.
02G103-3 Roustaie et al.: In situ synthesis of metallic nanowire arrays
02G103-3
FIG. 4. (Color online) Average deposition current. (a) Current increases by
diffusion of the ions in the pores. (b) Growth of plateaus in the cavity. (c)
Nanowire growth. The current drops owing to the decreasing deposition surface area. (d) Overgrowth of the wires.
saturation level when all of the overgrowths come together
as a plate and the deposition surface area remains constant.
To reach a specific wire length, the process must be stopped
at an appropriate time in region (c).
B. Fabrication process of the extractor grid
FIG. 3. (a) Deposition pulse applied on the working electrode. A pulse voltage
leads to a more homogeneous deposition compared to a direct current voltage.
(b) Current pulse. The deposition takes place at negative current flow.
The pulse voltage possessed a duty cycle of 20%, and the
80% resting pulse ensured that impoverishment of the metal
ions near the deposition layer did not occur. As shown in
Fig. 3(b), the current direction changes at the rest pulse
edge, which is owing to the exponential course discharge of
the capacitive charge that is stored in the double layer. For
base electrodes with different surface areas, we varied the
duty cycle in such a way that the discharging current reached
zero during the rest pulse.
For further process monitoring, the average deposition
current is calculated (Fig. 4), which gives information about
different deposition states. In region (a) in Fig. 4, the metal
ions diffuse into the foil pores and the current increases
while the concentration of the metal ions increases. In region
(b), metal fills in the cavities and forms plateaus.
When the plateaus reach the lower surface of the template
foil, the wires begin to grow in the pores. In this growth
phase, the total deposition surface area decreases and is associated with the current drop seen in region (c) of Fig. 4.
Finally, the current rise observed in region (d) is owing to
the fact that the wires are overgrown at the top side of the
foil, which causes the deposition area to increase again and
leads to an increase in current. The current reaches a
The next step is to remove the foil by plasma ashing or
dissolving in dichloromethane to reach the structure shown
in Fig. 1(e). To process the electron extraction grid as shown
in Fig. 5, we used the standard Si back-etch process. This is
done by covering the front surface of the Si plate with a passive layer and performing an anisotropic back-etch through
an electrochemical etch stop, after which the front side is deposited with gold. The vacuum gap between the extractor
and the emitting layer (wire tips) should be one to two times
the height of the wires. Copper spacers are grown on the
front side via an electroplating process, and the grid is subsequently fixed over the nanowire arrays using spring-loaded
clamping.
III. DESIGN AND MODELING
The electron source of an ion gauge whose lower pressure
measuring limit lies in the ultrahigh vacuum range must
FIG. 5. (Color online) Field emitter structure. The grid is fabricated through
a standard Si back-etch process. A metallic spacer is grown on the grid
through the electroplating process. The structure is clamped over the nanowire array after the spacers heights are balanced through polishing.
JVST B - Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena
02G103-4 Roustaie et al.: In situ synthesis of metallic nanowire arrays
necessarily deliver a net emission current of at least 200 lA.
This requirement results from the fact that at low pressures
the gauge output current (i.e., ion current) reaches extremely
low values in the pA range or even lower. This current,
which is directly converted into a pressure reading, must
therefore still be measurable by means of conventional electrometer and picoammeter. A second important aspect to be
considered is the kinetic energy of electrons leaving the cathode. Because the probability of ionization of gas molecules
by electron impact reaches its maximum at kinetic energies
of about 100–250 eV, the voltage to be applied to the extractor grid should not exceed 250 V. The field enhancement factor, b, of nanowire-based emitters depends strongly upon
their morphology, spatial distribution, and interspacing of
the wires.14,15 Moreover, the dependence of b upon the vacuum gap, d, has become a topic of ongoing research for optimizing the performance of field emission devices. A gold
nanowire density of 1.6 106 cm2 has an average interwire
distance of a ¼ 8 lm and, to reach a maximum field enhancement, the maximum length, h, of the wires should be the
same as their distance15 (i.e., h a ¼ 8 lm). The theoretically expected emission behavior for a wire array with a
wire diameter of 200 nm and a vacuum gap d ¼ 15 lm is calculated using the Fowler–Nordheim approximation and is
shown in Fig. 6.
As can be seen in Fig. 6, an electrical field strength of
2.2 109 V/m must be applied at the wire tips to obtain a
current density of 0.2 A/m2, which corresponds to an extraction voltage of 200 V. For an available emitting area of
12 cm2, the net emission current reaches 240 lA, which
meets the requirements.
IV. RESULTS AND DISCUSSION
In Fig. 7, an array of wires is shown that is obtained with
the lamination technique, which enables the fabrication of
broad area arrays. The covered area shown in Fig. 7(a) is
10 30 mm2, and we use four times this area to reach the
required emission current.
Although we have shown previously that the lamination
process works properly,12 expanding the process to fabricate
large area arrays has caused an increase in the amount of
FIG. 6. Analytical calculation of the current density J as a function of the
local electric field strength E at the tip of the nanowires.
J. Vac. Sci. Technol. B, Vol. 34, No. 2, Mar/Apr 2016
02G103-4
FIG. 7. (Color online) (a) Metallic nanowire cathode. The cathode consists
of plateau blocks with an area of 500 500 lm2. (b) Scanning electron
microscope (SEM) image of blocks fully covered with nanowires. (c) SEM
image of nanowires on one block. The wires have 400 nm diameters and
lengths of 25 lm, and the arrays have a wire density of 1.6 106 cm2. (d)
Sharper tips of the nanowires after removing the overgrowths.
fabrication waste. One of the problems observed is a different growth rate for the wires at the middle of the plateaus
than for those at the edges. This faster growth at the edges
can be explained by two phenomena: (1) field enhancement
at the sharp edges of the plateaus that induces a higher ion
current at this region, and (2) interfacial tension between the
supporting walls and the electrolyte that causes an impoverishment of the ions in the middle. These effects make it
impossible to stop the deposition at a definite time in region
(c) of Fig. 4 to reach the desired 8 lm-long wires over the
entire structure [Fig. 8(a)].
We solve the problem of varying growth rates by changing the geometry of the supporting walls to one of communicative vessels and by pressing the foil with a homogeneous
force on the supporting walls during the deposition process.
This made it possible to stop the process at the proper time
in region (c) of the current curve, at a time when most of the
wires have the same length. Pressing the foil simultaneously
solved another problem. We have always observed a slight
growth of metal film over the supporting walls and under the
foil in the form of a bending beam at the edge of the structures [Fig. 8(b)] that occur when the electrolyte delaminates
the template foil. Using the foil pressing technique, however,
causes the growth of the beam to be prevented. The nanowire array fabricated by pressing the foil on the supporting
walls is shown in In Fig. 9(a), where it can be seen that the
wire length deviation is much more affected by the interfacial tension than by the field enhancement at the plateau
edges.
Thermomechanical instability of the nanowires is one of
the major problems of such structures, however. The wires
shown in Fig. 10(a) are made by a different fabrication
method, where the back side of the porous template foil is
sputtered with a thin gold layer (100 nm thickness).13 In
this method, the nanowires are grown in the pores of the
template foil using the sputtered layer as the deposition starting layer.
02G103-5 Roustaie et al.: In situ synthesis of metallic nanowire arrays
FIG. 8. (Color online) Defects observed during the lamination technique development. (a) Faster growth of the wires at the edges of the plateaus caused
by field enhancement. (b) Free areas in the middle of the plateaus owing to
interfacial tension. Metallic beams that have crawled under the foil can be
seen at the edge of the plateaus.
During the sputtering process, the internal walls of the
pores are covered with gold atoms, which create hollow cylindrical wires during the deposition and, consequently, a
reduced contact area between the wires and the substrate.
This reduced contact area is the main reason for the thermomechanical instability of the wires, and it furthermore
increases the contact resistance of the nanowires, which can
lessen the emission current. Using the process we developed,
the starting deposition layer lies in the cavities, which is not
in contact with the foil. First, the plateau fills up the cavity
02G103-5
FIG. 10. (a) Cross-section of hollow wires fabricated by sputtering the back
side of the foils as the deposition start layer (Ref. 16). (b) Full cylinder fabricated with the lamination process.
and the wires then grow continuously in the pores. This leads
to a full cylindrical shape that produces a better electrical
contact and an improved thermomechanical stability compared with other methods. Changing the shape of the wires
from a cylindrical to a conical geometry will also increase
the mechanical stability of the wires further;17 so, for this
purpose, we performed an asymmetric etching of heavy iontracked polycarbonate foils.18–20 To apply asymmetric etching on the foils, we used the double membrane technique
wherein 9 M sodium hydroxide was used as the etchant and
methanol was used to strengthen the etchant. A 10% acetic
acid was used on the opposite side of the membrane as an
etch stop, whereby the acidic solution neutralized the etchant
at the pore opening tips, causing a decrease in the etching
rate at the very tip to promote the conical shape of the pores.
Varying the percentage of the methanol from 0% to 80%,
the percentage of the acid and other parameters such as temperature and etching time enabled us to vary the sizes of the
bases and tips of the conical pores. The fabrication process
used for the metallic cones is the same as for the wires, and
Fig. 11 shows fabricated metallic cones with a base diameter
of 2 lm and a tip diameter of about 300 nm on an area about
1 cm2. Most of the cones have the same length and their tips
lie on a plane.
V. SUMMARY AND CONCLUSIONS
FIG. 9. Arrays of nanowires fabricated by pressing the foil on the supporting
walls. The predefined electrodes are completely covered and have sharper
edges than those of the electrodes shown in Figs. 7(b) and 8(b).
By means of a novel fabrication approach, we successfully prepared microstructured field emission cathodes in the
JVST B - Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena
02G103-6 Roustaie et al.: In situ synthesis of metallic nanowire arrays
02G103-6
used for the cylindrical nanowires, which offers the opportunity to selectively deposit wires and cones on a predefined
area. This technique is compatible with semiconductor fabrication technology whereby, in contrast to CNT fabrication,
these metallic nanowires and cones can be deposited at a low
temperature of about 60 C. It is possible with this lamination process to fabricate nanowires on flexible substrates or
on the external and internal walls of metallic tubes. This fabrication technique also enables the integration of nanowires
in 3D complex microsystems.
ACKNOWLEDGMENTS
This project was funded by the Helmholtz International
Centre for Heavy Ion Research for the construction of the
accelerator complex in the context of the Facility for
Antiproton and Ion Research (FAIR) Project, Darmstadt,
Germany.
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2
FIG. 11. (a) Nanocones fabricated by pressing the foil on the supporting
walls. The large area of a predefined electrode enclosed with nanocones.
The supporting walls are structured as columns to shape communicative vessels. (b) The cones have a base diameter of 2 lm and a tip diameter of about
300 nm.
form of gold nanowire arrays for potential use in an ionization gauge. To overcome the thermomechanical instability
of the wires at higher extraction voltages, an in situ deposition of the nanowire arrays was used that could improve the
stability of the wires and provide a better electrical contact
to the substrate. We fabricated nanowire arrays with larger
areas and a low wire density. By pressing on the template
foil, we eliminated some defects that occurred in the structure during electrodeposition of the wires. We obtained
nanowires with a reduced standard deviation in their height
and obtained an increased homogeneity in the electrode
coverage.
Alternatively, we modified the shape of the cylindrical
wires to a conical geometry, which is believed to significantly increase the mechanical stability of the nanowires.
We demonstrated that in situ fabrication of the nanocones
could be accomplished with the same lamination process
J. Vac. Sci. Technol. B, Vol. 34, No. 2, Mar/Apr 2016