Fabrication and characterization of gated field emitter arrays with self-aligned carbon nanotubes grown by chemical vapor deposition

Article abstract of DOI:10.1063/1.1506408

Field emitter arrays with multiwall carbon nanotubes (CNTs) grown inside their gated holes were fabricated on glass substrates. The Fe-Ni-Co alloy catalyst dots on which the CNTs would be grown were deposited into the gated holes by a self-aligned method to maintain a constant distance between CNT emitters and gate electrodes. The CNTs were synthesized by thermal chemical vapor deposition using a gas mixture of CO and H₂ at 500°C. The CNT lengths were controlled by changing ratios of CO to H₂. Field emission currents and images were monitored as a function of gate and anode voltages. It was shown that the CNT emitters grown just up to the gate electrode height operated best in a triode mode.

Full text of DOI:10.1063/1.1506408

Fabrication and characterization of gated field emitter arrays with self-aligned carbon
nanotubes grown by chemical vapor deposition
In Taek Han, Ha Jin Kim, Young-Jun Park, Naesung Lee, Jae Eun Jang, Jung Woo Kim, Jae Eun Jung, and
Jong Min Kim
Citation: Applied Physics Letters 81, 2070 (2002); doi: 10.1063/1.1506408
View online: http://dx.doi.org/10.1063/1.1506408
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APPLIED PHYSICS LETTERS
VOLUME 81, NUMBER 11
9 SEPTEMBER 2002
Fabrication and characterization of gated field emitter arrays with self-
aligned carbon nanotubes grown by chemical vapor deposition
In Taek Han,a) Ha Jin Kim, Young-Jun Park, Naesung Lee,b) Jae Eun Jang,
Jung Woo Kim, Jae Eun Jung, and Jong Min Kimc)
FED Project Team, Samsung Advanced Institute of Technology, P.O. Box 111 Suwon 440-600, Korea
͑Received 1 May 2002; accepted for publication 17 July 2002͒
Field emitter arrays with multiwall carbon nanotubes ͑CNTs͒ grown inside their gated holes were
fabricated on glass substrates. The Fe–Ni–Co alloy catalyst dots on which the CNTs would be
grown were deposited into the gated holes by a self-aligned method to maintain a constant distance
between CNT emitters and gate electrodes. The CNTs were synthesized by thermal chemical vapor
deposition using a gas mixture of CO and H2 at 500 °C. The CNT lengths were controlled by
changing ratios of CO to H2 . Field emission currents and images were monitored as a function of
gate and anode voltages. It was shown that the CNT emitters grown just up to the gate electrode
height operated best in a triode mode. © 2002 American Institute of Physics.
͓DOI: 10.1063/1.1506408͔
It has been a hot issue for about last ten years to explore
new materials for the field emitters which can easily emit
electrons, from metals having high electron densities near the
Fermi level to wide band gap materials having negative elec-
tron affinity.1–3 Since the discovery of carbon nanotubes
͑CNTs͒, they have attracted much attention due to their ex-
cellent field emission characteristics for vacuum microelec-
tronics. The CNTs possess inherent geometry of high aspect
ratios ͑small diameter and large length͒ such that many sci-
entists have investigated their field emission properties, re-
porting lower electric fields for electron emission than any
other materials.4–6 Beyond such scientific interest, the efforts
to apply the CNTs to vacuum microelectronics have been
intensively made, among which gated CNT field emitter ar-
rays ͑FEAs͒, in particular, for the display applications, are
most interesting.
based on glass substrates. One of the requirements to de-
velop the gated CNT FEAs is controllability of their dimen-
sional parameters such as gate hole diameters, CNT lengths,
CNT-to-gate electrode distances, etc. Here we developed a
self-aligned growth method of CNTs inside the gated holes
to easily keep a designed distance between CNT emitters and
gate electrodes. The CNTs were deposited into the gated
holes as low as 500 °C by thermal CVD on the gated FEAs
of glass.
The gated FEA templates with the 2.5 in. diagonal were
fabricated by photolithography processes, as illustrated in
Fig. 1. The larger gate holes in diameter than expected by
photoresist ͑PR͒ are defined by overetching. The SiO2 insu-
lator is wet-etched through the gate holes as well as the PR
holes. Thereafter, the Invar alloy (Fe:Ni:Coϭ52:42:6) is
deposited to be catalyst for the CNT synthesis by electron
Two methods have been developed to put CNTs into the
gated holes so far. One is to squeeze the CNT mixtures7,8 ͑in
most cases with organic vehicles͒ into the holes, and the
other is to directly grow the CNTs inside the holes by chemi-
cal vapor deposition ͑CVD͒.9–12 The former approach has
already demonstrated color moving pictures with the gated
CNT FEAs.8 This has shown a strong potential of application
of the CNTs to the field emission displays, but seems to be
limited to large area and low resolution displays. Thus, to
achieve high-resolution displays with extremely low opera-
tion voltages requires direct growth of CNTs inside of the
gated holes and precise control of gate-to-CNT emitter dis-
tances. Several groups9–12 have fabricated and operated the
gated FEAs with the CNTs directly grown by CVD, how-
ever, those were of primitive cells and moreover fabricated at
high temperatures over 600 °C. Higher temperatures than
500 °C are usually not allowable to field emission displays
a͒
Also at: Department of Chemistry, Seoul National Univ., Sillim, KwanAck,
Seoul 151-742, Korea; electronic mail: ithan@sait.samsung.co.kr
b͒
Department of Nano Science and Technology, Sejong University,
Kwangjin-ku, Seoul 143-747, Korea.
Author to whom correspondence should be addressed; electronic mail:
c͒
jmkim@sait.samsung.co.kr
FIG. 1. Schematics of fabrication processes of FEA templates.
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0003-6951/2002/81(11)/2070/3/$19.00 2070 © 2002 American Institute of Physics
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Appl. Phys. Lett., Vol. 81, No. 11, 9 September 2002
Han et al.
2071
FIG. 2. ͑a͒ SEM image of an FEA with the CNT dots grown inside gate
holes at the CO/H2 flow rates of 80/1270 sccm and ͑b͒ cross-sectional SEM
image of CNTs in a dot.
beam evaporation using the PR layer as a mask to define the
catalyst dots inside the gate holes. The subsequent PR strip-
ping produces the gated FEA template. The electron beam
deposition that supplies evaporated metal beams in the ver-
tical direction, forms inside the gate holes the catalyst dots
with the same diameter with that of the PR holes. This pro-
cess results in concentric, circular catalyst dots self-aligned
with each gate holes. No contamination by catalyst on the
sidewall surfaces of insulator holes prevents electrical leak-
age between the cathode and gate electrodes. The self-
aligned formation of catalyst dots provides an easiness of
fabrication by reducing the processing steps. The diameters
of catalyst dots, gate holes, and insulator holes at the bottom
are 9, 13, and 15 ␮m, respectively in this experiment. The
as-fabricated FEA template was characteristic of a resolution
of 120 linesϫ40 linesϫ3 ͑red, green, blue colors͒ with to-
tally 1 800 000 gate holes for an active area of 3.96 cm
(gate electrodes)ϫ5.28 cm ͑cathode electrodes͒. The gated
FEA template was loaded into a vacuum chamber to grow
FIG. 3. Cross-sectional SEM images of the CNT dots grown inside gate
holes with the CO/H2 flow rates of ͑a͒ 50/1300, ͑b͒ 80/1270, ͑c͒ 100/1250,
and ͑d͒ 150/1300 sccm, where the insets are their tilted images.
of substrate from metallic to black started to occur at 350 °C
and CNTs were not grown during ramp up to 500 °C. Con-
clusively, the amorphous carbon starts to form at 350 °C
where the color change increases the substrate temperature
by absorbing more energies. The amorphous carbon deposi-
tion continues until CNTs start to grow at 500 °C, finally
leading to the configuration of the CNT layer enclosing the
amorphous carbon layer. A transmission electron microscopy
investigation of CNTs shows the herringbone, multiwalled
structure.
Figure 3 shows cross-sectional and tilted SEM images of
the CNT dots grown inside the gated holes with different
flow rates of CO while keeping the same total gas flow rate
of 1350 sccm. The CNT dot heights become larger with an
increase of the CO flow rates from 50 to 150 sccm. At 50
sccm, a small amount of CNTs, probably including the amor-
phous carbon, are deposited at the bottom of the gate hole
͓Fig. 3͑a͔͒. At 80 sccm, the CNTs are grown to just fit the
gate electrode height ͓Fig. 3͑b͔͒, whereas the CNTs are over-
grown slightly above the gate electrode at 100 sccm ͓Fig.
3͑c͔͒. It is observed that the CNTs are more uniformly grown
CNTs. After evacuating the chamber down to
1
ϫ10Ϫ2 Torr, a CO and H2 gas mixture was introduced up to
an atmospheric pressure with the total flow rate of 1350
sccm. Then, the substrate was heated up to 500 °C by an
infrared light radiation. The peak temperature of 500 °C was
kept for 20 min for CNT growth and thereafter ramped down
to room temperature. The substrate temperatures were moni-
tored by a thermocouple located just underneath the sub-
strate, which was well calibrated by a CNT-coated thermo-
couple in contact with the topmost surface of the substrate.
CNT deposition method and system were described in a pre-
vious report.13
Figure 2͑a͒ shows scanning electron microscopy ͑SEM͒
images of a resultant CNT FEA. Hemispherical dots of CNTs
are selectively grown only on the catalyst areas. The CNT
dots are kept uniformly apart from the gate hole edges by the
designed distance of 2 ␮m so that the electrical insulation is
maintained between the cathode and gate electrodes. As
shown in Fig. 2͑b͒, the hemispherical CNT dots are com-
posed of an inner amorphous carbon layer enclosed by an
outer CNT layer. The CNTs are randomly oriented. The
amorphous carbon and CNT layers are about 0.8 and 0.2 ␮m
thick, respectively. It seems that the inner amorphous carbon
forms because infrared light heating is not enough to synthe-
size CNTs at the initial stage of growth. Insufficient decom-
position of CO and H2 on the catalyst due to low thermal
energies would result in an amorphous phase of carbon. As
the amorphous carbon layer forms, the Invar alloy catalyst
dots undergo the color change from shiny metallic to black.
Such color change enables them to absorb more energy from
an infrared light, consequently giving rise to a temperature
FIG. 4. Field emission currents measured at anode electrodes as a function
of gate voltages for constant anode biases of 500, 800, 1000 V from the
CNT FEAs with the CO/H2 flow rates of ͑a͒ 80/1270 and ͑b͒ 100/1250
sccm, where the insets show their Fowler–Nordheim plots. The correspond-
ing emission images were observed at the gate voltages of ͑a͒ 60 and ͑b͒ 30
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increase of the substrate. In our observation, the color change
V for the anode voltage of 800 V.
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2072
Appl. Phys. Lett., Vol. 81, No. 11, 9 September 2002
Han et al.
at 80 sccm than 100 sccm. When the CO flow rate increases
to 150 sccm, the CNT dot as high as 1.7 ␮m occurs ͓Fig.
3͑d͔͒. The growth rates of CNTs ͑as well as amorphous car-
bon͒ strongly depend on the supplied carbon quantities.
Emission characteristics of our gated CNT FEAs were
measured for the cathode-to-anode gap of 1.1 mm. Measure-
ment conditions are descried previously.8 Figures 4͑a͒ and
4͑b͒ gives anode currents from the 2.5 in. diagonal FEAs
with the CNTs grown with the CO gas flow rates of 80 and
100 sccm ͓Figs. 3͑b͒ and 3͑c͔͒, respectively, with gate biases
for constant anode voltages of 500, 800, and 1000 V while
the cathode electrodes were grounded. Sufficient electron
emission was not observed from the 50 sccm CO grown
sample, probably due to the low CNT dot height. The sample
grown with 150 sccm CO has too low electrical resistance
between the cathode and gate electrodes to measure field
emission properties. It seems that the low resistance comes
from carbonaceous deposit formed on the sidewall surface of
the insulator holes at the high CO concentration. It is already
reported that high carbon concentrations give rise to lower
cathode-gate resistances in the CNT FEAs made by CVD.14
The FEAs with the CNTs reaching the gate electrode height
of approximately 1 ␮m show a successful operation in a
triode mode. As shown in Fig. 4͑a͒, the sample grown with
80 sccm CO turns on at the gate voltage of around 40 V, and
shows a linearity in the Fowler–Nordheim plot, representing
that measured anode currents occur by field emission. The
phosphor emission image shows a quite good uniform emis-
sion over a diagonal 2.5 in. area. When the flow rate in-
creases to 100 sccm, the turn-on gate voltage drops to 7 V,
but several bright spots are observed in an emission image. It
is considered that most emission currents are contributed
from these bright spots, giving rise to such a low turn-on
gate voltage. An appearance of the bright spots would result
from shorter distances between the CNT dots and gate elec-
trodes in the corresponding gate holes. This seems to agree
with the nonuniform growth of CNTs in a hole, as shown in
Fig. 3͑c͒. It is noted that emission currents do not signifi-
cantly change with anode voltages from 500 to 1000 V for
the cathode-to-anode gap of 1.1 mm. This implies that elec-
tron emissions from the CNTs are controlled only by gate
voltages without any diode electron emission by anode bi-
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