Corner-Promoted Focus Enhancement of Light in Conical Holes for Extraordinary Optical Transmission
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IEEE SENSORS JOURNAL, VOL. 21, NO. 7, APRIL 1, 2021 9081
Corner-Promoted Focus Enhancement of Light
in Conical Holes for Extraordinary Optical
Transmission
Simon Chun Kiat Goh , Li Lynn Shiau , Liangxing Hu, Nan Chen, Zhihao Ren,
Chengkuo Lee , Senior Member, IEEE, and Chuan Seng Tan , Member, IEEE
Abstract —Extraordinary optical transmission (EOT) is gen-
erated when light transmits through an array of subwave-
length holes on a metallic sheet. Most studies have focused
on the EOT effect with ideal cylindrical holes. Subsequently,
it has been recognized that imperfection in hole fabrication
could alter light-matter interaction. Later, adiabatic taper is
reported to promote light nano-focusing effect in the optical
and near infrared wavelength regime. Due to geometrical and
fabrication constraints, it is difficult to fabricate adiabatic
taper for mid-infrared responsive nanohole array. In this work,
we report the construction of nanohole arrays with varying
near-adiabatic taper holes. We demonstrate that when the
positive (inverted conic) angle increases from 10◦ to 20◦ ,
the peak full width half maximum (FWHM) improves by 14% from 800 to 700nm, respectively. Furthermore, inverted
conical hole is shown to be superior to negative conic angle. Unlike cylindrical hole array, conical hole array maintains
high transmission throughput for any given hole dimension while achieving better FWHM. The betterment in performance
for conical over cylindrical holes is attributed to a localized plasmonic effect at the terminal exit. Subsequently, the device
is demonstrated as a CO2 low flow sensor. The highest sensor sensitivity is determined to be between 1–4sccm.
Index Terms — Carbon dioxide, gas sensor, nanophotonics, nanostructures, nanofabrication.
I. I NTRODUCTION In the presence of an array of holes drilled into metal
films, coupled with geometrical design, maxima and minima
M ANY different sensing platforms, such as plasmonic
sensing, have been realized and evaluated for a mul-
titude of applications [1]–[4]. In 1998, Ebbesen et al dis-
peaks are revealed. Maxima peaks correspond to surface plas-
mons resonant excitation while the minima is associated with
covered the interesting phenomenon of light enhancement Wood’s anomaly [7]. Today, the phenomenon is commonly
by means of perforated holes in metallic thin film [5]. The described as the extraordinary optical transmission (EOT).
experiment demonstrated light-matter interaction between the EOT phenomenon is brought about by the surface plas-
incidental light and that of surface plasmons. The pioneering mon polariton, localized surface plasmons and transmission
work showcased the possibility of the 0th order transmission Fabry-Perot effects [8]–[10].
surpassing unity. In turn, the new discovery overturned the Ever since then, many research works have confirmed
then understanding based on Bethe’s theory of diffraction [6]. the essentiality of geometrical parameters to manipulate the
EOT transmission profile. These efforts include the alter-
Manuscript received January 12, 2021; accepted January 18, 2021. ation of period, diameter of the holes, hole shapes, unit
Date of publication February 18, 2021; date of current version March 5, cell arrangement, thickness of the metallic layer and the use
2021. This work was supported by the A∗STAR Science and Engi-
neering Research Council, Advanced Manufacturing and Engineering of different metals depending on the intended transmission
(SERC AME) Programmatic Fund under Grant A18A44b0055. The work frequency [11]. In particular, it has been shown by theoretical
of Simon Chun Kiat Goh was supported by Excelitas Technologies. and experimental works that a change in period could shift the
The associate editor coordinating the review of this article and approv-
ing it for publication was Prof. Hsin-Ying Lee. (Corresponding author: transmission wavelength. Earlier works focused on elucidating
Simon Chun Kiat Goh.) the nature of EOT that spanned localized EOT from isolated
Simon Chun Kiat Goh, Li Lynn Shiau, Liangxing Hu, and and randomized holes placement, light-molecule interaction
Chuan Seng Tan are with the School of Electrical and Electronic
Engineering, Nanyang Technological University, Singapore 639798 induced absorption transparency, Brewster angle EOT and
(e-mail: gohc0075@e.ntu.edu.sg; tancs@ntu.edu.sg). single light hole aperture [12]. In recent years, published works
Nan Chen, Zhihao Ren, and Chengkuo Lee are with the Department of are largely application-based [13], [14]. In general, EOT for
Electrical and Computer Engineering, National University of Singapore,
Singapore 117583. sensing applications is widely reported [15]–[17]. Broadly,
Digital Object Identifier 10.1109/JSEN.2021.3053273 these molecular sensors are centered on surface plasmonic
1558-1748 © 2021 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See https://www.ieee.org/publications/rights/index.html for more information.
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9082 IEEE SENSORS JOURNAL, VOL. 21, NO. 7, APRIL 1, 2021
resonance as well as fluorescence measurement enhancement thickness of the metal layer [31]. Due to the difficulty in fab-
for single molecule detection. Besides that, color filtering, ricating such feature for longer wavelength, conical nanohole
perfect absorber, meta lenses, optical trapping, lasers, etc have array is rarely reported for mid infrared and longer wavelength
also been demonstrated [18], [19]. radiation. Recently, Chen et al investigated the effect of
The myriad of applications mentioned is generally realized stepped and sloped taper nanohole array by simulation [33].
in the optical to the near infrared regime. Due to the sub- Despite the increase in transmission intensity for stepped taper,
wavelength hole size and other stringent geometrical require- its output response suffers from low quality (Q) factor.
ments, high precision nanolithography tools have to be used. In this work, mid infrared responsive tapered (conical)
A majority of the reported works in the literature are fabricated nanohole array is demonstrated. Through simulation and
using either single-step focused ion beam (FIB) or electron experiment works, an enhancement of EOT effect, in terms
beam lithography (EBL) followed by metal deposition and of transmission peak full width half maximum (FWHM)
liftoff [20]–[22]. Apart from that, other fabrication techniques and transmission intensity, brought about by the change in
such as nanoimprint, nanosphere lithography, laser interference nanohole taper angle will be investigated. At the same time,
lithography, etc have been proposed as alternatives to FIB and the fabricated sensor is validated as a low flow CO2 gas sensor.
EBL techniques [23]–[25].
Amongst the techniques, FIB, EBL and laser are typical II. E XPERIMENTAL D ETAILS
processes Comparing EBL, FIB and laser-based techniques, Nanohole array was patterned on CaF2 substrate by FIB or
the latter is operable at non-vacuum condition. On the con- EBL+liftoff to create nanoholes with different taper angles.
trary, EBL and FIB have to be operated under high vac- CaF2 (100) wafers were first cleaned in acetone and iso-
uum in specialized chambers, driving up fabrication and propanol under sonication. For FIB fabrication, 50 nm of Au
operating costs. On the other hand, EBL is more scalable was deposited directly on CaF2 by electron beam evaporation.
for possible production while the laser and FIB approaches The nanohole array was then etched with Ga as source. In this
being more time consuming are generally suitable for small case, samples of dimensions 50 μm x 50 μm were FIB-ed by a
scale manufacturing and research. The main shortcoming of ZEISS Crossbeam 540 FIB-SEM. The FIB probe acceleration
laser-based lithography is that the technique is hampered by voltage and current used for creating such array was set to
its wavelength-dependent optical diffraction limit. However, 30 kV and 50 pA. Then the surface was further etched to
recent advancements, as reported in a few publications, has remove the possible Ga remnants.
highlighted the performance of laser lithography technique For EBL process, a thin coating of AR-PC5090 con-
with the potential of challenging that of traditional FIB tech- ductive polymer was applied. Then, A4 PMMA was spin
nique in the future [26], [27]. coated onto the CaF2 substrate and processed according to
On the other hand, the focus of light in a single narrowing recommended specifications. It was then subjected to EBL
light pipe have been investigated previously [28], [29]. Such using 1400 μC/cm2 . The pattern was carefully developed in
field distributions and its associated localized field enhance- MIBK-IPA (1:3) for 1 min. Thereafter, 5 nm Ti and 50 nm Au
ment can be achieved through the modification of the structural were evaporated in sequence. Finally, metal liftoff was carried
design to realize Re [εm + 2εd ] → 0 [11]. So far, tapered out in acetone.
metal rods, glass sphere tip and tapered air gap have been The simulation was setup as follows. The material properties
reported[30] Many of those studies were carried out with of Au and Ti, used as-is from the default material database,
adiabatic taper where the critical angle, were obtained from the CRC Handbook of Chemistry &
Physics. On the other hand, CaF2 is allocated a constant refrac-
2εd tive index of 1.4 while the circular hole was set to “etched”
ζc ≈ | | (1)
εm with refractive index of one. The period (P) was set by varying
where εd and εm are the permittivity of the dielectric medium the x and y spans to be between 2.1 and 2.9 μm. The radius
in the taper and the real part of the metal permittivity. In addi- of the top hole (r1 ) was fixed at 0.625 μm throughout. Then,
tion, any design deviation from the critical taper angle would a plane-wave source was fixed at a location 15 μm away from
result in a decline in coupling efficiency along the length of the nanohole. The model mesh accuracy was set to 2 to balance
the taper. As a consequence, large angle (ζ ζc ) results in between accuracy and system requirement which affects the
scattering by mismatched waves. On the other hand, absorption overall runtime. To improve the simulated results accuracy,
losses dominate low coupling efficiency [31]. However, it has a high accuracy mesh was assigned to the nanohole with
to be noted that the laid out adiabatic condition is a mere a buffer of 300 nm. Finally, the simulation was performed
reference that could well underestimate the actual critical angle using Lumerical finite difference time domain (FDTD) module
value [29]. with a simulation time of 12000 fs at 300K. The boundary
So far, most studies focused on realizing a single nanotip conditions of x, y and z were set to anti-symmetric, symmetric
output port [32]. The output terminal is typically much smaller and PML, respectively with an auto shutoff criterion of 1e-5.
than the input terminal. As such, the critical angle for adiabatic
surface plasmon propagation decreases with an increase in III. R ESULTS AND D ISCUSSION
real part permittivity of metals as a result of a decrease in To verify the effects of non-vertical shape etch holes, conical
SPP frequency at longer wavelength. In addition, as reported shapes hole was simulated with top and bottom radii, r1 and r2 ,
by Choo et al, the critical angle is also determined by the as shown in Figure 1.
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GOH et al.: CORNER-PROMOTED FOCUS ENHANCEMENT OF LIGHT IN CONICAL HOLES 9083
Fig. 1. Schematic illustration of a conical hole with taper θ and radii r1 , r2
for the formation of an array of nanoholes where the top radius is larger
than the bottom hole radius.
The simulated results of the conical holes (r1 > r2 ) are
compared to that of perfect cylindrical holes where r1 = r2 .
The latter is typical described geometry for most EOT design
experiment. Although desirable, it is all but highly challenging
to fabricate due to equipment and process limitations. Typi-
cally, a positive or negative slope would be obtained experi-
mentally. From a practical perspective, it is more meaningful
to simulate sloped features. Thus, a simulation was carried
out to understand the effect of varying r1 and r2 . As shown in
Figure 2, a combination of different r1 and r2 scenarios were
simulated with P = 2.5μm.
It can be observed from Figure 2A that variations in r1
and r2 values with values less than 4% do not change the
transmission wavelength. On the other hand, when r2 < r1 ,
the peak full width half maximum (FWHM) decreases. Similar Fig. 2. A) Simulated results for different r1 and r2 values and B) shift in
passband, ιpeak , as a result of period change.
enhancement is obtained when r1 < r2 . However, the FWHM
enhancement (which corresponds to a smaller FWHM value) is
greater for r2 < r1 than r1 < r2 . A change from perfect cylin- Therefore, FDTD simulations involving cylindrical and conical
der to inverted conical hole array brings about an improvement hole arrays of various bottom hole radius, r2 , are carried out.
in transmission Q factor. Further reduction of r2 (>4%) with With reference to Figure 3A, it can be observed that the
fixed r1 leads to a significant decrease in transmission intensity transmission profiles of conical holes (r1 > r2 ) are differ-
albeit incremental improvement in FWHM. ent from that of cylindrical holes. Namely, the simulated
Generally, the transmission profile of a nanohole array can transmission intensity response of conical holes is greater
be approximated by the following equation (2) than that of cylindrical holes (Inset I). It is expected that a
smaller hole dimension will result in a lower transmission
P εdεm intensity. According to Figure 3A (Inset II), the decrease in
λpeak = Re( √ ) (2)
i 2 + j 2 εd + εm intensity is less for conical than cylindrical nanohole arrays.
Moreover, the difference is more explicit for smaller r2 radius.
where P is the nanohole array period, i and j are integers, εd As illustrated in Figure 3B, a localized plasmonic effect can
and εm are the relative permittivity of the dielectric and metal be induced with a change in the edge angle. With a progressive
layers. A change in period, as shown in Figure 2B, would difference in radii, r1 and r2 , greater localized plasmonic
affect the transmission passband, λ peak . In this case, smaller effect is promoted. As a result, localized plasmonic ‘hotspot’
period corresponds to shorter λ peak while larger period corre- at the exit terminal end enhances the overall electric field.
sponds to longer λ peak . As simulated, transmission intensity Comparing the case of r1 = r2 = 0.625μm with that of
decreases and a smaller FWHM at longer λ peak . r1 = 0.625μm, r2 = 0.55μm hole arrays, the field inten-
Possibly, the EOT quality can be enhanced by decreasing sity enhancement of the conical system is 40 % more than
the hole radius while keeping the period constant. It has that of the cylindrical system. Similar to previous reports,
been shown in earlier works that a decrease in hole radius by using noble metal nanotip, field enhancement can be
improves the transmission peak FWHM. However, a conse- further improved with the alteration of the cone geometrical
quential decrease in transmission intensity is then expected. design [34].
In this particular case, it is interesting to determine if the On the other hand, with reference to r1 = r2 = 0.625μm
enhancement is a result of hole narrowing and/or tapering. and r1 = r2 = 0.550μm, the “squeezing” effect due to a
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9084 IEEE SENSORS JOURNAL, VOL. 21, NO. 7, APRIL 1, 2021
Fig. 4. Simulated EOT for both cylindrical and conical hole array of
P = 2.5μm in the presence/absence of Ti interlayer necessary for
Au adhesion to substrate. The presence (absence) of a Ti interlayers,
up to 10 nm, improves (worsens) the performance of conical (cylindrical)
nanohole array.
pendent of hole radius. In which, transmission profile can be
finetuned in addition to hole radius and period manipulation.
Previously, the effect of Ti adhesion layer has been investi-
gated. Ti is commonly used as an interlayer thin film to attach
Au film onto an underlying substrate or thin film. In many
cases, Ti of up to 10 nm is sputtered or evaporated. It has been
shown that the inclusion of Ti interlayer would 1) redshift and
2) reduce the plasmonic peak attributed to its high damping
frequency [35].
According FDTD simulation as shown in Fig 4, significant
peak shift by 50 nm and the decrease in transmission amplitude
associated with Ti interlayer are observed. The phenomenon
can only be observed when the Ti thickness is more than
10 nm. On the contrary, a 5 nm Ti layer would only cause an
approximate 10% decrease in transmission. It does not cause
an observable redshift in peak position.
Similar to results documented in published reports, the pres-
ence of a Ti interlayer worsens the transmission peak. By using
FDTD simulation, it is proven that the FWHM increases
from 0.210 μm (Ti 0 nm) to 0.249 μm (Ti 10 nm) for
a cylindrical hole array. Interestingly, the inclusion of a Ti
Fig. 3. A) Transmittance profile differs between conical and cylindri- interlayer improves the transmission peak FWHM for a conical
cal holes Inset I: Zoomed in profile of peaks. Inset II: Improvement hole array. For a Ti 5nm interlayer thickness, the FWHM
in transmittance of conical holes over that of cylindrical holes with decreases from 0.210 μm (0.625 μm – 0.625 μm) to 0.187 and
larger transmittance difference accentuated by smaller r2 . B) Change
in electric field with respect to an increase in the conical angle resulting 0.163 μm for (0.625 μm – 0.600 μm) and (0.625 μm –
in an increase n localized enhancement as a consequence of greater 0.580 μm) conical holes, respectively.
light-matter interaction with an increase in taper angle. To demonstrate the effect experimentally, holes were fab-
ricated by both 1) EBL followed by liftoff (denoted as
EBL+liftoff) and 2) FIB. The former technique typically
narrower aperture diameter can also affect the transmission yields almost-vertical thin film profile after deposition and
response, such as the FWHM. This fact illustrates that conical liftoff. On the contrary, like other dry etching techniques,
hole might influence the transmission profile differently as FIB-ed structure can be moderated according to the milling
compared to cylindrical hole. Therefore, the taper angle is an process conditions. Subsequently, EBL and FIB prepared
additional parameter for the alteration of optical response inde- samples were subjected to transmission electron microscopy
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GOH et al.: CORNER-PROMOTED FOCUS ENHANCEMENT OF LIGHT IN CONICAL HOLES 9085
It can be seen in Figure 6A and B that the transmission
passband is λpeak period dependent. Similar to simulation,
the λpeak red shifted with larger period. Both EBL and FIB
samples have similar λpeak between 3.5 and 5.0 μm. The main
difference between transmission profile of EBL and FIB is that
of the transmission intensity. Comparing the λpeak when P =
2.1μm, the transmission intensity is approximately 17.5 % and
30% for EBL and FIB samples, respectively. It can be observed
that the FIB peaks intensities are approximately twice that
of EBL peaks intensities. The decline in transmission for
EBL-made samples is attributed to the decrease in plasmonic
propagation length caused by the Ti layer [38]. In addition,
we report an improvement in Q factor when the hole is
conically shaped. According to Figure 6C and 6D, a decrease
in slope gradient of about 20◦ translates to a decrease in
FWHM from 800 nm (EBL+liftoff) to 700 nm (FIB). The
outcome is similar to simulation and previous reports such
as [28].
Nanohole array can be fabricated for a multitude of EOT
sensors. One such use cases is the fabrication of miniaturized
spectrometers. The variation in period, as shown in Fig 2B,
allows one to construct a 1D or 2D sensors. For simplicity,
the FIB samples were later explored as an absorption-based
CO2 gas flow sensor. The steady-state flow rate of CO2
was passed through the overhead space, between that of the
sample and the light source. In this case, CO2 molecules
which absorb incidental radiation at 4.26 μm, would attenuate
the transmission intensity according to the volume of CO2
present. As shown in Figure 7A, CO2 gas flow is moderated
between 4 and 10 sccm by a mass flow controller. The best
fit curve, as plotted in Figure 7B, reveals a R-squared value
of 0.996. It can be observed that the maximum sensitivity,
determined by the slope of the curve at a given point, of the
sensor is when the flow rate is between 0 and 4 sccm.
Within the range, due to the sensitivity of the sensor, small
change in flow rate could be precisely discriminated. On the
other hand, the maximum detection limit is extrapolated to
Fig. 5. Cross-sectional TEM images of A) EBL+liftoff and B) FIB be approximately 23.6 sccm. This work epitomizes the use of
samples where EBL+liftoff (71◦ ) results in steeper sidewall profile than EOT sensors as a potential high sensitivity low flow gas sensor
FIB (50.7◦ ).
for industrial and healthcare uses. Potential use cases include
insufflation gas flow monitoring during laparoscopic surgery
and CO2 level moderation in greenhouses.
(TEM) for cross sectional analysis as shown in Figure 5. While numerous mid infrared plasmonic filters have been
It can be seen that EBL+liftoff process results in Au with published, their performances are rarely tabulated and com-
high vertical profile of 71◦ . More digital vertical profile can pared. As shown in Figure 7C, best reported experimental
be obtained by performing liftoff using thicker resists or FWHM from recent publications on free space plasmonic
by introducing a triple layer liftoff [36] On the other hand, filters are included and compared. A smaller FWHM which
the slope of FIB created sidewall is shown to be approximately translate to better sensor quality, is preferred. Generally,
50.7◦. In this case, r2,EBL > r2,FIB . Generally, both processes from Figure 7C, plasmonic filters FWHM increases at longer
result in the formation of smooth etched sidewalls [37]. wavelength. This work showcases an improved FWHM for
Thereafter, the fabricated samples were subjected to optical both EBL and FIB fabricated devices. However, reports that
testing using a Fourier transform infrared spectrometer (FTIR) utilize molecule gas traps such as metal organic framework,
with an attached microscope unit. The effective measurement polyethyleneimine as well as works which do not reveal the
range is from 2–8 μm. The aperture can be adjusted to accom- transmission / reflection profiles are excluded. As compared
modate sampling dimensions between 12.5 μm ×12.5μm and to other state of the art sensors, the developed sensor’s quality
200 μm ×200μm. Prior to transmission mode measurement, factor (FWHM) at 4.26μm is at least 2.4 times higher.
a background reference in air was taken. The results could be The enhancements in FIB transmission intensity and
seen in Figure 6. FWHM are further investigated. Recently, gallium has been
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9086 IEEE SENSORS JOURNAL, VOL. 21, NO. 7, APRIL 1, 2021
Fig. 6. Experimentally obtained transmission period specific passbands for A) EBL and B) FIB samples with periods between 2.1 and 2.9 μm.
Comparison of passband for period 2.5 μm for C) EBL and D) FIB showing a 14% enhancement of FWHM for the latter.
reported to display plasmonic effect [46]. Thus, one of the as follows. As shown in Figure 9, the nanohole array was
main questions is whether the presence of a thin layer of divided into four quadrants (I – IV). Thereafter, the deposited
Ga contamination introduced during the milling process is Au film on the bottom half of the hole array (III, IV) was
responsible for the enhancement [47]. Firstly, area scanned gently scrapped off using a sharp tip tweezer. The left-half
TEM energy dispersion spectroscopy (TEM-EDX) was carried of the sample (I, III) was then subjected to low KV cleaning.
on EBL+liftoff and FIB samples as shown in Figure 8. The quadrants were then optically measured. The aperture size
For EBL sample, the presence of Ca, F and Au and the was set to be 12.5 μm x 12.5 μm. As shown in Figure 9B,
absence of Ga as marked by the red circle suggest the the transmission between quadrants (I, II) were compared.
following. 1) FIB milling with Ga source used during surface Marginal change to FWHM and transmission intensity could
investigation does not contribute significantly to surface Ga be observed. Both Ga only quadrants (III, IV) were optically
contamination. 2) Serve as a confirmation reference for FIB measured to be non- plasmonic responsive between 2 – 8 μm.
sample. In contrast, the FIB sample, as shown in Figure 8B, This fact indicates that the implanted Ga exerts low influence
reveals a detectable amount of Ga as detected by TEM-EDX. on the infrared radiation transmission. As such, it is deter-
Therefore, surface contamination by Ga must be accounted for mined to be non-mandatory in attempting to remove Ga post
in processes that involve Ga ion milling. As such, alternatives FIB milling.
such as noble gas milling have been developed. Although it Essentially, a thick metal layer is required for effective
can eliminate Ga contamination, one drawback is that the light pipe focusing in the mid infrared regime. In which,
process is time-consuming and uneconomical for large area strict adherence to the critical angle rule would require the
hole milling operation. deposition of a very thick gold layer. To circumvent the
Secondary to ion milling, FIB sample was further processed limitation, instead of forming a fine taper output terminal,
to remove the deposited Ga. In this case, a low acceleration the structure is allowed to taper non-adiabatically with ζ > ζc .
voltage (KV) at 5 kV and current at 10 pA etching step was In this instance, the taper angle of interest has to be achievable
used. The main purpose of this step is to remove the implanted by a typical etching process. With a half cone angle of about
Ga by means of a physical process. The setup of the steps is 10◦ and 20◦ as demonstrated by the EBL and FIB samples, the
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GOH et al.: CORNER-PROMOTED FOCUS ENHANCEMENT OF LIGHT IN CONICAL HOLES 9087
Fig. 8. Cross-sectional TEM-EDX performed on ion milled A) EBL and
B) FIB samples to assess the extent of Ga contamination during the
hole milling procedure. The latter is presented with a more significant Ga
presence than the former in which FIB was used as part of the sample
preparation step.
Fig. 7. A) Transmission characteristics of FIB samples when mea- loss in transmission intensity is obvious with an increase in
suring CO2 gas flow at different flow rates. B) Transmission and r2 radius. Despite its ability to preserve transmission intensity,
CO2 flow rate relationship with. C) Performance comparison table further increase in taper angle would contribute to a decrease
showing the FWHM (FOM) of the proposed sensor against that of
recent state-of-the-art mid infrared plasmonic filters with references in the overall transmission intensity.
obtained from [39]–[45]. PE–polyethylene, PVC–polyvinyl chloride, In order to increase the EOT effect, an overall evaluation of
PEI–polyethylene imine. all participating variables have to be collectively considered.
Importantly, the optimal thickness of the gold layer to support
non-adiabatic taper results in loss as evidenced by the decrease the required taper angle has to be studied. In addition, this
in measured transmission intensity. work informs the need to optimize etching/milling process
However, the local field is then enhanced by the sharp to achieve repeatable sidewall profile. This is to ensure that
corner at the terminal end of the conical hole mitigating further sensors from different manufacturing batches can be fabricated
transmission loss. Comparing the simulated outcome between with similar performance as specified.
cylindrical and conical hole arrays, the latter suffers lower Previously, the differences between EBL and FIB fabri-
transmission loss (Figure 3A, Inset II). In particular, the lower cated holes have been reported by Michal et al [48]. While
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9088 IEEE SENSORS JOURNAL, VOL. 21, NO. 7, APRIL 1, 2021
etching process could offer a low sticking coefficient surface.
In the absence of molecule accumulation on the passivated
surface, the sensor is capable of performing transient mole-
cule flow monitoring. Furthermore, the highly precise etching
profile can be accomplished by dry etching. Depending on the
requirement, the nanohole array can be designed and fabricated
accordingly. Therefore, EOT sensors could be manufactured
with commercially available etchers in the near future.
V. C ONCLUSION
In this work, conical nanohole array fabrication techniques
are showcased. It has been demonstrated that EBL and sub-
sequent liftoff, and FIB processes result in 71◦ and 50.6◦
sloping metal sidewalls, respectively. The conical hole in Au
gold amplifies the localized field enhancement at the acute
corners resulting in the enhancement of FHWM and trans-
mission intensity. Also, an inverted shape conical hole, where
r1 > r2 , yields a better EOT enhancement than when r1 < r2 .
Comparing r1 = r2 and r1 = 0.625, r2 = 0.55 nanohole
arrays, the field enhancement of the latter is approximately
40 times that of the former. The FWHM is enhanced by 14 %
from 800 nm to 700 nm at 4.2 μm for EBL+liftoff and FIB
processed nanohole array, respectively. Moreover, it has been
verified that the presence of Ga on the FIB sample surface,
does not contribute to a decline in the transmission profile.
A CO2 gas flow sensor is demonstrated with the highest
sensitivity for CO2 gas flow between 1–4 sccm. Furthermore,
the demonstrated FIB etching methodology can be extended to
reactive ion dry etching techniques where the taper angle can
be precisely controlled. This case, large area nanohole array
Fig. 9. A) SEM image of a partially destructed FIB hole array to study the
effects of Ga stripping with segments, with/without Au and with/without fabrication for mass production can be realized. Finally, taper
Ga stripping. B) Optical measurement and comparison of transmission angle control can be used as a new parameter to regulate the
profiles of fresh FIB sample and Ga stripped sample. Inset I and II show performance of EOT devices.
the SEM-EDX of the respective samples.
the authors heralded the merits of FIB, rounded hole top ACKNOWLEDGMENT
was showcased to negatively affect the EOT performance. The authors acknowledge the Facility for Analysis, Char-
In contrast, as depicted in Figure 8B, greater light confinement acterization, Testing and Simulation (FACTS), Nanyang Tech-
occurs at the terminal exit end. Thus, this work demonstrates nological University, Singapore, for the use of their electron
that the case of rounded conical hole top exerts minimal impact microscopy facilities. The authors declare no conflicts of
on the overall field enhancement. Furthermore, cross sectional interest.
TEM analysis in Figure 5 confirms that the diameter of the
terminal exit end for the FIB milled hole is comparable to R EFERENCES
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