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Electron beam lithography of microbowtie
structures for next-generation optical probe
Ampere A. Tseng
Arizona State University
Department of Mechanical
& Aerospace Engineering
Tempe, Arizona 85287-6106
E-mail: [email protected]
Chii D. Chen
C. S. Wu
Institute of Physics
Academia Sinica
Nankang, Taipei, Taiwan, 11529
Rodolfo E. Diaz
Michael E. Watts
Arizona State University
Department of Electrical Engineering
Tempe, Arizona 85287-5706
Abstract. The development of microbowtie structures for a nextgeneration optical probe called the Wave Interrogated Near-Field Array
(WINFA) is presented. The WINFA combines the sensitivity of near-field
detection with the speed of optical scanning. The microbowties are designed to act as resonant elements to provide spatial resolution well
below the diffraction limit with a transmission efficiency approaching
unity. Following an introduction of the concept and background information, the design of the microbowtie is presented. A numerical electromagnetic scattering model is developed and used for better designs of the
bowtie structures. The electron-beam lithography process is then used to
fabricate the final designed bowties structure. Special fabrication procedures have been developed to cope with the charge dissipation problem
that arises when lithographing an insulating substrate as is required in
the present probe design. Two types of substrates and two types of
resists are considered in the present study. The fabricated microstructures have 40 nm bowtie gaps that are more than 200 000 times smaller
than the one built previously. All fabricated bowtie microstructures are
examined and the results are compared. It has been found that, in addition to the relative ease in fabrication, the bowties on indium–tin–oxide
coated glass substrate can not only minimize the charge accumulation in
a glass substrate, but also satisfy the functional requirement of optical
transparency to the incident wave. Recommendations for making a
bowtie structure in the even smaller bowtie array are also included.
© 2002 Society of Photo-Optical Instrumentation Engineers. [DOI: 10.1117/1.1479707]
Subject terms: electron-beam lithography; microbowtie; microfabrication; micro/
nano; inspection; optical probe.
Paper JM3 01028 received Nov. 12, 2001; revised manuscript received Dec. 19,
2001 and Feb. 7, 2002; accepted for publication Feb. 8, 2002.
1 Introduction
Small debris or defects in micrometer and nanometer scales
are the leading cause of failure of components and end
products in widely diverse industries.1 Nanoscale debris on
semiconductor substrates, such as process contaminants or
postpolishing substrate fragments, need to be accurately located and identified in order to obtain the cleanliness required for the next generation of integrated microcircuits.
The requirements for magnetic recording media surfaces
are becoming similarly stringent. In addition to resolving
defects of such a size, the on-line inspection tools required
to support this technology would still be required to meet
the current inspection rates of 50 wafers/hr. Of all current
inspection tools, optical probes are still the fastest in the
market. However, as the objects become of the order of the
wavelength 共␭兲, they become impossible to be imaged, and
below the diffraction limit 共on the order of ␭/␲兲 eventually
the signal to noise ratio becomes prohibitively low to support fast, reliable on-line inspection based on optical scattering. Therefore, with standard optical approaches 共assuming blue light兲, the smallest objects that can be reliably
examined are on the order of 500 nm, with the detectability
limit lying around 100 nm. A new, quick, and reliable
method of detecting defects significantly smaller than the
diffraction limit is needed.
JM3 1(2) 123–135 (July 2002)
1537-1646/2002/$15.00
Switching to shorter wavelengths 共ultraviolet, x ray,
e-beam兲 for detecting and identifying particles much
smaller than the diffraction limit would be a viable alternative, if mature scatterometry 共that is, dark-field兲 systems
existed in those regions of spectrum. But such systems capable of scanning production-sized semiconductor wafers
do not exist because of many technical and cost problems.
The use of shorter wavelengths in a microscopy mode 共or
bright-field system兲 is also precluded by the prohibitively
long time it would take to detect a nanoscale spot size in a
typical 200 mm 共in diameter兲 wafer. Thus a system that
sidesteps the diffraction limit but at the same time exploits
existing mature and reliable optical technology becomes an
attractive alternative.
Near-field optics, enabling optical imaging with spatial
resolution significantly better than the diffraction limit, has
recently found application in many fields.2,3 However, its
transmission efficiency is still orders of magnitude smaller
than unity. This low efficiency limit makes the near-field
optics unacceptably slow for practical applications. Recently, using a microwave setup as shown in Fig. 1, Grober
et al.4 showed that a bowtie antenna could be adopted to
realize a near-field optical probe that combines spatial resolution well below the diffraction limit with dramatically
increased transmission efficiency. The open-circuited
© 2002 Society of Photo-Optical Instrumentation Engineers
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to study the near field that will be generated in the vicinity
of these bowtie structures. The numerical results have been
used to optimize the transmission efficiency and for better
designs of the shape of the bowties. The fabrication process
using electron-beam lithography 共EBL兲 is reported in detail. Since transparent properties are required for the bowtie
substrate, SK7 glass is selected as the prime material considered for the substrate. Because of the insulating character of the glass, special fabrication procedures have been
developed to cope with the charge accumulation problem
encountered in lithographing an insulating substrate. Other
special considerations for fabricating microbowties are also
discussed. The fabricated bowtie microstructures are evaluated and studied.
Fig. 1 Schematic of wave concentration by a bowtie resonant element.
bowtie antenna intercepts the incident wave and concentrates its energy into the quasistatic dipole field in its gap
region. The result is an illuminated area on the order of
1/5–1/10 of the wavelength across, with a transmission efficiency ranging fom 2% to as high as 30%. Grober’s concept was demonstrated with a single-bowtie system operated at 2.2 GHz frequency or 13.6 cm wavelength; the
corresponding bowtie structure is 36 cm long with a 1 cm
gap. To operate at optical frequencies, the bowtie structure
has to be scaled down to the submicron level. If the reported efficiencies hold, it would represent an increase of
1–2 orders of magnitude over that of the current technology in near-field optics.
Consequently, the present research aims to extend the
concept proposed by Grober et al. from the microwave to
the visible range, and to demonstrate the feasibility of creating arrays of optical antennas to realize the Wave Interrogated Near-Field Array 共WINFA兲 concept proposed by
Diaz et al.5 As a major step in the development of the system, an array of microbowtie structures with 40 nm gap has
been built, appropriate for use in the visible range. A numerical electromagnetic scattering modeling is developed
2 Microbowtie Design
In this section the concept of the WINFA system and special considerations in the design of microbowties are described.
2.1 WINFA System
The possible optical probe system based on the WINFA
concept can be illustrated in Fig. 2. The system consists of
an array of open-circuited bowtie antennas, an incident
wave source, a system of Fourier optics, and a computercontrolled scanning stage. The substrate to be examined is
placed on the scanning stage. In this system, an array of
bowtie resonant elements is scanned over the surface of the
substrate. The array is illuminated by an interrogating optical wave so that its reflection can be continuously monitored by the detector array. The presence of a contaminant
on the surface, in close proximity to a given resonant
bowtie, will shift that bowtie’s resonance; thus perturbing
its contribution to the bowtie pattern’s reflected signal. Pattern recognition and holographic filters are then used with
the detector array to identify the perturbed bowtie element,
and gauge the properties of the contaminant.
As mentioned earlier, the feasibility of the WINFA concept has been recently demonstrated in the microwave
Fig. 2 Schematic illustration of wave interrogated near-field array concept.
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and time. For the preliminary analysis of the bowtie antennas it was assumed that all materials considered are nondispersive. This allows for the computational code to use
the standard update equation shown below:
␴ ⌬t
⌬t
2␧
␧
En⫹1 ⫽
En ⫹
共 ⵜ⫻Hn⫹ 共 1/2兲 兲 ,
␴ ⌬t
␴ ⌬t
1⫹
1⫹
2␧
2␧
共2a兲
⌬t
共 ⵜ⫻En 兲 .
␮0
共2b兲
1⫺
Hn⫹ 共 1/2兲 ⫽H n ⫺
Fig. 3 Numerical grid used in three-dimensional FDTD code.
range using a single bowtie structure. In the present research, the concept will be eventually extended to the visible or optical range. As a result, the corresponding bowtie
antenna to be built should be scaled down from 36 mm
originally tested by Grober et al.4 to less than 1 ␮m with a
40 nm gap. In the proposed optical probe, the length of the
bowtie and its gap size are extremely important since the
gap size represents the detecting resolution and the length
of a single bowtie structure directly dictates the wavelength
of the incident probing source. The EBL technique is used
to fabricate this microbowtie structure. Certainly, fabrication of such a micronscale structure itself is a major technical challenge.
2.2 Numerical Modeling
To analyze the complex problem at hand, a full wave solution must be conducted using a numerical method. The
finite-difference time domain 共FDTD兲 was the method of
choice since it can properly model the dispersive materials
that occur at optical frequencies. A discrete Fourier transform on the fly was used to convert the time domain solution to the steady-state frequency domain. Since FDTD is a
near-field solver, a far-field transformation must be conducted to get the far-field radiation patterns. This is accomplished by using the reaction theorem6 on the element currents calculated by the FDTD simulation. A standard grid7
has been used for the present FDTD code as is shown in
Fig. 3. For the update equations, the FDTD method uses a
Taylor expansion on Maxwell’s curl equations:
⳵D
⫽⫺Je ⫹ 共 ⵜ⫻H兲
⳵t
⳵B
⫽⫺ 共 ⵜ⫻E兲
⳵t
D⫽␧ r ␧ 0 E
B⫽ ␮ 0 H,
Je ⫽ ␴ E,
共1a兲
共1b兲
where ␴, ␧, and ␮ are the electric conductivity, permittivity,
and magnetic permeability, respectively; B, D, E, H, and J
are the magnetic-flux density, electric displacement,
electric-field intensity, magnetic field intensity, and the current density, respectively, and t denotes time. The above
partial differential equations are then discretized in space
Since the bowtie works like a focusing lens, increasing
the fields across the feed terminals will allow an increase in
the illumination of the defect. In turn, perturbation of these
high field strengths will increase the detuning of the radiating bowtie element. The figure of merit that is used in
determining the element of choice is the spot size. The spot
size is characterized by the distance between half-power
points of the field 共3 dB beamwidth兲. Defining a to be the 3
dB beamwidth parallel to the electric field 共E plane兲 and b
that parallel to the magnetic field 共H plane兲, we can say that
the power is concentrated in an area on the order of ␲ ab,
and therefore we define the focusing factor as: factor
⫽1/( ␲ ab) 共m⫺2兲 and the figure of merit as the ratio of this
factor to the inverse square of the wavelength.
To resolve subwavelength defects the diameter of the
spot size should be minimal. Three different shapes of
bowties were analyzed to determine the parameters affecting this figure of merit. The three different shapes considered for the micro bowties were concaved, convex, and
straight flared elements 共Fig. 4兲. Initial analysis of the
bowties indicated that the concaved element had the smallest 3 dB spot size when measured ␭/8 below the bowtie as
shown in Fig. 5, which is desirable, but due to the ease of
manufacturing the straight bowtie is the better candidate for
current technology. The results are summarized in Table 1
and also indicate that for straight bowties, as the half-angle
is increased from 22.5° to 60°, the figure of merit is maximized in the range of 30°– 45°, that is for bowties of included angle 60°–90°.
There is an additional figure of merit to be considered in
this analysis. As the flare angle increases, the capture area
of the element also increases. This allows the element to
reradiate more efficiently into the far field. This is an attractive characteristic, but as the angle increases so does the
spot size. Therefore, there is a tradeoff between the two
parameters.
As a result of the preliminary tradeoff analysis a bowtie
with 60° included angle 共30° half angle兲 was chosen for
fabrication. Further analysis was then performed on this
configuration to determine the effect of the supporting dielectric on the spot size, and also the effect of close proximity to the substrate to be examined. The full wave simulations conducted were for bowtie elements with flare half
angles of 20° and 30° suspended in free space and ␭/8
above the silicon substrate. Properties of the chromium,
silicon8 and BK-7 glass9 were modeled for a wavelength of
632.8 nm 共HeNe laser兲. The physical lengths of the bowtie
elements were adjusted to account for the properties of the
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Fig. 4 Three possible bowtie shapes considered in tradeoff study.
dielectric support. Figure 6 shows the focus energy at the
feed of the SK-7 supported 40° and 60° bowtie elements
suspended in free space. Figure 7 shows the corresponding
results in the presence of the silicon substrate under the
bowties. In operation, the silicon substrate is the sample to
be examined and the SK-7 carrier acts as a superstrate to
hold the bowties. The fields were perturbed and broadened
but still have the resolution to image subwavelength defects. Considering the 3 dB spot size above the surface, the
30° half-angle element or 60° bowtie has a narrower 3 dB
beam width and smaller fringing of the fields at the tips as
shown in Fig. 8.
Fig. 5 Field strength at several locations ␭/8 below bowties supported by BK-7 superstrate.
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Tseng et al.: Electron beam lithography of microbowtie structures . . .
Table 1 Focused spot size at ␭/8 below various bowtie elements.
Free space bowtie element
E plane
H plane
Height
Lambda/8
Element
3 dB beam width
Area
Lambda ˆ 2
Factor
1/lambda ˆ 2
BT 22.5 d
BT 30 d
BT 45 d
⬎Lambda/2
0.28
0.24
0.42
0.32
0.2
n/a
0.2815
0.2155
n/a
3.55
4.64
BT 60 d
Concave BT
⬎Lambda/2
0.21
⬎Lambda/2
0.21
n/a
0.1385
n/a
7.22
Convex BT
0.3
0.35
0.3518
2.842
3 Microbowtie Fabrication
To scale down the bowtie from the microwave range all the
way down to the optical range will require the fabrication
of submicron scale structures with details in the tens of nm
range, itself a major technical challenge. EBL is the technique of choice for fabricating such structures. We have
chosen to follow a stepping stone process in our development, and initially designed a micron-scale structure suitable for use in the infrared range.
Based on the tradeoff analysis described above and results presented in the section of numerical modeling, the
target design of the visible range microbowtie structure for
the manufacturability studies was chosen to be a 30° halfangle bowtie approximately 680 nm long 共one wavelength兲
having a 40 nm bowtie gap as shown in Fig. 9. It is expected that 40 nm sized voids or particles could be identified through the system explained in Fig. 2. Since only a
few prototypes are to be fabricated to demonstrate the
WINFA concept, the direct writing EBL on a resist-coated
substrate is used in the present research. The lithography
system used in the present study is converted from the Hitachi S4200 scanning electron microscope.
3.1 Materials
Special consideration should be given in selection of materials. To generate a strong field at the gap, the bowtie itself
should be opaque and of high electrical conductivity. The
carrier of the bowtie, on the other hand, should be transparent to the incident wave source to be used in the system.
Certainly, the materials selected should be also suitable for
micro- and even nanoscale manufacturing.
3.2 Nonconducting Substrate
The carrier or substrate of the bowties should be transparent to the incident wave source. However, most materials
that satisfy the transparency requirement are nonconducting
or insulating. The common problem in the exposure of resist on an insulating substrate is that in the absence of
charge dissipation, substrate charging causes considerable
distortion of the pattern.10,11 In the present study, two techniques have been used to cope with the problem of charge
accumulation or the absence of charge dissipation. The first
one uses a substrate coated with an indium–tin–oxide
共ITO兲 film, while the other employs a trilayer resist system.
In the former, the ITO coating is not only transparent but
also electrically conducting; in the latter, the trilayer resist
Fig. 6 Power intensity at free space ␭/8 below bowties supported by
BK-7 superstrate.
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Tseng et al.: Electron beam lithography of microbowtie structures . . .
Fig. 8 Cross sections of field intensity for E and H planes.
Fig. 7 Power intensity on silicon substrate surface ␭/8 below
bowties supported by BK-7 superstrate.
system consists of a thin germanium layer sandwiched by
two resist layers. As a result, electrons can travel through
the germanium or the ITO layer with minimal scattering
during resist exposure. A summary of the properties of ITO
is given in the Appendix.
3.3 ITO Coated Substrate Using Single Layer
Resist
Since the ITO coating is transparent and electrically conductive, the ITO-coated SK7 glass is selected as the substrate to minimize the charge accumulation problem. The
direct writing process used in the present study is very
similar to the conventional photolithography process; it
normally consists of several steps, including resist coating,
exposure, development, and etch/liftoff. The electron beam
directly writes on the resist the pattern that will serve as a
template for subsequent deposition of a material. The resist
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and all material deposited on the resist are ‘‘lifted off’’ in a
solvent. The deposited material that remains after liftoff
serves as the electronically and optically active bowtie
structure. The whole process is also known as the liftoff
process.
The usual e-beam resists are high molecular weight
polymers dissolved in a liquid solvent. Poly共methylmethacrylate兲 共PMMA兲 is the most commonly used positive resist for EBL and is selected for the present study
because of its superior resolution property.12 The 950 K
PMMA resist 共2% in Anisole兲 has been used. The PMMA
comes in powder form, and is dissolved in solvent 共such as
Anisole or chlorobenzene兲 to the desired concentration. The
resist liquid is dropped onto the substrate and then spun at
a high speed to form a thin coating. This is followed by soft
bake processing at a temperature of about 150°C to remove
the casting solvent. The final resist thickness is determined
by the PMMA concentration and by the spin speed. In the
present study, 3000 rpm spinning has resulted in a thickness
of about 200 nm. The bowtie pattern was written with an
area dose at 250 ␮C/cm2. The exposed region was then
developed and removed in methyl isobutyl ketone: isopropanol 共MIBK:IPA兲, typically 1:3, for 1 min and rinsed in
pure IPA for 30 s.
After developing, the sample is subjected to an oxygen
plasma cleaning process 共85 mTorr, 40 W, 45 s兲 to insure a
residue-free image. The desired material, in the present
case, 40 nm thick Cr, is then deposited from a small source
by a high-vacuum evaporator onto the substrate and resist.
The deposited layer thickness is controlled by a quartz–
crystal thickness monitor. The final step of the liftoff process is accomplished by soaking the substrate in an acetone
bath to wash away the remaining resist and unwanted material. This procedure is illustrated in Fig. 10, culminating
in the final structure of the bowtie pattern on an ITO carrier
as shown in Fig. 10共d兲. It is noteworthy that due to inevitable backscattering of electrons, along the edges of the
pattern the lower part of the resist receives more dose than
the surface, resulting in a slight undercut profile as shown
in Figs. 10共b兲 and 10共c兲. This undercut profile is essential
Tseng et al.: Electron beam lithography of microbowtie structures . . .
Fig. 9 Microbowtie design for fabrication.
because it provides a clean separation of the deposited material and hence a sharp liftoff pattern.
3.4 Nonconducting Glass Substrate Using Trilayer
Resist
Using a trilayer resist in EBL, the nonconducting SK7 glass
can be directly used as the substrate without the charge
accumulation problem. The trilayer resist technique combines the liftoff and dry etching processes as illustrated in
Fig. 11. The preparation of this resist system is shown in
Fig. 6共a兲. A thick bottom layer, acting as a spacer, is spun
coated on the substrate and baked dry. In the present study,
a copolymer, P共MMA-MAA兲 6% in chlorobenzene, is used
as the bottom resist. With a coating spin speed of 4000 rpm,
and a baking temperature of 165°C, the resulting resist film
is about 400 nm thick. This film is then coated with a 20
nm thick germanium 共Ge兲 layer by the normal thermal
evaporating process. Ge is a good candidate not only for its
conducting ability to cope with the charge accumulation
problem but also for its small granular size, allowing for
generation of fine patterns. A thin PMMA resist film is then
spin coated on top of the Ge layer. Since the pattern is
defined on this top layer, a thinner resist film is preferred
because it produces better pattern sharpness. Presently, a
diluted PMMA resist 共2% in Anisole兲, spun at 5000 rpm, is
used to form a 100 nm thick film after soft baking at 135°C.
The trilayer resist is then exposed with an area dose of
approximately 250 ␮C/cm2 as shown in Fig. 11共b兲; then the
top PMMA resist is developed in MIBK:IPA⫽1:3 solution
for 1 min, and rinsed in isopropyl alcohol for 30 s shown in
Fig. 11共c兲. The image is then transferred to the Ge layer
using a CF4 plasma etch process 关Fig. 11共d兲兴. In this process, the top PMMA layer acts as a mask, and the unprotected Ge layer can be etched away in a 70 W, 110 mTorr
CF4 plasma in 2 min. Then, the patterned Ge layer is used
as a mask for patterning of the bottom copolymer layer
关Fig. 11共e兲兴. In this step, a 60 W, 85 mTorr oxygen plasma
is employed, and the sample is tilted slightly to allow for
creation of a larger undercut along the pattern edge. The
rest of the steps that follow are then standard metal evaporation shown in Fig. 11共f兲 and the liftoff processing as indicated in Fig. 11共g兲.
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Fig. 10 Schematic of e-beam liftoff process.
3.5 Scattering and Pattern Variation
While e-beam diameters on the order of 1 nm are possible, beam–material interaction degrades this limit
significantly.13 When the electron beam strikes the resist
solid, many of the electrons experience small-angle forward
scattering, which tends to enlarge the initial beam size. As
the electrons penetrate through the resist into the substrate,
some of them undergo large-angle scattering events leading
to backscattering, in which these electrons return back
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through the resist in a region far from the desired exposure.
This causes additional exposure in the resist and is also
known as the e-beam proximity effect.
Also, as the primary electrons slow down, much of their
energy is dissipated in the form of secondary electrons in
which a small portion may have significant energies, on the
order of 1 keV. These so-called fast electrons are responsible for the bulk of actual resist exposure and can contribute to the proximity effect in the range of few tenths of a
Tseng et al.: Electron beam lithography of microbowtie structures . . .
Fig. 11 Schematic of liftoff/etching process using trilayer resist.
␮m.11 The net results of scattering electrons is to cause the
dose delivered by the e beam to extend beyond to the original shape, resulting in pattern variations. Other sources for
pattern variations include the path-butting error, resolution
of the resist, nonuniform resist temperature, beam current
instability, spot size instability, and beam deflection error.10
In the present study, the dimensions of the final bowtie
structures made by different processes will be compared for
assessing the magnitudes of the pattern variation. Eventually, the information obtained in the comparison will be
used to better control the final dimension of the pattern.
4 Fabrication Results
Two sets of the bowtie array are fabricated. The first set
having a 250 nm bowtie gap is fabricated for studying the
charge accumulation effects in the glass-based substrate.
The second set has the bowtie gap at 40 nm as required by
the system designed. Two types of substrates: one with an
ITO coating and one without the coating are considered.
4.1 ITO Coated Glass Substrate
Using the normal liftoff process illustrated in Fig. 10, the
bowtie array having a 250 nm gap was fabricated and
shown in an atomic force microscope image in Fig. 12. The
dimension of the fabricated bowtie in this substrate is measured and compared with the computer aided design 共CAD兲
profile used to control the e beam contour. As shown in Fig.
13, the actual structure made on the ITO substrate is in each
side, about 20 nm larger than the dimensions in the CAD
file. Although, this size enlargement from the input CAD
dimensions is relatively small, it is a combination of the
proximity effect and the over spread by the evaporation
process.
4.2 SK7 Substrate Using Trilayer Resist
In this case, a trilayer resist, 100 nm–PMMA/20 nm–Ge/
400 nm–PMMA, was used; one of the trilayer is germanium for conducting electrons. Thus, the 250 nm gap
bowtie structure can be fabricated on a 3-mm-thick SK7
glass substrate without ITO coating. The scanning electron
microscope 共SEM兲 image of the trilayer resist after development, before deposition is shown in Fig. 14共a兲, while the
final structure after the deposition of 40 nm thick Cr on the
glass substrate is shown in Fig. 14共b兲. The bright band
around the edge of the resist window shown in Fig. 14共a兲 is
a manifestation of the undercut structure illustrated in Fig.
11共e兲.
The dimensions of the resist opening and the fabricated
bowtie are measured and shown in Fig. 13. Also shown in
the figure, the shape deviation between the fabricated
bowtie and the resist opening is about 5 nm in each side. In
other words, the variation caused by the thermal deposition
is relatively small 共5 nm兲 and this is consistent with the
statements mentioned earlier that the undercut profile in the
resist layer does provide a clean separation of the deposited
material and, hence, a sharp liftoff pattern.
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Fig. 12 Fabricated 250 nm gap bowtie array on ITO/SK7 glass substrate.
By comparing the dimensions of the two fabricated
bowtie structures shown in Fig. 13, the bowtie based on the
trilayer resist technique 共on the noncoated substrate兲 is 20
nm smaller than the one fabricated by using a single layer
resist 共on the ITO coated substrate兲, but 10 nm larger than
the CAD file used to control the e-beam contour. Since the
fabricated pattern is defined by the resist layer, the top layer
of the trilayer resist is 100 nm thick and is twice thinner
than the single layer resist 共200 nm thick兲 used. As a result,
the trilayer resist technique produces better pattern sharpness or the pattern closer the CAD profile, because thinner
resists yield less effects by scattering.
Fig. 14 Fabricated 250 nm gap bowtie profiles using trilayer resist.
4.3 Bowtie Having 40-nm Gap on ITO Coated
Glass Substrate
Based on the information obtained in making the above two
250 nm-gap bowtie structures, the charge accumulation
problems can be minimized by either using the trilayer resist or using the ITO coating substrate. Since the technique
Fig. 13 Geometry comparison of fabricated 250 nm gap bowtie
structures.
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J. Microlith., Microfab., Microsyst., Vol. 1 No. 2, July 2002
of using an ITO coated substrate is relatively simple and
less complicated, the final 40 nm gap bowtie is made on an
ITO coated SK7 substrate.
Again, using the normal liftoff process illustrated in Figure 10, the bowtie array having a 40 nm gap is fabricated
on a 3-mm-thick glass coated with a 70 nm layer of ITO. A
SEM image of the bowtie array and a single isolated bowtie
are shown in Figs. 15共a兲 and 15共b兲, respectively. The dimension comparison of the fabricated bowtie and the CAD
profile is plotted in Fig. 16. It was found that the actual
structure on ITO coated substrate is, on each side, about 15
nm larger than the dimensions on the CAD file. This 15 nm
enlargement is very close to the 20 nm size variation found
in the case of fabricating 250 gap bowties in the ITO coated
substrate. Since the amount of the enlargement is somewhat
consistent in both cases, in the future this enlargement can
be corrected by shrinking the dimension specified in the
CAD file by 10–15 nm in each side. In this way, the dimension accuracy of the fabricated bowtie can be controlled within 5 nm as compared with the design requirement and the process of using an ITO coated substrate can
be as accurate as that of using the trilayer resist.
Tseng et al.: Electron beam lithography of microbowtie structures . . .
Fig. 16 Geometry comparison of fabricated 40 nm gap bowtie
structures.
Fig. 15 40 nm gap bowtie structures on ITO-coated glass substrate.
It should be noted that the electron beam dose 共approximately 250 ␮C/cm2兲 required for exposure on the ITO
coated glass substrate is similar to that on silicon based
substrates and this may indicate that the influence of the
secondary electron exposure is quite the same in these two
substrates and the distortion caused by the proximity effect
may be similar too.
5 Concluding Remarks
Microbowtie structures having a gap of 40 nm have been
successfully fabricated by direct-write e-beam lithography.
Two EBL techniques have been developed to cope with the
charge accumulation problem caused by the insulating
glass substrate required; one technique uses a trilayer resist
while the other adopts a ITO coated glass substrate. The
ITO coated substrate is recommended and used for making
the final 40 nm gap bowtie because of its simplicity. In the
proposed optical probe, the detecting resolution is directly
dictated by the gap size and it is expected that the 40 nm
gap bowtie can be used for probing defects down to 40 nm
in size.
The causes of the pattern variation are studied and discussed by comparing them with the input CAD file dimensions. The present study shows that the pattern variation
can be controlled within a few nm from the desired or
designed geometry. The present study also indicated that
the electron beam dose required for exposure on an ITO
coated glass substrate is similar to that on silicon substrates,
indicating that the influence of the secondary electron exposure is similar in these two substrates.
As discussed in Sec. 2, modeling or computer simulation
is also critical in understanding the response of a bowtie
element in probing defects less than 40 nm. Initial results
show that 45 nm sized voids and particles can be distinguished with enough phase and amplitude perturbation to
be identified through Fourier transform techniques. At this
time, although the straight 60° bowtie has been used as the
baseline element, no single antenna design has been chosen
as the optimum structure. To find an optimal structure and
to minimize the number of prototypes to be fabricated and
tested, a series of full-field electromagnetic simulations14
should be further performed. Various microantenna designs
should be considered in the future. The frequency response
can be simulated on each element to determine its ability to
radiate efficiently at the operating frequency. The impact of
the dielectric, height, feed gap along with the polarization,
and angle of incidence on the resonance properties, and on
the determination of its final dimensions should all be studied. Minor frequency fluctuations of the laser source should
not be a problem since the bowtie is a broadband element.
Furthermore, the effect on resonant frequency due to the
incident polarization 共parallel P or perpendicular S兲 should
also be investigated.
Appendix
ITO coatings are transparent, electrically conductive films
with excellent durability, high visibility, and near infrared
transmission. They are extensively used in optical and other
applications, including touch panel contacts, electrodes for
liquid crystal display and electrochromic displays, energy
conserving architectural windows, defogging aircraft, and
automobile windows, heat-reflecting coatings to increase
J. Microlith., Microfab., Microsyst., Vol. 1 No. 2, July 2002
133
Tseng et al.: Electron beam lithography of microbowtie structures . . .
light bulb efficiency, gas sensors, antistatic window coatings, wear resistant layers on glass, etc. ITO is normally
made by doping tin–oxide on indium–oxide; its coatings
can be deposited by electron-beam evaporation or sputtering. The optical and electronic properties of ITO coatings
are highly dependent on the deposition parameters and the
starting composition of evaporation material used. The deposited coating must contain a high density of charge carriers for it to conduct. These carriers are free electron and
oxygen vacancies, and an excessive population produces
absorption. High conductivity 共or low sheet resistance兲 is
balanced against high transmission in the visible region.
Coating resistance can be less than 10 ⍀/sq with a visible
transmission of ⬎80%. ITO coatings behave as metals to
long wavelength light because of the presence of a plasma
resonance at a wavelength longer than the infrared ranges.
Based on the data presented by Optical Components of
Milwaukee, WI,15 the average transmittance of ITO is
higher than 80% for the wavelength ␭ considered 共from
400 nm to 1.2 ␮m兲. In fact, between the range from 500 to
900 nm, the average transmittance can be as high as 89%.
For longer wavelengths, the film becomes reflecting and for
example, the transmittance reduces to 60% at ␭⫽2.5 ␮ m.
Improved transmission characteristics can be obtained by
overlaying the ITO layer with an antireflection coating.16
‘‘Issues in nanolithography for quantum effect device manufacture,’’
in Handbook of Microlithography, Micromachining, and Microfabrication, P. Rai-Choudhury, Ed., Vol. 1, Chap. 8, pp. 681–763, SPIE
Optical Engineering, Bellingham, WA 共1997兲.
14. A. A. Tseng, ‘‘Electroheating modeling in metal industry: A review,’’
Adv. Eng. Software 10共2兲, 58 –71 共1988兲.
15. See http://www.cerac.com/pubs/proddate/ito.htm
16. I. Brodie and J. J. Muray, The Physics of Micro/Nano-Fabrication,
Plenum, New York 共1992兲.
Ampere A. Tseng received his MS degree
from University of Illinois at Champaign–
Urbana in 1974 and PdD degree from
Georgia Institute of Technology in 1978. Dr.
Tseng is a Professor of Engineering at Arizona State University. Before joining Arizona State University in 1996, Dr. Tseng
taught at Drexel University for more than
ten years and held various research and
development positions in Industry. He was
the founding Director of the Manufacturing
Institute at ASU from 1997 to 2000 and the Co-Director of Center of
Automation Technology at Drexel University from 1989 to 1993. He
was a recipient of the Superior Performance Award of Martin Marietta Laboratories (1979–1984), RCA Service Award (1985), and Alcoa Foundation Research Award (1987), and ASU 1999–2000 Faculty Award (2001). Prof. Tseng is heavily involved in professional
society activities and has been a member of the editorial boards of
several professional journals. Dr. Tseng was the Chair of the ASME
Materials Division (1991–1992) and of the 2000 NSF Workshop on
Manufacturing of Micro-Electro-Mechanical Systems Workshop.
Acknowledgments
The authors gratefully acknowledge the support of this
study by the US National Science Foundation under Grant
Nos. DMI-0002466 and CMS-0115828 and by ROC National Science Council under Grant No. NSC90-2811-E002-007. A special thanks goes to Professor P. H. Chen for
hosting the first author’s visit to National Taiwan University in the summer of 2001. The assistance from Bharath
Leeladharan of Arizona State University in preparing this
manuscript should be specifically acknowledged.
Dr. C. D. Chen received his PhD degree in
1994 from Department of Physics, Chalmers University, Gothenburg, Sweden. After
three years of postdoctoral work, he became an assistant research fellow in Academia Sinica. His current research interest
is in electron transport properties of nanoelectronics as well as optic properties of
nanostructures.
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electron beam lithography techniques and
is currently working on nanoelectronics and
nanoscale optic devices.
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J. Microlith., Microfab., Microsyst., Vol. 1 No. 2, July 2002
Rodolfo E. Diaz has worked on many aspects of the interaction between electromagnetic waves and materials, from lightning protection on the Space Shuttle to the
design and manufacture of Radar absorbing structures for Stealth applications during his 20 years in the aerospace industry
with such companies as Rockwell International and Northrop Grumman. From 1998
to 2001 he worked as Research Faculty in
the Mechanical and Aerospace Engineering Department of Arizona State in the Laser Diagnostics Laboratory. He is presently an Associate Professor in the Electrical Engineering Department of ASU and Associate Director of the
Consortium for Metrology of Semiconductor Nanodefects. Prof. Diaz
holds ten patents ranging from the design of Broadband Radomes
to the amplification of magnetic fields.
Tseng et al.: Electron beam lithography of microbowtie structures . . .
Michael E. Watts received the BSE degree in electrical engineering from Arizona
State University in May 2000. He is currently working towards the MS degree in
electrical engineering at the same university. His research interests are in the areas
of antennas, detection of subwavelength
particles, and improvements to finitedifference time domain methods.
J. Microlith., Microfab., Microsyst., Vol. 1 No. 2, July 2002
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