Electron Beam Lithography: Pushing Nanostructure Fabrication Below 10nm


Abstract—Electron beam lithography (EBL) is a lithographic
technique that is widely used in the semiconductor industry for
nanostructure fabrication. EBL is a high resolution technique that
recently pushed fabrication of these nanostructures to 10nm and
below. High-energy electrons (10-100keV) are used focused into a
narrow beam and used to expose regions on an electro-sensitive
resist. The critical dimensions of these patterns are limited to
electron scattering and its substrate.
Keywords— Electron Beam Lithography, Resolution limits,
Nanostructures, Critical Dimensions, proximity effect.
I. INTRODUCTION
Gordon Moore stated that the number of transistors on an
Integrated circuit would double every year since the integrated
circuit was invented, in recent times this law has been revised
to eighteen months to two years. It is important to understand
why this has slowed down and its effects on the
semiconductor industry. The reason behind this is down to the
ever shrinking geometries of these transistors; new and
ongoing techniques are being researched and developed all the
time to continue to reduce the geometries of these transistors
for the coming years. Performance improvements for these
devices are down to the minimum printable feature sizes
(critical dimension or CD) resulting in higher speed
transistors, higher packing densities and lower power
dissipation in CMOS circuits [1]
. Currently devices that are in
production contain substructures around the 16nm and 14nm
range. This document is aimed towards looking at Electron-
beam lithography to be used to produce substructures 10nm
and below, while also giving an understanding of its resolution
limits as well as applications where EBL can be used.
II. OVERVIEW
Electron Beam lithography (EBL) is a key technique for
fabricating patterns at the nanoscale level. EBL tools were
developed in the late 1960s by modifying the Scanning
Electron Microscope (SEM) design. The difference between
an EBL system and an SEM is that in the EBL case, a pattern
generator is used to give instructions to the beam to be
scanned onto the sample while in SEM, an image is formed
using the beam to collect secondary electrons as it is raster
scanned across the sample [2]
. SEM tools can operate up to
30kev but for high resolution structures, EBL operates up to
100kv [2]
. EBL is used to deposit electrons to expose certain
regions of resist to give desired patterns.
It operates on the basis of condensing electrons to a finely
focused beam to expose regions of negative or positive
photoresist to alter the resists solubility [5]
. An EBL system
consists of a chamber, an electron gun and a column and an
example of a typical EBL system is shown in figure 1.
Figure 1. Typical schematic of an EBL system (Image
courtesy of “Electron Beam Technology in microelectronic
fabrication” [4]
)
III. ELECTRON COLUMN
A set of pumps are used to maintain a high vacuum in which
the column and chamber are situated. The operation of this
vacuum is to stop the interaction of gas molecules with the
electron beam to minimize the risk of scattering the beam [2]
.
The column consists of an electron source/electron gun that is
used to produce the electrons, along with a set of
electromagnetic lenses used to focus and guide the beam. The
on/off operation is controlled by a blanker, this is usually a set
of electrostatic plates. The off operation occurs when a voltage
Electron Beam Lithography: Resolution limits
and applications
Ray Tyndall, School of electronic engineering, Dublin City University, Glasnevin Dublin 7, Ireland.
Ray.Tyndall3@mail.dcu.ie

is applied across these plates, this deflects the beam off its axis
and is then blocked by a downstream aperture [9]
.
Alignment systems are used to center the beam in the column.
A stigmator is a special type of lens used to correct any
astigmatism in the beam due to any imperfections in the
alignment of the EBL column or contamination in the optical
column. At different lens settings, the electron beam may
become focused in different directions, changing the shape of
the e-beam to a more oblong shape rather than the nominal
rounded shape. This causes the resulting image to become
smeared and damaged [9]
.
An electrostatic deflection system is used to control the
deflection angle of the incident e-beam, this determines the
positional accuracy of the beam as it irradiates the sample [10]
.
An electron detector is used to assist in locating marks on the
sample, lastly a wafer handling system is used for the loading
and unloading of wafers [9]
.
Figure 2.1 gives the cross section area of an electron beam
column with the ray-trace of the electrons as they pass through
each of the defined areas of the electron optical components.
Figure 2.1 EBL column (Image courtesy of
www.cornell.edu/cnf_spie2.html [9]
)
The focusing system for an EBL tool is comprised of a four-
tier set of electro-magnetic lenses. As the electron beam
passes through these lenses, the beam spot diameter when
entering a sample can be reduced to almost one tenth of its
original size [10]
. A 15 ~ 20nm diameter electron beam in turn
can be reduced to less than 2nm when entering a sample [10]
.
Figure 2.2 gives a cross-section drawing of a high resolution
electron beam lithography tool with thermionic field emission
source with ray trace of e-beam passing through four
electromagnetic lenses to reduce the spot diameter size.
Figure 2.2 Ray-trace of e-beam passing through the
electromagnetic lenses. (Image courtesy of “High-energy
Electron Beam Lithography for Nanoscale Fabrication” [10]
)
IV. ELECTRON SOURCE
The electron source is where the electrons are generated.
These electrons can be emitted from a conducting material in
two ways and can be categorized as the following:
1. Field emission: A high electric field is applied to a
solid (e.g. Tungsten), the free electrons of the solid
are able to escape through the barrier (work function)
into the vacuum by means of the quantum
mechanical tunneling effect [3, 9,]
.
2. Thermionic emission: conducting material is heated
to the point where the electrons are given enough
thermal energy to overcome the barrier (work
function) of the source material [3, 9]
, an electric
potential is combined with this to give these electrons
direction and velocity [12]
.
Thermal Field emitters: Thermionic emission compliments
field emission in that some thermal energy can be given to the
electrons, to lower the potential barrier and to increase the
successful escape of these electrons from the surface [12]
.
An electron source has three key parameters [3, 9, 13]
.
1. Brightness: The brighter the source, the higher the
current density is in the electron beam, this is measured
in A/cm2
/steradians.
2. Virtual source size: this determines how much
demagnification is to be done to achieve the desired
beam diameter.

3. Energy Spread: The energy spread of the emitted
electrons, this is measured in electron volts (eV)
These parameters for different electron source materials can
be found below in Table 1.
Table 1. Electron source materials and parameters (Table
courtesy of http://www.cnf.cornell.edu/cnf_spie2.html [9]
)
It can be seen from Table 1 that the thermal field emitter
otherwise known as the Schottky emitter, has a brightness level
that is very close to the cold field emitter source, with a small
source size and a moderate energy spread. Thermal field
emitters normally operate at 1800K and give better standards
for EBL over thermionic sources [3,9].
V. ELECTRON INTERACTIONS
The goal of EBL writing is to achieve patterns in the resist that
uniformly holds high density, high resolution and high
reliability substructures. For this to occur, there are certain
aspects that must be controlled and is governed by key
determinants such as the choice of resist (positive or negative),
electron beam energy and dose, the ability to create a finely
focused beam of electrons to minimize the point spread
function due to the quality of the electron optics[5]
.
Other factors include forward and backscattering of electrons
(proximity effects) in which the delocalization of electrons
occur causing fluctuations in the size of features due to line
edge roughness.
As an electron penetrates the resist/substrate, it can have two
types of interactions known as forward scattering and
backscattering. A series of low energy elastic collisions start
to occur between the electrons being projected into the
resist/substrate with that of electrons of atoms of the
resist/substrate. This collision will cause the incident electron
to alter its trajectory while also passing some of its energy to
the atom. This is known as forward scattering and is
detrimental when trying to create substructures of 10nm and
below.
The atom will then become excited due to gaining extra
energy from the incident electron, causing an electron of the
atom to be released resulting in a secondary electron in the
material[6]
.
Forward scattering causes the effective incident beam size to
broaden when entering into the material and is more
pronounced at low energies. In forward scattering, the
deflected angle in the change of trajectory of the incident
electron is as a rule, small [6]
.
Forward scattering can be reduced by increasing the
accelerating voltage of the incident beam [3]
.
To calculate the increase in beam diameter, An empirical
formula is given in (1) is used where Δd is the change in the
incident beam diameter as it enters the resist, tr is thickness of
the resist and Vb is the accelerating voltage of the electron
beam [3]
.
𝛥𝑑 = 0.9(
𝑡 𝑅
𝑉 𝑏
)1.5
(1)
Due to the primary electrons giving up a lot of their energies
due to the formation of these secondary electrons, they begin to
slow. These secondary electrons have energy ranges from 2 to
50eV and are mostly responsible for the actual exposure of the
resist [3,6]
. These secondary electrons are the primary reason for
the diameter of the incident beam to broaden and can cause
features to be developed larger than originally intended.
While in forward scattering, the incident electron gives up most
of its energy to the atom and slows down with a resulting small
change in trajectory; Backscattering is the result of an electron
projected into the resist/substrate, colliding with a much heavier
nucleus, causing the electrons trajectory to change dramatically.
The scattering angle in this case can be very large. The electrons
when colliding with the atom retain a lot of their energies and
in cases where the deflection angle is very large, electrons can
be scattered back into the resist exposing the resist in areas
outside of the intended exposure region. This effect is known as
the proximity effect and can be seen in Figure 3 where an
incident electron beam exposes a defined region A, but with
backscattering effects, an area outside this region (region B)
gets exposed to these electrons.
Figure 3 Proximity effect
(Image courtesy of
nanolithography.gatech.edu/proximity.htm[6]
)
This proximity effect can cause problems such as narrow lines
between two exposed areas to be completely developed in
positive resists, or a small feature does not get its correct dose
due to the loss of the scattering effect that it does not develop
completely [3,7]
.
To minimize backscattering, it is useful to use substrates with a

low atomic number.
It can be seen from Figure 4(a) the incident electron colliding
with an electron from the target atom resulting in a small change
in its trajectory while (b) shows the electron colliding with the
nucleus resulting in a large scattering angle that can lead to
backscattering.
Figure 4 (c) shows forward scattering and backscattering
resulting from an incident electron beam. From this figure it can
be seen when an electron is backscattered and retains most of
its energy can scatter back into the resist.
For head on collisions between the electron and the nucleus, the
energy transfer is given by the formula [6]
E=E0(1.02+E0/106
)/(465.7A) (2)
Where E0 is the incident beam energy, and A is the atom
number of the target.
Figure 4. (a) Forward scattering (b) Backscattering (Image
courtesy of nanolithography.gatech.edu/proximity.htm[6]
)
Figure 4 (c) Forward/Back scattering
(Image courtesy of www.slideplayer.com/slide/4312318/)
Forward and Backscattering can be expressed by the Point
Spread Function (PSF). This function represents the energy
deposited in the electron sensitive resist from a single point of
incidence [23]
.
The function is modelled using the sum of the two Gaussian
distributions which represent both the forward scattering and
backscattering electrons [6]
.
This sum of the forward and backscattered distributions is
known as the double Gaussian model and can be seen in (3).
𝑓(𝑟) =
1
1+𝑛
(
1
𝜋𝛼2 exp (−
𝑟2
𝛼2) +
𝑛
𝜋𝛽2 exp(−
𝑟2
𝛽2)) (3)
In this equation, n is the ratio between the backscattered energy
and the forward scattering energy, α is the forward scattering
range parameters and β is the backscattering range
parameters[6]
. Sometimes a third term may be required to take
into consideration fast secondary electrons in the intermediate
range [22]
. Figure 4 (d) shows a comparison of the point spread
function over a range of electron energies from 50keV to
100keV. It can be seen that using higher electron energy results
in a narrower PSF [23]
.
Figure 4(d). PSF over 50keV to 100keV (Image courtesy of
Nanofabrication:Principles, capabilities and Limit [23]
)
VI. Forward/Backscattering Correction and Avoidance
There are techniques being used to minimize the scattering
effects leading to forward and backscattering. To minimize the
proximity effect, the dose may need to be adjusted until the
pattern comes out the desired size. This method is normally
used for isolated gate structures.
Multilevel resists have been used also to combat this effect by
creating a top layer that is sensitive to electrons while the
pattern developed in this layer is transferred to an underlying
layer by dry etching. This adds complexity to the process but
reduces the forward scattering effect [7]
.
To minimize forward scattering, higher beam voltages can be
used but also can increase the chances of backscattering.
The proximity effect can be eliminated by using low beam
energies in cases where the electron range is smaller than the
minimum feature size[7]
, in this case the resist must be a single
layer resist and also must be made smaller than the minimum
feature size. This allows it to be possible for the electron to
expose the entire film thickness [3,7]
.
By changing the exposure process through dose modulation, a
different exposure amount can be used to expose different
features on a pattern, this means that large features would
require a smaller dose of electrons while smaller features
would require higher doses.

A technique has been developed called GHOST that is used in
proximity correction. This technique uses a defocused beam to
mimic the shape of the backscatter distribution and the inverse
of the pattern is written.
GHOST uses this inverse pattern to give an additional
exposure dose to correct the backscatter in the primary dose
exposure. Through this technique, the combined distribution
leads to excellent line width control and can be seen in Figure
4.
Figure 4: Example of GHOST used to counteract the
proximity effect. (Image courtesy Electron Beam from past to
present [3]
)
This technique requires no computation to implement but
because it requires an additional exposure dose, this results in
a loss of throughput [11]
.
In conclusion, it can be seen that the amount of proximity
effects depend directly on the substrate/resist material used,
the accelerating voltage, the process used and the location and
size of the features being written [8]
.
VII. EBL RESISTS
In Electron Beam Lithography, there are two types of resist
that can are used that can be chemically changed under
exposure of an electron beam. Positive tone resists and
negative tone resists. These resists are polymers and
depending on their chemical structure, will either become
crosslinked or chain-scissioned under exposure to an electron
beam [14]
.
Positive tone resists when subject to an incident electron
beam, converts the solubility of the resist from a low to a high,
enabling the exposed area to be removed by a solvent.
In a positive tone resist, this is called chain scission as the
polymer chains are broken into smaller molecular fragments
which in turn reduces the molecular weight of the substance
allowing it to be dissolved easily by a developer that attacks
the low molecular weight substance.
An example of this would be PMMA (polymethyl
methacrylate). PMMA is a polymeric material that under
exposure from the electron beam causes the cutting of these
polymer chains.
PMMA is one of the first resist materials created and it still
holds true today that PMMA resists are regularly used in EBL
tools.
PMMA has several different molecular weight forms and if it
becomes overexposed, it can in fact change to a negative tone
e-beam resist.
Negative tone resists converts the material to a low solubility
state. In a negative tone resist, areas that are exposed to the
electron beam become cross-linked. Crosslinking is the term
used for when these polymer bonds under electron beam
exposure to become bonded together, this creates a three
dimensional polymer with a molecular weight higher than the
area not under exposure. When a developer is added, the area
not exposed will be removed due to its lower molecular
weight.
Examples of this would be Polyglycidylmethacrylate-
coethylacrylate (COP), or hydrogen silsesquixane (HSQ).
Negative aspects of COP is that it can swell during
development so it in turn lowers the resolution below that of a
positive photoresist [16]
.
The difference in the resists when exposed to an e-beam can
be seen in figure 5 when developed after exposure.
Figure 5. (a)Positive resists become Cross-scissioned (b)
Negative resists become chain-linked
(Image courtesy of
www.optics.rochester.edu/workgroups/cml/opt307/spr10/xiaos
hu/Lithography.html#Pattern_design )
VIII. EBL SYSTEMS
E-beam lithography systems can be broken into two main
categories: (1) electron beam projection lithography and (2)
direct write electron beam lithography.
A breakdown of this can be seen in figure 6.

Figure 6. Electron Beam Systems
(Image courtesy of www.hendersonresearchgroup/helpful-
primers-introductions/intro-to-ebeam-litho)
In direct-write systems, there are two methods used to scan the
beam across the sample [16]
.
1. Raster scan: The electron beam is passed over the
entire sample sequentially (vertically orientated), the
blanking system operates to turn on and off the e-
beam to expose the desired regions and can be seen
in figure 7(a).
2. Vector scan: The main advantage of vector scan over
raster scan is that the electron beam does not scan the
whole sample but in turn jumps from feature to
feature to expose and can be seen in figure 7 (b). This
saves time as not all of the sample may need to be
exposed.
Figure 7 (a) Raster scan (b) Vector Scan
(Image courtesy of www.hendersonresearchgroup/helpful-
primers-introductions/intro-to-ebeam-litho)
E-beam lithography tools regularly give better resolution and
depth of focus over other industry based lithography
techniques but its main disadvantage is that it cannot cope in
throughput and cost over its optical systems rivals.
One such advancement has been proposed by TSMC (Taiwan
SemiConductor Manufacturing Company) along with KLA-
Tencor to allow Multiple-electron direct patterning of features
to allow for high volume production [17]
.
This is governed under certain advancements such as in digital
electronics, this has allowed for affordable pricing in
equipment to enable very high throughput, an increase in
beam number with high speed writing can be supported by
new micro electrical and packaging systems [17]
.
To achieve this, a reflective e-beam lithography (REBL)
system was designed comprising of a reflective electron
optics, a digital pattern generator, temporal dose integration,
optical wafer alignment, and magnetic levitation stage
technologies [17]
.
This system can be seen in figure 8.
Figure 8 (a). REBL Tool
(Image courtesy of Multiple-electron beam direct-write comes
of age [17]
)
It operates by an electron beam being projected from an
electron gun into a series of lenses that bend and project the e-
beam onto the digital pattern generator to illuminate it at a 90
degree angle.
This digital pattern generator is comprised of a CMOS ASIC
chip with an array of small, independently controllable
metallic cells or pixels facing this incoming e-beam.
This array is designed to either absorb incident electrons or
reflect them back through the optics to etch the desired pattern
on the wafer.

This operates on the basis that the incoming e-beam
illuminates an array of small electrodes on the digital pattern
generator. A negative bias of 1-2V is applied to this entire
array [19]
.
The incoming electrons are slowed down to approximately
1ev before illuminating the electrodes as they pass through the
decelerating field of the electrostatic digital pattern lens. The
incoming decelerated electrons are repulsed by this negative
potential are reflected back through the digital pattern lens
causing them to re-accelerate as they pass through the
accelerating electrostatic field. These accelerated reflected
electrons are used to pattern the wafer [19]
. The ray trace
diagram can be seen in Figure 8(b) of a typical REBL system.
Figure 8(b) REBL Ray diagram indicating (i) The illuminating
beam from the Electron gun to the Digital Pattern Generator
are shown in blue, (ii) The modulated reflected beam from the
Digital Pattern Generator is shown in red.
(Image courtesy of [19])
A small positive bias can be applied to some of the electrodes,
allowing them to absorb the incoming electrons, these
absorbed electrons do not get reflected. A high speed data
processor is used to switch between a negative/positive biased
state. The image reflected will have both bright (reflected) or
dark (absorbed) areas corresponding to the positive or
negative biased state of the electrode [19]
.
This digital pattern generator allows the REBL system to
produce a parallel lithographic exposure using over a million
electron beams at extremely high data rates. The switching of
the voltages on these pads to absorb or reflect electrons is
controlled and limited by the data processor. The switching
can only be as fast as the data processor in operation, but can
enable fast and smooth direct–write operation.
The electron column for this tool is designed for 10cm and
allows for good column clustering on the wafer [17]
and can be
seen in figure 8(c).
Figure 8 (c). Linear stages for multi-column high-volume
manufacturing.
(Image courtesy of Multiple-electron beam direct-write comes
of age [17]
)
Research and development is currently in operation for a
multi-column high volume lithography REBL tool for the
10nm technology node with 100 digital pattern generators and
columns [19]
.
This new design has the potential to exceed 193nm immersion
lithography technology in wafer throughput per hour making it
a serious contender in the semiconductor industry [19]
.
A technique created by AT&T Bell labs in the early 90’s
called Scattering with Angular Limitation Projection Electron
–beam Lithography (SCALPEL) was believed to be a
candidate for the next generation lithography process [20]
. As
e-beam technology is deemed a maskless technique to pattern
wafers, this SCALPEL technique uses what is called a
scattering mask in its process. These scattering masks are used
to scatter the electrons and not absorb the electrons, resulting
in no thermal instability in the mask [3]
.
SCALPEL was researched using 100 keV electrons, this
technique was researched using electrons as they do not hold
to diffraction characteristics as in optical lithography. It is a
reduction image technique which uses high energy electrons
and their scattering ability. This system uses a mask consisting
of a basic structure of a low atomic membrane covered by a
high atomic number material patterned scattered layer. This
techniques, since using high energy electrons, utilize the
difference in the electron scattering characteristics between
these two layers [21]
. The low atomic number membrane layer
scatters the electrons weakly and to small angles, while the
high atomic number material pattern layer scatters the strongly
and to high angles [21]
.
An electron beam is projected towards the scattering mask, the
incident e-beam is scattered to high angles when passing
through the patterned layer, these scattered electrons are then
focused and blocked by a downstream aperture. The aperture
blocks unwanted energy from being projected to the wafer
plane. Since the low atomic number membrane is near
transparent to the high energy incident e-beam, the electrons
undergo narrow scattering angles. These electrons are focused
and allowed to pass through the aperture. A magnetic lens
focuses these electrons, and a small amount of the electron
beams energy is transferred to the wafer. Figure 9(a) shows a
SCALPEL system scattering electrons at high and low angles
as it passes through the scattering mask. A reduced image of

the mask is produced on the wafer plane by reduction-
projection optic that demagnifies the image at 4:1 [21]
. The
benefit of this technique over optical technologies is that this
system is aberration limited, not diffraction limited.
Figure 9(a). SCALPEL principle
(Image courtesy of
www.images.slideplayer.com/14/4312318/slides/slide
_25.jpg)
The design of this SCALPEL tool was based on a step-and-
scan architecture. The mask pattern is exposed on the wafer as
a stationary electron beam is passed through a moving mask at
a constant velocity. A step-and scan system can be seen in
figure 9(b).
Figure 9 (b) Schematic diagram of the SCALPEL step-and-
scan system (Image courtesy of [21])
Using this step-and-scan architecture, the images are
assembled (or stitched) from many small pieces on the mask,
through a combination of electron deflections in the beam and
the motion of the mask and wafer stages [1]
. This technique
was successful in producing 80nm isolated gate structure in a
positive tone DUV resist [21]
. The masks produced were
relatively cheaper than its optical lithography counterparts,
$27,000 compared to an EUV mask priced at £59,000[1]
. Bell
labs allied themselves with ASML to research this technique
but was discontinued in the late 2000 and never
commercialized.
IX. SUB 10NM NANOSTRUCTURES
It was mentioned in section VII that a positive PMMA when
overexposed can be operated as a negative tone resist. An
experiment was conducted and published in the Journal of
Vacuum Science & Technology [24]
where PMMA after
overexposure operated as a negative tone resist to produce
sub-10nm half-pitch dense nanostructures at energies as low
as 2keV. Hydrogen silsesquioxane (HSQ) and calixarene are
two negative resists regularly used to produce these high
resolution nanostructures. This experiment was to prove that
PMMA could be used instead in applications where HSQ was
incompatible to use and to better understand the resolution
limits of EBL. A disadvantage of using HSQ is that
hydrofluoric acid is used in the development process. HF has
the disadvantage that it can attack some metals.
PMMA has been used previously in fabrication techniques to
develop sub-10nm structures using isopropanol (IPA): water
developer. Overexposed PMMA can be transformed into
graphitic or carbon nanostructures by ion or electron beam
radiation [24]
, as also having applications as mechanical
building blocks, masks and dielectric layers or gaps [24]
.
In this experiment, PMMA with a molecular weight of 950 K
was used. This was exposed at different energies, from 30keV
to 2keV at a working distance of ~6mm. The dose range was
taken to be from 3fC to µ3C from a sparse dot array of 15 µm
pitch. This was exposed onto a PMMA layer of 40 nm
thickness on a silicon substrate using methy isobutyl ketone
(MIBK): IPA as a developer. Figure 10(a) shows the
transformation of PMMA from a positive tone resist to a
negative under different dosages and the resulting sub 10 nm
feature created can be seen with an exposure dose of 90
fC/dot.
Figure 10 (a). Range of exposure dosage given to PMMA and
the resulting transformation of PMMA into a negative resist.
(Image courtesy of Sub-10nm half-pitch lithography using
poly(methyl methacrylate) as a negative resist [24]
)
The point spread function was calculated through the
measurement of the diameters at different doses of both
positive and negative structures through increasing the dosage
and can be seen in figure 10(b).

It can be clearly seen that the PSF for negative and positive
tone PMMA are very similar but it was found from this
experiment that although the PSF is very similar, it was found
that the main difference of these two resists was their
sensitivity as the onset dose was about 30 times higher of a
negative tone PMMA compared to the PMMA positive tone.
Since the PSF for both is very similar, the interaction of
electrons have similar roles for the cross linking or cross
scission of these polymer based resists [24]
.
Figure 10(c) demonstrates the formation of 10nm half-pitch
and 12nm half-pitch hexagonally closed packed dots
fabricated using PMMA as a negative resist using MIBK as
the developer.
Figure 10(c) Hexagonally closed packed dots (a) 10nm half-
pitch (b) 12nm half-pitch
(Image courtesy of Sub-10nm half-pitch lithography using
poly(methyl methacrylate) as a negative resist [24]
)
This experiment was successful in creating sub 10nm half –
pitch features using this technique and although the results are
not published, it is stated that sub 6nm half pitch still showed
good response[24]
. Not only was the experiment a success in
proving PMMA as negative resist can be used for high
resolution patterning instead of its negative counter parts such
as HSQ, but can be used also for dense structure patterning
using appropriate developer as seen in figure 10(c).
In order to improve the resolution of an EBL system, an
experiment was conducted using a scanning transmission
electron microscope (STEM) to investigate higher resolution in
an EBL system. This system uses an aberration-corrected
STEM to reduce the scattering effects of high energy electrons
while also having the advantage of a reduced spot size [25]
.
The STEM uses high energy electrons at 200keV to reduce
forward scattering and allows for a spot size of 0.15nm. The
resist used is the HSQ negative tone resist as previously
discussed with a 10nm thick SiNx substrate. Transmission
electron microscopy (TEM) metrology method was used to
assess the resolution limits [25]
. Figure 11 illustrates the three
stages of this STEM system.
Figure 11 (a). Schematic diagram of the STEM system
(Image courtesy of Resolution Limits of Electron-Beam
Lithography toward the Atomic Scale [25]
)
Using this STEM exposure method and development
process, it can be seen in figure 11(b) that 5nm half pitch
dot arrays and patterning as low as 2nm is possible using
HSQ as its resist. It can be seen that using higher electron
energy exposures result in higher resolution over using
30keV as the exposure level.
Figure 11(b) (i)5nm half pitch dot array at dose level of
18fC/dot
(ii) 2nm feature is the minimum feature achieved by this
technique at a linear dose of 8 nC/cm
(Image courtesy of Resolution Limits of Electron-Beam
Lithography toward the Atomic Scale [25]
)

X. CONCLUSION
Many approaches have been researched into the resolution
limits of electron beam lithography. Resolution limits that
govern high resolution patterning include spot size, electron
scattering, resist development as well as the mechanical
stability of the resist. The main disadvantage of electron beam
lithography comes down to its slow throughput of wafers
compared to such rivals as Immersion lithography. Research
and development is currently being explored by companies to
combat this disadvantage and promising results can be seen
with the REBL nanowriter as discussed above. REBL has the
potential to be the first High Volume electron beam lithography
patterning resolutions to 16nm and beyond.
There have been failed attempts such as SCALPEL that have
not been commercialized but promising results indicate REBL
may not be one of them.
Advancements have also been realized using an aberration-
corrected STEM to produce to date one of the smallest patterned
features using electron beam technology.
XI. ACKNOWLEDGEMENTS
The author would like to thank Mr. Paul Ahern of the School of
Electronic Engineering in recommending this technology as an
avenue to research for this review paper, as well as taking the
time to explain many of the principles used in this technology I
would like to acknowledge Dublin City University for their
excellent library and facilities as well as the IEEE Digital
library to obtain relevant and cited papers of old and new
technologies where EBL is adapted into.
REFERENCES
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[3] David J Grant “Electron-Beam Lithography from past to present” 2003
[4] George Brewer “ Electron Beam Technology in Microelectronic
fabrication” 2012
[5] Mohammad Ali Mohammad, Mustafa Muhammad, Steven K. Dew, and
Maria Stepanova “Fundamentals of Electron beam exposure and
development” DOI 10.1007/978-3-7091-0424-8_2
[6] Araldo Van de Kraats “Proximity effect in E-beam lithography” 2005
[7] SPIE Handbook of microlithography, micromachining and
Microfabrication, Vol.1 Section 2.4 Proximity effect
[8] Charles A. Mack “Electron-beam lithography simulation for
maskmaking, part V: Impact of GHOST proximity effect correction on
process window” 1999
[9] SPIE Handbook of microlithography, micromachining and
Microfabrication, Vol.1 Section 2.2 Elements of Electron Optics
[10] Cen Shawn Wu1, Yoshiyuki Makiuchi2 and ChiiDong Chen3 “High-
energy Electron Beam Lithography for Nanoscale Fabrication” 2010
[11] Marc A. McCord “Electron beam lithography for 0.13µm manufacturing”
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[12] Bostjan Bercic “Introduction to Electron Beam Lithography”
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[14] Nicholas P. Cheremisino “Handbook of Polymer Science and Technology
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[19] Alan Brodie, Allen Carroll, Luca Grella, Marek Zywno “ REBL
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[21] L.R Harriott, Bell Laboratories “SCALPEL :Projection electron beam
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[22] Stefano Cabrini, Satoshi Kawata “Nanofabrication handbook” 2012
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[24] Huigao Duan, Donald Winston, Joel K. W. Yang, Bryan M. Cord, Vitor
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[25] Vitor R. Manfrinato†, Lihua Zhang‡, Dong Su‡, Huigao Duan§, Richard
G. Hobbs†, Eric A. Stach‡, and Karl K. Berggren “Resolution Limits of
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Lett., 2013, 13 (4), pp 1555–1558 DOI: 10.1021/nl304715p

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