Optical fiber communication scientific presentation renjith mathew roy
Traineeship Melbourne University - Michael Beljaars
1. High energy ion beam masking by
nano-apertures
Michael Beljaars Bsc.
February 7, 2007
CQT 2007-01
Micro Analytical Research Centre (MARC)
University of Melbourne
Melbourne, Australia
Supervisors:
MARC
prof David N. Jamieson
RMIT
prof. Peter Johnston
TU/e
dr. ir. Peter Mutsaers
d.jamieson@unimelb.edu.au
peter.johnston@rmit.edu.au
p.h.a.mutsaers@tue.nl
2. Abstract
The lower limit of the beam spot on the Melbourne Microprobe 2 beam line is
currently around 1 µm. Because of this limit the spatial resolution achievable
with Ion Beam Lithography is stuck at the same barrier. To avoid costly and
time consuming upgrading of the system an alternative technique to enhance
the beam spot is investigated.
The general idea is to fabricate a small aperture in a Silicon AFM cantilever by
means of Focussed Ion Beam (FIB) milling and to use this nano-aperture as a
mask for the ion beam. The nano-aperture is mounted in front of a photodiode
which is used to detect the energy of incident ions by using Ion Beam Induced
Charge (IBIC) measurements. On the photodiode a 50 nm thick layer of Poly-
methyl methacrylate (PMMA) is spun in order to research the damage done by
the ions passing through the aperture.
The milling of the nano-apertures in the AFM cantilevers proved to be more
difficult than expected. Several attempt to mill an aperture are made, but only
one aperture proved to be useful for experimenting. During the research it is
found that various parts of the Microprobe beam line are in desperate need of
maintenance. Nonetheless a nano-aperture is successfully used to mask a 1.5
MeV Helium ion beam.
4. Chapter 1
Introduction
The focus of scientific research has shifted in the last 20 years from macro to
micro and more recently to the behavior of the physical world in the nanoregime.
Because the behavior on such a scale is different from the macro world it enables
a wide variety of new applications. One of these applications is the use of
nanostructures to manipulate electromagnetic radiation in ways that are not
possible with conventional optics. Such nanostructures are in general referred
to as photonic crystals.
1.1 Photonic crystals
Photonic crystals enable manipulations such as optic bandfiltering (filtering of
optical frequencies), waveguidance (the guidance of light along curvatures in the
order of the wavelength) and channel drop filtering (sort of optic transistors).
The functioning of all photonic crystals relies on the periodicity of the nanos-
tructures out of which they consist. It is this periodicity that determines which
wavelengths are affected and which are not. For the manipulation of optical fre-
quencies, spacings between structures should smaller than the wavelength of the
used light (400 - 800nm). Furthermore for a photonic crystal to be effective the
structures themselves should be much smaller than the spacing between them,
say of the order of 10 nm. Moreover the structures require a certain minimum
height of several wavelengths to effectively couple the light into the photonic
crystal. These requirements put dramatically limit the amount of techniques
capable of creating these nanostructures.
1.2 Motivation
The motivation for this research is to develop a technique for creating photonic
crystals. As described in the previous section this application of nanostructures
does put an upper limit to the in-plane dimensions, for both spacings and di-
mensions of the nanostructures, as well as a lower limit to their height. With
these boundary conditions the structures will have to have a aspect ratio around
1 to 100. Because of this height-width aspect ratio the number of techniques
capable of creating these structures becomes very limited.
2
5. Figure 1.1: Simulation of a waveguide. The spacing between the poles is 1
5 of
the wavelength of the used light. Image taken from http://ab-initio.mit.edu/
1.3 Nano-structuring techniques
There are multiple ways of creating nanostructures, each with its advantages
and disadvantages. A quick outline of the most important techniques and their
advantages / disadvantages in creating photonic crystals is given here. For a
more extensive overview it is recommended to read [1].
Self assembly provides an easy and accurate manner of creating a periodic struc-
ture, but lacks control over the size and type of structuring. An accurate but
slow technique is stacking atoms one by one using a scanning tunneling micro-
scope (STM). Although this provides a useful tool for academic purposes, it is
unfit for large scale applications.
A new kid on the block is 2 photon polymerization (2PP). By means of two
lasers a monomer solution is locally polymerized and forms a solid. The total
object is build layer by layer. In this way virtually every sort of structure can be
created as it is possible to address every grid point separately (see figure 1.2).
The limitation of this technique is currently the spatial resolution, but also this
technique is evolving. Recent work at the Laser Zentrum Hannover has shown
that dimensions below 100 nm are achievable [2].
One of the most common ways of creating nanostructures is by lithography
and subsequent etching of an appropriate resist layer. Lithography can be done
with either electro-magnetic radiation, an electron or ion beam, depending on
the composition of the target material and the desired dimensions. In general
it is possible to achieve a resolution of a few tens of nanometers with either
one of the lithography beams, but it is the aspect ratio that is very limited
for the electro-magnetic radiation and electron beam. The reason for this is the
relatively short penetration depth of the electrons and photons in the resist layer.
A technique which enables a high aspect ratio is called LIGA, which is a Ger-
man acronym for X-ray lithography (X-ray Lithographie), Electroplating (Gal-
3
6. Figure 1.2: Nanorabbits created with the 2PP technique
vanoformung), and Molding (Abformung). The general idea is to use a mask
directly attached to the substrate with resist layer to determine the area that
will be exposed. Exposing can either be done with Synchrotron X-rays, UV
radiation or with reactive chemicals. The last technique is called Deep reactive-
ion etching (DRIE) and doesn’t need a resist layer as the right chemicals are
capable of etching the silicon substrate itself. Although these techniques have
proved to be able to create structures with ratios up to 20:1 or more, there are
practical limits in their application as intended in this report. One of those
limitations is the need for a nano mask for every different structure. Also with
LIGA there is the continuing search to improve the spatial resolution. At the
time of writing this resolution is still limited to a few hundred nanometers.
The focus of this research is the potential of Ion Beam Lithography (IBL) in
creating high aspect ratio nanostructures. IBL must not be confused with Fo-
cussed Ion Beam (FIB) milling. Where FIB uses nuclear stopping power to
actually sputter out the atoms, IBL uses electron stopping power, affecting the
electrons that form the bonds between the atoms. This explains why with IBL
an extra etching process is required to actually remove the exposed resist. The
general advantage of IBL is the minor amount of lateral spread and the great
penetration depth, resulting in high aspect ratio structures. This makes IBL
interesting for creating photonic crystals. FIB milling provides an accurate tool
to create structures on a nanometer scale, but does not allow for high aspect
ratio because of the large lateral scattering due to the nuclear collisions.
This concludes the short overview of methods for creating nano-structures and
explains why for IBL is chosen in this research. In the next chapter IBL will be
explained more extensively.
4
7. 1.4 Objective
The aim of the research is to explore the possibilities of IBL a a tool for cre-
ating photonic crystals and to improve its capabilities by enhancing the spatial
resolution. As will become clear in the next chapter there are currently cer-
tain limitations to IBL that complicate the application as a tool for creating
nano-structures. It is believed that with time these limitations can be overcome
and a true nano-structuring tool can be created: a tool capable of producing
structures of just a few nanometer wide and several hundred nanometers high.
5
8. Chapter 2
Background
2.1 Ion Beam Lithography
Ion Beam Lithography (IBL) uses light high energy ions to expose a resist layer.
By moving the point of focus of the ion beam over the resist layer arbitrary
shapes can be ’written’.
The typical energy of the ions is of the order of a 1-3 MeV. To accelerate ions to
such energies a particle accelerator is required. Because of this the magnitude
of the setup needed for IBL is far more massive than the previous discussed
Focussed Ion Beam (FIB).
The elements that are generally used are Hydrogen and Helium, the lightest
and second lightest element in the periodic table. By choosing a light element
the probability for nuclear collisions is small compared to the probability for
electron collisions. This in contrast to FIB, where deliberately a heavy element
is chosen to create a large probability for nuclear collisions. These collisions
lead to the actual sputtering of the surface atoms.
Because IBL affects mainly the electrons and thus the bonds in the target ma-
terial, the composition of the target surface is of great importance in order to
do any damage. A very popular resist layer for IBL is Polymethyl methacrylate
(PMMA), which is also used in this experiment. After exposure of the resist
layer the affected material can be removed by specific chemicals.
Ion beam lithography is a very promising technique for creating high aspect
ratio nanostructures, which makes it an interesting as a tool for developing pho-
tonic crystals. Nonetheless the spatial resolution of IBL on the setup used is
limited due to the size of the beam spot. Because of non-ideal ionic optics,
interference of stray fields and brightness and chromaticity limitations [3] in
general the beam spot is limited to a few hundred nanometers [4] for the high
current regime (¿50pA). On the setup used in the experiment the beam spot
even proved to be more in the order of 1 µm. in diameter
One way to narrow the beamspot is to invest in a better setup with better
6
9. quality ion optics, object and image apertures. The Singapore group has al-
ready proved that for the low current regime ( 1pA) a beam spot of 75 x 35 nm
is achievable [5], provided a superior quality setup is used. Such an adjustment
of the setup is a huge investment and takes the setup offline for an extensive
period of time.
In stead of trying to narrow the beam spot by investing in the setup a mask can
be used in between the ion beam and the resist layer. In this case the spatial
resolution is determined by the size of the mask. The application of such a mask
is called Ion beam masking.
2.2 Ion beam masking
In theory the application of such a mask is relatively simple (see figure 2.1)
and has a lot of advantages. Due to the use of high energy ions diffraction
is negligible. Not only is such a mask a far less expensive option than the
upgrading of the system, also it is expected to easily enable beam spots in the
order of 10 nm by simply using a smaller mask.
1 µm
d
substrate
nano-aperture
ion beam
Figure 2.1: By using a mask the size of the beam spot is determined by the
diameter d.
A lot of pioneering work on the masking of ion beams was done by the Nuclear
Microscopy Group at the National University of Singapore [6]. They were the
first to come up with the idea of Ion Beam Masking. In their first experiment
they used a X-ray mask, which proved to be quite fragile and difficult to handle.
Nonetheless they proved the effectiveness of Ion Beam Masking for improving
the spatial resolution.
Ideally a mask for IBL consists of a single very small cylindrical hole. Be-
cause its diameter is in the nanometer regime, such a hole is referred to as a
nano-aperture.
7
10. 2.3 Nano-aperture
As explained in the previous sections, a possible solution to overcome the limi-
tation to the beam spot size is the use of a nano-aperture as a mask for the ion
beam [7][8].Recent work [9] on this subject has shown that enhancement of the
spatial resolution is possible. The use of an nano-aperture not only enhances
the resolution of ion beam lithography, but also is a promising tool for other
applications such as singe ion implantation [10].
Because of their availability and easy application AFM cantilevers are chosen
to mask the ion beam. Therefore nano-apertures are milled in the cantilevers
using a Focussed Ion Beam (FIB) (see 3.1). The use of cantilevers also has
the advantage of future in situ monitoring of the IBL process by using them
in an actual Atomic Force Microscope (AFM) and enables easy step-and-repeat
methods.
The application of AFM cantilever as a mask for IBL imposes certain limits
on the type and energy of the used ions. For ions of the same energy applies:
the lighter the ion, the greater its penetration depth and the lower its lateral
spread. Assuming this, the highest aspect ratio would be achieved with high
energy hydrogen ions. Nonetheless for the mask to be effective, the ions should
be stopped within the 8 µm Silicon of the cantilever. For Hydrogen ions this
would restrict their energy to 500 keV, whereas Helium ions are still stopped
completely up till 2100 keV, according to simulations in SRIM [11]. Because
there is a preference to work with high energy ions in order to be certain about
the exposure of the resist layer, the experiments are conducted using a 1.5 MeV
Helium beam.
The ultimate goal and fundamental limit of Ion Beam Lithography is to use
a single ion as the machining tool. By making the step to smaller apertures
the resolution is enhanced. Because of an influence radius in the low nanome-
ter regime structures with atomic dimensions will be within arm’s reach. The
difficulties in achieving this limit lie not only in manufacturing an single ion
aperture, but also in creating enough damage in the resist layer with only one
ion. Nonetheless it is believed that within time these difficulties will be con-
quered and a true nanotool will be created.
2.4 Ion beam alignment
One of the difficulties of using a nano-aperture to effectively mask an ion beam
is the high sensitivity of the alignment of the beam with the axis of the aperture.
A slight deviation of this alignment stops ions from passing through because of
the high aspect ratio of the aperture (see figure 2.2).
For an aperture of diameter d and a depth of 8 µm the angle at which no ions
can pass through without scattering is given by tan(θ) = d
8000 . For small an-
gles tan(θ) can be approximated by θ, leading to a total angle of acceptance
(the angular range within which at least some ions can pass through without
scattering) of 2θ = 180
Π · 2d
8000 (180
Π for conversion to degrees). So in general
the total admission angle is twice the aspect ratio. For the desired dimension
8
11. 8 µm
d
è
Figure 2.2: Dark grey 8 µm thick cantilever with nano-aperture of diameter d
width a light grey negligible divergence beam.
for the nano-aperture of 10 nm this leads to a admission angle of 0.14 degrees.
The goniometer used in the setup has an uncertainty of 0.5 degrees, which is
inadequate for aligning the beam with such an aperture.
One possible solution to the alignment issue is the application of a technique
called beam rocking. This technique has already been applied for angular adjust-
ment in ion beam channeling experiments [12]. The idea behind beam rocking
is quite simple. The ion beam is bend off the axis of the system by a set of
scanning coils and is brought back parallel to the axis by a second set (see fig-
ure 2.3). By doing this the ion beam enters the quadrupole lenses at a different
point and is focussed in the image plane, as is the undisturbed beam. In figure
2.3 it is clearly visible that the location of the beam on the image plane is left
unchanged by the beam rocking, but the incident angle is different.
Only for ideal systems containing no higher order magnetic poles there is no
spacial displacement associated with the beam rocking. Beam rocking on sys-
tems which have focussing lenses containing sextupole and higher multipole
contamination inevitably will displace and deform the beam spot [13]. When
using beam rocking for channeling experiments a displacement of a few hundred
micrometer is not a problem as the sample is supposed to be homogenous. So
as long as the ion beam targets the sample, the desired results are obtained.
In the nano-aperture experiment though, a shift of a few micrometer will put
the beam spot outside the aperture area. The effect of a deviation from such
an ideal system has to be researched on the Melbourne system [14]. However
implementation of this technique was beyond the scope of this project owing to
numerous technical difficulties.
2.5 Simulation programs
To support the experiments a couple of simulation programs are used. These
programs are addressed here briefly.
2.5.1 Propagate Rays & Aberrations by Matrices
Propagate Rays & Aberrations by Matrices (PRAM) is written by D.N. Jamieson
of the Micro Analytical Research Centre (MARC) at Melbourne University. As
9
12. 1
2
a b c d
Figure 2.3: Principle of beam rocking, (1) corresponds to respectively -2 and 2
Gauss, (2) corresponds to both scanning coil sets set to 0 Gauss. Explanation
of the elements: a) image aperture, b) first set of scanning coils, c) second set
of scanning coils, d) magnetic lenses.
the name suggests the program uses matrices to calculate the propagation of
rays in a ion beam system. The program allows for all kinds of ion beam op-
tical elements such as collimators, magnetic and electric dipole, quadrupole,
sextupole and octupole elements. The theory behind the program is described
in Chapter 3 of [15]. The program is freely available on the web [16].
2.5.2 Stopping & Range of Ions in Matter
Stopping & Range of Ions in Matter (SRIM) is a simulation program to deter-
mine the range of ions in matter. The program has a lot of degrees of freedom,
ranging from the energy and species of the incident ions to the thickness, com-
position and amount of layers of the target. It uses Monte Carlo simulation to
determine the actual path of the ions. More information on the program and
the program itself is available on the internet [11].
10
13. Chapter 3
Experimental setup
3.1 Focussed Ion Beam
Two different Focussed Ion Beam (FIB) instruments have been used during this
research, a Jeol dual beam system and a FEI Nova 200 Nanolab (see figure
3.1). The latter represents state-of-the-art technology as will become clear in
the results on the milling of the apertures.
Figure 3.1: FEI Nova 200 Nanolab
A FIB uses a source of liquid metal, mostly Galium heated by a Tungsten ele-
ment. The liquid Galium is then ionized by applying a electric field. The Galium
ions are accelerated through a set of electrically charged plates (the extractor)
and are eventually focussed on the substrate by a column of electrostatic lenses.
The final energy of the Galium ions can be chosen between 5 and 50 keV. The
ideal energy to work with depends on the composition of the surface. In between
the lenses a aperture is mounted which determines the ion beam current.
11
14. On the substrate the Galium ions collide with surface atoms and transfer enough
energy to sputter out the substrate atoms, thereby leaving a gap. By continu-
ing this process while shifting the substrate a spatial structure can be milled.
Figure 3.2 gives a schematic overview of how a FIB works.
liquid-metal source
lens 1
lens 2
suppressor
extractor
aperture
substrate
Figure 3.2: Schematic overview of a FIB
Both the FEI and the JEOL are dual beam systems, meaning that both have a
build in Scanning Electron Microscope (SEM) to image the created structures
without damaging the substrate.
3.2 Ion beam line
The experiments are performed on the Microprobe 2 (MP2) beam line at Mel-
bourne University. Figure 3.3 gives a schematic overview of the system and the
setup used.
Outside the field of view of figure 3.3 is the actual creation and acceleration
of the ions. The ions are created by ionizing a gas of the desired composition
using RF coupling. With RF coupling electromagnetic radiation of a specific
wavelength is used to excite the electrons in the gas enough to make them com-
pletely unbound to the atoms of the gas (i.e. ionization). What is left after this
12
15. process is a soup of charged particles, called a plasma.
The positively charged particles in the plasma will be accelerated through an
electric field, which is set to the approximate desired energy. After the beam
leaves the accelerator it is bent by a magnet, which is set to exactly select the
ions with the desired energy. Ions with a higher energy will be bent more than
90 ◦
, ions with a lower energy will be bent less than 90 ◦
. These ions will hit
the high and low energy slits (not drawn in figure 3.3) which provide a feedback
signal for the electric field used for acceleration. If to many ions hit the low
energy slit, the electric field has to be increased to create higher energy ions and
vise versa.
A first set of coils, called the steering coils, is used to successfully steer the
ions through the pre-collimator and object box. In the object box the beam is
cut down using apertures. The sizes range from 500 µm to 10 µm in diameter. If
desired the beam can even be cut down further by using the micro V slit. Unfor-
tunately these are very sensitive to distortion and have a huge hysteresis, which
makes it difficult to control the beam. Further up the line the beam passes the
image box, containing apertures from 6 to 0.25 mm to limit the divergence of
the ion beam. On the Melbourne setup they also serve the purpose of filtering
out scattered ions, as there occurs a lot of ion scattering at the object apertures.
The fluorescent screen and the Faraday cup are both tools for setting up the
beam. The fluorescent screen makes the beam spot visible, whereas the Faraday
cup measures the beam current.
Two sets of scanning coils are present on the beam line, of which only one
is currently in use. The other is meant to be used for the beam rocking as de-
scribed in 2.4. A set of scanning coils consists of four coils, two for each direction
perpendicular to the beam line. As the name suggest these coils are used to
perform scans with the beam across the surface of the target. For example by
applying a magnetic field in the positive x-direction, the ions will feel a Lorentz
force in the y-direction.
Finally the beam passes through a setup of four magnetic lenses. These magnets
are placed in a quadrupole arrangement to focus the beam.
3.3 Target chamber
The target chamber is located at the end of the beam line, containing the mount-
ing assembly with the actual target. This assembly consists of multiple items as
shown in figure 3.3. A piece of glass is used to make the beam spot visible and
focus it. Furthermore the assembly contains a Nanonics piezo movable stage on
which two Hamamatsu S1223-01 photodiodes are mounted. The photodiodes
have their protective caps removed. One of the diodes is provided with three
copper grids to determine the scan range of the beam and evaluate the quality
of its focus. The copper grids have a 500, 1000 and 2000 mesh. The other
diode is mounted underneath the cantilever and used in the experiments with
the aperture. The photodiode has been spin coated with an approximately 50
13
16. nm thick layer of Polymethyl methacrylate (PMMA) resist layer in order to
research the damage done by the ions passing through the aperture. The can-
tilever is mounted on a mechanical arm which can be lowered to the photodiode
by means of a screw as to set the separation distance between the aperture and
the resist layer.
The photodiodes are not position sensitive, but only detect the energy deposited
by an ion strike. The energy of the ions is measured by means of the photo-
diodes, using a phenomenon called Ion Beam Induced Charge (IBIC) [17]. An
incident ion creates a number of electron-hole pairs in the same way as electro
magnetic radiation does. The number of electron-hole pairs created depends on
the energy of the incident ion and is equal to the energy of the ion divided by
the energy needed for the creation of a single pair. Thus the eventual current
is a measure for the ion energy.
14
18. To get spatial information the currents through the scanning coils are used as x
and y reference. Because of the linear relation between the fields of the scanning
coils and the position on the sample, the fields can be used to acquire a spatial
map of energy.
The whole assembly is mounted on the arm of a goniometer (see figure 3.4) which
allows for the whole to be moved in the directions perpendicular to the beam
and to be rotated along both corresponding axes. The assembly is mounted
on the goniometer as such that the aperture is approximately in the center of
rotation. This ensures that the aperture stays within the area exposed to the
beam when adjusting the angle.
Figure 3.4: Assembly mounted on the goniometer. Indicated are the translation
and rotation directions.
16
19. Chapter 4
Results
This chapter is divided in three sections. The first section is about simulating the
beam rocking. In the second section the manufacturing of the nano-apertures is
described, whereas the second section deals with the experiments on the particle
accelerator using the nano-aperture.
4.1 Beam rocking
One of the issues of the ion beam - aperture system is the aligning of the ion
beam with the axes of the aperture. As discussed especially for apertures with
a high aspect ratio this is an important issue. The beam rocking has been sim-
ulated using PRAM to research its potential for application on the MP2 line.
The simulation has extensively tested PRAM and found a couple of bugs that
have been fixed along the way. The simulation proved beam rocking is a subtle
way of controlling the incident angle. Figure 4.1 shows that the incident angle
can be controlled within 1 ◦
(β = 0.5) around the axis for the MP2 line when
the scanning coils are separated by a distance of 430 mm and a maximum field
of 200 Gauss is used.
The angular range might be increased by either increasing the separation dis-
tance of the scanning coils or increasing the applied field. Both enhancements
have their limitations; the length of the section suitable for mounting the scan-
ning coils is 1000 mm and the scanning coils can cope with a maximum field of
600 Gauss for a few seconds.
An additional program to PRAM is a program called Oxtrace. Oxtrace has
the possibility to map the incident angles θ and φ (the angles with the x and
y axis), which is useful to get an idea about the amount of spread and diver-
gence in the beam. Unfortunately Oxtrace can not handle dipole elements. This
makes the program unsuitable for researching beam rocking. A useful future
change to the program would be to incorporate the use of dipole elements, so
that beam rocking can be simulated.
Although the beam rocking has extensively been investigated by simulating,
the beam rocking unfortunately hasn’t been tested experimentally on the MP2
17
20. scanning
coils
quadrupole
lenses
focal point
â
1
2
Figure 4.1: Beam rocking simulation; (1) corresponds to both scanning coils set
to 0 Gauss; (2) corresponds to scanning coils set to respectively 200 and -200
Gauss; β is in this case 0.5, leading to a range of 1 ◦
within which the angle of
incidence can be controlled.
system due to a couple of setbacks and only a limited amount of time.
4.2 Manufacturing apertures
All the milled cantilevers are milled with the tip facing upward, towards the
incoming ion beam. This side is referred to as the top side, whereas the other
side is referred to as the bottom side.
In a first attempt a design with multiple apertures, differing in size and two
calibration slots was milled in a cantilever (#1) (see figure 4.2) by means of a
Focussed Ion Beam (FIB) FEI Nova 200 Nanolab, using 30 keV Ga ions. Milling
of the small apertures proved to be quite difficult, whereas the 10 µm long slots
appeared to be milled very neatly all the way through, measuring approximately
200 nm at the bottom side.
18
21. 35mμ
130 mμ
Figure 4.2: Design of cantilever
After milling the supposedly 15 µm thick AFM cantilevers turned out to be only
3 µm thick (see figure 4.3). Because the range for 1.5 MeV Helium is around 6
µm in Silicon this is far too thin to stop the ions (figure 4.4a).
30µm
2.91µm(tiltcorrected)
(a) SEM image of the slots milled in the
cantilever
1µm
(b) SEM image of the largest aperture,
which is approximately 800 nm in diameter;
inlay: backside image of the same aperture
with the dimensions 238 by 57 nm.
Figure 4.3: SEM images of cantilever #1
With SRIM the usefulness and feasibility of depositing a layer of platinum on
top of the cantilever was examined 4.4b. Judging from these simulations at least
a 1 µm thick layer of platinum is needed to stop the ions. Although the depo-
sition of such a thick layer might be possible, the more simple and promising
approach of finding and milling thicker cantilevers was chosen.
A supply of used cantilever appeared to be 8 µm thick after imaging in a SEM
(see figure 4.5). Using the same design and FIB as for cantilever #1 one of these
cantilevers (#2) is milled. After turning over the cantilever to examine the other
side, it turned out that the slots weren’t visible, while all the three apertures
were (see figure 4.6). It is clear that the two apertures most close to the tip (on
the left) are all the way through, whereas the other aperture (on the right) is not.
19
22. (a) 3 µm silicon (b) 3 µm silicon with 1 µm Pt on top
Figure 4.4: SRIM simulation of 1.5 MeV He ions
(a) SEM image of the entire cantilever (b) SEM image of the cantilever near the
tip
Figure 4.5: SEM images of old used cantilever
Because of intensive use of the FEI FIB and consequent unavailability an other
FIB, a JEOL dual beam system, was used to mill one of the 8 µm thick can-
tilevers (#3) (see figure 4.7). The resolution of the JEOL FIB is lower than
the FEI FIB, but it might provide insights in the approximate best settings to
mill all the way through. Unfortunately it turned out after inspection with the
optical microscope that none of the apertures nor any of the slots is visible on
the other side of the cantilever. What can be see in figure 4.7 is that the resolu-
tion of the JEOL system is indeed much lower than the FEI system as the slots
appear as huge canals in the left image. The right image shows an attempt to
gain insight in what it takes to drill al the way through. It appears that creating
a side cut in the cantilever is much easier than creating a slot in the middle of
the cantilever. This is due to the fact that the sputtered material easily can
escape when milling on the side of a cantilever. This in contrast to milling in
the center of a cantilever, where sputtered material can easily redeposit.
Even a fourth cantilever (#4) is milled, using the FEI FIB. Although exten-
sive time is spend on the milling of just a single aperture, no useful aperture is
created. Figure 4.8 gives an overview of the four cantilevers.
20
23. 80µm
(a) SEM image of top side of the cantilever.
20µm
(b) SEM image of bottom side of cantilever.
The left aperture is 400 nm and the middle
aperture is 600 nm in diameter. The right
aperture is not all the way through.
Figure 4.6: SEM images of cantilever #2
(a) Overview of cantilever #3 from top side (b) Zoom in of side cut in cantilever
Figure 4.7: SEM images of cantilever #3
21
25. 4.3 Microprobe 2 beam line
4.3.1 Calibration grid
For the detection of the energy of the ions through the aperture it is not partic-
ulary important to calibrate the scan area. Nonetheless it is very useful to scan
the photodiode with the copper grids as this gives a good indication of how well
focussed the beam is and where exactly it is targeted (see figure 4.9). Using
this information the camera is aligned as such that the beam spot is targeted
in the center of the view. As the position of the beam spot and the center of
the view of the camera now coincide, it is relatively easy to target the beam on
the aperture by just moving the area of interest in the center of the view.
(a) IBIC image of the copper grids on the
photodiode
(b) IBIC image of the 2000 mesh grid
Figure 4.9: Calibration photodiode
Judging from the smallest features in figure 4.9b the size of the beam spot
can be estimated. A 2000 mesh has a grid period of 12.7 µm. The grid lines
themselves are in the order of 2 µm. To resolve such details the beam spot has
to quantitatively be smaller than this and will be of the order of 1 µm.
4.3.2 Sensitivity of angular alignment
First experiments were done to align the high aspect ratio apertures with the
ion beam direction. By means of the goniometer a angular sweep in θ from 13.0
to 17.0 ◦
in steps of 0.5 ◦
was performed using the middle aperture on cantilever
#2. This aperture measures 800 nm at the top and 600 nm at the bottom (top
and bottom side defined as in 4.2). Because the cantilever is mounted with the
bottom side facing the incident ion beam, the ions encounter a conical aperture
that starts small (600 nm) and then widens to 800 nm.
A calculation of the maximum angle that ions can pass through the aperture
without scattering can be made using just simple geometry (as described in 2.4).
23
26. For an conical aperture of dimensions as described above the maximum angle
of incidence is 6.4 ◦
in both directions, resulting in a admission angle of approx-
imately 13 ◦
. This suggests that even for a deviation from perfect alignment of
a few degrees there would still be ions passing through without scattering and
thus maintaining their energy.
In figure 4.10 the energy spectrum of the ions passing through the aperture
is shown for different angles of incidence. Strikingly there are almost no full
energy ions passing through the aperture, but there are a lot of scattered ions
coming through. This suggest that there might still be some material partially
blocking the beam. From the shift of the peak it can be deduced that there
would be an equivalent of 2 µm of Silicon (see figure 4.12). The SEM images
showed that at least the middle and top aperture were all the way through,
which makes it unlikely that it was Silicon blocking the beam. Inspection of the
cantilever under an optical microscope revealed that the aperture was blocked
by some of the glue that was used for mounting it (see picture 4.11. The top side
(milling side, side facing the photodiode when mounted) of the cantilever looks
fine, but the bottom side clearly has a layer of glue on it. The low resolution of
figure 4.11b compared to 4.11a is due to the fact that the height of the mounting
block of the cantilever doesn’t permit the use of the highest magnification lens.
Hence a digital magnification is used.
Apart from a mean offset in energy which is the equivalent of 2 µm Silicon there
is a angle dependent energy shift as shown in figure 4.13. There is a local mini-
mum in energy shift at a theta of 15 degrees, which would correspond with the
optimal alignment of the aperture with the ion beam. An increasing deviation
from this alignment would lead to a larger and larger shift in energy due to a
continuing increase in the amount of scattered ions and a longer trajectory of
the ions through the silicon. Strangely enough a decrease is observed for angles
of 13 and 17 degrees. This can be understood by realizing for a high aspect ratio
aperture the angular sensitivity is high, meaning that a small change in angle
the shift in energy is already quite big. Because the backlash in the goniometer
gives an uncertainty of at least 0.5 degrees this effect causes an uncertainty of
several hundred keV in the energy shift.
One would expect to see a shift to lower energies for angles greater and smaller
than the alignment angle because of more scattering of the ions. The shift of
the full energy peak as a function of the angle is not unambiguous though, for
a couple of reasons. First of all, the shift of the peak is very small around the
alignment angle. Even a offset of a few degrees doesn’t affect the full energy
ions much as the total angular range is approximately 13 ◦
. Secondly, a shift
of the peak can also be due to degradation of the detector by the hitting ions.
As this process is dependent on the amount of striking ions and their position,
results can be irregularly masked by this effect. Therefore it is of importance
to quantify the degradation of the detector and measure the region affected by
the ions that passed through the aperture. Last, the goniometer has backlash,
which leads to an uncertainty of 0.5 ◦
. This last problem can be resolved by
using the beam rocking technique to fine tune the angle after course alignment
with the goniometer.
24
28. (a) Photo of the top side of the cantilever.
The slots and apertures appear clearly on
the surface.
(b) Photo of the bottom side of cantilever.
There is seemingly some glue stuck on the
cantilever.
Figure 4.11: Photos of cantilever #2.
It appears from calculations that angular alignment of the aperture is not an
important issue as the acceptance angle for the middle aperture is 13 ◦
. The
outlook of the research though is effective masking of an ion beam for use with
lithography. For that purpose a smaller aperture is needed. Preferable dimen-
sions are in the order of 10 nm diameter and 8 µm depth. For such an aperture
the admission angle is just 0.14 ◦
and it becomes apparent why accurate angular
alignment is of great importance.
4.3.3 Full energy ions
Attempts were made to clean the glue on the cantilever. At first the cantilever
was given an acetone ultrasonic bath. Viewing the cantilever under an opti-
cal microscope revealed that the glue showed some cracks but is not removed.
Therefore a more aggressive remover, piranha etch, was subsequently used to
clean the cantilever. This proved to be more successful as there was even coming
some light through the holes when viewed under the microscope.
After the cleaning the cantilever was mounted on the stage using carbon tape
for further experiments. Scanning of the copper grid photodiode proved that
the beam was highly fluctuating (see figure 4.14). The highly deformed grid like
pattern on the left side of the figure was caused by the pulsating character of
the beam. As the beam scanned the area the photodiode picked up ion strikes
in bursts, which consequently resulted in the pattern.
A couple of scans of the copper grids were made to determine the scan area and
location and align the camera view. Subsequently the stage was moved to put
the cantilever in the field of view. After decreasing the gain of the scan coils
amplifier to reduce the scan area figure 4.15 was obtained. The image shows
the middle and the bottom aperture. The fact that the apertures appear to be
26
29. 0 200 400 600 800 1000 1200 1400 1600 1800 2000
0
0.002
0.004
0.006
0.008
0.01
0.012
Energy (keV)
equivalent of
2 µm Silicon
Figure 4.12: The energy gap is the equivalent of 2 µm Silicon
elliptical can be explained by a difference in gain of the scan amplifier between
X and Y, an elliptical shape of the beam spot or the shape of the aperture. In
figure 4.5b can be seen that the aperture is not a perfect cylinder.
For three different regions spectra are extracted from the data shown in fig-
ure 4.10. These spectra are corrected for their corresponding area and plotted
in figure 4.16. The red curve corresponds to the spectrum of the total area, the
green curve corresponds to an area outside the apertures and the blue curve
is the spectrum of both the apertures. The lower graph displays the energy
spectrum for the apertures, corrected for the background noise by subtracting
the green curve from the blue curve in the upper graph. The X axis is scaled
as such that the highest peak in the red curve corresponds to 1500 keV, the full
energy of the Helium ions.
It is clearly visible that even after subtracting the full energy counts striking
elsewhere on the detector there are some full energy counts in the spectrum
of the aperture. This means that there are actually full energy ions passing
through the aperture. The reason that most of the ions seems to strike with
an energy just below 1500 keV can be explained by taking the damage to the
detector into account. Figure 4.15 was captured during the 11th run, meaning
that the detector already had been exposed to many ion strikes during the first
10 scans. Because the beam was pulsating, it was hard to estimate the ion count
and impossible to adjust the micro slit as to control it.
27
30. 12 13 14 15 16 17 18
500
550
600
650
700
750
800
850
900
950
1000
0 200 400 600 800 1000 1200 1400 1600 1800 2000
0
0.012
Energy (keV) theta
energy shift
Aperture spectrum
(corrected for background)
Energy shift
for different theta
normalizedcounts
Energyshift(keV)
Figure 4.13: Shift in energy as indicated in left figure are mapped for different
angles of theta in the right figure
Figure 4.14: IBIC image of photo-
diode with the copper grids
Figure 4.15: IBIC image of the left
and middle aperture as seen in 4.6b.
The right aperture was not visible.
28
32. Chapter 5
Conclusions
The simulations prove that beam rocking is a accurate technique to control the
incident angle of the beam within at least 1 ◦
around the beam axis and provides
an excellent addition to the coarse mechanism of the goniometer.
Although the angular alignment issue proved to be none relevant for the used
aperture, beam rocking can be very useful when working with higher aspect
ratio apertures.
The implementing of beam rocking is relatively easy, as a second set of scan-
ning coils is available and already mounted on the beam line. The way beam
rocking is meant to be used for experiments with the nano-aperture is different
from the conventional beam rocking used in channeling experiments. Instead of
sweeping the beam rocking angle, the beam itself is scanned across the surface
of the cantilever with fixed beam rocking angle. By means of multiple scans
differing in beam rocking angle the optimal angle can be determined. For this
type of scanning the two sets of scanning coils need to be provided with an
DC current to set the beam rocking angle. For the scanning itself a AC cur-
rent needs to be applied to either one of the two sets or to a separate third set
of scanning coils. A possible reason for using a third set of scanning coils in
stead of superimposing a AC signal on the DC signal of one of the sets used
for the beam rocking is to avoid damage to the coils by applying to large cur-
rents. Mounting a third set though might be difficult because there is only few
space available. The beam rocking requires fields of the order of 150 Gauss,
while the scanning coils are capable of handling up to 600 Gauss. If the scan
area is kept small enough the coils should be able to cope with such fields. How
well this technique in practice will work on the MP2 line is still to be researched.
The creation of a useful aperture for ion beam masking proved to be a project
on its own. The initial focus of this research was to make the step to smaller
apertures, but it already proved difficult enough to create a large aperture that
was al the way through the cantilever. As the aperture is a crucial part of the
ion beam masking more research on how to create the perfect aperture would
be very useful.
Despite of the high background noise of scattered full energy ions, full energy
ions have been observed passing through a conical 800 to 600 µm aperture with
a depth of 8 µm. Most likely the full energy ions background noise is due to
30
33. scattering of the ions at the object aperture. The background noise in the sec-
ond experiment, using a 750 µm image aperture, proved to be more than 50
% of the detected ions whereas for the first experiment, using a 250 µm image
aperture this was less than 20 %. To reduce the scattered ions to a minimum
it is thus of importance to work with a small image aperture. Also the use of
a pre-collimator just before the nano-aperture might cut down the number of
scattered ions. Such pre-collimators have been manufactured, but have not yet
been used in experiments.
There are still a few minor problems to be overcome before nano-aperture mask-
ing can be applied for step and repeat ion beam lithography. Nonetheless it
proves to be one of the most promising tools in the continuing quest to create
smaller and smaller structures. Not only enhances the nano-aperture the spatial
resolution for IBL, it is also potentially a tool for precise single ion implantation.
Especially when the manufacturing of a suitable aperture is under control, it
is believed that technique strives past conventional IBL regarding the spatial
resolution.
5.1 Outlook
Once the milling of small holes and slots in cantilevers is under control, attempts
can be made to close them down using platinum deposition in order to achieve
the desired dimensions of the order of 50 nm or less as suggested in [8]. For
the moment it is still a problem to determine if the holes and slots are milled
all the way through, apart from the fact that it takes up a lot of time to mill
them. For example, milling a 800 nm hole in a 8 µm thick Silicon cantilever
takes more than half an hour, using a 300 pA current.
5.2 Recommendations
One important conclusion involves a couple important possible improvements
of the setup used. To avoid too much damage to the photodiode and to get the
best results, it is of great importance to have a stable beam of a few thousand
ions per second. To achieve this the stable and reliable operation of different
parts of the beam line is essential. In the next subsections the most vital parts
and their possible improvements are discussed.
5.2.1 Plasma source
Indispensable for stable ion beam is a stable plasma source. As the accelerator
at the moment of writing is already a few months overdue for maintenance, the
coupling of the RF source on the gas is very weak due to deterioration of the
glass of the source. This leads to spontaneous fluctuating or vanishing of the
plasma.
31
34. 5.2.2 Object aperture box
The best way to cut down the beam to the desired current is by using a small
object aperture. The smallest aperture currently available on the MP2 line is 10
µm in diameter. This should be sufficient, but unfortunately this aperture is not
well aligned, making it nearly impossible to get the beam through. Realignment
of the object apertures would be very useful.
5.2.3 Steering coils
Apart from the misalignment of some of the object apertures, the lack of sen-
sitivity of the power supply of the steering coils makes it hard to accurately
steer the beam through the apertures. A more stable fine tunable power supply
would make this a lot easier.
5.2.4 Micro V slit
After cutting down the beam with the object aperture and filtering out most of
the scattered ions by using the smallest image aperture (250 µm) the only way
to reduce the beam current further is by means of the micro V slit. As already
remarked earlier in this report the mechanism to control the micro slit is very
coarse and has a huge hysteresis. Revision of this part is highly recommended as
it should provide an accurate control over the beam current in order to prevent
high rate deterioration of the photodiode.
32
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34
