1. Deposition of very thin films by
thermal evaporation using a heated
substrate
Conor Browne
Undergraduate Student of the School of Physics, National university of Ireland, Galway
Dr. Ger O’ Connor (Supervisor)
NCLA, National University of Ireland, Galway
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Deposition of very thin films by thermal evaporation using a
heated substrate
Conor Browne
School of Physics, National University of Ireland, Galway.
Abstract
The results of depositing very thin films by thermal evaporation using a heated substrate are
presented in this paper. The qualities of the aluminium thin films have been enhanced
through a series of improvements made to the standard method. Upon heating the
substrate, prior to deposition, a decrease of 41.6% in sample roughness was obtained from
Atomic Force Microscope (AFM) results, when compared to the results of a non-heated
substrate. A reduction in surface roughness of the heated sample, when compared to the
non-heated sample was also observed in the results gathered from the White Light
Interferometer (WLI).The grain size of the heated sample was found to be 3.5 times larger
than the grain size of the non-heated sample, from the AFM grain size analysis. The increase
of grain size upon heating was verified using two techniques in the image processing tool
ImageJ. The reflectance of the heated sample performed better over the range of
wavelengths 410 – 540 nm than the non-heated sample, however its performance was
significantly worse in comparison over the rest of the spectrum, in particular the Infra-red
(IR) region. The reasons behind this are discussed within the report.
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Acknowledgements
I would like to thank my supervisor Dr. Ger O’ Connor for all of his help and guidance
throughout the duration of this project.
Special thanks to James Nallen for all his help with the apparatus, and his help in improving
the quality of the samples.
To Conor Mc Brierty, thank you for designing and implementing the glass slide heater, this
was invaluable to obtaining the main results for the project.
I would also like to thank Clare Mc Daniel and Pinaki Das Gupta for their help using the
Atomic force Microscope, and for their advice and help with the presentation.
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Appendix A………………………………………………………………………………………………………………….. 29
Appendix B………………………………………………………………………………………………………………….. 31
Appendix C………………………………………………………………………………………………………………….. 33
1. Introduction
A thin film is a layer of material that has minimal thickness ranging from just fractions of a
nanometre (nm) to several micrometres (µm) [1]. Very thin film coatings are generally
regarded to be less than 50 nm thick. Very thin film materials are hugely important in the
realisation of many key future technologies such as capacitive touch panels, photovoltaic
cells and thin filmbatteries. The properties of the deposited thin film depend on process
parameters such as the surface energy of the films and substrates, the substrate
temperature and the growth rate.
In this project the material used for deposition of thin films was aluminium, which was
deposited onto both glass microscope slides and glass cover slips, which were used
throughout the project. The method of deposition used was thermal evaporation, using the
Quorum Emitech K975X thermal evaporator. The initial goals of the project were to learn
how to use the thermal evaporator, and then to deposit a range of aluminium thin films of
specific thickness onto the substrate. These films were to then be characterised using
Atomic Force Microscopy (ATM), White light Interferometry (WLI) and Spectral Reflectivity
(SR). The main objective of this project was to develop a high temperature substrate
capability within the thin filmevaporative apparatus. This would allow the substrate to be
heated before deposition began, which would increase the thermally promoted process of
surface diffusion, which would improve the ‘smoothness’ of the aluminium thin films.
Following this development a range of aluminium layers were once again deposited but
onto the heated substrate, these samples were then characterised using the AFM, WLI and
SR. The heated and non-heated samples were compared and contrasted using the results
gathered from the AFM, WLI and SR.
Some of the most common examples of aluminium thin films are in mirrors, space blankets
and spaceship insulation [2]. These applications are all due to aluminium’s high reflectivity
of both visible and infra-red light, even in the form of a thin-film. This project will also
observe how having heated the substrate before deposition will affect this important
characteristic of aluminium thin films.
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2. Background
2.1 What is a thin film?
A thin film is a layer of very thin solid material or a liquid, deposited onto a substrate,
ranging from just fractions of a nanometre to several micrometres. Electronic semi-
conductor devices and optical coatings are just some of the main applications benefiting
from thin films.
The act of applying a thin filmonto a substrate is thin filmdeposition which is broadly
categorised into chemical and physical deposition. The technique used in this project is
thermal evaporation, a type of physical deposition which we will also discuss [3].
2.2 Aluminium
Aluminium (Al) is a chemical element in the boron group with an atomic number of 13, an
atomic mass of 26.9815385u and has a face-centred cubic (FCC) crystal structure. It is the
third most abundant element within the Earth’s crust, and most abundant metal in it. Due
to its high reflectance in both the visible light and Infra-Red spectrums it is often used as a
reflective coating, and in spaceship insulation. Due to the materials light weight and
strength it is also widely used in aircraft construction.
2.3 Thermal evaporation
Thermal evaporation involves heating a solid material in a crucible until it melts inside a high
vacuum chamber and raises its vapour pressure up to a useful range [3]. Inside a high
vacuum even a relatively low vapour pressure is sufficient to raise a vapour cloud inside the
chamber. This evaporated material now constitutes a vapour stream, can reach the
substrate without interacting with other gas phase atoms within the chamber. Upon
reaching the substrate the material will condense back to the solid state forming a thin film.
2.4 Vacuum
A vacuum refers to a space from which a gas or a mixture of gases such as air has been
partially removed so that it has a relatively low pressure [4]. A low vacuum has a pressure
that is slightly less than atmospheric pressure, and a high vacuum has an even lower
pressure. In a high vacuum such as is used in thermal evaporation, the evaporated particles
have a long mean free path and can travel directly to the substrate without interacting with
background gases.
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2.5 Surface diffusion
Surface diffusion involves the motion of molecules and atomic clusters at solid material
surfaces; the motion of these is a thermally promoted process with rates increasing with
temperature. The process can be thought of as atoms moving between adjacent adsorption
sites on the substrate surface. Increasing the temperature of the substrate should increase
surface diffusivity and produce a smoother thin film [5].
2.6 Reflectance
Reflectance is the ratio of the amount of light (Intensity) that is reflected by a surface in
relation to the amount of light incident on the surface. Spectral reflectance is the fraction of
light reflected from a surface as a function of wavelength; this can be measured by a
spectrometer such as that in the Filmetrics F20 thin film analyser (Section 3.8). Spectral
reflectance is an important parameter in many materials including aluminium thin films.
2.7 Surface roughness
The roughness of a surface is a component of surface texture that is defined by deviations
on the surface [6]. The average roughness is the average height between these deviations in
the form of peaks and troughs. If these deviations are small the sample is considered to be
smooth. Smooth surfaces have benefits of wearing less quickly than rough surfaces and
scatter less light.
2.8 What is a grain?
A grain is a collection of atoms lined up in a specific pattern that depends on the crystal
structure of the metal [7]. As the grains grow they will eventually impinge on other grains
and form an interface where the atomic orientations of the grains are different, these
interfaces are known as grain boundaries. Grain size is an important factor in terms of light
scattering as a larger grain size scatters, rather than reflects more light.
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3. Instrumentation
3.1 K975XTurbo evaporator
Figure 1: K975X set-up, including the rotary vane pump. The turbo-molecular pump is
located behind the evaporator chamber.
The K975X is a bench mounted turbo-pumped thermal evaporator used in the vacuum
deposition of thin films for a variety of metals. The set-up includes a rotary vane pump and
turbo-molecular pump which are used to obtain a high vacuum with the chamber. A Pirani
gauge provides a measurement of the vacuum pressure. The chamber cavity is 228 mm in
diameter and the chamber housing consists of boro-silicate glass; this houses the tungsten
crucible in which aluminium is heated, the film thickness monitor (FTM) and the glass slide
holder in which the glass slides are placed. Later throughout the project it also housed the
glass slide heater.
In the chamber there are 4 electrodes which can be used to supply power to parts within
the chamber, but only one at a time. The B and C terminals were used to power the
tungsten crucible and the glass slide heater respectively. The menu controls allow a variety
of parameters to be adjusted such as the mode used, or the density of the material used in
calculating the thickness with the FTM.
The current control is used to control the current coming from the electrode that would be
in use at that time. Each electrode has a constant voltage across it during usage. This current
control is used to supply enough power to the tungsten crucible to turn the aluminium into
a vapour which can then form a thin film on the glass slide within the vacuum.
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3.2 Rotary vane pump
Figure 2: RV3 Rotary vane pump.
A rotary vane pump is a mechanical pump that consists of vanes mounted to a rotor which
rotates within a cavity. As the pump turns it causes a cyclical change in the suction volume.
It takes in air from the evaporator chamber and pumps this through the pump cavity and
forces it out a discharge outlet [8]. This will create a rough vacuum at which point the turbo-
molecular pump will take over to bring the chamber to the required pressure of a high
vacuum.
3.3 Turbo-molecular pump
Figure 3: EXT 70H 24V Turbo-molecular pump.
This is used to obtain and maintain a high vacuum within the evaporator chamber. It works
on the principle that gas molecules can be given momentum in a desired direction by
repeated collisions with a moving solid surface. This solid surface is in the form of rapidly
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spinning turbine blades which deflect gas molecules from the inlet of the pump towards the
exhaust [9].
3.4 Filmthickness monitor (Quartz crystal microbalance)
Figure 4: K150x Film thickness monitor (FTM) also known as a Quartz crystal microbalance
(QCM).
The FTM measures the mass per unit area by measuring the change in frequency of a quartz
crystal resonator. As mass is deposited on the surface of the crystal, the frequency of
oscillation decreases, this is due to the thickness of the thin film increasing [10]. The FTM is
useful for monitoring the rate of thin filmdeposition in a vacuum.
3.5 Glass slide heater
Figure 5: Glass slide heater connected to electrode C (Power supply).
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The glass slide heater consists of a mild steel frame and pitched roof with two small bars to
hold the glass slide, the main heating element is the nichrome wire wrapped around the
ceramic bars. The glass slide heater works by supplying a large current causing the nichrome
wire and mild steel to radiatively heat the glass slide within the chamber as there is no
convection within a vacuum.
3.6 Atomic Force Microscope (AFM)
Figure 6: Agilent 5500 Atomic Force Microscope [11].
The AFM provides Nano-scale resolution of samples. It consists of a cantilever with a sharp
tip at its end that is used to scan the specimen surface. The tip is brought into close contact
with the sample surface and the forces that interact between the tip and sample lead to a
deflection of the cantilever [12]. This deflection is measured using a laser spot reflected
from the top of the cantilever. As the cantilever bends the position of the spot on an array
of photodiodes changes and the resulting signal is the deflection which is used to produce
3D images of the sample surface.
AFM parameters
(Contact mode) Size: 10µm I-Gain: 1.5 P-Gain: 1.5 Speed: 0.5 Resolution: X: 512 Y: 512
Tip: Nanosensors Type: PPP-CONTR-10 Serial Number: 78932F7L995 Thickness: 2 ± 1 µm
Length: 450 ± 10 µm Width: 50 ± 7.5 µm Resonance frequency: 6-21 kHz
Force constant: 0.02 – 0.77 Nm Tip height: 10-15 µm
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3.7 White Light Interferometer (WLI)
Figure 7: Zygo NewView 100 surface profiler.
The Zygo NewView 100 surface profiler is a White Light Interferometer that can produce
surface profiles of samples. Interferometry uses the superposition principle to combine
waves in way that will cause the results of their combination to extract information from the
instantaneous wave fronts. An interference pattern is generated on the sample which
corresponds to the profile of the sample surface [13]. This is then scanned and the
computer produces a surface profile of the sample.
WLI parameters:
Microscope Lens: Mirau 10x lens Image zoom: 0.75x Scan length: 20 µm bipolar (14 sec)
3.8 Spectral Reflectivity (SR)
Figure 8: F20 Thin film analyser.
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The F20 is used to obtain spectral reflectance graphs for thin filmsamples, using the
materials provided and the thickness of the filmas an input. The percentage of light
reflected from the sample in relation to the incident light, is passed through the lens
assembly and is transmitted through a fibre optic wire to a spectrometer which splits the
light into its component wavelengths [14]. A graph of the reflectance percentage versus
wavelength is then plotted on the computer.
4. Experimental procedure
4.1 K975XTurbo evaporator method [15]
Turn on the turbo evaporator.
Open the lid of the evaporator chamber and remove the glass cylinder.
Place a glass slide in the slide holder.
Get a small amount of aluminium foil and compress it before placing into the
tungsten crucible.
Once these steps are completed, return the glass cylinder to its original position and
close the lid, ensuring a secure fit.
Pressing the enter button will allow a change of parameters. The ‘Sputter coating’
option will not be used in this project.
Under the ‘Evaporate’ option on the menu, only ‘normal mode’ was used during the
duration of this project. ‘Evaporate time’ was initially set to 6 minutes but this was
later changed to 10 minutes.
Under the ‘FTM’ option the FTM was set to ‘Enabled’ and the FTM mode to ‘Manual’,
all other settings can be ignored, except for ‘Sources’ which is set to the density of
aluminium (2.70 g cm¯³).
The last option on the menu is ‘Miscellaneous’. The operating mode is set to
‘Evaporating’, the ‘Turbo pumping’ is enabled, the ‘Vent time’ is set to 75 seconds
and the ‘Purge time‘ is set to 15 seconds, all other parameters are not used.
Once these parameters have been initially verified to be correct, the actual
evaporation cycle can begin. Pressing start on the front panel will prompt the user to
press enter for ‘Run a coating cycle’ and then again for ‘Manual cycle’. Once enter
has been pressed twice the cycle will begin.
The vacuum sequence will now commence where the chambers pressure is reduced
to a high vacuum, this takes around 10 minutes.
Once the vacuum sequence is finished the option to select a source will appear.
Source B provides the power to the crucible so press enter on this option.
Dial the current up as necessary (Do not go over 15 Amps) and monitor the
deposition rate onscreen.
Once you have reached the desired deposition thickness, dial down the current and
press the stop button twice to return the chamber to atmospheric pressure.
Press stop once this has finished to purge the chamber.
Once the chamber has been purged the glass cylinder can be removed and the
sample can also be removed.
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Due to the often poor quality of the samples in the initial weeks, a series of modifications
were made to the standard method and this enhanced the quality of the thin films
significantly.
Improvements:
The position of the slide and FTM were altered to see which produced the best
quality films while also providing a good deposition reading.
Use of a constant mass of aluminium (5.9 mg).
The current was increased by one amp every ten seconds.
The glass slides were cleaned with Isoclene before use.
The samples were handled wearing latex gloves.
Any residual aluminium that had built up on the crucible was evaporated off.
After the installation of the glass slide heater, adjustments to the method were made due to
only one source being able to supply power within the chamber at a given time.
Heated method:
Once the vacuum sequence is finished select source C. This is the electrode that
supplies the power to the glass slide heater.
Increase the current by 3-4 Amps every 10 seconds, aiming to have the current at 35
Amps after 100 seconds.
Maintain this current for 300 seconds before ramping down the current over 100
seconds.
Pressing stop and then enter will bring up the source menu. Now source B can be
selected and the deposition process can begin. The deposition process is identical to
the normal method.
4.2 Atomic Force Microscope method [16]
Turn on the AFM and computer.
Start the AFM program.
Enter in parameters as given in section 3.6.
Align laser onto probe and maximise reflection onto the photodiode.
Place sample into the holder and close the switch on the box.
If possible move the stage to the position of interest before pressing the approach
button on the software.
Double check all parameters have been entered correctly and that the AFM is in
contact mode before pressing start.
Upon completion of a scan the AFM image analysis tool can produce a variety of
different images of the scanned image including 3D surface profiles and surface side
profiles.
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4.3 White Light Interferometer method [17]
Turn on the computer, fringe monitor (second screen) and the WLI.
Launch the MetroPro program and click the micro.app button when the option
appears on screen.
Check the microscope lens and make sure it is the 10x Mirau lens, this also needs to
be selected within the program.
Verify all the parameters given in section 3.7 have been inputted into the program.
Position the slide under the area of interest under the microscope light.
Adjust the rough and then fine zoom control until a clear image in seen on the fringe
monitor.
Adjust the fine zoom control again until interference fringes can be observed.
The interference fringes can be spread out further using the tilt controls if desired.
Pressing F4 allows the light level to be adjusted, saturation must be reduced by
pressing the ‘-‘ button until the side bar appears green.
Pressing F1 or clicking the ‘Analyse’ button will begin the analysis.
Data files can be saved using the save button, or bitmap images of the control screen
can be saved by clicking Zygo and printing these to file.
4.4 Spectral Reflectance method [18]
Turn on the computer and Filmetrics box.
Launch the Filmetrics program by clicking on the icon of the same name.
Click on the set-up button and then on the data acquisition button.
Position the light stage slightly off of the table while holding securely, and then click
‘Take dark’ on the data acquisition window.
Placing the stage back on the table, take a piece of the reference silicon and place it
below the light source. Click ‘Take reference’.
Now close the data acquisition window and open the edit structure window on the
right side of the program.
Make sure the medium used is air and aluminium is selected for ‘Option 1’. Enter in
the sample thickness given by the FTM in the thickness bar to the right.
Once this is finished click ‘Measure’ and a reflectance spectrum will be produced,
this can then be saved.
4.5 ImageJ analysis method [19]
4.5.1 Particle analysis
Open the AFM image of the sample.
Crop the image using the built-in crop tool. Set the image scale to 5 µm.
Click on ‘Process’ and click ‘Enhance contrast’. Set the value to 15%.
Click on ‘Process’ again, then ‘Binary’, then ‘Make binary’.
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Click on ‘Process’, then ‘Binary’, then ‘Watershed’ to add lines in-between two
particles (grains).
Click on ‘Analyse’, and then on ‘Analyse particles’.
4.5.2 Random particle average
Open the image, crop it and set the image scale to 5 µm as in section 4.5.1.
Using the free draw tool, encircle the selected grain.
Click ‘Analyse’, then ‘Measure’. This will produce a value for the area of the selected
grain.
Repeat this until you have 10 values, then average the 10 values to get the average
grain size.
5 Results
5.1 Depositionrates
Figure 9: Average deposition rate of aluminium onto the FTM over a period of 60 seconds
during thermal evaporation.
The first value taken is always set to zero for the purpose of the graph. The average
deposition rates give a good idea of how useful certain currents will be for creating thin
films of certain thickness over 60 seconds. After the vacuum sequence finished, the vacuum
pressure would be 9x10¯⁴ mbar.
In the effort to improve the quality of the thin film samples a constant mass of aluminium
was used in the evaporation. This was a 15 mm x 10 mm (Area = 150 mm²) strip of
0
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0 10 20 30 40 50 60
DepositedAlthickness(nm)
Time (s)
Average deposition rates
12 Amps
Averaged
11 Amps
Averaged
10 Amps
Averaged
9 Amps
Averaged
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aluminium foil. The average mass of these constant samples was found to be 5.875 ± 0.125
mg over a set of 4 samples.
The effect of temperature on the FTM was investigated as the instruments are known to be
sensitive to changes in temperature. The operating temperature of a FTM is between 20 –
250 °C [10], so it was possible that the heat radiated by the crucible may have affected the
FTM. This was carried out over a series of 4 trials with no aluminium in the crucible bar
some residual aluminium which gave a reading in the first run. There was no deposition in
the others, barring a glitch that sometimes displays a random value from the FTM at
random times throughout the experiment.
With the FTM placed directly over the crucible the average deposited thickness of the
constant mass of aluminium was measured. The average deposited thickness over 6 samples
was found to 27.7 ± 2.2 nm.
The position of the FTM, and the glass slides position on the glass slide holder was varied in
an effort to improve the quality of the samples. It was found that the optimal position was
having the slide directly over the crucible with the FTM positioned close to the right of the
glass slide.
Figure 10: Average deposition rates following the changing of the slide and FTM positions to
ones that improved thin film quality.
The deposition rates from figure 10 above also include the beginning of the use of the
constant mass of aluminium. Due to the use of the small constant mass some of the
aluminium would have evaporated off as the current was dialled up slowly, which may make
these deposition rates look slower than they were as data would only be taken once the
desired current was reached. This also explains why some values seemto level off after a
time.
0
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0 10 20 30 40 50 60
DeposititedAlthickness(nm)
Time (s)
New positioning average deposition rates
12 Amps
averaged
11 Amps
averaged
10 Amps
averaged
9 Amps
averaged
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5.2 COMSOL model
Figure 11: COMSOL model of a glass microscope slide.
The glass slide has an initial temperature of 600 K in a room with an ambient temperature of
300 K, cooling radiatively. The colour bar represents the temperature of different regions of
the slide after 100 seconds of cooling.
5.3 White Light Interferometer results
5.3.1 Non-heatedsample
Figure 12: Coloured contour map of the surface of a non-heated sample.
Large amount of red/pink areas represent a large build-up of excess aluminium at these
points. According to the associated scale in nanometres these can be as high as 440 nm
which is high for a thin filmsample.
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Figure 13: Side profile of a non-heated sample.
The highest peak above the average line is 17 nm in figure 13. Final roughness value
obtained over 3 sample measurements of the non-heated sample was 8.667 ± 0.667 nm.
5.3.2 Heatedsample
Figure 14: Coloured contour map of the surface of a heated sample.
A reduction of the red/pink areas of unusually high aluminium build-up can be seen when
compared to the non-heated sample. The highest value on this scan according to the
associated scale in nanometres is 17 nm. This is significantly smaller than the non-heated
samples highest peak value.
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Figure 15: Side profile of a heated sample.
The highest peak above the average line is roughly 6 nm. Over 3 sample measurements of
the same heated sample the average roughness was found to be 1.333 ± 0.333 nm.
5.4 AFM results
5.4.1 Non-heated sample
Figure 16: ATM produced 3D profile of the non-heated sample.
The scale bar range is from 0 to 56 nm. A lot of variation on the surface of the sample can be
seen, this can also be seen in the side profile slice below (Figure 17).
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Figure 17: Side profile of the non-heated sample.
This data was taken from 3 separate AFM side profile files and imported into excel where it
was averaged and graphed. Over 5 samples of AFM roughness values, the average
roughness was found to be 5.07 ± 0.38 nm for the non-heated samples.
Figure 18: AFM grain size analysis image of the non-heated sample.
The AFM calculates the mean area of the grains by differentiating the different grain
boundaries from each other and averaging the resulting sizes. The average grain size of the
non-heated sample over 4 samples was calculated to be 0.0238 ± 0.0003 µm².
0
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0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Height(nm)
Width of scan (µm)
Non-heated average side profile
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5.4.2 Heated sample
Figure 19: 3D profile of the heated sample.
The scale bar range is from 0 to 95 nm. The large peaks are most likely dirt (due to unusual
height and infrequency) that got onto to the film. Excluding these the heated sample is
much more uniform than the non-heated sample; this can also be seen in figure 20 below.
Figure 20: Side profile of the heated sample.
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0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Height(nm)
Width of scan (µm)
Heated average side profile
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The average side profile of the heated sample has the same scale as the non-heated sample
side profile. A significant reduction in the roughness can be observed. The average
roughness obtained for the heated sample was 2.96 ± 0.26 nm over 5 samples.
Figure 21: AFM grain size analysis image of the heated sample.
The sizes of the grains in the heated sample have increased in comparison to the non-
heated. Over 4 AFM roughness scans, the average grain size was found to be 0.0811 ±
0.0074 µm² for the heated sample.
5.5 ImageJ grain size analysis
Figure 22: Histogram distribution of the grain sizes of a non-heated sample in µm²
produced using the ImageJ particle analysis tool.
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Grain sizes range from 0.000481 µm² to 0.0952 µm². The average grain size of the
sample is 0.0174 µm² with a mode value of 0.00574 µm² appearing 176 times out of
668 grain size counts.
Figure 23: Histogram distribution of grain sizes in µm² for a heated sample produced
by the ImageJ particle analysis tool.
Grain sizes range from 0.000485 µm² to 0.249 µm². The average grain size of the sample is
0.03 µm², though the larger grains such as the 0.249 µm² grain skew this value. The non-
heated sample has a more even distribution of sizes, with outliers significantly closer to the
mean value. The smallest grain size of 0.000485 µm² is also the value of the mode statistic
for the heated sample, appearing 115 times in a count of 315 grain sizes.
Heated grain size (µm²) Non-Heatedgrain size (µm²)
0.02 0.041
0.046 0.006
0.058 0.016
0.073 0.009
0.03 0.012
0.023 0.036
0.059 0.01
0.02 0.009
0.041 0.016
0.064 0.014
Average grain size (µm²) Average grain size (µm²)
0.0434 0.0169
Standard deviation(µm²) Standard deviation(µm²)
0.019608388 0.011883228
Standard error of the mean (µm²) Standard error of the mean (µm²)
0.006200717 0.003757807
10 particle average grain size (µm²) 10 particle average grain size (µm²)
0.0434 ± 0.0062 0.0169 ± 0.0038
ImageJ Particle analysis value (µm²) ImageJ Particle analysis value (µm²)
0.03 0.016
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Table 1: Shows the results for the heated and non-heated samples using the 10 random
particles method, and the built-in particle analysis tool in ImageJ.
These results confirm that the grain size has increased for the heated samples.
5.6 Spectral reflectanceprofiles
Figure 24: Reflectance spectrum of the heated and non-heated samples.
3 different heated and non-heated samples were used to produce this graph. The heated
sample has a higher average reflectance between 415 – 540 nm, which corresponds to the
violet to green wavelengths of light in the visible spectrum. The non-heated sample
outperforms the heated sample in all other areas of the spectrum shown. Heated
reflectance peaks at 500 nm, whereas the non-heated reflectance peaks at 990 nm.
6 Discussion
Deposition rates have been observed to increase with increasing current. This is due to extra
power being supplied to the tungsten crucible, which in turn supplies more heat to the
aluminium in order to create the aluminium vapour. The deposition rates are useful to know
in terms of desired thickness of a thin film and the current or time needed to produce the
desired thickness. The graph is almost identical to the deposition rate graph included in the
K975X manual [15]. The accuracy of the FTM was calculated to be 0.325nm due to the
smallest measurement that the FTM can achieve.
0.05
0.08
0.11
0.14
0.17
0.2
0.23
0.26
0.29
0.32
0.35
0.38
0.41
0.44
350 400 450 500 550 600 650 700 750 800 850 900 950 1000
Reflectance(%)
Wavelength (nm)
Average reflectance profiles for Al thin films
Non-Heated
Reflectance
Heated
Reflectance
Glass slide
Reflectance
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The COMSOL model was created to show, roughly, how a heated glass slide would lose heat
in a vacuum. Unfortunately the glass slide heater radiatively heating the glass slide could not
be modelled.
All error estimates use the standard error of the mean. Comparing the WLI surface profiles
and side profiles of the heated and non-heated samples visually, the reduction in roughness
is evident. The highest peak in the non-heated sample (Figure 13) above the average line is
17 nm compared to just 6 nm in the heated sample (Figure 15). Comparing the average
roughness of the non-heated sample which was calculated to be 8.667 ± 0.667 nm, to the
heated value of 1.333 ± 0.333 nm, gives a reduction of 84.6% in roughness of the thin film
when the sample is heated.
The 3D AFM surface profiles display a 5 µm x 5 µm section of the samples, displaying surface
features and variations in uniformity on the surface. The difference in surface roughness of
the non-heated (Figure 16) and the heated (Figure 19) is clear. This is highlighted further in
the average side-profiles of the samples which use the same scale (X: 0–5 nm, Y: 0-22 nm).
The average roughness obtained from the AFM measurements for the non-heated sample
was 5.07 ± 0.38 nm, and for the heated 2.96 ± 0.26 nm. This shows a reduction of 41.6% in
the sample roughness, a more reasonable value than the WLI results, given the AFM also
has a higher level of accuracy [12].
The grain size analysis images show how the AFM grain size analysis tool differentiates the
different grains in 5 µm x 5 µm scans, in order to calculate the average grain size. Over a
series of 4 measurements the average grain size of the non-heated sample was 0.0238 ±
0.0003 µm², and the heated sample average grain size found was 0.0811 ± 0.0074 µm². This
shows that the grain size increased by a factor of 3.5 upon heating of the substrate. This is
due to the enhanced surface diffusion effects.
The image analysis tool ImageJ was also used to compare the grain size results from the
AFM images. The heated grain size values gathered were 0.030 µm² (No error given for the
tool within the program) for the ImageJ particle analysis tool, and 0.0434 ± 0.0062 µm²
when 10 random grains were measured and averaged. For the non-heated sample the
particle analysis gave a grain size of 0.0174 µm², and 0.0169 ± 0.0038 µm² for 10 random
grains. A disparity of 63% and 46.5% for the particle analysis and 10 random grain
measurements were observed when compared to the AFM results for the heated sample. A
disparity of 29% and 26.9% for the particle analysis and 10 random grain measurements was
observed for the non-heated sample when compared to the AFM results. The larger
disparity for the heated grains when compared to the non-heated is due to the lack of
increased uniformity of the grain sizes which make it difficult to accurately calculate larger
grain sizes in the particle analysis tool, even after enhancing the contrast and using the
binary process. Due to the AFMs higher accuracy and more efficient grain size analysis
algorithm, the AFM results are considered more accurate. However the ImageJ results do
verify an increase in grain size for the heated samples.
In the spectral reflectance profile (Figure 24) the heated sample exhibits a higher
reflectance between the wavelengths of 415 – 540 nm, but is significantly lower elsewhere
in the spectrum when compared to the non-heated reflectance. The heated sample has
poor reflectance in the infra-red (IR) region, a high reflectivity in this region is key to many
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aluminium thin films applications. Overall the non-heated reflectance curve is much higher
than the heated, and follows more closely to the expected reflectance profile for aluminium.
A decrease in roughness will generally lead to a smoother surface which will be more
reflective; however larger grain sizes scatter more light away from the surface rather than
directly reflecting light, leading to a lower reflectivity. Due to an increase in grain sizes by a
factor of 3.5 compared to a roughness reduction of 41.6%, the scattering becomes more
prominent, resulting in the reduced reflectivity of the heated sample, except at certain
wavelengths where this effect is reduced. This would limit the heated samples usefulness to
applications where wavelengths of 415 – 540 nm are used, such as in reflective mirrors for
laser applications.
7 Conclusions
The uniformity of the aluminium thin films has been improved by making a series of
adjustments to the standard method such as using a constant mass of Al and cleaning the
glass slides before usage. Upon heating the slides a reduction of 41.6% in roughness was
obtained when compared to non-heated slides using the AFM results. A reduction in
roughness between the non-heated and heated samples was verified by WLI results. The
grain size was found to increase upon heating of the substrate prior to deposition, due to
enhanced surface diffusion effects. The average grain size of the heated sample increased to
3.5 times the size of the non-heated average grain size. The increase in grain size was
verified using the ImageJ particle analysis tool, and also calculating the average of 10
random grain sizes using ImageJ.
In terms of reflectance the heated sample performs better over wavelengths ranging from
410 to 540 nm, but performs significantly worse elsewhere in the spectrum, in particular the
IR region. Though reflectance should be improved with decreasing roughness, scattering at
the surface of the thin film increases with increasing grain size, and this is the dominant
factor in the heated samples. As many thin film applications of aluminium are due to its
high reflectivity in the IR region, the usefulness of the heated sample is limited in
comparison to the non-heated. The heated thin films would be useful in reflective mirrors
for laser applications between the violet and green wavelengths (410 - 540 nm) whereas the
non-heated thin films could still be used for thin film applications such as space blankets and
spaceship insulation.
Future work
There was a more efficient design for a glass slide heater involving industrial heating
cartridges, designed by Conor Mc Brierty, which would heat the slide conductively, but the
parts did not arrive before the end of the project. If finished, it would be useful to see how
much of a difference this would make to the thin film grain size and roughness. It would also
be worthwhile to investigate how different cool down times, after heating and before
deposition, would affect the grain size and roughness of the thin films. The effect of
deposition rate on roughness and grain size could also be investigated.
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References
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3. Chen, E. 'Thin Film Deposition'. 2004. Presentation.
4. Chambers, Austin. ‘Modern Vacuum Physics’. 2004. Boca Raton: CRC Press. 2014.
5. BORDO, Kirill, and Horst-Günter RUBAHN. 'Effect of Deposition Rate on Structure And
Surface Morphology Of Thin Evaporated Al Films On Dielectrics And
Semiconductors'. ms 18.4 (2012).
6. Mfg.mtu.edu. '1. Roughness'. N.p., 2015.
7. Metallography.com. 'ASTM and Grain Size Measurements'. N.p., 2015.
8. Rotary vane vacuum pump, Pfeiffer. Pfeiffer Vacuum. N.p., 2015.
9. Turbo-molecular vacuum pump, Pfeiffer. Pfeiffer Vacuum. N.p., 2015.
10. ‘K150X Film Thickness Monitor’. Lab manual. Quorum technologies. Kent, UK. 2015.
11. Nanogune.eu,. 'Atomic-Force Microscope (AFM 5500 Agilent/Nano Observer CSI
Instruments) | CIC Nanogune'. Nanogune.eu. N.p. 2015.
12. ‘Atomic Force Microscope – Agilent 5500’. Nuigalway.ie. 'NCLA'. N.p., 2015.
13. ‘White Light Interferometer – Zygo NewView 100 Surface profiler’. Nuigalway.ie,.
'NCLA'. N.p., 2015.
14. 'Ellipsometers and Spectral Reflectance: Thin Film Measurement Guide'. Filmetrics,
Inc. Filmetrics.com. N.p., 2015.
15. ‘K975X Turbo Evaporator Instruction Manual’. Lab manual. Quorum Technologies.
Kent, UK. 2015. Print.
16. Haustrup, Natalie. ‘AFM of a new sample’. Lab manual. NUIG. 2012. Print.
17. ‘NewView 100 Interferometer User Manual’. Lab manual/ Zygo. Connecticut, USA.
2014. Print.
18. ‘F20 Thin film analyser’. Lab manual. Filmetrics. California, USA. 2015. Print.
19. Ferreira, Tiago. 'Imagej User Guide - IJ 1.46R | Analyse Menu'. Rsbweb.nih.gov. N.p.,
2015. Print.
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Appendices
Appendix A - Additional tables
Non-Heatedsample Heated sample
Roughness(nm) Roughness(nm)
4.74 3.05
4.35 2.09
4.38 3.51
5.58 3.42
6.29 2.72
Average roughness(nm) Average roughness(nm)
5.068 2.958
Standard deviation(nm) Standard deviation(nm)
0.844375509 0.57807439
Standard error of the mean (nm) Standard error of the mean (nm)
0.377616207 0.258522726
Overall average Roughness(nm) Overall average Roughness(nm)
5.068 ± 0.378 2.958 ± 0.259
Table 2: AFM roughness results in table format.
Non-Heatedsample Heated sample
Grain size (µm²) Grain size (µm²)
0.024 0.0706
0.0232 0.103
0.0244 0.0735
0.0237 0.0774
Average grain size (µm²) Average grain size (µm²)
0.023825 0.081125
Standard deviation(µm²) Standard deviation(µm²)
0.000506 0.014847082
Standard error of the mean (µm²) Standard error of the mean (µm²)
0.0002529 0.007423541
Overall average grain size (µm²) Overall average grain size (µm²)
0.0238 ± 0.0003 0.0811 ± 0.0074
Table 3: AFM grain size results in table format.
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Appendix A - Additional tables
Heated roughness (µm) Non-Heated roughness (µm)
0.001 0.01
0.001 0.008
0.002 0.008
Average roughness(µm) Average roughness(µm)
0.001333333 0.008666667
Standard dev(µm) Standard dev(µm)
0.00057735 0.001154701
Standard error of the mean (µm) Standard error of the mean (µm)
0.000333333 0.000666667
Overall average roughnessvalue (µm) Overall average roughnessvalue (µm)
0.001333 ± 0.000333 0.008667 ± 0.000667
Table 4: WLI roughness results in table format.
The overall average roughness values for the WLI results have been converted to
nanometres for the results section of the report and given to 3 decimal places due to the
WLI accuracy.
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Appendix B – Additional WLI scan images
Figure 25: Coloured contour map of the surface of a non-heated sample where a piece of
tape was kept on the sample during deposition and removed before taking WLI scans.
Tape was used in order to obtain a step height measurement for the thin films.
Figure 26: Side profile of a heated sample after removal of tape which was left on the slide
during deposition.
Figure 27: 3D surface profile of a non-heated sample after the removal of tape which was
left on the slide during deposition.
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Appendix B – Additional WLI scan images
Figure 28: Coloured contour map of the surface of a non-heated sample at the thin films
edge. The edge is due to the slide holder bar covering part of the slide.
Figure 29: Side profile of a non-heated sample at the thin films edge.
Figure 30: 3D surface profile of a non-heated sample at the thin films edge.
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Appendix C - Additional AFM scan images
Figure 5: Example of one set of non-heated sample roughness values.
Ra represents the average roughness.
Figure 31: Another example of an AFM grain size analysis image of the non-heated sample.
Figure 32: Non-heated AFM grain size image with height scale.
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Appendix C - Additional AFM scan images
Table 6: Example of one set of heated sample roughness values.
Figure 33: Another example of an AFM grain size analysis image of the heated sample.
Figure 34: Heated AFM grain size image with height scale.
