1) Researchers developed an ultrasonic atomizer system to deposit graphene oxide thin films on silicon wafers for applications in semiconductor manufacturing. 2) Graphene oxide was spray coated using the ultrasonic atomizer and then thermally reduced to improve its electrical and thermal conductivity. 3) Characterization of the reduced graphene oxide thin films found they have high coverage area above 90% with uniform thickness, low sheet resistance below 3 kΩ/sq, and thermal conductivity over 1000 W/mK making it suitable as a heat spreader material.

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Materials Today: Proceedings 7 (2019) 763–769 www.materialstoday.com/proceedings
2214-7853 © 2018 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Selection and peer-review under responsibility of the scientific committee of the Nanotech Malaysia 2018.
Nanotech Malaysia 2018
Ultrasonic atomization of graphene derivatives for heat spreader
thin film deposition on silicon substrate
Mohd Rofei Mat Hussina,
*, Siti Aishah Mohamad Badaruddina
,
Nik Mohd Razali Mohd Nora
, Mohd Hilmy Azuan Hamzaha
a
MIMOS Berhad, Technology Park Malaysia, 57000 Kuala Lumpur, Malaysia
Abstract
We report the development of graphene derivative deposition on silicon wafers through ultrasonic atomization process. The study
includes the ultrasonic atomizer system design, process development, characterization of the nanomaterial, and its heat transfer
analysis. In this study, ultrasonic atomizer system was developed to deposit graphene oxides on 8”silicon wafer for seamless
integration with semiconductor processes. The atomization process was performed in a chamber under control environment to
ensure high quality of deposition process.
© 2018 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Selection and peer-review under responsibility of the scientific committee of the Nanotech Malaysia 2018.
Keywords: Graphene; Graphene Oxide; Reduced Graphene Oxide; Ultrasonic Atomizer; Heat Spreader
1. Introduction
Graphene is a one-atom-thick layer of carbon atoms, exhibits remarkably high specific surface area, thermal
conductivity, charge carrier mobility, transparency, and impermeability to gases [1-5]. Therefore, it has attracted a
lot of attention in various technological areas, especially in electronic applications. One of the challenges to integrate
graphene film into semiconductor devices is process compatibility issues. Deposition of graphene film on a silicon
substrate for commercial products is not straightforward and require thorough process investigation. There are
several methods of deposition techniques, which can be categorized as a gas phase deposition and a liquid phase
* Corresponding author. Tel.: +603-8995-5000.
E-mail address: rofei@mimos.my

2. 764 M.R.M. Hussin et al./ Materials Today: Proceedings 7 (2019) 763–769
deposition. The most commonly used deposition technique for high-quality graphene film is through gas phase
Chemical Vapour Deposition (CVD). However, CVD is an expensive process that requires a special substrate for
synthesis with subsequent transferal to another material for practical applications. Another method is liquid phase
approaches which are significantly cheaper than CVD but the final material typically contains defects and impurities.
Spray coating process is one of the liquid phase deposition techniques, which is very attractive for nanomaterials
deposition since it has various advantageous including cost-effective, high-throughput, and industrial scalable
process. This technique also offers a relatively good control of film uniformity and morphology, suitable for large-
area deposition, and flexibility in process parameters adjustment for different types of solvent.
Ultrasonic spray coating process is more advantageous as compared to air pressurized spray coating technique,
especially for applications, which involve expensive materials that require accurate and uniform deposition of
suspensions on substrates. Ultrasonic atomization is a technique to produce micro size of droplets during spray
coating process through vibrational energy of ultrasonic nozzle. In this paper, we will discuss about the development
of ultrasonic atomizer system and process development of thin film graphene derivatives deposition on silicon wafer.
We have also studied the heat transfer characteristic of graphene derivatives deposited on silicon to understand its
thermal effect behavior. As graphene material has remarkable properties with extremely high thermal conductivity, it
has a great potential to be used as a heat spreader material in electronic applications [6-8].
2. Ultrasonic Atomizer System
Ultrasonic atomizer system was developed to improve the process of depositing nanomaterials on silicon wafer
for seamless integration with semiconductor processes. Ultrasonic atomization is a proven spray coating technology
for efficient deposition of expensive chemical solutions on substrate. It helps to matched functional and economic
performance, especially for nanomaterials deposition, which requires accurate and uniform deposition on substrates.
Fig. 1. shows the image of MIMOS Ultrasonic Atomizer System (Mi-Atomizer) that was developed for research and
development in ultrasonic atomization process of nanomaterials. The atomizer system has built-in automatic control
system with graphical user interface (GUI) for users to optimize their process recipes. The main parts of the system
are ultrasonic nozzles, syringe pump, spray chamber and heater plate. The system enables optimization of process
recipes for layer-by-layer deposition to get high uniformity thin film with large percentage of coverage areas.
Ultrasonic nozzle with power rating between 0.5W to 2W and fixed frequency at 120kHz was used in this study
to produce median droplet size about 13um. When the graphene derivatives solution film is placed on a smooth
surface of nozzle outlet (as shown in Fig. 1.) that is set into vibrating motion such that the direction of vibration is
perpendicular to the surface, the solution absorbs some of the vibrational energy, which is transformed into standing
waves. The amplitude of the capillary waves increase when the underlying vibration increased in amplitude. Tiny
drops of solution are ejected from the tops of degenerating waves once the critical amplitude is reached. In
comparison to air pressurized spray techniques, ultrasonic atomization can provide a large reduction in material
usage because they form droplets through low ultrasonic vibration; thereby, imparting lower kinetic energy to the
droplet. The liquid can be dispensed to the spray nozzle by either gravity or a small low pressure metering pump
[9,10].
Fig. 1. (a) Mi-Atomizer system for nanomaterials deposition on silicon wafer and (b) the illustration of ultrasonic atomization process

3. M.R.M. Hussin et al./ Materials Today: Proceedings 7 (2019) 763–769 765
3. Experimental procedure
Test samples were prepared by depositing 3000A of silicon dioxide (SiO2) film onto 8” bare silicon wafer
through Plasma Enhanced Chemical Vapor Deposition (PECVD) process. Deposition of graphene oxide (GO) was
then performed through ultrasonic atomization process in a chamber under control environment. The film thickness
was controlled with the amount of spray volume and the flow rate. This is to prepare the test samples for thin film
heat spreader on silicon wafer. The samples were then annealed under high temperature (>700℃) for thermal
reduction of GO. Sheet resistance of the film were measured using four-point probes system. Atomic Force
Microscopy (AFM) measurements were performed to characterize the film thickness and surface roughness for
correlation study with spray parameters. Field Emission Scanning Electron Microscopy (FESEM) images were
captured from samples to study the percentage of coverage areas by using a scientific image analysis software
(Image J). The chemical and structural parameters of the thin film was analyzed by Raman spectroscopy and X-ray
Photoelectron Spectroscopy (XPS) studies. The transparency of the film was also studied by using UV-Vis
Spectroscopy measurement. Heat transfer characteristics of the thin film was analyzed by using IR thermal imaging
system.
3.1. Test samples and GO solution preparation
There are several sample types were prepared for this study to cater for different analysis requirement. Test
samples for thickness measurement, percentage of coverage areas, sheet resistance measurement, material analysis,
and heat transfer characterization were prepared on 8” silicon wafer. The test sample for UV-Vis measurement on
the other hand are prepared on glass substrate. In this study, GO was used as the coating material. One of the
advantages of GO is its easy dispersability in water and other organic solvents, as well as in different matrixes. This
is due to the presence of the oxygen functionality groups [11]. A modified Hummers’ method was used to prepare
the GO. The GO was dispersed in ethanol (EMSURE® ACS,ISO,Reag. Ph Eur) with concentration of 1mg/ml.
3.2. Ultrasonic Spray coating of GO on Silicon and Glass Substrate
The ultrasonic spray coating of GO were performed using Mi-Atomizer system. Prior to spray coating process,
GO solution was sonicated in ultrasonic bath for 30 to 60 minutes at frequency of 37 kHz. The sonicated solution
was then being filled in the system’s syringe pump and about 10 ml of the solution was needed in order to fill up the
solution line up to the ultrasonic nozzle tip. The solution’s flow rate was setup up to 1 ml/min with nozzle height
(distance from wafer stage) up to 60 mm and hot plate temperature up to 70 °C. Ultrasonic generator power was set
at 1.6 W. The test wafer was then placed onto wafer stage (heater plate), and the spraying process was activated by
choosing the desired recipe. The thickness of GO film deposited was controlled by the number of spray’s passes
onto the wafer.
3.3. Thermal Reduction of Graphene Oxide
GO is known as an electrical insulator with low thermal conductivity, due to the disruption of its sp2
bonding
networks. A reduction of the graphene oxide has to be done in order to recover the honeycomb hexagonal lattice and
the electrical conductivity. The reduction of graphene oxide was thermally done in a high temperature vacuum
chamber. At temperature of 700°C, the reduction process took about 30 minutes while at 1000°C the process time
can be shorten to 10 minutes.

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4. Results and Discussions
4.1. Process and Material Characterizations
The sheet resistance (Rs) measurement for reduced GO (rGO) films was carried out using Jandel RM3-AR four-
point probes system. The sheet resistance for different spray passes (representing different thickness) and the
temperature of reduction process were measured and presented in table 1. It was observed that the thicker the film
the lower the sheet resistance value recorded. It was also noticed that GO reduction process at 1000°C produces
significantly lower sheet resistance value than the reduction process at 700°C. To study the uniformity of thin film
coated on silicon wafer, sheet resistance measurement was measured at multiple locations (49 points) on the wafer.
Table 2 showed the average values of sheet resistance for rGO films on different wafers and its percentage of non-
uniformity, which shows the process stability. In summary, the percentage of non-uniformity (%NU) for the rGO
film within wafer is ~18% and between wafers is ~13%.
Table 1. Sheet Resistance of rGO for different thickness
Spray (pass) Reduction
T=700C ; 2hr
Reduction
T=1000C ; 10 min
6X ~ 40 – 50 kΩ/sq ~ 2.6-2.9 kΩ/sq
12X ~ 9.5 – 11 kΩ/sq ~ 1.04 - 1.44 kΩ/sq
18X ~ 6.5 – 8 kΩ/sq ~ 620 - 907.9 Ω/sq
24X ~ 5 – 5.5 kΩ/sq ~ 489-518 Ω/sq
30X ~ 3.5 – 4 kΩ/sq ~ 359 - 397 Ω/sq
Table 2. Sheet Resistance of rGO for different wafers
Wafer # Rs (Ω/sq) %NU
1 2971.61 14.08
2 2875.98 16.83
3 2776.32 21.2
4 2703.96 21.55
5 2204.21 20.69
6 2508.99 16.87
7 2037.69 15.46
8 2358.64 20.49
AVG 2554.68 18.4
%NU between wafers: 13.06%
The thickness of a single droplet of GO solution sprayed on SiO2/Si substrate was measured using Atomic Force
Microscope (AFM) system as shown in Fig. 2 below. This shows that a single droplet of sprayed GO solution can
have a thickness of ~9.43nm. Thickness measurement was performed on several samples with different spray
passes. In average the thickness for single spray pass, double spray pass, and triple spray pass are 11nm, 22nm and
33nm respectively.

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Fig. 2. 2D and 3D AFM images for a single GO droplet on SiO2/Si substrate
The percentage of coverage areas was investigated using FESEM system. A good spray coverage area was
achieved by applying optimum recipe parameters for the spray process, which includes solution’s flow rate, nozzle
height, N2 pressure, and spray passes. The visual observation of FESEM images are shown in Fig. 3. It was observed
that for single, double and triple passes of spray, the percentage coverage areas increased significantly from around
82.86% to 96.23%. Raman Spectroscopy, a tool using light scattering technique in identifying and qualifying any
given material was used to characterize the GO and rGO samples. The Raman spectrum was obtained by
illuminating the samples with a laser beam of 473 nm (blue laser). For GO and rGO the Raman spectrum can be
referred to Fig. 4. For GO, the Raman shift of D, G and 2D peak occurs at 1359 cm-1
, 1606 cm-1
, and 2701cm-1
respectively. A significant D shift peak for GO may relates to the nature of GO flakes themselves, which constitute
of large edge areas, and ripples. For rGO, the D, G, and 2D peak observed at 1365 cm-1
, 1593 cm-1
, and 2698 cm-1
respectively. The rGO’s ID/IG=0.841 is lower compared to GO’s ID/IG=0.958, which indicate that the recovery of sp2
carbon happened during the reduction process using high temperature.
The Carbon to Oxygen atomic ratio (C/O) of GO and rGO was obtained from the X-ray Photoelectron
Spectrometry (XPS) analysis. In Fig. 5, four components that correspond to carbon atoms in different functional
groups are the non-oxygenated ring C (~284.8 eV), the C in C–O bonds (~ 286 eV), the carbonyl C (~287 eV), and
the carboxylate carbon (O–C = O, ~ 289 eV). The C1s XPS spectrum of GO in Fig. 5(a) shows a considerable
degree of oxidation while the XPS spectrum of rGO in Fig. 5(b) displays similar oxygen functional groups but with
lower peak intensities than that of GO. It is observed that the content of oxygen (O) atoms of rGO is significantly
lower than that of GO which indicates the successful reduction process of GO.
Fig. 3. FESEM images of sprayed GO with (a) 1 pass = 82.86%, (b) 2 passes = 94.72%, and (c) 3 passes = 96.23% coverage area.
(a) (b)
Fig. 4. Raman spectra of (a) GO and (b) rGO

6. 768 M.R.M. Hussin et al./ Materials Today: Proceedings 7 (2019) 763–769
(a) (b)
Fig. 5. The C1s XPS spectra of (a) GO and (b) rGO
GO was sprayed onto glass substrate and the transmittance data was obtained using the UV-Vis system. The UV-
Visible spectrum of the GO and rGO is depicted in Fig. 6. The spectrum is a plot of Percent Transmittance (%T) as a
function of Wavelength (λ). It was observed that rGO has lower percent transmittance compared to GO. It was also
observed that the thicker and denser the film (GO or rGO) the lower the percent transmittance.
Fig. 6. UV-Visible spectrum plot of %Transmittance vs Wavelength.
4.2. Heat Transfer Characteristic
Thermal conductivity describes the transport of energy in the form of heat through a body of mass as the result of
a temperature gradient. Heat conduction in solid materials usually takes place through acoustic phonons and by
electrons. In carbon based materials i.e. Graphene, GO, rGO, the heat conduction is dominated by phonons. As
reported in the literature, graphene and rGO have high thermal conductivity of >3000 W/mK and >1000 W/mK
respectively [2, 8]. Therefore, these materials are suitable to be used as a heat spreader in semiconductor devices. It
can helps to cool down the devices by reducing the localized self-heating effect. Heat produced by the
semiconductor devices can be removed and effectively transferred to the surrounding air as quickly as possible. In
this study, it is found that the functionalized rGO thin film deposited on silicon substrate showed good heat transfer
characteristic. The test results are shown in Fig. 7, where comparison has been made between samples coated with
rGO thin film heat spreader and without heat spreader. Sample with rGO thin film heat spreader showed uniform
temperature across the whole sample when heated at the center. The test system used for this study consist of a
micro-heater plate, a programmable switching D.C. power supply (RS Pro IPS 603), and FLIR IR thermal imaging
system (ETS320).

7. M.R.M. Hussin et al./ Materials Today: Proceedings 7 (2019) 763–769 769
(a) (b)
Fig. 7. Thermal images of test samples (a) with functionalized rGO heat spreader, (b) without heat spreader.
5. Conclusions
The advantages of using Ultrasonic Atomizer System for graphene derivatives deposition are high transfer
efficiency which reduces materials usage, non-clogging, and highly uniform distribution of droplet sizes to help
reduce coating imperfections. Ultrasonic atomization of graphene oxide on 8” silicon wafer showed more than 95%
coverage area with less than 20% non-uniformity with average thickness of about 22 nm. Reduced Graphene Oxide
is also suitable for heat spreader application.
Acknowledgements
The authors acknowledge funding from Government of Malaysia under 11MP (MOSTI) program and thanks to
Multimedia University (MMU), Universiti Perguruan Sultan Idris (UPSI), Universiti Malaysia Perlis (UniMAP),
Universiti Putra Malaysia (UPM) and NSW Automation Sdn. Bhd. for supporting this research.
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