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1. Available online at www.sciencedirect.com
ScienceDirect
[IConMEAS 2020]
Effect of Cu thickness and temperature on growth of graphene on 8-
inch Cu/SiO2/Si wafer using cold-wall CVD reactor
Nurhidaya Soriadi*, Mohd Faizol Abdullah, Firzalaila Syarina Md Yakin, Siti Aishah
Mohamad Badaruddin, Mohd Ismahadi Syono
Advanced Devices Lab, MIMOS Berhad, Technology Park Malaysia, Kuala Lumpur 57000, Malaysia
Abstract
The growth of graphene by chemical vapor deposition (CVD) method on Cu thin film using a cold-wall CVD reactor is desirable
for developing a compatible graphene transfer process on large SiO2/Si wafer. We reported the growth of graphene on the
evaporated Cu on an 8-inch SiO2/Si wafer for the varied thickness of 100–600 nm. We found the temperature for the cold-wall
CVD reactor needs to be 825 o
C or higher for methane decomposition and graphene growth. The high-temperature process results
in the formation of isolated and interconnected Cu islands due to the dewetting mechanism. The evaporated Cu film on SiO2/Si
wafer needs to be sufficiently thick minimum of 600 nm to mitigate the dewetting during the 825 o
C process. Successfully grown
graphene on 8-inch Cu/SiO2/Si wafer using a cold-wall reactor was monolayer without crystal defect. The grain size of graphene
was at an average of 2.5 µm, correlated to the grain size of Cu film which is slightly smaller at an average of 2 µm. We anticipate
further enlargement of Cu grain size along with graphene grain size by the thickening of Cu film and by using higher process
temperature.
[copyright information to be updated in production process]
Keywords: Cold-wall CVD; Cu thin film dewetting; graphene growth; grain size
1. Introduction
Chemical vapor deposition (CVD) growth of graphene on Cu thin film is one step closer towards the development
of a compatible graphene transfer method onto a large-area Si wafer. At present, graphene is commonly grown on 25
µm-thick Cu foil or thicker with an acceptably good crystal quality [1]. However, the transfer of graphene from Cu
foil onto target substrate e.g. Si and SiO2/Si wafer for device fabrication was proven challenging without inducing
breakages, wrinkles, and residual impurities [1]. On the other hand, an alternative approach of growing graphene on
Cu thin film evaporated on SiO2/Si wafer may offer a solution for developing a more robust graphene transfer
methodology. This is very promising considering the technological compatibility of Cu/SiO2/Si wafer with a cold-wall
CVD reactor, which is slowly replacing the use of conventional hot-wall CVD for growing graphene [2]. Such a home-
made hot-wall CVD furnace consumes more power but requires longer process time due to poor temperature control,
thus not desirable for the industrial-scale synthesis of graphene.
Tao et al. demonstrated the growth of monolayer graphene on the evaporated Cu on SiO2/Si substrate using a cold-
wall CVD reactor [3]. They managed to grow graphene at 875 o
C and found grain size of graphene is correlated to the
grain of Cu(111). Their average Cu grain size on 1 µm-thick film was within 10–20 µm. In recently, Huet et al. reported
on the growth of monolayer graphene on evaporated Cu on a 3-inch SiO2/Si wafer using a hot-wall CVD furnace [4].
Their grown graphene on 1 µm-thick Cu film at 1050 o
C was reported with a large grain size up to 1 mm, far larger
* Corresponding author. E-mail address: nurhidaya.soriadi@mimos.my

2. than the Cu grain size that approximately within 100 µm. In terms of thin film, 1 µm-thick Cu film can be considered
as thick and not desirable for the graphene transfer process which involves Cu etching/removal. Thinning of Cu film
however invites another problem which is dewetting of Cu, especially at high-temperature process ~1000 o
C [5]. In
this work, we investigated the use of a significantly thin Cu film from 100–600 nm to grow the monolayer graphene
using the cold-wall CVD reactor at a lower temperature, just above the effective decomposition of methane to reduce
the dewetting. We discussed the quality and grain size of the grown graphene according to the dewetting and grain
size of the evaporated Cu thin film.
2. Experimental
The substrate for growth in this work was an 8-inch SiO2/Si (100 nm-thick SiO2) wafer. The Cu film catalyst was
deposited on SiO2/Si wafer by electron beam evaporation system (Evatec BAK-641) for the varied thickness of 100–
600 nm. The source for evaporation was 99.99% purity Cu pellets from Kurt J. Lesker Co. Next, graphene was grown
on the Cu/SiO2/Si wafer using the cold-wall CVD system (Aixtron Black Magic). Before the growth, the evaporated
Cu film was annealed in H2/Ar (750/250 sccm) ambient at either 725 o
C or 825 o
C for 5 min to remove the native oxide
and to enlarge its grain size. In the following, CH4/H2/Ar (5/40/960 sccm) flowed into the reaction chamber for 4 min
to grow the graphene. The sample was then slowly cooled to 130 o
C in continuous Ar flow before vented to be taken
out from the reaction chamber. The surface morphology of Cu film and graphene was investigated using the field
emission scanning electron microscopy (FESEM) system (JEOL JSM-7500F) at a magnification of 10k. The crystal
quality of graphene was inspected using the Raman spectroscopy system (NTEGRA Spectra II) using 473 nm laser
excitation at room temperature.
3. Results and discussion
3.1. Thickness and temperature dependence on dewetting of Cu film
Fig. 1(a)–(f) respectively show the FESEM image of Cu film with varied thickness of 100–600 nm after the 725 o
C
process. We found solid-state dewetting was obvious for the thin Cu with a thickness of 100–300 nm, where the Cu
particles form the isolated and interconnected islands instead of forming a continuous film. Dewetting of Cu was
driven by the high interfacial energy between Cu and SiO2/Si. Consequently, Cu rearranges itself as the islands to
minimize the area in contact with the substrate [5]. The dewetting of Cu was gradually suppressed after the thickening
of the evaporated Cu layer from 400–600 nm. The enlargement of grain size during the heat treatment formed a
continuous film on 600 nm-thick Cu. The average grain size for this sample was around 2 µm.
The mechanism and suppression of Cu dewetting can be explained by considering the Young-Laplace equation
with  as the equilibrium contact angle:
cos
s i f
   
  (1)
where γs, γi, and γf is the isotropic energy surface per unit area for SiO2/Si substrate, the interface of Cu-SiO2/Si,
and the Cu thin film, respectively [6]. For the Cu thickness of 400–600 nm, many areas did not satisfy the condition
of γs > γi + γf, therefore did not dewetted. During the high-temperature process of 725 o
C, grain boundaries of Cu were
enlarged and ruptures of Cu film started at the grain boundary when:
 
3
3
3sin
2 3cos cos
R
t

 

 
(2)
where R is the radius of the Cu grain, t is the thickness of Cu film, and  = sin–1
(γgb/2γf) is the pitch angle for groove
separation at grain boundaries with γgb is the energies of the Cu grain boundaries [6]. Therefore, decreasing t results in
the accelerated formation of the void and eventually ruptures of the Cu film.
Fig. 2(a)–(f) respectively show the FESEM image of Cu film with varied thickness of 100–600 nm after the 825 o
C
process. We observed higher temperature heat treatment on 100 nm-thick Cu caused the film to agglomerated into
microparticles. It is known that the dewetting of metal including Cu dominated by the surface diffusion kinetic rather

3. than evaporation-condensation kinetic [7]. The increase in temperature exponentially increase the surface diffusion of
Cu, therefore undesirably results in a severe dewetting condition. For the film thickness of 200–400 nm, the dewetting
of Cu results in isolated and interconnected islands. The dewetting was reduced when the thickness of the Cu film was
increased to 500 nm. The average grain size for the 600 nm-thick Cu was around 2 µm and comparable to the previous
CVD process at 725 o
C.
Fig. 1. Surface condition of Cu film after 725 o
C process for the varied thickness of the evaporated Cu: (a) 100 nm. (b) 200 nm. (c) 300 nm. (d) 400
nm. (e) 500 nm. (f) 600 nm.
Fig. 2. Surface condition of Cu film after 825 o
C process for the varied thickness of the evaporated Cu: (a) 100 nm. (b) 200 nm. (c) 300 nm. (d) 400
nm. (e) 500 nm. (f) 600 nm.
(a) (c)
(b)
(d) (e) (f)
(a)
(e)
(d)
(b) (c)
(f)

4. 3.2. Graphene growth on the Cu film
Fig. 3(a) and (b) show the Raman spectra from the Cu film with a thickness of 100–600 nm from the CVD process
of 725 and 825 o
C, respectively. From the plots, there was no response from the sample of 100 nm-thick Cu due to the
sparse distribution of Cu microparticles and islands. We observed a sharp peak at 1285 cm–1
appeared on every sample
of 200–600 nm-thick Cu. This peak corresponds to the CO2 molecules on the Cu surface. No graphene peak found
from the 725 o
C process, indicating this temperature was infective to decompose the methane for the growth of
graphene. When the temperature increased to 825 o
C, prominent G-peak intensity (IG) at ~1580 cm–1
and 2D-peak
intensity (I2D) at ~2700 cm–1
were observed from the sample of 200–600 nm-thick Cu. It is known that IG originated
from the first-order phonons scattering at the Γ-point, while I2D is the second-order of zone-boundary phonons of the
graphitic materials [8]. The ratio of I2D/ IG > 2 obtained in the plot of Fig. 4(b) indicates the process managed to grow
monolayer graphene on all the 200–600 nm-thick Cu films. The absence of defect-related intensity ~1350 cm–1
suggests the high quality of CVD grown graphene on the Cu films [9–11].
Fig. 3. Raman spectra from the sample with varied Cu thickness after the CVD growth: (a) 725 o
C process. (b) 825 o
C process.
Fig. 4(a) shows the FESEM image of graphene on 600 nm-thick Cu film after the 825 o
C process. We noticed there
were several white dots on graphene, especially at the grain boundaries. The dots were most probably the location
where monolayer-limited growth of graphene occurred. This condition was explained in the previous report in ref.
[12], where high hydrogen concentration in the cold-wall reaction chamber deactivated the nucleation sites for the bi-
layer graphene. The formation of an adlayer is prohibited by the desorption of the hydrogenation species [13]. Fig.
4(b) compares the grain sizes between monolayer graphene to the 600 nm-thick Cu film (substrate) after the 825 o
C
process. The average grain size of monolayer graphene was around 2.5 µm and can reach up to 6 µm, while the average
grain size of Cu was around 2 µm. The grain size of graphene was certainly influenced by the grain size of the catalytic
metal substrate. Typical growth on Cu foil with relatively large grain size compared to the Cu thin film has a great
advantage in terms of obtaining graphene with large grain size, for example up to 5.9 mm as reported in ref. [14]. We
are expecting further grain size enlargement of Cu film without dewetting issues, potentially up to 100 µm by
increasing the thickness of Cu and applying higher temperature heat treatment.
Fig. 5(a)–(f) shows the optical microscopy image for the graphene/Cu/SiO2/Si for the varied thickness of the
evaporated Cu of 100–600 nm. Large-area imaging enables the evaluation of pores distribution. Note that the definition
of pore does not apply for 100 and 200 nm-thick Cu due to the completely disconnected Cu islands. The average
distance between neighboring pores for the 300–600 nm-thick Cu was 4.70 µm, 7.51 µm, 10.16 µm, and 23.74 µm,
respectively. The standard deviation for each data was 2.07 µm, 2.54 µm, 3.95 µm, and 14.80 µm, respectively. A
significant increase in pore distance (more than double) was observed between 500 nm to 600 nm-thick Cu. This
suggests that the thickness of Cu needs to be 600 nm and above to effectively reduce the effect of dewetting during
the 825 o
C process for obtaining a long-order continuous layer of graphene. Those pores, fortunately, did not induce
defects on the monolayer graphene as no defect peak was detected in the previous Raman spectra.
(b)
(a)

5. Fig. 4. Monolayer graphene grown after the 825 o
C process on 600 nm-thick Cu film: (a) Surface condition. (b) Grain size distribution of graphene
and Cu film.
Fig. 5. Large-area pore distribution on graphene/Cu/SiO2/Si after 825 o
C process on the varied Cu thickness: (a) 100 nm. (b) 200 nm. (c) 300 nm.
(d) 400 nm. (e) 500 nm. (f) 600 nm.
4. Conclusion
The growth temperature of graphene using a cold-wall CVD reactor need not lower than 825 o
C. Even though the
lower temperature of 725 o
C helps reduced the dewetting of Cu film, no graphene was obtained after the CVD process
due to the ineffective decomposition of methane. The thickness of Cu film needs to be sufficiently thick more than
600 nm to enable continuous Cu film after the high-temperature process. Isolated and interconnected Cu islands result
in a non-continuous graphene layer. Successfully grown graphene at 825 o
C on 8-inch Cu/SiO2/Si wafer using a cold-
(a) (b)
(a)
(d) (e)
(b)
(f)
(c)

6. wall CVD reactor was monolayer without crystal defect. The grain size of graphene was at an average of 2.5 µm. It
was associated with the grain size of the Cu film, which was at an average of 2 µm.
Acknowledgments
This project is internally funded by MIMOS Berhad.
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