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Kaplas, Tommi; Bera, Arijit; Matikainen, Antti; Pääkkönen, Pertti; Lipsanen, HarriTransfer and patterning of chemical vapor deposited graphene by a multifunctional polymerfilm

Published in:Applied Physics Letters

DOI:10.1063/1.5012526

Published: 12/02/2018

Document VersionPublisher's PDF, also known as Version of record

Please cite the original version:Kaplas, T., Bera, A., Matikainen, A., Pääkkönen, P., & Lipsanen, H. (2018). Transfer and patterning of chemicalvapor deposited graphene by a multifunctional polymer film. Applied Physics Letters, 112(7), [073107].https://doi.org/10.1063/1.5012526

Transfer and patterning of chemical vapor deposited graphene by a multifunctionalpolymer filmTommi Kaplas, Arijit Bera, Antti Matikainen, Pertti Pääkkönen, and Harri Lipsanen

Citation: Appl. Phys. Lett. 112, 073107 (2018); doi: 10.1063/1.5012526View online: https://doi.org/10.1063/1.5012526View Table of Contents: http://aip.scitation.org/toc/apl/112/7Published by the American Institute of Physics

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Transfer and patterning of chemical vapor deposited grapheneby a multifunctional polymer film

Tommi Kaplas,1,a) Arijit Bera,1 Antti Matikainen,2 Pertti P€a€akk€onen,1 and Harri Lipsanen21Institute of Photonics, University of Eastern Finland, P.O. Box 111, FI-80101 Joensuu, Finland2Department of Electronics and Nanoengineering, Aalto University, FI-00076 Aalto, Finland

(Received 6 November 2017; accepted 30 January 2018; published online 15 February 2018)

Graphene is seeking pathways towards applications, but there are still plenty of unresolved

problems on the way. Many of those obstacles are related to synthesis and processing of graphene.

Chemical vapor deposition (CVD) of graphene is currently one of the most promising techniques

that enable scalable synthesis of high quality graphene on a copper substrate. From the transient

metal substrate, the CVD graphene film is transferred to the desired dielectric substrate. Most

often, the transfer process is done by using a supporting poly(methyl methacrylate) (PMMA) film,

which is also a widely used electron beam resist. Conventionally, after graphene is transferred to

the substrate, the supporting PMMA film is removed by organic solvents. Hence, the potential of

using the same PMMA layer as a resist mask remains unexplored. Since PMMA is an electron

beam resist, the same polymer film can be useful both for transferring and for patterning of

graphene. In this work, we demonstrate simultaneous transfer and patterning of graphene by using

the same PMMA film. With our demonstrated method, we are able to receive sub-micron resolution

very easily. The graphene transfer and its subsequent patterning with the same resist layer may

help developing device applications based on graphene and other 2D materials in the near future.

Published by AIP Publishing. https://doi.org/10.1063/1.5012526

Chemical vapor deposition (CVD) of graphene on a cop-

per foil has become a standard technique in graphene synthe-

sis.1,2 It allows the synthesis of large-area, monocrystalline

graphene with high charge carrier mobility, which makes

CVD one of the top candidates for the scalable and cost-

effective graphene production.3–5 However, the well-known

drawback of the CVD technique is the requirement of gra-

phene to be transferred from a transient copper foil to a

dielectric substrate.2,6

The transfer of an atomically thin film has become a

routine technique nowadays. This delicate process requires

high precision in order to achieve good quality graphene

films without wrinkles or fractures.2,6,7 One of the key ele-

ments of a successful transfer is a supporting polymer film,

which is deposited on graphene before the transfer.6 The

polymer film supports and protects graphene during copper

foil etching and subsequent deposition of graphene to a

dielectric substrate. After a successful transfer, the polymer

layer is usually removed. However, since the use of the poly-

mer film seems to be unavoidable for graphene transfer, it

would be reasonable to get maximal benefit from it.

Conventionally, the most popular polymer for graphene

transfer has been poly(methyl methacrylate) (PMMA).2,6–10

This is because PMMA is a long chained polymer and provides

a robust support for graphene transfer. Although there are some

issues with PMMA residues, the PMMA support layer is reason-

ably easy to remove with organic solvents, e.g., by acetone.6–10

This makes PMMA a nice candidate to be a support layer for

the transfer or heterostructure stacking of the 2D materials.6–15

After graphene has been successfully transferred, it is

often required to be patterned for micro- and nanoscale device

applications. During recent years, many intriguing patterning

techniques for graphene have been introduced.16–24 However,

many of these interesting techniques require rather compli-

cated procedures, which involve either pre- or post-transfer

patterning. Since these proposed techniques increase the total

number of process steps, it would be more desirable to per-

form patterning simultaneously along with the transfer.

Since PMMA is widely used as an electron beam resist,

it gives an attractive opportunity to use the same PMMA

layer for both graphene transfer and patterning. In this paper,

we demonstrate how the same PMMA support film can be

used for (i) graphene transfer, (ii) electron beam lithography,

and (iii) as a sacrificial resist film for a metal mask lift-off.

The proposed technique allows us to obtain a metallic mask

for graphene etching, which yields micron and sub-micron

scale graphene structures. The technique reduces the amount

of process steps required for patterned graphene (or other

transferrable 2D materials), which is a major step forward

when pushing the 2D materials towards the application level.

Graphene was grown on a copper foil (99.99% pure) by

a conventional hot wall CVD technique.2,25,26 We performed

60min of annealing at 960 �C in hydrogen (20 sccm,

1mBar), followed by 20min of graphene growth in the

H2þCH4 (20 sccmþ 10 sccm, 1.4 mBar) gas atmosphere.

The graphitization process was followed by chamber cooling

in the static H2 atmosphere at 5 mBar pressure (overnight).

For supporting the graphene film during the transfer, we

chose a positive tone, long chained, PMMA based e-beam resist

AR-P 672.11.27 Graphene/copper was spin-coated with an

approximately 500nm thick resist film and baked at 60 �C for

5min. After the spin coating and baking, the backside graphene

was removed from the copper substrate by oxygen plasma

(1min, 20 sccm, 100W) and the copper was wet etched bya)tommi.kaplas@uef.fi

0003-6951/2018/112(7)/073107/5/$30.00 Published by AIP Publishing.112, 073107-1

APPLIED PHYSICS LETTERS 112, 073107 (2018)

aqueous FeCl3 solution (3:20, FeCl3:H2O dilution, overnight).

The sample was rinsed with distilled (DI) water for 1 h, and the

remaining iron particles were then removed by the modified

RCA cleaning solution, which consisted of H2O:HCl:H2O2

(20:1:1).28 Next, the PMMA/graphene was subtly rinsed with

deionized water again. Finally, the PMMA/graphene was trans-

ferred on an oxidized (280nm) silicon (Si/SiO2) substrate and

left to dry (overnight).

When the transferred PMMA/graphene sandwich on Si/

SiO2 was dry, the sample was ready for electron beam lithogra-

phy. Micron and sub-micron test patterns were exposed to the

PMMA film by e-beam (Vistec EBG 5000þES, 100 kV). The

electron doses were varied systematically from 500lC/cm2 to

1050lC/cm2 (with 50lC/cm2 steps), which is a typical dose

range for the AR-P 67X resists with 100kV acceleration volt-

age.27 The e-beam exposure was followed by 60-s developing

in methyl isobutyl ketone (MIBK, development done by

OPTISpin SST20).

After development, the sample was ready for lift-off

(see Fig. 1). By evaporating an 80 nm thick Cu film on the

PMMA resist and then removing the sacrificial PMMA in

acetone (overnight), we received a patterned copper mask.

This mask was then used to etch the exposed areas of the

graphene with oxygen plasma (20 sccm O2 flow, 100W RF-

power, 30 s). Following the graphene etching, we removed

the copper mask in FeCl3 solution (1 min) and rinsed the

sample with DI water for 5min. Finally, the sample was

cleaned again in the modified RCA solution for 15min and

then in water for 5min. A schematic illustration of the pro-

cess is shown in Fig. 1.

Roughness of the PMMA/graphene film on the SiO2/Si

substrate was characterized using a Veeco Dektak 150 stylus

profiler. In Fig. 1(a), the interference fluctuation in the micro-

scopic image of the PMMA film can be clearly seen, indicating

rather a rough film. This roughness originates from the copper

substrate. The process temperature of almost 1000 �C is already

high enough for re-arranging copper surface morphology.2 This

not only increases the copper grain size but also affects the sur-

face roughness of the copper foil.2 When the resist layer is

spin-coated on the copper surface, the copper foil roughness is

imprinted onto the resist film. This imprinted roughness pre-

vails in PMMA when the Cu substrate is etched and the gra-

phene/PMMA stack is transferred to the dielectric substrate. By

measuring the height differences on Si/SiO2/graphene/PMMA,

we observed rather high (about 100–150nm) peak-to-peak

roughness. This is almost one third of the thickness of the

PMMA film. Although the PMMA film itself is rather rough,

the e-beam exposed areas were precisely patterned as it can be

seen in Fig. 1(b). Thus, the roughness of the PMMA film,

which originates from the copper grain boundaries, did not sig-

nificantly affect the patterning resolution of our test grating

structures (line width varied from 100nm to 2lm). However, it

is worth keeping in mind that when the structure size is reduced

below 100 nm, the resist film thickness deviation may have an

impact on the patterning resolution.

The fabricated graphene structures were characterized

using an optical microscope, a Raman microscope (Renishaw

inVia Raman microscope, 514 nm excitation wavelength), a

scanning electron microscope (SEM, Leo 1550 Gemini), and

an atomic force microscope (AFM, AutoProbe M5).

Raman spectroscopy is a powerful tool for recognizing

the disorders and defects in the graphitic lattice.29,30 Thus, in

order to study the effect of the e-beam exposure on graphene,

we characterized the sample by Raman spectroscopy. We

FIG. 1. A schematic illustration and the corresponding optical microscopy images of a lift-off process with PMMA/graphene sandwich. (a) In the supporting resist

layer, copper grain boundaries are clearly visible as thickness deviation in the resist. (b) Despite the thickness deviation, e-beam exposed areas are well patterned. (c)

Copper evaporation on the sample (d) and lift-off result in a copper mask for oxygen etching. (e) After graphene etching and (f) after copper removal, the patterned

graphene sample was ready. (The scale bar in optical microscopy images is 20lm.)

FIG. 2. Raman characterization of the patterned graphene, exposed with a

dose of 850lC/cm2. (a) An optical microscopy image of 2lm wide gra-

phene ribbons on the Si/SiO2 substrate. (b) A typical Raman spectrum of the

patterned graphene ribbons (and reference graphene). Raman mapping of

patterned graphene ribbons for (c) D-, (d) G-, and (e) 2D-peaks from the

dashed area in (a).

073107-2 Kaplas et al. Appl. Phys. Lett. 112, 073107 (2018)

used 514 nm excitation wavelength to probe the patterned

graphene ribbons (the intensity of the laser beam was kept

low to avoid self-heating in graphene31). For comparison, we

used graphene areas with and without electron beam expo-

sure as the sample and the reference, respectively.

Typical Raman characteristics of the patterned graphene

structure are shown in Fig. 2. More precisely, Fig. 2(b)

shows D, G, and 2D peaks of graphene, which are located at

1350 cm�1, 1595 cm�1, and 2696 cm�1, respectively. These

peaks are found at the same position and almost with the

same intensity in the electron beam exposed sample and in

the reference. This implies that electron beam exposure does

not significantly harm the graphene layer. Even with a rela-

tively high 1050 lC/cm2 dose, we were unable to detect any

significant changes in the Raman signal.

In the pristine, single layer graphene, the G and 2D peaks

are located at 1581 cm�1 and �2685 cm�1, respectively.29,32

However, here in our experiments, these two peaks are

slightly shifted both in the e-beam exposed sample and in the

reference sample. The shift of the G peak is expected to origi-

nate from the doping during FeCl3 etching.32 Moreover, the

shift of the 2D peak indicates doping and the presence of bi-

layered graphene. This conclusion is supported by the full

width at half maximum of the 2D peak, typically around

35 cm�1, which implies the presence of bi-layered graphene

areas.32,33

An electron bombardment may have a destructive effect

in carbon bonds. However, in order to pattern the resist layer,

a sufficient dose of electrons is required. If the dose is too

small, a fair amount of resist will be left behind after devel-

opment (see Fig. 3). When the dose is high enough, those

resist residues from the exposed area disappear. Usually,

PMMA dissolves in MIBK much slower than in acetone.

However, with a sufficient e-beam dose, the molecular

weight of the polymer can be reduced from 950 kg/mol

down to 5–10 kg/mol, which will assist PMMA solubility in

MIBK.27 In our experiment, the resist layer was well

exposed with doses above 700 lC/cm2, although the best

results were obtained with the dose of 850 lC/cm2.

If the dose is too high, it will also expose nearby resist

film areas, due to the proximity effect. Most importantly, it

may increase the risk of breaking the sp2 bonds in graphene.

The average bonding energy of the sp2 hybridized material is

much greater in comparison to sp3 hybridized C-C, C-O, or

C-H bonds in PMMA.34 Moreover, the atomically thin gra-

phene layer is almost invisible for the electron beam.35

FIG. 3. An optical microscopy image of resist remains on graphene after e-

beam exposure and development. When the dose is increased from (a)

500lC/cm2, (b) 600lC/cm2 to (c) 700 lC/cm2, the amount of resist residues

decreases.

FIG. 4. SEM and AFM characteriza-

tions of the patterned graphene,

exposed with a dose of 850lC/cm2. (a)

SEM image of a patterned graphene

structure. (b) Atomic force microscope

(AFM) height mapping shows the pres-

ence of some particles on graphene rib-

bons. (c) A height profile plot shows

that the graphene ribbon is roughly one

nanometer thick. (d) With the presented

technique, sub-micron resolution was

easily achieved (LW denotes the line

width of the pattern). (e) Despite the

resist layer thickness of 500 nm, even

100 nm linewidth resolution was

achieved. However sometimes, poor

adhesion resulted in graphene ribbons

to peel out from the SiO2 surface

(dashed line).

073107-3 Kaplas et al. Appl. Phys. Lett. 112, 073107 (2018)

Therefore, it could be expected that the disintegration of the

graphene requires higher doses in comparison to the dose

required for the PMMA resist film. By increasing the dose

up to 1050lC/cm2, we were unable to detect any change in

the Raman spectrum even though this dose was much higher

than that is required for the PMMA film.

The SEM images [Figs. 4(a), 4(d), and 4(e)] and AFM

height map [Fig. 4(b)] show the amount of defects at the gra-

phene surface. By using the methods introduced in Ref. 21,

we were able to remove the copper remains and perform

rather clean transfer. However, as it can be seen from Fig.

4(b), there was a fair amount of defects on the surface of the

patterned graphene. These defects are believed to originate

either from copper oxide or from PMMA. Despite these

defects, the height plot [Fig. 4(c)] shows sub-nanometer

thick ribbons, indicating the presence of mono/bi-layered

graphene.

In addition to the defects, we occasionally observed the

patterned graphene being detached from the substrate surface

[see, e.g., Fig. 4(e)]. For a monoatomic graphene layer, van

der Waals force is sufficient to hold the graphene film on the

SiO2 surface. However, graphene was masked with an 80 nm

thick copper film before the lift-off and plasma etching steps.

Such a metallic film is more than two orders of magnitude

thicker than graphene, which greatly increases the aspect

ratio as well as the total weight of patterns. This caused the

patterned areas sometimes to peel out from the substrate.

Unfortunately, we were unable to reduce the load because

according to our observations, a thinner (e.g., 30 nm) Cu film

was not sufficient for protecting graphene during plasma

etching. However, since the peeling is likely associated with

the defects and wrinkles in graphene, we believe that the

issue can be resolved by further process optimization.

In conclusion, we have demonstrated that the same

PMMA support film can be used for graphene transfer and

patterning. Despite the PMMA film roughness, sub-micron

resolutions can be achieved very easily with the presented

technique. Since graphene, as well as the 2D material hetero-

structure stacking, relies strongly on the transfer techniques,

we believe that our technique will provide a simple yet use-

ful route toward efficient sample processing for the realiza-

tion of various electrical, optical, or thermal devices based

on 2D materials.

We thank Professor Yuri Svirko for helpful discussions.

This work was financially supported by Academy of Finland

(Grant Nos. 287886 and 298298), NP-Nano FidiPro by the

Finnish Funding Agency for Innovation (TEKES), and the

European Union H2020 program (Grant No. 604391

Graphene Flagship and No. 64407 CoExAn).

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