# Resist and Transfer Free Patterned CVD Graphene Growth on ALD $MoC_{x}$ Nano
Layers

Eldad Grady  Chenhui Li  Institute for Photonic Integration Eindhoven
University of Technology Den Dolech 2, 5612 AZ Eindhoven, the Netherlands
Oded Raz  Institute for Photonic Integration Eindhoven University of
Technology Den Dolech 2, 5612 AZ Eindhoven, the Netherlands  W.M.M. Kessels
Ageeth A. Bol

###### Abstract

Multilayer graphene (MLG) films were grown by chemical vapour deposition (CVD)
on molybdenum carbide ($MoC_{x}$) substrates. We fabricated the catalytic
$MoC_{x}$ films by plasma enhanced atomic layer deposition (PEALD). The
mechanism of graphene growth is studied and analysed for amorphous and
crystalline $MoC_{x}$ films. In addition, the unique advantages of catalytic
substrate PEALD are demonstrated in two approaches to graphene device
fabrication. First, we present a complete bottom up, resist-free patterned
graphene growth (GG) on pre-patterned $MoC_{x}$ PEALD performed at
50${}^{\circ}C$. Selective CVD GG eliminates the need to pattern or transfer
the graphene film to retain its pristine, as grown, qualities. Furthermore, we
fabricated MLG directly on PEALD $MoC_{x}$ on 100 nm suspended SiN membrane.
We characterise the MLG qualities using Raman spectroscopy, and analyse the
samples by optical microscopy, scanning electron microscopy and X-ray
diffraction measurements. The techniques of graphene device manufacturing
demonstrated here pave the path for large scale production of graphene
applications.

##  1 Introduction

Next generation graphene-based applications would require a resist and
transfer free graphene growth in order to realise the promise of graphene for
flexible electronics[1, 2], and sub 10nm nodes’ interconnects [3]. Graphene
based interconnects would significantly reduce the RC delay and thermal budget
which is currently the bottle neck in sub 10nm transistor nodes [4, 5] due to
the excellent room temperature carrier mobility and thermal conductivity.
However, graphene based devices exhibit far lower carrier mobility than the
theoretical predictions promise. Encapsulation of graphene with lattice
matching dielectric material such as hBN shows remarkable improvement, but
performance is still subpar [6]. The main challenges of current commercial
chemical vapour deposition (CVD) grown graphene are resist residue due to
target transfer process [7, 8], and patterning of graphene, which degrade the
graphene quality, resulting in the far lower carrier mobility than theoretical
predictions. Known techniques include photolithography, ion beam milling [9],
shadow masking [10], area selective passivation layer [11] and stencil mask
[12]. Despite these tremendous efforts, the goal of a scalable, low cost, high
quality patterned graphene using commercially available tools, has not been
achieved. Photolithography methods leave resist residue, and ion beam etching
causes broad lateral damage close to pattern area due to scattering ions [13,
14]. Patterning graphene with oxygen plasma inevitably oxides graphene edges
[15], and the stencil mask poses resolution limitation . Furthermore, transfer
of graphene from one substrate onto another degrades the graphene quality due
to induced film compression and tension, and potential contamination between
the graphene and target interfaces. A wholesome solution that avoids both the
need to pattern the graphene or to transfer a pre patterned graphene onto a
target, has yet to be introduced. While advancements have been made toward
transfer free graphene grown on patterned Mo catalytic layer [16], resist
residue on the underlying growth substrate cannot be fully removed. As
defects, non uniformity and contamination will imminently translate to
graphene defects, it is vital to achieve a resist free catalytic surface, in a
uniform and conformal layer deposition. Atomic layer deposition (ALD) is a
cyclic, broad temperature window soft deposition, with precise thickness
control due to its self-limiting nature. It allows for unmatching conformal
deposition on high-aspect ratio (HAR) objects. Recently, we demonstrated the
plasma enhanced ALD (PEALD) of $MoC_{x}$, with excellent composition control
[17]. Consequently, we presented the growth of multilayer graphene (MLG) films
on these substrates, and the correlation between the catalytic substrate
physical and chemical properties to the grown MLG [18]. In this work, we
describe the growth mechanism of graphene on $MoC_{x}$, and compare between
amorphous and highly crystalline catalytic substrates for graphene growth.
Finally, we demonstrate a proof of concept for the merits of ALD based
graphene growth for future interconnects (IC) in the form of low temperature
patterned PEALD of $MoC_{x}$, for selective CVD growth of MLG, so that no
resist residue is present between the graphene and the catalytic material
interface. Additionally, we show the advantage of the ALD soft deposition on
100nm thick suspended SiN membrane. Thereafter, MLG was grown on the suspended
SiN membranes to achieve suspended graphene based heterostructure without
exposing the suspended MLG to wet chemicals or corrosive acids.

##  2 Experimental methods

$MoC_{x}$ thin films have been deposited by plasma enhanced atomic layer
deposition (PEALD) at various temperatures and plasma conditions, as described
elsewhere [17].

PEALD was performed on 100 mm Si (100) wafers coated with 450 nm of thermally
grown $SiO_{2}$. The depositions were performed in an Oxford instruments
FlexAL2 ALD reactor, which is equipped with an inductively coupled remote RF
plasma (ICP) source (13.56 MHz) with alumina dielectric tube. $MoC_{x}$ thin
films have been deposited by PEALD at various temperatures and plasma
conditions, with $MoC_{x}$ films varying from 15$\mu{m}$ to 30$\mu{m}$ in
thickness. MLG was grown by low-pressure CVD (LPCVD) in a quartz tube (d=50mm,
l=60cm) furnace with 3 heat zones set to 1050∘C. The typical base pressure
when evacuated is $10^{-3}$ mbar. The furnace is set on cart wheels, to allow
samples to be rapid annealed, as furnace temperature stabilises within 3.5
minutes after tube insertion. When moved away from the furnace, sample cooling
down duration is typically 15 minutes. Carbon feedstock gas ($CH_{4}$) is fed
along with Argon through a quartz inner tube of 5 mm in diameter to the sealed
side of the outer tube. $MoC_{x}$ films have been saturated with carbon by
annealing at temperatures between 500${}^{\circ}C$ to 800${}^{\circ}C$ with
100 sccm $CH_{4}$ gas flow at 4 mbar pressure. Then, graphene films have been
grown under similar conditions at 1100∘C for 10 minutes. The samples were then
promptly extracted from the furnace and allowed to cool down at ambient room
temperature under Ar gas flow in the quartz tube. As shown in figure 3(a)
photoresist (PR) ma-N 400 with 4.1 $\mu{m}$ thickness on 90 nm $SiO_{2}$ on Si
2” wafers were used for low temperature PEALD of $MoC_{x}$ film. After
deposition, $MoC_{x}$ was patterned by lift-off process, and rinsed in
isopropyl alcohol (IPA). 100 nm and 50 nm SiN membranes were supplied by
Philips Innovation Services (PInS) foundry. $MoC_{x}$ were also deposited at
300${}^{\circ}C$ on 100 nm SiN membranes suspended on Si (5x5 mm suspended
rectangular area). MLG were then grown the $MoC_{x}$ films as illustrated in
figure 5. While deposition was successful for both SiN sample thicknesses, due
to the brittle nature of the thinner membranes we present here results
measured on the thicker 100nm based membranes. Raman spectroscopy was
performed with Reinshaw InVia 514 nm laser. Film crystallinity and preferred
crystal orientation was studied by Gonio x-ray diffraction. Experiments were
conducted with PanAnlytical X’pert PROMRD diffractometer operated using
$CuK\alpha(\lambda=1.54A)$.

##  3 Characterisation and Results

This section is divided to three parts: first part deals with MLG growth
mechanism in $MoC_{x}$ films and comparison between amorphous and crystalline
catalytic substrate. The second part demonstrate a technique for resist and
transfer free patterned graphene device. In the third part we fabricate MLG on
a thin suspended membrane using ALD and CVD. We show the limitation of Raman
measurements on suspended membranes low thermal conductivity.

###  3.1 Carburisation of $MoC_{x}$

We studied the growth mechanism of graphene on $MoC_{x}$ substrates, to
optimise growth process for the various film compositions. The importance of
saturating the catalytic $MoC_{x}$ with free carbon is demonstrated and the
effects of film crystallinity on carbon precipitation to the surface during
growth is shown by Raman and XRD measurements. The mechanism of graphene
growth on $MoC_{x}$ films is explained in this section. When we subjected the
$MoC_{x}$ film to direct growth at 1100${}^{\circ}C$ for 10 minutes. SEM
images show ablation on the film surface, that has a characteristic graphene
Raman signature [1(a)]. Outside these areas, no indication of graphene growth
was measured. We added then a carburisation step in order to saturate the film
with carbon and examined different temperatures around the crystalline phase
change temperature ($\sim{650}^{\circ}C$). Figure 1(c) indicates that for
crystalline $MoC_{x}$ film, ideal saturation takes place above the crystalline
phase change, for MLG grown at the same growth time. The lack of sufficient
carburisation affects the graphene growth significantly, as can be seen in
figure 1(c). This effect is less dominant with amorphous, low mass density
$MoC_{x}$ films, which can be carburised at lower temperatures as well. After
establishing an optimal carburisation temperature, we examined the ideal
growth time for various $MoC_{x}$ types. As seen in figure 2, $MoC_{x}$ with
rich carbon content, typically low mass density and crystallinity exhibit good
quality MLG growth after 10 minutes at 1100${}^{\circ}C$. $MoC_{x}$ films with
higher mass density and crystallinity display no graphene growth at this time
duration, but rather require a longer exposure time to $CH_{4}$ at the growth
temperature of 20 minutes. Longer exposure begin to deteriorate the graphene
film. We study the physical alteration for this crystalline film during the
carburisation and graphene growth process, as seen in figure 2. XRD
diffraction peak typical to cubic- $MoC_{0.75}$ are dominant for the deposited
film. After 2 hours carburisation at 800${}^{\circ}C$, a transition
Orthorhombic crystalline phase is noted, along with sharp graphite (101) plane
diffraction peak. After 10 minutes graphene growth at 1100${}^{\circ}C$, the
orthorhombic phase crystallinity increases, while no significant change in the
graphitic peak is noticed.

###  3.2 Patterned MLG Growth

Graphene film has been grown on patterned catalytic substrates. Patterning of
$MoC_{x}$ has been performed by a lift-off process, so that no exposed
$MoC_{x}$ surface needed to be coated with PR. When depositing $MoC_{x}$ at
150${}^{\circ}C$ we found resist residues on the $SiO_{2}$, due to hard baking
of the photoresist at this temperature, as the thermal stability limits of the
PR (110${}^{\circ}C$). Moreover, after the CVD growth we found random patches
of MLG coverage on the exposed $MoC_{x}$, and no continuous MLG coverage.
However, when the PEALD is performed at 50${}^{\circ}C$, we could seamlessly
remove the PR and no significant traces were found after lift-off. The CVD
growth of graphene on these samples showed full MLG coverage with excellent
uniformity, albeit a relatively high D/G peak ratio. Moreover, as can be seen
in figure 4(a), D and G peaks were detected throughout the exposed $SiO_{2}$
areas, but no significant 2D peak. We have seen that a transfer free release
of MLG on $MoC_{x}$ membranes is a direct result of wet etching the Mo based
catalyst. This observation is valid for ultra thin layers with thicknesses
below 30 nm. Thicker layers ($>50nm$) release the MLG film such that the
graphene membrane is afloat on the liquid surface.

###  3.3 MLG Growth on Suspended Thin Film

SiN membranes were suspended on Si substrate with 5x5 mm openings. $MoC_{x}$
was then deposited by PEALD process. There after, MLG was grown by a CVD
growth process. Thus, a suspended heterostructure of MLG/$MoC_{x}$/SiN was
fabricated on the supporting Si frame. MLG film grown on SiN membrane were
characterised by Raman spectroscopy, as can be seen in 6. We measured the
Raman signal at the edge of the suspended membrane where the SiN was supported
by the underlying Si, and at the centre of the membrane, where the MLG was on
top of $\sim{15}$nm $MoC_{x}$ and 100 nm SiN. The Raman spectrum was fitted
and baseline corrected due to enhanced SiN background signal. Although Raman
spectroscopy is considered a non-destructive measurement, we discovered that
suspended MLG/SiN membranes were highly sensitive to the Raman laser power. As
figure 6(c) shows, when 10% power of the 20mW Laser was used, an increase in
the D peak was measured, and the 2D peak was quenched in comparison to 5%
power. With 50% power of the Raman’s laser, we could punch a hole through the
heterostructure suspended membrane as seen with an optical microscopy image
[see figure 6(d) ].

##  4 Discussion and Conclusions

We have demonstrated in this work the growth mechanism and conditions of MLG
on PEALD $MoC_{x}$ films. In order to achieve full graphene film coverage, and
a uniform graphene growth, carburisation of the substrate is essential. The
carburisation step allows for saturation of the catalytic film with free
carbon. The free carbon then precipitate to the surface during annealing at
1100∘C. We found that in order to saturate $MoC_{x}$ film with higher mass
density, carburisation temperature has to exceed the crystalline phase change
point. For $MoC_{x}$ film we found that point to be around $\sim{650}$.
Amorphous films with lower mass density could be saturated at lower
carburisation temperatures.
We compared MLG growth time on amorphous film and on a highly crystalline one.
The prolonged annealing time necessary to grow graphene on crystalline film is
stipulated to result from low carbon saturation during carburisation, due to
the dense crystal structure and the resulting low bulk precipitation of free
carbon. Defects ratio of MLG on crystalline film could be explained with the
transition from a single crystalline cubic-$MoC_{0.75}$ to a polycrystalline
orthorhombic phase, which results in multiple strain points in the MLG film.
By contrast, graphene growth on an amorphous carbon rich film is more facile
and results in a lower D/G peak ratios. After understanding the growth
mechanism, we demonstrated the advantages of graphene growth on PEALD
catalytic films. The low temperature, soft and atomic precise deposition
allows for pre patterning of catalytic substrates, such that no PR coating of
the growth surface is needed. We demonstrated here an initial proof of
concept, which showed a full film coverage with excellent uniformity. We
speculate that decomposition of the $CH_{4}$ molecules on the $SiO_{2}$
surface with PR residue is the reason for the carbide formation outside the
patterned $MoC_{x}$ areas. The relatively high D/G peak could be addressed by
growth process optimisation, and different approaches to selective ALD which
was out of the scope of this work. We cannot rule out potential surface
impurities during the lift-off process, that could result in additional
amorphous carbon during the CVD growth process. A complete bottom up selective
area ALD process will avoid any potential resist contamination and should
allow for a broader deposition window. We have also used ALD to deposit
$MoC_{x}$ films on brittle SiN membranes of 50 and 100 nm. The MLG growth
demonstrated on 100 nm suspended membrane shows a viable fabrication route for
sensor applications, and graphene based resonators for a wide range of
frequencies. We showed for the first time damage to a suspended
heterostructure caused directly by Raman measurements. The low thermal
conductivity of the SiN is likely the cause for the local damage to the
suspended heterostructure, with no directly available heat sink - as oppose to
the membrane edges. This however could prove useful, when punctured suspended
graphene heterostructures are required, for water filtration and desalination
applications for example. Building on the capabilities demonstrated here, the
route for future application based on patterned graphene or suspended graphene
heterostructure is clearly marked. Further steps, such as $Al_{2}O_{3}$
encapsulation, could be readily performed by ALD directly on the MLG without
damage by a process of hydrogenation and post ALD annealing [19]. Moreover,
one can combine recent developments in area selective ALD to realise a
complete bottom up fabrication of the catalytic substrate resist free, with
atomic-scale precision alignment.

### Acknowledgements

This research is supported by the Dutch Technology Foundation STW (project
number 140930), which is part of the Netherlands Organization for Scientific
Research (NWO), and partly funded by the Ministry of Economic Affairs as well
as ASML and ZEISS. E. Grady thanks Cristian Helvoirt, Janneke Zeegbregts,
Jeroen van Gerwen and the lab technical staff for their support.

## References

  * [1] Veronica Strong, Sergey Dubin, Maher F El-Kady, Andrew Lech, Yue Wang, Bruce H Weiller, and Richard B Kaner.  Patterning and electronic tuning of laser scribed graphene for flexible all-carbon devices.  ACS nano, 6(2):1395–1403, 2012.
  * [2] Rak-Hwan Kim, Myung-Ho Bae, Dae Gon Kim, Huanyu Cheng, Bong Hoon Kim, Dae-Hyeong Kim, Ming Li, Jian Wu, Frank Du, Hoon-Sik Kim, et al.  Stretchable, transparent graphene interconnects for arrays of microscale inorganic light emitting diodes on rubber substrates.  Nano letters, 11(9):3881–3886, 2011.
  * [3] Ning C Wang, Saurabh Sinha, Brian Cline, Chris D English, Greg Yeric, and Eric Pop.  Replacing copper interconnects with graphene at a 7-nm node.  In 2017 IEEE International Interconnect Technology Conference (IITC), pages 1–3. IEEE, 2017.
  * [4] Maria Politou, Xiangyu Wu, Inge Asselberghs, Antonino Contino, Bart Soree, Iuliana Radu, Cedric Huyghebaert, Zsolt Tokei, Stefan De Gendt, and Marc Heyns.  Evaluation of multilayer graphene for advanced interconnects.  Microelectronic Engineering, 167:1–5, 2017.
  * [5] Yuji Awano.  Graphene for vlsi: Fet and interconnect applications.  In 2009 IEEE International Electron Devices Meeting (IEDM), pages 1–4. IEEE, 2009.
  * [6] Shaloo Rakheja, Vachan Kumar, and Azad Naeemi.  Evaluation of the potential performance of graphene nanoribbons as on-chip interconnects.  Proceedings of the IEEE, 101(7):1740–1765, 2013.
  * [7] Junmo Kang, Dolly Shin, Sukang Bae, and Byung Hee Hong.  Graphene transfer: key for applications.  Nanoscale, 4(18):5527–5537, 2012.
  * [8] Woosuk Choi, Muhammad Arslan Shehzad, Sanghoon Park, and Yongho Seo.  Influence of removing pmma residues on surface of cvd graphene using a contact-mode atomic force microscope.  RSC Advances, 7(12):6943–6949, 2017.
  * [9] Max C Lemme, David C Bell, James R Williams, Lewis A Stern, Britton WH Baugher, Pablo Jarillo-Herrero, and Charles M Marcus.  Etching of graphene devices with a helium ion beam.  ACS nano, 3(9):2674–2676, 2009.
  * [10] Yong Seung Kim, Kisu Joo, Sahng-Kyoon Jerng, Jae Hong Lee, Euijoon Yoon, and Seung-Hyun Chun.  Direct growth of patterned graphene on sio 2 substrates without the use of catalysts or lithography.  Nanoscale, 6(17):10100–10105, 2014.
  * [11] Mario Hofmann, Ya-Ping Hsieh, Allen L Hsu, and Jing Kong.  Scalable, flexible and high resolution patterning of cvd graphene.  Nanoscale, 6(1):289–292, 2014.
  * [12] Keong Yong, Ali Ashraf, Pilgyu Kang, and SungWoo Nam.  Rapid stencil mask fabrication enabled one-step polymer-free graphene patterning and direct transfer for flexible graphene devices.  Scientific reports, 6:24890, 2016.
  * [13] L Gustavo Cançado, A Jorio, EH Martins Ferreira, F Stavale, Carlos Alberto Achete, Rodrigo Barbosa Capaz, Marcus Vinicius de Oliveira Moutinho, Antonio Lombardo, TS Kulmala, and Andrea Carlo Ferrari.  Quantifying defects in graphene via raman spectroscopy at different excitation energies.  Nano letters, 11(8):3190–3196, 2011.
  * [14] Nick FW Thissen, RHJ Vervuurt, JJL Mulders, JW Weber, WMM Kessels, and AA Bol.  The effect of residual gas scattering on ga ion beam patterning of graphene.  Applied Physics Letters, 107(21):213101, 2015.
  * [15] Isaac Childres, Luis A Jauregui, Jifa Tian, and Yong P Chen.  Effect of oxygen plasma etching on graphene studied using raman spectroscopy and electronic transport measurements.  New Journal of Physics, 13(2):025008, 2011.
  * [16] S Vollebregt, B Alfano, F Ricciardella, AJM Giesbers, Y Grachova, HW van Zeijl, T Polichetti, and PM Sarro.  A transfer-free wafer-scale cvd graphene fabrication process for mems/nems sensors.  In 2016 IEEE 29th International Conference on Micro Electro Mechanical Systems (MEMS), pages 17–20. IEEE, 2016.
  * [17] Eldad Grady, Marcel Verheijen, Tahsin Faraz, Saurabh Karwal, W. M. M. Kessels, and Ageeth A. Bol.  Tailored molybdenum carbide properties and graphitic nano layer formation by plasma and ion energy control during plasma enhanced ald, 2019.
  * [18] Eldad Grady, W. M. M. Kessels, and Ageeth A. Bol.  Control of graphene layer thickness grown on plasma enhanced atomic layer deposition of molybdenum carbide, 2019.
  * [19] Rene HJ Vervuurt, Bora Karasulu, Marcel A Verheijen, Wilhelmus (Erwin) MM Kessels, and Ageeth A Bol.  Uniform atomic layer deposition of al2o3 on graphene by reversible hydrogen plasma functionalization.  Chemistry of Materials, 29(5):2090–2100, 2017.

## Figures

(a) SEM: $MoC_{x}$ ablation

(b) SEM: MLG grown on $MoC_{x}$

(c) Raman spectra (carb. temp)

Figure 1: SEM images and Raman spectrum of $MoC_{x}$ after $CH_{4}$ annealing
at 1100${}^{\circ}C$. Top: (a) SEM image of films that were not carburised
prior to growth. Bottom: (b) SEM image of MLG grown on carburised $MoC_{x}$
film. Smooth surface no signs of inhomogeneity. (c) Raman spectrum after
graphene growth on crystalline $MoC_{x}$, as a function of carburisation
temperature. Growth time remained identical for all 3 experiments. Figure 2:
Raman spectra and XRD measurements. Left: Raman measurements of MLG grown on
(a) amorphous $MoC_{x}$ film and (b) Single crystalline $MoC_{x}$ after 10 and
25 minutes growth at 1100${}^{\circ}C$. Left: (c) XRD measurements of
crystalline $MoC_{x}$ for each process step from ALD, to carburisation, to
graphene growth

(a) Fabrication flow graphics

(b) Optical microscope

Figure 3: Fabrication of patterned MLG. (a) Fabrication flow schematics:
computer grphics and illustration of fabrication steps. (b) Top right: optical
microscopy of pre patterned $MoC_{x}$ ALD at 50${}^{\circ}C$ after deposition.

(a) D/G peak ratios mapping

(b) 2D/G peak ratios mapping

Figure 4: Characterisation of patterned MLG: Raman mapping scan of
40$\mu{m}$x40$\mu{m}$ area with 1$\mu{m}$ step resolution of MLG grown on 20
$\mu{m}$ thick $MoC_{x}$ ALD film . (a) left: D/G peak ratio shows a high D/G
peak ratio, albeit uniformal continuous coverage. (b) Right: 2D/G peak ratio
show a uniform continuous MLG film. Variation in colour is inversely
proportional to MLG uniformity Figure 5: Illustration of suspended
heterostructure and optical images. (a)+(b)+(c) Computer graphics of the
fabrication schematics of suspended graphene heterostructure. (d) Optical
images of SiN membranes before and after PEALD of $\sim{15}$ nm $MoC_{x}$ film
on top

(a) MLG/SiN/Si substrate

(b) 5% laser power

(c) 10% laser power

(d) Optical microscope: 50% laser power

Figure 6: Raman spectrum of MLG CVD grown on ALD of $MoC_{x}$ on SiN membrane.
Measurements taken on the membrane’s edge with underlying Si, and at various
spots around the centre of the membrane with increasing laser power. (a) MLG
on SiN membrane’s edge with underlying Si substrate. (b) Raman scan measured
with 5% laser power shows diminished 2D peak and a rise in D peak. (c) Raman
scan measured with 10% laser power shows quenched 2D peak and a high D peak.
(d) Optical microscope image: after Raman scan measured with 50% laser power.
A hole is punched through the MLG/MoC/SiN suspended heterostructure