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Title

Low pull‑in voltage graphene electromechanicalswitch fabricated with a polymer sacrificial
spacerAuthor(s)Sun, Jian; Wang, Wenzhen; Muruganathan,
Manoharan; Mizuta, Hiroshi

Citation

Applied Physics Letters, 105(3): 033103‑01‑
033103‑04

Issue Date

2014‑07‑21

Type

Journal Article

Text version

publisher

URL

http://hdl.handle.net/10119/12243

RightsCopyright 2014 American Institute of Physics.
This article may be downloaded for personal use
only. Any other use requires prior permission of
the author and the American Institute of Physics.
The following article appeared in Jian Sun,
Wenzhen Wang, Manoharan Muruganathan and Hiroshi
Mizuta, Applied Physics Letters, 105(3), 033103(2014) and may be found at
http://dx.doi.org/10.1063/1.4891055

Description

Japan Advanced Institute of Science and Technology

APPLIED PHYSICS LETTERS 105, 033103 (2014)

Low pull-in voltage graphene electromechanical switch fabricatedwith a polymer sacrificial spacer
Jian Sun,1,a) Wenzhen Wang,1 Manoharan Muruganathan,1 and Hiroshi Mizuta1,2,b)
1

School of Materials Science, Japan Advanced Institute of Science and Technology, Nomi 923-1211, Japan
Nano Research Group, Faculty of Physical Sciences and Engineering, University of Southampton, Highfield,
Southampton SO17 1BJ, United Kingdom
2(Received 29 April 2014; accepted 13 July 2014; published online 21 July 2014)
A simple bottom-up procedure using a polymer sacrificial spacer is presented to fabricate graphene
electromechanical contact switch devices without using acid etching. Low pull-in voltage of below
2 V is achieved with good consistency on a run-to-run basis, which is compatible with the conventional, complementary metal-oxide-semiconductor circuit requirements. In addition, the formation ofcarbon-gold bonds at the contact position is proposed as another important mechanism for the irreC 2014 AIP Publishing LLC.
versible switch—other than the well-known irreversible static friction. V[http://dx.doi.org/10.1063/1.4891055]

Nanoelectromechanical (NEM) contact switches are
attractive components in data storage, high frequency communication, and logic circuits.1–3 Graphene, as an atomic
sheet of graphite, has an ultra-high Young’s modulus of
1 TPa, making it a promising candidate for future NEM
applications. The graphene NEM switches showed minimized electrical leakage, sharp switching response, lowactuation voltage, and high on/off ratio.4–7 Despite thesepotential advantages, graphene has yet to outperform conventional materials because of its low reliability.4,7 So far,only few reports have shown multiple cycles of switching
operation.5 The common failure is that graphene is stuck on
the electrode and not reversible after retracting actuation
voltage, which is simply ascribed to an irreversible static
friction. However, the details are still unclear. In addition,
the switch with suspended graphene achieved using conventional buffered hydrofluoric (BHF) acid etching suffers fromsome drawbacks.8 First, BHF attacks titanium, a frequently
used adhesive material for Ohmic contact to graphene.
Second, as an isotropic process, SiO2 below metal contacts is
also removed rapidly. Both of them lead to mechanicallyunstable structures.5 Therefore, it is crucial to find a method
to produce a suspended graphene device that excludes thisaggressive BHF etching step. Recently, several methods
have been proposed to realize the suspended graphene on organic polymers without using acid etching.9,10 However,they all require, at least, two lithography processes.
Moreover, the suspended graphene is anchored on a flexible
polymer substrate, which may cause damages to the graphene through strain effects.9
In this Letter, we report a simple bottom-up procedure
to get suspended graphene with a polymer sacrificial spacer,
which requires no acid etching and only one lithography process. The two-terminal graphene switches fabricated using
the proposed method are characterized. Furthermore, thefailure of the irreversible graphene switch is studied to gain abetter understanding of the mechanism behind it.
a)sun-jian@jaist.ac.jp
mizuta@jaist.ac.jpb)0003-6951/2014/105(3)/033103/4/$30.00

The bottom-up process is illustrated in Fig. 1. First, the35-nm-thick bottom electrode is defined with a Ti/Au stackon the highly p-doped silicon substrate covered with 300 nm
of SiO2 via conventional nanofabrication techniques.
Afterwards, a thin polymethyl methacrylate (PMMA) polymer (495 K A4) spacer is spun onto the substrate (Fig. 1(a)),
which shows a thickness of 170 nm after baking. Later, itdefines the distance between the suspended graphene andsubstrate/bottom electrode. Next, graphene is mechanicallyexfoliated from highly oriented pyrolytic graphite onto the
PMMA spacer (Fig. 1(b)). Possible bilayer graphene flakes
located on top of the bottom electrode are identified using
optical microscopy (left inset of Fig. 2). The bilayer flakes
are chosen over monolayer for the higher mechanicalstrength. Raman spectroscopy (633 nm excitation) is utilized
to verify the actual number of layers of the exfoliated graphene (Fig. 2). Four Lorentzian sub-peaks can be fitted to the2D band (right inset of Fig. 2(b)), showing a clear signature

FIG. 1. Schematics of fabrication procedure of graphene switch. (a) PMMA
spacer is spin-coated on substrate with pre-defined bottom electrodes. (b)Graphene flakes are mechanically exfoliated onto PMMA spacer. (c) MMA
copolymer and PMMA layers are spun in sequence onto substrate, and thenEBL is utilized to pattern contacts. (d) Development of the exposed contact
areas. PMMA under graphene is protected from development. (e) Metalevaporation. (f) Lift-off and critical point release.

105, 033103-1

C 2014 AIP Publishing LLC
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FIG. 2. Raman spectrum of a bilayer graphene on PMMA spacer probed at
the blue dot marked in the inset image. Left inset: the microscopic imageof graphene flake identified, dot-line shows its edges. The gold strip is thebottom electrode. Scale bar: 5 lm. Right inset: the Lorentzian peak analysisof 2D band (shadowed). Blue solid curves are the Lorentzian fitted subpeaks.

of the bilayer graphene.11,12 The ratio of the intensity of 2D
over the G band, I2D/IG, is found to be 1.83, which accords
with the previously published data.14 The value is several
times larger than that of the graphene on SiO2 substrate, indicating less influence to graphene from PMMA polymer thanfrom SiO2. Furthermore, other valuable information is
deduced from the Raman spectrum. D peak is absent, showing a negligible lattice defect in the probed flake. G and 2D
bands are found at 1580.3 and 2650.2 cm 1, respectively,
without obvious shifting compared with that of the pristinegraphene.11 It manifests that no/weak strain is built in the
exfoliated graphene on the PMMA spacer.13
After exfoliation, the sample is coated with methylmethacrylate (MMA) 8.5 methacrylic-acid (MAA) EL9 copolymer and PMMA in sequence (Fig. 1(c)). Short baking of
90 s at 180  C is used for both polymer layers to avoid scrolling and folding in graphene during long thermal treatment.14
We note that copolymer is first spun rather than PMMA
because the solvent of PMMA—anisole—dissolves the
PMMA spacer very quickly, hence dislocating graphene during spinning. Top contacts to graphene are defined by using
electron beam lithography (Fig. 1(c)). A large electron beamdose is chosen to expose through all polymer layers. After
exposure the development process with well-controlled duration opens exposed areas, but leaves the part of the PMMA
spacer underneath graphene undeveloped. Top contacts aredeposited with 10 nm of Ti and 190 nm of Au in an electron
beam evaporator (Fig. 1(e)). The thick metal stack assuresthe continuity of electrical contacts and the good mechanicalstability of the structure. Later, hot N-methyl-2-pyrrolidone(NMP) solvent is used to dissolve all polymers and lift off
metal. Finally, the sample is dried using a critical point drier
to prevent surface tension induced collapse of suspended
graphene (Fig. 1(f)). In principle, this bottom-up method canbe applied to any substrates which are insoluble in NMP.
Figure 3(a) displays a bilayer graphene switch fabricatedusing the aforementioned method. The topographic image isacquired for it using a tapping mode atomic force microscopy (AFM) (Fig. 3(b)). The height profiles in Fig. 3(c) demonstrate that the graphene locates at 170 nm above theAppl. Phys. Lett. 105, 033103 (2014)

FIG. 3. (a) Microscopic photo of a graphene switch. Red dot-lines indicate
the edges of suspended graphene. T and B denote the top and bottom electrode, respectively. Scale bar is 5 lm. (b) AFM image of the dash-box in (a).
Scale bar is 1 lm. (c) Height profiles measured along the green, red, andblue lines in (b). (d) Longitudinal line scans along suspended graphene withrespect to two ends at varied locations (dot-lines in (b)) showing concave
buckling. Thick black line is the fitted parabolic curve.

SiO2 substrate near two top electrodes, thereby verifying thegraphene’s suspension. However, in the center of the graphene, it only exhibits a distance of 90 nm from the bottom
electrode, but not 135 nm calculated with the 170-nm-thick
PMMA spacer and 35-nm-thick bottom electrode for a tabular suspend graphene. This difference is from the concave
buckling of the suspended graphene (Fig. 3(d)). It is known
as a result of built-in compressive strain and attributed to thenegative thermal expansion of graphene in the thermal process.15 In our fabrication, the 180  C baking (above the polymer’s glass temperature) could lead to relative sliding ofgraphene with respect to the underlying substrate. Whencooled down to room temperature, graphene expands whilesubstrate shrinks, resulting in the contracted carbon bonds,
namely, the compressive strain. A similar convex feature
was also reported earlier in an acid etching released graphene
after thermal treatment.16 The buckling can be controlled byvarying the baking temperature.
We are now to characterize the graphene switch (dimension of graphene: 2.5 lm  1 lm). All measurements are performed at a low pressure of 0.1 Pa to reduce the impacts
from ambience, e.g., the molecular absorption and moistureinduced capillary force. First of all, the electrical contact of
two top electrodes and leakage current between top and bottom electrodes are examined. Figure 4(a) plots the currentvoltage (I–V) measurements at top-top and top-bottom electrodes, respectively. The linear response between two top
electrodes presents Ohmic contact. A low current of 3 pA
is measured between top and bottom electrodes as leakage,
which is another evidence of the suspension of graphene.
Figure 4(b) exhibits the performance of the above switch
characterized with the two-terminal configuration (inset of
Fig. 4(c)). A voltage Vtb increased from 0 V is applied
between top and bottom electrodes, which electrostaticallydeflects graphene toward the bottom electrode. At low Vtb,
only leakage current is measured, which is normally referred
to as the “switch-off” status. At 3.5 V, the current abruptly

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FIG. 4. (a) I–V responses at top-top and top-bottom electrodes before switch
operation. Inset shows the zoom-in of the leakage current between top andbottom electrodes. Itb Vtb characteristics of the same graphene switch in(a): (b) in operation and (c) after switch’s failure; inset of (b) plots the datausing a low compliance current. Inset of (c) illustrates the two-terminal configuration. (d) I Vtb characteristics for a different graphene switch at multiple switching cycles. Result of a reverse scan after failure is also provided.
Thick arrows show the direction of scan. Insets illustrate different statuses:
switch-on, failure, and switch-off (from top).

increases, indicating the physical pull-in of graphene onto
the bottom electrode; this is also known as “switch-on.”Compared to these using back gates as actuation electrodes,1
the stronger electrostatic force generated by the local bottom
electrode brings about the low pull-in voltage. Interestingly,
as voltage continuously increases, we observe a unique transition in current response. The current first increases following a slow slope after pull-in; at 4.2 V, a transition to a
sharper slope suddenly occurs and current dramaticallyreaches the high compliance current of 0.9 mA. However,this behavior does not emerge if a low compliance is set(inset of Fig. 4(b)). The slow increase of current at the beginning of “switch-on” is probably attributed to the poor electrical contact between graphene and the bottom electrode. A
large contact resistance may exist at the interface, as a resultof the barrier of adsorbates at the contact interface and small
contact area of an incomplete physical pull-in. The Joule
heating generated by the current could gradually improve thecontact condition. At 4.2 V, the high current density of1010 A/m2 generates high temperature, which could createcarbon-gold (C-Au) bonds by removal of absorbates at thecontact interface.17 Hence, the sudden transition arises. The
switch is not reversible after this operation; graphene is stuckon the bottom electrode. Later, a symmetric linear Itb Vtb
behavior shows well-established Ohmic contact at the graphene/bottom electrode interface, implying the presence of
C-Au bonds (Fig. 4(c)).
A different switch device is also characterized. Here, theapplied voltage Vtb is immediately retracted once physicalpull-in is observed. The Joule heating is limited by shortening
the “switch-on” state to prevent the formation of C-Au bonds
at the improved contact interface as far as possible. Several
cycles of reversible switching are achieved (Fig. 4(d)). Dueto a narrower width of graphene (2.5 lm  0.5 lm), a much
lower pull-in voltage (1.85 V) is measured for it comparedwith the previous one. The consistency of pull-in voltages

Appl. Phys. Lett. 105, 033103 (2014)

observed in different cycles is noted, which highlights a goodmechanical stability of the structure. A reverse scan of Vtb isconducted after the switch’s failure. An abrupt drop of current at 1.65 V, known as pull-out, is still pronounced in thefailed switch. However, the current does not promptly reduceto the leakage level, but gradually decreases to zero following
a quasi-linear curve, which clearly reveals that contact still
partially remains between graphene and the bottom electrode(middle inset of Fig. 4). A large contact resistance read fromthe small slope of the Itb Vtb curve echoes the little contact
area. In this case, we propose that, despite the limited Joule
heating, the large potential difference between graphene andthe bottom electrode at the moment of pull-in could cause a
voltage pulse which is also sufficient to remove absorbates at
the first contact position and creates C-Au bonds locally.17 It
interprets the failure as only a small part of graphene remaining in contact, but not as completely collapsed graphene on
the bottom electrode. This type of failure could probably beeliminated by coating thin dielectric film on the bottom electrode5 or using different materials as the bottom electrode.18
In summary, we demonstrate the mechanically stablegraphene switch fabricated using a simple etching-freemethod with a polymer sacrificial spacer. A low pull-in voltage of below 2 V is realized. It is compatible with the conventional
complementary
metal-oxide-semiconductor(CMOS) circuit requirements, showing the possibility ofintegrating graphene switches into CMOS circuits. A goodconsistency of the pull-in voltages in multiple cycles highlights the mechanical stability of the switch’s structure. The
failure mechanism of the irreversible switch is studied. The
formation of C-Au bonds at the contact interface is suggested
as a possible factor of failure, besides the irreversible static
frictions. By limiting them, multiple switching cycles are
realized. However, more efforts are still needed to further
enhance the reliability of the graphene switch.
This work was supported by Grant-in-Aid for Scientific
Research No. 25220904 from Japan Society for the
Promotion of Science.
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