How to make a cheap,simple transfer machine for VdW heterostructures assembly

Article
How to make a cheap, simple and convenient transfer
machine for Van der Waals heterostructures assembly.
Sergey G. Martanov 1 , Natalia K. Zhurbina 1 , Mikhail V. Pugachev2 , Aleksandr I. Duleba2 ,
Aleksandr Yu. Kuntsevich 3,†
1
National Research University Higher School of Economics, Moscow, 101000; Russia
Moscow Institute of Physics and Technology, Dolgoprudny, Moscow Region, 141701, Russia 3
Institute of the RAS, Moscow,119991; Russia [email protected]
* Correspondence: [email protected]
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P.N. Lebedev
Version October 3, 2020 submitted to Nanomaterials
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Abstract: Van-der Waals hetorstuctures assembled from one or few atomic layer thickness layered
crystals become increasingly more popular in condensed matter physics. These structures are
assembled using transfer machines, typically based on mask aligner, probe station or custom device,
assembled from the sketch. Thus for many laboratories it is vital to build a simple, convenient and
universal transfer machine. In this paper we discuss guiding principles for the design for such a
machine and demonstrate our own construction, that is cheap, powerful and fast-in-operation.
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Keywords: 2D materials; dry transfer; Van-der Waals; home-made setup
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0. Introduction
Two-dimensional materials and Van der Waals heterostructures area is developing explosively[1–
4]. The main idea of this field is to obtain new functionality by mechanical joining atomically thin
flakes of layered materials. As a rule, the flakes are obtained by splitting a single crystal using the
"scotch tape" method, CVD [5,6] or PVT growth [7] or by liquid phase exfoliation[8]. Atomically thin
flakes most commonly are transferred onto PDMS or an oxidized silicon wafer, where they are held by
Van der Waals forces and could be optically identified[9]. A heterostructure is then assembled out of
the flakes using various transfer methods.
Figure 1. Stages of the dry transfer process for VdW heterostructure assembly.
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The fragility of the flakes and the tendency to form wrinkles led to development of different
methods of dry [10–16] and wet transfer [17–20], see also [21] for review.
Dry transfer methods have become much more common because of their convenience. They are
carried out using transparent temporary substrates (TTS), sometimes called stamps. For dry transfer, a
piece or a drop of viscoelastic transparent material, as a rule poly-dimethysiloxane (PDMS), is placed
on the surface of the slide. Then, using a spinner, the drop is covered with a gluing compound,
usually PPC (polypropylene carbonate) [14,15] polycarbonate [12]), PMMA [10,16] or polyvililchloride
(PVC)[22]). The adhesive properties of these polymers vary with temperature.
The transfer typically proceeds as follows: The substrate with the flakes is fixed on a substrate
holder (SH). It is positioned in microscope field of view and at a certain temperature (about 40◦ C for
PPC [14]) the stamp touches a desirable flake on a substrate (see Figure 1a). The flake stick to the
surface of the drop. Then the drop with the flake is lifted (see Figure 1b). Next, the substrate is replaced
with the next one containing the other flake (see Figure 1c). The operator positions the TTS relative to
substrate in a way that the previously collected flake overlaps with a new desired one (see Figure 1d).
This operation is repeated several times until a whole stack is collected on the drop. Then the stack is
transferred onto a final substrate (see Figure 1e). The temperature is set high to make the glue more
liquid, the droplet is raised and the sample peels off, remaining on a substrate surface.
To carry out these heating and positioning operations, a transfer machine is required. Mask
aligners, e.g. like in Ref.[23] have all the required positioning functions. There are also several reports
of home-made machines [10,12,24–27] from the simplest for k$ [25] with limited functionality, to fully
robotic for 1 M$ [26]. Such mashines are now produced commercially e.g. by HQGraphene and
Creative Devices. Laboratories are often faced the task of building such a machine within quite a
limited budget and time. Despite several reviews on transfer processes [4,21,28], we did not find a
description of the basic guiding opto-electro-mechanical principles those would help to avoid mistakes
and assemble a powerful and cheap machine. This paper aims to fill this gap.
Figure 2. (a) Schematics of the transfer machine (b) photo of our machine.
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1. Device design and guiding principles
Any transfer machine should necessarily contain three main components, shown in Figure 2a:
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1.
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Substrate holder
TTS holder
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Microscope
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We describe below requirements to each of them using our own home-made transfer machine as
an example. Note that for a focusing, the machine should be rigid and unaffected by vibrations,
therefore it should be placed onto an optical plate. Moreover, single basement allows relocation of the
transfer machine as a whole into the glove-box to operate with materials those are unstable at ambient
conditions. In our machine we used a 200×300 mm2 anodized aluminum plate with an array of M6
screwholes equipped with rubber legs.
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1.1. Microscope
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The operator needs to have a good view of the working area (typically 100x100 µm2 -20x Plan
objective) during the transfer process. Therefore, the device must contain a microscope. To find the
flakes on a substrate a lens with smaller magnification is needed (field of view more than 500x500 µm2
- 5x Plan objective). It is also desirable to inspect the wafers with high magnification (field of view
below than 50x50 µm2 - 50x Plan Objective). The objectives should have large working distance to
avoid touching the glass (more than 4 mm) and should be placed on the revolver head. Exactly this
set of objectives is realized in the top class laboratory [27,29] and commercial [HQGraphene] transfer
machines. Note that the high-magnification objective is useless for transfer, because its depth of focus
is too small, that makes it impossible due to TTS to have all field of view focused. Moreover it is
time consuming to find focus with high-magnification objective. The microscope should be equipped
with fast (more than 30 fps) and high resolution (more than 20 MP) camera for real-time monitoring
and taking photos of all stages of the process. Such a camera is now inexpensive, below 100 $, it can
be connected to PC or monitor. Binoculars are convenient and very useful for adjustment purpose,
however they hardly could be applied in the glovebox. Since the substrate is generally not transparent,
the illumination should be fed through the objective.
Most of high-magnification microscopes have stationary objectives attached to heavy base.
However, for transfer machines it is necessary to move TSS and substrate holder with respect to
each other focusing alternately on both of them. It is vital therefore to have objective height adjustable.
There are not so many commercially available metallurgical microscopes with the revolver head and
objective movable in z−direction. The models by e.g. Zeiss and Olympus are superior in quality but
pretty expensive (several 10 k$). Recently there appeared much more available microscopes ICM-100
(China, 1.5 k$) and Radical (India, 400$).
The last one was used in our setup. Besides the price its advantage is small size, that will allow a
subsequent placement into the glove-box, similarly to this machine. Large working-distance objectives
for this microscope were purchased separately here. A disadvantage of this microscope is not very
precise coincidence of axes of the revolver head objective slots.
A microscope with a single objective and zoom, like this one may also be considered as an option,
as in Ref.[25]. Note that zoom adjustment automatically de-focus the microscope.
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1.2. Transparent temporary substrate holder
The function of this part is to hold the glass slide with a drop/stamp. The glass should be
fixed by the clips in the space between the sample and the microscope. Atomically thin flakes are
practically invisible on TTS, therefore, when creating stacks, the microscope and TTS holder should
remain mutually immobile, while the substrates with flakes should be movable and easily changeable,
otherwise the stack on TTS could be lost. However, before the transfer, the center of the stamp or
PDMS drop should be located close to the optical axis and its top should placed in the microscope
focus.
Thus, TTS holder should have at least two degrees of freedom along the x and y axes. If the
microscope has a stationary objective, then TTS holder must also have z-degree of freedom. There are
many options for the design of the TTS holder. For example, a manipulator from the probe-station
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might be used for this purpose[14], like this one. In the model by HQGraphene the glass might
additionally be deflected from the horizontal orientation using a corresponding goniometer.
Figure 3. Problem of too small glass slide (a) and its possible solutions: (b) using large glass e.g. like in
mask aligner; (c) making a small sample holder table; (d) gluing a glass slide or stamp to the end of the
other slide.
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We have used XYZ-linear stage and attached a glass slide using the cheeseplates for photography.
The main challenge with the TTS holder is illustrated in Figure 3a. Standard glass slides are rather
short (≈ 76mm). In order to spin-coat the PDMS drop with PPC, the drop should be located in the
center of the glass. Typically the radius of the SH table exceeds half of the glass slide length (∼ 3 cm),
and the table touches the clamp, that is not acceptable. There are some approaches to overcome this
issue, illustrated in Figure 3b-d. If a piece of glass is taken large, it can be safely positioned as it is in
mask-aligner based machines[23] (Figure 3b). The table size could also be minimized (Figure 3c), for
example in our machine the diameter of the table is less than 6 cm, so the stamp fits the center of the
table. Most commonly, the stamp is either relocated to the end of the glass slide or to another glass
slide [27,29] that is glued to the bottom of the holder, as shown in Figure 3d. The combination of these
approaches is also possible.
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1.3. Substrate holder
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Substrate holder should allow positioning with µm resolution along the x,y, and z directions. It is
also necessary to rotate the substrate in the horizontal plane with a fractions of a degree precision, that
is crucial e.g. for creating the magic-angle structures[30]. Corresponding manipulators are available
commercially and relatively cheap ( $ 200).
Note that all manual motion and rotation stages could be upgraded to the motorized ones, that is
useful for glovebox operation and helps to avoid vibrations[27].
The Van der Waals stack on the TTS during the assembly should be located near the optical
axis and should not move laterally, otherwise it might be lost. Therefore, it is necessary to remove
the substrate holder, replace the substrate and return the holder to exactly the same place without
displacement of the stack. In Ref. [25] a magnetic base was proposed for that. It is more reproducible
to place the manipulator on a single-axis slide with a locking screw, like this one. Such a slide allows
to change a substrate in a few seconds (see Supplementary Video).
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The substrate is placed on the table, i.e the top plane of the substrate holder, that is attached to the
manipulator. The dry transfer pick-up process is based on sensitivity of the polymer to temperature.
So the table should contain a heating element and a thermometer. It is also necessary to seal the
substrate. Entry-level solution is fixing the substrate with glue or double-sided scotch-tape. The
obvious disadvantages are contamination of the substrate, and in the case of glue, a long waiting time.
Moreover the glue might become unstable with temperature. The fastest and most convenient sealing
is vacuum suction of the substrate.
The heating table should therefore have the following components:
1. A vacuumed holder with metallic polished top plate with a hole for reliable vacuum fixation of
the the substrate.
2. A heater to change the sample temperature
3. A thermocouple to control these changes.
Commercially available heated vacuum chucks are expensive and massive, see e.g. here and here.
Making a customised one in a mechanic workshop from the sketch might be time consuming. We
suggest here making a cheap and compact one out of commercially available PC Motherboard Bridge
waterblock as shown in Figures 4a,b.
We place a heater and a thermocouple inside a chamber of the waterblock and drill a hole in the
center of the copper plate for vacuum sealing of the substrate. Heater and thermocouple wires pass
through one the water entrances and 90◦ adaptors and are sealed hermetically. In order to make a path
for vacuum above the heater a groove was milled to the pumping hole, as shown in Figure 4a. The
tube for vacuum pumping passes through the second water entrance.
Figure 4. (a) Schematics of the substrate holder, a.1 pumping trench, a.2 sealing, (b) photo of the
substrate holder, (c) photo of our electronic box
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All electronic components for the table operation are placed into a box shown in Figure 4c. We
use a cheap (∼2$) aquarium diaphragm pump for vacuum and standard PID-controller to measure
the temperature. One common mobile phone AC/DC adaptor (5V, 2A) is used for both pump and
PID-controller. The pump has a switch on the front panel. We use the laboratory power source (0-32V,
0-5A by Gophert) for the heater instead of PID-controller, since the latter is too slow due to thermal
inertia of the table.
Using a computer control similarly to HQGraphene machine makes the process faster and more
controllable and probably makes sense if the mass production of the Van der Waals heterostructures is
required. Such automatization is not required for a small scientific group with limited measurement
facilities where the bottleneck is time-consuming stamp preparation, exfoliation and search.
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2. Examples of operation
Figure 5. (a) Schematics of the substrate holder, a.1 pumping trench, a.2 sealing, (b) photo of the
substrate holder, (c) photo of our electronic box
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Figure 5 shows examples of the systems assembled with the aid of our setup using the standard
PPC on PDMS drop route, similar to Ref.[14]. An approximately 50 nm thick flake of Srx Bi2 Se3 was
obtained by scotch tape exfoliation onto an oxidized Si wafer of the single crystal, the growth and
characterization of which are described in Refs. [31,32]. The photography of the flake was taken (see
Figure 5a) and the corresponding 30 nm thick gold electrodes with contact pads were patterned on
a fresh oxidized Si wafer using photolitography with laser pattern generator Heidelberg muPG101,
gold evaporation and a standard lift-off process (see Figure 5b). We locate the flake with a drop, collect
it onto a TTS, change a substrate and find the contact electrodes. Figures 5c and 5d show the flake
with the contacts before and after the peel off, respectively. The whole transfer process took about 5
minutes.
Figure 5e-h shows a Van der Waals heterostructure boron nitride / monolayer MoS2 / boron
nitride. We used natural mineral molybdenite and commercially available hexagonal boron nitride.
Boron nitride was scotch-tape exfoliated onto an oxidized Si wafer (285 nm oxide thickness). To extract
MoS2 monolayer we used gold layer, similarly to Ref. [ADD]. In order to locate the flakes we exfoliate
them onto an array of markers. Figure 5 shows all elements and the assembly. The whole transfer
process starting from the insulating flakes took about 15 minutes. In the Supplementary Video the
transfer process is shown.
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3. Discussion: advantages, limitations, further improvements
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It should be noted that the suggested machine uses all the design principles implemented in
top-level transfer machines, such as:
1. Independent and precise positioning of all three components of the machine.
2. Presence of lenses with different magnification for transfer, location and analysis.
3. Vacuum chuck and temperature control on the substrate holder.
Moreover this design has some advantages:
1. Compact size for easy integration into a glovebox;
2. Low cost, about $ 1k for all components;
3. Modular design of the device;
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4. Cheap and compact substrate holder table.
5. Convenient substrate replacement without touching the TTS holder and the microscope.
Some other technical aspects also should be discussed:
• Vacuum sealing of the substrate is never perfect, there is some air underflow that cools the
sample down making a thermometer misleading. This effect should be taken into account.
• It is highly desirable not to lose a focus during the substrate positioninig and rotation. Therefore
the sample holder table plane and microscope plane of view should be parallel and aligned
perpendicular to the rotational axis of the holder. The latter axis should be rather close to the
optical axis, otherwise it will take extra time to find the flakes after the rotation.
• The construction by HQGraphene is a bit different from the one considered in this paper as its
xy-manipulator (TTS Holder) is placed onto another bigger manipulator (substrate holder). Such
a placement is potentially a fruitful idea, as it suppresses mutual vibrations of the substrate and
TTS and can help to avoid extra degrees of freedom.
• Note that the machine is assembled from very cheap components because there is no need in
sub-micrometer resolution positioning. For our machine we estimate the precision of flakes
alignment as 2 micrometers, that comes from diffraction limit of the microscope, vibrations
and viscoelastic properties of PDMS, those lead to slight shift of flakes in the process of
TTS-to-substrate contact.
• The transfer with the optimized machine might be very fast and takes just a few minutes
provided that the stamp is properly prepared. One drop op PDMS might be used many times.
However, sometimes the transfer might be unsuccsessfull. This is because there are hidden
controlling parameters those crucially affect the properties of thermosensitive polymer, such
as humidity, thickness of the polymer layer, freshness of the PDMS drop, preparation of the
substrate surface etc. These parameters in turn determine the optimal temperature regime and
should be maintained stable for a reproducible process. Seaching for flakes and controlling these
parameters takes much more time than the transfer itself.
• In Refs. [10,16] a Peltier element, like this one, is used instead of resistive heater. This element is
faster, though it is not compatible with vacuum fixation, and the authors have to use double-sided
scotch-tape.
4. Conclusions
We demonstrate the guiding principles for assembly of cheap and multi-functional transfer
machines for Van der Waals heterostructures taking our own machine as an example. There are many
fruitful ideas that can help to improve such a machine. This paper aims to help those who want to
enter the Van der Waals heterostructures field with maximal outcome per minimal expenses.
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Author Contributions: S.G.M. and N.K.Z - methodology, visualization and writing the manuscript, M.V.P. and
A.I.D. - [TO ADD], A.Y.K. - conceptualization, supervision. All authors have read and agreed to the published
version of the manuscript.
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Funding: The work is supported by Russian Foundation of Basic research (RFBR) Grant No. 18-32-20202.
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Acknowledgments: The work was done in Shared Facility Center of the P.N. Lebedev Physical institute.
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Conflicts of Interest: “The authors declare no conflict of interest. The funder had no role in the design of the
study, the decision to publish the results; assembling the setup and writing of the manuscript.
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Abbreviations
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The following abbreviations are used in this manuscript:
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SH
TTS
PVA
CVD
PPC
PID
PVC
PMMA
PC
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Substrate holder
Transparent temporay substrate
Polyvinylalcohol
chemical vapour deposition
Poly-propilene carbonate
Proportional-integral-differential (controller)
poly-vinyl chloride
poly-methyl meta acrylate
personal computer
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Submitted to Nanomaterials for possible open access publication
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(http://creativecommons.org/licenses/by/4.0/).