Abstract— This paper presents a nanorobotic approach facilitating the transfer and characterization of individual graphene flakes that are grown by different fabrication techniques. The approach makes use of a nanorobotic atomic force microscope system that is integrated into a high resolution scanning electron microscope and focused ion beam device. This combination is used to perform both, the nanorobotic transfer and the mechanical characterization of the graphene flake allowing to systematically analyze different sample areas and to optimize the fabrication processes. Furthermore, the nanorobotic system enables the reliable pick-and-place handling and processing of graphene flakes to realize more comprehensive analysis steps or even the prototyping of graphene-based devices.

I. INTRODUCTION

One- and two-dimensional materials have become an emerging research direction in nanotechnology. Especially carbon-based materials such as carbon nanotubes (CNTs) [1] and graphene [2] are promising materials in semiconductor industry. The latest edition of the international technology roadmap for semiconductors (ITRS) 2011 clearly identifies these two materials for beyond CMOS logic [3], where graphene and CNTs are not only explored as electrical interconnects to potentially dramatically reduce electrical resistance and improve energy efficiency of computing but also considered as alternate channel materials with higher field effect mobility. Compared to CNTs, graphene has the big advantage and ability of being processed in a planar form. For these reasons, graphene has an exceptional potential for various applications [4].

The preferred long-term goal for a direct fabrication and integration of graphene would be a CMOS compatible growth process on a silicon wafer. Different fabrication techniques have been demonstrated so far. The most common techniques are exfoliation from highly oriented pyrolytic graphite (HOPG) [5], epitaxial sublimation of silicon from silicon carbide [6] or chemical vapor deposition (CVD) on metal substrates [7]. While mechanical exfoliation offers a cheap and simple way to get comparable high-quality graphene, it is limited in terms of flake size and scalability. On the other hand, CVD and epitaxial growth can be realized on wafer-scale but still lead to graphene islands with limited size and different lattice orientations of several grains. Furthermore, additional transfer techniques are required to place graphene onto the desired target substrate [8].

S. Zimmermann is with the Technology Cluster Automated

Nanohandling (TCANH), Institute for Information Technology (OFFIS), Oldenburg, Germany (phone: +49-441-798-4331; fax: +49-441-798-4267; e-mail: zimmermann@offis.de).

V. Eichhorn and S. Fatikow are with the Division Microrobotics and Control Engineering (AMiR), Department of Computing Science, University of Oldenburg, Germany (e-mail: volkmar.eichhorn@uni-oldenburg.de).

Nevertheless, the CVD-based growth seems to be the most promising approach to become compatible with standard CMOS processes in the near future. For example, [9] and [10] reported the possibility of transfer-free CVD-based device fabrication of transistors.

Before these CVD processes become capable of being fully integrated into CMOS production lines, nanorobotic handling systems and techniques are a promising approach to rapidly close this gap between bottom-up and top-down fabrication methods and to facilitate the prototypic integration of novel nanomaterials into existing microsystems [11]. This can speed up the required time for iteration loops in the development of nanoelectronic devices and thus can support the entering of novel nanomaterials into the silicon-based CMOS era [12].

In this paper, a nanorobotic technology is presented that facilitates a flexible manipulation and transfer of individual CVD-grown graphene flakes as well as their systematic mechanical characterization. This approach will not only help to improve the fabrication technique itself but can also be used to build up prototypic graphene-based devices. The paper is structured as follows: In section II the nanorobotic technology is introduced by describing the robotic system and its microscopic environment. The novel transfer and characterization strategies for graphene flakes are described in section III and IV. The experimental results for the transfer and mechanical characterization are presented in section V. Finally, a conclusion is given in section VI.

II. NANOROBOTIC TECHNOLOGY

The handling of two-dimensional nanomaterials such as few-layer graphene flakes requires a high-resolution microscopic and nanorobotic setup. Due to the extraordinary high surface to volume ratio and the resulting adhesion forces, the manipulation endeffectors and strategies have to be selected carefully. The graphene samples used in this paper are commercially available and usually applied as a transmission electron microscope (TEM) sample holder. First, the graphene is being synthesized by chemical vapor deposition on a nickel substrate and then transferred onto a standard aluminum TEM grid with a mesh size of 50 μm. The TEM grid is covered with a supporting film of lacey carbon, so that the transferred graphene layer is freely suspended and can be used to analyze the graphene without the direct influence of an underlying substrate. The suspended graphene membranes typically consist of one to six monolayers with a corresponding thickness of 0.3 to about 1.8 nm. Since the robotic system is integrated into a dual beam scanning electron microscope (SEM) and focused ion beam (FIB) device, the inspection of graphene samples and identification of a suitable sample area as well as the preparation and structuring of graphene nanomembranes can be easily performed within the same environment.

Nanorobotic Transfer and Characterization of Graphene Flakes Sören Zimmermann, Volkmar Eichhorn, and Sergej Fatikow, Member, IEEE

2012 IEEE/RSJ International Conference onIntelligent Robots and SystemsOctober 7-12, 2012. Vilamoura, Algarve, Portugal

978-1-4673-1736-8/12/S31.00 ©2012 IEEE 640

Subsequently these graphene flakes can be handled by the nanorobotic system using a tungsten tip as endeffector, which is prepared by an electrochemical etching process in sodium hydroxide. Once the graphene flake is transferred from the TEM grid onto a special testing substrate, it can be mechanically characterized by using commercially available piezoresistive AFM probes to perform bending and indentation experiments on the suspended graphene flake.

A. Microscopic Environment The whole nanorobotic manipulation system is placed in

the vacuum chamber of a high resolution scanning electron microscope (HRSEM). The HRSEM enables the inspection and observation of samples and handling procedures by providing real-time visual feedback. This ability can be used to compensate for drift and creep of the entire experimental setup that may occurs during the nanohandling procedure in order to perform the nanorobotic assembly task with high accuracy. In addition to the electron microscope, the microscopic device is equipped with a focused ion beam (FIB) and gas injection system (GIS).

The FIB uses gallium ions which are accelerated by a high voltage of up to 30 kV and focused onto a small area of the sample surface. By carefully adjusting the ion current the sample can be structured by FIB milling. The GIS provides different precursor gases that can be fed into the vacuum chamber to the immediate proximity of the sample surface. The electron beam can then be focused onto a defined sample area in order to generate secondary electrons. Subsequently these secondary electrons decompose the precursor gas resulting in a selective deposition process. Different precursor gases are available to deposit tungsten, platinum as well as silicon oxide. In the experiments presented in section V, such an electron beam induced deposition (EBiD) of platinum is used to fix the graphene flake on the specially shaped testing substrate.

B. Nanorobotic System The nanorobotic system is reported in [13] and consists of

two independent positioning stages. Both stages are mounted onto the motorized stage of the scanning electron microscope facilitating the alignment of the whole nanorobotic system with respect to the electron and ion beam. The two robotic stages have a different positioning accuracy and travel range so that they can serve as coarse and fine positioning units, respectively:

The coarse positioning stage has three linear axes that are orientated perpendicular to each other. All axes are slip-stick-driven piezo actuators equipped with internal sensors enabling a closed-loop mode with an accuracy below 50 nm. The lateral x- and y- axes have a stroke of 35 mm whereas the z-axis has a stroke of 27 mm. This coarse positioning stage serves as a sample holder and enables fast switching between different areas of interest on one sample or even between two different samples. This is of particular advantage when performing pick-and-place operations of nanoobjects that are transferred from a sample substrate onto a target substrate.

The fine positioning stage is implemented as a stack of three orthogonally arranged axes. The lateral axes

have a stroke of 100 μm, while the z-axis offers a stroke of 50 μm. The piezo-driven stack is a friction-free and extremely stiff flexure actuator and equipped with capacitive positioning sensors allowing a closed-loop accuracy of 1.6 nm. This fine positioning stage carries an etched tungsten tip as manipulation tool and is used to perform the final handling or characterization tasks of the graphene flake.

This combination of a fine and coarse positioning stage promotes the interplay of working on macroscopic work spaces with nanometer precision, which is of significant importance for the transfer, characterization and processing of two-dimensional nanomaterials such as graphene.

III. HANDLING STRATEGY FOR GRAPHENE FLAKES

The aim of the nanorobotic handling and transfer of graphene flakes is the placement of selected membrane fragments onto specially shaped testing substrates for characterization experiments as well as device prototyping. Such a handling strategy can be divided into six subsequences as illustrated in Fig. 1:

A. Inspection The TEM grid contains only a few areas of graphene

membranes that are suitable for the handling and characterization experiments. The graphene membrane has to be undamaged, free of contaminations, of appropriate size and easily accessible. For locating such sample areas, the TEM grid is systematically inspected using the HRSEM.

B. Separation The selected graphene membrane fragment is separated

from the surrounding lacey carbon film using focused ion beam cutting. In a first step, finger holes for the later nanohandling process with the tungsten tip are created. Subsequently the graphene flake is cut out until only a small bond bridge is remaining and holding the flake.

Figure 1. Strategy for the nanorobotic transfer of the graphene

nanomembrane: A. Inspection and locating of a suitable graphene flake on the lacey carbon film. B. Separation procedure of the flake and creation of the finger holes using FIB cutting. C. Picking up the graphene flake with a

tungsten tip. D. Transferring the flake to the testing substrate. E. Placing the graphene flake onto the characterization test bed. F. Attaching the graphene

flake to the substrate by using EBiD.

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C. Picking The etched tungsten tip is approached to graphene flake

by moving the coarse positioning stage. The graphene flake is then picked up by inserting the tungsten tip into the finger hole using the fine positioning stage. This nanorobotic sequence is supported by visual feedback of the HRSEM. After the bond bridge has been removed, the completely cut out flake sticks to the tungsten tip.

D. Transfer The graphene flake is then carefully lifted by the fine

positioning stage and transferred to the target characterization substrate by exchanging the samples using the coarse positioning stage.

E. Placement The graphene flake is placed onto the desired substrate

area with high accuracy. For this purpose, the flake is carefully approached to the target substrate. Due to the small contact surface between the tungsten tip and the flake, the adhesive forces between substrate and flake exceed those between tip and flake as soon as the graphene flake touches the substrate surface. For this reason, the tungsten tip can be easily retracted from the graphene flake.

F. Attachment EBiD of platinum or tungsten can be used to

mechanically and electrically fix the graphene flake to the characterization test bed. Compared to FIB-induced deposition, EBiD is preferred allowing the deposition of more precise structures and avoiding the damaging of the graphene flake. Different manipulation and characterization measurements can be performed afterwards. In this work, we focus on the mechanical characterization of the graphene flake using self-sensing piezoresistive AFM probes.

IV. CHARACTERIZATION STRATEGY FOR GRAPHENE FLAKES

The mechanical characterization of the graphene flakes is realized by performing deflection and indentation experiments on the suspended graphene flakes. The prior nanorobotic transfer onto a specially structured substrate provides an excellent test bed with well- known geometric parameters. For the bending and indentation experiments, the nanorobotic system is equipped with a self-sensing piezoresistive AFM probe (SEIKO PRC 400) having a tip radius smaller than 20 nm. Fig. 2 illustrates the mechanical characterization procedure for a transferred and suspended graphene flake.

Figure 2. Test bed for the mechanical characterization of suspended graphene flakes. A self-sensing piezoresistive AFM probe is used to

measure characteristic force-displacement curves.

The tip of the AFM probe is placed above the center of the hole covered with the graphene flake. Afterwards a non-destructive deflection of the flake is performed and characteristic force-displacement curves are recorded. Finally, the piezoresistive AFM probe is calibrated on a hard reference substrate in order to estimate the bending of the cantilever itself depending on the applied force. The indentation measurements can then be corrected via subtracting from the measured values.

In order to determine the elastic properties of nanomembranes several approaches were proposed with slightly different models [14-16]. According to [14] we expect the relationship between the force , deflection at the center point of the membrane, membrane diameter , pretension of the graphene membrane and the Young’s modulus as follows:

(1)

The dimensionless constant results of the assumption that the Poisson's ratio of a graphene layer can be described as graphite in the basal plane.

V. EXPERIMENTAL RESULTS

A. Transfer of Graphene Flakes In order to enable FIB cutting, HRSEM observation and

nanorobotic manipulation at the same time the SEM stage is tilted to an angle of 55° towards the ion gun of the microscopic system corresponding to a perpendicular orientation of the sample surface towards the gallium ion gun. After a suitable graphene flake is found and centered in the SEM image, the tungsten tip is positioned near the TEM grid so that the graphene flake is located within the range of the fine positioning system. Fig. 3 (a) shows the SEM image of the selected graphene flake. This flake is characterized by a homogeneous layer and is surrounded by a lacey carbon net supporting the graphene flake. A focused ion beam image has to be captured in order to align the ion gun. This image should be recorded with low ion current to reduce the damage and defect generation on the graphene sample. For this purpose, several focused ion beam images were captured on different test graphene flakes, before starting the planned experiments, to ensure that the chosen ion beam current is appropriate to reach a sufficient resolution without creating observable damages.

Figure 3. SEM images of the graphene flake on a lacey carbon film before

(a) and after the FIB milling of the finger holes (b). The tungsten tip is already placed close to the graphene flake. Three finger holes are created to

have different points of contact.

(a) (b)

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The next step is the generation of finger holes for the nanorobotic handling using FIB milling (compare Fig. 3 (b)). After this FIB milling step a straightening of the graphene flake can be clearly observed. This increased surface tension can be caused by the modification of the surrounding area.

The graphene flake is then separated from the overall lacey carbon film by performing FIB line cutting around the area of interest. In this case two supporting bond bridges are maintained still supporting the graphene flake. Afterwards the tungsten tip is aligned to the nanomembrane and inserted into the finger hole as shown in Fig. 4(a). The last separation steps are not performed by FIB line cutting due to the fact that the graphene flake tends to bend while using high ion current densities, which might result in damaging the graphene membrane. To avoid this, the last separation steps are realized by FIB area milling which minimizes the ion current density and prevents the bending of the almost freestanding nanomembrane. Finally, the graphene flake is completely separated from the lacey carbon film and sticks to the tungsten tip.

Now the graphene flake is transferred to the specially structured testing substrate, which is a p-doped silicon wafer with a 300 nm thick silicon dioxide surface layer. A 5 μm diameter hole has been previously FIB-milled into this substrate providing a well-known test bed for the mechanical characterization. The transfer procedure is a combination of movements of the fine and coarse positioning stage. The fine positioning stage lifts the graphene flake while the coarse positioning stage exchanges the substrate. The attached graphene flake is approached to the testing substrate and then placed onto the hole using the fine positioning stage (compare Fig. 4 (b)). Since the adhesion between the graphene flake and the silicon dioxide substrate surface is very strong [17] the graphene flake can be easily released from the tungsten tip. Nevertheless, the tungsten tip can be used to position and align the graphene flake on the testing structure afterwards to perfectly center the graphene flake on the hole. Since the nanomembrane is specially intended to be mechanically characterized, the graphene flake is additionally fixed to the substrate using the GIS and EBiD of platinum. In this way, the graphene flake is mechanically connected to the substrate as illustrated in Fig. 4 (c).

B. Mechanical Characterization of Graphene Flakes For the mechanical characterization of graphene flakes

the tungsten tip is replaced by a piezoresistive AFM probe. The whole nanorobotic system is then tilted up to 80° using the motorized SEM stage to provide free access and to observe the tip sample interaction. The tip of the AFM probe is then approached to the graphene flake by using the coarse and fine positioning stage. After the AFM probe tip is centered over the circular test structure, the position of the sample is kept fix and only the fine positioning stage is moved to perform the bending and indentation of the graphene flake. Fig. 5 shows two SEM images of the mechanical characterization of graphene flakes. It has to be mentioned that the SEM images show the deflection of a graphene flake suspended on a TEM grid. Fig. 5 (a) shows the AFM probe in close proximity to the graphene flake, while the graphene flake is already slightly deflected by the AFM probe in Fig. 5 (b).

Figure 4. SEM image sequence showing the nanorobotic transfer of the graphene flake: (a) Picking up the graphene flake by the tungsten tip. (b)

Placing the flake onto the target structure. (c) Attaching the graphene flake using EBiD of platinum bonds.

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Figure 5. SEM images of a typical mechanical characterization experiment

on a graphene flake suspended on a TEM grid. The whole nanorobotic system is tilted by 80°. (a) The tip of the self-sensing piezoresistive AFM probe is in close proximity to flake. (b) The AFM probe is deflecting the

graphene flake while measuring the acting forces.

Characteristic force-displacement curves are recorded. For a deflection of up to 300 nm the typical force acting between graphene flake and AFM probe is in the range of hundreds of nN and single μN. Fig. 6 shows the graphical representation of such a characteristic force-displacement curve. The measurements are evaluated using a cubic fit function with fixed coefficients equal to zero for the constant and the quadratic term and free coefficients for the linear and the cubic term. The results of the fit are given in Fig. 6 as well and can be used to calculate the Young’s modulus and pretension of the suspended graphene flake.

Based on equation (1) the resulting Young’s modulus is E = (1690 ± 30) N/m and pretension is = (0.51 ± 0.01) N/m. In comparison to previous results for monolayer graphene [14], the value of the pretension is in the same range whereas the Young’s modulus is significantly larger by a factor of five, which might be caused by the fact that the graphene fake consists of several layers. No detailed layer-dependent analysis could be done in this proof-of-concept experiment since the exact thickness of the graphene flakes was not determined previously. For this reason, more systematic experiments will be performed in the near future on different graphene flake samples with well-known number of monolayers. The number of layers can be either determined by micro Raman spectroscopy or by topographic AFM analysis.

Figure 6. Characteristic force-displacement curve for the mechanical

characterization of graphene suspended on the circular test substrate. The measured values are evaluated with a cubic fit function.

C. AFM probe-based Handling of Graphene Flakes The piezoresistive AFM probe used for the mechanical

characterization can also be applied as an alternative nanohandling endeffector. For this purpose, a haptic device can be coupled to the nanorobotic system facilitating the AFM-based handling of graphene flakes with force feedback provided by the AFM probe and visual feedback provided by the SEM. Fig. 7 shows SEM images of an AFM probe-based pick-and-place operation of a freestanding vertically aligned graphene flake that is produced by mechanical exfoliation. A fracture of the graphene flake is separated and released on the substrate surface again.

Figure 7. AFM probe-based handling of graphene flakes with both visual

and force feedback. A fraction of the freestanding vertically aligned graphene flake is separated (a) and released again on the substrate (b).

(a)

(b)

(a)

(b)

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VI. CONCLUSION The presented nanorobotic strategy facilitates the reliable

handling, characterization and processing of micrometer sized graphene flakes. This is achieved by combining a nanorobotic system and a focused ion beam, high resolution scanning electron microscope and gas injection system. After a suitable area of the graphene sample has been identified, the graphene flake is separated by focus ion beam milling and transferred onto a special test bed by nanorobotic pick-and-place handling using a tungsten tip as endeffector. Subsequently, this test bed enables the mechanical characterization of the graphene flake using a self-sensing piezoresistive AFM probe. Characteristic force-displacement curves are recorded and analyzed to calculate the pretension and Young’s modulus of the suspended graphene flake. In addition, the AFM probe can be applied as manipulation endeffector facilitating the AFM probe-based nanohandling of graphene flakes with both force and visual feedback.

The presented nanorobotic approach can not only be used for the mechanical characterization of graphene as discussed in this paper, but also enables a flexible technique for the assembly of novel prototypic graphene-based devices. In particular, this allows the assembly of novel devices based on different nanomaterials that cannot be achieved and combined by standard CMOS technologies.

Upcoming work will focus on the optimization and automation of the presented transfer and mechanical characterization process providing the basis for more systematic measurements that vice versa enable the optimization of the fabrication process itself. Furthermore, the quality of the graphene flakes before and after the transfer process has to be carefully analyzed to check if the FIB treatment causes any damages in the graphene membrane. A special test bed will be designed enabling the electro-mechanical characterization of graphene flakes. In addition, the use of graphene samples with a well-known number of layers will lead to more significant results in the near future.

ACKNOWLEDGMENT The authors thank Uwe Mick for his work on AFM

probe-based nanohandling experiments and for providing the SEM images shown in Fig. 7.

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