Nanorobotic handling of few-layer graphene membranes using a combined AFM/SEM/FIB setup

Sören ZimmermannTechnology Cluster Automated NanohandlingOFFIS – Institute for Information Technology

D – 26121 Oldenburg, Germanyzimmermann@offis.de

Sergej FatikowDivision Microrobotics and Control Engineering

University of OldenburgD – 26111 Oldenburg, Germany

fatikow@uni-oldenburg.de

Abstract - This paper presents a new experimental approach for pick and place handling of few-layer graphene nanomembranes with side length of 10 µm and below. In the as-received condition, the membranes are freely suspended on a grid which is covered with a lacey carbon film. Using focused ion beam cutting and a nanorobotic driven tungsten tip, selected membrane-fragmentscan be separated from the grid and transferred to any chosen substrate with high accuracy. As an example, one membrane is placed on a hole with a diameter of 5 µm on a Si/SiO2 sample.Subsequently, the membrane is fixed and mechanically characterized. The paper investigates the advantages, the opportunities and the limits of this technique in particular with regard to possible applications.

Keywords- nanorobotic assembly, microscopic graphenetransfer, AFM based indentation

I. INTRODUCTION

The interest in two-dimensional (2D) materials - especially graphene, a monolayer sheet of covalently bonded carbon atoms arranged in a honeycomb lattice - continuously rises due to its unique electrical and mechanical properties [1, 2].Although the discovery [3] lies only few years back, graphene is one of the most active research areas and seems to be predestined for a wide range of applications, e.g. nano-electronics [4, 5], supercapacitors [6] and biological devices [7]. Despite of the significant progress achieved in recent years, graphene-based technology is still in its infancy and needs further research and development. The reproducible and non-destructive handling of graphene membranes towards manufacturing and testing of prototype like structures is important for the ongoing evolution of this technology. Until now, most research groups use conventional production procedures for graphene based device fabrication well-known from semiconductor technology. These techniques are established and compatible with mass production but they suffer from time consuming, rigid and irreversible production steps. Within this paper, a first approach towards nanorobotic handling of graphene nanomembranes is presented, which is amore flexible alternative for prototypic graphene device production. Freely suspended few-layer graphene nanomembranes placed on a lacey carbon film were cut out and transferred to a desired test structure without loss or damage of the target membrane. Subsequently, the membrane was placed, fixed and mechanically characterized. Afterwards, the membrane can be easily transferred and manipulated again.

The paper is structured as follows: Section II gives an introduction to the experimental setup and environment. Section III explains the handling strategies, and their experimental execution is shown in section IV. Finally, section V summarizes the result.

II. SETUP FOR MANIPULATION TASKS

Handling of nanosized objects like few-layer graphene membranes requires an adapted experimental setup. Due to the extraordinary high surface to volume ratio and the resulting adhesive forces, the manipulation instruments have to be selected carefully. The used graphene samples are commercially available (Graphene Laboratories Inc.) and usually applied as a transmission electron microscope (TEM) sample holder. After the synthesis by chemical vapordeposition on Nickel substrates, the graphene is transferred on an aluminum grid with a mesh size of 50 µm using a macroscopic transfer process [8]. The grid is covered with a lacey carbon net, so that the transferred graphene film is partially freely suspended. This is advantageous for preparation of suitable specimen. The freely suspended graphene film consists of one to six monolayers, which corresponds to athickness of 0.3 to 2 nm. Preparation of suitable graphene nanomembranes is done by ion beam cutting of the graphene film. Subsequently, these nanomembranes can be handled by a nanorobotic setup using a tungsten tip, which is prepared by an electrochemical etching process in sodium hydroxide, or a commercially available piezoresistive atomic force microscope (AFM) cantilever.

The robotic handling setup and the experimental environment are described in detail in the following.

A. Robotic Handling SetupThe robotic manipulation setup consists of two independent

stages. Both stages are mounted on the positioning platform of a scanning electron microscope as illustrated in Figure 1. This platform places the whole setup into the electron beam. The two stages have different positioning accuracy and travel range, so that they can serve as a coarse or a fine positioning system, respectively:

� The coarse positioning stage has three linear axes (SmarAct SLC) which are orientated perpendicular to each other. All axes are slip-stick driven and equipped with an internal sensor allowing a closed-loop mode

2012 International Conference on Manipulation, Manufacturing and Measurement on the Nanoscale (3M-NANO) 29 August - 1 September 2012, Xi’an, China

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with an accuracy of a few nanometers. The x- and the y-direction axes have a maximal stroke of 35 mm, whereas the z-direction has a maximal stroke of 27 mm. This stage serves as a sample holder and enables fast switching between different areas of interest on one sample or between two different samples as well. This is of particular advantage because treated objects attached to the fine positioning system are not exposed to considerable vibrations which can occur using the slip-stick mode.

� The fine positioning stage (PI Hera) is implemented as a stack and has also three orthogonally arranged axes with a stroke of 50 µm for the z-direction and 100 µm for the x- and the y-direction. The stack is piezo-driven and equipped with capacitive positioning sensors allowing a closed-loop accuracy of at least 1.6 nm. This stage is used for nanomanipulation treatments of the graphene membranes. The respective handling-instrument can be easily affixed to this stage and exchanged in order to facilitate different manipulation tasks.

The fine positioning system in tandem with the coarse positioning system promotes the interplay of processing macroscopic work spaces (millimeter range) with nanometer precision. This capacity is of significant importance for transfer, characterization and manufacturing tasks of nanoscale objects.

Figure 1 Technical drawing of the experimental setup for nanorobotic manipulation. The entire setup is mounted inside an HR-SEM/FIB system.

B. Experimental EnvironmentThe whole robotic manipulation setup is placed in the

vacuum chamber of a high resolution scanning electron microscope (HR-SEM) with a Schottky field emission gun(“Lyra” by TESCAN). The HR-SEM enables the observation of the handling procedures giving fast and direct visual feedback. This ability is necessary because if drift and creeping of the entire experimental setup occurs during the handling procedure the high accuracy of the nanorobotic assembly cannot be exploited sufficiently. Additionally, to the electron microscope, the chamber is equipped with a focused

ion beam (FIB) and a gas injection system (GIS). The FIB uses a liquid gallium metal source as reservoir for the generation of gallium ions which are accelerated by high voltage (20 kV) and focused on a small area on the target surface. By careful adjustment of the ion current the target surface can be structured by FIB area milling or FIB line cutting, respectively. The GIS system provides different precursor gases which are emitted in the immediate proximity of the target surface. The electron beam can be used to generate a huge amount of secondary electrons near a defined surface area. Subsequently, these secondary electrons decompose the precursor gas, which results in a deposition process. In this case (trimethyl)-methyl-cyclo-penta-dienylplatinum serves as the precursor gas. The products of the decomposition reaction form an amorphous solid layer on the target surface. This electron beam induced deposition (EBiD) can be applied to fix nanomembranes on previously structured sample areas.

III. HANDLING STRATEGIES

The aim of the nanorobotic handling of graphene layers is the placement of selected membrane fragments on predefined substrates for manipulation and characterization experiments. This handling strategy is illustrated in Figure 2 and essentially includes six process steps:

A. InspectionThe TEM grid contains only a few membranes which are

suitable for the handling experiment. The membrane has to be undamaged, of an appropriate size and easy accessible. For finding such samples the TEM grid is systematically inspected using the HR-SEM.

B. SeparationThe selected membrane is separated from the surrounding

film using focused ion beam cutting. In a first step anchor holes for handling with the tungsten tip are created. Subsequently, the membrane is cut out and attached to the tungsten tip using the nanorobotic fine positioning stage and the visual feedback of the HR-SEM.

Figure 2 Strategy for the transfer handling of the graphene nanomembrane: (a) - a suitable membrane on the lacey carbon film, (b) - the separation

procedure using the focused ion beam, (c) - the transfer process using the tungsten tip and (d) - the transferred membrane on the target substrate.

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C. TransferThe membrane sticks to the tip after the separation

procedure and is lifted by the fine positioning stage. Afterwards, the coarse positioning system can be used to exchange the substrate within a few seconds.

D. PlacementThe graphene layer can be placed on any desired substrate

with high accuracy. To achieve this, the membrane is carefully approached to the substrate. Due to the small contact surface between tip and membrane, the adhesive forces between the substrate and the membrane exceed those between tip and membrane as soon as the membrane touches the substrate. In this way the tip can be easily removed from the graphene layer.

E. AttachementDepending on the substrate and the planned experiment, the

adhesive forces itself can be sufficient to attach the membrane. In addition EBiD can be applied to fix the layer mechanically or to connect it electrically.

F. ManipulationFinally, different manipulation and characterization

measurements can be performed. Therefore different manipulation tools can be applied, e.g. a piezoresistive cantilever.

IV. EXPERIMENTAL PROCEDURE

In order to enable FIB cutting, SEM observation and nanorobotic manipulation at the same time without reorientation, the SEM stage is tilted to an angle of 55° towards the electron gun of the SEM which corresponds to a perpendicular orientation towards the gallium ion gun. The tungsten tip is positioned close to the TEM Grid so that suitable membranes are located within the range of the fine positioning system.

Once a suitable membrane is found, a single focused ion beam image has to be aquired in order to identify the position of the target membrane and to get orientated with the focused ion beam. This image should be performed with low ioncurrent (approximately 10 pA with a scanning speed of 0.6 µs/pxl) to avoid damage of the membrane and to reduce defect generation. Before this experiment, several ion beam images were captured on different test membranes to ensure that the chosen ion beam current is appropriate to reach a sufficient resolution and to avoid observable damage. A SEM image of the selected membrane is shown in Figure 3. This membrane is characterized by a homogeneous layer, a round shape but also by a slightly wrinkled surface. Furthermore it is surrounded by a lacey carbon net which supports the graphene layer.

The first step in the FIB cutting procedure is the generation of the anchor holes for the nanorobotic handling. Directly after this step it can be clearly observed that the membrane is tightened which might be due to an increased surface tension caused by the modification of the surrounding area. In the further course of the cut-out-process the layer is severed near the anchor holes using FIB line cutting. Afterwards, the tungsten tip can already be inserted into the anchor hole which is shown in Figure 4 (a). The last separation steps are not

performed with line cutting steps, due to the fact that the graphene layer tends to erect while using high ion current densities. Subsequently, this can result in damage of the target membrane. To avoid this, the last cutting steps are realized using FIB area-milling which minimizes the ion current density

Figure 3 SEM images of the graphene nanomembrane on the lacey carbon film before (above) and after the generation of the handling holes. The

tungsten tip is already placed close to the membrane.

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Figure 4 SEM images of the membrane: (a) - attached to the tip, (b) - after thetransfer process and (c) - after the EBiD fixation procedure.

and prevents the erecting tendency of the almost freestanding membrane. Finally, the graphene layer is completely separated from the lacey carbon net, sticks to the tungsten tip and is ready for the transfer process.

The transfer procedure is a combination of movements of fine and coarse positioning system. The fine positioning system lifts the membrane fragment carefully and the coarse positioning system exchanges the substrate. The attached membrane is approached to a slightly p-doped silicon substrate with a 300 nm thick silicon dioxide surface layer. A 5 µm diameter hole has been previously milled into this substrate using the focused ion beam. The graphene layer is positioned on this hole and laid down with the fine positioning system. As the adhesive forces between graphene and silicon dioxide are quite high [9] the graphene can be easily fixed on the substrate and the tip can be removed. Subsequently, the tip can be used to position and align the membrane on the hole. As the membrane is specially intended to be mechanically characterized, the membrane is additionally fixed to the substrate using EBiD. For this purpose the precursor gas (trimethyl)-methyl-cyclo-penta-dienylplatinum was fed into the vacuum chamber. In the presence of secondary electrons which are emitted when the electron gun is focused on the substrate, the precursor gas decomposes and forms a solid mixture of platinum and carbon on the edge of the membrane. In this way the membrane is mechanically connected to the substrate which is illustrated in Figure 4 (c).

With regard to the planned indentation measurement, the tungsten tip is replaced by a self-sensitive cantilever (SEIKO PRC 400) with a tip radius smaller than 20 nm.

The cantilever’s tip is placed above the center of the hole which is covered with the graphene membrane. Afterwards, anon-destructive indentation of the membrane is performed several times. Before the indentation, the cantilever has been calibrated on the silicon substrate in order to determine the bending of the cantilever �z, as illustrated in Figure 5. The indentation measurements are corrected via subtracting �zfrom the measured values. A resulting graph showing the applied force versus the indentation depth is presented in Figure 5.

For the determination of the elastic properties of nanomembranes, several approaches were proposed with slightly different models [10, 11, 12]. In accordance with [10],the relationship between force F, the deflection at the center point of the membrane �, the membrane diameter a, the pretension in the film ���and the Young’s modulus E is expected as follows:

The dimensionless constant q = 1.02 results from the assumption that the Poisson’s ratio of a graphene layer can be described as graphite in the basal plane. The fact that the membrane might consist of more than one monolayer is neglected as the exact thickness has not been previously

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determined. 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 shown in Figure 5.

Figure 5 Illustration of the measurement principle of the AFM based indentation (above) and measured values evaluated with a cubic fit function.

Based on (1) the results for the Young’s modulus E and the pretension ��are determined as:

E = (1690 ± 157) N/m,

��������±� ����m.

In comparison to previous results for monolayer graphene [10] 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 due to the fact that the graphene membrane consists of several layers. The pretension value is mainly influenced by the alignment and fixation procedure using EBiD. The value is comparable to that measured on larger graphene flakes with strong adhesive bonding to the substrate.This indicates high stability of the platinum depositions aswell as the absence of strong prestresses inside the transferred membrane.

V. CONCLUSION AND OUTLOOK

The handling of micrometer size graphene nanomembranes using a nanorobotic setup in combination with a focused ion beam and visual feedback of a scanning electron microscope has been presented. This method facilitates a reliable way to

pick and place selected membrane fragments on predefined substrates with high accuracy. Prerequisites for this handling strategy are freely suspended graphene membranes that are accessible to a tungsten tip. The transferred membrane can be characterized and manipulated using the same setup.

This handling strategy was found to be highly reproducible in further experiments. However, the potential number of membranes that can be transferred using this approach is limited due to several time-consuming manual worksteps. On the contrary, the advantages of this technique are obvious: Graphene membranes can be characterized before being transferred to their final position with high accuracy. This aspect is of great importance for the prototyping of graphene based devices. Macroscopic graphene-transfer-processes [13, 14] enable transfer of large-area graphene on any desired substrate, but they lack precision and selectivity. In addition, the nanorobotic approach is promising, since the setup can also be used to handle different micro- and nanosized objects that are not accessible to the planar semiconductor technology, e.g. nanowires [15]. This allows the assembly of devices com-bining one and two-dimensional nanomaterials, which is a unique feature of nanorobotic handling.

Upcoming work will focus on the evaluation of the electrical properties of the membrane before and after the transfer process. In this way, it can be evaluated whether the FIB treatment causes any damage to the membrane. Based on these results the handling procedure can be reconsidered and modified. Furthermore, future work will focus on the production of devices combining one-dimensional materials, such as nanowires, and two-dimensional materials such as nanomembranes.

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