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Manipulation and in situ transmission electron microscope characterization of sub-100 nm

nanostructures using a microfabricated nanogripper

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2010 J. Micromech. Microeng. 20 035009

(http://iopscience.iop.org/0960-1317/20/3/035009)

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IOP PUBLISHING JOURNAL OF MICROMECHANICS AND MICROENGINEERING

J. Micromech. Microeng. 20 (2010) 035009 (7pp) doi:10.1088/0960-1317/20/3/035009

Manipulation and in situ transmissionelectron microscope characterization ofsub-100 nm nanostructures using amicrofabricated nanogripperAlberto Cagliani1,4, Rafal Wierzbicki1, Luigi Occhipinti2,Dirch Hjorth Petersen1, Karin Nordstrøm Dyvelkov1,5,Ozlem Sardan Sukas1, Berit G Herstrøm3, Tim Booth1 and Peter Bøggild1

1 DTU Nanotech—Technical University of Denmark, Building 345 east, DK-2800 Kgs. Lyngby,Denmark2 STMicroelectronics, Str. Primsole, 50-95121 Catania, Italy3 DTU Danchip—Technical University of Denmark, Building 347, DK-2800 Kgs. Lyngby, Denmark

E-mail: Alberto.Cagliani@nanotech.dtu.dk

Received 8 December 2009, in final form 10 December 2009Published 9 February 2010Online at stacks.iop.org/JMM/20/035009

AbstractWe present here a polysilicon electrothermal microfabricated nanogripper capable ofmanipulating nanowires and nanotubes in the sub-100 nm range. The nanogripper wasfabricated with a mix and match microfabrication process, combining high throughput ofphotolithography with 10 nm resolution of electron beam lithography. Vertically grown III–Vnanowires with a diameter of 70 nm were picked up using the nanogripper, allowing directtransfer of the nanogripper-nanowire ensemble into a transmission electron microscope (TEM)for structural characterization. By refining the end-effectors with focused ion beam millingand subsequently coating these with Au, the nanogripper could lift up laterally alignedsingle-walled carbon nanotubes from a 1 μm wide trench, while immediately making goodelectrical contact. One such carbon nanotube was structurally and electrically characterizedreal-time in TEM, showing a breakdown current density of approximately 0.5 × 1012Am−2.The nanogripper is the smallest microfabricated gripper to date and is the first tool showingrepeatable, 3D nanomanipulation of sub-100 nm structures.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

While robotic grippers are today a widespread concept inindustrial scale assembly, the relevance of direct deviceassembly using gripper-like tools on the micro- and nanoscaledepends on the emergence of compact, mechanically stable,user-friendly and reliable tools. Despite the availability ofcommercial micromanipulation tools [1], direct manipulation

4 Author to whom any correspondence should be addressed.5 Present address: Danish Technological Institute, Center forMicrotechnology and Surface Analysis, Gregersensvej, DK-2630 Taastrup,Denmark.

has not yet been established as a viable technique forhandling of structures in the sub-100 nm range, such ascarbon nanotubes [2, 3] and semiconductor nanowires [4, 5].Furthermore, an important issue for progress in utilization ofnanomaterials is the correlation of the structural and electricalproperties of an individual nanostructure, rather than just thecharacterization of representative specimens from a greaterensemble. Nanomanipulation offers a fast and direct approachto the prototyping of nanodevices, as well as electrical andstructural characterization inside scanning or transmissionelectron microscopes [6].

0960-1317/10/035009+07$30.00 1 © 2010 IOP Publishing Ltd Printed in the UK

J. Micromech. Microeng. 20 (2010) 035009 A Cagliani et al

Recently, microfabricated microgrippers were used todetach carbon nanotubes and nanowires from a fixed positionand transfer these to a TEM grid or a tip of an AFM probe[6, 7]. In these experiments, however, the manipulated wireshad a diameter in the 100–400 nm range, which is too largefor many 1D nanoelectronic applications, such as nanowiresensors [8, 9]. Kim and Lieber reported a gripper consistingof two multiwalled CNTs attached to a glass pipette, whichhandled sub-100 nm wires and beads [10]. The gripping forceof the nanotube-based gripper is limited by the low stiffness ofthe thin nanotube arms, and the electrostatic actuation principleemployed results in unwanted currents through conductivemanipulated objects. Finally, there is currently no knownway to mass-produce such grippers. Complex manipulatorsystems with several independently controlled sharp tips havealso been used for manipulation and mechanical and electricalcharacterization of carbon nanotubes [11–13]. Althoughthis method allows great flexibility in terms of adaptationto a specific manipulation task, simultaneous alignment ofseveral nanomanipulators is complex. Moreover, the methoddoes not allow transfer of the gripped objects outside ofthe manipulation setup, limiting the possibilities of furthercharacterization and use of the nano-object.

We report here a mass-produceable electrothermallyactuated nanogripper, specifically designed for manipulatingobjects in the sub-100 nm range with rigidity and grippingforce sufficient for detaching nanotubes and nanowires, fromthe substrates on which they are initially grown. Moreoverby focused ion beam milling the tips of the nanogripper, itcan be made small enough to enter sub-μm cavities. Finallywe demonstrate a powerful approach to TEM characterization,where the nanogrippers act to detach, transport and electricallycharacterize a nanostructure, as well as playing the role of theTEM grid.

2. Design

We use a simple mix and match process to scale down thetopology optimized microactuator design reported previously[6]. This allows us to manipulate objects an order ofmagnitude smaller than previously reported with a mass-produced gripper. This actuator geometry provides sufficientmechanical strength to break off nanotubes and nanowires asgrown on a substrate. We consider two variants of the topologyoptimized design, termed A and B (figure 1), with scaled-downactuator length, device layer thickness, end-effector gap andminimum line width. The arms were shortened from 200to either 50 or 35 μm, to keep the nanogripper as compactas possible. To reduce out-of-plane bending, the aspectratio was kept above 2 in all designs. If the actuator armsbecome too narrow, the spring constant and thus the availablegripping force may be reduced. As a compromise between asmall overall size, a small out-of-plane bending and a largegripping force, a device layer thickness of 1 μm was chosen,as compared to 5 μm in [6, 7, 14]. Keeping the height ofthe actuator arms relatively small is moreover crucial to avoidblocking of line-of-sight during manipulation. As a result of

(a)

(b)

Figure 1. (a) SEM micrographs of a 50 μm long nanogripper(design A). (b) A 35 μm long nanogripper (design B). The anchorpoints of the nanogrippers are marked with a dashed line.

the scaling down the minimum line width was reduced from 2to 0.3 μm.

Compared to [6], the gap between the actuator arms wasreduced from 2 to 0.5 and 0.3 μm. A smaller actuatorgap size g directly leads to a larger gripping force throughFgrip = k (A − g/2 + R), where k is the spring constant, A isthe free actuation range of the actuator and R is the radius ofthe gripped object [14].

3. Fabrication

The nanogrippers were fabricated in a 1 μm thick layerof highly boron-doped polysilicon. The actuator structureswere defined with electron beam lithography, while the large-scale electrical connections and bond pads were realizedvia standard photolithography (see figure 2), using a hybridaluminium/photoresist etch mask described below. Thiscombines the high throughput of photolithography with the10 nm resolution of a JEOL JBX9300FS e-beam writer.Focused ion beam milling was used to further refine the shapeof the end-effector.

LPCVD was used to deposit a 1 μm thick boron-dopedpolysilicon device layer on top of a thermally oxidized Si wafer(250 nm SiO2/500 μm Si). A 60 nm thick Al etch mask forthe actuators was defined on the device layer using electronbeam lithography and lift-off. Photolithography was then usedto produce a photoresist etch mask for the electrical contactsand bond pads. The pattern of this Al/photoresist hybridetch mask was then transferred to the device layer via a two-step RIE process (described below), and the mask removed.A protective low-stress conformal LPCVD SixNy layer was

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J. Micromech. Microeng. 20 (2010) 035009 A Cagliani et al

(a)

(b)

(c)

(d )

(e)(i )

(f )

(g)

(h)

Figure 2. Mix-and-match fabrication process. (a) Poly-SOI wafer (1 μm polysilicon and 250 nm SiO2). (b)–(d) Deposition and lift-off of60 nm Al defining the nanoscale part (actuator structures) of the hybrid etch mask. (e) The remaining part (electrical connections) of thehybrid mask is defined by UV lithography. (f ) The pattern is transferred to the polysilicon with a two-step RIE process, and (g) the mask isstripped. (h) A conformal SixNy low stress layer is deposited for front- and backside protection in the following release. The remaining partof the process is described in [6, 7, 14]. (i) Illustration of areas defined by electron beam and photolithography.

deposited, and the structures released from the back side via acombination of UV lithography, RIE, KOH, phosphoric acidand buffered HF etches as described in [6, 7, 14].

A two-step RIE process is required to optimize thefunctioning nanogripper yield due to the high aspect ratioof some of the nanogripper features, e.g. the actuator gap.First, an SF6/O2 plasma RIE process is used to remove themajority of polysilicon—whilst this is effective in trencheswith low aspect ratios some residual polysilicon remains inhigh aspect ratio trenches. Extended etch times in SF6/O2

plasma results in undesired isotropic underetching due to thebuild-up of F∗ radicals in the plasma. Instead, a physicaletching with a CHF3/CF4/O2 plasma is used, once the SiO2

etch stop layer is reached in the low aspect ratio trenches,to remove the residual polysilicon whilst maintaining verticalsidewalls. The CHF3/CF4/O2 plasma could not be used toetch the entire device layer thickness due to low polysilicon:Al selectivity. The minimum actuator gap achieved throughlithographic means was 480 nm, with a yield of 30%, using a60 nm layer of Al for optimal resolution. Gaps of 300 nm andless were achieved by FIB milling (FEI Quanta 3D) of residualpolysilicon (figures 3(a) and (b)). FIB milling also allows therefinement of the end-effector geometry (figures 3(c) and (d))beyond that achievable via lithography.

4. Results and discussion

4.1. Actuation

The temperature distribution of the design B has beensimulated with COMOSOL R© and results very similar to thedistribution of the micro version with a temperature tip of325 ◦C when the jaws are closed and the gap is 300 nm(see figure 4) and 462 ◦C when the gap is 500 nm. Thistemperature range precludes manipulation of polymers, but is

(a) (b)

(c) (d )

Figure 3. (a) Tip of the nanogripper with a nominal gap of 300 nm.The polysilicon is not completely removed after the two-step RIEprocess. (b) Tip of the nanogripper after the FIB modification. Thepolysilicon is removed inside the gap. The modified gap is 330 nm.(c) A nanogripper which is simultaneously released and shaped byFIB milling. (d) The nanogripper after sharpening.

compatible with many metals, carbon nanostructures, as wellas the large range of nanowires such as group IV, group III–Vand ZnO nanowires. Figure 4 presents actuation curves forthree different nanogrippers of design A (see figure 1), as wellas a FIB-modified B-type nanogripper with an initial gap sizeof 280 nm, as measured by SEM. The data fit quadratic curvescharacteristic of electrothermal actuators [15, 16]. By repeatedapplication of bias voltage the nanogrippers can be closed

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J. Micromech. Microeng. 20 (2010) 035009 A Cagliani et al

(a)

(b)

Figure 4. (a) Temperature distribution of design B with 300 nm gapwhen the nanogripper is closed. (b) Three actuation curves (gapversus voltage) of three different nanogrippers of design A fit aquadratic curve. The full diamonds are the actuation data of aFIB-modified nanogripper of design B, with a gap size of 280 nm.

and opened reproducibly several times without any observablestiction between the jaws. Moreover we did not observe anykind of coupling between the mechanical vibrations of themanipulation system and the motion of the nanogripper. Thedifferent curvatures of actuation data are due to the fact thatthe total resistance of the device can vary from 20 k� up to80 k� layer. The reason is that a gold e-beam-depositedlayer was used to metalize just the contact pads, and thealignment of the shadow mask for this deposition is donefor every nanogripper manually under the microscope. Hencethe uncovered leads have a varying resistance. This, however,does not influence the operation of the nanogripper; it merelyrequires a higher voltage to be applied to some nanogrippersfor closing.

4.2. Nanomanipulation and TEM imaging of verticallyaligned nanowires

The performance of the nanogrippers was tested bymanipulation of metal organic vapour phase epitaxially grownGaAs-GaP-GaAs heterostructure nanowires [17, 18] withdiameters ranging from 30 to 80 nm. The nanomanipulationsetup was realized inside an FEI Quanta FEG 200 SEMmicroscope, using a Smaract 13 degrees of freedom (DOF)nanopositioning system. Two 3DOF positioners of the systemwere used; one for handling the nanowire substrate, and theother to position the nanogripper. Figure 5 shows imagesof a 70 nm diameter nanowire being picked from a growthsubstrate and the TEM micrographs (FEI Tecnai F20) of thenanowire. The pick operation was performed using a seriesof steps: (i) the nanogripper was aligned to the horizontal

(a)

(b)

(c)

Figure 5. (a) A gallium phosphide nanowire is picked up by ananogripper. (b) Low magnification TEM image showing the wireadhering to one of the end-effectors after transfer to the TEMchamber. (c) The nanowire is at this point encapsulated in a crust ofcarbonaceous material due to EBiD, which does not preventimaging of its crystal structure. The marked region in (c) is shownin the inset.

plane of the nanowire; (ii) the end-effectors were aligned sothat the nanowire was between the jaws; (iii) the nanogripperwas actuated, creating a mechanical contact to the nanowire;(iv) shear force eventually broke the nanowire at the base(figure 5(a)). A question arises with regard to current flowingthrough the nanowire via the potential. First of all, the voltageis supplied symmetrically, which means that the potentialdifference between the jaws is small. Second, the naturalpolysilicon oxide of the jaws is likely to reduce the currenteven further. We did not observe any indication of damagethat could be related to such scenario. After this processthe nanowire could be placed on a TEM grid as in [6],or alternatively, left adhering to the end-effector for direct

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J. Micromech. Microeng. 20 (2010) 035009 A Cagliani et al

(a) (b)

(c) (d )

Figure 6. Manipulation of a single wall nanotube bundle using a FIB-sharpened nanogripper. (a) The nanogripper approaches the trench.(b) The lower arm touches the sidewall and is deflected sideways to fit in the trench. (c) The nanogripper is positioned under the nanotubeand lifts it up. The nanotube sticks via van der Waals forces and possible carbonaceous deposition induced by the electron beam duringimaging.

transfer to the TEM chamber. While strong adhesion betweenthe nano-objects and the manipulator system is normally aserious problem for nanomanipulation [19], the adhesion inthis case facilitated a safe transfer from the manipulation setupinside the SEM chamber into the TEM chamber for structuralcharacterization, see figures 5(b) and (c). For manipulationof nano-objects in this size range, the release step is oftendifficult and unpredictable due to charging, adhesion, etc. Thisis avoided entirely by moving the nanogripper and nano-objectensemble directly to the TEM.

In general, SEM image distortion and drift due toelectrical field and thermal expansion appear more severethan that observed with larger microgrippers [6], simply dueto the relatively higher magnification required to operate thenanogripper.

4.3. Nanomanipulation and TEM imaging of laterallyaligned single walled nanotubes

Another characteristic type of nanostructure is the lateralbridge [3]. Since two terminal CNT bridges are oftenmade as prototypic or actual electrical devices, electricalcharacterization of specific devices is critical. We hereconsider laterally aligned single-walled carbon nanotubes,embedded in a 20–30 nm thick carbon crust. The nanotubeswere grown by Fe or Co-catalysed chemical vapour depositionacross 0.9–4 μm wide, 400 nm deep lithographically defined

SiO2 or polysilicon/SiO2 trenches on Si following the methoddescribed in [20], and were picked up and transferred to aTEM chamber for structural and electrical characterization.

In this case, a FIB-refined nanogripper was used as amechanically flexible two-point-probe with an electrode gap of200 nm. The gripping arms were coated with 70 nm Au by anAlcatel e-beam evaporator, to provide a good electrical contactto the CNTs. Picking up the nanowire with the sharpenedtips turned out to be straightforward. Figure 6(a) shows thenanogripper approaching the nanotube, while figures 6(b) and(c) show how first the upper arm, and then the lower arm entersthe trench below the nanotube. In figure 6(d) the nanotube islifted clear of the trench and is straightened out due to the gaprecovering its equilibrium size after release from the trench.

The nanogripper with the attached nanotube was removedfrom the SEM and transferred to an FEI Tecnai T20 TEM.The microscope was equipped with an electrically connectedTEM holder designed to accommodate the 1.5 × 3 mmsilicon microchip. Figure 7(a) shows a CNT embedded inthe carbonaceous crust, which formed on the nanotube duringmanipulation and imaging in the SEM. This electron beam-induced deposition is a well-known phenomenon in bothtransmission and scanning electron microscopy [7, 21]. Afterinitial structural inspection, the electric current was rampedslowly up, while TEM images were continuously recorded at200 keV with a frame rate of 1.5 s. At a current of 2 μA, thenanotube failed abruptly between two image frames, leaving

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J. Micromech. Microeng. 20 (2010) 035009 A Cagliani et al

(a)

(b)

(c)

Figure 7. (a) Nanogripper holding a nanotube. The nanotube isbridging two end-effectors which act as electrodes, while a currentis passed through (excerpt from a film of roughly 100 frames).(b) Left side of burned-off nanotube showing the single wallnanotube core embedded in the more irregular carbon crust.(c) Right side of burned-off nanotube with a short piece of materialwhich is possibly recrystallized (graphitized) carbon material fromthe heat generated by the current.

a gap of several hundred nm. Closer inspection of the TEMimages (figures 7(b) and (c)) indicates a carbon nanotube witha diameter of roughly 4 nm, embedded in a 25 nm wide crust ofcarbonaceous material. While the image does not clearly showif the CNT is single or double walled, the sample from whichthe nanotube was picked up contains mainly single-wall carbonnanotubes. Assuming a single wall carbon nanotube with across-section of A = 2πRd, where R = 2 nm is the radius andd = 0.33nm is the thickness of the wall, the current density canbe roughly estimated as J = I/A ≈ 0.5 × 1012Am−2, basedon a breakdown current of 2 μA. This value is comparable toother experimental and theoretical data on nanotube currentfailure [22–24].

The carbonaceous coating originating from contaminationin the chamber is in the literature often reported to have ahigh resistivity, while the exact properties of the depositedmaterial is instrument dependent. The contribution of thecarbonaceous crust to the conductance cannot be ruled out—this would reduce the current density however. Furthermore,the acceleration voltage of 200 keV for TEM imaging is abovethe threshold for knock-on damage of roughly 90 keV [25],which could introduce vacancy defects in the nanotube, andthereby reduce the maximal critical current density throughincreased scattering.

5. Conclusion

We have here presented a mass-producible electrothermalnanogripper, which is capable of manipulating nanotubes andnanowires in the 10–100 nm range, as well as providingpositionable contacts for in situ electrical characterizationin the TEM. While aligning the nanogripper to verticalnanowires in three dimensions turned out to be challenging,the manipulation of carbon-clad SWNTs was straightforward.In this work the actuators are defined with e-beam lithography,which could be replaced by nanoimprint lithography leadingto a more parallel microfabrication process. Moreover, withthe added flexibility of focused ion beam milling equipmentavailable at most nano- and microfabrication laboratories, thenanogripper can be adapted to a range of different tasks, whichcould well contribute to the faster development of nanodevices.

Acknowledgments

We acknowledge financial support from European projectsNANOHAND (IP034274) and NANORAC (STREP 013680),as well as the Danish Research Agency (FTP). We alsoacknowledge Christian Kallesøe for supplying the III–Vnanowire samples and the DTU-Danchip staff for their valuedhelp.

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