IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 18 (2007) 125303 (5pp) doi:10.1088/0957-4484/18/12/125303

Forging of metallic nano-objects for thefabrication of submicron-size componentsJ Rosler1, D Mukherji1,3, K Schock2 and S Kleindiek2

1 Technical University Braunschweig, IfW, Langer Kamp 8, 38106 Braunschweig, Germany2 Kleindiek Nanotechnik GmbH, Aspenhaustraße 25, 72770 Reutlingen, Germany

E-mail: d.mukherji@tu-bs.de

Received 11 December 2006, in final form 29 January 2007Published 21 February 2007Online at stacks.iop.org/Nano/18/125303

AbstractIn recent years, nanoscale fabrication has developed considerably, but thefabrication of free-standing nanosize components is still a great challenge.The fabrication of metallic nanocomponents utilizing three basic steps isdemonstrated here. First, metallic alloys are used as factories to produce ametallic raw stock of nano-objects/nanoparticles in large numbers. Theseobjects are then isolated from the powder containing thousands of suchobjects inside a scanning electron microscope using manipulators, and placedon a micro-anvil or a die. Finally, the shape of the individual nano-object ischanged by nanoforging using a microhammer. In this way free-standing,high-strength, metallic nano-objects may be shaped into components withdimensions in the 100 nm range. By assembling such nanocomponents,high-performance microsystems can be fabricated, which are truly in themicrometre scale (the size ratio of a system to its component is typically10:1).

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1. Introduction

The miniaturization of components and systems is advancingsteadily in many areas of engineering, and the trend towardsever smaller components and devices is relentless. Today,technology is available for the fabrication of microsystems,like the microscale motion sensor that is installed in carsready to deploy airbags. While engineers and scientists raceto shrink the size of integrated circuits (ICs) and MEMS4

through nanotechnology to create the next generation of high-performance electronic and mechanical devices, biologists andlife scientists have just begun to employ micropatterning andnanofabrication which enables revolutionary ways of building

3 Author to whom any correspondence should be addressed.4 There is a confusion in the acronym MEMS as it is used today. Due tothe enormous breadth and diversity of the devices and systems that are beingminiaturized, the acronym MEMS is not a particularly apt one (i.e., the fieldis more than simply micro, electrical and mechanical systems). However,the acronym MEMS is used almost universally to refer to the entire field(i.e., all devices produced by microfabrication except ICs). Other names forthis general field of miniaturization include microsystems technology (MST),popular in Europe, and micromachines, popular in Asia.

complex systems, e.g. for drug delivery and other implantablediagnostic and therapeutic devices.

Microsystems can be very broadly classified into twocategories:

(i) the systems which are curved out of a monolithic substrate(MEMS and NEMS (nano-electro-mechanical systems))and

(ii) those which are assembled from free-standing compo-nents.

In both these cases, the complete system dimensions generallyrange from micrometre to millimetre scale, although in manycases the structural features within individual components (butnot the free-standing components themselves) have reacheddimensions lower than 10 nm [1]. Irrespective of the sizeof a complex machine, a typical relation exists between thesize of the device and the components from which they arebuilt. This is demonstrated in figure 1, where the size of atoothed gear pinion in different devices, from a very big churchclock to a very small MEMS, are compared. Typically, thesystem to component size ratio is about 10:1. On extrapolating

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Nanotechnology 18 (2007) 125303 J Rosler et al

Figure 1. The size of toothed gear pinions is compared with the dimensions of the devices they are built in. Over a wide size range, thesystem to component size ratio is about 10:1. The images of the microgear pinion and the MEMS rotary motor are courtesy of SandiaNational Laboratories, SUMMiT Technologies, www.mems.sandia.gov.

this typical system/component size ratio to smaller devices, itbecomes clear that components with submicron dimensions,i.e. in the nanometre range, are needed to make micron-sizedevices feasible. Today, free-standing microcomponents arestill 10–1000 µm in size [2], and the fabrication of individualfree-standing components in the nanometre size range is still achallenge. Recently we explored the possibilities of fabricatingcomponents with dimensions in the 100 nm range with somesuccess. In this paper we report on our effort. This allows usfor the first time to produce high-strength metallic componentswith dimensions on the nanoscale by forging nano-objects ofsimilar dimensions.

2. Shaping nano-objects

Several techniques, such as micro-injection moulding [3], mi-crocasting [4], or lithographic fabrication [5], to name only afew, are available today to manufacture miniaturized compo-nents from metals, polymers, ceramics and semiconductors.The smallest dimensions of the components that can be pro-duced depend on the process used, but are not smaller thantens of microns. On the other hand, there is also the conceptof ‘nano-assembly’, or ‘bottom-up’ approach, i.e. the synthe-sis of a nanostructured material by assembly of its previouslyprepared nano-building-blocks (nanoscopic particles, or evenatoms and molecules). A number of nano-assembly techniquesare currently being investigated, but most are for producingnanostructures on substrates or in monolithic form and notfor producing free-standing components with specific shapes.Some futurists (like Eric Drexler) have proposed nanoscopic‘assemblers’ (nanobots) which would compose a nanoscale ob-ject or other nanobots on a molecule-by-molecule basis [6].Molecular manufacturing is a method conceived for the pro-cessing and rearrangement of atoms to fabricate custom prod-ucts, e.g. a planetary gear with 3000–6000 atoms. It relies

on the use of a large number of molecular robotic subsys-tems working in parallel and using commonly available chem-icals. Drexler’s most compelling argument that such radicalnanotechnology must be possible is that cell biology gives usendless examples of sophisticated nanoscale machines. Theseinclude molecular motors of the kind that make up our mus-cles, which can convert chemical energy to mechanical energywith astonishingly high efficiencies [7]. Fabricating such de-vices is still a speculative fiction at the present state of scienceand technology.

From both the fundamental and applied points of view,there are interesting questions about scaling down to themicro- and nano-world. In particular, effects negligible at themacroscopic level become important at the micro/nanometrescale, and vice versa. For example, when the characteristicdimensions of an object decrease from the macroscopic tomicrometre size, the effects of gravity as compared withadhesive and friction effects become negligible and surfacetension dominates gravity [8]. When the characteristic sizedecreases further to attain the nanometre range, another limitis encountered. While the macroscopic properties of matterremain generally valid at the micrometre size, surface effectsbecome dominant at the nanometre scale and the physical,mechanical, electrical, optical and magnetic properties of thematerial might change [9]. Over and above this, objects inthese dimensions are no longer visible by the naked eye oreven by an optical microscope, and gripping and manipulatingthe nano-object requires special techniques. The boundarybetween the macroscopic and micro/nanoscopic levels is notsharp; it depends on the effect to be considered.

We demonstrate here the fabrication of metallic nanocom-ponents, utilizing three basic steps (schematically depicted infigure 2). First, metallic alloys are used as factories to producemetallic raw stock in the form of nano-objects/nanoparticles.

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Nanotechnology 18 (2007) 125303 J Rosler et al

Figure 2. Schematic representation of the steps involved in nanoforging. A metallic alloy is used as a factory to produce metallic raw stock inthe form of nano-objects/nanoparticles. One of these nano-objects is isolated and forged to shape a component.

Figure 3. Cube-shaped nano-objects in the size range of 300–500 nmare isolated from the alloy SX1 by selective phase dissolution(electrochemical processing) [11].

These objects are then isolated from the nanopowder contain-ing thousands of such objects inside a scanning electron micro-scope (SEM) using manipulators and placed on a micro-anvilor die. Finally, a microhammer and anvil are used to change theshape of the individual nanoparticles. In this way free-standingmetallic nano-objects could be shaped into components.

3. Results

Nickel-based superalloys, containing Ni3Al-type orderedintermetallic precipitates, are particularly well suited for theproduction of the metallic nano-objects, as the precipitatescan be controlled to extremely uniform shape and size inthe nanometre scale. To demonstrate the above-mentionedprocessing route, we used a single-crystal alloy designated SX-1 (nominal composition Ni–base with 11.6Al, 2.4Ta, 3.5W,6.0Cr, 1.3Mo in at.%) which was heat treated accordingto standard practice [10] to obtain Ni3Al-type coherentprecipitates with a narrow size distribution. These precipitateswere then isolated from the alloy by an electrochemicalprocess to selectively dissolve the matrix phase embeddingthe precipitates [11], thereby obtaining cubic particles withan edge length of 300–500 nm (figure 3). A powdersample containing the nano-objects was then loaded in theSEM. Micromanipulators mounted inside the microscope andequipped with sharp tungsten needles were then used toisolate one cubic nanoparticle and place it on an anvil. Thearrangement inside the SEM is schematically depicted infigure 4. Four micromanipulators are used to manipulate andforge the metallic nano-object. They are positioned at a 90◦angle relative to each other. Two of these manipulators are

Nano-object Manipulator

Hammer

Anvil

(b)(a)

Figure 4. The arrangement of the micromanipulators (MM3A-EM from Kleindiek Nanotechnik) inside the SEM. (a) Schematic view. Twomanipulators fitted with sharp tungsten needles are used for the handling of the nano-object. A third manipulator is equipped with a siliconcantilever arm which serves as micro-anvil and a force senor at the same time. A fourth manipulator holds a tungsten needle with flattened tipwhich acts as the microhammer. (b) In the inset the actual manipulator arrangement is seen.

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Nanotechnology 18 (2007) 125303 J Rosler et al

equipped with a silicon cantilever arm (which serves as theanvil and force senor at the same time) and a tungsten needle(which serves as the hammer), respectively. The tungstenhammer has a flattened surface (1–5 µm diameter) preparedby ion-beam cutting. Like in conventional forging, the metalnano-object is shaped by deformation at room temperature.

While micromanipulation with tools such as micropipettesor lasers is a mature technique for handling microscopicsamples in liquids, micro- and nano-manipulation in dryenvironments is still a challenging task due to the absenceof liquids to reduce the often strong adhesion forces [12].In dry environments, scanning probe microscopy (SPM)equipment and mobile microrobots have been previously usedto push, slide, and roll nanostructures in two dimensions acrosssurfaces [13], but this has limited application. Apart fromstandard SPM tips, more advanced tools like microfabricatedgrippers [14] could provide better control in manipulation. Weused micromanipulators (MM3A from Kleindiek NanotechnikGmbH) fitted with tungsten probe needles, having 20 nmtip radius, for handling the nano-objects. Two manipulatorsmounted on the specimen stage inside a SEM were used forisolating a single cubic-shaped nano-object from the powdersample. When the probe tips were moved to the area ofinterest, the high surface area of the nano-object provides astrong attractive force so that a cubic nanoparticle could beeasily picked up by a single needle tip from a group of particles(figure 5). No gripping action is needed. In fact the adhesionforce is so strong that a second probe tip is required to removethe nano-object from the tungsten needle (supplementaryvideo 1 available at stacks.iop.org/Nano/18/125303). Precisecontrol (0.5 nm positional resolution) of the probe tips anddirect visual feedback in the SEM made working with themanipulator like using chopsticks. The manipulators werecontrolled from outside with the help of a joystick. Oncea nanocube is picked up by the manipulator, the SEM stagetable is moved out and the anvil and the hammer mounted ontwo other micromanipulators are brought into position. Theraw stock is then placed on the silicon anvil and betweenthe flattened plateau of the tungsten hammer. The nano-object is deformed by the movement of the hammer onto thesurface of the anvil (figure 5). The piezoelectric nanomotordrive of the manipulator allows displacement control of thetungsten hammer with an accuracy of 1 nm, so the degree ofdeformation during forging can be precisely controlled.

Figure 6 shows a sequence of the forging process.Note that a high degree of deformation was achievedat room temperature (supplementary video 2 available atstacks.iop.org/Nano/18/125303) and a load of up to 2 mNcould be applied. The high-strength Ni3Al intermetallic phasein its bulk form is considerably brittle and is considereddifficult to deform at room temperature [15]. Appreciabledeformation, however, sets in at a load of 150 µN, which isequivalent to a compressive stress of approximately 940 MPaon a 400 nm size nanocube. In comparison the yield strengthof a bulk single-crystal Ni3Al alloy containing Cr and B isabout 300 MPa [16]. The force was continuously monitoredduring deformation by a force sensor mounted on the siliconcantilever arm. Figures 6(e) and (f) show that the nano-objects are freely detached from the tungsten-tip hammer afterthe deformation, suggesting that the forging process does notproduce any contamination.

Figure 5. Illustration of the nanoforging process. (a) Themanipulator tip approaches a powder sample of extractednanoparticles. (b) The manipulator is isolating and picking up ananocubic particle. (c) and (d) The nano-object is being placed on aSi cantilever beam, serving as a micro-anvil. (e) The nano-object isdeformed by a tungsten microhammer on the anvil. (Pictures taken inan FEI Quanta 3D SEM at Kleindiek Nanotechnik.)

4. Discussions

The novel experiment clearly demonstrates the following:

(i) metallic nano-objects with shape and size suitable forforging can be produced from simple alloys,

(ii) a single nano-object can be handled and manipulatedeasily inside an SEM,

(iii) nanocubes of the high-strength Ni3Al intermetallic phasecan be plastically deformed to large extent by compressiveloading at room temperature without inducing cracking.

Bulk polycrystalline Ni3Al shows brittle grain boundaryfracture without appreciable tensile ductility at room temper-ature [15]. An extrinsic source of grain-boundary brittlenesshas been discovered in Ni3Al, namely, the moisture (H2O)present in ambient air. The embrittlement mechanism is sim-ilar to that found in many ordered intermetallic alloys con-taining reactive elements (e.g. Al, Ti, Si) [15]. The nanopar-

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Nanotechnology 18 (2007) 125303 J Rosler et al

Figure 6. Two examples of forging experiments at ambienttemperature. (a), (b) A nano-object is deformed by a tungstenhammer (plateau diameter 1 µm) and flattening of the particlesurface facing the silicon anvil indicates the onset of plasticdeformation. (c)–(f) Severe plastic deformation of the nano-object ata load of up to 2 mN (plateau diameter of the hammer 5 µm), resultsin the change of the cubic-shaped nanoparticle. The images are takenfrom a video sequence5 available on-line (supplementary video-2available at stacks.iop.org/Nano/18/125303). (Pictures taken in anFEI Quanta 3D SEM at Kleindiek Nanotechnik.)

ticles of Ni3Al used by us are single crystals, and deforma-tion was carried out under high vacuum inside the SEM cham-ber. At this point it is not clear whether the high ductilityof Ni3Al nanocubes is a result of the nanosize or becausea grain-boundary-free material was deformed in a moisture-free environment. Irrespective of the actual reason for theplasticity, the result is encouraging, and shows promise fordeforming metallic nano-objects into complex shapes.

5 The reason for the poor quality of the pictures is that they are taken in a SEMwith a tungsten filament (low resolution), and for the deformation experimenta high frame rate was used to record the video sequence.

To form complex shapes (e.g. a gear wheel) from thenanocubes, a shaped die will be needed. The required shapecan be engraved, for example by electron-beam lithography,onto the silicon cantilever anvil. A micro V-groove on(100) Si with 200 nm width, formed by electron-beamlithography has been used to test the microformability ofamorphous alloys [17]. The lithographic technique hasadvanced sufficiently and is now able to produce detailedfeatures on the nanometre scale on monolithic substrates [1]. Acounterpart of the die curved out of a Si wafer can, for example,be fixed to the flat surface of the tungsten hammer, therebybuilding a closed die forging set up. The size distribution ofthe cubic Ni3Al particles is relatively narrow (520 ± 140 nm).Moreover, the raw stock for the forging can be carefully chosenfrom many available nanocubes, and a proper die design canensure adequate die filling. Such experiments are planned inthe future.

5. Conclusions

The present study shows the feasibility of forging metallicnano-objects (∼400 nm size) to form shapes. The ability toproduce high-strength metallic components with characteristicdimensions of 10−7 m by nanoforging opens up newpossibilities to eventually produce complex microsystems with10−6–10−5 m size by assembling free-standing nanoscopiccomponents. At these sizes they are of the same dimensions asmicro-organisms and therefore sufficiently small even to travelthrough the human body.

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