IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 18 (2007) 125303 (5pp)
doi:10.1088/0957-4484/18/12/125303
Forging of metallic nano-objects for the fabrication of submicron-size components J R¨osler1 , D Mukherji1,3 , K Schock2 and S Kleindiek2 1 2
Technical University Braunschweig, IfW, Langer Kamp 8, 38106 Braunschweig, Germany Kleindiek Nanotechnik GmbH, Aspenhaustraße 25, 72770 Reutlingen, Germany
E-mail:
[email protected]
Received 11 December 2006, in final form 29 January 2007 Published 21 February 2007 Online at stacks.iop.org/Nano/18/125303 Abstract In recent years, nanoscale fabrication has developed considerably, but the fabrication of free-standing nanosize components is still a great challenge. The fabrication of metallic nanocomponents utilizing three basic steps is demonstrated here. First, metallic alloys are used as factories to produce a metallic raw stock of nano-objects/nanoparticles in large numbers. These objects are then isolated from the powder containing thousands of such objects inside a scanning electron microscope using manipulators, and placed on a micro-anvil or a die. Finally, the shape of the individual nano-object is changed by nanoforging using a microhammer. In this way free-standing, high-strength, metallic nano-objects may be shaped into components with dimensions in the 100 nm range. By assembling such nanocomponents, high-performance microsystems can be fabricated, which are truly in the micrometre scale (the size ratio of a system to its component is typically 10:1). M This article features online multimedia enhancements (Some figures in this article are in colour only in the electronic version)
complex systems, e.g. for drug delivery and other implantable diagnostic and therapeutic devices. Microsystems can be very broadly classified into two categories:
1. Introduction The miniaturization of components and systems is advancing steadily in many areas of engineering, and the trend towards ever smaller components and devices is relentless. Today, technology is available for the fabrication of microsystems, like the microscale motion sensor that is installed in cars ready to deploy airbags. While engineers and scientists race to shrink the size of integrated circuits (ICs) and MEMS4 through nanotechnology to create the next generation of highperformance electronic and mechanical devices, biologists and life scientists have just begun to employ micropatterning and nanofabrication which enables revolutionary ways of building
(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 components. In both these cases, the complete system dimensions generally range from micrometre to millimetre scale, although in many cases the structural features within individual components (but not the free-standing components themselves) have reached dimensions lower than 10 nm [1]. Irrespective of the size of a complex machine, a typical relation exists between the size of the device and the components from which they are built. This is demonstrated in figure 1, where the size of a toothed gear pinion in different devices, from a very big church clock to a very small MEMS, are compared. Typically, the system to component size ratio is about 10:1. On extrapolating
3 Author to whom any correspondence should be addressed. 4 There is a confusion in the acronym MEMS as it is used today. Due to the enormous breadth and diversity of the devices and systems that are being miniaturized, the acronym MEMS is not a particularly apt one (i.e., the field is 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 for this general field of miniaturization include microsystems technology (MST), popular in Europe, and micromachines, popular in Asia.
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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, the system to component size ratio is about 10:1. The images of the microgear pinion and the MEMS rotary motor are courtesy of Sandia National Laboratories, SUMMiT Technologies, www.mems.sandia.gov.
on the use of a large number of molecular robotic subsystems working in parallel and using commonly available chemicals. Drexler’s most compelling argument that such radical nanotechnology must be possible is that cell biology gives us endless examples of sophisticated nanoscale machines. These include molecular motors of the kind that make up our muscles, which can convert chemical energy to mechanical energy with astonishingly high efficiencies [7]. Fabricating such devices is still a speculative fiction at the present state of science and technology. From both the fundamental and applied points of view, there are interesting questions about scaling down to the micro- and nano-world. In particular, effects negligible at the macroscopic level become important at the micro/nanometre scale, and vice versa. For example, when the characteristic dimensions of an object decrease from the macroscopic to micrometre size, the effects of gravity as compared with adhesive and friction effects become negligible and surface tension dominates gravity [8]. When the characteristic size decreases further to attain the nanometre range, another limit is encountered. While the macroscopic properties of matter remain generally valid at the micrometre size, surface effects become dominant at the nanometre scale and the physical, mechanical, electrical, optical and magnetic properties of the material might change [9]. Over and above this, objects in these dimensions are no longer visible by the naked eye or even by an optical microscope, and gripping and manipulating the nano-object requires special techniques. The boundary between the macroscopic and micro/nanoscopic levels is not sharp; it depends on the effect to be considered. We demonstrate here the fabrication of metallic nanocomponents, utilizing three basic steps (schematically depicted in figure 2). First, metallic alloys are used as factories to produce metallic raw stock in the form of nano-objects/nanoparticles.
this typical system/component size ratio to smaller devices, it becomes clear that components with submicron dimensions, i.e. in the nanometre range, are needed to make micron-size devices feasible. Today, free-standing microcomponents are still 10–1000 µm in size [2], and the fabrication of individual free-standing components in the nanometre size range is still a challenge. Recently we explored the possibilities of fabricating components with dimensions in the 100 nm range with some success. In this paper we report on our effort. This allows us for the first time to produce high-strength metallic components with dimensions on the nanoscale by forging nano-objects of similar dimensions.
2. Shaping nano-objects Several techniques, such as micro-injection moulding [3], microcasting [4], or lithographic fabrication [5], to name only a few, are available today to manufacture miniaturized components from metals, polymers, ceramics and semiconductors. The smallest dimensions of the components that can be produced depend on the process used, but are not smaller than tens of microns. On the other hand, there is also the concept of ‘nano-assembly’, or ‘bottom-up’ approach, i.e. the synthesis of a nanostructured material by assembly of its previously prepared nano-building-blocks (nanoscopic particles, or even atoms and molecules). A number of nano-assembly techniques are currently being investigated, but most are for producing nanostructures on substrates or in monolithic form and not for producing free-standing components with specific shapes. Some futurists (like Eric Drexler) have proposed nanoscopic ‘assemblers’ (nanobots) which would compose a nanoscale object or other nanobots on a molecule-by-molecule basis [6]. Molecular manufacturing is a method conceived for the processing and rearrangement of atoms to fabricate custom products, e.g. a planetary gear with 3000–6000 atoms. It relies 2
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Figure 2. Schematic representation of the steps involved in nanoforging. A metallic alloy is used as a factory to produce metallic raw stock in the form of nano-objects/nanoparticles. One of these nano-objects is isolated and forged to shape a component.
3. Results Nickel-based superalloys, containing Ni3 Al-type ordered intermetallic precipitates, are particularly well suited for the production of the metallic nano-objects, as the precipitates can be controlled to extremely uniform shape and size in the nanometre scale. To demonstrate the above-mentioned processing route, we used a single-crystal alloy designated SX1 (nominal composition Ni–base with 11.6Al, 2.4Ta, 3.5W, 6.0Cr, 1.3Mo in at.%) which was heat treated according to standard practice [10] to obtain Ni3 Al-type coherent precipitates with a narrow size distribution. These precipitates were then isolated from the alloy by an electrochemical process to selectively dissolve the matrix phase embedding the precipitates [11], thereby obtaining cubic particles with an edge length of 300–500 nm (figure 3). A powder sample containing the nano-objects was then loaded in the SEM. Micromanipulators mounted inside the microscope and equipped with sharp tungsten needles were then used to isolate one cubic nanoparticle and place it on an anvil. The arrangement inside the SEM is schematically depicted in figure 4. Four micromanipulators are used to manipulate and forge the metallic nano-object. They are positioned at a 90◦ angle relative to each other. Two of these manipulators are
Figure 3. Cube-shaped nano-objects in the size range of 300–500 nm are isolated from the alloy SX1 by selective phase dissolution (electrochemical processing) [11].
These objects are then isolated from the nanopowder containing thousands of such objects inside a scanning electron microscope (SEM) using manipulators and placed on a micro-anvil or die. Finally, a microhammer and anvil are used to change the shape of the individual nanoparticles. In this way free-standing metallic nano-objects could be shaped into components.
Nano-object
Manipulator Hammer
Anvil
(a)
(b)
Figure 4. The arrangement of the micromanipulators (MM3A-EM from Kleindiek Nanotechnik) inside the SEM. (a) Schematic view. Two manipulators fitted with sharp tungsten needles are used for the handling of the nano-object. A third manipulator is equipped with a silicon cantilever arm which serves as micro-anvil and a force senor at the same time. A fourth manipulator holds a tungsten needle with flattened tip which acts as the microhammer. (b) In the inset the actual manipulator arrangement is seen.
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equipped with a silicon cantilever arm (which serves as the anvil and force senor at the same time) and a tungsten needle (which serves as the hammer), respectively. The tungsten hammer has a flattened surface (1–5 µm diameter) prepared by ion-beam cutting. Like in conventional forging, the metal nano-object is shaped by deformation at room temperature. While micromanipulation with tools such as micropipettes or lasers is a mature technique for handling microscopic samples in liquids, micro- and nano-manipulation in dry environments is still a challenging task due to the absence of liquids to reduce the often strong adhesion forces [12]. In dry environments, scanning probe microscopy (SPM) equipment and mobile microrobots have been previously used to push, slide, and roll nanostructures in two dimensions across surfaces [13], but this has limited application. Apart from standard SPM tips, more advanced tools like microfabricated grippers [14] could provide better control in manipulation. We used micromanipulators (MM3A from Kleindiek Nanotechnik GmbH) fitted with tungsten probe needles, having 20 nm tip radius, for handling the nano-objects. Two manipulators mounted on the specimen stage inside a SEM were used for isolating a single cubic-shaped nano-object from the powder sample. When the probe tips were moved to the area of interest, the high surface area of the nano-object provides a strong attractive force so that a cubic nanoparticle could be easily picked up by a single needle tip from a group of particles (figure 5). No gripping action is needed. In fact the adhesion force is so strong that a second probe tip is required to remove the nano-object from the tungsten needle (supplementary video 1 available at stacks.iop.org/Nano/18/125303). Precise control (0.5 nm positional resolution) of the probe tips and direct visual feedback in the SEM made working with the manipulator like using chopsticks. The manipulators were controlled from outside with the help of a joystick. Once a nanocube is picked up by the manipulator, the SEM stage table is moved out and the anvil and the hammer mounted on two other micromanipulators are brought into position. The raw stock is then placed on the silicon anvil and between the flattened plateau of the tungsten hammer. The nanoobject is deformed by the movement of the hammer onto the surface of the anvil (figure 5). The piezoelectric nanomotor drive of the manipulator allows displacement control of the tungsten hammer with an accuracy of 1 nm, so the degree of deformation during forging can be precisely controlled. Figure 6 shows a sequence of the forging process. Note that a high degree of deformation was achieved at room temperature (supplementary video 2 available at stacks.iop.org/Nano/18/125303) and a load of up to 2 mN could be applied. The high-strength Ni3 Al intermetallic phase in its bulk form is considerably brittle and is considered difficult to deform at room temperature [15]. Appreciable deformation, however, sets in at a load of 150 µN, which is equivalent to a compressive stress of approximately 940 MPa on a 400 nm size nanocube. In comparison the yield strength of a bulk single-crystal Ni3 Al alloy containing Cr and B is about 300 MPa [16]. The force was continuously monitored during deformation by a force sensor mounted on the silicon cantilever arm. Figures 6(e) and (f) show that the nanoobjects are freely detached from the tungsten-tip hammer after the deformation, suggesting that the forging process does not produce any contamination.
Figure 5. Illustration of the nanoforging process. (a) The manipulator tip approaches a powder sample of extracted nanoparticles. (b) The manipulator is isolating and picking up a nanocubic particle. (c) and (d) The nano-object is being placed on a Si cantilever beam, serving as a micro-anvil. (e) The nano-object is deformed by a tungsten microhammer on the anvil. (Pictures taken in an 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 for forging can be produced from simple alloys, (ii) a single nano-object can be handled and manipulated easily inside an SEM, (iii) nanocubes of the high-strength Ni3 Al intermetallic phase can be plastically deformed to large extent by compressive loading at room temperature without inducing cracking. Bulk polycrystalline Ni3 Al shows brittle grain boundary fracture without appreciable tensile ductility at room temperature [15]. An extrinsic source of grain-boundary brittleness has been discovered in Ni3 Al, namely, the moisture (H2 O) present in ambient air. The embrittlement mechanism is similar to that found in many ordered intermetallic alloys containing reactive elements (e.g. Al, Ti, Si) [15]. The nanopar4
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To form complex shapes (e.g. a gear wheel) from the nanocubes, a shaped die will be needed. The required shape can 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-beam lithography has been used to test the microformability of amorphous alloys [17]. The lithographic technique has advanced sufficiently and is now able to produce detailed features on the nanometre scale on monolithic substrates [1]. A counterpart of the die curved out of a Si wafer can, for example, be fixed to the flat surface of the tungsten hammer, thereby building a closed die forging set up. The size distribution of the cubic Ni3 Al particles is relatively narrow (520 ± 140 nm). Moreover, the raw stock for the forging can be carefully chosen from many available nanocubes, and a proper die design can ensure adequate die filling. Such experiments are planned in the future.
5. Conclusions The present study shows the feasibility of forging metallic nano-objects (∼400 nm size) to form shapes. The ability to produce high-strength metallic components with characteristic dimensions of 10−7 m by nanoforging opens up new possibilities to eventually produce complex microsystems with 10−6 –10−5 m size by assembling free-standing nanoscopic components. At these sizes they are of the same dimensions as micro-organisms and therefore sufficiently small even to travel through the human body.
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Figure 6. Two examples of forging experiments at ambient temperature. (a), (b) A nano-object is deformed by a tungsten hammer (plateau diameter 1 µm) and flattening of the particle surface facing the silicon anvil indicates the onset of plastic deformation. (c)–(f) Severe plastic deformation of the nano-object at a load of up to 2 mN (plateau diameter of the hammer 5 µm), results in the change of the cubic-shaped nanoparticle. The images are taken from a video sequence5 available on-line (supplementary video-2 available at stacks.iop.org/Nano/18/125303). (Pictures taken in an FEI Quanta 3D SEM at Kleindiek Nanotechnik.)
ticles of Ni3 Al used by us are single crystals, and deformation was carried out under high vacuum inside the SEM chamber. At this point it is not clear whether the high ductility of Ni3 Al nanocubes is a result of the nanosize or because a grain-boundary-free material was deformed in a moisturefree environment. Irrespective of the actual reason for the plasticity, the result is encouraging, and shows promise for deforming metallic nano-objects into complex shapes.
5 The reason for the poor quality of the pictures is that they are taken in a SEM
with a tungsten filament (low resolution), and for the deformation experiment a high frame rate was used to record the video sequence.
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