Tip-based nanomanufacturing by electrical, chemical, mechanical and thermal processes

CIRP Annals - Manufacturing Technology 59 (2010) 628–651

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Tip-based nanomanufacturing by electrical, chemical, mechanical and thermal processes A.P. Malshe (2)a,*, K.P. Rajurkar (1)b, K.R. Virwani c, C.R. Taylor d, D.L. Bourell e, G. Levy (1)f, M.M. Sundaram g, J.A. McGeough (1)h, V. Kalyanasundaram a, A.N. Samant a a

University of Arkansas, Fayetteville, AR, USA University of Nebraska, Lincoln, NE, USA c IBM Almaden Research Center, San Jose, CA, USA d University of Florida, Gainesville, FL, USA e University of Texas, Austin, TX, USA f InspireAG, St. Gallen, Switzerland g University of Cincinnati, Cincinnati, OH, USA h University of Edinburgh, Edinburgh, UK b

A R T I C L E I N F O

A B S T R A C T

Keywords: Manufacturing Nano manufacturing Tip-based nano manufacturing

Nanomanufactured products with higher complexities in function, materials, scales and their integration demand an increasing need for advanced manufacturing tools. It is driven by applications such as ultradense memory, individualized biomedicine and drug delivery, molecular reading and sorting, and nanoscale circuitry. The tip-based nanomanufacturing (TBN) platform represents a potent gamut of processes for such applications – performing various nanoscale manufacturing operations including machining, depositing, patterning, and assembling with in situ metrology and visualization. This keynote paper presents a comprehensive overview of TBN processes based upon ‘‘nanotool tips’’ applying electrical, electrochemical, mechanical, electromagnetic and other forces to perform manufacturing operations. ß 2010 CIRP.

1. Introduction Today, early success has shown that nanotechnology holds a large potential to meet many of the world’s greatest challenges, spawning revolutions in energy production, affordable health care, safety and food, and better air and water quality. The ability to atomically engineer and manufacture structures that exploit the unique properties at the nanoscale will enable quantum leaps and improvement in high-performance technologies – from new sensors, high-density data storage, and drug delivery to high strength materials and energy efficient solar cells. Although much progress has been made in identifying nanoscale materials and approaches that can be used in products, current nanoscale manufacturing capabilities limit their commercialization for nanosized components and systems. The grand challenge is to build upon and expand the capabilities in nanofabrication to produce a wider range of structures, with greater complexity, improved precision and accuracy, and with increasingly higher performance [1]. Nanoscale tool tips are emerging as unique enablers to address this challenge. Although many papers exist that address the traditional material characterization capabilities of nanoscale tips in atomic force microscopy (AFM), scanning

* Corresponding author at: Mechanical Engineering, 204 Mechanical Engineering Bldg., West Dickson Street, Fayetteville, AR 72701, USA. Tel.: +1 479 575 6561; fax: +1 479 575 6982. E-mail address: [email protected] (A.P. Malshe). 0007-8506/$ – see front matter ß 2010 CIRP. doi:10.1016/j.cirp.2010.05.006

tunneling microscopy (STM), and nanoindentation, this paper comprehensively focuses on the processes and application of nanoscale tips in manufacturing applications. 1.1. Tip-based nanomanufacturing (TBN) The origin of the word nano, the prefix in nanotechnology, stems from a Greek word n[TD$INLE] noz, meaning ‘‘dwarf.’’ Nanotechnology began to garner the attention of the scientific community after a lecture, ‘There’s Plenty of Room at the Bottom’ [2], given by Dr. Richard Feynman, a joint recipient of the 1965 Nobel Prize in Physics, to the American Physical Society on December 29, 1959. Over the time, there are multiple definitions of the word nanotechnology. For example, Taniguchi defined it as the processing, separation, consolidation and deformation of materials by one atom or by one molecule [3]. In recent times, defined in its simplest form [4], ‘‘Nanotechnology is the understanding and control of matter at dimensions between approximately 1 and 100 nanometers, where unique phenomena enable novel applications. Encompassing nanoscale science, engineering, and technology, nanotechnology involves imaging, measuring, modeling, and manipulating matter at this length scale.’’ The development and production of structures and sub-systems at the nanoscale to produce systems of scientific and technological importance are of great interest due to their underlying fundamental advantages. In particular, the interaction of matter at the atomic and molecular level produces new chemistries [5], defectfree materials [6], high surface-to-volume ratios enhancing surface

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Fig. 2. Diagram of nanomanufacturing hierarchy.

Fig. 1. Diagram of advances in nanomanufacturing that have facilitated initial successes in the production of nanotechnologies.

reactivity [7], higher compaction density per unit volume [8], and many other unique properties that can be exploited to deliver novel materials, devices, and systems. In summary, nanoscale fabrication is based on the fact that material properties change as a function of physical dimension (<100 nm), and focuses on building nanoscale structures in large quantities with potential costeffective manufacturability. To accomplish this task, it is imperative to have tools that can image (i.e. microscopy), interrogate (i.e. spectroscopy) and predict (i.e. model) physical, chemical, and other processes occurring at the nanoscale. In the past decade, initial successes in nanotechnology have been primarily due to advances in nanomanufacturing (see Fig. 1), herein defined as the science and engineering of design, deposition, machining, metrology, assembly, and integration of heterogeneous materials to produce millions of nanoscale devices, components, and systems in a sustainable manner. Today, the results of nanomanufacturing are evident in many commercial sectors. For example, nanoscale components are used in scratch-free paints, high efficiency catalysts and batteries, sunscreen lotions, nanoparticle-based cancer detection, advanced lubricants, and high wear-resistant nanocomposite coatings for cutting tools [9–14]. Nanomanufacturing can be envisioned in three generations with ascending levels of complexity for manufacturing and metrology (Fig. 2). The first generation of nanomanufacturing, which is currently an economic generator, includes nanostructures

integrated into micro- and macroscopic systems, such as nanoparticle-based bulk composites and coatings. The second generation of nanomanufacturing includes nano and sub-micron sized feature integrated sub-systems, such as quantum dot layered integrated light emitting diodes (LEDs) and lasers. The third generation of nanomanufacturing encompasses complete systems that are nanoscale in size (<100 nm) such as molecular circuitry, sensor and device systems. In particular, there is an immense need for a spectrum of high derivative manufacturing processes and metrological tools at sub-micro, nano, and angstrom scales for realization of second and third generations of nanomanufactured products. More importantly, the nanomanufacturing community envisions and projects that industries will be working to mass produce second and third generations of nanoscale systems commercially by 2025. Examples of such nanoscale systems include single-DNA (biological bar code) detection devices, highly selective molecular detection devices for chemical and biological agents such as anthrax, nanoscale robots for detection and single cell surgery/ extraction, single electron transistors, high-density molecular memory devices, ultrafast quantum dot and quantum wire based chip-to-chip communication, single nanowire based accelerometers, chip-based nanowire power generators, and many others. However, the available choices of nanomanufacturing processes (see Table 1) [15–28] including optical lithography, nanoimprinting, and focused electron/ion beam technologies for the production of these kinds of second and third generation systems could be limited in their abilities to machine diverse sets of materials with the required dimensions and ultra-high tolerances in three dimensions along with sub-20 nm scale metrology. From a manufacturing process perspective, a complete system smaller than 100 nm typically requires subcomponents that are less than 20 nm with a tolerance of less than 1 nm. These needs were clearly highlighted by Defence Advanced Research Projects Agency (DARPA,

Table 1 Comparison of nanomanufacturing processes [15–28]. Nanomanufacturing process

Minimum 3D feature size and materials

Defects/contamination

Type and speed of processing

Scalability

Cost

Tip-based nanomanufacturing [15–17]

<10–80 nm; all materials

Occasional pile-up

Yes

Low

Nanoimprint lithography [18,19]

10 nm; polymers and silicon

Residue buildup/contamination

Deposition, machining, assembly and measurement; fast Molding; fast

Yes

Focused ion beam (FIB) machining/lithography [20,21] Femtosecond laser machining [22,23] UV lithography [24,25] Electron beam lithography (EBL) [26,27] X-ray lithography [28]

30 nm; all 100 nm; all 90 nm; polymers and semiconductors 5 nm; all 1 nm; all

Ga ion implantation/material r edeposition Laser redeposition Photoresist/development residue

Machining; slow

No

Low in volume High

Machining; fast Machining; slow

Yes Yes

High High

No No

Machining; slow Machining; slow

No No

High High

+

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Table 2 Targeted metric for tip-based nanofabrication [29]. Metric

Unit

Phase I

Phase II

Phase III

Feature position control Feature size controla Heterogenityb

nm % of dimension

50 10 2 values of one parameter

25 3 5 values of 2 parameters

1/min single tip

5/min/tip 5-tip array Height < 5 Radius < 10 1000 operations 10

5 1 Continuous control over 2 parameters 60/min/tip 30-tip array Height < 1 Radius < 3 1e6 operations 2

Feature ratec Tip shape variation

Tip height sensing

d

% of dimension

nm

Height < 10 Radius < 20 100 operations 20

a

This metric specifies the length of a nanowire or the diameter of a quantum dot. For example, a 1000 nm nanowire needs to be grown with variations of less than 100 nm in the first phase. b This metric is a statement of the requirement that nanostructures be fabricated with controlled variations in parameters. For example, nanowires must be grown with 2 different lengths (or radii). Specifically, 1000 nm  10% and 2000 nm  10% would meet the first phase milestone. These must be grown on the same substrate using the same apparatus, one after the other. c In Phase 1, a single tip is expected to be used. In Phases 2 and 3, linear arrays of 5 and 30 are required, respectively. d This metric is a statement that the shape of the tip should not significantly change during operation. The underlying assumption is a conical-shaped tip with a spherical end. Different tip geometries should still be parameterized by an overall ‘‘height’’ and the radius of curvature of the end, and these metrics can apply.

U.S.) in 2007 (see Table 2) [29], in reference to the development of nanoscale tool tips, hereafter referred to as tip-based nanomanufacturing (TBN). TBN has emerged from the well established scanning probe microscopy (SPM) platform [30,31] to manufacture nanointegrated systems of the second and third generations. In particular, it delivers capabilities for sub-20 nm manufacturing with process flexibility, integrated processing, assembly, metrology and visualization in a cluster tool through the use of different nanoscale tool tips. This paper specifically discusses the role of these tool tips in various processes that use electrical, electrochemical, mechanical and photonic energy and possibly related process mechanisms. The goal of this keynote paper is to comprehensively present driver manufacturing applications and the state-of-the-art in TBN processes, including the historical perspective, fundamentals, instrumentation and results for top-down, bottom-up and hybrid manufacturing, metrology, and visualization.

material depolarizes. The other important parameter used to characterize scanner tubes is resonance frequency (nscanner), as it is possible for an SPM to scan only at frequencies that are lower than nscanner. The resonant frequency depends upon the tube’s length and thickness, the material’s density (used to determine the system inertial mass) and Young’s modulus (used to estimate the tube’s equivalent spring constant). Another essential feature of all SPMs is the feedback control loop. This electronic system maintains the tip–sample interaction at a preset value by controlling the z position (or deflection) of the probe relative to the surface (or to an undeflected position). Commercial feedback loops generally incorporate integral and proportional gain stages. It is usually desirable to set the gain as high as possible, but not so high as to drive the system into oscillation. The discussion of specific SPM platforms will follow in the next section. 2.1. Scanning tunneling microscopy (STM)

2. Scanning probe measurement techniques and components of SPM and related TBN platform The major components of an SPM instrument include a sensing tip (also called probe), a piezoelectric actuator that controls the tip’s or sample’s location in all three dimensions, a voltage source for piezoelectric actuation, a means to measure the current flowing through the tip or force experienced at the sample–tip interface, and finally, the necessary computing power to control tip movement (i.e. feedback control system) and transform scanned data into an image [32]. The tip is typically a stylus-shaped material facing the sample surface. The tip–sample interaction mechanism, in either contact or non-contact mode, can be used to gain information about the distance of separation between the two. This can be fed back into the distance control unit, usually the scanner tube, in order to obtain topographic information about the surface. Tips are made of different functional materials such as diamond, silicon, silicon nitride, platinum–iridium, or tungsten. The typical tip radius is 2– 10 nm up to tens of nanometers. Various tip geometries and aspect ratios are commonly used, depending upon the purpose of operation. Examples of these geometries include spherical, conical, and pyramidal tips. Although manufacturing a tip still remains an art to some extent, there is a greater need to develop a methodology for in-line ‘‘health monitoring’’ of a tip tool. Scanner tubes, fabricated using piezoelectric materials like lead zirconate titanate (PZT) or other polarized ceramics, allow the tip to be precisely positioned and scanned (within 0.1 A´˚ in x, y and z) over the sample surface. One of the most important selection considerations for a piezo tube is its scan range, which is determined by the material’s piezoelectric constant, tube size and the maximum voltage that can be applied before the piezo

In STM, an atomically sharp conducting tip (usually tungsten or platinum–iridium alloy) is brought within a few atomic diameters (1 nm) of the surface under investigation [33,34]. Scanning is accomplished without actual physical contact, as such, there is a very small overlap of the wave functions of the surface with the nearest atom of the tip (see Fig. 3). When a small bias voltage (usually in the mV range) less than the work functions of the tip and sample is applied between these electrodes, electrons tunnel across this gap with a probability that increases exponentially as the tip approaches the sample. This provides an extremely sensitive way of detecting small changes in the surface height of a specific sample material due to the individual atoms and their clusters. It is an empirical observation that about 90% of the tunneling current is carried by the apex atom because of the difference in distance between it and the atoms at its base. The stream of tunneling electrons between a conducting tip and a conducting substrate may also be viewed as the narrowest low-energy electron beam. Due to its high sensitivity (typically 0.005 nm) to gap width, the tunneling current is a suitable signal to control the gap by means of an electronic feedback loop. If the tip is scanned across the surface while a current is sensed, a topograph of the surface can be obtained in two ways. In the constant current mode, the tip height is adjusted with the feedback loop, maintaining a constant current [35,36]. The topographic image is obtained with the aid of computer imaging software as a map of the tip height z (x, y) versus the lateral coordinates x and y. Alternately, in the constant height mode the tip is scanned at a constant height and the current I (x, y) is recorded as a function of x and y [37]. Interestingly, it has also been found that the imaging mechanism of STM at atomic resolution is a process of making and breaking partial chemical bonds between the electronic states on the tip and the sample [38].

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the cantilever displacement was detected using a tunneling tip fabricated on the back of the cantilever. Among the subsequently developed techniques for detecting the cantilever displacement, the optical beam deflection technique [39] measures the displacement by detecting the deflection of a laser beam reflected from the backside of the microcantilever, which is directed onto a position sensitive detector (photodiode sensor). The microcantilever’s elastic deflection can be converted to force by applying Hooke’s law F = kz, where k is the cantilever spring constant and z is the measured deflection. The AFM image is typically generated by performing a raster scan of the tip over the surface in x and y while recording the z deflection required to maintain the setpoint (predefined force, amplitude, or deflection). In contact mode operation, either the constant height or the constant force mode can be used. In noncontact or tapping mode operation, the cantilever tip is made to vibrate near the sample surface with spacing on the order of a few nm or intermittently touching the surface at the lowest deflection. 2.3. Nanoindenter

Fig. 3. Schematic illustration (figure by Michael Schmid, TU Wien) [33] and operation mechanism [34] of an STM.

2.2. Atomic force microscopy (AFM) In AFM, the sample to be imaged is placed on the scanner tube and brought close to a very sharp tip mounted at the free end of a thin compliant beam (microcantilever). The beam bends in proportion to the attractive or repulsive force acting on the atom(s) at the apex of [(Fig._4)TD$IG]the tip to atom(s) on the substrate (see Fig. 4). In the original design,

A nanoindenter is capable of performing four operations: (1) apply load (or displacement), (2) measure displacement (or load) with very high accuracy, resolution and precision; (3) position and perform indentations at any desired location on a sample, and (4) interpret load and displacement data to obtain hardness, elastic modulus, adhesion force, fracture toughness, and other mechanical properties. In principle, if a very sharp tip is used, the contact area between the sample and the tip can be made arbitrarily small, thus enabling testing of a nanoscale volume of material. As the issue of determining the exact indentation area arises in such a case, depth-sensing indentation methods were developed. Subsequently, the indenter’s load and displacement are recorded during indentation to obtain the contact area without having to image the actual indentations. Hence in a typical nanoindentation test, a tip or indenter, typically made of diamond, is pressed into the test sample with a known load followed by an intermediate holding period if necessary. After some time when the load is removed, the area of the projected residual indentation in the sample is measured and the hardness, H, is obtained as: H¼

P max Ar

(1)

where Pmax is the maximum load applied and Ar is the projected residual indentation area. It is important to note that the indenter employed has an axisymmetric or pyramidal geometry with a small radius of curvature at the apex. A generalized schematic illustration of a nanoindenter is shown in Fig. 5 [41]. [(Fig._5)TD$IG]

Fig. 4. Schematic illustration [40] and operation mechanism (inset) [34] of an AFM for the optical beam deflection method.

Fig. 5. Schematic of a nanoindenter [41].

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Available commercial and research grade nanoindenters include those developed by MTS Systems Corporation (now part of Agilent Technologies) [42], Hysitron, Inc. (Minneapolis, MN) [43], CSIRO (Australia) [44], CSM Instruments (Switzerland) [45] and Micro Materials Ltd. (UK) [46]. 3. Tip-based nanomanufacturing processes based upon SPM platforms The force or interactions measured between the tip and substrate in SPMs can come from various sources. Other specialized methods have been designed based upon the origin of the force. For example, these methods have been designed to take specific advantage of optical properties (scanning near-field optical microscope or SNOM) [47], friction forces (lateral force microscopy or LFM), electric field responses (electrostatic force microscopy or EFM), magnetic properties (magnetic force and resonance microscope or MFM), thermal properties (scanning thermal microscope, SThM) [48], torsional resonance (TR mode AFM), phase imaging, electrical conduction (C-AFM), and tunneling characteristics (TUN-AFM or TUNA). Scanning force/tunneling microscopy (AFM/STM) [49] has enabled investigation of both the surface corrugation (by AFM) and the local conductance (by STM) simultaneously in the same microscopic area with atomic resolution. Scanning electrochemical microscopy (SECM) [50] is an SPM technique based on the changes in the Faradaic current (electrochemical reactions) as the tip moves across the sample and is used for chemical imaging of the surface. Other innovative methods like scanning surface harmonic microscopy [51] and alternating current STM [52] have also been developed to image insulating films with STM at high spatial resolution. The fundamental understanding and advances in STM, AFM and nanoindentation have allowed progress to be made in new tipbased manufacturing process spin-offs, not only for metrology but also for novel nanomanufacturing processes. These tip-based nanomanufacturing processes use electrical, electrochemical, physical, thermal, optical and other forms of interactions between the tool tip and the substrate to perform deposition, machining, patterning, annealing, and other operations. The following is a discussion detailing these processes and describing the fundamental principles, instrumentation, understanding and applications for each. 3.1. Dip pen nanolithography 3.1.1. Operational principle For nanoscale additive patterning of a broad spectrum of hard and soft materials, direct-write methods, generally known for high-throughput lithography, are preferred as structures are fabricated without the need for additional processing steps allowing rapid and cost-effective prototyping. Often these applications need a lithography process in clean and wet/humid conditions [53,54], and conventional lithography techniques are not always appropriate for writing, especially on soft surfaces. To address this need, modification and manipulation of substrate surface atoms with extremely fine line widths (10–100 nm) using SPM tips have become ubiquitous as a method for controlled nanoscale patterning on such systems. In 1995, Jaschke and Butt [55] reported the deposition of organic molecules from an AFM tip onto a mica surface. Subsequently in 1999, research in this domain lead to the invention of a powerful high-resolution and high-registration direct-write lithographic technology based on bottom-up patterning of selfassembled monolayers (SAMs). Dip pen nanolithography (DPN) was developed by Mirkin and co-workers for thiol molecule lithography on a freshly prepared gold surface [56], and since then DPN has been used to pattern surfaces spanning insulating, semiconducting, and metallic substrates with a wide variety of functional ink materials [57].

In DPN, direct writing involves transfer of the writing material to the substrate of interest from the tip of an AFM via raster scanning, through the process of molecular diffusion in a positive printing mode. The AFM tip is analagous to a pen, the writing material acts as ink, and the substrate surface acts as paper for nanostructures to be drawn on (see Fig. 6(a)). Since this method involves the dropping-in of the ink material onto the substrate surface on a user-defined position, it can also be referred to as ‘‘drop-on-demand’’ fabrication methodology. DPN offers 10 nm line-width resolution, 5 nm spatial resolution, and the typical minimum feature size achievable is 15 nm. Substrates that have been employed include gold [57–59], silicon, silicon oxide, gallium arsenide (see Fig. 6(b)) [60,61], glass [62], etc. Inks that have been studied include small [63–66] and macromolecular organic medium [67,68], sol gels [69,70], bio- and conductive polymers [67,71], metals [72,73], pH-based buffer solutions [74], and proteins [75]. The most commonly used inks in DPN are 1octadecanethiol (ODT) and 16-mercaptohexadecanoic acid (MHA) that form a hydrophobic and hydrophilic SAM on gold surface, respectively. Well-defined nanoscale features fabricated by DPN have been extensively used as nanoscale sensors [76], nanoscale plotters [77], and molecular erasers [78]. They have been employed in manipulation of quantum dots [79], molecular electronics [53,80–83], photonic devices [84,85], and magnetic nanoparticles [86]. Nanoscale templates made via DPN are used for studying polymer crystallization [87], individual virus particles [88], deoxyribonucleic acid (DNA) [63,89,90], carbon nanotubes [91], biorecognition [80,92], and optical and electrical transport in a wide variety of nanostructures [93,94]. 3.1.2. Process mechanism and understanding In the DPN process, ink molecules diffuse from a delivering AFM tip to a surface and self-assemble because of chemisorption or other electromagnetic interactions [96]. It should be noted that deposition does not occur unless the tip contacts the surface, either physically or through a meniscus [97]. As a multi-step process (e.g. writing using a polymer resist) is prone to concerns like cross-contamination, the ability to directly write in a single step is a great advantage, enhancing the chances of process scale-up to multi-pen and multiink systems [61,98–101]. Fundamentally, DPN can be applied to pattern large set of material on virtually wide choice of substrates, provided there is a driving force for moving the molecules from the AFM tip to the substrate. In practice, DPN experiments are generally limited by factors such as the solubility of the desired ink, the transfer and stability of the material within the water meniscus, and the adsorption of the writing material on the substrate surface. Thus, the selection of stable ink dispersions those can be diffused from the tip to the substrate is critical to DPN. The relative humidity and the water meniscus [102,103] that forms naturally under ambient conditions at the point of contact between the AFM probe and substrate also play a critical role in patterning and have been used to regulate the ink transport process [104,105]. Other researchers have demonstrated that hydrocarbons are essentially insoluble molecules in water, making it unlikely that they could be transferred via the water meniscus [106]. It has also been shown that the ink’s diffusion transport rates vary with its deposition time onto the substrate [107]. Another remarkable observation of this diffusion process is that all the deposited molecules obey the same functional form for ink drop-area with respect to time, area ¼ kt þ b;

(2)

where k depends on ink, temperature and in some cases humidity, while b reflects the tip size/coating dependence, and t is the time. 3.1.3. Process scale-up potential The conventional DPN patterning process utilizes a single AFM probe (pen) to perform the writing operation, which results in low writing speed. Typical write speed ranges from 0.1 to 5 mm/s and the throughput is limited by the serial nature of the process. Eq. (2)

[(Fig._6)TD$IG]

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Fig. 6. (a) A schematic diagram of the DPN process where the ink molecules transfer from an AFM probe to the substrate surface [56]. (b) TMAFM topographic images of etched MHA/Au/Ti/SiOx/Si nanogaps [95].

[(Fig._7)TD$IG]

also underlines some of the limitations of a serial-mode DPN. For practical reasons, imaging and deposition are performed with the same tip wherein the inked tips inherently cause contamination on the surface. The overall writing speed can be significantly increased by developing DPN probe arrays to realize parallel writing. To increase the patterning throughput, parallel DPN

patterns have been realized using passive high-density arrays of silicon and silicon nitride DPN pens (as many as 50,000 thus far) with an inter-pen spacing of 1.4 mm [77,108–111], wherein all pens in the array move in unison and draw the same pattern. A schematic illustration of a DPN nanoplotter and two-pen DPN system is shown in Fig. 7 where one tip, designated ‘‘Imaging tip,’’

Fig. 7. Schematic illustration of DPN nanoplotter and the multi-pen DPN system [111].

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is used for both imaging and writing while the other, designated ‘‘Writing tip,’’ is used simply for writing. The imaging tip is used like an AFM tip and is connected with force sensors giving feedback. On the contrary, there is no need of feedback systems for writing tips. The imaging tip identifies alignment marks generated by DPN, predicts overall surface topology, and generates molecules lithographically in an area with coordinates defined with respect to the alignment marks. In this manner, the writing tip(s) reproduce the structure produced by the imaging tip at a distance governed by the arrangement of the tips in the cantilever array. For parallel patterning, the stage and thus the substrate is titled in such a way that the writing tip is 0.4 mm closer to the sample than the imaging tip. In order to ensure that both tips touch the surface during patterning, the laser is positioned on the imaging tip. Thus, in order to implement parallel DPN processes, the stage and substrate are tilted as it is essential that the apex-tips of all the pens in the array contact the writing surface simultaneously. The alignment is quite challenging due to small tolerances and large, high-density arrays. Hence, a contact sensing method has been developed based on the detection of electrical continuity between a scanning pen and a writing surface [112]. Recently, a massively parallel method (called 2D DPN) has been developed to pattern large (1 cm2) areas with 80–100 nm sized nanostructures (see Fig. 8) arranged in complex patterns fabricated within 30 minutes [99]. An additional capability of DPN, which is referred to as ‘‘overwriting,’’ involves generating one soft structure out of one type of ink and then filling in with a second type of ink by raster scanning across the original nanostructure (see Fig. 9). In another, less-used version of DPN, ink is stored in a cantilever at the back of the tip and delivered through a pore pre-fabricated at the apex of the tip [113]. 3.1.4. Thermal DPN (tDPN) Thermal DPN (tDPN) is a spin-off of DPN, where the tip is heated during DPN. tDPN allows the operator to control the deposited amount of the ink onto the substrate surface and to limit the flow of the ink at a defined substrate location by controlling the temperate at the tip–substrate interface. At low temperatures, the ink could remain frozen on the tip/cantilever, preventing transfer to the substrate. At high temperatures, the ink

Fig. 9. Self-assembled monolayers (SAMs) in the shapes of polygons drawn by DPN with 16-mercaptohexadecanoic acid (MHA) on an amorphous Au surface. A 1octadecanethiol (ODT) SAM has been overwritten around the polygons [109].

melts and transfers from the tip to the substrate with certain ease. Lines may be drawn as the tip (pen) is translated across the substrate. Also, by tuning experimental conditions, tDPN allows direct nanopatterning of a number of high-melting-temperature molecules through smooth transport of medium from the AFM tip tool to the substrate at room temperature without preheating the tip. tDPN may also be important for inks that do not have appreciable water solubility (e.g. metals below their melting points). It has been found that heating the probe above the ink’s melting temperature is not a prerequisite for ink delivery, thus extending the ‘‘ink–substrate’’ combinations available through this process [101]. Similar to parallel DPN, tDPN can also be made to operate in a parallel fashion to write multiple nanostructures at

[(Fig._8)TD$IG]

Fig. 8. Massively parallel or 2D DPN: (a) artist’s impression of the arrays of cantilevers (shown in part b) writing nanoscale features; (c and d) demonstration of the formation of complex features using 80 nm diameter dots [99].

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Fig. 10. (a) An array of ten thermally actuated DPN probes showing the power lead and heater layout. Each probe is 300 mm long, 80 mm wide, and 1.3 mm thick. (b) LFM scans, 8 mm square, of ten simultaneously generated ODT patterns on a gold surface. Each numeral is 6 mm tall, 4 mm wide and written at 1 mm/s [115].

the same time using individually addressable cantilever arrays (see Fig. 10). The development of tDPN also opens several opportunities for DPN-based nanofabrication. Thermal control should in principle allow deposition of a wide variety of solid inks that were previously inaccessible to DPN. For instance, metals of appropriate melting temperatures could be patterned with this technique; for example, a nanometer scale ‘‘soldering iron’’ is possible. It is also important to note that the ink molecules in tDPN can be solid at room temperature, making it possible to build multilayer multicomponent patterns to create true three-dimensional nanostructures [114]. 3.2. Tip-based nanoembossing and related mechanical manufacturing 3.2.1. Operational principle Nanoembossing uses a nanoscale tip to make mechanical contact with a surface in order to create nanoscale features of defined size, shape, and spatial position. The operational principle is much like that of a traditional macroscale sheet metal press or embossing process where a stamp is forced downward and impressed into a metal surface [116]. The nanoscale tool tip is of a predefined geometry, chosen for the desired feature shape, and the embossing tip is forced downward into and perpendicular to the substrate surface. The embossing force and applied stress of the tip must be high enough to create plastic deformation and transfer of the pattern into the substrate. Generally, the applied stress of the tip displaces atoms in compression just underneath the tip and tensile perpendicular to the axis of loading. The nanoembossing process is depicted in Fig. 11(a).

[(Fig._1)TD$IG]

Fig. 11. Diagram of (a) nanoembossing process where the tip is forced downward into the substrate surface to create an impressed nanoscale pattern. (b) A typical nanoembossing platform and key components.

3.2.2. Instrumentation Instrumentation for nanoembossing requires nanometer precise control of applied force, actuation, and positioning of the tool tip. Additionally, the instrument must be isolated from environmental noise sources including thermal gradients, mechanical vibration, and electromagnetic interference. A diagram of a typical nanoembossing platform is given in Fig. 11(b). Currently, commercially available AFMs and nanoindenters provide an excellent means for small scale proof-of-concept nanoembossing processes. Applied forces and actuation of the embossing tool tip can be measured and controlled to less than 200 nN and 0.2 nm, respectively, via nanoindentation platforms and less than 1 nN and 0.1 nm for AFM platforms. Both AFM and nanoindenters use a transducer to generate motion and force of the tool tip. A variety of transducing mechanisms are used to generate force, including piezoelectric, electromagnetic [(linear variable differential transformer (LVDT)], and electrostatic (capacitive) transducers. Force is measured via the displacement of a flexure or cantilever beam (of known stiffness) attached to the transducer. Displacement is measured via a position sensor that is integrated into the transducer or independently mounted along the axis of motion. Placement of embossing features is performed by either lateral movement of the tool tip or substrate. Lateral positioning of the substrate along the X–Y axes is performed using a nanopositioning stage or scanner. Nanopositioning scanners are predominantly piezoelectric actuated flexures. The stages allow for positioning over areas up to 100 mm  100 mm. Integrated position sensors (i.e. LVDT, strain gauge and capacitive) coupled with closed-loop control schemes and low noise environments can push lateral positioning resolution to less than 1 nm [117]. For high-load capacity applications, accurate positioning of the substrate is quite difficult and thus positioning of the tool tip is a much better method. For movement of the tip, the force transducer can be mounted to a piezoelectric tube scanner or other type of scanner that allows for precise lateral positioning of the tip. One of the innovative features of several nanoindentation systems is the capability of in situ imaging and metrology via scanning force microscopy of embossed features using the same embossing tip. Such systems are primarily for imaging and are not suitable for nanoscale metrology due to the large embossing tip radius that reduces microscopy resolution. Some systems have dual heads – one embossing head and one AFM head – that allows for both fabrication and high-resolution metrology. In situ metrology is critical to understanding the repeatability of the embossing process and assessing the overall quality of the feature generation process. Additionally, it avoids the difficult issue of relocating nanoembossed features on large substrates (1 cm  1 cm or greater). 3.2.3. Tool tip designs, materials, fabrication processes, analysis Nanoscale tool tips used for embossing have generally been the same tips used in scanning probe microscopy and nanoindentation. Cantilever based tips are typically made out of silicon and

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have a sphero-conical geometry with a spherical end radius less than 30 nm. These tips generate hole-like spherical impressions or features [118,119]. Although the use of cantilever tips allows for embossing at forces less than 1 nN, one key issue of any cantilever based tip is that the tip will never make contact perpendicular with the workpiece surface. Therefore, embossed features will have an asymmetric geometry. Non-cantilever based tool tips, traditionally used in nanoindentation, are available in a variety of geometries such as pyramids, wedges, cones, cylinders, or spheres. The end of the tip can be made sharp, flat, or rounded. For nanoembossing, sharp pyramidal tips are used, as these tips have an end radius less than 100 nm. Three-faceted pyramidal Berkovich and cube corner tips have end radii less than 50 nm with the cube corner being the sharpest (r < 20 nm). Rounded end conical tips are also available with end radii less than 50 nm. Commercially available wedge and flat end tips have square sides or corner radii of 200 nm or greater. The geometry and therefore contact area of the pyramidal tip is defined by the end radius and the face angle, the angle between the tip central axis and any of the pyramid facets given by theta (u in Fig. 12). For Berkovich and cube corner, the face angle is 65.3 and 35.268, respectively. The tips are attached to a tip holder via bonding or press fitted into a ring in the holder. The tip holder can be steel, titanium, or another suitable material. The embossing tips are primarily made from single crystalline silicon or diamond. Sharp silicon tips are fabricated via anisotropic etching [120]. When etching the (1 0 0) surface via appropriate etchants (i.e. KOH, etc.), the (1 1 1) crystal plane has a slower etch rate than the other planes, thereby forming trench or pyramid shapes in the (1 0 0) surface. Silicon allows for the fabrication of a sharper end radius than diamond, but it wears more readily in comparison to diamond [121]. Several anti-wear coatings have been explored to minimize wear of silicon tips including diamondlike carbon (DLC) and SiN [122–124]. Diamond tips have advantages over silicon, such as high hardness, wear resistance, high thermal conductivity, low thermal expansion and chemical inertness. Diamond tips are usually shaped via mechanical grinding or abrasive processes [125]. Other fabrication processes have been explored, including chemical vapor deposition (CVD) growth in silicon molds, and growth of diamond films on etched silicon tips [117,126,127]. Tip shapes other than conical, spherical, and pyramidal can be formed in silicon or diamond by focused ion beam machining [20]. Analysis of tool tip wear has primarily been performed by direct characterization using scanning and transmission electron microscopy [20,122,125]. Indirect characterization methods include a combination of experimental and continuum mechanics theory, such as the Oliver and Pharr method [128], to determine the tip contact area and geometry. Recent indirect methods include

[(Fig._12)TD$IG]

commercially available calibration samples and software routines known as ‘tip characterizers’ – samples are usually well-defined spike-like features, which degraded tips cannot adequately resolve. One of the most reliable and less cumbersome direct methods involves AFM imaging of the tip with ultra sharp (2 nm) probes, which provides one of the most accurate methods of measuring tip geometry [129,130]. 3.2.4. Manufacturing process understanding Nanoembossing can be used as a process to reproducibly create nanoscale features in a surface with control over feature shape, size, and spatial position. The tip geometry and emboss depth into the surface determines feature shape and size. It is important to note that for shallow depth features (<10 nm) using a non-flat tip, the embossed feature geometry will be spherical due to the spherical end radius of the tool tip. The sharpest diamond tips have an end radius of 20 nm while silicon tips can have radii down to 2 nm. However, embossing operations quickly dull the silicon tip to a larger radius. Above a critical depth of emboss (>10 nm), the embossed feature will take on the specific geometry of the tip (pyramidal, conical, etc.) [131]. Thus for low depth embossing, the sharpest tips should be used in order to create well-defined feature geometries. The ultimate minimum feature size is determined by the tip radius and applied stress; currently, features as small as 5 nm in width and 1–2 nm in depth can be produced. Applied stress is an important consideration in the nanoembossing process because in order for the feature to be impressed into the material surface, the applied stress must be greater than the critical stress to induce plastic deformation. The tip geometry, size, and applied force determine the applied stress and strain gradient underneath the tip. These three parameters must be carefully chosen and controlled to ensure process repeatability. Also, the stress and strain gradients may cause changes in the material atomic structure including phase transformations, work hardening, or fracture, all of which may be undesired effects [132–134]. The stress fields and depth of emboss can produce surface effects near the embossed feature due to the flow of atoms, including pile-up and sink-in [131]. Spatial positioning of the embossed features is controlled by either positioning of the tip or sample. While piezoscanners allow for lateral positioning resolution to <1 nm, the minimum embossed resolution is dictated by the embossed feature size. The feature center-to-center positioning of non-overlapping features can be no smaller than d = 2r, which is twice the feature radius. A significant error in positioning embossed features using piezoelectric materials is the inherent hysteresis and creep of the material, which results in non-linearities and undesired offsets in the commanded position and pattern geometries [135]. In

Fig. 12. (a) Drawing of nanoindenter tip; (b) 3D AFM image of diamond Berkovich tip showing the face angle given by theta; (c) 2D AFM image of Berkovich tip; (d) line profile along the X-axis of boxed area in (c) AFM image, indicating tip radius 47 nm.

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addition, positioning systems are subject to errors due to thermal drift and vibrational noise sources. The above mentioned errors can be attenuated and compensated for by using integrated position sensors with closed-loop and control schemes such as feedforward and iterative-based methods [135]. Repeatability of creating embossed features is governed by the ability to control the applied force, depth of emboss and tip shape. The use of closed-loop feedback control of applied force and depth of emboss allows for highly repeatable embossing operations with force uncertainty to 1 uN or less and depth uncertainty to 1 nm. Note that these values are subject to low noise floors and transducers with stiffness <800 N/m. Comparison of force versus depth curves provides an excellent measure to assess repeatability, as shown in Fig. 13. Tip shape variation is another significant factor that increases embossed feature uncertainty. Tip shape after a certain number of embossing operations can change significantly, depending on the tip–substrate material and loading conditions, resulting in featureto-feature uncertainty. Even diamond tips undergo considerable wear. Thus it is important to monitor tip shape periodically to ensure reproducibility of embossed features. Current feature rates of embossing vary by instrument platforms. This rate is primarily governed by the speed of tip positioning, tip–surface approach, contact detection, and desired force profile. Theoretically, features can be embossed at rates much less than 1 feature/s. Current feature rates on AFM and nanoindenters are in the range of 1 feature/2–12 s. Various geometries of embossed patterns can be made, including circles, polygons, linear n x n arrays and hexagonal arrays (see Fig. 14). Scripted numerically controlled patterns can be easily programmed into the instruments to provide custom pattern geometries. The magnitude of embossed features is limited by the memory processing/data storage of the instrument and maximum scan size of the nanopositioning scanner. Typically, arrays no larger than 100 mm  100 mm can be made in one batch operation. However, combining the nanopositioning scanner with larger course positioning stages provides for patterning of samples much larger (1 cm  1 cm or greater). The use of multiple probe embossing systems in the future should provide relatively larger scale patterning of surfaces as a routine manufacturing operation [136]. 3.2.5. Specific case study and industrial application Nanoembossing enables a generation of nanoscale surface features with control over individual feature size, shape and spatial registration. The applications and advantages of this approach are (1) it provides low-cost nanofabrication and patterning since no cleanroom processes are required; (2) it provides in situ metrology of embossed features for quality control using AFM and potential for in situ characterization using a variety of SPM modes including

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Fig. 13. Load depth curves of embossed features on GaAs (1 0 0) using a cube corner tip showing the repeatability of embossed features as shown the overlapping portions of the loading curves.

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Fig. 14. (a) AFM image of nanoembossed pattern on fused quartz made by a Berkovich tip. (b) Line profile of embossed features in (a), features are 100 nm in width, 10 nm in depth, and 190 nm spacing. (Imaged with the MFP-3DTM AFM, Asylum Research, Santa Barbara, CA).

magnetic force, capacitance and electron spectroscopy; (3) it allows for minimal substrate preparation as embossing is done on surfaces with no chemical processing needed; (4) it allows for subnanometer positional control of fabricated features; (5) it enables highly localized modulation of surface properties; (6) it allows patterning of a wide range of both soft and hard materials; (7) it can be applied to non-planar surfaces; and (8) it enables nanoscale rapid prototyping of nanostructures and devices. Epitaxially self-assembled quantum dot (QD) nanostructures offer tremendous potential for fabricating phenomenally powerful electronic and optical devices. Due to their small size (<80 nm), the electron energy levels in QDs are highly discrete (quantized), resulting in unique electronic properties such as the confinement of a single electron and highly efficient electron transport. These unique properties can be exploited to create innovative quantum electronic circuitry, including chip-to-chip wireless interconnection, highly coherent and wavelength tunable QD lasers, ultra-high resolution color displays, photodetectors and spintronic-based logic processors capable of high computational speeds. The development of such devices and advantages of quantum confinement have not been fully realized, however, due to the difficulty in reproducibly fabricating precise patterns of QDs, including control over dot size, shape, positioning and density during self-assembly. For example, under typical self-assembly conditions, the QDs are observed to be significantly non-uniform in size (10% variation), shape (two to three different shapes) and position (nearly totally randomly distributed). In recent years, significant progress has been made toward improving both uniformity and positioning of QDs using various patterning techniques [137]. However, these techniques lack in their ability to deliver well-engineered and patterned structures with nanometer precision required for quantum circuitry, reproducibility, heterogeneity, chemical cleanliness, repeatability and throughput. Additionally, many of these techniques are complex and costly, for example,

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[(Fig._15)TD$IG]

requiring multiple-step cleanroom processes or energetic beams to create patterns. Nanoembossing is potentially an effective, low-cost, scalable and robust process to enable precision QD patterning. One route to patterning or directed assembly of QDs is made possible by introducing energetically favorable areas on epitaxial films, such as a strain-relaxed site or where there is a substantial density of seed material at a given site. QDs nucleate from these favorable sites. Recent work at IBM has shown that directed assembly of QDs can be achieved by the formation of subsurface misfit dislocations [138]. Elasticity calculations and transmission electron microscopy (TEM) reveal that the mechanisms by which patterning occurs can be attributed to the underlying dislocation strain field and possibly by steps formed at the surface by the propagation of the dislocations through the epitaxial layer. However, irregular patterns were formed because precise control of misfit dislocations is not feasible due to the arbitrary formation of dislocation sources at the heterointerface. But if the placement and formation of subsurface dislocations can be controlled, then it would be a highly effective patterning mechanism to regulate growth surface strain. Nanomechanical contact by a probe tip on the substrate surface creates dislocation-mediated deformation; and thus it can be used as a tool to regulate surface strain with sub-nanometer precision. In the QD formation process, due to the difference in lattice parameter (7%) of the deposited thin film material (InAs) on the substrate material (GaAs), the thin film is in a state of strain. As a result of this unstable, high-energy strain state, upon reaching a critical film thickness, the thin film morphs itself into threedimensional islands or quantum dots that are stable and of a lower energy strain state. This process is a balance between the recovery of strain energy from the psuedomorphic growth and the formation of the new facets [139]. Therefore, the reduction of strain due to lattice mismatch is one of the predominant driving forces for QD nucleation during heteroepitaxy and the other is control of the facet geometry. Mechanical contact by a single diamond probe tip has the potential to produce both of these features. The diamond tip creates a local tensile strain gradient perpendicular to the axis of loading just underneath the tip and dislocation-mediated deformation over a region of a few nanometers in size as depicted in Fig. 15(a) and (b). This tensile strain gradient, at an atomic scale, represents a local perturbation or increase in the substrate lattice parameter thereby locally lowering strain due to lattice mismatch. As a result, the lower strain biases adatom diffusion making nucleation of QDs more favorable at these sites. In addition to this factor, the tip modifies the surface structure locally by changing the surface free energy. This factor has the potential to dramatically influence the reorganization of deposited strain layers. By precisely controlling the applied force and actuation of the probe tip, nanoscale surface

strain patterns or templates with new surface facets can be manufactured. Followed by molecular beam epitaxy deposition on these templates or annealing, remarkable arrays of patterned QDs can be produced. Scaling-up this approach using highly parallel tips in a nanoembossing modality is extremely promising as a high-precision, low-cost and low complexity QD (and other related nanostructures) manufacturing technology. The nanoembossing approach has been demonstrated to pattern InAs QDs on GaAs (1 0 0) substrates [140,141], as shown in Fig. 15. It is clear from Fig. 15(c) that large dot structures were formed in a linear array following the underlying nanoembossed template. The nanoembossed features provide a strong surface bias to direct the diffusion of deposited atoms on the surface to form linear arrays. This approach is continuing to be refined and investigated for the fabrication of highly uniform QD superlattices for multiband optical detector applications. 3.3. Tip-based nano-electromachining (Nano-EM) 3.3.1. Operational principle Under the application of a very high electric field between two surfaces separated by a dielectric (such as a vacuum), the gap between the surfaces breaks down [142], and material removal occurs. As early as the year 1912 [143] Calka and Wexler demonstrated mechanical milling with electrical discharges. The credit of the invention of the macroscale electric discharge machining [144] process goes to Lazarenko B.R. and Lazarenko N.I. in the year 1947. Since then, there have been numerous studies in the area of electric discharge machining, details about which are in the following references [145–155]. With the increasing need for miniaturization, microscale electric discharge machining was demonstrated, and is summarized in a paper [142] by Kuneida et al. The natural progression led from the microscale to the nanoscale adoption of a discharge based machining process. With the invention of the STM and subsequently of SPMs a common theme emerged of probe-based machining. Some of these probebased machining techniques rely on the application of an electrical bias between a probe and a sample to cause material removal. Based upon this principle, the goal for nanoscale machining [156] is to develop a process capable of nanomanufacturing difficult-tomachine/fabricate heterogeneous materials such as specialty steels, titanium alloys, gold, invar, etc. The breakdown of liquid dielectrics confined between a nanometrically sharp tool tip and workpiece surface results in surface machining. The extent of nanoconfinement (varies from a few angstroms to tens of nanometers) not only dictates the field strength but also controls the divergence of the electric field from a sharp electrode and the nature of dielectric between the tool and the workpiece [157]. From a manufacturing perspective, it is important to note that end radius and profile of the tool governs the size of features produced

Fig. 15. (a) Diagram of nanoembossing of GaAs (1 0 0) and dislocation-mediated deformation; (b) diagram of surface strain resulting from nanoembossed template that acts to spatially bias nucleation of InAs QDs; and (c) AFM image of InAs QDs on unpatterned and nanoembossed patterned regions.

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on the workpiece surface. In a typical nano-EM setup, an electric field of the order of 1 V/nm is applied across a gap filled with ndecane dielectric between a 35 nm sharp tool [platinum–iridium (Pt–Ir) or tungsten (W)] and an atomically flat gold surface to produce nanoscale features on the workpiece. 3.3.2. Instrumentation and specific process conditions An established STM with an advanced controller was modified to perform nano-EM as per the schematic shown in Fig. 16. The SPM platform is chosen to operate in STM [158] mode as it provides a system with two electrodes (similar to a conventional die-sinking EDM) that can be operated in air and liquid media, and also be used for in situ surface-scanning and monitoring of tool tip quality [159]. Nano-EM is performed at room temperature and pressure [160], and the setup is equipped with a signal access module that enables monitoring of the power required for machining. The distance between the tool and the workpiece is controlled using the tunneling current feedback as well as with voltages applied directly to the piezo-ceramic tubes. Based upon the separation between the tool and the workpiece, nano-EM can be performed under two different machining configurations: near-field and farfield. In the near-field configuration, it is possible for electrons to directly tunnel from the tool tip to the workpiece and vice versa under the application of a bias less than or equal to 200 mV. STM’s scanning feedback loop typically maintains the tunneling current between the tool and the workpiece at 500 pA for a 50 mV applied bias. In this mode of nano-EM, the applied voltage is then increased to allow machining of the smallest possible features (about 8 nm in diameter and 2 nm deep) on the gold surface. As the tool–workpiece separation is between 3 and 25 nm in the far-field configuration, it is not possible for electrons to directly tunnel from the tool tip to the workpiece and vice versa under the application of a bias less than or equal to 200 mV. Far-field nanoEM, in regards to instrumentation, hence bears a close resemblance to macroscale die-sink EDM [161], where the tool–workpiece separation is on the order of a few microns and the electron tunneling probability is close to zero. At 200 mV bias, the electrons could approximately tunnel until they reach a tool–workpiece

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separation of about 1 nm in air. Thus the lower limit of 3 nm of tool–workpiece separation is chosen to avoid any effects of electron tunneling in machining and the upper limit of 25 nm is imposed by the capacity of the power supply. There are two submodes of far-field nano-EM, namely direct current (DC) and pulse mode. In DC mode, linearly increasing voltage is applied until the tool–workpiece gap breaks down resulting in surface machining; whereas in pulse mode, 2 ms long pulses are applied for surface machining. Although both DC and pulse mode far-field nano-EM machine features on gold, the tool wear in the latter case is noticeably smaller than that in the former. 3.3.3. Tooling 80% Pt-20% Ir and 99.99% pure polycrystalline W tool tips are used for nano-EM, where Pt–Ir tips are generally produced by mechanical shearing and W tips by a special etching setup. A commercially available electrochemical tool etcher from Obbligato Objectives Inc., Canada, is used for etching W tips. The instrument consists of a high purity (>99.99%) gold dual ring (8–9 mm dia. upper ring and 3–4 mm dia. lower ring) setup for etching, a tip mounting holder attached to a micrometer, electronic circuits with abilities to apply DC and pulsed DC voltage and to monitor the current during the etching process. Although sodium hydroxide (2–3 M) and potassium hydroxide are commonly used to electrochemically etch W tool tips, the former is preferred as previous literature [162–169] suggests that etching with KOH results in greater amount of residue on the tool tips after etching. As voltage is applied between the tool tip wire and upper ring, the following chemical reactions take place [162] resulting in etching of tip at the upper meniscus, while the lower ring serves as part of the monitoring circuit (see Fig. 17): At the ring or cathode: 6H2 O þ 6e ¼ 3H2 ðgÞ þ 6OH

(3)

At the wire or anode: WðsÞ þ 8OH ¼ ðWO4 Þ2 þ 4H2 O þ 6e WðsÞ þ 2OH þ 2H2 O ¼ ðWO4 Þ2 þ 3H2 ðgÞ

(4)

Two tool tips are obtained (one retained in the tool tip wire holder and the other drops off) and the net tool shape is a result of two processes: chemical etching of W wire and drawing of the tool due to the weight of the lower part of the wire. Since the lower tool drops off under gravity, this tool preparation method is commonly referred to as the ‘etch drop-off’ technique. Once tool etching is complete, tool quality is quantified based upon STM current– displacement (I–Z) spectroscopy curves which depict the variation of tunneling current as the tool is moved away from the workpiece surface under the application of a small constant DC bias. For a detailed discussion of this method to obtain I–Z curves, refer to [159].

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Fig. 16. Schematic of the nano-EM setup.

Fig. 17. Schematic of the tool etching setup.

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Fig. 18. Near-field nano-EM with long DC pulses [159].

3.3.4. Nano-EM process understanding In near-field nano-EM, the tool and workpiece are in the near field of each other such that the instrument always maintains feedback with 500 pA of feedback current for a bias voltage of up to 200 mV. Near-field nano-EM is performed at room temperature and pressure with long DC pulses and Fig. 18 shows the letters ‘NSF’ machined using nano-EM with a Pt–Ir tool in an oil environment. As Fig. 18 clearly illustrates, the tool retained its ability to machine as well as to resolve atomic steps on the surface after machining. This clearly establishes nano-EM as a potential nanoscale machining process with in situ metrology capability. Fig. 19 shows that material removal rate (MRR) increased linearly with the length of the pulse duration under DC bias for increasing machining times. The fit of MRR with time is a straight line with a 0.95 correlation coefficient suggesting that near-field nano-EM is a predictable machining process. This behavior was expected as longer times enabled deposition of greater amount of energy in the tool–workpiece gap. The error in the measurements in Fig. 19 rises due to the fact that depth of the feature could not be measured with sufficient accuracy using state-of-the-art STM tips. The instrument’s feedback system also prevents the tip from accidentally crashing into the workpiece thereby limiting the depth resolution. In far-field nano-EM with both DC and pulse bias, a known tool– workpiece gap is maintained by applying an external bias to the Zpiezo-ceramic tube. This bias is then linearly increased until a breakdown of the gap and subsequent machining of the workpiece occurs. The atomic numbers of transition metals W, Pt and Ir are 74, 78 and 77, respectively. The work function of tungsten is about 4.5 eV and that of Pt–Ir alloy is about 5.5 eV. The electric field strength required for dielectric breakdown varies within 10% with the change in cathode materials for W and Pt–Ir, unlike a 100% variation at the macro- and microscales. This phenomenon suggests that the breakdown is related to the confinement of the molecular dielectric in nanoscale gaps and the

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Fig. 19. Material removal rate of near-field nano-EM.

net applied electric field stress. The following model is created to understand the dielectric breakdown behavior in the nanometer regime. Upon increasing the applied voltage to generate field strength of about 0.9 V/nm, the gold atoms from the atomic steps [170] and W tool atoms [171] started to migrate into the tool– workpiece gap. Due to this high field strength, positive space charges [172] form at the gold anode. It has been shown that at the nanoscale, the primary energy input into the gap is from the field emission process [173,174]. These field emitted electrons cause chemical ionization of the dielectric species [175]. The ions produced as a result of this phenomenon, in addition to the gold and tungsten ions produced due to atom migration, are a possible cause of the high current (low resistance of the gap) at breakdown. Due to such high current, the process hereafter is referred to as a ‘field avalanche’. Even though the breakdown field predicted by the existing spark discharge models [176] is about 7.5  107 V/m, the experimentally measured field is about 1  109 V/m. The breakdown field strengths obtained for n-decane and n-undecane compares within 10% with the field strength required to initiate breakdown for W covered with a layer of epoxy (1 V/nm) [177]. Thus, the nanoconfinement of the dielectric liquid molecular medium masks the effect of work function of W and Pt–Ir alloy electrode materials. Another interesting observation about the dielectric recovery is that the gap did not recover its strength at the voltage the breakdown occurred in the case of DC breakdown. The current continues to flow through the gap even when the voltage has dropped to 50% of the breakdown voltage. This is attributed to ionized species still present in the gap in the voltage decay region. The study of pulse mode of far-field nano-EM provides a better understanding of the gap strength recovery [178]. 3.3.5. Nano-EM tools: wear analysis In addition to machining of features in gold, wear effects are also observed in the nano-EM tools. Unlike the wear effects in macroscale EDM operations, the tool wear in nano-EM led to the eventual sharpening of the tools. These effects are attributed to the unique initial profile of the tool as well as the nature of the electric fields present across nanoscale gaps. With the use of high capacity power supplies (200 W) the entire tool vaporizes due to large amount of heat and current flow through the gap [171]. A lower capacity (50 W) power supply allows a very controlled understanding of the evolution of tool wear process at the nanoscale. The machining process is interrupted at three different field strengths of 0.675, 0.8 and 1.1 V/nm and each experiment is performed for a tool–workpiece separation of 8 nm. The electric field required for the breakdown of the dielectric is about the same magnitude of 1 V/nm for both high power and low power conditions for machining. Upon increasing the applied voltage to cause field strength of about 0.675 V/nm, the gold workpiece atoms from the atomic steps [179] and the W tool atoms migrate into the tool–workpiece gap. The oxygen molecules, which are invariably adsorbed on the workpiece, react chemically with the tool. A second source of oxygen is from n-decane, which also contains dissolved oxygen and other impurities in trace quantities [175]. These chemical reactions between the migrating tungsten atoms and oxygen molecules form the nanocrystalline coating on the top of the tool. Simultaneous to this chemical reaction, the dielectric n-decane also undergoes ionization. The primary products of the ionization reaction are carbon and hydrogen, the constituent molecules forming n-decane, which led to the formation of a nanocrystalline matrix of tungsten oxide, tungsten, graphite and amorphous carbon on the tool as is revealed by EDX and EELS analysis. Previous studies by Tanabe et al. [180] on micro-EDM electrodes show that both a high electric field and a high current are required for sharpening cathode W electrodes. However, in nano-EM such effects are observed primarily as a result of the electric field. For a field strength of 0.8 V/nm, the current densities for material migration are two orders of magnitude smaller when

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compared with this study (8  105 A/m2 compared with 4  107 A/ m2). Finally, when the voltage increases to cause field strength of 1.1 V/nm, the entire interface vaporizes and the nanocrystalline coating on W becomes a very stable coating covering the entire tool apex nearly uniformly. Therefore, electric field driven chemical and mechanical changes of the tool–workpiece surface leads to tool sharpening and a stabilization of a conducting tungsten oxide protective coating on the tool. 3.3.6. Other studies and nano-EM Micron and sub-micron scale features such as holes and mounds have been produced with setups similar to a scanning tunneling microscope. Rokhlin and co-workers [181] in their studies reported on the formation of micron sized features in gold films with etched platinum–iridium probes. Vallance et al. [182] have presented studies on using carbon nanotubes as tools for nanoscale machining of materials. Numerous other studies on the creation of nanoscale features have been reported with scanning tunneling microscopes [16,183–204]. Nanoscale electromachining has the potential of the following features: (a) introduction of a consistent medium (liquid dielectric) between the tool and the workpiece eliminating the effects of factors such as relative humidity, (b) control of tool–workpiece gaps from sub-nanometer to tens of nanometer scales and the development of Paschen curves which enable the translation of the process across laboratories, (c) room temperature and pressure operation of the instrument reducing cost and improving ease of operation, (d) in situ metrology of the machined features, (e) in situ tool quality monitoring, (f) in situ tool sharpening, (g) use of environmentally benign chemicals during machining, and (h) generation of consistent and predictable machined features in materials. These characteristics of nano-EM make it an attractive process for probebased nanomanufacturing. AFM has been used as a platform to generate holes, cavities and grooves at nanoscale in copper (Fig. 20(a)) and polymer films (Fig. 20(b)) by applying current pulses of 2–10 s. A nanogroove machined using an atomic force microscope is shown in Fig. 20(a). Machined features were found to be stable within the period of study. However, it was noticed that destruction was caused when larger scanning forces were used. Further, electrostatic forces were found to be of concern in maintaining interelectrode gap distance during electromachining [205–209]. 3.3.7. Nano-EM: scale-up potential The current findings are vital for a controlled nanomanufacturing process that is capable of machining materials such as III–V compounds, metallic nanoparticles, thin diaphragms, and other at sub-100 nm scales for applications such as the fabrication of optical sensors, ultra-dense memories, drug delivery orifices, and other. Further work on reproducibility and repeatability of nanoEM has demonstrated excellent performance [211,212]. At this time, nano-EM is being developed to produce functional components and structures of scientific and technological importance. It has to be stated that for nano-EM to be a potential nanomanufacturing process, further scientific and engineering challenges need to be addressed and resolved for the success of the process in a mass-production environment. 3.4. Nano-electrochemical machining, deposition and transformation 3.4.1. Fundamental operation principle Micro- and nano-electrochemical machining and deposition, in ideal situations, can be viewed as the application of the principles of electrolysis at nanoscale for material removal and deposition. The operation principle is based on the fact that by reducing the duration of the current applied between the electrodes, the electrochemical reactions can be confined well within the close proximity of the electrodes [213]. These electrochemical reactions in corresponding processes at macroscale are governed by Faraday’s laws of electrolysis [206–208]. At nanoscale, however,

Fig. 20. (a) Nanogroove (1000 nm  150 nm  2.4 nm) machined in copper using atomic force microscope [205]. (b) AFM images of polymer structures corresponding to the zeroth/first/second order EHD wave patterns [210].

based on the workpiece materials and potential applied, the fundamental operation principle of nanoscale electro/chemical related processes involve several mechanisms, including electrochemical transformation and anodic oxidation. Anodic oxidation, though a chemical process, is often viewed as a nanolithographic process and is not discussed here. 3.4.2. Instrumentation The traditional approach involves downsizing the electrochemical cell to nanoscale. It includes characteristic features such as bulk liquid, potentiostat and balance electrode [209]. An alternate approach builds a liquid meniscus between the tip and the substrate. Electrochemical reactions are initiated in the meniscus by the direct bias of the tip and the substrate. AFM/SPM has been used in both approaches for nanoscale modification of surface morphology by a material addition on the substrate, as well as a removal and/or manipulation of the substrate material [214]. Electrochemical nanomanufacturing by the first approach involving AFM is generally known as electrochemical AFM (ECAFM) and the second approach can be considered an electrochemical version of the DPN process (Section 3.1). The schematics of both

[(Fig._21)TD$IG]

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Fig. 22. Electrochemical deposition of iron nanoclusters on Au (1 1 1) [219]. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.

is shown in Fig. 20(b). In addition to EHD, any pre-existing defects, such as buried cavities in the polymer, may cause nanoblister formation and the creation of hollow pillar-like structures by electrostatic detachment mechanism. It is reported in [220] that the occurrences of the two modes of mechanisms depend on the field strength and the efficiency of probe-induced joule heating. In summary, nanoscale electrochemical processes allow machining as well as deposition, offering opportunities to work with electrically nonconducting materials, unlike nano-EM. 3.5. Tip-based laser assisted nanomanufacturing

Fig. 21. Schematic of AFM/SPM based electrochemical nanomanufacturing: (a) Approach 1 and (b) Approach 2.

approaches are shown in Fig. 21. It is also feasible to use microliquid droplets instead of the above mentioned bulk liquid or meniscus approaches [215]. 3.4.3. Tooling Commercially available AFM tips are used as tool electrodes in machining/deposition and also as an AFM probe for the measurement of those machined features. Often, coated tips with tip radius of 40 nm are used in such experiments. In the case of experiments in STM mode, tool tips are prepared as described in Section 3.3.3. 3.4.4. Process understanding Material deposition studies were attempted at nanoscale as early as 1988. STM tip induced chemical vapor deposition of 10 nm wide metallic features by the decomposition of organometallic tungsten and gold is reported [216]. A chemically related nanomachining process reported in 1989 involves the application of short voltage pulses in the range of 3–8 V for 10–100 ms between an STM tip and graphite substrate to machine Ø 4 nm holes [217]. Using ionic liquid electrolytes in situ STM, light and transition metals as well as compound semiconductors were recently deposited on metal and semiconductor substrates [218]. Iron nanostructures deposited on Au (1 1 1) from 1-butyl-3methyl-imidazolium tetrafluoroborate (BMI-BF4) ionic liquid by electrochemical deposition is shown in Fig. 22, demonstrating good control at nanoscale. In addition to metals, other materials have also been tried as work materials. Micro- and nanoscale patterns were formed on polymer films by combining the target specific capability of AFM and the surface wave amplification of electrohydrodynamic (EHD) instability [210]. AFM images of well-defined polymeric wave patterns were formed by the surface waves caused by the electrohydrodynamic destabilization mechanism on a 8 nm thick molten polymer film due to the application of 12 V tip bias for 10 s

3.5.1. Fundamental operating principle Lasers are generally not amenable to nanoprocessing (direct writing) due to the optical diffraction limit which constrains the spot size to be no finer than the half-wavelength. Visible and IR lasers have wavelengths on the order of 400–800 nm and therefore are normally not suitable. While it is possible to achieve nanosized spots by further reducing the wavelength, working in the deep ultraviolet and X-ray regimes is less attractive due to cost and safety factors. Ion beam and electron beam processes are similarly less attractive. Other factors include speed of processing, simplicity of setup, and large area processing [221]. The diffraction limit does not apply to near-field laser assisted nanoprocessing which offers the capacity to produce surface features as small as 10–50 nm. First demonstrated by Gorbunov and Pompe [222], the method involves interaction of a laser with a nanosized electrically conducting particle or nanotipped probe. Shown in the Fig. 23(a) inset is an oblate spheroid. When a laser beam of intensity Eo traveling parallel to the major axis impinges on the spheroid, the resultant electric field on the opposite surface near the major axis E is intensified according to the graph. For the case of spherical nanoparticulate probes, the electric field intensification also occurs along a line parallel to the optic axis of the incident laser beam [221,223,224]. The size of this concentrated field is approximately the minor radius [225] and is smaller than predicted by the Mie Solution for light interactions with fine particulate [223]. Fig. 23(b) is a calculation of this electric field intensity enhancement based on the Mie Solution as a function of the size parameter 2pa/l, where a is the probe radius and l is the laser wavelength [221]. The electric field is generally intensified by a factor of 40–150 times the incident laser electric field intensity [226]. STM probes are well suited as manufacturing tools when the laser impinges in near orthogonal fashion on the probe tip (i.e. a vertical STM probe illuminated on its tip by a nearly horizontal laser beam). This hybrid tip is based on a nanomanufacturing process. The field intensification effect has been attributed to an interplay of electrostatic, surface plasmon and electromagnetic resonance effects [222]. While the effect may be photolytic, for much nanoprocessing, the thermal effect is used. Mai et al. [225] described the thermal state in the near-field region of an intensified electric field. They

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Fig. 23. (a) Electric field enhancement compared to the incident laser beam intensity, measured at surface major axes. The laser beam optic axis is shown by the arrow on the inset [222]. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission. (b) Mie Solution for the laser intensity enhancement under a particle irradiated by a laser. a is the particle radius and l is the laser wavelength [221].

calculated an increase in temperature DT equal to pffiffiffiffi ð1  Ru ÞF sin u paFð1  Ru Þwo cos u ffi þ DT ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 8kT t 2 lopt þ 4Dt

(5)

where u is the laser incident angle, Ru is the u-dependent sample reflectance, F is the laser fluence, lopt is the optical absorption length, D is the thermal diffusivity, t is the laser pulse duration, a is the electric field enhancement factor equal to E/Eo, wo is the spot size of the near field and kT is the thermal conductivity. The temperature rise has been computed to be on the order of 100– 800 8C which for tips results in an expansion relative to the sample of 2–5 nm [226,227]. Since the temperature increase is proportional to the laser intensity F/t, a short pulse duration with a high repetition rate is preferred. Grigoropoulos and co-workers [226–228] have produced features as small as 10 nm with femtosecond lasers. The added advantage of femtosecond processing is minimization of adjacent thermal damage as the laser–material interaction time is shorter than the electron thermal diffusion duration. 3.5.2. Instrumentation and specific process conditions related to the manufacturing technique An experimental nanotip apparatus is shown as Fig. 24. The [(Fig._24)TD$IG]laser impinges on a tool tip in near proximity of a surface. The

Fig. 24. Experimental apparatus for near-field laser assisted nanoprocessing. Similar to standard STM, a sample is oriented relative to a probe. The laser is focused onto the probe tip, typically at an angle u equal to 5–158 [221].

electric field is strongly focused in the region between the tip and sample. The intense electric field acts as an energy source that may be used to create pits and other features. By translating the probe relative to the surface, it is possible to create trenches. Interactions of the intense field with species in the atmosphere may be used for an additive (deposition) nanomanufacturing process. Several researchers have placed nanosized particles in a random or self-assembled array on a surface and radiated them with a pulsed laser [221,223,224]. Fig. 25 shows self-assembly nanoprocessing using 640 nm silica spheres on a borosilicate glass substrate back-radiated with a 1064 nm laser using a fluence of 3 J/ cm2. The holes are 350 nm in diameter. 3.5.3. Tip designs, materials, specific fabrication process and analysis Near-field laser assisted nanoprocessing probes may be either STM tips or particulates spread randomly or by self-assembly on the sample surface. Tip materials to date have been standard STM tips: tungsten [225,229,230], doped/coated or untreated silicon [221,223,225–227,231] or standard coated fibers [228,232]. To maximize the near-field effect, a silver tip has also been employed [222]. Particulate probes include silicon [223], silica [221,223,224] and polystyrene [221,223,224]. A wide variety of sample materials have been nanoprocessed. Material removal processes have been performed on gold [221,223,226,227,231], silicon [221,226], copper [223], aluminum [221,223], glass [224] and silicon carbide [224]. Materials deposition is also possible by additive mechanisms. Currently, the following materials have been processed using nanodeposition: gold [222], silicon [225,228], tungsten [229,230] and aluminum [228]. Laser operating parameters vary widely. In some cases, single pulses are used with power densities reported at 7–11 MW/cm2 [223]. Typical multiple pulsed laser power ranges from 100 to 800 mW/cm2 [222,223]. Energy fluencies range from a threshold value of 12 mJ/cm2 [226] to 6 J/cm2. Pulse durations vary from 80 fs [226,227] to 5 ms [222], although more typical values range between 5 and 25 ns. Samples have been laser assisted near-field nanoprocessed with feature sizes as small as 10 nm [221,226,227] to as large as 450 nm [224,232]. Specific process features are typically holes produced under stationary probes. Moving the probe slowly at rates of 0.01–1 mm/s results in formation of nanosized troughs [221,223,226]. Deposition processes include formation of oxide [225] as well as generation of nanocoatings on tungsten tips [229,230]. Under low-energy modes, autogenous formation of unstable, nanosized hillocks has been observed [222,226]. The

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Fig. 25. (a) Schematic showing the back irradiation of 640 nm silica nanospheres by a 1064 nm wavelength laser. (b) Self-assembled array of silica particles on a borosilicate glass substrate. (c) Hexagonally arrayed indentations on the glass surface after a single pulse laser irradiation with a fluence of 3 J/cm2 [224].

[(Fig._26)TD$IG]

Fig. 26. Near-field laser assisted nanoprocessing of pits using an 800 nm femtosecond laser and an AFM silicon tip. The plot abscissa is the horizontal distance profile, and the ordinate on the plot is the nanopit depth in nm. The laser fluence was 100 mJ/cm2, operated at 0.5 kHz for 2 s. Feature size as small as 10 nm was obtained with nanopit depths as deep as 10.5 nm [227].

hillocks were approximately 20 nm tall and degraded temporally, presumably due to surface diffusion. 3.5.4. Specific case studies and related industrial applications Mai et al. [225] deposited oxide onto silicon. The formation of nanopits or holes on gold using an 800 nm femtosecond laser and

[(Fig._27)TD$IG]

an AFM silicon tip is shown in Fig. 26 [227]. The abscissa is the horizontal distance, and the ordinate on the plot is the nanopit depth in nm. The laser fluence was 100 mJ/cm2, operated at 0.5 kHz for 2 s. A feature size as small as 10 nm was obtained with nanopit depths as deep as 10.5 nm. Fig. 27 illustrates the formation of a nanoline array on gold with a trough width of 17 nm and depth of 4.4 nm [223]. These AFM image features were produced using a 532 nm Nd:YAG laser at 8 MW/cm2, a pulse width of 7 ns coupled to a boron doped silicon STM tip. Industrial applications of near-field laser assisted nanoprocessing focus on increased data recording capacity for data storage, although, like other tip-based nanomanufacturing techniques, the applications extend to contamination removal, mask repair, magnetic disk surface texturing/tagging, surface treatment for optical uses such as Fresnel optics, nanobiotechnology and optical lithography [221,226]. The process has applications in the micro/ nanomachining of inert dielectric materials such as glass and silicon carbide which have low absorbance in the visible range [224]. Researchers at the University of Nebraska – Lincoln and the National University of Singapore have shown that near-field laser assisted nanoprocessing is an effective method for coating tungsten nanoprobe tips with diamond-like carbon for wear resistance [229,230]. The extension to the deposition of wearresistant diamond-like carbon coatings onto MEMS and NEMS devices, such as gears and other rubbing contacts, is desirable since associated degradation is minimal. 4. Driver applications for TBN

Fig. 27. A nanoline array produced on gold with a trough width of 17 nm and depth of 4.4 nm. These AFM image features were produced using a 532 nm Nd:YAG laser at 8 MW/cm2, a pulse width of 7 ns coupled to a boron doped silicon STM tip [232].

Today, the TBN manufacturing processes stand at a verge to be integrated in one ‘‘universal’’ platform to realize ‘‘TBN nano factory on a chip,’’ like a ‘‘lab on a chip’’ [233], which will be the factory of the future, to interface with CAD tools. There are a range of

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Fig. 28. SEM images of the cantilever array section with approaching and thermal sensors in the corners, array and single cantilever details, and tip apex (image reproduced by permission of IBM Research) [234].

established and emerging product applications for TBN in various industry sectors, including: ultra-high density memory, biomolecular sorting and manufacturing, deposition of sensors, assembly and manipulation of nanostructures and optical grating device fabrication. Following are highlights of such established and emerging driver applications. 4.1. Ultra-high density memory [234] In this TBN prototype product, named Millipede (IBM Co., Zurich, Switzerland), a two-dimensional array of v-shaped silicon cantilevers that are 0.5 mm thick and 70 mm long is the framework for nanotips used for writing and re-writing the memory using thermo-mechanical forces (Fig. 28). At the end of each cantilever is a downward-pointing tip less than 2 mm long. The tips and cantilevers are fabricated using silicon surface micromachining. A terabit demonstration using this technology employed a single nanotip making indentations only 10 nm in diameter. The few nanometers thick polymer film deposited on silicon is used as the memory medium. This device can read, write, erase, and rewrite. Tips are brought into contact with the polymer medium, and an indent (a ‘‘bit’’) is written using a resistor built on the cantilever and heated typically at 400 8C. The heated nanotip softens the polymer medium and for a short time sinks into it resulting in an indent. For reading the memory, the tip is heated at lower temperature, such as 300 8C, and does not allow polymer softening, but when the tip is dropped into an indent, the resistor is cooled by the resulting better heat transport and notable change in the resistance occurs. For re-writing the data, the nanotool tip applies a series of new indents those overlap so closely with previous indents that their edges fill in the previous indents, removing the undesirable data. More than 100,000 write/overwrite cycles have been demonstrated and about 1–2 megabits per second of data rates could be supported by an individual tip. A unique design of this product delivers desired leveling of the tip array with respect to the surface of storage medium and reduces vibrations and noise. Time-multiplexed electronics, like DRAM chips, allow addressing each tip independently for

parallel writing. Electromagnetic actuation accurately translates the writing medium beneath the array of tips in the x- and y- directions, facilitating each Millipede tip independently to read–write in the accessible storage field. Prototype of Millipede is built using thousands of tips and such dimensions enable a complete ultra-high density data storage system to be integrated into the smallest footprint, like flash memory. This TBN process is a significant advance to write and read and rewrite ultra-high density memory, e.g. 25 DVDs on a surface the size of a postage stamp. 4.2. Dip pen nanolithography for life science, medicine and semiconductor manufacturing [235,236] Recently DPN platform has delivered scalability of the manufacturing process with arrays of tips for true massive production with more than 50,000 tips. This established TBN manufacturing platform can reproducibly print various materials like organic reagents, proteins, and other organic and inorganic materials accessible in molecular liquid form (Fig. 29). For example, traditional protein deposition units that use pin spotting or ink jetting technology often print non-reproducible features in foot print, leading to less than acceptable sensitivity and reproducibility of results. Also pin arrays are typically limited in number of sub-arrays per slide. Other protein detection processes yield slow reaction kinetics and/or demand significant amounts of samples. Unlike, DPN manufacturing process yields highly consistent features. This consistency in printed features allows higher sensitivity and increased reproducibility. As a result, even specialized protein sample studies are possible for biomanufacturing. DPN’s ability to print many sub-arrays per batch translates to high output protein analysis on a single test slide. Also since DPN technology can deposit minute (femtoscale) quantities of samples such as proteins and reagents using nanotip, it facilitates rapid reaction kinetics and significant cost-savings. This TBN application offers a tremendous breakthrough in manufacturing of biosensors, semiconductor devices, and nanoscale interconnects. In a recent development, Nanoink has

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Fig. 29. Schematic of a human HER2 receptor and EGF receptor kinase array in a NanoDiscovery Fluorescent Instrument System Assay and a Fluorescent Instrument System [236].

introduced a security encryption technology, called NanoGuardianTM, using DPL to address counterfeiting for medicinal tablets, capsules and vial caps [236].

surface. This emerging process has additional applications in molecular electronics or nanolithography features for more conventional nanoelectronics.

4.3. Molecular sorting and delivery for biomanufacturing [237]

4.4. nProber and measurement tool and actuation [238,239]

Molecular sorting and delivery is an important area of nanobiomanufacturing due to its application in reducing DNA sample size and accelerating throughput in a sequencing procedure in medicine. In this emerging TBN application (IBM-Almaden Co.), a tool tip is modified to undertake ultrafast differentiation of molecules, using electrophoretic effect, on populations of <0.1 zeptomoles (1022 moles) on the surface of a tip tool (Fig. 30). The driving force for differentiation of molecules is a high electric field applied over the length of a nanotool tip that results in enhanced differential mobilities of molecules stemming from the confinement of the water layer on the tip surface. In this TBN process, one can deliver DNA oligonucleotides, a 5-mer and a 16mer, with migration times of 15 and 5 ms, respectively. This is approximately five orders of magnitude faster than in conventional capillary electrophoresis. This TBN process can also be used for molecular manipulation and deposition. The process is scalable using a multi-tip approach. The writing only occurs when electric field is applied and not continuously while in contact with the

In addition to machining (like Millipede) and deposition (DPN), TBN can also be used as nanoscale measurement tools. This TBN product tool by a division of DCG (formerly Zyvex Corporation) is designed and optimized to electrically probe sub-100 nm features on semiconductor devices with superior throughput. This nanotipbased product consists of a nanomanipulator integrated with SEM observation, a standard parametric measurement analyzer, an advanced clean system, and special software to manipulate and integrate specific component. This is a semi-automated system and has more than 5 encoded sample positioners for increased probing capability and production throughput. The XYZ encoded center stage provides step and repeat capability allowing the probes to remain in place while the sample is moved to the next bit. The nProber product also provides vision feedback capability for point-and-click positioning of the probes as well as center stage. All of this can be combined with a CAD guidance software suited to locate and move to the device area of interest for measurement during the batch process. This nanotool tip-based product has

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Fig. 30. (a) Scheme of an AFM mode used in this study with associated electric field and (b) SEM image of modified cantilever [237].

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Table 3 Concise summary of merits of TBN processes. Merits Techniques

Process mechanism

Type of process

Materials

Feature sizes

Integrated metrology and scalability

Other (ability for interfacing with CAD and patterning)

Nano-EM

Electric field assisted material transport in dielectric medium in atomic and molecular form Electrochemical field assisted material transport in ionic form Physical material transport due to capillary force at sub-microscales Pre-planned plastic deformation caused by physical and its hybrid processes Tip relayed photonic or EM transport of material at atomic and molecular scale

Subtractive and additive

Metals, semiconductors, conducting polymers

As small as few nm

STM and conducting AFM – scalable

CAD and patterning in 0- and 1-dimensional structures in X–Y plane

Subtractive and additive

Metals, semiconductors

As small as few nm

STM and conducting AFM – scalable

Additive

Biomolecules, metals

15 nm

AFM – scalable

CAD and patterning in 0- and 1-dimensional structures in X–Y plane CAD and patterning in 0- and 1-dimensional structures in X–Y plane

Subtractive and additive

Metals, ceramics, semiconductors, polymers, composites

AFM and nanoindentation – scalable

CAD and patterning in 0-, 1- and 2-dimensional structures

Subtractive and additive (potentially)

Metals, ceramics, semiconductors, polymers, composites

As small as 5 nm in width and 1–2 nm in depth 100 nm

AFM, STM and nanoindentation – scalable

CAD and patterning in 0- to 3-dimensional structures

ECM

DPN

Nanoembossing

TBN – laser

[(Fig._31)TD$IG]

Fig. 31. Eight probes characterize the stability of a 6T SRAM bitcell [238].

applications in electrical characterization for device quality or failure analysis, temperature characterization, CV characterization and pulsing IV characterization at nanoscale (Fig. 31). In a recent development, arrays of such nanoprobes have been applied as actuators to generate energy in nanogenerators. Nanogenerators use very small piezoelectric discharges created when zinc oxide nanowires are bent and then released using array of AFM tip actuators. By building interconnected arrays containing millions of such wires, it is projected that the device could produce enough current to power nanoscale devices [239]. Table 3 provides a concise summary of merits of the above discussed TBN processes. In addition to the above established processes, and developed and emerging major applications in memory, electronics, and biomanufacturing, many other technologies based upon TBN are under active development. 5. Summary and future directions TBN techniques represent a potent gamut of processes for performing various manufacturing operations, including machining, deposition, patterning, assembly, in situ measurements and visualization. The versatility of the nanotip platform inspired from earlier developments in STM, AFM and nanoindentation allows the teaming of tip tools with electrical, chemical, mechanical, optical

and their hybrid forces to apply the desired bias for precision manufacturing at low cost. Also, TBN provides the opportunity to work with a wide range of materials, including metals, semiconductors, ceramics, polymers and their combinations to obtain heterogeneity in integration and multifunctionality. Further TBN platform allows 1D, 2D and 3D precision nanomanufacturing, unlike optical (X-ray) lithography, focused ion beam, and other lithography processes. These processes are capable of achieving sub-10 nm scale additive and subtractive structures for device and system applications, including vias, channels, quantum dots, nanowires, ultra-dense memory structures, and others. These processes may be coupled with new upcoming applications including, but not limited to, single molecular detection devices, single electron transistors and memory structures, nanoscale fuel cells, ultra-high density 3D memory structures, molecular assemblies for genetic drugs, nanomanufacturing factory on a chip. Future advancements in nanotip-based manufacturing will continue to address fundamental science and engineering challenges in manufacturing. Following is a list summarizing current challenges and future opportunities in TBN:  Fundamental understanding of interaction between tip and substrate through applied force and processing medium.  Design and fabrication of application specific tool tips.  Increasing robustness and wear resistance of tool tips.  Integration of in-line tool tip health and wear monitoring.  Nanotool tip changer for multifunctional cluster tools [240].  Increasing speed of tip actuation/scanning in three dimensions over larger areas [241].  Integration of multiple TBN processes along with metrology in one platform.  Scaling to multiple tips platform with interdependent control.  Tool tips compatible with application specific environments.  Multi-axes actuation of tips for fabrication and metrology.  Ability to synthesize individual nanostructures directly from a tip with control of structure size, shape, composition, and position [29].  Automation of tip-based processes/programmable on-demand nanofabrication.  Heterogeneous materials integration and assembly via ‘‘pickand-place’’ and ‘‘plug-and-play’’ approaches.  Tip-based drilling, milling and other related processes [242].  Precision stages for workpiece translation with sub-1 nm scale accuracy [243].

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Packaging of multiple tip platform for large scale production. Measuring of in situ and ex situ rate of production. Measuring dimensions of high aspect ratio structures. Development of TBN factory on a chip. Development of portable TBN factory. Development of applications in optics, high-density electronics, biomedicine, sensors and other areas.

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