Fabrication of nanostructures using atomic force microscope assisted nanolithography 2

pulsed-laser evaporation MAPLE, laser chemical vapor deposition, micro pen, ink jet printer, scanning probe microscope have been developed.37 These techniques differ in the way that they

Chapter 1

Introduction

During the last few decades, a research related to nanoscience and nanotechnology

have become one of the most important and exciting forefronts in Physics, Chemistry, Engineering and Biology It is the science and engineering of making materials, functional structures, and devices at the nanometer scale with a dimensions less than 100 nm by controlling the matter at atomic and molecular dimension.1 Advances in the development

of new methods for measuring, manipulating, and constructing objects in the nanoscale dimension provide many opportunities for scientific and technological developments for creating active nanostructures and nanosystems with improved functionality Identification

of the concept of nanotechnology has been attributed to Richard Feynman, who presented

a speech in 1959 entitled “There’s Plenty of Room at the Bottom.” In his speech, Feynman predicted manipulating atoms to make materials.2 Although, his bold vision had been considered a bit eccentric at that time, this vision was eventually realized by IBM scientists The invention of scanning tunneling microscope (STM) 3, 4 and atomic force microscope (AFM) 5 enabled the visualization and then manipulation of atoms and molecules Later, Eric Drexler stated that nanotechnology would dramatically change the world in the 1980s.1 Nanotechnology attracted interest among scientific and engineering communities, not only as a scientific challenge but also for practical reasons

There have been a number of driving forces facilitating the revolution of nanoscience and nanotechnology Some of the primary factors which advanced in this field are the need for faster and smaller-scale electronic devices, biochips, and sensors for the fabrication of nano objects or devices A leap in the data storage industry also helps in the fast advancement of nanoscience and nanotechnology; according to Gordon Moore,

co-founder of Intel, the data density would double approximately every 18 months Moreover the limits of photolithographic process to make computer chips are limited by the diffraction limit of the wavelength of light used to pattern a resist.6 The limitation of current technology warrants new processing methods and protocols Likewise in biological systems the scientists have studied the basic mechanisms happening for a long time without knowing how and why they work But the current technological advancement made it easy to mimic the fundamental activities or mechanisms involved in biological systems7 by constructing active nanomachines of living cells, which are chiefly made of proteins

In physics, the wave function of electrons becomes significantly confined when the size of an object approaches atomic dimension This electron confinement leads to different chemical properties because the quantum mechanical (wavelike) properties of electrons inside matter are strongly influenced by variations on the nanoscale.8-11However, the observation and manipulation of materials at the atomic scale was not very easy owing to the unavailability of proper technological tools The invention of the scanning probe microscope (SPM) enabled us to overcome this issue by manipulating materials at the atomic scale.12 AFM is also a key tool for researchers in the fields of bioscience, life science and materials science In addition, AFM is now used as a tool for the fabrication of nanometer scale features from different materials.13-15 The use of AFM for fabrication of nanostructures from organic/polymer materials is the focus of this thesis

1.1 Evolution of Nanotechnology

Nanotechnology is the term used to cover the design, development and utilization

of functional structures with at least one characteristic dimension measured in nanometers.16-17 Though the term nanotechnology is relatively new, the existence of functional devices and structures of nanometer dimension are not new In the fourth–century AD, glass makers fabricated glasses with nano-sized metals, in particular the Lycurgus cup which now resides in the British museum in London.18 The cup is made from soda lime glass which contained silver and gold nanoparticles The color of the cup changes from green to deep red when a light source is placed inside due to the presence of metal nanoparticles in the glass.19 In the eighteenth century, the photographic film was developed on a thin layer of gelatin containing silver halide, such as silver bromide, and a base of transparent cellulose acetate.20 The light decomposes the silver halides into nanoparticles of silver which are the pixels of the image.21 Later in 1857, Michel Faraday attempted to explain how metal particles affect the color of church windows.22 However the first explanation for the dependence of color of the glasses on metal size was provided

by Gustav Mie.23 Later, in 1960, Richard Feynman, speculated that functional nanoscale devices could be constructed from atomic components However, it was not before the 1990s that the field of nanotechnology took on as a significant movement.24 Particularly nanostructure characterization was made possible by the invention of scanning probe microscopy Scanning probe microscopes such as STM, AFM provide imaging and manipulation at the atomic scale and can be used as a characterization tool for high-resolution microscopes and nanoindenters.25 At the same time, a rapidly growing level of activity has been directed towards improving the synthesis and assembly of nanoscale

building blocks with nanodimensions

Objects at the nanoscale may display physical attributes substantially different from those displayed by either atoms or bulk materials, which may lead to new technological opportunities as well as new challenges.26-28 The dependence of the behavior

on the particle sizes can allow one to engineer their properties indicating that this technology has potential to create advances over a wide and diverse range of technological areas.29-33 One good example is increasing the storage capacity in magnetic tapes and provide faster switches for feedback Thus, the nature of research in nanotechnology is interdisciplinary, covering a wide range of subjects from the chemistry of catalysis in nanoparticles to the physics of quantum dot lasers.34 Researchers in any one particular area need to reach beyond their expertise in order to appreciate the broader implication of nanotechnology and learn how to contribute to this exciting new field

1.2 Development of Micro and Nanoscale Fabrication

Fabrication techniques in the semiconductor industry depend largely on lithographic process to make microelectronic circuits and devices.35-36 The schematic shown in Figure 1.1 explains the necessity of the lithographic process for the evolution of electronic devices All electronic devices are developed on the semiconducting chip which

is essentially built from lithographic process In the conventional technique, the first step

in the nanofabrication process involves the formation of a pattern on the resistive layer on top of a substrate by selectively irradiating with electrons, ions, or photons on the surface followed by a chemical etching These indirect patterning approaches can compromise the chemical purity of the structure generated and have limitation on type of materials and number of materials that can be patterned Most of these conventional lithographic

methods either break down or poorly controlled when the feature sizes go down to

nanometer scale.37

Lithography

Electronic devices

Semiconductor chips Lithography

Electronic devices

Semiconductor chips

Figure 1.1 Schematic diagram shows the outline of the evolution of electronic devices

from lithographic techniques

These drawbacks in the conventional technique lead to the development of new techniques such as direct-write method, where materials can be patterned directly on the substrate at nanoscale Using this direct write approach, 3D structures can be built directly without the use of masks, allowing for rapid prototyping, product development, and cost effective manufacturing with superior capabilities But the increased capabilities come with limited flexibility as well as increased complexity, time, and cost A wide range of direct-write technologies are being developed with approaches to write or transfer patterned materials onto a substrate Each technique has its own strengths and shortcoming

in terms of capabilities such as, resolution, writing speed, 3-dimensional and multimaterial capabilities, operational environment, and the type of final structure to be obtained Different direct write lithographic techniques which includeplasma spray, matrix-assisted

pulsed-laser evaporation (MAPLE), laser chemical vapor deposition, micro pen, ink jet printer, scanning probe microscope have been developed.37 These techniques differ in the way that they transfer, deposit or dispense materials for forming nano/micro patterns and they can be compared principally in terms of cost, speed, resolution, and flexibility to work with different materials, final material properties, and processing temperature Among these techniques, SPM based lithography such as oxidation lithography is regarded as direct write and “resist-free” lithography where a conductive probe is used to provide a local intense electric filed under the vicinity of tip to modify the surface through local anodic oxidation process 38

Since the invention of scanning probe methods, 38-42 considerable efforts have been undertaken to find methods for manipulating and structuring matter at nano and molecular level For manipulating matter under ambient conditions or in liquid, scanning probe methods are very convenient Due to the small size of the probes, which interacts directly with surfaces, a resolution of a few nanometers should be achievable Furthermore, the nature of the interaction can comprise a multitude of physical and chemical quantities.The scanning-probe-based direct structuring of polymer films has been performed by scratching or indentation experiments.43-47 Local topographical features can also be created

by thermo-mechanical writing by using a heated AFM tip for locally melting a polymer substrate.48 -58 The localized deposition of a foreign substance on a surface has been demonstrated with a method called “dip-pen" nanolithography (DPN).59

1.2.1 Necessity of nanoscale fabrication

The task of creating devices at the nanoscale requires robust methods for controlled deposition of functional materials on the surface Nanofabrication can be

achieved both by extension of current microfabrication techniques into smaller size regime and by the development of new techniques such as scanning probe microscope patterning Basic research in nanotechnology should open more potential applications in the semiconductor industry The conventional lithographic process ranges from current photolithography to next generation lithographies such as extreme ultraviolet lithography,60 -63 electron lithography,64-65 ion lithography66-67 and X-ray lithography.68

It is known that soft X-ray and extreme ultra violet (EUV) lithography employs short wavelength radiation (13 nm) such that normal optical lenses become opaque and an alternative reflective method of focusing and masking must be used Conventional lithographic methods are still far away from this limit of resolution, the best one being the electron beam lithography process, which achieves a resolution of about 5 to 10 nm Furthermore, lithographic methods require clean room equipment, vacuum technology, optical systems, etc and apply only to specific material combinations for the resists, the utilized chemical substances as well as the substrates.69-72 In addition, the down scaling of device dimension due to miniaturization causes the cost of fabrication and processes increases dramatically Also, there are many basic physics and reliability related issues which demand improved fabrication techniques and smart components for the continued downsizing of electronics devices For example, the large capacitance and dielectric breakdown in the large electric fields imposed by small lateral dimensions make conventional silicon metal oxide semiconductor (MOS) and biopolar devices impractical These limitations of existing technique and the current trend of miniaturizing the electronic component has been the driving force for new lithographic techniques in micro/nano technology and nanoelectronics.73-75

The long-term vision is to assemble matter at the atomic scale to obtain full control

over the physical and chemical properties of a material Single atom manipulation is the ultimate limit and it is restricted to specific systems requiring special conditions such as vacuum and low temperatures.76-79 However, in biological systems the build-up of structures on a cellular scale, i.e in the range of a few micrometers, takes place through self-assembly processes thereby achieving a high degree of order.80-84 It is likely that in the future an important part of micro- and nanostructure techniques will involve ambient liquid environment, which is not compatible with conventional clean room and vacuum techniques Moreover, these novel techniques based on bottom up processes take advantage of new findings from chemistry, biology and materials science, i.e they do not have to rely only on downscaling of macroscopic techniques.85-87

1.2.2 Nanofabrication

Nanostructures can be made in numerous ways In the broad classification, it divides into either those which build from the “bottom-up”, like atom by atom, or those which construct from the “top-down” using processes that involve the removal or reformation of atoms to create the desired structure.86 These two methods (both top-down and bottom-up) have evolved separately and converge to nanodimensions (figure 1.2) In the bottom-up approach, atoms, molecules and nanoparticles can be used as the building blocks for the creation of complex nanostructures; the useful size of the building blocks depends on the properties to be engineered By altering the size of the building blocks, controlling their surface functionality, constituents, organization and assembly, it is possible to engineer the properties and functionalities of the nanostructured solids or systems.88-90

Figure 1.2 Schematic representation of evolution of top-down and bottom approach for

nanofabrication

On the other hand, top-down approaches are inherently simpler and rely either on the removal or division of bulk material, or on the miniaturization of bulk fabrication processes to produce the desired structure with the appropriate properties Both top-down and bottom-up methods may be viewed as essentially different forms of structural and molecular engineering Molecular electronics and biomaterials are the two areas that offer the greatest opportunity and challenges for the development, implementation and commercialization of these nanoscale fabrication techniques.91-95 A brief overview of some of the more common fabrication methods using lithographic technique for the fabrication of nanostructures are discussed below

1.3 Lithographic Techniques

The invention of new nanofabrication and characterization tools are essential to nanoscience and nanoengineering In fact, the development of nanoscience and nanotechnology has been mainly experimental-driven which is often related with applications of new fabrication and characterization techniques Among the various fabrication techniques, SPM lithography, 38-42 optical lithography and electron beam lithography have been playing particularly important roles

1.3.1 Optical lithography

The desire for faster and smaller-scale devices has continuously stimulated the development of lithography on ever-smaller scales.103 Smaller devices offer the promise of low cost, higher speed, and greater convenience Lithographic techniques such as microcontact printing,96-99 photolithography,100-101 micromachining,99 and microwriting97,99,102 can produce micrometer- or sub-micrometer sized patterns 104-107

Extreme ultraviolet lithography (EUV) can produce nanometer-scale circuit patterns on chip surfaces The main difference between EUV and photolithography is the wavelength of the light used to create the circuit features EUV will allow chipmakers to make smaller circuits because of the smaller wavelengths of light

Photolithography

Photolithography is the process of transferring features on a mask to the surface of

a silicon wafer Photopatterning is a convenient method for the fabrication of sub micrometer patterns The shortcoming of this method is the requirement for multiple photolithographic steps The steps include wafer cleaning, barrier layer formation, photo

resist application, soft baking, mask alignment, exposure, development and hard baking The aim of this procedure is the selective removal of silicon dioxide to produce a patterned feature on the silicon wafer The wafer is first coated with a photoresist polymer that is sensitive to ultraviolet light A mask is aligned with the wafer, so that the pattern can be transferred onto the wafer surface Mask alignment is one of the most important steps in photolithography A mask or photomask is typically a glass plate with a patterned metal film on one side The photoresist is then exposed through the pattern on the mask with a high intensity ultraviolet light There are two types of photoresist, positive and negative When the light hits the positive resist, it causes a chemical change that enables the resist to be removed with a solvent This step transfers a positive image of the mask to the resist layer The opposite occurs with negative resist materials The exposed oxide is then removed by using a chemical Finally the resist is removed leaving the patterned oxide

Laser ablation lithography

Laser ablation of a polymer was first reported in 1982 Since this process has the advantages of fewer processing steps and fewer design limitations, laser ablation can be used as an alternative or complementary technique to conventional photolithography Thin-metal films can be patterned directly by laser ablation without the need for the multi-step processes involved in photolithography (mask preparation, resist spinning, exposure, developing, etching, and resist removal) A high intensity laser pulse vaporizes a small area on the surface of a target The ablated materials deposit on a substrate under controlled conditions of temperature and pressure, forming a thin film If the substrate has

a uniform crystal lattice structure, the film may be induced to grow on top of it as a crystal

with a preferred orientation Microfabrication by laser ablation of thin films is readily accomplished with commercially available microscope adaptations.103

Wei et al.104 investigated the ablation properties of various polymers at low and high fluence lasers High fluence produces high ablation rates in small areas, e.g., drilling

or cutting At low fluence, the ablation rate depends on the structural parameters of the polymer The low fluence range is important for lithographic applications Fluence and/or pulse number can control the depth of channels

1.3.2 Electron beam lithography

Electron beam lithography (EBL) is a technique for creating extremely fine patterns for integrated circuits The electron beam has a wavelength so small that the diffraction no longer defines the lithographic resolution However, EBL requires alternation of environments between vacuum and ambient conditions, 103because electron beam exposures are conducted in a high vacuum and deposition occurs in a solution This feature makes the procedure slow and more expensive EBL differs from conventional UV lithography in such a way that it is a maskless process and the dose may be varied throughout the pattern EBL can produce high-resolution nanometer sized patterns The electron beam has a wavelength so small that diffraction no longer defines the lithographic resolution and the mask is not required

Harnett et al105developed patterned self assembled monolayer (SAMs) using EBL and fabricated biomolecules on a selected area By attaching flourescent polystyrene spheres to mercaptohexadecanoic acid - cysteamine templates, they could observe the pattern with a florescence microscope In order to apply this technique to SAMs, low energy, sub-2 kV, electron beam was used Low voltage electrons deposit energy

efficiently on the top surface of materials, thereby exposing monolayer without

transferring excess energy to the substrate

1.3.3 SPM lithography

Scanning probe microscope (SPM) lithography is a promising method for fabricating nanostructures using a sharp probe The high resolution pattern formation depends on the tip shape and does not require a high-vacuum system like EBL It has provided a set of tools for the direct nanomanipulation and modification of objects on surfaces Various atomic force microscope (AFM) and scanning tunneling microscope (STM) based nanofabrication methods have been reported.106-113 Representative methods are nanografting,107 electric filed assisted anodic oxidation,111 and dip-pen nanolithography.59 STM-based lithography includes tip-assisted electrochemical etching and field-induced desorption

Broadly, AFM based lithography can be classified into two categories according to the probe operation on substrate (a) force induced AFM nanolithography and (b) electric field induced AFM nanolithography Various successful approaches have been developed which includes nanoindentation,106 nanografting,107,108 nanomanipulation,109 dip pen nanolithography,110 anodic oxidation,111 electrochemical deposition,112 field induced deposition113 and thermomechanial writing 114

induce various patterns on soft films deposited on the substrate when suitable force, such

as mechanical or electrical field, applied between the probe and substrate

1.3.4.1 Electric-field assisted oxidation

Local oxidation of silicon surfaces using scanning probe microscopic (SPM) techniques in air was observed for the first time in 1990 by Dagata et al.119-120 An AFM can be used on a freshly hydrogen-passivated Si (111) surface so as to induce chemical modifications of the surface Patterns were formed on the silicon surface by applying a voltage The patterns involve incorporation of ambient oxygen into the substrate results the formation of SiOx on the patterned areas

Silicon surface oxidation is observed when a positive bias is applied on the hydrogenated surface with respect to the SPM tip which is generally grounded. 121-137 The height and width characteristics of the induced oxide depend on the tip – surface bias value which gives evidence that the oxidation mechanism is linked to the tip – surface electric field An electric field of 109 Vm-1 is commonly utilized as the threshold value to produce oxidation On the other hand, ambient humidity is essential to realize the oxidation139-144 which highlights the importance of the film of water molecules and ionic species adsorbed onto the surface The role of oxygen is obviously important for the oxidation process By choosing controlled experimental conditions, Marchi et al.138proved

that the oxide growth process was limited both by thermodynamic (as evidenced by the threshold voltage) and kinetic (as shown by a significant increase of the oxide growth by

an ozone supply in the ambient air) factors They propose a qualitative three steps mechanism: (1) surface de-passivation by desorption of hydrogen atoms assisted by the electric field above the threshold voltage, (2) formation of oxyanions in the water meniscus between the tip and the surface; a complex step which may involve both

electrolytic and dissolution processes and (3) diffusion of the oxyanions assisted by the electric field toward the SiOx – Si interface Such an oxidation mechanism is also expected to produce a current flow Marchi et al.138 did not detect any current; however Avouris et al.139 measured the current flow in the order of 10- 14 A They suggested that an electrochemical reaction for the anodic oxidation of silicon as given below:

Si + 4 H + + 2 OH - SiO 2 + 2 H +

This reaction involves an interaction between ionic species OH- and electronic carriers H+(holes) The schematic representation of nanooxidation using AFM is shown in figure 1.3 The oxide growth mechanism has been addressed by many researchers and it is suggested that the oxide formation is related to the applied electrical field, water meniscus formation and OH- diffusion Various theoretical models including space charge model, Caberera-Mott model and power-law model have been proposed to explain the oxidation behavior.139-149 The oxide growth brings about an elevation of the surface due to the fact that the molar volume of silicon oxide is roughly twice that of silicon

Si

Figure 1.3 Schematic representation of the nano-oxidation process on Si substrate

In addition to the oxide pattern on silicon surface, AFM local oxidation can be used with different types of metals such as Ti,150-152 Ni,153-154 Nb,155-156 Ta,157 Mo,158

Zr,159-160 Al,161 Cr,162 and Co.163 In another work, oxidation process was demonstrated using organic meniscus such as octane and 1-octene instead of water meniscus.164-165 Here, these menisci have been used to confine chemical reactions that gave rise to the fabrication of nanometer-size structures Using these organic solvents, silicon carbide nanostructures have been fabricated on the silicon surface instead of oxide pattern

1.3.4.2 Dip-pen nanolithography (DPN) 59

DPNuses an AFM tip to deliver small molecules to a surface through a solvent meniscus, which is naturally formed in the ambient atmosphere In this technique, an AFM tip is coated with a thin film of thiol derivative (or ink) which is to be transferred When the tip is placed close to the surface in an atmosphere containing a high concentration of water vapor, a minute drop of water condenses between the tip and surface, and the thiol molecules migrate from the tip to the surface through the water meniscus The tip is then scanned across the surface to deliver the molecules in a predetermined pattern.177-191 The chemisorption of ink molecules to the substrate is the driving force for the transportation of the ink from the AFM tip through the water to the substrate Due to the high reactivity of thiol functional groups with gold, a compact monolayer can be formed as the ink molecules migrate onto the substrate following a scanning track (Figure 1.4)

Au Substrate

Water meniscus

Scan direction

Ink

Figure 1.4 Schematic representation of DPN

The success of DPN depends on two main factors, a narrow spatial deposition of patterning molecules, and self-assembly of the functionalized ink molecules to form a compact monolayer.175 The transport rate and the line width of the pattern depend on the size of the water meniscus, which can be controlled by adjusting the relative humidity The tip-substrate contact time and scan speed can also influence DPN resolution Faster scan speeds can produce narrower lines, however, very high speed results in a failure of effective transportation of the ink molecules.176

Piner et al prepared the dot patterns by DPN to demonstrate the diffusion properties of the ink molecules.59 The tip was coated with 1-octadecanethiol or 16-mercaptohexadecanoic acids and brought into contact with the gold substrate for a set period of time The size of the dot pattern depends on the contact time and the diffusion properties of ink molecules on the substrate The uniform shape of the dots reflects an even flow of molecules in all directions from the tip to the surface

DPN is capable of creating structures made of various materials such as metal,

inorganic compounds, organic molecules, and biological species166-191 The fabrication of

a wide range of functional structures by DPN has been demonstrated and some of the typical structures include high resolution organic features, metallic structures, magnetic patterns, polymer brush arrays, and biological devices Ali et al demonstrated the direct transfer of Au nanoparticles from the tip to a silica substrate172 The direct deposition of

Au and Pd nanocrystals onto a mica substrate using hydrosol inks was also described.177The deposition of Au nanoparticles on SAM-covered Au surface through S-Au linkage was presented by Garno et al.187 The conversion of soluble Au (III) ion to insoluble Au(0) metal was attributed to surface-induced reduction of noble metal ions during the adsorption process Ivanisevic and Mirkin used hexamethyldisilazane (HMDS) as ink to pattern organic materials on Si/SiOx and oxidized GaAs semiconducting substrates.183

In addition to inorganic and organic nanostructure, biological molecules can also

be patterned using DPN process Lim et al directly wrote antirabbit IgG (immunoglobulin) patterns on different classes of silicon oxide substrates.178 A straightforward methods for the preparation of biologically active protein nanoarrays on nickel oxide surfaces was reported by Nam et al.179 In addition, the DPN process is also extended to the delivery of many organic materials such as thiols, silazanes, alkynes, polymer and proteins.178-180, 183-185

1.3.4.3 Thermomechanical writing

The surface modification capability of scanning probes, such as mechanical scratch, manipulation of molecules and nanotube has motivated efforts to create data storage devise based on scanning probe technique for storing data at much higher densities The method, known as thermomechanical writing, is based on the indentation of

an AFM tip on a polymer surface, which is softened locally by the combined effect of heat

and mechanical force from the AFM tip This technique was developed by the IBM Zurich research group,48-51and they achieved a storage density of up to 30 Gb/in2 In this process, AFM can typically operate on the microsecond time scale, whereas the conventional magnetic storage operates on the nanosecond time scale To improve the data storage speed, MEMS based array of cantilever operating in parallel were used.52-58 In this technique, high data transfer rates are achievable through the massive parallel operations

of such tips

The contact between the probe arrays and the polymer sample is realized by adjusting the z-piezo voltage, and only one feedback is used to control all the probes Additional approaching sensors are integrated into the corners of the array chip to control the approach of the chip to the storage medium These sensors can provide feedback signals to adjust the z-actuator until uniform contact with the medium is established During data writing, the chip is scanned over an area called the storage field, and each cantilever of the array writes and reads data only in its own storage field For parallel operation on a large scale, they developed a micro scanner which is able to control the parallelism between the probe array and the sample.52 Their computer controlled write/read scheme addresses the 32 cantilevers of one row in parallel Writing is performed by connecting the addressed row for 20 µs to a high, negative voltage and simultaneously applying data inputs to the 32 column lines The row-enabling and column addressing scheme supplies a heater current to all cantilevers Selected cantilevers heated

to high temperatures generate an indentation, while those with low temperatures made no indentation The scan stage was moved to the next bit position and the process repeated until the line scan was finished

1.3.4.4 Nanofabrication using self assembled monolayer

Self-assembled monolayers offer a flexible and convenient way to give surface functionality in a chemically defined way This technique provides relatively ordered structure at the molecular scale with interesting functional group at the surface To exploit this chemical functionality at the nanometer scale, it is attractive to take advantage of the nano manipulating ability of SPM to create functional nanoscale structure The development of nanoscale structure on SAM can be divided into three categories such as elimination lithography, 192-200 addition lithography and substitution lithography 201-210

In elimination lithography, the patterns are created using an AFM tip by removing the film from the substrate Addition lithography involves molecular materials deposited

on the substrate directly from a tip A probe coated with molecular ink is brought into contact with a “bare” substrate The ink gets transferred from the probe to the surface and this process was called as dip-pen nanolithography

Substitution lithography denotes in-situ mechanical or electrochemical pattern fabrication strategies Substitution approaches are further subdivided into two categories: (1) initial elimination of the SAM film, followed by (2) in-situ addition of another component to the exposed substrate

Various AFM assisted lithographic techniques have been developed using self assembled monolayer In this lithography process, SAM is first imaged with low contact force and then fabrications are chosen The patterns are then created on surface either by deposition or removing the materials on the surface using the AFM probe

1.3.4.5 Probe assisted patterning using organic resist

The formations of organic films on certain substrates through chemisorptions of organic molecules have attracted considerable attention, owing to their structures with highly ordered molecular arrangement and the simplicity of preparation processes Patterning of these organic thin films is of special importance due to their application as lithographic resists There have been numerous reports describing the SPM based patterning of organic molecular films.211-212, 216-220 Organic thiol SAM on gold or gallium arsenide substrate have been demonstrated as AFM patternable films213 and SAM of organosilane molecules showed great potential as ultra thin resist fims.214 Sugimura, et al, have demonstrated the fabrication of nanoscale patterns on an alkylsilane monolayer on Si and fluoroalkylsilane monlayer on Ti under ambient atmosphere.215 The monolayer prepared on a Si substrate was locally degraded in a region where the SPM tip had been scanned with bias and patterns were transferred to the Si substrate by chemical etching using the degraded regions as etching windows Also the effect of atmospheric environment on tip-induced degradation of monlayer was studied

In another approach, AFM anodization lithography offers excellent alternative to fabricate highly resolved nanopatterns at high speed through chemical modification of surface Lee et al reported the effect of the surface chemical group on AFM anodization using well-defined self-assembled monolayers (SAMs).221-224 The role of surface chemical functional groups of the molecules in anodization lithography was studied using the SAMs

of 1,12 - diaminododecane dihydrochloride (DAD.2HCl) and n - tridecylamine

hydrochloride (TDA.HCl) as resist molecules.225

1.3.4.6 Constructive nanolithography

The formation of nanostructure through hierarchical construction of self assembled layer using probe-induced local electrochemical modification of organic molecules is known as constructive lithography It is a generic chemical approach that combines processes of self-assembly and surface chemical modification with a nondestructive electrochemical patterning technique using conductive AFM tips.226-229This electro-oxidative reaction developed by Sagiv and co-workers converts the terminal methyl group of an OTS-coated silicon surface to a hydrophilic, carboxylic acid-terminated (OTSox) terminated surface by a voltage-biased conducting AFM tip.226, 228-229

An additional molecular layer was deposited ex situ on the oxidized regions by dipping the OTSox patterned surface in a solvent containing the desired molecules

Liu et al showed the template guided self-assembly of a water soluble derivative [Au55(Ph2PC6H4SO3Na)12Cl6]2 on bilayer patterns with top thiol (–SH) functionality.230-231Besides metallic, semiconductor and organic nanostructures, magnetic Fe particles were also assembled on electrochemically oxidized monolayer templates.234 Fresco et al reported the localized chemical activation of a protected amine surface by triggering heterolytic bond cleavage of dendrons via the application of a voltage bias between the AFM tip and a silicon substrate.232-233 The latent amine patterns were visualized by the self-assembly of dendritic carboxylates

The electrochemical cathodic electrografting reaction was demonstrated on the nanometer scale by Hurley et al235 Electrografting allows the easy formation of covalently bonded thin films on the surface of an electrode In electro pen nanolithography (EPN) method, demonstrated by Cai and Ocko, an underlying thin organic film was oxidized, and

the ink molecules were transferred simultaneously to the oxidized regions by scanning an ink-coated AFM tip 236

1.3.4.7 Catalytic probe lithography

The catalytic probe lithography is used to fabricate nanostructure on the assembled monolayers (SAMs) of reactive adsorbates by a catalytic probe In this approach, functionalized AFM tips were employed as a catalytic tip for the fabrication of nanopattern on monolayer through a chemical process.237-241 In an early attempt, Schultz

self-et al showed that by using Pt coated AFM tips, the terminal azide groups of a monolayer

on glass substrate could be locally reduced to amino groups.37 In another approach, an acid-functionalized AFM tip was used to locally induce a surface hydrolysis reaction on trimethylsilyl (TMS) ether SAM and then create patterns at nanometer level 42

Similarly, the catalytic approach was carried out by writing over bis(ö-

2-mercapto-5-benzimidazole sulfonic acid functionalized gold-coated AFM tips.43-44 Here, the acidic tips induced local hydrolysis of the silyl ether moieties in the contacted areas and patterns were created In another work, Suzuki reaction initiated on an aryl bromide monolayer using a Pd nanoparticle functionalized AFM probe in a methanolic solution of phenylboronic acid.245

1.4 Scanning Probe Microscopy

Scanning probe microscopy (SPM) enables researchers to obtain three-dimensional images of surfaces at the nanometer to atomic scale using a nanoprobe The physical, electrical or chemical information of the surface can be acquired with high spatial resolution using SPM, by monitoring the interaction between a sharp tip and a sample

surface There are many types of SPMs used for diverse applications ranging from surface science to semiconductor metrology to biological probing

1.4.1 Scanning tunneling microscope (STM)

The first type of SPM developed was the scanning tunneling microscope (STM) It was developed by Binnig and Rohrer working at IBM in Switzerland in 1981.3 In STM, an extremely fine conducting probe is held close to the sample A tunneling current generated between the surface and the tip produces an electrical signal while the tip slowly scans across the surface at a distance of about an atom’s diameter The tip is raised and lowered

in order to keep the signal constant and maintain the distance between the tip and the surface Recording the vertical movement of the tip produces a computer-generated contour map of the surface.186

The second type of SPM was atomic force microscopy (AFM) It was developed

by Binnig and Quate in 1986.5 Although the ability of STM to obtain an atomic-scale image is outstanding, both the tip and the surface must be good conductors The invention

of AFM satisfied the need for imaging an insulating surface In AFM, the sum of range attractive interactions and short-range repulsive interactions between the probe and the sample surface provides the contrast mechanism instead of a tunneling current The probe tip is brought into continuous or intermittent contact with the surface by a piezoelectric scanner A constant force between the tip and the sample is maintained while the tip is scanned across the surface Then the system produces a three-dimensional surface image by mapping scanner movements in the z direction relative to the x-y scan direction AFM enables not only imaging of the surface in atomic resolution but also measurements of the force between the tip and the surface up to the nano-newton scale

long-The force between the tip and surface is measured by tracking the deflection of the cantilever.246

Although the features and the characteristics of each method are different, a common instrumental principle follows the whole families of SPM The measurements are performed with a sharp probe scanning over the surface at very short distances As the tip can be modified in many different ways, the number of techniques reported in literature is continuously growing Of the many SPMs, AFM is currently the most widely used variant

It is the focus of this section

1.4.2 Atomic force microscope (AFM)

AFM was designed to measure the strong short-range repulsive forces between a tip and surface implemented as contact mode imaging The probe (tip) is brought into contact with the surface (hence the name) and repulsive van-der waals forces result in the deflection of the cantilever A feedback loop keeps the deflection constant by adjusting the vertical position of the cantilever while scanning along a surface The feedback signal provides the topographical profile of the surface Operation in contact mode typically implies relatively large shear forces that can damage the tip and the surface, thus limit the range of samples that can be imaged

AFM tapping mode operation has an advantage over contact mode in that it minimizes damage to a specimen It also provides a tool for investigating forces between the surfaces, such as van der Waals forces, electrostatic forces and magnetostatic forces A useful aspect in the application of AFM is that it can be operated in various environments such as in aqueous solution, which allows imaging of enzymes in their native configuration

Apart from being a nondestructive method, AFM is also a tool for modifying and functionalizing samples on a nanometer scale A huge advantage of this application is that, fabrication and subsequent imaging can be carried out with the same instrument by just changing parameters of operation

1.4.2.1 Basic components of an AFM

Detector

In order to detect local forces and maintain a narrow spacing, the sharp probe has

to be linked to a force sensor which detects the force between the probe and the surface

By keeping a constant force between the tip and surface, it provides a correction signal to the piezoelectric scanner to keep the spacing constant A light beam deflection system is widely used for AFM In this system, a laser beam is directed onto the back of a cantilever A segmented photodiode measures the deflection of the cantilever spring by detecting the reflected laser beam (figure 1.5) The light signal detected is then converted into force according to Hook’s law

∆z = ∆F/c

where ∆z is the deflection of the cantilever, ∆F is the acting force and c is the spring constant A position-sensitive photodiode detector, segmented into four quadrants, detects the spatial variation of the reflected laser beam The relative displacement of a reflected laser beam is then converted into the cantilever deflection (∆z)

Figure 1.5 Components of a scanning probe instrument

If the reflected laser beam moves between the upper and the lower part of the segmented detector as the tip scans over the surface, the relative displacement of the laser beam can be represented as, A B C D position–sensitive photodetector In standard imaging, a triangular cantilever is normally used to minimize the torsion effect The torsional stiffness is actually greater for rectangular beams However, this torsion effect can be used for mapping frictional forces between the probe and sample surface in lateral force microscopy (LFM)

when the temperature changes Silicon is sometimes used as an alternative because it is reflective and can be used without gold coating Anisotropic etching of silicon can also be used to make sharper tips The best tip may have a radius of curvature around 5 nm, however, silicon is more brittle than silicon nitride

Each cantilever has its own spring constant usually less than the inter-atomic bond strength (about 1 Newton/m) One can achieve the topographic image of a surface by sliding the cantilever/tip across the surface and monitoring the cantilever deflection Due

to the continuing growth of SPM technology, new kinds of probes are constantly developed For example, a wide range of chemically functionalized tips are commercially available today

Electronics and control system

The electronic unit provides the interface between the scanning system and the computer It supplies voltages to the scanner to position the tip in the correct place and synchronize the signal from the position-sensing units It includes the feedback control system for maintaining a constant spacing between the tip and sample surface

The feedback loop is designed to keep the force between the tip and sample at a constant value The feedback loop compares the actual force to a “set-point” force If the two are not identical, a signal is sent to the z-component of the piezoelectric scanner to move the tip closer to or further away from the sample Thus, the feedback loop causes a constant force as desired by the user In tapping mode AFM, the feedback loop is used to maintain a constant root mean square (RMS) amplitude of vibration With control of a suitable feedback-loop system, a probe can scan along the x and y directions upon continuously varying the z direction, so that the interaction is kept constant A three-

dimensional image of the surface can be constructed by plotting the z driving voltage of the piezoelectric actuator as a function of x-y coordinates

Acoustic vibration isolation

AFM images are very sensitive to vibration If vibration from the environment causes a tip-sample displacement, then this will appear as noise in the AFM image Vibration isolation must be used to keep tip-sample vibration below 1 Å The AFM must

be made small and rigid in order to have a high resonance frequency Low frequency vibrations are removed by connecting the AFM to the surroundings by a mechanical low-pass filter.The AFM can also be isolated from airborne acoustic vibrations by placement within an enclosure

1.4.2.2 AFM imaging modes

Various imaging modes are available in AFM to scan the surface of the sample Appropriate method should be used to image the sample without disturbing the surface Contact mode and tapping mode are most commonly used modes in scanning the surface

Contact mode

Contact mode is the most common operational method of AFM As the name suggests, the tip stays within a few angstroms of the sample during imaging While scanning across the surface, the cantilever exhibits a locally varying deflection, which represents the corrugation of the sample surface Because the deflection of the cantilever

is directly proportional to the force between the two surfaces, the cantilever deflection represented by the laser spot intensity for quadrants, ((A+B)-(C+D)), can be regarded as the vertical force signal (Figure 1.6)

Figure 1.6 Beam-deflection set-up for the detection of interacting force in an AFM

The feedback loop constantly measures the local tip-sample interaction by recording the deflection of the tip and sends the correction signal to the scanner to keep the cantilever deflection constant Therefore, the sample-tip spacing is increased if the local force exerted on the tip becomes higher than the set-point value and it is decreased if the force falls below the set point Three-dimensional height image of the surface is then constructed by plotting the vertical movements of the piezoelectric scanner versus horizontal coordinates Color mapping is widely used for displaying local height differences, for example black for the lowest features and white for the highest features One of the most important factors for obtaining true atomic resolution with an AFM is the sharpness of the tip The need for a sharp tip is well explained in terms of tip convolution The main influences of the tip on an image are broadening, compression and aspect ratio

Tip broadening arises when the radius of curvature of the probe is greater than or comparable to the size of the object being imaged As the probe scans over the object, the side of the probe touches the specimen before the apex does, which results in the

broadening of the image and is called tip convolution Compression occurs when the probe scans over some soft biological polymers such as DNA Even though the force between the tip and sample may only be in nN, the pressure onto the sample surface can

be MPa The aspect ratio (cone angle) of the tip affects on the image when observing a steep feature

Tapping mode

The biggest disadvantage of contact mode imaging is the lateral motion of the tip

in contact with the sample produces friction and wear of both the tip and the sample Wear

of the tip causes loss of resolution and wear of the sample obviously modifies the surface Sample alteration is particularly acute for soft or weakly bound samples such as biological

or polymer film surfaces

Tapping mode imaging overcomes these limitations by alternately placing the tip

on the surface and lifting the tip off the surface during scanning If the rate of oscillation is high relative to the frequency of scanning, then the tip effectively does not slide in contact with the sample.247 When tapping mode imaging is implemented in ambient air, the cantilever is oscillated at or near it’s resonant frequency using a piezoelectric crystal and positioned above the surface, so that it only taps the surface for a very small fraction of its oscillation period As the oscillating cantilever begins to intermittently contact the surface, the cantilever oscillation is reduced and the phase is changed due to energy loss caused by the tip contacting the surface The magnitude of amplitude damping and the amount of phase change depend on the surface chemical compositions as well as the physical properties of the surface Thus, the reduction in oscillation amplitude can be used to observe the contrast between regions of varying mechanical or chemical composition of

the sample surface A feedback loop maintains the cantilever oscillation amplitude during

an imaging Under ambient conditions, amplitudes as large as 10 –100 nm are frequently used for cantilevers with resonant frequency of 100 kHz or more When the probe passes over a bump in a surface, the cantilever has less room for oscillation, so the amplitude decreases On the other hand, when the probe meets a depression, the cantilever has more room for oscillation and the amplitude increases The oscillation amplitude is measured by the detector and the signal is sent to the controller electronics The feedback loop then adjusts the probe sample spacing by moving the scanner up and down to maintain constant amplitude and the force on the sample surface Under liquid immersion, the amplitude is set much smaller because of damping by the fluid Operation of tapping mode in a fluid provides the same advantages of tapping mode in air, with the additional ability to image samples under native liquid conditions, which is enormously advantageous for the observation of various bio-samples

Recently, there has been considerable interest in phase imaging,248which works by monitoring the phase differences between the driving oscillation of the cantilever and the actual cantilever oscillation during imaging In phase imaging, the phase lag of the cantilever oscillation relative to the driving signal is simultaneously monitored The phase lag is very sensitive to the variations in material properties of the surface such as elastic, electrostatic, magnetic, and thermal properties Therefore, it provides us information on the variation in composition, adhesion, viscoelasticity and others

1.4.2.3 Lateral force microscopy (LFM)

Friction is a well-known phenomenon occurring when two surfaces in contact are

in relative motion and an AFM is a very suitable tool for studying friction at a nanometer

scale In contact mode AFM, it is possible to measure the torsional motion of the cantilever during the scanning The torsion of the cantilever depends on the lateral force or friction acting on the scanning probe This interaction in turn depends on chemical properties of surface as well as topography Even if a surface exhibits only minor topographical variations but pronounced variations in chemical composition, AFM friction mode can still map the variation in the material properties at the surface Lateral force microscopy is a special application of contact mode AFM that is suitable for studying both the microscopic foundations of friction and the local variations in chemical composition The cantilever probe is most susceptible to the frictional effect when the scan direction is perpendicular to the major axis of the cantilever Therefore, the scan angle must be set to 90°or 270°to get a high quality frictional image

1.4.2.4 Force curve measurements

In addition to topographic measurements, the AFM can also record the amount of force felt by the tip as a function of the separation between the tip and sample A force curve (i.e force-versus-distance curve) can be constructed by monitoring deflection of the cantilever as a function of the distance normal to the sample surface plane

Force measurements are more interesting in liquids, where the electrostatic interactions between the dissolved ions and other charged groups play an important role in determining the forces sensed by an AFM cantilever.246, 249One can control many details

of the probe - surface force interaction by changing the properties of liquid The electrostatic tip - sample forces strongly depend on pH and salt concentration It is often possible to adjust the pH or salt concentration such that repulsive electrostatic forces effectively negate the attractive van der waals forces

Figure 1.7 Distance dependence of Van Der Waals and electrostatic forces compared to

the typical tip-surface separations in the contact mode (CM), non-contact mode (NCM), and intermittent contact mode

This force will be increased when the separation is less than hundreds of angstroms AFM

utilizes the force – distance relationship to provide the surface properties of the sample when scanning the sample The forces essentially experienced by the tip are

i) Van Der Walls force ii) surface tension iii) electrostatic force iv) chemical force

At large distances and in the absence of a meniscus force, the interaction is dominated by the van der waals forces results from electromagnetic dipole interactions This force increases in magnitude when the probe approaches the sample surface Surface tension is experience by the tip due to the influence of the water layer Once meniscus is formed, it dominates the interaction, extending far from the sample as the meniscus is stretched Electrostatic forces are dominant when there is a voltage difference between the tip and the sample; an attractive Coulomb force pulls them together

Chemical forces result from the interactions of the electron clouds and nuclei at the apex of the tip with those at the sample surface They are the same forces that are responsible for covalent or hydrogen bonds and cause repulsion, which is measured in contact AFM The attractive part of the potential decays very rapidly with distance, and it

is this behavior that enables atomic resolution in an AFM, since it means that only the microtip coming closer to the sample contributes significantly to the interaction

1.5 Applications and Challenges of SPM

The first use of STM was in atomic-scale imaging of crystalline materials by recording the variation in tunneling current between a sharp tip and the sample as the tip passes over the atomically corrugated surface However, the restriction to conductive samples limited the choice of materials investigated using STM In the AFM, the van der Waals force acting between the tip and surface provides the contrast mechanism instead of the tunneling current In addition to imaging the surface morphology of the insulating surfaces, AFM is also used for measuring the force between two surfaces

The use of AFM in nano-scale fabrication of surfaces has been accelerated with the development of nanografting techniques in 1997.107 In nanografting, an AFM tip is used to create a pattern in an organic self-assembled monolayer (SAM) formed on a gold surface

A force is applied to a selected area of the film to remove the adsorbed thiol molecules A different thiol is adsorbed to the exposed gold to form a nanometer-scale feature on the surface Gold - thiol SAMs (a thin, uniform, and chemically stable monolayer) provide us with a promising platform to construct chemically specific nanostructures By introducing proper terminal functional groups, the surface properties of the SAM can be modified

In another application, scanning probe based data storage concept was developed

by IBM researcher using a heated AFM probe In this thermomechanical writing process,

an AFM probe writes a data bit over a polymer surface through combined heat and mechanical force of the tip, which cause the polymer to soften, thereby facilitating the writing Information is stored as sequences of ‘indentation’ and ‘no indentation’ written on

a polymer films using an array of AFM cantilevers.52-55 This probe-based data storage combine ultrahigh density, small form factor and high data rates by means of highly

parallel operation of a large number of probes

In 1999 Mirkin developed a different scanning probe fabrication method called Dip-Pen Nanolithography (DPN).59With this methodology, an AFM tip is coated with a thin film of thiol molecules When the tip is placed close to the Au surface in an atmosphere containing a high concentration of water vapor, a minute drop of water condenses between the tip and the surface, and the thiol molecules migrate from the tip to the surface through the water meniscus as the tip moves across the surface DPN can also

be extended to silicon and semiconductor surfaces by choosing a suitable ink molecule with the meniscus ink-transport medium

The primary advantage of AFM lithography towards the nanofabrication by using scanning probes has been discussed in several reviews.38-42, 250 However, there are two important technological issues that should be overcome to get rid of the limits of this method First, the writing speed of the AFM lithography systems is too slow for mass production because of the serial patterning process Typical writing speed for a single tip

is about 2 µm/sec Researchers are making a huge effort to overcome this drawback The

aim is to produce an array of multiple tips and to write using all the tips simultaneously,

thereby increasing the speed It has been reported that anisotropic wet etching can make arrays of thousands of cantilevers.The second obstacle involves the friction-induced wear

of the SPM tip during fabrication One potential solution to this problem may be the use of nanotube probes, which are very robust and do not increase in diameter during wear owing

to the cylindrical geometry Such probes will enable us to write extremely thin patterns of several nanometers width without noticeable wear In spite of speed and wear problems, the use of SPM families in nanoscience is increasing continuously The growing number

of publications is good evidence for the widespread use of these techniques

1.6 Nanolithography of Polymer Films

Nanolithography is an art and science which includes etching, writing or printing

on the surface at the nanoscopic level The conventional lithographic method was introduced in 1798 as a printing tool by Alois Senefelder, who found that ink adsorbed to

an image drawn with a greasy fluid onto limestone (lithographic stones) could be used to transfer the image to a piece of paper pressed against the stone.251 Nowadays, this printing technique is not only used for printing, but also for the chemical modification of substrates

on a sub-micrometer scale The most prominent application of lithography is that of mask lithography, which is widely used in the semiconductor industry With the development of (computer) chips and their increasing complexity, the requirement for techniques that can create smaller structures is ongoing Both industry and academia are investing intensive efforts in developing UV and deep-UV lithographic techniques, which can provide submicron resolution

UV-One of the current challenges in nanotechnology is the fabrication of functional nanostructure and inter-connection of these structures into nanodevices on semiconducting surface Fabrication of nanostructures using organic/polymer materials has attracted much attention owing to the recent interest in organic electronics, including field effect transistors, light emitting diodes (LED), solar cells, and other devices, due to their favorable processibility, reasonable stability and low cost In addition, the possibility of tailoring the structure of the polymer at the molecular scale allows fine – tuning the properties The ability to tailor the surface with chemical functionality in nanometer scale

is a fundamental and exciting challenge in nanofabrication The SPM based patterning technique is one among the many lithographic techniques, which allow us to pattern the

surface on nanometer scale as well as visualize the surface at molecular scale

1.6.1 Electrostatic nanolithography

Electrostatic nanolithography is an SPM based lithographic technique in which patterns are drawn on thin polymer/organic films on a solid substrate using a biased AFM tip.113 Electric field has proven to be a suitable tool for contactless, rapid and parallel manipulation of small particles by using electrophoretic forces such as in electro-deposition The polymer/organic film can be modified directly at nanoscale with the combined action of AFM and electric fields with its nanoscale manipulation capability In this method, nanopatterns are created on the substrate due to Joule heating of the polymer film without any chemical modification when the electric field is applied between the tip and the film.113

1.6.2 Chemical nanolithography

In chemical lithography, patterns are created on the substrate through chemical modification of the surface coated with polymer/organic film Recently, Geyer et al introduced the possibility for chemical lithography using an electron beam.252 They used focused electrons to chemically modify the terminal nitro groups of self-assembled monolayers (SAMs) to amines, followed by subsequent functionalization with carboxylic acid anhydrides (Figure 1.8) Reports also show the preparation of ordered arrays of metallic nanoparticles and metallic rods using the self-assembly of metal-containing block copolymer micelles onto electron-beam patterned substrates, by effectively combining techniques from the bottom-up and the top-down approaches In another approach,

nanopatterns of thermosensitive poly(N-isopropylacrylamide) brush on gold substrate