Atomic force microscopy (AFM) based nanopatterning and nanocharacterization

Table 1.1 Comparison of AFM nanolithography techniques with other lithographic techniques Lithographic Patterning Operation Resolution Advantages Limitations AFM Based on tip-sample A

Chapter 1 Introduction

1.1 Nanofabrication for miniaturized devices

Nanotechnology is vital for the continued miniaturization of components such as integrated circuits, memory devices, display units, biochips and biosensors The advancement of nanotechnology involves the control of matter and fabrication of meaningful structures at the nanometer scale One of the key processes in nanofabrication

is the construction of functional units in the size regime less than 100 nm Top-down and bottom-up approaches have been used to generate nanostructures The former involves the application of various lithographical techniques to create nanoscale patterns starting from a featureless bulk material, while the latter uses the interactions of molecules and colloidal particles to assemble two- and three-dimensional structures The conventional techniques for nanofabrication are based on various lithographical methods in the top-down approach, including photolithography,[1,2] electron beam lithography[3,4] and focused ion beam lithography.[5,6] However, the applicability of these techniques is often limited by their high capital and operating cost, multiple-step processes, and poor accessibility In recent years, novel methods such as nano-imprint lithography (NIL),[7-9]soft lithography,[10-11] and atomic force microscopy (AFM) nanolithography[12,13] have emerged as flexible alternatives for nanoscale patterning and fabrication These novel methods have the potential to be future low-cost techniques for nanoscale pattern formation and replication

Among these newer techniques, AFM nanolithography has shown itself to be a unique tool for materials structuring and patterning with nanometer precision In this technique, the probe can be used to: (i) transfer chemicals to a surface; (ii) induce localized reactions

by applying a bias on the tip; (iii) mechanically scratch a surface; and (iv) manipulate molecules, nanotubes via pushing, sliding and rotating The working principle of AFM nanolithography is based on the interaction between the probe and substrate The typical radius of curvature of the probe is 20–60 nm, and the probe–substrate separation in close contact condition is <1 nm When suitable forces are exerted, and/or external fields applied, the probe can induce various physical and chemical processes on the substrate surface Consequently, localized nanostructures are generated through physical modifications and/or chemical reactions of the surface materials AFM nanolithography possesses the versatility to pattern a wide range of materials including metals, semiconductors, polymers and biological molecules in different media Due to its nanoscale positioning and imaging capability, AFM nanolithography is uniquely able to create site-specific and localized functional structures Moreover, the morphological and physical properties of patterns formed can be immediately characterized with AFM by integrating additional measurement modules This combined fabrication and

characterization function in AFM nanolithography allows convenient in situ and in-line

pattern creation and characterization Table 1.1 compares the characteristics of AFM nanolithography and other nanofabrication techniques

Table 1.1 Comparison of AFM nanolithography techniques with other lithographic techniques

Lithographic Patterning Operation Resolution Advantages Limitations

AFM Based on tip-sample Ambient ,vacuum, ≤ 10 nm structures [120] – Easy to operate – Serial patterning

interaction or liquid phase to atomic – Low cost – Controllability and

Photolithography Selectively exposing Vacuum Sub 100 nm [1,2] – Parallel patterning – High operation cost

parts of substrate to – Good controllability – Multiple process

E-beam lithography Interaction between Vacuum ≤ 50 nm [3,4] – Well developed for – High operation cost

Focus ion beam Interaction between Vacuum ~ 50 nm nanogap [5,6] – High sensibility – High operation cost

(FIB) lithography ion-beam and – Good controllability – Multiple process

research – Poor accessibility

Nanoimprint Mechanical Vacuum or ambient ~ 100 nm regime [7-9] – Low cost – Precision issue

1.2 Development of AFM nanolithographic techniques

Numerous AFM-based lithographic techniques have been developed in the last two decades Generally, these techniques can be classified into two groups in terms of their operational principles: (i) force-assisted AFM nanolithography; (ii) bias-assisted AFM nanolithography (Table 1.2).[14] In force-assisted AFM nanolithography, a large force is applied to the tip for pattern fabrication, and the tip–surface interaction is mainly mechanical Typical methods in this category include mechanical indentation and plowing,[15] thermomechanical writing,[16] nanomanipulation,[17] and dip-pen nanolithography (DPN).[18] During force-assisted nanolithography, forces larger than those used for AFM imaging are loaded onto the tip The initially featureless surface is then patterned by mechanically scratching, pulling, or pushing the surface atoms and molecules with the probe In DPN, instead of manipulating the existing molecules on the surfaces, the tip is used as a nanoscale pen to directly deposit collections of ink materials onto the substrate to define a functional structure As for bias-assisted AFM nanolithography, the AFM tip is biased to create a localized electric field in the regime of

108 V/m to 1010 V/m, and the tip acts as a nanoscale electrode for current injection or collection Under such a high localized field, electrostatic, electrochemical, field emission, dielectric breakdown and explosive gas discharge processes can be initiated to facilitate pattern formation through nanooxidation,[13] electrochemical deposition,[19]electrostatic attraction,[20] and nanoscale explosion and shock wave propagation.[21,22] In anodic oxidation, the tip is negatively biased, and the local field induces the ionic dissociation of a water meniscus formed between the tip and sample surface The

oxidative OH- anions migrate along the field and react with the substrates to form oxide structures Electrochemical deposition is capable of generating positive structures with distinct physico-chemical properties from the precursor materials through bias-induced local chemical reactions In this section, we will discuss some important AFM based nanolithographic techniques relevant to this thesis

1.2.1 Dip-pen nanolithography (DPN)

In 1999, Mirkin’s group invented the DPN technique to deliver collections of molecules

in a positive printing mode.[18] DPN is an AFM-based direct-write lithographic technique

in which the AFM probe is used as a pen to directly deliver materials (inks) to a nanoscopic region on a target substrate (Fig 1.1) In most cases, the transport of ink molecules from the tip to the substrate is mediated by a water meniscus which is formed through capillary condensation.[23-28] Depending on the selection of ink molecules, DPN

is capable of creating structures made of various materials such as metal, inorganic compounds, organic molecules, and biological species (Table 1.3).[23-53] 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 and magnetic patterns, polymer brush arrays, and biological devices A comprehensive description on the evolution of DPN can be found in a recent review article by Mirkin and co-workers.[53]while significant progress has been achieved in DPN-based nanofabrication of patterns and devices, there is an ongoing debate on the detailed mechanism for ink transport and diffusion Theoretical and experimental results showed that the transport and deposition

Table 1.2 Comparison of force-assisted and bias assisted AFM nanolithography

techniques

Force-assisted AFM nanolithography Bias-assisted AFM nanolithography Operational

principle Large force applied to tip Bias applied to tip create

Tip acts as nanoscale electrode and induce physical and/or chemical processes

Tip-surface

interaction Largely mechanical

Under high field, electrostatic, electrochemical, field emission, dielectric breakdown and explosive gas discharge processes can be initiated to facilitate pattern formation

Table 1.3 Types of inks used in DPN (adapted from ref[53])

Types of inks Applications References

Organics – Study of diffusion dynamics of ODT on Au 24,25

– Effects of temperature and humidity on patterning 26

of ODT and MHA molecules – Redox-active ferrocenylalkylthiol inks on Au 43 – Rhodamine 6G(R6G) dye deposition 51 – Patterning of poly-EDOT nanowires 31 – Patterining of conducting polymer 40

– Patterning of oligonucleotides on Au and SiO 2 48

Inorganics – Au nanocluster and nanoparticles 33,35

– Deposition of Pt nanofeatures by E-DPN 19 – Hard magnetic barium hexaferrite pattern 38 – Composite nanostructures consisting of 39

Al 2 O 3 , SiO 2 and SnO 2

Fig 1.1 Schematic showing the transport of ink from the AFM tip to the substrate through the water meniscus (Source: ref[18])

(a)

(b)

Fig 1.2 (a) Schematic showing the configuration of the AFM probe arrays (b) Photograph of fabricated chip with the 32 x 32 cantilever array located at the center (Source: refs[64,65])

of inks depends on several factors such as the formation of water meniscus, the properties

of the tip and the substrate, and the ink deposition time and temperature.[23-32]

1.2.2 Thermomechanical writing and millipede techniques

In thermomechanical writing, a resistively heated AFM probe writes a data bit by scanning over a polymer surface The combined heat and mechanical force of the tip causes the polymer to soften and flow, thus facilitating the writing of data bits in a storage medium This technique was pioneered by the IBM Almaden research group, and systematic descriptions on its working principles and applications can be found in their numerous publications.[16,54-66] In this technique, an electrical current is passed through the cantilever, and a force is loaded on the hot tip to indent the polymer medium Thermomechanical writing is a reversible nanofabrication technique in which data bits can be written, erased, and re-written.[56] Data erasing is simply achieved by thermal reflow of the storage field as a whole No alteration of the polymer film was observed after repeated writing and erasing With this approach, storage fields of a few hundred

microns can be erased en bloc Subsequently, a data-storage concept named the

"millipede" was introduced that combines ultrahigh density, terabit capacity, small form factor, and high data rate Fig 1.2(a) shows the configuration of a 2D AFM cantilever array fabricated for the millipede system by the IBM group.[64,65] A 32 x 32 array chip can generate 1024 storage fields on an area of less than 3 mm x 3 mm The corresponding data capacity of the 1024 storage fields is 0.9 Gb assuming an areal density of 500 Gb/in.2 Fig 1.2(b) shows a photograph of the fabricated chip, on which the 32 x 32

cantilever array is located at the center with bond pads distributed on either side In general, the storage capacity of the system scales with the areal density, the cantilever pitch, and the number of cantilevers in the array

1.2.3 AFM nanooxidation

AFM nanooxidation is one of the earliest and most extensively studied techniques in bias-assisted AFM nanolithography In this method, the water meniscus formed in the tip–sample gap is dissociated by the negative tip bias, and the O- and OH- oxidative ions react with the substrate to form localized oxide nanostructures (Fig 1.3) Since the molecular volume of the oxides is usually larger than that of the substrate materials, raised nanopatterns are formed after the oxidation reaction Research in this area is aimed

at fabricating nanoscale devices such as metal-oxide-semiconductor (MOS) transistors through the precise control of the local oxide growth by the AFM probe In addition, the anodic oxide features can act as reactive sites for the further assembly of molecules and nanoparticles through chemical linkages and affinities Alternatively, the oxides can also

be etched to produce negative structures on the substrates for pattern transfer

The mechanism of probe nanooxidation has been addressed by several authors Generally, it is suggested that the oxidation mechanism and kinetics are closely related to electrical field, surface stress, water meniscus formation, and OH- diffusion.[67-80]

Specifically, various models such as the Cabrera–Mott model,[67] power-law model,[68]log kinetic model,[69-72] and space charge model[74-77] have been proposed to account for

the oxidation behavior (Table 1.4) There is an ongoing debate on the limiting-factors of oxide growth due to the complexity of such nanoscale oxidation processes In addition, several studies focused on oxidation in non-contact AFM mode, and in particular the formation of a water bridge between the tip and surface.[81-82]

AFM nanooxidation has been applied to produce oxide structures on semiconductors,

[83-118] metals[119-132] and molecularly functionalized/passivated surfaces[133-146] (Table 1.5) Examples of AFM oxidation on semiconductors surfaces include studies on Si[85] and Ga[Al]As substrates [111-113] AFM local oxidation has been applied to different metals including Ti,[119-121] Al,[122] Cr,[123] Nb,[124,125] Ni,[126,127] Ta,[127] Mo,[129] Zr,[130,131] and

Co.[132] Furthermore, various approaches using self-assembled monolayer (SAM)[133-143]

or Langmuir–Blodgett (LB) films[144-146] as a resist for AFM local oxidation have been proposed For instance, early work by Sugimura et al involved the anodic oxidation of Si covered by organosilane TMS monolayer.[133] This resist was locally degraded during oxidation due to electrochemical reactions induced in the tip– sample junction The oxide patterns generated were then transferred to the Si substrate by chemical etching Work pertaining to the oxidation of molecularly functionalized/ passivated surfaces have also been previously reviewed.[147,148] In recent years, interesting works of AFM oxidation through organic meniscus formation instead of water bridge have also been carried out.[149,150] García and co-workers extended the oxidation nanolithography to produce nanostructures made of materials other than oxides.[149,150] Their strategy is based on the formation of organic menisci to adjust the chemical composition of structures formed

Si substrate

AFM oxide

O - ion

Negative bias

Fig 1.3 Schematic description of AFM nanooxidation

Table 1.4 Summarized AFM oxidation model developed by various researchers

Cabrera–Mott model – Thickness of oxide is governed by diffusion limited 67

electric field

Power-law model – AFM oxidaton is only observed for voltages 68

exceeding a doping dependent threshold above which oxidation kinetics follows a power law

log kinetic model – Height, h proportional to log (1/v), v-1/2 and v2-1/4 and 69-72

linear behavior between 1/h vs log v

Space charge model – Varied space-charge dependence of oxidation 74-77

process as a function of substrate doping type/level

– Space-charge effects are consistent with the rapid decline of high initial growth rates

– Alberty–Miller scheme to describe direct pathway for reaction of oxyanions with silicon at the Si/SiO2 interface and an indirect reaction pathway, mediated

by trapped charge defects at the interface

Table 1.5 Examples of application of AFM oxidation on different substrates

Type of Substrate Nanostructures Fabricated References

Semiconductors – Fabrication of nanometer-scale side-gated silicon 85

field effect transistors – High speed and large area writing, e.g 97-100 fabrication of 0.1 mm metal oxide semiconductor

field effect transistors on amorphous silicon (α:Si) films – Fabrication of high quality antidot lattices, e.g 110 20x20 antidot array with a lattice period of 300 nm

– Electrical conduction on hydrogenated diamond 116

Metals – Fabrication of metal-oxide devices on thin Ti films 119

(~ 7 nm) – Probe-grown nickel oxide as a catalytic template for 127 selective growth of CNTs

– Oxidation of molybdenum (Mo) film to form MoO3 129 patterns

Molecularly – Oxidation of Si covered by organosilane TMS 133

functionalized/ monolayer

passivated surfaces – Poly(benzylether) dendrimers terminated with both 136

benzyl and tert-butyldiphenylsilyl ether groups as resists for AFM oxidation lithography

– Oxidation of surfaces passivated by mixed SAM 141 layer comprising 1,12-diaminododecane dihydrochloride (DAD• 2HCl) and n-tridecylamineahydrochloride (TDA•HCl)

The water meniscus is replaced by an organic meniscus, and the oxidation reaction is eliminated or significantly suppressed

1.2.4 Electrostatic deformation and electrohydrodynamic nanofluidic motion

The ability to directly pattern and write polymers at the nanometer scale is crucial to applications in data storage and molecular electronics.[20,151-153] Lyuksyutov et al introduced AFM electrostatic nanolithography (AFMEN) to generate nanoscale polymeric features by Joule heating and mass transport on initially featureless polymer films.[20] In this technique, current flow generated by tip biasing produces effective Joule heating which locally softens the polymer film The extremely non-uniform electric field gradient polarizes the viscoelastic polymer and attracts it towards the tip apex, leading to the formation of protruding structures on the film (see Fig 1.4) The attractive force arises from the imbalance between the Laplace, viscous, and electrostatic pressures When the electrostatic pressure overcomes the combination of Laplace and viscous pressures, electrostatic deformation of the polymer melt takes place The optimal polymer film for patterning relies on materials selection and processing that provide gradual

dielectric breakdown under the electric field Lyuksyutov et al also presented a detailed

theoretical investigation of the behavior of dielectric materials under electrostatic pressure.[153] In their studies, the method of images is applied to solve the electric field configuration produced by charge distribution in the presence of a thin polymer film coated on a conductive substrate When electric breakdown takes place inside the film, polymer features are generated by softened polymer mass transport in a single step

Au-Pd

Softened polymer Polymer

AFM tip

Fig 1.4 Schematic presentation of AFM electrostatic nanolithography for polymer pattern formation (Source: ref[20])

process without external heating either in contact or amplitude-modulated AFM modes The feature size does not depend substantially on the tip shape or polymer composition while the process is dependent on the glass transition temperature of the planar polymer film When the AFM tip comes closer to the polymer surface (e.g., <1 nm), the electrostatic pressure overcomes the threshold of the material plasticity, thus creating the conditions necessary for irreversible changes in the polymer surface This process is dependent strongly on the AFM tip shape and can be implemented only in contact AFM mode

1.3 Motivation and objectives of this work

The scope of this work covers different nanofabrication techniques in AFM nanolithography, from nanooxidation of inorganic semiconducting materials to the nanopatterning of insulating polymers The work includes extensions of existing techniques and explorations of new lithographic methods with the aim to expand the capabilities of AFM-based nanofabrication It is hoped that this work could provide new insights and solutions to the understanding and development of AFM nanolithography The specific work presented in the thesis and their motivations are elaborated below

1.3.1 AFM nanooxidation of semiconductors

Despite the extensive studies of AFM nanooxidation of Si,[67-82] the understanding of nanooxide formation on Si is still incomplete, and there is an on-going debate as to the

mechanism and kinetics of nanooxidation Si is still the principal material for most semiconductor-based device fabrication The nanooxidation of Si is crucial to the fabrication of Si-based nanodevices such as field effect transistors,[85,97] nanocapacitors,

[154] biosensors,[155] and quantum wires.[156] In such devices, the nanooxides act as dielectric barriers to modulate carrier mobility and electrical conductions In addition, the oxide nanostructures can also be used as templates for selective molecular assembly through chemical interactions.[133-148] In chapter 3, we set out to further investigate the probe-induced oxidation behaviors of Si substrates A thorough understanding of the key factors affecting nanooxidation is essential in controlling the stability and reproducibility

of nanooxidation and the aspect ratio of the oxide structures These factors include tip bias application, meniscus formation and the spreading modes of OH- oxidants, etc.[67-82,

157-158] For example, the spreading modes of OH- oxidants in AFM nanooxidation could critically influence the growth of mechanism of nanooxides, leading to a series of distinct oxide configurations.[67,73-75,77,86,157,158]

1.3.2 AFM nanooxidation of silicon carbide (SiC) semiconductor

SiC is a wide band gap semiconductor which exhibits high breakdown field, good thermal conductivity, and chemical inertness.[159] SiC is advantageous over Si for high-power, high-frequency, and high temperature applications Moreover, SiC is the only compound semiconductor on which oxide can be thermally grown for metal–oxide–semiconductor field effect transistor devices.[160,161] However, thermal oxidation and subsequent device fabrication require complicated multiple steps including sample

annealing under dry or wet oxygen conditions, followed by ex-situ etching and

lithographic treatments.[162-164] More importantly, although many semiconductors have been used for AFM nanooxidation (see Table 1.4), there is no report on the AFM probe oxidation of SiC materials, probably due to its physical hardness and chemical inertness

In Chapter 4, we present results pertaining to the native oxidation decomposition and localized oxide growth on SiC induced by a biased AFM probe We also discuss the aspect ratio and growth kinetics of oxides on SiC in comparison to Si

1.3.3 AFM nanocharacterization of local oxides

AFM nanooxidation is capable of producing oxides of sub-10 nm critical dimension with precisely tailored electric properties.[119] However, due to their high locality and nanoscale sizes, the characterization of the chemo-physical properties of AFM nanooxides is difficult Since AFM-fabricated nanooxides are expected to be gate oxides for metal-oxide-semiconductor (MOS) devices, it is essential to understand their chemical stability and dielectric characteristics Etching experiments are often performed

to evaluate the stability of large scale oxides using the quartz microbalance[165] and spectroscopic techniques.[166-168] However, these techniques are not suitable to characterize the etching of localized AFM nanooxides due to (i) the limits of lateral and vertical resolutions and (ii) the lack of precision in relocating the same single

nanostructure after ex situ treatments This motivates us to develop novel in-situ

AFM-based nanocharacterization technique to directly analyze the etching behavior of ultrathin (≤5 nm) nanooxides produced by AFM nanooxidation As for the dielectric property of

nanooxides, it is viable to use conductive AFM (cAFM) to study the current leakage and

breakdown of nanooxides by monitoring their current-voltage (I-V) curves In Chapter 5,

we will present the chemical and electrical stability of nanooxides fabricated by AFM nanooxidation on Si and SiC semiconductors

1.3.4 AFM nanofabrication of polymeric materials

Micro- or nanoscale patterning of polymers is crucial for manufacturing advanced electronic,[171-173] optical,[172,174] mechanical,[175-177] and memory[54-66] devices Polymers are commonly used in lithographic and fabrication processes either as masks or matrix materials due to their in chemical and physical properties.[178] Polymers can be used either as nanotemplates with different morphologies and tunable sizes, or further modified with different functional groups to enhance the interactions Numerous AFM nanolithographic techniques[16,20,54-66,151-153,177-181] have been demonstrated for the patterning of polymer thin films For example, in AFM electrostatic nanolithography (AFMEN), raised polymer patterns are fabricated through the electrostatic attraction of softened polymer melts to a biased AFM probe.[20,151-153] In Chapter 6, we address the initiation of local EHD instability by a biased probe and its application in fabricating Taylor cone-like structures on polymer matrix The ionic conduction mechanism in the formation of conical patterns will also be discussed

1.3.5 AFM nanolithography in acidic thin layers

Most AFM nanolithography is carried out in air, and the formation of the water meniscus[23,81,182-184] between the tip-substrate gap plays a crucial role in pattern formation Such nanoscale water meniscus can facilitate molecular transport and deposition,[18,185] assist electric conduction,[180] and mediate chemical reactions in the nanometer-sized gap.[76,119,183,185] Since the water meniscus is formed through the capillary and field-enhanced condensation of ambient water vapor, it is neutral in acidity, and can only supply species containing H and O atoms This may limit the applications of water meniscus-based techniques, as many fabrication processes require the use of acidic solutions which contain etching species such as F- and HF.[186,187] Recent works have reported the formation of nanostructures by extending the water meniscus to acidic and organic menisci.[150,151,188] Alternatively, the operation of AFM nanolithography in bulk solutions was also demonstrated.[189,190] In chapter 7, we present the operation of AFM nanolithography in acidic thin layers which provides an intermediate working state between local meniscus-based and bulk solution-based operations We show the formation of acidic thin layers by microscale droplet and AFM probe scanning, and the unique features of Si nanostructuring by performing AFM patterning in thin layers

1.4 Strategies and approaches of this work

Based on the objectives and motivations explained in 1.3, we further describe various approaches that we use for our patterning and characterization studies The details of our experimental setup/ techniques are explained in chapter 2

1.4.1 AFM nanolithography

The work in this thesis mainly involves bias-assisted AFM nanolithography[14] and in-situ

AFM nanocharacterization The nanolithography methodology is usually based on the application of electrical (negative) bias to the tip, which subsequently creates a localized high field region (~108 to 109 V/m) between the tip apex and sample As reviewed earlier, such extreme high field favors the initiation of physical and/or chemical processes that lead to the formation of various patterns on different substrates We use this method (i) to create oxide patterns on semiconductor surfaces with AFM nanooxidation (chapter 3 to 5), (ii) to induce structure formation on various polymer surfaces based on the localized joule heating and EHD instabilities under the biased tip (chapter 6), and (iii) to form nanostructures on Si by biased-assisted AFM patterning in dilute acidic thin layers (chapter 7) In case (iii), oxidation and dissolution process can be induced depending on the concentration of the thin acidic layer Contact mode AFM is used in all of the above patterning process due to its stability as the tip is maintained in close contact throughout the process Previous studies have pointed out the importance of electrical field induced water bridge formation between the tip and surface especially in the case of dynamic

mode AFM oxidation.[82] Due to the existence of the nanogap, a threshold voltage is needed to form the water bridge in order to initiate oxidation process However, in contact mode, a close contact between the tip and the sample in ambient drives the spontaneous formation of a water meniscus Although the dimension of such meniscii may be larger than previously reported refs[81,82], an optimized experimental setup can improve the spatial resolution Furthermore, due to the experimental condition in ambient, contact mode AFM provides better controllability and reproducibility as the water bridge formation at tip/substrate interface is also affected by surrounding humidity

1.4.2 I-V electrical characterization by cAFM

To further understand the electrical properties of AFM-fabricated nanooxide, the I-V

characteristic of the oxide is measured using the conducting AFM (cAFM) setup In

chapter 5, immediately after AFM oxidation, current–time (I–t) or current–voltage (I–V) curves are collected in situ by switching on the integrated current sensor The electrical

characterizations are carried out mainly to study the stress-induced leakage current (SILC) and breakdown (BD) in fresh and thermally annealed ultrathin AFM oxide on Si and SiC In chapter 4, we will discuss the electron transport in the above AFM oxide on SiC in terms of interface barrier height, while in chapter 6, we will explore the humidity dependent of electrical behavior of water assisted ionic conduction in polymer substrate

1.4.3 Force curve measurement

AFM force curves have great impact on the theoretical studies of tip-surface interactions for numerous materials properties.[191] Generally, the force curve records the tip-sample interaction by monitoring the deflection of the cantilever as a function of the position of

the piezo, while the force F is obtained from the relation F = kx, where k is the force constant of the cantilever and x is the deflection of the cantilever.[191] The force curve may be used to analyze local surface properties such as elasticity, hardness, Hamaker constant, adhesion and surface charge densities.[191] In chapter 7, we study the force curve

to further understand the tip-surface interaction, particularly the probe-induced adhesive forces in liquid layers for nanocluster collection and assembly Force curves were collected during the approach and withdrawal of the tip from the liquid layer The force measurements provide vital information on the magnitude of the adhesive force under the influence of liquid thickness and tip withdrawing velocity Force curve measurements are also used to monitor the transformation of water droplet to uniform layer in the AFM tip scanned area

1.4.4 Theoretical simulations and other supporting methods

Theoretical simulations are also demonstrated with collaboration from other research groups to verify our observations in AFM patterning experiments In chapter 3, MSC Dytran package was used for numerical hydrodynamic simulation Finite element method (FEM) was also applied to simulate the electrical field distribution between the tip and

surface during AFM nanooxidation BEM (boundary element method), and MOC (method of characteristics) were used to obtain the solution of the Laplacian electric field where the space charge was assumed to be zero and only the applied static voltage was considered In chapter 4, Maxwell 2D (SV) package is used to simulate the electrostatic field distribution In chapter 6, density functional theory (DFT) calculation is used to support the crosslinking of poly(N-vinyl carbazole) (PVK)

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Chapter 2 Experimental

2.1 Atomic Force Microscopy

The nanopatterning and nanocharacterization work in this thesis is mainly performed using a NanoMan AFM system (Nanoscope IV, Veeco Instruments and Process Metrology) The NanoMan (a software and hardware configuration) system allows us to perform high-resolution imaging, high-definition nanolithography, and direct nanoscale manipulation.[1] The NanoMan AFM system consists of a Dimension 3100 Scanning Probe Microscope (SPM) which provides a basic imaging platform; a Dimension Closed Loop XY Scanning Head (90 μm x 90 μm scan size) for precise lateral probe positioning for patterning with improved accuracy and repeatability; a NanoScope IV controller that supports closed loop scans; and a NanoMan User Interface software for setting experimental parameters such the pulse (limited within -12 V to 12 V) and the tip lateral speed in the range of 1x10-6 to 103 µm.[1] The Dimension 3100 Scanning Probe Microscope together with the scanner are shown in Fig 2.1 (right)

2.1.1 Working principle and key components of AFM

Working principle: AFM was invented in 1986 and is capable of imaging the surface

morphology with atomic and molecular resolution.[2] AFM operates by measuring attractive or repulsive forces between the tip and sample Typical contact forces are in the range of nN to μN in ambient The key components of the AFM are shown in Fig 2.1

Controller electronics

Detector

electronics

Laser

X, Scanne

or constant oscillation Amplitude (tapping mode)

or constant frequency (non-contact mode)

Integrated frequency synthesizer for tapping

or non-contact mode

piezo Cantilever and Sample

Fig 2.1 Schematic representations of the key components of the NanoMan AFM system (left) The photo on the right displays the actual setup of the microscope

AFM cantilevers (usually made of Si or Si3N4) with holder are mounted onto a scanner which responds to the vertical movement of the cantilever The piezo-scanner expands or contracts in proportion to the applied voltage The piezo-scanner has an AC voltage range of +220 V to -220V for each scan axis During operation, a laser beam is positioned on the cantilever by a tilt stage in the scanner head This laser spot is then reflected and monitored by a quad photodetector The photodetector consists of four elements that provide different information depending on the AFM operation mode, and the differential signal between the top two elements and the bottom two elements measures cantilever deflection The detector and controller electronics maintain the feedback and adjust the piezo scanner depending on the operation mode Simultaneously, the data collected in each scan is sent to a digital signal processor Required images are then formed by summing up each (x, y) data point collected

piezo-Operation modes: There are three main classes of AFM based on the tip-substrates

interaction: contact mode, tapping mode (or intermittent mode) and non-contact mode In

contact mode, the tip and sample remains in close contact as the tip scans across the surface.[2] The feedback loop maintains a constant deflection (hence maintaining a constant force) between the tip and sample through the vertical movement of scanner at

each (x,y) data point The force, F, is measured according to Hooke’s Law, F = kx where

k is the spring (force) constant and x is the cantilever deflection The recorded vertical

heights at each data point are then processed by the computer to form the topographic image of the scanned surface

The “Tapping Mode” is a patented technique (Veeco Instruments) that maps topography

as the tip is oscillated at its resonant frequency above the sample surface.[3] The feedback loop maintains a constant oscillation amplitude and the laser beam is then detected by the split photodiode detector Tapping mode AFM can be operated in ambient or liquid environment It is developed for non-destructive imaging, in particular for soft samples such as biological molecules[4] as lower force is applied during imaging while the lateral force is virtually eliminated The resolution obtained rivals that of contact mode AFM (lateral resolution of ~1nm and vertical resolution of ~ 0.1nm) The high resolution is achieved through the constant oscillation amplitude which allows the tip to contact the surface without getting ‘stuck’ in the absorbed fluid layer as compared to contact mode AFM (especially at high humidity ~70% RH in most of our experiments)

In non-contact mode AFM, the tip does not contact the surface, but oscillates above at a frequency slightly higher than the cantilever resonant frequency In this mode, the tip oscillates at a frequency which is slightly above the cantilever’s resonance frequency typically with an amplitude of a few nanometers (<10nm) in order to obtain an AC signal from the cantilever, while Tapping mode AFM will have the tip oscillating with an amplitude ranging typically from 20nm to 100nm Non-contact mode AFM now has achieved atomic resolution.[5]

AFM tips/cantilevers: In our experiments, several commercially available tips are used

for imaging, patterning and characterization A typical AFM Si3N4 contact mode cantilever (NP series, Veeco Instruments and Process Metrology) is used for imaging

(Fig 2.2(a)) The tip can be coated with ~ 30-50 nm Au layer by e-beam evaporation for bias-assisted patterning The NP series consists of 4 triangular probes with different force

constants (k = 0.06, 0.12, 0.38 and 0.52 N/m respectively) with a typical tip height is ~2.5

- 3.5 μm with a nominal radius of ~ 20 nm AFM probes with k = 0.12 and 0.52 N/m are

often used for the AFM oxidation

Tapping mode tips (Tap300 metrology probe, Veeco Instruments and Process Metrology) are used for imaging, particularly on softer surfaces (e.g polymer) before or after patterning The phosphorus doped (n-type) Si tip has a nominal force constant k ~ 40 N/m and frequency in the range of 300 kHz (Fig 2.2(b)) The tip height is ~ 15 - 20 μm with < 10 nm nominal tip radius

For contact mode AFM patterning and characterization, we used the commercially

available conductive AFM tips A Cr/Pt coated conductive tip (ElectriCont,

BudgetSensors) with 5 nm Cr is usually used The front side Cr/Pt coating provides a conductive layer while the back side coating serves as a reflective coating and compensates the stress created by the front side coating (Fig 2.2 (c)) The typical tip

height is ~ 10 - 15 μm with a nominal radius of ~ 20 nm and force constant k = 0.2 N/m

2.1.2 Conductive AFM module for electrical nanocharacterization

In cAFM, a current sensor is integrated to the AFM system to collect current flowing in the tip-sample nanojunction The cAFM sensor contains the current amplification circuit

Fig 2.2 Schematic drawing of (a) Si3N4 contact mode cantilevers, (b) Tapping mode tip,

and (c) conductive AFM tip

Keithley

237 DC Power supply

LABVIEW Software

Of I-t/I-V collection

Sample

Features to be

characterized

Au-coated / PtIr-coated tip

Fig 2.3 (a) cAFM setup in this work (b) The external Keithley module is used for

higher bias range

For I-V measurements, the tip is either positioned in the centre of a particular scanned image (point of measurement) after a scan or immobilized and engaged directly the preferred location The experiment parameters are then defined, which include the start/end of ramp (-12 V to 12 V), the scan rate, and the X and Y position offset The ramp start/ramp end function allows the user to control the start and end values of a ramp cycle (either positive or negative value) The scan rate represents the ramp velocity in a complete ramp cycle (indicated in Hz or V/s) The X and Y offsets allow us to move the tip to a specific location for a new measurement The spectrum obtained during the measurement is then saved and further analyzed using the software

In some parts of this work, an external Keithley 237 High-Voltage Source-Measure Unit with 10 fA and 10 μV measurement sensitivity is integrated into the AFM system whenever higher biases (≥ 12 V) are needed, e.g for studying high-bias regime oxidation (Fig 2.3(b)).[6]

2.1.3 Nanolithographic software

Two patterning techniques can be performed with the NanoMan software, (i) mechanical scratching of soft surfaces such as polymers, and (ii) AFM nanooxidation The lithography program directs the microscope to predefined lines or polygon shapes either

by creating a path or modifying the script in the software (we only use the former in our work) Fig 2.4 shows the software operating window where a path is defined by drawing

lines The control of the probe-induced patterning is generally based on a NanoMan Manual tab and the Path tab in the software The former consists of a few important experimental parameters such as XY move, Z distance, Z velocity, pulse voltage, and pulse duration XY move is used to define the tip lateral speed (1x10-6 to 103 μm/s), the voltage applied to the tip (-12 V to 12 V), and tip voltage ramp (0-5 s) Z distance defines the distance that the tip is lifted from or pushed to the sample surface (in nm step) Z velocity controls the vertical speed of tip during either retraction from or compression to the sample surface (0.1 – 1000 nm/s) The pulse voltage represents the tip voltage applied during a pulse (-12 - 12 V) while the pulse duration defines the time interval when the pulse voltage is maintained (10 ms - 1000 s) The Path tab menu is used to define a sequence of straight lines of tip motion When the command is executed, the tip traverses along the recorded path according to the parameter setting of each segment

Fig 2.4 (a) Image of the operating windows of NanoMan lithographic software used in the patterning works (b) Two examples of AFM oxide fabricated by using the software