Atomic force microscopy (AFM) based nanopatterning and nanocharacterization 1

Acknowledgements This works has been made possible by the advices and inspirations of many individuals. First, I would like to take this opportunity to thank Prof. Andrew Wee Thye Shen (Department of Physics) and Dr. Xie Xianning (NUSNNI, NUS Nanoscience and Nanotechnology Initiative) for their great supports and invaluable guidance throughout these years. I would also like to acknowledge the fantastic and enthusiastic contributions from my colleagues and friends who help me in sample preparations, additional experimental and analysis works. Special thanks are dedicated to my fiancée and family for their love and encouragement. And last but no least, I am grateful for the financial supports from the Department of Physics (NUS Research Scholarship award) and NUSNNI. i Contents Acknowledgements Table of Contents Summary List of Tables List of Figures Chapter Chapter Introduction 1.1 Nanofabrication for miniaturized devices 1.2 Development of AFM nanolithographic techniques 1.2.1 Dip-pen nanolithography (DPN) 1.2.2 Thermomechanical writing and millipede Techniques 1.2.3 AFM nanooxidation 1.2.4 Electrostatic deformation and electrohydrodynamic nanofluidic motion 14 1.3 Motivation and objectives of this work 1.3.1 AFM nanooxidation of semiconductors 16 1.3.2 AFM nanooxidation of silicon carbide (SiC) semiconductor 17 1.3.3 AFM nanocharacterization of local oxides 18 1.3.4 AFM nanofabrication of polymeric materials 19 1.3.5 AFM nanolithography in acidic thin layers 20 1.4 Strategies and approaches of this work 1.4.1 AFM nanolithography 21 1.4.2 I-V electrical characterization by cAFM 22 1.4.3 Force curve measurement 23 1.4.4 Theoretical simulations and other supporting Methods 23 References Experimental 2.1 Atomic Force Microscopy 2.1.1 Working principle and key components of AFM 32 2.1.2 Conductive AFM module for electrical nanocharacterization 36 2.1.3 Nanolithographic software 38 2.2 Experimental procedures 2.2.1 Semiconductor sample preparation 41 2.2.2 Polymer sample preparation 41 i ii vi vii viii 1 16 21 25 32 32 41 ii 2.2.3 AFM nanofabrication 42 2.2.4 cAFM nanocharacterization 43 2.2.5 TOF-SIMS and SEM characterizations 2.2.6 Theoretical simulations 45 References Chapter Chapter Chapter 44 AFM nanooxidation of semiconductor surface (I): Oxidation of Si 3.1 Introduction 3.2 AFM nanooxidation 3.2.1 Influence of space charge in AFM Nanooxidation 50 3.3 Nanoexplosion and shock wave propagation generated by electrical discharge under high field 3.3.1 Electrical discharge observed with AFM 54 3.3.2 Oxide disks and diffusion model 61 3.3.3 Numerical hydrodynamic simulation of the propagation of transient wave 66 3.4 Conclusions References AFM nanooxidation of semiconductors (II): Native oxide decomposition and localized oxidation of 6H-SiC (0001) 4.1 Introduction 4.2 Native oxide decomposition on 6H-SiC (0001) 4.2.1 Oxide decomposition 77 4.2.2 The kinetics of oxide decomposition 80 4.3 Local oxide growth on 6H-SiC(0001) 4.3.1 Direct oxidation of SiC 87 4.3.2 Simulation and discussions 88 4.3.3 Influences of humidity, stress and diffusion 91 4.3.4 Comparison of nanooxidation on 6H-SiC (0001) − and 4H-SiC (11 ) surfaces 97 4.4 Conclusions References Nanocharacterization of probe-grown oxides by atomic force microscope 5.1 Introduction 5.2 In-situ chemical etching study of AFM ultrathin oxides by atomic force microscopy 5.2.1 Chemical etching behavior of probe grown 48 49 49 50 54 72 74 76 76 77 87 98 99 101 101 102 iii oxides 102 5.2.2 Comparative study on the chemical properties of ultrathin oxides grown by AFM and SEB techniques 111 5.3 Dielectric characteristics of ultrathin AFM oxides 5.3.1 Electrical properties of AFM oxides 116 5.3.2 Soft and hard breakdown of AFM oxides 119 5.3.3 Comparative study on the electrical properties of ultrathin oxides grown by AFM and SEB techniques 122 5.3.4 I-V characteristics of AFM oxide on 6H-SiC (0001) 124 5.4 Conclusions References Chapter Chapter AFM nanolithography of polymeric materials 6.1 Introduction 6.2 Polymer patterning by probe-induced electrohydrodynamic (EHD) instability and water-assisted ionic conduction 6.2.1 Conical structure formation on PMMA 134 6.2.2 Electrical characteristics and conduction mechanism during conical formation 142 6.3 Poly(N-vinyl carbazole)(PVK) patterning by probe-induced nanoexplosion 6.3.1 Experimental results 149 6.3.2 Characterization and simulations 152 6.4 Conclusions References 116 129 131 133 133 134 149 164 166 Thin liquid layer assisted atomic force microscope (AFM) manipulation and structure formation 168 7.1 Introduction 168 7.2 Formation of microscale droplets and their conversion to thin aqueous layers by AFM probe scanning 169 7.2.1 Preparation method 169 7.2.2 Results and discussions 169 7.3 Nanostructuring of Si in DHF acidic layer 179 7.3.1 AFM oxidation in DHF layer 179 7.3.2 Dissolution of AFM oxide 183 7.4 Localized collection and assembly of nanoclusters by probe-induced adhesive forces in thin liquid layers 189 7.4.1 Collection and assembly of Au nanoclusters 189 7.4.2 Discussion on the probe-induced adhesive force 194 7.5 Conclusions 197 iv Chapter Publications References 199 Conclusions and future works 8.1 Conclusions 8.1.1 AFM nanooxidation on Si 200 8.1.2 AFM nanooxidation on SiC 201 8.1.3 In-situ nanocharacterization of AFM oxides 8.1.4 EHD motion assisted patterning of polymer 8.1.5 Thin liquid film based AFM nanopattering technique 205 8.2 Future plans 8.2.1 Future research plans 206 References 200 200 202 203 205 211 212 v Summary This thesis aims to provide a comprehensive studies and discussions on the atomic force microscopy (AFM)-based nanopatterning and in-situ nanocharacterization techniques. The direct view of features and control of tip motion at the nanoscale make AFM nanolithography especially useful in generating site-specific and localized structures. Extensive works have been carried out on the development of AFM nanolithography for structuring and fabrication in recent years. The many AFM nanolithographic techniques can be generally classified into (i) force-assisted and (ii) bias-assisted nanolithography on the basis of their mechanistic and operational principles. Force-assisted AFM nanolithography includes mechanical indentation and plowing, thermomechanical writing, manipulation and dip-pen nanolithography. Bias-assisted AFM nanolithography encompasses probe anodic oxidation, field evaporation, electrochemical deposition and modification, electrical cutting and nicking, electrostatic deformation and electrohydrodynamic nanofluidic motion, nanoexplosion and shock wave generation, and charge deposition and manipulation. The scope of this thesis covers different nanofabrication techniques in the bias-assisted AFM nanolithography. We focus on the understanding of AFM nanooxidation on semiconductor surfaces (silicon and silicon carbide), the studies of patterning on polymer surfaces and finally, a discussion on the thin liquid assisted manipulation and structuring techniques. It is our hope that the various methods described in this thesis may contribute more insights to the development and understanding of the probe-based nanopatterning research. vi List of Tables Table 1.1 Comparison of AFM nanolithography techniques with other lithographic techniques……… page Table 1.2 Comparison of force-assisted and bias assisted AFM nanolithography techniques………. page Table 1.3 Types of inks used in DPN……… page Table 1.4 Summarized AFM oxidation model developed by various researchers……… page 12 Table 1.5 Examples of application of AFM oxidation on different substrates……… page 13 Table 3.1 Calculated maximum shock propagation distances Rmax and corresponding parameters of e and Pmax. The experimentally observed radius Rox of disk oxide patterns generated under power densities W are also tabulated for comparison……… page 70 Table 3.2 Calculated maximum pressure Pmax and shock propagation distance Rmax for the air/water ionization medium with different relative density ρ/ρ0……… page 70 Table 4.1 Typical variables of interface barrier height, φ, field enhancement factor, β, and scaling factor, α, used in the F-N fitting………. page 84 vii List of Figures Fig. 1.1 Schematic showing the transport of ink from the AFM tip to the substrate through the water meniscus……… page 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……… page Fig. 1.3 Schematic description of AFM nanooxidation……… page 11 Fig. 1.4 Schematic presentation of AFM electrostatic nanolithography for polymer pattern formation……… page 15 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……… page 33 Fig. 2.2 Schematic drawing of (a) Si3N4 contact mode cantilevers, (b) Tapping mode tip, and (c) conductive AFM tip……… page 37 Fig. 2.3 (a) cAFM setup in this work. (b) The external Keithley module is used for higher bias range……… page 37 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……… page 40 Fig. 3.1 Examples of the formation of (a) “NUSNNI” pattern formed by scanning the tip across the region (scan rate ~ 1μm/s and 8V negative bias); (b) Oxide platform (0.5μm/s and 8V) and (c) Oxide dots (pulse duration ~ 1s at 8V)……… page 51 Fig. 3.2 Examples of (a) height, (b) width and (c) aspect ratio (h/d) of AFM oxide as the function of negative tip bias for AFM oxide dot (filled box) and AFM oxide lines (unfilled box), respectively. The oxide dot is grown pulse duration of 10-3 s while the line is created with a scan rate of 0.1μm/s……… page 52 Fig. 3.3 AFM height images showing the central- and outer-structures produced on (a) Si with 18 V bias and s (second) duration, (b) AFM image of oxide structures A, B and C produced on Si with 12, 15, 18 V bias voltage and s bias duration, respectively……… page 56 viii Fig. 3.4 Left column: hybrid BEM/FEM/MOC simulation of field/charge distribution and ionic wind in the nanometer-sized discharge gap. (a) Geometry of the discharge device consisting of the AFM tip (cathode), substrate (anode) and the discharge cylinder with a height of h=5 nm and basal area of A=πr2 where r= 50 nm. (b) Calculated field/charge density radial distributions in the local discharge gap. (c) Calculated velocity vectors of the airflow in the discharge gap. Right column: proposed shock wave generation and lateral expansion of charge density by shock front. (d) Propagation of shock waves induced by the nano-explosion. (e) Outerring formation by shock wave assisted charge density expansion. (f) Comparison of charge density radial distribution between explosions with and without shock wave formation……… page 57 Fig. 3.5 Calculated airflow velocity in the radial direction and 2.5 nm above the substrate surface, respectively……… page 60 Fig. 3.6 (a-f) AFM images of various oxide patterns: (a) single-disk oxide SD (diameter DSD ≈ 2.2 μm), (b) double-disk oxide consisting of disks DD1 and DD2 (DDD1 ≈ 4.0 μm), (c) outer-disk/central-dot oxide (outer-disk diameter DOD ≈ 3.5 μm), (d) square oxide S, (e) square oxide surrounded by single-disk oxide, (f) square oxide surrounded by double-disk oxide. (g-i) AFM images recorded for the etching of oxide patterns: (g) an oxide pattern similar to that shown in (f) recorded before HF etching, (h) the same oxide pattern recorded after HF etching for s, (i) the same oxide pattern recorded after prolonged HF etching……… page 63 Fig. 3.7 (a-d) illustrates oxide pattern formation under the SW-assisted ionic spreading mechanism: (a) OH- spreading by shock wave propagation, (b) single-disk oxide pattern, (c) double-disk oxide pattern, and (d) outerdisk/central-dot oxide pattern……… page 64 Fig. 3.8 (a) The geometry of the ionization zone in the tip-sample junction for hydrodynamic simulation. (b) Shock pressure calculated for different propagation time t. (c) Pressure pattern generated by a single SW event which is responsible for the formation of the single-disk oxide structure. (d) Pressure pattern generated by two SW events leading to the formation of the double-disk oxide structure……… page 67 Fig. 4.1 (a) Six 1×1 μm2 oxide squares grown on Si; (b) Protruded lines (e.g. AB line) generated by scanning 1×1 μm2 area on SiC with different tip bias; (c) and (d) Height and friction images of four lines fabricated at various tip bias; (e) and (f) Height and friction images of three lines formed by scanning 1×1 μm2 areas at different scan rates……… page 79 Fig. 4.2 (a) Plots of H and I curves based on Eq. (1) and (2) without scaling factor α. After introducing α, the mismatch between H and I curves is only ix 5.4%; (b) Variation of oxide height as a function of τ, the reciprocal of scan rate……… page 81 Fig. 4.3 (a) Oxide dots R, S and T grown with 12, and V negative tip biases, respectively; (b) Oxide dots U, V and W grown at 2, 1.7 and 1.4 seconds’ bias duration; (c) × arrays of oxide dots grown with similar conditions; (d) and (e) Variation in oxide height versus bias voltage and bias duration; (f) Plots of vertical and lateral dimensions of oxide dots grown on SiC and Si surfaces under identical conditions; (g) Comparison of aspect ratio of AFM oxide grown on SiC and Si surfaces……… page 86 Fig. 4.4 (a) The configuration of the tip/water/substrate model used in the simulation; (b) Simulated electrical field distribution for the models with vacuum, R=20 nm and R=60 nm water meniscus between AFM tip and substrate, respectively; (c) Simulated electrical field distribution for the models with vacuum, R=100 nm and R=200 nm water meniscus between AFM tip and substrate, respectively……… page 89 Fig. 4.5 Top: AFM height images of dot oxide grown at various humidity of (a)-(i) 75%, (a)-(ii) 60%, (a)-(iii) 45% and (a)-(iv) 30% respectively. Bottom: Plots of oxide (b) vertical dimension, (c) lateral dimension, and (d) aspect ratio as a function of humidity……… page 92 Fig. 4.6 (a) Vertical growth of AFM oxide involving OH- diffusion in the polar [0001] direction; (b) Lateral growth of AFM oxide involving OHdiffusion in the non-polar directions like [11 0] axis……… page 93 Fig. 4.7 Top: LEED (low energy electron diffraction) patterns and corresponding reciprocal lattice structures of SiC (a) (0001) and (b) (11 0) surfaces. Bottom: AFM height images of oxide dots grown on (c) (0001) and (d) (11 0) surfaces. The vertical lattice structures are also shown in Fig. 4.6……… page 96 Fig. 5.1 (a)–(d) AFM height images of an oxide square (1 x1 μm2, apparent thickness dA =3 nm) subjected to cumulative HF etching for 0, 40, 80, and 120 s, respectively. In (a), the square R is the reference area, from which the average thickness of the oxide square S was determined. (e) Representative AFM cross sections showing the heights of the oxide square recorded at etching stages of (a)–(d)……… page 103 Fig. 5.2 (a) AFM cross sections showing the ratio of dA/dI ≈ observed for oxides with dA =5 nm (solid and dashed profiles in red) and dA = nm (solid and dashed profiles in blue), respectively. (b) The ratio of dA/dI ≈ for the oxide formation by simple oxygen incorporation into Si and subsequent volume expansion by a factor of 2. (c) The etching kinetics recorded for oxides of dA = and nm, respectively……… page 105 x Fig. 5.3 Propose oxide growth mechanism for AFM anodic oxidation: (a) at the initial stage, reactive O inserts into Si substrate, and (b) at the growth stage, Si diffuses outwards through the oxides and reacts with incoming reactive O to form oxides on the top……… page 106 Fig. 5.4 (a) Etching kinetics recorded for the annealed oxides of dA =5 and nm, respectively. Significant surface passivation was observed for the initial 80 s etching duration. (b) Proposed etching mechanism of ultrathin AFM oxides. The reactive sites are the SiOH silanols in the oxides……… page 109 Fig. 5.5 (a) AFM height image of AFM oxide pattern “NUS 2003”; (b) SEM image of sad recorded simultaneously when the pattern was exposed to SEB; (c) AFM height image showing the formation of a 534 mm2 SEB oxide rectangle; (d) friction image of (c); (e) AFM height image showing the preferential etching of AFM oxide; (f) AFM close-up height image for the oxide patterns after further etching……… page 112 Fig. 5.6 (a)-(b) Peak intensity of cluster ions; (c)-(f) Time profiles of cluster ions. The ratio of total ion yield between AFM and SEB oxides, YAFM/YSEB, is also shown……… page 113 Fig. 5.7 (a) Arrays of AFM oxide dots grown on a Si substrate. (b) I–t curves recorded for 3.0-nm-thick AFM oxide dots stressed under constant voltage of Vs=3.0, 4.5, and 6.0 V, respectively. (c) Variation of TBD recorded for 1.5-, 2.0-, and 3.0-nm-thick AFM oxide dots after annealing for h at elevated temperatures. Inset shows the plots of normalized Pt vs. annealing temperatures……… page 117 Fig. 5.8 (a) AFM height image of one AFM oxide dot after BD. (b) and (c) Current images collected with ±2 V tip bias for the oxide after BD. (d) and (e) Current images collected with ±1 V tip bias for the same oxide dot as in (b) and (c). (f) Cross-sectional schematic showing the coexistence of hard BD (HBD) and soft BD (SBD) in oxide. The thickness of SBD spots, tox’, may vary from site to site……… page 120 Fig. 5.9 (a) I-V characteristics recorded for nm thick AFM and SEB oxides. The inset shows the format of F-N equation and interface barrier height φ, derived by fitting experimental data (empty and solid circles) to F-N equation (solid line). The I-V characteristic of oxides after breakdown is also shown. (b) Comparison of statistic breakdown occurrence between AFM and SEB oxides……… page 123 Fig. 5.10 I-V characteristics collected for AFM oxide on SiC with different height. The barrier height for oxide/SiC interface is shown in the inset……… page 125 xi Fig. 5.11 Current shooting events and the I-V curve recorded after dielectrical breakdown……… page 127 Fig. 5.12 Comparison of the occurrence of current shooting and breakdown events among various AFM oxide on SiC……… page 127 Fig. 6.1 (a) AFM close-up height image and its corresponding cross section profile of a conical structure generated with 40 V tip bias and 12 s duration. (b) Three conical structure R, S, and T grown with identical bias and duration (40 V, 12 s), but under different humidity condition of 70, 55 and 40 %, respectively……… page 135 Fig. 6.2 (a) AFM image of a cone (~50 nm high) created by biasing tip at 40 V for 16 s. Six nanodots (~8 nm high) produced using low tip voltage (10–30 V) are also shown beside the cone. (b) Three cones Q, R, and S, and one pit T created by biasing tip at 50 V for 6, 10, 14, and 22 s, respectively……… page 139 Fig. 6.3 AFM images of (a) single, (b) double, and (c) triple cones created under identical tip voltage and bias duration by AFM tips with estimated end radius of R~50 (SEM image, top left) and R~130 nm (SEM image, lower right), respectively……… page 140 Fig. 6.4 (a) Temporal evolution of relative tip repulsion recorded for cone growth under 55 and 35 V tip voltages, respectively. (b) Tips with different force constant k were used to create 1–5 nm dots, 10–20 nm cones, and 50–80 nm cones. The probability of generating the earlier structures by these tips under identical conditions is compared……… page 143 Fig. 6.5 (a) I-V curve collected simultaneously in the formation of conical structures on PMMA. The inset display the I-V characteristic in the range of – 60 V tip bias. the data were obtained under 65 % RH. (b) I-V characteristic recorded for pattern formation under various humidity conditions. The I-V curve recorded under 40 V tip bias and 40 % humidity are shown in the inset. (c) Plot of conductivity, σ vs. humidity, H……… page 144 Fig. 6.6 Schematic showing the sequential formation of the conical structure on polymer film: (a) ionic dissociation of water bridge upon the application of bias on an AFM tip; (b) water-assisted ion conduction in the conical polymer melts generated by local heating; (c) formation of conical pattern after deactivating the tip bias……… page 148 Fig. 6.7 (a) Raised central cone and outer ring formation on PVK by applying a 10 V tip bias for s duration. (b) Proposed nano-discharge and shock wave generation for the formation of central and outer structures. (c) Central and xii outer structures R, S and T produced by maintaining different tip voltages of 12, 10 and V for the same duration, respectively……… page 151 Fig. 6.8 (a) AFM morphology image of a central and outer structure produced with 12 V tip voltage. The dots N1-N5 outside the ring are native dots initially present on the PVK film before AFM patterning. (b) AFM current image of (a) recorded simultaneously with a scanning voltage of 4V. (c) AFM current image of (a) recorded with a scanning voltage of V……… page 153 Fig. 6.9 (a) AFM morphology and (b) current map recorded for PVK samples exposed to large scale discharge in ambient air. The raised PVK structures observed in (a) correspond to the conducting sites in (b). A scanning voltage of V was used to obtain the current image……… page 154 Fig. 6.10 (a) FTIR spectra collected for PVK samples (i) unexposed and (ii) exposed to discharges. Spectrum (iii) is the calculated vibration frequency for the Ph-O-Ph configuration in the crosslinked carbazole groups shown below. (b) PES spectra of O 1s core level emission recorded for PVK samples unexposed and exposed to discharges, respectively. The difference spectrum of the two peaks is also shown. The spectra were obtained with photons of energy hv = 649.2 eV. (c) Top: calculated HOMO for the structure displayed in (a) showing the overlap of O 2p states with the π-states of the benzene ring. Bottom: comparison of HOMO levels calculated for crosslinked PVK (left) and silicon oxide (right). The O 1s core level was assumed to be the same in the two molecules. (d) Proposed reactions between the nano-discharge and native PVK for the production of crosslinked carbazole groups by one or two bridge oxygen……… page 157 Fig. 6.11 (a) Calculated HOMO-LUMO gap for structures shown in (b). The inset shows the further gap narrowing with increased number of crosslinked carbazole pairs in structure (v). (c) I-V curves collected for 20-nm-high, 10-nm-high PVK patterns and native PVK, respectively. The inset shows the energy diagram of the Si/PVK/Au (tip) configuration used in AFM I-V measurements. ε0LUMO, ε0HOMO and Δ0 denote the LOMO, HOMO and tunneling barrier of native PVK, while εLUMO, εHOMO and Δ denote the LUMO, HOMO and tunneling barrier of the PVK patterns. (d) Linear ln(I/V2) ~ 1/V plots of I-V curves shown in (c). The solid circles are experimental data, and the solid lines are the fitted linearity with a slope of β. The inset displays the ln(I/V2) ~ 1/V plot for native PVK……… page 160 Fig. 7.1 CCD camera-captured image of (a) the bare Si surface, and (b) DHFtreated surface showing the formation of large domains of microdroplets xiii on Si. The inset of (b) displays the close-up image of individual droplets……… page 170 Fig. 7.2 (a) Plot of droplet formation time versus DHF concentration. (b) The size distribution of droplets recorded for a development time of 30 and 50 minutes, respectively. (c) Proposed mechanism for the droplet formation: (i) the initial hydrophilic bare Si surface; (ii) creation of hydrophobic sites (open circles) X, Y, and Z by gentle DHF etching, and the co-existence of hydrophilic sites D, E, and F at less or unetched sites; (iii) confinement of the remaining DHF at the hydrophilic sites D, E and F to form microdroplets (filled circles) . page 171 Fig. 7.3 (a) CCD camera-captured image showing the creation of DHF thin layers by scanning the droplets using an AFM probe. (b)-(d) Force curves collected on the bare, droplet-covered, and thin layer-covered surfaces, respectively……… page 175 Fig. 7.4 (a) Plots of pull-off force and Z-hysteresis versus cumulative scan round. (b) The decrease in DHF layer thickness as the cumulative scan round is increased……… page 178 Fig. 7.5 (a) AFM image and line profile of oxide dots grown by AFM in thin acidic layers. (b) Plot of oxide height versus tip voltage recorded for AFM anodic oxidation in 0.01% DHF layers of 400 nm nominal thickness. (c) The variation of oxide height with DHF concentration obtained at fixed tip voltages. The nominal thickness of these different DHF layers is ~400 nm. (d) Calculated KE / K0 curve in the E range of 1-5×109 V/m. (e) I-E characteristic collected in situ during the oxidation. (f) Comparison between calculated KE/Kmax and experimental I / Imax values……… page 180 Fig. 7.6 (a) AFM image of two oxide dots fabricated in DHF thin layers by a negative tip bias of V. (b) Dissolution of the oxide dot P by applying a higher tip bias of V……… page 185 Fig. 7.7 (a) The apparent height dA and buried height dB of oxides grown by AFM anodic oxidation. (b) When the oxide is etched by conventional HF dipping, the depth of the pore dP is approximately equivalent to the buried height, dP ≈ dB. (c) When Si underneath the oxide is also etched by fieldassisted dissolution, the depth of the pore dP’ is larger than the buried height, dP’ > dB. (d) Three pores P1, P2 and P3 obtained by field-assisted dissolution of Si substrate……… page 186 Fig. 7.8 (a) Thin layers of the Au dispersion solution. (b) Attraction of Au nanoclusters towards the tip apex by the adhesive force when the tip xiv withdraws from the thin layers. (c) Formation of Au-nanocluster assembly after the evaporation of the solvent……… page 190 Fig. 7.9 (a) Optical microscope image of microdroplets formed globally on Si by DHF treatment. (b) AFM image (14.5×14.5 μm2) of random Au clusters left over after the evaporation of the solvent. (c) Optical microscope image of single droplet created by the attractive force during tip withdrawal. (d) AFM image of collection of Au nanoclusters assembled after the evaporation of the single droplet shown in (c). (e-g) Three Au assemblies fabricated with decreasing adhesive forces……… page 191 Fig. 7.10 (a) Disordered CaCO3 islands observed after the evaporation of the random microdroplets formed globally on the surface after DHF treatment (image size: 20 × 20 μm2). (b) A Single CaCO3 assembly (2.4 μm wide, and 124 nm high) created by tip pulling of the CaCO3-containing solution……… page 192 Fig. 7.11 (a) Dependence of droplet diameter D and Au assembly volume V (inset) on the adhesive force Fad. (b) The impact of liquid layer thickness d and tip retraction velocity v on adhesive force Fad……… page 195 Fig. 8.1 AFM oxide dots created by dc ramp mode with light and flexible probe (0.2 N/m, 1Hz, (a) to (c)) and stiff probe (3 N/m, 1Hz, (d) to (e)). The aspect ratio (h/d) of the oxide created by light probe at 0-4 V, 0-6 V, and 0-8 V ramp are ~4.5, and times higher than the stiff probe, respectively (Fig. 1(g))……… page 207 Fig. 8.2 Ring structures instead of conical cones are formed on thin polymer film with 9V negative tip bias for 25 seconds………. page 208 xv [...]... by applying a 10 V tip bias for 1 s duration (b) Proposed nano-discharge and shock wave generation for the formation of central and outer structures (c) Central and xii outer structures R, S and T produced by maintaining different tip voltages of 12 , 10 and 8 V for the same duration, respectively……… page 15 1 Fig 6.8 (a) AFM morphology image of a central and outer structure produced with 12 V tip voltage... layer thickness d and tip retraction velocity v on adhesive force Fad……… page 19 5 Fig 8 .1 AFM oxide dots created by dc ramp mode with light and flexible probe (0.2 N/m, 1Hz, (a) to (c)) and stiff probe (3 N/m, 1Hz, (d) to (e)) The aspect ratio (h/d) of the oxide created by light probe at 0-4 V, 0-6 V, and 0-8 V ramp are ~4.5, 6 and 9 times higher than the stiff probe, respectively (Fig 1( g))……… page 207... created by biasing tip at 40 V for 16 s Six nanodots (~8 nm high) produced using low tip voltage (10 –30 V) are also shown beside the cone (b) Three cones Q, R, and S, and one pit T created by biasing tip at 50 V for 6, 10 , 14 , and 22 s, respectively……… page 13 9 Fig 6.3 AFM images of (a) single, (b) double, and (c) triple cones created under identical tip voltage and bias duration by AFM tips with estimated... breakdown occurrence between AFM and SEB oxides……… page 12 3 Fig 5 .10 I-V characteristics collected for AFM oxide on SiC with different height The barrier height for oxide/SiC interface is shown in the inset……… page 12 5 xi Fig 5 .11 Current shooting events and the I-V curve recorded after dielectrical breakdown……… page 12 7 Fig 5 .12 Comparison of the occurrence of current shooting and breakdown events among... among various AFM oxide on SiC……… page 12 7 Fig 6 .1 (a) AFM close-up height image and its corresponding cross section profile of a conical structure generated with 40 V tip bias and 12 s duration (b) Three conical structure R, S, and T grown with identical bias and duration (40 V, 12 s), but under different humidity condition of 70, 55 and 40 %, respectively……… page 13 5 Fig 6.2 (a) AFM image of a cone... Disordered CaCO3 islands observed after the evaporation of the random microdroplets formed globally on the surface after DHF treatment (image size: 20 × 20 μm2) (b) A Single CaCO3 assembly (2.4 μm wide, and 12 4 nm high) created by tip pulling of the CaCO3-containing solution……… page 19 2 Fig 7 .11 (a) Dependence of droplet diameter D and Au assembly volume V (inset) on the adhesive force Fad (b) The impact... image (14 .5 14 .5 μm2) of random Au clusters left over after the evaporation of the solvent (c) Optical microscope image of single droplet created by the attractive force during tip withdrawal (d) AFM image of collection of Au nanoclusters assembled after the evaporation of the single droplet shown in (c) (e-g) Three Au assemblies fabricated with decreasing adhesive forces……… page 19 1 Fig 7 .10 (a) Disordered... tips with estimated end radius of R~50 (SEM image, top left) and R ~13 0 nm (SEM image, lower right), respectively……… page 14 0 Fig 6.4 (a) Temporal evolution of relative tip repulsion recorded for cone growth under 55 and 35 V tip voltages, respectively (b) Tips with different force constant k were used to create 1 5 nm dots, 10 –20 nm cones, and 50–80 nm cones The probability of generating the earlier... for 1. 5-, 2.0-, and 3.0-nm-thick AFM oxide dots after annealing for 6 h at elevated temperatures Inset shows the plots of normalized Pt vs annealing temperatures……… page 11 7 Fig 5.8 (a) AFM height image of one AFM oxide dot after BD (b) and (c) Current images collected with ±2 V tip bias for the oxide after BD (d) and (e) Current images collected with 1 V tip bias for the same oxide dot as in (b) and. .. sites D, E and F to form microdroplets (filled circles) page 17 1 Fig 7.3 (a) CCD camera-captured image showing the creation of DHF thin layers by scanning the droplets using an AFM probe (b)-(d) Force curves collected on the bare, droplet-covered, and thin layer-covered surfaces, respectively……… page 17 5 Fig 7.4 (a) Plots of pull-off force and Z-hysteresis versus cumulative scan round (b) The decrease . SEB techniques 11 1 5.3 Dielectric characteristics of ultrathin AFM oxides 11 6 5.3 .1 Electrical properties of AFM oxides 11 6 5.3.2 Soft and hard breakdown of AFM oxides 11 9 5.3.3 Comparative. 4H-SiC ( ) surfaces 97 0 211 − 4.4 Conclusions 98 References 99 Chapter 5 Nanocharacterization of probe-grown oxides by atomic force microscope 10 1 5 .1 Introduction 10 1 5.2 In-situ chemical. thin layers 20 1. 4 Strategies and approaches of this work 21 1. 4 .1 AFM nanolithography 21 1. 4.2 I-V electrical characterization by cAFM 22 1. 4.3 Force curve measurement 23 1. 4.4 Theoretical