Atomic force microscopy (AFM) based nanopatterning and nanocharacterization 1

2.2.3 AFM nanofabrication 42 2.2.4 cAFM nanocharacterization 43 2.2.5 TOF-SIMS and SEM characterizations 44 2.2.6 Theoretical simulations 45 Chapter 3 AFM nanooxidation of semiconductor

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

Contents

Acknowledgements i

Table of Contents ii

Summary vi

List of Tables vii

List of Figures viii

Chapter 1 Introduction 1

1.1 Nanofabrication for miniaturized devices 1

1.2 Development of AFM nanolithographic techniques 4

1.2.1 Dip-pen nanolithography (DPN) 5

1.2.2 Thermomechanical writing and millipede Techniques 8

1.2.3 AFM nanooxidation 9

1.2.4 Electrostatic deformation and electrohydrodynamic nanofluidic motion 14

1.3 Motivation and objectives of this work 16

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 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 simulations and other supporting Methods 23

References 25

Chapter 2 Experimental 32

2.1 Atomic Force Microscopy 32

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 41

2.2.1 Semiconductor sample preparation 41

2.2.2 Polymer sample preparation 41

2.2.3 AFM nanofabrication 42 2.2.4 cAFM nanocharacterization 43 2.2.5 TOF-SIMS and SEM characterizations 44 2.2.6 Theoretical simulations 45

Chapter 3 AFM nanooxidation of semiconductor surface (I):

3.1 Introduction 49 3.2 AFM nanooxidation 50

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 54

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 72 References 74

Chapter 4 AFM nanooxidation of semiconductors (II): Native

oxide decomposition and localized oxidation of

4.2 Native oxide decomposition on 6H-SiC (0001) 77

4.2.1 Oxide decomposition 77 4.2.2 The kinetics of oxide decomposition 80

4.3 Local oxide growth on 6H-SiC(0001) 87

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 (11−0) surfaces 97

Chapter 5 Nanocharacterization of probe-grown oxides by

5.1 Introduction 101 5.2 In-situ chemical etching study of AFM ultrathin oxides

by atomic force microscopy 102

5.2.1 Chemical etching behavior of probe grown

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 116

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

Chapter 6 AFM nanolithography of polymeric materials 133

6.2 Polymer patterning by probe-induced electrohydrodynamic

(EHD) instability and water-assisted ionic conduction 134

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 149

6.3.1 Experimental results 149 6.3.2 Characterization and simulations 152

Chapter 7 Thin liquid layer assisted atomic force microscope (AFM)

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

References 199

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 202 8.1.4 EHD motion assisted patterning of polymer 203 8.1.5 Thin liquid film based AFM nanopattering

technique 205

8.2.1 Future research plans 206

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

List of Tables

Table 1.1 Comparison of AFM nanolithography techniques with other lithographic

techniques……… page 3

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

techniques……… page 6

Table 1.3 Types of inks used in DPN……… page 6

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 R max

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

List of Figures

Fig 1.1 Schematic showing the transport of ink from the AFM tip to the substrate

through the water meniscus……… page 7

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 7

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 2 s (second) duration, (b) AFM image of oxide structures A, B and C produced on Si with 12, 15, 18 V bias voltage and 5

s bias duration, respectively……… page 56

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=πr 2 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) Outer-ring 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 1 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 3 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)

outer-disk/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

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, 8 and 6 V negative tip biases,

respectively; (b) Oxide dots U, V and W grown at 2, 1.7 and 1.4 seconds’ bias duration; (c) 3 × 3 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 OH

-diffusion in the non-polar directions like [11 2 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 2 0) surfaces Bottom: AFM height images of oxide dots grown on (c) (0001) and (d) (11 2 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 d A =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 d A /d I ≈ 3 observed for oxides

with d A =5 nm (solid and dashed profiles in red) and d A = 3 nm (solid and

dashed profiles in blue), respectively (b) The ratio of d A /d I ≈ 1 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 = 5 and 3 nm, respectively……… page 105