Interconnection Of Electrodes Using Field Emission Induced Growth Of Nanowires FAIZHAL BIN BAKAR B ENG (Hons), NUS A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE NANOSCIENCE AND NANOTECHNOLOGY INTIATIVE 2009 a Acknowledgement I would like to thank my Supervisor, Assoc Prof Dr John Thong for his ideas and patience in the completion of this work I would also like to thank the staff of the Center for Integrated Circuit Failure Analysis and Reliability (CICFAR), Mrs CM Ho and Mr CK Koo, for their logistical support in the execution of these experiments I would also like to thank Dr CH Oon for sharing his expertise in the experimental section of this work i Contents Page Acknowledgements i Contents ii Abstract v List of figures vi Chapter Introduction 1.1 Nanowire fabrication 1.2 Integration of Nanowires 1.3 Objective and thesis outline Chapter Literature Review 2.1 Dielectrophoresis 2.1.1 Bridging using nanoparticles 10 2.1.2 Bridging using CNTs and nanowire 13 2.2 Electroplating 15 2.3 Chalcogenide Based Electrical Switching 17 2.4 Field Emission Induced Growth 19 Chapter Cathode Growth Simulation 3.1 Simulation 25 3.1.2 Simulation parameters 26 3.2 Simulation results 33 3.2.1 Simulation of emitted rays 33 ii 3.2.2 Simulation of W ions 3.3 Summary 36 45 Chapter Fabrication of Lateral Field Emission Structures 4.1 Cathode Requirements 47 4.2 Fabrication Of Planar Si Electrodes 50 4.2.1 Si Die Fabrication 50 4.2.2 Si tips through FIB milling 53 4.2.3 Directly-grown ZnO tips 55 4.2.4 CNT Tips Through Dielectrophoresis 57 4.3 Summary 58 Chapter Experimental Study of FEIG Growth 5.1 Bridging Of Cathode Nanowire Growth 59 5.1.1 Experimental Setup 59 5.1.2 Experimental Results 61 5.2 Anode Growth 68 5.3 Simulation Of Anode Growth 72 5.3.1 Simulation set up 73 5.3.2 Simulation results 74 5.4 TEM Imaging Of Anode Growth 77 5.5 Summary 80 iii Chapter Conclusion 6.1 Summary 82 6.2 Future Work 83 References 85 iv Abstract This thesis concerns the interconnection of electrodes using field emission induced growth (FEIG) of W nanowires We seek to understand the process through simulation of the growth phenomena seen at both the cathode and the anode Better experimentation and understanding of FEIG will bring a step closer to understanding of the formation of nanosized connection between electrodes Results of the simulation of electron and W ions trajectories at the beginning stage of the nanowire growth suggests a formation of a conical supporting structure at the base of the nanowire as well the formation of a nanowire with a diameter that grows at a much smaller rate than its length Results of simulation at the end stage of the nanowire growth suggest that nanowire growth speed reduces as the nanowire nears the anode The measured resistance of W nanowire bridge however shows a large ohmic value due to the high resistance of the anode growth due W supply-limited deposition The anode growth observed is thicker and has a tree-like structure and is deduced to arise from the continual fusing and growth of the cathode which encourages the anode growth as the thicker anode is unlikely to fuse Preliminary anode simulations suggest that formation of protrusions at the anode will influence incoming electrons and thus the fractal growth of the anode observed From TEM images, it was deduced that the tree-like anode growth is made up of metal agglomerates, enclosed in a carbonaceous matrix The low crystallinity observed is deduced to be due to the low electron energy used v List of Figures Caption Page Figure 1.1 Schematic illustrations of six different methods of achieving 1D growth: (A) dictation by the anisotropic crystallographic structure of a solid; (B) confinement by a liquid droplet as in the vapor-liquid-solid process; (C) direction through the use of a template; (D) kinetic control provide by a capping reagent; (E) self-assembly of 0-Dimensional nanostructures; and (F) size reduction of a 1D microstructure Source: Xia et al (2003) Figure 1.2 (a) Schematic illustration of the fabrication of the VLS grown Si nanowire bridge between two vertical Si{111} surfaces (b) SEM image of nanowire bridges grown in the microtrenches He R et al (2005) Figure 1.3 (a) SEM top view of a hexagonal network of SWNTs (line- like structures) suspended on top of silicon posts (bright dots) (b) SEM top view of a square network of suspended SWNTs (c) Side view of a suspended SWNT power line on silicon posts (bright) (d) SWNTs suspended by silicon structures (bright regions) The nanotubes are aligned along the electric field direction Source: Dai et al (2002) Figure 1.4 A nanowire was grown via FEIG and was allowed to straddle across two metal electrodes Source: Oon et al (2004) Figure 1.5 Electrons from field-emitting tip (illustrated by the dashed lines) dissociates the precursor W(CO)6 into W+ ions which accelerates towards the field emitting tip to form the nanowire Neutral carbon atoms intercepted by the wire forms the amorphous overcoat Source: Oon et al (2006) Figure 2.1 (a) Pt electrodes separated by a ~14 nm gap (b) After electrostatic trapping of a ~17 nm Pd nanoparticle Source: Bezryadin and Dekker (1997) 10 Figure 2.2 (a) 400 nm electrode gap bridged by fused 50 nm Au nanoparticles (b) sub-10 nm gap after electromigration failure due to DC biasing Source: Khondaker and Yao (2002) 11 Figure 2.3 500 nm electrode gap bridged by 120 ± 20 nm Au 11 vi nanoparticles at an applied voltage of (a) V and (b) V Source: Bernard et al (2007) Figure 2.4 (a) Schematic of 3D platform (b) Process flow of 3D 12 platform (b)(i) Thermally grown oxide and Au deposition patterned using lift-off (b)(ii) Deposition of Parylene and Au patterned using lift-off (b)(iii) Etching of Parylene layer (b)(iv) Assembly of Au nanoparticles using dielectrophoresis Source: Nishant et al (2007) Figure 2.5 SEM micrograph of assembled Au nanoparticles on 3D 12 platform using dielectrophoresis Source: Nishant et al (2007) Figure 2.6 A higher AC frequency improves the degree of orientation 13 and reduces the amount of the CNT to carbon particulate ratio Source: Yamamoto et al (1998) Figure 2.7 A higher AC peak-to-peak voltage gives a larger amount of aligned CNTs Source: Chen et al (2001) Figure 2.8 A single CNT bundle trapped through dielectrophoresis 15 Source: Krupke et al (2003) Figure 2.9 3-electrode configuration with the ‘source’ (left), ‘latch’ 15 (middle) and ‘drain’ (right) (a), (b) and (e) assembly of nanowires using ‘source’ and ‘latch’ relative phase of 180º (c) and (d) assembly of nanowires using ‘source’ and ‘latch’ relative phase of 0º Source: Wissener-Gross et al (2006) Figure 2.10 The experimental set-up, Vdc controls deposition or removal 16 of material while Vac is used to monitor the conductance and thus the conductance Source: Morpurgo et al (1999) Figure 2.11 (A) Schematic and SEM images of electrodes before deposition (B) Schematic and SEM images of electrodes after deposition Source: Morpurgo et al (1999) Figure 2.12 (a) Schematic illustration lateral device with Au-Ag 18 electrodes on As2S3 Film (b) Ag dendrite grows from the Au electrode towards the Ag electrode (c) Ag dendrite bridged the electrodes Source: Yooichi et al (2005) Figure 2.13 Schematic of (a) synthesis of Ag/Ag2S heteronanowire 18 arrays and (b) electrochemical sulfurization growth of Ag2S Source: Liang et al (2005) 14 16 vii Figure 2.14 Schematic of the switching on and off by breaking the 19 nanobridge connections through application of reversible voltages Source: Liang et al (2005) Figure 2.15 Glow discharge growths on the cathode having the structure 20 of a thin core with unoriented microcrystals grown on the core Source: Okuyama (1980) Figure 2.16 A grown nanowire of 1-2µm in length is attracted to an 22 adjacently biased electrode without fusing Source: Thong et al (2002) Figure 3.1 Dissociation of W(CO)6 occurs due to electron 26 bombardment originating from the field emission tip Positively charged W+ ions accelerate towards the cathode tip and form the nanowire Dissociated C accounts for the carbonaceous coating Figure 3.2 (a) 2D drawn structure with an axial symmetry (b) Equivalent 3D structure 27 Figure 3.3 2D drawn schematic with the simulated electric field 27 Figure 3.4 Illustrating the method of approximating the area occupied by rays of electrons 28 Figure 3.5 Discretizing emitted rays into 20nm sections 31 Figure 3.6 Ray data from ‘crosses’ are collected by test planes and ray data from ‘dots’ will be spline interpolated The ray data collected are its current magnitude, its location, and its electron energy 32 Figure 3.7 Ionization cross-section curves of CH4, Si(CH3)4 and SF6 having a similar hill like shape but with differing energies for the onset of ionization and ionization turning point Sources: CH4 - Y.-K Kim et al (1997), Si(CH3)4 - M A Ali et al (1997), SF6 - M A Ali et al (2000) 33 Figure 3.8 Current density reduces with perpendicular distance from the cathode due to diverging rays The highest and lowest current density recorded differs by 10 orders of magnitude 35 viii Figure 3.9 Areas with filled dots are locations where W ions will be located and launched from Areas that are not traversed by electron rays will not have W ions and are indicated with open dots 37 Figure 3.10 Trajectory of launched W ions moving towards the Cathode 38 Figure 3.11 The cathode is discretized into 20nm sized sections where landed W ions are taken count A magnified view of the pointed end of the cathode showing where the Tip of the cathode is defined as another discrete section 39 Figure 3.12 Simulation done for W ions distribution Voltage applied is 245V with an applied current of ~120nA (a) W ions distribution along the base of the nanowire (A) (b) W ions distribution along the nanowire (B) (c) Schematic of the simulation set up 41 Figure 3.13 Simulation done for W ions distribution Voltage applied is 93V with an applied current of ~120nA (a) W ions distribution along the base of the nanowire (A) (b) W ions distribution along the nanowire (B) (c) Schematic of the simulation set up 44 Figure 3.14 From work done by Yeong et al (2006), the growth rate of W nanowire decreases with time 45 Figure 4.1 A simple 3D schematic of a field emission structure to be fabricated 47 Figure 4.2 The field enhancement factor, β, is expressed as β = (h/r) h is the height of the protrusion and r is the radius of the tip of the protrusion 49 Figure 4.3 Cross-sectional schematic of the Si die showing the 51 anisotropic profile due to wet etching using KOH:IPA:DIW (200g : 63.5 cm3 : 250 cm3) at 85 °C A 1500 Å patterned nitride was used in the wet etch as a bottom-side mask Figure 4.4 Electrode patterns for (a) 3x3 mm2 die and (b) 5x5 mm2 chip 51 Figure 4.5 E-Beam Lithography pattern with the smallest line width at 5μm 52 ix Cathode Protrusion Anode Figure 5.11 A 3D drawn schematic in CPO The highlighted dotted area signifies the quadrant where the landing coordinates of the electron trajectories are taken before and after the protrusions are placed 75 Figure 5.12 Simulation results of quadrant shown in Figure 5.5 The Coordinate (0, 0) represents the center of the 3D structure The ‘o’ represents simulation results without perturbation while the ‘+’ represents simulation results with perturbation 76 Cathode Affected electron trajectory Unaffected electron trajectory X Y Anode Figure 5.13 Cross-section illustrates how electron trajectories nearer to the protrusion at the anode are affected while those further away from the protrusions are least affected The dotted lines represent trajectories with a protrusion present while the solid lines represent trajectories without the protrusion 5.4 TEM imaging of Anode Growth TEM analysis of cathode growth previously carried out by Oon et al (2006) revealed a metal core surrounded by a thin carbonaceous coating In the present work, TEM imaging was carried out on the tree-like anode growth to observe its microstructure Figure 5.14(b) shows that the branching anode growth is made up of metal agglomerates enclosed in a carbonaceous matrix Higher resolution imaging of one of the branches clearly shows the metal agglomerates and the carbonaceous matrix An inset in Figure 5.14(b) of the electron diffraction pattern shows that the W is comprises randomly-oriented nanocrystals Xie et al (2005) compared the EBID W nanodendrite growth at different 77 electron acceleration voltages ranging from 400 kV to 1000 kV and found that the degree of micro-crystallinity reduces as the electron energy reduces from 1000 keV to 400 keV There reduction in the level of crystallinity at lower electron energy was attributed to the smaller amount of energy available to the dissociated W to move into crystalline sites Reproduced from Xie et al.’s work, Figure 5.15 shows the microstructure of the EBID W nanodendtrites at electron acceleration voltages and the reduction in micro-crystallinity as the deposition electron acceleration voltage decreases Comparing Figure 5.14 to Figure 5.15, the level of crystallinity is clearly poorer in the current work This is expected since the electron energy for EBID is in the range of tens to hundreds of eV, as opposed to hundreds of keV (a) (b) Figure 5.14 (a) Branching anode growth is made up of metal agglomerates enclosed in a carbonaceous matrix Dotted region shows the location where the high magnification TEM is taken (b) is a higher resolution image of one of the branches showing the metal agglomerates and the carbonaceous matrix Inset shows the electron diffraction pattern 78 Figure 5.15 The micro crystallinity of the W nanodendrites grown via EBID increases with increased electron acceleration voltage Reproduced from Xie et al (2005) 79 5.5 Summary Bridging of the cathode and anode via FEIG growth is possible but due to capacitive discharge which continually shortens the growth at the cathode, the thicker and tree-like anode growth begins to approach the cathode and form the larger part of the connection This is more likely to happen if the voltage fluctuations between subsequent nanowire fusing and growth brings the average electron energy above the expected W(CO)6 dissociation threshold energy of 10 – 50 V for EBID to occur The bridged nanowire structure has an unexpectedly high resistance of 30 MΩ, beyond that of a FEIG nanowire This is attributed to the high resistivity of the material deposited at the anode by EBID Calculations based on the highest resistivity value reported in the literature for EBID tungsten material give an upper bound of around 20 MΩ This could be explained by higher material resistivity than those previously reported, as the conditions of EBID in this work favour supply-limited deposition of W, and hence lower W concentration, due to the higher electron density and lower electron deposition voltage The final bridging is suspected to be from neither EBID nor FEIG growth as these growth mechanisms would cease at biasing conditions of 10V and 8.5V, respectively Electrostatic attraction between the FEIG nanowire and the EBID growth is also unlikely as the nanowire is already being stretched for the entire duration of the growth, and there is no additional tensile force that comes into play to cause the final bridging Another mechanism that might have contributed to the bridging process when both EBID and 80 FEIG growth stops is similar to that which occurs in STM assisted nanostructure formation By decomposition of the precursor molecules through dissociation by an electron attachment process and field-induced decomposition in the presence of high electric fields the growth process is able to continue until bridging occurs Preliminary anode growth simulation using CPO-3Ds also provides evidence that branching or fractal growth at the anode occurs where there is irregularity in the surface texture of the anode which leads to a focused form of EBID TEM images ware also taken of the anode growth which revealed differences in microstructure between the growth at the cathode due to FEIG and the growth at the anode due to EBID Growth at the anode is made of metallic agglomerates in a carbonaceous matrix unlike the FEIG nanowire which is made up of a continuous metal core with a carbonaceous thin coating Comparison with other EBID work has also revealed the poor crystallinity of the anode growth observed due to the low electron energy in this study 81 Chapter – Conclusions 6.1 Summary To elucidate the growth of nanowires via field-emission induced growth (FEIG), simulations were carried starting from the basis of the nanowire formation mechanism previously proposed by Oon et al (2006) The simulations of cathode growth performed at the start and end stage of the nanowire growth have provided some insight into the growth processes observed experimentally Simulation of electron and W ion trajectories at the beginning stage of the nanowire growth point to the formation of a conical supporting structure at the base of the nanowire as well the formation of a nanowire with a diameter that grows at a much smaller rate to its length This is shown by the larger rate of W ion deposition at the base of the nanowire as well as a larger rate of W ion deposition at the tip of the nanowire as compared to the shaft Results of the simulation performed at the end stage of the nanowire growth show that the rate of W ion deposition at the tip of the nanowire is still high but lower than that at the initial stage The rate of deposition at the base and shaft of the nanowire however has decreased to a negligible level This suggests that nanowire growth speed reduces as the nanowire approaches the anode Several attempts at fabricating lateral field-emission structures were made and the options of using conventional Si fabrication techniques to form the lateral electrodes and using dielectrophoresis for placement of CNTs for field emission were chosen Growth experiments carried out on these structures showed that bridging of the nanowire across 82 the cathode and anode can occur However, the mechanism responsible for the final bridging remains unclear as both FEIG and EBID mechanisms are inoperable at low electron energies expected when the cathode tip approaches the anode Precursor dissociation from electron attachment and field-induced dissociation are potential mechanisms that can account for material deposition at the end-stage of bridging The measured resistance of this bridge however shows a large ohmic value due to the high resistance of the anode growth due to supply-limited deposition of tungsten material The anode growth observed is thicker and has a tree-like structure This morphology is different to that observed for cathode growth and is deduced to be from the continual fusing and growth of the cathode which encourages anode growth as the thicker anode is unlikely to fuse Preliminary anode simulations suggest that formation of protrusions at the anode will influence incoming electrons and thus the fractal growth of the anode observed From TEM imaging, the tree-like anode growth is made up of metal agglomerates embedded in a carbonaceous matrix The low micro-crystallinity observed is deduced to be due to the low electron acceleration energy used 6.2 Future Work Nanowire growth that terminates in a bridge might lead to possible destructive capacitive discharge It would be useful to understand if such a phenomenon plays a part in the tree-like formation of the anode Capacitive discharge of the cathode grown nanowire might randomly deposit perturbations on the opposing anode and thus trigger the observed anode growth as demonstrated by simulations 83 Although the rate of W ions that land at the locations of the base and nanowire has been shown, details of the original locations of these W ions were not analyzed Different locations of the W ions will correspond to different 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89 [...]... through the technique of field emission induced growth (FEIG), as shown in Figure 1.4 In FEIG of nanowires, an electric field high enough for field emission is applied on a sharp tip which ionizes the ambient organometallic precursors which then accelerates to the field emission tip (cathode) thus forming the nanowires Figure 1.5 illustrates the FEIG mechanism for W nanowire growth using a W(CO)6 precursor... constant of 3.14 Å An fcc crystal structure with lattice constant of 3.85 Å would have been expected and this deviation of crystal structure and stretched lattice on the molybdenum needles was attributed to the high tensile stress during growth from the high electric field Cold -field emission induced growth of single ultrathin nanowires of tens of microns long was first reported by Thong et al (2002) By using. .. (a) Schematic of 3D platform (b) Process flow of 3D platform (b)(i) Thermally grown oxide and Au deposition patterned using lift-off (b)(ii) Deposition of Parylene and Au patterned using lift-off (b)(iii) Etching of Parylene layer (b)(iv) Assembly of Au nanoparticles using dielectrophoresis Source: Nishant et al (2007) Figure 2.5 SEM micrograph of assembled Au nanoparticles on 3D platform using dielectrophoresis... using high voltage to initiate field emission from a sharp tip followed by continued field emission from the grown nanowire, nanowire of several tens of micrometers could be grown Low field emission currents of ~100nA were used to initiate and grow the nanowire For growth initiation, the voltage would start off very high, typically up to a few kilovolts depending on the emission tip radius, but this... review of alternate methods of forming nanosized bridges between electrodes including nanowires formed by FEIG Chapter 3 discusses the 7 simulation set up and results of simulations of growth at the cathode Chapter 4 discusses the sample preparation of FEIG structures for experimentation Chapter 5 discusses about the experimental results of the anode and cathode growth as well as discussions on anode growth. .. and electrical properties of FEIG grown nanowires have been studied, there is still work to be done in understanding the observed growth seen both at the cathode and anode, leading finally to the bridging of the two electrodes This thesis aims to elucidate the nanowire growth through a combination of simulation of FEIG nanowire growth, and experimentation Simulation of the growth at the cathode and... Examples of structures milled using a donut fill FIB mask with an ion bean in the direction shown by the thick arrows 55 Figure 4.10 ZnO nanowire grown from a lithographically patterned zinc island on the top of Si electrodes 56 Figure 4.11 Field emission caused patches of ZnO to be detached from the electrodes 56 Figure 4.12 CNTs placed onto Si electrodes via dielectrophoresis 57 Figure 5.1 Sketch of a... permission 2.4 Field Emission Induced Growth Formation of metallic dendritic growth at the cathode in a glow discharge in tungsten hexacarbonyl W(CO)6 ambient was first observed by Linden et al (1978) Such dendritic growth in W(CO)6 was studied in greater detail by Okuyama (1991) These growths on the cathode, termed needles, are mainly dendritic-like growth and each needle has the structure of a thin core... hundreds and tens of volts as the nanowire grows Higher currents provide higher growth rates and thicker nanowires but can give rise to random forking and the formation of multiple nanowires Various nanowire materials can be grown depending on the precursor used Growth from W(CO)6 yields W nanowires with core diameters as small as 3-5nm, Fe nanowires from Fe(CO)5 of 10-15nm in diameter, Co nanowires from... assembly of nanowires Examples of bottom-up assembly of nanowires onto prefabricated substrates include those demonstrated by He et al (2005) with vapor-liquid solid (VLS) grown silicon nanowires between silicon blocks as shown in Figure 1.2, and by Dai et al (2002) with the growth of single walled carbon nanotubes (SWNT) shown in Figure 1.3 Specifically, these examples cited address the problem of making ... stress during growth from the high electric field Cold -field emission induced growth of single ultrathin nanowires of tens of microns long was first reported by Thong et al (2002) By using high... nanotube) through the technique of field emission induced growth (FEIG), as shown in Figure 1.4 In FEIG of nanowires, an electric field high enough for field emission is applied on a sharp tip... initiate field emission from a sharp tip followed by continued field emission from the grown nanowire, nanowire of several tens of micrometers could be grown Low field emission currents of ~100nA