NANOSTRUCTURED COLUMNAR THIN FILMS USING OBLIQUE ANGLE DEPOSITION: GROWTH, SERS CHARACTERIZATION AND LITHOGRAPHIC PROCESSING A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy By Piyush J. Shah M.S., Wright State University, 2005 2012 Wright State University COPYRIGHT BY PIYUSH JAYANT SHAH 2012 WRIGHT STATE UNIVERSITY SCHOOL OF GRADUATE STUDIES May 30, 2012 I HEREBY RECOMMEND THAT THE DISSERTATION PREPARED UNDER MY SUPERVISION BY Piyush Jayant Shah ENTITLED Nanostructured Columnar Thin Films using Oblique Angle Deposition: Growth, SERS Characterization and Lithographic Processing BE ACCEPTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF Doctor of Philosophy. _____________________________ Kefu Xue, Ph.D. Dissertation Director _____________________________ Ramana V. Grandhi, Ph.D. Director, Ph.D. in Engineering Program _____________________________ Andrew Hsu, Ph.D. Dean, School of Graduate Studies Committee on Final Examination __________________________ Kefu Xue, Ph.D. __________________________ Andrew M. Sarangan, Ph.D. __________________________ Saiyu Ren, Ph.D. __________________________ Yan Zhuang, Ph.D. __________________________ Ronald A. Coutu, Ph. Abstract Shah, Piyush Jayant. Ph.D; Engineering Ph.D. Program Wright State University, 2012. Nanostructured Columnar Thin Films using Oblique Angle Deposition: Growth, SERS Characterization and Lithographic processing Oblique angle physical vapor deposition technique has led to the evolution of new class of nanostructured thin films. These films posses’ novel anisotropic electrical, magnetic, optical, properties which could be potentially engineered based on the growth conditions and the deposition parameters. The technique is based on the atomistic level self-shadowing principle. In the oblique angle deposition (OAD) technique, the substrate is held at an oblique angle with respect to the incoming vapor flux. As the vapor atoms condense and nucleate on the substrate, the shadowed regions behind each site stop receiving the subsequent vapor atoms. Instead, they land on the previously formed droplets, resulting in the evolution of a columnar morphology. In this dissertation research, three aspects of nanostructured thin films grown using this method are investigated. When grown at room temperatures, soft metals such as silver (Ag), copper (Cu), gold (Au) etc; generally produce low aspect ratio collapsed columnar structures compared to high melting temperature metals such as titanium (Ti), chromium (Cr), nickel (Ni), etc. Using e-beam evaporation and a custom built cryogenic substrate cooling apparatus; we investigate the growth of nanostructured thin films made from soft metals at near 100 K and 300 K side-by-side. Growth of these films at cryogenic substrate temperatures has resulted in high aspect ratio nanocolumns thin film morphology. iv Ag nanorods (AgNRs) thin films are known to have surface enhanced Raman scattering (SERS) response. In this study, AgNRs thin films are incubated in liquid and vapor phase with SERS test probe molecules. The SERS response of AgNRs thin films grown at room and cryogenic substrate temperature is compared. The hypothesized improved SERS response of cryogenically grown AgNRs is attributed to the morphological differences and higher surface of AgNRs at this growth conditions. Rigorous SERS enhancement factor (SEF) calculations are discussed by estimating the number of molecules absorbed on the surface of AgNRs through the use of fluorescence spectrophotometer measurements. The third aspect of this research is to investigate the effect of liquids exposure and lithographic processing of these films. It is known that exposure of nanorods thin films to liquids and solvents, permanently changes the physical structure of these films. This prevents conventional lithographic patterning of such thin films since it involves wet processing. In this study, we show the use of CO based critical point drying (CPD) 2 technique to mitigate the structural collapse of nanorods thin films after liquids exposure. Further, we discuss dry lift-off based lithographic process to pattern these thin films. v Table of Contents Chapter 1 Introduction ..................................................................................................... 1 1.1 Dissertation research ............................................................................................ 5 1.1.1 Growth .......................................................................................................... 5 1.1.2 SERS characterization .................................................................................. 7 1.1.3 Lithographic processing ................................................................................ 9 1.2 Dissertation contributions .................................................................................. 11 Chapter 2 Growth ............................................................................................................ 13 2.1 Introduction ........................................................................................................ 13 2.2 Surface diffusion theory and analytical modeling.............................................. 21 2.2.1 Results and discussion ................................................................................ 23 2.3 Experimental details ........................................................................................... 27 2.3.1 Custom built cryogenically cooled substrate holder ................................... 27 2.3.2 Fabrication of nanorods thin films using OAD technique .......................... 30 2.3.3 SEM and Image analysis ............................................................................. 31 2.4 Results and discussion ........................................................................................ 32 2.4.1 Physical morphology .................................................................................. 32 2.4.2 Surface diffusion and nucleation study ....................................................... 40 2.4.3 Chemisorption theory.................................................................................. 48 Chapter 3 SERS characterization – Liquid phase sensing .............................................. 53 vi 3.1 Raman and SERS spectroscopy ......................................................................... 53 3.2 Introduction ........................................................................................................ 56 3.3 Experimental details ........................................................................................... 59 3.3.1 Fabrication of Ag nanorods thin films using OAD technique .................... 59 3.3.2 SEM measurements .................................................................................... 60 3.3.3 SERS substrates incubation with R6G molecules ...................................... 60 3.3.4 Micro-Raman and SERS measurements ..................................................... 60 3.3.5 Fluorescence measurements........................................................................ 61 3.3.6 Raman and SERS data analysis .................................................................. 61 3.4 Results and Discussion ....................................................................................... 63 3.4.1 Quantitative and qualitative SERS response comparison ........................... 63 3.4.2 Analysis of fluorescence emission spectra ................................................. 72 3.4.3 Estimation of the SERS enhancement factors ............................................ 75 Chapter 4 SERS characterization – Vapor phase sensing ............................................... 78 4.1 Introduction ........................................................................................................ 78 4.2 Experimental details ........................................................................................... 80 4.2.1 Vapor phase incubation using 4-ABT......................................................... 80 4.2.2 Vapor phase incubation using CEES vapors .............................................. 81 4.2.3 Micro-Raman and SERS measurements ..................................................... 81 4.3 Results and discussion ........................................................................................ 83 vii Chapter 5 Effects of liquids exposure on nanorods thin films ........................................ 87 5.1 Introduction ........................................................................................................ 87 5.2 Experimental details ........................................................................................... 91 5.2.1 Fabrication of SiO nanorods thin films ..................................................... 91 2 5.2.2 Liquids exposure with SiO nanorods thin films ........................................ 92 2 5.2.3 SEM and image analysis ............................................................................. 93 5.3 Results and discussion ........................................................................................ 94 Chapter 6 Dry lift-off based nanorods thin films patterning ......................................... 101 6.1 Introduction ...................................................................................................... 101 6.2 Experimental details ......................................................................................... 105 6.3 Results and discussion ...................................................................................... 107 Chapter 7 Summary and Conclusions ........................................................................... 111 Chapter 8 Future work................................................................................................... 114 Appendix A ................................................................................................................... 116 A.1 Nanorods thin films on Flexible substrate (Polyimide) ................................... 116 A.2 Nanorods thin films on 1-D gratings structure ................................................. 118 A.3 MEMS Cantilever based volatile organic compound sensing using SiO 2 nanorods thin films ...................................................................................................... 120 References ...................................................................................................................... 121 viii List of Figures Figure 1 Schematic and SEM images showing the difference between normal incidence thin film and OAD thin film deposition techniques ......................................... 2 Figure 2 SEM images showing SiO STF grown using OAD based on (a) random 2 nucleation (b) periodic nucleation. ................................................................... 3 Figure 3 Schematic showing GLAD scheme and resultant difference in morphology of thin films (a) tilted nanorods (b) spiral nanorods ............................................. 4 Figure 4 Comparative SEM results showing the physical morphology of nanorods thin films deposited using low and high melting materials. Figure shows near collapsed or small aspect ratio nanocolumns grown from Ag and Cu versus distinctly separate, high aspect ratio nanorods thin films from Ti, Ni, SiO and 2 Cr .................................................................................................................... 15 Figure 5 Average surface diffusion distance as a function of substrate temperature and deposition rate ................................................................................................. 23 Figure 6 Co and Ag adatom hop time on Co and AG surface respectively as a function of substrate temperature .................................................................................. 25 Figure 7 Schematic shows custom built cryogenic substrate cooling setup. The setup allows concurrent fabrication of STF at 100 K and 300 K substrate temperature. .................................................................................................... 27 Figure 8 Image of custom built cryogenic substrate cooling setup. (a) Inside of e-beam deposition chamber and (b) cryogenic cooling setup (c) Copper and Teflon sample mounting stage for concurrent fabrication of nanorods thin films at near 100 K and 300 K temperature ................................................................. 29 ix Figure 9 Top and side view SEM results showing Ag nanorods thin films grown at 300 K and 100 K substrate temperature. Film thickness is 300 nm. (a) top view at 100 K (b) top view at 300 K (c) side view at 100 K (d) side view at 300 K .. 32 Figure 10 Top and side view SEM results showing Ag nanorods thin films grown at 180 K substrate temperature. Film thickness is 300 nm. (a) Top view and(b) side view................................................................................................................. 36 Figure 11 Top and side view SEM results showing Cu nanorods thin films grown at 300 K and 100 K substrate temperature. Film thickness is 300 nm. (a) top view at 100 K (b) top view at 300 K (c) side view at 100 K (d) side view at 300 K .. 37 Figure 12 Top and side view SEM results showing Au nanorods thin films grown at 300 K and 100 K substrate temperature. Film thickness is 300 nm. (a) top view at 100 K (b) top view at 300 K (c) side view at 100 K (d) side view at 300 K .. 38 Figure 13 Schematic showing Ag nanorods grown on Si and glass substrates (a) with normal incidence deposited barrier layer thin film (as seen in literature) (b) without barrier layer thin film (current study). ............................................... 41 Figure 14 Top view SEM results showing early stage nucleation study by terminating the Ag nanorods thin films growth at 100 Å (top row) and 300Å (bottom row) at 300 K and 100 K substrate temperatures. (a) 100 Å at 100 K(b) 100 Å at 300 K (c) 300 Å at 100 K (d) 300 Å at 300 K. ............................................... 42 Figure 15 Plot showing particle size distribution of nucleated Ag islands from the SEM results shown in Figure 14. (a) 100 Å at 100 K (b) 100 Å at 300 K(c) 300 Å at 100 K (d) 300 Å at 300 K ........................................................................... 43 x