FEMTOSECOND LASER MATERIAL PROCESSING FOR MICRO-/NANO-SCALE FABRICATION AND BIOMEDICAL APPLICATIONS DISSERTATION Presented in Partial Fulfillment of the Requirements for The Degree Doctor of Philosophy in Graduate School of the Ohio State Univeristy By Hae Woon Choi, M.S. ***** The Ohio State University 2007 Dissertation Committee: Professor Dave F. Farson, Adviser Approved by Professor Charles E. Albright Professor L. James Lee ________________________________ Welding Engineering Graduate Program ABSTRACT Femtosecond laser ablation has interesting characteristics for micromachining, notably non-thermal interaction with materials, high peak intensity, precision and flexibility. In this dissertation, the potential of femtosecond laser ablation for fabrication of biomedical and electronic devices is studied. In a preliminary background discussion, some key literature regarding the basic physics and mechanisms that govern ultrafast laser pulse interaction with conductive materials and dielectric materials are summarized. In the dissertation work, results from systematic experiments were used characterize laser ablation of ITO (Indium Tin Oxide), stainless steel (hot embossing applications), polymers (PMMA, PDMS, PET, and PCL), glass, and fused quartz. Measured parameters and results include ablation threshold, damage threshold, surface roughness, single- and multiple-pulse ablation shapes and ablation efficiency. In addition to solid material, femtosecond laser light interaction with electrospun nano-fiber fiber mesh was analyzed and studied by optical property measurements. Ablation of channels in nano-fiber mesh was studied experimentally. Internal channel fabrication in glass and PMMA polymers was also demonstrated and studied experimentally. In summary, it is concluded that femtosecond laser ablation is a useful process for micromachining of materials to produce ii microfluidic channels commonly needed in biomedical devices such as micro-molecular magnetic separators and DNA stretching micro arrays. The surface roughness of ablated materials was found to be the primary issue for femtosecond laser fabrication of microfluid channels. Improved surface quality of channels by surface coating with HEMA polymer was demonstrated. iii Dedicated to God, my wife, my families and teachers iv ACKNOWLEDGMENTS There are many people whom I would like to thank for their support of my research at The Ohio State Univeristy. Edison Welding Institute (EWI) and the Nano Science and Engineering Center (NSEC) program at OSU have been providing a comfortable research environment as well as their financial support in my life of graduate study. I would like to thank to my advisor, Dr. Dave Farson, for his invaluable advice, endless patience and support. As committee members, Dr. Stanislav Rokhlin, Dr. Charles Albright, Dr. James Lee provided me a lot of references and research considerations. I also thank to our lab colleagues, Yong Chae Lim, Jian Chen, and Min Hyun Cho for their support and encouragement. I would like to thank God for the successful completion of this dissertation. I dedicate this work to my wife, Jin Sun, for all of her love, help and encouragement. This dissertation is also dedicated to my parents, and my lovely brothers and sisters, for providing me with the opportunities and support to achieve my goals. My mentor, Pastor Jay Chun encouraged me a lot during Ph. D study. Chungnyun boo mokjang family were my greatest supporters in prayer. This dissertation is a result of much collaboration with other NSEC fellows. In chapter 3, femtosecond laser for metallic materials, I had collaboration with Prof. Allen v Yi and Prof. James Lee. Lei Li, Chunhe Zhang and Chunmeng were dedicated to getting the results and I appreciate their time and effort. In chapter 4, femtosecond laser for dielectric materials, Prof. Jeff Chalmers, Prof. James Lee, Prof. John Lannutti, and Prof. Susan Olesik supported me to complete the research. Burr Zimmerman and Jeremy Steach are the men whom I should specially thank. I also thank to Nick Farrell for his support on characterization of channels and other valuable advice. We enjoyed the research together and shared much information. Jed Johnson, Sarah Drilling, and Ruth Li supported my research by providing PCL fiber and the results of cell growth. vi VITA Oct. 16th, 1972 …………………………… Born – Taegu, South Korea 1992 – 1996 ……………………………. BSME, Keimyung University Taegu, South Korea 2001- 2003 ……………………………. MSME, Univ. Central Florida Orlando, South Korea 1989 – 1991 ……………………………. Engineer, Samsung Heavy Industries 1995 – 2000 ……………………………. Engineer, Daehyun Tech 2000 – 2003 ……………………………… Eng. Manager, Laser Tech USA 2003 – present …………………………… Ph. D study at Ohio State University vii PUBLICATIONS A. Journal Articles 1. H W Choi, D F Farson, M Cho, “A hybrid laser+GMAW process for control of the bead humping defect”, Welding Journal 85 (8): 174S-179S Aug. 2006 2. D F Farson, H W Choi, S I Rokhlin, “Electrical discharges between platinum nanoprobe tips and gold films at nanometer gap lengths” Nanotechnology, 17(1):132-139 Jan. 2006. 3. D F Farson, H W Choi, C Lu, and L. James Lee “Femtosecond bulk laser micromachining of microfluid channels in PMMA” Journal of Laser Applications, 18 (3): 210-215 Aug. 2006 4. H W Choi, D F Farson, J Bovatsek, A Arai, D Ashkenasi, “Direct-write patterning of ITO film by high pulse repetition rate femtosecond laser ablation” , Applied Optics 46 (23), Aug. 2007 FIELD OF STUDY Major: Welding engineering Minor: Biomedical engineering viii TABLE OF CONTENTS ABSTRACT..................................................................................................................... ii ACKNOWLEDGEMENTS................................................................................................ v VITA.............................................................................................................................. vii LIST OF FIGURES ...................................................................................................... xiii LIST OF TABLES....................................................................................................... xvii Chapter 1.......................................................................................................................... 1 1.1 Introduction................................................................................................................... 1 1.2 Organization of dissertation.......................................................................................... 2 Chapter 2 Femtosecond laser for micro/nano-scale machining.......................................... 8 2.1 Micro-/nano-scale machining technologies .................................................................. 8 2.2 The mechanism of sub-picosecond ablation............................................................... 11 2.3 Femtosecond laser system for experiments ................................................................ 12 2.3.1 Femtosecond laser system........................................................................................ 12 2.3.2 Beam delivery system.............................................................................................. 16 2.4 References................................................................................................................... 18 ix Chpater 3 Femtosecond laser interaction with metallic materials .................................... 21 3.1 ITO ablation................................................................................................................ 21 3.1.1 Introduction...................................................................................................... 21 3.1.2 Experiments ..................................................................................................... 27 3.1.3 Results and Discussion .................................................................................... 28 3.1.4 Conclusions and future works.......................................................................... 49 3.1.5 References........................................................................................................ 51 3.2 Hot embossing ............................................................................................................ 53 3.2.1 Introduction..................................................................................................... 53 3.2.2 Experiment procedure..................................................................................... 56 3.2.3 Laser ablation.................................................................................................. 61 3.2.4 Ablation and damage thresholds and pattern resolution................................. 61 3.2.5 Mold micromachining..................................................................................... 74 3.2.6 Hot embossing ................................................................................................ 79 3.2.7 Conclusions and future works......................................................................... 85 3.2.8 References....................................................................................................... 87 Chapter 4 Femtosecond laser interaction with dielectric materials .................................. 90 4.1 Poly-caprolactone (PCL) machining........................................................................... 90 x 4.1.1 Introduction.....................................................................................................