ULTRAHIGH RESOLUTION ELECTRON BEAM LITHOGRAPHY FOR MOLECULAR ELECTRONICS A Dissertation Submitted to the Graduate School of the University of Notre Dame in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy By Wenchuang (Walter) Hu, B.S., M.S.E.E. Gary H. Bernstein, Director Graduate Program in Electrical Engineering Notre Dame, Indiana May 2004 © Copyright by WENCHUANG (WALTER) HU 2004 All rights reserved ULTRAHIGH RESOLUTION ELECTRON BEAM LITHOGRAPHY FOR MOLECULAR ELECTRONICS Abstract By Wenchuang (Walter) Hu As the downscaling of conventional CMOS IC technology approaches its fundamental limits of power dissipation and fabrication difficulties, alternative computing approaches must be exploited. One of the promising candidates is molecular electronics. Significant progress for molecular devices and related theoretical calculations have been achieved during the last decade. Fabrication methods, however, are still a big challenge. In this work, we study ultrahigh resolution electron beam lithography (system, resist capability, and process) for the fabrication of molecular electronics, and molecular quantum-dot cellular automata (QCA) in particular. The major work contains three parts: the development of an EBL system based on a cold cathode field emission scanning electron microscope (Hitachi S-4500 SEM); the development of an ultrahigh-resolution lithography process with cold development of PMMA; and the application to the fabrication of molecular QCA, which involves the Wenchuang Hu patterning of self-assembled monolayers (SAMs), the patterning of DNA rafts, and the patterning of PMMA tracks for molecular liftoff of QCA molecules. The nanofabrication for molecular QCA is a close collaboration with Dr. Marya Lieberman’s group (Dr. Xuejun Wang and Dr. Koshala Sarveswaran) in the Department of Chemistry and Biochemistry of the University of Notre Dame. The ultrahigh resolution (5 nm) EBL in PMMA was achieved by the use of the SEM converted cold cathode field emission system and a cold development process, which proved to reduce the resist residue in the sub-10 nm lithographic patterns. Such high resolution has been successfully transferred to metal and nanoparticle structures by liftoff. The fabrication of 5-nm-wide metal and Au-nanoparticle lines has been demonstrated. Dense line patterns with sub-30 nm pitches in PMMA have been obtained. In addition, we have developed a novel precision wafer cross-sectioning technique to study EBL in resists. Besides PMMA, we have studied a chemical amplified resist called “XP9947” (Shipley Co.) for its capability for the 90 nm lithography node. 50-nm single- pass lines and 70-nm half-pitch resolution are achieved on the XP9947 resist. Application to molecular QCA involves the patterning of SAMs and DNA rafts. EBL is performed to cleave the embedded disulfide bonds in the SAMs to define nanometer scale patterns for chemical binding of molecules. Sub-30 nm lines are fabricated, and attachment of fluorescent molecules, N-(1-pyrene)maleimide is demonstrated. Also, a well-controlled molecular liftoff process was developed to pattern 4-tile DNA rafts onto silicon substrates with low level defects and low surface roughness. The goal is to use an appropriate combination of lithography and self assembly technology to fabricate molecular systems. To my family… ii CONTENTS FIGURES……………………………………………………………………...…...…..vii TABLES……………………………………………………………………….…….…xii ACKNOWLEDGMENTS……………………………………………………………xiii CHAPTER 1 INTRODUCTION……………………………………………...…………1 1.1 Molecular electronics and electron beam lithography ………..…………1 1.2 EBL for molecular quantum-dot cellular automata ……………..………3 CHAPTER 2 COLD CATHODE FIELD EMISSION EBL SYSTEM………………...10 2.1 Introduction to CCFE EBL system…………………………………...…11 2.2 CCFE EBL system overview………………………………...………….11 2.3 System modules………………………………..………………………..15 2.4 System optimization……………………………...……………………...21 2.4.1 Beam blanking problem……………………………..…………..21 2.4.2 X-Y dimension ratio calibration…………………………….…..25 2.4.3 Design-actual dimension ratio calibration………………..……..26 2.4.4 Noise elimination………………………………..………………26 iii 2.4.5 V5.2 update………………………………………..…………….30 2.4.6 Future improvements……………………………..……………..31 2.5 Pattern exposure………………………………………………....………31 2.5.1 Magnification and scan field………………..…………………..31 2.5.2 Beam energy and current……………………………………..…32 2.5.3 Spot size …………………………………………..…………….36 2.5.4 Objective lens aperture……………………………………..…...36 CHAPTER 3 NANOLITHOGRAPHY WITH COLD DEVELOPMENT OF PMMA ……………...................................39 3.1 Introduction to e-beam lithography……….…………………...………..39 3.1.1 E-beam resists………………………………………………..….40 3.1.2 Substrate………………………………………..……………….43 3.1.3 EBL theory and Monte Carlo simulation…………………..…...43 3.1.4 Developer …………………..…………………………………..52 3.1.5 Pattern transfer………………………..………………………...52 3.2 EBL process with cold development…………………………..……….54 3.2.1 Substrate and cleaning……………………….…………………55 3.2.2 PMMA preparation…………………………….…………..…...56 3.2.3 Cold development……………………………………………...56 3.2.4 Comparison of cold and ultrasonic development………………66 3.2.5 Liftoff of metal…………………………………………………69 3.2.6 Liftoff of Au nanoparticle ……………………………………..76 3.3 EBL process discussion………………………………………………..78 iv 3.3.1 Resolution …………………..…………………………….…...78 3.3.2 Metal liftoff…………………………..………………….……..81 3.3.3 Pattern quality………………..………………………….……..85 3.3.4 Aspect ratio and cross-sectional shape ……………….….…....88 3.3.5 PMMA resist……………………………………..…….………88 3.4 Applications…………..………………………………………………..91 CHAPTER 4 STUDY OF RESISTS…………………………………..……………….95 4.1 Introduction to EBL resists………………………………………….…96 4.1.1 Polymer resists for EBL………………….…………………….96 4.1.2 Chemically amplified resists for NGL…………………………98 4.2 Precise wafer breaking technique……………………………………...102 4.2.1 Process overview…………………………..…………………..103 4.2.2 Process details…………………..……………………………..107 4.3 Cross-section protection process………………………………………109 4.4 Aging effect study of PMMA ……………….……………..…………112 4.5 Study of chemically amplified resist (XP 9947)………………………113 4.6 Other applications……………………………………………..………117 CHAPTER 5 EBL OF SELF ASSEMBLE MONOLAYERS…………………….......118 5.1 Introduction of SAMs…………………………………..……………..118 5.2 SAMs synthesis process………………………………...……………..120 5.3 Chemical characterization of monolayer resist………………………..121 5.4 EBL process ………………………....………………………………..123 v 5.5 Molecular attachment………………..………………………………...126 5.6 Discussion and conclusions………………………...………………….129 CHAPTER 6 DNA PATTERNING FOR MOLECULAR QCA………………..…….132 6.1 Introduction………………...………………………………………….132 6.2 Synthesis of DNA raft….………………………………………..…….133 6.3 Molecular liftoff of DNA……………………….…..………..………..134 6.4 Conclusions……………..……………………………………………..141 CHAPTER 7 SUMMARY AND FUTURE WORK………………………..………...143 7.1 Summary………………………………………………………………143 7.2 Future work……………………………………………………………147 REFERENCES………………………………………………………………..………..150 Chapter 1……………………………………………………………………….150 Chapter 2……………………………………………………………………….153 Chapter 3……………………………………………………………………….154 Chapter 4……………………………………………………………………….160 Chapter 5……………………………………………………………………….164 Chapter 6……………………………………………………………………….166 Chapter 7……………………………………………………………………….167 vi FIGURES 1.1 Schematic of four-dot QCA cells and QCA devices…………………………….4 1.2 Molecule as QCA cell……………………………………………………….…..6 1.3 Crossbar molecular electronics……………………………………………….…7 1.4 Nanowires fabricated by nonlithographic super-lattice nanowires pattern transfer (SNAP) process…..……………………………………………………………...7 1.5 Schematic illustration of proposed fabrication process for M-QCA using EBL and molecular liftoff…………………………………………………………………9 2.1 Schematic diagram of the FESEM EBL system…………………..……………13 2.2 Hitachi S-4500 CCFE EBL system…………………………….………………14 2.3 Schematic view of Hitachi S-4500 field emission SEM………………….……16 2.4 Architecture of the V5 software……...….……………………………………..18 2.5 Schematic of beam blanking circuit ……….…………………………………..19 2.6 One-bit QCA full adder patterns……………………….………………………22 2.7 Schematic diagram of the blanking circuit…………………….……………….23 2.8 AFM image of an one-bit QCA full adder pattern in PMMA……………….…24 2.9 EBL patterns of one-bit QCA full adders…………………………….………...24 2.10 Calibrated X-Y dimension ratio (X:Y~1:1)……………………….…...……….25 2.11 Combined noise on the EBL system……………………………………………26 vii 2.12 Ground diagram of Hitachi CCFE EBL system…………………………….…..27 2.13 Cartoon of isolation box made for roughing pumps……………….………..…..28 2.14 Beam current stabilization process……………………….…………………..…29 2.15 Lithography pattern of line grating after noise reduction…………….……..…..30 2.16 Calculations using Gaussian exposure model to determine pixel spacing requirements, pixel spacing higher than the beam diameter will cause uniformity problem and result in higher LER and more resist residue……………………...33 2.17 Electron scattering in electron resist exposure. The curves at the top of the figure show the exposure distributions due to the incident and backscattered electrons …………………………………………...34 3.1 Schematic illustration of positive and negative resists…………………………..41 3.2 Electron scattering during EBL on a resist-coated substrate (Si substrate with 80- nm-thick PMMA resist), simulated by Monte Carlo simulation software………45 3.3 Exposure distributions of forward-scattered and backscattered electrons for a 1-µ m-thick resist layer on a silicon substrate for 10 keV.…………………………..46 3.4 Trajectories of (a) forward-scattered electrons and (b) secondary electrons in 400- nm-thick PMMA resist with