UNIVERSITY OF CINCINNATI Date:___________________ I, _________________________________________________________, hereby submit this work as part of the requirements for the degree of: in: It is entitled: This work and its defense approved by: Chair: _______________________________ _______________________________ _______________________________ _______________________________ _______________________________ AMPEROMETRIC CHARACTERIZATION OF A NANO INTERDIGITATED ARRAY (nIDA) ELECTRODE AS AN ELECTROCHEMICAL SENSOR A thesis submitted to The Division of Research and Advanced Studies of the University of Cincinnati In partial fulfillment of the requirements for the degree of MASTER OF SCIENCE In the Department of Electrical and Computer Engineering and Computer Science of the College of Engineering August 1, 2006 By Ashwin Kumar Samarao B.E. (Hons.) Electrical and Electronics Birla Institute of Technology and Science, India, 2004 Committee chairman Dr.Chong H. Ahn ABSTRACT The main goal of this research is to amperometrically characterize a ring type nano interdigitated array (nIDA) electrode as an electrochemical sensor and to verify the enhancements in the sensitivity of such a sensor when compared to its micro counterparts. Each electrode was fabricated in gold with 275 fingers, each of width 100 nm and spacing 200 nm, using electron beam lithography and nano lift-off processes on a SiO2/Si wafer. The reference and counter electrodes were fabricated using electroplating. P – Aminophenol (PAP) was used as the redox species to be detected by the nano- IDA electrochemical sensor. Using Chronoamperometry, concentrations of PAP as low as 10 pM were successfully detected using the fabricated sensor. The current output by the sensor for such low concentrations was in the pico-ampere range and was measured using a very sensitive pico-ammeter. An instrumentation circuit was designed and fabricated to reliably convert the pico-ampere currents to corresponding voltage levels for further signal processing. The lowest concentration detected by the nano-IDA electrochemical sensor was three orders of magnitude less than that detected by the micro-IDA. This proves the enhanced sensitivity at lower dimensions for an electrochemical sensor which will find very wide application in a variety of fields, the main one being the rapidly emerging field of biosensors. ACKNOWLEDGEMENT I am deeply indebted to my advisor, Dr. Chong H. Ahn for giving me an opportunity to do research under his guidance. The experience of working under his encouragement and being part of his esteemed research group has helped me grow into a better individual, both as a researcher and as a person. I would like to thank my other committee members, Dr. Ian Papautsky and Dr. Joseph Nevin, who have helped me in this research through the courses they offered and the discussions we had at various stages of my work. The successful completion of my thesis within a relatively short span of time involved a lot of help from many of my lab mates and friends. I would like to express my sincere gratitude to Michael Rust for helping me extensively with the e-beam lithography process. I will definitely miss all the interesting conversations with him in the clean room while the e-beam write was in progress. Besides the nano sensor, I have also won myself a very good friend. Another great friend who got me started off on the circuit design part of my thesis was Lakshminarayanan Ramasamy. I am very thankful to him for giving so many valuable suggestions on designing circuits for pico-ampere measurements and also for letting me share a portion of his chip for fabricating my circuit. I wonder at Jaephil Do’s patience while he taught me to prepare the different concentrations of PAP and I sincerely appreciate his guidance for helping me quickly finish the characterization process. Without these three people, it would have been impossible to complete my research work and I am very grateful for all their kind help and guidance. I am thankful to Jeff Simkins and Robert Jones for helping me with the Clean room equipments and for their valuable suggestions for my processes. I am also thankful to Ron Flenniken for readily agreeing to deposit gold on my wafers whenever I asked for it. I learnt the art of working alone in the Clean rooms during my TA experience with Zhiwei Zhou who gave me the freedom to learn at my own pace and helped me out whenever needed. Pei-Ming Wu was instrumental in teaching me to do research at a faster pace and always showed true concern in the success of my work. I earnestly thank both of them for they have indirectly played very important roles in motivating me to do better research. My stay for over a year in this research group would have not been half as entertaining but for Nathaniel Hadlock, Andrew Browne and Matthew Estes, who always had their own ways of cheering up everyone in the lab. I thank them for all the fun we had and I will miss them. I immensely thank my dearest friend Padma Priya Venkata for helping me out with the experimental setup and for being a patient listener to all my research ramblings though most of it rarely would have made any sense to a mechanical engineer like her. Above all, I express sincere gratitude to my parents who are completely responsible for all my achievements and interests. I thank them for their love and support with which everything in this world seems possible to me. TABLE OF CONTENTS Table of contents 1 List of figures 3 List of tables 5 1. Introduction 6 1.1 Introduction………………………………………………………………… 7 1.2 Previous work……………………………………………………………… 12 1.3 Research motivation……………………………………………………….. 13 1.4 Objective of this thesis…………………………………………………….. 14 References…………………………………………………………………. 15 2. Fabrication of the ring type nano-IDA electrochemical sensor 18 2.1 Introduction………………………………………………………………. 19 2.2 Electron beam lithography using the Raith 150 system…………………. 23 2.3 Design of the ring type nano-IDA pattern……………………................. 29 2.4 Nanofabrication of the ring type nano-IDA electrodes… ………........... 32 2.3 Conclusion………………………………………………………………. 47 References……………………………………………………………..…. 48 3. Circuit design for current-to-voltage conversion of pico-ampere currents 50 3.1 Introduction……………………………………………………………… 51 3.2 Design of an I-V converter for pico-ampere………..……………………. 53 3.3 Implementation…………………………………………………………… 60 3.4 Conclusion………………………………………………………………... 64 References………………………………………………………………… 65 1 4. Amperometric characterization of the nano-IDA electrochemical sensor 66 4.1 Introduction……………………………………………………………. 67 4.2 Instrumentation ……………………………………………………….. 68 4.3 Chronoamperometry experimental results……………………………. 70 4.4 Comparison of the ring type nano-IDA with the micro-IDA………… 74 4.5 Conclusion…………………………………………………………….. 77 References …………………………………………………………….. 78 5. Conclusion and future work 79 5.1 Summary………………………………………………………………... 80 5.2 Future work……………………………………………………………... 81 2 LIST OF FIGURES Figure 1.1 Illustration of an IDA setup………………………………………….. 8 Figure 1.2 Redox cycling in an IDA…………………………………………….. 10 Figure 2.1 The Raith 150 e-beam lithography machine at the University of Cincinnati………………………. 24 Figure 2.2 The electron gun, wafer and stage inside the Raith 150……………… 26 Figure 2.3 Illustration of a complete nano-pattern as a group of misaligned write fields………………………………. 28 Figure 2.4 Design of a circular IDA electrode…………….……………………. 29 Figure 2.5 AutoCAD image of the nano-pattern………………………………… 30 Figure 2.6 GDSII image of the nano-pattern on the Raith software…………….. 31 Figure 2.7 Nano-patterns with undercut on the PMMA layer…………………… 34 Figure 2.8 Metal lift-off process after e-beam write…………………………….. 35 Figure 2.9 Gold nanoelectrodes after the metal lift-off process…………………. 36 Figure 2.10 Quality of gold lines for a dose of 900 µC / cm2…………………….. 37 Figure 2.11 Quality of gold lines for a dose of 1000 µC / cm2…………………… 37 Figure 2.12 Quality of gold lines for a dose of 1200 µC / cm2…………………... 38 Figure 2.13 SEM image showing the exposed and developed PMMA layer along with the unexposed regions in the nano-pattern in the shape of ‘X’………………………………………………………… 40 Figure 2.14 5.48k times magnified image of the underexposed ‘X’ region…….. 41 Figure 2.15 20.67k times magnified image of the underexposed ‘X’ region…… 41 Figure 2.16 Straight and jagged lines of the nano-pattern in the AutoCAD file... 42 3 Figure 2.17 Magnified image of the fabricated nano-IDA electrodes………….. 43 Figure 2.18 Mask used for photolithography after creating nano-IDA………….. 44 Figure 2.19 Photolithography for the reference and working electrodes………… 45 Figure 2.20 Snapshot of the aligned photoresist patterns for reference and counter electrodes with the nanoelectrodes…………………….. 45 Figure 2.21 Gold depositions for the reference and working electrodes………… 46 Figure 2.22 Electroplating of Ag/AgCl reference electrode…………………….. 46 Figure 2.23 Snap-shot of the complete nano-electrochemical sensor…………... 47 Figure 3.1 A common current-to-voltage converter…………………………… 51 Figure 3.2 Current-to-voltage converter for pico-ampere currents…………….. 54 Figure 3.3 I-V converter repeated twice to nullify systematic faults in fabrication……………... 57 Figure 3.4 Layout of the designed I-V converter for pico-ampere………..…… 58 Figure 3.5 Simulation results of the designed I-V converter…………………... 58 Figure 3.6 Modified pad-frame layout for the current output from nano-sensor…… 59 Figure 3.7 Snap shot of the fabricated I-V converter circuit for pico-ampere… 60 Figure 3.8 Plot of the current input Vs voltage output of the I-V converter……. 63 Figure 4.1 Redox reaction of P – Aminophenol……………………………….. 67 Figure 4.2 Instrumentation for amperometric characterization of the sensor….. 68 Figure 4.3 Plot of current output from the sensor Vs concentration of PAP….. 73 Figure 4.4 Comparison of nano-IDA with micro-IDA………………………… 75 4 LIST OF TABLES Table 2.1 E-beam exposure parameters………………………………………... 39 Table 3.1 Transistor sizes of the current-to-voltage circuit….………………… 55 Table 3.2 Current input and the voltage output of the designed I-V converter..