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... 61
Table 3.3 Current input and the mean and standard deviation of the
voltage output of the designed I-V converter………………………… 62
Table 4.1 Current measured from the sensor for various concentrations of PAP 70
Table 4.2: The mean and standard deviations of the currents measured from the
sensor for various concentrations of PAP…………………………. 72
5
Chapter 1
INTRODUCTION
6 1.1 Introduction
Electrochemistry is the branch of chemistry concerned with the interrelation of electrical and chemical effects [1]. The resulting electrical effects from the chemical
phenomena correspond to the qualitative and quantitative properties of the chemicals
undergoing reaction. This is particularly fascinating for Chemists and Electrical
Engineers, who have been working together for many years now to utilize
electrochemistry and to gain valuable insights about the chemicals being analyzed. This
field of work is better known as Electroanalytical Chemistry [2-4]
One such information of interest about the chemicals is their concentration. The
easy and accurate measurement of concentration using electrochemical methods is very
desirable for the numerous electrochemical sensors available in the market today.
Measurement of concentration of species relies on the chemical phenomena that involve
charge transfer (like redox reactions) from or to the electrode. The amount of charge
transferred is a direct indication of the concentration of the species and this charge can be
measured as the charge itself (Coulometry) [5-8] or as a current (Amperometry) [9-12] or
as a voltage (Potentiometry) [13-16]. It is clear that the amount of charge transferred will
get lower as the concentrations gets lower and so, this need for measuring lower
concentrations has been a very important challenge for circuit designers over the years.
Also, it has been realized that for phenomena like charge transfer which occurs at the
molecular level, it would be more conducive to carry out the measurements if the systems
were scaled down. This scaling down of electrochemical systems is expected to make the
sample size smaller and the electron transfer faster as compared to its macro counterparts.
7 The scaling down is also expected to enable the measurements of even lower
concentrations that were once considered impossible for electrochemical measurements.
The micro fabrication technology comes as a boon to this scaling down venture of
electrochemical systems. Recent advancement in micro/nano fabrication technology
offers the possibility of reducing the electrode dimensions to micro-meters and even
nano-meters. The advancement in recent nano technologies has also made possible the
realization of different electrode geometries that increase the possibility of electron
transfer with a minuscule sample and thus pushed down the measurable limits of concentrations. One such innovative concept of Interdigitated Array (IDA) electrodes in micro scale has been explored since the 1980s [17, 18].
Figure 1.1: Illustration of an IDA setup
8 IDA electrode [19-21], shown in Figure 1, is specific for the reversible redox
species detection with higher sensitivity. The reversible redox species which widely
exists in the various chemical and biochemical reactions is defined as the kind of species
that is reduced or oxidized at the electrode surface with a potential change. The IDA
electrode is mainly named due to its geometry or shape. The electrode materials could be
Pt, Cu, Carbon, or other inert conductive materials. The IDA electrode works in a dual
mode to achieve steady-state voltammetry. In a dual-mode configuration, one band of
electrodes is set to the reduction potential to drive the reaction O + ne Æ R and the other
band is applied to the species oxidation potential to drive the reaction R Æ O + ne. The species diffuses back and forth between the two bands of electrodes to be reduced and oxidized (redox cycling) [22-25]. Such a species redox cycling between the two bands of electrodes enhances current greatly. Once the redox cycling process becomes saturated, a steady state limiting current is achieved from the two bands of electrodes. The steady- state limiting current is proportional to the species concentration, and thus the measured current can determine the concentration.
A pictorial representation of Redox Cycling is best described in the picture drawn
by Xiaoshan Zhu [26] and is shown in Figure 1.2. One of the two working electrode is
fixed at the reduction potential of the species to be detected. The other electrode is varied
from the reduction potential extreme to the oxidation potential extreme and even beyond.
9 Initially, when both electrodes are at the same reduction potential, all the redox
Figure 1.2: Redox Cycling in an IDA
species get reduced and there is an excess of reduced species in the solution. Now, when one of the working electrode’s potential is swept towards the oxidation potential, the redox species in its neighborhood gets oxidized first. These oxidized species now start diffusing towards the reducing electrode and the reduced species far away from the oxidizing electrode will start diffusing towards the oxidizing electrode. At a point when the voltage of the electrode that is being swept, reaches the exact oxidation potential of the species, a equilibrium is reached between the oxidized and reduced species and both the oxidation and reduction occurs at the same rate. At this stage, the redox cycling is
10 only limited by the length the species have to diffuse to reach the other electrode. The
oxidation or cathodic current that has been on the rise since the beginning of the potential
sweep in the oxidizing electrode saturates when the equilibrium is reached. The value of
this saturated current depends upon the concentration of the species that is undergoing
redox cycling. The aim of this research work is to measure this current reliably and
convert it to voltage for further signal processing.
The magnitude of the current depends on how fast the species can diffuse in the
solution to reach the other electrode. If this distance to be traveled by the species during
the redox cycling can be reduced, then the response time of the sensor as well as the
magnitude of the saturated current is higher. This demands the necessity for micro and
nano electrodes. The redox cycling is reported to be many folds efficient in nano-IDA than in micro-IDA [27]. This necessity opens up a new challenge of identifying the conditions and methodology needed for successfully fabricating a nano-IDA and interfacing that with the macro world. This drive towards minimizing the inter-electrode spacing to nano dimensions has also forced the currents measured in through this sensor to be many folds lower than it was in the micro-IDA sensors. This effect of nano-IDA has made the circuit design community understand the need for designing circuits that measure currents in the pico-ampere and even in the femto-ampere range [28].
It can be understood from the aforesaid explanations that the voltages applied and the currents measured are the most important signals involved in electrochemical detection. So, there has to be a very high level of accuracy in these parameters to tap the complete potential of the sensor. Hence, this necessitates the use of Reference electrodes
[29-31] to measure the voltage of the working electrodes against a fixed reference
11 potential. As the constancy of the reference potential is a must, the Reference electrode cannot be allowed to conduct a current through it and we need another electrode to bypass this current. This work is done by the Counter Electrode [32].
1.2 Previous work
Xiaoshan Zhu [26] has developed a nano-IDA electrochemical sensor and its measurement electronics with a dynamic transduction mechanism. The key approach of this dynamic transduction mechanism is the charge injection method where a charged capacitor is used as a charge supplier for the electrochemical reaction of the reversible redox species at the interdigitated array electrodes, and the characteristics of the capacitor
(or electrode) potential decay are recorded and analyzed to evaluate the concentration of reversible redox species. A small surface area IDA nanoelectrode with a finger width of
100 nm and a finger spacing of 200 nm has been fabricated using e-beam lithography and nano-lift off processes. These IDA nanoelectrodes achieved a detection limit of around 6 x 10-9 M for reversible redox species. An interface application-specific-integrated-circuit
(ASIC) for IDA nanoelectrode was also design, fabricated and fully characterized.
This comprehensive work was very successful in tapping the potential of the nano-electrochemical sensor to a very good extent. But nevertheless it is imperative to understand that the charge induction method involves the charging and discharging of a capacitor which inherently suffers the problem of charge leakage. As the amount of charge available in this nano-sensor design is very minimal, any charge leakage will lead us to severely underestimate the detection limits of such a sensor. This necessitates the need to directly measure the low currents output from the sensor. In other words, an
12 Amperometric Characterization of the sensor is a must to harness the true potential of a
nano-sensor.
1.3 Research Motivation
The challenges in realizing a successful nano-IDA electrochemical sensor and in
designing circuits for converting currents as low as pico-amperes from the nano- electrochemical sensor reliably to voltages are the true motivation for this research.
Though the general circular-IDA pattern has already been realized by Xiaoshan Zhu [26],
research efforts were focused on experimenting with different parameters during the e-
beam lithography process to obtain a deeper understanding of the electron-beam
lithography process. Experiments were done with different doses of electron-beam and
an optimal recipe for the successful fabrication of the nano patterns is presented. A
comprehensive documentation of the nano-fabrication process is presented to enable any
new user to get a thorough feel for the process. For measuring the pico-ampere currents
from this fabricated sensor, a simple but efficient integrated circuit is designed and was
fabricated by MOSIS. The circuit can be integrated with the sensor and the signals from
the electrochemical reaction can be converted to measurable ranges of voltages which can be further digitized using an ADC and fed to a Digital Signal Processor for display.
13 1.4 Objective of this thesis
There are four important objectives for this thesis. Firstly, to successfully
fabricate a relatively large area ring-type nano-IDA electrode pattern in gold. Secondly, to use the fabricated nano-IDA as an electrochemical sensor for detecting redox species using Chronoamperometry and determine the lowest possible detection limit of the sensor.
Thirdly, to compare the performances of the nano-IDA with the reported values of the micro-IDA and analyze the differences. For lower concentrations, the nano-IDA is expected to generate currents in the pico-ampere range. Hence, the last objective is to design and fabricate a circuit to reliably convert pico-ampere currents to corresponding voltage levels, to enable further signal processing on the signals.
Chapter 2 explains in detail about the fabrication steps involved in realizing this sensor. The fabrication involves both nanofabrication and microfabrication steps. The choice of each step and the advantages of the chosen steps over the other methods available are thoroughly discussed. Chapter 3 dwells on the Circuit Design part of the thesis. It explains the challenges in designing a circuit for measuring smaller currents and the design proposed for the nano-sensor along with simulation results. Chapter 4 presents the characterization results obtained by performing the Chronoamperometry experiment on the sensor. This chapter will analyze the efficiency of the sensor in measuring low concentrations of the analyte. Chapter 5 presents the conclusions derived from this work and also the directions for the future work.
14 References
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17
Chapter 2
FABRICATION OF THE RING TYPE NANO-IDA
ELECTROCHEMICAL SENSOR
18 2.1 Introduction
The proposed sensor design involves both nano and micro patterns. The micro
patterns include the fabrication of Reference and Counter Electrodes and also the
electrical connection of all four electrodes to their respective bonding pads. The
Interdigitated Array of electrodes is fabricated using the nano fabrication. As nano-
fabrication has been utilized for fabricating numerous devices including electronic devices [1-4], nanofluidic devices [5-8] and nano electromechanical systems (NEMS) [9-
12], there has been considerable amount of innovative fabrication methods reported in the last decade. The nano fabrication methods available as of today can be broadly classified into four categories. First category consists of using Self-Assembled Monolayer (SAM)
[13]. Research in this area began in 1983 [14] and has seen an increasing number of published papers every year since then. In principle, molecule which is essentially an alkane chain, typically with 10-20 methylene units, is given a head group with a strong
preferential adsorption to the substrate used. Thiol (S-H) head groups and Au (111)
substrates have been shown to work excellently. The thiol molecules adsorb readily from
solution onto the gold, creating a dense monolayer with the tail group pointing outwards
from the surface. By using thiol molecules with different tail groups, the resulting
chemical surface functionality can be varied within wide limits. Alternatively, it is also
possible to chemically functionalize the tail groups by performing reactions after
assembly of the SAM [15]. Langmuir-Blodgett (LB) film [16, 17] also falls into this first category of nano-fabrication methods. Langmuir-Blodgett (LB) film is a set of monolayers, or layers of organic material one molecule thick, deposited on a solid substrate. An LB film can consist of a single layer or many, up to a depth of several
19 visible-light wavelengths. The term Langmuir-Blodgett comes from the names of a
research scientist and his assistant, Irving Langmuir and Katherine Blodgett, who discovered unique properties of thin films in the early 1900s. Langmuir's original work involved the transfer of monolayers from liquid to solid substrates. Several years later,
Blodgett expanded on Langmuir's research to include the deposition of multi-layer films on solid substrates. By transferring monolayers of organic material from a liquid to a
solid substrate, the structure of the film can be controlled at the molecular level. Such
films exhibit various electrochemical and photochemical properties. This has led some
researchers to pursue LB films as a possible structure for integrated circuits (ICs).
Ultimately, it might be possible to construct an LB-film memory chip in which each data
bit is represented by a single molecule. Complex switching networks might be fabricated
onto multilayer LB-films chips [18]. It can be clearly seen that these two methods involve
manipulation at the molecular level and they will be immense use in constructing
structures with dimensions of tens of nanometers. However, these two methods are at a
very primitive stage still and standardized methods of fabrication using these are not yet
available. The nano-electrochemical sensor’s Interdigitated Array electrodes should be
many in number so that the redox cycling occurs at many places, thereby amplifying the
output current. So, we can consider that the design of such a nano-sensor is going to be
relatively complex. Such structures may be very challenging to fabricate using SAM and
LB methods. Even if fabricated, the stability of such structures is not guaranteed. Hence,
these methods were ruled out for the fabrication of the sensor.
The second category was to use the naturally available nano-structured materials
like the Buckyball (C60) and the Carbon NanoTube (CNT) [19]. Though a lot of research
20 has been done in analyzing the properties of CNTs and manipulating and controlling the
growth of them on substrates, the research can still be considered to be at a rudimentary
stage. This prohibits the usage of CNTs for realizing our electrochemical sensor. The
third category is the Nano Imprint Lithography (NIL) [20, 21] which has evolved very
rapidly in the past five years and holds a lot of promise in the semiconductor industry for
the future. Stephen Y. Chou [22] is the inventor of Nano Imprint Lithography technique
and is considered as one of the emerging technologies that will change the future of the world. The first step involves creating a mold with the nano patterns on it. One method of preparing this mold is using Molecular Beam Epitaxy (MBE) wherein two different metal layers are grown one over the other repeatedly to form a sizeable sandwich. Then the edge of this sandwich is dipped in a developing solution that etches away one of the two metals selectively. This process will enable the development of a mold with teeth like patterns of nano dimensions. For more complex patterns, the MBE layers have to be grown appropriately. The second method of developing molds for Nano Imprinting is by using Electron Beam Lithography. On some wafer, PMMA is spin coated and the nano patterns are written on the PMMA layer using electron beam. The electron beam exposed
areas of the PMMA layer is then developed using a solution of Methyl Isobutyl Ketone
solution and the resulting pattern forms the PMMA mold. If a layer of metal is deposited
on this PMMA layer and lift off is done using Acetone, then the patterns can be obtained
in metal to serve as the mold. Clearly, this Electron Beam Lithography is a relatively
easier method of obtaining a mold than the MBE method. Once the mold is prepared, the
pattern is transferred to a thin resist spun on a wafer by just pressing the mold into it. The
selective removal of resist occurs because of this imprinting step instead of exposing the
21 resist to a light source as is done in Photolithography. Hence this method is called Nano
Imprint Lithography. This method is very clearly suited for batch productions as this
imprinting technique will have a very high throughput. All the time is usually invested to
create the mold. As the electrochemical sensor is primarily at its research stage, it is not
necessary to use Nano Imprint Lithography for the process.
The fourth category of nanofabrication methods is a part of the Nano Imprint
Lithography. The method of using Electron Beam Lithography for forming patterns on a
PMMA layer and then using metal deposition and lift-off for realizing the nano patterns
in metal is itself an established method of nanofabrication. This method is relatively
simple and very accurate for realizing complex patterns. And as this method helps
realizing patterns in metals at nano-scale, it plays an important role in making nano-
electrodes. Fortunately, the Clean Rooms of the University of Cincinnati have an
Electron Beam nano lithography system – the Raith 150 machine installed. This
definitely favors the choice of using this system for fabricating our nano-electrochemical
sensor.
22 2.2 Electron Beam Lithography using Raith 150 system
Electron Beam Lithography [23] involves the usage of an electron beam to attack
selected portions of PMMA and changing its property in those exposed regions. In the
exposed regions, the polymer chains of PMMA are broken and become soluble in Methyl
Isobutyl Ketone (MIBK) solution. This is analogous to using a positive photoresist and
exposing it to ultraviolet radiations to make it soluble in the corresponding developer
solutions, as is done in Photolithography. To venture down to nano-patterns instead of
micro-patterns, electron beam is used instead of ultraviolet radiations and PMMA is used instead of Photoresist as PMMA is sensitive to electron beam. There are many formulations of PMMA available depending on the molecular weight of the PMMA and the solvent added to the PMMA to modulate its viscosity. Microchem PMMA 495k with
Aerosol 6 (A6) solvent was chosen as the candidate for the sensor owing to its proven success. It can be visualized that the handling of electron beam has to be extremely precise for writing nano-patterns with it and hence the Raith 150 System that performs this action costs millions of dollars and requires efficient operations skills. Once the
PMMA layer is patterned, the patterns are realized in metal using a metal deposition and metal lift-off process, explained in greater detail later in this chapter.
23
Figure 2.1: The Raith 150 e-beam lithography machine at the University of Cincinnati
The figure 2.1 shows the Raith 150 System installed at the University of
Cincinnati. It consists of the Raith system and two computers for the user interface to control the system. One of the computers communicates with the Raith machine and controls the operations like loading the stage with wafer, unloading the wafer from the stage, controlling the power of the electron beam that is used to write on the PMMA layer, feeding the data about the nano-pattern to the electron beam controller and similar control operations. The second computer is the visual interface to the inbuilt Scanning Electron
Microscope (SEM) of the Raith System through which the user can see the electron beam gun and the wafer on the stage. This visual interface is also used for capturing images of the patterns on the wafer for presentation purposes. Most of the illustrative pictures in
24 this chapter are from this SEM and were captured using this second computer. The Raith
system houses all the precise equipments for generating and controlling the electron beam,
which forms the heart of the electron beam lithography.
The block diagram of the system shown in the figure 2.1 explains the different components of the Raith 150 System. There is a robotic arm in this system (not shown in figure) which holds the main stage. The user can instruct the system to use the robotic arm and bring this stage outside the main system, so that the user can place the wafer on the stage. Once this is done, the robotic arm gradually takes in the stage into the system and places it in the vicinity of the electron beam gun. The wafer and the electron beam gun are viewed using the second computer through the interface it provides. Using the first computer, the strength of the electron beam in terms of Kilo Volts (kV) of Voltage to
be applied on the electron beam, in terms of the aperture size that lets the beam out of the
gun onto the wafer and in terms of the working distance which is the distance from the tip
of the electron gun to the wafer surface can be controlled by the user. Once all these
preliminary setup are done, the nano-pattern file in the GDSII format is fed to the second computer and is in turn sent to the Raith System’s memory unit. Then, a Write Field alignment step is done, the details of which are explained later in this chapter. The exact spot on the wafer at which the patterns are to be written is brought underneath the electron gun using the stage control jockey. The stage control consists of laser- interferometric positioning system that has a resolution of 5 nm along with combinations
of servomotors and piezoelectric actuators. After all these steps, the Raith System is
instructed to write the patterns on the wafer.
25 An extremely high negative voltage is applied to the filament in the electron gun
which makes the filament eject out electrons. This ocean of electrons is focused into a
fine beam of infinitesimally small diameter using an Electron Blanking Control which
applies selective positive voltage all around the electron beam compressing the beam to a
fine narrow stream of electrons. This fine beam of electrons gets out of the electron gun
through a small aperture, the size of which varies from 30 microns to around 120 microns and is set by the user during the preliminary setup. The electron beam that gets out of this
aperture hits the wafer that is kept right below this electron gun (Figure 2.2). The distance
from the aperture to the electron wafer (called the Working Distance) is adjusted by the
user in such a way that the focal point of this electron beam coincides exactly with the
wafer surface. By this adjustment, the patterns written on the wafer can also be made to
be as precise, without exposing more PMMA than the beam is supposed to. To write the
patterns on the wafer, the electron beam need to be steered like the pen tip while writing
on a paper. This is done by the Electron Deflection Control shown in the block diagram
Figure 2.2: The electron gun, wafer and stage inside the Raith 150
26 of the Figure 2.1. It has both X-direction and Y-direction deflection plates for a 2-
dimensional steering of the electron beam on the wafer. The high-precision pattern
generator of the Raith Machine efficiently controls this electron beam steering to exactly
reproduce the pattern data fed to the Raith onto the wafer. This control is the main
functionality of the system. This process can be considered analogous to holding a pen at
one spot and writing on the paper below it. The maximum area that can be written on the
paper is limited to the maximum deflection of the pen provided by the hand holding it.
Similarly, there is a limitation to the deflection provided by the Electron Deflection
Control. For more accuracy, the stage is not moved when the electron beam write is in progress. As a result, for a given position of the wafer stage, there is a limited area of
wafer that the electron beam can reach and write. This area is defined as a ‘Write Field’.
For patterns extending for more than one Write Field (WF), the patterns in one WF is
written first and then the stage control moves the wafer so that the next WF comes under
the electron gun and the patterns can be written in that WF. This method relies heavily on
the capability of the stage control to precisely move the wafer by one write field. Any
error in this motion will lead to discontinuities between the write fields and as a result the
overall pattern may be highly disjoint and split. The Figure 2.3 shows a cartoon of the
discontinuities in a nano-pattern if the write fields are not accurately aligned next to each
27
Figure 2.3: Illustration of a complete nano-pattern as a group of misaligned write fields
other. It also shows the ways in which a nano-pattern is broken down to several write fields and how the electron gun writes one write field at a given time.
Because of this undesirable effect of mismatch in write fields, the Write Field
Alignment is a very important preliminary step that needs to be done. It involves teaching the stage control to move the wafer precisely by one write field so that there is no error while stitching the fields together to form the complete pattern.
The Electron Deflection Control can be made to deflect more thereby increasing the size of the write fields. But larger the write fields, the coarser are the patterns written.
Usually, a write field of size 100 microns by 100 microns is preferred.
28 2.3 Design of the ring type nano-IDA pattern
A cartoon of a circular IDA pattern with just 3 electrode fingers per electrode is shown below for a clear understanding of the circular nano-IDA design.
Figure 2.4: Design of a circular IDA electrode
The redox cycling that happens between the IDA electrodes causes an
amplification of the current. To achieve currents in measurable levels, more redox
cycling should happen and this demands a considerable number of electrodes. This
number is chosen to be 275 per electrode. The smallest radius of the circular IDA is
chosen to be 30 microns and the largest radius is 250 microns, thereby including all 550
electrode fingers within this area.
29 Raith System comes with a Software interface, which in addition to letting the user do the preliminary steps before writing the patterns, also lets the user create the nano patterns with an editor analogous to AutoCAD. But this editor is not as flexible and user- friendly as AutoCAD and hence, the nano-IDA pattern was created using AutoCAD and then imported into the Raith software. The figure 2.5 shows the circular IDA patterns in the AutoCAD editor. The Interdigitated electrode fingers can be seen. The two long lines coming out of the dense circular region are the main working electrodes 1 and 2.
Figure 2.5: AutoCAD image of the nano-pattern
This design is converted from the AutoCAD format to GDSII format and is imported into the Raith software, which is shown in the figure 2.6.
30
Figure 2.6: GDSII image of the nano-pattern on the Raith software
From figure 2.6, it is clear that the patterns look skewed and disarranged in the
Raith software. Upon investigation, it was clear that in the Raith software, a circle has been defined as a 48 sided polygon unlike the AutoCAD editor where the circles are smooth. This mismatch in the definition of circular geometries skews the pattern as shown in the figure when the nano patterns are imported into the Raith software from
AutoCAD. As there was no way for the user to manipulate the definitions of geometries in the Raith software, the entire pattern was redrawn with 48 sided polygons instead of circles and this problem was circumvented. A 48 sided polygon can be considered as a circle for all practical purposes as far as the nano-sensor is concerned.
Every electrode finger has a fixed width and a fixed spacing from the adjacent electrode fingers of the other working electrode. The choice of these dimensions is dependant upon the electron beam parameters and are discussed in the next section.
31 2.4 Nanofabrication of the ring type nano-IDA electrodes
The IDA should be fabricated on an insulating layer to avoid conduction through
the substrate. Hence, on a 2 inch Silicon wafer, 100 nm of Silicon Dioxide is grown as
the insulating layer and the electrodes are fabricated atop this SiO2 layer. Using the Raith,
the patterns can be written as a set of dots or set of lines or set of areas. Once any of these three options are chosen, the dose needed for it should also be finalized. For our design, we chose to draw the patterns using lines. When the electron writes a line on the PMMA layer, the line has a width depending on the strength of the electron beam. As the beam gets stronger, it tends to penetrate more into the neighborhood of the line it is exposing, ending up with a line pattern that has more width than it is supposed to have. This
strength is measured in µC / cm2 as the dose given by the electron beam. To know how
closely these lines can be drawn and to know the requisite dose to expose the PMMA
layer to its full thickness, a dose matrix was created. The dose matrix has five columns.
The first column has lines with a spacing of 100 nm between them. The second column
has lines with spacing or 200 nm between them. Similarly, the third, fourth and fifth
columns have 300 nm, 400 nm and 500 nm respectively. The dose matrix also had 15
rows. The first row was given a dose of 100 µC / cm2 and each consecutive row was given doses in steps of 100 such that the 15th row had a dose of 1500 µC / cm2. With these said conditions of line spacing and doses, the e-beam write process was done. Then the wafers were developed in Methyl Isobutyl Ketone (MIBK) solution to remove the exposed PMMA.
32 The first five rows corresponding to doses of 100 µC / cm2 to 500 µC / cm2 were underexposed for all line spacing and the PMMA was not removed completely. In the columns corresponding to line spacing of 100 nm and 200 nm, in the remaining rows that had doses from 600 µC / cm2 to 1500 µC / cm2, the patterns were completely washed out
due to overexposure. This suggested that the line spacing of 100 nm and 200 nm could
not be realized for our design. For the third column with line spacing of 300 nm, the
results were good for a dose range of 600 µC / cm2 to 1200 µC / cm2 but overexposure
resulted beyond this range. The fourth and fifth columns corresponding to the line
spacing of 400 nm and 500 nm respectively had good patterns for doses from 600 µC /
cm2 to 1500 µC / cm2. This study ruled out the usage of doses less than 500 µC / cm2 for any line spacing. The study also suggested that line spacing of 100 nm and 200 nm were totally impossible to realize and suggested that the line spacing of 400 nm and 500 nm were very successful. Line spacing of 300 nm was on the border line and it is the closest the lines can be. The closer the fingers of the Interdigitated Array of electrodes are the better is the redox cycling and higher is the current output from the sensor. So, the line spacing of 300 nm is chosen. As the IDA pattern is going to be very dense, a dose of higher value is to be chosen as the complete removal of PMMA upon exposure must be guaranteed.
Another important aspect of e-beam lithography is the proximity effect of the electron beam. The electrons of the electron beam are accelerated at a very high voltage that upon contact with the PMMA layer, they tend to scatter to the neighborhood of their
actual point of contact. This increases the width of the line written by the electron beam, and makes it typically larger than the width of the electron beam itself. The smallest
33 width with which an electrode finger can be fabricated using the Raith 150 system is 10
nm. As the nano-IDA pattern is relatively large, care has to be taken to avoid any kind of discontinuities in the electrode finger. Hence, a finger width of 100nm, which is 10 times the minimum width possible, is chosen as it is a safe option. So, the choice of dose in this research work is made in such a way that 50 nm on either side of the written line are exposed due to the proximity effect. Hence, writing lines with 300 nm spacing results in a
‘line width’ of 100nm and the ‘line spacing’ reduces to 200nm.
To verify the removal of PMMA upon exposure and choose a dose value, the
metal deposition was done on PMMA with just gold and the metal lift off was done using
acetone. As the electron beam penetrates through the PMMA layer, it breaks the polymer
chains and changes the properties of PMMA. The deeper the electron beam penetrates,
the more it scatters. Owing to this scattering, the amount of PMMA exposed increases as
the depth of penetration of the electron beam increases. Hence, when the PMMA layer is
developed with MIBK solution after the e-beam lithography, the cross-section of the
patterns in the PMMA layer have an inverted ‘V’ shape as seen in the figure 2.7.
Figure 2.7: Nano-patterns with undercut on the PMMA layer
34 This ‘undercut’ cross section is the key factor that aids in the metal lift-off process.
Now the metal Gold is evaporated on this PMMA. The figure 2.8 illustrates the metal lift off process. When this wafer is dipped in acetone, it dissolves the PMMA and the metal on top of the PMMA is also removed with the PMMA. This is the metal lift-off process.
The points of attack of Acetone that favors the lift-off process are indicated in the figure
2.8. As a rule, the thickness of the gold layer is made to be at most half of the thickness of the PMMA layer for a successful lift off process at nano-dimensions. In this case, the
PMMA layer is 300 nm thick and the Gold layer is 100 nm thick.
Figure 2.8: Metal lift-off process after e-beam write
Gold was the chosen metal owing to its wonderful properties as an electrode.
Gold is deposited on the PMMA layer through an evaporation process. Once the metal
lift off is done, the wafer looks as shown in figure 2.9.
35
Figure 2.9: Gold nanoelectrodes after the metal lift-off process
The quality of these gold lines on the wafer is the key to choose the required dose for the e-beam lithography. As discussed before, a line spacing of 300 nm is chosen and the dose is expected to be at the higher end of the available range to produce the desired
‘line width’ of 100nm. The following figures show the gold lines for doses of 900 µC / cm2, 1000 µC / cm2 and 1200 µC / cm2 respectively.
36
Figure 2.10: Quality of gold lines for a dose of 900 µC / cm2
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
In all the three cases shown above, the line width is approximately 100 nm and
the line spacing is 200 nm. It can be seen that for a dose of 900 µC / cm2, the gold
deposition is less uniform and is visible as chunks of gold. This texture is not preferred
for the complex circular IDA pattern as it might lead to discontinuities in the electrodes.
The figures corresponding to the doses 1000 µC / cm2 and 1200 µC / cm2 show that the
gold lines become more uniform with increasing dose. At a dose of 1400 µC / cm2 (data not shown), the line width of the gold lines drop from 100 nm to 80 nm. This suggests an onset of overexposure of the PMMA layer at some dose between 1200 µC / cm2 and 1400
µC / cm2. Therefore, to have a good and uniform quality of gold lines and to avoid any kind of overexposure that leads to unpredictable line widths, a dose level of 1200 µC / cm2 was chosen as the final candidate.
38 The nano-IDA pattern is relatively complex and expands over a large area. Hence, to reduce the e-beam write time for such a pattern, a low electron beam acceleration voltage of 10 KeV is chosen. Further, as the pattern can be considered intricate for nano- dimensions, a thinner electron beam is preferred. Hence, a low electron beam current value of 226 pA was chosen. All other parameters were chosen to be the default values as they are not critical.
The following table states the Raith parameters used to write this nano-pattern on the PMMA layer of the wafer.
Table 2.1: E-beam exposure parameters
Electron beam current 226 pA
Write field size 100 µm by 100 µm
Accelerating voltage 10 KeV
Line step size 2 nm
Line dwell time 1.057 µs
Electron gun aperture size 30 µm
Electron dose 1200 µC / cm2
The figure 2.13 shows the complete written pattern on the PMMA layer of the wafer. The following observations were made upon examination. The PMMA was underexposed in certain parts of the pattern. These underexposed regions looked like the symbol “X” in the unmagnified image of the sensor as is seen in the figure 2.13.
39
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’
The following two figures show the magnified images of the sensor that give a better view on the underexposed regions.
40
Figure 2.14: 5.48k times magnified image of the underexposed ‘X’ region
Figure 2.15: 20.67k times magnified image of the underexposed ‘X’ region
41 The reasons for these underexposed patterns are not obvious. The same dose of
electron beam is applied for the entire pattern and so the formation of such underexposed
pattern seems very strange. The reasons for this pattern can be explained by looking at
the 48 sided polygons in the AutoCAD editor, shown in the figure below.
Figure 2.16: Straight and jagged lines of the nano-pattern in the AutoCAD file
In fact, the AutoCAD editor also shows the “X” pattern while displaying the pattern on the screen. The sides of the 48-sided polygon which correspond to the “X” pattern happen to be perfect straight lines unlike the other sides which are jagged. While exposing a jagged line with the electron beam, the beam dwells for a longer time to expose such a line than to expose a line that is perfectly straight. This additional dwell time succeeds in exposing the entire thickness of the PMMA layer and hence the patterns are clear and well-defined. Whereas, in the regions of straight lines, the lesser dwell time
42 leads to underexposed patterns. To circumvent this problem, the exposure was repeated
with higher dose and was found that this problem disappears at a dose level of 1250 µC /
cm2.
Before depositing gold by evaporation on the developed PMMA later, 100 Å of
titanium is deposited for promoting the adhesion of gold. On top of this, 1000 Å of gold
was deposited. A magnified image of the fabricated IDA electrode is shown below.
Figure 2.17: Magnified image of the fabricated nano-IDA electrodes
Once the nano-IDA is fabricated, the reference and the counter electrodes have to be fabricated along with the bonding pads for all these two electrodes and the two working electrodes. These patterns can be efficiently fabricated using photolithography, the various steps of which are explained below.
43
First the Shipley 1818 positive photoresist is spun on the entire wafer and pre- baked for 20 seconds at 90oC. The photoresist was spread for 10 seconds and spun for 40 seconds, thereby covering the wafer with a near uniform thickness of 1.8 µm. A mask is designed to pattern the photoresist for connecting the working electrodes to the bonding pads as well as the reference and counter electrodes. An image of the mask is shown in the figure 2.18. Using this mask, the wafer was exposed to UV light (~ 350 nm) for 12 seconds. The wafer is then developed using the Microposit 351 developer solution which
Figure 2.18: Mask used for photolithography after creating nano-IDA
44 removes the exposed photoresist. A post-bake for 30 seconds at 120oC is also done. A cartoon of the cross-section of the wafer after photolithography is shown in the figure
2.19.
Figure 2.19: Photolithography for the reference and working electrodes
A snap shot of the wafer at this stage of fabrication is shown in the figure 2.20 below.
Figure 2.20: Snapshot of the aligned photoresist patterns for reference and counter
electrodes with the nanoelectrodes
45 100 Å of titanium followed by 1000 Å of gold are again evaporated on this, as shown in the figure 2.21. The metal lift-off was done using acetone which dissolves and removes the Shipley S1818 photoresist.
Figure 2.21: Gold deposition for the reference and working electrodes
Finally, Ag and AgCl are sequentially electroplated on one Ti/Au electrode using
Silver Cy-less solution (Technic, Inc) and 0.1 M KCl solution to form the reference electrode, as shown below.
Figure 2.22: Electroplating of Ag/AgCl reference electrode
46 2.5 Conclusion
Thus the nano-IDA electrochemical sensor was successfully fabricated. A snap shot of the fabricated sensor is presented in the figure 2.23.
Figure 2.23: Snap-shot of the complete nano-electrochemical sensor
Using P-Aminophenol (PAP), a redox species, Chronoamperometry was performed on
this sensor. The details of this process and the results are explained in the Chapter 4.
47 References
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48 [13] H. Zhang and N. Li, "The direct electrochemistry of myoglobin at a dl-homocysteine self-assembled gold electrode," Bioelectrochem. Bioenerget., vol. 53, pp. 97-101, January 1, 2001. 2001.
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49
Chapter 3
CIRCUIT DESIGN FOR CURRENT-TO-VOLTAGE
CONVERSION OF
PICO-AMPERE
50 3.1 Introduction
In an electrochemical sensor, a current is output and this current can be measured
against time (Chronoamperometry) or against voltage applied on the working electrodes
(Voltammetry). In this research work, one of the working electrodes is maintained at the
oxidation potential and the other is maintained at the reduction potential. The steady
current that is output at such a condition is directly proportional to the concentration of
the redox species. This current is measured against time (Chronoamperometry) in this
experiment. This current is expected to be in pico-amperes and can be measured using a
pico-ammeter. But for further signal processing, the current should be reliably converted
to a corresponding voltage level. The figure below shows a simple circuit that is
generally used for this current-voltage conversion.
Figure 3.1: A common current-to-voltage converter
A high-input impedance Operational Amplifier (OPA) is chosen and one of the input terminals is applied the sweeping voltage V (WE) of the working electrode. As the
voltage difference between the two inputs of the OPA is considered to be infinitesimally
51 small, the other input terminal follows V (WE) and this terminal is connected to the working electrode. The current ‘i’is generated in the working electrode owing to the
redox cycling and this current flows in, as shown in the figure 3.1. The input impedance
of an OPAMP is infinitely high and hence the current cannot enter the OPAMP. Instead it
flows through the resistor R and the voltage V (= iR) develops at the output of the
OPAMP which is measured. Thus the working electrode is applied a sweeping voltage of
desired range and the current produced is instantly converted to voltage through this
circuit.
High input impedance OPAMP typically contains a MOSFET pair as the input
differential pair and the MOS capacitance of the Field-Effect Transistor provides the high
impedance. Therefore, the current from the working electrode is expected to be many
orders of magnitude higher than this MOS capacitance’s leakage current and the leakage
current of the resistor R. If the current being measured is very small and comparable to
the leakage currents of the MOS capacitor and the resistor, then the current signal is lost
in the processing and the voltage V measured at the output of the OPAMP will not
correspond to the current being generated through the electrochemical reaction. This
condition poses a lower limit for the current value that can be measured using such
circuits using MOS transistors and resistors.
If it is assumed that the inter electrode spacing is as small as 10 nm and the
diffusion coefficient is in the typical range of 4 x 10-6 cm2/s to 7 x 10-6 cm2/s, say the
value chosen is 6 x 10-6 cm2/s, and say two electrons are transferred in the electrochemical reaction, then the total amount of charge transferred by a single molecule
in one second would be Q = 2ef = 2 x 1.6 x 10-19 x 3 x 106 C = 10-12 C. For such a case,
52 the reaction current would be 1 pA or lower [1]. If the instrumentation circuit discussed
above has to be used for measuring currents as low as a pico-ampere, then the leakage
currents of the Resistor and the MOSFET gate capacitor must me orders of magnitude
lower than a pico-ampere. But typically, the leakage currents of Resistors and MOS
capacitances are in the order of pico-amperes and hence these are not reliable candidates
for measuring the low currents.
3.2 Design of an I-V converter circuit for pico-ampere
Circuits dealing with pico-amperes are not new to the Electrical Engineering
community and substantial research work has been in progress since 1971 [2, 3]. The
goal in those days was to design a pico-ampere current generator. Around 1985, the main
aim was to measure currents with a resolution at the pico-ampere range, for the purpose
of measuring leakage currents in Integrated Circuits [4]. Many groups have built
successful circuits for the aforementioned functionalities. This increased precision in
current generation and current measurement circuits have encouraged production of
devices that produced signals at low current levels. This has been the main reason for the progression of the dimensions of the amperometric sensors from the macro to the micro level and now to the nano level. With this progressive reduction in dimensions for
sensors like the electrochemical sensors, there is a corresponding reduction in the magnitude of current being generated. This has fueled research for designing circuits that can measure pico-ampere currents [5-10]. Though these circuits provided good functionality, the designs were complex and did not suit perfectly the needs of an electrochemical sensor. Hence, a simple circuit has been designed for converting reliably
53 the low sensor current to voltage. Once this conversion is done, the voltage can be handled by signal processing circuits. The figure 3.2 shows the circuit designed.
Figure 3.2: Current-to-voltage converter for pico-ampere currents
It is assumed that the working electrode connected to this circuit is at the oxidation potential and the other working electrode is at the reduction potential of the redox species of interest. From the figure, it can be seen that the node connected to the oxidizing electrode has a potential VX. To adjust the potential of this working electrode to the oxidizing potential, the voltage of the reference electrode VREF is adjusted in such a
54 way that Vox = VX – VREF. Similarly, the potential of the reducing electrode, VY is
chosen such that Vred = VY – VREF.
The W/Ls of the different transistors are given below.
Table 3.1: Transistor sizes of the current-to-voltage circuit
Transistor name W/L
M1 4 µ/110.4 µ
M2 4 µ/40 µ
M3 4 µ/40 µ
M4 4 µ/40 µ
M5 4 µ/40 µ
M6 4 µ/40 µ
M7 4 µ/40 µ
M8 4 µ/40 µ
M9 4 µ/119.2 µ
The transistor M9 is the current source, which produces the bias current, Ibias for the entire circuit. This bias current is mirrored by the three stage current mirror formed by the transistors M3, M4, M5, M6, M7, and M8. The reason for three stages in a current mirror is explained later. This mirrored bias current appearing at the node, labeled as VX, then flows through the branch that contains the transistors M2 and M1. This is the sensing branch. This bias current maintains the node Vout of the sensing branch at a
reference voltage, say Vreference. The branch carrying the sensor current, Isensor is also
connected at the node VX as explained before. The pico-ampere current flowing out of
55 the sensor is expected to flow through the sensing branch containing M1 and M2 and
thereby produce a linear variation at the Vout node and thereby convert the current to
voltage.
As the pico-ampere is expected to create a variation at the node Vout, the bias current in the circuit must be comparable to Isensor. Hence the bias current sourcing
transistor M9 is long, to produce a current of approximately 5 nA. When this current is
mirrored and enters the sensing branch, the Vreference is at 2.86 V. Both M1 and M2 are
always in the linear region of operation. The mirrored bias current from the transistor M4
will enter the sensing branch preferentially over the nano-sensor branch because the
output impedance of the nano-sensor is very high. Now when the redox cycling drives the
pico-ampere current, Isensor, this current can flow into the sensing branch and the current
mirror branch made of M4, M6 and M8. As all the transistors in the current mirror are in
saturation region of operation and are identical, they have an ON resistance of ro. As the current mirror here is made of three stages, the impedance seen by Isensor looking into the
3 current mirror branch is roughly ro . This is many orders of magnitude higher than the
impedance offered by the linearly operating transistors M1 and M2. Hence, almost the
entire current flows into the sensing branch. This also explains the need for three stages
of transistors for the current mirror.
As Isensor varies from 1 pA to 100 pA, the Vout varies from Vreference (2.86V) to
-1.36 V. This variation of 4.22V over 100pA corresponds to a sensitivity of
approximately 40mV/pA. To measure the variation of Vout with reference to Vreference and to nullify the errors due to systematic faults during fabrication, the circuit is
56 replicated twice, with just one of them connected to the sensor. This is shown in the figure below.
Figure 3.3: I-V converter repeated twice to nullify systematic faults in fabrication
The designed circuit is laid out using Magic Layout editor using AMI 1.5 micron technology. The corresponding layout is shown below in figure 3.4.
57
Figure 3.4: Layout of the designed I-V converter for pico-ampere
The plot of Vout and Vreference (2.86V) is shown below in figure 3.5. The X axis shows the current Isensor from 0 to 100pA.
Figure 3.5: Simulation results of the designed I-V converter
58 The most important concern is fabricating a circuit for handling pico-ampere
currents are the electrostatic protection circuits in the pad-frames of the layout. These circuits typically run on bias currents many folds higher than pico-amperes and hence it is
prohibitive to input the pico-ampere currents from the nano-electrochemical sensor
through such an I/O pad. For this reason, only a layer of metal-1 was used as the input
pad for the current signal from the sensor, as is shown in figure 3.6. This demands very
careful handling of the fabricated chip to avoid any accidental electrostatic discharge
through this pad.
Figure 3.6: Modified pad-frame layout for the current output from nano-sensor
59 3.3 Implementation
The designed circuit was fabricated by MOSIS. A snapshot of the complete
circuit on the chip is shown below in figure 3.7. It can easily be compared with the layout
shown in figure 3.6.
Figure 3.7: Snap shot of the fabricated I-V converter circuit for pico-ampere
Noise plays a very important role in interfering with measurements involving pico-ampere currents. Hence, the voltage output of the I-V converter was recorded for three different runs to observe the variation range in the value of the voltage measured.
The following table 3.2 shows the measured voltage values corresponding to pico-ampere current inputs. Such low currents were sourced using Keithley 220 Programmable
Current Source which was capable of sourcing currents as low as 1 pA.
60 Table 3.2: Current input and the voltage output of the designed I-V converter
Input current from current source Output voltage measured
Run 1 Run-2 Run-3
0 pA 4.5012 V 4.4013 V 4.4017 V
1 pA 4.3347 V 4.3712 V 4.2247 V
2 pA 4.2740 V 4.2414 V 4.1734 V
10 pA 3.9724 V 3.8742 V 3.8442 V
20 pA 3.5444 V 3.4749 V 3.6747 V
30 pA 3.2247 V 3.3442 V 3.1424 V
40 pA 3.0012 V 2.9432 V 3.0002 V
50 pA 2.8474 V 2.7994 V 2.8012 V
60 pA 2.5943 V 2.5992 V 2.6400 V
70 pA 2.5624 V 2.5424 V 2.5547 V
80 pA 2.5249 V 2.5113 V 2.5224 V
90 pA 2.2100 V 2.2512 V 2.1100 V
100 pA 1.9972 V 1.9240 V 1.9643 V
200 pA 1.8942 V 1.7442 V 1.7012 V
500 pA 1.7934 V 1.6437 V 1.6444 V
1000 pA 1.7642 V 1.6324 V 1.6442 V
To understand the degree of variation between the different runs and to observe the trend in the values, the mean and standard deviation of the values are calculated and tabulated in the following table 3.3.
61 Table 3.3: Current input and the mean and standard deviation of the voltage
output of the designed I-V converter
Input current from current source Output voltage measured
Mean Standard deviation
0 pA 4.4347 V 0.0576
1 pA 4.3102 V 0.0763
2 pA 4.2296 V 0.0513
10 pA 3.8969 V 0.0671
20 pA 3.5647 V 0.1014
30 pA 3.2371 V 0.1015
40 pA 2.9815 V 0.0332
50 pA 2.8160 V 0.0272
60 pA 2.6112 V 0.0251
70 pA 2.5532 V 0.0101
80 pA 2.5195 V 0.0072
90 pA 2.1904 V 0.0726
100 pA 1.9618 V 0.0367
200 pA 1.7799 V 0.1013
500 pA 1.6938 V 0.0862
1000 pA 1.6803 V 0.0729
62 From the table, it is clear that for currents above 100 pA, there is no appreciable
change in the output voltage measured, suggesting a saturation in the circuit. A graph is plotted for the current input from 0 to 100 pA in the figure 3.8.
Picoampere current input Vs Voltage output
5.0000
4.5000
4.0000
3.5000
3.0000
2.5000
2.0000
Voltage output /V 1.5000
1.0000
0.5000
0.0000 0 20 40 60 80 100 120 Picoampere current /pA
Figure 3.8: Plot of the current input Vs voltage output of the I-V converter
The plot does not suggest a very strong variation in the voltage levels corresponding to the input values of the current. The visible distortion in the values of the voltage output can be attributed to the noise present in the experimental surroundings.
The circuits fabricated had no noise cancellation circuits and hence, are susceptible to their interference. But it is clear that there is a progressive variation in the voltage levels in response to the current input. The mean value of the voltage at no current input (0 pA) is 4.4347 V where it is expected to be 2.86 V from the simulations. Though the
63 simulations suggest a sensitivity of 40 mV/pA in the current input range of 1pA - 100pA, it is found that in real time, the sensitivity is approximately 24 mV/pA. These results suggest that this circuit is just a first step in processing pico-ampere currents and the circuit design in this field demands many modifications before it becomes part of a commercial nano-electrochemical sensor. This circuit was not integrated with the fabricated nano-IDA electrochemical sensor as the circuit does not ensure a robust conversion of pico-ampere currents to corresponding voltage levels. Hence, the voltage output measured in response to the current signals from the sensor may not be accurate.
However, the sensor was characterized by measuring its pico-ampere current output using a very sensitive pico-ammeter. The details of this amperometric characterization of the fabricated nano-sensor are explained in detail in Chapter 4.
3.4 Conclusion
Thus, the pico-ampere current from the nano-IDA electrochemical sensor can be reliably converted to voltage using this circuit. It can be used to interface the nano-IDA electrochemical sensor to the signal processing circuits. This circuit is just a first step in designing circuits processing pico-ampere currents. The circuit design in this area demands many modifications before it can be a part of commercial electrochemical sensors.
64 References
[1] Xiaoshan Zhu, "Nano electrochemical sensor and its measurement electronics with a dynamic transduction mechanism," 2004.
[2] SHAPIRO EG, "MOSFET current- to- frequency converter with a linear sub picoampere- to- microampere range," IEEE Trans. Nucl. Sci., vol. NS-18, pp. 155-159, 1971.
[3] L. Grillo, P. F. Manfredi and R. Marchesini, "High-performance current generator with one-picoampere resolution," Nucl. Instrum. Methods, vol. 127, pp. 435-440, 1975.
[4] S. D. Malaviya, "Picoampere current measurements by on-chip amplifier," IBM Technical Disclosure Bulletin, vol. 27, pp. 4707-4709, 1985.
[5] B. Linares-Barranco, T. Serrano-Gotarredona, R. Serrano-Gotarredona and C. Serrano-Gotarredona, "Current Mode Techniques for Sub-pico-Ampere Circuit Design," Analog Integr. Cir. Signal Proc., vol. 38, pp. 103-119, February 2004 - March 2004. 2004.
[6] B. Linares-Barranco, T. Serrano-Gotarredona, R. Serrano-Gotarredona and L. A. Camun?as, "On leakage current temperature characterization using sub-pico-ampere circuit techniques," in 2004,
[7] K. Bult and G. J. G. M. Geelen, "An inherently linear and compact MOST-only current division technique," IEEE J Solid State Circuits, vol. 27, pp. 1730-1735, 1992.
[8] T. Reimann, F. Krummenacher, B. Willing, P. Muralt and M. Declercq, "CMOS readout circuit for pico-ampere thin film pyroelectric array detectors," in 2000, pp. 395- 398.
[9] H. Kawaguchi, K. Nose and T. Sakurai, "CMOS scheme for 0.5 V supply voltage with pico-ampere standby current," in 1998, pp. 192-193, 436.
[10] M. Breten, T. Lehmann and E. Bruun, "Integrating data converters for picoampere currents from electrochemical transducers," in 2000,
65
Chapter 4
AMPEROMETRIC CHARACTERIZATION OF THE
NANO-IDA ELECTROCHEMCIAL SENSOR
66 4.1 Introduction
There are many redox species that can be used for the Chronoamperometry
experiment with the nano-IDA. My research group at the University of Cincinnati has
been working extensively on research in Electrochemical Immunoassay [1] and has
gained a good expertise in handling the redox species P-Aminophenol (PAP). Hence,
PAP was chosen to be the redox species of interest for this experiment. The redox
reaction of PAP is explained in the figure below.
Figure 4.1: Redox reaction of P - Aminophenol
As shown in the figure, 2 electrons are transferred per redox cycling. PAP gets oxidized to PQI which in turn gets reduced to PAP. The current at the oxidizing and reducing electrodes are the cathodic and anodic currents respectively. Different concentrations of PAP, ranging from 10-3 M to 10-12 M, were prepared by dissolving
calculated quantities of PAP in Phosphate Buffer Solution (PBS). The reversibility of
PAP/PQI, which is calculated as the ratio of cathodic current to anodic current, is most
efficient at a PH of 7.5. Hence a PBS solution is chosen such that its PH is closest to 7.5.
67 PAP is very sensitive to air and light and hence, the entire experiment was done in a dark
N2 environment. The oxidation and reduction potentials of PAP are 350mV and -350mV
respectively.
4.2 Instrumentation
The working, reference and counter electrodes are connected as shown in the
figure 4.2.
Figure 4.2: Instrumentation for amperometric characterization of the sensor
A pico-ammeter (Keithley 6487 Picoammeter Voltage Source) is used to record the current output from the sensor. It is connected to either the oxidizing electrode to
measure the cathodic current or to reducing electrode to measure the anodic current.
68 A discussion on the steady state currents of IDA electrodes must include the works of Sanderson and Anderson [2] and many others [3, 4] who have devised empirical
formulae for predicting the steady state currents in twin interdigitated electrodes way
back in 1985. Their predictions are not accurate but they have been used as a yardstick
for designing linear IDA electrochemical sensors. There are no known formulae for
calculating the current in circular IDA as fabricated in this research but still, the same
equation can be used as an approximation. According to the equation, the limiting steady
state current is given by
lim bulk I = n . F . A . DR . C R. / (k W)
bulk where n is the number of electrons in the redox reaction, F is the Faraday constant, C R. is the concentration of the reduced species in bulk solution, A is the electrode area, DR is
the diffusion coefficient of the reduced species, k is a dimensionless constant dependant
only on the geometry of the electrodes, and W is the spacing between electrode fingers.
Since this calculation involves the parameters of reduced species, the current calculated using this equation will be the cathodic current. If the parameters of reduced species are replaced by the parameters for oxidized species, the anodic current can be calculated. For
the geometry of the sensor fabricated and for the redox species chosen, the values of the
-6 2 4 -5 2 variables are: n = 2, DR = 10 x 10 cm /s, F = 9.6 x 10 C/mol, A = 8 x 10 cm , W =
200nm, k = 0.5 (k is in the range of 0~1). The equation suggests that the limiting current
is directly proportional to the concentration of the redox species. This would mean that a
variation in the concentration over nine orders of magnitude from 10-3M to 10-12M would
also produce nine orders of magnitude change in the current generated. But this does not
69 happen in real time due to various interferences in the form of light, electrical and other noise, impurity in the solutions, inaccuracy in concentrations etc.
4.3 Chronoamperometry experimental results
The following table shows the current measured from the sensor for the various concentrations.
Table 4.1: Current measured from the sensor for various concentrations of PAP
Steady current output from sensor Concentration of PAP Run-1 Run-2 Run-3
10-3 M 271.473 nA 270.043 nA 270.012 nA
10-4 M 140.432 nA 140.543 nA 140.022 nA
10-5 M 64.322 nA 63.983 nA 64.004 nA
10-6 M 11.933 nA 12.325 nA 12.321 nA
10-7 M 1.432 nA 1.472 nA 1.436 nA
10-8 M 247.532 pA 247.329 pA 247.487 pA
10-9 M 57.924 pA 57.532 pA 57.542 pA
10-10 M 14.543 pA 14.546 pA 14.535 pA
10-11 M 1.257 pA 1.259 pA 1.273 pA
10-12 M -- too low to be measured --
70 The current measured from the sensor stabilizes after approximately one minute
after the prepared concentration of PAP is pipetted on to the nano-sensor. The entire
reaction was carried out in an absolutely dark environment as the PAP species are very
reactive in the presence of light and get transformed to other species that do not undergo redox cycling. The pico-ammeter used has an inbuilt noise canceling circuit to filter out the signal from the noise. Hence, the current value measured was extremely stable after the one minute settling time and approximately the same current values were recorded for all three runs of this experiment, as can be seen from table 4.1.
The following table 4.2 shows the mean and standard deviation of the currents measured during the three runs of the experiment for every concentration of PAP. The
same is plotted in figure 4.3.
71 Table 4.2: The mean and standard deviations of the currents measured from the sensor for
various concentrations of PAP
Steady current output from sensor Concentration of PAP Mean Standard Deviation
10-3 M 270.509 nA 834.704 pA
10-4 M 140.332 nA 274.427 pA
10-5 M 60.770 nA 189.950 pA
10-6 M 12.193 nA 225.175 pA
10-7 M 1.447 nA 22.030 pA
10-8 M 247.449 pA 106.613 fA
10-9 M 57.666 pA 223.490 fA
10-10 M 14.541 pA 5.686 fA
10-11 M 1.263 pA 8.717 fA
10-12 M -- too low to be measured --
A plot of the same is shown in figure 4.3 for easier understanding of the trend in the current measured.
72 Sensor current Vs Concentration of PAP
1.00E+00 1.00E-01 1.00E-02 1.00E-03 1.00E-04 1.00E-05 1.00E-06 1.00E-07 Current /A Current 1.00E-08 1.00E-09 1.00E-10 1.00E-11 1.00E-12 1.00E-12 1.00E-10 1.00E-08 1.00E-06 1.00E-04 1.00E-02 1.00E+00
Concentration /M
Figure 4.3: Plot of current output from the sensor Vs concentration of PAP
From the plot in figure 4.3, it is clear that the current output from the sensor is indeed directly proportional to the concentration of the PAP species as expected. The lowest concentration measured by the charge injection method as proposed by Xiaoshan
Zhu [5] was 6 x 10-9 M whereas a direct Chronoamperometry measurement as done in
this work can measure concentrations down to 10 pM as can be seen in the plot. Though a stable instrumentation for a nano-IDA electrochemical sensor will demand the usage of
the charge injection method, it is clear that the leakage currents through the capacitor
used in the charge injection method reduces the lowest concentration that can be
measured using the nano-sensor by two orders of magnitude.
73 The standard deviations of the current measured in different runs are atleast three
orders of magnitude lesser than the current being measured. Hence the error bars
depicting these standard deviations are not even visible in the plot in figure 4.3. These
concordant values prove that the current measured are very stable and definitely
correspond to the concentration of the species. It also proves that this sensor is a very
reliable device and can be used to distinguish between minute variations of
concentrations even at the nano and the pico molar concentrations.
4.4 Comparison of the ring-type nano-IDA with micro-IDA
It is important to compare the performance of the nano IDA with micro IDA to
understand the improvements in sensitivity of the nano-sensor in comparison to its micro
counterpart. The Chronoamperometry results from a similar experimental setup on a micro IDA [6] are compared with the results of this research work in the Figure 4.4. The main difference between the micro-IDA and the nano-IDA are their areas on the wafer.
The micro-IDA that has been used to compare has an area of 4 mm2 whereas the nano-
IDA fabricated in this work has an area of 500 µm2. Each electrode finger in the micro-
IDA has a length of 2 mm, width of 1 µm and spacing of 0.5 µm.
74 Nanoelectrode IDA Vs Microelectrode IDA
1.00E+00 1.00E-01 1.00E-02 1.00E-03 1.00E-04 Micro-IDA 1.00E-05 Nano-IDA 1.00E-06 1.00E-07 Current /A 1.00E-08 1.00E-09 1.00E-10 1.00E-11 1.00E-12 1.00E-12 1.00E-10 1.00E-08 1.00E-06 1.00E-04 1.00E-02 1.00E+00
Concentration /M
Figure 4.4: Comparison of nano-IDA with micro-IDA
The results obtained prove the enhancement of redox cycling in a nano-IDA when compared to the micro-IDA. It is clear that, using Chronoamperometry, the lowest concentration that can be measured using this micro IDA is 10 nM whereas it is 10 pM using a nano IDA. This shows an improvement in the detection limit by three orders of magnitude while using a nano-IDA electrochemical sensor. But as explained before, the nano-IDA is 640000 times smaller in area when compared to the micro-IDA it is compared with. If the nano-IDA is made as large as the micro-IDA in this comparison, then the nano-IDA is expected to detect even lower concentrations than 10 pM.
75 Another main difference between the micro and nano IDA is the magnitude of the
current measured. It is clear that, as the sensor dimensions become smaller and smaller,
so does the magnitude of the current generated. The 10 pM concentration measured using
the nano-IDA corresponds to the lowest current generated that could be reliably measured using the pico-ammeter and is not the lowest detection limit of the sensor. The usage of devices to measures currents lower than one pico-ampere reliably can in fact show the real detection limit of the nano sensor. This clearly demands the need for circuits to measure such low currents so that such signals can be measured and processed.
It is also reported that, by using substitutional stripping voltammetry [7] instead
of Chronoamperometry, the micro-IDA can be used to detect concentrations as low as 10
pM. This involves a highly complex circuit and a sophisticated experimental setup in
comparison to Chronoamperometry.
Hence, it is clear that, to push down the lower detection limits using
electrochemical sensors, it either requires a reduction in dimensions of the sensors by
orders of magnitude with instrumentation circuits for measuring low currents or it requires a very sophisticated and an expensive experimental setup. In this era of nanotechnology, where the fabrication of nano-IDA has been made easy and less time consuming, it is a wise choice to prefer nano-IDA as electrochemical sensors for
detecting concentrations down to the pico-molar range.
76 For concentrations around 10 nM, the current measured using the nano-IDA enters the pico-ampere range. The main motivation towards building a nano-sensor is to measure concentrations lower than 1 nM which cannot be done efficiently by micro-IDA electrochemical sensors. Hence, it is reasonable to expect that the currents from a concentration the nano-sensor detects will always be in the pico-ampere range. The circuit explained in detail in Chapter 3 can be interfaced with the nano-sensor to convert the pico-ampere currents to voltages, which can further be digitized and processed for displays.
4.5 Conclusion
Thus the fabricated nano-sensor is amperometrically characterized. It is observed that concentrations as low as 10 pM can be successfully detected using such a sensor.
This motivates the usage of such an electrochemical nano-sensor for immunoassay purposes where the main challenge is to detect the miniscule amounts of biological molecules. The size of such a nano-sensor system makes it easier to integrate the sensor with micro and nano fluidics, thereby enabling the construction of portable lab-on-chips.
77 References
[1] N. J. Ronkainen-Matsuno, J. H. Thomas, H. B. Halsall and W. R. Heineman, "Electrochemical immunoassay moving into the fast lane," TrAC Trends in Analytical Chemistry, vol. 21, pp. 213-225, April, 2002. 2002.
[2] D. G. Sanderson and L. B. Anderson, "Filar electrodes: steady-state currents and spectroelectrochemistry at twin interdigitated electrodes." Analytical Chemistry, vol. 57, pp. 2388-2393, 1985.
[3] Koichi Aoki, "Theory of stationary current - potential curves at interdigitated microarray electrodes for quasi-reversible and totally irreversible electrode reactions," Electroanalysis, vol. 2, pp. 229-233, 1990.
[4] Osamu Niwa, Masao Morita,Hisao Tabei, "Highly selective electrochemical detection of dopamine using interdigitated array electrodes modified with nafion/polyester lonomer layered film," Electroanalysis, vol. 6, pp. 237-243, 1994.
[5] Xiaoshan Zhu, "Nano electrochemical sensor and its measurement electronics with a dynamic transduction mechanism," 2004.
[6] A. E. Cohen and R. R. Kunz, "Large-area interdigitated array microelectrodes for electrochemical sensing," Sensors and Actuators, B: Chemical, vol. 62, pp. 23-29, 2000.
[7] M. Morita, O. Niwa and T. Horiuchi, "Interdigitated array microelectrodes as electrochemical sensors," Electrochim. Acta, vol. 42, pp. 3177-3183, 1997.
78
Chapter 5
CONCLUSION AND FUTURE WORK
79 5.1 Summary
The goal of this research work is to amperometrically characterize a nano-
Interdigitated Array of electrodes when used for electrochemical detection. With 275
fingers per electrode, each of width 100nm and spacing of 200nm, the entire sensor was
fabricated on a SiO2/Si wafer using e-beam lithography. Using a very sensitive pico-
ammeter, the low current signals from the sensor in response to the electrochemical redox
cycling at its electrodes, was recorded. A circuit was designed to reliable convert this low
current signal to voltages to enable further signal processing on the signals. If these voltage signals could be digitized and interfaced with a display unit, the entire sensor can be visualized as a portable system.
The nano-patterns for this sensor are relatively complex and demanded precise values of different parameters for the e-beam lithography process. As the pattern extended over a wide area in comparison to the area of write fields, utmost care has been taken to avoid stitch field problems in spite of the e-beam drifts over time, inherent to the process. The patterns were redesigned to adapt to the limitations of the Raith software.
Once the nano patterns were fabricated, microfabrication was used to realize the gold
patterns for reference and working electrodes. The Ag/AgCl reference electrode was
achieved by the process of electroplating.
P-Aminophenol (PAP) was chosen to be the redox species of interest. Using
phosphate buffer solution (PBS), various concentrations of PAP ranging from 1mM to
1pM were prepared. Chronoamperometry was performed in a completely dark
environment on the sensor using PAP and the miniscule currents were successfully
measured using a very sensitive Keithley 6487 pico-ammeter. Concentrations of PAP as
80 low was 10 pM were detected by the fabricated sensor. The Chronoamperometry taps the
complete potential of the nano-IDA electrochemical sensor and pushes the lowest
detection limit of the sensor to two orders of magnitude below that measured by the
charge injection method.
5.2 Future work
Firstly, enzyme based immunoassay that are typically being performed on micro-
IDA electrochemical sensors can now be done on nano-IDA instead, thereby enabling the detection of very low concentrations of bio-molecules as is required by most of the biosensors.
Secondly, the issues in packaging such a sensor for making it a portable system needs to be further investigated. As a complete isolation between the reaction solutions and the instrumentation electronics is necessary for an electrochemical sensor, packaging involves a careful design.
Thirdly, the circuit to convert the pico-ampere currents to voltages presented in this work is a very simple first step in this regard. It holds a lot of scope for further design before a sturdy and reliable circuit is achieved.
These suggested future works will lead to the development of a commercial nano-
IDA electrochemical sensor which will find very wide application in a variety of fields, the main one being the rapidly emerging field of biosensors.
81
82