graphene on silicon carbide chip for biosensing applications730121/fulltext01.pdf · graphensic ab...

Department of Science and Technology Institutionen för teknik och naturvetenskap Linköping University Linköpings universitet

gnipökrroN 47 106 nedewS ,gnipökrroN 47 106-ES

LiU-ITN-TEK-A-14/008--SE

Graphene on Silicon CarbideChip for Biosensing

Applications Albert Skog

Karl Westerberg

2014-05-27

LiU-ITN-TEK-A-14/008--SE

Graphene on Silicon CarbideChip for Biosensing

Applications Examensarbete utfört i Elektroteknik

vid Tekniska högskolan vidLinköpings universitet

Albert SkogKarl Westerberg

Handledare Amir BaranzahiExaminator Igor Zozoulenko

Norrköping 2014-05-27

Upphovsrätt

Detta dokument hålls tillgängligt på Internet – eller dess framtida ersättare –under en längre tid från publiceringsdatum under förutsättning att inga extra-ordinära omständigheter uppstår.

Tillgång till dokumentet innebär tillstånd för var och en att läsa, ladda ner,skriva ut enstaka kopior för enskilt bruk och att använda det oförändrat förickekommersiell forskning och för undervisning. Överföring av upphovsrättenvid en senare tidpunkt kan inte upphäva detta tillstånd. All annan användning avdokumentet kräver upphovsmannens medgivande. För att garantera äktheten,säkerheten och tillgängligheten finns det lösningar av teknisk och administrativart.

Upphovsmannens ideella rätt innefattar rätt att bli nämnd som upphovsman iden omfattning som god sed kräver vid användning av dokumentet på ovanbeskrivna sätt samt skydd mot att dokumentet ändras eller presenteras i sådanform eller i sådant sammanhang som är kränkande för upphovsmannens litteräraeller konstnärliga anseende eller egenart.

För ytterligare information om Linköping University Electronic Press seförlagets hemsida http://www.ep.liu.se/

Copyright

The publishers will keep this document online on the Internet - or its possiblereplacement - for a considerable time from the date of publication barringexceptional circumstances.

The online availability of the document implies a permanent permission foranyone to read, to download, to print out single copies for your own use and touse it unchanged for any non-commercial research and educational purpose.Subsequent transfers of copyright cannot revoke this permission. All other usesof the document are conditional on the consent of the copyright owner. Thepublisher has taken technical and administrative measures to assure authenticity,security and accessibility.

According to intellectual property law the author has the right to bementioned when his/her work is accessed as described above and to be protectedagainst infringement.

For additional information about the Linköping University Electronic Pressand its procedures for publication and for assurance of document integrity,please refer to its WWW home page: http://www.ep.liu.se/

© Albert Skog, Karl Westerberg

Abstract

Graphene is a single layer of carbon atoms, laid out in a hexagonal lattice. The materialhas remarkable properties that opened up several new research areas since its discovery in2004. One promising field is graphene based biosensors, where researchers hope to create newdevices that are smaller, cheaper and more reliable than those based on today’s technology.Among several manufacturing methods, graphene grown on silicon carbide is one of thepromising ones for biosensing. A chip design has been developed in order to support researchinto graphene on silicon carbide as a base material for biosensors. Along with the chip, aholder for electrochemical measurements has been designed and an investigation into therequirements of a custom measurement device for the sensor has been undertaken.

Acknowledgements

We would like to thank Graphensic AB for the opportunity to make this project with them.We also thank all researchers who provided us with feedback along the way and and othersinvolved in helping us for their support.

Abbreviations

2D Two dimantionalAC Alternate currentADC Analog-to-digital converterAFM Atom force spectroscopyBLG Bi layer of grapheneBNC Bayonet Neill-ConcelmanCV Cyclic voltammetryCVD Chemical vapour depositionDAC Digital-to-analog converterDC Dircect currentDMF DimethylformamideEDL Electrical double layerEIS Electrochemical impedance spectroscopyFLG Few layer of grapheneFR-4 Flame retardant 4 (circuit board material)IFM Department of Physics, Chemistry and BiologyISFET Ion sensitive field effect transistorLEEM Low energy electron microscopyLiU Linkoping UniversityMOSFET Metal oxide semiconductor field effect transistorPCB Printed circuit boardPOM PolyoxymethylenePVD Physical vapor depositionQFN Quad Flat No leadsQHBR Quantum hall bar resistorsSLG Single layer of grapheneSoC System-on-a-chipSPR Surface plasmon resonance

Chemical Symbols

Ag SilverAgCl Silver chlorideAr ArgonAr+ Argon ionCr ChromiumH+ Hydrogen ionHCl Hydrogen chlorideNaCl Sodium chlorideNH2 AmidogenPt PlatinumSi SiliconSiC Silicon carbideSiO2 Silicon dioxideTi Titanium

Contents

1 Introduction 1

1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Purpose and Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.3 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Theory 5

2.1 Graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1.1 Graphene on Silicon Carbide . . . . . . . . . . . . . . . . . . . . . . . 8

2.1.2 Surface Characterisation . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.1.3 Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.2 Graphene Biosensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.2.1 Ion Sensitive Field Effect Transistors . . . . . . . . . . . . . . . . . . 13

2.3 Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.3.1 The Three Electrode Setup . . . . . . . . . . . . . . . . . . . . . . . 15

2.3.2 Cyclic Voltammetry . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.3.3 Electrochemical Impedance Spectroscopy . . . . . . . . . . . . . . . . 17

3 Requirements Analysis 19

3.1 Graphensic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.2 Resistance Sensor Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 20

5

3.3 ISFET Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.4 Electrochemical Cell Requirements . . . . . . . . . . . . . . . . . . . . . . . 22

3.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

4 Implementation 25

4.1 Chip Holder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4.2 Sensor Chip . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.2.1 Process Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4.3 Measurement circuitry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

5 Results 37

5.1 Chip Holder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

5.2 Sensor Chip . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

5.3 Measurement Circuitry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

6 Discussion and Conclusion 45

6.1 Holder and Chip Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

6.2 Chip and Mask Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

6.3 Measuring Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

6.4 Ethical and Social Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

6.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

A Holder Design 57

A.1 Lid Drawings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

A.2 Cavity Drawings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

A.3 Bottom Plate Drawings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

B Masks and Chip Design 63

B.1 Chip . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

B.2 Masks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

C Circuit Board Designs 69

C.1 Holder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

C.2 BNC Breakout Board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

Chapter 1

Introduction

In 2004 the two scientists, Andre Geim and Konstantin Novoselov, showed that single layersof graphite, graphene, could be isolated and studied [1] without collapsing. In fact, thematerial was not only stable but also showed remarkable electrical and thermal properties.As a sign of the magnitude of this discovery, the duo was awarded the Nobel Prize in Physicsin 2010 [1; 2], sparking a huge interest among researchers. The discovery of graphene isbelieved to bring a number of technological advances, such as smaller and faster transistors,more efficient solar-cells, better biosensors and probably others still unheard of. Today, weare still only on the verge of finding out which of these applications are going to make itinto commercial products. The use of graphene for biosensing has caught the interest ofresearchers hoping to create more accurate sensors and ones capable of detecting diseasesthat today require tedious and expensive lab tests.

1.1 Background

Graphensic AB is a manufacturer of graphene grown on silicon carbide (SiC) and was foundedin 2011 as a spin-off form the Department of Physics, Chemistry and Biology (IFM) atLinkoping University (LiU). The company has a patented method for growing large-areamono-layer graphene on SiC and is one of only a handful commercial manufacturers world-wide.

Today Graphensic has a limited clientele consisting of researchers within different areas ofmaterial science and biochemistry. The company sees several upcoming applications in futureproducts that would expand their market, one of which is biosensors. For one, a so calledquantified-self movement is emerging, where athletes and enthusiasts monitor and measureevery possible aspect of their body [3], implying a growing interest in health monitoring.

1

2 CHAPTER 1. INTRODUCTION

The desire to measure health is also visible in the pharmaceutical market, where self-testsfor lactose intolerance or bacterial infections currently sell for upwards of 400 SEK [4].Furthermore, the Internet of Things era is approaching with its promises of ever growingnumbers of connected devices, many of which are predicted to be sensors. Looking aheadin this context, Graphensic envisions that within a few decades, graphene based biosensorswill be part of daily life in the form of small hand held self-tests that can, for example, tellwhether the user suffers from a specific disease.

Working towards this, the current focus is on supporting scientists by offering graphene chipsdesigned for biosensing and a development kit with hardware and software geared towardsbiochemists. The vision may still be far away and intermediate steps where graphene sensorsare incorporated into larger medical equipment are probably necessary. Graphensic wants tostart out by becoming the natural supplier of research samples and hopes to grow with themarket into also becoming the natural partner in future mass production.

One possible stepping stone for graphene based biosensors is replacing or complementing to-day’s surface plasmon resonance (SPR) equipment. In SPR, small differences in light diffrac-tion caused by binding of certain molecules at the sensor surface are utilised [5, pp. 62-63].Because SPR needs a high quality laser beam, light diffraction sensors and over all stableconditions, graphene sensors have the potential to be smaller, cheaper and faster.

1.2 Purpose and Objectives

This thesis shall investigate the requirements of and propose a design for a graphene basedbiosensor chip and investigate the requirements of a measurement device for it. The resultsshould include the designs and manufactured parts of the sensor and some basic conceptsfor a future measurement device.

The development kit can be divided into three parts; sensor, measurement electronics anduser interface software. Development begins with the sensor followed by the measurementelectronics and last a computer interface. This thesis will focus on the sensor design and therequirements of the measurement circuit.

Graphensic will use the results to learn about biosensing and work towards expandingtheir market beyond researchers who pattern their graphene themselves, into more focusedgraphene based biosensing research. The designs produced will be used as a starting pointfor products directed especially towards biochemists interested in this.

CHAPTER 1. INTRODUCTION 3

1.3 Methodology

Designing a graphene biosensor requires some theoretical foundation of graphene physics,general electrochemistry and microfabrication. Detailed requirements are not presented asan input at the beginning of the project, but derived as a part of it. Because the require-ments process is an integral part of the project, work on the design and requirements willbe iterative and informal. Graphensic has no deeper knowledge of biosensors beyond thegraphene manufacturing, hence the project begins with theoretical studies to understandthe application and the different electrochemical measurements that might be used or testedon biosensors. Meetings and discussions with researchers in the field are then used to formu-late the specification. Based on the needs of these scientists and the most suitable solutionfor Graphensic as a business, a graphene sensor will be designed and manufactured. All workis done with the entire development kit in mind to enable future work on the product.

The parts of the sensor will be manufactured by professionals, then assembled, evaluated andpresented as the results of this thesis. For the graphene patterning, an ongoing collaborationbetween Graphensic and Chalmers University, who have experience in this area, is in place.Other parts will be manufactured or purchased from suitable suppliers.

After completion, the sensor will be put into a development kit containing measurementcircuitry for working with the sensor. While the development kit as a whole is outside thescope of this thesis, requirements for such a circuit are also derived from the informationgathered for the sensor design and presented. The information, again, may be used as astarting point for future development.

Using researchers, who are also potential buyers of the final product, to get feedback meansthe result is likely to suit their needs. There is, however, a risk of over-specialising the design,making it less usable to others. One counter measure to this is consulting several researchersfrom different universities (and countries) and with interest in different implementations.However, there is only so much feedback that can go into one design. It is likely that thisfirst sensor will be followed by iterations specialised to suit both more general needs andmore specialised ones. Because of this, design decisions that make relevant parts easy tomodify in future work have been given priority.

Three major software tools have been used for the design:

Altium Designer for laying out circuit boards.

Agillent Advanced Design System for drawing lithography masks.

Google Sketchup for making design sketches for mechanical parts and illustrations andanimations of proposed designs.

4 CHAPTER 1. INTRODUCTION

The authors have worked collaboratively on most parts of the project. Chip and mask design,however, was mainly done by Karl and circuit board layout by Albert. Working together onthe theoretical background studies and requirements analysis ensured a good understandingof the concepts and a coherent view on the needs.

Chapter 2

Theory

In order to understand what requirements a graphene based biosensor needs to complywith, the theoretical background of graphene physics, biosensors and electrochemistry hasbeen explored. This chapter will present relevant portions of these to lay a foundation forterms and concepts used in upcoming chapters. Common measurements for biosensors arealso presented as a background to sensor design and some concepts of microfabrication arecovered to s discussions about chip processing. Readers are expected to have a fundamentalunderstanding of electronics and modern physics.

2.1 Graphene

For many years, researchers have studied the electrical properties of graphite, the carbonbased mineral that is used in a normal pencil. Already in 1947 Phillip Wallace conductedstudies [6] on the properties of what he called graphite crystals, predicting some of theremarkable properties of single layer graphite. In 2003 Andre Geim and Kostya Novoselov [7]started to investigate how thin a layer of graphite can be. They used a method that becameknown as ”The Scotch Tape Technique”, which is performed by placing a piece of scotch tapeon graphite and pulling it off. A few layers of graphite will adhere to the tape and can thenbe separated further by double folding the tape and pulling it apart again. This is repeateda number of times until flakes of a single layer of carbon atoms, graphene, are found.

The unique properties of graphene makes it interesting for many different fields and researchinto finding applications has exploded during the last few years [8, p. 1182]. Most notably,graphene forms a two dimensional (2D) structure, is a very good charge- and thermal con-ductor and its in-plane bonds are stronger than steel [9; 10]. Sought applications range fromtransistors [11] to highly accurate resistors [12], loudspeakers [13] and biosensors [14–22].

5

6 CHAPTER 2. THEORY

The 2D structure of graphene arises through a certain hybridisation of the carbon atoms.Hybridisation is a wildly used term in the area of chemistry and describes how the atomicorbitals in atoms interact with each other and affects how the atom can bind to formmolecules [23, pp. 419-421]. The carbon atom contains 6 electrons and, according to thePauli exclusion principle and Hund’s rule of maximum multiplicity [24, pp. 340-342] hasthe electron configuration 1s22s22p2. This is illustrated in Figure 2.1a where the 1s and 2sorbitals are each occupied by two electrons of opposite spin and 2p (divided into 2px, 2py and2pz), which is occupied by two electrons of the same spin [24, pp. 340-342; 25, pp. 49-51].

Energy

Level

1s

2s2px 2py 2pz

(a) Carbon ground state

Energy

Level

1s

2s 2px 2py 2pz

(b) sp3 hybrid carbon

Energy

Level

1s

2s 2px 2py2pz

(c) sp2 hybrid carbon

Figure 2.1: Energy diagrams of atomic orbital hybridisation for carbon. In the ground state (a), thelowest possible energy states are occupied. Depending on surrounding conditions, one electron mayleave the 2s orbital and move to 2p, forming a sp3 (b) or sp2 (c) hybrid.

In the case of carbon, the energy levels 2s and 2p are located close to each other with respectto energy, resulting in an environmental dependence [23, pp. 418-421]. If carbon is surroundedby electron-hungry atoms like hydrogen or oxygen, the 2s-orbital will release one electronto the higher 2p energy level [23, pp. 418-421]. The same phenomenon occurs also at hightemperature and pressure, for example when carbon forms into diamond. This hybridisationof carbon is denoted sp3 since it is able to bind to four orbitals; once with the 2s orbitaland three times in 2p (2px, 2py and 2pz). In Figure 2.1b, the second electron previouslyoccupying the 2s orbital moved to 2pz, giving the atom the electron orbital hybridisationssp3 [23, pp. 418-421]. The binding structure of sp3 carbon can be illustrated geometricallyas a tetrahedron as seen in Figure 2.2a.

CHAPTER 2. THEORY 7

(a) sp3 hybrid carbon (b) sp2 hybrid carbon

Figure 2.2: Carbon hybridisations. In sp3 carbon (a), the orbitals are of equal energy and forma tetrahedron. In sp2 carbon (b), three orbitals separated by 120 ° form strong bonds in a plane,leaving the fourth free to interact.

In the case of graphene, two of the 2p orbitals (2px and 2py) are mixed with the 2s orbitaland the 2pz orbital is left free, as illustrated in Figure 2.1c. This hybridisation is denotedsp2. The molecular structure is based on three strong bonds in a plane separated by anangle of 120° [24, p. 391] as illustrated in Figure 2.2b. Together, these carbon atoms formthe hexagonal structure which builds up a single layer of graphene (SLG). The layer has thethickness of one carbon atom - usually negligible compared to the width and length, whichis how the graphene layer can be claimed to be two-dimensional. The free pz electron has theability to bind to other materials, or to another graphene layer, forming bi- or a few layersof graphene (BLG, FLG). The pz electron also has the ability to move over the graphenelayer and thus transport current [8].

Graphene is a semi-metal, or a zero-bandgap semiconductor [8; 26; 27; 25, p. 19]. Thisis because of the phenomenon of Dirac points. At these points in the graphene lattice,the difference in energy between the valence and conduction bands is zero, resulting ingraphene’s metal properties [27; 25, pp. 49-51]. The further implications of Dirac points andthe mechanisms of how graphene conducts current are not within the scope of this thesisand will not be discussed in any more detail.

Graphene has shown high carrier mobility due to its the 2D structure. The 2D structureprovides a large scattering length since the electrons travel upon the carbon layer in anelectron gas without any interference from the carbon atoms [7; 27; 8, p. 1208]. This makescharge carrier mobility of graphene very sensitive to defects, a fact which will play a centralrole in biosensing applications [16; 17; 19; 20]. Defects may be introduced by physicallybreaking the layer apart, or locally changing the hybridisation of the atoms from sp2 to sp3

by introducing other atoms. Measurements on SLG, suspended between two gold plates tominimise scattering effects from the underlying material, have shown carrier mobility up to200 000 cm2/Vs [11]. Graphene grown on SiC and exfoliated graphene placed on a SiO2 havemobilities in the range of 1000 - 5000 cm2/Vs and 40 000 - 70 000 cm2/Vs [11].

8 CHAPTER 2. THEORY

2.1.1 Graphene on Silicon Carbide

There are several methods for forming graphene. Geim and Novoselov recieved their Nobelprice in 2010 for the work [28; 29] on micromechanical cleaving (The Scotch Tape Technique).Others deposit graphene on copper in a chemical vapour deposition (CVD) process [22] orreduce graphene oxide [14]. A fourth possibility is forming graphene from silicon carbide (SiC)by removing silicon atoms from the topmost layer. Graphitisation is a common unwantedeffect in SiC processing [30]; heating the material in vacuum causes Si atoms to sublimefrom the surface and leave behind layers of graphite. The thickness of the graphite is hard tocontrol and the continuity of the material is poor. A method for controlling the sublimationto get only a few layers of graphene was presented by LiU researchers in 2008 [31]. Thesuggested process heats the sample, with minimal spatial temperature gradients, up to 2000C using argon (Ar) gas to control the speed of the sublimation. In this manner, large areaswith 1-4 layers of graphene were created.

Producing graphene on SiC may be a suitable method for future large scale production. Thetechnique should scale well, although the raw material (SiC) is still rather expensive. Alter-natives to epitaxial SiC growth are tedious and time consuming. Cleaving creates irregularshapes and requires looking for the small flakes [31; 32]. The CVD graphene on copper maybe better for some applications, but requires transferring the graphene onto an insulatingsubstrate [22] for biosensing applications.

2.1.2 Surface Characterisation

One of the challenges working with graphene is how to measure the number of layers and thequality of the surface. Novoselov et. al. [29] discovered that even SLG flakes become visiblein an optical microscope when placed on SiO2. They also went on to study the flakes furtherusing atomic force microscopy (AFM). Since then, scientists have started to employ severalother tools, including most notably Raman spectroscopy and low energy electron microscopy(LEEM) to identify FLG and SLG.

In Raman spectroscopy, a laser beam is directed towards the sample and the scatteredphotons are detected [24, p. 432]. The functionality depends on the fact that for certain(anisotropically polarizable [24, p. 449]) materials, when photons hit the surface, some arereflected with higher and some with lower energy (frequency) than before [24, p. 431]. ForSLG, three peaks called 2D, G and D are visible [25, p. 52]. 2D is due to phonon excitation,G is common also to multilayer graphene and graphite and D is caused by defects [25, p. 52].

CHAPTER 2. THEORY 9

2.1.3 Fabrication

Conventional lithography- and metallisation techniques from semiconductor manufacturingcan be used to pattern graphene on SiC [11; 19; 26; 32; 33]. Generally, these techniques involvecovering the sample with photoresist by spin coating, followed by patterning by optical- orelectron-beam lithography and finally deposition or etching to add or remove material inexposed areas [34].

Devices have been made from graphene on SiC before, for example biosensors [19] andquantum hall bar resistors [35] (QHBR). A QHBR is used to measure quantum resistance,potentially creating a new universal standard for the resistance unit, a niche applicationwhich is not in the scope of this thesis. The QHBR (Figure 2.3) consists of two gold padson each side of a graphene channel and two additional legs along its sides. To observe thequantum hall effect, the sensor is placed in a magnetic field, the left and right pads are usedto pass a current and measurements are done at the other four. This type of device has beenmanufactured in the MC2 laboratory at Chalmers [35].

Figure 2.3: Quantum hall bar resistor. The pads on the left and right are used to pass a currentand the top and bottom pads are used for observing the quantum hall effect.

Photoresist

The first step in most semiconductor processing is to add a layer of light sensitive film –photoresist. Film is normally added by placing droplets of the liquid onto the sample andspinning it to create a uniform layer, a method known as spin-coating. The spinning ensurescontinuous resist thickness without risk of damaging the surface. [34, pp. 55-56]

10 CHAPTER 2. THEORY

Photoresist Patterning

After photoresist has been added, the desired features on the surface need to be eitherexposed or protected. This is done by selectively hardening the photoresist through lightexposure (photolithography). It is also possible to use an electron beam (e-beam lithography),a method which has a resolution several orders of magnitude better than optical [34, p. 93].It is common to make masks for optical lithography with e-beam lithography, typically fromchromium on glass [34, p. 94]. The mask may be manufactured on a larger scale in order tocreate finer details [34, p. 99]. In this case, the light shining through the mask will need tobe optically reduced down to the right size. With the undeveloped parts of the photoresistwashed away, the surface is now ready for deposition or etching.

Photoresist can be positive or negative, depending on whether areas exposed to light will becured and covered areas remain liquified or vice versa, as illustrated in Figure 2.4. Figure 2.4ashows a substrate with negative photoresist, where the exposed areas become cured whileFigure 2.4b shows a substrate with positive photoresist, where instead the protected areasbecome cured. Because of the properties of light, some of the positive resist will remain curedoutside of the mask and form an outward slant, while the negative gets an inward slant.

Light

Resist

Mask

Substrate

(a) Negative photoresist

Light

Resist

Mask

Substrate

(b) Positive photoresist

Figure 2.4: Photoresist types. Negative photoresist (a) is cured by light exposure while positivephotoresist (b) is dissolved. Depending on the resist type, the edge will have either positive ornegative slant at the edges.

For some types of deposition, it is more viable to use a shadow mask or stencil with physicalcutouts where deposition is desired. This is only possible for rather simple layouts as thestencil cannot have free-standing details or long protrusions (see Figure 2.5). Since the maskis not directly in contact with the sample, this method is normally used for applicationsdown to ±10 µm accuracy. [34, p. 229]

CHAPTER 2. THEORY 11

Figure 2.5: A shadow mask is a stencil with physical cutouts, meaning completely free-standingstructures are not possible, but have to be suspended by tabs.

Deposition

If the desired action is to add material on top of the exposed areas from the patterning,deposition is the next step. Various types of chemical vapour deposition (CVD) and physicalvapour deposition (PVD) exist, enabling deposition of a wide range of materials - bothmetals, insulators and semiconductors. [34, pp. 49-52]

The most common form of PVD is sputtering. Here, argon ions (Ar+) hit a solid piece ofthe material that is to be deposited, ejecting single atoms. By applying an electrical fieldbetween the substrate and deposition target, the atoms move to the substrate. [34, p. 50]Finally, the photoresist may be removed, leaving only the deposited areas. This is commonfor creating metal contacts and is known as lift-off metallisation [34, p. 228].

In previous work, graphene has been interfaced with gold pads [35]. However, noble metalslike gold have poor adhesion to substrates and therefore an additional adhesion layer oftitanium (Ti) or chromium (Cr) is often added before the gold [34, p. 56,57].

Etching

If the desired action after patterning is to remove the exposed parts of the pattern, etching isthe next step. Depending on the underlying layer(s) it may be favourable to use wet etchingwhere the sample is submerged or sprayed with an etchant solution, or dry etching wherethe liquid etchant is replaced with gas plasma. [34, pp. 119-120]


2.2 Graphene Biosensors

Biosensors use biological sensing elements to measure the concentration of some specific sub-stance [36]. Common to all biosensors is the translation of the presence of this substance intoan electrical signal [5], be it in an electrical circuit or inside the human body. The biologicalsensing element connects to a transducer which does the translation from biological responseto electrical current or voltage [36]. The transducer in its turn connects to a measuring deviceconverting the electrical signal to a measurable quantity [5]. In graphene based biosensors,graphene is used as the transducer, changing its electrical characteristics upon interactionwith the attached sensing elements [37].

Graphene can easily attach to atoms, ions or molecules because of the free pz electrons. Thismeans untreated graphene can not be used for detection of a certain molecule or ion since itis not possible to tell what has attached. To make the biosensor selective, sensing elementsneed to be attached and the rest of the surface needs to be passivated [20]. The process ofattaching sensing elements to the graphene surface is referred to as functionalisation. Severaltypes of sensing elements have been used for functionalisation [17; 18; 20; 25], some of whichrequire an intermediate layer, first binding an amidogen (NH2) group to the graphene whichin turn binds to an antibody [20], DNA string or protein. Others are attaching antigensdirectly to the graphene [16]. The details of these previous experiments are not relevant inthemselves, but are brought up to show that many variations exist. Simplifying the work ofexploring what types of functionalisation are viable is one of the core motivations for thisproject and the current state of research in this field is in no way extensively covered by thisbrief overview.

On a chemical level, functionalisation can be either covalent or non-covalent, dependingon how the molecules attach to the graphene surface [10]. Non-covalent functionalisationincludes ionic bonding and van der Waals interaction between the surface and moleculeswhile the covalent bond share electrons with the carbon atoms [10; 20]. The non-covalentattachment causes a change of the electrical properties by introducing or absorbing chargecarriers from the surface (essentially doping), while the covalent attachment causes a changeof the 2D surface. This change in electron concentration or surface structure changes theresistance of the graphene.

When a covalent bond is introduced to the graphene layer, the 2D structure is damagedbecause of the local change of hybridisation. As discussed in Section 2.1, carbon can changethe atomic structure in specific environments. To make a covalent bond possible the hybridi-sation for a specific atom goes from sp2 to sp3 [38]. This transformation breaks the planarstructure of the graphene and decreases the charge mobility [10; 20; 39].

With the sensing element in place, the sensor is ready for operation. While each implemen-tation differs in the details and all of the mechanisms are far from understood, all graphene


based biosensors achieve a change in charge concentration near the surface or otherwisechange their characteristics when exposed to the substance it is supposed to detect. Thechange can be measured by observing the impedance of the interface between the function-alised graphene and a liquid containing the substance in question, meaning the graphene isused as an electrode [22; 37]. Alternatively, the impedance of the graphene can be measuredby passing a current through it [20]. This type of sensor may also be covered by a liquidwith an electrode, creating a transistor setup called an ion sensitive field effect transistor(ISFET) [22].

2.2.1 Ion Sensitive Field Effect Transistors

Biosensors can be implemented as ISFETs, where the concentration of ions affects thetransconductance of the transistor channel [15; 22]. An ISFET is structurally similar toa metal oxide field effect transistor (MOSFET), but with the gate metal replaced by anelectrochemical solution and a gate electrode in contact with the solution [40–42]. Both usu-ally need a gate voltage to be applied and exceed a certain threshold voltage to open up aconduction channel. For the ISFET, the gate voltage is applied at the electrode. Figure 2.6shows the cross sections of a MOSFET and an ISFET to illustrate the difference.

Gate metal Gate oxide

DrainSource

Substrate

(a) MOSFET cross section.

Gate electrode

Solution Gate insulator

DrainSource

Substrate

(b) ISFET cross section.

Figure 2.6: Cross section of MOSFET and ISFET transistor types.

ISFETs can be used to detect biomolecules by measuring the change in threshold voltagesince it depends on the interface between solution and gate insulator, known as the electricaldouble layer (EDL). The EDL is illustrated in Figure 2.7, which includes the equivalentcapacitances of electrode-channel interface. The double layer capacitance occurs because ofmolecules or ions near the gate insulator that attract or repel charges in the channel. Thepresence of such molecules can be controlled and made selective by adding a functionalisationlayer to the gate insulator. [42; 43]


- - - - - - - - - -

- - - - - - - - -

-

-

-

+ + + + + + + + + +

+

+

+

+ Bulk fluid

Electrical

double-layer

Gate insulator

Substrate

CEDL

COX

Figure 2.7: Electrical double-layer. When a current is passed through the channel of an ISFET, adouble-layer of ions will be formed in the fluid. The EDL will act as a capacitance in series withthe capacitance of the gate insulator. This equivalent circuit is shown to the left.

In graphene based ISFETs, the conducting channel and the gate insulator is replaced withgraphene. As mentioned before, graphene is a zero-band gap semiconductor, meaning it isimpossible to turn off the transistor with the gate electrode, and thus meaning it has nothreshold voltage. Instead, channel resistance measurements can be done at different gateelectrode voltages.

Researchers have also shown that capacitive impact from the oxide capacitance Cox andbulk capacitance CSi (the capacitance of the substrate itself) in silicon based ISFETs canbe circumvented by using graphene instead. This makes it possible to use the ISFET tomeasure the electrical double layer capacitance CEDL, since CEDL becomes of similar orderof magnitude as CSi. This is typically done by using the ISFET as an electrochemical cellas described in section 2.3.1, where a alternate current (AC) signal is applied at the gateelectrode [22].

The graphene ISFET may be simplified by not applying a gate voltage. The result is asensor with only two connections, used simply by measuring resistance. The resistance canbe measured by applying a voltage over the channel and measuring the current. Whenintroducing the specific molecule to detect, the functionalised surface will bind to it andchange the resistance of the channel [20]. Detection is done by comparing the measuredresistance of the graphene and functionalisation with that of graphene and functionalisationwith the attatched molecule [10; 20; 39].


2.3 Measurements

A number of different measurements are typical for graphene biosensors, some for creat-ing and characterising and some for actual use of the sensors. Most processes involvedin functionalisation treats the graphene as an electrode. In sensing, apart from using thegraphene electrochemically as an electrode [37], it may also be used as the channel of anISFET [22; 37; 44].

Voltammetry; measuring voltage at constant current and vice versa, amperometry - to mea-sure current at constant potential [36, p. 27] are the most rudimentary measurements ofinterest for graphene biosensors. For example, current- or voltage steps can be used to ob-serve transients and learn about interface properties [45, p. 173]. In electrochemical mea-surements, the electrode potential (working electrode - reference electrode) and current aremeasured [45, p. 173], but it is also of interest to observe current/voltage characteristicthrough the channel [20].

2.3.1 The Three Electrode Setup

Working

electrode

Reference

electrode

Counter

electrode

I+ −

V

Figure 2.8: Three electrode measurement setup.

Electrodes used in electrochemical measurements require some additional precautions whenaccurate measurements are needed. The interface between electrode and solution will have avoltage drop [45, p. 5]. When using a graphene sample as an electrode, this interface can beinvestigated [45, p. 17], which is useful when carrying out the functionalisation. In order tocomplete a circuit, at least one more electrode has to be added, meaning there will alwaysbe at least two liquid-electrode interfaces at play. To investigate a single interface, chemistsuse a measurement setup known as the three electrode setup. In the setup, three types of


electrodes exist: working electrode (the graphene sample), counter electrode and referenceelectrode [45] (Figure 2.8). To calculate the absolute electrode potential, a “work function”Φsol for a solutions is derived [45, p. 16] from electrochemical relations. It is found to beapproximately the same as the electrochemical potential [45, p. 16]. For the measurementsetup in Figure 2.8, at equilibrium, this yields [45, p. 19]:

V = Φwork − Φref (2.1)

Having a known electrochemical potential Φref for the reference and a measured value for V ,the electrochemical potential Φwork can be easily calculated [45]. For this reason it is common[5; 36] to use a silver/silver chloride (Ag/AgCl) reference electrode, as its electrochemicalpotential is well known and reproducible [36, p. 17]. In practice, this means the Ag referenceelectrode is coated with AgCl and submerged in sodium chloride (NaCl) [36, p. 17]. TheNaCl solution makes contact with the surrounding solution through a capillary tube [45,p. 18] and since no current is flowing, the NaCl should not affect nor mix with the othersolution. Alternatively, the electrode consists of hydrogen chloride (HCl) and a silver wireinside a glass container where the glass is made so thin that hydrogen ions (H+) can travelthrough it [46, p. 690]. A third possibility is to simply coat a silver electrode in AgCl.

Finally, there should be a way to introduce a current to the system and observe how thepotential at the working electrode changes. For this, a counter electrode is introduced (Figure2.8). The counter electrode is commonly made from platinum (Pt), for different reasons. Thethree-electrode setup allows for a constant reference voltage while the current varies [45,p. 18].

Electrochemical setups generally contain a potentiostat, a waveform generator and a mea-surement device [47, p. 632]. The potentiostat is connected to the counter, working andreference electrode and its task is to ensure constant potential between the reference andworking electrode. In its simplest form, a potentiostat is realised with an operational am-plifier (Op-amp) supplying more current to the counter electrode when the potential of thereference electrode falls under the expected value and vice versa [47, p. 641].

2.3.2 Cyclic Voltammetry

One use for the three-electrode setup is cyclic voltammetry (CV). Generally, a voltammogram

is a plot of the current through the solution as the potential between the reference- andworking electrode is varied linearly [36, p. 27]. In CV, a triangle wave potential (Figure 2.9a)is applied and current is measured [45, p. 178]. Typical rise time of the input signal is in theorder of seconds or minutes. As current starts to flow, ions will travel to the anode whereoxide will build up as electrons transition into the electrode and at the cathode molecules getreduced and go back into the solution [36, p. 30]. This type of measurement reveals at which


voltages the oxidation and reduction peaks. If the reduction or oxidation is not reversible,the resulting graph will change with each cycle, making CV a useful quantification of, forexample, functionalisation of an electrode.

Potential

Time

(a) Potential of a cyclic voltammogram. (b) Typical cyclic voltammogram.

Figure 2.9: In- and output of a cyclic voltammogram for a typical redox solution.

2.3.3 Electrochemical Impedance Spectroscopy

In electrochemical impedance spectroscopy (EIS), the impedance of the system at differentfrequencies is recorded by applying a sinusodial potential and recording the current [45, p.181]. EIS is how researchers are planning on measuring the EDL capacitance in grapheneISFETs.

ZEDL

Zct ZW Rb

(a) EIS equivalent circuit.

Current

Voltage

Semi-linear

region

(b) Semi-linear region of IV-curve.

Figure 2.10: EIS equivalent circuit and suitable characterisation region.

As different processes and materials have a different impact on the impedance [45, p. 181],the output may be used as a mean of characterisation. For a simple redox reaction, thesystem may be represented electrically by an equivalent circuit [45; 48, p. 182] as shown in


Figure 2.10a. In the circuit, ZEDL is the capacitance of the EDL [45, p. 5]. Zct is the chargetransfer resistance, ZW the Wartburg impedance due to diffusion at lower frequencies andRb the bulk resistance measured between the two electrodes [45, p. 182]. At low frequencies,impedance due to diffusion (Wartburg impedance) dominates [45, p. 182]. In a Nyquist plot,this is seen as a straight line and the double layer capacitance forms a semi-circle.

To be able to measure the impedance, the electrochemical system needs to be placed in apseudo-linear region (see Figure 2.10b). This is due to the introduction of noise for non-linearities [49; 50, pp. 3-5], which can be derived from the Taylor series way of expressingthe non-linearity – by expanding the Taylor series different frequencies will be introducedand affect the measurement [49].

Chapter 3

Requirements Analysis

The following chapter is a compilation of meetings with Graphensic AB and three researchgroups in the area of graphene based biosensors. The analysis is a supplement to the studiedtheory and will be a key part of the sensor and development kit design. The research groupsare current or presumptive Graphensic customers and their opinions and requests are there-fore important in the design. The groups have different focus in their research, providinga more general foundation for the analysis. During the meetings, the researchers describedtheir current work and future research and also discussed general measurement techniquesand chip designs for the development kit. Out of respect for their ongoing research theresearch groups will not be named.

The research groups have different focus in their research; one group uses the chip as aresistance sensor to detect proteins, the second wants to use an ISFET to detect DNAstrands and the third uses the chip as a graphene electrode in an electrochemical cell to detectproteins. The detection methods give different requirements in chip design, chip interfacingand measuring instruments and will result in a compilation at the end of this chapter.

3.1 Graphensic

Graphenisc wants to support electro- and biochemists by supplying a development kit forbiosensing applications. As mentioned in the introduction, this thesis will treat the sensordesign and the start of the measuring electronics. Graphensic has expressed some generalrequirements for the design:

• The sensor chip size should be minimised to reduce cost.

19

20 CHAPTER 3. REQUIREMENTS ANALYSIS

• The sensor should have a wide range of applications and should, if possible, not bespecialised for any single application.

• The sensor should save time for researchers.

• The sensor should be designed for future extension and modification.

As an initial idea, the company has come up with a proposed chip design. This chip is of thesize 7x7 mm and contains four graphene channels.

3.2 Resistance Sensor Requirements

One research group is working on detecting proteins by functionalising a graphene channelwith a specific antibody and measuring the resistance of the channel. The measurements areperformed with a regular voltage generator and a multimeter, on both graphene on SiO2

and graphene grown on SiC. The measurements are done in dry conditions to avoid metalto solution contact but will in the future be performed in solutions.

Sensor Chip

In experiments, a larger concentration of functionalisation molecules have been obtained atthe edge of the channel. Therefore, narrow channels were used to get a greater edge to arearatio; length 1-3 mmand width 50-70 µm . The channels are interfaced with gold pads at eachend. In general it was concluded that, for experimental applications, the pads and substrateshould be large enough to use without a microscope.

This group have thoughts on a commercial product where the user places a liquid over thegraphene channels for some type of diagnostics. There is currently a working prototype andthoughts on improving it, for example by mounting the sensor with flip-chip technology. Forthis type of application it is of interest to integrate platinum and silver electrodes on thechip.

Measurements

The detection is done by current-voltage (IV) measurements (Section 2.2.1), where a suffi-ciently large difference is observed in the voltage range of ±2V. During the functionalisationof the channel, a CV measurement is run for about 10 minutes. This is performed in astandard electrochemical cell, with a platinum counter electrode and Ag/AgCl reference.

CHAPTER 3. REQUIREMENTS ANALYSIS 21

In addition, it was suggested that chemists are interested in EIS, chronoamperometry, opencircuit potential and temperature measurements. It may also be interesting to do AC mea-surements and channel cross-talk measurements but that is not in their current scope.

3.3 ISFET Requirements

Initial experiments by a second research group have been performed using a graphene IS-FET as a DNA sensor. The experiments have been performed with CVD graphene on copper,where the copper is etched away and the graphene placed on a piece of SiO2. This compli-cates the experiment since the graphene can be ruined during the transfer to SiO2. Anotherproblem occurs when the graphene channel is placed in a solution; the sheet is not bound tothe SiO2 substrate and can therefore float away. This problem does not exist with graphenegrown on SiC since the graphene is atomically bounded to the SiC substrate.

To be able to detect DNA, the graphene surface needs to be functionalised, in this case usingdimethylformamide (DMF). DMF is a common solvent for chemical applications, but putshigh demands on passivation. The passivation is necessary to isolate the active area of thesensor from the connection pads.

When a DNA strand attaches to the functionalised surface, a change in capacitance betweenthe reference electrode and the graphene surface is introduced (Section 2.2.1). It is also ofinterest to measure the resistance for different gate voltages.In this stage of the research project, many things are still untested. This results in a lot ofrequests for the chip design:

• Large chip size to simplify handling (∼15x15 mm)

• Sufficiently large cavity for ISFET liquid (they previously used 4x5 mm)

• Pad size larger than 100x100 µm in gold or aluminium

• 20 channels per chip

• Various size of the active region; width-length relation of 1:7-1:20

• Channel separation of at least 100 µm

• Passivation resistant to DFM

• Minimised distance between active region of graphene and gold conductor to minimisepotential Schottky effects in the contacts

• Additional gold channel as reference surface

22 CHAPTER 3. REQUIREMENTS ANALYSIS

3.4 Electrochemical Cell Requirements

A third research group has started to study the properties of graphene as an electrochemicalelectrode. They have developed a holder for graphene grown on SiC. A graphene sample isplaced on a piece on aluminium foil and is secured with copper tape. The top of the holderhas a hole sealed at the bottom with an o-ring. The o-ring makes contact with the grapheneand forms a cylinder with the active graphene area at the bottom. Liquids and electrodescan be placed into the hole to conduct electrochemical measurements. The graphene workingelectrode is connected by applying an aligator clip to the foil. The setup is the same as thestandard electrochemical cell described in section 2.3.1. Figure 3.1 is a cross section view ofthe construction. So far, the measurements of interest for this application are CV and EIS.

Figure 3.1: Cross section of graphene electrode electrochemical cell.

3.5 Summary

The three research groups use graphene in different ways, resulting in a requirement onthe chip design to enable measurements both as a electrode and a channel sensor. Anotherrequirement related to the chip design is the size. Two of three researchers mention that thechip should be sufficiently large to be handled by hand and one specifies a size of ∼15x15 mm.Since this is not compatible with the company opinion the chip size will become a trade-offbetween usability, number of channels per chip and size.

In order to use the chip as an ISFET or electrochemical cell, a cavity is required. Thefirst group mentioned that their ambition is to perform experiments with fluids, but thatthey are afraid of contact in gold-to-solution interface. The gold-solution contact can beavoided through passivation, which was a requirement from the ISFET researchers. Since theapplications are in the area of medicine, the used materials should be resistant to chemicals.

To concretise the main requirements to start the design is presented in the following list:

CHAPTER 3. REQUIREMENTS ANALYSIS 23

• Suitable size to match the company and customer requirements

• Design for both electrode and channel application

• A cavity for holding liquids is needed for electrochemical measurements

• Chip-level passivation separating the channels from the connection pads

• Consider and adapt the design for medical applications

Chapter 4

Implementation

This chapter describes the design process of the graphene biosensor. The sensor will bepart of a future development kit and need therefore be designed with further developmentand usability in mind. The design incorporates requirements from the various electrochemicalmeasurement techniques that have been studied in section 2.3 and also regards other requestsfrom the Requirements Analysis (Chapter 3). Some basic concepts for a future measurementdevice are also presented.

The initial, most fundamental specification of the sensor was to supply a graphene sensorsurface and means to connect it to a measurement device. Thus, the sensor consists of thesensor chip, which is a patterned graphene on SiC sample, and a chip holder. The holderprovides electrical contacts to the channels on the chip surface and holds the chip in place.

Three design decisions were paramount for starting the design process. First, the request tohave passivation able to withstand DMF (see section 3.3) meant initial research into bondingthe chip onto a PCB or a ceramic chip was abandoned in favour of a holder where the chipwould not be permanently attached. Secondly, since the majority of biosensing applicationsmean the sensor is subjected to liquids, it was decided that the holder should feature a cavityon top of the sensor chip. Two mechanical solutions for this type of holder were considered:

1. A PCB based design with plastic parts pressing on the sensor to make contact andform the cavity.

2. A Test-rig-based design consisting of a bed of needles-type of tester with an addedplastic cylinder to be pressed onto the sensor along with the probe needles.

The third design decision was discarding the second alternative due to the specialist nature oftest-rig manufacturing. Designing the holder around a PCB meant more common componentsand methods could be used.

25

26 CHAPTER 4. IMPLEMENTATION

4.1 Chip Holder

The longer term motivation for designing a chip holder was to interface with the developmentkit measurement electronics. Although, in a short time-frame, it also added value by removingthe need for a wafer prober or similar instruments for connecting test equipment to the sensorchip.

Because of certain design decisions (discussed later), the sensor chip was designed withcontacts on two sides. This meant it could not be dipped into a solution, leading up to thedesign decision to provide a cavity to hold liquids as described above.

Several methods for physically connecting the chip to the holder were evaluated. As men-tioned before, bonding was an early alternative, but after the design decision of adding acontainer, three possible methods were investigated:

1. QFN open-top socket or ”burn-in” test-socket (Figure 4.1a)

2. Single sided board-to-board spring connectors (Figure 4.1b)

3. Spring header connectors (Figure 4.1c)

(a) QFN open-top (b) Board-to-board connector (c) Pin headers

Figure 4.1: Connectors alternative

To ensure good connection, each contact needed to be spring-loaded. Furthermore, the padsand sensing area are on the same side of the chip (as discussed later). This fact made it almostimpossible to find a suitable QFN socket without ordering a custom made one. Board-to-board connectors had the advantages of being flexible, but the spring contacts either had tobe soldered individually or had the soldering pads located on the wrong side of the contact,making this solution overly complicated or restraining access to the active sensing area.Spring-loaded headers were found to be the most feasible solution as they come in fairly

CHAPTER 4. IMPLEMENTATION 27

dense 1.27 mm pitch and do not require much width to be mounted, enabling maximalcut-out in the PCB for accessing the sensing area.

In the initial design, standard 2.54 mm pitch angled pin headers were used to connect thechip holder to measurement equipment. The header would be connected with a ribbon cableto a breakout board, and in the future directly to custom measurement equipment. Figure4.2 shows drafts of the first version of the chip holder, where a bottom PCB was used to fixthe chip, positioning the pads of the chip under the spring headers.

(a) Top view (b) Side view (c) 3D view

Figure 4.2: First version of the chip holder.

The requirements analysis showed cavities of the size 4x5 mm and holes as small as 2.5 mmin diameter have been used to hold liquids in experimental setups. The cavity size is a trade-off between chip size, number of channels and channel size and will be further discussed inSection 4.2.

The cavity design was iterative and started with deciding a preferred size, initially 10x10 mm.With a known circumference, the next step was to ensure the existence of a suitable o-ring forsealing against the sensor surface. Because of other design decisions which will be discussedbelow, the final dimensions were shrunk to 8x4 mm. The resulting sketch is shown in Figure4.3.


(a) Top view (b) Side view (c) 3D view

Figure 4.3: Initial container design sketch.

Feedback and additional information from researchers was gathered in order to improve theinitial design. Some comments were:

• To reduce the risk of interferences while measuring the small currents in question, theinterface cable should be a 50 Ω coaxial cable.

• Vibrations have to be minimised with a more stable bottom plate.

• The chip needs to be connected with 50 Ω BNC connectors to be compatible withcommon measuring equipment.

• A lid for the cavity is needed to fix the electrodes and reduce evaporation.

Corresponding changes in the design were made. The pin headers and ribbon cable werereplaced with a high speed connector and coaxial ribbon cable, and a 0.5 m cable was usedto connect to a breakout board with 20 BNC connectors. In this way, different channelscan be connected without moving the chip holder itself, minimising vibrations. Since bothcable and connectors are designed for 50 Ω it was also decided to make all traces to 50 Ω

matched as well, opening up the possibility of going higher in frequency. In order to increasethe stability, the bottom PCB was replaced with a larger plastic piece. The resulting holderis shown in Figure 4.4, where 4.4a is the plastic bottom plate with PCB and lid, 4.4b showsthe sensor interface PCB and 4.4c is the BNC breakout PCB.


(a) Assembled view (b) Holder PCB (c) Breakout board

Figure 4.4: Second version holder design sketches.

4.2 Sensor Chip

In the previous chapter (Chapter 3) two different chip structures were used, i.e, ISFET andresistor sensor containing a number of graphene channels, or the electrochemical cell wherethe entire graphene surface is used as an electrode. The ISFET researchers were interested inupwards of twenty channels in varying sizes. Different channel shapes were also investigatedin early stages.

Before starting the chip design, the decision had already been made to use 1.27 mmpitchheaders for interfacing and the preferred measurements of the cavity (10x10 mm) had beenestablished. Graphensic’s standard sample size of 7x7 mmwas quickly discarded and mea-surements upwards of 15x15 mmwere considered. Weighing in the aspect that such longchannels would have very large resistance, and the financial incentive to reduce SiC area,the final size was reduced to 13x10 mm. The height was to a large extent guided by thefact that 13 mmwill accomodate 10 pads with the desired spacing and sufficient padding.The width could have been further reduced had it not been for the desire to keep the cav-ity at least 4 mmwide. Knowing the final cavity footprint, a passivated area was definedwhere the o-ring would meet the sensor. Figure 4.5 shows the initial sensor design, withyellow interconnection gold pads, red passivation and the black patterned area defining thegraphene channels. Gray areas are unpassivated SiC and are left uncovered because of theshadow-mask limitation of not being able to have completely free-standing structures (seesection 2.1.3).


Figure 4.5: Initial sensor design sketch.

In order to accurately determine the active area of the sensor channel (marked with red inFigure 4.5), chip level passivation was desired. The passivated area would make contact withan o-ring at the bottom of the chip holder cavity, creating a waterproof seal. While passi-vation is common practise in semiconductor manufacturing, there was a request recorded inthe requirements analysis to have a passivation capable of withstanding graphene surfaceactivation in solutions of DMF. DMF is a strong solvent and finding a suitable passivationwas claimed to be difficult. However, plastics able to withstand DMF up to 100C are avail-able and more notably, DMF is inferior to most oxides. Oxides like SiO2 are often used forsemiconductor passivation and posed a viable candidate. Because oxidation of the existingsurface might destroy the SLG layer, SiO2 sputtering was a better option. The idea of sput-tering SiO2 onto gold where discussed with different manufacturing experts, who expressedscepticism towards SiO2 attaching well enough to gold and graphene. Gold’s adhesion can beincreased by adding a adhesive layer of titanium, but after seeking feedback from researchers,the passivation layer was discarded. The added complexity did not increase functionality ac-cordingly as the o-ring seal around the cavity should be sufficient to define the active area.From a manufacturing standpoint, adding passivation selectively on top of graphene, SiC and


gold would mean adding previously untested steps into the patterning process. Passivationmay still be added as an additional final step with the updated design. With the basic designof the sensor locked down, work continued towards producing the chip.

4.2.1 Process Description

The sensor chips were to be manufactured at the Chalmers MC2 laboratory. To be able toproduce them, masks and a corresponding a process description had to be developed. Thedevelopment of the process description was based on a process description from a manu-factured quantum hall bar resistor (QHBR) at Chalmers. The QHBR was manufactured ongraphene grown on SiC from Graphensic and consists of a graphene channel interfaced withgold pads on each side. The proven process description can be divided into three major steps:

1. Anchoring provides mechanical stability for the gold pads by etching away grapheneand depositing gold directly on the SiC.

2. Gold-graphene creates electrical contact between gold and graphene by depositinggold on the anchored gold and the graphene.

3. Graphene channels defines the channels between the gold pads by etching.

In order to move forward, several changes needed to be made in the process description toadjust it to the larger scales used in the sensor. The necessary changes were identified incollaboration with the MC2 laboratory and incorporated into the final process description.The next step was then to design the masks needed for the processing.

Mask development

To be able to start mask development, the process limitations needed to be determined, bothfrom Graphensic and the manufacturer laboratory at Chalmers. In contact with both partiesthe following requirements were established for the process:

• Spin coating requires circular wafers

• It is recommended to leave 6 mm from the edge of the wafer, because of the resistquality

• Chip separation should be at least 200 µmfor cutting

• The largest size Graphensic are currently able to produce is 2 inch wafers


The design started by drawing an entire wafer and marking the 6 mm margin at the edge.In order to maximise the number of chips per wafer, the initial chip design in Figure 4.5 wasused to find out how the mask should be structured to fit as many as possible. This resultedin 5 chip per wafer with a large amount of free space. To prevent material waste, four 7x7mm samples where added to the wafer as well.

After defining the chip placement, the right separation for cutting was verified. To simplifythe cutting process, all chips where aligned to the same lines to minimise the number of cuts.As described in the previous section, the process requires three masks e.g. step 1 anchor mask,step 2 gold pad mask and step 3 graphene channel mask. The first thing to establish waswhether the resist should be negative or positive. For positive resist, the exposed area willbe cured and for negative resist the covered area will be cured. For the metal deposition,a lift-off process was used. In order to simplify the lift-off step, negative resist was used.Because of its profile, negative resist makes it easier to grip the resist and start the lift-off.For the channel mask, positive resist was used for similar reasons; the profile should alowthe etchant to flow directly down onto the wafer without passing around any overhang.

For the design of the anchoring mask, two things must be taken into account. The first is toensure that the metal-substrate area is sufficiently large to give enough mechanical stability.The second is to ensure ohmic contact between gold and graphene. A recommendation formthe researchers who implemented the QHBR was that the metal-substrate overlap should begreater than 5 µm. To provide ohmic contact, a contact area grater than 10 000 µm2 havebeen used to obtain a contact resistance of ∼ 1 Ω [12; 35]. Since this chip is likely to besubject to more physical stress, the anchoring was increased to improve mechanical stability.

To align the masks to the right position on the wafer, alignment marks were added. Thealignment marks were implemented as a cross with the hight 580 µmand the thickens of 180µm(Figure 4.6a) and 20 µmspacing was added for each mask. A problem with this alignmentmethod is when using negative resist twice, the second alignment mark will cover the first.This problem was solved by adding a new mark beside the old one as illustrated in Figure4.6.

(a) Anchor mask (b) Gold mask (c) Channel mask

Figure 4.6: Alignment marks for the different masks. Because the gold mask (b) covers the markfrom the anchor mask (a), a new mark is added beside it to enable alignment in the final channelpatterning step (c).


4.3 Measurement circuitry

In order to use the sensor described in this chapter, a measurement device is needed. Thefollowing section is a brief overview of what functionality would be needed in a single devicedesigned specifically for use with the sensor chip. As mentioned in Section 2.3, two typesof measurements need to be taken into account; electrochemical and classical electrical. Forelectrochemical measurements, the graphene channel is connected as an electrode, with asingle lead connected to one or both ends. For electrical measurements of the channel itself,current shall pass through it from one end to the other. A circuit capable of doing both thesetypes of measurements thus needs to support both these operating modes. Furthermore, therelevant measurements discussed in Section 2.3 need different input signals. CV measure-ments are done with a slow triangle wave as input, EIS requires a reasonably fast sinusodalwave and measuring the resistance of the channel requires a DC voltage to be applied acrossit. Figure 4.7 illustrates these three different operating modes in an overview.

CVTriangle

wavePotentiostat Ammeter

EIS Sine wave Potentiostat Ammeter

Resistance DC voltageGraphene

channelAmmeter

Figure 4.7: Functional comparison of CV, EIS and resistance measurement setups

To enable all of this functionality, an input signal generator needs to be able to generatearbitrary waveforms and feed them into either the potentiostat or the graphene channel. Inboth cases the output goes to an ammeter capable of microampere resolution.

The ramp signal used in CV is generated by sweeping the input voltage of the potentiostat.Potentiostats range from large rack-mount units to small system-on-chip (SOC) implementa-tions. One such SOC has been presented by Huang [51]. Figure 4.8 shows a simplified blockdiagram of their implementation. In the circuit, a microcontroller has been used to con-


trol a digital-to-analog converter (DAC), which in turn controls the potentiostat. The rampsignal is generated by sweeping the applied voltage at the potentiostat. By using a analog-to-digital converter (DAC), the electrolyte potential is fed back to the microcontroller in orderto measure the voltage of the three electrode setup and to maintain the constant slope of thetriangular wave [51]. The output signal is measured by an ADC connected to the workingelectrode.

Figure 4.8: Block diagram for CV measuring implementations

EIS measurements are performed by biasing the system in a semi-linear region to avoid noiseand then applying a sine wave, usually with a frequency in the order of kilohertz. In order toavoid large shifts in the biasing point, the signal needs to have a low amplitude. This placescertain demands on the measuring circuit to be able to measure low voltage and currentsignals. Figure 4.9 shows a block diagram of an EIS measurement circuit, which includes afrequency generator, biasing circuit and the measuring circuit. EIS analysis is usually doneby measuring the real and complex impedance for certain frequencies. The measurementscan be implemented in software or with more complex analog solutions. To make use of thesame measuring circuit as for CV, it can be advantageous to solve it in software. In Figure4.9, the frequency generator and the biasing block (DAC) are separated to get a solution assimilar as possible to Figure 4.8. Separating the blocks has the added advantage of enablingindependent control of the bias and the properties of the signal generator.


Figure 4.9: Block diagram for EIS measuring implementations

The resistance measurement is what would distinguish this measuring equipment form otherexisting EIS and CV measuring stations. The measurement is performed by applying avoltage over the channel and measuring the voltage drop. The requirements analysis hasshown voltages range of ±2 V and currents in the magnitude of microamperes are of interest.

Figure 4.10: Block diagram for resistance measurement implementations

Chapter 5

Results

The following chapter is a compilation of the produced results. The content is divided intothree main parts; chip holder, chip design and measurement circuitry. Chip holder resultsinclude the developed drawings and the manufactured product, while chip design consists ofthe lithography masks for that will be used for manufacturing of the chip. Finally, system-level considerations of measurement circuitry for a future development kit are presented.

5.1 Chip Holder

The sensor chip holder, as introduced in the previous chapter, holds the chip in place, provideselectrical contact to the chip channels and has a sealed cavity for testing with liquids. Figure5.1 shows the plastic parts of the holder:

• Bottom part for holding the chip (Figure 5.1c)

• Top part with cavity (Figure 5.1b)

• Lid for sealing the cavity and holding electrodes (Figure 5.1a)

37

38 CHAPTER 5. RESULTS

(a) Lid (b) Cavity

(c) Bottom part

Figure 5.1: Plastic parts of the chip holder.

All parts are made from polyoxymethylene (POM) and have a total weight of approximately270 grams. The cavity can hold 480 µL of liquid with the lid and 1280 µL without. Drawingsfor the plastic parts can be found in Appendix A.

Figure 5.2 shows the chip interface board and the coaxial connector breakout board. In bothcases, four-layer boards are used in order to make 50 Ω traces of reasonable width. The

CHAPTER 5. RESULTS 39

layer stack has a 1 mm FR-4 substrate in the middle and additional signal layers with 200µm separation on each side. All traces have been automatically dimensioned by the layoutsoftware to have 50 Ω characteristic impedance and are 0.285 mm wide. Table 5.1 shows thecomplete component list for both the coaxial breakout board and chip interface board. Thecircuit board layouts are available in Appendix C.

(a) Chip interface board (b) Coaxial breakout board

Figure 5.2: Circuit boards for the chip holder.

Table 5.1: Component list for the chip holder circuit boards.

Component Manufacturer Part nameSpring-loaded pin header Mill-Max 854-22-010-10-00110Coaxial ribbon cable Samtec ERCD-010-20.00-TED-TEU-1-DConnector Samtac ERF8-010-01-L-D-RA-L-TRBNC connector TE Connectivity AMP 5227699-1

The final holder is shown in Figure 5.3. Four bolts locked in place by slots in the bottom partare used to guide and secure both the interface board and the cavity. The lid fits into thewider top part of the cavity and the breakout board is connected via 0.5 m of coaxial ribboncable. The separation makes it easier to change connections without causing vibrations inthe sensor and thus makes measurements more stable and reliable and also enables differentbreakout boards or adaptor cables to be used in future work.


Figure 5.3: Eniter holder, mounted with Interface PCB connected to the coaxial breakout board

5.2 Sensor Chip

Three different sensor chip variants and masks for optical lithography have been designed.The masks are made for 2-inch wafers and adapted to be manufactured at the MC2 laboratoryat Chalmers. The mask set consists of three masks; pad anchoring mask (Figure 5.4a), goldpad mask (Figure 5.4b) and channel mask (Figure 5.4c). The pad anchoring and the goldpad masks are developed for negative resist and the channel mask for positive. At the timeof writing, the masks and subsequent chips are still in production and can unfortunately notbe presented.


(a) Anchor mask for negative resist

(b) Gold pad mask for negative resist (c) Channel mask for positive resist

Figure 5.4: Resulting mask for manufacturing

The 2-inch wafer size can fit five sensor chips and four 7x7 mm samples. Three of the chips aredesigned with the different channel width-length ratios 1:7, 1:10 and 1:20. The remaining twocombine three of each ratio on the same sensor. Detailed channel dimensions are presentedin Table 5.2. Figure 5.5 shows the four different sensor designs, where gray areas representthe SiC substrate, yellow gold and anchoring pads and the cross-hatched areas the graphenechannel. The mask designs and dimensions for the individual chips are available in AppendixB.


Table 5.2: Channel dimension of the resulting sensor chips

Ratio [width:length] Width [µm] Length [µm]1:7 857 60001:10 600 60001.20 300 6000

(a) 1:7 ratio (b) 1:10 ratio

(c) 1:20 ratio (d) Alternating ratios

Figure 5.5: Final chip designs.

In order to ensure mechanical stability and provide a good contact resistance, the chip has


been designed with extensive pad anchoring as seen in Figure 5.6. Contact pads all have ananchoring area ∼0.64 mm2 and the metal to graphene area is ∼0.36 mm2. At the edge tothe channel a 150 µm anchoring has been added to give a better stability where the o-ringwill be placed. The total area varies slightly depending on pad placement because the leadlength varies.

Figure 5.6: Dimensioned pad anchoring

5.3 Measurement Circuitry

Some requirements for a future development kit measurement circuit have been presented.More work is needed to establish how and if such a device should be implemented (as willbe discussed in the next chapter). A number of statements constitute the main results of theinvestigations. These statements are presented as-is and can be used as a starting point forfurther development:

1. A device should be capable of at least CV and EIS measurements in electrochemicalmode and resistance measurements in electrical mode.

2. It is possible to do small-size potentiostats, which are a central part in electrochemicalmeasurements. This confirms that a development kit for desktop use is technicallypossible.

3. It is possible to incorporate the through-channel DC measurements of interest, inaddition to electrochemical measurements, to such a device.

4. More work is needed to determine if the development costs of a measurement device areviable with regard to accuracy and the added work in for example customer support –other equipment already available on the market may be an alternate route.

Chapter 6

Discussion and Conclusion

The results presented in the previous chapter are a developed sensor where the associatedchip is still in manufacturing. Although the function of the product is hard to fully evaluatewithout the chips, it can be discussed and evaluated based on the requirement analysis. Apartfrom the requirements, this chapter will also discuss improvements for further developmentand some of the design decisions that were made. Some ethical and social aspects of a theproject are also covered.

Graphene is still very much a research area. To make progress within the timeframe ofa thesis, previously tested methods have generally been preferred to new ones. Instead,focus has been on the implementation – optical lithography for making biosensor chips fromgraphene on SiC in the way presented in this report has not been found in any other research.This direction also helped keeping the project from veering off into materials physics andmicrofabrication technology.

6.1 Holder and Chip Interface

The resulting holder and chip design depends a lot on the early decision not to use chipbonding or test-fixture design. Bonding was rejected mainly because of problems with findinga suitable passivation of the bond wires that would be chemically resistant to the harshsolvent DMF. Both methods also require specialist equipment and competence. Conversely,they could result in a smaller chip with a larger number of channels per area. In this case,other requirements were deemed more important. Apart from the DMF issue, the chip shouldbe large enough to handle by hand and liquid contact to the metal pads should be avoided.The current solution makes the chip removable using a simple PCB and some plastic parts.Making the holder non-permanent with mechanical sealing of the cavity partially moots the

45

46 CHAPTER 6. DISCUSSION AND CONCLUSION

requirement of chemical stability, since the chip can be functionalised in solvents separatelyand then placed into the holder.

A requirement that remains to be evaluated is the stability, or the amount of vibrations,in the holder. While the bottom part has been enlarged several times from the initial de-signs, the holder is still fairly small and therefore light. The importance of stable conditionswas only understood late in the design process and should have been a larger factor fromthe start. Should vibrations become a problem, the size of the bottom part can easily beincreased further and/or heavier materials like metal could be introduced. In a future ver-sion, the plastic design can also be improved to minimise time, cost and material waste inmanufacturing.

Another requirement which was raised late in the project was high frequency measurementcompatibility. The motivations for this are somewhat vague since the highest frequencies inquestion are in EIS, which is typically done in the kilohertz range. Of course, all aspects ofgraphene are interesting to investigate at the current state of research, but most informa-tion suggests the high frequency compatibility will mainly be a marketing feature until the”normal” frequency ranges have been extensively investigated. From an electronics perspec-tive, a high frequency design has been hard to achieve since the impedance of the channelis presently unknown and it was therefore impossible to design a matching network for it.Furthermore, the whole point of the senor is its changing impedance as a sign of detection.The effects on performance caused by high frequency and matching networks has not beenconsidered in this work but should definitely be investigated for future implementations. Inthe current design, the trance width is adjusted to obtain 50 Ω characteristic impedance. Inorder to improve this further the spring loaded headers could be replaced by connectors withcloser contacts and larger contact area to minimise parasitic capacitances. An even bettersolution would be to design a bonding solution with matching networks for each channel.

The coaxial ribbon cable used for connecting the sensor to the BNC breakout board presentedan unexpected problem. Initially, the manufacturer provided an offer for the specific cablesrequired, quoting a price and delivery time for a cable with the same connector at eachend. At the time of order, however, said cable was only available with one of the connectorsrotated upside-down, even though they are built to order. This has resulted in the silkscreenof the BNC breakout board not matching the interface board and means the cable mustbe twisted to connect the boards. Additional labeling can be done to cover the incorrectmarkings.

CHAPTER 6. DISCUSSION AND CONCLUSION 47

6.2 Chip and Mask Design

At the time of writing, the chip has not yet been manufactured and therefore the functionalityhas not been verified. In order to avoid problems during and after manufacturing, a previouslytested manufacturing process has been used. A lot of work has gone into understandingand defining all the processing steps from an initial draft of the previously used processdescription. Since a more detailed process description was found in a later stage, some ofthe investigations could have been avoided. Understanding the process was a big part of themask development, since it resulted in knowledge when to use negative or positive resist,and the functionality and implementation of the anchoring.

The choice of using already proven methods resulted in rejecting the idea of chip levelpassivation. It is likely chip level passivation will be needed in a later stage if the chips areincluded in a consumer product. A main question was how well the passivation will attachto gold. This has been solved before in microfabrication and should not pose an insolvableproblem. It can for example be of interest to add more adhesion metal on top of the gold. Ifthis affects the contact resistance, the anchoring area can be minimised to provide a largeroverlap between gold and graphene. It is suggested microfabrication experts are consultedfor further exploration of the possibilities for chip level passivation.

The chips are designed to enable electrode, ISFET and resistance measurements. In order toevaluate the functionality, the manufactured chips need to be tested by both the companyand by researchers. If overall functionality is good, the width-length ratios are the featuremost likely to be subject to change. Current rations were taken straight from the requirementanalysis. Some contradictory theories regarding the effects of different ratios have prevailedthroughout the project. Testing is required to establish if the ratio is important or not.Continued close collaboration with researchers and other presumptive customers is suggestedas the best way to find out what ratios to use.

Not all requirements from the Requirements Analysis have been fulfilled with the presentedchip. For example, the chip has 10 channels instead of 20 and no chip-level passivation. Someof the reasoning behind this has been presented previously; the spring loaded pin headerpitch dictated the channel count and passivation was abandoned to ensure the previouslyused process description could be followed.

An improvement that should be implemented in future mask versions is to add cutting marksto define where each chip is placed. The marks can be added to the anchoring mask for goodadhesion to the SiC and could then also be used as additional alignment marks.

48 CHAPTER 6. DISCUSSION AND CONCLUSION

6.3 Measuring Equipment

Information about electrochemistry gathered for the Theory chapter and requirements re-lated to measurements have been summarised to lay a foundation for continued work onmeasurement equipment for the planned development kit based around the presented sen-sor. While a measurement device implementing CV, EIS and resistance measurements ispossible, potentiostats and other electrochemical test equipment is already present in themarket. The interesting questions become whether Graphensic have, or want to have, theknowledge and resources to design their own? Is the customer base sufficiently large to sup-port it? Are there existing products that can be used? A proper market analysis is suggestedto establish the necessity of custom hardware for the development kit and further define therequirements for it.

6.4 Ethical and Social Aspects

Working with devices that can be used for medical equipment raises some ethical issues. If,in the future, the graphene biosensor can be used to create the envisioned self-tests, shouldthere be any limitations to what can be diagnosed? The consequences and implications ofsome diagnoses, for example cancer or terminal diseases like AIDS, maybe are too vast notto be made by a professional doctor? What will happen if these advanced self tests becomecommonly available? Surely it will not be possible to completely avoid false positives, oreven worse – false negatives. These types of questions need to be taken into consideration bythe future customers of Graphensic. The work done in this thesis is merely a foundation forthe research of sensor chips that will potentially go in to these types of devices. As such, theacademic strive to see what is possible is far over shadowing any of these hypothetical issues.Furthermore, before arriving at self-tests, graphene sensors will probably be used in largerlab equipment as suggested in the introduction. Here the outlook seems purely positive -if graphene biosensors can be used to create newer, cheaper and maybe more capable andaccurate lab equipment, the benefits for all of medical care should be obvious.

From an economic perspective, the possible impact of graphene biosensors seems to be equallypositive. If a cheap and accessible way to diagnose flu or other infectious diseases existed,even the smallest workplace epidemics could potentially be avoided. This would financiallybenefit companies as well as individuals and to some extent decrease the strain on healthcare,potentially even having an impact on the macroeconomic level. To reach this state, futureresearch based on the sensor chip presented in this thesis and similar projects needs to besuccessful in creating functioning sensors that are viable for mass production. Near time im-pact will presumably be limited to Graphensic itself. If the sensor is in fact useful in research,Graphensic will dominate the market - providing a good base for sales and marketing.

CHAPTER 6. DISCUSSION AND CONCLUSION 49

6.5 Conclusion

Requirements for a graphene based biosensor chip, a chip holder and potential measurementdevice have been investigated and designs for the first two have been presented. The sensoris still to be evaluated, but regardless of its function, the development process has broughtGraphensic closer to the biosensor research area. The presented sensor and holder can beused as a tool for investigating graphene on SiC as a base material for biosensors and willhopefully be used by researchers to create unprecedented devices.

Bibliography

[1] “The story of graphene,” May 2014. [Online]. Available: http://www.graphene.manchester.ac.uk/story/

[2] “The nobel prize in physics 2010,” May 2014. [Online]. Available: http://www.nobelprize.org/nobel prizes/physics/laureates/2010/

[3] E. Waltz, “How i quantified myself,” IEEE Spectrum, vol. 49, no. 9, p. 42, 2012.[Online]. Available: https://lt.ltag.bibl.liu.se/login?url=http://search.ebscohost.com/login.aspx?direct=true&db=edb&AN=79467035&site=eds-live

[4] “Apoteket,” May 2014. [Online]. Available: http://www.apoteket.se/privatpersoner/common/search.aspx?q=sj%C3%A4lvtest

[5] R. Comeaux, Biosensors properties, materials and applications. New York: Nova Sci-ence Publishers, 2009.

[6] P. R. Wallace, “The band theory of graphite,” Phys. Rev., vol. 71, pp. 622–634, May1947. [Online]. Available: http://link.aps.org/doi/10.1103/PhysRev.71.622

[7] A. K. Geim and P. Kim, “Carbon Wonderland,” Scientific American, vol.298, no. 4, pp. 90–97, Apr. 2008. [Online]. Available: http://dx.doi.org/10.1038/scientificamerican0408-90

[8] V. Singh, D. Joung, L. Zhai, S. Das, S. I. Khondaker, and S. Seal, “Graphene basedmaterials: Past, present and future,” Progress in Materials Science, vol. 56, no. 8, pp.1178 – 1271, 2011. [Online]. Available: http://www.sciencedirect.com/science/article/pii/S0079642511000442

[9] N. Savage, “Materials science: Super carbon,” Nature, vol. 483, pp. 30–31, March 2012.

[10] V. Georgakilas, M. Otyepka, A. B. Bourlinos, V. Chandra, N. Kim, K. C.Kemp, P. Hobza, R. Zboril, and K. S. Kim, “Functionalization of graphene:Covalent and non-covalent approaches, derivatives and applications,” Chemical

51

52 BIBLIOGRAPHY

Reviews, vol. 112, no. 11, pp. 6156–6214, 2012. [Online]. Available: http://pubs.acs.org/doi/abs/10.1021/cr3000412

[11] F. Schwierz, “Graphene transistors,” NATURE, vol. 5, pp. 487–496, July 2010.

[12] A. Tzalenchuk, O. Kazakova, T. Janssen, S. Lara-Avila, A. Kalaboukhov,S. Kubatkin, S. Paolillo, M. Syvajarvi, R. Yakimova, and V. Fal’Ko,“Towards a quantum resistance standard based on epitaxial graphene.”Nature Nanotechnology, vol. 5, no. 3, pp. 186–189, 2010. [Online].Available: https://lt.ltag.bibl.liu.se/login?url=http://search.ebscohost.com/login.aspx?direct=true&db=edselc&AN=edselc.2-52.0-77949267645&site=eds-live

[13] Q. Zhou and A. Zettl, “Electrostatic graphene loudspeaker,”Applied Physics Letters, vol. 102, no. 22, 2013. [Online].Available: https://lt.ltag.bibl.liu.se/login?url=http://search.ebscohost.com/login.aspx?direct=true&db=edselc&AN=edselc.2-52.0-84879107875&site=eds-live

[14] Y. Shao, J. Wang, H. Wu, J. Liu, I. Aksay, and Y. Lin, “Graphene basedelectrochemical sensors and biosensors: A review,” Electroanalysis, vol. 22, no. 10, pp.1027–1036, 2010. [Online]. Available: http://dx.doi.org/10.1002/elan.200900571

[15] H.-J. Park, S. K. Kim, K. Park, H.-K. Lyu, C.-S. Lee, S. J. Chung, W. S.Yun, M. Kim, and B. H. Chung, “An isfet biosensor for the monitoring ofmaltose-induced conformational changes in mbp,” FEBS Letters, vol. 583, no. 1, pp.157 – 162, 2009. [Online]. Available: http://www.sciencedirect.com/science/article/pii/S0014579308009514

[16] S. Okamoto, Y. Ohno, K. Maehashi, K. Inoue, and K. Matsumoto, “Fragment-modifiedgraphene fet for highly sensitive detection of antigen-antibody reaction,” in 14th

International Meeting on Chemical Sensors - IMCS 2012. AMA Service, 2012, pp.519 – 522. [Online]. Available: http://www.ama-science.org/home/details/967

[17] S. Mao, K. Yu, G. Lu, and J. Chen, “Highly sensitive protein sensor based on thermally-reduced graphene oxide field-effect transistor,” Nano Research, vol. 4, no. 10, pp. 921–930, 2011.

[18] Y. Huang, X. Dong, Y. Liu, L.-J. Li, and P. Chen, “Graphene-based biosensors fordetection of bacteria and their metabolic activities,” Journal of Materials Chemistry,vol. 21, no. 33, pp. 12 358–12 362, 2011, cited By (since 1996)46. [Online]. Avail-able: http://www.scopus.com/inward/record.url?eid=2-s2.0-80051618011&partnerID=40&md5=ab675ee86700dea3c1f459fb50ca8e2d

BIBLIOGRAPHY 53

[19] Fabrication of Ultrasensitive Graphene Nanobiosensors., Swansea Univer-sity, School of Engineering, Singleton Park, Swansea, UK, 2010. [Online].Available: https://lt.ltag.bibl.liu.se/login?url=http://search.ebscohost.com/login.aspx?direct=true&db=inh&AN=11831729&site=eds-live

[20] O. J. Guy, G. Burwell, Z. Tehrani, A. Castaing, K.-A. Walker, and S. Doak, “Graphenenano-biosensors for detection of cancer risk,” Materials Science Forum, vol. 711, pp.246–252, January 2012.

[21] W. Fu, C. Nef, A. Tarasov, M. Wipf, R. Stoop, O. Knopfmacher, M. Weiss, M. Calame,and C. Schonenberger, “High mobility graphene ion-sensitive field-effect transistors bynoncovalent functionalization,” Nanoscale, vol. 5, pp. 12 104–12 110, 2013. [Online].Available: http://dx.doi.org/10.1039/C3NR03940D

[22] S. Chen, Z.-B. Zhang, P. Ahlberg, X. Gao, S.-L. Zhang, L. Ma, W. Ren,H.-M. Cheng, Z. Qiu, and D. Wu, “A graphene field-effect capacitor sen-sor in electrolyte,” Applied Physics Letters, vol. 101, no. 15, 2012. [Online].Available: https://lt.ltag.bibl.liu.se/login?url=http://search.ebscohost.com/login.aspx?direct=true&db=edselc&AN=edselc.2-52.0-84867525394&site=eds-live

[23] R. Harris, Modern Physics, 2nd ed. Addison Wesley, 2007.

[24] P. Atkins and J. de Paula, Atkins Physical Chemistry, 8th ed. New York: W.H. Freeman,2006.

[25] C. N. R. Rao, Graphene synthesis, properties, and phenomena. Weinheim: Wiley-VCH,2013.

[26] A. K. Geim and K. S. Novoselov, “The rise of graphene,” Nature Materials, vol. 6,no. 3, pp. 183–191, 2007. [Online]. Available: http://dx.doi.org/10.1038/nmat1849

[27] A. H. Castro Neto, F. Guinea, N. M. R. Peres, K. S. Novoselov, and A. K. Geim, “Theelectronic properties of graphene,” Rev. Mod. Phys., vol. 81, pp. 109–162, Jan 2009.[Online]. Available: http://link.aps.org/doi/10.1103/RevModPhys.81.109

[28] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V.Dubonos, I. V. Crigorieva, and A. A. Firsov, “Electric field effect in atomicallythin carbon films,” Science, vol. 306, no. 5696, pp. 666 – 669, 2004. [Online].Available: https://lt.ltag.bibl.liu.se/login?url=http://search.ebscohost.com/login.aspx?direct=true&db=aph&AN=14884266&site=ehost-live

[29] K. Novoselov, D. Jiang, F. Schedin, T. Booth, V. Khotkevich, A. Geim, andS. Morozov, “Two-dimensional atomic crystals.” Proceedings of the National Academy

54 BIBLIOGRAPHY

of Sciences of the United States of America, vol. 102, no. 30, pp. 10 451–10 453, 2005.[Online]. Available: https://lt.ltag.bibl.liu.se/login?url=http://search.ebscohost.com/login.aspx?direct=true&db=edselc&AN=edselc.2-52.0-23044442056&site=eds-live

[30] T. Seyller, L. Ley, F. Speck, A. Bostwick, J. McChesney, E. Rotenberg, K. Horn, T. Ohta,J. Riley, and K. Emtsev, Epitaxial Graphene: A New Material. Universitat Erlangen-Nurnberg, Institut fur Physik der Kondensierten Materie: Wiley-VCH, 2011, vol. 1.[Online]. Available: https://lt.ltag.bibl.liu.se/login?url=http://search.ebscohost.com/login.aspx?direct=true&db=edselc&AN=edselc.2-52.0-84885490027&site=eds-live

[31] C. Virojanadara, R. Yakimova, A. A. Zakharov, and L. Johansson, “Large homogeneousmono-/bi-layer graphene on 6h-sic(0001) and buffer layer elimination,” JOURNAL OF

PHYSICS D-APPLIED PHYSICS, vol. 43, no. 37, pp. 374 010–, 2010.

[32] H.-C. Kang, H. Karasawa, Y. Miyamoto, H. Handa, T. Suemitsu, M. Suemitsu,and T. Otsuji, “Epitaxial graphene field-effect transistors on silicon substrates,”Solid-State Electronics, vol. 54, no. 9, pp. 1010 – 1014, 2010. [Online]. Available:http://www.sciencedirect.com/science/article/pii/S0038110110001358

[33] E. Pallecchi, M. Ridene, D. Kazazis, C. Mathieu, F. Schopfer, W. Poirier, D. Mailly,and A. Ouerghi, “Observation of the quantum hall effect in epitaxial graphene onsic(0001) with oxygen adsorption.” Applied Physics Letters, vol. 100, no. 25, p. 253109,2012. [Online]. Available: https://lt.ltag.bibl.liu.se/login?url=http://search.ebscohost.com/login.aspx?direct=true&db=aph&AN=77330043&site=eds-live

[34] S. Franssila, Introduction to Microfabrication, 2nd ed. Wiley & Sons, 2007.

[35] S. Avila, Magnetotransport characterization of epitaxial graphene on SiC. Goteborg:Chalmers University of Technology, 2012.

[36] B. Eggins, Chemical sensors and biosensors. Chichester Hoboken, NJ: J. Wiley, 2002.

[37] M. Pumera, “Graphene in biosensing,” Materials Today, vol. 14, no. 7–8, pp. 308– 315, 2011. [Online]. Available: http://www.sciencedirect.com/science/article/pii/S1369702111701602

[38] V. Krasnenko, J. Kikas, and M. G. Brik, “Modification of the structural andelectronic properties of graphene by the benzene molecule adsorption,” Physica B:

Condensed Matter, vol. 407, no. 23, pp. 4557 – 4561, 2012. [Online]. Available:http://www.sciencedirect.com/science/article/pii/S0921452612008022

[39] S. P. Economopoulos and N. Tagmatarchis, “Chemical functionalization of exfoliatedgraphene,” Chemistry – A European Journal, vol. 19, no. 39, pp. 12 930–12 936, 2013.[Online]. Available: http://dx.doi.org/10.1002/chem.201302358

BIBLIOGRAPHY 55

[40] W. Wroblewski, “Field effect transistors (fets) as transducers in electrochemicalsensors,” 2005. [Online]. Available: http://csrg.ch.pw.edu.pl/tutorials/isfet/

[41] S. Chen, “Electronic sensors based on nanostructured field-effect devices,” Ph.D. dis-sertation, Uppsala University, Solid State Electronics, 2013.

[42] M. Yuqing, G. Jianguo, and C. Jianrong, “Ion sensitive field effect transducer-basedbiosensors,” Biotechnology Advances, vol. 21, no. 6, pp. 527 – 534, 2003. [Online].Available: http://www.sciencedirect.com/science/article/pii/S0734975003001034

[43] P. Bergveld, “Thirty years of isfetology: What happened in the past 30years and what may happen in the next 30 years,” Sensors and Actuators

B: Chemical, vol. 88, no. 1, pp. 1 – 20, 2003. [Online]. Available: http://www.sciencedirect.com/science/article/pii/S0925400502003015

[44] B. Zhang and T. Cui, “An ultrasensitive and low-cost graphene sensor based onlayer-by-layer nano self-assembly.” Applied Physics Letters, vol. 98, no. 7, p. 073116,2011. [Online]. Available: https://lt.ltag.bibl.liu.se/login?url=http://search.ebscohost.com/login.aspx?direct=true&db=aph&AN=58509087&site=ehost-live

[45] W. Schmickler, Interfacial electrochemistry. New York: Oxford University Press, 1996.

[46] B. Laird, University chemistry. Dubuque, IA: McGraw-Hill, 2009.

[47] A. Bard, Electrochemical methods : fundamentals and applications. New York: Wiley,2001.

[48] S. Chen, J. Zhu, and X. Wang, “One-step synthesis of graphene-cobalthydroxide nanocomposites and their electrochemical properties.” Journal of

Physical Chemistry C, vol. 114, no. 27, pp. 11 829–11 834, 2010. [Online].Available: https://lt.ltag.bibl.liu.se/login?url=http://search.ebscohost.com/login.aspx?direct=true&db=edselc&AN=edselc.2-52.0-77955315017&site=eds-live

[49] M. Orazem, Electrochemical impedance spectroscopy. Hoboken, N.J: Wiley, 2008.

[50] V. Lvovich, Impedance spectroscopy applications to electrochemical and dielectric phe-

nomena. Hoboken, N.J: Wiley, 2012.

[51] C.-Y. Huang, Y.-C. Huang, T.-Y. Lin, X.-F. Li, and C.-H. Chang, “Ansoc-based portable cyclic voltammetry potentiostat for micro-albumin biosen-sors.” in ICIEA 2007: 2007 Second IEEE Conference on Industrial Elec-

tronics and Applications, no. ICIEA 2007: 2007 Second IEEE Conferenceon Industrial Electronics and Applications, 2007, pp. 604–609. [Online].Available: https://lt.ltag.bibl.liu.se/login?url=http://search.ebscohost.com/login.aspx?direct=true&db=edselc&AN=edselc.2-52.0-35248901965&site=eds-live

Appendix A

Holder Design

A.1 Lid Drawings

Figure A.1: Lid overview

57

58 APPENDIX A. HOLDER DESIGN

Figure A.2: Lid top-view drawing

Figure A.3: Lid side-view drawing

APPENDIX A. HOLDER DESIGN 59

A.2 Cavity Drawings

(a) Top (b) Bottom

Figure A.4: Cavity overview

Figure A.5: Cavity top-view drawing


Figure A.6: Cavity bottom-view drawing

Figure A.7: Cavity side-view drawing

APPENDIX A. HOLDER DESIGN 61

A.3 Bottom Plate Drawings

Figure A.8: Bottom overview

Figure A.9: Bottom top-view drawing


Figure A.10: Bottom side-view drawing

Appendix B

Masks and Chip Design

B.1 Chip

Figure B.1: Dimensioned sensor chip with ratio 1:7

63

64 APPENDIX B. MASKS AND CHIP DESIGN



APPENDIX B. MASKS AND CHIP DESIGN 65

Figure B.4: Dimensioned sensor chip with alternating ratio 1:7, 1:10 and 1:20


B.2 Masks

Figure B.5: Anchor mask for negative resist

APPENDIX B. MASKS AND CHIP DESIGN 67

Figure B.6: Gold pad mask for negative resist


Figure B.7: Channel mask for positive resist

70 APPENDIX C. CIRCUIT BOARD DESIGNS

Appendix C

Circuit Board Designs

C.1 Holder

Figure C.1: Holder PCB

APPENDIX C. CIRCUIT BOARD DESIGNS 71

Figure C.2: Holder PCB top layer


Figure C.3: Holder PCB ground planes

Figure C.4: Holder PCB bottom layer


C.2 BNC Breakout Board

Figure C.5: BNC Breakout Board


Figure C.6: BNC Breakout Board top layer


Figure C.7: BNC Breakout Board ground planes


Figure C.8: BNC Breakout Board bottom layer

graphene on silicon carbide chip for biosensing applications730121/fulltext01.pdf · graphensic ab...

Documents