Development of a Oxygen Sensor for Marine ... - DTU Nanotech

Development of a Oxygen 

Sensor for Marine Environment 

Studies 

M.Sc. Thesis 

Peter Kastberg Hansen s958291 

Supervisor: Erik V Thomsen 

September 20, 2005

ii 

MIC - Department of Micro and Nanotechnology 

Technical University of Denmark 

DTU Building 345east 

DK-2800 Kgs. Lyngby

Abstract 

This thesis described the first steps towards creating a oxygen sensor for 

measuring dissolved oxygen in the oceans, with the intended use as part of 

a larger project concerning fish behavior studies. The sensor is a Clark type 

oxygen sensor, with an thermal platinum sensor added as well. 

The first part describes the reasons for choosing the Clark sensor and the 

theory behind. 

The second part describes the design considerations and the fabrication steps 

that leads to the layout. 

While the final part deals with problems encountered, solutions to them, 

results gathered, and ways of improving the design. 

i

Acknowledgements 

I would like to say thanks the following people for their help, inspiration and 

patience; Erik V. Thomsen (my supervisor), Anders Hyldg˚ard (effectively the 

co-supervisor), and the rest of the Applied Sensors group at MIC. Also thanks 

to Oliver Geschke for giving some valuable tips and insight on the workings 

of the Clark sensor early on. I would also like to thank the Laboratory 

Technicians for help in the Clean room, and the other master students in 

room 119 at MIC for a fun and spirited atmosphere, and for giving a hand 

with a lot of small things. 

iii

Contents 

1 Introduction 1 

1.1 Dissolved Oxygen Level . . . . . . . . . . . . . . . . . . . . . 2 

1.2 Thesis Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

2 Optodes and ISFET 7 

2.1 Optodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

2.2 ISFET (Ion Selective Field Effect Transistor) . . . . . . . . . . 9 

2.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

3 Theory of the Clark Sensor 13 

3.1 Chemistry of the Clark Sensor . . . . . . . . . . . . . . . . . . 14 

3.1.1 One layer electrode model . . . . . . . . . . . . . . . . 16 

3.1.2 Two layer model . . . . . . . . . . . . . . . . . . . . . 19 

3.2 The Potentiostat . . . . . . . . . . . . . . . . . . . . . . . . . 22 

3.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 

4 System Design 27 

4.1 General Design . . . . . . . . . . . . . . . . . . . . . . . . . . 27 

4.2 Actual Design . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 

4.2.1 Minor Design Variations . . . . . . . . . . . . . . . . . 32 

4.3 Temperature Sensor . . . . . . . . . . . . . . . . . . . . . . . . 33 

v

vi CONTENTS 

4.4 Packaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 

4.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 

5 Fabrication 41 

5.1 Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 

5.2 Back-end Processing . . . . . . . . . . . . . . . . . . . . . . . 45 

5.2.1 Packaging . . . . . . . . . . . . . . . . . . . . . . . . . 45 

5.2.2 Ag to Ag/AgCl . . . . . . . . . . . . . . . . . . . . . . 47 

5.2.3 Electrolyte and Membrane . . . . . . . . . . . . . . . . 47 

5.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 

6 Problems and Solutions 49 

6.1 Wirebonding . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 

6.1.1 Conducting Glue . . . . . . . . . . . . . . . . . . . . . 50 

6.2 The flex print . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 

6.2.1 Non Conducting Glue . . . . . . . . . . . . . . . . . . 54 

6.3 The ’O’-ring . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 

6.3.1 Changing the resting area . . . . . . . . . . . . . . . . 55 

6.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 

7 Evaluation of Results and Measurements 57 

7.1 Fabrication results . . . . . . . . . . . . . . . . . . . . . . . . 57 

7.2 Temperature Sensor . . . . . . . . . . . . . . . . . . . . . . . . 58 

7.3 Clark Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 

7.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 

8 Conclusion 63 

A Silicide Recipe 69

CONTENTS vii 

B Fabrication Process 71 

C Process Sequence 75

viii CONTENTS

Chapter 1 

Introduction 

Roughly 70 % of the world is covered by water[1], and have been an inspiration 

for mankind over the ages, from explorers such as Columbus to works of 

fiction, such as Hemingway’s ’The Old Man and the Sea’. Several time the 

oceans have shown the tremendous forces that rest within in various forms 

of natural disasters such as tidal waves, floods, tsunamis and many other. 

However the oceans is also a source of life, in fact the original source of life, 

not just for us but for the fish that lives below. 

This thesis have been subpart of a larger project (Fish and Chips), where 

the main goal is the development of a multi-sensor for marine environmental 

studies. In that, the prototype for a Micro Electronic Mechanical System 

(MEMS) have been developed, with then intended use for fish tagging. While 

this thesis does not have quite as wide a scope, it does focus on one of the 

more important aspects of it, namely oxygen, or rather dissolved oxygen. 

There are commercially available dissolved oxygen sensors already today, 

however these are more often than not macro scale sensors attached to a 

buoy, and therefore only measuring in one place. They can also be attached 

to ships, however they will only measure the oxygen level at the surface, and 

therefore not give a full accurate picture of the oxygen level. 

Why dissolved oxygen? Well there are many reasons, mainly like us the 

fish needs to breathe, and the gills of a fish are not all that different form 

our lungs[2]. Basically there needs to be enough dissolved oxygen in the 

water to favor diffusion of oxygen from the water, through the gills of the 

fish and into the blood. When the oxygen falls below a certain level this 

passive diffusion fails to be able to drive, the oxygen into the blood and the 

fish suffocates much like a human might at high altitudes. So by monitoring 

1

2 CHAPTER 1. INTRODUCTION 

the oxygen level it is possible to learn more about the behavior and health 

of the fish. Also given the relationship between dissolved oxygen levels and 

temperature(more about this later), it provides an idea of where the fish have 

been.[3, 4] 

Figure 1.1: Sketch of a fish’s gills, with oxygen leaving the water, and entering 

the bloodstream, and then subsequently moved around to 

the rest of the fish’s body. Reproduced from [5] 

Furthermore given the amount of pollution, algae growth and other problems 

in the world oceans today, it supplies additional info on the general 

health state of the oceans. The use of the sensor doesn’t need be limited 

to the oceans however, as it in theory should also be useful for on site measurement 

of the dissolved oxygen levels in rivers, creeks and lakes. Not to 

mention the medical uses involving the oxygen levels in blood. Also in these 

cases having a small easy to use sensor is preferable over a macro scale one.[6] 

1.1 Dissolved Oxygen Level 

How much dissolved oxygen is there in seawater? Under the assumption that 

the only source for the dissolved oxygen in water is the atmosphere (two 

other sources being by rapid movement(aeration), and as a waste product 

of photosynthesis. Both are natural processes, which however is affected by 

pollution.), the relationship between air and water can be represented with 

the equation: 

This has the equilibrium constant 

O2(g) ⇔ O2(aq) (1.1)

1.1. DISSOLVED OXYGEN LEVEL 3 

K = aO2 

fO2 

(1.2) 

Where aO2 is the activity of oxygen and fO2 the fugacity 1 of oxygen in 

the gas phase. 

However when the fugacity of oxygen is approximated by its partial pressure 

PO2 and the activity of oxygen in water by its concentration CO2(aq), 

Henry’s law 2 will be expressed as: 

CH = CO2(aq) 

PO2 

(1.3) 

Where CH is the Henry’s law constant, CH(O2) = 1.3 × 10 −3 Mol/atm. 

PO2 can be calculated by multiplying the fraction volume of oxygen in the 

air (21 %) with the pressure at sea level ( 760 mmHg, atmospheric pressure). 

Also in dilute solutions and for perfect gases K = CH, hence the 

concentration of oxygen in the ocean is 

C = CHPO2n = 1.3 × 10 −3 · 0.21 · 760 · 32 = 6.6mg/L (1.4) 

6.6 mg/L might not tell much on its own, however is the level of dissolved 

oxygen water drops below 5.0 mg/L, fish and other aquatic life will start to 

suffocate. The lower it drops the worse it gets, once the oxygen levels drop 

below 1-2 mg/L it will only take a few hours before most or all the fish are 

dead.[8] 

Now as mentioned earlier there is also a connection between the concentration 

of dissolved oxygen and temperature, or rather between water 

temperature and the saturation of oxygen. This is the point where water 

have the maximum amount of oxygen it can have, relative to pressure and 

temperature. It is calculated from the following equation[9]: 

Cp = C ∗ 

PW V (1 − )(1 − ΘP ) 

P P 

(1 − PW V )(1 − Θ) 

(1.5) 

1 The measure of the tendency of a gas to escape or expand[7] 

2 The mass of a gas that dissolves in a definite volume of liquid is directly proportional 

to the pressure of the gas provided the gas does not react with the solvent.

4 CHAPTER 1. INTRODUCTION 

Where 

CP = equilibrium oxygen concentration at nonstandard pressure. mg/L. 

C ∗ = equilibrium oxygen concentration at 1 atm, mg/L. 

P = nonstandard pressure, atm. 

PW V = partial pressure of water vapor, atm, calculated from 

lnPW V = 11.8571 − 3840.70 

T 

T = temperature, Kelvin 

t = temperature, o C. 

− 216.961 

T 2 

Θ = 0.000975 − 1.426 × 10 −5 t + 6.436 × 10 −8 t 2 

(1.6) 

(1.7) 

As can be seen from the equation, the warmer it gets, the less oxygen 

there is the water, as it becomes saturated faster. 

Summed up all this gives an idea of how high, or low depending on how 

you look at it, the measured values can be expected to be. 

1.2 Thesis Outline 

In Chapter 2 other ways of using micro technology to measure oxygen is 

looked at, and the reason for picking the Clark type sensor. 

In Chapter 3 the theory behind the Clark Oxygen sensor, amperometry, 

and the potentionstat is explained. 

In Chapter 4 the reasons for why the sensor ended up looking as it did 

is explained. 

In Chapter 5 contains a description of the process to create the sensor. 

In Chapter 6 the problems encountered while fabricating the sensor is discussed, 

as well as how they were solved. 

In Chapter 7 various measurements, tests, and results are presented.

1.2. THESIS OUTLINE 5 

In Chapter 8 the conclusion for the whole project is made.

6 CHAPTER 1. INTRODUCTION

Chapter 2 

Optodes and ISFET 

There are several ways to measure oxygen, and the concept of doing it is 

not entirely new, and have been done for several decades, in oceanography, 

medicine, mountaineering as well as in many other areas. However while 

several ways exists on the macro scale, there are not quite as many ways on 

the micro scale. In this chapter the pros and cons of two of the ways are 

described, the theory behind them is briefly described, though without going 

deeply into the details. A comparison to the Clark sensor is also made (the 

theory of Clark sensor is described in Chapter 3). 

2.1 Optodes 

Figure 2.1: Commercial oxygen optode from Aanderaa, actual size of the 

sensor is 36 × 86mm. Reproduced from [10] 

Traditional oxygen sensors, such as the Clark sensor described in Chapter 

3 are based on the reduction at a working electrode, which is immersed a 

electrolyte and then separated from other interfering gasses by a membrane 

7

8 CHAPTER 2. OPTODES AND ISFET 

only permeable to oxygen. The oxygen flow through the membrane being 

induced by the consumption of O2 at this electrode. With the electrical flow 

between the electrodes being proportional to this flow of oxygen. Problem 

is that this flow is highly dependant on the diffusion though the membrane, 

and in seawater these can change due to temperature and biological fouling. 

Especially the biological fouling can be a problem, not only because they 

can cause a change in the diffusion characteristics of the membrane. But 

also because biological processes quite often involves the production or the 

consumption of oxygen, and as such will influence the amount measured.[11] 

Optodes (also know as optoelectrodes or optrodes[12]) are the optical equivalent 

of electrodes, and represent a theoretical solution to this. These are 

usually based on the quenching of a fluorescent indicator by oxygen, and 

described by the Stern-Volmer equation[13]: 

I0 

I = 1 + KSV [O2] (2.1) 

Where I and I0 are the fluorescence intensities with and without oxygen 

present, KSV the Stern-Volmer quenching constant and [O2] the concentration 

of dissolved oxygen. The sensitivity depends on the value of KSV , 

which determines the change in the fluorescent intensity with various oxygen 

concentrations.[14] 

Figure 2.2: Sketch of the optical system of the optode.[10] 

The way the optical sensor, as shown on figure 2.2, works is: 

1. The blue LED emits light, at 475 nm, to an optical fiber.

2.2. ISFET (ION SELECTIVE FIELD EFFECT TRANSISTOR) 9 

2. The optical fiber then carries the light to the probe. The end of the 

probe tip consists of a thin layer of a hydrophobic material, with ruthenium 

inside it, protected from the liquid measured on. 

3. The light from the LED excites the ruthenium at the probe tip. 

4. The excited ruthenium then fluoresces, emitting energy at 600 nm. 

5. If the excited ruthenium hits an oxygen molecule, the excess energy 

is transferred to the oxygen molecule in a non-radiative transfer, decreasing 

or quenching the fluorescence). The degree of quenching corresponds 

to the level of oxygen concentration or to oxygen partial pressure 

in the film, which is in dynamic equilibrium with oxygen in the liquid. 

6. The energy is collected by the probe and carried through the optical 

fiber to the data storage/PC. Which in turn translates the data into a 

graph or a similar output, from which the oxygen level can be read. 

Some of the advantages of the optodes is that unlike traditional electrodes 

the optodes doesn’t consume oxygen, and is therefore not sensitive to stirring. 

Also the partial pressure of the oxygen in the probe is in equilibrium 

with the partial pressure outside, therefore it is less effected by pressure and 

the pressure behavior becomes easier to predict. Finally some of the commercially 

available optodes comes with warranties with up to two years, which 

allows for long term measurements.[10] 

Disadvantages of the optodes however is that they tend to be relatively 

large (the optode itself is small, but the electronics and power supply adds 

considerably to the size), and is therefor often attached to ships or buoys. 

It also have a relatively large power consumption in comparison to a micro 

Clark sensor. 

2.2 ISFET (Ion Selective Field Effect Transistor) 

Electrochemical sensors are one of the older and larger groups of chemical 

sensor, the sensor type includes those based on electrochemical and charge 

transfer reactions, meaning the transfer of charge from an electrode to a 

solid or liquid phase, or the other way around. The chemical process takes 

place at the electrodes or in the probed volume, and the current from this

10 CHAPTER 2. OPTODES AND ISFET 

is measured. A electrochemical sensor are always made up of a minimum 

of two electrodes, one that allows for connection through the probed sample 

and the other via measuring equipment or a transducer. 

There are various electrochemical sensors, which can be defined by their 

analytical principles[15]: 

1. Voltammetric sensors; which measures the current-voltage relationship. 

The principal behind it, is having a potential applied, and measuring 

a current proportional to the concentration of the electro-active substance 

(a variant of this is amperometry[16], wherein the potential is 

kept constant.). 

2. Conductometric sensors; Which measures the conductance, by applying 

a small amplitude AC potential to a pair of electrodes to stop 

polarization. The charge carries will then determine the conductance. 

3. Potentiometric sensors; which measures potential of an electrode at 

equilibrium state, meaning no current is allowed to flow at the time of 

the measurement. 

While all three can use CMOS based technology, it is especially in the 

case of the later, potentiometric sensors, that ISFET are used.[17] 

Figure 2.3: (a) diagram of a MOSFET (b) diagram of a ISFET. The metal 

gate of the MOSFET is replaced by the metal of a reference electrode, 

and a liquid to make contact with the bare gate insulator. 

Reproduced from [18]. 

The ISFET is a variant of a MOSFET (see figure 2.3), where the MOS- 

FET is used for gas sensing, the ISFET is used for liquids. The more traditional 

of these are the pH-ISFET, where the basic principle is the electrolysis

2.3. SUMMARY 11 

of oxygen molecules, which is similar to that of a Clark sensor (more about 

this in Chapter 3). However the way the ISFET detects it, is by determining 

the surface potential at the insulator/electrolyte interface.[19] Also in 

the case of the pH-ISFET the surface of the gate oxide will contain OH − 

molecules, which can be protonated and deprotonated, so when the gate oxide 

contacts an aqueous solution, the change in pH will cause the silicon 

surface potential to change as well. 

The advantages of the ISFET is that the CMOS and MOSFET are tried 

and used technologies. However chemical sensors are still a relative new area 

for this to be used in, and as such suffers from ’child’-diseases. 

Disadvantages have included light sensitivity, lack of solid-state reference 

electrodes, and packaging integrity difficulties. [20] 

2.3 Summary 

While both the Optodes and the ISFET are viable ways of creating a oxygen 

sensor, both had certain distinct disadvantages that made the Clark type a 

better choice. However of the two, the optodes might in the future become a 

superior solution, once the problems with size and power supply have been 

solved.

12 CHAPTER 2. OPTODES AND ISFET

Chapter 3 

Theory of the Clark Sensor 

The Clark sensor was named after its creator Leland C. Clark[21], who in 

the late fifties and mid sixties studied the electrochemistry of oxygen gas 

reduction at platinum (Pt) metal electrodes. Clark began pioneering the 

use of what would later be an oxygen- (and therefore chemical-) sensor. In 

fact, Pt electrodes used to detect oxygen electrochemically are often referred 

to generically as ’Clark electrodes’. While the sensor he originally came up 

with was intended for the measurement of glucose concentration in blood, 

the sensor itself was the start of biosensor history, and the Clark sensor is 

still around today with many different shapes and several applications far 

from the original design. 

Liquid measured 

on (Blood, water, 

etc.) 

Membrane 

Ag Wire 

Pt 

KCl Solution (electrolyte) 

Figure 3.1: Sketch of a basic 2 electrode Clark Oxygen Sensor, with a Ag 

reference electrode (the anode) and a Pt working electrode (the 

cathode). 

The electrodes as seen in figure 3.1 have a thin organic membrane covering 

a layer of electrolyte and two metallic electrodes. Oxygen diffuses through 

13 

A 

V

14 CHAPTER 3. THEORY OF THE CLARK SENSOR 

the membrane and is electrochemically reduced at the cathode. There is a 

carefully fixed voltage between the cathode and an anode so that only oxygen 

is reduced (explained further a little later in this chapter). The greater 

the oxygen partial pressure is, the higher the amount of oxygen that diffuses 

through the membrane in a given time. This results in a current that is proportional 

to the oxygen concentration in the sample, which can be measured 

with an external device, between the anode and the cathode. Temperature 

sensors can be built into the probe to allow compensation for the membrane 

and sample temperatures, as the temperature of these affect diffusion speed 

through the membrane and solubility in the electrolyte. The potentiostat 

(an instrument commonly used for measuring differences in electric potential. 

Essentially, this instrument balances the unknown voltage against a 

known, adjustable voltage. A potentionstat is frequently used in conjunction 

with a thermocoupler for measuring temperature. More about this in section 

3.2) uses cathode current, sample temperature, membrane temperature, 

barometric pressure and salinity information to calculate the dissolved oxygen 

content of the sample in either concentration (ppm) or percent saturation 

(% Sat). The voltage for the reduction can either be supplied electronically 

by the meter (potentiometric oxygen electrode), or by dissimilar metals being 

used for the two electrodes, picked in a way so that the correct voltage 

is generated between them (galvanic electrode)[22]. 

So how does this all work? In the following chapter I will cover the theory 

behind the Clark sensor, and explain what is happening when it is used. First 

a look at the chemistry in the Clark sensor, then the diffusion through the 

membrane, and finally the potentiostat. 

3.1 Chemistry of the Clark Sensor 

When an electrode of noble metal such as platinum or gold is biased between 

-0.6 and -0.8 V [23] with respect to a reference electrode such as Ag/AgCl or 

a calomel electrode in a KCl solution, the oxygen dissolved in the liquid is 

reduced at the surface of the noble metal (the cathode). This phenomenon 

can be observed from a current-voltage diagram (called a polarogram, example 

of this can be seen on figure 3.2) of the electrode. When the negative 

voltage applied to the noble metal electrode (called the cathode) is increased, 

the current increases initially but soon it becomes saturated. In this plateau 

region of the polarogram, the reaction of oxygen at the cathode is so fast that 

the rate of reaction is limited by the diffusion of oxygen to the cathode sur-

3.1. CHEMISTRY OF THE CLARK SENSOR 15 

face. When the negative bias voltage is further increased, the current output 

of the electrode increases rapidly due to other reactions. If a fixed voltage in 

the plateau region (for instance, -0.7V) is applied to the cathode, the current 

output of the electrode can be linearly calibrated to the dissolved oxygen 

(short termed DO). It should be noted that the current is proportional not 

to the actual concentration but to the activity or equivalent partial pressure 

of dissolved oxygen, which is often referred to as oxygen tension. A fixed 

voltage between -0.6 and -0.8 V (due to the cell potential, and the onset of 

other processes) is usually selected as the polarization voltage when using 

Ag/AgCl as the reference electrode. 

Figure 3.2: Polarogram: Current to voltage at different oxygen tensions and 

calibration obtained at a fixed polarization voltage. Reproduced 

from [24]. The shape of the curves indicates the nature of the 

substance in the sample that is being reduced; where the height 

is equivalent to the concentrations of oxygen. Also note that 

convention usually have -V along the x-axis and not V, as is 

otherwise the norm. 

The cathode, the reference electrode, and the electrolyte are separated 

from the measurement medium by a membrane, which is permeable to the 

dissolved gas but not to most of the ions and other species. When at the 

same time most of the mass transfer resistance is confined in the membrane, 

the electrode system can measure oxygen tension in various liquids. This is 

the basic operating principle of the membrane covered DO probe[25]. 

Electrode Reactions: The reaction can proceed as follows (a 4e − reaction 

with a Ag/AgCl electrode is used as an example) 

Cathodic reaction:


Anodic reaction: 

Overall reaction: 

O2 + 2H2O + 2e − → H2O2 + 2OH − 

H2O2 + 2e − → 2OH − 

O2 + H2O + 4e − → 4OH − 

Ag + Cl − → AgCl + e − 

4Ag + O2 + 2H2O + 4Cl − → 4AgCl + 4OH − 

Two pathways are possible for the reduction of oxygen at the noble metal 

surface. One is a 4-electron pathway where the oxygen in the bulk diffuses 

to the surface of the cathode and is converted to H2O via H2O2. The other 

is a 2-electron pathway where the H2O2 diffuses directly from the cathode 

surface into the bulk liquid. 

Also as can be seen above from the reaction, hydroxyl ions are constantly 

being substituted for chloride ions once the reaction starts, therefore KCl 

or NaCl have to be used as an electrolyte. However this electrolyte will 

slowly be depleted of Cl − , so something has to be done to keep this depletion 

from occurring to fast, or to replenish it somehow. Hence this is one of the 

problems that have to be faced when building a Clark sensor, as it can be 

one of the main limits for the lifetime of the sensor. 

3.1.1 One layer electrode model 

Now three assumptions will be used to look at a one dimensional diffusion 

equation[26]: 

1. The cathode is well polished and the membrane is tightly fit over the 

cathode surface such that the thickness of the electrolyte layer between 

the membrane and the cathode is negligible. Therefore the membrane 

will define the oxygen flux to the cathode surface. 

2. The liquid around the sensor is well agitated so the partial pressure of 

oxygen at the membrane surface is the same as that of the bulk liquid. 

3. Oxygen diffusion occurs only in one direction, perpendicular to the 

cathode surface.


PSfrag replacements 

y 

Cathode 

Membrane Liquid 

x = 0 x = dm 

Figure 3.3: One layer electrode model to illustrate the three assumptions, as 

well as to help derive the equations based on them. 

Using Fick’s second law and the coordinate system from Figure 3.3, the 

unsteady diffusion in the membrane will be described by: 

∂p 

∂t 

∂ 

= Dm 

2p ∂x2 p0 

x 

(3.1) 

Where Dm is the oxygen diffusivity in the membrane, p the partial pressure 

of oxygen in the membrane, and x is the distance from the cathode 

surface. 

Under the assumption that diffusion have yet to take place, the initial 

boundary conditions are: 

p = 0 at t = 0 (3.2) 

p = 0 at x = 0 (3.3) 

p = p0 at x = dm 

(3.4) 

where dm is the membrane thickness and po is the partial pressure of oxygen 

in the bulk liquid.


The first boundary condition assumes that we insert the membrane and cathode 

in the liquid, and as such no pressure have yet to build up at the cathode. 

The second boundary condition assumes a fast reaction at the cathode surface, 

which is generally achieved when the cathode is properly polarized. 

The third boundary condition assumes there is a state of equilibrium inside 

the membrane, hence no pressure is being applied from within so that the 

only pressure at the surface of the membrane is coming from the outside 

liquid. 

Under these boundary conditions the solution[27] of Eq. 3.1 is 

p 

p0 

= x 

+ 

dm 

∞ 2 

nπ (−1)nsin nxπ 

 

2 2 −n π Dmt 

exp 

n=1 

dm 

d 2 m 

(3.5) 

The current output I of the electrode is proportional to the oxygen flux 

at the cathode surface: 

 

∂C 

I = NF ADm 

∂x 

x=0 

 

∂p 

= NF APm 

∂x 

x=0 

(3.6) 

Where N is the number of electrons per mole of reduced oxygen, F is 

Faraday’s constant, A the surface area of the cathode, C the concentration 1 

and Pm the oxygen permeability 2 of the membrane. 

Pm is related to diffusivity by 

Pm = DmSm 

where Sm is the oxygen solubility 3 of the membrane. 

From Eq. 3.5 and Eq. 3.6 the current output I of the electrode is 

I = NF A Pm 

p0 

dm 

 

1 + 2 

∞ 

(−1) n 2 −n 2π Dmt 

exp 

 

n=1 

d 2 m 

(3.7) 

(3.8) 

 

1 ∂C 

J = Dm ∂x being the oxygen flux in the x direction 

x=0 

2The passage or diffusion of a gas, vapor, liquid, or solid through a barrier without 

physically or chemically affecting it. 

3Ability of a substance to dissolve in another substance


The pressure profile within the membrane and the current output under 

steady state conditions can then be obtained from Eq. 3.5 and Eq. 3.8: 

and 

p 

p0 

= x 

dm 

I = NF A Pm 

dm 

· p0 

(3.9) 

(3.10) 

At steady state, the pressure profile is linear and the current output is 

proportional to the oxygen partial pressure in the bulk liquid, where Eq. 3.10 

forms the basis for DO measurement by the sensor. The response time of the 

sensor as seen from Eq. 3.8 is: 

τ = d2 m 

Dm 

(3.11) 

Where τ determines how fast the sensor responds, due to the thickness 

of the membrane or a high Dm. However these conditions tend to weaken 

the assumption of membrane-controlled diffusion. Therefore, a compromise 

has to be made for optimum sensor performance. Changing dm (rather than 

Dm) is more effective in adjusting τ (since it depends on the square of dm). 

Eq. 3.10 and Eq. 3.11 gives the indication, that the design variables for a 

DO sensor is Pm, dm, Dm, and A. 

3.1.2 Two layer model 

The problem is, however, that the second assumption, made earlier in this 

chapter, is not entirely accurate. A stagnant liquid film will almost always 

exists right outside the membrane even at high liquid velocities.[28] Hence 

a more realistic model have to be made, in order to account for this film as 

shown on Figure 3.4. This take into consideration the effect of the film, as 

it slows down the diffusion through the membrane. 

The effect of the liquid layer on the sensor current can be calculated by 

expanding the previous model, at steady state the oxygen flux J through 

each layer should be the same.


PSfrag replacements 

y 

Cathode 

Membrane Liquid 

High Velocity 

Low velocity 

Figure 3.4: Two layer model for DO sensor to illustrate the changed conditions. 

J = Kp0 = kL(p0 − pm) = kmpm 

p0 

x 

(3.12) 

Where K is the overall is the overall mass transfer coefficient and kL and 

km are the mass transfer coefficient for the liquid film and the membrane. 

The inverse of the mass transfer coefficient can be termed as the mass 

transfer resistance. If kL and km is thought of as two parallel resistors, we 

get: 

1 

K 

1 

= + 

kL 

1 

km 

(3.13) 

What this equation says is that the overall mass transfer resistance is the 

sum of mass transfer resistance and the membrane phase transfer resistance. 

The derivation is similar to Ohm’s law, where J is equivalent of the current, 

and p of the voltage. 

The individual resistance can be replaced by more commonly known parameters: 

1 

K 

dL 

= + 

PL 

dm 

Pm 

(3.14)


Where dL is the liquid film thickness and PL the oxygen permeability of 

the liquid film. 

When the individual mass transfer resistances are included, the steady 

state sensor output becomes: 

Where ¯ d is defined as: 

I = NF A Pm 

¯d p0 

dL 

¯d = dm + Pm 

PL 

The time constant τ from Eq. 3.11 can be modified to: 

with d defined as: 

τ = d2 

Dm 

d = dm + dL 

Dm 

DL 

(3.15) 

(3.16) 

(3.17) 

(3.18) 

The DO sensor, when placed in a stagnant liquid, produces a diffusion 

gradient extending outside the membrane and farther into the liquid. When 

the liquid is stirred, the diffusion gradient can no longer be extended beyond 

the liquid film around the membrane. Since the diffusion gradient becomes 

steeper with decreasing liquid film thickness, the current output of the sensor 

increases with increase in liquid velocity, also that the response time of the 

sensor increases as the liquid velocity decreases. This ’flow sensitivity’ is 

greater for a sensor with a larger cathode because the size of the stagnant 

diffusion field is proportionally greater with a larger cathode. Hence there 

is an advantage in going from macro to micro scale, as the cathode will 

naturally be smaller here. 

Eq. 3.15 can be written as: 

I = NF A p0 

dm 

Pm 

+ dL 

PL 

(3.19)


And from this the conditions for membrane controlled diffusion becomes: 

dm 

Pm 

≫ dL 

PL 

(3.20) 

In order to achieve this condition, a relatively thick membrane with a 

low oxygen permeability have to be used. When this condition is achieved, 

the oxygen sensor output depends only on membrane properties as given by 

Eq. 3.10 and the sensor calibrated in one liquid can be used in other liquids 

without recalibration. However there is always a liquid film (however thin it 

may be) and this causes variations in calibration in different liquids. 

3.2 The Potentiostat 

Most of us can probably recall from the early chemistry lessons in school , 

that if you take a piece of iron and dips it into a sulphuric acid, it will start to 

dissolve, or rather corrode. Though in the 17th century Sir Humphrey Davy, 

following Galvani’s experiments [29] discovered that if a piece of non corrosive 

metal, like Platinum, was put in the acid, and then connected to a positive 

pole. While at the same time connecting the piece of iron to the negative pole 

of the same current source. The corrosion of the iron would slow down, or 

even come to a complete halt if a high enough amount of voltage was applied. 

Vice versa the corrosion would increase with the voltage, if the iron was 

connected to the positive pole and the platinum to the negative pole. 

Though above a specific current(the exact one varying with the temperature, 

composition, and size of the metals) the current will suddenly drop to 

low values and the iron would cease to corrode (A phenomenon discovered 

first by Michael Faraday.[30]). The understanding and explanation for this 

was later brought on by the invention of the potentiostat. 

As mentioned at the beginning of this chapter, a potentiostat is used in 

order to measure the current, so what does this device do, and what is the 

principle behind it. Basically the potentiostat, is an amplifier that sets and 

maintains a voltage between two electrodes at a specific value, a Working 

Electrode (also sometimes referred to as the Indicator Electrode[31]) and a 

Reference electrode.

3.2. THE POTENTIOSTAT 23 

Reference 

Electrode 

Constant E required 

Working 

Electrode 

Figure 3.5: A basic electrochemical cell with a cathode (Working Electrode) 

and a anode (Reference Electrode). 

However in order for this to be accomplished some conditions have to be 

fulfilled[32]. The Reference electrode maintains a constant voltage referred to 

as the hydrogen electrode potential, an often used metal for this (as is also the 

case in this project) is Ag covered with a layer of AgCl. The moment a current 

passes through this electrode it will become polarized, eg its potential will 

vary accordingly to the current. Therefore in order for a steady potentential 

can be maintained, no current must be allowed to pass through it. This 

is accomplished by having a constant potential difference between the two 

electrodes. However in order to achieve this a third electrode have to be 

added to the cell, a Counter Electrode (also known as a Auxiliary Electrode). 

Hence a current is forced between the Counter Electrode and the Working 

Electrode, this current is high enough and in the proper polarity to keep 

the potential of the Working Electrode at a set value in comparison to the 

Reference Electrode. 

There will be two focuses of the Potentiostat[33]: 

1. Measuring the potential difference between the Reference and Working 

Electrode, in a way that leaves the Reference Electrode unpolarized, 

while comparing this potential difference to an already preset voltage. 

2. Force a current between the Counter Electrode and the Working Electrode 

to counterbalance the difference between the Working Electrode 

potential and the preset voltage. 

This is realized electronically by using a operational amplifier also known 

as an OPAMP, the operational amplifier will has two inputs, an inverting and


Working 

Electrode 

Constant E required 

Reference 

Electrode 

Current 

Input 

Counter 

Electrode 

Figure 3.6: A basic electrochemical cell with a third electrode (Counter Electrode) 

added to control the potential. 

a non-inverting. If a current/voltage is feed into the non-inverting part, an 

amplified current/volage will be produced, while feeding it into the inverting 

part will maintain the magnitude, while switching the sign (+ to − or − 

to +). By closing the loop between in- and output, the voltage difference 

between the two inputs of the amplifier will fall. An increase in the voltage in 

the inverting input will force a complementary current on the output, which 

will counteract the difference in input voltage. 

Connecting the Reference Electrode to the inverting input, the Working 

Electrode to the non-inverting, and the Counter Electrode to the output will 

give this: 

Between the Working Electrode and the Reference Electrode the difference 

will now be amplified and inverted by the amplifier, the circuit will 

be closed by the electrochemical cell, where the current runs through the 

electrolyte from the Counter Electrode to the Working Electrode. This will 

polarize the Working Electrode, so that the input difference between that and 

the Reference Electrode will be zero. In doing so, the Working Electrode and 

the Reference Electrode potential is kept level. To shift the Working Electrode 

potential to some other reference value in regards to the Reference 

Electrode, a voltage in series between the input to the Reference Electrode 

and the Reference Electrode itself must be inserted. Also to measure the current 

running through the Counter Electrode, a resistor is used in the wiring 

of the Counter Electrode, over which a voltage that is proportional to the

3.2. THE POTENTIOSTAT 25 

Working 

Electrode 

Reference 

Electrode 

+ 

OPAMP 

− 

Counter 

Electrode 

Figure 3.7: Using the OPAMP as a potentiostat. 

current is measured (See Figure 3.8) 

Working 

Electrode 

Reference 

Electrode 

+ 

OPAMP 

− 

R 

Counter 

Electrode 

Figure 3.8: Resistor added to measure the current running through the 

Counter Electrode. 

Of course a more complex electronic system is still needed to protect 

the potential amplifier from voltage shocks, to counteract noise and other 

miscellaneous problems with electrical systems. 

V


3.3 Summary 

The theories upon which the Clark sensor is based, as well as the potentiostat, 

have been highlighted, and it is now a matter of trying to use these in order 

to design and build a micro oxygen sensor. While trying to find a way around 

the problems that electrolyte depletion and membrane fouling presents.

Chapter 4 

System Design 

The sensor layout is one of the more essential parts of the project, as the 

design and general setup will be the deciding factor, in how well the sensor 

will work. At such at least one design should be made that is reasonably 

safe, which based on the general theory should work. 

In this chapter the thoughts and ideas, concerning why the first designs of 

the sensor(s) looks as they do, are given and explained. 

4.1 General Design 

Based on a previous project by Anders Hyldg˚ard [34], an ’O’-ring based 

design was decided on, this was done in part to take advantage of the already 

existing packaging scheme 1 . The setup uses an ’O’-ring as pressure sealing, 

which leaves the active part of the sensor exposed, while protecting the rest 

of the chip from the corrosive effects of seawater[35]. Of course care still has 

to be taken, to ensure that it is tightly sealed, since even a small roughness 

on the surface of the chip might allow seawater to seep through. Furthermore 

a lot of the clean room steps from that project can be duplicated to create 

the various layers in this design. 

Making the sensor circular however presents other problems, as the sensor 

needs a membrane covering it as mentioned in a Chapter 3, so that only 

oxygen diffusing through the membrane is measured, as well as an electrolyte. 

This in itself may not seem like a major obstacle, though given that the 

membrane (Siloprene) is liquid to begin with, it needs a small ’container’ 

1 See the Packaging section for more info on this 

27

28 CHAPTER 4. SYSTEM DESIGN 

until it solidifies. Experiments have been made by M. Dawgul et. al. [36], in 

which a membrane has been spin coated on the wafer. The results from this 

however were not good, as the membrane often ended up sliding off, or at 

best varied greatly in thickness, thereby lacking in regards to the feasibility 

of being able to create a preset, uniform membrane thickness. 

A logical solution to this has been used by M. Wittkampf et. al [37] where 

a second wafer is anodic bonded to the first, with the second wafer having a 

hole created with a wet isotropic etch where the exposed part of the sensor is, 

there by creating a hole that the membrane liquid can be ’poured’ into. This 

is a fairly simple process, however the basic of this design makes it a little 

more difficult, as a circular rather than a rectangular hole have to be made. 

However it is a problem that can be overcome. In the initial design, the 

’O’-ring will be used as a ’container’ for the electrolyte and the membrane. 

Also it should be mentioned that a micro electrode have the advantage over 

a macro electrode, in that the diffusion occurs in all directions, where as with 

the macro it only have diffusion in one direction[38]. Hence at macro scale 

the oxygen will be consumed faster at the electrode, and thereby influence 

the measurements. (See Figure 4.1) 

Figure 4.1: Micro electrodes have diffusion to all directions while Macro only 

have to one, why at macro scale setup the oxygen is consumed 

faster at the electrode. 

4.2 Actual Design 

Four ’major’ designs, each with some variant sub design, have been created, 

taking into consideration some of the experiences that have already been 

made by others[37, 39, 36, 38, 40, 41]. The idea behind the main one as

4.2. ACTUAL DESIGN 29 

seen on figure 4.2 have been to create a layout, which as mentioned earlier, 

would have a high chance of working. Thereby enabling me to do some 

measurements with it, so that some practical experience could be gathered. 

Also of course to see if it did indeed work, and what response could be 

expected from it. The design is basically an array of Pt working electrodes 

located in the middle, then a torus shaped Pt counter electrode surrounding 

it, and finally a circular Ag/AgCl reference electrode around the two of them. 

Extreme care has been taken to avoid, or at the very least minimize, the 

chances of any short circuits. Put in other words each electrode have its own 

connection to the outside via a silicide ’wire’, that touches neither the other 

electrodes, nor the other ’wires’. Also there is a temperature sensor at the 

top, more about this later however. 

Figure 4.2: Primary/Safe Design, the white circles are where the ’O’-ring will 

be placed on the chip, showing both the actual inner and outer 

diameter of it, as well as where it minimally will touch the chip. 

The number refer to the design, while the roman numerals refer 

to the size and distance between the electrodes in the array. For 

a closeup of the Working Array see Figure 4.6


Based on this design, three others were created, a second where the aforementioned 

short circuits is assumed not to take place. A third, where the 

counter electrode and the counter array are switched, and a fourth, where 

the size of the reference electrode is increased. 

The second (See Figure 4.3) is intended to replace the first if it works, since 

as can be seen on the figure, it is basically the same as the first, except that 

I have here assumed no short circuits will occur. 

Figure 4.3: Second Design, note that the electrodes are now closed rings and 

lies over the contact layer, albeit with a Nitride layer inbetween

4.2. ACTUAL DESIGN 31 

The third design (See Figure 4.4) is partly done to see if the right choice 

was made in placing the working array in the center, instead it have here 

switched place with the counter electrode. As such it is more done out of 

curiosity to learn more about the workings of a Clark sensor, rather than a 

design in which great expectations is placed. 

Figure 4.4: Third Design, the working array is now the inner ring, rather 

than the center circle


The reference electrode is always placed furthest out in the circle, due 

to the silver being slowly consumed as the sensor is used. Therefore it is 

preferable to have as much of it to consume as possible, in order to extend 

the lifetime of the sensor, which is the basis of the fourth design (See Figure 

4.5). 

Figure 4.5: Fourth Design, the reference electrode has been enlarged. 

4.2.1 Minor Design Variations 

In order to get a better understanding of the sensor, as well as trying to 

optimize it, some variations have be made on various aspects of the sensor, 

such as distance between each electrode in the array (See Figure 4.6), as well 

as the size of each electrode in the array (See Figure 4.7). Other possible 

variations could include the distance between the reference electrode, the 

counter electrode and the working array, as well as making the already used 

variations smaller or larger. 

Why these variations? Well as the reference electrode (Ag/AgCl) is slowly 

consumed, it will be deposited at the working array, the speed at which this 

occur will in part depend on the size and numbers of array electrodes. So if 

there is too few, or they are too small, they will be covered faster. On the

4.3. TEMPERATURE SENSOR 33 

Figure 4.6: Closeup of the working array, there is 150 µm between each electrode 

on the left array, and 100 µm between each on the right. 

The radius of the green circle (contact area for the working array) 

is in both cases 350 µm. 

other hand if there is too many in the array, there is a risk of the array starting 

to act like a macro scale electrode, where the oxygen will be consumed, hence 

corrupt the obtained measurements. Not to mention that the sensor itself 

will be used underwater, while being attached to a fish, hence no external 

power source is possible. Therefore it is an advantage if it uses as little power 

as possible, this equals a smaller battery which in turn will make the overall 

size of the finished device as small as possible, and hence it can be attached 

to smaller fish. Finally the size and distance also influence the signal output 

of the device, as the measured current is partly dependable on these two 

parameters.[42, 43] 

So it is in large part a question of finding a balance between all the factors, 

to ensure the lifetime is as long as possible, without sacrificing accuracy. 

Therefore variations on all conceivable variables should be looked into. 

4.3 Temperature Sensor 

Since the diffusion through the membrane among other things is dependent 

on temperature, and there was room to spare on the chip, it was deemed 

an advantage to add a temperature sensor (platinum resistance temperature


Figure 4.7: Closeup of electrode in a working array, the one on the left have 

a radius of 5 µm, while the right have a radius of 10 µm, to get a 

more accurate idea of the size compare it to the distance between 

the brown area (the counter electrode) and the green area (the 

contact layer for the electrode array), which is 20 µm in both 

cases. The black diagonal area is the nitride contact holes. 

sensor). Since the relationship between temperature and resistance for platinum 

is approximately linear over a small temperature range, and since the 

properties for platinum is well known and documented this was a preferable 

choice. The resistance is calculated from Eq. 4.1 

R = ρ l 

wh 

(4.1) 

Where R is the resistance, ρ the resistivity of the material (ρ = 1.06 × 

10 −8 Ωm for Pt), and l, w and h the length, width and height of the resistor. 

Figure 4.8: Close up of the l-edit design for the Pt50 

3 versions of this have been implemented on the various designs, a Pt50,


a Pt300, and a Pt600 (where the number is the resistance of the sensor. 

Figure 4.9: Close up of the l-edit design for the Pt300 

Of these 3, the Pt50 is the safest design. The reason for this is that the 

The Pt300 and Pt600 have a width of 10 µm, while the Pt50 have a width 

of 20 µm. Hence if some error occurs during the metallization part of the 

process for the sensor, and you end up with a width of 9 µm instead, which is 

merely 1 µm off. It would translate into a 10% shift in the resistance, while it 

would merely be 5% with the Pt50. Hence having a reasonably wide resistor 

can minimize a potential error, unfortunately however the higher the width, 

the lower the resistance is as well. So once again it is a matter of balancing 

one advantage against another disadvantage. 

Figure 4.10: Close up of the l-edit design for the Pt600, it is basically the 

Pt300 done twice. 

Why the 4 contacts, instead of merely 2? Well consider if there was only 

2, each serving as a contact for both current and voltage, the total resistance 

between the two can then be calculated from: 

R = V 

I = 2Rc + 2Rsp + Rs 

(4.2) 

Where Rc is the contact resistance between the chip and the equipment 

connected to it, Rsp the spreading resistance, and Rs the chip resistance (and


the resistance that I want to assign). This in itself may seem straightforward, 

however neither Rc nor Rsp can be calculated precise, so as a result it isn’t 

possible to measure Rs accurately. 

The 4-contact approach solves this problem however; 2 carry the current 

while the other 2 are used to measure the voltage. While Rc and Rsp is still 

there, they are now negligible due to the very small current that flows through 

them, as the voltage is now measured with a potentiometer that draws no 

current, or with a high impedance voltmeter that draws little current. 

Temperature Calculated Width Effective Heigth 

Sensor Resistance (Ω) (µm) Length (µm) (µm) 

Pt50 47.8 20 18019 0.2 

Pt300 287 10 54170 0.2 

Pt600 590 10 113207 0.2 

4.4 Packaging 

Table 4.1: Overview of the temperature sensors. 

Shielding the interface electronic part of the chip from the corrosive effect 

of seawater, is as mentioned earlier the ’O’-ring and packaging. How is this 

built? 

The package or casing consists of three different layers (hereafter referred 

to as the lower, middle and upper layer) of laser 2 cut PMMA (PolyMethyl- 

MethAcrylate), that each have their own structure. While there are materials 

that are not as porous, and have better chemical properties, PMMA 

have the advantage that several can be fabricated within minutes once the 

actual structure is in place. Which have a higher priority than long term 

usage as far as testing purposes goes. Also PMMA can generally be used in 

the temperature area from -40 o C to 70 o C, as well as being electric isolating. 

So what do these layers look like, and what purpose does each serve? 

2 Laser used is a SYNRAD laser

4.4. PACKAGING 37 

Figure 4.11: Close up photography of a flex print. 

1 mm 

1.7 mm 

11 mm 

6.5 mm 

66.5 mm 

Figure 4.12: Rough sketch of the middle layer of the encapsulation, sizes 

listed to give a clearer picture of actual size 

Figure 4.13: Pictures taken of the middle layer, to show what it looks like 

once fabricated, the layer is tilted slight on the picture to the 

right to compensate for the light. 

The middle layer as seen on figure 4.12 and 4.13 is where the chip is 

located, 4 areas are cut into the PMMA with a laser. The area with the


chip, 2 adjourning not quite as deep areas (a short and a long area) where 

the flex print (This provide the electronic wiring to devices outside the sensor, 

see Figure 4.11 for a picture of the flex print) is located, and a hole for the 

flex print on the short side (So that the connection to the outside system is 

located in one end, with the sensor at the other.) 

The lower layer as seen on figure 4.14 and 4.15 have been cut to provide 

a a grove path where the flex print sticking through the hole on the middle 

can run back. 

6 mm 

Figure 4.14: Rough sketch of the lower layer of the encapsulation, sizes listed 

to give a clearer picture of actual size 

1.5 mm 

Figure 4.15: Pictures taken of the lower layer. 

The upper layer as seen on figure 4.16 and 4.17 consist of 3 parts, 2 

square holes that provides room for the connection between then chip and 

the flex print since there isn’t a direct contact between the two (More on this 

connection and how the 3 layers are made to stick together in Chapter 5).

4.4. PACKAGING 39 

90 mm 

15 mm 

6 mm 

4.75 mm 

4.5 mm 

4.75 mm 

Figure 4.16: Rough sketch of the middle layer of the encapsulation, sizes 

listed to give a clearer picture of actual size 

2 mm 

0.5 mm 

Figure 4.17: Pictures taken of the upper layer. 

Located between the two squares are a circle with a small hole cut entirely 

through the PMMA. The ’O’-ring is located in the circle, see Figure 4.18, 

while the hole will be located directly above the center of the center on the 

chip. Thereby allowing the dissolved oxygen to interact with the 3 electrodes 

(Reference, Working and Counter) on the chip. Once the three layers are 

put together, the ’O’-ring should be compressed roughly 20%. The ’O’-ring 

itself should be chosen, so that it can resist the water, acids, oils and other 

biological and non-biological substances in the ocean.


Figure 4.18: Pictures taken of the ’O’-ring, on the picture to the right the 

’O’-ring is placed in the PMMA. 

4.5 Summary 

4 different sensor layouts have been designed, along with variations on electrode 

size and distance, as well as 3 different Pt-temperature sensors. Combing 

into a total of 13 different chips, opening up for the possibility to test 

various aspect of the overall design of the Clark sensor. Finally the encapsulation 

have been chosen so that it is easy and fast to produce, while still 

being durable. Table 4.2 shows the various chips, and which design variation 

that can be found on each. 

Chip Electrode Electrode Temperature 

Design Distance (µm) Radius (µm) Sensor 

1CVA 100 5 Pt50 

1CVB 100 5 Pt300 

1CXA 100 10 Pt50 

1CXB 100 10 Pt300 

1CLV 150 5 Pt300 

1CLXA 150 10 Pt50 

1CLXB 150 10 Pt300 

2CV 100 5 Pt600 

2X 100 10 Pt600 

3CV 100 5 Pt300 

3CX 100 10 Pt300 

4CV 100 5 Pt300 

4CX 100 10 Pt300 

Table 4.2: Overview of the chips, listing the size and distance between the 

electrodes in the working electrode array, and what temperature 

sensor there is on it.

Chapter 5 

Fabrication 

The fabrication 1 process is a fairly straight forward process, with 5 masks 

being used in total. Only one side of the wafer is used, so the potential problems 

with double sided wafers is avoided. Within this chapter is a description 

of the steps in the process, as well as some of the thoughts about each step. 

The starting material is single crystalline n-type silicon (100) wafers. Further, 

several steps in the fabrication process could be duplicated from the 

project which this is subpart of [34]. 

Figure 5.1: Illustration of a bare Silicon wafer, for the complete process, and 

a better understanding of this illustration and those that will 

follow, refer to Appendix B. 

5.1 Processing 

Silicon Dioxide Layer 

First a Silicon Dioxide layer is of 2000 ˚A is grown with thermal oxidation 

using a wet oxidation, the layer will serve to protect the wafer from the later 

processes, as well as working as an insulator. The reason wet oxidation is 

1 For the step by step cleanroom process recipe, see Appendix C 

41

42 CHAPTER 5. FABRICATION 

used is mainly that it is faster, basically it have a faster growth rate, and 

that the chip doesn’t require the higher breakdown voltage of dry oxidation. 

Figure 5.2: A layer of Silicon Dioxide have been added. 

Silicide Layer 

Next comes the Silicide layer, the silicide is going to serve as the conduction 

layer, between the actual sensor and the contacts. Silicide 2 (or rather 

TiSi2), resistivity of 13-16mΩcm is well suited for this due to its low resistivity. 

Which is an advantage since the finished Clark Sensor have to have 

an internal power supply, due to the sensor being attached to a fish, where 

the battery cannot be exchanged as one see fit. 

Figure 5.3: Illustration of the wafer after the silicide step. 

The Silicide layer is created by first depositing a layer of undoped Polysilicon 

with Low Pressure Chemical Vapor Deposition (LPCVD). It is also at 

this point the first alignments marks are made (See Figure 5.6). After this a 

Titanium layer is deposited on top of the Polysilicon, which is the patterned 

as well. Finally by using Rapid Thermal Annealing (RTA), the desired TiSi2 

is made. 

2 The silicide recipe is in Appendix A

5.1. PROCESSING 43 

Figure 5.4: Alignment marks from Silicide. 

Silicon Nitride Layer 

The Si3N4 layer is used to protect the bulk chip (not the actual sensor itself of 

course) from exposure to seawater, as seawater is highly corrosive and could 

otherwise quickly corrode the metals. 

Figure 5.5: Illustration of the wafer after the LPCVD and RIE of the Nitride. 

LPCVD is used to deposit the Si3N4, after which a RIE is used to make 

the contact holes to the silicide layer for the metals. 

Figure 5.6: Alignment marks after Si3N4. Since a dark mask is used in this 

step, the alignment mark is here made relatively big, so that 

there is hole to peek through to to the wafer in order to locate 

the alignment marks from the previous step. 

Metal(s) Layer 

Finally the metals, Pt (Working Electrode Array, Counter Electrode and


Thermometer), Ag (Reference Electrode), and Au (Contacts) are deposited. 

Why these 3? 

As discussed previously Pt and Ag have been used almost exclusively for 

Clark Sensors, and referenced as the best suited metals from a dissolved oxygen 

sensor[37]. 

Where as Au gold provides an excellent surface for bonding to wirebond, as 

well as being able resist some exposure to saltwater. 

Figure 5.7: Illustration of the wafer with the three metals added. 

Figure 5.8: Alignment marks after the metals are added. From left to right, 

ignoring the first one, it is Pt(brown), Ag(light brown), Au(red). 

Each metal have double alignment marks, two boxes and a crisscross 

pattern. This is mainly to have a little extra safety when 

aligning the masks, so that there is always another alignment 

mark to check with. 

Pt will be put on first for two reasons; It is the metal with the smallest 

structure, and hence the metal where the lithography step is most sensitive, 

and therefore where it is most like to go wrong. Also since the Au-Contacts 

and the Pt-Thermometer crosses, having the PT buried beneath the Au will 

ensure an unbroken line (Especially as the Au, 3000 ˚A, layer is over 10 times 

as thick as the Pt, 200 ˚A, layer). 

A minor problem however is that a lot of metals have poor adhesion to 

many forms of Silicon (including Silicide), hence a titanium layer is added

5.2. BACK-END PROCESSING 45 

under each to improve adhesion. 

The 3 metals are added and patterned with 3 double rounds (First Ti, then 

the actual metal) of metalization and then lift-off. 

5.2 Back-end Processing 

Once the cleanroom processes are done, there is still a few steps that have to 

be completed. These steps involves chemicals usually not found in a cleanroom, 

and the chip have to soak in them for a while, therefore they are done 

outside the cleanroom, also there is no real contamination danger for the 

electronics part of the wafer at this point. 

5.2.1 Packaging 

First the chip have to be packaged, this is done in three steps. 

The Flexprint and then Chip is glued to the middle layer of the PMMA. The 

Flexprint is attached first as to avoid getting glue onto the chip, however 

should it occur, the glue used (Super Attakgel) can be removed using 

ethanol, provided it is only a thin layer of glue. 

Figure 5.9: Flex print and Chip glued to the PMMA. 

Once the glue have dried (for 1-2 hours to be sure that it is completely 

dry), the flex print and the Au-contacts have to be connected. This can be 

done with either wirebonding, which will be the first choice, or by using a


conducting glue. 

The final step in the packaging, will be to bond the 3 layers of PMMA 

together (the ’O’-ring is inserted here as well), this is accomplished by putting 

them in a oven at 115 o C for 1 hour, which will cause them to bond. To hold 

then layers in place, two pieces of metal are used as well as a screw clamp. 

Care should be taken not to put to much pressure on the PMMA with the 

screw clamp, nor to little. However applying less than needed pressure is 

preferable, as it can be put back into the oven, if the bonding have not taken 

place. While to much pressure can cause the chip to break (a closer look is 

taken on this problem in Chapter 6). 

Figure 5.10: Preparation for the oven. Top left: The screw clamp. Top 

right: The two pieces of metal that holds the PMMA, chip and 

flexprint in place and together. Bottom Left: The 3 layers of 

PMMA have been placed in the the metal holder. Bottom right: 

Screw clamp have been closed tightly around the metal holding 

the PMMA.

5.2. BACK-END PROCESSING 47 

5.2.2 Ag to Ag/AgCl 

Ag has to be turned into AgCl. This can be done in several ways, one of the 

easier ways[44] however consists of using: 

1. 0.1 M HCl 

2. A flashlight battery. 

3. A few centimeters of pure silver. 

Figure 5.11: Ag turned into Ag/AgCl. 

The packaged chip is dipped down into the HCl along with one end of the 

silver wire. While the battery is connected with the negative terminal to the 

silver wire and the positive to the Ag output on the flexprint. Slowly the Ag 

electrode will turn a light tan and then a dark brown, at which point it will 

have a suitable thick layer of AgCl. 

5.2.3 Electrolyte and Membrane 

Finally the electrolyte and the membrane is added. The material used is 

Siloprene from Fluka-Chemie. It consists of 3 liquids Siloprene Crosslinking 

Agent K-11, Siloprene K 1000 and Hexane. All three liquids are dripped into 

the small hole on the device, where the chip is located, by using a syringe 

(The amounts used are of the order of 100:10:1 for Siloprene Crosslinking 

Agent K-11 : Siloprene K 1000 : Hexane). Finally the sensor is soaked in a 

liquid containing a large amount of Cl − ions (the time it needs to be soaked 

depends on the exact concentration, the higher the concentration is, the less 

time it needs. Care should be taken however if a strong acid is used, as it 

might damage the membrane) to add the electrolyte. Other membranes have 

been used with micro electrode oxygen sensor(such as Nafion[45]). However 

this was chosen, due to the recommendation and experiences of a professor 

at MIC.


5.3 Summary 

The fabrication steps have been outlined and explained, and while there are 

always the possibility of changes or variations in parameters in each step, the 

process as described in this chapter, should be duplicable by others.

Chapter 6 

Problems and Solutions 

In almost every project some unexpected problems appears, ranging from 

merely inconvenient problems such as cleanroom machinery being out of 

work, to more actual problems, such as fabrications steps being more complicated 

than they appear on paper. In this chapter some of the problems 

encountered during the process of creating the Clark sensor will be highlighted, 

as well as how they were solved when possible. 

6.1 Wirebonding 

As mentioned in Chapter 5, the first, and at the time the only, choice for 

connecting the chip to the flex print was wirebonding. Wirebonding is basically 

done by having a needle, with a metal (Al) thread through it, pressing 

down on the contact pad of the chip, and then through the use of ultrasound 

and vibrations attach the thread. The similar process is then repeated on 

the flex print, at which time the rest of the thread is snapped off. However 

this would soon prove to be a lot more complicated than it sounds. For some 

unexplainable reasons the thread refused to perform the second attachment, 

whether it was on the gold on the chip, or on the flex print. Albeit some 

were wirebondings were successful, they were few and there was far between 

them. 

Why this is so, remained a mystery for a while, however there was some 

problems with the machine at the time. However a later examination of the 

flex print revealed that the metal on it was tin covered copper. Therefor it 

was impossible to bond to the flex print, unless the tin was scraped off, as 

49

50 CHAPTER 6. PROBLEMS AND SOLUTIONS 

Figure 6.1: Picture of the wirebonder used to attach the wire to the chip and 

the flex print. 

wiring bonding is not possible to tin. Also people in the industry have suggested 

that the Au layer on the chip might have been to thin, and should be 

increased to 10k ˚A(it was 3000 ˚A), however others at MIC have successfully 

bonded to this thickness before. However this was only discovered late in the 

project, so some other way of connecting the chip and the flex print had to 

be found at the time. 

6.1.1 Conducting Glue 

As is often the case the simplest solution is also the most viable, in this case 

a conducting glue. 1 While it may look and sound less refined than a a metal 

wire, it is relatively easy to apply with the tip of a needle dipped in the glue. 

That is if as long as some distance exists between the output electrodes. 

The glue used is, like most glues, somewhat viscous and hence if the output 

electrodes are to close to each other, it can be very hard if not impossible to 

1 The glue is a conductive paste (H9807) from Namics, viscosity: 20 P a · 

s, V olumeresistivity : 0.7 × 10 −4 Ω · cm

6.1. WIREBONDING 51 

make a line between the chip and the flex print without a short circuit being 

created. So while the conducting glue is a possible solution, it should only 

be considered a temporary solution. 

Figure 6.2: One of the chips with the silver colored electric glue on, as can 

be seen by comparing the left and the right side, there is a decent 

amount of distance between the output electrodes (left) for the 

Clark Sensor. While the output electrodes (right) of the temperature 

sensor is closer and harder to connect to with the conducting 

glue. 

Figure 6.3: Closeup of the design, the arrows point towards the platinum line 

that will cause the short circuit. 

While the conducting glue was a very viable solution one minor problem 

had to be overcome first. In order to make the wafer easier to dice, the chip 

design (as seen in Chapter 4 had a platinum line near the edge, see picture 

6.3, which if the conducting glue was just put on as it were, would work as a 

short circuit, hence a small procedure was developed to apply the glue. This 

procedure was as follows: 

1. Put a piece of ordinary tape over the sensor, to prevent it from being 

covered with glue, leaving roughly half of the output electrodes free.


2. Put a thick layer of non-conducting glue over the exposed half of the 

output electrodes and the platinum wire, and remove the tape. Then 

leave it to dry for at least a few hours. Then check under a microscope 

if more needs to be applied. 

Figure 6.4: Non conducting glue have been put on the chip, and the tape 

have been removed. 

3. Connect the output electrodes and the flex print with the conducting 

glue, using the tip of a needle dipped in it. Check for short circuits 

under a microscope, and then leave it overnight to dry. 

Figure 6.5: Both the conducting and the non conducting glue have been 

added, and the non conducting is now separating the conducting 

glue from the platinum line.

6.2. THE FLEX PRINT 53 

The reason behind letting the conducting glue dry slowly in step 3, rather 

than putting it in the oven right away, was that tests showed a lack of connection 

if the glue hadn’t dried properly first. 

Also it should be noted that while the solution with the conducting glue 

does work, it is not the most optimal solution, as the yield on working sensors 

with this solution is discouragingly low. 

6.2 The flex print 

There was also another problem with the wirebonds, or rather the flex print, 

even if the wire had been successfully attached to both the output electrodes 

and the flex print. After taking the packaged device out of the oven, the flex 

print appeared to bend upwards, despite the glue. This had the side effect 

of making the thread snap off at one of the ends. Hence cutting off the connection 

between the chip and the flex print. The reason for this appears to 

be that the flex print used, isn’t suitable for use with temperatures over 80 C. 

Figure 6.6: Picture from a chip, where conducting glue have been used to 

form the connection between chip and flex print, as can be seen 

the flex print have bend and caused the connection to break. 

So in order to solve this, a way to keep the wire attached or to prevent 

the flex print from bending had to be found.


6.2.1 Non Conducting Glue 

Once again the solution involves glue, albeit a non-conducting one this time, 

the idea for this came to mind when a fellow master student 2 had to encapsulate 

a chip fast, and hence used glue instead to protect the thin metal wire 

from being torn off. This made me thinking what if a similar glue was put in 

the hole in the upper PMMA piece? Once it was dry it should ensure that 

the flex print is unable to bend, as there is no room for it to bend to. Also 

as an added bonus it would help protect the more sensitive electronics from 

any seawater that might seep past the ’O’-ring. 

Figure 6.7: Glue covering both the flex print, wirebonds, and chip, thereby 

making the flex print and wirebond durable. 

6.3 The ’O’-ring 

Another problem with the design was the ’O’-ring, or rather making the 

’O’-ring stay where it was supposed to in the PMMA. The hole which the 

’O’-ring rest in had to have the right depth, since if it was to deep, the ’O’ring 

wouldn’t fully protect the rest of the chip from the seawater. While 

making it to shallow would cause the ’O’-ring to be squeezed out of the hole, 

or outside the area it is supposed to cover, as seen on picture 6.8. 

2 Sune Duun

6.3. THE ’O’-RING 55 

Figure 6.8: The ’O’-ring is being squeezed out of where it is supposed to be. 

The picture to the left shows it sticking out of the hole, while the 

picture to the right shows it being squeezed over the chip. 

6.3.1 Changing the resting area 

Now the original design for the packaging had the ’O’-ring resting on a flat 

area as show on picture 6.9. Also while the ’O’- was supposed to be squeezed 

together it was only supposed to contract about 20 % percent, in order to 

have a tight fit. So how was this solved? First off the resting area for the 

’O’-ring was changed, so that it became a grove instead of a flat, as show on 

picture 6.9. 

Figure 6.9: The sketch to the left shows the original flat area, which the ’O’ring 

rested upon, while the sketch to the right shows the changed 

design, where a grove is added for the ’O’-ring. 

This, along with a optimization of the process where the laser cuts the 

grove, ensured that the the ’O’-ring no longer did this in most cases. Also it 

was better to have the screw clamp give a smaller amount of pressure, and 

then have the packing inside the oven a little longer if the PMMA layers 

hadn’t bonded properly, rather than applying much pressure and cause the 

’O’-ring to slip out. Since everything in the design should be able to survive 

the extended duration inside the oven.


6.4 Summary 

While many problems can occur in creating a sensor, it is not safe to assume 

that merely because the cleanroom processing is done that no further problems 

will be encountered. The problems shown and solved in this chapter, 

illustrates this very well.

Chapter 7 

Evaluation of Results and 

Measurements 

Shown in this chapter is the results gained during this project, as well as how 

they might be improved upon. While the process itself was successful, the 

problem that showed up with the packaging and the wirebonder, as detailed 

in Chapter refsec:problems, there is not as many measurements on the Clark 

Sensor as hoped, and therefore these will have to be obtained after this thesis 

is handed in. 

Figure 7.1: Pictures showing the missing Pt. On the picture on the left, 

some residues of platinum can be seen, where as the middle one 

shows a hole in the temperature sensor, while on the picture to 

the right the temperature sensor is missing entirely. 

7.1 Fabrication results 

The fabrication of the chips was successful, and one wafer made it through 

the whole clean room process, while a few others were postponed at an early 

57

58 CHAPTER 7. EVALUATION OF RESULTS AND MEASUREMENTS 

process step, in case problems with the rest of the process should arise. 

There was one small problem with the Pt, the exposure time in the photolithography 

was originally set to 7 seconds instead of 4. As a result this, 

both the working array and the temperature sensor had huge gaps, or were 

missing entire as seen on Figure 7.1. 

However once the exposure time was lowered to 4, the problem was solved, 

and the fine structure and small dots of Pt was clearly evident, as seen on 

Figure 7.2. 

Figure 7.2: Pt are no longer missing and both the dots in the working and 

the lines in the temperature sensor looks fine. 

Beyond this however there was no real problem in fabricating the chip, 

except for the problems already mentioned, also while the recipe for the 

titanium silicide was new, it formed successfully. 

7.2 Temperature Sensor 

The temperature sensor was tested to see if it had the required linear dependency, 

and to see if the TCR (Temperature Coefficient of Resistance) varied 

from the standard and if so, how much it did. This is done in order to know 

how much that needs to be compensated, since there is a titanium strip beneath 

the platinum, and the gold contacts can also be expected to change it 

a bit. 

The experiment was done using 2 multimeter’s, a probestation, and a 

heating plate. The chip measured on was placed in the probestation, and 

the 2 multimeter’s was connected, one measuring the temperature of the chip, 

and the other measuring the resistance through the temperature sensor. The 

heating plate was then used to raise the temperature to around 50 o C, after 

which it was cooled down. Measurements was made both while heating and 

cooling.


Figure 7.3: Graphs showing the relative temperature against the resistance, 

TCR is equal to the slope of the lines. 

As can be seen from Figure 7.3 the measured TCR’s (3.33×10 −3 , 4.09×10 −3 , 

4.09×10 −3 , 4.11×10 −3 )are very close to the standard[46] 3.8×10 −3 for platinum.

60 CHAPTER 7. EVALUATION OF RESULTS AND MEASUREMENTS 

7.3 Clark Sensor 

While there as mentioned was problems with the wire bonding and packaging, 

there was one sensor that was both bonded and packaged, and therefore 

could be measured upon. The measurements was done using a Gamry FAS2 

Femtostat, and a beaker of water with the sensor in it, finally grounded metal 

plates was put up around the beaker, in order to create the equivalent of a 

Faraday cage. This was done in order to minimize any outside noise (See 

Figure 7.4 for a sketch of the setup. 

PC 

Ground WE CE RE 

Potentiostat 

Constructed Faraday cage 

Figure 7.4: Sketch of the setup used to measure on the Clark sensor, 3 wires 

connects the working, reference and counter electrodes respectively 

to the potentiostat. The potentiostat collects the data and 

sends it to a nearby pc, which translates the signal into a graph. 

Unfortunately due to the faulty wirebonding one of the wires broke after 

a few test measurements, so no detailed measurements with varying amounts 

of dissolved oxygen could be made. 

However, as can be seen on Figure 7.5, there is the plateau shaped curve, 

which indicates that the sensor itself works. Of course since the a wire 

broke during the test measurements, no conclusion can be made in regards 

to whether it is measuring the right amount of oxygen or not. Hence further 

tests have to be made to confirm this. That the right shape of curve was 

obtained, is taken as a good sign, especially as there seems to be no noticeable 

internal noise generated. 

Beaker 

with 

Clark 

sensor 

and 

water

7.4. SUMMARY 61 

Figure 7.5: Polarogram from the test measurement with the designed Clark 

Sensor (Design 1CLX). 

7.4 Summary 

While there was a lack of measurements on the Clark sensor itself, the results 

from the measurements that have been obtained, as well as pictures of the 

chips, indicates that once the packaging problems have been solved, the Clark 

sensor should in all likelihood work as well.

62 CHAPTER 7. EVALUATION OF RESULTS AND MEASUREMENTS

Chapter 8 

Conclusion 

The primary goal for this project was to create a first generation oxygen 

sensor to compliment the Fish ’n Chip sensor. While a sensor have been 

created, it have not been tested, due to problems encountered during the 

projects, and as such the primary goal have not been fully fulfilled. However 

important steps towards a functioning dissolved oxygen sensor have been 

taken. 

Possible ways of measuring dissolved oxygen have been investigated, and 

a Clark type chosen. 

The theory behind the Clark sensor have been detailed, a further study 

into membrane materials and electrolytes could allow for improvements, as 

some of the potential optimization lies hidden here. Albeit this fall outside 

what knowledge of chemistry that I have. 

Various designs and the considerations behind them have been described, 

and with future results these should provide the basis for improving the 

sensor. This should also give a basic idea of what design outlay that is the 

most advantageous. 

The packaging concept, which proved to be the main problem despite not 

being within the focus of this project, have several flaws and will have to be 

given some thorough investigation. 

The temperature sensor seems to be working within the expected norms, 

and while not being revolutionary or even new, it is needed for the Clark 

sensor, due to the influence of temperature. An interesting aspect yet to 

explore however, is whether the Clark sensor influence it or vice versa. 

63

64 CHAPTER 8. CONCLUSION 

In summary this project still have a lot of potential, once the results from 

the Clark sensors have been obtained. Though from the gathered results and 

the design theory behind it, I feel confident that it does work, and plans have 

been made to measure on the chips to verify this and gather date, once the 

packaging problem is solved, even if it unfortunately falls outside the time 

frame of this project.

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Appendix A 

Silicide Recipe 

The following two recipes are for the Jipelec RTP machine at MIC. 

Step Initial Temperature Temperature Duration Thermocoupler Open Vent N2 

or Power or Power Control Valve (sccm) 

1 20 0 30 Y Y 0 

2 0 0 30 Y Y 200 

3 0 0 30 Y Y 0 

4 0 0 100 Y Y 200 

5 0 0 100 Y Y 0 

6 0 Power 20 30 N Y 0 

7 Power 20 Power 20 2400 N Y 0 

8 Power 20 0 10 N Y 0 

Table A.1: Prebake Settings to remove any oxygen from the ceramic wafer 

holder and prepare it (Recipe name Bake Clark). 

69

70 APPENDIX A. SILICIDE RECIPE 

Step Initial Temperature Temperature Duration TC Open Vent N2 

or Power or Power Control Valve (sccm) 

1 20 20 40 Y N 0 

2 20 20 30 N Y 400 

3 20 20 40 Y N 0 

4 20 20 30 N Y 400 

5 20 20 40 Y N 0 

6 20 200 20 Y N 0 

7 200 400 20 Y N 0 

8 400 600 20 Y N 0 

9 600 800 20 Y N 0 

10 800 850 10 Y N 0 

11 850 850 30 Y N 0 

12 850 20 600 Y N 0 

13 20 20 120 N Y 400 

Table A.2: Silicide Settings to make the actual Silicide(Recipe name PKH 

1).

Appendix B 

Fabrication Process 

71

72 APPENDIX B. FABRICATION PROCESS

74 APPENDIX B. FABRICATION PROCESS

Appendix C 

Process Sequence 

75

76 APPENDIX C. PROCESS SEQUENCE

78 APPENDIX C. PROCESS SEQUENCE

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