Development of a Oxygen Sensor for Marine ... - DTU Nanotech
Development of a Oxygen Sensor for Marine ... - DTU Nanotech
Development of a Oxygen Sensor for Marine ... - DTU Nanotech
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
<strong>Development</strong> <strong>of</strong> a <strong>Oxygen</strong><br />
<strong>Sensor</strong> <strong>for</strong> <strong>Marine</strong> Environment<br />
Studies<br />
M.Sc. Thesis<br />
Peter Kastberg Hansen s958291<br />
Supervisor: Erik V Thomsen<br />
September 20, 2005
ii<br />
MIC - Department <strong>of</strong> Micro and <strong>Nanotech</strong>nology<br />
Technical University <strong>of</strong> Denmark<br />
<strong>DTU</strong> Building 345east<br />
DK-2800 Kgs. Lyngby
Abstract<br />
This thesis described the first steps towards creating a oxygen sensor <strong>for</strong><br />
measuring dissolved oxygen in the oceans, with the intended use as part <strong>of</strong><br />
a larger project concerning fish behavior studies. The sensor is a Clark type<br />
oxygen sensor, with an thermal platinum sensor added as well.<br />
The first part describes the reasons <strong>for</strong> choosing the Clark sensor and the<br />
theory behind.<br />
The second part describes the design considerations and the fabrication steps<br />
that leads to the layout.<br />
While the final part deals with problems encountered, solutions to them,<br />
results gathered, and ways <strong>of</strong> improving the design.<br />
i
Acknowledgements<br />
I would like to say thanks the following people <strong>for</strong> their help, inspiration and<br />
patience; Erik V. Thomsen (my supervisor), Anders Hyldg˚ard (effectively the<br />
co-supervisor), and the rest <strong>of</strong> the Applied <strong>Sensor</strong>s group at MIC. Also thanks<br />
to Oliver Geschke <strong>for</strong> giving some valuable tips and insight on the workings<br />
<strong>of</strong> the Clark sensor early on. I would also like to thank the Laboratory<br />
Technicians <strong>for</strong> help in the Clean room, and the other master students in<br />
room 119 at MIC <strong>for</strong> a fun and spirited atmosphere, and <strong>for</strong> giving a hand<br />
with a lot <strong>of</strong> small things.<br />
iii
Contents<br />
1 Introduction 1<br />
1.1 Dissolved <strong>Oxygen</strong> Level . . . . . . . . . . . . . . . . . . . . . 2<br />
1.2 Thesis Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . 4<br />
2 Optodes and ISFET 7<br />
2.1 Optodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7<br />
2.2 ISFET (Ion Selective Field Effect Transistor) . . . . . . . . . . 9<br />
2.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11<br />
3 Theory <strong>of</strong> the Clark <strong>Sensor</strong> 13<br />
3.1 Chemistry <strong>of</strong> the Clark <strong>Sensor</strong> . . . . . . . . . . . . . . . . . . 14<br />
3.1.1 One layer electrode model . . . . . . . . . . . . . . . . 16<br />
3.1.2 Two layer model . . . . . . . . . . . . . . . . . . . . . 19<br />
3.2 The Potentiostat . . . . . . . . . . . . . . . . . . . . . . . . . 22<br />
3.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26<br />
4 System Design 27<br />
4.1 General Design . . . . . . . . . . . . . . . . . . . . . . . . . . 27<br />
4.2 Actual Design . . . . . . . . . . . . . . . . . . . . . . . . . . . 28<br />
4.2.1 Minor Design Variations . . . . . . . . . . . . . . . . . 32<br />
4.3 Temperature <strong>Sensor</strong> . . . . . . . . . . . . . . . . . . . . . . . . 33<br />
v
vi CONTENTS<br />
4.4 Packaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36<br />
4.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40<br />
5 Fabrication 41<br />
5.1 Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41<br />
5.2 Back-end Processing . . . . . . . . . . . . . . . . . . . . . . . 45<br />
5.2.1 Packaging . . . . . . . . . . . . . . . . . . . . . . . . . 45<br />
5.2.2 Ag to Ag/AgCl . . . . . . . . . . . . . . . . . . . . . . 47<br />
5.2.3 Electrolyte and Membrane . . . . . . . . . . . . . . . . 47<br />
5.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48<br />
6 Problems and Solutions 49<br />
6.1 Wirebonding . . . . . . . . . . . . . . . . . . . . . . . . . . . 49<br />
6.1.1 Conducting Glue . . . . . . . . . . . . . . . . . . . . . 50<br />
6.2 The flex print . . . . . . . . . . . . . . . . . . . . . . . . . . . 53<br />
6.2.1 Non Conducting Glue . . . . . . . . . . . . . . . . . . 54<br />
6.3 The ’O’-ring . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54<br />
6.3.1 Changing the resting area . . . . . . . . . . . . . . . . 55<br />
6.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56<br />
7 Evaluation <strong>of</strong> Results and Measurements 57<br />
7.1 Fabrication results . . . . . . . . . . . . . . . . . . . . . . . . 57<br />
7.2 Temperature <strong>Sensor</strong> . . . . . . . . . . . . . . . . . . . . . . . . 58<br />
7.3 Clark <strong>Sensor</strong> . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60<br />
7.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61<br />
8 Conclusion 63<br />
A Silicide Recipe 69
CONTENTS vii<br />
B Fabrication Process 71<br />
C Process Sequence 75
viii CONTENTS
Chapter 1<br />
Introduction<br />
Roughly 70 % <strong>of</strong> the world is covered by water[1], and have been an inspiration<br />
<strong>for</strong> mankind over the ages, from explorers such as Columbus to works <strong>of</strong><br />
fiction, such as Hemingway’s ’The Old Man and the Sea’. Several time the<br />
oceans have shown the tremendous <strong>for</strong>ces that rest within in various <strong>for</strong>ms<br />
<strong>of</strong> natural disasters such as tidal waves, floods, tsunamis and many other.<br />
However the oceans is also a source <strong>of</strong> life, in fact the original source <strong>of</strong> life,<br />
not just <strong>for</strong> us but <strong>for</strong> the fish that lives below.<br />
This thesis have been subpart <strong>of</strong> a larger project (Fish and Chips), where<br />
the main goal is the development <strong>of</strong> a multi-sensor <strong>for</strong> marine environmental<br />
studies. In that, the prototype <strong>for</strong> a Micro Electronic Mechanical System<br />
(MEMS) have been developed, with then intended use <strong>for</strong> fish tagging. While<br />
this thesis does not have quite as wide a scope, it does focus on one <strong>of</strong> the<br />
more important aspects <strong>of</strong> it, namely oxygen, or rather dissolved oxygen.<br />
There are commercially available dissolved oxygen sensors already today,<br />
however these are more <strong>of</strong>ten than not macro scale sensors attached to a<br />
buoy, and there<strong>for</strong>e only measuring in one place. They can also be attached<br />
to ships, however they will only measure the oxygen level at the surface, and<br />
there<strong>for</strong>e not give a full accurate picture <strong>of</strong> the oxygen level.<br />
Why dissolved oxygen? Well there are many reasons, mainly like us the<br />
fish needs to breathe, and the gills <strong>of</strong> a fish are not all that different <strong>for</strong>m<br />
our lungs[2]. Basically there needs to be enough dissolved oxygen in the<br />
water to favor diffusion <strong>of</strong> oxygen from the water, through the gills <strong>of</strong> the<br />
fish and into the blood. When the oxygen falls below a certain level this<br />
passive diffusion fails to be able to drive, the oxygen into the blood and the<br />
fish suffocates much like a human might at high altitudes. So by monitoring<br />
1
2 CHAPTER 1. INTRODUCTION<br />
the oxygen level it is possible to learn more about the behavior and health<br />
<strong>of</strong> the fish. Also given the relationship between dissolved oxygen levels and<br />
temperature(more about this later), it provides an idea <strong>of</strong> where the fish have<br />
been.[3, 4]<br />
Figure 1.1: Sketch <strong>of</strong> a fish’s gills, with oxygen leaving the water, and entering<br />
the bloodstream, and then subsequently moved around to<br />
the rest <strong>of</strong> the fish’s body. Reproduced from [5]<br />
Furthermore given the amount <strong>of</strong> pollution, algae growth and other problems<br />
in the world oceans today, it supplies additional info on the general<br />
health state <strong>of</strong> the oceans. The use <strong>of</strong> the sensor doesn’t need be limited<br />
to the oceans however, as it in theory should also be useful <strong>for</strong> on site measurement<br />
<strong>of</strong> the dissolved oxygen levels in rivers, creeks and lakes. Not to<br />
mention the medical uses involving the oxygen levels in blood. Also in these<br />
cases having a small easy to use sensor is preferable over a macro scale one.[6]<br />
1.1 Dissolved <strong>Oxygen</strong> Level<br />
How much dissolved oxygen is there in seawater? Under the assumption that<br />
the only source <strong>for</strong> the dissolved oxygen in water is the atmosphere (two<br />
other sources being by rapid movement(aeration), and as a waste product<br />
<strong>of</strong> photosynthesis. Both are natural processes, which however is affected by<br />
pollution.), the relationship between air and water can be represented with<br />
the equation:<br />
This has the equilibrium constant<br />
O2(g) ⇔ O2(aq) (1.1)
1.1. DISSOLVED OXYGEN LEVEL 3<br />
K = aO2<br />
fO2<br />
(1.2)<br />
Where aO2 is the activity <strong>of</strong> oxygen and fO2 the fugacity 1 <strong>of</strong> oxygen in<br />
the gas phase.<br />
However when the fugacity <strong>of</strong> oxygen is approximated by its partial pressure<br />
PO2 and the activity <strong>of</strong> oxygen in water by its concentration CO2(aq),<br />
Henry’s law 2 will be expressed as:<br />
CH = CO2(aq)<br />
PO2<br />
(1.3)<br />
Where CH is the Henry’s law constant, CH(O2) = 1.3 × 10 −3 Mol/atm.<br />
PO2 can be calculated by multiplying the fraction volume <strong>of</strong> oxygen in the<br />
air (21 %) with the pressure at sea level ( 760 mmHg, atmospheric pressure).<br />
Also in dilute solutions and <strong>for</strong> perfect gases K = CH, hence the<br />
concentration <strong>of</strong> oxygen in the ocean is<br />
C = CHPO2n = 1.3 × 10 −3 · 0.21 · 760 · 32 = 6.6mg/L (1.4)<br />
6.6 mg/L might not tell much on its own, however is the level <strong>of</strong> dissolved<br />
oxygen water drops below 5.0 mg/L, fish and other aquatic life will start to<br />
suffocate. The lower it drops the worse it gets, once the oxygen levels drop<br />
below 1-2 mg/L it will only take a few hours be<strong>for</strong>e most or all the fish are<br />
dead.[8]<br />
Now as mentioned earlier there is also a connection between the concentration<br />
<strong>of</strong> dissolved oxygen and temperature, or rather between water<br />
temperature and the saturation <strong>of</strong> oxygen. This is the point where water<br />
have the maximum amount <strong>of</strong> oxygen it can have, relative to pressure and<br />
temperature. It is calculated from the following equation[9]:<br />
Cp = C ∗ <br />
PW V (1 − )(1 − ΘP )<br />
P P<br />
(1 − PW V )(1 − Θ)<br />
(1.5)<br />
1 The measure <strong>of</strong> the tendency <strong>of</strong> a gas to escape or expand[7]<br />
2 The mass <strong>of</strong> a gas that dissolves in a definite volume <strong>of</strong> liquid is directly proportional<br />
to the pressure <strong>of</strong> the gas provided the gas does not react with the solvent.
4 CHAPTER 1. INTRODUCTION<br />
Where<br />
CP = equilibrium oxygen concentration at nonstandard pressure. mg/L.<br />
C ∗ = equilibrium oxygen concentration at 1 atm, mg/L.<br />
P = nonstandard pressure, atm.<br />
PW V = partial pressure <strong>of</strong> water vapor, atm, calculated from<br />
lnPW V = 11.8571 − 3840.70<br />
T<br />
T = temperature, Kelvin<br />
t = temperature, o C.<br />
− 216.961<br />
T 2<br />
Θ = 0.000975 − 1.426 × 10 −5 t + 6.436 × 10 −8 t 2<br />
(1.6)<br />
(1.7)<br />
As can be seen from the equation, the warmer it gets, the less oxygen<br />
there is the water, as it becomes saturated faster.<br />
Summed up all this gives an idea <strong>of</strong> how high, or low depending on how<br />
you look at it, the measured values can be expected to be.<br />
1.2 Thesis Outline<br />
In Chapter 2 other ways <strong>of</strong> using micro technology to measure oxygen is<br />
looked at, and the reason <strong>for</strong> picking the Clark type sensor.<br />
In Chapter 3 the theory behind the Clark <strong>Oxygen</strong> sensor, amperometry,<br />
and the potentionstat is explained.<br />
In Chapter 4 the reasons <strong>for</strong> why the sensor ended up looking as it did<br />
is explained.<br />
In Chapter 5 contains a description <strong>of</strong> the process to create the sensor.<br />
In Chapter 6 the problems encountered while fabricating the sensor is discussed,<br />
as well as how they were solved.<br />
In Chapter 7 various measurements, tests, and results are presented.
1.2. THESIS OUTLINE 5<br />
In Chapter 8 the conclusion <strong>for</strong> the whole project is made.
6 CHAPTER 1. INTRODUCTION
Chapter 2<br />
Optodes and ISFET<br />
There are several ways to measure oxygen, and the concept <strong>of</strong> doing it is<br />
not entirely new, and have been done <strong>for</strong> several decades, in oceanography,<br />
medicine, mountaineering as well as in many other areas. However while<br />
several ways exists on the macro scale, there are not quite as many ways on<br />
the micro scale. In this chapter the pros and cons <strong>of</strong> two <strong>of</strong> the ways are<br />
described, the theory behind them is briefly described, though without going<br />
deeply into the details. A comparison to the Clark sensor is also made (the<br />
theory <strong>of</strong> Clark sensor is described in Chapter 3).<br />
2.1 Optodes<br />
Figure 2.1: Commercial oxygen optode from Aanderaa, actual size <strong>of</strong> the<br />
sensor is 36 × 86mm. Reproduced from [10]<br />
Traditional oxygen sensors, such as the Clark sensor described in Chapter<br />
3 are based on the reduction at a working electrode, which is immersed a<br />
electrolyte and then separated from other interfering gasses by a membrane<br />
7
8 CHAPTER 2. OPTODES AND ISFET<br />
only permeable to oxygen. The oxygen flow through the membrane being<br />
induced by the consumption <strong>of</strong> O2 at this electrode. With the electrical flow<br />
between the electrodes being proportional to this flow <strong>of</strong> oxygen. Problem<br />
is that this flow is highly dependant on the diffusion though the membrane,<br />
and in seawater these can change due to temperature and biological fouling.<br />
Especially the biological fouling can be a problem, not only because they<br />
can cause a change in the diffusion characteristics <strong>of</strong> the membrane. But<br />
also because biological processes quite <strong>of</strong>ten involves the production or the<br />
consumption <strong>of</strong> oxygen, and as such will influence the amount measured.[11]<br />
Optodes (also know as optoelectrodes or optrodes[12]) are the optical equivalent<br />
<strong>of</strong> electrodes, and represent a theoretical solution to this. These are<br />
usually based on the quenching <strong>of</strong> a fluorescent indicator by oxygen, and<br />
described by the Stern-Volmer equation[13]:<br />
I0<br />
I = 1 + KSV [O2] (2.1)<br />
Where I and I0 are the fluorescence intensities with and without oxygen<br />
present, KSV the Stern-Volmer quenching constant and [O2] the concentration<br />
<strong>of</strong> dissolved oxygen. The sensitivity depends on the value <strong>of</strong> KSV ,<br />
which determines the change in the fluorescent intensity with various oxygen<br />
concentrations.[14]<br />
Figure 2.2: Sketch <strong>of</strong> the optical system <strong>of</strong> the optode.[10]<br />
The way the optical sensor, as shown on figure 2.2, works is:<br />
1. The blue LED emits light, at 475 nm, to an optical fiber.
2.2. ISFET (ION SELECTIVE FIELD EFFECT TRANSISTOR) 9<br />
2. The optical fiber then carries the light to the probe. The end <strong>of</strong> the<br />
probe tip consists <strong>of</strong> a thin layer <strong>of</strong> a hydrophobic material, with ruthenium<br />
inside it, protected from the liquid measured on.<br />
3. The light from the LED excites the ruthenium at the probe tip.<br />
4. The excited ruthenium then fluoresces, emitting energy at 600 nm.<br />
5. If the excited ruthenium hits an oxygen molecule, the excess energy<br />
is transferred to the oxygen molecule in a non-radiative transfer, decreasing<br />
or quenching the fluorescence). The degree <strong>of</strong> quenching corresponds<br />
to the level <strong>of</strong> oxygen concentration or to oxygen partial pressure<br />
in the film, which is in dynamic equilibrium with oxygen in the liquid.<br />
6. The energy is collected by the probe and carried through the optical<br />
fiber to the data storage/PC. Which in turn translates the data into a<br />
graph or a similar output, from which the oxygen level can be read.<br />
Some <strong>of</strong> the advantages <strong>of</strong> the optodes is that unlike traditional electrodes<br />
the optodes doesn’t consume oxygen, and is there<strong>for</strong>e not sensitive to stirring.<br />
Also the partial pressure <strong>of</strong> the oxygen in the probe is in equilibrium<br />
with the partial pressure outside, there<strong>for</strong>e it is less effected by pressure and<br />
the pressure behavior becomes easier to predict. Finally some <strong>of</strong> the commercially<br />
available optodes comes with warranties with up to two years, which<br />
allows <strong>for</strong> long term measurements.[10]<br />
Disadvantages <strong>of</strong> the optodes however is that they tend to be relatively<br />
large (the optode itself is small, but the electronics and power supply adds<br />
considerably to the size), and is there<strong>for</strong> <strong>of</strong>ten attached to ships or buoys.<br />
It also have a relatively large power consumption in comparison to a micro<br />
Clark sensor.<br />
2.2 ISFET (Ion Selective Field Effect Transistor)<br />
Electrochemical sensors are one <strong>of</strong> the older and larger groups <strong>of</strong> chemical<br />
sensor, the sensor type includes those based on electrochemical and charge<br />
transfer reactions, meaning the transfer <strong>of</strong> charge from an electrode to a<br />
solid or liquid phase, or the other way around. The chemical process takes<br />
place at the electrodes or in the probed volume, and the current from this
10 CHAPTER 2. OPTODES AND ISFET<br />
is measured. A electrochemical sensor are always made up <strong>of</strong> a minimum<br />
<strong>of</strong> two electrodes, one that allows <strong>for</strong> connection through the probed sample<br />
and the other via measuring equipment or a transducer.<br />
There are various electrochemical sensors, which can be defined by their<br />
analytical principles[15]:<br />
1. Voltammetric sensors; which measures the current-voltage relationship.<br />
The principal behind it, is having a potential applied, and measuring<br />
a current proportional to the concentration <strong>of</strong> the electro-active substance<br />
(a variant <strong>of</strong> this is amperometry[16], wherein the potential is<br />
kept constant.).<br />
2. Conductometric sensors; Which measures the conductance, by applying<br />
a small amplitude AC potential to a pair <strong>of</strong> electrodes to stop<br />
polarization. The charge carries will then determine the conductance.<br />
3. Potentiometric sensors; which measures potential <strong>of</strong> an electrode at<br />
equilibrium state, meaning no current is allowed to flow at the time <strong>of</strong><br />
the measurement.<br />
While all three can use CMOS based technology, it is especially in the<br />
case <strong>of</strong> the later, potentiometric sensors, that ISFET are used.[17]<br />
Figure 2.3: (a) diagram <strong>of</strong> a MOSFET (b) diagram <strong>of</strong> a ISFET. The metal<br />
gate <strong>of</strong> the MOSFET is replaced by the metal <strong>of</strong> a reference electrode,<br />
and a liquid to make contact with the bare gate insulator.<br />
Reproduced from [18].<br />
The ISFET is a variant <strong>of</strong> a MOSFET (see figure 2.3), where the MOS-<br />
FET is used <strong>for</strong> gas sensing, the ISFET is used <strong>for</strong> liquids. The more traditional<br />
<strong>of</strong> these are the pH-ISFET, where the basic principle is the electrolysis
2.3. SUMMARY 11<br />
<strong>of</strong> oxygen molecules, which is similar to that <strong>of</strong> a Clark sensor (more about<br />
this in Chapter 3). However the way the ISFET detects it, is by determining<br />
the surface potential at the insulator/electrolyte interface.[19] Also in<br />
the case <strong>of</strong> the pH-ISFET the surface <strong>of</strong> the gate oxide will contain OH −<br />
molecules, which can be protonated and deprotonated, so when the gate oxide<br />
contacts an aqueous solution, the change in pH will cause the silicon<br />
surface potential to change as well.<br />
The advantages <strong>of</strong> the ISFET is that the CMOS and MOSFET are tried<br />
and used technologies. However chemical sensors are still a relative new area<br />
<strong>for</strong> this to be used in, and as such suffers from ’child’-diseases.<br />
Disadvantages have included light sensitivity, lack <strong>of</strong> solid-state reference<br />
electrodes, and packaging integrity difficulties. [20]<br />
2.3 Summary<br />
While both the Optodes and the ISFET are viable ways <strong>of</strong> creating a oxygen<br />
sensor, both had certain distinct disadvantages that made the Clark type a<br />
better choice. However <strong>of</strong> the two, the optodes might in the future become a<br />
superior solution, once the problems with size and power supply have been<br />
solved.
12 CHAPTER 2. OPTODES AND ISFET
Chapter 3<br />
Theory <strong>of</strong> the Clark <strong>Sensor</strong><br />
The Clark sensor was named after its creator Leland C. Clark[21], who in<br />
the late fifties and mid sixties studied the electrochemistry <strong>of</strong> oxygen gas<br />
reduction at platinum (Pt) metal electrodes. Clark began pioneering the<br />
use <strong>of</strong> what would later be an oxygen- (and there<strong>for</strong>e chemical-) sensor. In<br />
fact, Pt electrodes used to detect oxygen electrochemically are <strong>of</strong>ten referred<br />
to generically as ’Clark electrodes’. While the sensor he originally came up<br />
with was intended <strong>for</strong> the measurement <strong>of</strong> glucose concentration in blood,<br />
the sensor itself was the start <strong>of</strong> biosensor history, and the Clark sensor is<br />
still around today with many different shapes and several applications far<br />
from the original design.<br />
Liquid measured<br />
on (Blood, water,<br />
etc.)<br />
Membrane<br />
Ag Wire<br />
Pt<br />
KCl Solution (electrolyte)<br />
Figure 3.1: Sketch <strong>of</strong> a basic 2 electrode Clark <strong>Oxygen</strong> <strong>Sensor</strong>, with a Ag<br />
reference electrode (the anode) and a Pt working electrode (the<br />
cathode).<br />
The electrodes as seen in figure 3.1 have a thin organic membrane covering<br />
a layer <strong>of</strong> electrolyte and two metallic electrodes. <strong>Oxygen</strong> diffuses through<br />
13<br />
A<br />
V
14 CHAPTER 3. THEORY OF THE CLARK SENSOR<br />
the membrane and is electrochemically reduced at the cathode. There is a<br />
carefully fixed voltage between the cathode and an anode so that only oxygen<br />
is reduced (explained further a little later in this chapter). The greater<br />
the oxygen partial pressure is, the higher the amount <strong>of</strong> oxygen that diffuses<br />
through the membrane in a given time. This results in a current that is proportional<br />
to the oxygen concentration in the sample, which can be measured<br />
with an external device, between the anode and the cathode. Temperature<br />
sensors can be built into the probe to allow compensation <strong>for</strong> the membrane<br />
and sample temperatures, as the temperature <strong>of</strong> these affect diffusion speed<br />
through the membrane and solubility in the electrolyte. The potentiostat<br />
(an instrument commonly used <strong>for</strong> measuring differences in electric potential.<br />
Essentially, this instrument balances the unknown voltage against a<br />
known, adjustable voltage. A potentionstat is frequently used in conjunction<br />
with a thermocoupler <strong>for</strong> measuring temperature. More about this in section<br />
3.2) uses cathode current, sample temperature, membrane temperature,<br />
barometric pressure and salinity in<strong>for</strong>mation to calculate the dissolved oxygen<br />
content <strong>of</strong> the sample in either concentration (ppm) or percent saturation<br />
(% Sat). The voltage <strong>for</strong> the reduction can either be supplied electronically<br />
by the meter (potentiometric oxygen electrode), or by dissimilar metals being<br />
used <strong>for</strong> the two electrodes, picked in a way so that the correct voltage<br />
is generated between them (galvanic electrode)[22].<br />
So how does this all work? In the following chapter I will cover the theory<br />
behind the Clark sensor, and explain what is happening when it is used. First<br />
a look at the chemistry in the Clark sensor, then the diffusion through the<br />
membrane, and finally the potentiostat.<br />
3.1 Chemistry <strong>of</strong> the Clark <strong>Sensor</strong><br />
When an electrode <strong>of</strong> noble metal such as platinum or gold is biased between<br />
-0.6 and -0.8 V [23] with respect to a reference electrode such as Ag/AgCl or<br />
a calomel electrode in a KCl solution, the oxygen dissolved in the liquid is<br />
reduced at the surface <strong>of</strong> the noble metal (the cathode). This phenomenon<br />
can be observed from a current-voltage diagram (called a polarogram, example<br />
<strong>of</strong> this can be seen on figure 3.2) <strong>of</strong> the electrode. When the negative<br />
voltage applied to the noble metal electrode (called the cathode) is increased,<br />
the current increases initially but soon it becomes saturated. In this plateau<br />
region <strong>of</strong> the polarogram, the reaction <strong>of</strong> oxygen at the cathode is so fast that<br />
the rate <strong>of</strong> reaction is limited by the diffusion <strong>of</strong> oxygen to the cathode sur-
3.1. CHEMISTRY OF THE CLARK SENSOR 15<br />
face. When the negative bias voltage is further increased, the current output<br />
<strong>of</strong> the electrode increases rapidly due to other reactions. If a fixed voltage in<br />
the plateau region (<strong>for</strong> instance, -0.7V) is applied to the cathode, the current<br />
output <strong>of</strong> the electrode can be linearly calibrated to the dissolved oxygen<br />
(short termed DO). It should be noted that the current is proportional not<br />
to the actual concentration but to the activity or equivalent partial pressure<br />
<strong>of</strong> dissolved oxygen, which is <strong>of</strong>ten referred to as oxygen tension. A fixed<br />
voltage between -0.6 and -0.8 V (due to the cell potential, and the onset <strong>of</strong><br />
other processes) is usually selected as the polarization voltage when using<br />
Ag/AgCl as the reference electrode.<br />
Figure 3.2: Polarogram: Current to voltage at different oxygen tensions and<br />
calibration obtained at a fixed polarization voltage. Reproduced<br />
from [24]. The shape <strong>of</strong> the curves indicates the nature <strong>of</strong> the<br />
substance in the sample that is being reduced; where the height<br />
is equivalent to the concentrations <strong>of</strong> oxygen. Also note that<br />
convention usually have -V along the x-axis and not V, as is<br />
otherwise the norm.<br />
The cathode, the reference electrode, and the electrolyte are separated<br />
from the measurement medium by a membrane, which is permeable to the<br />
dissolved gas but not to most <strong>of</strong> the ions and other species. When at the<br />
same time most <strong>of</strong> the mass transfer resistance is confined in the membrane,<br />
the electrode system can measure oxygen tension in various liquids. This is<br />
the basic operating principle <strong>of</strong> the membrane covered DO probe[25].<br />
Electrode Reactions: The reaction can proceed as follows (a 4e − reaction<br />
with a Ag/AgCl electrode is used as an example)<br />
Cathodic reaction:
16 CHAPTER 3. THEORY OF THE CLARK SENSOR<br />
Anodic reaction:<br />
Overall reaction:<br />
O2 + 2H2O + 2e − → H2O2 + 2OH −<br />
H2O2 + 2e − → 2OH −<br />
O2 + H2O + 4e − → 4OH −<br />
Ag + Cl − → AgCl + e −<br />
4Ag + O2 + 2H2O + 4Cl − → 4AgCl + 4OH −<br />
Two pathways are possible <strong>for</strong> the reduction <strong>of</strong> oxygen at the noble metal<br />
surface. One is a 4-electron pathway where the oxygen in the bulk diffuses<br />
to the surface <strong>of</strong> the cathode and is converted to H2O via H2O2. The other<br />
is a 2-electron pathway where the H2O2 diffuses directly from the cathode<br />
surface into the bulk liquid.<br />
Also as can be seen above from the reaction, hydroxyl ions are constantly<br />
being substituted <strong>for</strong> chloride ions once the reaction starts, there<strong>for</strong>e KCl<br />
or NaCl have to be used as an electrolyte. However this electrolyte will<br />
slowly be depleted <strong>of</strong> Cl − , so something has to be done to keep this depletion<br />
from occurring to fast, or to replenish it somehow. Hence this is one <strong>of</strong> the<br />
problems that have to be faced when building a Clark sensor, as it can be<br />
one <strong>of</strong> the main limits <strong>for</strong> the lifetime <strong>of</strong> the sensor.<br />
3.1.1 One layer electrode model<br />
Now three assumptions will be used to look at a one dimensional diffusion<br />
equation[26]:<br />
1. The cathode is well polished and the membrane is tightly fit over the<br />
cathode surface such that the thickness <strong>of</strong> the electrolyte layer between<br />
the membrane and the cathode is negligible. There<strong>for</strong>e the membrane<br />
will define the oxygen flux to the cathode surface.<br />
2. The liquid around the sensor is well agitated so the partial pressure <strong>of</strong><br />
oxygen at the membrane surface is the same as that <strong>of</strong> the bulk liquid.<br />
3. <strong>Oxygen</strong> diffusion occurs only in one direction, perpendicular to the<br />
cathode surface.
3.1. CHEMISTRY OF THE CLARK SENSOR 17<br />
PSfrag replacements<br />
y<br />
Cathode<br />
Membrane Liquid<br />
x = 0 x = dm<br />
Figure 3.3: One layer electrode model to illustrate the three assumptions, as<br />
well as to help derive the equations based on them.<br />
Using Fick’s second law and the coordinate system from Figure 3.3, the<br />
unsteady diffusion in the membrane will be described by:<br />
∂p<br />
∂t<br />
∂<br />
= Dm<br />
2p ∂x2 p0<br />
x<br />
(3.1)<br />
Where Dm is the oxygen diffusivity in the membrane, p the partial pressure<br />
<strong>of</strong> oxygen in the membrane, and x is the distance from the cathode<br />
surface.<br />
Under the assumption that diffusion have yet to take place, the initial<br />
boundary conditions are:<br />
p = 0 at t = 0 (3.2)<br />
p = 0 at x = 0 (3.3)<br />
p = p0 at x = dm<br />
(3.4)<br />
where dm is the membrane thickness and po is the partial pressure <strong>of</strong> oxygen<br />
in the bulk liquid.
18 CHAPTER 3. THEORY OF THE CLARK SENSOR<br />
The first boundary condition assumes that we insert the membrane and cathode<br />
in the liquid, and as such no pressure have yet to build up at the cathode.<br />
The second boundary condition assumes a fast reaction at the cathode surface,<br />
which is generally achieved when the cathode is properly polarized.<br />
The third boundary condition assumes there is a state <strong>of</strong> equilibrium inside<br />
the membrane, hence no pressure is being applied from within so that the<br />
only pressure at the surface <strong>of</strong> the membrane is coming from the outside<br />
liquid.<br />
Under these boundary conditions the solution[27] <strong>of</strong> Eq. 3.1 is<br />
p<br />
p0<br />
= x<br />
+<br />
dm<br />
∞ 2<br />
nπ (−1)nsin nxπ<br />
<br />
2 2 −n π Dmt<br />
exp<br />
n=1<br />
dm<br />
d 2 m<br />
(3.5)<br />
The current output I <strong>of</strong> the electrode is proportional to the oxygen flux<br />
at the cathode surface:<br />
<br />
∂C<br />
I = NF ADm<br />
∂x<br />
x=0<br />
<br />
∂p<br />
= NF APm<br />
∂x<br />
x=0<br />
(3.6)<br />
Where N is the number <strong>of</strong> electrons per mole <strong>of</strong> reduced oxygen, F is<br />
Faraday’s constant, A the surface area <strong>of</strong> the cathode, C the concentration 1<br />
and Pm the oxygen permeability 2 <strong>of</strong> the membrane.<br />
Pm is related to diffusivity by<br />
Pm = DmSm<br />
where Sm is the oxygen solubility 3 <strong>of</strong> the membrane.<br />
From Eq. 3.5 and Eq. 3.6 the current output I <strong>of</strong> the electrode is<br />
I = NF A Pm<br />
p0<br />
dm<br />
<br />
1 + 2<br />
∞<br />
(−1) n 2 −n 2π Dmt<br />
exp<br />
<br />
n=1<br />
d 2 m<br />
(3.7)<br />
(3.8)<br />
<br />
1 ∂C<br />
J = Dm ∂x being the oxygen flux in the x direction<br />
x=0<br />
2The passage or diffusion <strong>of</strong> a gas, vapor, liquid, or solid through a barrier without<br />
physically or chemically affecting it.<br />
3Ability <strong>of</strong> a substance to dissolve in another substance
3.1. CHEMISTRY OF THE CLARK SENSOR 19<br />
The pressure pr<strong>of</strong>ile within the membrane and the current output under<br />
steady state conditions can then be obtained from Eq. 3.5 and Eq. 3.8:<br />
and<br />
p<br />
p0<br />
= x<br />
dm<br />
I = NF A Pm<br />
dm<br />
· p0<br />
(3.9)<br />
(3.10)<br />
At steady state, the pressure pr<strong>of</strong>ile is linear and the current output is<br />
proportional to the oxygen partial pressure in the bulk liquid, where Eq. 3.10<br />
<strong>for</strong>ms the basis <strong>for</strong> DO measurement by the sensor. The response time <strong>of</strong> the<br />
sensor as seen from Eq. 3.8 is:<br />
τ = d2 m<br />
Dm<br />
(3.11)<br />
Where τ determines how fast the sensor responds, due to the thickness<br />
<strong>of</strong> the membrane or a high Dm. However these conditions tend to weaken<br />
the assumption <strong>of</strong> membrane-controlled diffusion. There<strong>for</strong>e, a compromise<br />
has to be made <strong>for</strong> optimum sensor per<strong>for</strong>mance. Changing dm (rather than<br />
Dm) is more effective in adjusting τ (since it depends on the square <strong>of</strong> dm).<br />
Eq. 3.10 and Eq. 3.11 gives the indication, that the design variables <strong>for</strong> a<br />
DO sensor is Pm, dm, Dm, and A.<br />
3.1.2 Two layer model<br />
The problem is, however, that the second assumption, made earlier in this<br />
chapter, is not entirely accurate. A stagnant liquid film will almost always<br />
exists right outside the membrane even at high liquid velocities.[28] Hence<br />
a more realistic model have to be made, in order to account <strong>for</strong> this film as<br />
shown on Figure 3.4. This take into consideration the effect <strong>of</strong> the film, as<br />
it slows down the diffusion through the membrane.<br />
The effect <strong>of</strong> the liquid layer on the sensor current can be calculated by<br />
expanding the previous model, at steady state the oxygen flux J through<br />
each layer should be the same.
20 CHAPTER 3. THEORY OF THE CLARK SENSOR<br />
PSfrag replacements<br />
y<br />
Cathode<br />
Membrane Liquid<br />
High Velocity<br />
Low velocity<br />
Figure 3.4: Two layer model <strong>for</strong> DO sensor to illustrate the changed conditions.<br />
J = Kp0 = kL(p0 − pm) = kmpm<br />
p0<br />
x<br />
(3.12)<br />
Where K is the overall is the overall mass transfer coefficient and kL and<br />
km are the mass transfer coefficient <strong>for</strong> the liquid film and the membrane.<br />
The inverse <strong>of</strong> the mass transfer coefficient can be termed as the mass<br />
transfer resistance. If kL and km is thought <strong>of</strong> as two parallel resistors, we<br />
get:<br />
1<br />
K<br />
1<br />
= +<br />
kL<br />
1<br />
km<br />
(3.13)<br />
What this equation says is that the overall mass transfer resistance is the<br />
sum <strong>of</strong> mass transfer resistance and the membrane phase transfer resistance.<br />
The derivation is similar to Ohm’s law, where J is equivalent <strong>of</strong> the current,<br />
and p <strong>of</strong> the voltage.<br />
The individual resistance can be replaced by more commonly known parameters:<br />
1<br />
K<br />
dL<br />
= +<br />
PL<br />
dm<br />
Pm<br />
(3.14)
3.1. CHEMISTRY OF THE CLARK SENSOR 21<br />
Where dL is the liquid film thickness and PL the oxygen permeability <strong>of</strong><br />
the liquid film.<br />
When the individual mass transfer resistances are included, the steady<br />
state sensor output becomes:<br />
Where ¯ d is defined as:<br />
I = NF A Pm<br />
¯d p0<br />
dL<br />
¯d = dm + Pm<br />
PL<br />
The time constant τ from Eq. 3.11 can be modified to:<br />
with d defined as:<br />
τ = d2<br />
Dm<br />
d = dm + dL<br />
Dm<br />
DL<br />
(3.15)<br />
(3.16)<br />
(3.17)<br />
(3.18)<br />
The DO sensor, when placed in a stagnant liquid, produces a diffusion<br />
gradient extending outside the membrane and farther into the liquid. When<br />
the liquid is stirred, the diffusion gradient can no longer be extended beyond<br />
the liquid film around the membrane. Since the diffusion gradient becomes<br />
steeper with decreasing liquid film thickness, the current output <strong>of</strong> the sensor<br />
increases with increase in liquid velocity, also that the response time <strong>of</strong> the<br />
sensor increases as the liquid velocity decreases. This ’flow sensitivity’ is<br />
greater <strong>for</strong> a sensor with a larger cathode because the size <strong>of</strong> the stagnant<br />
diffusion field is proportionally greater with a larger cathode. Hence there<br />
is an advantage in going from macro to micro scale, as the cathode will<br />
naturally be smaller here.<br />
Eq. 3.15 can be written as:<br />
I = NF A p0<br />
dm<br />
Pm<br />
+ dL<br />
PL<br />
(3.19)
22 CHAPTER 3. THEORY OF THE CLARK SENSOR<br />
And from this the conditions <strong>for</strong> membrane controlled diffusion becomes:<br />
dm<br />
Pm<br />
≫ dL<br />
PL<br />
(3.20)<br />
In order to achieve this condition, a relatively thick membrane with a<br />
low oxygen permeability have to be used. When this condition is achieved,<br />
the oxygen sensor output depends only on membrane properties as given by<br />
Eq. 3.10 and the sensor calibrated in one liquid can be used in other liquids<br />
without recalibration. However there is always a liquid film (however thin it<br />
may be) and this causes variations in calibration in different liquids.<br />
3.2 The Potentiostat<br />
Most <strong>of</strong> us can probably recall from the early chemistry lessons in school ,<br />
that if you take a piece <strong>of</strong> iron and dips it into a sulphuric acid, it will start to<br />
dissolve, or rather corrode. Though in the 17th century Sir Humphrey Davy,<br />
following Galvani’s experiments [29] discovered that if a piece <strong>of</strong> non corrosive<br />
metal, like Platinum, was put in the acid, and then connected to a positive<br />
pole. While at the same time connecting the piece <strong>of</strong> iron to the negative pole<br />
<strong>of</strong> the same current source. The corrosion <strong>of</strong> the iron would slow down, or<br />
even come to a complete halt if a high enough amount <strong>of</strong> voltage was applied.<br />
Vice versa the corrosion would increase with the voltage, if the iron was<br />
connected to the positive pole and the platinum to the negative pole.<br />
Though above a specific current(the exact one varying with the temperature,<br />
composition, and size <strong>of</strong> the metals) the current will suddenly drop to<br />
low values and the iron would cease to corrode (A phenomenon discovered<br />
first by Michael Faraday.[30]). The understanding and explanation <strong>for</strong> this<br />
was later brought on by the invention <strong>of</strong> the potentiostat.<br />
As mentioned at the beginning <strong>of</strong> this chapter, a potentiostat is used in<br />
order to measure the current, so what does this device do, and what is the<br />
principle behind it. Basically the potentiostat, is an amplifier that sets and<br />
maintains a voltage between two electrodes at a specific value, a Working<br />
Electrode (also sometimes referred to as the Indicator Electrode[31]) and a<br />
Reference electrode.
3.2. THE POTENTIOSTAT 23<br />
Reference<br />
Electrode<br />
Constant E required<br />
Working<br />
Electrode<br />
Figure 3.5: A basic electrochemical cell with a cathode (Working Electrode)<br />
and a anode (Reference Electrode).<br />
However in order <strong>for</strong> this to be accomplished some conditions have to be<br />
fulfilled[32]. The Reference electrode maintains a constant voltage referred to<br />
as the hydrogen electrode potential, an <strong>of</strong>ten used metal <strong>for</strong> this (as is also the<br />
case in this project) is Ag covered with a layer <strong>of</strong> AgCl. The moment a current<br />
passes through this electrode it will become polarized, eg its potential will<br />
vary accordingly to the current. There<strong>for</strong>e in order <strong>for</strong> a steady potentential<br />
can be maintained, no current must be allowed to pass through it. This<br />
is accomplished by having a constant potential difference between the two<br />
electrodes. However in order to achieve this a third electrode have to be<br />
added to the cell, a Counter Electrode (also known as a Auxiliary Electrode).<br />
Hence a current is <strong>for</strong>ced between the Counter Electrode and the Working<br />
Electrode, this current is high enough and in the proper polarity to keep<br />
the potential <strong>of</strong> the Working Electrode at a set value in comparison to the<br />
Reference Electrode.<br />
There will be two focuses <strong>of</strong> the Potentiostat[33]:<br />
1. Measuring the potential difference between the Reference and Working<br />
Electrode, in a way that leaves the Reference Electrode unpolarized,<br />
while comparing this potential difference to an already preset voltage.<br />
2. Force a current between the Counter Electrode and the Working Electrode<br />
to counterbalance the difference between the Working Electrode<br />
potential and the preset voltage.<br />
This is realized electronically by using a operational amplifier also known<br />
as an OPAMP, the operational amplifier will has two inputs, an inverting and
24 CHAPTER 3. THEORY OF THE CLARK SENSOR<br />
Working<br />
Electrode<br />
Constant E required<br />
Reference<br />
Electrode<br />
Current<br />
Input<br />
Counter<br />
Electrode<br />
Figure 3.6: A basic electrochemical cell with a third electrode (Counter Electrode)<br />
added to control the potential.<br />
a non-inverting. If a current/voltage is feed into the non-inverting part, an<br />
amplified current/volage will be produced, while feeding it into the inverting<br />
part will maintain the magnitude, while switching the sign (+ to − or −<br />
to +). By closing the loop between in- and output, the voltage difference<br />
between the two inputs <strong>of</strong> the amplifier will fall. An increase in the voltage in<br />
the inverting input will <strong>for</strong>ce a complementary current on the output, which<br />
will counteract the difference in input voltage.<br />
Connecting the Reference Electrode to the inverting input, the Working<br />
Electrode to the non-inverting, and the Counter Electrode to the output will<br />
give this:<br />
Between the Working Electrode and the Reference Electrode the difference<br />
will now be amplified and inverted by the amplifier, the circuit will<br />
be closed by the electrochemical cell, where the current runs through the<br />
electrolyte from the Counter Electrode to the Working Electrode. This will<br />
polarize the Working Electrode, so that the input difference between that and<br />
the Reference Electrode will be zero. In doing so, the Working Electrode and<br />
the Reference Electrode potential is kept level. To shift the Working Electrode<br />
potential to some other reference value in regards to the Reference<br />
Electrode, a voltage in series between the input to the Reference Electrode<br />
and the Reference Electrode itself must be inserted. Also to measure the current<br />
running through the Counter Electrode, a resistor is used in the wiring<br />
<strong>of</strong> the Counter Electrode, over which a voltage that is proportional to the
3.2. THE POTENTIOSTAT 25<br />
Working<br />
Electrode<br />
Reference<br />
Electrode<br />
+<br />
OPAMP<br />
−<br />
Counter<br />
Electrode<br />
Figure 3.7: Using the OPAMP as a potentiostat.<br />
current is measured (See Figure 3.8)<br />
Working<br />
Electrode<br />
Reference<br />
Electrode<br />
+<br />
OPAMP<br />
−<br />
R<br />
Counter<br />
Electrode<br />
Figure 3.8: Resistor added to measure the current running through the<br />
Counter Electrode.<br />
Of course a more complex electronic system is still needed to protect<br />
the potential amplifier from voltage shocks, to counteract noise and other<br />
miscellaneous problems with electrical systems.<br />
V
26 CHAPTER 3. THEORY OF THE CLARK SENSOR<br />
3.3 Summary<br />
The theories upon which the Clark sensor is based, as well as the potentiostat,<br />
have been highlighted, and it is now a matter <strong>of</strong> trying to use these in order<br />
to design and build a micro oxygen sensor. While trying to find a way around<br />
the problems that electrolyte depletion and membrane fouling presents.
Chapter 4<br />
System Design<br />
The sensor layout is one <strong>of</strong> the more essential parts <strong>of</strong> the project, as the<br />
design and general setup will be the deciding factor, in how well the sensor<br />
will work. At such at least one design should be made that is reasonably<br />
safe, which based on the general theory should work.<br />
In this chapter the thoughts and ideas, concerning why the first designs <strong>of</strong><br />
the sensor(s) looks as they do, are given and explained.<br />
4.1 General Design<br />
Based on a previous project by Anders Hyldg˚ard [34], an ’O’-ring based<br />
design was decided on, this was done in part to take advantage <strong>of</strong> the already<br />
existing packaging scheme 1 . The setup uses an ’O’-ring as pressure sealing,<br />
which leaves the active part <strong>of</strong> the sensor exposed, while protecting the rest<br />
<strong>of</strong> the chip from the corrosive effects <strong>of</strong> seawater[35]. Of course care still has<br />
to be taken, to ensure that it is tightly sealed, since even a small roughness<br />
on the surface <strong>of</strong> the chip might allow seawater to seep through. Furthermore<br />
a lot <strong>of</strong> the clean room steps from that project can be duplicated to create<br />
the various layers in this design.<br />
Making the sensor circular however presents other problems, as the sensor<br />
needs a membrane covering it as mentioned in a Chapter 3, so that only<br />
oxygen diffusing through the membrane is measured, as well as an electrolyte.<br />
This in itself may not seem like a major obstacle, though given that the<br />
membrane (Siloprene) is liquid to begin with, it needs a small ’container’<br />
1 See the Packaging section <strong>for</strong> more info on this<br />
27
28 CHAPTER 4. SYSTEM DESIGN<br />
until it solidifies. Experiments have been made by M. Dawgul et. al. [36], in<br />
which a membrane has been spin coated on the wafer. The results from this<br />
however were not good, as the membrane <strong>of</strong>ten ended up sliding <strong>of</strong>f, or at<br />
best varied greatly in thickness, thereby lacking in regards to the feasibility<br />
<strong>of</strong> being able to create a preset, uni<strong>for</strong>m membrane thickness.<br />
A logical solution to this has been used by M. Wittkampf et. al [37] where<br />
a second wafer is anodic bonded to the first, with the second wafer having a<br />
hole created with a wet isotropic etch where the exposed part <strong>of</strong> the sensor is,<br />
there by creating a hole that the membrane liquid can be ’poured’ into. This<br />
is a fairly simple process, however the basic <strong>of</strong> this design makes it a little<br />
more difficult, as a circular rather than a rectangular hole have to be made.<br />
However it is a problem that can be overcome. In the initial design, the<br />
’O’-ring will be used as a ’container’ <strong>for</strong> the electrolyte and the membrane.<br />
Also it should be mentioned that a micro electrode have the advantage over<br />
a macro electrode, in that the diffusion occurs in all directions, where as with<br />
the macro it only have diffusion in one direction[38]. Hence at macro scale<br />
the oxygen will be consumed faster at the electrode, and thereby influence<br />
the measurements. (See Figure 4.1)<br />
Figure 4.1: Micro electrodes have diffusion to all directions while Macro only<br />
have to one, why at macro scale setup the oxygen is consumed<br />
faster at the electrode.<br />
4.2 Actual Design<br />
Four ’major’ designs, each with some variant sub design, have been created,<br />
taking into consideration some <strong>of</strong> the experiences that have already been<br />
made by others[37, 39, 36, 38, 40, 41]. The idea behind the main one as
4.2. ACTUAL DESIGN 29<br />
seen on figure 4.2 have been to create a layout, which as mentioned earlier,<br />
would have a high chance <strong>of</strong> working. Thereby enabling me to do some<br />
measurements with it, so that some practical experience could be gathered.<br />
Also <strong>of</strong> course to see if it did indeed work, and what response could be<br />
expected from it. The design is basically an array <strong>of</strong> Pt working electrodes<br />
located in the middle, then a torus shaped Pt counter electrode surrounding<br />
it, and finally a circular Ag/AgCl reference electrode around the two <strong>of</strong> them.<br />
Extreme care has been taken to avoid, or at the very least minimize, the<br />
chances <strong>of</strong> any short circuits. Put in other words each electrode have its own<br />
connection to the outside via a silicide ’wire’, that touches neither the other<br />
electrodes, nor the other ’wires’. Also there is a temperature sensor at the<br />
top, more about this later however.<br />
Figure 4.2: Primary/Safe Design, the white circles are where the ’O’-ring will<br />
be placed on the chip, showing both the actual inner and outer<br />
diameter <strong>of</strong> it, as well as where it minimally will touch the chip.<br />
The number refer to the design, while the roman numerals refer<br />
to the size and distance between the electrodes in the array. For<br />
a closeup <strong>of</strong> the Working Array see Figure 4.6
30 CHAPTER 4. SYSTEM DESIGN<br />
Based on this design, three others were created, a second where the a<strong>for</strong>ementioned<br />
short circuits is assumed not to take place. A third, where the<br />
counter electrode and the counter array are switched, and a fourth, where<br />
the size <strong>of</strong> the reference electrode is increased.<br />
The second (See Figure 4.3) is intended to replace the first if it works, since<br />
as can be seen on the figure, it is basically the same as the first, except that<br />
I have here assumed no short circuits will occur.<br />
Figure 4.3: Second Design, note that the electrodes are now closed rings and<br />
lies over the contact layer, albeit with a Nitride layer inbetween
4.2. ACTUAL DESIGN 31<br />
The third design (See Figure 4.4) is partly done to see if the right choice<br />
was made in placing the working array in the center, instead it have here<br />
switched place with the counter electrode. As such it is more done out <strong>of</strong><br />
curiosity to learn more about the workings <strong>of</strong> a Clark sensor, rather than a<br />
design in which great expectations is placed.<br />
Figure 4.4: Third Design, the working array is now the inner ring, rather<br />
than the center circle
32 CHAPTER 4. SYSTEM DESIGN<br />
The reference electrode is always placed furthest out in the circle, due<br />
to the silver being slowly consumed as the sensor is used. There<strong>for</strong>e it is<br />
preferable to have as much <strong>of</strong> it to consume as possible, in order to extend<br />
the lifetime <strong>of</strong> the sensor, which is the basis <strong>of</strong> the fourth design (See Figure<br />
4.5).<br />
Figure 4.5: Fourth Design, the reference electrode has been enlarged.<br />
4.2.1 Minor Design Variations<br />
In order to get a better understanding <strong>of</strong> the sensor, as well as trying to<br />
optimize it, some variations have be made on various aspects <strong>of</strong> the sensor,<br />
such as distance between each electrode in the array (See Figure 4.6), as well<br />
as the size <strong>of</strong> each electrode in the array (See Figure 4.7). Other possible<br />
variations could include the distance between the reference electrode, the<br />
counter electrode and the working array, as well as making the already used<br />
variations smaller or larger.<br />
Why these variations? Well as the reference electrode (Ag/AgCl) is slowly<br />
consumed, it will be deposited at the working array, the speed at which this<br />
occur will in part depend on the size and numbers <strong>of</strong> array electrodes. So if<br />
there is too few, or they are too small, they will be covered faster. On the
4.3. TEMPERATURE SENSOR 33<br />
Figure 4.6: Closeup <strong>of</strong> the working array, there is 150 µm between each electrode<br />
on the left array, and 100 µm between each on the right.<br />
The radius <strong>of</strong> the green circle (contact area <strong>for</strong> the working array)<br />
is in both cases 350 µm.<br />
other hand if there is too many in the array, there is a risk <strong>of</strong> the array starting<br />
to act like a macro scale electrode, where the oxygen will be consumed, hence<br />
corrupt the obtained measurements. Not to mention that the sensor itself<br />
will be used underwater, while being attached to a fish, hence no external<br />
power source is possible. There<strong>for</strong>e it is an advantage if it uses as little power<br />
as possible, this equals a smaller battery which in turn will make the overall<br />
size <strong>of</strong> the finished device as small as possible, and hence it can be attached<br />
to smaller fish. Finally the size and distance also influence the signal output<br />
<strong>of</strong> the device, as the measured current is partly dependable on these two<br />
parameters.[42, 43]<br />
So it is in large part a question <strong>of</strong> finding a balance between all the factors,<br />
to ensure the lifetime is as long as possible, without sacrificing accuracy.<br />
There<strong>for</strong>e variations on all conceivable variables should be looked into.<br />
4.3 Temperature <strong>Sensor</strong><br />
Since the diffusion through the membrane among other things is dependent<br />
on temperature, and there was room to spare on the chip, it was deemed<br />
an advantage to add a temperature sensor (platinum resistance temperature
34 CHAPTER 4. SYSTEM DESIGN<br />
Figure 4.7: Closeup <strong>of</strong> electrode in a working array, the one on the left have<br />
a radius <strong>of</strong> 5 µm, while the right have a radius <strong>of</strong> 10 µm, to get a<br />
more accurate idea <strong>of</strong> the size compare it to the distance between<br />
the brown area (the counter electrode) and the green area (the<br />
contact layer <strong>for</strong> the electrode array), which is 20 µm in both<br />
cases. The black diagonal area is the nitride contact holes.<br />
sensor). Since the relationship between temperature and resistance <strong>for</strong> platinum<br />
is approximately linear over a small temperature range, and since the<br />
properties <strong>for</strong> platinum is well known and documented this was a preferable<br />
choice. The resistance is calculated from Eq. 4.1<br />
R = ρ l<br />
wh<br />
(4.1)<br />
Where R is the resistance, ρ the resistivity <strong>of</strong> the material (ρ = 1.06 ×<br />
10 −8 Ωm <strong>for</strong> Pt), and l, w and h the length, width and height <strong>of</strong> the resistor.<br />
Figure 4.8: Close up <strong>of</strong> the l-edit design <strong>for</strong> the Pt50<br />
3 versions <strong>of</strong> this have been implemented on the various designs, a Pt50,
4.3. TEMPERATURE SENSOR 35<br />
a Pt300, and a Pt600 (where the number is the resistance <strong>of</strong> the sensor.<br />
Figure 4.9: Close up <strong>of</strong> the l-edit design <strong>for</strong> the Pt300<br />
Of these 3, the Pt50 is the safest design. The reason <strong>for</strong> this is that the<br />
The Pt300 and Pt600 have a width <strong>of</strong> 10 µm, while the Pt50 have a width<br />
<strong>of</strong> 20 µm. Hence if some error occurs during the metallization part <strong>of</strong> the<br />
process <strong>for</strong> the sensor, and you end up with a width <strong>of</strong> 9 µm instead, which is<br />
merely 1 µm <strong>of</strong>f. It would translate into a 10% shift in the resistance, while it<br />
would merely be 5% with the Pt50. Hence having a reasonably wide resistor<br />
can minimize a potential error, un<strong>for</strong>tunately however the higher the width,<br />
the lower the resistance is as well. So once again it is a matter <strong>of</strong> balancing<br />
one advantage against another disadvantage.<br />
Figure 4.10: Close up <strong>of</strong> the l-edit design <strong>for</strong> the Pt600, it is basically the<br />
Pt300 done twice.<br />
Why the 4 contacts, instead <strong>of</strong> merely 2? Well consider if there was only<br />
2, each serving as a contact <strong>for</strong> both current and voltage, the total resistance<br />
between the two can then be calculated from:<br />
R = V<br />
I = 2Rc + 2Rsp + Rs<br />
(4.2)<br />
Where Rc is the contact resistance between the chip and the equipment<br />
connected to it, Rsp the spreading resistance, and Rs the chip resistance (and
36 CHAPTER 4. SYSTEM DESIGN<br />
the resistance that I want to assign). This in itself may seem straight<strong>for</strong>ward,<br />
however neither Rc nor Rsp can be calculated precise, so as a result it isn’t<br />
possible to measure Rs accurately.<br />
The 4-contact approach solves this problem however; 2 carry the current<br />
while the other 2 are used to measure the voltage. While Rc and Rsp is still<br />
there, they are now negligible due to the very small current that flows through<br />
them, as the voltage is now measured with a potentiometer that draws no<br />
current, or with a high impedance voltmeter that draws little current.<br />
Temperature Calculated Width Effective Heigth<br />
<strong>Sensor</strong> Resistance (Ω) (µm) Length (µm) (µm)<br />
Pt50 47.8 20 18019 0.2<br />
Pt300 287 10 54170 0.2<br />
Pt600 590 10 113207 0.2<br />
4.4 Packaging<br />
Table 4.1: Overview <strong>of</strong> the temperature sensors.<br />
Shielding the interface electronic part <strong>of</strong> the chip from the corrosive effect<br />
<strong>of</strong> seawater, is as mentioned earlier the ’O’-ring and packaging. How is this<br />
built?<br />
The package or casing consists <strong>of</strong> three different layers (hereafter referred<br />
to as the lower, middle and upper layer) <strong>of</strong> laser 2 cut PMMA (PolyMethyl-<br />
MethAcrylate), that each have their own structure. While there are materials<br />
that are not as porous, and have better chemical properties, PMMA<br />
have the advantage that several can be fabricated within minutes once the<br />
actual structure is in place. Which have a higher priority than long term<br />
usage as far as testing purposes goes. Also PMMA can generally be used in<br />
the temperature area from -40 o C to 70 o C, as well as being electric isolating.<br />
So what do these layers look like, and what purpose does each serve?<br />
2 Laser used is a SYNRAD laser
4.4. PACKAGING 37<br />
Figure 4.11: Close up photography <strong>of</strong> a flex print.<br />
1 mm<br />
1.7 mm<br />
11 mm<br />
6.5 mm<br />
66.5 mm<br />
Figure 4.12: Rough sketch <strong>of</strong> the middle layer <strong>of</strong> the encapsulation, sizes<br />
listed to give a clearer picture <strong>of</strong> actual size<br />
Figure 4.13: Pictures taken <strong>of</strong> the middle layer, to show what it looks like<br />
once fabricated, the layer is tilted slight on the picture to the<br />
right to compensate <strong>for</strong> the light.<br />
The middle layer as seen on figure 4.12 and 4.13 is where the chip is<br />
located, 4 areas are cut into the PMMA with a laser. The area with the
38 CHAPTER 4. SYSTEM DESIGN<br />
chip, 2 adjourning not quite as deep areas (a short and a long area) where<br />
the flex print (This provide the electronic wiring to devices outside the sensor,<br />
see Figure 4.11 <strong>for</strong> a picture <strong>of</strong> the flex print) is located, and a hole <strong>for</strong> the<br />
flex print on the short side (So that the connection to the outside system is<br />
located in one end, with the sensor at the other.)<br />
The lower layer as seen on figure 4.14 and 4.15 have been cut to provide<br />
a a grove path where the flex print sticking through the hole on the middle<br />
can run back.<br />
6 mm<br />
Figure 4.14: Rough sketch <strong>of</strong> the lower layer <strong>of</strong> the encapsulation, sizes listed<br />
to give a clearer picture <strong>of</strong> actual size<br />
1.5 mm<br />
Figure 4.15: Pictures taken <strong>of</strong> the lower layer.<br />
The upper layer as seen on figure 4.16 and 4.17 consist <strong>of</strong> 3 parts, 2<br />
square holes that provides room <strong>for</strong> the connection between then chip and<br />
the flex print since there isn’t a direct contact between the two (More on this<br />
connection and how the 3 layers are made to stick together in Chapter 5).
4.4. PACKAGING 39<br />
90 mm<br />
15 mm<br />
6 mm<br />
4.75 mm<br />
4.5 mm<br />
4.75 mm<br />
Figure 4.16: Rough sketch <strong>of</strong> the middle layer <strong>of</strong> the encapsulation, sizes<br />
listed to give a clearer picture <strong>of</strong> actual size<br />
2 mm<br />
0.5 mm<br />
Figure 4.17: Pictures taken <strong>of</strong> the upper layer.<br />
Located between the two squares are a circle with a small hole cut entirely<br />
through the PMMA. The ’O’-ring is located in the circle, see Figure 4.18,<br />
while the hole will be located directly above the center <strong>of</strong> the center on the<br />
chip. Thereby allowing the dissolved oxygen to interact with the 3 electrodes<br />
(Reference, Working and Counter) on the chip. Once the three layers are<br />
put together, the ’O’-ring should be compressed roughly 20%. The ’O’-ring<br />
itself should be chosen, so that it can resist the water, acids, oils and other<br />
biological and non-biological substances in the ocean.
40 CHAPTER 4. SYSTEM DESIGN<br />
Figure 4.18: Pictures taken <strong>of</strong> the ’O’-ring, on the picture to the right the<br />
’O’-ring is placed in the PMMA.<br />
4.5 Summary<br />
4 different sensor layouts have been designed, along with variations on electrode<br />
size and distance, as well as 3 different Pt-temperature sensors. Combing<br />
into a total <strong>of</strong> 13 different chips, opening up <strong>for</strong> the possibility to test<br />
various aspect <strong>of</strong> the overall design <strong>of</strong> the Clark sensor. Finally the encapsulation<br />
have been chosen so that it is easy and fast to produce, while still<br />
being durable. Table 4.2 shows the various chips, and which design variation<br />
that can be found on each.<br />
Chip Electrode Electrode Temperature<br />
Design Distance (µm) Radius (µm) <strong>Sensor</strong><br />
1CVA 100 5 Pt50<br />
1CVB 100 5 Pt300<br />
1CXA 100 10 Pt50<br />
1CXB 100 10 Pt300<br />
1CLV 150 5 Pt300<br />
1CLXA 150 10 Pt50<br />
1CLXB 150 10 Pt300<br />
2CV 100 5 Pt600<br />
2X 100 10 Pt600<br />
3CV 100 5 Pt300<br />
3CX 100 10 Pt300<br />
4CV 100 5 Pt300<br />
4CX 100 10 Pt300<br />
Table 4.2: Overview <strong>of</strong> the chips, listing the size and distance between the<br />
electrodes in the working electrode array, and what temperature<br />
sensor there is on it.
Chapter 5<br />
Fabrication<br />
The fabrication 1 process is a fairly straight <strong>for</strong>ward process, with 5 masks<br />
being used in total. Only one side <strong>of</strong> the wafer is used, so the potential problems<br />
with double sided wafers is avoided. Within this chapter is a description<br />
<strong>of</strong> the steps in the process, as well as some <strong>of</strong> the thoughts about each step.<br />
The starting material is single crystalline n-type silicon (100) wafers. Further,<br />
several steps in the fabrication process could be duplicated from the<br />
project which this is subpart <strong>of</strong> [34].<br />
Figure 5.1: Illustration <strong>of</strong> a bare Silicon wafer, <strong>for</strong> the complete process, and<br />
a better understanding <strong>of</strong> this illustration and those that will<br />
follow, refer to Appendix B.<br />
5.1 Processing<br />
Silicon Dioxide Layer<br />
First a Silicon Dioxide layer is <strong>of</strong> 2000 ˚A is grown with thermal oxidation<br />
using a wet oxidation, the layer will serve to protect the wafer from the later<br />
processes, as well as working as an insulator. The reason wet oxidation is<br />
1 For the step by step cleanroom process recipe, see Appendix C<br />
41
42 CHAPTER 5. FABRICATION<br />
used is mainly that it is faster, basically it have a faster growth rate, and<br />
that the chip doesn’t require the higher breakdown voltage <strong>of</strong> dry oxidation.<br />
Figure 5.2: A layer <strong>of</strong> Silicon Dioxide have been added.<br />
Silicide Layer<br />
Next comes the Silicide layer, the silicide is going to serve as the conduction<br />
layer, between the actual sensor and the contacts. Silicide 2 (or rather<br />
TiSi2), resistivity <strong>of</strong> 13-16mΩcm is well suited <strong>for</strong> this due to its low resistivity.<br />
Which is an advantage since the finished Clark <strong>Sensor</strong> have to have<br />
an internal power supply, due to the sensor being attached to a fish, where<br />
the battery cannot be exchanged as one see fit.<br />
Figure 5.3: Illustration <strong>of</strong> the wafer after the silicide step.<br />
The Silicide layer is created by first depositing a layer <strong>of</strong> undoped Polysilicon<br />
with Low Pressure Chemical Vapor Deposition (LPCVD). It is also at<br />
this point the first alignments marks are made (See Figure 5.6). After this a<br />
Titanium layer is deposited on top <strong>of</strong> the Polysilicon, which is the patterned<br />
as well. Finally by using Rapid Thermal Annealing (RTA), the desired TiSi2<br />
is made.<br />
2 The silicide recipe is in Appendix A
5.1. PROCESSING 43<br />
Figure 5.4: Alignment marks from Silicide.<br />
Silicon Nitride Layer<br />
The Si3N4 layer is used to protect the bulk chip (not the actual sensor itself <strong>of</strong><br />
course) from exposure to seawater, as seawater is highly corrosive and could<br />
otherwise quickly corrode the metals.<br />
Figure 5.5: Illustration <strong>of</strong> the wafer after the LPCVD and RIE <strong>of</strong> the Nitride.<br />
LPCVD is used to deposit the Si3N4, after which a RIE is used to make<br />
the contact holes to the silicide layer <strong>for</strong> the metals.<br />
Figure 5.6: Alignment marks after Si3N4. Since a dark mask is used in this<br />
step, the alignment mark is here made relatively big, so that<br />
there is hole to peek through to to the wafer in order to locate<br />
the alignment marks from the previous step.<br />
Metal(s) Layer<br />
Finally the metals, Pt (Working Electrode Array, Counter Electrode and
44 CHAPTER 5. FABRICATION<br />
Thermometer), Ag (Reference Electrode), and Au (Contacts) are deposited.<br />
Why these 3?<br />
As discussed previously Pt and Ag have been used almost exclusively <strong>for</strong><br />
Clark <strong>Sensor</strong>s, and referenced as the best suited metals from a dissolved oxygen<br />
sensor[37].<br />
Where as Au gold provides an excellent surface <strong>for</strong> bonding to wirebond, as<br />
well as being able resist some exposure to saltwater.<br />
Figure 5.7: Illustration <strong>of</strong> the wafer with the three metals added.<br />
Figure 5.8: Alignment marks after the metals are added. From left to right,<br />
ignoring the first one, it is Pt(brown), Ag(light brown), Au(red).<br />
Each metal have double alignment marks, two boxes and a crisscross<br />
pattern. This is mainly to have a little extra safety when<br />
aligning the masks, so that there is always another alignment<br />
mark to check with.<br />
Pt will be put on first <strong>for</strong> two reasons; It is the metal with the smallest<br />
structure, and hence the metal where the lithography step is most sensitive,<br />
and there<strong>for</strong>e where it is most like to go wrong. Also since the Au-Contacts<br />
and the Pt-Thermometer crosses, having the PT buried beneath the Au will<br />
ensure an unbroken line (Especially as the Au, 3000 ˚A, layer is over 10 times<br />
as thick as the Pt, 200 ˚A, layer).<br />
A minor problem however is that a lot <strong>of</strong> metals have poor adhesion to<br />
many <strong>for</strong>ms <strong>of</strong> Silicon (including Silicide), hence a titanium layer is added
5.2. BACK-END PROCESSING 45<br />
under each to improve adhesion.<br />
The 3 metals are added and patterned with 3 double rounds (First Ti, then<br />
the actual metal) <strong>of</strong> metalization and then lift-<strong>of</strong>f.<br />
5.2 Back-end Processing<br />
Once the cleanroom processes are done, there is still a few steps that have to<br />
be completed. These steps involves chemicals usually not found in a cleanroom,<br />
and the chip have to soak in them <strong>for</strong> a while, there<strong>for</strong>e they are done<br />
outside the cleanroom, also there is no real contamination danger <strong>for</strong> the<br />
electronics part <strong>of</strong> the wafer at this point.<br />
5.2.1 Packaging<br />
First the chip have to be packaged, this is done in three steps.<br />
The Flexprint and then Chip is glued to the middle layer <strong>of</strong> the PMMA. The<br />
Flexprint is attached first as to avoid getting glue onto the chip, however<br />
should it occur, the glue used (Super Attakgel) can be removed using<br />
ethanol, provided it is only a thin layer <strong>of</strong> glue.<br />
Figure 5.9: Flex print and Chip glued to the PMMA.<br />
Once the glue have dried (<strong>for</strong> 1-2 hours to be sure that it is completely<br />
dry), the flex print and the Au-contacts have to be connected. This can be<br />
done with either wirebonding, which will be the first choice, or by using a
46 CHAPTER 5. FABRICATION<br />
conducting glue.<br />
The final step in the packaging, will be to bond the 3 layers <strong>of</strong> PMMA<br />
together (the ’O’-ring is inserted here as well), this is accomplished by putting<br />
them in a oven at 115 o C <strong>for</strong> 1 hour, which will cause them to bond. To hold<br />
then layers in place, two pieces <strong>of</strong> metal are used as well as a screw clamp.<br />
Care should be taken not to put to much pressure on the PMMA with the<br />
screw clamp, nor to little. However applying less than needed pressure is<br />
preferable, as it can be put back into the oven, if the bonding have not taken<br />
place. While to much pressure can cause the chip to break (a closer look is<br />
taken on this problem in Chapter 6).<br />
Figure 5.10: Preparation <strong>for</strong> the oven. Top left: The screw clamp. Top<br />
right: The two pieces <strong>of</strong> metal that holds the PMMA, chip and<br />
flexprint in place and together. Bottom Left: The 3 layers <strong>of</strong><br />
PMMA have been placed in the the metal holder. Bottom right:<br />
Screw clamp have been closed tightly around the metal holding<br />
the PMMA.
5.2. BACK-END PROCESSING 47<br />
5.2.2 Ag to Ag/AgCl<br />
Ag has to be turned into AgCl. This can be done in several ways, one <strong>of</strong> the<br />
easier ways[44] however consists <strong>of</strong> using:<br />
1. 0.1 M HCl<br />
2. A flashlight battery.<br />
3. A few centimeters <strong>of</strong> pure silver.<br />
Figure 5.11: Ag turned into Ag/AgCl.<br />
The packaged chip is dipped down into the HCl along with one end <strong>of</strong> the<br />
silver wire. While the battery is connected with the negative terminal to the<br />
silver wire and the positive to the Ag output on the flexprint. Slowly the Ag<br />
electrode will turn a light tan and then a dark brown, at which point it will<br />
have a suitable thick layer <strong>of</strong> AgCl.<br />
5.2.3 Electrolyte and Membrane<br />
Finally the electrolyte and the membrane is added. The material used is<br />
Siloprene from Fluka-Chemie. It consists <strong>of</strong> 3 liquids Siloprene Crosslinking<br />
Agent K-11, Siloprene K 1000 and Hexane. All three liquids are dripped into<br />
the small hole on the device, where the chip is located, by using a syringe<br />
(The amounts used are <strong>of</strong> the order <strong>of</strong> 100:10:1 <strong>for</strong> Siloprene Crosslinking<br />
Agent K-11 : Siloprene K 1000 : Hexane). Finally the sensor is soaked in a<br />
liquid containing a large amount <strong>of</strong> Cl − ions (the time it needs to be soaked<br />
depends on the exact concentration, the higher the concentration is, the less<br />
time it needs. Care should be taken however if a strong acid is used, as it<br />
might damage the membrane) to add the electrolyte. Other membranes have<br />
been used with micro electrode oxygen sensor(such as Nafion[45]). However<br />
this was chosen, due to the recommendation and experiences <strong>of</strong> a pr<strong>of</strong>essor<br />
at MIC.
48 CHAPTER 5. FABRICATION<br />
5.3 Summary<br />
The fabrication steps have been outlined and explained, and while there are<br />
always the possibility <strong>of</strong> changes or variations in parameters in each step, the<br />
process as described in this chapter, should be duplicable by others.
Chapter 6<br />
Problems and Solutions<br />
In almost every project some unexpected problems appears, ranging from<br />
merely inconvenient problems such as cleanroom machinery being out <strong>of</strong><br />
work, to more actual problems, such as fabrications steps being more complicated<br />
than they appear on paper. In this chapter some <strong>of</strong> the problems<br />
encountered during the process <strong>of</strong> creating the Clark sensor will be highlighted,<br />
as well as how they were solved when possible.<br />
6.1 Wirebonding<br />
As mentioned in Chapter 5, the first, and at the time the only, choice <strong>for</strong><br />
connecting the chip to the flex print was wirebonding. Wirebonding is basically<br />
done by having a needle, with a metal (Al) thread through it, pressing<br />
down on the contact pad <strong>of</strong> the chip, and then through the use <strong>of</strong> ultrasound<br />
and vibrations attach the thread. The similar process is then repeated on<br />
the flex print, at which time the rest <strong>of</strong> the thread is snapped <strong>of</strong>f. However<br />
this would soon prove to be a lot more complicated than it sounds. For some<br />
unexplainable reasons the thread refused to per<strong>for</strong>m the second attachment,<br />
whether it was on the gold on the chip, or on the flex print. Albeit some<br />
were wirebondings were successful, they were few and there was far between<br />
them.<br />
Why this is so, remained a mystery <strong>for</strong> a while, however there was some<br />
problems with the machine at the time. However a later examination <strong>of</strong> the<br />
flex print revealed that the metal on it was tin covered copper. There<strong>for</strong> it<br />
was impossible to bond to the flex print, unless the tin was scraped <strong>of</strong>f, as<br />
49
50 CHAPTER 6. PROBLEMS AND SOLUTIONS<br />
Figure 6.1: Picture <strong>of</strong> the wirebonder used to attach the wire to the chip and<br />
the flex print.<br />
wiring bonding is not possible to tin. Also people in the industry have suggested<br />
that the Au layer on the chip might have been to thin, and should be<br />
increased to 10k ˚A(it was 3000 ˚A), however others at MIC have successfully<br />
bonded to this thickness be<strong>for</strong>e. However this was only discovered late in the<br />
project, so some other way <strong>of</strong> connecting the chip and the flex print had to<br />
be found at the time.<br />
6.1.1 Conducting Glue<br />
As is <strong>of</strong>ten the case the simplest solution is also the most viable, in this case<br />
a conducting glue. 1 While it may look and sound less refined than a a metal<br />
wire, it is relatively easy to apply with the tip <strong>of</strong> a needle dipped in the glue.<br />
That is if as long as some distance exists between the output electrodes.<br />
The glue used is, like most glues, somewhat viscous and hence if the output<br />
electrodes are to close to each other, it can be very hard if not impossible to<br />
1 The glue is a conductive paste (H9807) from Namics, viscosity: 20 P a ·<br />
s, V olumeresistivity : 0.7 × 10 −4 Ω · cm
6.1. WIREBONDING 51<br />
make a line between the chip and the flex print without a short circuit being<br />
created. So while the conducting glue is a possible solution, it should only<br />
be considered a temporary solution.<br />
Figure 6.2: One <strong>of</strong> the chips with the silver colored electric glue on, as can<br />
be seen by comparing the left and the right side, there is a decent<br />
amount <strong>of</strong> distance between the output electrodes (left) <strong>for</strong> the<br />
Clark <strong>Sensor</strong>. While the output electrodes (right) <strong>of</strong> the temperature<br />
sensor is closer and harder to connect to with the conducting<br />
glue.<br />
Figure 6.3: Closeup <strong>of</strong> the design, the arrows point towards the platinum line<br />
that will cause the short circuit.<br />
While the conducting glue was a very viable solution one minor problem<br />
had to be overcome first. In order to make the wafer easier to dice, the chip<br />
design (as seen in Chapter 4 had a platinum line near the edge, see picture<br />
6.3, which if the conducting glue was just put on as it were, would work as a<br />
short circuit, hence a small procedure was developed to apply the glue. This<br />
procedure was as follows:<br />
1. Put a piece <strong>of</strong> ordinary tape over the sensor, to prevent it from being<br />
covered with glue, leaving roughly half <strong>of</strong> the output electrodes free.
52 CHAPTER 6. PROBLEMS AND SOLUTIONS<br />
2. Put a thick layer <strong>of</strong> non-conducting glue over the exposed half <strong>of</strong> the<br />
output electrodes and the platinum wire, and remove the tape. Then<br />
leave it to dry <strong>for</strong> at least a few hours. Then check under a microscope<br />
if more needs to be applied.<br />
Figure 6.4: Non conducting glue have been put on the chip, and the tape<br />
have been removed.<br />
3. Connect the output electrodes and the flex print with the conducting<br />
glue, using the tip <strong>of</strong> a needle dipped in it. Check <strong>for</strong> short circuits<br />
under a microscope, and then leave it overnight to dry.<br />
Figure 6.5: Both the conducting and the non conducting glue have been<br />
added, and the non conducting is now separating the conducting<br />
glue from the platinum line.
6.2. THE FLEX PRINT 53<br />
The reason behind letting the conducting glue dry slowly in step 3, rather<br />
than putting it in the oven right away, was that tests showed a lack <strong>of</strong> connection<br />
if the glue hadn’t dried properly first.<br />
Also it should be noted that while the solution with the conducting glue<br />
does work, it is not the most optimal solution, as the yield on working sensors<br />
with this solution is discouragingly low.<br />
6.2 The flex print<br />
There was also another problem with the wirebonds, or rather the flex print,<br />
even if the wire had been successfully attached to both the output electrodes<br />
and the flex print. After taking the packaged device out <strong>of</strong> the oven, the flex<br />
print appeared to bend upwards, despite the glue. This had the side effect<br />
<strong>of</strong> making the thread snap <strong>of</strong>f at one <strong>of</strong> the ends. Hence cutting <strong>of</strong>f the connection<br />
between the chip and the flex print. The reason <strong>for</strong> this appears to<br />
be that the flex print used, isn’t suitable <strong>for</strong> use with temperatures over 80 C.<br />
Figure 6.6: Picture from a chip, where conducting glue have been used to<br />
<strong>for</strong>m the connection between chip and flex print, as can be seen<br />
the flex print have bend and caused the connection to break.<br />
So in order to solve this, a way to keep the wire attached or to prevent<br />
the flex print from bending had to be found.
54 CHAPTER 6. PROBLEMS AND SOLUTIONS<br />
6.2.1 Non Conducting Glue<br />
Once again the solution involves glue, albeit a non-conducting one this time,<br />
the idea <strong>for</strong> this came to mind when a fellow master student 2 had to encapsulate<br />
a chip fast, and hence used glue instead to protect the thin metal wire<br />
from being torn <strong>of</strong>f. This made me thinking what if a similar glue was put in<br />
the hole in the upper PMMA piece? Once it was dry it should ensure that<br />
the flex print is unable to bend, as there is no room <strong>for</strong> it to bend to. Also<br />
as an added bonus it would help protect the more sensitive electronics from<br />
any seawater that might seep past the ’O’-ring.<br />
Figure 6.7: Glue covering both the flex print, wirebonds, and chip, thereby<br />
making the flex print and wirebond durable.<br />
6.3 The ’O’-ring<br />
Another problem with the design was the ’O’-ring, or rather making the<br />
’O’-ring stay where it was supposed to in the PMMA. The hole which the<br />
’O’-ring rest in had to have the right depth, since if it was to deep, the ’O’ring<br />
wouldn’t fully protect the rest <strong>of</strong> the chip from the seawater. While<br />
making it to shallow would cause the ’O’-ring to be squeezed out <strong>of</strong> the hole,<br />
or outside the area it is supposed to cover, as seen on picture 6.8.<br />
2 Sune Duun
6.3. THE ’O’-RING 55<br />
Figure 6.8: The ’O’-ring is being squeezed out <strong>of</strong> where it is supposed to be.<br />
The picture to the left shows it sticking out <strong>of</strong> the hole, while the<br />
picture to the right shows it being squeezed over the chip.<br />
6.3.1 Changing the resting area<br />
Now the original design <strong>for</strong> the packaging had the ’O’-ring resting on a flat<br />
area as show on picture 6.9. Also while the ’O’- was supposed to be squeezed<br />
together it was only supposed to contract about 20 % percent, in order to<br />
have a tight fit. So how was this solved? First <strong>of</strong>f the resting area <strong>for</strong> the<br />
’O’-ring was changed, so that it became a grove instead <strong>of</strong> a flat, as show on<br />
picture 6.9.<br />
Figure 6.9: The sketch to the left shows the original flat area, which the ’O’ring<br />
rested upon, while the sketch to the right shows the changed<br />
design, where a grove is added <strong>for</strong> the ’O’-ring.<br />
This, along with a optimization <strong>of</strong> the process where the laser cuts the<br />
grove, ensured that the the ’O’-ring no longer did this in most cases. Also it<br />
was better to have the screw clamp give a smaller amount <strong>of</strong> pressure, and<br />
then have the packing inside the oven a little longer if the PMMA layers<br />
hadn’t bonded properly, rather than applying much pressure and cause the<br />
’O’-ring to slip out. Since everything in the design should be able to survive<br />
the extended duration inside the oven.
56 CHAPTER 6. PROBLEMS AND SOLUTIONS<br />
6.4 Summary<br />
While many problems can occur in creating a sensor, it is not safe to assume<br />
that merely because the cleanroom processing is done that no further problems<br />
will be encountered. The problems shown and solved in this chapter,<br />
illustrates this very well.
Chapter 7<br />
Evaluation <strong>of</strong> Results and<br />
Measurements<br />
Shown in this chapter is the results gained during this project, as well as how<br />
they might be improved upon. While the process itself was successful, the<br />
problem that showed up with the packaging and the wirebonder, as detailed<br />
in Chapter refsec:problems, there is not as many measurements on the Clark<br />
<strong>Sensor</strong> as hoped, and there<strong>for</strong>e these will have to be obtained after this thesis<br />
is handed in.<br />
Figure 7.1: Pictures showing the missing Pt. On the picture on the left,<br />
some residues <strong>of</strong> platinum can be seen, where as the middle one<br />
shows a hole in the temperature sensor, while on the picture to<br />
the right the temperature sensor is missing entirely.<br />
7.1 Fabrication results<br />
The fabrication <strong>of</strong> the chips was successful, and one wafer made it through<br />
the whole clean room process, while a few others were postponed at an early<br />
57
58 CHAPTER 7. EVALUATION OF RESULTS AND MEASUREMENTS<br />
process step, in case problems with the rest <strong>of</strong> the process should arise.<br />
There was one small problem with the Pt, the exposure time in the photolithography<br />
was originally set to 7 seconds instead <strong>of</strong> 4. As a result this,<br />
both the working array and the temperature sensor had huge gaps, or were<br />
missing entire as seen on Figure 7.1.<br />
However once the exposure time was lowered to 4, the problem was solved,<br />
and the fine structure and small dots <strong>of</strong> Pt was clearly evident, as seen on<br />
Figure 7.2.<br />
Figure 7.2: Pt are no longer missing and both the dots in the working and<br />
the lines in the temperature sensor looks fine.<br />
Beyond this however there was no real problem in fabricating the chip,<br />
except <strong>for</strong> the problems already mentioned, also while the recipe <strong>for</strong> the<br />
titanium silicide was new, it <strong>for</strong>med successfully.<br />
7.2 Temperature <strong>Sensor</strong><br />
The temperature sensor was tested to see if it had the required linear dependency,<br />
and to see if the TCR (Temperature Coefficient <strong>of</strong> Resistance) varied<br />
from the standard and if so, how much it did. This is done in order to know<br />
how much that needs to be compensated, since there is a titanium strip beneath<br />
the platinum, and the gold contacts can also be expected to change it<br />
a bit.<br />
The experiment was done using 2 multimeter’s, a probestation, and a<br />
heating plate. The chip measured on was placed in the probestation, and<br />
the 2 multimeter’s was connected, one measuring the temperature <strong>of</strong> the chip,<br />
and the other measuring the resistance through the temperature sensor. The<br />
heating plate was then used to raise the temperature to around 50 o C, after<br />
which it was cooled down. Measurements was made both while heating and<br />
cooling.
7.2. TEMPERATURE SENSOR 59<br />
Figure 7.3: Graphs showing the relative temperature against the resistance,<br />
TCR is equal to the slope <strong>of</strong> the lines.<br />
As can be seen from Figure 7.3 the measured TCR’s (3.33×10 −3 , 4.09×10 −3 ,<br />
4.09×10 −3 , 4.11×10 −3 )are very close to the standard[46] 3.8×10 −3 <strong>for</strong> platinum.
60 CHAPTER 7. EVALUATION OF RESULTS AND MEASUREMENTS<br />
7.3 Clark <strong>Sensor</strong><br />
While there as mentioned was problems with the wire bonding and packaging,<br />
there was one sensor that was both bonded and packaged, and there<strong>for</strong>e<br />
could be measured upon. The measurements was done using a Gamry FAS2<br />
Femtostat, and a beaker <strong>of</strong> water with the sensor in it, finally grounded metal<br />
plates was put up around the beaker, in order to create the equivalent <strong>of</strong> a<br />
Faraday cage. This was done in order to minimize any outside noise (See<br />
Figure 7.4 <strong>for</strong> a sketch <strong>of</strong> the setup.<br />
PC<br />
Ground WE CE RE<br />
Potentiostat<br />
Constructed Faraday cage<br />
Figure 7.4: Sketch <strong>of</strong> the setup used to measure on the Clark sensor, 3 wires<br />
connects the working, reference and counter electrodes respectively<br />
to the potentiostat. The potentiostat collects the data and<br />
sends it to a nearby pc, which translates the signal into a graph.<br />
Un<strong>for</strong>tunately due to the faulty wirebonding one <strong>of</strong> the wires broke after<br />
a few test measurements, so no detailed measurements with varying amounts<br />
<strong>of</strong> dissolved oxygen could be made.<br />
However, as can be seen on Figure 7.5, there is the plateau shaped curve,<br />
which indicates that the sensor itself works. Of course since the a wire<br />
broke during the test measurements, no conclusion can be made in regards<br />
to whether it is measuring the right amount <strong>of</strong> oxygen or not. Hence further<br />
tests have to be made to confirm this. That the right shape <strong>of</strong> curve was<br />
obtained, is taken as a good sign, especially as there seems to be no noticeable<br />
internal noise generated.<br />
Beaker<br />
with<br />
Clark<br />
sensor<br />
and<br />
water
7.4. SUMMARY 61<br />
Figure 7.5: Polarogram from the test measurement with the designed Clark<br />
<strong>Sensor</strong> (Design 1CLX).<br />
7.4 Summary<br />
While there was a lack <strong>of</strong> measurements on the Clark sensor itself, the results<br />
from the measurements that have been obtained, as well as pictures <strong>of</strong> the<br />
chips, indicates that once the packaging problems have been solved, the Clark<br />
sensor should in all likelihood work as well.
62 CHAPTER 7. EVALUATION OF RESULTS AND MEASUREMENTS
Chapter 8<br />
Conclusion<br />
The primary goal <strong>for</strong> this project was to create a first generation oxygen<br />
sensor to compliment the Fish ’n Chip sensor. While a sensor have been<br />
created, it have not been tested, due to problems encountered during the<br />
projects, and as such the primary goal have not been fully fulfilled. However<br />
important steps towards a functioning dissolved oxygen sensor have been<br />
taken.<br />
Possible ways <strong>of</strong> measuring dissolved oxygen have been investigated, and<br />
a Clark type chosen.<br />
The theory behind the Clark sensor have been detailed, a further study<br />
into membrane materials and electrolytes could allow <strong>for</strong> improvements, as<br />
some <strong>of</strong> the potential optimization lies hidden here. Albeit this fall outside<br />
what knowledge <strong>of</strong> chemistry that I have.<br />
Various designs and the considerations behind them have been described,<br />
and with future results these should provide the basis <strong>for</strong> improving the<br />
sensor. This should also give a basic idea <strong>of</strong> what design outlay that is the<br />
most advantageous.<br />
The packaging concept, which proved to be the main problem despite not<br />
being within the focus <strong>of</strong> this project, have several flaws and will have to be<br />
given some thorough investigation.<br />
The temperature sensor seems to be working within the expected norms,<br />
and while not being revolutionary or even new, it is needed <strong>for</strong> the Clark<br />
sensor, due to the influence <strong>of</strong> temperature. An interesting aspect yet to<br />
explore however, is whether the Clark sensor influence it or vice versa.<br />
63
64 CHAPTER 8. CONCLUSION<br />
In summary this project still have a lot <strong>of</strong> potential, once the results from<br />
the Clark sensors have been obtained. Though from the gathered results and<br />
the design theory behind it, I feel confident that it does work, and plans have<br />
been made to measure on the chips to verify this and gather date, once the<br />
packaging problem is solved, even if it un<strong>for</strong>tunately falls outside the time<br />
frame <strong>of</strong> this project.
Bibliography<br />
[1] http://earthobservatory.nasa.gov/study/weighingwater/.<br />
[2] http://www.fisheriesmanagement.co.uk/fish%20studies/bloodflow countercurrent.htm.<br />
[3] Fortunat Joos, Gian-Kasper Plattner, Thomas F. Stocker, Arne Kortzinger,<br />
and Douglas W.R. Wallace. Trends in marine dissolved oxygen:<br />
Implications <strong>for</strong> ocean circulation changes and the carbon budget. EOS,<br />
84:197–204, May 2003.<br />
[4] E. Hunter, J.N. Aldridge, J.D. Metcalfe, and G.P. Arnold. Geolocation<br />
<strong>of</strong> free-ranging fish on the european continental shelf as determined from<br />
enviromental variables. <strong>Marine</strong> Biology, 142:601–609, 2003.<br />
[5] http://www.sk.lung.ca/content.cfm.<br />
[6] Chen Yu-Quan and Li Guang. An auto-calbrated miniature microhole<br />
cathode array sensor system <strong>for</strong> measuring dissolved oxygen. <strong>Sensor</strong>s<br />
and Actuators B, 10:219–222, 1993.<br />
[7] http://scienceworld.wolfram.com/chemistry/fugacity.html.<br />
[8] http://www.state.ky.us/nrepc/water/wcpdo.htm.<br />
[9] http://waterontheweb.org/under/waterquality/oxygen.html.<br />
[10] Td 218 operating manual oxygen optode 3830 and 3930.<br />
[11] S. Gatti, T. Brey, W.E.G. Muller, O. Heilmayer, and G. Holst. <strong>Oxygen</strong><br />
microoptodes: a new tool <strong>for</strong> oxygen measurement in aquatic animal<br />
ecology. <strong>Marine</strong> Biology, 140:1075–1085, 2002.<br />
[12] I. Klimant, M. Kuhl, R.N. Glud, and G. Holst. Optical measurements<br />
<strong>of</strong> oxygen and temperature in microscale: strategies and biological applications.<br />
<strong>Sensor</strong>s and Actuators B, 38-39:29–37, 1997.<br />
65
66 BIBLIOGRAPHY<br />
[13] S. McCulloch and D. Uttamchandani. Fibre optic micro-optrode <strong>for</strong><br />
dissolved oxygen measurements. IEE Proc.-Sci Meas. Technol., 146:123–<br />
127, May 1993.<br />
[14] S.McCulloch and D.Uttamchandi. Fibre optic micro-optrode <strong>for</strong> dissolved<br />
oxygen measurements. IEEE Proc.-Sci. Meas. Tecnol, 146:123–<br />
127, May 1999.<br />
[15] P. Bergveld. Isfet, theory and practise. IEEE <strong>Sensor</strong> Conference Toronto,<br />
October 2003.<br />
[16] R. Ramamoorthy, P.K. Dutta, and S.A. Akbar. <strong>Oxygen</strong> sensors: Materials,<br />
methods, designs and applications. Journal <strong>of</strong> Materials Science,<br />
38:4271–4282, 2003.<br />
[17] Andreas Hierleman and Henry Baltes. Cmos-based chemical microsensors.<br />
The Analyst, pages 15–28, November 2002.<br />
[18] P. Bergveld. Isfet, theory and practice. IEEE <strong>Sensor</strong> Conference<br />
Toronto, October 2003.<br />
[19] Byung-Ki Sohn and Chang-Soo Kim. A new ph-isfet based dissolved oxygen<br />
sensor by employing electrolysis <strong>of</strong> oxygen. <strong>Sensor</strong>s and Actuators<br />
B, 34:435–440, 1996.<br />
[20] W. Oelssner, J. Zosel, U. Guth, T. Pechstein, W. Babel, J.G. Connery,<br />
C. Demuth, M. Grote Gansey, and J.B. Verburg. Encapsulation <strong>of</strong> isfet<br />
sensor chips. <strong>Sensor</strong>s and Actuators B, pages 1–13, 2004.<br />
[21] L.C. Clark. Monitor and control <strong>of</strong> blood and tissue tensions. Trans.<br />
Am. Soc. Artif. Intern. Organs., 2:41–48, 1956.<br />
[22] Cynthia G. Zoski and Nafeesa Simjee. Addressable microelectrode arrays:<br />
Characterization by imaging with scanning electrochemical microscopy.<br />
Analytical Chemistry, 76:62–72, January 2004.<br />
[23] Fumio Hine. Electrode Processes and Electrochemical Engineering.<br />
Plenum Press, 1985.<br />
[24] http://www.eidusa.com/theory do.htm.<br />
[25] R. Mark Wightman. Voltammetry with microscopic electrodes in new<br />
domains. Science, 240:415–420, April 1988.
BIBLIOGRAPHY 67<br />
[26] W.E. Morf. The Principles <strong>of</strong> Ion-Selective Electrodes and <strong>of</strong> Membrane<br />
Transport. Elsevier, 1981.<br />
[27] T. R. Yu and G. L. Ji. Electrochemical Methods in Soil and Water<br />
Research. Pergamon Press, 1993.<br />
[28] A.F. Albantov and A.L. Levin. New functional possibilities <strong>for</strong> amperometric<br />
dissolved oxygen sensors. Biosensors & Bioelectronics, 9:515–526,<br />
1994.<br />
[29] http://www.corrosion-doctors.org/biographies/davybio.htm.<br />
[30] http://www.rigb.org/rimain/heritage/faradaypage.jsp.<br />
[31] D.L. Short and G.S.G. Shell. Fundamentals <strong>of</strong> clark membrane configuration<br />
oxygen sensors: some confussion clarified. J. Phys. E. Sci.<br />
Instrum., 17:1085–1092, 1984.<br />
[32] Alexander Frey, Martin Jenker, Meinrad Schienle, Christian Paulus, Birgit<br />
Holzapfl, Petra Schindler-Bauer, Franz H<strong>of</strong>mann, Dirk Kuhlmeier,<br />
Jurgen Krause, Jorg Albers, Walter Gumbrecht, Doris Schmitt-<br />
Lansiedel, and Roland Thewes. Design <strong>of</strong> an integrated potentiostat<br />
circuit <strong>for</strong> cmos bio sensor chips. IEEE, 2003.<br />
[33] Slawomir Kalinowski and Zbigniew Figaszewski. A four-electrode<br />
potentiostat-galvanostat <strong>for</strong> studies <strong>of</strong> bilayer lipid membranes. Meas.<br />
Sci. Technol, 6:1050–1055, 1995.<br />
[34] Anders Hyldg˚ard. Developement <strong>of</strong> a multi-sensor <strong>for</strong> marine environment<br />
studies. Master’s thesis, MIC, April 2004.<br />
[35] U. Guth, W. Oelssner, and W. Vonau. Investigation <strong>of</strong> corrosion phenomena<br />
on chemical microsensors. Electrochimica Acta, 47:201–210,<br />
2001.<br />
[36] Marek Dawgul, Dorota G. Pijanowska, Alfred Krzyskow, Jerzy Kruk,<br />
and Wladyslaw Torbicz. An influence <strong>of</strong> polyHEMA gate layer on properties<br />
<strong>of</strong> chemFETs. <strong>Sensor</strong>s, 3:146–159, 2003.<br />
[37] M. Wittkampf, K. Cammann, M. Amrein, and R. Reichelt. Characterization<br />
<strong>of</strong> microelectrode arrays by means <strong>of</strong> electrochemical and surface<br />
analysis methods. <strong>Sensor</strong>s and Actuators B, 40:79–84, 1997.<br />
[38] Yuzuru Iwasaki and Masao Morita. Electrochemical measurements with<br />
interdigitated array microelectrodes. Current Seperations, 14, 1995.
68 BIBLIOGRAPHY<br />
[39] Mairi E. Sandison, Natalie Anicet, Andrew Glidle, and Jonathan M.<br />
Cooper. Optimization <strong>of</strong> the geometry and porosity <strong>of</strong> microelectrode<br />
arrays <strong>for</strong> sensor design. Analytical Chemistry, 74:5717–5725, November<br />
2002.<br />
[40] Hiroaki Suzuki. Advances in the micr<strong>of</strong>abrication <strong>of</strong> electrochemical<br />
sensors and systems. Electroanalysis, 12:703–715, 2000.<br />
[41] B. Ross, K. Cammann, W. Mokwa, and M. Rospert. Ultramicroelectrode<br />
arrays as tranducers <strong>for</strong> new amperometric oxygen sensors. <strong>Sensor</strong>s<br />
and Actuators B, 7:758–762, 1992.<br />
[42] Glen W. McLaughlin, Katie Braden, Benjamin Franc, and Gregory T.A.<br />
Kovacs. Micr<strong>of</strong>abricated solid-state dissolved oxygen sensor. <strong>Sensor</strong>s and<br />
Actuators B, 83:138–148, 2002.<br />
[43] Chen Yu-Quan and Li Guang. A mathematical model with finiteelement<br />
analysis <strong>of</strong> recessed dissolved-oxygen cathode array. <strong>Sensor</strong>s<br />
and Actuators B, 10:223–228, 1993.<br />
[44] http://www.sablesys.com/oxrechlo.htm.<br />
[45] Sotiris Sotiropoulos and Kirsi Wallgren. Solid-state microelectrode oxygen<br />
sensors. Analytica Chimica Acta, 388:51–62, 1999.<br />
[46] http://www.microwaves101.com/encyclopedia/temperature.cfm.
Appendix A<br />
Silicide Recipe<br />
The following two recipes are <strong>for</strong> the Jipelec RTP machine at MIC.<br />
Step Initial Temperature Temperature Duration Thermocoupler Open Vent N2<br />
or Power or Power Control Valve (sccm)<br />
1 20 0 30 Y Y 0<br />
2 0 0 30 Y Y 200<br />
3 0 0 30 Y Y 0<br />
4 0 0 100 Y Y 200<br />
5 0 0 100 Y Y 0<br />
6 0 Power 20 30 N Y 0<br />
7 Power 20 Power 20 2400 N Y 0<br />
8 Power 20 0 10 N Y 0<br />
Table A.1: Prebake Settings to remove any oxygen from the ceramic wafer<br />
holder and prepare it (Recipe name Bake Clark).<br />
69
70 APPENDIX A. SILICIDE RECIPE<br />
Step Initial Temperature Temperature Duration TC Open Vent N2<br />
or Power or Power Control Valve (sccm)<br />
1 20 20 40 Y N 0<br />
2 20 20 30 N Y 400<br />
3 20 20 40 Y N 0<br />
4 20 20 30 N Y 400<br />
5 20 20 40 Y N 0<br />
6 20 200 20 Y N 0<br />
7 200 400 20 Y N 0<br />
8 400 600 20 Y N 0<br />
9 600 800 20 Y N 0<br />
10 800 850 10 Y N 0<br />
11 850 850 30 Y N 0<br />
12 850 20 600 Y N 0<br />
13 20 20 120 N Y 400<br />
Table A.2: Silicide Settings to make the actual Silicide(Recipe name PKH<br />
1).
Appendix B<br />
Fabrication Process<br />
71
72 APPENDIX B. FABRICATION PROCESS
74 APPENDIX B. FABRICATION PROCESS
Appendix C<br />
Process Sequence<br />
75
76 APPENDIX C. PROCESS SEQUENCE
78 APPENDIX C. PROCESS SEQUENCE