Fluidic channel detection system using a differential c4d structure = hệ thống phát hiện kênh chất lỏng sử dụng cấu trúc c4d vi sai

The DC4D output voltage when a 4.18 l tin particle crosses electrodes in machine oil channel.. The DC4D capacitance change when a 4.18 l tin particle crosses electrodes in machine oil

VIETNAM NATIONAL UNIVERSITY, HANOI

UNIVERSITY OF ENGINEERING AND TECHNOLOGY

Nguyen Ngoc Viet

FLUIDIC CHANNEL DETECTION SYSTEM USING A DIFFERENTIAL C4D STRUCTURE

Branch : Electronics and Telecommunications Technology

Major : Electronics Technology

Code : 60520203

MASTER THESIS ELECTRONICS AND TELECOMMUNICATIONS TECHNOLOGY

SUPERVISOR: Assoc Prof Dr Chu Duc Trinh

HA NOI - 2015

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Acknowledgements

I would first like to express my sincere gratitude towards my research supervisor, Assoc Prof Dr Chu Duc Trinh, who has helped me throughout my research work The teacher was always by my side for interesting discussions and for giving some fruitful advice

In addition, I would also like to thank T Chu Duc’s research group members from MEMS Laboratory for their valuable inputs towards my research And last but not least, I am grateful to the Faculty of Electronics and Telecommunications, UET-VNU, Hanoi for their willingness to offer help and suggestions whenever needed

Finally, I want to express the deepest gratitude to my family and my friends for their love and encouragements during my study

Ha Noi, November 1 st , 2015

Nguyen Ngoc Viet

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Declaration

I certify that the research described in this dissertation has not already been submitted for any other degree

I certify that to the best of my knowledge all sources used and any help received

in the preparation of this dissertation has been acknowledged

Ha Noi, November 1 st , 2015

Nguyen Ngoc Viet

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Table of contents

List of figures 6

List of tables 8

List of symbols and abbreviations 9

Summary 10

Chapter 1 INTRODUCTION 12

1.1 Background and Overview 12

1.2 Research Objectives 13

Chapter 2 THEORY OF CAPACITIVE SENSOR 17

2.1 Capacitance 17

2.2 Dielectric constant 19

2.3 Capacitive sensor applications 19

2.3.1 Proximity sensor 20

2.3.2 Position sensor 21

2.3.3 Humidity sensor 22

2.3.4 Pressure sensor 22

2.3.5 Tilt sensors 23

2.4 Basic principles of C4D structure 23

2.5 Coplanar capacitive sensor in CMOS chip 26

Chapter 3 DIFFERENTIAL C4D STRUCTURE FOR DETECTION OF OBJECT IN FLUIDIC CHANNEL 29

3.1 DC4D sensor for Conductive and Non-conductive Fluidic Channel 29

3.1.1 Design and operation 29

3.1.2 DC4D simulations for non-conductive fluidic channel 31

3.1.3 Modelling of DC4D for conductive fluidic channel 34

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3.1.4 Fabrication and measurement setup 36

3.2 Developing DC4D sensor for microfluidic channel 37

Chapter 4 RESULTS AND DISCUSSIONS 43

4.1 DC4D sensor system using U-shaped electrodes 43

4.1.1 DC4D for non-conductive fluidic channel 43

4.1.2 DC4D for conductive fluidic channel 47

4.2 DC4D sensor system using microelectrodes 52

Conclusions 56

References 57

Figure 2.4 Electric field formed between positive and negative electrodes for

different pitch lengths, (l1, l2 and l3) 26

Figure 2.5 Sensing possibilities to detect various characteristic of samples: (a)

sensing density, (b) sensing distance, (c) sensing texture, (d) sensing moisture 27

Figure 2.6 A simplified diagram of a capacitive sensing based LoC 28 Figure 3.1 Block diagram design of the DC4D fluidic sensor 29

Figure 3.2 The DC4D based on three-electrode configuration; (b) The equivalent diagram 30

Figure 3.3 The interface of the structure simulation process using COMSOL

Multiphysics 31

Figure 3.4 Simulated picture of the electric field norm when a plastic particle inside

the fresh water channel 32

Figure 3.5 Simulated picture of the electric field norm when a tin particle inside oil

channel 33

Figure 3.6 Capacitance change versus particle position inside a single C4D 34

Figure 3.7 The equivalent circuit of the DC4D for conductive fluidic channel The circuit diagram of the suggested structure 35

Figure 3.8 The equivalent circuit of the DC4D fluidic sensor 35

Figure 3.9 The single C4D admittance change when a particle moves though electrode inside conductivity solution 36

Figure 3.10 Measurement system setup of the DC4D fluidic sensor 37

Figure 3.11 Proposal of a DC4D sensor 38

Figure 3.12 Cross-sessional view (a), side view (b), and DC4D sensor model (c) 39

Figure 3.13 Fabrication process 40 Figure 3.14 The fabricated chip 41 Figure 3.15 Block diagram of the measurement system 42 Figure 4.1 Capacitance change versus particle position inside a single C4D, when the air bubble and tin particle move through machine oil channel, respectively 43

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Figure 4.2 Single C4D capacitance change versus volume of the particles in oil

channel 44

Figure 4.3 The DC4D output voltage when a 4.18 l air bubble crosses electrodes in machine oil channel 45

Figure 4.4 The DC4D output voltage when a 4.18 l tin particle crosses electrodes in machine oil channel 45

Figure 4.5 The DC4D capacitance change when a 4.18 l air bubble crosses electrodes in machine oil channel 46

Figure 4.6 The DC4D capacitance change when a 4.18 l tin particle crosses electrodes in machine oil channel 46

Figure 4.7 The DC4D output voltage response versus tin particle volume in oil channel 47

Figure 4.8 The DC4D output capacitance change versus tin particle volume in oil channel 47

Figure 4.9 The DC4D output voltage response when a plastic particle crosses electrodes: (a) water channel; and (b) salt solution channel 48

Figure 4.10 The DC4D admittance change when a plastic particle crosses electrodes: (a) water channel; and (b) salt solution channel 48

Figure 4.11 The DC4D output voltage amplitude versus particle volume in salt solution and water 49

Figure 4.12 The DC4D output voltage amplitude versus particle volume in various concentration of salt solution 50

Figure 4.13 The DC4D output voltage change’s amplitude versus conductive fluidic resistivity 51

Figure 4.14 Velocity of investigated particle inside fluidic channel calculation 51

Figure 4.15 Meshed region 52

Figure 4.16 Capacitance output of the DC4D sensor 53

Figure 4.17 Maximum differential capacitance output versus particle’s volume 54

Figure 4.18 Maximum differential capacitance output and electrical field distribution in 3e positions of object inside water fresh flow: (a), (b), (c): air bubble; (d), (e), (f): tin particle 55

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List of tables

Table 3.1 Geometry parameters of the proposed DC4D structure 31

Table 3.2 Parameters of capacitive fluidic microsensor 38 Table 4.1 The DC4D output voltage amplitude versus particle volume in salt solution and water 49

Table 4.2 The DC4D output voltage amplitude versus particle volume in various concentration of salt solution 50

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List of symbols and abbreviations

C4D : Capacitively Coupled Contactless Conductivity Detection (or C4D)

CMOS: Complementary Metal-Oxide-Semiconductor

CTCs : Circulating Tumor Cells

DC4D : Differential Capacitively Coupled Contactless Conductivity Detection

i

d : Size parameters of the pipe (i=1,2,3) (m)

E : Electric field intensity (V/m)

L l : Size parameters of the electrodes (i=1,2,3)

MEMS: Micro Electro-Mechanical Systems

PCB : Printed Circuit Board

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Summary

Detection of the presence of strange particles in fluidic channels is important, due to their potential in chemical analysis, biology, pharmacology and especially in medical The appearance of air bubble in the patient’s blood vessels is dangerous in case of the unpredictable of cerebral embolism can lead to instant death The detection

of strange cell in the blood vessel plays a crucial role in diagnosis or early detection of some diseases including cancer In MEMS, the appearance of a particle in the microfluidic channel can affect significantly to the response of the flow such as the flow velocity, the fluidic pure quality

Among the different physical techniques for detection of objects in fluidic channel such as optical, ultrasonic, electrical sensing based on contact and contactless mechanism, capacitive sensing emerged as the best technique Capacitive sensor has been developed and applied in many field of technology due to simple fabricate and setup measurement, as well as minimization capability Additionally, there are many advantages of capacitive sensors in micro fabrication and integration on systems Capacitively coupled contactless conductivity detection (C4D) is a new detection technique has been developed in recent years and used mainly in capillary electrophoresis and microchip electrophoresis The characteristics of C4D detector are simple in structure, easy in miniaturization and integration, and free of electrodes contamination, which are common problems in an electrochemical detection

This thesis presents a novel design of a differential C4D (DC4D) structure based

on three U-shaped electrodes which can apply to the fluidic channel detection systems

at millimeter size This structure consists of two single C4D with an applied carrier sinusoidal signal to the center electrode as the excitation electrode The electrodes are directly bonded on the PCB with built-in differential amplifier and signal processing circuit in order to reduce the parasitic component and common noise The proposed structure can be used for both conductive and non-conductive fluidic channel Simulations and experimental measurements are performed Experimental results show that a good agreement with the simulation Air bubbles and tin particles are pumped through electrodes for characterizing non-conductive fluidic case Plastic particles with various sizes are employed in the conductive fluidic configuration Changes in both particles position and volume result in changes in the capacitance, the admittance or the output voltage between the electrodes are investigated In the non-conductive fluidic channel, the output voltage and capacitance changes 214.39 mV

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and 14 fF, respectively when a 4.18 µl tin particle crosses an oil channel In conductive fluidic channel, the output voltage and admittance change up to 200 mV and 0.05 µS for the movement of a 4.63 µl plastic particle through 0.75% salt solution channel The measured results indicated the linear relation between the output voltage and the particle volume Beside particle detection, this sensor system allows measuring velocity of the particle inside fluidic channel thanks to distance and travel time between the two single C4D structure

In addition, a microsensor based on DC4D structure is also designed, modeled, simulated and fabricated The four-electrode capacitor is covered by thin PDMS protective layer Coplanar electrodes configuration is made of gold on glass substrate, which are arranged to form differential coplanar capacitor structures in order to achieve a high sensitivity and robust operation The differential capacitance is changed when a micro particle (air bubble, particles or living cell for instance) crosses the microfluidic channel The output capacitance changes versus object’s volume and position are simulated by using FEM tool The simulation inspection reveals that the sensor can detect an object with diameter down to 10 µm in a 50×100 µm cross-section channel The capacitance change up to 0.375 fF amd 0.3 fF when a 30 µm diameter air bubble and a same diameter tin particle move through in fresh water channel, respectively A measurement setup was designed and implemented to monitor the capacitance change The DC4D sensor is also fabricated by micro machining The measurement with particles and living cell is in progress This proposed DC4D sensor can be used for detection of strange particle, air bubble in microfluidic flow or cell in medical devices and systems This novel design can not only detect the present of an object but also volume, velocity, as well as electrical property (conduct/non-conduct) of the investigated object

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Chapter 1 INTRODUCTION

1.1 Background and Overview

The development of particle detectors practically starts with the discovery of radioactivity in the year 1896 Henri Becquerel noticed that the radiation emanating from uranium salts could blacken photosensitive paper Almost at the same time X rays, which originated from materials after the bombardment by energetic electrons, were discovered by Wilhelm Conrad Röntgen The first detectors were used to detect radiation particles based on optical measurement The trend of particle detection has shifted in the course of time from optical measurement to purely electronic means [9] The particle detectors have found applications not only in nuclear and particle physics, but also in oil exploration, civil engineering, archaeology, environmental science, medicine, biology, etc The methods and principles of these devices are also increasingly diversified and improved, with resolution and speed increasing

Nowadays, many advanced technologies in this evolving world are moving towards miniaturization of products It resulted in the rapidly development of a new technology called MEMS (Micro Electro Mechanical Systems) MEMS (also called as Micro Systems or Micro Machines) is a technology which integrates mechanical elements, electronics, sensors and actuators on a common silicon substrate using micro fabrication technology [22] The processes are a result of merging of advanced micromechanical and integrated circuit technologies While it may seem that the only advantage for MEMS are their small size, there are indeed many additional benefits Small sizes imply that less material is used and less energy is consumed Their small size allows for the construction of arrays of hundreds of them on a single chip In addition, the prominent advantage to MEMS is the financial factor By being able to produce thousands of devices on each individual silicon wafer, the cost per unit can be driven down to affordable prices MEMS devices are rapidly making their way into every aspect of modern life The future is getting smaller, more accurate, and quicker, and MEMS technology is aiding in the development of NEMS (Nano Electro Mechanical Systems) technology MEMS has been creating more and more interests

in many application fields Microstructures find applications in the optical systems, communications, RF devices, analytical, and biology

In fact, the most common applications of MEMS was as a sensor They have become varied in their applications and can be found almost everywhere in everyday

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life The popularity of these sensors is mostly due to the great advantages that they possess In addition to their small size, MEMS sensors consume very little power and are capable of delivering accurate measurements, which are unparalleled with macro-sized sensors Methods of operation of MEMS sensors are different depending on the use All sensors measure a change and MEMS devices do it with either one or a combination of the following six detection methods: mechanical, optical, electrical, magnetic, thermal and chemical These methods are generalizations for the basic system in which a MEMS device gathers information from the surrounding environment

Various sensors built by different mechanisms for particles detection and manipulation have been reported, such as mechanical structures based micro-tweezers for manipulation and micro-cantilever for detection [11], magnetic field based magnetic beads labeled bioparticles manipulation and detection [35], as well as optical field based light beam for manipulation and fluorescence labeled bioparticles detection [7], electric field based dielectrophoresis (DEP) manipulation and resistive/capacitive detection [15, 31] Meanwhile, the other devices are expensive, energy intense and barely portable, the resistive/capacitive detection as labels of bioparticles have been attracting great attention in many environmental and health applications In recent years, the MEMS capacitive sensors have become one of the most important application components The capacitive sensor has been obtained the research interests because its structure is the simplicity of design and fabrication, easy

to measure and inexpensive These include, many sensors have the capabilities to detect presence of particles, tissues or cells in fluidic channel

1.2 Research Objectives

Fluidic flow detection has been developed for many practical applications in different areas, such as in pharmacology, MEMS, biology, analytical chemistry, food analysis, water-quality control and especially in medical [32, 33, 37] For instance, appearance of air bubble in the patient’s blood vessels is dangerous in case of the unpredictable of cerebral embolism can lead to instant death Bubbles may appear in the blood of the body tube when dialysis or air bubbles can be created when intravenous infusion of the patient's body, so the detection of air bubbles in the blood

or in the pipe conduit body fluids is essential Another typical example is detecting the circulating tumor cells (CTCs) in the blood stream, for early cancer detection and diagnosis CTCs are tumor cells circulating through normal vessels and capillaries

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Metastases result from tumor cells migrating from the primary tumor sites to distant organs, and are directly responsible for most cancer-related deaths The detection of CTCs has raised considerable interest over the last decade In MEMS, the appearance

of a particle in the microfluidic channel can affect significantly to the response of the flow such as the flow velocity, the fluidic pure quality Many fundamental methods have been applied for fluidic flow detection such as optical, ultrasonic, electrical sensing based on contact and contactless mechanisms

Fluidic channel sensor can be used electrical conductivity parameter of material and channel geometry based on the direct contact technique [16] In this technique, the electrodes are directly in contact with the fluidic, liquid or electrolyte solution The polarization effect and electrochemical erosion effect in the solution or the electrodes are unavoidable in this way Besides, the contamination of the electrodes usually causes errors in conductivity measurement These disadvantages limit the practical applications of the conventional contact conductivity detection techniques

[17]

The capacitive contactless sensor structures are developed in order to avoid the direct contact technique issues [16, 34, 45, 46] Capacitive sensor structures as the contactless mechanism are often used to measure the phase flow detection such as air-water-oil [6, 42, 44] However, the sensor sensitivity of the capacitive configuration is low in case of high conductivity liquid due to the much small resistance value of the conductive fluidic channel in comparison with the sensor capacitance [42] Jaworek,

et al presents a high frequency capacitance sensor to solve the conductive effects of water using an 80 MHz oscillator However, that device requires an extremely short electrodes for a quasi-local measurement and a rather complicated circuit [20]

Capacitively coupled contactless conductivity detection, abbreviated as C4D, which was proposed by Fracassi da Silva, et al and Zemann, et al., independently in

1998 [1, 19], as a detection technique for capillary electrophoretic systems [5, 18] This kind of technique is applied in many areas and has brought an undeniable advantage into detection and measurement field The C4D structures consist of two electrodes separated by a gap Based on the conductivity of liquid, the flow will transmit the signal from an exciting electrode through the dielectric of a pipe and bring the information of the liquid’s conductivity to the pick-up electrode [1, 17, 26,

27, 29, 37, 45, 47, 48] C4D can be used for detecting oil in the water and impurities in tap water (electrical conductivity liquid) Hence, this application can become an

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excellent method in solving problems in the oil and gas industry [6] Moreover, up to date, the C4D technique is studied and used in the research field of Analytical Chemistry for ion concentration/conductivity detection in the capillary and the conductivity of fluidic channels [17] Another useful application of this technique is estimating the velocity of the conduct fluidic flow and measurement of bubble velocity in gas-liquid two-phase flow in millimeter-scale pipe, which is a fundamental problem existing in many industries, such as chemical, pharmaceutical, petroleum, energy and power engineering [42] Application based on C4D technique in detecting impurities and estimating their velocity in fluidic channel is researched and developed

by many research groups despite of its difficulties and limitations [26, 27, 41] There are several measurement methods that are developed to against these difficulties and limitations of the conventional C4D technique A grounded shield between the excitation electrode and the pick-up electrode can be used to prevent stray capacitance

of stray capacitance and coupling capacitances [40] Some designs use this resonator method to measure the conductivity and flow detection [17, 28] but in that case, the permittivity could not be recognized, for example the case of full oil or the air inside pipe

C4D technique offers great promise for microfluidic systems, with features that include high sensitivity, intrinsic miniaturization capability, low-power requirements, compatibility with advanced microfabrication technologies and low cost In this thesis, a C4D structure was designed and constructed suitable for fluidic channel detection system The results obtained primarily in MEMS laboratories are discussed Some applications of C4D sensor and tentative directions for the future are also outlined

This study employs a differential amplifier to avoid the above difficulties with a sensor system including three U-shape electrodes on the top of a printed circuit board (PCB) in order to reduce the parasitic capacitance and increase the sensitivity not only

in the conductivity liquid but also in the non-conductivity liquid This DC4D structure consists of two sensing electrodes and one exciting electrode The electrodes are layout as a coplanar capacitive sensor This proposed structure and measurement setup can detect two-phase flow channel for both case of conductive liquid and non-conductive liquid

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Among the different physical techniques for detection of objects in micro fluidic channel, capacitive sensing emerged as the best technique thanks to the simple fabrication and measurement setup, as well as minimization capability [18, 44] Another method is to take advantage of differential technique in 3-electrode C4D structure (DC4D) to increase the sensitivity of sensor [4] but the 3-electrode structure can bring the disadvantage because when there is an object inside the channel, it can influence to all 3 electrodes This issue causes the erroneous result in measurement The 4-electrode structure, which are covered with a thin PDMS protective layer, are arranged in pair in order to reduce the interference between them An optimal design employing the differential capacitance between coplanar electrodes inside the micro fluidic channel is proposed Four adjacent electrodes are laid on glass substrate and arranged close together in pair to form differential coplanar structures The micro-channel structure is fabricated inside the PDMS substrate We take advantage of the self-bonding ability of these two substrate’s material in order to attach the PDMS substrate on the glass substrate When an object goes through the active area of the sensor in the channel, the differential capacitance will be changed, which leads to the change in the output signal of measurement device

electric field, with dl an elementary length along a flux line of displacement current

This integral gives the capacitance of an elementary volume surrounding the flux line, and must be repeated for all flux lines emanating from one of the plates and terminating in the other plate

Figure 2.1 shows two parallel conducting plates connected by wires to a battery, separated by an insulator (e.g., air) and the electric field lines For the parallel two-electrode geometry, capacitance is the measure of the amount of charge that a capacitor can hold at a given voltage [39]

+ U -

E Q

-Figure 2.1 Charged parallel plates separated by an insulating medium [13].

Capacitance can be defined in the unit coulomb per volt as:

Q C U

in which, Q is the magnitude of charge stored on each plate (coulomb),

U is the voltage applied to the plates (volts)

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A capacitor with the capacitance of one farad can store one coulomb of charge when the voltage across its terminals is 1 V Typical capacitance values range from about 1 fF (10-15 F) to about 1000 μF (10-3 F) An electric field will exist between the two plates of a capacitor if the voltage is applied to one of the plates The resulting electric field is due to the difference between the electric charges stored on the surfaces of each plate The capacitance describes the effects on the electric field due

to the space between the two plates

The capacitance depends on the geometry of the conductors and not on an external source of charge or potential difference The space between the two plates of the capacitor is covered with dielectric material In general, the capacitance value is determined by the dielectric material, distance between the plates, and the area of each plate The capacitance of a capacitor can be expressed in terms of its geometry and dielectric constant as:

0 r A C

A is the area of each plate, in square meters,

d is the separation distance (in meters) of the two plates

The capacitance phenomenon is related to the electric field between the two plates of the capacitor The electric field strength between the two plates decreases as the distance between the two conducting plates increases Lower field strength or greater separation distance will lower the capacitance value The conducting plates with larger surface area are able to store more electrical charge; therefore, a larger capacitance value is obtained with greater surface area

In addition, the sensing electrodes of the capacitive sensor could be shaped into various forms and structures The geometry of the sensing electrodes influences the electric field between them In fact, a few types of sensing electrodes are designed and fabricated, such as cylindrical rods, cylindrical tubes, rectangular plates, helixical wires, coplanar plates and tubular shaped capacitors

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2.2 Dielectric constant

The gap between the two surfaces of a capacitor is filled with a non-conducting material such as rubber, glass or, particle or fluid that separates the two electrodes of the capacitor [21] This material has a certain dielectric constant The dielectric constant is the measure of a material’s influence on the electric field The net capacitance will increase or decrease depending on the type of dielectric material Permittivity relates to a material’s ability to transmit an electric field In the capacitors, an increased permittivity allows the same charge to be stored with a smaller electric field, leading to an increased capacitance

According to Eq 2.3, the capacitance is proportional to the amount of dielectric constant As the dielectric constant between the capacitive plates of a capacitor rises, the capacitance will also increase accordingly The capacitance can be stated in terms

of the dielectric constant, as:

the water as dielectric will increase by a factor of 80 This factor is called Relative

dielectric constant or Relative electric permittivity

2.3 Capacitive sensor applications

A typical capacitive sensor bases on a change of parameters in capacitor structure lead to change its capacitance during the sensing time It converts a change

in position, or properties of the dielectric material into an electrical signal [10] According to the Eq 2.3, capacitive sensors are realized by varying any of the three

parameters of a capacitor: distance (d), area of capacitive plates (A), and dielectric

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functionalities range from humidity sensing, through level sensing, to displacement sensing [9] A number of different kinds of capacitance based sensors are used in a variety of industrial and automotive applications, such as proximity sensor, position sensor, humidity sensor, and pressure sensor [43]

2.3.1 Proximity sensor

A proximity sensor is able to detect the presence of nearby objects without any physical contact Normally a proximity sensor emits an electromagnetic or electrostatic field, and detects any change in the field or return signal Capacitive type proximity sensors consist of an oscillator whose frequency is determined by an LC circuit to which a metal plate is connected When a conducting or partially conducting object comes near the plate, the mutual capacitance changes the oscillator frequency This change is detected and sent to the controller unit The object being sensed is often referred to as the proximity sensor’s target As the distance between the proximity sensor and the target object gets smaller, the electric field distributed around the capacitor experiences a change, which is detected by the controller unit

The maximum distance that a proximity sensor can detect is defined as ‘nominal range’ Some sensors have adjustments of the nominal range or ways to report a graduated detection distance A proximity sensor adjusted to a very short range is often used as a touch switch Capacitive proximity detectors have a range twice that of inductive sensors, while they detect not only metal objects but also dielectrics such as paper, glass, wood, and plastics They can even detect through a wall or cardboard box Because the human body behaves as an electric conductor at low frequencies, capacitive sensors have been used for human tremor measurement and in intrusion alarms Capacitive type proximity sensors have a high reliability and long functional life because of the absence of mechanical parts and lack of physical contact between sensor and the sensed object

An example of a proximity sensor is a limit switch [23], which is a mechanical pushbutton switch that is mounted in such a way that it is activated when a mechanical part or lever arm gets to the end of its intended travel It can be implemented in an automatic garage door opener; where the controller needs to know

if the door is all the way open or all the way closed Other applications of the capacitive proximity sensors are [2]:

+ Spacing: If a metal object is near a capacitor electrode, the mutual capacitance

is a very sensitive measure of spacing

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+ Thickness measurement: Two plates in contact with an insulator will measure the insulator thickness if its dielectric constant is known, or the dielectric constant if the thickness is known

+ Pressure sensing: A diaphragm with stable deflection properties can measure pressure with a spacing-sensitive detector

2.3.2 Position sensor

A position sensor is a device that allows performing position and displacement measurements Position can be either an absolute position or a relative one [38] Linear as well as angular position can be measured using position sensors Position sensors are used in many industrial applications such as fluid level measurement, shaft angle measurement, gear position sensing, digital encoders and counters, and touch screen coordinate systems Traditionally, resistive type potentiometers were used to determine rotary and linear position However, the limited functional life of these sensors caused by mechanical wear has made resistive sensors less attractive for industrial applications Capacitive type position sensors are normally non-mechanical devices that determine the position based on the physical parameters of the capacitor Position measurement using a capacitive position sensor can be performed by varying the three capacitive parameters: Area of the capacitive plate, Dielectric constant, and Distance between the plates The following applications are some examples of the utilization of capacitive position sensors in:

+ Liquid level sensing: Capacitive liquid level detectors sense the liquid level in

a reservoir by measuring changes in capacitance between conducting plates which are immersed in the liquid, or applied to the outside of a non-conducting tank

+ Shaft angle or linear position: Capacitive sensors can measure angle or position with a multi-plate scheme giving high accuracy and digital output, or with an analogue output with less absolute accuracy but faster response and simpler circuitry

+ X–Y tablet: Capacitive graphic input tablets of different sizes can replace the computer mouse as an x–y coordinate input device Finger-touch-sensitive devices such as iPhone [38], z-axis-sensitive and stylus-activated devices are available

+ Flow meter: Many types of flow meters convert flow to pressure or displacement, using an orifice for volume flow or Coriolis Effect force for mass flow Capacitive sensors can then measure the displacement

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2.3.3 Humidity sensor

The dielectric constant of air or some materials is affected by humidity As humidity increases the dielectric increases The permittivities of atmospheric air, of some gases, and of many solid materials are functions of moisture content and temperature Capacitive humidity devices are based on the changes in the permittivity

of the dielectric material between plates of capacitors Capacitive humidity sensors commonly contain layers of hydrophilic inorganic oxides which act as a dielectric

[14] Absorption of polar water molecules has a strong effect on the dielectric constant

of the material The magnitude of this effect increases with a large inner surface which can accept large amounts of water

The ability of the capacitive humidity sensors to function accurately and reliably extends over a wide range of temperatures and pressures They also exhibit low hysteresis and high stability with minimal maintenance requirements These features make capacitive humidity sensors viable for many specific operating conditions and ideally suitable for a system where uncertainty of unaccounted conditions exists during operations There are many types of capacitive humidity sensors, which are mainly formed with aluminum, tantalum, silicon, and polymer types

Pressure sensors can be classified in terms of pressure ranges they measure, temperature ranges of operation, and most importantly the type of pressure they measure Pressure sensors are variously named according to their purpose, as:

+ Absolute pressure sensor: measures the pressure relative to perfect vacuum

+ Gauge pressure sensor: measures the pressure relative to atmospheric pressure

A tire pressure gauge is an example of gauge pressure measurement; when it indicates zero, then the pressure it is measuring is the same as the ambient pressure

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+ Vacuum pressure sensor: measures pressures below atmospheric pressure, showing the difference between that low pressure and atmospheric pressure (i.e negative gauge pressure); or measures low pressure relative to perfect vacuum (i.e absolute pressure)

+ Differential pressure sensor: measures the difference between two pressures, one connected to each side of the sensor Differential pressure sensors are used to measure many properties, such as pressure drops across oil filters or air filters, fluid levels (by comparing the pressure above and below the liquid) or flow rates (by measuring the change in pressure across a restriction)

+ Sealed pressure sensor: is similar to a gauge pressure sensor except that it measures pressure relative to some fixed pressure rather than the ambient atmospheric pressure (which varies according to the location and the weather)

2.3.5 Tilt sensors

In recent years, capacitive-type micro-machined accelerometers are gaining popularity These accelerometers use the proof mass as one plate of the capacitor and use the other plate as the base When the sensor is accelerated, the proof mass tends to move; thus, the voltage across the capacitor changes This change in voltage corresponds to the applied acceleration Micromachined accelerometers have found their way into automotive airbags, automotive suspension systems, stabilization systems for video equipment, transportation shock recorders, and activity responsive pacemakers [36]

Capacitive silicon accelerometers are available in a wide range of specifications

A typical lightweight sensor will have a frequency range of 0 - 1000 Hz, and a dynamic range of acceleration of ±2 to ±500 g Analogue Devices, Inc has introduced integrated accelerometer circuits with a sensitivity of over 1.5 g With this sensitivity, the device can be used as a tilt meter

2.4 Basic principles of C 4 D structure

Currently, C4D is mainly studied and applied in research of bioMEMS for concentration/ conductivity detection in fluidic channel or capillaries [45] Typical examples of the C4D geometries can be seen in Figure 2.2 Tubular and semi tubular electrodes (Figure 2.2a,b) are common in flow-through applications, primarily liquid chromatography and capillary electrophoresis Co-planar geometries (Figure 2.2c) are useful in microfluidic systems, chip electrophoresis, or lab-on-chip systems

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Figure 2.2 Examples of C 4 D designs used mostly for conduct metric detection [34]

Figure 2.3(a) illustrates design of a single C4D fluidic sensor, which consists of two electrodes A sinusoidal signal is applied to left electrode as the excitation electrode and the sensing is the right one, as the pick-up electrode, usually in the form

of an alternating current signal Both electrodes sandwich the fluidic channel, which

produces two wall capacitors through dielectric layer of shell of channel (Cw1, Cw2)

Figure 2.3. Design of a single C 4 D structure: (a) excitation and pick-up electrodes; (b) The

equivalent circuits

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A simplified electrical equivalent circuits of a single C4D structure are shown in Figure 2.3(b). The resistance of conductivity solution inside channel is R S and the

solution capacitance is C s The wall capacitances Cw1, Cw2 depend on the thickness

and permittivity of the dielectric layer and the size of the electrode These two

electrodes also make a stray capacitance C 0 parallel to the main passageway along the fluidic channel The parasitic effect of the stray capacitance is sometimes eliminated

by taking the grounded shield [12, 24-26] or placing a shield foil between the electrodes [3, 12]

The impedance of the first electric equivalent circuit can be calculated:

1 2

1 2

Z Z Z

of the circuit, and 2

0

1

Z

j C

 is the impedance determined by stray capacitance C 0

BecauseR s C the sensor works primarily as a conductivity detector, the s,

effect of the solution capacitance can be neglected, and Cw1, Cw2 are simplified to

C w The analytical form of the cell impedance, Z, defined by the familiar general

where R 1 and X C are the real and imaginary component of the impedance of C4D,

R S is the solution resistance,2 f is the angular frequency, with f is the ordinary

frequency, and j 1 is the imaginary unit, respectively

When an alternating actuator voltage is applied to a C4D, the detection current is

proportional to the magnitude of its admittance, Y , which is expressed as:

 2 2 2 2 2 4 2

2 2 2

2 2 1

in which, G s 1/R s is the solution conductance It can be seen that in the case

of high conductivity solution, G s C w, the equation (2.8) can be simplified as:

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 0 

2 2 1

1

w C

G S can be ignored in this case Therefore, equation (2.9) shows that the Y value

mainly depends on the value of wall and stray capacitance at a specific frequency In order to increase the sensitivity of the measurement, the value of solution resistance

R S and wall capacitance C w1 , C w2 have to be at the same level in correlation with each

other This can be done by increasing R S or decreasing

Z must be decreased by making the distance between two electrodes become longer,

or increasing the C w by increasing the length of electrodes

2.5 Coplanar capacitive sensor in CMOS chip

Figure 2.4 shows the side view of a coplanar capacitive sensor showing how electric field was formed between positive and negative electrodes Gold is used extensively as sensing electrodes for biomedical applications due to be biocompatible Gold is also a far better electron conductor than aluminum, copper or even silver This highly defined and conductive surface of gold may be ideal for several biosensing applications including bacterial growth monitoring, virus detection, and DNA detection The gold layer can readily be fabricated using commercially available lithographic technologies on chip using CMOS compatible micromachining procedures at low temperature

Substrate

l1 l2

l3

Figure 2.4 Electric field formed between positive and negative electrodes for different pitch

lengths, (l1, l2 and l3) [30]

It is seen clearly in Fig 2.4 that the penetration depths of the electric field lines

is different for different pitch length The pitch length of the conventional capacitive

27

sensors is the distance between two consecutive electrodes of the same polarity Also

in Fig 2.4, there are three pitch length (l 1 , l 2 and l 3) showing the different penetration depths with respect to the pitch length of the sensor The penetration depth can be increased by increasing the pitch length, but the electric field strength generated at the neighboring electrodes will be weak Planar capacitive sensors can be used for different sensing application

Figure 2.5 illustrates on the sensing possibilities of planar capacitive sensors These sensing possibilities for various characteristic of samples have given us the opportunity to design and fabricate new type of planar capacitive sensors

Figure 2.5 Sensing possibilities to detect various characteristic of samples [30]: (a) sensing

density, (b) sensing distance, (c) sensing texture, (d) sensing moisture

The working conventional capacitive sensor typically based on the change of parameters in the capacitor structure, which results in the change of its capacitance There are many developed capacitive sensor structures basing on two parallel electrodes structure In micro fabrication, the capacitive sensor structure is mainly coplanar structure The capacitance of two parallel, coplanar and semi-infinite

conducting plates separated by a gap distance of 2a are embedded within a uniform dielectric medium of permittivity ε r is:

Where ε0 is the vacuum permittivity, l and w is the length and the width of the

electrode pair, respectively [43] Recently, most of capacitive fluidic sensor based on

28

the mechanism: a change of capacitance, which caused by a change of permittivity and conductivity of material between electrodes, can be caused by a change in fluidic channel The dielectric is different for each material or different liquids Hence, the change of material inside channel can lead to the change of sensor’s capacitance Therefore, an object in a homogeneous fluidic flow can be easily detected

To date, several capacitive readout techniques, of various complexity, have been reported for autonomous MEMS-based capacitive sensor (MBCS) systems but there is

a little published literature on the custom design of an on-chip capacitive sensor for LoC applications The sensing electrodes are usually realized on the same chip of capacitive interface circuit and a microfluidic channel is used to direct the biological fluid toward sensing site as seen in Figure 2.6 However, some rapid prototyping methods have alternatively been reported to detect the bioparticles through the capacitive sensors created in between one electrode on the chip and another electrode between the chip and a grounded electrode above the chip

Lab-on-Chip

Electrodes

Microfluidic

Figure 2.6 A simplified diagram of a capacitive sensing based LoC [8]

A variety of on-chip capacitive sensors have been fabricated for many biological and chemical applications, including DNA detection, antibody-antigen recognition, cell monitoring, organic solvent detection, bacteria growth monitoring and ultrathin polyelectrolyte layer detection and detection of protein conformation, toxic chemical gases

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OBJECT IN FLUIDIC CHANNEL

3.1 DC 4 D sensor for Conductive and Non-conductive Fluidic Channel

In this section, a sensor structure consists of three U-shape electrodes on the top

of a printed circuit board (PCB) is presented It consists of two sensing electrodes and one exciting electrode The electrodes are layout as a coplanar capacitive sensor This structure employs a differential amplifier in order to reduce the parasitic capacitance and increase the sensitivity This proposed structure and measurement setup can detect two-phase flow channel for both case of conductive liquid and non-conductive liquid

3.1.1 Design and operation

Figure 3.1 shows a block diagram design of the DC4D fluidic sensor based on three electrodes for detecting particles inside both conductive and non-conductive liquid channel This sensor is equivalent to two single C4D structures with an applied carrier sinusoidal signal to the center electrode as the excitation electrode The differential signal between the top and bottom electrodes is then amplified and demodulated for removing the carrier components The output signal indicates the different response between two single C4D structures This proposed sensor could detect a particle like plastic particle, air bubble, metal particle and so on inside channel when it passes the electrodes

V in

V out LPF

Reservoir

AC

Source

Differential amplifier

R0

580 KHz sine wave

Particle Cylinder

R0

Figure 3.1 Block diagram design of the DC 4 D fluidic sensor

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In this design, there are two pick up electrodes, which are outside electrodes The center electrode is excited electrode (see Figure 3.2) The differential signal between the two pick up electrodes indicates changing inside the fluidic channel

AC

Figure 3.2 The DC 4 D based on three-electrode configuration; (b) The equivalent diagram

The distance between two electrodes is L2 to make a so-called co-planar capacitor at this point L1 and L3 is the length and the height of each electrode,

respectively The U-shape geometry held tightly the fluidic channel along the sensor This proposed U-shape is convenient in order to setup and can be used for various size

of fluidic channel A pipe with outlet diameter d1 is laid inside three electrodes as

shown in Figure 3.2(a) The system is modeled, simulated and fabricated with some essential parameters Geometry parameters of the DC4D are listed in the Table 3.1

Figure 3.2(b) shows the equivalent circuit of the sensor R S and Cw are the resistance

of conductivity solution and wall capacitance of each single C4D segment,

respectively C 0 is the stray capacitance between two adjacent electrodes This DC4D structure reduces the effect of common noise inside the fluidic channel and amplifies the differential signal between the two single C4D structures