Capacitive pH-Sensors using pH sensitive...

Department of Physics, Chemistry and Biology

Master Thesis in Molecular Electronic and System Design at

Linkoping’s University

Capacitive pH-Sensors using pH sensitive polymer

by

Krishna Chytanya Chinnam

Reg Nr: LITH-IFM-A-EX--08/2031-- SE

Linköpings University Department of Physics, Chemistry and Biology

581 83 Linköping

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Keywords

Capacitive pH Sensors, pH Sensors, Capacitive Sensors & Sensors.

Publication Title Capacitive pH-Sensors using pH sensitive polymer

Author(s) Krishna Chytanya Chinnam

Abstract

This project aims in building a new experimental setup for capacitive measurements of a pH-Sensor. PAA-IOA (Poly Acrylic Acid co – Iso Octyl Acrylate) is the dielectric material over the in-plane interdigitated gold electrodes where PAA IOA acts as an H+ ion sensing layer. The changes in the capacitance of the sensor when the sensor is dipped into different pH solutions will be quantized accordingly. The dipping setup is built in such a way that the electrodes (containing the polymer layer) can be easily dipped into different pH liquids and to eliminate any contact between the polymer and set-up (e.g. pressure effects on the sensor). From the setup it is visible that the gold electrodes are not subjected to any external force as in the case of the setup used previously. Three phases of experiments have been used in this project to get a clear view on the working principle of the polymer. The effect of pH is only considered in this project, as we already have the evidences for the salt sensitiveness of PAA IOA from the work done in the past. The influence of various pH on polymer is observed as capacitance measurements. Response time is more than 5 minutes for PAA IOA. ∆C decreases with frequency and frequency choice depends on application/electronics. The degree of other ions influence is

not clear but they have a minor influence in the resistance.

URL, Electronic Version

http://www.ep.liu.se

Presentation Date

2008-09-25

Publishing Date (Electronic version)

2009-01-15

Division, Department

Chemistry

Department of Physics, Chemistry and

Biology

Linköping University

Number of Pages

56

ISBN (Licentiate thesis) _____________________________________________

ISRN: LITH-IFM-A-EX--08/2031-- SE

_____________________________________________

Title of series (Licentiate thesis) _____________________________________________

Series number/ISSN (Licentiate thesis)

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Internship Report

On

Capacitive pH-Sensors using pH sensitive polymer

By,

Krishna chytanya chinnam

Student: Molecular Electronics & System Design

Electrical Engineering

Linköping University, Sweden

[email protected]

Supervisor at Imego, Supervisor at Linköping University,

Dag Ilver Prof. Mats Fahlman

Research Scientist Surface Physics and Chemistry Division

R&D, Bio- and Chemical Sensors Department of Physics, Chemistry and Biology

Imego AB, Sweden Linköping University, Sweden

[email protected] [email protected]

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Abstract

This project aims in building a new experimental setup for capacitive measurements of a pH-Sensor. PAA-IOA (Poly Acrylic Acid co – Iso Octyl Acrylate) is the dielectric material over the in-plane interdigitated gold electrodes where PAA IOA acts as an H+ ion sensing layer. The changes in the capacitance of the sensor when the sensor is dipped into different pH solutions will be quantized accordingly. The dipping setup is built in such a way that the electrodes (containing the polymer layer) can be easily dipped into different pH liquids and to eliminate any contact between the polymer and set-up (e.g. pressure effects on the sensor). From the setup it is visible that the gold electrodes are not subjected to any external force as in the case of the setup used previously. Three phases of experiments have been used in this project to get a clear view on the working principle of the polymer. The effect of pH is only considered in this project, as we already have the evidences for the salt sensitiveness of PAA IOA from the work done in the past. The influence of various pH on polymer is observed as capacitance measurements. Response time is more than 5 minutes for PAA IOA. ∆C decreases with frequency and frequency choice depends on application /electronics. The degree of other ions influence is not clear but they have a minor influence in the resistance.

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Acknowledgement

The project work for a period of 6 months as an Intern at Imego, Göteborg has been documented in the form of this report. This report will be submitted at Linköping University, Sweden as my Master’s Thesis work. As a student working in an industrial and professional environment, I have gained great experience.

I would like to personally thank the most important people who have been involved with my project. Cristina Rusu who is a project leader at Imego, for giving me the opportunity to work in the company and her inputs, Dag Ilver being my daily supervisor has always been motivating me as a mentor with his ideas, suggestions, Anatol Krozer for his valuable conceptual inputs, Linus Lofgren who has been efficiently helping me in the clean room for the sensors, Katrin Persson for her sensor assembly suggestions and I would like to thank the affable staff of Imego that has always treated me as a part of their Imego family.

I would like to thank Mats Fahlman who is my Thesis supervisor at Linköping University for his suggestions regarding the project. I would like to thank Sri Krishna Chanakya and my friends who have been giving me constant support and encouragement.

Finally last but not the least I would like to thank my parents: Ch.V.Subrahmanyam and D.Shyamala, my brothers: Ch.Suman and Ch.Vishal who have always been encouraging me in all aspects of life.

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Contents Abstract..................................................................................................................................................................................4 Acknowledgement..........................................................................................................................................................5 SI units.............. .......................................................................................................................................................................................... 7 1. Introduction .................................................................................................................................................. 8 2. Theory ............................................................................................................................................................. 9 2.1. Acids and Base in Chemistry ........................................................................................................................... 9 2.2. Defining pH ................................................................................................................................................................. 9 2.3. Capacitive sensing .............................................................................................................................................. 11

2.3.1. Interdigitated gold electrodes ............................................................................................................. 12 2.4. Sensor design ........................................................................................................................................................ 13

2.4.1. Typical dimensions ..................................................................................................................................... 13 2.4.2. Choosing electrode Geometry ............................................................................................................ 14 2.4.3. Electrode material ...................................................................................................................................... 14 2.4.4. Substrate material ..................................................................................................................................... 15

2.5. Sensitive Polymer ................................................................................................................................................ 15 2.6. Fabrication ............................................................................................................................................................... 16

2.6.1. Polymer Deposition.................................................................................................................................... 17 2.6.2. Contacts for the electrodes ................................................................................................................. 19

3. Measurement Setup for pH-Sensor .................................................................................................. 20 3.1. Measurement Setup Description .............................................................................................................. 20 3.2. Polymer deposition on the sensor chip .................................................................................................. 23 3.3. Preparation of the buffered pH Solutions ............................................................................................. 24 4. Measurements & Results .................................................................................................................... 25 4.1. Measurement techniques employed in the project ......................................................................... 25 4.2. Results & Discussions ...................................................................................................................................... 26

4.2.1. Phase1 ............................................................................................................................................................. 27 4.2.2. Phase2 ............................................................................................................................................................. 31 4.2.3. Phase3 ............................................................................................................................................................. 36

5. Conclusions ............................................................................................................................................... 41 6. Recommendations for future work ................................................................................................... 44 7. References................................................................................................................................................. 45 8. APPENDIX .................................................................................................................................................. 46

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SI units

Symbol Name of Quantity Derived Units Unit Base Units

I Electrical Current ampere (SI base unit) A A = W/V = C/s

q Electric charge, Quantity of electricity coulomb C A·s

V Potential difference volt V J/C = kg·m2·s−3·A−1

R, Z, X Resistance, Impedance, Reactance ohm Ω V/A = kg·m2·s−3·A−2

ρ Resistivity ohm meter Ω·m kg·m3·s−3·A−2

P Power, Electrical watt W V·A = kg·m2·s−3

C Capacitance farad F C/V = kg−1·m−2·A2·s4

ε Permittivity farad per meter F/m kg−1·m−3·A2·s4

σ Conductivity siemens per meter S/m kg−1·m−3·s3·A2

μ Permeability henry per meter H/m kg·m·s−2·A−2

Retrieved from "http://www.iupac.org/publications/books/gbook/green_book_2ed.pdf "

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1. Introduction

pH is measured in different applications: medical, environmental, food production, safety appliances and in industry for process control (ex. where controlling pH helps ensure product quality and reduces corrosion and scaling in plant equipment and protects the environment by helping wastewater dischargers) [1]. pH can be measured using Litmus paper, pH sensitive glass electrode, sensors with ion selective layers on FETs, etc.

The main objective of this project is to develop a capacitive pH-Sensor that requires very low power consumption and low packaging space. These characteristics offer the sensor some advantages in remote applications. The basic idea to construct a capacitive pH sensor has been developed by Imego within the Intellisense RFID project (Nordic program). The layout and working principles of the sensor chip was designed in such a way that it is the same for several sensors (e.g. Humidity, pH, etc). Only the sensing layers are selected different for different sensors according to their usage for detection.

Capacitive pH sensor is a device which is used to indicate the H+ ion concentration of any aqueous liquid in terms of capacitance. It is hard to calculate the H+ ion concentration and the pH at the same time, so quantization of H+ ion concentration in terms of the capacitance is done by constructing the calibration curves. This curve consists of the capacitance ranges whose pH is already known. Using this curve one can determine the concentration of the H+ ion concentration in various unknown solutions available without calculating the pH of the solution.

The most common pH sensing device used in these days is the Standard pH Meter. Its functional mechanism is described in the section APPENDIX-1, [1]. Capacitive sensing is becoming very popular due to their low power consumption requirements and less packaging space. The special swelling and shrinking properties of the polymer PAA IOA which is a pH sensitive polymer has been used in the resonance frequency measurements [2][3][4], and conductimetric measurements [5]. We have used the pH sensitive polymer PAA IOA on a capacitor consisting of interdigitated gold electrodes on a glass substrate to develop the capacitive pH sensor. The kinetics about swelling & shrinking of polymers and the hydro gels have been discussed in [6].

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2. Theory

2.1. Acids and Base in Chemistry According to Bronsted-Lowry theory an acid is a proton donor and base is a proton acceptor. An acid when dissolved in water would produce H+ ions (Hydrogen nuclei consisting of protons) and counter negative ions and still the whole solution maintains its neutral nature. Therefore water containing excess H+ ions becomes more acidic. Similarly when a base is dissolved in water it would produce OH- ions in turn makes the whole solution more basic indicating a proton accepting behaviour. Similarly when acids and bases are mixed water and salt are formed; by evaporating the water one can extract the salt.

This theory works well for water and other proton solvents but it doesn’t really explain all the acid- base reactions. E.g. If a salt MgCl2 is added to water a slightly acidic solution is obtained. Magnesium coordinates with water molecules around itself in the solution which ionizes the water molecules and protons will be released. Clearly Mg2

+ ion cannot donate protons hence it is not considered as an acid according to this theory.

According to the Lewis Theory Mg2+ is positively charged it forces the water molecules to

aggregate around it to minimize the positive charge. Those water molecules behave as small bar–magnets dangling around the powerful electromagnet, hence the more electronegative oxygen end points towards positively charged Mg2

+ and the electropositive Hydrogen end points away. There is a strong interaction between the metal ion and the water molecules which makes the water molecule to donate an electron pair between the coordination bond and it loses a proton which makes the solution acidic. According to this theory an electron pair acceptor is an acid and an electron pair donor is a base. [20]

2.2. Defining pH pH = - log [H+]

pH is a combination of two entirely different terms, ‘p’ belonging to Mathematics and ‘H’ belonging to Chemistry. In Mathematics lower case alphabet ‘p’ represents the negative logarithm coefficient and in Chemistry ‘H’ represents the hydrogen ion concentration. E.g.: pH, pCO2, pOH etc. The measurement of pH is quite essential in the day to day life to judge the acidity and alkalinity of any solution. pH is a measure of the relative amount of hydrogen and hydroxide ions in an aqueous solution. In any collection of water molecules a very small number will have dissociated to form hydrogen (H+) and hydroxide (OH-) ions:

H2O = H+ + OH-

The number of ions formed is small, at 25°C less than 2 x 10-7 % of the water molecules dissociate. In terms of molar concentrations, water at 25°C contains 1x10-7 moles per litre of hydrogen ions and the same concentration of hydroxide ions. In any aqueous solution the concentration of hydrogen ions multiplied by the concentration of hydroxide ions is constant.

Kw = [H+] * [OH-]

Where the square brackets signify molar concentrations and Kw is the dissociation constant for water. The value of Kw depends on temperature. For example, at 25°C Kw = 1.00 x 10-14 and

at 35°C Kw = 1.47 x 10-14.Acids and bases, when dissolved in water, simply alter the relative

amounts of H+ and OH- in solution. Acids increase the hydrogen ion concentration, and, because the product [H+] [OH-] must remain constant, acids decrease the hydroxide ion concentration. Bases have the opposite effect. They increase hydroxide ion concentration and

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decrease hydrogen ion concentration. For example, suppose an acid is added to water at 25°C and the acid raises the H+ concentration to 1.0 x 10-4 moles/litre. Because [H+] [OH-] must always equal 1.00 x 10-14, [OH-] will be 1.0 x 10-10 moles/litre. pH is another way of expressing the hydrogen ion concentration. Therefore, if the hydrogen ion concentration is 1.0 x 10-4 moles/litre, the pH is 4.00. The term neutral is often used in discussions about acids, bases, and pH. A neutral solution is one in which the hydrogen ion concentration exactly equals the hydroxide ion concentration. At 25°C, a neutral solution has pH 7.00. At 35°C, a neutral solution has pH 6.92. The common assertion that neutral solutions have pH 7 is not true. The statement is true only if the temperature is 25°C. [1]

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2.3. Capacitive sensing

The project intends to use the concept of capacitance to measure pH. If a dielectric polymer is applied onto the gold electrodes, pH change would induce changes in the volume (swelling & shrinking) or the permittivity of the polymer. This pH change is detected as a change in capacitance. The amount of charge that can be stored on a capacitor determines the capacitance. This capacitance is:

C = (ε0 εr A)/d

Where εr is the relative permittivity, ε0 is the vacuum permittivity, A is the area of the capacitor

and d the distance between the electrodes/plates. This capacitance changes depend upon the change in the dielectric constant of the material over the electrodes as the field changes according to the different media. In this project the change in the dielectric constant is basically due to the influence of different liquids. The field lines tend to change every solution being used which in turn changes the capacitance. Dielectric material (on the gold electrodes) sensitive to pH swells and shrinks, which changes the thickness and the permittivity of the polymer thus the capacitance. Capacitive sensing involves supplying the gold electrodes with an input voltage and measuring the changes in the capacitance when the sensor chip is exposed to different pH solutions. The general principle of a capacitor is shown in the Figure 1.

Figure 1: Physical model of the capacitive sensor with sandwiched structures

Figure 2: physical model of the capacitive sensor with interdigitated electrodes

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The non mobile charges that are formed as a consequence of pH i.e. COOH to COO- & H+ followed by a volume change which is induced depending on the hydrphobicity which induces charging and hydrophillicity induces discharging. In the case of the pH sensors we are developing, due to the in-plane interdigitated electrode structure the phenomenon of capacitance can be explained using the figure 2. When an input voltage is applied to the capacitor, charges (electrons) accumulate on the inner surface of the electrode. The oppositely charges (holes) in the polymer layer gets attracted to the charges that are accumulated on the gold electrodes, they can never recombine due to the difference in the energy gaps. These accumulated charges generate and electric field due to the presence of the dielectric material between them. These field lines change due to the change in the permittivity of the dielectric material due to absorption of different liquids. Depending on this new media formation when the sensor is dipped into different solutions the field lines tend to change accordingly. This changes the potential across the electrode which in turn changes the capacitance.

2.3.1. Interdigitated gold electrodes

Electrodes used in this project are made of gold material and they are not in the form of

parallel plates. Interdigitated electrodes have fringed structure which looks like fingers. The

figure 4 represents the gold interdgitated electrodes. These structures are being used as we

could not use the sandwiched structures for this project and they measure approximately the

same capacitance as the sandwiched structures. These gold electrodes can be easily printed

onto the substrate. Because interdigitated electrodes are used, the calculation and simulation

of the capacitance is not easy. For example, with interdigitated electrodes, the homogeneity of

the field is much less, compared to parallel plate capacitors, mainly because the height of the

fingers is much smaller than the width and the spacing of the fingers. This is illustrated in

Figure 3: There are methods to convert calculations on interdigitated electrodes to parallel

plate calculations, for example with conformal mapping. These interdigitated electrodes are

printed on glass substrate.

Figure 3: differences in field lines between various capacitor layouts

Figure 4: interdigitated gold electrodes with fringed structure used in this project

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Electrodes made of aluminium are easily attacked by humidity. This moisture problem can be resolved as mentioned in [9] using other methods but the cost effectiveness is reduced and higher power inputs would be required. The interdigitated structure with polymer on top is used for capacitive sensing because of its ease in constructing and integrating with the other electronics compared to the sandwiched structures. The simulations and calculations for the design of the electrodes and their fabrication are not part of this project.

2.4. Sensor design

2.4.1. Typical dimensions

Finger length L = 400 µm, finger width W = 0.5 µm, finger spacing g = 0.5 µm, spatial wavelength d = 2(g + w), height of the electrode he = 100 nm, thickness of the nitride layer hn = 50 nm, thickness of the polymer layer hp = ca. 2µm of PAA IOA, N=1000 (500 on each side). The base capacitance of the electrodes is dependent on ‘N’ which is the number of fingers.

Figure 5: Top view and cross-sectional view of the sensor chip

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2.4.2. Choosing electrode Geometry

An important parameter for the sensor is the ratio between the thickness of the polymer layer and the spacing of the fingers. When the spacing of the fingers plus the width of the fingers is equal to the thickness of the polymer, 95% of the electrical field lines are within the polymer [10] [11] [12]; stated differently, the ratio between the polymer thickness and the electrodes finger spacing has to be two, provided that the finger spacing equals the width of the fingers, as in this project. In that case, one can say that the ratio between layer thickness and spatial wavelength, or electrode periodicity, has to be a minimum of ½.

In figure X it is illustrated how electrical field lines look like with different spatial wavelengths.

When one has a fixed layer thickness, it is clear that with different ratios between this

thickness and the spatial wavelengths, more or less field lines are within the layer. This ratio is

important because the sensor for pH involves measurements in a liquid. Water has a dielectric

constant of 70-80 around room temperature. So this is a large difference compared to the

dielectric constant of air (= 1) or a polymer (= 3 to 4). Therefore it will have a very significant

contribution to the measurement when it is not guaranteed that most field lines are within the

polymer. If this requirement is met, the capacitance will mainly be dependent on the dielectric

constant of the polymer and analytic mixture within the polymer.

2.4.3. Electrode material

In this project, the electrodes are made of gold material, because of the high chemical stability of gold. However, aluminium is commonly used for electrodes but aluminium is known to be easily attacked by humidity. Because the sensors in some way will be exposed to different liquid environments, it is better to use a different material. There are other ways to solve this problem like reported in [9], where a sensor is reported without any contact between humidity and electrodes, because no sensing layer is required. But most of these solutions make the sensors difficult to integrate or cost too much power.

Figure 6: Field lines represented for different spatial wavelengths, at higher spatial wavelengths the ratio of

polymer thickness and spatial wavelength is less and the field lines tend to lie within the layer.

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2.4.4. Substrate material

The choice for glass was made because of its better isolating properties compared to silicon and SiO2. This reduces parasitic resistances and capacitances towards the substrate. Between the electrodes and the substrate, a polyimide insulator might give good results as is shown in [13] for humidity sensors on a silicon substrate. Glass has better isolating properties compared to silicon and other regularly used substrates. Glass is chemical resistant and temperature stable and usable for pH and humidity sensors, so glass is first choice for the sensors made at Imego.

2.5. Sensitive Polymer

The dielectric material used in this project is a polymer called poly (acrylic acid-co- isooctyl

acrylate). This is prepared by co-polymerizing AA acrylic acid with IOA isooctly acrylate. Not

much data is known about the polymer. Generally, the pH-responsive hydro gels contain two

monomer components: one is the pH-sensitive component, in this case AA, and the other one

is the pH inert component that is related to the mechanical properties of the hydro gel, in this

case IOA [14]. The pH dependency is caused by dissociation-induced swelling or shrinking of

the AA group under influence of the pH level. In [4] a change of 0.5% in mass is reported per

pH level in the resonance frequency measurements. Also salt dependence is reported. For a

polymer immersed in a solution, in which an acid group has been introduced, increasing salt

concentration or pH level results in polymer shrinking and a basic solution causes the polymer

to swell [2][14]. This dissociation occurs particularly in the presence of carboxyl-groups. The

whole principle of swelling and shrinking is driven by the difference in the osmotic pressure

between the polymer ions and solution ions and the ionisation of the groups in the polymer

which changes the hydrophobicity that causes swelling.

The effect of salt concentration on polymer swelling is dependent upon the swelling ratio of the

specific polymer. The swelling ratio is defined as the weight of the wet polymer, divided by the

weight of the dry polymer. PAA-IOA has a swelling ratio of 5.8, which indicates a relative large

water uptake. Due to this water uptake, the dielectric constant might increase also quite a lot,

just because of the water in the polymer. The dielectric constant of water is 70-80 and this is

quite high compared to polymers. Unfortunately, the exact dielectric constant of pAA-IOA is

unknown. PAA-IOA with a swelling ratio Q of 5.8, increasing salt concentration in relation to a

fully hydrated polymer results in polymer shrinking. Polymers with small Q values (around 1)

show no significant response. But polymers with high Q values (around 90) swell with

increasing salt concentration. This should indicate that beyond a certain degree of swelling the

osmotic pressure does not further affect the swelling process and inter-chain actions can be

ignored [2]. Furthermore, the swelling of depends upon the solution ionic strength but not the

ion species and the effect on swelling is more significant at low ion concentrations than at high

ion concentrations. The effect of salt concentration results is a non-linear response [2]. At this

point it is not exactly known, what the contribution is of the positive and negative salt ions in the

solution to swelling or shrinking.

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The response time for the polymer is dependent on several parameters such as layer

thickness and the porosity. It is expected the response time will be in the order of several

minutes, since for this polymer in [2] it is stated that the response time is about 5 to 7

minutes. However, this is for mass resonance frequency measurements with different

sensors, layers and fabrication processes, so an exact response time cannot be given. But at

least several minutes are expected to be required to make the solution diffuse into and interact

with the polymer.

2.6. Fabrication

The fabrication of the interdigitated capacitor relies on using a negative e-beam resist and dry etching a metal layer with an argon beam to form the fingers as well as the pads in one step. In general, the thinner the metal layer, the smaller it is possible to make the structures. This is due to the relatively poor etching edge of the ion milling of metals, which limits to minimum feature size to roughly the etched layer thickness. More details can be found from the report written by Bertjan.

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2.6.1. Polymer Deposition

The polymer is deposited onto the electrodes first by spin coating as shown in figures 7a, 7b &

8a, 8b and then drop pipeting the polymer drop by drop onto the spun layer of polymer. A

polymer layer of 1µm was deposited by spin coating and 1µm with drop pipetting. Totally seven

sets of chips have been used and it is explained in the next section how the polymer was

deposited and what was the initial thickness of the polymer layer and which sets of chips have

been used for further measurements.

Set- I: These chips were spun with the polymer dissolved in ether: hexane 1:1 ratio solvent at 3000rpm. The thickness was measured after drying the sensor chips in vacuum oven at 80°C for 90min, using the Tencor Profiler. A very thin layer of polymer was deposited onto the electrodes which wouldn’t be of much use for the project. A second layer of polymer was spin coated onto the polymer layer formed already. This polymer was dissolved in a different solvent called Tetrahydrofuran (THF) overnight. With this solvent the polymer formed a sticky thick jelly that was then spun onto the electrodes with the thinner layer of polymer at 2000rpm. The chips were left for baking for 1hour in a vacuum oven at 80°C and then the thickness was measured. A relatively thick 1µm layer of polymer was estimated by the measurements in some parts on the electrode surface.

These chips were taken onto the probe station measuring device, a very low voltage of 100mV were set at a frequency of 10 KHz was applied and the probes were connected to the pads of the gold electrodes. The capacitance on the left chip on the first set was found to be 10.8pF and then the right chip was connected and then the capacitance was found to be in µ Farads. The sensor chips should be measuring a capacitance around 10pF so the left chip was considered for the further measurements from the first set.

Figure 7a: The glass chip (gray) supports the gold (red) pads and interdigitated capacitor structure.

Figure 7b: The pads are covered by blue tape to prevent the resist from covering them during spinning.

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Set II: These chips were spun with the polymer dissolved in Ether: hexane solution in the ratio of 1:1 at 3000rpm. The thickness was measured after drying using the Tencor profiler. A very thin layer of polymer was deposited onto the electrodes which wouldn’t be of much use for the project. Another layer of polymer was deposited onto the electrodes by dissolving the polymer in a different solvent called Tetrahydrofuran (THF) overnight.

This polymer with the solution formed a sticky thick jelly; it was then mixed with more solvent and was made into a loose jelly which was then spun onto the gold electrodes at 2000rpm. The chips were left for baking for 90mins in a vacuum oven at 80°C and then the thickness was measured. A relatively thick 500nm layer of polymer was estimated by the measurements at least in some parts on the electrodes surface. This set had three chips on it and the capacitance was tested for all the three. First chips showed a value of 15.48pF and the others -350nF and 100pF. So only the first chip was considered for the further measurements.

Set III: These chips were spun with the polymer dissolved in Ether: hexane solution in the ratio of 1:1 at 3000rpm. The thickness was measured after drying using the Tencor profiler. A very thin layer of polymer was deposited onto the electrodes which wouldn’t be of much use for the project. Then the polymer solution was pipetted 10 times drop by drop onto the chip and then was baked for 30mins to evaporate the solvent. A relatively thicker and sticky layer was formed and it was not possible to measure the thickness as the sticky nature made the polymer made the chip stick to the measuring needle. But a visibly thick layer was formed by this technique. This set had two chips out of which one of them showed 15.6pF and the other 80µF, the first chip was considered for further measurements from this set.

Set IV: These chips were spun with the polymer dissolved in Ether: hexane solution in the ratio of 1:1 at 3000 rpm. The thickness was measured after drying using the Tencor Profiler. A very thin layer of polymer was deposited onto the electrodes, too thin for use in the project.

Figure 8a: The chip is placed on a 2”chuck (for the spinner) that has a small hole in the middle. The

fingers should be on one side of the hole and the tape on the opposite side of the hole.

Figure 8b: The chip after removing the blue tape. The PAA IOA in this figure is orange in colour.

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Set IX: These electrodes were insulated with a 50nm layer of Silicon Nitride to avoid corrosive effects if any. These chips were spun with the polymer dissolved in Ether: hexane solution in the ratio of 1:1 at 3000rpm. The thickness was measured after drying using the Tencor profiler. A very thin layer of polymer was deposited onto the electrodes, too thin for use in the project.

Set X: These electrodes were insulated with a 50nm layer of Silicon Nitride to avoid corrosive effects if any. These chips were spun with the polymer dissolved in Ether: hexane solution in the ratio of 1:1 at 3000 rpm. The thickness was measured after drying using the Tencor profiler. A very thin layer of polymer was deposited onto the electrodes, too thin for use in the project.

Set XI: These electrodes were insulated with a 200nm layer of Silicon Nitride to avoid corrosive effects if any. These chips were spun with the polymer dissolved in Ether: hexane solution in the ratio of 1:1 at 3000rpm. The thickness was measured after drying using the Tencor profiler. A very thin layer of polymer was deposited onto the electrodes which wouldn’t be of much use for the project. Another layer of polymer was deposited onto the electrodes by dissolving the polymer in a different solvent called Tetrahydrofuran (THF) overnight. This polymer with the solution formed a sticky thick jelly that was then spun onto the gold electrodes at 2000rpm. The chips were left for baking for 90mins in a vacuum oven at 80°C and then the thickness was measured. A relatively thick 1µm layer of polymer was estimated by the measurements in some parts on the electrode surface. This set had two chips which showed 10µF and 14.8pF. Only one chip was considered for further measurements.

The Sensor chips after drying were analysed under the probe-station microscope to determine the resultant capacitance. The electrode pads were scratched with the probes so that the nitride layer was taken off to make contact with the gold material for the measurements. Finally only four sets of Sensor chips were selected with thicker polymer layers i.e. Ist, IInd, III rd and the XIth set of chips as per the requirements of the Intellisense project requirements.

2.6.2. Contacts for the electrodes

The chip that was found to be in a good working condition (having a capacitance range of 10 to 20 pF) was equipped with 2mm contacts for quick connections to the measurement setup. Contacts were glued onto the 2mm gold pads using conducting glue Ecobond CE850. It was really tricky to glue the contacts as they were in the micro metre range. This conducting glue is a one component, low stress, electrically conductive modified epoxy adhesive. This glue was taken out from the freezer and was kept at room temperature for 2 hours before usage. Once the conducting glue is melted it is carefully applied to the contacts with needle of a syringe of 0.7*50mm dimension. Glue was applied on the contact and then without disturbing the electrodes the contacts have been pressed onto the gold pads. The contacts have been examined with the Multi-meter for short-circuiting due to the glue. The chips were placed in the Convection-Oven and are heated for 2 hours at 110˚C. The mechanical sticking strength of the contacts to the substrate has been improved by applying a special glob top material called Amicon S 7503 which is commonly used for glob top applications. After applying the glue these chips were baked in the convection oven for 90 minutes at 120˚C. Once the contacts were dry it was clearly visible that the contacts were glued properly to the electrodes and then they were tested for any short circuiting.

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3. Measurement Setup for pH-Sensor

3.1. Measurement Setup Description The Sensor chip is fit to a plastic holder which contains electrical contacts and allows also for mechanical stability (see Figure 9a). BNC (Bayonet Neill Concelman) cables from the LCR meter (LCR Meter is electronic test equipment used to measure the impedance (inductance, capacitance, resistance) of a component) were connected to the 2mm socket of a PCB card by soldering. The PCB (Printed Circuit Board) card was filed smoothly to fit in the plastic holder. At one end of the PCB the socket was soldered to have a good mechanical support. The holder is a plastic Centrifuge tube which was cut at the bottom tip with a blade to make up the space for the socket to expose from the holder. This holder gave a reasonably good grip to hold the chip to dip into the liquids without causing much movement, it was very flexible to plug in and plug out the chips to the socket. But later the plug damaged the glue in the contacts.

Figure 9a: Measurement dip setup for capacitance

measurements

21

Figure 9b: Photo of the measurement dip set-up showing the sensor chip, the plastic holder and

the sample tube.

The different pH buffers pH-6, pH-7 and pH-8 were prepared (see below Section 3.3) and tested with a Standard pH meter This buffer was filled into smaller Eppendorf tubes with lids for further usage. These sample tubes were fixed to a plastic holder for stability that is placed on a mechanical lift for easy adjustment of the height to immerse the sensor in the liquid sample. This arrangement of moving the sample tube upwards helps to minimize any disturbances that might influence the measurements.

A HP 4284A Precision LCR meter is connected with a GPIB bus to a Laptop with interface software in the form of a Lab View program. This software helps in recording the capacitances for set sample time and different intervals of time. The LCR meter is a device that measures the unknown capacitance from the sensor. This LCR meter is constructed internally with a comparator circuit which helps to find the unknown capacitance of the sensor chips.

One electrode of the sensor is connected to the high potential and high current points in the LCR meter and the other electrode is connected to the low potential and the low current points onto the LCR meter. The GPIB address of the LCR meter was extracted onto the interface program, and then the computer started talking and listening to the LCR meter.

The LCR meter was calibrated before starting the measurements. The open and short calibrating was done and then the parallel capacitance (Cp) and parallel resistance (Rp) mode was calibrated. The parallel modelling is less complex than the series model so we tried to use this measurement model. The mode of the measurements was changed to series capacitance and series resistance and the frequency was adjusted to 10 KHz and the voltage was set to 0.1V. Initial measurements were done using the known capacitances, and then the sensor

22

chips have been used to check the capacitances. Chip I, II and III showed negative reactive component in the reading while chip XI failed to show any reactance, so chip XI was taken out from further investigation. The other chips have shown high capacitance and lower layer thickness so they were not considered. The negative reactance indicates the presence of capacitive component in the ‘R+JX’ impedance measurement. Where ‘X’ is the reactive component and in this sensor it is ‘Xc’ which is the capacitive reactance.

Figure 9c: Photo of the sensor chip and electrical connections covered by glob top

Figure 9c represents the sensor chip which is plugged into the above measurement setup. The interdigitated gold electrodes are printed on the glass substrate. Then a layer of polymer is deposited over these electrodes using the above mentioned techniques of polymer deposition in section 2.6.1, section 3.2. These micro level electrodes are connected with 2mm gold contacts by gluing the electrode pads to one end of the gold contact using conducting glue first and then a glob top material. The 2mm gold contacts are glued to the electrodes with conducting glue called Ecobond CE850. It was really tricky to glue the contacts as they were in the ‘µm’ range. This conducting glue is a one component, low stress, electrically conductive modified epoxy adhesive. This glue was taken out from the freezer and was kept at room temperature for 2 hours before usage. Once the conducting glue is melted it is carefully applied to the contacts with needle of a syringe of 0.7*50mm dimension. Glue was applied on the contact and then without disturbing the electrodes the contacts have been pressed onto the gold pads. The contacts have been examined with the Multi-meter for short-circuiting due to the glue. The chips are placed in the Convection-Oven and are heated for 2 hours at 110˚C. The mechanical sticking strength of the contacts to the substrate has been improved by applying a special glob top material called Amicon S 7503 which is usually used in the glob top applications. After applying the glue these chips were baked in the convection oven for 90 minutes at 120˚C. Once the contacts are dry it can be seen that the contacts were glued properly to the electrodes and then they were tested for any short circuiting. There have been some problems with the glues used because of their reactive nature with the solvents being used. A recommendation has been made later at the end of the report to change the glues that were used in this project.

23

3.2. Polymer deposition on the sensor chip

With a Polymer thickness to the spatial wavelength ratio of 1 and more, almost all the field lines are expected to lie within the polymer. Out of all the chips, Set III showed convincingly low capacitance. These chips were spun with the polymer dissolved in Ether: hexane solution in the ratio of 1:1 at 3000rpm as shown in the Figures 7a, 7b and Figures 8a, 8b. The chips were left for baking for 90mins in a vacuum oven at 80°C and then the thickness was measured. The thickness was measured after drying using the Tencor profiler. A very thin layer of polymer was deposited onto the electrodes which wouldn’t be of much use for the project. Then the polymer solution was pipetted 10 times drop by drop onto the chip and then was baked for 30mins to solidify the wet polymer. A relatively thicker and sticky layer was formed and it was not possible to measure the thickness as the sticky nature made the polymer made the chip stick to the measuring needle. But visibly thick layer was formed by this technique. This set had two chips out of which one of them showed 15.6pF and the other 80µF, the first chip was considered for further measurements from this set. Only 3 chips out of 7 chips worked in the required range for the Intellisense project.

Figure 10a: High Resolution Microscopic view of the polymer layer deposited over the gold electrodes

Figure 10b: Low Resolution Microscopic view of the polymer layer deposited over the gold electrodes

24

Figure 10a shows the polymer layer deposited on the electrodes especially near the figure like structure. This is a high resolution microscopic picture of the polymer layer which was first spun onto the electrodes and then the other layer of polymer was deposited using the polymer processed with Ether: Hexane solvent. We can clearly see that the whole gold material of the fingers is covered with the polymer, but the only drawback being its non uniformity in the layer formation after drying. Similarly Figure 10b shows a lower resolution microscopic picture is taken from the same microscope but with a low resolution lens.

Solvents being used for polymer processing are: Tetrahydrofuran: 50mg of the polymer was dissolved in 5ml of the solvent and was left to dissolve overnight. A visibly sticky Jelly kind of polymer was formed when a drop of processed polymer was dropped on a sample plate and kept in a vacuum chamber overnight until the solvent evaporates. N-Hexane: 50mg of the polymer was dissolved in 5ml of the solvent and was left to dissolve overnight. Relatively thin layer of polymer layer was formed when a drop of processed polymer was dropped on a sample plate and kept in a vacuum chamber overnight until the solvent evaporates. Ether: Hexane 1:1: Higher volatility gives quicker evaporation. 50 mg of the polymer was dissolved in 5ml of the solvent and was left to dissolve overnight. A visibly thick layer of polymer layer was formed when a drop of processed polymer was dropped on a sample plate and kept in a vacuum chamber overnight until the solvent evaporates. DMSO: Less volatility, harder it would be to evaporate all the solvent. 50 mg of the polymer was dissolved in 5ml of the solvent and was left to dissolve overnight. A non uniform layer with a lot of left over solvent is formed even after evaporating the solvent. The solvent doesn’t evaporate totally.

3.3. Preparation of the buffered pH Solutions

13.8grams of sodium phosphate in was dissolved in 0.5l of pure water to prepare a 100mM mono salt component. 7.2grams of sodium hydrogen phosphate is dissolved in 0.5l of pure water to prepare a 100mM Di salt component. The measured pH has large divergence from pH6, 7, and 8. If the pH is in the correct range the curves can be easily modified. We are actually looking for the principle. Preparation of 50mM (milli Molar) pH solutions: PH-6: 44ml of the mono salt is mixed with 6.15ml of di salt and then the solution is stirred well and mixed with 50ml of pure water. Then the pH of the solution has been checked on the PHM210 Standard PH Meter by Radiometer Copenhagen. It indicated a pH of 5.65.

PH-7: 19.5ml of the mono salt is mixed with 30.5ml of di salt and then the solution is stirred well and mixed with 50ml of pure water. Then the pH of the solution has been checked on the PHM210 Standard PH Meter by Radiometer Copenhagen. It indicated a pH of 6.68.

PH-8: 5ml of the mono salt is mixed with 47.35ml of di salt and then the solution is stirred well and mixed with 50ml of pure water. Then the pH of the solution has been checked on the PHM210 Standard pH-Meter by Radiometer Copenhagen. It indicated a pH of 7.71.

The calculated (Na+) concentration was: pH6 ≈ 28mM, pH7 ≈ 40mM and pH8 ≈ 48mM.

25

4. Measurements & Results

4.1. Measurement techniques employed in the project

All the measurements have been done with the LCR meter and the results are stored in the form of LAB View data. The LCR Meter is always calibrated for open and short circuit connections before starting any measurements. Several measurements have been done to analyze the polymer behaviour as described below:

Phase 1: These measurements have been done on different chips with different layer thicknesses. The thickness and the deposition techniques have been discussed in the section 3.6.1 about polymer deposition. These measurements have been done at various frequencies and voltages to look for the optimal combination of voltage and frequency for further measurements. We could see that distinct results could be obtained at the input voltages of 0.1V and a frequency of 10 KHz and 1 MHz.

Phase 2: As it was really hard to judge the exact phenomenon going on in the polymer, simultaneous CPRP and CSRS measurements were done manually. The results of these measurements are presented in the results section 5.2.2 in the form of tabular columns (Table 2). These measurements have been done by tabulating the CPRP and CSRS for different chips at 2 frequencies (10 KHz & 100 KHz) at V = 0.1V, 1V. These measurements have been done by interchanging water with different pH solutions and some of the measurements were done by interchanging water and pH6 solution in a series of 10 cycles.

Phase3: Measurements were done on the new electrodes with no nitride and with 50nm, 100nm, 200 nm thick silicon nitride (deposited by PEVCD). A CPRP measurement was done before the polymer layer was deposited. These measurements were also done at 2 different frequencies 10 KHz and 1 MHz. The resistance and capacitance through air, liquid, substrate and the polymer are the basic unknowns. The capacitance and resistance through the substrate, air and liquids can be measured collectively by the capacitance CPRP measurements on the LCR meter by doing the measurements on the naked electrodes. These measurements were done with an input voltage of 0.1V. Then by repeating the same set of measurements after depositing the polymer one can almost judge the change in the capacitance due to the polymer. To get a better base for the modelling, the measurements have been done without the polymer at the beginning and later we intended to deposit the polymer and to conduct the same measurements.

26

4.2. Results & Discussions

Polymers generally used as dielectrics have very high resistivity and they always exist as insulator in the ground state. When such polymers are exposed to the electric field, the electric charges in the polymer along with the permanent and induced dipoles move making the material polar. Usually the dielectric constant characterizes the dielectric properties. When an A.C field is applied then the whole dielectric constant is a complex combination of the dielectric constant which is the real part and the dissipation factor which is the complex part. Dissipation factor has greater influence on the magnitude of dielectric constant at higher frequencies.

The dielectric constant basically depends on the polar charges and the degree of mobility of

the charges. As mobility is temperature dependant dielectric constant is also temperature

dependant. Since polarization is time dependant frequency plays an important role. [15] [16].

Based on this theory the results are being discussed.

27

4.2.1. Phase1 In this phase measurements have been done on various sensor chips. The best of the results

in the case of Set III chips with approximately 2µm polymer applied by spinning and pipetting

are being discussed in this phase. The signal from the other chips was not reproducible, or the

sensor didn’t work after few measurements. In this section we are going to discuss the results

from the measurements on the chips that have been subjected to different pH solutions for

different time periods and the corresponding changes in the sensor responses.

Figure 11 shows the capacitance curve and resistance curve for 2µm PAA IOA layer sensor

chip at 1 MHz and 0.1V without nitride. The sequence of treatment is air-water-pH6-water-

pH7-water-pH8 three times continuously, with a 5minutes time period for each stage. Changes

of capacitance and resistances are visible in this graph due to pH variation (change in the ionic

mobility due to the swelling/ shrinking phenomenon and the number of ions depending on the

pH). The resistance decreases drastically the dry polymer is dipped in the water due to the flow

of ions into the polymer layer which increases the number of ions in the polymer. The drift in

the signal may be due to the polymer hydration (perhaps the polymer did not reach ‘hydration’

equilibrium). Comparing the magnitudes of the changes in the capacitances and resistances

we cannot thoroughly say that for a given pH this would have reflected on a given capacitance

or resistance values. This has to be due to the drift seen in the capacitance curve. For water

sequence, the change in capacitance is dependent on the ions that remained from the

previous solution used.

Figure 11: Capacitance (blue) & resistance (pink) curves of 2µm PAA IOA layer sensor chip at 1 MHz and

0.1V without nitride. The sequence of treatment is air-water-pH6-water-pH7-water-pH8 three times

continuously, with a 5minutes time period for each stage. This measurement was done when the polymer

was initially in a dry state, when the chip was left to dry over a period of a 7 days.

28

Figure 12 represents the capacitance and the resistance curves of the same chip as used in

figure 11 the differences are the frequency of 10 KHz, its time period and air and water are

used as an intermediate stage. The polymer was already wet which means that it has been

through a set of experiments and was not specially dried before the measurement, just let dry

in air overnight. The capacitance values are higher than for those in Figure 11 which almost

proves that the dielectric used here is a polar type where the dipoles have enough time to align

with the field lines. With other words, the lower capacitance values at 1 MHz (figure 11)

indicate that the permanent (dipoles in the polymer) and the induced dipoles (due to the

various liquids) do not align with the field lines due to the less time available at higher

frequency. The water absorbing nature of the polymer also indicates that it can be a polar

polymer. The capacitance of the chip in air is around 18pF; water is around 230pF at the

beginning, for pH6 it is around 100pF, and water come to almost the same level if the drift is

not considered, around 60pF for pH7 and 140pF for pH8.

Figure 12: Capacitance (blue) & resistance (pink) curves of 2µm PAA IOA layer sensor chip at 10 KHz and 0.1V

without nitride. The sequence of treatment is air-water-pH6-water-air-pH7-water-air-pH8- water-air, with a 2

minutes time period for each stage. This measurement started when the polymer was not in the perfectly dry state.

If we measure the decrease in capacitance from pH6 to water is 60pF, for pH7 to water is

around 25pF and for pH8 to water is 80pF. We can probably use the decrease in the

capacitance value from this graph to estimate the pH influence. The R values are quite low

which may suggest the polymer may not be perfectly dry. Negligible change in R can be

observed with pH solutions and slightly decrease of R when exposed to air.

29

Figure 13 represents the capacitance and the resistance curves of the same chip used in

figure 11 measurements, the difference being the time period and the frequency of 500 KHz.

If we compare this capacitance curve with the others in Figure 11 and Figure 12 it is clearly

evident that the capacitance values lie between the other two, and it depends on the frequency.

So it has lower capacitance compared to figure 12 and higher capacitance compared to

Figure 11 which indicates the time is required for the dipoles to align with the field lines. In this

case we could see good changes in the resistance curve with pH compared to the previous

graphs. The resistance curve seems to be linearly decreasing with the increase in the pH

values. The decrease in the resistance is around 20Ω for pH6, 40Ω for pH7 and 60Ω for pH8.

The capacitance curve also seems to be linear if the drift is not taken into account. If we

consider the increase in the capacitance for pH6 is around 30pF, for pH7 is 40pF and for pH8

is 50 but the slopes of increase are different. We find the decrease in capacitance from pH to

water in this case we see that for pH6 it is around 25pF, for pH7 around 20pF and for pH8 it

cannot be seen. The drift in the capacitance curve is making it really difficult to measure the

capacitance changes.

From figure 13 we can see that we are quite far away from the equilibrium state of the polymer. This is because: Observing the first slope for water we see that it is steeper than the second and the third slopes due to water where the slope gets flatter. For pH8 the slope is very steep and we don’t see a stable value. After pH8 we don’t see that we have totally got rid of the pH8 buffer and we also observe that the resistance is low.

Figure 13: Capacitance (blue) & resistance (pink) curves of 2µm PAA IOA layer sensor chip at 500 KHz

and 0.1V without nitride. The sequence of treatment is air- water-air- pH6- water-air-pH7-water-air-pH8-

water, with a 2 minutes time period for each stage. This measurement started when the polymer was not in

the perfectly dry state.

30

Summary from Phase 1 results:

• The influence of pH is clearly visible but the degree of other ions influence is not clear.

• There is no need to wash the electrodes in between pH solutions (not much difference on pH behaviour by using air and water as intermediates).

• Resistance trend understandable but values are not.

• Interchanging the sequence shows fairly consistent changes.

• The”drift” in the signal is always visible as the hydration of polymer is not at equilibrium.

31

4.2.2. Phase2

These measurements have been done manually so the readings have to be tabulated in

column form (see Table below). The frequency has been changed manually for every stage and

the capacitance and resistance values have been recorded. Some of the results have been

included in the Appendix in the section 9.2.

Table1: Capacitance & resistance responses of 2µm PAA IOA layer sensor chip at 10 KHz and 100 K Hz

and 0.1V. This measurement was done when the polymer was not perfectly in the dry state.

Environment

CP

RP

CS

RS

10KHz

100KHz

10KHz

100KHz

10KHz

100KHz

10KHz

100KHz

AIR

13.8pF

11.22pF

462.82Ω

462.68Ω

85.24µF

1.05µF

462.75Ω

462.72Ω

WATER

286.12pF

75.62pF

461.04Ω

452.58Ω

4.24µF

143.5nF

460.98Ω

451.56Ω

pH6

674.12pF

215.31pF

458.57Ω

433.78Ω

1.72µF

62.75nF 458.22Ω

432.78Ω

WATER

308.22pF

100.56pF

461.06Ω

449.76Ω

3.82µF

127.5nF

460.81Ω

450.12Ω

pH6

820.86pF

229.08pF

456.83Ω

429.25Ω

1.43µF

60.04nF

456.27Ω

427.96Ω

WATER

282.62pF

93.43pF

461.29Ω

451.98Ω

4.16µF

134.9nF

461.22Ω

451.68Ω

Ph6

504.22pF

172.12pF

459.87Ω

442.99Ω

2.31µF

75nF

459.68Ω

442.04Ω

WATER

280.32pF

90.22pF

461.30Ω

451.98Ω

4.15µF

140.1nF

461.24Ω

452.26Ω

Ph6

526.79pF

174.35pF

459.60Ω

441.82Ω

2.23µF

74.89nF

459.37Ω

441:132Ω

WATER

260.20pF

83.67pF

461.25Ω

453.28Ω

4.53µF

149nF

461.23Ω

453.22Ω

Ph6

438.56pF

138.08pF

460.45Ω

446.45Ω

2.70µF

92.08nF

460.34Ω

445.82Ω

WATER

263.38pF

81.39pF

461.21Ω

452.97Ω

4.43µF

153.6nF

461:18Ω

453.31Ω

Ph6

403.32pF

119.86pF

460.14Ω

447.44Ω

2.94µF

105.5nF

459.97Ω

446.95Ω

WATER

242.17pF

78.5pF

461:16Ω

454.10Ω

4.84 µF

159.9nF

461:16Ω

454.18Ω

Ph6

383.72pF

114.53pF

460.01Ω

448.01Ω

3.09µF

110.2nF

459.92Ω

447.60Ω

WATER

250.32pF

78.06pF

461.22Ω

453.69Ω

4.63µF

161.3nF

461:14Ω

453.91Ω

Ph6

508.40pF

170.81pF

458.96Ω

440.12Ω

2.20µF

78.36nF

458.77Ω

439.86Ω

WATER

247.93pF

77.15pF

461:12Ω

453.56Ω

4.68µF

165.3nF

461.07Ω

453.87Ω

Ph6

380.22pF

114.15pF

460.07Ω

448.57Ω

3.15µF

110.6nF

460.01Ω

448.16Ω

WATER

236.10pF

75.29pF

461:14Ω

454.28Ω

4.85µF

169.2nF

460Ω

454.43Ω

Ph6

371.93pF

111.40pF

460.12Ω

448.81Ω

3.20µF

112.8nF

460.02Ω

448.42Ω

WATER

233.22pF

74.42pF

460.81Ω

454.08Ω

4.95µF

174.8nF

460.72Ω

454.34Ω

32

Figure 14a represents the capacitance and the resistance curves formed by using the data

from Table 1, where the measurement is done at a frequency of 10 KHz by interchanging

water and pH 6 for 10 times to see the pH influence. Figure 14a shows the parallel C and R.

We can see the drift in the capacitance curve however, after some time it looks that may get a

little more stable. The capacitance on average due to water is around 230pF. We may

consider the capacitance change pH6 to water around 200pF (on the more stable behaviour).

So if one could repeat the same measurement with various pH solutions one could judge the

capacitance change relative to the other pH solutions. Almost similar changes in capacitance

are seen for pH 6 at 10 KHz as in the previous case (Figure 12). We can see big changes in

the capacitance at the beginning but later we see uniform changes in the capacitance values.

This may be due to the time is taken by the polymer to hydrate uniformly. The R value are again

quite low but consistent with those in figures 12 and 13, since it is the same chip, and perhaps

is the same reason, the polymer may not be perfectly dry. Very small change in R can be

observed with pH solution but somehow follows the corresponding changes in C.

Figure 14a: Parallel capacitance (blue) & resistance (pink) curve of 2µm PAA IOA layer sensor chip at

10 KHz and 0.1V. The sequence of treatment is air-(water-pH6, 10 times interchanging), with a 1minute

time period of each stage. This measurement was done when the polymer was not perfectly in the dry state.

These curves are constructed using the data in Table2.

33

Figure 14b represents the capacitance curve formed by using the data from Table 1, where

the measurement is done at a frequency of 10 KHz by interchanging water and pH 6 for 10

times to see the pH influence. Figure 14b represents the trend in the changes in the parallel

capacitance. We can see the drift in the capacitance curve but in an opposite direction

compared to the parallel modelling. The capacitance on average due to water is around 4µF.

The capacitance change pH6 to water around 2µF but in the other way compared to the

parallel modelling. We can see that almost all the time changes in the capacitance values

remain the same.

Figure 14b: Series model of C and R for the same sequence of measurements as in Figure 14a.

The series capacitance values are very high compared to the parallel one. The Cs variation pH

to water looks more stable than for Cp. However, Cs cannot be used for IntelliSense project

since the interface electronics can handle just few pF values.

34

Figure 15a represents the capacitance and the resistance curves formed by using the data

from Table 1 at 100 KHz by interchanging water and pH 6 for 10 times to see the changes.

From Figure 15a, we can see the drift in the parallel capacitance curve but over a period time

it is more stable. The change in capacitance due to water on an average is around 80pF. We

may consider the increase in capacitance from water to pH of around 60pF on the average.

The capacitance range is comparatively low from the previous case due to the higher

frequency which has to do with the dipole alignments in the polymer. We can see that we get

rough changes in the capacitance at the beginning but later we almost have more reproducible

changes in the capacitance values.

So the preliminary values cannot be considered as time is taken by the polymer to hydrate

uniformly because the polymer needs more time to get to a state of equilibrium. The changes

in resistance are small but bigger than for 10 kHz, and also follow the corresponding changes

in C. The increase in R changes with frequency is consistent with those observed in figures 12

and 13. However, R should not depend on frequency. Moreover the higher the frequency the

less is the taken time for ions to travel into the polymer.

Figure 15a: Parallel capacitance (blue) & resistance (pink) curve of 2µm PAA IOA layered sensor chip

at 100 KHz and 0.1V. The sequence of treatment is air-(water-pH6, repeated 10 times), with a 3minute

time period of each stage. This measurement was done when the polymer was not perfectly in the dry

state. These curves are constructed using the data in Table2.

35

Figure 15b represents the capacitance curve formed by using the data from Table 1 at 100

KHz by interchanging water and pH 6 for 10 times to see the changes. From Figure 15b, we

can see the drift in the series capacitance curve but over a period time but in an opposite way

compared to the parallel capacitance. The series capacitance due to water on an average is

around 80nF. The change in capacitance from water to pH is around 60nF. The capacitance

range is comparatively low from the previous case due to the higher frequency which has to do

with the dipole alignments in the polymer. We can see that we get rough changes in the

capacitance at the beginning but later we almost have the similar changes in the capacitance

values. So the preliminary values cannot be considered as time is taken by the polymer to

hydrate uniformly. However, if we compare the series model for capacitance at 10 KHz and

100 KHz, we see a drastic change in the capacitance. It is rather increasing compared to the

parallel modelling and the increase is by almost 1000 units which is irrelevant. The series

capacitance values are very high compared to the parallel one. The Cs variation pH to water

looks more stable than for Cp. However, Cs cannot be used for IntelliSense project since the

interface electronics can handle just few pF values.

Figure 15b: Series C and R for the same measurements as in Figure 15a.


• Good change range of Cp for both frequencies 10 and 100 KHz.

• Interchanging of water and pH6 curve: C variation for pH to water decreases.

• From Cs - Rs measurements the signal looks nicer and different models in LCR meter are used thus some extra information for (Cp-Rp) can be found.

36

4.2.3. Phase3 These measurements have been made automatically at 10 KHz, 1 MHz, input voltage of 0.1V

and in the sequence mentioned in section 5.1:1. In this phase we measured on new set of

chips that have the same geometry as the ones used for the previous measurements but with

no polymer layer to understand better the changes due to the liquids. The chips used are of

four different types 1) without SiN 2) 50nm SiN layer3) 100nm SiN layer and 4) 200nm SiN

layer. The basic use of nitride layer in this project is to insulate the gold material from liquids.

The available chips with bare Au electrodes and with 100 nm SiN have unfortunately very low

R, 100 Ω, and 370 Ω, respectively that suggests electrodes short circuit or other processing

fault. The capacitance behaviour of these chips is not trustful and therefore it is not presented

here. For the chips with bare electrodes, it could be seen their deterioration after many

experiments (change in colour and removal from the glass substrate).

Figure 16 corresponds to the responses from the 50nm SiN layer on gold electrodes without

polymer. These measurements have been done at a frequency of 10 KHz and 0.1V. The base

capacitance of this chip in air is around 5 pF; the capacitance change due to water is 55 pF

and increases slightly during measurements. It can be clearly seen in the resistance curve that

the resistance decreases drastically when the sensor is dipped in water in the beginning, but

stays in the M values, thus we may assume that the SiN layer is of quite good quality (it does

not have many porous).It is strange to see that the resistance decreases for water and not for

pH solutions.

0

20

40

60

80

100

120

140

160

180

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

Time(Minutes)

Cap

acit

ance

(pF

)

5,00E+05

2,50E+06

4,50E+06

6,50E+06

8,50E+06

1,05E+07

1,25E+07

1,45E+07

6

a

w

7

86

7

8 6 7

8

w w w w w w w w w

Res

ista

nce

(Ω

)

CAPACITANCE CURVE

RESISTANCE CURVE

Figure 16: Capacitance (blue) & resistance (pink) curve of 50nm SiN layer on gold electrodes of the

chip without polymer at 10 KHz and 0.1V. The sequence of treatment is air-water-pH6-water-pH7-

water-pH8-water repeating for 3 times, with a 5minutes time period of each stage.

37

Figure 17 corresponds to the responses from the 50nm SiN layered chips over gold

electrodes without polymer at 1 MHz The base capacitance in air of this chip is around 12pF

as the chip used is a different one as the chip used previously was damaged due to short

circuit connections, the capacitance change due to water is 40 pF decreasing slightly after 30

min of measurements and staying stable after that. These capacitance values are quite similar

to the ones measured at 10 KHz. When comparing the observed changes in capacitances at

different frequencies for electrodes with and without polymer, we find that measurements

performed on electrodes with polymer are strongly dependant on frequency, while the

measurements on electrodes without polymer are independent of frequency. This might also

be a proof to say that the polymer has polar nature. The resistance curve follows a more

expected behaviour, decrease when pH solutions are used. This figure clearly shows the

changes in the behaviour of the field lines due to changes in the dielectric media. The field lines

tend to behave in a different way for every test solution which in turn would change the

potential around the sensor and this would correspondingly influence the value of capacitance.

One could also observe that visibly similar changes are seen in the values of capacitance when

the measurements are repeated over time. Talking about the changes in resistance they

remain almost the same in all the cases. We observe changes in capacitance when pH is

changed which might be an effect of pH directly on the nitride and also the pin holes play a role.

0

20

40

60

80

100

120

140

0 10 20 30 40 50 60 70 80 90 100

Time(Minutes)

Cap

acit

ance

(pF

)

1100

1105

1110

1115

1120

1125

Res

ista

nce

(Ω

)

a

w

6 7

8

w w w w w ww

ww

6

78

67

8

CAPACITANCE CURVE

RESISTANCE CURVE

Figure 17: Capacitance (blue) & resistance (pink) curve of 50nm SiN layer on gold electrodes of the chip

without polymer at 1 MHz and 0.1V. The sequence of treatment is air-water-(pH6-water-pH7-water-

pH8-water repeating for 3 times), with a 5minutes time period of each stage.

38

Figure 18 corresponds to the responses from the 200nm SiN layered chips over gold

electrodes without polymer at 10 KHz The base capacitance in air of this chip is around 12pF

as the chip used is a different one as the chip used previously was damaged due to short

circuit connections, the capacitance change due to water is 40 pF decreasing slightly after 30

min of measurements and staying stable after that. These capacitance values are quite similar

to the ones measured at 10 KHz. This might also be a proof to say that the polymer has polar

nature. The resistance curve follows a more expected behaviour as it decreases when pH

solutions are used. We could clearly see the resistance curves are not smoother like the ones

in the previous cases this has to do mainly with the gluing of the contacts, if the conducting

glue loses its hold on the gold electrodes it probably shows the behaviour as seen. This would

also a reason to see minimal changes in the resistances when the sensor is dipped in different

solutions. The base capacitance of the chip used here is around 12pF. We can also observe

that the capacitance in water is quite low compared to the one used in Figure 19 with a 50 nm

nitride layer. This confirms that when one layer of nitride is deposited over the other the pin

holes in the layer tend to fill up and which makes it difficult for water to get into the pores. In

the case of the thinner layers as the nitride layers are deposited once the presence of the pin

holes makes the water to rush in. This is a basic reason to see an increase in the capacitance

when water is used. Similarly we can say that due to the porous nature of the polymer the

changes in capacitances are not appropriate in the beginning for the first set of

measurements, so the measurements have to be repeated several times to obtain

appropriate vales of capacitances.


without polymer at 10 KHz and 0.1V. The sequence of treatment is air-water-(pH6-water-pH7-water-


39

Figure 19 represents the resistance and the capacitance curves of the chip with 200nm

silicon nitride layer which is operated at 1 MHz with 0.1v as input voltage. Very smooth trends

of changes are visible in the capacitance and the resistance curves. We could clearly make out

the changes in the capacitance and resistances in this measurement which confirms that the

conducting glue acts properly compared to the previous chip which was worked out at 10 KHz.

These two chips are not the same due to the problems with the gluing of contacts another chip

with the same geometry is being used when measuring at 1 MHz frequency. This chip has the

same base capacitance as used in the previous case. In this graph we could see that the

resistance is almost constant when dipped in water which is quite an obvious behaviour. We

could also observe linearity in the capacitance changes when different pH solutions are used

which can be very a very useful feedback for future work. It can also be observed that the

capacitance change when different pH solutions are used is quite low compared to the chip

operated at 10 KHz which could also imply that the dipolar nature is different for water and pH

solutions which play a vital role in the capacitance changes. It can be also seen that as in the

previous case the capacitances are low compared to the ones 50nm and no polymer chips

which confirms that the capacitance decrease for thicker layers of the nitride layers.

0

10

20

30

40

50

60

70

80

90

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

Time(Minutes)

Cap

acit

ance

(pF

)

750

800

850

900

950

1000

1050

1100

1150

1200

Res

ista

nce

(Ω

)

RESISTANCE CURVE

CAPACITANCE CURVEa

w

6 7 8 6 7

8

67

8

w w w w w w w w w


without polymer at 1 MHz and 0.1V. The sequence of treatment is air-water-(pH6-water-pH7-water-


40

We also intended to measure on chips with polymer on SiN but we could not manage due to

some complications with the gold contacts that were falling apart probably due to the

interaction of the solvent in the processed polymer and the conducting glues used to glue the

gold contacts to the electrode pads.


• Gold electrodes are influenced by the liquids when they are used repeatedly.

• Silicon nitride layer:

1) Resistance curves do not show any significant changes with pH. 2) Drift in the signal is insignificant which indicates that it is mainly related to the

polymer swelling over time.

41

5. Conclusions

Summary tables for capacitance and resistance from all the measurements are shown in

Figure 20a & Figure 20b which corresponds to the linearity curves for pH versus capacitance

and resistance. In this plot we try to implement all the sensors outputs to see their sensitivity

for the different pH solutions. The different type of points used in the graph show the different

kinds of frequencies that the sensors are being operated. A clear linearity is observed in the

case when the sensor is operated at a frequency of 10 KHz which is within a few hundreds of

Pico Farads range. Similarly the almost linear behaviour is seen in the case of the resistance

curve when the sensor is operated at 10 KHz. So it can be concluded that 10 KHz would be

the frequency that would give linearity in the sensor operation.

Figure 20a: Linearity Curves for pH vs. Capacitance for the pH sensor used in this project

It can also be seen that the magnitude of change in capacitance and resistance is not known

but in future if the work is done with different polymers or with more specific designing we

could expect to get more similar values for capacitance and the resistance. It was difficult to

reproduce the results as a problem with contact gluing was being faced. If one could overcome

this problem it would be easier to reproduce the results when the polymer is deposited over

42

the electrodes. Finally in this project after all the observations it can be concluded by talking

about the polymer thickness and the deposition techniques employed and the measurement

setup and the measurement techniques employed in the project.

Figure 20b: Linearity Curves for pH vs. Resistance for the pH sensor used in this project

Using an alternative polymer deposition technique and new solvents we were able to achieve useful polymer coating over the gold electrodes. The measurement setup was build for dipping the sensors in the sample liquids being very flexible and reduced the complexities created by the previous measurement setup. In this project we managed to get the ratio of polymer layer thickness and spatial wavelength to approximately 1:1. We are not sure whether this is an optimal thickness regarding the electric filed lines in the polymer and the liquids. An apparent “drift” in the signal is seen in all the measurements with polymer and negligible in the cases when there is no polymer over the electrodes. We propose that this is not any drift but rather is a sign that the polymer does not reach equilibrium during the incubation time. When starting with a dry polymer we observe a “start jump” in the signal, this might be caused by the water uptake in the polymer. The start jump in the signal when the sensor is first dipped in water is not because of field lines through the liquid, but can be also due to the water uptake of the polymer. Therefore, the sensor needs to be stored in a “neutral” liquid for at least 24 hours before the measurements are done. By comparing the resistance and the capacitance curves, it is hard to judge the exact amount of changes in the magnitudes of capacitances for different pH liquids. The resistance is very high when started with really dry polymer compared to the one which is already used. The capacitance gets back to its base value once the liquid is taken out but the resistance remains very low and doesn’t increase until the polymer is totally dry.

43

The influence of the various liquids on the polymer is observed in the capacitance measurements. Because of the complexity of the polymer and the different contradictory responses that occur at the same time, it is very difficult to determine the influence of the various pH solutions. The degree of other ions influence is not clear. The model building based on either parallel or series capacitance is not sufficient. The changes in the capacitance with frequency and the frequency choice depend on application and/or interface electronics (i.e. if pF or mF has to be measured).

The electrical properties from the results indicate the polymer PAA IOA to be a polar type of dielectric polymer, but it is cannot be confirmed at the moment. The usage of mixture of ether & hexane as a solvent also has an influence on the polymer properties. The non-uniform changes in the capacitances are dependent on the uniformity of the polymer layer as mobility of ions should be uniform to have a constant capacitance. In this project we could see that the response time of the polymer is more than the incubation times being used.

44

6. Recommendations for future work

Future work has to be done by considering the following aspects:

Chip Assembly & Polymer deposition

The interaction of the solvent with the conducting glue and the glob top material have made it really difficult to deposit the polymer over the electrodes once the contacts are glued. An alternative strategy could be to take a set of chips with the same base capacitance, deposit the polymer on some, and glue the contacts; the rest of the chips should be glued with contacts without depositing the polymer. Measure in different pH solutions and compare the results. One could also look for different glues to avoid complications. We also found that the contacts used slide into the sockets connected to the setup. This has a greater influence on the conducting glue of the contacts. We recommend the usage of better and more rigid contacts.

A new measurement protocol should be developed with revised longer time periods and measuring sequences. The sensor should be stored in a neutral liquid to equilibrate the capacitance to the environment. Measurements have to be repeated to confirm if resistance is frequency dependant in the case of the 50nm nitride layered chips. At this point, a combination of several measurements like capacitive and resistive measurements is necessary to determine the composition of the solution. Some measurements have to be designed to calculate the response time of the polymer. More complex model for parallel and series capacitance to fit the data has to be developed for a deeper view about the performance. The usage of LCR meter also might be a drawback as the results shown by it are not perfect always.

An expansion if the pH range would be interesting as one could study what happens at the extremes to analyse the behaviour of the polymer. This would imply the usage of another pH buffering system than phosphate to achieve a large pH span. An accurate and thorough calibration will be necessary to determine the pH of a solution by the difference in capacitance.

One has to calculate/measure the total ion concentration of the liquids used to have an idea about the number of ions. Measurements should always be done both on the naked electrodes and electrodes with polymer. Before the polymer is deposited the base capacitance must be checked to confirm that they are equal on different chips being used. One has to check the resistance of the chips before the nitride layer is deposited. It would be good to have different chips with differently processed polymers to see if they respond in the same way. The volume of the analytic solution used for every measurement has to be the same in all the cases. One has to work on the uniformity of the polymer layer as the ionic mobility is totally dependent on the uniformity of the polymer.

Ion selective membranes can be used to enhance the properties of the polymer but they might have some limitations as well. This membrane would be semi-permeable and would not allow salt ions to pass to the polymer. There is one polymer that is claimed to be non-salt sensitive [3], but this polymer is even more complex than pAA-IOA. It is confirmed that it will take much efforts and resources to create this polymer by some external partner. Still it is an attractive alternative for the present polymer. Finally, a purely salt sensitive reference electrode could be a way to compensate the salt sensitivity of the sensor. There are polymers known that are claimed to be sensitive to salt only [2] and the capacitance of both sensors could be subtracted to determine the response to pH.

45

7. References

[1] Theory and Practice of pH Measurement, Rosemount Analytical, September 1999 [2] A salt-independent pH sensor, Q.Y. Cai, C.A. Grimes, Sensors and Actuators B79, April 2001 [3] A wireless pH sensor based on the use of salt-independent micro-scale polymer spheres, C. Ruan, et

al, Sensors and Actuators B96, may 2003 [4] A wireless micro-sensor for simultaneous measurements of pH, temperature and pressure, M.K.

Jain, Q. Cai, C.A. Grimes, Smart Materials and Structures 10, 2001 [5] Micro fabricated conductimetric pH sensor, N.F. Sheppard Jr. et al, Sensors and Actuators B28,

August 1994

[6] Review on Hydrogel-based pH Sensors and Microsensors, Andreas Richter 1,2,

*, Georgi Paschew 1,2

,

Stephan Klatt 1,2

, Jens Lienig 1

, Karl-Friedrich Arndt 2

and Hans-Jürgen P. Adler ,3

25 January 2008

[7]Analytical evaluation of the interdigitated electrodes capacitance for a multi-layered structure, Rui Igreja, C.J. Dias, January 2004 [8] Interdigitated sensors and transducers, A.V. Mamishev et al., Proceedings of the IEEE, vol92 may

2004 [9] Influence of the electrodes configuration on a differential capacitive rain sensor performances, Isabelle Bord, Pascal Tardy, and Francis Menil. June 2005

[10] Novel Technique for Measuring Through-Plane Modulus in Thin Polymer Films, K.S. Patel, P.A. Kohl, S.A. Bidstrup-Allen. May 1998

[11] High-Sensitivity Capacitive Humidity Sensor Using 3-Layer Patterned Polyimide Sensing Film, J. Laconte, V. Wilmart, D. Flandre, J.-P. Raskin, 2003

[12] Configurable Electrodes for capacitive-type sensors and chemical sensors, A. M. Kummer, A. Hierlemann, IEEE Sensors Journal, vol6, No1, February 2006

[13] High-Sensitivity Capacitive Humidity Sensor Using 3-Layer Patterned Polyimide Sensing Film, J. Laconte, at al. IEEE, 2003

[14] A polymeric microfluidic valve employing a pH-responsive hydrogel microsphere as an actuating source, J.Y Park et al, Journal of micromechanics and microengineering, January 2006

[15]Thin Film Polymer Dielectrics for High-Voltage Applications under Severe Environments, James R.Webster, 26 May 1998.

[16] TECHNICAL WHITEPAPER Dielectric Properties of Polymers, Copyright ©2005 Zeus Industrial Products, Inc.

[17] Wikipedia, Carboxyl groups, accessed November 2008

[18] Wikipedia, Alkyl groups, accessed November 2008

[19]Wikipedia, Acryl groups, accessed November 2008,

[20]http://www.bbc.co.uk/dna/h2g2/A686702

46

8. APPENDIX

1. Standard pH Meter Nowadays the very usual and the standard way of measuring pH is usage of the Standard pH Meter. It consists of three different electrodes: reference electrode, measuring electrode and a temperature sensing electrode in a test cell constituting the liquid. The voltage of the cell varies linearly with the change in pH of the liquid. The total cell voltage is the algebraic sum of the potentials of the measuring electrode, reference electrode and the liquid junction. The measuring electrode potential depends totally on the changes in the pH and the reference potential remains unchanged and the junction potential is usually very small. All the three potentials are temperature dependant; therefore temperature compensation is made in the final pH to voltage conversion relation. This is the basic working principle of the Standard pH meter.

The measuring electrode has high internal impedance. This electrode has a glass membrane which is sensitive to changes in pH. The silver wire plated with silver chloride contacts the potassium chloride solution kept at a constant pH of 7 in the glass electrode. The potential at the outside surface of the membrane depends on the pH of the solution and the potential of the inner surface is a constant. Therefore the overall potential measured depends solely on the pH of the solution. [1]

47

2. Components of PAA IOA

Poly acrylic acid isooctyl acrylate has three basic groups which constitute the structure.

Carboxyl groups: The carboxyl group is (C=O)-OH, and is usually written as –COOH. The salts and anions of carboxylic acids are called carboxylates. The structure of a

carboxyl group is shown in the figure. Carboxylic acids are weak acids, because only 0.02% of all acetic acid molecules dissociate into H+ cations and

RCOO− anions in aqueous solution, at room temperature. The carboxyl groups

tend to donate the H+ ions into the solution and act as acids. The two electronegative oxygen atoms tend to pull the electron located within the hydrogen atom away, and the remaining proton H+ can leave more easily. The result of the dissociation of a carboxylic acid is a product in which the negative charge is shared (delocalized) between the two oxygen atoms. Each of the carbon-oxygen bonds has what is called a partial double bond characteristic. The presence of electronegative groups (such as OH- or Cl-) next to the carboxylic group increases the acidity. Carboxylic acids react with bases to form carboxylate salts, in which the hydrogen of the hydroxyl (-OH) group is replaced with a metal ion. [17]

Alkyl groups: An alkyl is a univalent radical containing only carbon and hydrogen atoms arranged in a chain. The alkyls form a homologous series with the general formula CnH2n+1. The structure is shown in figure. They are normally found as part of larger branched chain organic molecules. On their own they are free radicals and therefore extremely reactive, only three out of four valence electrons are used in bonding on one of the carbon atoms. The structure of an alkyl is the same as its straight chained alkenes counterpart but with one less hydrogen atom. [18]

Acryl groups: The acryl group is derived from acrylic acid and has the structure H2C=CH-C(=O)-. This group is illustrated in the figure shown. Compounds containing an acryl group can be

referred to as "acrylic compounds". The acryl group is an α, β-unsaturated carbonyl

compound, because it contains a carbon-carbon double bond and a carbon-oxygen double bond, separated by a carbon-carbon single bond. As a result, the acryl group has the properties characteristic of both functional groups. At the C=C bond: electrophilic addition of acids and halogens, hydrogenation, hydroxylation and cleavage of the bond. At the C=O bond: nucleophilic substitution (such as in esters) or nucleophilic addition (such as in ketones). [19]

Structure of a Carboxyl

group

Structure of an Alkyl

Group

Structure of an Acryl

Group

48

3. Data tabulated from Phase 2 measurements for different chips

Environment

CP

RP

CS

RS

10KHz

100KHz

10KHz

100KHz

10KHz

100KHz

10KHz

100KHz

AIR

016.5pF

015.8pF

34.95MΩ

0.54MΩ

16.55pF

15.89pF

26.29KΩ

02.83KΩ

WATER

239.7pF

129.7pF

84.42KΩ

25.02KΩ

390.5pF

157.8pF

32.26KΩ

04.98KΩ

pH6

1.08nF

240.1pF

14.31KΩ

5.47KΩ

2.27nF

595.3pF 7.20KΩ

3.18KΩ

WATER

88.48pF

174.5pF

36.68KΩ

100.6KΩ

110.4pF

317.5pF

6.91KΩ

44.60KΩ

pH7

1.66nF

380.8pF

9.80KΩ

3.58KΩ

3.29nF

893.6pF

4.72KΩ

2.12KΩ

WATER

304.4pF

136.56pF

60.84KΩ

18.77KΩ

538.2pF

187.2pF

24.76KΩ

5.44KΩ

Ph8

3.46nF

1:14nF

4.72KΩ

1.53KΩ

7.08nF

1.75nF

2.08KΩ

950.60Ω

WATER

1.36nF

624.4pF

22.56KΩ

4.46KΩ

1.897nF

829.9pF

4.62KΩ

1:10KΩ

Environment

CP

RP

CS

RS

10KHz

100KH

z

10KHz

100KHz

10KHz

100KHz

10KHz

100KHz

AIR

17.26pF

16.0pF

15.1MΩ

2.86MΩ

17.08pF

16.06pF

49.52KΩ

3.52KΩ

WATER

3.77nF

2.92nF

26.05KΩ

1.51KΩ

3.81nF

3.07nF

677.49Ω

185.85Ω

pH6

2.43pF

633.pF

9.21KΩ

2.05KΩ

3.60nF

1.71nF 3.16KΩ

1.79KΩ

WATER

840.4pF

228pF

23.32KΩ

9.25KΩ

1.65nF

322.8pF

8.02KΩ

3.63KΩ

pH7

4.10nF

1.07nF

5.50KΩ

1.30KΩ

6.23nF

2.12nF

1.76KΩ

855.62Ω

WATER

1.99nF

720pF

15.97KΩ

2.89KΩ

2.66nF

1:12nF

3.20KΩ

1.07KΩ

Ph8

10.21nF

2.63nF

1.83KΩ

679.7KΩ

17.55nF

4.66nF

766.70Ω

301.57Ω

WATER

3.34nF

2.59nF

21.45KΩ

1.99KΩ

3.60nF

2.80nF

1.09KΩ

175.68Ω

Table 2: Capacitance & resistance responses of 2µm PAA IOA layer sensor chip at 10 KHz and 100 K Hz

and 0.1V. This measurement was done when the polymer was not perfectly in the dry state.

Table 3: Capacitance & resistance responses of 0.5µm PAA IOA layer sensor chip at 10 KHz and 100 K

Hz and 0.1V. This measurement was done when the polymer was perfectly in the dry state.

49

4. Steps of fabrication

a) Plasma cleaning the wafer. b) Coat a Pyrex wafer with 5 nm Ti, and 100 nm Au in the FHR sputter. c) Immediately before e-beam exposure, spin coat the spin resist diluted with 1:1 resist on the wafer. The thickness of the resist is approximately 150 nm. d) Soft bake the wafer on a hotplate. e) Expose the course and the fine pattern in the new e-beam. To ensure that there are not field lines in the resist structure, the pattern should be exposed twice with a small shift. f) Post bake the wafer immediately after unloading the cassette for 2min at 90deg on a hotplate. g) Develop the resist, to ensure rapid and complete development, use ultrasonic agitation during development. h) After development, do not rinse the wafer, but put it in a 4-inch wafer cassette. This cassette should be in a portable water tank, filled with water. i) Carry the water tank with the soaked wafer to the silicon area, and put the cassette in an R&D unit. Run a standard program of rinse and dry. The finger like structures and pads should be visible on top of the gold surface. j) Immediately before the dry etching, ash the wafer to ensure that there are no resist residues in between the fingers. k) After the ion milling, inspect the wafer in an optical microscope. All areas surrounding the capacitor structures should be transparent. A greyish colour signifies residues of titanium, if there are residues etch for some more time. Also, to ensure that the titanium removed between the fingers, measure the resistance pad to pad, it should be high. l) Remove the resist with oxygen plasma in the batch top. m) The top of the wafer should be protected with a thick photo resist layer. Dice the wafer with a diamond saw and break the chips from the wafer, and bond the chip to the PCB-chip holder.

Environment

CP

RP

CS

RS

10KHz

100KHz

10KHz

100KHz

10KHz

100KHz

10KHz

100KHz

AIR

19.43pF

18.08pF

12.4MΩ

2.05MΩ

19.56pF

18.09pF

53.33KΩ

3.72KΩ

WATER

10.12nF

4.32nF

4.54MΩ

740.84Ω

10.75nF

5.27nF

478.92Ω

152.72Ω

pH6

2.76nF

528.3pF

5.65KΩ

2.059KΩ

5.60nF

1.69nF 2.90KΩ

1.39KΩ

WATER

336.4pF

194.2pF

51.38KΩ

9.33KΩ

667.92pF

262.6pF

23.42KΩ

4.54KΩ

pH7

8.54nF

1.72nF

1.78KΩ

623.75Ω

18.17nF

5.43nF

894.24Ω

441.32Ω

WATER

4.32nF

1.25nF

5.46KΩ

1.46KΩ

6.65nF

2.61nF

1.55KΩ

696.67Ω

Ph8

19.32nF

4.24nF

1.04KΩ

407.23Ω

32.24nF

8.06nF

391.01Ω

183.96Ω

WATER

12.91nF

4.45nF

2.42KΩ

633.7Ω

15.24nF

5.69nF

488.12Ω

157.68Ω

Table 4: Capacitance & resistance responses of 0.5µm PAA IOA layer sensor chip at 10 KHz and 100 K Hz

and 1V. This measurement was done when the polymer was perfectly in the dry state.

50

5. Finger images SEM

To make SEM images of the inter-digital capacitor, one may evaporate a thin layer (10 nm) of chromium on top of the fingers. This layer will remove the accumulated charge from the surface when using the SEM. After imaging, the chromium can be safely removed by using chrome etch, which will etch neither the Ti under layer nor the top gold layer.

As seen in the two figures below, the line roughness if fair in the 250 nm wide fingers. The roughness is likely partly due to the granularity of the resist, or possibly the large step during e-beam exposure. It would be nice to use a smaller step size during the exposure, but the minimum dwell time (40 ns) and the high sensitivity of the resist (26 micro coulomb/cm2) makes it difficult, as long as a short exposure time is important.

51

6. Break through voltage of 0.5 um capacitors covered by Si3N4

LCR-meter is used to apply an AC voltage to the capacitors, and registered the capacitance. At the break through voltage, the capacitance increased markedly, and in the case of the water covered interdigitated capacitor, this voltage was accompanied by a strong electrolysis of the water. The Si3N4 chips were covered with 50 nm Si3N4 to prevent breakthrough (PECVD, HFSIN). Since we were not able to take SEM pictures of the cross section, we were not able to measure the thickness. For the first chip, we merely checked that it could withstand the full 20 V swing of the LCR-meter, both in the dry and the wet condition. By drying the chip when wet, the original capacitance could be re-obtained. This indicates that the nitride works fine to protect the interdigitated capacitors.

There was no change in the capacitance in the dry state when switching from 10 kHz to 1 MHz. In the wet state, however, the 10 kHz was approximately a factor 2 larger than the capacitance at 1 MHz. This may have to do with ion species of the water and their mobility. More surprising was the fact that all capacitors could withstand very large fields, i.e. up to 40 MV/cm in the case of the silicon nitride chips. This may be due to a requirement that a minimum ionization voltage has to be supplied if a breakthrough should happen. Regardless of the origin of the surprisingly high breakthrough voltages, it seems clear that applying 2 V to the silicon nitride covered chips should pose very little hazard to the integrity of the capacitor. Protecting the chip with silicon oxide may also work, if the voltages are kept below 4 V.

7. Microscopic view of the Electrodes with polymer layer

The above figures are taken from the high precision microscopes to check the uniformity of the polymer layer. One could clearly see how non uniform the layer is, which could one of the reasons for the complications in measuring the exact capacitance value in different pH solutions. These are the pictures of the sensor chip we have been used in the phase 1 and phase 2 measurements with a polymer thickness layer of 2µm. It is also visible after conducting so many measurements the gold fingers remains unharmed due the moisture.

52

8. Block Model of the LCR Meter

The LCR meter reduces complexity but as said, information is thrown away as well. From an engineering point of view, the LCR analysis is simple and sufficient to design the circuit electronics around the sensor, but it does not contribute to any understanding of the principle.

For the LCR measurements, different pH solutions are used with the interdigitated capacitive electrode. The setup used is the one used in the previous chapter NO2 of the report. Settings for the capacitive measurement are a frequency of 10 kHz and 1 MHz, a voltage of 0.1V, a 1hour 40mins run time of the experiment and a sample time of 2s. A result is obtained that increases more than exponential with pH. This is because pH is a logarithmic scale and the ion concentration of H+ increases also exponential. Because of the very small ratio between layer thickness and finger spacing, the influence of the ions in the solution is very dominant. Even with a polymer on top it, the capacitance increases so much due to the solution.

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9. Lab View measurement program

The Lab View program is written in Lab View 7 and is based on a driver VI of National Instruments, the developer of Lab View. There are several standard drivers available that can be chosen, depending on the desired application. The one chosen here was a continuous measurement VI that just constantly copied the measurements values of the LCR-meter to the VI and displays them, until the stop-button is pressed. This loop runs continuously and outside the loop some basic and required National Instruments VI’s were used to initialize the meter, read the signals and so on. In the next section a scheme is to be found of the used Lab View program. Also the user interface and an example of an output file are added. The pictograms in the lower part are basic functions and remain unchanged. The automatic generated file with measurement values is created by adding the standard Lab View tool to write a file. Three columns are generated in this file: 1) Time registration of each measurement 2) the primary value 3) the secondary value. These primary and secondary values are the output of the LCR-meter and it can be chosen what the equipment exactly needs to measure, like capacity and resistance or capacity.

This file is written only when the loop that reads the measurements values stops. Therefore, the values that are displayed by default are not only displayed, but also put in an array. Now, with each new loop (initiated by a new measurement value) a measurement value is added to the array. By default, a new loop is started when the measurements value change. Because of the accuracy of the LCR meter, it is sensitive up to several digits that change easily so usually a loop can be generated each tenth of a second for example. This is much too often for the desired purpose here, because the circumstances in the chamber will change during a time span of several minutes to hours and the expected changes in the sensors are slow. This would create an unnecessarily large output file. Therefore, an adjustable sample timer is added. In seconds, this value is taken as input, multiplied by 1000 because the timer needs milliseconds and then put into the loop where it arrives at the timer. The loop will only start again when all parts in it are finished, so only when the timer is finished, a new measurement is done and a new series is added to the arrays. Besides the measurements value, an array is created with the exact times the measurement is performed. This time is directed from the upper loop, where the elapsed time of the measurement run is determined.

This upper loop has three tasks. It stops the measurement run when the run time has passed, it stops the run if the STOP button is pushed and it provides the elapsed time for the output file and as active data to the user. It is important to see, that the upper loop, controls the lower one. The lower loop stops only when the upper one stops, because the signal that takes care for the stop signal in the upper loop is copied to the lower one. The upper loop stops when the STOP button is pushed or when the chosen run time has passed. This is calculated by subtracting an internal timer of this chosen run time. As long as this is larger than zero, the loop continuous. The two time-counters making up the internal timer, one inside and one outside the loop, are necessary to be able to start the run at a relative time of zero. The time counters start at a (apparently) random but equal value, so when the loop is started they are subtracted from each other, so the result is approximately zero. Because one timer is outside the loop, this is only done by initialization and from that point the output is just a counter inside the loop, starting at zero. Now a clean calculation is possible to determine if the run time has passed and the timer value can be shown to the user as the elapsed time of the measurement run. The number of loops performed in the lower loop is shown to the user. This is in fact equal to the number of measurements, because each loop represents a measurement. And it is mentioned again that the file is written when the upper loop stops and thereby stopping the lower loop as well. At that moment, all the content of the arrays collected during the total run time, is passed through to the write function and the total file is generated at once.

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10. Lab View Capacitance Measurement (block scheme)

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11. Lab View Capacitance Measurement readout file from LCR Meter display (user interface)

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12. Lab View Capacitance Measurement (output file)

LabVIEW Measurement Writer_Version 0.92 Reader_Version 1 Separator Tab Multi_Headings Yes X_Columns No Time_Pref Relative Operator labuser Date 2006/08/30 Time 17:21:31.574999 ***End_of_Header*** Channels 3 Samples 31 31 31 Date 2006/08/30 2006/08/30 2006/08/30 Time 17:21:31.594999 17:21:31.594999 17:21:31.594999 X_Dimension Time Time Time X0 0.0000000000000000E+0 0.0000000000000000E+0 0.0000000000000000E+0 Delta_X 1.000000 1.000000 1.000000 ***End_of_Header*** X_Value Comment 0.361000 0.577236 -1.743610 30.360000 0.422716 -1.380850 60.360000 1.060980 -0.865947 90.360000 0.373721 -1.806310 120.378000 -0.021047 -30.183100 150.378000 0.078524 -9.901980 180.378000 0.176360 -2.508660 210.378000 0.508555 -0.983041 240.378000 0.301133 -1.430350 270.378000 0.803860 -1.084450 300.386000 0.172628 -3.440340 330.386000 0.456503 -0.624571 360.386000 0.416582 -1.369280 390.386000 0.326948 -1.952070 420.386000 0.444146 -1.093800 450.390000 0.424772 -2.521350 480.395000 0.268230 -2.219350 510.378000 0.566048 -0.952581 540.399000 -0.200113 -12.007800 570.399000 -0.246697 -20.425500 600.399000 0.614507 -1.327110 630.411000 0.502895 -1.225980 660.413000 0.609556 -1:197820 690.186000 0.221391 -3.088200 720.440000 0.395947 -1.800730 750.440000 -0.030763 -23.583400 780.440000 0.244987 -2.698880 810.440000 0.286195 -1.811860 840.440000 0.360502 -1.615850

870.440000 0.254735 -1.740480

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