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)
_____________________________________________
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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 Krich541@student.liu.se
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
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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 ... 92.1. 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
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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
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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
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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
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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, thicknessof 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.
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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.
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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
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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
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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.
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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
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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+
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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.
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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.
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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.
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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.
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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.
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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.
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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
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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.
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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.
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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.
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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.
Summary from Phase 2 results:
• 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.
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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) C ap ac it an ce (p F ) 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 8 6 7 8 6 7 8 w w w w w w w w w Resis ta n ce ( 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.
