Current-Voltage (I-V) measurement set-up with solid

In this situation, we replace the glass reference electrode with solid reference electrode. The long-term stability of the output voltage (VG) is what we concern about. Hence, as the same with measuring for sensitivity, ever pH value is immersed for 30 seconds before IDS-VGS measurement. Then the long-term stability of output voltage is measured for 60 seconds with 15 seconds a measurement point.

3.4 References

[1]`K. M. Chang, K. Y. Chao, T. W. Chou, and C. T. Chang, ”Characteristics of Zirconium Oxide Gate Ion-sensitive Field-Effect Transistors” Japanese Journal of Applied Physics Vol. 46 No. 7A pp. 4334-4338 2007.

[2] Paik-Kyun Shin, ”The pH-sensing and light-induced drift properties of titanium

dioxide thin films deposited by MOCVD”, Applied Surface Science, vol. 214, pp.214-221, 2003

Chapter 4

Results and Discussions

Part I: Polymer-based material applied as the sensing layer of REFET

4.1 H

⁺

and Na

⁺

Sensitivity of different polymer-based material

For solving the unstable voltage problem of solid-state reference electrode, a REFET (Reference ion-sensitive Field Effect Transistor) co-fabricated with ISFET is applied. The REFET is identical with ISFET but does not react on the ion concen- tration to be measured. By applying a differential measurement between ISFET and REFET, the unknown and unstable electrode potential manifests itself as common mode signal and is thus suppressed by the CMRR (common-mode-rejection-ratio) of the system.

According to Chapter 2, the sensitivity of ISFET is related to the numbers of surface sites, i.e., the more numbers of surface sites the larger sensitivity it has. For the sake of reducing sensitivity, we need to find an effective diffusion barrier for protons, thus reducing the pH sensitivity of the device. In this study, we want to build up a REFET which is suitable for the ZrO2-pH- ISFET, so to find a REFET with low H⁺ sensitivity is our main object. But a REFET must also insensitive to the other ion concentration, or else the H⁺ sensitivity of ISFET will be decreased by REFET in differential measurement. The alkali metal ions are easy to be found in electrolytes, so in this study, we also study the Na⁺ sensitivity of ISFET and REFET.

4.1.1 H

⁺

and Na

⁺

Sensitivity of ZrO

2

Fig. 4-1 and Fig. 4-2 show theH⁺ and Na⁺ Sensitivity of ZrO2 is 57.89mV/pH and 15.88mV/pNa, respectively. It shows that the ZrO2 sensing film is more selective to H⁺ ions than Na⁺ ions. The results are agreed with Ref. [1]. The H⁺ sensitivity of ZrO2 sensing film will be a little decreased by Na⁺ ions in the electrolyte. So, if we find a REFET with Na⁺ sensitivity is similar to ZrO2, then the H⁺ sensitivity decreased by Na⁺ will also be eliminated in differential measurement.

4.1.2 H

⁺

and Na

⁺

Sensitivity of NF coated ZrO

2

In our experimental experience, the polymer-based material, PR and P3HT, are easy to damage in electrolyte, and the film properties will be unstable in measurement.

In order to solve this problem, we apply Nafion developed by Dr. Walther Groot at DuPont in the late 1960’s.

Nafion is a perfluorinated polymer that contains small proportions of sulfonic or carboxy ionic functional groups and with this functional groups, Nafion has the feature of unique equilibrium ionic selectivities and the ionic transport properties. Fig.

4-3 shows the its chemical structure and model, we can see that Nafion can be divided into three parts: (A) a hydrophobic fluorocarbon backbone C-F (B) an interfacial region of relatively large fractional void volume (C) the clustered regions where the majority of the ionic exchange sites, counter ions, and absorbed water exists [2] [3] [4]

[5].Because of the high concentration SO3^－ ion clusters, Nafion exhibits a high cation conductivity, i.e., high cation exchange. And Nafion is modified from Teflon, so Nafion is extremely resistant to chemical attack and high working temperature.

Based on the above knowledge, we use Nafion to be the protective layer which

coated above PR or P3HT. In order to see if the coated Nafion will influence any H⁺ sensitivity, we coated Nafion above ZrO2 sensing layer. Fig. 4-4 shows the NF-coated ZrO2 H⁺ sensitivity and Na⁺ sensitivity are 58.92mV/pH and 16.34mV/pNa, respectively. The sensitivity is a little increased by Nafion, because of its high cation conductivity property thus more cations will be easily trapped at the surface sites.

This result means that Nafion will be exact a protective role, and will not decrease any sensitivity, so in the following experiment any decreased sensitivity will be from the contribution of PR or P3HT.

4.1.3 H

⁺

and Na

⁺

Sensitivity of NF-P3HT-ZrO

2

According to Ref. [6], deposition of hydrophobic ion-unblocking polymer layers is a way to design a REFET and such membranes exhibit slight conductivity. Based on this idea, we using the popular organic material, P3HT (poly (3-hexylthiophene) ) as the REFET sensing layer. P3HT is used as the semiconductor layer in FET, because it is easily processable and is compatible with plastic substrates. Fig. 4-6 shows the chemical structure of P3HT, we can see that P3HT is synthesized from polythiophene (PHT) with 3-alky substitutes in head to tail regioregular structure [7]. Because of ordering and crystallinity in its solid states, the mobility in P3HT will be improved.

We think that the ordering structure of P3HT maybe an effective diffusion barrier for.

protons. Table 4-1 and Fig. 4-7 ~ Fig. 4-9 is the H⁺ sensitivity and Na⁺ sensitivity of P3HT-ZrO2 structure. From Table 4-1, the sensitivity of P3HT is very unstable during measurement, it maybe come from the poor adhesion between P3HT and ZrO2. P3HT is very hydrophobic with contact angle more than 90°, and the original gate surface is hydrophilic, so the attachment will be weak. But we believe P3HT has the potential to reduce sensitivity because the once low sensitivity during measurement.

4.1.4 H

⁺

and Na

⁺

Sensitivity of NF-P3HT-HMDS-ZrO

2

In order to improve the attachment, we apply HMDS (hexamethyldisilazane) to the gate region before coating P3HT. Table 4-2 and Fig.4-10 ~Fig. 4-12 show the H⁺ sensitivity and Na⁺ sensitivity of P3HT-HMDS-ZrO2. By applying HMDS as the silylating reagent, the sensitivity of P3HT film becomes more stable with the average of 8.07mV/pH. This means P3HT is an effective film for REFET. But Fig. 4-12 shows the Na+ sensitivity is 39.21mV/pNa which is too large to be a REFET sensing film for ZrO2-pH-ISFET.

4.1.5 H

⁺

and Na

⁺

Sensitivity of NF-PR-ZrO

2

Positive photo resistor (FH6400) is also a test polymer material in our experiment.

FH6400 is composed of three parts: (1) resin (2) sensitizer,DNQ (3)solvent. Among them, resin is a polymer with heat-resistant property and is widely used as the etching protective film. Table 4-3 and Fig. 4-13 ~ Fig. 4-15 show the average H⁺ sensitivity and Na⁺ sensitivity of NF-PR-ZrO2 are 9.92mV/pH and 9.9mV/pNa, respectively. It shows PR is a good sensing film for REFET. Without coating HMDS, The sensitivity of PR is more stable compared with P3HT w/o HMDS. This is because the adhesion of PR is stronger than P3HT at ZrO2 surface.

4.1.6 H

⁺

and Na

⁺

Sensitivity of NF-PR-HMDS-ZrO

2

With applying HMDS at gate surface before coating PR, from Table 4-4 and Fig.

4-16 ~ Fig. 4-17, the H⁺ sensitivity is more stable than PR w/o HMDS. But from Fig.

4-18 the Na⁺ sensitivity is 28.27mV/pH which is too large to be a sensing layer of REFET.

4.1.7 H

⁺

and Na

⁺

Sensitivity of NF-mix-PR-ZrO

2

The more uniform and careful film structure will be one of the methods to form a more effective film to reduce the numbers of surface sites. As we test in previous, the film structure by the coating process maybe not in ordering. In section 4.1.2, Nafion has many voids in its chemical structure, and if we mix PR with Nafion, the voids will be filled by PR and become a more ordering and stronger structure. Table 4-5 and Fig.

4-19 ~ Fig. 4-20 show the H⁺ sensitivity of NF-mix-PR ZrO2. The average H⁺ sensitivity is 8.47mV/pH and is more stable and lower than Nafion cap PR structure presented in section 4.1.5. This means that our supposition is working, but Fig.4-21 shows the Na⁺ sensitivity of this structure is 19.89mV/pNa, it is a little larger than 15.88mV/pH, the Na⁺ sensitivity of ZrO2.

4.1.8 H

⁺

and Na

⁺

Sensitivity of NF-mix-PR-HMDS-ZrO

2

With applying HMDS at the gate surface before coating NF-mix-PR solution, from Table 4-6 and Fig. 4-22 ~Fig. 4-23, the H⁺ sensitivity is stable and lower than NF-mix-PR-ZrO2 structure. Fig. 4-23 shows the Na⁺ sensitivity is 11.27mV/pNa which is very close to 15.88mV/pH, the Na⁺ sensitivity of ZrO2. This structure is suitable for REFETT as the sensing layer.

The results of Na⁺ sensitivity in section 4.1.6 and 4.1.7 with structure NF-mix-PR is different from other sections. In the previous sections, Na⁺ sensitivity is larger when coating HMDS, but in this structure, NF-mix-PR, is contrary. We guess this come

from the structure difference, and in previous sections the REFET sensing layer is only PR or P3HT, the trend of Na⁺ sensitivity with or with out HMDS is the same. In the NF-mix-PR structure, the mechanism of trapping Na⁺ ions maybe differ with PR or P3HT, but this mechanism nowadays is still complex for us to explain. Although the mechanism we are still not very clear, but this results of low H⁺ and Na⁺ sensitivity imply the structure has a big potential in the application for REFET.

4.1.9 Summary of Part I

In Part I, we have tried various REFET sensing layer structure for ZrO2- pH – ISFET. Fig. 4-25 and Table 4-7 summary the results of Part I, we find the NF-mix-PR-HMDS-ZrO2 structure exhibit a potential being a sensing layer of REFET. Table 4-8 list the sensitivity results for the differential ISFET/REFET measurement.

Part II: Solid-State Reference Electrode integrated with ISFET

4.2 Solid-State Reference Electrode

The potential at the solid/liquid interface is thermodynamically undefined and will lead to significant errors in pH measurement. The electrode modifying methods has been tried at the Ag/AgCl reference electrode by KCl and Nafion [4] [8]. It shows with the Nafion coating, the electrode potential will be more stable. This is because the Cl^－ ions of the Ag/AgCl reference electrode will be trapped within the KCl membrane by Nafon which is only permeable for cation ions. Therefore it will maintain a constant chloride activity and stable potential on the Ag/AgCl reference

electrode surface.

According to our previous experiment shows that the polymer-based materials make REFET have low sensitivity, it means that the surface potential is a constant value, i.e. the surface potential will be stable. Based on this idea, we coat the polymer-based materials at the solid-state reference electrode. We choose NF-PR and NF-mix-PR-HMDS structures, the better structures applied in REFET, to coat at the solid-state reference electrode. For having a chemical-resistant electrode, we deposit noble metal, Pt, as the reference electrode by sputtering. Comparing with the Ag/AgCl reference electrode, the Pt reference electrode will not have the problem of chloride diffusion.

4.2.1 The glass reference electrode

In this experiment, for the sake of having a standard reference, we first measure the sensitivity of ZrO2-pH-ISFET by glass reference electrode. In chapter 1, we have introduce the potential of glass electrode is very stable. Fig.4-26 shows the sensitivity of ZrO2-pH-ISFET measured by glass reference electrode is 56.67mV/pH.

4.2.2 The bare solid-state reference electrode

Fig. 4-27 shows the long-term stability and sensitivity of ZrO2-pH-ISFET measured by bare solid-state reference electrode. We can see the gate voltage (VG) of each pH value is very unstable within 60 seconds and the sensitivity linearity is also very bad. As the lectures said, this is the main problem of solid-state reference electrode and prevent from realizing.

4.2.3 The NF coated solid-state reference electrode

Fig. 4-28 shows the long-term stability and sensitivity of ZrO2-pH-ISFET measured by Nafion coated solid-state reference electrode. We can see with the polymer coated, the linearity of sensitivity becomes better. We also find that the long-term stability in acid electrolyte is more stable than in basic electrolyte. This maybe the reasons that Nafion is a cation exchange membrane, and the H⁺ ions in acid are more than in basic, so the thermodynamic equilibrium at the solid/liquid in acid electrolyte will achieve quickly than in basic electrolyte. Although the linearity is better by Nafion-coated, but the sensitivity is only 26.78mV/pH and is still difficult in application.

4.2.4 The NF-PR coated solid-state reference electrode

Fig. 4-29 shows the long-term stability and sensitivity of ZrO2-pH-ISFET measured by the NF-PR coated solid-state reference electrode. We can see NF-PR could greatly improve the performance. The long-term stability and linearity is very stable between pH 3 to pH 11 and the sensitivity can reach to 45.4mV/pH which is better than NF-coated reference electrode. These results mean that the low sensitivity sensing layer of REFET also work at the solid-state reference electrode.

4.2.5 The NF-mix-PR-HMDS coated solid-state reference electrode

Fig. 4-30 shows the long-term stability and sensitivity of ZrO2-pH-ISFET measured by the NF-mix-PR-HMDS coated solid-state reference electrode. The stability and linearity are much better than the bare solid-state reference electrode and the sensitivity is 55.9mV/pH which is very close to 56.67mV/pH, the sensitivity

measured by glass reference electrode. In order to check the stability, we measure this two times, Fig. 4-31 is the second measurement by NF-mix-PR-HMDS coated solid-state reference electrode. It still exhibits a stab output voltage and good linearity.

The sensitivity is still at the order of 55.67mV/pH. From the experimental results, the NF-mix-PR-HMDS structure seems to have the potential to solve the unstable problem of the solid-state reference electrode.

4.2.6 Summary of Part II

In this experiment, we prove the polymer-based material can work at the solid-state reference electrode. The long-term stability and linearity can be improved by polymer-based structure. We verify that the most simple and compact structure of ISFET can put into come true by modifying the troublesome solid-state reference electrode with NF-mix-PR-HMDS.

4.3 Conclusion

In this study, we firstly apply the Nafion mix PR polymer-based material for REFET and solid-state reference electrode. The sensitivity of ZrO2-pH-ISFET is 57.89mV/pH without any polymer modifying, but after treating with Nafion mix PR, the sensitivity can decrease to 5.8mV/pH at average. The Na⁺ sensitivity of this structure is only a little influenced by different Na⁺ concentrations with the value 11.27mV/pH. It means modifying by the Nafion mix PR polymer material is a successful treating method for REFET. The process is simple and easy to fabricate.

The unstable voltage problem of solid-state reference electrode is also solved by the polymer material, Nafion mix PR. The long-term stability within 60 seconds is

very steady and the linearity of sensitivity is extremely close to the results of glass reference electrode.

From the experimental results, we confirm the kind of polymer-based material have a big potential in surface modifying of REFET and solid-state reference electrode.

4.4 References

[1] K. M. Chang, K. Y. Chao, T. W. Chou, and C. T. Chang, ”Characteristics of Zirconium Oxide Gate Ion-sensitive Field-Effect Transistors” Japanese Journal of Applied Physics Vol. 46 No. 7A pp. 4334-4338 2007.

[2] John Payne ”Nafion® - Perfluorosulfonate Ionomer”, April, 2005 from http://www.psrc.usm.edu/mauritz/nafion.html

[3] Daivd T.V. Anh, W. Olthuis, P. Bergveld, ”Hydrogen peroxide detection with improved selectivity and sensitivity using constant current potentiometry”, Sensors and Actuators B, vol. 91, pp.1-4, 2003.

[4] Patrick J. Kinlen, John E. Heider, David E. Hubbard, ”A solid-state pH sensors based on a Nafion-coated iridium oxide indicator electrode and a polymer-based silver chloride reference electrode” Sensors and Actuators B, vol. 22, pp.13-25, 1994.

[5] J.P. Tsao, C.W. Lin, ”Preparations and Characterizations of the Nafion/SiO2 Proton Exchange Composite Membrane”, Journal of Materials Science and Engineering, vol. 34, No. 1, pp.17-26, 2002.

[6] Michal Chudy, Wojciech Wroblewski, Zbigniew Brzozka, ”Towards REFET”, Sensors and actuators B, vol. 57, pp.47-50, 1999.

[7] Zhnan Bao, Ananth Dodabalapur, and Andrew J. Lovinger, ” Soluble and

processable regioregular poly(3-hexylthiophene) for thin film field-effect transistor applications with high mobility” Appl. Phys. Lett. vol. 69, pp.4108-4110, Dec. 1996.

[8] Chen Dong-chu, et al., ”Preparation of Nafion Coated Ag/AgCl Reference Electrode and Its Application in the pH Electrochemical Sensor”, Journal of Analysis Science, vol. 21, pp. 432-434, Aug. 2005.

Chapter 5 Future Work

In our experiment, sensitivity of various sensing material and structure are studied.

Based on the observed result, Nafion mix PR is a good candidate for REFET and solid-state reference electrode. But the coating process is not optimized in this experiment and the long-term endurance for the material is also not studied.

For the purpose of mass manufacture, the yield and reliability are the most important issues. When in experiment, the performance sometimes will fail and unstable during coating and measurement. So, the optimized coating method, including the influence of baking temperature or dropping manners, and the accurate thickness, needs to be studied further. The properties of these polymer materials and other new polymer stuffs are also need to be more understood.

Glass Membrane Internal

Reference Electrode Internal

Buffer Solution

Internal Conducting Line

Fig 1-1 Conventional pH glass electrode

Fig 1-2 ISFET Cross-Section Structure

Source Drain

Reference electrode

Fig 1-3 Encapsulation of the ISFET chip in the sensor for in vivo measurements in gynaecology: (a) chip encapsulation, (b) ISFET chip, (c)Ag/AgCl reference electrode, (d) reference electrolyte, (e) ceramic diaphragm,

(f) prefabricated moulded thermoplastic part, (g) sensor shaft (stainless steel) [9].

Fig. 1-4 Block diagram of a differential ISFET/REFET measuring system [8].

Fig 2-1 MOSFET and ISFET cross-section structure

Fig 2-2 ID-VDS curve of an ISFET with Vgs(a), and pH (b) as a parameter.

Source Drain

Reference electrode

Gate

Source Drain

Fig 2-3 Site-binding model

Fig. 2-4 Hemholtz model [2-23].

Fig. 2-4 Gouy-Chapman model [2-23].

Fig. 2-6 Gouy-Chapman-Stern model [2-23].

ψ

1

Diffuse layer Helmholtz layer

X

H

Fig 2-7 Potential profile and charge distribution at an oxide electrolyte solution interface

Fig 2-8 Electrode and electrolyte interface

C _i,d C _i,st

C _i,dl

Fig 2-9 Schematic representation of carriers hopping through a random array of sites.

P

P

t ty yp p e

e

S Si i S S ub

u

bs st tr ra at te e

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Fig. 3-1 Fabrication Process Flow

parameters of ZrO2 sputter power : 110 W Ar / O₂ : 24 / 8 ( sccm )

Density : 6.51 Acoustic impendance : 14.72

Tooling factor : 0.533 Rate : 0.01 Å / s

pre sputter 60W for 10 min Pressure : 7.6×10^-3

Table 3-1 ZrO2 Sputtering parameters

Test structures

NF-ZrO2 NF-P3HT NF-PR NF mix PR

NF-P3HT-HMDS NF-PR-HMDS NF mix PR-HMDS

Table 3-2 The test structures of REFET, NF=Nafion

Na

N

a fi

f

io on n

Fig 3-2 Measurement set-up

0.0 0.5 1.0 1.5 2.0 2.5 3.0

1.1 1.2 1.3 1.4 1.5 1.6

ZrO2

H⁺ Sensitivity = 57.89 mV/pH

IDS (mA)

VG (V)

pH = 13 pH = 5 pH = 11 pH = 3 pH = 9 pH = 1 pH = 7

Fig 3-3 Extraction method of sensitivity

Dark Box Gate

V

GS

V

DS1

V

DS2

Drain1 Source Drain2

Fig. 4-1 H⁺ sensitivity of ZrO2-ISFET

Na⁺ Sensitivity = 15.88 mV/pNa

IDS (mA)

Fig. 4-3 Nafion chemical structure and model

Fig. 4-4 H⁺ sensitivity of NF-ZrO2-ISFET

Fig. 4-5 Na⁺ sensitivity of NF-ZrO2-ISFET

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Na⁺ Sensitivity = 16.34 mV/pH

-1 0 1 2 3 4

Na⁺ Sensitivity = 16.34 mV/pNa

Fig. 4-6 The chemical structure of P3HT

Table 4-1 H⁺ sensitivity of NF-P3HT-ZrO2

# 1 # 2

0 10 20 30 40 50 60 70 80

Sensitivity (mV/pH)

P3HT w/o HMDS

Fig. 4-7 H⁺ sensitivity error bar of NF-P3HT-ZrO2

Fig. 4-8 H⁺ sensitivity of NF-P3HT-ZrO2

Fig. 4-9 Na⁺ sensitivity of NF-P3HT-ZrO2

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Na⁺ Sensitivity = 20.17 mV/pH

VG (V)

Na⁺ Sensitivity = 20.17 mV/pH

IDS (uA)

Table 4-2 H⁺ sensitivity of NF-P3HT-HMDS-ZrO2

# 3 # 4

0 10 20 30 40 50 60 70 80

P3HT with HMDS

Sensitivity (mV/pH)

Fig. 4-10 H⁺ sensitivity error bar of NF-P3HT-HMDS-ZrO2

Fig. 4-11 H⁺ sensitivity of NF-P3HT-HMDS-ZrO2

Na⁺ Sensitivity = 39.21 mV/pNa

IDS (uA)

Na⁺ Sensitivity = 39.21 mV/pNa

VG (V)

pNa value

Table 4-3 H⁺ sensitivity of NF-PR-ZrO2

# 5 # 6

-5 0 5 10 15 20 25

Sensitivity (mV/pH)

PR w/o HMDS

Fig. 4-13 H⁺ sensitivity error bar of NF-PR-ZrO2

Fig. 4-14 H⁺ sensitivity of NF-PR-ZrO2

Table 4-4 H⁺ sensitivity of NF-PR-HMDS-ZrO2

# 7

---5 0 5 10 15 20 25

PR with HMDS

Sensitivity (mV/pH)

Fig. 4-16 H⁺ sensitivity error bar of NF-PR-HMDS-ZrO2

Fig. 4-17 H⁺ sensitivity of NF-PR-HMDS-ZrO2

Na⁺ Sensitivity = 28.27 mV/pNa

IDS (uA)

Table 4-5 H⁺ sensitivity of NF-mix-PR ZrO2

# 8 # 9

0 5 10 15 20

Sensitivity (mV/pH)

NF-mix-PR-ZrO2

Fig. 4-19 H⁺ sensitivity error bar of NF-mix-PR ZrO2

Fig. 4-20 H⁺ sensitivity of NF-mix-PR ZrO2

Na⁺ Sensitivity = 19.89 mV/pNa

IDS (uA)

Na⁺ Sensitivity = 19.89 mV/pNa

VG (V)

pNa value

Table 4-6 H⁺ sensitivity of NF-mix-PR-HMDS-ZrO2

# 10 # 11

0 5 10 15 20

NF-mix-PR-HMDS-ZrO2

Sensitivity (mV/pH)

Fig. 4-22 H⁺ sensitivity error bar of NF-mix-PR-HMDS- ZrO2

在文檔中 以NafionTM/高分子材料為結構的感測層應用在pH-ISFET離子選擇場效電晶體之研究 (頁 43-0)