Chapter 2 Theory Description
2.4 Summary
In this chapter, we have introduced the basic concepts and theories of ISFET. The ISFET operational mechanism is similar to the MOSFET. In a constant drain to source voltage VDS, the drain current will only controlled by VG, but in ISFET, the drain current will also controlled by different pH values. Because there will be a chemical reaction at the oxide/electrolyte interface then build up charges and potential to change the drain current ID. We introduce the charging mechanism at the surface layer by site- binding model introduced by Yate et al.. We also describe the background electrolyte will also influence the surface charge density, and we use the Gouy-Chapman-Stern model to describe this mechanism. By these two models, we can get the critical parameters βint, Cdif and derive the expression equation of sensitivity o 2.3 non-ideal phenomena of ISFET, drift and hysteresis, which prevent the widespread application.
2.5 References
[1] Y. Q. Miao, J. R. Chen and K. M. Fang, “New technology for the detection of pH”, J. Biochem. Biophys. Methods, vol. 63, pp. 1-9, 2005.
[2] P. Bergveld, “Thirty years of ISFETOLOGY What happened in the past 30 years and what happen in the next 30 years”, Sensors and Actuators B, vol. 88, pp. 1-20, 2003.
[3] P. Bergveld, “ISFET, Theory and Practice”, in IEEE Sensor Conference, Toronto, Oct. 2003.
[4] R.E.G. van Hal et al. , “A general model to describe the electrostatic potential at electrolyte oxide interface”, Advance in Colloid and Interface Science, vol.69, pp.31-62, 1996.
[5] Miao Yuqing , Guan Jianguo, Chen Jianrong, “Ion sensitive field transducer-based biosensors”, Biotechnology Advances, vol. 21, pp.527-534, 2003.
[6] W. M. Siu, R. S. C. Cobbold, “Basic Properties of the Electrolyte-SiO2-Si System:
Physical and Theoretical Aspects”, IEEE Transactions on Electron Device, vol.
ED-26, NO. 11, Nov., 1979
[7] H.K. Liao, et al. ”Study on pHpzc and surface potential of tin oxide gate ISFET”, Materials Chemistry and Physics, vol. 59, pp.6-11, 1999
[8] R.E.G. van Hal, J.C.T. Eijkel, P.Bergveld, “A novel description of ISFET sensi- tivity with the buffer capacity and double layer capacitance as key parameters”, Sensors and Actuators B, vol. 24-25, pp.201-205, 1995.
[9] Tadayuki Matsuo, Masayoshi Esashi, ”Methods of ISFET Fabrication”, Sensors and Actuators, vol. 1, pp.77-96, 1981.
[10] Imants R. Lauks, Jay N. Zemel, “The Si3N4/Si Ion-Sensitive Semiconductor Electrode ”, IEEE Transactions on Electron Devices, vol. ED-26, no.12, pp. 1959- 1964, Dec., 1979.
[11] J.C. Chou, C.Y. Weng, “Sensitivity and hysteresis effect in AL2O3 gate pH- ISFET ”, Materials Chemistry and Physics, vol. 71, pp.120-124, 2001.
[12] P.D. van der Wal et al. ,”High-K Dielectrics for Use as ISFET Gate Oxide”, in Sensors, Proceedings of IEEE.2004.
[13] H.K.Liao et al.,” Study of amorphous tin oxide thin films for ISFET applications”, Sensors and Actuators B, vol.50, pp.104-109, 1998.
[14] Luc Bousse, Piet Bergveld, “The Role Of Buried OH Sites In The Response Mechanism Of Inorganic-Gate pH-Sensitive ISFETs”, Sensors and Actuators, vol. 6, pp.65-78, 1984.
[15] P. Woias, L.Meixner, P. Frostl, ”Slow pH response effects of silicon nitride ISFET sensors”, Sensors and Actuators B, vol. 48, pp.501-504, 1998.
[16] J.C. Chou, K.Y. Huang, J.S. Lin, ”Simulation of time-dependent effects of pH-ISFETs ” Sensors and Actuators B, vol. 62, pp.88-91, 2000.
[17] Luc Bousse et al. , ”Comparison of the hysteresis of Ta2O5 and Si3N4 pH-sensing insulators”, Sensors and Actuators B, vol.17, pp. 157-164,1994.
[18] J.C. Chou, Y.F. Wang, ”Preparation and study on the drift and hysteresis properties of the tin oxide gate ISFET by the sol-gel method”, Sensors and Actuators B, vol.86, pp.58-62, 2002.
[19] S. Jamasb, S. D. Collins, R. L. Smith, ”A Physical Model for Threshold Voltage Instability in Si3N4-Gate H+-Sensitive FET’S ( pH ISFET’s )”, IEEE Transactions on Electron Devices, vol. 45, no. 6, pp.1239-1245, Jun, 1998.
[20] H. Scher, Elliott W. Montroll, ”Anomalous transit-time dispersion in amorphous solid”, Physical Review B, vol. 12, no.6, pp.2455-2477, Sep., 1975.
[21] G. Pfister, H. Scher, ” Time-dependent electrical transport in amorphous solid:
As2Se3”, Physical Review B, vol. 15, no. 4, pp.2062-2082, Feb., 1977.
[22] J. Kakalios, R. A. Street, W. B. Jackson, ”Stretched-Exponential Relaxation Arising from Dispersive Diffusion of Hydrogen in Amorphous Sillicon”, Physical Review Letters, vol. 59, no.9, pp.1037-1040, Aug. 1987.
[23] 吳浩青, 李永舫, ＂電化學動力學＂, 科技圖書公司, 2001 年 2 月
[24] S. Jamasb, S. D. Collins, R. L. Smith, ”A Physically-based Model for Drift in Al2O3-gate pH ISFETs ” in International Conference on Solid-State Sensors and Actuators Chicago, June, 1997.
[25] S. Jamasb, S. D. Collins, R. L. Smith, ”A physical model for drift in pH ISFET ”, Sensors and Actuators B, vol. 49, pp.146-155, 1998.
[26] George T. Yu, S.K. Yeh, ”Hydrogen ion diffusion coefficient of silicon nitride thin films”, Applied Surface Science, vol. 202, pp.68-72, 2002.
Chapter 3
Experiment and Measurement
3.1 ISFET and REFET fabrication Process flow
To investigate the properties of P3HT, PR, Nafion as the REFET up most sensing layers, the co-fabricated ISFET/REFET is presented. All processes were accomplished in Nano Facility Center. The fabrication procedures are listed as follows and the process is illustrated in Fig 3-1:
（a）
1. RCA clean.
2. Wet oxide growth 6000Ǻ, 1050°C, 65mins.
（b）
3. Mask#1 to define Source/Drain region.
4. BOE wet etching of silicon dioxide.
5. Dry (Screening) oxide growth 300 Ǻ, 1050°C, 12mins.
6. Source/Drain implantation, Dose=5E15(1/cm2), Energy=25Kev.
7. Source/Drain annealing, 950°C, 30mins.
（c）
8. PECVD Oxide deposition 1μ m.
（d）
9. Mask#2 to define contact hole and gate region.
10. BOE wet etching of silicon dioxide.
11. Dry oxide growth 100 Ǻ, 850°C, 60mins.
（e）
12. Mask#3 to define the sensing layer region.
13. Sensing layer (ZrO2) deposition by Sputtering.300 Ǻ.
14. ZrO2 sintering 600°C, 30mins.
（f）
15. Mak#4 to define the contact and solid reference electrode region.
16. Ti/Pt deposition by Sputtering 150 Ǻ /350 Ǻ.
（g）
17. Backside Al evaporation 5000 Ǻ.
18. Pt and Al sintering 400°C, 30mins
（h）
19. Coating the polymer-based material as REFET sensing layer.
3.2 Key steps illustration
In step 8, we deposit SiO2 film of 1μm thickness by PECVD, The 1μm SiO2 film can protect the structure of a pH-ISFET. During a long period of electrolyte immersing, ions may diffuse and affect the ISFET’s electrical characterization and a thick SiO2 can eliminate the effect.
In step 14, a ZrO2 sensing layer is deposited by sputtering. It has been proved the ZrO2 film deposited by sputtering has good characteristics as a pH-ISFET sensing layer in our lab. Therefore, in this study we still use ZrO2 as the sensing layer and research the the suitable REFET for it. Table 3-1 is the sputtering parameters. In step16, before depositing Pt we have to deposit Ti as the adhesion layer. The adhesion between Pt/SiO2 is very poor, so we have to deposit Ti to improve the adhesion.
In step 19, a key step in our experiment, we coat the polymer-based material as the
REFET sensing layer. In this experiment, a three-layer structure is presented. Above the original sensing layer (ZrO2), a polymer-based material PR(FH6400) or P3HT is applied. After that, we coat Nafion above PR or P3HT as a passivation layer. In order to improve the adhesion of PR and P3HT with ZrO2, we apply HMDS before coating PR or P3HT. Besides the three-layer structure, a mix composition of Nafion and PR is also coating above the ZrO2 sening layer, we call this is an entrapment method. We think that this will reduce one coating time, and the interface induced uncertain problems of three-layer will also be reduced. The experiment test structures are listed in Table 3-2. Why we did not mix Nafion with P3HT is because they can not dissolve together.
According to the lectures, Nafon is extremely resistant to chemical attack and has relatively high working temperatures, so Nafion can protect the underlying polymer-based layer from damaging. There is also an additional advantage for using Nafion. That is Nafion is a cation exchange polymer, i.e. Nafion will not affect the hydrogen ions to pass and any sensitivity decreased will be from the contribution of polymer-based material. But, in our experimental experience, P3HT is very hydrophobic and PR(FH6400) is easy to dissolve in Nafion solutions, so the coating method and the quantity of liquid have to be controlled carefully, following is our process flow:
1. Dropping the PR or P3HT solution at the sensing layer region.
2. Dried in air for 6 hours, the solution becomes colloid with thickness of 5-10um.
3. Bake at 90°C for 5mins for the purpose of annealing and having a flat surface.
4. Dropping Nafion 2% solution above PR or P3HT with thickness of 2-3um.
5. Dried in air for 30mins
6. Baked at 90°C for 5mins to anneal Nafion film.
When dropping Nafion above PR(FH6400), we have to pay attention that Nafion solution must simultaneously cap the all parts of PR which coated at previous step, otherwise some part of PR will be etched entirely and cause damage to PR. This may be that PR is easy to dissolve in Nafion even PR become solid. If we cover all parts of PR, the dissolve stress will be equal at PR and PR film will be still fine, otherwise some parts of PR will against more stress and easy to break.
3.3 Measurement system
To investigate the characteristics of different polymer-based sensing layer, we measure the I-V curves for the pH-ISFET by using HP4156 as measurement tool and the system is shown in Fig 3-2. For preventing from light influence [2], the entire measurement procedures were executed in a dark box. The measured pH values are 1,3,5,7,9,11,13, and the pH buffer solution were supplied by Riedel-deHaen corp.
Preparation of NaCl solution is also needed in Na+ ions measurement. The NaCl salts are electronic grade and solutions are prepared in DI water with different mole concentrations, 10-3M, 10-2M, 10-1M, 1M .When preparation of NaCl solution, it have to know the solubility of Nacl at room temperature is about 37g Nacl per 100g water.
In order to reduce the preparation error of mole concentration, we prepare 1M solution, and the other mole concentrations are diluted by DI water.
3.3.1 Current-Voltage (I-V) measurement set-up
A HP-4156 semiconductor parameter analyzer system were set up to measure the I-V characteristic curves, in which include IDS-VGS and IDS-VDS curves at controlled
temperature. It have to take care when dropping the pH-buffer solution at the sensing region. Because the sensing areas are small, prevention of air bubbles generated at the interface is needed to pay attention. In order to measure at more stable situation, every pH value is immersed for 30 seconds before measurement.
From IDS-VGS curves, we can extract the pH sensitivity (mV/pH) of ZrO2 pH-ISFET. At first, finding the point of maximum transconductance, i.e. the maximum slope of IDS-VGS curves, and getting the current value (IDS). At the constant IDS of maximum transconductance, we can see that with different pH values the reference electrode voltage (VG) is shifted and the shifted voltage per pH value is the sensitivity, as illustrated in Fig. 3-3.
3.3.2 Current-Voltage (I-V) measurement set-up with solid reference electrode
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
2Fig. 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
2In 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
2Positive 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
2The 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
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