國 立 交 通 大 學
電子工程學系 電子研究所碩士班
碩 士 論 文
以二氧化鋯為感測層之離子場效電晶體其離子
以二氧化鋯為感測層之離子場效電晶體其離子
以二氧化鋯為感測層之離子場效電晶體其離子
以二氧化鋯為感測層之離子場效電晶體其離子偵測
偵測
偵測
偵測與
與
與
與
干擾之特性研究
干擾之特性研究
干擾之特性研究
干擾之特性研究
The study of ion detection and interference on ZrO
2gate pH-ISFETs
指導教授: 張國明 博士
桂正楣 博士
Advisor: Dr. Kow-Ming Chang Dr. Cheng-May Kwei
學生: 林卓慶
Student: Cho-Ching Lin
以二氧化鋯為感測層之離子場效電晶體其離子
以二氧化鋯為感測層之離子場效電晶體其離子
以二氧化鋯為感測層之離子場效電晶體其離子
以二氧化鋯為感測層之離子場效電晶體其離子偵測
偵測
偵測
偵測與
與
與
與
干擾之特性研究
干擾之特性研究
干擾之特性研究
干擾之特性研究
The study of ion detection and interference on ZrO
2gate pH-ISFETs
研 究 生：林卓慶 Student：Cho-Ching Lin
指導教授：張國明 博士 Advisor：Kow-Ming Chang
桂正楣 博士 Cheng-May Kwei
國 立 交 通 大 學
電子工程學系 電子研究所碩士班
碩 士 論 文
A ThesisSubmitted to Department of Electronics Engineering & Institute of Electronics College of Electrical and Computer Engineering
National Chiao Tung University in Partial Fulfillment of the Requirements
for the Degree of Master
in
Electronics Engineering August 2009
Hsinchu, Taiwan, Republic of China
以
以
以
以二氧化鋯
二氧化鋯
二氧化鋯
二氧化鋯為
為
為感測層
為
感測層之
感測層
感測層
之
之離子場效電晶體
之
離子場效電晶體
離子場效電晶體其離子偵測與干擾
離子場效電晶體
其離子偵測與干擾
其離子偵測與干擾
其離子偵測與干擾
之
之
之
之特性
特性
特性研究
特性
研究
研究
研究
學生：林卓慶 指導教授：張國明 博士
桂正楣 博士
國立交通大學
電子工程學系 電子研究所碩士班
摘
摘
摘
摘 要
要
要
要
這篇論文我們將利用一個建構基礎在傳統的參考電極(RE)以及金屬場效電 晶體(MOSFET)的電子感測元件，離子感測場效電晶體(ISFET)來處理一個多種離 子混合之反面議題；這個議題正是 ISFET 的離子干擾與離子偵測現象的研究。 其中由於感測層 ZrO2 對某些離子有較高的感測性與選擇性，以及氫離子(H+)、 鉀離子(K+ )與鈉離子(Na+)對人體機能的影響有相當的重要性，所以這篇文章將著 重探討於由薄膜二氧化鋯(ZrO2)做成的離子感測場效電晶體(ISFET)之技術製程 與其對於氫離子(H+ )、鉀離子(K+)以及鈉離子(Na+)偵測的電性分析。而該電子感 測元件對於多種的，甚至是不同的離子之感測度的獲得，是利用參考電極、電解 液及特定的感測層來取代傳統場效電晶體的金屬閘極結構，其中參考電極的部份 將會直接接觸並浸泡在溶液之中去決定待測溶液的酸鹼值(pH 值)的大小，甚至 是其他離子的濃度。 正如我們所知，這個由薄膜二氧化鋯(ZrO2)做成的離子感測場效電晶體(ISFET)化學電子元件的感測度表現出大約為 58 mV/pH，這個偵測能力與理論中 能士特方程式的吻合度高達百分之九十八左右。不過 pH 的響應並不是我們的主 要研究目標，並且 ZrO2這個薄膜(也就是感測層)對離子的選擇性在含有較多且 複雜的離子溶液之中，其主要離子的感測度將會被次要離子們所影響。因此，在 這篇論文中，我們將會先研究在酸鹼溶液中的鉀離子與鈉離子的電性量測與感測 度分析，也就是 pK 以及 pNa 的量測實驗。而經由實驗結果的討論與驗證，鈉離 子與鉀離子的感測度兩者大致上都低於 20 mV/pH 左右，並且氫離子的感測度在 較高的鈉離子與鉀離子濃度(pK 以及 pNa 的值低於 3)之中會比較不明顯。這個結 果也就表示，氫離子(H+ )、鉀離子(K+)以及鈉離子(Na+)的感測度會彼此相互影響 的原因就是高濃度離子所造成的飽和現象。
The study of ion detection and interference on ZrO
2gate
pH-ISFETs
Student: Cho-Ching Lin Advisor: Dr. Kow-Ming Chang
Dr. Cheng-May Kwei
Department of Electronics Engineering & Institute of Electronics
National Chiao Tung University
ABSTRACT
This article deals with an inverse problem of ion mixture composition estimation using electronic sensors based on conventional reference electrode (RE) and MOS transistors; this topic exactly is ion detection and interference of ISFET. As a result of higher sensitivity as well as selectivity of the sensing layer ZrO2 for some ion and the
importance of H+, K+ and Na+ ions to human body mechanism, it reports the technological fabrication and the electrical characterization of ZrO2 ion sensitive field
effect transistors (ISFET) for the detection of H+, K+ and Na+ ions. The device sensitivity to various ions is obtained by replacing the traditional transistor metal gate electrode with the series combination of the reference electrode, electrolyte and specific sensing layers, the first one is immersed the aqueous solution to detect the pH value and sensitive to the other ions in an electrolyte flowing over the gate.
So far as we know, ZrO2 ISFET chemical sensors show quasi-nernstian pH
the Nernst equation. However, it is not our main goal for pH response as well as selectivity of the membrane (i.e. sensing layer) ZrO2 is limited and ions other than the
main one also influence the measurement in complex solutions. Therefore, in this study, we will first investigate K+ and Na+ ions measurement in acid or base solution, that is, pK and pNa measurement. By way of evidencing, sensitivities of K+ and Na+ ions is lower than 20 mV/pH and non-nernstian pH-dependent phenomena for highest K+ or Na+ ions concentrations (pK and pNa lower than about 3). It is shown that the detection properties of H+, K+ and Na+ ions are dependent on each other, being responsible for saturation effects for the highest concentrations.
誌
誌
誌
誌 謝
謝
謝
謝
學生能完成此一論文，首先要感謝張國明老師以及桂正楣老師能讓我有機會 參與 ISFET 的研究，張老師以及桂老師就像明燈一般在學術上給予我許多關鍵 性的指導，使我在擁有最豐富資源的交通大學裡學習到專業上的知識，因而我才 能夠順利完成論文。 再來要感謝張知天學長對此論文理論及實驗部分的完善規劃，我在研究過程 中若遇到困惑，他的指導總讓我豁然開朗，使我有一有觀念上的謬誤，必能即時 修正，使得我的研究能夠順利進行並且達到預期的成果。 除此之外，我還要感謝士軒學長、庭嘉學長、同儕仲逸、秉燏以及昆謀在實 驗製程上的幫助與解惑，我們總會互相討論於教學相長之下，使我自己獲益良 多；當然還有感謝我女朋友珮如在我心情鬱悶的時候總是會鼓勵與支持我，使我 的態度由消極轉為積極，讓我得以在 ISFET 的領域了解其真實內涵。 最後還要感養育我的慈父林肇成先生，以及母親劉秀卿女士，由於兩位契而 不捨的栽培，小兒才能在此完成學業，很感謝他們倆的支持，將來期許自己能夠 為社會貢獻自己微薄的力量，不枉費老師以及父母對我的期許。 誌于 2009.08 林卓慶Contents
Abstract (in Chinese)
………... iAbstract (in English)
……….... iiiAcknowledgement
……….... vContents
……….... viFigure Captions
……….... viiiTable Captions
……….... xChapter 1
Introduction
………... 11.1 The cognition of pH-ISFETs………... 1
1.2 The characteristics of pH-ISFETs………... 1
1.3 1.4 1.5 The introduction to reference electrode……….……... Motivation of this work………... References……….. 2 3 4
Chapter 2
Theory Description
……….…... 52.1 2.1.1 2.1.2 Introduction to pH……...……….... Basic definition of pH………. How to detect pH……… 5 5 6 2.2 2.2.1 2.2.2 The realization of ISFET…………...………... Comparison between MOSFET and ISFET……… The Oxide-Electrolyte Interface pH response………. 7 8 10 2.3 The phenomenon of membrane selectivity………...….. 14
2.4 References..………... 15
Chapter 3
Experiment and Measurement
………... 183.1 The fabrication process of ISFET..………... 18 3.2
3.2.1
The key steps of the experiment……….. Na+ and K+ ion solution allocation………..
19 19
3.2.2 3.2.3
Gate region formation………. Sensing layer deposition……….
20 20 3.3 3.3.1 3.3.2 Measurement system……….………... Preparation of measurement……… Current-Voltage measurement set-up……….
21 21 21
3.4 References………... 22
Chapter 4
Results and Discussions
………... 244.1 Introduction………... 24
4.2 The glass reference electrode for pH measurements…..……... 24
4.3 pK and pNa measurements……….…... 25
4.4 4.5 4.6 4,7 Simultaneous pH, pK and pNa measurements…….…………... Simultaneous detection explanation of the H+, Na+ and K+ ions.. Conclusion……….. References……….. 25 26 27 28
Chapter 5
Future Work
………... 29Figure captions
Figure 1-1 Schematic representation of (a) MOSFET (b) ISFET. ………….…..30
Figure 2-1 Conventional pH glass electrode. ……….…..30
Figure 2-2 Electrode and electrolyte interface. ……….…...31
Figure 2-3 Schematic representation of the site-binding model. ……….31
Figure 3-1 Fabrication process flow. ………...32-36 Figure 3-2 Measurement setup. ………36
Figure 3-3 Detection principle of pH. ………..37
Figure 4-1 Id-Vg curve of ZrO2 to n-type ISFET. ………...38
Figure 4-2 Sensitivity characteristic of ZrO2 to n-type ISFET. ………...38
Figure 4-3 pK response of ZrO2 ISFET for pH=9 buffer solution. ………..39
Figure 4-4 pNa response of ZrO2 ISFET for pH=9 buffer solution. ………39
Figure 4-5 pK response of ZrO2 ISFET for pH=3 buffer solution. ………..40
Figure 4-6 pNa response of ZrO2 ISFET for pH=3 buffer solution. ………40
Figure 4-7 pK response of ZrO2 ISFET for pH=5 buffer solution. ………..41
Figure 4-8 pNa response of ZrO2 ISFET for pH=5 buffer solution. ………...….41
Figure 4-9 pK response of ZrO2 ISFET for pH=7 buffer solution. ………..42
Figure 4-10 pNa response of ZrO2 ISFET for pH=7 buffer solution. ………42
Figure 4-11 pK response of ZrO2 ISFET for pH=11 buffer solution. ………43
Figure 4-12 pNa response of ZrO2 ISFET for pH=11 buffer solution. …………..43
Figure 4-13 pK response of ZrO2 ISFET. ………..44
Figure 4-14 pNa response of ZrO2 ISFET. ………....44
Figure 4-15 normalized pK response of ZrO2 ISFET. ………...…45
Figure 4-17 pH response of ZrO2 ISFET in pK=1 solution. ……….46
Figure 4-18 pH response of ZrO2 ISFET in pK=3 solution. ……….46
Figure 4-19 pH response of ZrO2 ISFET in pK=5 solution. ……….47
Figure 4-20 pH response of ZrO2 ISFET in pK=7 solution. ……….47
Figure 4-21 pH response of ZrO2 ISFET in pK=9 solution. ……….48
Figure 4-22 pH response of ZrO2 ISFET in pNa=1 solution. ………...48
Figure 4-23 pH response of ZrO2 ISFET in pNa=3 solution. ………...49
Figure 4-24 pH response of ZrO2 ISFET in pNa=5 solution. ………...49
Figure 4-25 pH response of ZrO2 ISFET in pNa=7 solution. ………...50
Figure 4-26 pH response of ZrO2 ISFET in pNa=9 solution. ………...50
Figure 4-27 pH sensitivity of ZrO2 ISFET in different pK solutions. …………..51
Figure 4-28 pH sensitivity of ZrO2 ISFET in different pNa solutions. ………....51
Table captions
Table 1-1 Specifications of wafers. ………52 Table 1-2 Sensitivity for different sensing layers. ……….53 Table 3-1 Specifications of wafersParameters of sensing layers deposition with
Chapter 1
Introduction
1.1 The cognition of ISFET
Ion-sensitive-field-effect-transistor (ISFET), the most importance of lots of biomedical sensors, was first pretended by P.Bergveld in the early 1970s [1]. Its numerous properties are very like to metal-oxide-semiconductor-field-effect-transistor (MOSFET) and its basic wafer is in Table 1-1 because it just uses silica as gate and integrate MOSFET with the peculiarity of traditional ion-sensitive-electrode (ISE). Thus the obvious distinction of ISFET and MOSFET is that the MOSFET metal/poly gate is replaced by sensing (silica) layer exposed to the solution directly. It will make that ISFET can sense diverse pH solutions and detect a number of various ions by means of H+ or other ions accumulating on the top of sensing material. Furthermore, it also can be calculated by the current transformation caused by H+ ions difference on sensing layer..
1.2 The characteristics of pH-ISFETs
The development of ISFET has been on going for more than 35 years from the 1970s as an alternative to the fragile glass electrode for the measurement of pH or concentrations of ions (Na+, K+, Cl-, NH4+, Ca2+, etc.), and the first ISFET sensing
layer exploited was silicon dioxide (SiO2), which showed an unstable sensitivity and a
large drift. Recently, there are many materials have been investigated and applied for the ion sensing layer. Table 1-2 shows the sensitivity and test ranges of different
sensing layers. It is found that pH sensitivity is one of the important characteristic parameters of the ISFET device and the response of the ISFET is mainly determined with the type of the sensing layer, thus its material plays a significant role in ISFET field. So in this study, we will use zirconium oxide (ZrO2) as the ion sensing layer.
The structure of ISFET is similar to the MOSFET except for metal gate electrode removed and deposited a sensing film on the gate oxide, as shown in Fig. 1-1. The upmost sensing layer will have chemical reactions with the test or buffer solutions and build up charges at the surface so that the surface charges will induce surface potential as well as change the threshold voltage of the ISFET, hence the operation current ID
will also change.
Comparing with the traditional pH-meter using glass electrode, ISFET has following features:
(1) Small sample requirement (2) Short response time (3) Small size and weight
(4) Potential of mass production at low cost (5) Compatible with the standard CMOS process
Nevertheless, it is possible to generate a variety of chemical sensors with small size down to micrometer scale so that only a small amount of the test solution should be necessary, but this is useless owing to the lack of a miniaturized reference electrode [2].
1.3 The introduction of reference electrode
An ideal reference electrode for use as the ISFET gate terminal should provide [3]:
(1) An electrical contact to the solution from which to define the solution potential;
(2) An electrode/solution potential difference that does not vary with solution composition.
The conventional silver chloride or calomel electrode provides both of these functions by maintaining an electrochemical equilibrium with the solution. Novel techniques are to fabricate electrodes in miniaturized dimensions [4] [5]. The on-chip fabrication of a reference of a reference electrode with IC-compatible techniques would make ISFET suitable for biomedical sensing because of the low cost, small size and rigidity.
1.4 Motivation of this work
Although we previously come at that ion-sensitive-field-effect-transistors (ISFETs) could be developed as an alternative to the fragile glass electrode for the measurement of pH and concentrations of ions such as Na+, K+, Cl-, NH4+, Ca2+, etc
from many famous studies, however, we rarely focus on the ion interference for above-mentioned ions. Certainly, we also find some sensing layers for ISFETs with good hydrogen ion sensitivity, but we also did not concentrate on the sensing layers with good selectivity or not. It will be very essential for the simultaneous measurement or detection.
On the bases of upward description, in this article, we will show that ZrO2 can
provide a solution for the pH, pK or pNa simultaneous measurements since it is sensitive to H+, K+ and Na+ ions. And it also reports the technological fabrication and the electrical characterization of ZrO2 ISFET chemical sensors, investigating more
influences for simultaneous pH, pK and pNa measurement.
1.5 References
[1] P. Bergveld, “Development of an ion sensitive solid-state device for neurophysiological measurements,” IEEE Trans.Biomed. Eng.,vol. BME-17, p.70, 1970.
[2] A. Simonis, H. Luith, J. Wang, M.J. Schoning, “New concepts of miniaturized reference electrodes in silicon technology for potentiometric sensor system,” Sens. Actuators B 103, pp. 429-435, 2004.
[3] Paul A. Hammond, Danish Ali, and David R. S. Cumming, ”Design of a Single-Chip pH Sensor Using a Conventional 0.6-µm CMOS Process,” IEEE Sensors Journal, vol. 4, no. 6, Dec, 2004.
[4] R.L. Smith and D.C. Scott, “An integrated sensor for electrochemical measurements,” IEEE Trans. Biomed. Eng. BME 33 (1986) 83-90.
[5] I.Y. Huang and R.S. Huang, “Fabrication and characterization of a new plnar solid-state reference electrode for ISFET sensors,” Thin Solid Films 406 (2002) 225-261.
Chapter 2
Theory
Description
2.1 Introduction to pH
The word, pH, was form by two letters p and H, and the first means ‘Power’ as well as the second means ‘Hydrogen’ [1]. As implied in the name, pH is a basic measurement of how many H+ ions in a unknown solution is. In general, aqueous solutions with a pH value less than 7.0 are treated as acid solutions, greater than 7.0 are treated as basic solution. And equal to 7.0 are treated as neutral solution at room temperature 25 ℃ because the concentration of H3O+ equals the concentration of
OH− in pure water [2]. Detecting pH is essential in finding the chemical characteristics of material because it is one of the most common biomedical measurements.
2.1.1 Basic definition of pH
Generally speaking, there are acid (H+
)
and alkali (OH−) ions in any complex solution, and they are in equilibrium all the time.H2O ↔ H+
+
OH− (2-1)The basic definition of pH is expressed as
log log [ ] H pH a + γ H + = − = − (2-2) where H
a + is the hydrogen ion activity, γis the activity coefficient which equals to 1 when diluted solution, and [H+] is the molar concentration of solvated protons in
units of moles per liter. Actually, pH depends on a lot of factors, such as the concentration of the added acid and its dissociation constant [3].
2.1.2 How to detect pH
Previously, there are many methods measuring the pH value, including: (1) Indicator reagent
(2) pH test strips (3) Metal electrode (4) Glass electrode
There are critical drawbacks on above methods, except for glass electrode. For example, the first and second methods are differentiated from colors and both are impossible to reach high accurate pH value, and the third method is difficult for daily use due to the inconvenience of reproducing hydrogen gas [4]. The fourth method glass electrode modernly becomes the most extensively used method for the pH measurement on account of some limitations in practical applications of the first three methods. Therefore, we need to illustrate this topic glass electrode conscientiously.
The first pH glass electrode was improved by M. Cremer wih Fritz Haber. It composed of an electrode membrane that determines the pH value, and a stationary concentration of HCl or a buffered chloride solution inside in contact with an internal reference electrode, which use of Ag/AgCl, as shown in Fig. 2-1.
When the glass electrode is immersed in pH buffer solution, the outer bulb surface will be hydrated and exchange sodium ions for hydrogen ions to build up a surface layer of hydrogen ions [2], the build up of charges on the inside of the membrane is proportional to the amount of hydrogen ions in the outside solution. The potential difference between inside and outside can be derived by nernst equation:
+
0 H
E=E RTlna nF
+ (2-3)
where E = electrode potential, E0 = standard potential of the electrode, R = gas constant (8.31441JK mol-1 -1), T = temperature (in Kelvin), n = valance (n = 1 for hydrogen ions), F = Faraday constant and +
H
a = activity of hydrogen ions.
According to the equation, providing that side of the interface the activity of the ion of interest is kept constant, the electrode potential is direct logarithmic function of the ion activity on the other side [5]. As a result of ideal Nernstian response independent of redox interferences, short balancing time of electrical potential, high reproducibility, high selectivity, reliability, wide pH range, and long lifetime. It is most universal used for pH measurement. Nevertheless, glass electrode has some drawbacks for many industrial applications:
(1) Unstable in alkaline or HF solutions or at high temperature (2) Exhibiting a sluggish response and Difficult to miniaturize (3) Cannot be used in food due to their brittle nature
(4) Must be used at the vertical position for chemical reproducibility
Consequently, there is an increasing need for alternative pH sensor, the ISFET-based pH sensor is a new and appropriate technique for pH detection.
2.2 The realization of ISFET
Since the first ion-sensitive-field-effect transistors (ISFETS) study by P. Bergveld [6], ISFET had gone into a new potential type of chemical sensing device. This device is very similar to metal-oxide-semiconductor-field-effect transistor (MOSFET) except for the metal gate electrode replaced with a reference electrode inserted in a buffer solution which is in touched with the sensing layer above gate
oxide. Deserving to be mentioned, The ISFET used to measure or sense ion concentration in unknown solution. The brief construction of MOSFET and ISFET are shown in Fig. 1-1 once again.
2.2.1 Comparison between MOSFET and ISFET
Because there are numerous similarities between with ISFET and MOSFET and the main difference between ISFET and MOSFET is that MOSFET metal/poly gate electrode is replaced with sensing layers, the series combination of the reference electrode, electrolyte, and chemically sensitive insulator or membrane. The best method to interpret this ISFET device is to review the operation of the MOSFET device firstly.
While MOSFET is operated in the so-called non-saturated region, the general expression for the drain current ID is given by:
(
)
1 2 OX D GS T DS DS C W I V V V V L µ = − − (2-4) where COX is the gate insulator capacitance per unit area, W and L are the channel width and the channel length respectively such that W/L is the width-to-length ratio of the channel, µ is the electron mobility in the channel, VGS is gate to source voltage,VDS is drain to source voltage and VT is the threshold voltage. By the way, if the fabrication process is controlled and biased well in designed applied electronic circuit, we will hold the geometric sensitivity parameter = CoxW
L
β µ , VDS , and VT , the drain ID will be a unique function of the only variable VGS .
The so-called threshold voltage VT of the MOSFET is: 2 B T FB F OX Q V V C φ = − + (2-5)
ψF is the potential difference between the Fermi level and intrinsic Fermi level and it is dependent on the doped concentration. VFB also can be illustrated by the following expression: M Si OX SS FB OX Q Q V q C Φ −Φ + = − (2-6)
where the first term describes the difference between the gate metal work function ΦM and the silicon work function ΦSi, the second term is caused by the charge accumulated in the oxide QOX and at the oxide-silicon interface surface QSS.
Then we Substitute Eq. (2-5) in Eq (2-6), the common form of the threshold voltage of the MOSFET can be described by the following expression:
2 M Si OX SS B T F OX Q Q Q V φ q C Φ −Φ + + = − + (2-7)
The threshold voltage of ISFET contains terms that the interface between the liquid and oxide, the others between liquid and reference electrode when ISFET is immersed in a liquid. So that, The surface potential must take in into account. In a word, the threshold voltage becomes the following expression:
0 2 sol Si OX SS B T ref F OX Q Q Q V E χ Ψ φ q C Φ + + = + − − − + (2-8)
where Eref is the constant potential of the reference electrode, χsol is the surface dipole potential of the solution which also has a constant value. The surface dipole potential is different from aqueous solution, even though a little variation of surface dipole potential at disparity aqueous solution. The value compare to other term is too small to take as a constant. All terms are constant except Ψ0, it is the kernel of ISFET sensitivity to the electrolyte pH which is controlling the dissociation of the oxide surface [7]. In order to obtain an accuracy pH value, to investigate a high pH sensitivity ISFET on the electrode-electrolyte interface is necessary.
In brief, the ISFET is very similar to the MOSFET structurally and electronically, but with one more property: the possibility to chemically modify the threshold voltage
via the interfacial potential at the oxide-electrolyte interface. And we will focus on the key point of the ISFET, the oxide-electrolyte interface.
2.2.2 The Oxide-Electrolyte Interface pH response
Based on the site-binding model introduced by Yate et al [8], the oxide/electrolyte interface will build up charges and generate an extra electrostatic potential while we are immersing the ISFET in the pH buffer solution. That is, the properties of the ISFET are exactly controlled by the performance of the oxide-electrolyte interface, protonation/deprotonation of the gate material is influenced by the pH solution which dominate the surface potential.
The charging mechanism at the surface is the most well-known Site-Binding model introduced by Yate et al [8]. It describes the charging mechanism at the oxide/electrolyte interface in Fig. 2-2 and Fig. 2-3. The surface of any metal oxide always contains hydroxyl groups, for example, in the case of silicon dioxide is SiOH groups [9]. These groups consist of donate and accept a proton from the solution. Therefore, as ISFET sensing layer like SiO2 contact an aqueous solution, the change
of pH will change the SiO2 surface potential. These reactions can be expressed by:
1 2 K S H++SiOH←→SiOH+ (2-9) 2 K -S SiOH←→SiO +H+ (2-10)
where HS+ represents the protons at the surface of the oxide, K1 and K2 are
2 2 1 2 . [ ] (2-11) [ ][ ] . [ ][ ] [ ] s s SiOH H s SiOH SiO H s SiOH v a SiOH K or v SiOH H v a SiO H K or SiOH v + + − + + + − + = = (2-12)
where S denotes the surface.
The potential between the gate insulator surface and the electrolyte solution causes a proton concentration difference between bulk and surface that is according to Boltzmann [10]. 0 exp s B H H q a a kT + + − = Ψ (2-13) or 0 2.3 S B q pH pH kT = + Ψ (2-14) where + S H
a is the activity of the oxide surface and + B
H
a is the activity of the
solution bulk individually, q is the elementary charge, k is the Boltzmann constant, T is the absolute temperature, pHS is the pH value at the oxide surface and pHB is the pH value in the solution bulk, Ψ0 is surface potential. By the way, the subscripts
B and S refer to the bulk and the surface, respectively.
Now we can start with the fixed number of surface site per unit area NS:
2
S SiOH SiO SiOH
N =v +v − +v + (2-15)
Based on some electrochemical knowledge and math derivation, the surface charge density σ0[ /C m2] can be available:
2
0 q v( SiOH vSiO ) qB
where B is the number of negatively charged groups minus the number of positively charged groups in mole per unit area. It can show that when the number of positively and negatively charged groups on the surface is equal and consequently, there will be no net charge on the surface. In this situation, we can say the pH value at the point of zero charge is pHpzc. One more thing we have to know is that different
operations of ISFETs (flat band condition and linear region) will yield different value of pHpzc [11]. Then: 2 0 2 S S S H a b S a b b H H a K K qN K K K a a σ + + + − = + + (2-17)
where Ka and Kbare dissociation constant. And a detailed derivation can see the Ref. [2] [7]. After getting the surface charge density, then we can look for the intrinsic buffer capacity βint, the capability of the surface to store charge as result of
a small change in the H+ concentration, defined as 0 int S S B q q pH pH σ β ∂ ∂ = − = − ∂ ∂ (2-18) Now we can obtain the equation for intrinsic buffer capacity from above equation (2-17) and (2-18):
(
)
2 2 int 2 2 4 2.3 S S S S S b H a b H a b S H a b b H H K a K K a K K N a K K K a a β + + + + + + + = + + (2-19)Possibly owing to buffering small changes in surface pH (pHS) and not in the
bulk pH (pHB), so that it is called “intrinsic” buffer capacity. We also can see that the
value of NS, Ka, and Kb are oxide dependent. More surface sites will have greaterβint. In accordance with [7], Hydrolysis of the surface will create more surface
sites and thus a rise in the intrinsic buffer capacity and the sensitivity.
The surface charge density will affected by the surface reaction and the background electrolyte that result from variations in the double layer capacitance.
Because of charge neutrality, an equal but opposite charge is built up in the electrolyte solution side of the double layer σDL , shown in Figure 2-2. This charge can be
described as a function of the integral double layer capacitance, Ci, and the
electrostatic potential:
σDL=-CiΨ0=-σ0 (2-20)
The integral capacitance will be used later to calculate the total response of the ISFET on changes in pH. The ability of the electrolyte solution to adjust the amount the of stored charge as result of a small change in the electrostatic potential is the differential capacitance, CS: 0 0 0 DL S C σ σ ∆ ∆ = − = − ∆Ψ ∆Ψ (2-21)
As a result, combination of (2-18), (2-20), and (2-21) lead to an expression for the sensitivity of the the electrostatic potential change in
S H a : 0 s pH ∆Ψ ∆ = 0 0 σ ∆ ∆ Ψ 0 s pH σ ∆ ∆ = C S S qβ − = 0 0 ( ) 2.3 B q pH kT ∆Ψ Ψ ∆ + (2-22)
Rearrange Eq. 2-15 gives a general expression for the sensitivity of the electrostatic potential to changes in the bulk pH [9]:
0 2.3 B kT pH q α ∆Ψ = − ∆ (2-23) with 2 1 2.3 1 S S kTC q α β = + (2-24)
The parameter α is a dimensionless sensitivity parameter that varies between 0 and 1, depending on the intrinsic buffer capacity, βS , of the oxide surface and the differential capacitance CS . We can get the maximum value α so that the
sensitivity become -59.2 mV/pH at 298K which is called Nernstian sensitivity. Therefore, the intrinsic buffer capacity βS need to be the more higher or the double layer capacity CS to be the more lower. In ideal, the intrinsic buffer capacity βS=∞ or the double layer capacity
S
C =0 would be the best. It appears that the usual SiO2 from MOSFET does not fulfil
the requirements of a high vale of βS. The pH sensitivity is low depending also on the electrolyte concentration through CS. Therefore other films such as ZrO2 were
introduced to increase the values of βS. The higher the intrinsic buffer capacity so that the less important of the value of CS which means that independent of the electrolyte concentration a Nernstian sensitivity can be achieved over a pH range from 1 to 13.
In sum, to use SiO2 from the MOSFET process does not obtain the requirements
of a high value of βint. The pH sensitivity is only about 30mV/pH, so the research
nowadays is to find high sensitivity sensing film. The material found by high sensitivity is Si3N4, Al2O3, and SnO2 [13-17].
2.3 The phenomenon of membrane selectivity
The most important problem encountered during measurements of the ion concentration with CHEMFETs is related to the limited membrane selectivity. As a result, the membrane potential varies not only with the concentration of the main ion to be detected, but also it is dependent on the concentration of some other ions, called interfering ions. The most commonly employed model of the phenomena occurring in the sensor membrane is based on the semi-empirical Nikolski-Eisenman equation, derived from the Nernst equation [18]. The main advantage of this approach is that it is time efficient, quite accurate and it can be easily implemented in any simulation
environment. Moreover, the model parameters, such as the selectivity coefficients kij,
can be found analytically without any difficulties. According to this model, the membrane potential changes ∆VT in presence of various ions in the analyzed
solution can be expressed by the following equation [19]: i j i ij j j i i ln[ + ( ) ] z z T RT V a k T a z F ≠ ∆ =
∑
⋅ (2-25) where F is the Faraday constant, R is the gas constant, T is the absolute temperature, a is the ion activity, i, j are the main and interfering ion indices respectively and z is the ion electrovalence. The issue, which has to be commented on, is the difference between the ion concentration and its activity. These two quantities are related through the so-called ionic strength of the solution. However, except for very strong solutions, they are equal to each other and thus these terms will be used interchangeably although all the simulations the ion activity is always taken into account.2.4 References
[1] Sfrenson SPL. Enzyme studies II: the measurement and meaning of hydrogen ion concentration in enzymatic processes. Biochem Z 1909;21:131–200.
[2] P. Bergveld, “ISFET, Theory and Practice”, in IEEE Sensor Conference, Toronto, Oct. 2003
[3] D.A. Skoog, D.M. West, and F.J. Holler, Fundamentals of Analytical Chemistry, 7th ed., Philadelphia, PA: Saunders College Publishing, 1996.
[4] Y. Q. Miao, J. R. Chen and K. M. Fang, New technology for the detection of pH, J. Biochem. Biophys. Methods 63 (2005) 1-9.
and what may happen in the next 30 years” Sensors and Actautors B 88 (2003)1-20 [6] P. Bergveld, “Development of an ion sensitive solid-state device for neurophysiological measurements” IEEE Trans.Biomed. Eng.,vol. BME-17, p.70, 1970.
[7] R.E.G. van Hal, J.C.T. Eijkel and P. Bergveld, A general model to describe the electrostatic potential at electrolyte oxide interfaces, Adv. Coll. Interf. Sci. 69 (1996) 31-62.
[8] D.E. Yates, S. Levine and T.W. Healy, Site-binding model of the electrical double layer at the oxide/water interface, J. Chem. Soc., Faraday Trans. 70 (1974) 1807-1818.
[9] Dr. Ir. P.Bergveld Em University of Twente, ISFET, Theory and Practice, IEEE senor conference Toronto,October 2003 1-26.
[10] W. M. Siu and R. S. C. Cobbold. “Basic Properties of the Electrolyte-SiO2-Si
System: Physical and Theoretical Aspects, ”IEEE Trans. Electron Device, ED vol. 26, pp. 1805-1815, 1979.
[11] H.K. Liao, et al. “Study on and surface potential of tin oxide gate ISFET”, Materials Chemistry and Physics, vol. 59, pp.6-11, 1999.
[12] Dr. Ir. P.Bergveld Em University of Twente, ISFET, Theory and Practice, IEEE sensor conference Toronto, October 2003 1-26.
[13] 吳浩青, 李永舫, “電化學動力學”, 科技圖書公司, 2001 年 2 月
[14] Tadayuki Matsuo and Masayoshi Esashi, Methods of ISFET fabrication, Sens. Actuators 1 (1981) 77-96.
[15] Imants R. Lauks, Jay N. Zemel, “The Si3N4/Si Ion-Sensitive Semiconductor
Electrode”, IEEE Transaction on Electron Devices, vol. ED-26, no. 12, pp. 1959-1964, Dec., 1979.
pH-ISFET”, Materials Chemistry and Physics, vol. 71, pp. 120-124, 2001.
[17] 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.
[18] G.Eisenman, D.Rudin and J. Casby, Glass electrode for measuring sodium ion. Science (1957), 61, 831-834.
[19] M. JANICKI and A. NAPIERALSKI, Department of Microelectronics and Computer Science, Technical University of Lodz, Al. Politechniki 11, 93-590 Lodz, Poland.
Chapter 3
Experiment and Measurement
3.1 The fabrication process of ISFET
All procedures of experiment are done in NDL (National Nano Device Laboratory) and NFC (Nano Facility center), similar to the manufacturing process of MOSFET [1]. The process flow of ISFET is illustrated in Figure 3-1. The sensing layers ZrO2 is deposited onto the SiO2 gate ISFET which prepared by Sputter in Nano
Facility center. Before every step, besides after sensing membrane deposited onto SiO2 gate, the initial clean immersed in H2SO4+H2O2 about 5 minutes and dipped in
HF solution were done. The fabrication and its parameters are listed in Fig. 3-1 and the fabrication procedures are listed as follows:
1) RCA clean
2) Wet-oxidation of silicon dioxide(6000Å, 1050°C, 65 mins) 3) Defining of Source/Drain (Mask 1)
4) BOE wet-etching of SiO2
5) Dry-oxidation of SiO2 as screen oxide (300Å, 1000°C, 12 mins)
6) S/D ion implantation
(Dose=5e15 [1/cm2], energy=25Kev [Phosphorus] for p-type ISFET) 7) S/D annealing ( 950°C, 60min)
8) PECVD SiO2 for passivation (1µm)
9) Defining contact hole and gate region (Mask 2) 10) BOE wet-etching of SiO2
11) Dry growth of gate oxide (100Å, 850°C, 60mins) 12) Defining the sensing layer region ( mask3) 13) Sputtering ZrO2 as sensing layer, 300Å (mask 3)
14) ZrO2 sintering (600°C, 60min)
15) Define the contact hole region (mask4)
16) Contact hole and reference electrode Al deposition 17) Backside Al evaporation, 5000 Å
18) Al sintering (400°C, 30min)
3.2 The key steps of the experiment
3.2.1 Na+ and K+ ion solutions allocation
For a start, we need to artificially allocate Na+ and K+ ions solution with different pH buffer solutions because of no precise machine for titration, so that we can proceed ion detection.
1) Preparing 3M NaCl and KCl solution and pH=3,5,7,9,11 buffer solutions 2) Diluting NaCl and KCl with pH buffer solutions as following:
a) Taking 3M NaCl or KCl 1ml into pH=3,5,7,9,11 buffer solutions 30ml such that the concentration of the new solution is 0.1M Na+ or K+ ion solutions.
b) Taking just allocating solution 0.1M Na+ or K+ ion solutions 1ml into pH=3,5,7,9,11 buffer solution 100ml such that the concentration of the new solution is 000.1M Na+ or K+ ion solutions.
3) Beginning to measuring and detecting ion interference.
3.2.2 Gate region formation
RCA clean is usually performed at wafer starting to reduce the possible pollution such as particles, organics, diffusion ions and native oxide. Careful RCA clean will ensure the integrity of device electricity. The next step 600nm thickness wet oxide is deposited as barrier layer for S/D implant. The density and the energy of S/D implant is 5E15 (1/cm2) and 25Kev with phosphorous dopant for n-type ISFET. After S/D implanting, in order to activate the dopants, a 950℃ 30 min N+ anneal for n-type ISFET.
The extra 1μm thickness PECVD oxide deposition is essential, which protect the other pH-ISFET from aqueous solution overflowing pH-ISFET [2]. During a long period of electrolyte immersing, ions may diffuse and affect the ISFET’s electrical characterization [3]. It is a significant difference compare with standard MOSFET processes. A thick PECVD oxide deposition can eliminate the effect. Following the PECVD oxide deposition, 100Å thickness dry oxide was grown in a dry oven as gate oxide formed.
3.2.3 Sensing layer deposition
This procedure is the kernel of the pH-ISFET in our important part in our
experiment. The ZrO2 sensing film 300Å is growth by the sputter which appear a
good sensitivity nearly Nernstian sensitivity [4]. The detailed parameters of sputter are listed in Table 3.1.
3.3 Measurement system
3.3.1 Preparation of measurement
To investigate the characteristics of the ZrO2 as sensing layers, we measured the
I-V curves for the pH-ISFETs by using HP4156 as measurement tool and the system is shown in Fig. 3-2. For getting correct result of measurement, the entire measurement procedures were executed in a dark box to prevent light influence and the electromagnetic wave.
In order to make the sensing film immersed in the aqueous solution, some extra works on works on wafers must be done before measurement with HP4156. At first, we glued a container on the wafer. This step is very important for following complex and frequently solution change activities which also can protect the other ISFET from immersed aqueous solution. The container, to load the test electrolyte, was open at its bottom and covered the whole sensing region on wafer to keep electrolyte contact with sensing layers exactly.
The pH-standard solution is purchased by Riedel-deHaen corp. and the pH-values are 1, 3, 5, 7, 9, 11, 13. The electric potential of the pH-solution will be floating [5] during open-loop circuit. The disturbance from the environment would induce the electric potential variance of the solution. By eliminating this variance, a reference electrode is needed to immersion in the pH-solution to close the circuit loop.
3.3.2 Current-Voltage measurement set-up
A HP-4156 semiconductor parameter analyzer system were set up to measure the current-voltage (I-V) characteristics curves, in which included Ids-Vgs and Ids-Vds
curves at controlled temperature. All measurements were arranged in a dark box to minimize the effects of photoelectric and temperature.
In the I-V measurements, due to the sensing areas were so small, prevention of air bubbles from being generated between the sensing membrane and the buffer solution during the testing is needed to take care.
In the setup of HP-4156, substrate voltage is ground to avoid the body effect and the reference electrode is sweeping to different voltage. In the measurement of sensitivity, the response of the pH-ISFET is the function of time. According to P. Woias, the first equilibrium will achieves in a minute.
In order to obtain the sensitivity, at first we measure the Ids-Vds to observed the
linear area. Secondly, we make the Vds as constant to measure Ids-Vds from pH 13 to
pH 1 in turn. As changing the pH buffer solution, we diluted next butter solution which under test twice, and stay 1min to avoid the effect of the buffet solution that measured before. This step can make our the measurement of pH-ISFET more easily. The variation of the gate voltage exhibits the pH sensitivity of the sensing oxide. Fig. 3-3 illustrates the detection principle of pH
3.4 References
[1] T. Matsuo and M. Esashi, Methods of ISFET fabrication, Sensor. & Actuator 1 (1981) 77-96.
[2] U. Guth, “Investigation of corrosion phenomena on chemical microsensors”, Electrochimica Acta 47 pp. 201–210 , 2001.
[3] George T. Yu, “Hydrogen ion diffusion coefficient of silicon nitride thin films”, Applied Surface Science 202 pp.68–72, 2002.
Zirconium Oxide Gate Ion-sensitive Field-Effect Transistors,” Japanese Journal of Applied Physics Vol. 46 No. 7A pp. 4334-4338 2007.
Chapter 4
Results and Discussions
4.1 Introduction
We exactly know that ion sensitive field effect transistors (ISFETs) have shown good properties for the detection of numerous ions in solution from the above article, and it is an inevitable phenomenon of pH-ISFETs from other studies for the second time [1]. In spite of ISFET with many ions detecting, however, their use in the case of complex chemical solutions and simultaneous detection of different ions is characterized by different drawbacks:
(1) Few ionosensitive layers are able to detect several ions with a good and distinct sensitivity for each of them
(2) The measurement technique based on charge detection is unable to separate the influences of similar ions and is therefore responsible for poor selectivity. All in all, multi-ion detection in complex solutions of ISFET requires generally adapted characterization, multi-ISFET sensors and information processing [1].
In the following article, we will find out that the original sensitivity of pH-ISFETs compared with the later sensitivity after adding Na+ or K+ ions into solutions should be smaller because of the ion interference. And we will soon discuss and show this peculiar phenomenon with some powerful experience data.
4.2 The glass reference electrode for pH measurements
and accurate. Thus we will first discuss the sensitivity of ZrO2-pH-ISFET by glass
reference electrode. The good functioning of the ZrO2 ISFET chemical sensor has
been demonstrated by studying the H+ ion detection (see Fig. 4-1). And its sensitivity is in Fig. 4-2 [2]. A quasi-nernstian pH response has been obtained on a 1-13 pH range and pH sensitivity has been finally estimated to 58.73 mV/pH (detection yield of 99.2% compared to the Nernst law).
4.3 pK and pNa measurements
The pK and pNa responses of the ZrO2 ISFET chemical sensor have been studied
for buffered solutions of KCl or NaCl salts (pH 9), respectively (see Fig. 4-3 and Fig. 4-4). We can conclude that similar results have been obtained for both ions (i.e. K+ and Na+). For the lowest concentrations (pK > 5 or pNa > 3), no detection properties have almost been obtained. And for the highest concentrations (pK < 5 or pNa < 3), linear responses are evidenced and sensitivities to K+ and Na+ ions have been estimated to 12 mV/pX (X = Na or K ion and detection yield of 20.3% compared to the Nernst law).
4.4 Simultaneous pH, pK and pNa measurements
The simultaneous detection of the H+ ion with the K+ or the Na+ ion has been accomplished. This purpose has been performed by researching the ZrO2 ISFET
responses to pK or pNa for different solution pH (see Fig. 4-5 to 4-12). These may be not clear, so we show normalized pK and pNa response of ZrO2 ISFET in Fig. 13 and
14. As already shown in Section 4.3, K+ and Na+ ions are only detected for the highest concentrations. Nevertheless, as the solution pH decreases gradually, detection limits
also decrease by degrees and no detection properties are ultimately obtained for the lowest pH.
In order to clarify these phenomena, let us study the different sensitivities of the ZrO2 ISFET chemical sensor and more precisely the pK and pNa sensitivities as a
function of pH (see Fig. 4-29) and the pH sensitivity as a function of pK and pNa (see Fig. 4-15 to 4-28).
As a result, it shows that the sensitivity to the K+ or Na+ ion increases almost linearly with the solution pH. Maximal values around 12 mV/pX can be estimated for the more basic solutions (pH 11) while no detection properties (s < 3.5 mV/pX) are obtained for the more acidic solution (pH 3). By turns, when K+ or Na+ ions are present in solution, the sensitivity to the H+ ion is lowered around 45 mV/pH as well as 40 mV/pH and decreases further for the lowest pK or pNa values, reaching 40 mV/pH as well as 35 mV/pH when pK or pNa is equal to 1.
At least, the simultaneous detection of the K+ and Na+ ions have been studied for buffered solutions of mixed KCl and NaCl salts (pH 9). Thus, for the lowest pK and pNa values, electrical characterisation has effectively evidenced the detection of ions. However, it has been impossible to indicate any comprehensive results, to separate clearly the influences of the K+ and Na+ ions and to define precisely the different ionic selectivity and sensitivity.
All these results demonstrate that the use of ZrO2 as an ionosensitive layer for
the simultaneous detection of H+, K+ and Na+ ions is characterized by complex phenomena, saturation effects and mutual influences. In order to clarify the simultaneous detection mechanisms, the modeling of the EIS structure must be further improved. (EIS = electrolyte/insulator/semiconductor)
Until now, the theory of this phenomenon has considered two types of surface sites (silanol SiOH and silylamine SiNH2) and has neglected the sensitivity to others
ions like K+ or Na+ [6] [7]. This last assumption cannot be in agreement with the complex phenomena, saturation effects and mutual influences of the pH, pK and pNa simultaneous measurements described previously. For high pK or pNa values, even if the K+ or Na+ ions do not react directly with the ZrO2 ionosensitive layer, their
presence in the solution, and more precisely in the inner and outer Helmotz planes [8], should be responsible for electrostatic or/and chemical influences. These phenomena should lead to shifts of the equivalent capacitor or/and of the reactivity of the insulator interface. The presence of chloride ions Cl- in solution (from the KCl or/and NaCl salts) should also be taken into account in a similar way.
For low pK or pNa values, the K+ or Na+ ion binding with the silanol SiOH or silylamine SiNH2 sites of the ZrO2 ionosensitive layer occurs. These reactions should
be responsible for a decrease of the effective surface density of sites as well as for electrostatic repulsion of other cations such as H+ ions. In return, the same effect holds good for the detection of K+ or Na+ ions in the case of low pH values.
Therefore, by theoretically developing them and by improving the site-binding model, the pH, pK or pNa simultaneous measurement of the ZrO2 ISFET sensor will
be precisely described.
4.6 Conclusion
According to above results, we can find that ZrO2 ISFET chemical sensors have
been studied for the detection of H+, K+ and Na+ ions in aqueous solution, evidencing maximal sensitivities of 57.2 mV/pH, 12.22 mV/pK and 11.55 mV/pNa, respectively
(ranges available: 1 ≤ pH ≤ 13, 1 ≤ pK ≤ 4, 1 ≤ pNa ≤ 3). The study of the simultaneous detection evidence complex phenomena, saturation effects and mutual influences of the H+, K+ or Na+ ion detection properties for the highest ionic concentrations in solution.
Results finally demonstrate that ZrO2 ISFETs are well adapted to the H+ ion
detection, are usable for the K+ or Na+ ion detection separately in the case of buffered basic solutions, and are not suitable for the H+, K+ and Na+ ion simultaneous detection in the case of complex solutions or medical analysis [3] [4].
4.7 References
[1] Bergveld P, Sibbald A. Analytical and biomedical applications of ISFETs. Amsterdam: Elesevier, 1988.
[2] Tsukada K, Miyabata Y, Shibata Y, Miyagi H. Sensors and Actuators 1990; B2:291-5.
[3] Baccar ZM, Jaffrezic-Renault N, Martelet C, Jaffrezic H, Marest G, Plantier A. Sensors and Actuators 1996; B32:101-5.
[4] Baccar ZM, Jaffrezic-Renault N, Martelet C, Jaffrezic H, Marest G, Plantier A. Materials Chemistry and Physics 1997; 48:56-9.
[5] B. Hajji, P. Temple-Boyer, J. Launay, T. do Conto, A. Martinez,
Microelectronics Reliability 40 (2000) 783-786.
[6] Grattarola M, Massobrio G, Martinoia S. IEEE Trans Elec Devices 1992;39(4):813±9.
[7] Niu M, Ding X, Tong Q. Sensors and Actuators 1996; B37:13±7. [8] Meixner LK, Koch S. Sensors and Actuators 1992; B6: 315±8.
Chapter 5
Future Work
5.1 Future Work
In our experiment of this study, we find the truth that ZrO2 will be sensitive to H+,
Na+ and K+ ions. Based on this peculiar phenomenon, we can know that characteristics of some ion detection are researched. For these last applications of this study, multi-ions detection will only be possible by developing new characterization techniques or by realizing multi-ISFETs chemical sensors with specific ionosensitive layers for the different ions.
(a) (b)
Fig. 1-1 Schematic representation of (a) MOSFET (b) ISFET
Glass Membrane
Internal
Reference Electrode
Internal
Buffer Solution
Internal
Conducting Line
Fig. 2-1 Conventional pH glass electrode Gate Source Drain D r a i n S o u r c e G a t e
electrolyte
Fig. 2-2 Electrode and electrolyte interface
(a) (b) (c)
Substrate
Substrate
Substrate
(d) (e) (f)
Substrate
S D DSubstrate
S D DSubstrate
(g) (h)
Substrate
S D DSubstrate
S D D(i) (j)
Substrate
S D DSubstrate
S D D(k)
Fig. 3-1 Fabrication process flow
Fig. 3-2 Measurement setup
Substrate
S D
D
Gate
Drain1
Source
Drain2
V
GSFig. 3-3 Detection principle of pH
0.0 0.5 1.0 1.5 2.0 2.5 3.0 0 20 40 60 80 100 120 140 160 180 ID=71.7uA VG=1V I D (u A ) ZrO2 300Angstrom pH13 pH11 pH9 pH7 pH5 pH3 pH1 VG(V)
Fig. 4-1 ID-VG curve of ZrO2 to n-type ISFET before drift
0 2 4 6 8 10 12 14 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 V G (V )
Sensitivity=58.73mV/pH
pH value ID=71.7uA VG=1VFig. 4-3 pK response of ZrO2 ISFET for pH=9 buffer solution
Fig. 4-4 pNa response of ZrO2 ISFET for pH=9 buffer solution
0 1 2 3 4 5 6 7 8 9 10 1.93 1.94 1.95 1.96 1.97 1.98 1.99 2.00 2.01 2.02 2.03 s ~ 8.672 mV/pK
pK measurement (pH=9)
v
o
lt
ag
e (
V
)
pK unit
0 2 4 6 8 10 1.80 1.81 1.82 1.83 1.84 1.85 1.86 s ~ 7.503 mV/pNapNa measurement (pH=9)
v
o
lt
ag
e (
V
)
pNa unit
Fig. 4-5 pK response of ZrO2 ISFET for pH=3 buffer solution
Fig. 4-6 pNa response of ZrO2 ISFET for pH=3 buffer solution
pK measurement (pH=3)
0 1 2 3 4 5 6 7 8 9 10 1.686 1.688 1.690 1.692 1.694 1.696 1.698 1.700 1.702 1.704vo
lt
ag
e
(
V
)
pK unit
s ~ 3.141 mV/pK 0 1 2 3 4 5 6 7 8 9 10 1.590 1.592 1.594 1.596 1.598 1.600 1.602 1.604 1.606 1.608 1.610 1.612 1.614 s ~ 3.552 mV/pNapNa measurement (pH=3)
v
o
lt
ag
e (
V
)
pNa unit
Fig. 4-7 pK response of ZrO2 ISFET for pH=5 buffer solution
Fig. 4-8 pNa response of ZrO2 ISFET for pH=5 buffer solution
0 2 4 6 8 10 1.685 1.690 1.695 1.700 1.705 s ~ 3.758 mV/pNa
pNa measurement (pH=5)
v
o
lt
ag
e (
V
)
pNa unit
0 1 2 3 4 5 6 7 8 9 10 1.780 1.785 1.790 1.795 1.800 1.805 1.810 1.815 1.820v
ol
ta
g
e
(
V
)
s ~ 4.964 mV/pKpK measurement (pH=5)
pK unit
Fig. 4-9 pNa response of ZrO2 ISFET for pH=7 buffer solution
Fig. 4-10 pNa response of ZrO2 ISFET for pH=7 buffer solution
0 2 4 6 8 10 1.835 1.840 1.845 1.850 1.855 1.860 1.865 1.870 1.875 1.880 1.885 s ~ 6.747 mV/pK
pK measurement pH=7)
v
o
lta
ge
(
V
)
pK unit
0 2 4 6 8 10 1.750 1.752 1.754 1.756 1.758 1.760 1.762 1.764 1.766 1.768 1.770 1.772 1.774 1.776 1.778 1.780 s ~ 4.544 mV/pNapNa measurement (pH=7)
v
o
lta
ge
(
V
)
pNa unit
Fig. 4-11 pNa response of ZrO2 ISFET for pH=11 buffer solution
Fig. 4-12 pNa response of ZrO2 ISFET for pH=11 buffer solution
0 1 2 3 4 5 6 7 8 9 10 11 1.95 1.96 1.97 1.98 1.99 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 s ~ 11.534 mV/pK
pK measurement (pH=11)
v
o
lta
ge
(
V
)
pK unit
0 2 4 6 8 10 1.85 1.86 1.87 1.88 1.89 1.90 1.91 1.92 1.93 1.94 s ~ 10.621 mV/pNapNa measurement (pH=11)
v
o
lta
ge
(
V
)
pNa unit
Fig. 4-13 pK response of ZrO2 ISFET
Fig. 4-14 pNa response of ZrO2 ISFET
0 2 4 6 8 10 1.6 1.7 1.8 1.9 2.0 2.1
v
o
lta
ge
(
V
)
pK unit
pH = 11 pH = 9 pH = 7 pH = 5 pH = 3 0 2 4 6 8 10 1.5 1.6 1.7 1.8 1.9 2.0 pH = 11 pH = 9 pH = 7 pH = 5 pH = 3v
o
lta
ge
(
V
)
pNa unit
Fig. 4-15 normalized pK response of ZrO2 ISFET
Fig. 4-16 normalized pNa response of ZrO2 ISFET
1 2 3 4 5 0.00 0.01 0.02 0.03 0.04 pH = 11 pH = 9 pH = 7 pH = 5 pH = 3
v
o
lta
ge
(
V
)
pK unit
1 2 3 4 5 0.00 0.01 0.02 0.03 0.04 pH = 11 pH = 9 pH = 7 pH = 5 pH = 3v
o
lta
ge
(
V
)
pNa unit
Fig. 4-17 pH response of ZrO2 ISFET in pK=1 solution
Fig. 4-18 pH response of ZrO2 ISFET in pK=3 solution
2 4 6 8 10 12 1.65 1.70 1.75 1.80 1.85 1.90 1.95 2.00 2.05
pH measuement
fixed in pK=1
v
o
lta
ge
(
V
)
pH unit
s ~ 40.34 mV/pH 2 4 6 8 10 12 1.65 1.70 1.75 1.80 1.85 1.90 1.95 2.00 2.05pH measuement
fixed in pK=3
v
o
lta
ge
(
V
)
pH unit
42.24 mV/pHFig. 4-19 pH response of ZrO2 ISFET in pK=5 solution
Fig. 4-20 pH response of ZrO2 ISFET in pK=7 solution
2 4 6 8 10 12 1.65 1.70 1.75 1.80 1.85 1.90 1.95 2.00 2.05 2.10
pH measuement
fixed in pK=7
v
o
lta
ge
(
V
)
pH unit
45.42 mV/pH 2 4 6 8 10 12 1.65 1.70 1.75 1.80 1.85 1.90 1.95 2.00 2.05 2.10pH measuement
fixed in pK=5
v
o
lta
ge
(
V
)
pH unit
45.24 mV/pHFig. 4-21 pH response of ZrO2 ISFET in pK=9 solution
Fig. 4-22 pH response of ZrO2 ISFET in pNa=1 solution
2 4 6 8 10 12 1.65 1.70 1.75 1.80 1.85 1.90 1.95 2.00 2.05 2.10
pH measuement
fixed in pK=9
v
o
lta
ge
(
V
)
pH unit
45.42 mV/pH 2 4 6 8 10 12 1.60 1.65 1.70 1.75 1.80 1.85 1.90pH measuement
fixed in pNa=1
v
o
lta
ge
(
V
)
pH unit
s ~ 34.86 mv/pHFig. 4-23 pH response of ZrO2 ISFET in pNa=3 solution
Fig. 4-24 pH response of ZrO2 ISFET in pNa=5 solution
2 4 6 8 10 12 1.60 1.65 1.70 1.75 1.80 1.85 1.90 1.95
pH measuement
fixed in pNa=3
v
o
lta
ge
(
V
)
pH unit
37.5 mv/pH 2 4 6 8 10 12 1.60 1.65 1.70 1.75 1.80 1.85 1.90 1.95pH measuement
fixed in pNa=5
v
o
lta
ge
(
V
)
pH unit
s ~ 39.14 mV/pHFig. 4-25 pH response of ZrO2 ISFET in pNa=7 solution
Fig. 4-26 pH response of ZrO2 ISFET in pNa=9 solution
2 4 6 8 10 12 1.60 1.65 1.70 1.75 1.80 1.85 1.90 1.95 s ~ 38.77 mv/pH
