Chapter 3 Experiment and Measurement
3.4 Measurement system
3.4.3 Drift measurement set-up
The drift characteristics were measured with differential pH=7 value of aqueous solution and the same condition samples period of 30 seconds, 1 minute, 10 minutes and 1 hour. 33 sampling points in the time frame of 7 hours were observed for n-type and p-type ISFET with ZrO2. The detection principle is in a similar manner to that of the pH measurement and is shown in Fig. 3-4.
3.5 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.
[4] 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.
[5] P. Woias, “Slow pH response effects of silicon nitride ISFET sensors”, Sensors
Chapter 4
Results and Discussions 4.1 Introduction
According to chapter 2, the sensitivity of ISFET is related to the numbers of surface sites the more numbers of surface sites the larger sensitivity it has. To reduce the sensitivity, we use a high polymer. Polymer is a hydrophobic material which can reduce H ion in a large number. Finally, high/low sensitivities of membranes for ISFET and REFET are essential for getting higher resolution of pH measurement.
This experiment discusses a best REFET (Reference Electrode Field Effect Transistor) in different structure.
In this study we fabrication of a REFET based on a polymer membrane. The membrane has been coated on over the ZrO2 and SiO2 gate insulator surface of an ISFET. The polymer membrane we use polyimide and Nafion . Nafion is a proton exchange membrane, it can protect polymer and it become the mechanism of proton exchange to make ion absorb on the surface of polymer steadily. We use three different membrane polyimide, Nafion/polyimide and Nafion mix polyimide.
4.2 Comparison of different structure’s feature
The experiment utilizes different approach of entrapment to measure the comparison of its structure’s feature. It use Polyimide, ZrO2/Polyimide/Nafion and ZrO2-mix(Ployimide+Nafion) three different structure’s approach of entrapment, and get the common conclusion that sensitivity of both reduce from 58mV/p in ZrO2
(Fig.4-1) sensing layer to 12.5mV/pH (Fig.4-2) ,10mV/pH (Fig.4-3) and 9.75mV/pH (Fig.4-4) respectively.
The main reason is the entrapment Polyimide can reduce the sensitivity efficiently, and it can get different material’s site-binding from Gouy-Chapman-Stern Theory model and Site-binding model, which traps the number of ion are also different. the sensitivity can reduce largely about 80%. But ZrO2/Polyimide/Nafion and ZrO2-mix (Ployimide+Nafion) membrane can not sense pH11 and pH13 . Accordingly we baked the membrane in order to solve this problem. After baked we observed the membrane can sense pH11 and pH13, and sensitivity of both reduce to 10mV/pH (Fig 4-5), 7.5mV/pH (Fig4-6) and 8.4mV/pH (Fig4-7) respectively. The main reason is the solvent in polyimide be volatilized. It made polyimide hardness and polyimide is not easy to absorb H ion. Table 4.1 lists the corresponding sensitivity values.
4.3 The comparison of deposition on different structure
The comparison of this experiment utilize two different base on different Sensing film, ZrO2 and SiO2, and the material of both of them mix Polyimide and Nafion coast on sensing film to get the sensitivity are 7.5mV/pH and 8.175mV/pH (Fig.4-9) respectively. The sensitivity measured in previously ZrO2 and SiO2 experiment are about 58mV/pH (Fig.4-1) and 37.5mV/pH (Fig.4-8). It concludes from site-binding model and experiment data that the main reason affect sensitivity is ” hydrophobic”
from Polyimide material cause H ion uneasy to absorb, so the H ion mainly absorb on the surface of Polyimide and it not connect with the sensing layer in bottom.
Nevertheless, the structure base on SiO2 is inferior to ZrO2 on the improvement of sensitivity because interface of SiO2 and Polyimide is not as well as the interface of ZrO2 and Polyimide.
4.4 Drift Characteristics
Drift characteristics of REFET sensing layer structure for ZrO2- pH –ISFET are shown in Fig. 4-10~4-15. Measurements were carried out in pH 7, and the duration of each measurement was about 7 hours. Average drift rate per hour was calculated by averaging the gate voltage drift in the last 6 hour, we discovered after baked membrane have low Drift.
The performance of two structure on Drift since structure of baked ZrO2/ Polyimide/Nafion inferior to the structure of baked Polyimide mix Nafion the results were 2.298, 1.284/hour respectively. Table 4.2 lists the corresponding Drift values.
4.5 Quasi-Reference Electrode integrated with REFET/ISFET
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. In this experiment, we use ZrO2 membrane with QRE and the after baked NF-mix-polyimide/ ZrO2 ISFET/REFET sensor array system, and ZrO2 membrane with QRE. And the after baked NF/polyimide- ZrO2 ISFET/REFET sensor array system, and ZrO2 membrane with QRE.
4.5.1 The bare Quasi-Reference Electrode integrated with REFET/ISFET
Fig. 4.16 shows three times the the sensitivity of ZrO2-pH-ISFET measured by bare Quasi-Reference Electrode. We can see the gate voltage (VG) range of pH 1~13 value is very unstable, and the sensitivity linearity is very bad. This is the main problem of Quasi-Reference Electrode.
4.5.2 The NF/ polyimide-ZrO
2and NF-mix- polyimide/ZrO
2Quasi-Reference Electrode
Fig. 4.17 shows three times sensitivity of NF/polyimide- ZrO2-pH-REFET measured by Quasi-Reference Electrode. Fig. 4.18 shows three times the sensitivity of NF-mix-polyimide/ZrO2-pH-REFET measured by Quasi-Reference Electrode. We can see with the polymer coated, the gate voltage (VG) range of pH 1~13 value becomes better.
4.5.3 The coplanar ISFET/REFET sensor array system with Quasi-Reference Electrode
The sensor array system is composed by the coupled sensing membrane without polymer coated and with the polymer coated. Fig. 4.19 shows the sensitivity of differential ISFET/REFET with Quasi-Reference Electrode.
The linearity are much better than the bare Quasi-Reference Electrode. And the sensitivity is 54.47pH/mV and 49.56pH/mV. From the experimental results, the NF/polyimide-ZrO2 and NF-mix-polyimide/ZrO2 coplanar ISFET/REFET sensor array system seems to have the potential to solve the unstable problem of the Quasi-Reference Electrode. Table 4.3 lists the corresponding sensitivity values.
4.6 Conclusions
The results of this study show the Nafion and polyimide based membrane made a low sensitivity and made a low drift. When polyimide which after baked can have good sensitivity for pH1~13 and low drift. The sensitivity of ZrO2-pH-ISFET is 58mV/pH without any polymer modifying, but after treating with Nafion mix polyimide, the sensitivity can decrease to 7.5mV/pH. It means modifying by the Nafion mix polyimide material is a successful treating method for REFET. The process is simple and easy to fabricate. The unstable gate voltage (VG) range problem of Quasi-Reference Electrode is also solved by coplanar ISFET/REFET sensor array system .And we have the sensitivity of co-planar structure of ISFET/REFET are 54.47mV/pH by ZrO2/Polyimide/Nafion and 49.56mV/pH by ZrO2-mix(Ployimide+Nafion). This and the use glass electrode is similar. Used the Quasi-Reference Electrode to completed has been small.
A miniaturized ISFET/REFET pair with Quasi-Reference Electrode was demonstrated. It shows 54.47mV/pH sensitivity and 1.2841mV/hour drift with ZrO2/Polyimide/Nafion structure. The results are compatible with glass electrode.
Baking process steps are necessary for polyimide membrane, which can further obtain low sensitivity for pH1~13 and achieve lower drift.
Chapter 5 Future Work
5.1 Future Work
After this study, we find a REFET based on polyimide and Nafion membrane. This REFET is have a low sensitivity and low drift. But the coating process is not optimized in this experiment. 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. Final, we went to integrating differential electric circuit on chip.
parameters of ZrO2sputter 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
ZrO2 PI Nafion
ZrO2 NF mix PI ZrO2
PI
Table 3-2 The test structures of REFET, NF=Nafion PI=polyimide
Structures Sensitivity(mV/pH) SENSER RANGE
ZrO2 58 1~13
Polyimide/ZrO2 12.5 1~13
PLmixNF/ZrO2 9.75 1~9
PL/NF/ZrO2 10 1~9
baked PL/ZrO2 10 1~13
baked PImixNF/ZrO2 7.5 1~13
baked PI/NF/ZrO2 8.4 1~13
Table4.1 Sensitivity in different test structures PI=polyimide NF=Nafion Structures Drift(mv/hr)
ZrO2 0.6175
Polyimide/ZrO2 4.3575
PImixNF/ZrO2 3.7525
NF/PI/ZrO2 2.5
Baked PI/ZrO2 4.0533
Baked PImixNF/
ZrO2
2.298
Baked NF/PI/
ZrO2
1.2841
Table4.2 Drift in different test structures PI=polyimide NF=Nafion
Structures Sensitivity (mV/pH)
Drift (mV/hr) Linearity
ZrO2 with glass electrode
58 0.6175 0.9932
Baked NF/PI/
ZrO2-REFET/
ZrO2-ISFET
54.47 1.2841 0.9743
Baked PImixNF/
ZrO2-REFET/
ZrO2-ISFET
49.56 2.298 0.96371
Table4.3 Sensitivity in different electrode PI=polyimide NF=Nafion
electrolyte
Drain Source Gate
(a) (b)
Fig. 2-1 Schematic representation of(a) MOSFET, (b) ISFET
Fig. 2-2 Potential profile and charge distribution at an oxide electrolyte solution interface
Silicon
Thermal Oxide Sensing Layer Solution
Hydration
Fig. 2-3 Series combination of the (a) initial (b) hydrated insulator capacitance
Substrate
(1)
Substrate
(2)
Substrate
(3)
Substrate
(4)
Substrate
S D
D
(5)
Substrate
S D
D
(6)
Substrate
S D
D
(7)
Substrate
S D
D
(8)
Substrate
S D
D
(9)
Substrate
S D
D
(10)
(11)
Fig. 3-1 Fabrication process flow
Substrate
S D
D
Gate
Drain1 Source Drain2
V
GSV
DS1V
DS2Fig. 3-2 Measurement setup
Fig. 3-3 Detection principle of pH sensitivity
Fig. 3-4 Detection principle of drift
0 2
Fig. 4-1 H+ sensitivity of ZrO
2-ISFET
0 2
Fig. 4-2 H+ sensitivity of Polyimide/ZrO
2-REFET
0 2 4
NF/Polyimide/ZrO2-REFET sensitivity=10mV/pH for 1~9
Fig. 4-3 H+ sensitivity of NF/ Polyimide/ZrO
2-REFFT
-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
NFmixPolyimide /ZrO2-REFFT sensitivity=9.75mV/pH for1~9
Fig. 4-4 H+ sensitivity of NFmixPolyimide/ZrO
2-REFFT
0 2
Fig. 4-5 H+ sensitivity of baked Polyimide/ZrO
2-REFET
0 2
baked NF/ Polyimide /ZrO2-REFFT sensitivity=8.4mV/pH
Fig. 4-6 H+ sensitivity of baked NF/Polyimide/ZrO
2-REFFT
0 2
baked NFmixPolyimide /ZrO2-REFFT sensitivity=7.5mV/pH
Fig. 4-7 H+ sensitivity of baked NFmixPolyimide/ZrO
2-REFFT
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0.0000
0.0001 0.0002 0.0003 0.0004 0.0005
ID (A)
VG (V)
(pH1) (pH3) (pH5) (pH7) (pH9) (pH11) (pH13)
0 2 4 6 8 10 12 14
1.5 1.6 1.7 1.8 1.9 2.0
VG (V)
pH
SiO2-ISFET sensitivity=37.5mv/pH
Fig. 4-8 H+ sensitivity of SiO
2-ISFET
0 2 4
baked NFmixPolyimide /SiO2-REFFT sensitivity=8.125 mV/pH
Fig. 4-9 H+ sensitivity of baked NFmixPolyimide/SiO
2-REFFT
0 5000 10000 15000 20000 25000 30000 2.06
2.07 2.08 2.09 2.10 2.11 2.12
VG(V)
Time(s)
polyimide drift=4.3575mV/hr
Fig. 4-10 Drift of polyimide/ZrO2-REFET
0 5000 10000 15000 20000 25000 30000
1.360 1.365 1.370 1.375 1.380 1.385 1.390
VG(V)
Time(s)
baked polyimide drift=4.0533mV/hr
Fig. 4-11 Drift of baked polyimide/ZrO2-REFET
0 5000 10000 15000 20000 25000 30000 2.180
2.185 2.190 2.195 2.200 2.205 2.210 2.215 2.220 2.225
VG(V)
Time(s)
NF/Polymide drift=2.466mV/hr
Fig. 4-12 Drift of NF/polyimide/ZrO2-REFET
0 5000 10000 15000 20000 25000 30000
1.735 1.740 1.745 1.750 1.755 1.760 1.765 1.770
VG (V)
Time(s)
baked NF/Polymide drift=1.28416mV/hr
Fig. 4-13 Drift of baked NF/Polyimide/ZrO
2-REFFT
0 5000 10000 15000 20000 25000 30000 1.33
1.34 1.35 1.36 1.37
VG(V)
Time(s)
NFmixPolyimide drift=3.725mV/hr
Fig. 4-14 Drift of NFmixPolyimide/ZrO
2-REFFT
0 5000 10000 15000 20000 25000 30000
1.470 1.475 1.480 1.485 1.490 1.495
VG(V)
Time(s)
baked NFmixPolyimide drift=2.298mV/hr
Fig. 4-15 Drift of baked NFmixPolyimide/ZrO
2-REFFT
0 2 4 6 8 10 12 14
Fig. 4-16 ZrO2-ISFET with QRE (Pt)
0 2 4 6 8 10 12 14
Fig. 4-17 Baked NF/Polyimide/ ZrO
2-REFFT with QRE(Pt)
0 2 4 6 8 10 12 14
Fig. 4-18 Baked NFmixPolyimide/ZrO
2-REFFT with QRE(Pt)
0 2 4 6 8 10 12 14 -0.9
-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1
VG (V)
pH
NF/Polyimide REFET /ZrO2-ISFET sensitivity=54.47mV/pH
0 2 4 6 8 10 12 14
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
VG(V)
pH