Chapter 4 Results and Discussions
4.4 Conclusions
In the first part, we know that the ZrO2 gate ISFET is a good choice for circuit design because of its high sensitivity. The high sensitivity could provide the high resolution. The sensitivity is 57.08 mV/pH for the p-type ISFET and 58.73 mV/pH for the n-type ISFET. Fig. 4-15 and Fig. 4-16 are the figures of the amount and the rate of drift. We can reduce the drift influence by introducing a new variable T = N – P. N and P respectively represent the gate output voltage.
In the second part, we find the modulation of the compositions of the polymer based material can obtain good properties. In this situation, the 1/1 ratio of Nafion/polyimide is the better choice. From Table 4-4 and Fig. 4-28, the behavior of this structure is almost the same as ZrO2 film, and its sensitivity and drift rate are the acceptable quantities. Although the best selectivity and the drift are the pure polyimide, it is inconsistent with the ZrO2 film in the part of transconductance. We may increase the complexity of readout circuits if we use the pure polyimide as the sensing material. In this situation, it is difficult to produce the small size sensors. To select the similar material in transconductance is the significant consideration for the commercial applications.
4.5 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] 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.
[3] S. Jamasb, S. D. Collins, and R. L. Smith, ”A Physical Model for ThresholdVoltage 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.
[4] 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.
[5] C. S. Lai, C. E. Lue, C. M. Yang, M. Dawgul, and D. G. Pijanowska,
“Optimization of a PVC Membrane for Reference Field Effect Transistors”, Sensors, vol. 9, pp. 2076-2087, 2009.
Chapter 5 Future Work
Among this experiment, we propose a method to reduce the effect of drift. We also find the specific composition of the mixture of polyimide and nafion as the suitable REFET sensing layer material. To realize simplified compensation and readout circuits with matched transconductance for ISFET/REFET pair is the future objective.
Fig. 1-1 ISFET cross-section structure
Fig. 1-2 Important platform of Bio-sensors
Fig. 1-3 The electrode structures
Fig. 1-4 Conventional glass electrode
Fig. 1-5 Differential ISFET/REFET measuring system
Fig. 1-6 Direct (a) and indirect (b) feedback circuit
Fig. 1-7 Sketch of readout circuit with global feedback
Fig. 1-8 Sketch of an operational amplifier
Fig. 2-1 Structure of MOSFET and ISFET
Fig. 2-2 Site-binding model Gate
Source Drain
Fig. 2-3 The interface between silicon dioxide and electrolyte
Fig. 2-4 Potential profile and charge distribution at an oxide electrolyte solution interface
Fig. 2-5 Gouy-Chapman-Stern model
Fig. 2-6 Series combination of the (a) initial (b) hydrated insulator capacitance
silicon silicon
Thermal Oxide tL , εL Thermal Oxide tL , εL Sensing Layer tU , εU Sensing Layer tU-tHL(t) , εU Hydration tHL(t) , εHL
Solution Solution
Part 1
Part 2
Part 3
Part 4
Part 5
Part 6
Part 7
Part 8
Fig. 3-1 Fabrication process flow
Fig. 3-2 Chemical formula and sketch for Nafion
Fig. 3-3 Measurement set-up
Gate V
GSV
DS1V
DS2Drain1 Source Drain2
Dark Box
Fig. 3-4 Detection principle of pH
Fig. 3-5 Detection principle of drift
-3 -2 -1 0 1
Fig. 4-1 Id-Vg curve of ZrO2 for p-type ISFET before drift
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Fig. 4-2 Id-Vg curve of ZrO2 for n-type ISFET before drift
0 100 200 300 400 -1.2055
-1.2050 -1.2045 -1.2040 -1.2035 -1.2030
VG (V)
Time (mins)
Drift=1.92 mV
Fig. 4-3 Time to drift in pH 3 buffer solution of p-type ISFEET for 400 minutes
0 100 200 300 400
-1.1570 -1.1565 -1.1560 -1.1555 -1.1550 -1.1545 -1.1540 -1.1535
VG (V)
Time (mins)
Drift=-2.62 mV
Fig. 4-4 Time to drift in pH 5 buffer solution of p-type ISFEET for 400 minutes
0 100 200 300 400
Fig. 4-5 Time to drift in pH 7 buffer solution of p-type ISFEET for 400 minutes
0 100 200 300 400
Fig. 4-6 Time to drift in pH 9 buffer solution of p-type ISFEET for 400 minutes
0 100 200 300 400 -1.100
-1.099 -1.098 -1.097 -1.096 -1.095
VG (V)
Time (mins)
Drift=-4.02 mV
Fig. 4-7 Time to drift in pH 11 buffer solution of p-type ISFEET for 400 minutes
0 100 200 300 400
1.637 1.638 1.639 1.640 1.641 1.642 1.643 1.644
VG (V)
Time (mins)
Drift=-1.77 mV
Fig. 4-8 Time to drift in pH 3 buffer solution of n-type ISFEET for 400 minutes
0 100 200 300 400
Fig. 4-9 Time to drift in pH 5 buffer solution of n-type ISFEET for 400 minutes
0 100 200 300 400
Fig. 4-10 Time to drift in pH 7 buffer solution of n-type ISFEET for 400 minutes
0 100 200 300 400 1.696
1.697 1.698 1.699 1.700 1.701 1.702
V G (V)
Time (mins)
Drift=-5.19 mV
Fig. 4-11 Time to drift in pH 9 buffer solution of n-type ISFEET for 400 minutes
0 1 00 2 0 0 3 0 0 4 00
1 .4 42 1 .4 44 1 .4 46 1 .4 48 1 .4 50 1 .4 52
V G (V)
T im e (m ins)
D rift= -8 .03 m V
Fig. 4-12 Time to drift in pH 11 buffer solution of n-type ISFEET for 400 minutes
2 4 6 8 10 12 -4
-3 -2 -1 0 1 2
Drift amount (mV)
pH value
Fig. 4-13 Time to drift rate of p-type ISFET in various buffer solution
2 4 6 8 10 12
-9 -8 -7 -6 -5 -4 -3 -2 -1
Drift amount (mV)
pH value
Fig. 4-14 Time to drift rate of n-type ISFET in various buffer solution
2 4 6 8 10 12
Fig. 4-15 Comparison of drift amount between p-type and n-type ISFET in various buffer solutions
Fig. 4-16 Comparison of drift rate between p-type and n-type ISFET in various buffer solutions
-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Fig. 4-17 Selectivity of the HMDS/ZrO2 structure
-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Fig. 4-18 Comparison of gm between ZrO2 and the HMDS/ZrO2 structure
-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Fig. 4-19 Comparison of gm between ZrO2 and the test structures
-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Fig. 4-20 Selectivity of the PR/HMDS/ZrO2 structure
0 100 200 300 400
Fig. 4-21 Time to drift in pH 7 buffer solution of the PR/HMDS/ZrO2 structure for 400 minutes
Fig. 4-22 Sensitivity of the mixture (Nafion1:polyimide1)/HMDS/ZrO2 structure
0 100 200 300 400
Fig. 4-23 Time to drift in pH 7 buffer solution of the mixture (Nafion1:polyimide1)/HMDS/ZrO2 structure for 400 minutes
-0 .5 0.0 0 .5 1 .0 1 .5 2 .0 2 .5 3.0 3.5 4 .0
Fig. 4-24 Sensitivity of the mixture (Nafion1:polyimide3)/HMDS/ZrO2 structure
0 1 0 0 2 0 0 3 0 0 4 0 0
Fig. 4-25 Time to drift in pH 7 buffer solution of the mixture (Nafion1:polyimide3)/HMDS/ZrO2 structure for 400 minutes
-0 .5 0 .0 0 .5 1 .0 1 .5 2 .0 2.5 3.0 3.5 4.0
Fig. 4-26 Sensitivity of the polyimide/HMDS/ZrO2 structure
0 100 200 300 400
Fig. 4-27 Time to drift in pH 7 buffer solution of the polyimide/HMDS/ZrO2
structure for 400 minutes
Table 1-1 Sensitivities and test range for different sensing membranes Sensing
membrane
Test range(pH) Sensitivity(mV/pH) Reference
ZrO2 1-13 57.5 4
Diameter (mm): 100+/-0.5 Diameter (mm): 100+/-0.5 Type/Dopant : P/Boron Type/Dopant : P/Phosphorous
Orientation : <100> Orientation : <100>
Resistivity (ohm-cm):1-10 Resistivity (ohm-cm):1-12 Thickness (μm) ：505-545 Thickness (μm) ：515545
Grade : Prime Grade : Prime
Table 3-2 Parameters of sensing layers deposition with sputter parameters of ZrO2 sputter
pre sputter 60W for 10 min Pressure : 7.6×10-3
Table 3-3 The test structures of REFET, NF=Nafion, and PI=polyimide
Table 4-1 Sensitivity at the optimum operation current
p-type pH-ISFET n-type pH-ISFET
Table 4-2 Comparison between original and modified S / N ratio S/N| p-type
Table 4-3 Parameters of the test structures; NF=Nafion, PI=polyimide
Epoxy/HMDS/ZrO2
PR mixed NF/HMDS/ZrO2
PI mixed NF/HMDS/ZrO2
Sensitivity
(mV/pH) 14.03 5.61 8.12
Drift rate
(mV/hr) N.A. 27.96 9.74
Table 4-4 Parameters of the test structures; NF=Nafion, PI=polyimide mixture(NF1:PI1)
/HMDS/ZrO2
mixture(NF1:PI3)
/HMDS/ZrO2 PI /HMDS/ZrO2
Sensitivity
(mV/pH) 8.12 6.34 4.79
Drift rate
(mV/hr) 9.74 4.68 3.36