Chapter 3 Experiment
3.4 Measurement system
3.4.2 Measurement setup
HP-4156 is used as measurement tool in this experiment. All experiment proceeds in the dark-box, to avoid the light emission causes the electron-hole pair in the semiconductor and the unknown results in the sensing membrane.
The electrolyte buffer solutions were purchased form Riedel-deHaen and the pH-value is 1, 3, 5, 7, 9, 11, 13, respectively. First of all, we glued a container made by plastic on the wafer, which is covered the sensing layer, and the container’s volume is about 50 c.c. This step was important and complex, right here, an acknowledgement must be given for 3M company by the fine glue to reduce the glue proceed complexity.
A reference electrode is immersed in the electrolyte solution, to give the voltage to the EIS system same as the gate voltage controller in MOSFET. Where we fix the height from sensing layer surface to the reference electrode is trying to stabilize the electrical potential floating phenomenon. [7] Note that, the electrolyte must be injected into the container smoothly preventing the bubble produced near the sensing area.
Firstly, the electrical characteristic of drain current-voltage relation is used to determinate the FET quality. Fortunately, the device made in this work has a fine electrical property Fig.3.1. Next, the electrical relation between drain-current and gate-voltage is going on. The electrical characteristic of Id-Vg is used to determinate
the ISFET sensitivity. Electrolyte solutions change from the degree of pH=13 to the degree of pH=1 during the measurement of Id-Vg. We purpose the changing step of electrolyte by 3 times injecting and 3 times pumping to prevent the ion concentration over the fixed pH. When next to the following pH solution, first pumping electrolyte, and first injecting the target pH solution into container till the third injecting stop.
Secondly, the drift characteristic is measured with specific pH value of 7 and different sampling period of 30 seconds, 1 minute, 10 minutes and 1 hour. 33 sampling points in the time frame of 7 hours for each ISFET film.
3.5 References
[1]P. Bergveld, “Thirty years of ISFETOLOGY What happened in the past 30 years and what may happen in the next 30 years” Sensors and Actautors B 88
(2003)1-20
[2] T. Matsuo and M. Esashi, Methods of ISFET fabrication, Sensor. & Actuator 1 (1981) 77-96.
[3] Shiun-Sheng Jana, “Effect of Mg2+-dopant on the characteristics of lead titanate sensing membrane for ion-sensitive field-effect transistors” Sensors and Actautors B108(2005) 883-887
[4]Arnaldo D’Amico,” Sensors small and numerous: always a winning strategy?”Sensors and Actuator B 106(2005) 144-152
[5] Artur Dybko, “Errors in Chemical Sensor Measurements”, Sensors, ISSN 1424-8220, 2001 by MDPI
[6] Skoog, D.A., “Fundamentals of analytical chemistry”, Saunders College Publications, 1996
[7] P. Bergveld ,“ How electrical and chemical requirements for REFETs may coincide ＂, Sensors and Actuators 18 (1999) p.309-327
Chapter 4
Results and Discussions
4.1 Introduction
The pH-ISFET differs from a MOSFET on the structural components especially in the gate terminal part. At which, the metal covered over gate material is replaced by electrolyte solution and a reference electrode. Changing the pH values of the electrolyte produces a potential drop which is differ from each value, thus, the sensitivity is arising when increasing pH value. In the point of view, determining the potential drop, expect the electrolyte, several kinds of sensing membrane is considered. In this work, titanium dioxide (TiO2) and zirconium dioxide (ZrO2) are used to be the sensing membrane because of the higher sensitivity and the stable electrical characteristic.
The pH-ISFET, a potentiometric based sensor, also needs a reference field-effect-transistor (REFET) to eliminate the other affecting factors, such as temperature effects. In our experiment, differs from others, trying to co-manufacturing ISFET/REFET in the CMOS processing by the method of purposed plasma surface treatment. To reduce the sensitivity, several time intervals by plasma treatment we tried. Finally, high/ low sensitivities of membranes for ISFET and REFET are essential for getting higher resolution of pH measurement.
4.2 Sensitivity characteristic of sensing materials
In changing of the sensing membrane, the sensitivity differs from each other because of the different characteristics of the sensing membrane. And we knew TiO2
and ZrO2 membranes have stable electric characteristic. Firstly, we discuss the sensitivity of pure sensing membrane. Next, the plasma treated sensing membrane is discussed. Each of them treats under NH3 plasma surface treatment during 0, 5, 10, 20, 30, and 60 minutes.
4.2.1 Sensitivity characteristic of TiO2 membrane
The sensitivity of pure TiO2 membrane is the values of 56.7mV/pH, 58.3mV/pH, and 55mV/pH. It is presenting the stable sensitivity characteristic. And because of the better performance of TiO2 in previous study investigated by our team, the TiO2
membrane is used to be sensing membrane directly while the surface pretreatment of plasma finished. Fig.4.1 is the corresponding I-V curves with no plasma treatment of TiO2, and figure 4.2 is the corresponding sensitivity of TiO2 without plasma treatment.
The way we find out sensitivity here based on following steps, firstly we calculate the I-V curve of pH=13 in Fig.4.1, secondly we find the maximum conductance corresponding to the value of voltage from g-V curve of pH=13, finally we find the voltage value of pH=13 with maximum conductance and it’s corresponding current value by I-V curve of pH=13, then the fixed current value correspond to it’s own voltage value with each pH I-V curve, plotting in Fig.4.2 and finding the value of slope out, then we got the sensitivity value. Fig.4.3 is the I-V curves of TiO2 with NH3
plasma treatment during 5 minutes, the narrower interval is showed, Fig.4.4 shows the corresponding sensitivity of TiO2 with plasma treatment during 5 minutes. Fig.4.5 shows the TiO2 electrical characteristic with NH3 plasma treating during 10 minutes, and Fig.4.6 shows the corresponding sensitivity of TiO2 with NH3 plasma treatment
during 10 minutes. Fig.4.7 shows the I-V curves of TiO2 treated with NH3 plasma during 20 minutes, and the corresponding sensitivity is shown in Fig.4.8. The I-V curves of TiO2 membrane treated by NH3 plasma during 30 minutes is shown in Fig.4.9 and the corresponding sensitivity is shown in Fig.4.10. Obviously, there is a downstair tendency when the treating time increasing. Fig.4.11 shows the I-V curves of NH3 treating time during 60 minutes and the corresponding sensitivity is shown in Fig.4.12. The raising value of treating time during 60 minutes is considered by the increasing effective dangling bonds. Table 4.1 lists the corresponding sensitivity values and Fig.4.13 shows the tendency of various plasma treating time interval.
4.2.2 Sensitivity characteristic of ZrO2 membrane
Fig.4.14 shows the I-V curves of ZrO2 without plasma treatment and the corresponding sensitivity is shown in Fig.4.15. Fig.4.16 shows the I-V curves of ZrO2
treated by NH3 plasma during 5 minutes, the narrower interval can be seen, and the corresponding sensitivity is shown in Fig.4.17. Fig.4.18 shows the I-V curves of ZrO2
treated by NH3 plasma during 10 minutes and Fig.4.19 shows the corresponding sensitivity. Fig.4.20 shows the I-V curves of ZrO2 with NH3 plasma treatment during 20 minutes and sensitivity chart is shown in Fig.4.21. Fig.4.22 shows the I-V curves of ZrO2 with NH3 plasma treatment during 30 minutes and the corresponding sensitivity is shown in Fig.4.23. Fig.4.24 shows the I-V curves of ZrO2 treated by NH3
plasma during 60 minutes and the corresponding sensitivity is shown in Fig.4.25.
Table 4.2 lists the corresponding sensitivity values and Fig.4.26 shows the tendency of various plasma treating time interval. We have the downstair tendency in NH3
plasma treatment during longer spending time.
4.3 The coplanar ISFET/REFET sensor array system
The sensor array system is composed by the coupled sensing membrane without plasma treatment and with post-plasma-treatment after deposition. In this experiment, we use TiO2 membrane without and with NH3 post-plasma-treatment to form the ISFET/REFET sensor array system, and ZrO2 membrane without and with NH3
post-plasma-treatment as the ISFET/REFET sensor array system. Thus, the table 4.3 lists the difference of coplanar structure in TiO2 membrane and table 4.4 lists the difference of coplanar structure in ZrO2 membrane. Obviously, the difference of sensitivity increased with longer NH3 plasma treating time. The highest value we have about 34.2 mV/pH made by ZrO2 pair.
4.4 Conclusions
In this work, we are trying to study the influence factors with sensitivity and attending to simplify the manufacturing of REFET. For the further purpose, we are trying to realize the comanufacturing process of ISFET/REFET sensing array with compatible CMOS manufacturing process. And different from other gate materials of REFETs, the sensing materials are available in CMOS fabrication technology in this work. Fortunately, the purposed NH3 plasma surface treatment work for the purpose indeed. A novel fabrication of REFET with plasma surface treatments is demonstrated.
And we have the sensitivity of co-planar structure of ISFET/REFET are 30.8mV/pH by TiO2/NH3_plasma_treated_30mins_TiO2 and 34.2mV/pH by ZrO2/NH3_plasma_treated_60mins_ZrO2.
Chapter 5 Future work
5.1 Future work
We had introduced the method of co-manufacturing ISFET/REFET by plasma treatment successfully in this work. Most of the attempts to create a REFET are based on covering the gate oxide of an ISFET with an additional ion insensitive membrane.
Such as PVC membranes, but it is pity that they are not MOSFET fabrication compatible and the manufacturing of this material is complicated. So, the purposed methods of co-manufacturing ISFET/REFET are useful.
However, there still are lots of problems in ISFET. Such as the stability of sensing membrane, the reproducibility of ISFET (drift phenomenon ), and the minification of ISFET. The minfication of ISFET is purposed by taking the solid state electrode replace the huge reference electrode immersed in the electrolyte.
Finally, based on the knowledge of ion-sensitive field effect transistor, the ion-selective field effect transistor can be produced by the same MOSFET manufacturing process. The ion-selective field effect transistor, resulted from sensing specific ions with the specific membrane, is also a proper extensive topic for future study.
Diameter (mm): 100+/-0.5 Type / Dopant : P / Boron
Orientation : <100>
Resistivity (ohm-cm):1-10 Thickness (μm) ：505-545
Grade : Prime (a) Specifications of wafers
TiO2
Density 4.26 Z-ratio 0.4 Tooling 50.47 Current (mA) 1 ~ 60
Rate (Å/sec) 1.2 Pressure (Torr) 5*10-6
(b)
E – gunparameters of ZrO2 sputter power : 200 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 (c) Sputter
NH3
Pressure=200mtorr RF power=100w Flow rate=30sccm Temperature=300℃
(d) Plasma
Table 3.1 (a) Specifications of wafers
(b) Parameters of sensing layers deposition with E – gun (c) Parameters of sensing layers deposition with Sputter
(d) Condition of plasma treatment
Sensitivity (mV/pH) 1~7 7~13 1~13
0 min 58.3 53.3 55.8
5 mins 61.7 41.7 51.7
10 mins 55 20 61.7
20 mins 40 25 32.5
30 mins 28.3 16.7 22.5
TiO2_NH3 plasma treatment
60 mins 30 30 30
Table 4.1 Sensitivity values of TiO2 with NH3 plasma treatment during time interval
Sensitivity (mV/pH) 1~7 7~13 1~13
Table 4.2 Sensitivity values of ZrO2 with NH3 plasma treatment during time interval
Sensitivity(mV/pH) 1~7 7~13 1~13
5 mins -3.4 11.6 4.1 Table 4.3 The sensitivity of coplanar structure in TiO2 membrane
Sensitivity(mV/pH) 1~7 7~13 1~13
5 mins 6.7 1.6 4.2 Table 4.4 The sensitivity of coplanar structure in ZrO2 membrane
Fig 1.1 The schematic of ISFET
Fig 2.1 ISFET structure band diagram (Luc Bousse, J. Chem. Phys. , 76 )
Figure 2.2 Potential profile and charge distribution at oxide / electrolyte interface
`
(a) RCA clean bare silicon
(b) wet oxidation
(c) maskⅠ
(d) screening oxidation & implantation
Figure 3.1 Experimental process
(e) passivation layer deposition
(f) maskⅡ& dryoxidation
(g) sensing layer α deposition
Figure 3.1 Experimental process
(i) Surface plasma treatment
(j) Sensing layer β deposition & mask Ⅲ
(k) Al deposition
Figure 3.1 Experimental process
Fig 4.1 I-V curves of TiO2 without plasma treatment Sensitivity_TiO2_PH=1-13
y = 0.057x + 1.4841 R2 = 0.998
0 0.5 1 1.5 2 2.5
0 2 4 6 8 10 12 14
ph
Vg
Fig 4.2 Sensitivity chart of TiO2 without plasma treatment
Fig 4.3 I-V curves of TiO2 membrane with NH3 plasma treatment during 5 minutes
Sensitivity_TiO2_NH3 plasma 5mins_PH=1-13
y = 0.0525x + 1.4796 R2 = 0.9902
0 0.5 1 1.5 2 2.5
0 2 4 6 8 10 12 14
ph
Vg
Fig 4.4 Sensitivity chart of TiO2 with NH3 plasma treatment during 5 minutes
Fig 4.5 I-V curves of TiO2 membrane with NH3 plasma treatment during 10 minutes
Sensitivity_TiO2_NH3 plasma 10mins_PH=1-13
y = 0.038x + 1.7666 R2 = 0.9446
0 0.5 1 1.5 2 2.5
0 2 4 6 8 10 12 14
ph
Vg
Fig 4.6 Sensitivity chart of TiO2 with NH3 plasma treatment during 10 minutes
Fig 4.7 I-V curves of TiO2 membrane with NH3 plasma treatment during 20 minutes
Sensitivity_TiO2_NH3 plasma 20mins_PH=1-13
y = 0.0311x + 2.5382 R2 = 0.9691
2.5 2.55 2.6 2.65 2.7 2.75 2.8 2.85 2.9 2.95 3
0 2 4 6 8 10 12 14
ph
Vg
Fig 4.8 Sensitivity chart of TiO2 with NH3 plasma treatment during 20 minutes
Fig 4.9 I-V curves of TiO2 membrane with NH3 plasma treatment during 30 minutes
Sensitivity_TiO2_NH3 plasma 30mins_PH=1-13
y = 0.0241x + 1.7998 R2 = 0.9234
1.75 1.8 1.85 1.9 1.95 2 2.05 2.1 2.15
0 2 4 6 8 10 12 14
ph
Vg
Fig 4.10 Sensitivity chart of TiO2 with NH3 plasma treatment during 30 minutes
Fig 4.11 I-V curves of TiO2 membrane with NH3 plasma treatment during 60 minutes
Sensitivity_TiO2_NH3 plasma 60mins_PH=1-13
y = 0.0304x + 1.3475 R2 = 0.9868
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
0 2 4 6 8 10 12 14
ph
Vg
Fig 4.12 Sensitivity chart of TiO2 with NH3 plasma treatment during 60 minutes
TiO2
0 10 20 30 40 50 60
S en sitiv ity
w/o plasma treatment 55.8 55.8 55.8 55.8 55.8
with plasma treatment" 51.7 37.5 32.5 22.5 30
5mins 10mins 20mins 30mins 60mins
Fig 4.13 The tendency of various plasma treating time interval of TiO2
Fig 4.14 I-V curves of ZrO2 without plasma treatment
Sensitivity_ZrO2_PH=1-13
y = 0.063x + 2.1959 R2 = 0.9993
0 0.5 1 1.5 2 2.5 3 3.5
0 2 4 6 8 10 12 14
ph
vg
Fig 4.15 Sensitivity chart of ZrO2 without plasma treatment
Fig 4.16 I-V curves of ZrO2 membrane with NH3 plasma treatment during 5 minutes
Sensitivity_ZrO2_NH3 plasma 5mins_PH=1-13
y = 0.0589x + 2.0361 R2 = 0.9955
0 0.5 1 1.5 2 2.5 3
0 2 4 6 8 10 12 14
ph
Vg
Fig 4.17 Sensitivity chart of ZrO2 with NH3 plasma treatment during 5 minutes
Fig 4.18 I-V curves of ZrO2 membrane with NH3 plasma treatment during 10 minutes
Sensitivity_ZrO2_NH3 plasma 10mins_PH=1-13
y = 0.0475x + 1.6118 R2 = 0.9834
0 0.5 1 1.5 2 2.5
0 2 4 6 8 10 12 14
ph
Vg
Fig 4.19 Sensitivity chart of ZrO2 with NH3 plasma treatment during 10 minutes
Fig 4.20 I-V curves of ZrO2 membrane with NH3 plasma treatment during 20 minutes
Sensitivity_ZrO2_NH3 plasma 20mins_PH=1-13
y = 0.0482x + 2.6796 R2 = 0.9879
0 0.5 1 1.5 2 2.5 3 3.5
0 2 4 6 8 10 12 14
ph
Vg
Fig 4.21 Sensitivity chart of ZrO2 with NH3 plasma treatment during 20 minutes
Fig 4.22 I-V curves of ZrO2 membrane with NH3 plasma treatment during 30 minutes
Sensitivity_ZrO2_NH3 plasma 30mins_PH=1-13
y = 0.0393x + 1.4136 R2 = 0.9641
0 0.5 1 1.5 2 2.5
0 2 4 6 8 10 12 14
ph
Vg
Fig 4.23 Sensitivity chart of ZrO2 with NH3 plasma treatment during 30 minutes
Fig 4.24 I-V curves of ZrO2 membrane with NH3 plasma treatment during 60 minutes
Sensitivity_ZrO2_NH3 plasma 5mins_PH=1-13
Fig 4.25 Sensitivity chart of ZrO2 with NH3 plasma treatment during 60 minutes
ZrO2
w/o plasma treatment 62.5 62.5 62.5 62.5 62.5
with plasma treatment 58.3 46.7 47.5 38.3 28.3 5mins 10mins 20mins 30mins 60mins
Fig 4.26 The tendency of various plasma treating time intervals of ZrO2
ZrO2
treated
ZrO2 TiO2
treated
TiO2
Fig 4.27 The coplanar structure