Chapter 3 Procedures of the Experiment
3.3 Measurement System
3.3.3 Temperature Measurement Set-Up
For characterizing the temperature influence of ISFETs, we measured I-V curves for etch film with changing the pH-solution in order of pH 1, 3, 5, 7 buffer solutions and controlling the different ambient temperature about 25°C, 35°C, 45°C, 55°C, 65°C, 75°C, and 85°C. For each temperature value, we wait the 15 minutes then measured the ID-VG curves which the pH-ISFET had been covered by the pH-solution. The measurement system is showed in Figure 3-7. Figure 3-8 illustrates the detection principle of pH. Firstly, we obtain
the pH1 transconductance, it is purpose to get maximum gain. The second step we decide the pH1’s VG then decide the IDS. At last, we can obtain the pH3, pH5, pH7’s VG. When we change the different pH buffer solution, we must use the pH7 buffer solution first, then pH5, pH3, and pH1, it is purpose to get better performance which let the pH buffer solution concentration from low to high.
Chapter 4
Results and Discussions
4.1 Temperature Sensitivities of Various Membranes
4.1.1 ZrO2 membrane gate ISFET
Figure 4-1~4-14 are the ID-VG curves and sensitivities of ZrO2 gate ISFET in pH=1, 3, 5, 7 buffer solutions at 25℃, 35℃, 45℃, 55℃, 65℃, 75℃, and 85℃. The measuring data are sorted in table 4-1. Figure 4-15 is temperature sensitivity correlation coefficient. The measuring data are sorted in table 4-3. Figure 4-16 is normalize the temperature sensitivities curve. The measuring data are sorted in table 4-2. According to the data about table 4-3, we can find that ZrO2 gat ISFET temperature sensitivity is increase progressively.
Figure 4-17~4-20 the ID-VG curve at a specific pH declined with an increase in temperature. That can fine the isothermal point as show as Figure 4-21, it is near zero temperature coefficient, this indicates that a well-closen operating point can eliminate the temperature influence, the measuring data are sorted in table 4-4.
4.1.2 Ta2O5 membrane gate ISFET
Figure 4-22~4-35 are sensitivities of Ta2O5 gate ISFET in pH=1, 3, 5, 7 buffer solutions at 25℃, 35℃, 45℃, 55℃, 65℃, 75℃, and 85℃. The measuring data are sorted in table 4-1. Figure 4-36 is temperature sensitivity correlation coefficient. The measuring data are sorted in table 4-3. Figure 4-37 is normalize the temperature sensitivities curve. The measuring data are sorted in table 4-2. According to the data about table 4-3, we can find that
Ta2O5 gat ISFET temperature sensitivity is decrease progressively.
Figure 4-38~4-41 the ID-VG curve at a specific pH declined with an increase in temperature. That can fine the isothermal point as show as Figure 4-42, it is near zero temperature coefficient, this indicates that a well-closen operating point can eliminate the temperature influence, the measuring data are sorted in table 4-4.
4.1.3 Thermal Oxide membrane gate ISFET
Figure 4-43~4-56 are sensitivities of Thermal Oxidegate ISFET in pH=1, 3, 5, 7 buffer solutions at 25℃, 35℃, 45℃, 55℃, 65℃, 75℃, and 85℃. The measuring data are sorted in table 4-1. Figure 4-57 is temperature sensitivity correlation coefficient. The measuring data are sorted in table 4-3. Figure 4-58 is normalize the temperature sensitivities curve. The measuring data are sorted in table 4-2. According to the data about table 4-3, we can find that Thermal Oxidegat ISFET temperature sensitivity is increase progressively.
Figure 4-59~4-62 the ID-VG curve at a specific pH declined with an increase in temperature. That can fine the isothermal point as show as Figure 4-63, it is near zero temperature coefficient, this indicates that a well-closen operating point can eliminate the temperature influence, the measuring data are sorted in table 4-4.
4.1.4 PE Oxide membrane gate ISFET
Figure 4-64~4-77 are sensitivities of PE Oxidegate ISFET in pH=1, 3, 5, 7 buffer solutions at 25℃, 35℃, 45℃, 55℃, 65℃, 75℃, and 85℃. The measuring data are sorted in table 4-1. Figure 4-78 is temperature sensitivity correlation coefficient. The measuring data are sorted in table 4-3. Figure 4-79 is normalize the temperature sensitivities curve. The measuring data are sorted in table 4-2. According to the data about table 4-3, we can find that
PE Oxidegat ISFET temperature sensitivity is decrease progressively.
Figure 4-80~4-83 the ID-VG curve at a specific pH declined with an increase in temperature. That can fine the isothermal point as show as Figure 4-84, it is near zero temperature coefficient, this indicates that a well-closen operating point can eliminate the temperature influence, the measuring data are sorted in table 4-4.
4.1.5 Followings are the discussions of the measurement results:
It is very interested in sensitivity measurement result. Why, there are ZrO2 gate ISFET and Thermal Oxide gate ISFET temperature sensitivity increase progressively?
Why, there are Ta2O5 gat ISFET and PE Oxide gate ISFET temperature sensitivity decrease progressively?
According to the Eq. (2-6), the parameter α is a dimensionless sensitivity parameter and T is temperature parameter. If T is multiply α and product is increase that cause temperature sensitivity is upwardly. Otherwise, if T is multiply α and product is decrease that cause sensitivity is downward.
Accordingly, an isothermal point of the ZrO2, Ta2O5, Thermal Oxide, and PE Oxide in pH=1, 3, 5, 7 buffer solutions at 25℃, 35℃, 45℃, 55℃, 65℃, 75℃, and 85℃, the pH response increases monotonically as the temperature increases.
4.2 Drift Characteristics of Gate Stress Voltages in Various Membranes
4.2.1 ZrO2 membrane gate ISFET
Figure 4-85 shows the drift of ZrO2 gate ISFET with time. It is obviously shows a strong relation of gate drift and gate stress voltages. When the gate voltage is controlled as -0.5V, the drift voltage will decrease from -57.94 mV to -3.45mV in six hours measurement.
The improvement of the drift voltage reaches 94.05%. In order to confirm that the method works, a ZrO2 film ISFET has also measured by various gate voltages. Figure 4-86 shows the relation of drift voltages and gate stress voltages, and the data are sorted in table 4-5.
4.2.2 Ta2O5 membrane gate ISFET
Figure 4-87 shows the drift of Ta2O5 gate ISFET with time. It is obviously shows a strong relation of gate drift and gate stress voltages. When the gate voltage is controlled as -0.5V, the drift voltage will decrease from 40.6mV to 25.48mV in six hours measurement.
The improvement of the drift voltage reaches 37.24%. In order to confirm that the method works, a Ta2O5 film ISFET has also measured by various gate voltages. Figure 4-88 shows the relation of drift voltages and gate stress voltages, and the data are sorted in table 4-5.
4.2.3 Thermal Oxide membrane gate ISFET
Figure 4-89 shows the drift of Thermal Oxidegate ISFET with time. It is obviously shows a strong relation of gate drift and gate stress voltages. When the gate voltage is controlled as -0.5V, the drift voltage will decrease from 56.12mV to 2.94mV in six hours measurement. The improvement of the drift voltage reaches 94.76%. In order to confirm that
the method works, a Thermal Oxide film ISFET has also measured by various gate voltages.
Figure 4-90 shows the relation of drift voltages and gate stress voltages, and the data are sorted in table 4-5.
4.2.4 PE Oxide membrane gate ISFET
Figure 4-91 shows the drift of PE Oxidegate ISFET with time. It is obviously shows a strong relation of gate drift and gate stress voltages. When the gate voltage is controlled as 1V, the drift voltage will decrease from 45.54mV to 0.92mV in six hours measurement. The improvement of the drift voltage reaches 97.98%. In order to confirm that the method works, a PE Oxide film ISFET has also measured by various gate voltages. Figure 4-92 shows the relation of drift voltages and gate stress voltages, and the data are sorted in table 4-5.
4.2.5 Followings are the discussions of the measurement results:
According to the table 4-5, it is a simple and cheap way to solve the drift problem is presented which described the relation of drift and gate voltage. A constant various gate voltages are biased in the sensing layer with reference electrode. The improvement of drift voltages reaches higher. This may result from the gate electric field affecting the ions to diffusive into the gate insulator. To use this method, we can change different gate voltages which get a series of drift voltage characteristic. By this way, we can find the point about gate voltage which let drift approach to 0V for each membrane gate ISFET, it is very important to us for ISFET application. Then we will be commercialize the ISFET with a very low drift rate in a simple way.
4.3 Drift Characteristics Hysteresis of Cycle Time Test in Various Membranes
4.3.1 ZrO2 membrane gate ISFET
Figure 4-93~4-94 are drift hysteresis of ZrO2 gate ISFET in pH=7 buffer solution at either 25℃ or 85℃. The measuring data are sorted in table 4-6 and table 4-10. According to table 4-11, the percentages of accuracy after second cycle time are about 1.30% at 25℃ and 1.96% at 85℃.
4.3.2 Ta2O5 membrane gate ISFET
Figure 4-95~4-96 are drift hysteresis of Ta2O5 gat ISFET in pH=7 buffer solution at either 25℃ or 85℃. The measuring data are sorted in table 4-7 and table 4-10. According to table 4-11, the percentages of accuracy after second cycle time are about 1.60% at 25℃, and 1.69% at 85℃.
4.3.3 Thermal Oxide membrane gate ISFET
Figure 4-97~4-98 are drift hysteresis of Thermal Oxidegat ISFET in pH=7 buffer solution at either 25℃ or 85℃. The measuring data are sorted in table 4-8 and table 4-10.
According to table 4-11, the percentages of accuracy after second cycle time are about 3.66%
at 25℃, and 4.12% at 85℃.
4.3.4 PE Oxide membrane gate ISFET
Figure 4-99~4-100 are drift hysteresis of PE Oxidegat ISFET in pH=7 buffer solution at either 25℃ or 85℃. The measuring data are sorted in table 4-9 and table 4-10. According to table 4-11, the percentages of accuracy after second cycle time are about 3.68% at 25℃, and 4.19% at 85℃.
4.3.5 Followings are the discussions of the measurement results:
According to the Figure 4-94, Figure 4-96, Figure 4-98, Figure 4-100, we can see that cycle time 1 is toward gate voltage hysteresis larger then the other cycle time, it is initial drift characteristic of ISFET. If we want to know the ISFET gate voltage drift hysteresis at different temperatures, we must calculate hysteresis with beginning at second cycle.
Table 4-12 shows percentages of gate voltage drift hysteresis are smaller then 5% of all membranes. It is indirect identification that the confidence of this thesis experiment results are very high.
4.4 Conclusions
In this thesis of our experiment is to study and obtain the most suitable membrane which is compatible with CMOS fabrication processes to be a sensing layer for ISFET. We choose four membranes of ZrO2, Ta2O5, Thermal Oxide, and PE Oxide to be sensing films.
The first purpose is studying the temperature sensitivity characteristic. According to the Eq. (2-6), the parameter α is a dimensionless sensitivity parameter and T is temperature parameter. If T is multiply α and product is increase that cause temperature sensitivity is upwardly. Otherwise, if T is multiply α and product is decrease that cause sensitivity is
downward. There are ZrO2 gate ISFET and Thermal Oxide gate ISFET temperature sensitivity increase progressively. There are Ta2O5 gat ISFET and PE Oxide gate ISFET temperature sensitivity decrease progressively.
The second purpose is studying the temperature isothermal point of the four membranes in pH=1, 3, 5, 7 buffer solutions at 25℃, 35℃, 45℃, 55℃, 65℃, 75℃, and 85℃, the pH response increases monotonically as the temperature increases. In other words, the temperature isothermal point is temperature balance point in pH=1, 3, 5, 7 buffer solutions at 25℃, 35℃, 45℃, 55℃, 65℃, 75℃, and 85℃. We can obtain a well-chosen operating point that can eliminate the temperature influence in this region.
The third purpose is studying the gate voltage drift characteristic. According to the table 4-5, it is a simple and cheap way to solve the drift problem is presented which described the relation of drift and gate voltage. A constant various gate voltages are biased in the sensing layer with reference electrode. The improvement of drift voltages reaches higher. This may result from the gate electric field affecting the ions to diffusive into the gate insulator. To use this method, we can change different gate voltages which get a series of drift voltage characteristic. By this way, we can find the point about gate voltage which let drift approach to 0V for each membrane gate ISFET, it is very important to us for ISFET application. Then we will be commercialize the ISFET with a very low drift rate in a simple way.
The fourth purpose is studying the drift characteristics hysteresis of cycle time test in various membranes. According to the Figure 4-94, Figure 4-96, Figure 4-98, Figure 4-100, we can see that cycle time 1 is toward gate voltage hysteresis larger then the other cycle time, it is initial drift characteristic of ISFET. If we want to know the ISFET gate voltage drift hysteresis at different temperatures, we must calculate hysteresis with beginning at second cycle.
Table 4-12 shows percentages of gate voltage drift hysteresis are smaller then 5% of all membranes. It is indirect identification that the confidence of this thesis experiment results
are very high.
Chapter 5 Future Work
5.1 Temperature and Gate Voltage Stress Modulation influence
In our experiments, we investigated the ISFET’s reliability with temperature and gate voltage stress modulation individual. Therefore, how to link these factors influence range about temperature and gate voltage stress modulation together, and how to control the substrate voltage to make drift voltage become zero for each membrane gate ISFET, and how to design the circuit to realize our idea.
At present, the temperature hysteresis of the gate ISFET is not investigate deeply, and how to reduce it will be study more necessary. A perfect model of gate voltage stress with drift voltage should also be built in the future.
References
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Fig. 2-1 Electrode and electrolyte interface
(a) (b)
Fig. 2-2 Schematic representation of (a) MOSFET, (b) ISFET
Fig. 2-3 Schematic representation of the site-binding model
Drain Source Gate
electrolyte
Fig. 2-4 ID-VG characteristic of MOSFET and ISFET in different temperatures.
Fig. 2-5 ID-VG characteristic of MOSFET in different temperatures.
Isothermal point
Fig. 2-6 ID-VG characteristic of ISFET in different temperatures.
(a) (b)
Fig. 2-7 Series combination of the (a) initial (b) hydrated insulator capacitance
Silicon
Thermal Oxide Sensing Layer Solution
Hydration
(a)
(b)
`
(c)
(d) Si substrate Wet oxide
Si substrate BOE wet-etching
Si substrate S/D implant
Screen Oxide
PE Oxide
(e)
(f)
(g)
(h) Dry Oxide
Sensing layer 1
Sensing layer 2
(i)
Fig. 3-1 Corresponding graph for fabricate process flow.
Fig. 3-2 Shows the set up of measurement with the HP4156A Semiconductor Parameter Analyzer at room temperature.
Source
Drain Drain
Al electrode
Fig. 3-3 ID-VD curve of the 300Å ZrO2-gate ISFET.
Fig. 3-4 ID-VD curve of the 300Å Ta2O5-gate ISFET.
Fig. 3-5 ID-VD curve of the 300Å Thermal Oxide-gate ISFET.
Fig. 3-6 ID-VD curve of the 300Å PE Oxide-gate ISFET.
Fig. 3-7 Shows the set up of measurement with the HP4156A Semiconductor Parameter Fig. 3-8 Detection principle of different VG.
0
Fig. 4-1 ID-VG curves of the ZrO2 gate ISFET in pH=1, 3, 5, 7 buffer solutions at 25℃.
Fig. 4-2 Sensitivity of the ZrO2 gate ISFET in pH=1, 3, 5, 7 buffer solutions at 25℃.
Fig. 4-3 ID-VG curves of the ZrO2 gate ISFET in pH=1, 3, 5, 7 buffer solutions at 35℃.
Fig. 4-4 Sensitivity of the ZrO2 gate ISFET in pH=1, 3, 5, 7 buffer solutions at 35℃.
Fig. 4-5 ID-VG curves of the ZrO2 gate ISFET in pH=1, 3, 5, 7 buffer solutions at 45℃.
Fig. 4-6 Sensitivity of the ZrO2 gate ISFET at pH=1, 3, 5, 7 buffer solutions at 45℃.
Fig. 4-7 ID-VG curves of the ZrO2 gate ISFET in pH=1, 3, 5, 7 buffer solutions at 55℃
Fig. 4-8 Sensitivity of the ZrO2 gate ISFET at pH=1, 3, 5, 7 buffer solutions at 55℃.
Fig. 4-9 ID-VG curves of the ZrO2 gate ISFET in pH=1, 3, 5, 7 buffer solutions at 65℃.
Fig. 4-10 Sensitivity of the ZrO2 gate ISFET in pH=1, 3, 5, 7 buffer solutions at 65℃.
Fig. 4-11 ID-VG curves of the ZrO2 gate ISFET in pH=1, 3, 5, 7 buffer solutions at 75℃.
Fig. 4-12 Sensitivity of the ZrO2 gate ISFET in pH=1, 3, 5, 7 buffer solutions at 75℃.
Fig. 4-13 ID-VG curves of the ZrO2 gate ISFET in pH=1, 3, 5, 7 buffer solutions at 85℃.
Fig. 4-14 Sensitivity of the ZrO2 gate ISFET in pH=1, 3, 5, 7 buffer solutions at 85℃.
Fig. 4-15 Temperature sensitivity and correlation coefficient dependency of the ZrO2
gate ISFET.
Fig. 4-16 Normalize the temperature sensitivity curve of the ZrO2 gate ISFET.
Fig. 4-17 ID-VG curves of the ZrO2 gate ISFET in pH=1 buffer solution at temperatures of 25℃, 35℃, 45℃, 55℃, 65℃, 75℃, and 85℃.
Fig. 4-18 ID-VG curves of the ZrO2 gate ISFET in pH=3 buffer solution at temperatures of 25℃, 35℃, 45℃, 55℃, 65℃, 75℃, and 85℃.
Fig. 4-19 ID-VG curves of the ZrO2 gate ISFET in pH=5 buffer solution at temperatures of 25℃, 35℃, 45℃, 55℃, 65℃, 75℃, and 85℃.
Fig. 4-20 ID-VG curves of the ZrO2 gate ISFET in pH=7 buffer solution at temperatures of 25℃, 35℃, 45℃, 55℃, 65℃, 75℃, and 85℃.
Fig. 4-21 Iso-thermal point range of the ZrO2 gate ISFET in pH=1, 3, 5, 7 buffer solutions.
Fig. 4-22 ID-VG curves of the Ta2O5 gate ISFET in pH=1, 3, 5, 7 buffer solutions at 25℃.
Fig. 4-23 Sensitivity of the Ta2O5 gate ISFET in pH=1, 3, 5, 7 buffer solutions at 25℃.
Fig. 4-24 ID-VG curves of the Ta2O5 gate ISFET in pH=1, 3, 5, 7 buffer solutions at 35℃.
Fig. 4-25 Sensitivity of the Ta2O5 gate ISFET in pH=1, 3, 5, 7 buffer solutions at 35℃
Fig. 4-26 ID-VG curves of the Ta2O5 gate ISFET in pH=1, 3, 5, 7 buffer solutions at 45℃.
Fig. 4-27 Sensitivity of the Ta2O5 gate ISFET in pH=1, 3, 5, 7 buffer solutions at 45℃.
Fig. 4-28 ID-VG curves of the Ta2O5 gate ISFET in pH=1, 3, 5, 7 buffer solutions at 55℃.
Fig. 4-29 Sensitivity of the Ta2O5 gate ISFET in pH=1, 3, 5, 7 buffer solutions at 55℃
Fig. 4-30 ID-VG curves of the Ta2O5 gate ISFET in pH=1, 3, 5, 7 buffer solutions at 65℃.
Fig. 4-31 Sensitivity of the Ta2O5 gate ISFET in pH=1, 3, 5, 7 buffer solutions at 65℃.
Fig. 4-32 ID-VG curves of the Ta2O5 gate ISFET in pH=1, 3, 5, 7 buffer solutions at 75℃.
Fig. 4-33 Sensitivity of the Ta2O5 gate ISFET in pH=1, 3, 5, 7 buffer solutions at 75℃.
Fig. 4-34 ID-VG curves of the Ta2O5 gate ISFET in pH=1, 3, 5, 7 buffer solutions at 85℃.
Fig. 4-35 Sensitivity of the Ta2O5 gate ISFET in pH=1, 3, 5, 7 buffer solutions at 85℃.
Fig. 4-36 Temperature sensitivity and correlation coefficient dependency of the Ta2O5 gate ISFET.
Fig. 4-37 Normalize the temperature sensitivity curve of the Ta2O5 gate ISFET.
Fig. 4-37 Normalize the temperature sensitivity curve of the Ta2O5 gate ISFET.