Chapter 2 Theory and Principle
3.4 Conclusions
In this study, the ZrO2 gate ISFET is proposed as a pH-sensitive membrane. The sensing properties of the device such as sensitivity and drift are obtained by IDS-VGS
measurement in a series of buffer solutions. A pH response of about 57.5 mV/pH and a drift of 0.831 mV/h were obtained. Different sensitivities of various ratios of width to length in the channel were not observed. Thus, the ZrO2 gate ISFET can be used in the pH range of 1 to 13 with a perfect linear-fitted line that will enable the application of ISFETs to many fields.
3.5 References
[1] P. Bergveld, “Development of an ion sensitive solid-state device for neurophysiological measurements,” IEEE Trans. Biomed. Eng. vol. 17, pp. 70–71, 1970.
[2] S. D. Moss, C. C. Johnson, and J. Janata, “Hydrogen calcium and potassium ion sensitive FET transducers,” A preliminary report, IEEE Trans. Biomed. Eng. vol.
25 pp. 49–54, 1978.
[3] H. K. Liao, J. C. Chou, W. Y. Chung, T. P., and S. K. Hsiung, “Study on the interface trap density of the Si3N4/SiO2 gate ISFET,” Proceedings of the Third East Asian Conference on Chemical Sensors, Seoul, South Korea, pp. 394–400, November 1997.
[4] L. T. Yin, J. C. Chou, W. Y. Chung, T. P., and S. K. Hsiung, “Study of indium tin oxide thin film for separative extended gate ISFET,” Mater. Chem. Phys. vol.70 pp. 12-16, 2001.
[5] P. Gimmel, B. Gompf, D. Schmeiosser, H. D. Weimhofer, W. Gopel, and M.
Klein, “Ta2O5 gates of pH sensitive device comparative spectroscopic and electrical studies,” Sensors and Actuators B vol. 17 pp. 195–202, 1989.
[6] J. C. Chou and J. L. Chiang, “Study on the amorphous tungsten trioxide ion-sensitive field effect transistor,” Sensors and Actuators B vol. 66 pp. 106-108, 1998.
[7] T. matsuo and M. Esashi, “Methods of ISFET fabrication” Sens and Actuators vol.
1 pp. 77-96, 1981.
[8] J. C. Chou and J. L. Chiang,” Ion sensitive field effect transistor with amorphous tungsten trioxide gate for pH sensing ” Sens and Actuators B vol. 62 pp. 81-87 2000.
[9] 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.
Table 3-Ⅰ. Growth conditions for film deposition using sputtering system.
Parameter Conditions
Deposition rate (nm/min) 0.15 Deposition pressure (Torr) 20×10-3 Background pressure (Torr) 3×10-6 Ar/O2ratio (cm3/min) 24/8 Substrate temperature (℃) 150
P-type-silicon
N+ N+
ZrO2
PE-SiO2
Thermal-SiO2 Al
Figure 3-1 Schematic diagram of ZrO2 gate ISFET fabricated by the MOSFET technique.
P-type-silicon
N+ N+
Buffer Solution
Heating and Cooling Platform
Temperature Controller HP4156A Semiconductor
Parameter Analyzer
Reference Electrode
Figure 3-2 Setup of measurement using HP4156A semiconductor parameter analyzer and temperature controller.
Figure 3-3 IDS-VDS curves of ZrO2 gate ISFETs.
Figure 3-4 IDS-VGS curves that are shifted parallel with pH concentration of buffer solutions, and in nonsaturation region with VDS = 2 V.
Figure 3-5 VGS and pH values for obtaining pH response of 57.5 mV/pH from slope of linear-fitted line.
Figure 3-6 Sensitivities of three devices having different ratios of width to length.
Figure3-7 Drift of 5.82 mV during 7 h measurement in which a drift of 0.831 mV/h is obtained.
Figure 3-8 Long-term drift of about 85 mV during 200 h measurement in which a drift of 0.425 mV/h is obtained.
Figure 3-9 VGS and pH values in 1M NaCl solution for obtaining pH response of 52.5 mV/pH from slope of linear-fitted line.
Chapter 4
Drift Characteristics with Sensing Oxide Thickness Modulation by Co-fabricating ISFET and REFET
4.1 Backgrounds and Motivation
Because so many chemical and biological processes are dependent on pH, it is one of the most common measurements in the laboratories. In the past, the glass electrode was usually used to detect pH until a brainchild technique based on MOSFET called ISFET that had demonstrated by Bergveld in 1970 [1]. The ISFET is a special type of the MOSFET without a metal gate, by which, the gate is directly exposed to the buffer solution. When a change of the surface potential between the gate insulator and electrolyte, the electric field at the insulator semiconductor interface will be changed and the channel conductance that influences the drain current will be modulated, too. Since the channel conductance and drain current can be modulated, we may measure the changes by applying a fixed source to drain voltage or constant source to drain voltage current and different output gate voltages.
By these ways, we can plot a standard linear line between gate voltages and various pH values, and the standard linear line can be taken to measure unknown acid or alkaline solution. Comparing to the conventional method, the ISFET device owns some advantages that are small size, short response time, cheap and CMOS compatible processes. The development of ISFET has been over 35 years, and the first sensitive membrane is silicon dioxide (SiO2) which appeared instable sensitivity and large drift. Subsequently, Al2O3, Ta2O3, SnO2, TiN, a-WO3, ZrO2, and ITO were used as pH-sensitive membranes for the higher pH response and much stable drift voltage [2~9]. Table 2-I shows the sensitivities and test range of the different sensing
membranes. We can easily find that the sensitivity of SiO2 is not the largest one and owns a large drift voltage. But in the view of CMOS compatible and bio-compatible, the SiO2 is the most famous one. The most disadvantage of SiO2 gate ISFET is its unstable effect that is introduced by hydration. In order to understand the relation between thickness and drift, we overcome the difficulty in measuring the unstable effect of SiO2 gate ISFET by using the Ta2O5 gate REFET.
In this study, the SiO2 membrane was used as a pH sensitive layer for ISFET.
Ta2O5 was prepared by dc sputtering process as a pH-sensitive membrane for REFET that could estimate the effect of temperature and processing difference. The electrical characteristics and pH responses of the ISFET and REFET are studied by the standard MOSFET measurement with HP 4156A. The drift voltages are also calculated by the drain current to gate voltage (ID-VG) measurements.
4.2 Experimental
4.2.1 Device Fabrication
Figure 4-1 shows the schematic diagram of the SiO2 gate ISFET which is fabricated by the MOSFET technique. A 30nm thickness sensitive layer of SiO2
membrane was deposited onto the SiO2 gate ISFET by PECVD. And a 30 nm thickness sensitive layer of Ta2O5 membrane was also sputtering onto the SiO2 gate ISFET by sputtering system as a REFET. The SiO2 membrane is defined by photo resist and etched by BOE. The ISFETs were fabricated on the p-type silicon wafer with (100) orientation, and the manufacture processes were listed as follows:
(1) RCA cleaning of 4-inch, p-type silicon wafer
(2) Wet oxidation of silicon dioxide (600 nm, Figure 4-1(a))
(3) Defining of S/D areas with mask Ⅰ and wet-etching of silicon dioxide by BOE
(4) Thermal growth of silicon dioxide as screen oxide (30 nm, Figure 4-1 (b)) (5) Phosphorus ion implantation and post annealing at 950℃ (Figure 4-1 (c)) (6) PECVD silicon dioxide for passivation layer (Figure 4-1 (d))
(7) Defining of contact hole and gate region with mask Ⅱ and wet-etching of silicon dioxide by BOE
(8) Dry oxidation of gate oxide (30 nm)
(9) PECVD silicon dioxide for sensitive layer (30 nm, Figure 4-1 (e))
(10) Defining of gate region with mask Ⅲ and wet-etching of oxide by BOE (11) DC sputtering a 30 nm thickness of Ta2O5 film with hard contact mask Ⅳ (12) Aluminum sputtering with hard contact mask Ⅴ (600 nm, Figure 4-1 (f)) 4.2.2 Packaging and Measurement
A container is bonded on the gate region of the ISFET/REFET by epoxy resin.
Figure 4-2 shows the set up of measurement and the HP4156A Semiconductor Parameter Analyzer is used to measure the IDS-VGS characteristics of the ZrO2 gate ISFET/REFET devices in the buffer solutions. All the measurement processes are carried out at the room temperature of 25 ℃ by a temperature control system, and placed in the dark box. After the buffer solution is injected into the container, we will not measured until the ISFET/REFET is immersed in the buffer solution for 60 seconds to make sure that the devices are under steady situation.
4.3 Results and Discussion
4.3.1 pH Sensitivities of the ISFET/REFET
The pH sensitivities of the ISFET and REFET in pH = 1, 3, 5, 7, 9, 11 and 13 buffer solutions at room temperature are obtained by a HP4156A Semiconductor Parameter Analyzer. Figure 4-3 shows that the IDS-VGS curves are shifted in parallel with the pH concentration of the buffer solutions, and in the non-saturation region
with VDS = 2 V. The IDS-VGS curves represent the threshold voltage shift towards positive values with increasing pH values. After several times of measurements, a median linear pH response of 31.15 mV/pH and 55.66 mV/pH are obtained by calculating the shifts in the VGS of the ISFET and REFET, respectively, by a constant drain current measurement for different pH values. Figure 4-4 shows the VGS to pH values that can obtain the pH response of 31.15 mV/pH and 55.66 mV/pH by the slope of the linear fitted lines.
4.3.2 Drift of the SiO2 Gate ISFET
The drifts of the ISFET and REFET are also calculated by IDS-VGS
measurements for 7 hours. Figure 4-5 shows the drift during 7 hours measurement time that can be obtained a drift of 37 to 81 mV in ISFET and 8mV in REFET at the slow response region. The drift is saturation after 6 hours that implies the hydration will be stable too. In order to estimate the unwanted effect (e.g. temperature, light, process), a delta drift is defined as the differential drift of ISFET and REFET. The first hours always defined as surface reaction (fast response region) which shows a high value of drift, so we determined the last six hours as the effective times of hydration (slow response region). Figure 4-6 shows the delta drift that increases with increasing the thickness of SiO2 film, and the curve is saturation at about 50 nm.
According to the equation 43 in chapter 2, the thickness of hydration layer is about 50 nm.
4.4 Conclusion
The much stable sensing properties of sensitivity and drift are obtained by the correction of REFET. The pH response of about 28 to 32 mV/pH can be obtained. A depth of hydration layer in SiO2 gate ISFET is presented by modulating the oxide thickness. The predicted thickness of hydration layer is 50 nm of PECVD SiO2 during
7 hours immersing in a buffer solution. This study not only provide a simple way to determine the thickness of hydration layer without simulating, but also make the measurement of much stable drift in SiO2 gate ISFET possible.
4.5 References
[1] P. Bergveld, “Development of an ion sensitive solid-state device for neurophysiological measurements,” IEEE Trans. Biomed. Eng. vol. 17, pp. 70–71, 1970.
[2] S. D. Moss, C. C. Johnson, and J. Janata, “Hydrogen calcium and potassium ion sensitive FET transducers,” A preliminary report, IEEE Trans. Biomed. Eng. vol.
25 pp. 49–54, 1978.
[3] H. K. Liao, J. C. Chou, W. Y. Chung, T. P., and S. K. Hsiung, “Study on the interface trap density of the Si3N4/SiO2 gate ISFET,” Proceedings of the Third East Asian Conference on Chemical Sensors, Seoul, South Korea, pp. 394–400, November 1997.
[4] L. T. Yin, J. C. Chou, W. Y. Chung, T. P., and S. K. Hsiung, “Study of indium tin oxide thin film for separative extended gate ISFET,” Mater. Chem. Phys. vol.70 pp. 12-16, 2001.
[5] P. Gimmel, B. Gompf, D. Schmeiosser, H. D. Weimhofer, W. Gopel, and M.
Klein, “Ta2O5 gates of pH sensitive device comparative spectroscopic and electrical studies,” Sensors and Actuators B vol. 17 pp. 195–202, 1989.
[6] J. C. Chou and J. L. Chiang, “Study on the amorphous tungsten trioxide ion-sensitive field effect transistor,” Sensors and Actuators B vol. 66 pp. 106-108, 1998.
[7] T. matsuo and M. Esashi, “Methods of ISFET fabrication” Sens and Actuators vol.
1 pp. 77-96, 1981.
[8] J. C. Chou and J. L. Chiang,”Ion sensitive field effect transistor with amorphous tungsten trioxide gate for pH sensing ” Sens and Actuators B vol. 62 pp. 81-87, 2000.
[9] 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.
Figure 4-1 The schematic diagram of the SiO2 gate ISFET which is fabricated by the MOSFET technique.
Figure 4-2 The set up of measurement with the HP4156A Semiconductor Parameter Analyzer and temperature controller.
Figure 4-3 (a) The IDS-VGS curves of SiO2 gate ISFET devices (50 nm).
Figure 4-3 (b) The IDS-VGS curves of Ta2O5 gate ISFET devices (30 nm).
Figure 4-4 (a) Sensitivity of SiO2 gate ISFET device (50 nm).
Figure 4-4 (b) Sensitivity of Ta2O5 gate ISFET device (30 nm).
0 1 2 3 4 5 6 7
Figure 4-5 Drifts in ISFET and REFET during 7 hours.
0 20 40 60 80 100 0
10 20 30 40 50 60 70 80
ΔV GS (mV)
SiO2 Thickness (nm)
Figure 4-6 Delta drifts of SiO2 gate ISFET devices.
CHAPTER 5
Ultra-Low Drift Voltage by Using Gate Voltage control in Oxide-Based Gate ISFET
5.1 Backgrounds and Motivation
There are a lot of chemical and biological processes dependent on pH, and thus the pH detection device is one of the most common measurements in laboratories. In the past, a glass electrode was usually used for detecting pH until a brainchild technique based on metal-oxide-semiconductor field-effect transistor (MOSFET) called ion-sensitive field-effect transistor (ISFET) that was demonstrated by Bergveld in 1970 [1]. The first sensing membrane used is silicon dioxide (SiO2), which showed an unstable sensitivity due to a large drift. Subsequently Al2O3, Ta2O5, SnO2, TiN, α -WO3, ITO, and ZrO2 [2-4] have been tried to use as the pH-sensitive membranes to achieve a higher pH response and a more stable drift voltage. Although drift voltage is much smaller than sensitivity, the drawback is continuously affecting the ISFET to commercialize. There are some proposed explanations for drift including electric field enhanced ion migration within the gate insulator [5], some electrons creating space change inside the insulator films, and some surface effects [6]. There are also some methods to solve the drift problem, such as manufacturing with a reference field-effect transistor (REFET) or disposing a correct integrated circuit device to reduce the drift rate. However, these methods are always either having a difficult process or costing too expensive.
In this study, a simple and cheap way to solve the drift problem is presented which describes the relation of drift and gate voltage. Constant various gate voltages are biased in the sensing layer with reference electrode. It obviously shows a strong relation of gate drift and gate stress voltages. There are two sensing films of ZrO2 and
SiO2 to be considered in this study. A great improvement of drift voltage in these two materials as ISFETs can be achieved by using gate voltage controlling method.
From equation 39 in chapter 2, we observed that drift is directly proportional to the thickness of the hydrated layer which may contain both H+ and OH- ions. The series combinations of the capacitances are illustrated in Figure 5-1 (a). Figure 5-1 (b) shows series combinations of capacitances after ISFETs immersing in the buffer solution. It is proposed that ions will penetrate into sensitive layer and form a hydrated layer which will distribute either positive or negative charges.
According to Bousse and Bergveld [6], there are two responses of inorganic-gate ISFETs. One is fast response which is called sensitivity and another is slow response which is called drift. The slow response of drift has several models to explain. One model based on the reactivity of the insulator in the electrical double layer in the electrolyte at the interface with the insulator [7]. Another model based on the presence of H+ ions in the insulating layer [8]. The last one model based on the modification of the interface of Si and SiO2 through a pH controlled charge in the surface state density via transport of a hydrogen-bearing species [9]. All of these models point out that the drift voltages are affected by the charges either in insulator or on interface of insulator.
Thus, this study considered to apply a gate-to-silicon substrate voltage to create an electrical field to erase the drift effect caused by trapping charges. Figure 5-1(c) shows the schematic diagram of the electrical force will enhance or reduce the penetration of ions into sensitive film forming a hydrated layer. Thus, if we properly control the electrical force by gate voltage, then the drift effect by charges migrating will be solved. In this study, we try to control the gate voltages during measuring drift voltage to achieve a zero drift voltage measurement.
5.2 Experimental Procedure
5.2.1 Device Fabrication
Figure 5-2 shows a fabricating process of ISFET which is a CMOS compatible technique. A 30-nm-thick sensitive layer of the SiO2 membrane was deposited onto the SiO2 gate ISFET by PECVD. And a 30-nm-thick sensitive layer of the ZrO2
membrane was also sputtering onto the SiO2 gate ISFET using DC sputtering system.
The sensing membrane is defined by photoresist and etched by buffer oxide etch (BOE). The ISFETs were fabricated on the p-type silicon wafer with (100) orientation and the manufacturing processes are listed as follows:
(1) RCA cleaning of 4-inch p-type silicon wafer
(2) Wet oxidation of silicon dioxide (600 nm, Figure 5-2(a))
(3) Defining of S/D area with mask Ⅰ and wet-etching of silicon dioxide by BOE
(4) Thermal growth of silicon dioxide as screen oxide (30 nm, Figure5-2 (b)) (5) Phosphorus ion implantation and post annealing at 950℃ (Figure 5-2 (c)) (6) PECVD of silicon dioxide for passivation layer (Figure 5-2 (d))
(7) Defining of contact hole and gate region with mask Ⅱ and wet-etching of silicon dioxide by BOE
(8) Dry oxidation of gate oxide (30 nm)
(9) PECVD silicon dioxide or DC sputtering ZrO2 film for sensitive layer (30 nm, Figure 5-2 (e))
(10) Defining of gate region with mask Ⅲ and wet-etching of oxide by BOE (11) Aluminum sputtering with hard contact mask Ⅳ (600 nm, Figure 5-2 (f)) 5.2.2 Packaging and Measurement
A container is bonded to the gate region of ISFET using epoxy resin. Figure 5-3 shows the measurement setup and a HP4156A semiconductor parameter analyzer is used for measuring characteristics of ISFETs in the buffer solutions. All the
measurements are carried out at a room temperature of 25℃ using a temperature control system, and what was placed in a dark box. An Ag/AgCl reference electrode is used as a constant DC voltage reference in the measurement system. After the buffer solution is injected into the container, measurements are not carried on until the ISFETs are immersed in the buffer solution for 60 s to make sure that the devices are under a steady state
5.3 Results and Discussion
5.3.1 ID-VDS Characteristics of the ISFET Devices
As in the discussion of the previous section, the operation of ISFET is very similar to that of conventional MOSFET, except that a reference electrode and an electrolyte are used instead of a metal gate. In the ID-VDS measurement, a reference electrode is placed in buffer solutions with pHs = 1, 3, 5, 7, 9, 11 and 13, and IDS-VDS
curves are obtained by applying a series of step voltages at the reference electrode.
Figure 5-4 (a) shows the ID-VDS curves of the SiO2 gate ISFETs. And figure 5-4 (b) shows the ID-VDS curves of the ZrO2 gate ISFETs. There exhibits the similar ID-VDS
curves as the conventional MOSFET, and the non-saturation region is about 0 to 3 V at pH = 7. The perfect characteristics of ISFETs indicate that even the metal gate has already been replaced using a buffer solution and a reference electrode, the device still owned pretty good characteristics of MOSFET. Thus, all measurements are following conventional methods of MOSFET.
5.3.2 pH Sensitivity of the ISFETs
The pH sensitivities of SiO2 and ZrO2 gate ISFET in buffer solutions with pHs = 1, 3, 5, 7, 9, 11 and 13 at room temperature are obtained using a HP4156A semiconductor parameter analyzer. Figure 5-5 (a) shows that the ID-VGS curves of SiO2 gate ISFET are shifted parallel with the pH concentration of the buffer solutions,
and in the nonsaturation region with VDS = 1 V. The ID-VGS curves represent the threshold voltage shift towards positive values with increasing pH values. Figure 5-5 (b) shows that the ID-VGS curves of ZrO2 gate ISFET are shifted parallel with the pH concentration of the buffer solutions, and in the nonsaturation region with VDS = 2 V.
The ID-VGS curves also represent the threshold voltage shift towards positive values with increasing pH values. After several times measurements, median linear pH responses of 31.15 and 57.5 mV/pH are obtained by calculating the shifts in the VGS
of SiO2 and ZrO2 gate ISFETs using constant drain current measurements for different pH values. Figure 5-6 (a) shows the VGS and pH values for obtaining pH responses of 31.15 of SiO2 gate ISFET with fixed ID = 508 μA from the slope of the linear-fitted line. And Figure 5-6 (b) shows the VGS and pH values for obtaining pH responses of 57.5 mV/pH of ZrO2 gate ISFET with fixed ID = 527 μA from the slope of the linear-fitted line.
5.3.3 Drift of ISFET
The drift of the ISFETs are also calculated using ID-VGS measurement for 10 hours.
During the 10 hours measuring time, various DC biased voltages are applying between the reference electrode and substrate. To make sure that the reference pH devices are in a stable state, the devices must be immersed in the buffer solution for one hour before drift measuring. There are two sensing films of SiO2 and ZrO2 to be consideration in the study. Figure 5-7 (a) shows drift of SiO2 gate ISFET during 10 hours measurement time that can be obtained a drift of 56.12 to -27.15 mV at the slow response region. Figure 5-7 (b) shows drift of ZrO2 gate ISFET during 10 hours
During the 10 hours measuring time, various DC biased voltages are applying between the reference electrode and substrate. To make sure that the reference pH devices are in a stable state, the devices must be immersed in the buffer solution for one hour before drift measuring. There are two sensing films of SiO2 and ZrO2 to be consideration in the study. Figure 5-7 (a) shows drift of SiO2 gate ISFET during 10 hours measurement time that can be obtained a drift of 56.12 to -27.15 mV at the slow response region. Figure 5-7 (b) shows drift of ZrO2 gate ISFET during 10 hours