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 measurement time that can be obtained a drift of 0.76 to -57.94 mV at the slow response region. It obviously showed a strong relation of drift voltages and gate stress voltages. The values of drift voltages always show from a positive number to a negative number. In other words, a zero value of drift can always be found using this

method. Thus, a great improvement of drift voltage in these two membranes as ISFETs can be achieved by using the gate voltage controlling method. When gate voltage is controlled as 0.5 V, drift voltage of SiO2 gate ISFET will decrease from 56.12 to 2.94 mV in ten hours measurement. The improvement of the drift voltage reaches 94.8%. Also, when gate voltage is controlled as -1 V, drift voltage of ZrO2

gate ISFET will decrease from -57.94 to 0.76 mV in ten hours measurement. The improvement of the drift voltage reaches 98.7%. This may result from the gate electric field affecting the ions to diffusive into the gate insulator, and the gate voltage shift will be blocked. When the properly electric field was applied between reference electrode and substrate, the drift effect would be erased. Figure 5-8 shows the relation of drift voltages and the gate stress voltages. With the different gate stress voltages, both of the two sensing films have various differential gate voltages shift named drift voltages. We can always find a gate stress voltage that makes the differential gate voltage shift close to zero deviation. In this study, the gate voltage should be controlled as 0.5 V in the SiO2 gate ISFET and -1 V in the ZrO2 gate ISFET.

5.4 Conclusion

An ultra-low drift voltage by using gate voltage controlling method in oxide-based gate ISFETs is realized. There are two sensitive oxide membranes of ZrO2 and SiO2

using in this research. We can always find a gate stress voltage that makes the differential gate voltage shift be close to zero deviation. The improvements of drift voltage reach 98.7% of ZrO2 and 94.8% of SiO2. It is a very great improvement of drift that will improve the ISFETs more applications. When the optimum gate voltage was applied to the ISFETs, the drift problem of ISFET would be solved in the future.

5.5 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] T. matsuo and M. Esashi, “Method of ISFET fabrication,” Sens. Actuators, vol. 1, pp. 77-96, 1981.

[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] M. Esashi and T. Matsuo, “Integrated Micro Multi. Ion Sensor Using Field Effect of Semiconductor,” IEEE Trans. Biomed. Eng., vol. 25, pp. 184-192, 1978.

[6] L. Bousse and P. Bergveld, “Role of buried OH sites in the response mechanism of inorganic-gate pH-sensitive ISFETs, “ Sens. Actuators, vol. 6, pp. 65-78, 1984.

[7] R. G. Kelly,” Microelectronic Approaches to Solid State Ion Selective Electrodes,”

Electrochimica Acta, vol. 22, pp.1-8, 1977.

[8] P. Bergveld, N. F. de Rooij, and J. N. Zemel,” Physical mechanisms for chemically sensitive semiconductor devices,” Nature, vol. 273, pp. 438-443, 1978.

[9] A. G. Revesz,” The mechanism of the ion-sensitive. field effect transistor,” Thin Solid Films, vol. 41, pp. L43-L47, 1977.

Figure 5-1. Series combinations of (a) initial, (b) hydrated without gate voltage control, and (c) hydrated with gate voltage control

Figure 5-2. Fabricated processes of ISFET which is a CMOS compatible technique.

Figure 5-3. Setup of measurement Using HP4156A semiconductor parameter analyzer and temperature controller.

Figure 5-4 (a). ID-VDS curves of SiO2 gate ISFETs.

Figure 5-4 (b). ID-VDS curves of ZrO2 gate ISFETs.

Figure 5-5 (a). ID-VGS curves of SiO2 gate ISFETs.

Figure 5-5 (b). ID-VGS curves of ZrO2 gate ISFETs.

Figure 5-6 (a). Sensitivity of SiO2 gate ISFETs.

Figure 5-6 (b). Sensitivity of ZrO2 gate ISFETs.

Figure 5-7 (a). Drift of SiO2 gate ISFET with time.

Figure 5-7 (b). Drift of ZrO2 gate ISFET with time.

Figure 5-8 Relation of drift voltages and gate stress voltages.

Chapter 6

A Simple CMOS Compatible REFET for pH Detection by Post NH

Plasma Surface Treatment of ISFET

6.1 Backgrounds and Motivation

Ion-sensitive field effect transistor (ISFET) has been developed over 35 years, and the first sensitive membrane is silicon dioxide (SiO2) that has first demonstrated by Bergveld in 1970 [1]. Since the SiO2 gate ISFET appears instable sensitivity and large drift, a lot of sensitive layers, such as Al2O3, Ta2O5, SnO2, TiN, a-WO3, ITO and ZrO2, are used as pH-sensitive membranes for the higher pH response and much stable drift voltage [2-9]. A conventional reference electrode (e. g. Ag/AgCl electrode) is always used in the measurement system. If we want to integrate the ISFET devices into a chemical micro system for in vivo analysis or become a part of lab-on-a-chip, the huge conventional reference electrode will be the biggest challenge. For this reason, there are several approaches that have been investigated to solve this problem.

One method to solve this problem is co-fabricated an Ag/AgCl electrode with ISFET, including a gel filled cavity and a porous silicon plug [10]. But this solution has a leaking path from reference electrode to solution that will reduce the device lifetime.

Another method is applying a differential measurement between an ISFET and an identical FET, which does not react on the ion concentration to be measured, called REFET. This method is deposition on top of the ISFET’s surface one layer, which is an ion-unblocking membrane that is an insulating polymeric layer exhibiting independence on ionic strength. A commonly used material for ion-unblocking layer in REFET is polyvinylchloride (PVC) [11-12]. The REFET with PVC sensitive membrane has a smaller sensitivity of about 20mV/pH, but it will add some processes

to fabricate a REFET and is not compatible with integrated circuit (IC) technology.

The ZrO2 prepared by dc sputtering process as a pH-sensitive membrane for ISFET is first developed in our laboratory [9]. In this work, we report a simple CMOS compatible REFET by post NH3 plasma surface treatment on ZrO2 gate ISFET. The electrical characteristics and pH response of the ZrO2 gate ISFET is studied by the standard MOSFET measurement with HP 4156A. Without any unblocking layer to be deposited, the REFET also shows a very low sensitivity of about 28.3 mV/pH. With such ISFET/REFET differential pair, the conventional reference electrode can be replaced by a solid platinum electrode, which can fabricate in the same chip. By this way, a high integration of ISFET with IC fabricating can be realized in the future.

6.2 Experimental

6.2.1 Device Fabrication

Figure 6-1 shows the schematic diagram of the ZrO2 gate ISFET, which is fabricated by the MOSFET technique. DC sputtering from a 4-inch diameter, and 99.99% purity of Zr in oxygen atmosphere deposited a 30nm thickness sensitive layer of ZrO2 membrane onto the SiO2 gate ISFET. The sputtering total pressure was 20 mTorr in the mixed gases Ar and O2 for 200 minutes while the base pressure was 3×

10^-6 Torr, and the rf power was 200W which the operating frequency was 13.56MHz.

After a post NH3 plasma treatment of ZrO2 film, a REFET was completed with ISFET in a single chip. The detailed manufacturing processes were listed as follows:

(1) RCA cleaning of 4-inch, p-type silicon wafer

(2) Wet oxidation of silicon dioxide (600 nm, Figure 6-1(a))

(3) Defining of S/D area with mask Ⅰ and wet-etching of silicon dioxide by buffer oxide etcher (BOE)

(4) Thermal growth of silicon dioxide as screen oxide (30 nm, Figure6-1 (b))

(5) Phosphorus ion implantation and post annealing at 950℃ (Figure 6-1 (c)) (6) PECVD silicon dioxide for passivation layer (Figure 6-1 (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 (10 nm)

(9) DC sputtering 30nm thickness of ZrO2 film and post annealing at 600℃

(Figure 1 (e))

(10) Defining of gate region with mask Ⅲ and wet-etching of oxide by BOE (11) A post NH3 plasma treatment by high-density plasma reactive ion etching

(HDP-RIE) system

(12) DC sputtering a 30nm thickness of ZrO2 film with hard contact mask Ⅳ and post annealing at 600℃

(13) Aluminum sputtering with hard contact mask Ⅴ (600 nm, Figure 6-1 (f) 6.2.2 Packaging and Measurement

A container is bonded on the gate region of the ISFET/REFET by epoxy resin.

Figure 6-2 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. Originally, a platinum film is prepared for the reference electrode, but in order to estimate the effect of reference electrode. A conventional Ag/AgCl reference electrode is used as a DC reference voltage to measure the ISFET and REFET systems. 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.

6.3. Results and Discussion

6.3.1 The pH Sensitivity of the ZrO2 Gate ISFET

The pH sensitivity of the ZrO2 gate ISFET in pH = 1, 3, 5, 7, 9, 11 and 13 buffer solutions at room temperature is obtained by a HP4156A Semiconductor Parameter Analyzer. Figure 6-3 (a) shows that the IDS-VGS curves of ISFET 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 linear pH response of 56.7~58.3 mV/pH with the deviation of 3 % is obtained by calculating the shifts in the VGS of the ISFET by a constant drain current at 527 μA for different pH values. Figure 6-3 (b) shows the VGS to pH values that can obtain a median pH response of 57.5 mV/pH by the slope of the linear fitted line. A 99.74 % of the root mean square can be obtained that represents the perfect response of the ZrO2

gate ISFET.

6.3.2 The pH sensitivity of the ZrO2 gate REFET

Figure 6-4 shows that the sensitivities of REFET are decreasing with increasing of NH3 plasma treated times in 60 minutes. When the NH3 plasma treated times reach 90 minutes, the sensitivity of REFET will increase slightly for the plasma bombardment. The individual sensitivities errors of ISFET and REFET are both about 1.5 mV in several times of measurements, and the tendency of the errors is the same in ISFET and REFET. Thus, the differential sensitivities of ISFET/REFET pairs are very stable with an error bar of about 0.2 mV. The differential sensitivities accuracy of the ISFET/REFET is about 0.7% (0.2/28). With the low sensitivities of REFET, we suggest the reason is that the surface sites [13] are passivated by H⁺ of plasma, resulting in the lower sensitivity of REFET. As a result, after the surface sites are decreasing with increasing plasma times, the sensitivities will be decreased.

Unfortunately if the times reach 90 minutes, the sensitivity will increase for the damage of surface by the plasma bombardment. Thus, the optimum time of NH3

plasma process reaches 60 minutes in the experiment, the sensitivity of REFET will decrease to 27.6~29 mV/pH. And the differential sensitivity of the ISFET/REFET pair devices is 29.1~29.3 mV/pH.

6.4 Conclusion

A study of the ZrO2 gate ISFET and REFET are first proposed as a pH-sensitive membrane pair. The sensing property of sensitivity is obtained by the IDS-VGS

measurement in a series of buffer solutions. The optimum time of NH3 plasma process is 60 minutes for the REFET. The pH response is 56.7~58.3 mV/pH for ISFET and 27.6~29 mV/pH for REFET. The differential sensitivity about 29.1~29.3 mV/pH with an accuracy of 0.7% can be obtained for both the ISFET and REFET in the same drifting tendency. The ZrO2 gate ISFET and REFET can be used in the pH range of 1 to 13 with perfect linear fitted line that is able to enable the ISFET devices more applications in many fields.

6.5 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 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” Jpn. J. Appl. Phys.

Vol. 46 No. 7A pp. 4334-4338 2007.

[10] R. L. Smith and D. C. Scott, “An integrated sensor for electrochemical measurements,” IEEE Trans. Biomed. Eng. BME vol. 33 pp. 83-90, 1986.

[11] M. Chudy, W. Wroblewski, and Z. Brzozka, “Towards REFET,” Sensors and Actuators B 57 pp. 47-50, 1999.

[12] P. A. Hammond, D.R.S. Cumming, and D. Ali, “A single-Chip pH Sensor Fabricated by a Conventional CMOS Process,” Proceeding of IEEE Sensors vol. 1 pp. 350-355, 2002.

[13] D. E. Yates, S. Levine, and T. W. Healy, “Site-binding model of the electrical double layer at the oxide/water interface,” J. Chem. Soc. Faraday Trans. I vol. 70 pp.1807-1844, 1974.

(a)

30nm SiO₂

Silicon Thermal SiO₂ Screen SiO₂

PECVD SiO₂ ZrO₂ Aluminum

600nm SiO₂

Figure 6-1 The schematic diagram of the ZrO2 gate ISFET that is fabricated by the MOSFET technique.

Figure 6-2 The set up of measurement with the HP 4156A Semiconductor Parameter Analyzer and Temperature controller.

Figure 6-3 (a) The IDS-VGS curves that are shifted in parallel with the pH concentration of the buffer solutions, and in the non-saturation region with VDS = 2V.

Figure 6-3 (b) The VGS to pH values that can obtain a median pH response of 57.5 mV/pH by the slope of the linear fitted line.

0 10 20 30 40 50 60 70 80 90 0

10 20 30 40 50 60

Sensitivities (mV/pH)

NH

Plasma Time (min)

W/ PLASMA W/O PLASMA

Differential Sensitivities V_DS=2V, I_DS=527μA

Figure 6-4 The sensitivities to post NH3 plasma treatment time output characteristics of the ISFET(W/O Plasma), REFET(W/ Plasma) and ISFET/REFET pair (Differential Sensitivities) devices with VDS= 2V and drain current at 527 μA.

Chapter 7

Conclusion and Future work

7.1 Conclusion

This study has investigated the ZrO2 to be a membrane of ISFETs. And a REFET is used to control the effects of temperature and process deviation and proposed two methods to solve the unwanted effects.

The ZrO2 membrane has been successfully applied as a pH-sensitive layer for ISFETs in Chapter 3. It exhibited an excellent response range of 56.7 mV/pH to 58.3 mV/pH. The ZrO2 membrane prepared by DC sputteringwas used as a pH-sensitive film that showed good surface adsorption with oxide and silicon. The pH sensitivities slightly decreased in 1 M NaCl solution; however, the device showed a perfect linear response of 52.5 mV/pH.

Furthermore, a REFET is used to control the effects of temperature and process deviation. After the calibration of REFET, a very stable sensitivity and intrinsic drift of SiO2 gate ISFET can be obtained. It can be used to define the thickness of hydration layer that is introduced by the drift effect. Results of this study will show that the thickness of hydration is about 50 nm in SiO2 membrane ISFET. It exhibits a stable response of 28~32 mV/pH. This method is a really simple way to find the thickness of hydration layer, and it will be useful in the study of the real mechanism in drift effect.

Finally, when the phenomenon of drift is understood, two methods are designed to reduce this effect. One is using the gate voltage to control drift voltage. It is a simple and cheap way to solve the drift problem is presented which describes the relation of drift and gate voltage. When various constant gate voltages are biased in

sensing layers with reference electrode. It obviously shows a strong relation of gate drifts and gate stress voltages. When the gate voltage is controlled as 0.5 V, the drift voltage of SiO2 gate ISFET will decrease from 56.12 to 2.94 mV in ten hours measurement. The improvement of drift voltage reaches 94.8%. When the gate voltage is controlled as -1 V, the drift voltage of ZrO2 gate ISFET will also decrease from -57.94 to 0.76 mV. The improvement of drift voltage reaches 98.7%.

Another method is using a REFET to reduce the drift effect. A simple CMOS compatible REFET for pH detection by post NH3 plasma surface treatment of a ZrO2

membrane ISFET has been developed. It is a novel study that has latent capacity to integrate the ISFET devices into a chemical micro system for in vivo analysis or become a part of lab-on-a-chip. With the fixed current measurement by HP4156, we can get not only the individual sensitivities of ISFET and REFET, but also the differential sensitivities of ISFET/REFET pair. The ZrO2 membrane ISFET exhibits an excellent response of 56.7~58.3mV/pH with deviation of 3% and the REFET shows a small response of 27.6~29 mV/pH with a deviation of 5%. Using this ISFET/REFET differential pair, we can get a very stable differential sensitivities of 29.1~29.3 mV/pH with a small deviation of 0.7%. This result indicates that the research not only makes the ISFET integrate into a micro system in a simple way possible, but also increases the stability of sensitivity.

7.2 Future Work

Even though the drift voltage could be measured easily here, we did not collect enough data to develop a new model to predict how deep of the hydration layer would be. The mechanism of drift should be modeled and understood clearly. And we

proposed two methods to solve the drift effect to make the system become more stable and small. But we did not realize the system on one chip. We need to measure the out

put voltage by a HP4156A. It is not a good system to be used for everyone. Thus, the next step of our lab is going to develop a smart chip that integrates all the components in only one chip. That is the finally target of this study. When the single chip is

realized, we will change the sensing membranes to be a ChemFET that will have more applications in biology. Hence, the device will become a useful one in the future.