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 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

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