Sensing layer deposition and annealing

This step is the core of the pH-ISFET in our experiment. It has been proved the ZrO₂ sensing film deposited by sputtering has good characteristics as a pH-ISFET sensing layer in our laboratory [4]. The detailed parameters of sputter are listed in Table 3-1. Therefore, in this study we use ZrO₂ sensing film to investigate the effect of different annealing conditions. The ZrO2 sensing film will anneal in N2 gas for 30 min at temperature 600, 700, 800, and 900°C, respectively. Physical and electrical properties of the not-annealed and annealed ZrO2 sensing layers were investigated.

The annealing conditions are listed in Table 3-2.

3.3 Measurement system

3.3.1 Preparations before measuring

We utilize HP4156 to measure the I_DS-V_GS characteristic curves of the ZrO₂ gate pH-ISFET. The measurement system of our experiment was showed in Fig. 3-2. Since the pH-ISFET is easily influenced by light illumination [5], the pH-ISFET was kept in the dark box during the measurement.

In order to make sensing layer immersed in the buffer solution, we glue a

container on the wafer. To allow the opening window of entire sensing layer region under the container. The material of container is silica gel and the bottom is small enough to avoid touching the other devices. However, the opening on the top has to be big enough for inserting reference electrode.

The pH-solution that we use is purchased from Riedel-deHaen and the pH-values are 1, 3, 5, 7, 9, 11 and 13. The electric potential of the pH-solution is always floating [6]. The disturbance from the environment would induce the electric potential variance of the solution. By eliminating this variance, a reference electrode is needed to put into the pH-solution.

3.3.2 Setup of the I-V measuring system

To investigate characterizes of different annealing conditions for ZrO₂ sensing film. A HP4156 semiconductor parameter analyzer system was setup to measure the I-V curves, in which included I_DS-V_GS and I_DS-V_DS curves at controlled temperature. It needed to pay attention to drop pH-solution at the sensing region. Because the small sensing region. To avoid generating air bubbles at the sensing layers and electrolyte interface is important. In order to extract accurate values for different pH-solution, every pH value is immersed for 30 sec before measurement.

In the setup of HP4156, substrate is grounded and the reference electrode is sweeping to different voltage. In the measurement of sensitivity, the response of the pH-ISFET is the function of time. According to [6], the first equilibrium achieve in a minute.

In the beginning we measure IDS-VDS to find out the linear operating region. To define I_DS as constant, we extract the point of maximum transconductance from

I_DS-V_GS curves. We can observe that different pH-solution will cause the reference electrode voltage shifted and the shifted voltage per pH value is sensitivity. The detection principle of pH sensitivity was shown in Fig. 3-3.

3.3.3 Setup of hysteresis measuring system

In this study, we immerse the pH-ISFET in the pH7 buffer solution for 2 hr to keep the device in stable state. Then we measure the hysteresis curve in loop time 15 minutes in the pH = 737117 measuring loop, and another loop for pH = 711737. For each pH value we obtained 3 measure points with duration of 1 minute. The measuring step of the hysteresis curve was shown in Fig. 3-4.

3.3.4 Setup of drift measuring system

Before measurement, the ISFET’s sensing layer was contacted with the buffer solution about 13 hr in order to generate an equilibrium layer at interface between the electrolyte and the ZrO₂ sensing film. We measure 36 points with duration of 10 minutes in the same pH value of aqueous solution. The detection principle was shown in Fig 3-5.

3.4 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] U. Guth, “Investigation of corrosion phenomena on chemical microsensors”,

Electrochimica Acta 47 pp. 201–210 , 2001.

[3] George T. Yu, “Hydrogen ion diffusion coefficient of silicon nitride thin films”, Applied Surface Science 202 pp.68–72, 2002.

[4] 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.

[5] Paik-Kyun Shin, “The pH-sensing and light-induced drift properties of titanium dioxide thin films deposited by MOCVD”, Applied Surface Science, vol. 214, pp.214-221, 2000.

[6] P. Woias, “Slow pH response effects of silicon nitride ISFET sensors”, Sensors and Actuators B 48, pp.501-504, 1998.

Chapter 4

Results and Discussions

4.1 Introduction

In order to obtain high sensitivity, low drift and hysteresis, many pH-sensitive materials, such as, SiO2, Al2O3, Si3N4 and Ta2O5 have been investigated. In this work, we choose ZrO₂ as the sensing film and study the influences of different annealing temperatures on the ZrO2 sensing film for the pH-ISFET.

In addition, we utilize some theoretical analysis which based on site-binding model and MOSFET theory to describe the ZrO2 as the gate pH-ISFET characteristics.

From the experiment results, we can acquire the optimum annealing condition for ZrO2 gate pH-ISFET.

4.2 pH sensitivity

The pH sensitivity is one of the important factors of pH-ISFET devices, and it is influenced easily by sensing materials. In our experiment, we select ZrO₂ as a sensing layer to investigate a series of annealing conditions for the pH sensitivity properties.

The current-voltage (I_DS-V_GS) characteristics for the ZrO₂/SiO₂ gate pH-ISFET were shown in Fig. 4-1, and the VDS were set to be 1.5 V. We select the point of maximum g_m for extracting pH-sensitivity; it is because at this point the efficiency of amplifying signal is best. In the Fig. 4-1 it was found that the I-V curves can be shifted from left to right in the buffer solutions from pH = 1 to pH = 13, that is to say, the channel conductance can be modified by the concentration of hydrogen ion.

From the results, we can observe that the 600℃ sample presents the best sensitivity, 54.5mV/pH, and linear degree, 0.9996, as can be seen in Fig. 4-1, 4-2. The 700℃, 800℃, and 900℃ sample also has good linear degree, as shown in Fig.

4-3~4-5. In addition, the 700℃, 800℃, and 900℃ sample has a lower sensitivity, 42mV/pH, 45.65mV/pH, 48.17mV/pH , respectively, as shown in Fig. 4-6~4-8. Since the not annealed sample shows the worse sensitivity 39.54 mV/pH, and linearity, 0.9583, as shown in Fig 4-9, 4-10. We deduce that the surface of not annealing sample has many defects, as shown in Fig. 4-11. From Fig 4-12, 4-13, we can see the sensing layer surface of 600℃ sample seems to be more denser than 700℃ sample, and the sensitivity of 700℃ sample is reduced probably due to a smaller number of surface sites [1]. Furthermore, the pH sensitivity of the 800℃ and 900℃ may affect by the inhomogeneous state of the sensing films were shown in Fig. 4-14, 4-15. Fig. 16 shows the pH sensitivity of ZrO₂ gate pH-ISFET annealed at different temperature.

4.3 Hysteresis effect to pH-ISFET

The hysteresis effect may induce the inaccurate measurement of pH-ISFET devices. In this study, the hysteresis of ZrO2 gate ISFETs were first determined in pH cycles of pH = 7-3-7-11-7 and pH = 7-11-7-3-7 with the loop time of 15 min. Fig.

4-17~4-21 show hysteresis curve in pH loop 7-3-7-11-7 and Fig. 4-22~4-26 show hysteresis curve in another pH loop 7-11-7-3-7 for 600℃, 700℃, 800℃, 900℃, and not annealing sample, respectively.

It is found that the hysteresis width of 600℃ sample in pH loop 7-3-7-11-7 is smaller than that in pH loop 7-11-7-3-7, with the magnitude of 1.43 and 5.45mV, respectively. We also observe that the hysteresis of the acid side is smaller than basic

side. Since the hysteresis is caused by the slow response of the pH-ISFET [2], the ions diffuse from the surface of the sensing film into the buried site are very slow, and results in slow response. In addition, due to the different sizes of H⁺ and OH^- ions, the diffusion speed of H⁺ ions into the buried site are faster than that of OH^- ions, as described by Bousse et al. [3]. This causes the asymmetric hysteresis behavior of the pH-ISFET devices. And the asymmetric hysteresis behavior can be seen in the all samples.

It is presumed that the zirconium dioxide film annealed at 900℃ have more buried site. The large hysteresis of the 900℃ sample seems to be caused by the reactions of the diffused H⁺ and OH^- ions with buried site. Nevertheless, the higher pH-sensitivity of the 900℃ sample also affected by buried site. Table 4-1 shows the hysteresis at pH loop 7-3-7-11-7 and 7-11-7-3-7 for different annealing temperature.

From Table 4-1 we can observe that 600℃ sample shows the smallest hysteresis.

Hence, it has good hysteresis property that annealed at 600 ℃ for ZrO2 gate pH-ISFET.

4.4 Drift phenomenon to pH-ISFET

In the beginning, the pH-ISFET was immersed in the buffer solution pH = 7 for 13h to keep the device in a stable state. The surface response changes into stable after 5h with no pH variation, as described by Zhong et al. [4]. Hence, we select the data from 13 to 19h as the long-term drift.

Fig. 4-27~4-31 shows the drift of different annealing temperature condition in pH 7. And Table 4-2 shows the drift rate of the different sample in pH 7. From Table 4-2 we can obtain that the drift rate of the 600℃ sample is smaller than others. According

to Yule et al. [5] and Jamasb et al. [6], the drift is caused by the hydrated layer. The thickness of the hydrated layer is increased with time. Thus, the overall insulator capacitance would be decreased, results in the threshold voltage increases with time.

Since the sensing layer surface of 600℃ sample has a denser site, it is relatively difficult to generate the hydrated layer. Hence, the drift rate of the 600℃ sample is smaller.

4.5 Conclusion

The pH sensing characteristics of zirconium dioxide gate pH-ISFET were investigated for various annealing temperature of 30 min duration in nitrogen. It is found that annealing temperature of 600℃ has a maximal sensitivity of 54.5 mV/pH.

Because of the sensing layer surface of 600℃ sample has a denser site.

We can find that the hysteresis width of 600℃ sample in pH loop 7-3-7-11-7 is 1.43 mV, and the hysteresis in pH loop 7-11-7-3-7 is 5.45 mV. It can be observed that hysteresis in the acid side is smaller than basic side, results in asymmetric hysteresis.

For the application of the pH measurement, the maximum hysteresis of 600℃ sample is 5.45mV. It occupies 10％ of pH-sensitivity. The error causes by hysteresis can be accepted in pH measurement.

The drift rate of the ZrO2 gate pH-ISFET for not annealing and annealing temperature at 600, 700, 800, 900 ℃ are 2.4, 0.54, 1.9, 1.76 and 1.0 mV/h, respectively. Since the sensing layer surface of 600℃ sample has a denser site, it is relatively difficult to generate the hydrated layer. Therefore, annealing temperature at 600℃ shows the smallest drift rate.

In order to achieve the purposes for high pH-sensitivity, small hysteresis and low

drift. We can conclude the optimal annealing temperature is around 600℃. It reveals that ZrO2 gate pH-ISFET annealed at 600℃ is suitable for pH measurement.

4.6 References

[1] T. Mikolajick, Feldeffekttransistoren zur pH-Wert-Messung undals transducer fur Biosensoren, Thesis, University of Erlangen Nuremberg, 1996.

[2] L. Bousse, S. Mostarshed, B.van der schoot, N.F. de Rooij, Comparison of the hysteresis of Ta2O5 and Si3N4 pH-sensing insulators, Sens. Actuat. B 17 (1994) 157-164.

[3] L. Bousse, P. Bergveld, The role of buried OH^－ sites in the response mechanism of inorganic-gate pH-sensitive ISFETs, Sens. Actuat. 6 (1984) 65–78.

[4] Y. Zhong, S. Oho and T. Lin: Chinese J. Semicond. 12 (1994) 838.

[5] Z. Yule, Z. Shouan, L. Tao, Drift characteristic of pH-ISFET output, Chin. J.

Semicond. 12 (15) (1994) 838-843.

[6] S. Jamasb, S. Collins, R. L. Smith, A physical model for drift in pH ISFETs, Sens.

Actuat. B 49 (1998) 146-155.

Chapter 5 Future work

In this study, an optimum annealing condition for ZrO₂ gate pH-ISFET was investigated. The annealing temperature at 600 ℃ shows the higher sensitivity, smaller hysteresis and lower drift than others. Further investigation is attained to improve the pH-sensitivity properties. It can be achieved by optimizing the deposition condition, and regard to crystallographic properties of the sensing films and oxygen migration in the sensing film. In addition, we can anneal at different times to find out the properties of sensing film.

MOSFET ISFET

Fig. 1-1 Structure of MOSFET and ISFET

Fig. 1-2 Conventional glass electrode Gate

Source Drain

Source Drain

Reference electrode

Glass Membrane Internal Reference Electrode Internal

Buffer Solution

Internal Conducting Line

Fig. 2-1 Site-binding model

Fig. 2-2 Potential profile and charge distribution at an oxide electrolyte solution interface

(a) (b)

Figure 2-3 Series combination of the (a) initial (b) hydrated insulator capacitance

(a)

Silicon

Thermal Oxide Sensing Layer Solution

Hydration

(b)

(c)

(d)

(e)

(f)

(g)

Fig. 3-1 Fabrication process flow

Fig. 3-2 Measurement setup

Fig. 3-3 Extraction method of sensitivity

0 2 4 6 8 10 12 14 16 2

4 6 8 10 12

pH

Time (min)

Fig. 3-4 Measuring step of the hysteresis curve

Fig. 3-5 Detection principle of drift

Fig. 4-1 Sensitivity characteristic of ZrO2 gate ISFET at 600℃ sample

Fig. 4-2 Linearity characteristic of ZrO2 gate ISFET at 600℃ sample

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Fig. 4-3 Linearity characteristic of ZrO2 gate ISFET at 700℃ sample

Fig. 4-4 Linearity characteristic of ZrO2 gate ISFET at 800℃ sample

0 2 4 6 8 10 12 14

Fig. 4-5 Linearity characteristic of ZrO2 gate ISFET at 900℃ sample

Fig. 4-6 Sensitivity characteristic of ZrO2 gate ISFET at 700℃ sample

0 2 4 6 8 10 12 14

Fig. 4-7 Sensitivity characteristic of ZrO2 gate ISFET at 800℃ sample

Fig. 4-8 Sensitivity characteristic of ZrO2 gate ISFET at 900℃ sample

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Fig. 4-9 Sensitivity characteristic of ZrO2 gate ISFET at not annealed sample

Fig. 4-10 Linearity characteristic of ZrO2 gate ISFET at not annealed sample

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Fig. 4-11 SEM image of ZrO2 gate ISFET at not annealed sample

Fig. 4-12 SEM image of ZrO2 gate ISFET at 600℃ sample

Fig. 4-13 SEM image of ZrO2 gate ISFET at 700℃ sample

Fig. 4-14 SEM image of ZrO2 gate ISFET at 800℃ sample

Fig. 4-15 SEM image of ZrO2 gate ISFET at 900℃ sample

Fig. 4-16 The pH sensitivity of ZrO2 gate pH-ISFET annealed at different temperature

30 35 40 45 50 55 60

not annealed

600℃ 700℃ 800℃ 900℃

Sen sitiv ity (m V/p H)

Fig. 4-17 Hysteresis curves of ZrO2 gate ISFET at pH loop 7-3-7-11-7 to 600℃ sample

Fig. 4-18 Hysteresis curves of ZrO2 gate ISFET at pH loop 7-3-7-11-7 to 700℃ sample

1 2 3 4 5 6 7 8 9 10 11 12 13

1.6 1.7 1.8 1.9 2.0 2.1

V^G = 1.43 mV

VG(V)

pH Value

1 2 3 4 5 6 7 8 9 10 11 12 13

1.25 1.30 1.35 1.40 1.45 1.50 1.55 1.60

VG = -12.52 mV

VG(V)

pH Value

Fig. 4-19 Hysteresis curves of ZrO2 gate ISFET at pH loop 7-3-7-11-7 to 800℃ sample

Fig. 4-20 Hysteresis curves of ZrO2 gate ISFET at pH loop 7-3-7-11-7 to 900℃ sample

1 2 3 4 5 6 7 8 9 10 11 12 13

1.7 1.8 1.9 2.0 2.1

VG = 4.46 mV

VG(V)

pH Value

1 2 3 4 5 6 7 8 9 10 11 12 13

1.3 1.4 1.5 1.6 1.7

VG = 53 mV

VG(V)

pH Value

Fig. 4-21 Hysteresis curves of ZrO2 gate ISFET at pH loop 7-3-7-11-7 to not annealed sample

Fig. 4-22 Hysteresis curves of ZrO2 gate ISFET at pH loop 7-11-7-3-7 to 600℃ sample

1 2 3 4 5 6 7 8 9 10 11 12 13

1.6 1.7 1.8 1.9 2.0

V^G = -6.36 mV

VG(V)

pH Value

1 2 3 4 5 6 7 8 9 10 11 12 13

1.6 1.7 1.8 1.9 2.0 2.1

V^G = 5.45 mV

VG(V)

pH Value

Fig. 4-23 Hysteresis curves of ZrO2 gate ISFET at pH loop 7-11-7-3-7 to 700℃ sample

Fig. 4-24 Hysteresis curves of ZrO2 gate ISFET at pH loop 7-11-7-3-7 to 800℃ sample

1 2 3 4 5 6 7 8 9 10 11 12 13

1.7 1.8 1.9 2.0 2.1

VG = 8.23 mV

VG(V)

pH Value

1 2 3 4 5 6 7 8 9 10 11 12 13

1.30 1.35 1.40 1.45 1.50 1.55 1.60

V^G = 22.27 mV

VG(V)

pH Value

Fig. 4-25 Hysteresis curves of ZrO2 gate ISFET at pH loop 7-11-7-3-7 to 900℃ sample

Fig. 4-26 Hysteresis curves of ZrO2 gate ISFET at pH loop 7-11-7-3-7 to not annealed sample

1 2 3 4 5 6 7 8 9 10 11 12 13

1.3 1.4 1.5 1.6 1.7

V^G = 11.67 mV

VG(V)

pH Value

1 2 3 4 5 6 7 8 9 10 11 12 13

1.6 1.7 1.8 1.9 2.0

VG = 5.69 mV

VG(V)

pH Value

Fig. 4-27 Drift in pH 7 buffer solution of ZrO2 gate ISFET for 6 hours at 600℃ sample

Fig. 4-28 Drift in pH 7 buffer solution of ZrO2 gate ISFET for 6 hours at 700℃ sample

0 50 100 150 200 250 300 350 400 1.796

1.797 1.798 1.799 1.800 1.801

VG(V)

Time(mins)

Drift = 3.23mV

0 50 100 150 200 250 300 350 400 1.65

1.66 1.67 1.68 1.69 1.70 1.71

VG(V)

Time(mins)

Drift = 11.38mV

Fig. 4-29 Drift in pH 7 buffer solution of ZrO2 gate ISFET for 6 hours at 800℃ sample

Fig. 4-30 Drift in pH 7 buffer solution of ZrO2 gate ISFET for 6 hours at 900℃ sample

0 50 100 150 200 250 300 350 400 1.868

1.870 1.872 1.874 1.876 1.878 1.880 1.882 1.884

VG(V)

Time(mins)

Drift = 10.55mV

0 50 100 150 200 250 300 350 400 1.505

1.510 1.515 1.520 1.525

VG(V)

Time(mins)

Drift = 5.99mV

Fig. 4-31 Drift in pH 7 buffer solution of ZrO2 gate ISFET for 6 hours at not annealed sample

Table 3-1 Parameters of sensing layers deposition with Sputter

parameters of ZrO2 sputter power : 110 W

Ar / O₂ : 24 / 8 ( sccm ) Density : 6.51

Acoustic impendance : 14.72 Tooling factor : 0.533

Rate : 0.02 Å / s pre sputter 60W for 10 min

Pressure : 7.6×10^-3

0 50 100 150 200 250 300 350 400 1.86

1.87 1.88 1.89 1.90

VG(V)

Time(mins)

Drift = 14.4mV

Table 3-2 The different annealing condition of ZrO2 gate ISFET

Temperature (℃) Gas Time (min)

Not annealed N2 30

600 N₂ 30

700 N2 30

800 N2 30

900 N₂ 30

Table 4-1 The comparison of different test loop in hysteresis

Test loop Temperature

7 – 3 – 7 – 11 – 7 (mV)

7 – 11 – 7 – 3 – 7 (mV)

Not annealed -6.36 5.7

600℃ 1.43 5.45

700℃ -12.52 22.27

800℃ 4.46 8.23

900℃ 53 11.67

Table 4-2 Drift rate of ZrO2 gate ISFET at different annealing temperature

Temperature Drift rate (mV/h)

Not annealed 2.4

600℃ 0.54

700℃ 1.9

800℃ 1.76

900℃ 1.0

簡 歷

姓 名：詹秉燏 性 別：男

出生日期：民國 73 年 09 月 10 日 籍 貫：台灣省台中市

學 歷：國立彰化師範大學電機工程學系 國立交通大學電子工程研究所

碩士論文：二氧化鋯作為閘極之離子感測場效電晶體應用在 pH 量測之最佳化退火製程研究

The study of optimal annealing process for ZrO2 gate ISFETs in

pH measurement applications

在文檔中 二氧化鋯作為閘極之離子感測場效電晶體應用在pH量測之最佳化退火製程研究 (頁 35-0)