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Separate structure extended gate H+-ion sensitive field effect transistor on a glass substrate
Sensors and Actuators B 71 (2000) 106±111
Separate structure extended gate H-ion sensitive ®eld effect transistor on a glass substrate Li-Te Yina, Jung-Chuan Choub,*, Wen-Yaw Chungc, Tai-Ping Sund, Shen-Kan Hsiungc a
b
Department of Bio-Medical Engineering, Chung Yuan Christian University, 320 Chung-Li, Taiwan, ROC Institute of Electronic and Information Engineering, National Yunlin University of Science and Technology, 640 Toulin, Taiwan, ROC c Department of Electronic Engineering, Chung Yuan Christian University, 320 Chung-Li, Taiwan, ROC d Department of Electrical Engineering, National Chi Nan University, 545 Nantou, Taiwan, ROC Received 7 February 2000; received in revised form 16 June 2000; accepted 30 June 2000
Abstract In our research, glass was used as a substrate for an H ion sensitive ®eld effect transistor (ISFET). The sensitive characteristics of ®ve structures for separate extended gate ion sensitive ®eld effect transistors (EGFET) were studied. The components included tin oxide (SnO2)/ aluminum/micro slide glass, tin oxide/aluminum/corning glass, indium tin oxide (ITO) glass, tin oxide/indium tin oxide glass and tin oxide/ micro slide glass. Indium tin oxide (ITO) thin ®lm was ®rst time used as an H ion sensitive ®lm, which has a linear pH Nerstern response sensitivity, about 58 mV/pH, between pH 2 and 12. In addition, the sensing area effect of the tin oxide/glass, tin oxide/ITO glass and ITO glass structure is discussed. The results show that the tin oxide/ITO glass structure EGFET has the best drift, hysteresis and sensing area characteristics. # 2000 Elsevier Science B.V. All rights reserved. Keywords: pH-ISFET; EGFET; Indium tin oxide (ITO) glass; Tin oxide (SnO2); Contact area effect
1. Introduction Ion sensitive ®eld-effect transistors (ISFETs) have been developed on the basis of the MOSFET (metal oxide ®eld effect transistor). Since Bergveld [1] ®rst employed the ®eld-effect transistor in neurophysiological measurements in 1970, ISFETs have been developed into a new type of chemical-sensing electrode. Many theoretical and experimental studies have been published for describing the behavior of this chemical-sensing electronic device [2]. Silicon dioxide (SiO2) was ®rst used as a pH-sensitive dielectric for the ISFET [1]. Subsequently, Al2O3, Si3N4, Ta2O5, and SnO2 have been used as the pH-sensitive dielectric because of their higher pH responses [3±8]. However, an ISFET is a kind of transistor which works in saline water. Thus, a problem arises from the poor isolation between the device and solution. It is thus very important to develop an ISFET encapsulation process that is compatible with integrated circuit technology. Until now, several fabrication methods for ISFET-based biosensors have been reported. Esashi and Matsuo employed the anisotropic etching tech*
Corresponding author. Tel.: 886-55342601 ext. 2500; fax: 886-55312029. E-mail address: [email protected] (J.-C. Chou).
nique to make a needle-like ISFET device, which was completely isolated from the water [9]. However, the anisotropic etching is too complex to use in this process. Another method most frequently used is based on a silicon-onsapphire (SOS) structure [10±13]. However, SOS ISFETs have some disadvantages, such as characteristic instability because of the penetration of impurities (for example, Al) from the sapphire substrate, low current sensitivity and the high cost of a sapphire wafer. In addition, Poghossian solved the ISFET encapsulation problem using a Si-SiO2-Si (SIS) structure [14,15]. But, the silicon bonding process of the SIS structure ISFETs is still too complex. An extended gate ®eld effect transistor (EGFET) is another structure to produce FET isolation from the chemical environment, in which a chemically sensitive membrane is deposited on the end of a signal line extended from the FET gate electrode [16,17]. This structure has a lot of advantages, such as light insensitivity, simple to passivate and package, and the shape ¯exibility in the extended gate area, etc. Recently, Chi et al. have introduced an improved EGFET structure which is separated into two parts. One is a sensing part with a structure of SnO2/Al/Si and the other is a commercial MOSFET, CD4007UB [18]. This structure is suitable for application to a disposable biosensor, because the separate MOSFET is reusable when the sensing part is
0925-4005/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 0 ) 0 0 6 1 3 - 4
L.-T. Yin et al. / Sensors and Actuators B 71 (2000) 106±111
interchanged. However, as the charges could be leaked from the silicon layer into the conductor layer, Al, in the sensing part, critical encapsulation is still needed for this structure. In addition to the SnO2 sensing ®lm, we have applied Si3N4, Al2O3 and SiO2 to the separate structure, respectively. These kinds of insulating materials do not show a pH response. The insulating material may not be a proper sensing material for a separate EGFET. In this study, we have improved the separate EGFET structure in which the substrate of the sensing part is glass. Indium tin oxide (ITO) is popular for it's unique electrical and optical properties, that is, high electrical conductivity and high optical transmittance in the visible region [19]. These qualities have enabled it to be widely used as transparent conducting elements for addressing various kinds of alphanumeric displays [20]. In this study, the indium tin oxide glass electrode was used as a pHsensitivity material for a separate EGFET. 2. Experimental 2.1. Materials Tin oxide thin ®lms were formed by the R.F. sputtering system (tin oxide target: 99.9%) at a substrate temperature of 1508C. The ITO glasses (the sheet resistance: 50±100 O/&, Ê ) were supplied by the Wintek ITO coating thickness: 230 A corporation. The micro slide glass and corning 7059 glass were purchased from Kimax Glass Instrument and Tekstarter corporation. 2.2. Fabrication of sensing layer structure The sensing part of the separate EGFET is designed into ®ve kind of structures, which are SnO2/Al/micro slide glass, SnO2/Al/corning 7059 glass, ITO glass, SnO2/ITO glass and SnO2/glass. The Al and SnO2 were deposited by using thermal evaporation and sputtering method, 2000 and Ê , respectively. Before the glasses were deposited, 3000 A the Al or SnO2, were washed in methyl alcohol and D.I. water for 20 and 10 min, respectively. The complete encapsulation section diagram of SnO2/Al/micro slide glass sensing structure is shown in Fig. 1.
107
Fig. 2. EGFET measurement system.
the pH buffer solutions. The measurement system in this study is shown in Fig. 2. The sensing structure and calomel reference electrode were dipped into a buffer solution and connected with the gate of a commercial MOSFET device, CD4007UB. The VDS is ®xed at 0.2 V, where VDS is the voltage between drain and source of MOSFET. The distance between the reference electrode and the sensing electrode is about 0.5±2 cm, which will not affect the results during the measurement. A readout circuit based on an instrumentation ampli®er AD620, which is shown in Fig. 3, was used to study the drift and hysteresis characteristics of the separate EGFET. 3. Results and discussion 3.1. The sensing structures with Al In the structures with Al, the conductive layer Al became an unstable element in pH solutions. As the sensing gate was placed into the buffer solution, SnO2 and Al layers were peeled very soon. In our experiment, we demonstrated that a baking process can improve this peeling effect. Table 1 shows that the extended sensing gates with a micro slide glass and corning glass substrates must be baked for 3 and 18 h at 1508C, respectively, to avoid Al and SnO2 peeling in the buffer solution. The current±voltage (I±V) curves of the micro slide glass and corning glass with baking process are shown in Figs. 4
2.3. Measurement processes The HP4145B Semiconductor Parameter Analyzer was used to measure the threshold voltage (VT) of the EGFET in
Fig. 1. Cross-section of glass substrate sensing structure.
Fig. 3. Measurement circuit of the instrumentation amplifier AD620.
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Table 1 Baking (1508C) effect of structures for SnO2/Al/micro slide glass and SnO2/Al/corning glass, respectively Baking time
Without baking
3h
10 h
18 h
Structures SnO2/Al/micro slide glass SnO2/Al/corning glass
Failed Failed
54 mV/pH Failed
50 mV/pH Failed
49 mV/pH 46 mV/pH
and 5. In the Table 1 `Failed' means that the pH sensitivity cannot be extracted in the situations of Figs. 4(a) and 5(a). However, the pH sensitivity of SnO2 ®lm ISFET decreases with the baking time [7]. The micro slide glass substrate is more suitable for applications in SnO2 sensitive separate EGFET. 3.2. The characteristic of the ITO sensing gate The indium tin oxide (ITO) thin ®lm is a well-known material used as an electrical conductor of high optical transmittance but a novel material used as a pH-ISFET sensing ®lm. This section will show the fundamental characteristics of the ITO glass electrode applied as a separate EGFET sensing gate. 3.2.1. pH sensitivity of ITO gate ISFET The transconductance gives the same peak value in a concentration range between pH 2 and 12 under the same
Fig. 4. I±V curves of SnO2/Al/micro slide glass separate gate EGFET (a) without baking process (b) baking at 1508C for 3 h.
temperature, 258C as shown in Fig. 6. The slop of the same ID versus Vref can be obtained around the maximum transconductance. This slope has a concept with Vref that is the threshold voltage of ISFET. The pH sensitivity of the ISFET was investigated through a shift in the threshold voltage of an ISFET sensor. The result shows that the ITO gate separate EGFET sensor has a linear pH sensitivity of approximately 58 mV/pH in a concentration range between pH 2 and 12. In this study, the instrumentation ampli®er AD620 was used as a readout circuit, which is shown in Fig. 3. The connection diagram of AD620 is shown in Fig. 7. ITO was connected to one of the input terminal, that is terminal 2 or 3, and the other was connected to a ground, of which the output is shown in Fig. 7. Fig. 8(a) and (b) are the results of the ITO, which was connected to terminal 2 and 3, respectively. Both of the results show that the pH-sensitivity is about 58 mV/pH. This indicates that the AD620 can be used as a readout circuit for separate EGFET.
Fig. 5. I±V curves of SnO2/Al/corning glass separate gate EGFET (a) baking at 1508C for 10 h (b) baking at 1508C for 18 h.
L.-T. Yin et al. / Sensors and Actuators B 71 (2000) 106±111
109
Fig. 6. I±V characteristics of ITO sensing film EGFET. Fig. 9. Drift characteristics of ITO sensing gate EGFET.
Fig. 7. Connection diagram of AD620.
3.2.2. Drift and hysteresis of ITO sensing gate EGFET In the study of drift and hysteresis of the ITO sensing gate EGFET sensor, the ITO was connected to terminal 2 of instrumentation ampli®er AD620. The drift experiment described in Fig. 9 was tested for 18 h. The results indicated a drift of 110 mV in this time. It shows a serious drift in the ITO sensing gate EGFET sensor, especially nearly 1 mV min-1 during the ®rst hour. To evaluate the hysteresis of the EGFET, we measured the output offset voltage along the solution change such as pH 7 ! pH 4 ! pH 7 and pH 7 ! pH 10 ! pH 7. The result shows that the hysteresis of the ITO sensing gate EGFET is about 9.8 mV as shown in Fig. 10. 3.3. The structure of SnO2/glass and SnO2/ITO glass 3.3.1. The contact area effect of SnO2/glass and SnO2/ITO glass In this study, we have determined that the structure of a SnO2/glass sensing gate EGFET has a pH sensitivity of about 55 mV/pH when the contact window between the SnO2 layer and buffer solution is beyond 4 mm2. However the pH sensitivity is decreased as the contact window is
Fig. 8. Output voltage versus pH value for the ITO sensing gate connected with instrumentation amplifier AD620 (a) ITO connected to terminal 2 (b) ITO connected to terminal 3.
Fig. 10. Hysteresis characteristics of ITO sensing gate EGFET.
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contact window on the SnO2/glass gate EGFET becomes lower. 3.3.2. The drift effect of ITO glass, SnO2/glass and SnO2/ITO glass Fig. 12 shows the drift of ITO glass, SnO2/ITO glass and SnO2/glass sensing gate structure of EGFET. The results indicate drifts of 33.9 mV and 9.1 mV in 18 h for SnO2/glass and SnO2/ITO glass sensing gate EGFET, respectively. The sensing structures with SnO2 (SnO2/glass, SnO2/ITO glass) have an obviously smaller drift than a sensing structure with ITO glass. 4. Conclusions In summary, from the above results and discussion, the important conclusions are as follows:
Fig. 11. The relationship between contact area and pH-sensitivity of (a) SnO2/glass and (b) SnO2/ITO glass structure.
smaller than 4 mm2, of which the result is shown in Fig. 11(a). The structure of SnO2/ITO glass sensing gate EGFET has a pH sensitivity about 57 mV/pH, which is not so dependent on the contact window between the SnO2 and pH buffer solution. It has a normal pH-sensitivity as the contact window beyond 0.8 mm2, of which the result is shown in Fig. 11(b). Because the voltage sharing effect sets off a reducing of MOSFET output signal, the pH sensitivity of the small
1. The glass substrate sensing gate EGFETs have the advantages of easier fabrication processes than traditional ISFET and lower cost than SOS structure ISFETs or silicon based EGFET. 2. The structures of SnO2/Al/micro slide glass and SnO2/ Al/corning glass have a problem of peeling the Al and SnO2 into the buffer. This problem can be solved by baking the SnO2/Al/micro slide glass and SnO2/Al/ corning glass at 1508C for 3 and 18 h, respectively. 3. The indium tin oxide can be used as a pH-sensitive film of EGFET, and it has a linear sensitivity of about 58 mV/pH in a pH concentration in ranging from pH 2 to 12. 4. The instrumentation amplifier AD620 can be used as a readout circuit for separate gate EGFET. 5. The ITO gate EGFET has a drift characteristic of 110 mV for 18 h and a hysteresis characteristic of 9.8 mV along the solution change as pH 7 ! pH 4 ! pH 7 and pH 7 ! pH 10 ! pH 7. 6. The pH sensitivity of SnO2/glass sensing gate EGFET is decreased when the contact window smaller than 4 mm2. 7. The pH sensitivity of SnO2/ITO glass sensing gate EGFET is decreased as the contact window is smaller than 0.8 mm2. 8. The drift characteristics of SnO2/glass and SnO2/ITO glass sensing gate EGFET are 33.9 and 9.1 mV for 18 h, respectively. The two structures with SnO2 sensing film have a drift obviously smaller than that with ITO film.
Acknowledgements
Fig. 12. Drift characteristics of ITO glass, SnO2/ITO glass and SnO2/glass sensing gate EGFET.
This work was supported by National Science Council, the Republic of China under the contracts NSC88-2215E033-006.
L.-T. Yin et al. / Sensors and Actuators B 71 (2000) 106±111
References [1] P. Bergveld, Development of an ion sensitive solid-state device for neurophysiological measurements, IEEE Trans. Biomed. Eng. 17 (1970) 70±71. [2] Clifford D. Fung, Peter W. Cheung, Wen H. Ko, A generalized theory of an electrolyte-insulator-semiconductor field effect transistor, IEEE Trans. Electron Devices 33 (1986) 8±18. [3] Stanley D. Moss, Curtis C. Johnson, Jirijanata, Hydrogen, calcium and potassium ion sensitive FET transistors a preliminary report, IEEE Trans. Biomed. Eng. 25 (1978) 49±54. [4] Hung-Kwei Liao, Jung-Chuan Chou, Wen-Yaw Chung, Tai-Ping, Shen-Ken Hsiung, Study on the interface trap density of the Si3N4/SiO2 gate ISFET, in: Proceedings of the Third East Asian Conference on Chemical Sensors, Seoul, South Korea, 5±6 November 1997, pp. 394±400. [5] Li-Te Yin, Jung-Chuan Chou, Wen-Yaw Chung, Tai-Ping Shen-Ken Hsiung, Characteristics of silicon nitride after O2 plasma treatment for pH ISFET applications, in: Proceeding of 1998 International Electron Devices and Materials Symposia, B, C, National Cheng Kung University, Tainan, Taiwan, ROC, December 1998, pp. 267±270. [6] P. Gimmel, B. Gompf, D. Schmeiosser, H.D. Weimhofer, W. Gopel, M. Klein, Ta2O5 gates of pH sensitive device comparative spectroscopic and electrical studies, Sensors and Actuators B 17 (1989) 195±202. [7] Hung-Kwei Liao, Jung-Chuan Chou, Wen-Yaw Chung, Tai-Ping Sun, Shen-Kan Hsiung, Study of amorphous tin oxide thin films for ISFET applications, Sensors and Actuators B 50 (1998) 104±109. [8] Hung-Kwei Liao, Jung-Chuan Chou, Wen-Yaw Chung, Tai-Ping Sun, Shen-Kan Hsiung, Study of pHpzc and Surface Potential of Tin Oxide Gate ISFET, Mater. Chem. Phys. 2428 (1999) 1±6. [9] M. Esashi, T. Matsuo, Integrated micro multi ion sensor using field effect of semiconductor, IEEE Trans. Biomed. Eng. BME-25 (1978) 182±192. [10] B.H. van der Schoot, P. Bergveld, ISFET-based enzyme sensors, Biosensors 3 (1987) 161±186. [11] S. Nakamoto, N. Ito, T. Kuriyama, J. Kimura, A lift-off method for patterning enzyme-immobilized membranes in multi-biosensors, Sensors and Actuators 13 (1988) 165±172. [12] J. Kimara, T. Munakami, T. Kuriyama, An integrated multi-biosensor for simultaneous amperometric and potentiometric measurement, Sensors and Actuators 15 (1989) 435±443. [13] Y. Hanazato, M. Nakako, S. Shinon, M. Maeda, Integrated multibiosensors based on an ion-sensitive field-effect transistor using photolithographic technique, IEEE Trans. Electron Devices 36 (1989) 1303±1310. [14] A.S. Poghossian, Method of fabrication of ISFETs and CHEMFETs on an Si-SiO2-Si structure, Sensors and Actuators B 13±14 (1993) 653±654. [15] A.S. Poghossian, Method of fabrication of ISFET-based biosensors on an Si-SiO2-Si structure, Sensors and Actuators B 44 (1997) 361±364. [16] J. Van der Spiegel, I. Lauks, P. Chan, D. Babic, The extended gate chemical sensitive field effect transistor as multi-species microprobe, Sensors and Actuators B 4 (1983) 291±298. [17] T. Katsube, T. Araki, M. Hara, T. Yaji, Si Kobayashi, K. Suzuki, A multi-species biosensor with extended-gate field effect transistors, in: Proceedings of 6th Sensor Symposium, Tsukuba, Japan, 1986, 211±214. [18] Li-Lun Chi, Jung-Chuan Chou, Wen-Yaw Chung, Tai-Ping Sun, Shen-Kan Hsiung, New structure of ion sensitivite field effect transistor, in: Proceeding of the Biomedical Engineering Society 1998 Annual Symposium, Taiwan, 1998, 332±334. [19] M.A. Martinez, J. Herrero, M.T. Gutierrez, Electrochemical stability of indium tin oxide thin films, Electrochim. Acta 37 (1992) 2565±2571.
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Biographies Li-Te Yin was born in Taipei, Taiwan, Republic of China, on December 5, 1973. He received bachelor degree in electrical engineering and master degrees in aeronautical and astronautical engineering from Chung-Hua Polytechnic Institute, Hsinchu, Taiwan, in 1995 and 1997, respectively. Since 1997 he has been working toward the Ph.D. degree in the Department of Bio-Medical Engineering at Chung Yuan Christian University, Chung-Li, Taiwan. His research interests are in the biosensors. Jung-Chuan Chou was born in Tainan, Taiwan, Republic of China, on July 13, 1954. He received the BS degree in physics from Kaohisung Normal College, Kaohsiung, Taiwan, in 1976; the MS degree in applied physics from Chung Yuan Christian University, Chung-Li, Taiwan, in 1979; and the Ph.D. degree in electronics from National Chiao Tung University, Hsinchu, Taiwan, in 1988. He taught at Chung Yuan Christian University from 1979 to 1991. Since 1991 he has worked as an associate professor in the Department of Electronic Engineering at the National Youlin University of Science and Technology. His research interests are in the areas of amorphous materials and devices, electrographic photoreceptor materials and devices, electronic materials and devices, sensor devices, and science education. Wen-Yaw Chung was born in Hsin-Chu, Taiwan, ROC, on March 15, 1957. He received the BSEE and MS degrees from Chung Yuan Christian University, Chung Li, Taiwan, in 1979 and 1981, respectively, and the Ph.D. degree in Electrical and Computer Engineering from Mississippi State University, USA, in 1989. Subsequently, he joined the Advanced Microelectronics Division, Institute for Technology Development in Mississippi, where he was involved in the design of a bipolar optical data receiver. In 1990 he worked as a design manager for the Communication Product Division, United Microelectronics Corporation, Hsin-Chu, where he was involved in the design of analog CMOS data communication integrated circuits. Since 1991 he has been an associate professor in the Department of Electronic Engineering at Chung Yuan Christian University. His research interests include mixed-signal VLSI design, biomedical IC applications, and test-chip design methodology for deep submicron VLSI electronics. Tai-Ping Sun was born in Taiwan on March 20, 1950. He received the BSdegree in electrical engineering from Chung Cheng Institute of Technology, Taiwan, in 1974, the MS degree in material science engineering from National Tsing Hua University, Taiwan, in 1977, and Ph.D. degree in electrical engineering from National Taiwan University, Taiwan, in 1990. From 1977 to 1997, he worked at Institute of Science and Technology, Republic of China, concerning the development of Infrared device, circuit and system. He joined the Department of Management Information System, Chung-Yu college of Business Administration since 1997 as a associated professor. Since 1999 he has joined the Department of Electrical Engineering, National Chi Nan University as a professor and his research interests have been in Infrared detector and system, analog/digital mixed-mode integrated circuit design, special semiconductor sensor and their application. Shen-Kan Hsiung was born on June 14, 1942. He received the BS degree from Department of Electrical Engineering, National Cheng-Kung University, in 1965, the MS degree from Department of Electronic Engineering, National Chiao-Tung University, Taiwan, in 1968 and the Ph.D. degree from Material Science Engineering of USC, USA, in 1974. From 1974 to 1978, he was an associate professor in Department of Electrical Engineering, Chung Yuan Christian University. Since 1978 he has been a professor in Department of Electronic Engineering, Chung Yuan Christian University. His current interests are electronic materials, amorphous thin films and semiconductor sensors.
Separate structure extended gate H-ion sensitive ®eld effect transistor on a glass substrate Li-Te Yina, Jung-Chuan Choub,*, Wen-Yaw Chungc, Tai-Ping Sund, Shen-Kan Hsiungc a
b
Department of Bio-Medical Engineering, Chung Yuan Christian University, 320 Chung-Li, Taiwan, ROC Institute of Electronic and Information Engineering, National Yunlin University of Science and Technology, 640 Toulin, Taiwan, ROC c Department of Electronic Engineering, Chung Yuan Christian University, 320 Chung-Li, Taiwan, ROC d Department of Electrical Engineering, National Chi Nan University, 545 Nantou, Taiwan, ROC Received 7 February 2000; received in revised form 16 June 2000; accepted 30 June 2000
Abstract In our research, glass was used as a substrate for an H ion sensitive ®eld effect transistor (ISFET). The sensitive characteristics of ®ve structures for separate extended gate ion sensitive ®eld effect transistors (EGFET) were studied. The components included tin oxide (SnO2)/ aluminum/micro slide glass, tin oxide/aluminum/corning glass, indium tin oxide (ITO) glass, tin oxide/indium tin oxide glass and tin oxide/ micro slide glass. Indium tin oxide (ITO) thin ®lm was ®rst time used as an H ion sensitive ®lm, which has a linear pH Nerstern response sensitivity, about 58 mV/pH, between pH 2 and 12. In addition, the sensing area effect of the tin oxide/glass, tin oxide/ITO glass and ITO glass structure is discussed. The results show that the tin oxide/ITO glass structure EGFET has the best drift, hysteresis and sensing area characteristics. # 2000 Elsevier Science B.V. All rights reserved. Keywords: pH-ISFET; EGFET; Indium tin oxide (ITO) glass; Tin oxide (SnO2); Contact area effect
1. Introduction Ion sensitive ®eld-effect transistors (ISFETs) have been developed on the basis of the MOSFET (metal oxide ®eld effect transistor). Since Bergveld [1] ®rst employed the ®eld-effect transistor in neurophysiological measurements in 1970, ISFETs have been developed into a new type of chemical-sensing electrode. Many theoretical and experimental studies have been published for describing the behavior of this chemical-sensing electronic device [2]. Silicon dioxide (SiO2) was ®rst used as a pH-sensitive dielectric for the ISFET [1]. Subsequently, Al2O3, Si3N4, Ta2O5, and SnO2 have been used as the pH-sensitive dielectric because of their higher pH responses [3±8]. However, an ISFET is a kind of transistor which works in saline water. Thus, a problem arises from the poor isolation between the device and solution. It is thus very important to develop an ISFET encapsulation process that is compatible with integrated circuit technology. Until now, several fabrication methods for ISFET-based biosensors have been reported. Esashi and Matsuo employed the anisotropic etching tech*
Corresponding author. Tel.: 886-55342601 ext. 2500; fax: 886-55312029. E-mail address: [email protected] (J.-C. Chou).
nique to make a needle-like ISFET device, which was completely isolated from the water [9]. However, the anisotropic etching is too complex to use in this process. Another method most frequently used is based on a silicon-onsapphire (SOS) structure [10±13]. However, SOS ISFETs have some disadvantages, such as characteristic instability because of the penetration of impurities (for example, Al) from the sapphire substrate, low current sensitivity and the high cost of a sapphire wafer. In addition, Poghossian solved the ISFET encapsulation problem using a Si-SiO2-Si (SIS) structure [14,15]. But, the silicon bonding process of the SIS structure ISFETs is still too complex. An extended gate ®eld effect transistor (EGFET) is another structure to produce FET isolation from the chemical environment, in which a chemically sensitive membrane is deposited on the end of a signal line extended from the FET gate electrode [16,17]. This structure has a lot of advantages, such as light insensitivity, simple to passivate and package, and the shape ¯exibility in the extended gate area, etc. Recently, Chi et al. have introduced an improved EGFET structure which is separated into two parts. One is a sensing part with a structure of SnO2/Al/Si and the other is a commercial MOSFET, CD4007UB [18]. This structure is suitable for application to a disposable biosensor, because the separate MOSFET is reusable when the sensing part is
0925-4005/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 0 ) 0 0 6 1 3 - 4
L.-T. Yin et al. / Sensors and Actuators B 71 (2000) 106±111
interchanged. However, as the charges could be leaked from the silicon layer into the conductor layer, Al, in the sensing part, critical encapsulation is still needed for this structure. In addition to the SnO2 sensing ®lm, we have applied Si3N4, Al2O3 and SiO2 to the separate structure, respectively. These kinds of insulating materials do not show a pH response. The insulating material may not be a proper sensing material for a separate EGFET. In this study, we have improved the separate EGFET structure in which the substrate of the sensing part is glass. Indium tin oxide (ITO) is popular for it's unique electrical and optical properties, that is, high electrical conductivity and high optical transmittance in the visible region [19]. These qualities have enabled it to be widely used as transparent conducting elements for addressing various kinds of alphanumeric displays [20]. In this study, the indium tin oxide glass electrode was used as a pHsensitivity material for a separate EGFET. 2. Experimental 2.1. Materials Tin oxide thin ®lms were formed by the R.F. sputtering system (tin oxide target: 99.9%) at a substrate temperature of 1508C. The ITO glasses (the sheet resistance: 50±100 O/&, Ê ) were supplied by the Wintek ITO coating thickness: 230 A corporation. The micro slide glass and corning 7059 glass were purchased from Kimax Glass Instrument and Tekstarter corporation. 2.2. Fabrication of sensing layer structure The sensing part of the separate EGFET is designed into ®ve kind of structures, which are SnO2/Al/micro slide glass, SnO2/Al/corning 7059 glass, ITO glass, SnO2/ITO glass and SnO2/glass. The Al and SnO2 were deposited by using thermal evaporation and sputtering method, 2000 and Ê , respectively. Before the glasses were deposited, 3000 A the Al or SnO2, were washed in methyl alcohol and D.I. water for 20 and 10 min, respectively. The complete encapsulation section diagram of SnO2/Al/micro slide glass sensing structure is shown in Fig. 1.
107
Fig. 2. EGFET measurement system.
the pH buffer solutions. The measurement system in this study is shown in Fig. 2. The sensing structure and calomel reference electrode were dipped into a buffer solution and connected with the gate of a commercial MOSFET device, CD4007UB. The VDS is ®xed at 0.2 V, where VDS is the voltage between drain and source of MOSFET. The distance between the reference electrode and the sensing electrode is about 0.5±2 cm, which will not affect the results during the measurement. A readout circuit based on an instrumentation ampli®er AD620, which is shown in Fig. 3, was used to study the drift and hysteresis characteristics of the separate EGFET. 3. Results and discussion 3.1. The sensing structures with Al In the structures with Al, the conductive layer Al became an unstable element in pH solutions. As the sensing gate was placed into the buffer solution, SnO2 and Al layers were peeled very soon. In our experiment, we demonstrated that a baking process can improve this peeling effect. Table 1 shows that the extended sensing gates with a micro slide glass and corning glass substrates must be baked for 3 and 18 h at 1508C, respectively, to avoid Al and SnO2 peeling in the buffer solution. The current±voltage (I±V) curves of the micro slide glass and corning glass with baking process are shown in Figs. 4
2.3. Measurement processes The HP4145B Semiconductor Parameter Analyzer was used to measure the threshold voltage (VT) of the EGFET in
Fig. 1. Cross-section of glass substrate sensing structure.
Fig. 3. Measurement circuit of the instrumentation amplifier AD620.
108
L.-T. Yin et al. / Sensors and Actuators B 71 (2000) 106±111
Table 1 Baking (1508C) effect of structures for SnO2/Al/micro slide glass and SnO2/Al/corning glass, respectively Baking time
Without baking
3h
10 h
18 h
Structures SnO2/Al/micro slide glass SnO2/Al/corning glass
Failed Failed
54 mV/pH Failed
50 mV/pH Failed
49 mV/pH 46 mV/pH
and 5. In the Table 1 `Failed' means that the pH sensitivity cannot be extracted in the situations of Figs. 4(a) and 5(a). However, the pH sensitivity of SnO2 ®lm ISFET decreases with the baking time [7]. The micro slide glass substrate is more suitable for applications in SnO2 sensitive separate EGFET. 3.2. The characteristic of the ITO sensing gate The indium tin oxide (ITO) thin ®lm is a well-known material used as an electrical conductor of high optical transmittance but a novel material used as a pH-ISFET sensing ®lm. This section will show the fundamental characteristics of the ITO glass electrode applied as a separate EGFET sensing gate. 3.2.1. pH sensitivity of ITO gate ISFET The transconductance gives the same peak value in a concentration range between pH 2 and 12 under the same
Fig. 4. I±V curves of SnO2/Al/micro slide glass separate gate EGFET (a) without baking process (b) baking at 1508C for 3 h.
temperature, 258C as shown in Fig. 6. The slop of the same ID versus Vref can be obtained around the maximum transconductance. This slope has a concept with Vref that is the threshold voltage of ISFET. The pH sensitivity of the ISFET was investigated through a shift in the threshold voltage of an ISFET sensor. The result shows that the ITO gate separate EGFET sensor has a linear pH sensitivity of approximately 58 mV/pH in a concentration range between pH 2 and 12. In this study, the instrumentation ampli®er AD620 was used as a readout circuit, which is shown in Fig. 3. The connection diagram of AD620 is shown in Fig. 7. ITO was connected to one of the input terminal, that is terminal 2 or 3, and the other was connected to a ground, of which the output is shown in Fig. 7. Fig. 8(a) and (b) are the results of the ITO, which was connected to terminal 2 and 3, respectively. Both of the results show that the pH-sensitivity is about 58 mV/pH. This indicates that the AD620 can be used as a readout circuit for separate EGFET.
Fig. 5. I±V curves of SnO2/Al/corning glass separate gate EGFET (a) baking at 1508C for 10 h (b) baking at 1508C for 18 h.
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Fig. 6. I±V characteristics of ITO sensing film EGFET. Fig. 9. Drift characteristics of ITO sensing gate EGFET.
Fig. 7. Connection diagram of AD620.
3.2.2. Drift and hysteresis of ITO sensing gate EGFET In the study of drift and hysteresis of the ITO sensing gate EGFET sensor, the ITO was connected to terminal 2 of instrumentation ampli®er AD620. The drift experiment described in Fig. 9 was tested for 18 h. The results indicated a drift of 110 mV in this time. It shows a serious drift in the ITO sensing gate EGFET sensor, especially nearly 1 mV min-1 during the ®rst hour. To evaluate the hysteresis of the EGFET, we measured the output offset voltage along the solution change such as pH 7 ! pH 4 ! pH 7 and pH 7 ! pH 10 ! pH 7. The result shows that the hysteresis of the ITO sensing gate EGFET is about 9.8 mV as shown in Fig. 10. 3.3. The structure of SnO2/glass and SnO2/ITO glass 3.3.1. The contact area effect of SnO2/glass and SnO2/ITO glass In this study, we have determined that the structure of a SnO2/glass sensing gate EGFET has a pH sensitivity of about 55 mV/pH when the contact window between the SnO2 layer and buffer solution is beyond 4 mm2. However the pH sensitivity is decreased as the contact window is
Fig. 8. Output voltage versus pH value for the ITO sensing gate connected with instrumentation amplifier AD620 (a) ITO connected to terminal 2 (b) ITO connected to terminal 3.
Fig. 10. Hysteresis characteristics of ITO sensing gate EGFET.
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contact window on the SnO2/glass gate EGFET becomes lower. 3.3.2. The drift effect of ITO glass, SnO2/glass and SnO2/ITO glass Fig. 12 shows the drift of ITO glass, SnO2/ITO glass and SnO2/glass sensing gate structure of EGFET. The results indicate drifts of 33.9 mV and 9.1 mV in 18 h for SnO2/glass and SnO2/ITO glass sensing gate EGFET, respectively. The sensing structures with SnO2 (SnO2/glass, SnO2/ITO glass) have an obviously smaller drift than a sensing structure with ITO glass. 4. Conclusions In summary, from the above results and discussion, the important conclusions are as follows:
Fig. 11. The relationship between contact area and pH-sensitivity of (a) SnO2/glass and (b) SnO2/ITO glass structure.
smaller than 4 mm2, of which the result is shown in Fig. 11(a). The structure of SnO2/ITO glass sensing gate EGFET has a pH sensitivity about 57 mV/pH, which is not so dependent on the contact window between the SnO2 and pH buffer solution. It has a normal pH-sensitivity as the contact window beyond 0.8 mm2, of which the result is shown in Fig. 11(b). Because the voltage sharing effect sets off a reducing of MOSFET output signal, the pH sensitivity of the small
1. The glass substrate sensing gate EGFETs have the advantages of easier fabrication processes than traditional ISFET and lower cost than SOS structure ISFETs or silicon based EGFET. 2. The structures of SnO2/Al/micro slide glass and SnO2/ Al/corning glass have a problem of peeling the Al and SnO2 into the buffer. This problem can be solved by baking the SnO2/Al/micro slide glass and SnO2/Al/ corning glass at 1508C for 3 and 18 h, respectively. 3. The indium tin oxide can be used as a pH-sensitive film of EGFET, and it has a linear sensitivity of about 58 mV/pH in a pH concentration in ranging from pH 2 to 12. 4. The instrumentation amplifier AD620 can be used as a readout circuit for separate gate EGFET. 5. The ITO gate EGFET has a drift characteristic of 110 mV for 18 h and a hysteresis characteristic of 9.8 mV along the solution change as pH 7 ! pH 4 ! pH 7 and pH 7 ! pH 10 ! pH 7. 6. The pH sensitivity of SnO2/glass sensing gate EGFET is decreased when the contact window smaller than 4 mm2. 7. The pH sensitivity of SnO2/ITO glass sensing gate EGFET is decreased as the contact window is smaller than 0.8 mm2. 8. The drift characteristics of SnO2/glass and SnO2/ITO glass sensing gate EGFET are 33.9 and 9.1 mV for 18 h, respectively. The two structures with SnO2 sensing film have a drift obviously smaller than that with ITO film.
Acknowledgements
Fig. 12. Drift characteristics of ITO glass, SnO2/ITO glass and SnO2/glass sensing gate EGFET.
This work was supported by National Science Council, the Republic of China under the contracts NSC88-2215E033-006.
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Biographies Li-Te Yin was born in Taipei, Taiwan, Republic of China, on December 5, 1973. He received bachelor degree in electrical engineering and master degrees in aeronautical and astronautical engineering from Chung-Hua Polytechnic Institute, Hsinchu, Taiwan, in 1995 and 1997, respectively. Since 1997 he has been working toward the Ph.D. degree in the Department of Bio-Medical Engineering at Chung Yuan Christian University, Chung-Li, Taiwan. His research interests are in the biosensors. Jung-Chuan Chou was born in Tainan, Taiwan, Republic of China, on July 13, 1954. He received the BS degree in physics from Kaohisung Normal College, Kaohsiung, Taiwan, in 1976; the MS degree in applied physics from Chung Yuan Christian University, Chung-Li, Taiwan, in 1979; and the Ph.D. degree in electronics from National Chiao Tung University, Hsinchu, Taiwan, in 1988. He taught at Chung Yuan Christian University from 1979 to 1991. Since 1991 he has worked as an associate professor in the Department of Electronic Engineering at the National Youlin University of Science and Technology. His research interests are in the areas of amorphous materials and devices, electrographic photoreceptor materials and devices, electronic materials and devices, sensor devices, and science education. Wen-Yaw Chung was born in Hsin-Chu, Taiwan, ROC, on March 15, 1957. He received the BSEE and MS degrees from Chung Yuan Christian University, Chung Li, Taiwan, in 1979 and 1981, respectively, and the Ph.D. degree in Electrical and Computer Engineering from Mississippi State University, USA, in 1989. Subsequently, he joined the Advanced Microelectronics Division, Institute for Technology Development in Mississippi, where he was involved in the design of a bipolar optical data receiver. In 1990 he worked as a design manager for the Communication Product Division, United Microelectronics Corporation, Hsin-Chu, where he was involved in the design of analog CMOS data communication integrated circuits. Since 1991 he has been an associate professor in the Department of Electronic Engineering at Chung Yuan Christian University. His research interests include mixed-signal VLSI design, biomedical IC applications, and test-chip design methodology for deep submicron VLSI electronics. Tai-Ping Sun was born in Taiwan on March 20, 1950. He received the BSdegree in electrical engineering from Chung Cheng Institute of Technology, Taiwan, in 1974, the MS degree in material science engineering from National Tsing Hua University, Taiwan, in 1977, and Ph.D. degree in electrical engineering from National Taiwan University, Taiwan, in 1990. From 1977 to 1997, he worked at Institute of Science and Technology, Republic of China, concerning the development of Infrared device, circuit and system. He joined the Department of Management Information System, Chung-Yu college of Business Administration since 1997 as a associated professor. Since 1999 he has joined the Department of Electrical Engineering, National Chi Nan University as a professor and his research interests have been in Infrared detector and system, analog/digital mixed-mode integrated circuit design, special semiconductor sensor and their application. Shen-Kan Hsiung was born on June 14, 1942. He received the BS degree from Department of Electrical Engineering, National Cheng-Kung University, in 1965, the MS degree from Department of Electronic Engineering, National Chiao-Tung University, Taiwan, in 1968 and the Ph.D. degree from Material Science Engineering of USC, USA, in 1974. From 1974 to 1978, he was an associate professor in Department of Electrical Engineering, Chung Yuan Christian University. Since 1978 he has been a professor in Department of Electronic Engineering, Chung Yuan Christian University. His current interests are electronic materials, amorphous thin films and semiconductor sensors.