1. Field of Invention
This invention relates generally to a sensor, and more particularly to a method of fabricating an electrode assembly of a sensor.
2. Description of Related Art
Glass electrodes have many merits such as high linearity, good ion distinction, and stability. However, problems like the large size, high cost and long response time a reaction time have decreased their performance. In 1989 on pages 59-63, issue 1, volume 67 of the Int. J., B. D. Liu et al. illustrated the new direction in utilizing the mature field effect ion sensor developed by the mature silicon semiconductor integrated circuit process. The attempt was to replace the traditional glass electrode.
In 1970 on pages 70-71, volume BME-17 of IEEE Transactions Biomedical Engineering, Piet Bergveld first removed the metallic part of the poles of the metal-oxide-semiconductor field effect transistor (MOSFET). He then immerses the element into an aqueous solution; use the oxidation layer as an insulated ion sensor. This sensor produces different electrical potential at the interface when contacting solutions of different acidity, changing the electric current of the circuit to measure the pH or other ion concentration of the solution. Thus, Piet Bergveld named this sensor the ion sensitive field effect transistor (ISFET).
In the 70's, the development and application of the ISFET were still in an explorative stage. When the 80's arrived, the research on this field has reached a new dimension, whether in the basic theoretic research, key technologies, or practical applications. For example, dozens of ion and chemical field effect transistor based on the ISFET had been created, excelled in the microlization, modularization, and multi-function. The global popularity of the ISFET in a mere decade owed the credit from its distinctive characteristics described by D. Yu et al. in 1990 on pages 57-62, volume 1 of the Chemical Sensors, J. Sensor & Transducer Tech:
1. Minute size allowing micro solution measurements.
2. High input resistance and low output resistance.
3. Fast effect.
4. Compatible production process with the MOSFET technology.
Merits described above have fired a research fever on the ISFET within many research institutes in the past 2 decades. A brief outline of the international development of this element is noted below:
W. M. Siu and R. S. C. Cobbold reported an ISFET with silicon dioxide, silicon nitride, oxide, and aluminum oxide as ion sensors in 1979 on pages 1805 to 1815, issue 11, volume ED-26.
ISFET based on different elemental structures: such as back contact field effect ion sensor reported by A. S. Wong in his Ph.D. Thesis in Case Western Reserve University, 1985. Or the expanding ISFET reported by J. Van Der Spiegel et al. on pages 291-298, volume 4 of Sensors and Actuators B, 1983.
Microlization of the reference electrode reported by D. Yu on pages 53 to 57, volume 3 of Chemical Sensors, J. Sensor & Transducer Tech., 1991. Differential ISFET on pages 221 to 237, volume 11 of Sensors and Actuators, 1987.
On pages 237 to 239, volume 5 of Sensors and Actuators B, 1991, Atushi Saito reported the use of enzymes on the ISFET to detect metabolic messages in biology (for example: detection of glucose or oxygen level in the blood.) Theoretical research attachment bond module on pages 315 to 318 reported by L. K. Meixner on pages 315 to 318 on volume 6 of Sensors and Actuators B, 1992.
R. E. G. van Hal reported a study on wrapping materials on pages 17 to 26, volume 23 of Sensors and Actuators B, 1995. B. H. Van Der Schoot et al. reported an integration of measuring system and sensors on pages 239 to 241, volume 4 of Sensors and Actuators B, 1991.
M. Grattarola reported yet another study on the field effect ion sensor simulation on pages 813 to 819, issue 4, volume 39 of IEEE Transactions on Electron Devices, 1992.
Listed below are patents granted so far: U.S. Pat. No. 5,309,085 (May 3, 1994)—readout circuit as a biological ISFET. This circuit has a simple structure and easy integration. The circuit is composed of input terminals from two ISFET, one as an enzyme field effect transistor, the other as a reference field effect transistor. Immobilizing an enzyme to the electrode of the ISFET does the enzyme field effect transistor. This circuit has different magnifying functions to magnify and output the ion detection. The voltage effect of the ISFET is due to the temperature effect of unstable reference electrodes. Thus the benefits of the circuit can be recognized and the sensor can be adjusted. This ion sensitive filed effect transistor-biosensor can be integrated on one single chip with the measuring circuit, to minimize the size of the sensor.
U.S. Pat. No. 5,296,122 (Mar. 22, 1994)—hydrophobic thin film used as the reference electrode of the ion sensitive field effect transistor. This hydrophobic thin film can grow on the substrate via neutral electrolyte or electroplating. The apparatus includes a vacuum, an atom ray generator, a base, a cover board to control growth elements. This thin film is applicable to ion sensors such as ion sensitive field effect transistors and enzyme sensors.
U.S. Pat. No. 5,061,976 (Oct. 29, 1991)—ion sensitive field effect transistor with carbon gate insulated electrode. Conducting material, 2,6 xylenol is then coated. The ion sensitive field effect transistor exhibits high sensitivity to hydrogen ions, low time drift, high stability, and low light effect. If other ion selective thin film or enzymes are further coated on the 2,6 xylenol, different ions and metabolites of different concentrations can be detected.
U.S. Pat. No. 6,218,208 (Apr. 17, 2001)—hot steam plating or ratio frequency sputtering is used to produce a field effect ion sensor with a metal light cover. The structure: tin oxide/metal/silicon oxide multi-structure sensor and tin oxide/metal/silicon nitride/silicon nitride multi-structure sensor. Many excellent characteristics are associated with this device, such as Nernst Effect between pH 2 to pH 10—high linearity in the 56≈58 mV/pH range. One unique point is that this sensor effectively decreased light interference. Moreover, this process requires simple apparatus, low cost, and easy mass production. Inexpensive, disposable sensors can also be produced. Therefore this invention possesses extremely high feasibility and applicability among the ISFET.
U.S. Pat. No. 5,925,318 (Jul. 20, 1999)—an iron-detecting sensor. Iron compounds such as lactoferrin are immobilized on the surface of the potentiometric or acidic sensor. Reactions changes the potential or the pH value of the iron-detecting sensor, therefore this sensor detects such changes. This patent includes iron molecule ion compound ion sensitive field effect transistor and acidity paper tester.
U.S. Pat. No. 5,918,110 (Jun. 29, 1999)—this patent is on an multi-sensor including pressure and electrochemical sensor, based on the ion sensitive field effect transistor on a silicon substrate. A protective layer follows deposition of a nitride layer as an acidity sensor. Then a multi-silicon thin film is positioned on the top of the vacuum space. This area is the pressure sensor, and the readout of the sensor can be through the CMOS standard. The oxidized middle layer of the gaseous sensor is made by the removal of oxidized layer with the wet chemistry method. The platinum contact point and the attached protective layer are deposited by PECVD. The pressure sensor is made after the completion of the gaseous sensor layers.
U.S. Pat. No. 5,516,697 (May 14, 1996)—a simple, low-cost biosensor for detecting ion concentrations. Lactoferrin is immobilized on the sensor surface. Lactoferrin reacts with iron and expresses electricity, changing the electropotential or the surface potential of the acidic sensor. This property enables the biosensor to detect ion concentrations. The biosensor includes the ion sensitive field effect transistor, and acidity paper tester.
According to the current literature, there are some materials most frequently used as the sensing membrane of the pH sensor such as silicon dioxide, silicon nitride, Ta2O5, and aluminum oxide. Hung-Kwei Liao et al. reported the first-time completion of the ISFET with tin oxide as the sensing membrane in this laboratory on pages 410 to 415 of Proceedings of the 3rd East Asian Conference on Chemical Sensors (Seoul, Korea), 1997. Properties of this sensor include Nernst Effect. Within the range of 56˜58 mV/pH, a high linearity, time stability, low drift, and a reaction speed of lower than 0.1 second were all achieved. This laboratory also developed a multi-layered sensor, deterring light interference: sensor film/metal/silicon dioxide multi-layer sensor and film layer/metal/silicon nitride/silicon dioxide multi-structured sensor. This light deterring structure has inspired the structure of the EGFET. This apparatus views the metallic light deterring layer as a potential, and is pulled out of the field effect transistor with a conductor line, connecting to an ion sensitive film. The ion sensitive film is thus completely separate from the field effect transistor, only connected through a wire. Therefore, the ion sensitive film part can be seen as a low-cost, disposable ion sensitive electrode or bio-electrode. The field effect transistor part can then seen as a reusable front readout circuit. Our laboratory has discovered that the traditional highly insulative inorganic sensitive materials such as silicon nitride, aluminum oxide and tallium oxide cannot be used in this apparatus. The reason is that high insulation results in higher capacitance effect and an extremely unstable Transient response. However, this EGFET measuring apparatus performs rather well with the non-insulation sensitive materials below: tin oxide, ITO, titanium nitride. Therefore our laboratory has successfully completed this EGFET apparatus. To add to the strength of this new invention, it has very low light sensitivity and a linear, adjustable temperature coefficient.