Tadayuki Matsuo

Methods of isfet fabrication

The Ion Sensltlve Field-Effect Transistor (ISFET) 1s a new, integrated device composed of a conventional ion selective electrode and an InsulatedGate FET(IGFET) The device 1s slmllar to the conventional IGFET except that the metal gate electrode 1s removed m order to expose the underlying msulator layer to solution The gate msulator plays the role of an ion selective electrode Figure 1 shows the prmclple of the ISFET The first ISFET was reported by Bergveld [ 11, and V~IXOUS devices of this type have been described m recent papers [ 2 61 ISFETs have potential advantages over conventronal zon selective electrodes m theurapid response, small size, and low output impedance Addltlonally , the use of IC technology allows the fabrlcatlon of ISFETs for the detection of several ion species sunultaneously These advantages of ISFETs are extremely attractive for P1omedlca.l apphcations The nature of ISFETs requires that they are used m solutions of electrolytes which, from the electronic point of mew, are hostile enwonments The electrical mtegnty of the whole ISFET assembly 1s cntlcal and a prerequlslte for the stable operation of those devices, and, especially, the protection of the gate msulator from ion migration and hydration 1s crltlcally unportant For these reasons, the charactenstlcs of ISFETs are greatly dependent on the structure and the method of fabrication of the device

An Integrated Field-Effect Electrode for Biopotential Recording

An integrated liquid-oxide-semiconductor FET structure is described. The structure is similar to a conventional n-channel depletion mode MOSFET except the gate metal is omitted. Immersing the device in a saline solution, the drain current is controlled by the potential between the source and the solution. Operated as an active recording electrode, the structure has several properties useful in neurological research.

Integrated Micro Multi Ion Sensor Using Field Effect of Semiconductor

The fabrication of a microprobe for simultaneous, independent and in-vivo measurements of H+ and Na+ ion activities is described.

ISFET's using inorganic gate thin films

The characteristics of various types of ISFETs using inorganic gate films are described. The pH and pNa selectivities are investigated for SiO<inf>2</inf>, Si<inf>3</inf>N<inf>4</inf>, Al<inf>2</inf>O<inf>3</inf>, alumino-silicate, and sodium-aluminosilicate gate dielectrics. The transient response and device stability are also studied for different values of solution pH. The Al<inf>2</inf>O<inf>3</inf>gate shows a nearly ideal pH response, excellent stability, and selectivity to other cations. On the other hand, the Si<inf>3</inf>N<inf>4</inf>gate is also a good pH sensor, but it is proved by the studies of SiO<inf>2</inf>and SiO<inf>x</inf>N<inf>y</inf>films that the oxygen content in its surface degrades its properties as a pH sensor. Sodium-alumino-silicate, which is generally known as a material for pNa selective glass electrodes, is utilized as a gate film for the pNa ISFET. The pNa selectivity of this device is comparable to that of the conventional glass electrode. The alumino-silicate gate has also a pNa selectivity, but it is inferior to the sodium-alumino-silicate gate.

Prototype miniature blood gas analyser fabricated on a silicon wafer

Abstract The fabrication of a miniature blood pH monitoring system formed on a silicon wafer using a pH ISFET and a new type of micro-valve actuator is described. The system samples the blood from the vein, measures its pH and calibrates zero drift of the sensor automatically and periodically. The main body of the system, the micro-cell, is made of a silicon wafer and a Pyrex glass plate processed by IC technology (micro-machining). This cell is significantly smaller, lighter and more portable than presently available blood gas analysers, in particular, the sample volume reduction is a very distinctive feature of it. The total necessary sample volume of the system is about 10 μl.

Characteristics of reference electrodes using a polymer gate ISFET

A new type of ISFET with a Parylene gate as a site-free and ion-blocked membrane has been proposed for a reference electrode. The reference ISFET should be insensitive to the variation of ion concentrations of all ions or a specific ion in the electrolyte and should have a constant gate surface potential. In this paper, the operation of this reference ISFET is analysed by the site binding model and the conditions required to make the gate surface potential constant are examined. In general, the surface potential of an electrolyte-insulator-semiconductor (EIS) system is determined by two factors, i.e., the available surface site density of the insulator and the total charge density of the semiconductor, and it can be made constant by controlling the total charge density of the semiconductor. The preliminary experimental results on the electrolyte-Parylene-Si diode and ISFET are in good agreement with the theoretical calculations and suggest that a reference ISFET could be realized by using the Parylene gate.

Fabrication of catheter-tip and sidewall miniature pressure sensors

Silicon-diaphragm miniature pressure sensors, which use the piezoresistive effect, were developed for biomedical applications. We fabricated two types of sensors; that is, a catheter-tip (1.2-mm outside diameter, 0.17 mm thick) and a sidewall (1.4 × 3.45 × 0.22 mm) sensor, both having a thin circular diaphragm. Their diaphragms, 10 µm in thickness and 0.55 mm in diameter, were formed by an electrochemical etching method. Since the stability of pressure sensors is the most important requirement for precise pressure measurements, attractive approaches have been investigated to improve stability. The major instabilities of the present miniature pressure sensors are electrical drifts caused by leakage currents and thermal disturbances related to packaging stress. A shield and a guard plate can prevent the device from leakage currents. A thick supporting rim structure of the sensor and mounting on a stainless steel support with elastic material contribute to eliminate the packaging stress. For the purpose of easy lead attachment to the catheter-tip sensor, we use a unique structure having deep contact holes in deposited thick polysilicon layer (0.05 mm thick). Experimental results are as follows: Initial drift after power up was improved to about one tenth. Thermal disturbances, as temperature zero shift, thermal transient response, and temperature cycle hysteresis were greatly reduced. Low-temperature zero shift of 0.2 mmHg/°C was obtained using a simple temperature compensation method. Long-term drift was 0.6 mmHg/day. The catheter of 1.8-mm outer diameter having two sidewall sensors has been satisfactorily used for the study of urodynamics.

Integration of multi-microelectrode and interface circuits by silicon planar and three-dimensional fabrication technology

Abstract A multi-microelectrode for simultaneous recording of single-unit action potentials is a useful device for studying the organization and function of neural systems. We have fabricated a silicon probe which integrates a multi-microelectrode and interface circuits (preamplifier and analog switches) on a silicon chip by silicon planar and three-dimensional fabrication technology. This electrode has the following advantages: (1) it is easy to arrange the location of recording sites simply by changing photomasks; (2) crosstalk between the multi-electrodes can be reduced by the shielding effect of the silicon substrate; (3) integration of microelectrodes and preamplifiers on the same silicon chip eliminates the undesirable effects of stray lead capacitance; (4) analog swithces serve as selectors or multiplexers for a parallel to series conversion of multichannel signals of neural activities. In this paper, the design and fabrication processes of multi-microelectrode and interface circuits are described. In particular, the characteristics of polysilicon electrodes, design of a low-noise MOSFET for the preamplifier and silicon three-dimensional processes for the electrode probe are considered.

A Barium-Titanate-Ceramics Capacitive-Type EEG Electrode

A capacitive-type electrode using barium titanate ceramics is described. This electrode is not affected by the polarization potential. Because its surface is chemically inactive and mechanically strong, its noise voltage is minimum from the beginning of its installation. This device is especially suitable for EEG recording.

Stabilization of MISFET hydrogen sensors

Abstract Pd-gate MIS hydrogen sensors have serious drift problems and decreased sensitivity after long term operation. We have shown that the serious drift can be eliminated with Pd/Pt hydrogen sensing gate and Ag/Cu/Pd/Pt reference gate differential-type sensors. The decrease of sensitivity can be miniized by high temperature annealing in air. The decrease of sensitivity after 1000 hours of operation is less than 10%. It is considered that this decrease is due to a decrease of hydrogen adsorption sites on the Pd gates surface, which becomes oxidized during high temperature operation in air. This oxidation is observed with XPS and AES. The sensitivity abruptly decreases for operation at 100°C and below. This decrease is due to water adsorption on the Pd surface. H 2 O and OH peaks are observed in FTIR spectra for Pd powder in 1% hydrogen gas in synthetic air, and the solubility of Pd metal for hydrogen abruptly decreases because of water adsorption on the surface at 100 °C and below.