JP2009025124A - Electrode for isfet sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode for an ISFET sensor capable of achieving an increase in response speed and the stabilization of an indication value even when a non-aqueous solvent solution such as oil or the like is set as a measuring target and capable of easily and certainly washing off residue or a contaminant to stably keep measuring precision. <P>SOLUTION: The electrode 9 for the ISFET sensor is constituted by coating the surface of the gate part of a gate insulating type FET 4 with an ion-sensitive membrane 8 comprising tantalum pentoxide (Ta<SB>2</SB>O<SB>5</SB>) or the like. A porous titanium oxide (TiO<SB>2</SB>) is applied to and formed on at least the part corresponding to the gate part on the ion-sensitive membrane 8 of the electrode 9. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an electrode for an ISFET (ion sensitive FET) sensor using a gate insulating FET (field effect transistor), such as an ion sensor for measuring an ion concentration in a test liquid or a pH sensor for measuring pH. About.

As this type of ISFET sensor electrode, silicon nitride (Si ₂ N ₃ ) or tantalum pentoxide (Ta ₂ ) is used on the surface of the gate portion of the gate insulating FET instead of a glass electrode having high resistance and low mechanical strength. Oxide sensitive thin films containing inorganic substances including O ₅ ), alumina (Al ₂ O ₃ ), and the like are known (see, for example, Patent Document 1 and Patent Document 2).

Japanese Patent Publication No. 58-25221 Japanese Patent Laid-Open No. 9-178697

In the case of the conventional ISFET sensor electrode known in Patent Documents 1 and 2, the gate portion is not only useful as a microelectrode because it is superior in mechanical strength and has low resistance compared to a glass electrode. By covering the surface with an inorganic ion-sensitive thin film such as silicon nitride (Si ₂ N ₃ ), it can be used to measure various chemical properties such as pH, pNa, and pNH ₃ depending on the properties of the film. Is possible.

  However, in the case of the above-mentioned conventional ISFET sensor electrode, when a non-aqueous solvent solution such as oil or alcohol having a very small amount of water is used as a measurement target, it is easy to be charged due to friction between the solution and the ion-sensitive thin film on the surface of the electrode. An electric potential is generated, the response speed is slow due to this, and the indicated value is difficult to stabilize. In addition, when used for micro-measurement of enzyme activity, etc., the solution, especially protein, is likely to remain on the surface, and there is also the effect of dirt due to repeated use. As a result, there has been a problem that a decrease in measurement accuracy over time is unavoidable.

  The present invention has been made in view of the above circumstances, and its purpose is to improve the response speed and stabilize the indicated value even when a non-aqueous solvent solution such as oil or alcohol is used as a measurement object, and to provide a residue. It is an object of the present invention to provide an ISFET sensor electrode that can easily and reliably clean and maintain high measurement accuracy.

  In order to achieve the above object, an ISFET sensor electrode according to the present invention is for an ISFET sensor in which a gate portion surface of a gate insulating FET is coated with an ion sensitive thin film containing silicon nitride, tantalum pentoxide, and alumina. The electrode is characterized in that a porous titanium oxide film is coated on at least a portion corresponding to the gate portion on the ion-sensitive thin film.

According to the present invention in which the ion-sensitive thin film portion corresponding to at least the gate portion of the FET is coated with a titanium oxide (TiO ₂ ) film, the moisture content such as oil and alcohol is very small. Even when a non-aqueous solvent solution is a measurement object, a small amount of water can be agglomerated because the electrode surface is excellent in hydrophilicity with a small surface tension. The flow potential generated by the friction can be reduced, the response speed can be improved, and the indicated value can be stabilized. Moreover, even when it is used for micro-measurement of enzyme activity and the like, the self-cleaning function by the photocatalytic ability of the titanium oxide (TiO ₂ ) film can greatly reduce the residual adhesion of the solution, and some proteins such as enzymes Even if it remains, or even if it becomes dirty with repeated use, it can be easily and reliably removed by simple washing such as washing with water. Therefore, it is possible not only to expand the measurement target range of predetermined chemical physical properties such as ions and pH, including trace measurement such as non-aqueous solvent solution and enzyme activity, that is, not only can the application be expanded, There is an effect that contamination due to dirt is prevented, and high measurement accuracy can be stably maintained even in repeated use over a long period of time.

In the present invention, the porous titanium oxide (TiO ₂ ) film may be formed only in a portion corresponding to the surface of the gate portion. However, as described in claim 2, the porous titanium oxide (TiO ₂ ) is formed. ₂ ) It is desirable to coat the membrane over the entire surface of the ion sensitive thin film. In this case, it is easy to manufacture, that is, to form a porous titanium oxide (TiO ₂ ) film, and at the same time, the self-cleaning function is destroyed and reduced by the titanium oxide (TiO ₂ ) film as much as possible. The effect of maintaining accuracy can be further improved.

  In addition, the present invention may be applied to an ISFET sensor electrode having only a measurement FET. In particular, as described in claim 3, the gate-insulated FET is in contact with an object to be measured simultaneously. Two FETs, a measurement FET and a reference FET, arranged side by side on a single substrate so as to detect the difference between the detection voltages of the two, are provided with the ion sensitive thin film on the surface of the gate portion of the measurement FET. In addition, the surface of the gate portion of the reference FET is coated with a thin film having no ion sensitivity or low ion sensitivity, and at least on the ion sensitive thin film portion corresponding to the surface of the gate portion of the measurement FET. In addition, a porous titanium oxide film is coated, and on the thin film portion having low ion insensitivity or low ion sensitivity corresponding to at least the gate portion surface of the reference FET, It is desirable that the porous titanium oxide film is applied to what is covered.

  In this case, both the measurement electrode and the reference electrode use a gate insulation type FET to detect the detection voltage difference between them, for example, compared to a combination of metal electrodes such as a silver / silver chloride electrode, It is possible to further improve the measurement sensitivity and measurement accuracy by offsetting the drift characteristics of the potential over time, the sensitivity to various induced noises, and the temperature characteristics. A sufficient maintenance effect of measurement accuracy can be ensured.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 and FIG. 2 are a longitudinal sectional view of an essential part and an enlarged longitudinal sectional view of an essential part when applied to an electrode for an ISFET sensor having two FETs for measurement and a reference FET. The ISFET sensor 1 has two n-type (or p-type) regions 3 and 3 formed on a silicon (Si) wafer 2 which is a highly insulating one-chip substrate, and each of the n-type regions 3 and 3 has p. After forming the type (or n-type) channel FETs 4 and 4 and forming the aluminum wirings 5 and 6 between the respective drains and sources, the whole is covered with an oxide (insulating) film 7 such as SiO ₂ so that the same characteristics are obtained. The measuring ISFET electrode 9 and the reference ISFET electrode 10 are formed side by side on the one-chip silicon wafer 2.

Of the two FETs 4 and 4, ions such as tantalum pentoxide (Ta ₂ O ₅ ), silicon nitride (Si ₂ N ₃ ), and alumina (Al ₂ O ₃ ) are formed on the surface of the measurement ISFET electrode 9. The sensitive thin film 8A is deposited to form a film, and the surface of the gate portion of the reference ISFET electrode 10 is coated with a thin film 8B having low ion sensitivity, such as an organic monomolecular film or an organic film. ing. Further, a pseudo comparison electrode 11 made of, for example, a platinum (Pt) film is formed on a thin film corresponding to an intermediate position between the measurement ISFET electrode 9 and the reference ISFET electrode 10 arranged side by side on the silicon wafer 2. Has been.

A porous titanium oxide (TiO ₂ ) film 12A is formed on the ion-sensitive thin film 8A corresponding to the gate surface of the measurement ISFET electrode 9 by dip coating or CVD. On the thin film 8B corresponding to the surface of the gate portion of the reference ISFET electrode 10, the non-porous titanium oxide (TiO ₂ ) film 12B is formed by dip coating or CVD. The coating is formed by the method.

  FIG. 3 shows the configuration of an operation circuit example of the ISFET sensor 1 having the above-described structure. The measurement ISFET electrode 9 and the reference ISFET electrode 10 are configured to be driven by separate drive sources 13A and 13B. The operational amplifiers 14A and 14B for detecting the potential between the ISFET electrodes 9 and 10 and the pseudo comparison electrode 11 and the difference between the detected potentials of the operational amplifiers 14A and 14B are detected in a differential amplification manner. The differential amplifier 15, an AD converter (ADC) 16 that performs AD conversion on the output thereof, an arithmetic processing unit (MPU) 17 that performs arithmetic processing on both detected potential differences after AD conversion, and an ion concentration or pH that is the result of the arithmetic processing And a display unit 18 for outputting and displaying chemical characteristic values.

  The ISFET sensor 1 configured as described above is used so that the measurement ISFET electrode 9, each gate portion of the reference ISFET electrode 10, and the pseudo comparison electrode 11 are simultaneously in contact with a measurement target such as a test solution. . The potential between the electrodes 9 and 10 and the pseudo comparison electrode 11 is detected by the operational amplifiers 14A and 14B, and the difference between the detected potentials is detected by the differential amplifier 15. The output of the amplifier 15 (difference between both detection potentials) is digitized by the ADC 16 and taken into the MPU 17, and the chemical characteristic values such as ion concentration and pH are obtained by the arithmetic processing here and displayed on the display unit 18. It will be.

In the ISFET sensor 1 in which chemical property values such as ion concentration are measured and displayed in this way, the ion sensitive thin film 8A corresponding to the surface of the gate portion of the measurement ISFET electrode 9 and the gate portion of the reference ISFET electrode 10 are displayed. Since the thin film 8B corresponding to the surface has low ion sensitivity or low ion sensitivity is covered with the titanium oxide (TiO ₂ ) films 12A and 12B, a non-aqueous solvent solution with very little water such as oil or alcohol is used. When measuring, since the surface of the gate portion of the ISFET electrode 9 for measurement is excellent in hydrophilicity with a small surface tension, it is possible to agglomerate a minute amount of water, and is generated by friction between the solution and the ion sensitive thin film 8A. The streaming potential is very small, thereby improving the response speed and stabilizing the indicated value.

In addition, even when used for trace measurement of enzyme activity and the like, the self-cleaning function based on the photocatalytic ability of the titanium oxide (TiO ₂ ) films 12A and 12B can greatly reduce the residual adhesion of the solution as well as proteins such as enzymes. Even if it remains on the surface, or even if the surface becomes dirty with repeated use, it can be easily and reliably removed by simple washing such as washing with water. Therefore, it is possible not only to expand the measurement target range of predetermined chemical properties such as ions and pH, including trace measurements such as non-aqueous solvent solution and enzyme activity, that is, to expand the application, but also residue on the gate surface Contamination caused by dirt and dirt can be prevented, and high measurement accuracy can be stably maintained even during repeated use over a long period of time.

  In the above embodiment, the measurement ISFET electrode 9 and the reference ISFET electrode 10 having the same characteristics are applied to the electrodes 9 and 10 of the ISFET sensor 1 formed side by side on the one-chip silicon wafer 2. Although described above, the same effects as described above can be obtained even when applied to the measurement ISFET electrode (see FIG. 2) of the ISFET sensor having only the measurement ISFET electrode 9 and the comparison electrode.

  Further, in the above embodiment, the pseudo comparison electrode is used as the comparison electrode. Instead, a solid metal electrode such as gold or platinum may be used, and the measurement ISFET electrode 9 and the reference ISFET. The arrangement relationship of the electrodes 10 is not limited to the horizontal arrangement, and both the electrodes 9 and 10 may be arranged opposite to each other or arranged at an arbitrary angle.

It is a longitudinal cross-sectional view of the principal part at the time of applying to the ISFET sensor provided with two FET electrodes for a measurement, and a reference FET electrode as an ISFET sensor which concerns on this invention. It is an expansion vertical cross section of the electrode for ISFET sensors same as the above. It is a block diagram of the example of an operation circuit of an ISFET sensor same as the above.

Explanation of symbols

1 ISFET sensor 2 Silicon wafer (example of substrate)
8A Ion sensitive thin film 8B Ion insensitive or low ion sensitive thin film 9 ISFET electrode for measurement 10 ISFET electrode for reference 11 Pseudo comparison electrode (comparison electrode)
12A Porous titanium oxide (TiO ₂ ) film 12B Non-porous titanium oxide (TiO ₂ ) film

Claims (3)

In the electrode for ISFET sensor formed by coating the surface of the gate part of the gate insulating FET with an ion sensitive thin film containing silicon nitride, tantalum pentoxide, and alumina
An electrode for ISFET sensor, wherein a porous titanium oxide film is coated on at least a portion corresponding to the gate portion on the ion-sensitive thin film.   The ISFET sensor electrode according to claim 1, wherein the porous titanium oxide film is coated on the entire surface of the ion-sensitive thin film.   The gate-insulated FET comprises two measuring FETs and a reference FET, which are formed side by side on a single substrate so as to detect the difference in detection voltage between the two objects to be measured simultaneously. The surface of the gate part of the FET is coated with the ion sensitive thin film, and the surface of the gate part of the reference FET is coated with a thin film having no ion sensitivity or low ion sensitivity. A porous titanium oxide film is coated on at least the ion sensitive thin film portion corresponding to the surface of the gate portion, and the ion insensitivity or ion sensitivity corresponding to at least the surface of the gate portion of the reference FET. 3. The ISFET sensor electrode according to claim 1, wherein a non-porous titanium oxide film is coated on the low thin film portion.

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