JPS6041304B2 - Manufacturing method of FET reference electrode - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing an FET reference electrode.

Field effect transistors (hereinafter referred to as FETs), which have a special sensitive film on the gate part, are useful as sensors, and there are already ion sensors for hydrogen, potassium, sodium, calcium, etc., gas sensors for oxygen, hydrogen sulfide, etc., and enzyme sensors. Proposed. These FET sensors are easy to miniaturize and be multi-sensored by arranging multiple sensors on the same chip, and their low output impedance simplifies the measurement circuit and makes it easy to insulate wiring. It has the following characteristics, and is expected to be used as a medical sensor, in particular, as an application that takes advantage of such characteristics. However, in order to carry out measurements using the above-mentioned FET sensor, a reference electrode that exhibits a constant potential level regardless of the liquid to be measured is absolutely necessary. Such reference electrodes are usually electrodes in which a sweet potato electrode, a silver chloride electrode, a mercury oxide electrode, or the like is immersed in an internal standard solution, and the internal solution must be brought into contact with the liquid to be measured through a liquid junction. However, if the liquid junction is made smaller in order to prevent the internal standard solution from flowing into the liquid to be measured, the resistance increases and the measured value becomes unstable. Therefore, it is necessary to maintain the liquid junction to a certain size or more, and it is difficult to avoid outflow of the internal standard solution from the liquid junction. Therefore, the amount of internal standard solution cannot be reduced too much in consideration of the outflow of the solution, and therefore the reference electrode cannot be made too small. For this reason, even if a very small FET sensor is used, the shape of the reference electrode used at the same time is large, so it is often impossible to measure minute objects or small containers. The development of a reference electrode has been desired. Recently responded to the above request 2
~3 reference electrodes were proposed.

For example, the 30th Annual Conference
Engineer in Medicine a
Elbow In the Biolo game, J. JaMta proposed a reference electrode in which an FET sensor was placed in the internal standard solution instead of the silver chloride electrode used in the conventional reference electrode, and it was brought into contact with the test solution through a liquid junction. There is. The electrode is connected to the secondary reference electrode by FE.
Although the size reduction was achieved to some extent by using T, there was a limit to the size reduction due to the use of an internal standard solution. Furthermore, it is extremely difficult to fabricate a microelectrode with such a structure. On the other hand, Japanese Patent No. 52-26292 discloses the use of an FET sensitive to a constant concentration of a substance in a test liquid as a reference electrode. As one example, a comparison electrode has been proposed in which the surface of the gate portion of an FET is coated with Ag-Agcl. The electrode is a sensor that selectively detects chloride ion concentration. The electrode for this purpose is used as a reference electrode in a living body where the chloride ion concentration is constant. However, since the potential of this electrode changes depending on the chlorine ion concentration,
The drawback is that it can only be applied to limited applications.
The present inventors have developed a new FET reference electrode that eliminates the drawbacks of the reference electrode using the above-mentioned FET.
No. 3 (Special Publication No. 58-25221). The reference electrode is an FET reference electrode in which at least the surface of the gate portion of a gate insulated field effect transistor is coated with a non-porous hydrophobic organic polymer film. In an FET used as a reference electrode, at least the surface of the gate part must be covered with a non-porous hydrophobic organic polymer film. 52-26
It may have the ion-sensitive layer shown in No. 292, or it may have two layers of silicon oxide and silicon oxide or a silicon oxide layer may be coated with a hydrophobic organic polymer film. Such a FET reference electrode can be made to be approximately the same size as the FET sensor, and has the advantage of being able to be made on the same wafer as the FET sensor, which is very advantageous for miniaturizing the sensor. be. However, when the FET reference electrode, in which the polymer coating is formed on the gate surface by the immersion method, is immersed in a measuring solution, it temporarily shows an output corresponding to the potential of the solution, but when immersed in the aqueous solution for a long time, The potential gradually drifts and becomes felt as PH, etc. It is thought that the cracking is due to water entering between the FET gate surface and the hydrophobic film due to long-term heating, and the film peeling off. However, the majority of applications for such FET sensors are for measurements in aqueous solutions, and many need to be stored in a humid state to prevent drift at the start of the measurement, which means that FET sensors can be used at the same time. F
This is a fatal drawback for ET reference electrodes. Such FE
In order to eliminate the drawbacks of the T comparison electrode, a comparison electrode in which a polyparaxylylene film is formed on the surface of the FET gate portion has been proposed. Although the reference electrode on which the film was formed was greatly improved in terms of water resistance, its long-term stability was insufficient, and the equipment and operation for forming the film were complicated. FET sensors with such a structure have the drawback of being extremely expensive and impractical. The present inventors have arrived at the present invention as a result of intensive studies in order to eliminate the drawbacks of conventional FET reference electrodes, provide an inexpensive FET reference electrode that has excellent water resistance and can perform stable measurements over a long period of time. be. That is, the present invention is a method for manufacturing an FET reference electrode, in which the surface of the gate part of a gate insulated field effect transistor is treated with silane, and then a nonionic polymer film is coated with a thickness of 300 to 5,000 △ by ultraviolet polymerization. be. The FET used in the present invention is a gate insulation type FET, and preferably has an insulating film such as Si02, Si3N4, etc. or a metal layer exposed on the gate surface.

These FETs are sensitive to hydrogen ions, sodium, potassium ions, etc. depending on the nature of the antinode of their top layer. The specific form of such an FET sensor is described in detail in, for example, Japanese Patent Application No. 13257/1983. In the method of the present invention, the gate of this FET is treated with silane and then coated with a thin film by ultraviolet polymerization. A specific method of ultraviolet polymerization is described in, for example, J. J. As detailed in ``Plastic Coating for Plastics'' by Licari (translated by Hideo Nagasaka, Danshobo, 1974), first, the silanized FET gate surface is thoroughly washed, and then 2.
The system is placed in a container that transmits ultraviolet rays of 000 to 3,000 A or has a window thereof, and after the system is evacuated to vacuum, a predetermined amount of nonionic monomer vapor is introduced and ultraviolet rays are irradiated. At this time, it is possible to coat only the necessary parts by masking the FETs that are to be used as bonding parts or other sensors that do not want to be used as reference electrodes, or by using an appropriate method to prevent exposure to light. This is particularly advantageous when making a reference electrode along with one or more sensors on the same chip. The monomer used to form the nonionic polymer membrane is preferably hydrophobic and insulating, and particularly preferably has a double bond.

Examples of such monomers include olefins such as ethylene, propylene, butene, and isobutene, styrene, divinylbenzene, acrylonitrile, acrolein, vinyl acetate, methyl methacrylate, butyl acrylate, vinylidene cyanide, vinylene carbonate, and those of the general formula (
However, X. ~X4 are atoms selected from the group consisting of hydrogen, chlorine, and fluorine, but not all of them are hydrogen atoms at the same time.

), halogenated ethylene such as vinyl chloride, vinylidene chloride, vinyl fluoride, tetrafluoroethylene, and tetrachloroethylene; An atom or atomic group selected from the group consisting of alkyl groups.

) butadiene, chloroprene, isof.

Examples include dienes such as ene, hexachlorobutadiene, and the like. A copolymer of these monomers may also be used. However, monomers having dissociative groups such as 1COO and 1NH2 are not preferable because they are sensitive to water and cause the potential to fluctuate depending on the ions in the measurement solution. Among the above monomers, halogenated ethylenes such as pinyl chloride, vinylidene chloride, and tetrafluoroethylene, and gens such as butadiene and hexachlorobutadiene, as well as copolymers of these, are preferred in terms of PI-1 dependence and membrane stability. Copolymers of these and other nonionic monomers in an amount not exceeding 50% by weight are preferred. Among them, general formula (1)
A homopolymer of a monomer represented by (0) or a copolymer of a plurality of monomers represented by the same formula is preferred.
The vapor pressure of these monomers is preferably 1 Hg to 100 Hg, and the thickness of the film formed by polymerization of the monomers on the gate surface is preferably thin.

However, if it is too thin, pinballs will easily form, so a thickness of 300 to 5,000 Å is preferred for practical purposes. If the film becomes thicker than this, the mutual conductance of the FET becomes smaller, and the stability and ability to follow changes in the liquid potential and drain voltage are reduced. The thickness of the film is determined by the vapor pressure of the monomer,
It can be controlled by polymerization time, polymerization temperature, and light intensity. Of these, the monomer vapor pressure is
g to 10 Hg is preferable, and the polymerization temperature is 20 to 500 g.
oC is preferred, particularly 100-300qC.
The polymerization time can take any value, but if the polymerization rate is too fast, the results may become uneven, so
It is preferable to polymerize over one feeding period. Preferred light sources include xenon lamps and mercury lamps, which can be used as they are, and can be filtered to cut out undesirable wavelengths if necessary. In this case, a filter that transmits light having a wavelength of at least 2,000 to 3,000 A must be used. Also, 1,0
When a film of 00A or higher is formed, interference light is visible, so the thickness can be controlled while observing the color and intensity of the interference light. In order to improve water resistance, it is absolutely necessary to treat the gate portion with an appropriate silane treatment agent before UV polymerization.

If this treatment is not performed, if the FET is immersed in an aqueous solution for a long time, the aqueous solution will easily enter between the FET gate surface and the polymer coating layer, and the sensitivity to ions will gradually decrease during long-term measurement or storage. It becomes large and becomes unsuitable as a reference electrode. When treated with such a silane treatment agent, even if the reference electrode has pinholes in the coating layer and has slight pH dependence immediately after manufacture, it will remain stable even after being immersed in an aqueous solution for a long time. Since its ion dependence does not change, it can be used as a comparison electrode. As such a silane treatment agent, one having a vinyl group is preferable, for example, CH2=CHSi(OR)2
, CH2=C(CH3)COOCH2CH2CH2Si
(OR)2 (where R represents an alkyl group), and the like. These treatment conditions may be carried out in an aqueous solution or in an organic solvent such as hexane or toluene. In either case, place the FET in the silanizing agent solution at room temperature to 10,000 ℃.
The gate section may be washed with soot after bonding. When using hydrogen, it is preferable to carry out the reaction in the presence of a suitable catalyst such as acetic acid.
Further, it is preferable to carry out heat treatment at 60 to 150°C for 2 to 10 hours after the completion of polymerization. In order for the FET created in this way to be useful as a reference electrode, it must have a sensitivity to ions of 4.0 mm, no change over time even after prolonged immersion, and no sensitivity to other ions. is necessary.

However, in order to use it as a reference electrode, the change in potential with respect to pH does not necessarily have to be zero. This is because pH is proportional to the difference in output between the pH sensor FET and the comparison electrode FET. However, considering the accuracy and the case of use in other ion sensors, the pH dependence of the reference electrode is preferably 8 hV/pH or less, and very good if it is 1 mV/pH or less. In order to create such a low pH-dependent FET using ultraviolet polymerization, it is necessary to thoroughly refine the FET chip and handle it in a clean room, taking great care to prevent impurities and dirt from getting onto the FET gate surface. It is. The FET reference electrode obtained in this way has a pH of
It exhibits a constant output that is almost unaffected by the presence and concentration of other ions, dyes, and surfactants.

Therefore, by combining a pseudo reference electrode and an FET sensor, even if the potential of the pseudo electrode changes, the FET
By measuring the difference between the outputs of the sensor and the FET reference electrode, the concentration of the desired substance can be determined. The present invention will be specifically explained below using Examples.

The FET sensor used in this example is a pH-sensitive FET whose gate sensitive film is made of boron nitride, and all measurements are performed using l
Vcs is measured at d=30AA and Vos=3V.
Examples 1 to 3 and Comparative Examples 1 to 5 After polishing the pH-sensitive FETs whose gates were treated with silane with trichlene, they were fixed in a dry quartz cell and degassed to 10-3 Hg using a vacuum line.

Next, monomer was introduced at a predetermined pressure, and irradiation was performed using an IKW canoshi lamp. At this time, the FET gate face faces the xenon lamp and is fixed to the wall on the lamp side. This irradiation continues until light due to interference becomes visible on a glass near the FET. The thickness of the film at this time is 1,000 to 2,000A. The FETs thus obtained were immersed in buffer solutions of PH4, PH7, and pH9, respectively, and the gate-source voltage (Vos) was measured. The results are shown in Table 1. As a comparative example, the results of the FET before such ultraviolet polymerization and the sample obtained by soaking it in a 2% THF solution of vinyl chloride and vinylidene chloride copolymer (2:8) were combined. show. As is clear from the table, the FET comparison electrode treated with silane before being treated with ultraviolet light has excellent water resistance.

Table-1,

Claims (1)

[Claims] 1. A method for manufacturing an FET reference electrode, which comprises treating the surface of the gate portion of a gate insulated field effect transistor with silane and then coating the surface with a nonionic polymer film to a thickness of 300 to 5,000 Å using an ultraviolet polymerization method.