JPS6129666B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of manufacturing a reference electrode made of an FET. Field-effect transistors (hereinafter referred to as FETs), which have a special sensitive film on their gates, are useful as sensors, and have already been used for hydrogen, potassium,
Ion sensors for sodium, calcium, etc., gas sensors for oxygen, hydrogen sulfide, etc., enzyme sensors, etc. have been 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 in applications that take advantage of these characteristics, and in particular, as medical sensors. However, in order to perform measurements using the above-mentioned FET sensor, a comparison electrode that exhibits a constant potential difference regardless of the liquid to be measured is absolutely necessary. As such reference electrodes, electrodes in which a sweet tooth electrode, a wrought silver chloride electrode, a mercury oxide electrode, etc. are immersed in an internal standard solution are usually used, and in this case, the internal solution is brought into contact with the liquid to be measured through a liquid junction. There is a need. 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 size of the reference electrode used at the same time is large, making it often impossible to measure minute objects or small containers. The development of a reference electrode has been desired. Recently, a few reference electrodes have been proposed that meet the above requirements. For example, the 30th Annual Conference on
J in Engineering in Medicine and Biology.
Instead of the silver-silver chloride electrode used in conventional reference electrodes, Janata proposes a reference electrode in which an FET sensor is placed in an internal standard solution and brought into contact with the test solution through a liquid junction. The electrode is a conventional reference electrode.
Although we succeeded in reducing the size to a certain extent by using FEE, there were limits to miniaturization 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 Laid-Open No. 52-26292 discloses the use of an FET sensitive to a certain concentration of a substance in a test liquid as a reference electrode. As an example, a comparison electrode has been proposed in which the gate surface 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, it has the disadvantage that it can only be applied to limited applications. The present inventors filed a patent application for a new FET reference electrode that eliminates the drawbacks of the reference electrode using the above-mentioned FET.
-150093 (Japanese Unexamined Patent Publication No. 51-81897).
The reference electrode is an FET reference electrode formed by covering at least the surface of the gate portion of a gate insulated field effect transistor with a hydrophobic organic polymer film. In the FET used as such a reference electrode, at least the surface of the gate part must be covered with a hydrophobic polymer film. Therefore, even if the lower layer of the film has a metal film, It may have the ion-sensitive layer disclosed in 2003, or it may have a hydrophobic organic polymer film coated on the two layers consisting of silicon oxide and silicon nitride or on the silicon oxide layer. Such a FET reference electrode can be made to be approximately the same size as the FET sensor, and
It has the advantage of being able to be manufactured on the same wafer as the FET sensor, which is very advantageous for making the sensor smaller. However, when the FET reference electrode formed on the gate surface using the above-mentioned polymer coating immersion method is immersed in the measurement solution, it temporarily shows an output corresponding to the potential of the solution, but after prolonged immersion in the aqueous solution,
Gradually, the potential drifts and you start to feel something like PH. This is thought to be due to water entering between the FET gate surface and the hydrophobic film due to long-term immersion, causing the film to peel off. However, the majority of applications for such FET sensors are for measurements in aqueous solutions, and many need to be stored in a wet state to prevent drift at the start of measurements, which
This is a fatal flaw for the FET reference electrode used at the same time as the FET sensor. The inventors of the present invention solved the shortcomings of conventional FET comparison electrodes, and as a result of intensive studies, developed a special plasma reaction device to provide an inexpensive FET comparison electrode that has excellent water resistance and provides stable measurements over a long period of time. They discovered that a highly water-resistant coating could be easily produced using a plasma polymerization method. That is, the present invention provides a method in which at least an electrode for applying a high frequency to the surface of a gate portion of a gate insulated field effect transistor is separated from a discharge atmosphere by an insulator,
Below the plasma discharge port generated in the discharge atmosphere, there is a reactive gas nozzle whose upper end is ring-shaped and has a large number of small holes for distributing the reactive gas inside the nozzle. Thickness can be measured using a plasma generator with a sample stand
This is a method for manufacturing a comparison electrode consisting of an FET in which a hydrophobic polymer film of 300 to 10,000 Å is formed by plasma polymerization. The FET used in the present invention is a gate insulated FET
Therefore, it is preferable that an insulating film such as SiO ₂ or Si ₃ N ₄ or a metal film is exposed on the gate surface. These FETs are sensitive to hydrogen ions, sodium, potassium ions, etc. depending on the properties of their top layer membrane. The specific form of such a FET sensor is disclosed in, for example, Japanese Patent Application No. 52-132579 (Japanese Unexamined Patent Publication No. 54
-66194). The present invention uses an afterglow plasma generation system in which a reactant gas outlet is provided above the sample stage at the gate of the FET, the upper end of which is ring-shaped, and which has many small holes that distribute the reactant gas inside. Coat a thin film using the device. Conventionally used plasma reaction devices of this type include those in which, for example, an inert gas and a reactive gas are introduced into a vacuum chamber and a chemical reaction is caused by discharge from a metal internal electrode. In the case of such an internal electrode method, if at least the metal surface of the electrode that applies high frequency among the electrodes that generate plasma is exposed in the plasma reactor, ions and electrons will be emitted from the electrode surface. Decreased uniformity of the film formed due to reasons such as
This also causes deterioration such as cracking or peeling of the produced film. This thing is FET
Changes in PH insensitivity and decreases in durability occur, which is the most important for reference electrodes consisting of Here, in order to maintain PH insensitivity and increase durability, a very high level of coating uniformity and stability is required, so at least high frequency waves of the plasma generator used in the present invention are applied. It is necessary that the electrode be separated from the discharge atmosphere by a material such as glass or ceramic. If the plasma generating device of the present invention is based on the internal electrode method, at least the electrode for applying high frequency must be coated with an insulator. Also, in this case, it is extremely desirable that the sample (FET) be placed between the electrodes. On the other hand, as long as the device is based on the external electrode method, either the coil induction type or the capacitance addition type can be used. However, if the sample (FET) on which the plasma-polymerized film is to be formed is located within both electrode sections, it will hinder the formation of a uniform and stable film, so it is desirable to use the afterglow method in an external electrode system. On the other hand, the apparatus of the present invention also includes a system in which only the ground side electrode of the external electrode system is in the plasma atmosphere, which can be said to be an intermediate between the two systems. What is required is that the electrode to which the high frequency is applied be insulated from the plasma atmosphere. FIG. 1 shows an example of a plasma generation device based on the external electrode method. In the figure, reference numeral 1 denotes a bell gear made of glass in the shape of a hanging bell and detachably mounted on a base plate 2.
This bell gear 1 has an inert gas inflow pipe 3 at its upper end.
is connected to an inert gas cylinder 4 such as nitrogen, helium, or argon. Further, a reactive gas supply pipe 5 is installed vertically through the base plate 2 and is switchably connected to several reactive gas cylinders 6 such as benzene and tetrafluoroethylene. Further, a vacuum suction pipe 7 is erected through the center of the base plate 2, and suction holes 8 are arranged around the middle part thereof, and the other end of this pipe is connected to a vacuum pump 10 through a cooling medium 9 such as liquid air. are connected. The top end of this vacuum suction tube 7 is closed and formed into a sample holder 11 . Electrodes 12 and 13 are attached in a ring shape inside the bell gear 1 and connected to a power source via an inductance matching 14. Further, the upper end of the reactant gas supply pipe 5 is formed into a ring shape, and a large number of small holes 15' are formed around the circumference in vertical and diagonal directions to form reactant gas jet ports 15. The neck of the bell gear 1 directly above this is formed into a narrowed nozzle 16. This invention is constructed as described above, and when an inert gas is fed into the bell gear 1 from the cylinder 4 through the inert gas inlet pipe 8 and passed between the electrodes 12 and 13, plasma is formed, and this plasma is transferred to the nozzle 16.
At the same time, a reactive gas (benzene, tetrafluoroethylene, etc.) is supplied from the cylinder 6 through the reactive gas supply pipe 5, and a reactive gas outlet 15 consisting of a large number of small holes in the ring-shaped portion at the upper end thereof The polymer is ejected from the plasma toward the plasma to form a polymer coating on the surface of a desired sample a placed on the sample mount 11 in advance. Conditions such as degree of vacuum, temperature, plasma intensity, polymerization time, carrier gas, etc. during plasma polymerization are not particularly defined, and are appropriately selected from conventional conditions. For example, using the above-mentioned apparatus, place the sample on sample stage a, evacuate to below 10 ^-3 Torr, and then reduce the partial pressure of argon and tetrafluoroethylene.
0.2 Torr, flow rate 2 c.c./min (STP), SWR correction voltage
A good polytetrafluoroethylene film can be obtained by performing plasma polymerization at 50W.
When the reaction gas is supplied here, if it is gaseous, it can be supplied as is, but if it is liquid or solid, it may be evaporated by heating. Of course, it is also possible to use a material that decomposes when heated to generate a reactive gas, such as Teflon, which generates a gas when heated. At this time, it is possible to coat only the necessary parts by masking the bonding part of the FET and the gate part of the FET, which is to be used as another sensor that does not want to be used as a reference electrode in the case of multiple sensors. . This is advantageous when making a reference electrode along with one or more sensors on the same chip. The monomer used to form the hydrophobic polymer film is preferably hydrophobic and insulating, and particularly preferably one that does not have an ion-dissociable group such as --COOH or --NH ₂ group. In the present invention, a hydrophobic membrane refers to a membrane having a contact angle with water of 65° or more at 35°C. Such monomers include paraffins such as ethane and propane, olefins such as ethylene, propylene butene and isobutene, vinyl compounds such as styrene, divinylbenzene, vinyl chloride, vinylidene chloride, tetrafluoroethylene and hexafluoropyrene, benzene, toluene, Examples include aromatic compounds such as xylene. These monomers may be used alone or in copolymerization. In particular, fluorine-containing monomers such as tetrafluoroethylene, chlorotrifluoroethylene, hexafluoropropylene, vinylidene fluoride, and various freons of trifluoroethylene exhibit a constant charge over a wide PH range and have chemical resistance to solvents. It is preferable because it is excellent in water and does not stain easily. These monomers may be copolymerized alone or with the above-mentioned monomers containing or not containing fluorine, but if the fluorine content in the resulting polymer film is 30
% or more, these effects become particularly significant. Note that after plasma polymerization, it is preferable to perform annealing or cooling in an inert gas or hydrogen gas in order to prevent the formation of ionic pressure-sensitive groups due to the reaction of residual radicals with air. In addition, the use of the pulsed discharge method allows the formation of films with excellent deterioration resistance, and the use of various methods to remove ions and electrons using electric fields, magnetic fields, etc. to prevent good film formation. This is preferable in that it can prevent deterioration due to impact. Further, before plasma polymerization, it is preferable to clean the surface with plasma of oxygen or the like in order to avoid formation of pinholes and deterioration of adhesive properties due to impurities.
The thickness of the film formed by polymerization of the monomer on the gate surface is preferably thin. However, if it is too thin, pinholes will easily form, so for practical purposes, the thickness is 300 to 10,000.
A thickness of Å is preferred. If the film is thicker than this
The mutual conductance of the FET is small, and the stability and ability to follow changes in the liquid potential and drain voltage are reduced. The thickness of the coating can be controlled by controlling the vapor pressure of the monomer, polymerization time, merging temperature, plasma strength, etc. Furthermore, if a film of 1000 Å or more is formed, the interference light can be seen, so the film thickness can be controlled while observing the color and intensity of the interference light. In order for the FET created in this way to be useful as a comparison electrode, it must have low sensitivity to ions, no change over time even when immersed for a long time, and no sensitivity to other ions. . 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, from the viewpoint of accuracy and when used in other ion sensors, the pH dependence of the reference electrode is preferably 5 mV/PH or less, and is very good if it is 1 mV/PH or less. In order to make a FET with such a low pH dependence using the plasma polymerization method, it is necessary to thoroughly wash the FET chip and handle it in a clean room, taking great care to prevent impurities and dust from getting on the FET gate surface. be. The reference electrode made of the FET thus obtained exhibits a constant output that is almost unaffected by the presence and concentration of pH, other ions, dyes, and surfactants. Therefore, by combining a pseudo reference electrode and a FET sensor, even if the potential of the pseudo reference electrode changes, it is possible to determine the concentration of the substance by taking the difference in output between the FET sensor and the reference electrode made up of FET. It can be done. 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 silicon nitride, and all measurements are performed at Id = 30 μA and V _DS = 3 V to measure V _GS . Example 1 After washing a PH-sensitive FET with trichlene, a polytetrafluoroethylene film was formed by plasma polymerization as follows. Namely, Proceedings of the 26th Polymer Symposium S3 F10
Using the equipment described in , the FET gate surface is brought into contact with tetrafluoroethylene activated by argon plasma afterglow, and the film thickness is
Films of 2000 Å and 5000 Å were formed. The thickness of the film was determined by measuring the simultaneously polymerized silicon nitride chip using an interference microscope. The contact angle between silicon nitride chips treated with tetrafluoroethylene and water was 105°. The pressure of alcon and tetrafluoroethylene during polymerization is
0.2 Tor (Piran vacuum gauge) flow rate 2 c.c./min (STP)
The SWR correction power was 50W. The FETs thus obtained were immersed in commercially available buffer solutions of PH4, PH7, and PH9 (each meeting JS standards), and the gate-source voltage (V _GS ) was measured. The results are shown in Table 1. As a comparative example, the results of an FET before such plasma coupling and a sample obtained by immersing the FET in a 2% polyvinylidene fluoride DMF solution are shown. From the table, the comparison electrode made of FET obtained by plasma polymerization clearly has excellent water resistance. 【table】

[Brief explanation of the drawing]

The drawings show an example of a plasma reaction apparatus used in the present invention, with FIG. 1 being an overall explanatory view, and FIG. 2 being an explanatory view of a reactant gas outlet. 1... Belgear, 3... Inert gas inflow pipe,
7... Vacuum suction tube, 8... Suction tube, 11... Sample placement stage, 12, 13... Electrode, 15... Reaction gas outlet, 15'... Small hole, 16... Nozzle.

Claims (1)

[Claims] 1. The electrode for applying high frequency is separated from the discharge atmosphere by an insulator, and below the plasma emission opening generated in the discharge atmosphere, a large number of small holes with a ring-shaped upper end and distributing the reactive gas inside the hole are provided. Using a plasma generator having a reactant gas outlet and a sample stage below the reactant gas outlet, a 300 mm thick film was deposited on the gate insulating film of a gate insulated field effect transistor using a plasma polymerization method. ~10000
A method for manufacturing a reference electrode consisting of an FET characterized by forming a hydrophobic polymer film of .