Ion sensors and their divided parts
TECHNICAL FIELD 
     The present invention relates to an ion sensor having an improved ion sensitive portion and its divided part. 
     BACKGROUND OF THE INVENTION 
     Ion sensors are designed to detect chemical substances with the use of a membrane having a cyclic compound called the ionophore or ion exchange resin fixed thereon. Such sensors detect chemical substances with excellent specificity. As one example, there is available an ion sensor for the measurement of the concentration of ions in a sample under examination, which uses an ion sensitive field-effect type transistor (hereinafter often abbreviated as ISFET). 
     The ISFET makes use of a conductivity change in the vicinity of a semiconductor surface, which occurs depending upon a field change on the interface of a solution and a sensitive membrane provided on the surface of that semiconductor. 
     Such ISFET&#39;s use of ion sensitive membrane formed on an insulating film on the gate of a field-effect type transistor (hereinafter often abbreviated as FET). There is also known an extended gate type ISFET of a structure where the ion sensitive membrane is isolated from the FET on the same semiconductor substrate. 
     As the ion sensitive membranes for these ISFET&#39;s , there are known hydrogen ion sensitive membranes formed of an inorganic material such as silicon nitride, aluminum oxide or indium oxide. Alternatively, use is made of an ion sensitive membrane formed of an organic material such as, for instance, a polyvinyl chloride membrane carrying thereon an active substance--called an ionophore--which selectively entrains ions. For instance, potassium and sodium ion sensitive membranes may be formed by using as the ionophores valinomycin and bis-(12-crown-4), respectively. 
     Just after preparation, an electrode coated with an ion sensitive membrane needs to be immersed and stored in an aqueous ion solution of the type to be examined. This is necessary to insure the stable operation of the ion sensitive membrane. Unfortunately, however, a problem may develop in that the polyvinyl chloride membrane does not adhere well to the surface of the electrode. This often causes the ion sensitive membrane to peel. Alternatively, vesicles or blisters may form along the interface between the membrane and the surface on which it is coated. Either of these problems serves to degrade the overall characteristics of the ion sensor. This is especially true of an ion sensor of the extended gate type ISFET structure where the gate electrode of the FET is extended to isolate the ion sensitive membrane from the FET portion through an extended connecting line. Where peeling or blisters occur, the floating capacity of the gate electrode is increased, resulting in decreases in its response rate. 
     In addition to the problems with the aforesaid ion sensitive membranes, there has been another problem with gate electrodes. 
     Namely, a metal such as silver or gold or an oxide such as IrO 2  or SnO 2  have heretobefore been used for an extended gate formed by extending the gate of the FET. With such an extended gate ISFET, it is virtually impossible to obtain a certain or higher current, e.g., on the order of micro-amperes due to the fact that a potential occurring on the ion sensitive membrane depends upon polarization. For that reason, there is a need of using a transducer having a high input impedance, while a floating capacity from the ion sensitive membrane to the gate electrode correlates with response rates. The latter point is of no substantial significance in the case of mounting the ion sensitive membrane on the gate insulating film but, in the case of the structure where the ion sensitive membrane is separated from the FET portion through a long connecting line, offers a problem that the floating capacity is too increased to decrease the response rates. 
     SUMMARY OF THE INVENTION 
     A first object of the present invention is to provide an ion sensor which includes an ion sensitive membrane showing an improved adhesiveness to electrodes, inter alia, an extended gate electrode and can be uniformly formed as well as a divided part including an extended gate having such an ion sensitive membrane. 
     A second object of the present invention is to provide an ion sensor which can obtain an increased current from potential occurring on an ion sensitive membrane and respond more rapidly even to a structure wherein a gate electrode portion having an ion sensitive membrane formed thereon is isolated from an FET portion through an extended connecting line as well as a divided part including such an extended gate electrode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view showing a main part of a first embodiment of the ion sensor according to the present invention; 
     FIG. 2 is a sectional view taken along the line II--II of FIG. 1; 
     FIG. 3 is a measuring circuit diagram for the embodiments; 
     FIG. 4 is a sectional view corresponding to FIG. 2, showing a main part of a second embodiment of the ion sensor according to the present invention; 
     FIG. 5 is a diagram illustrating the results of measurement obtained with such an ion sensor; 
     FIG. 6 is a diagram illustrating the results of measurement obtained with a conventional ion sensor for the purpose of comparison; 
     FIG. 7 is a plan view showing an ion sensing chip that is a divided part for an ion sensor used in a third embodiment of the present invention; 
     FIG. 8 is a plan view showing a circuit board; 
     FIG. 9 is a plan view showing the circuit board into which the ion sensing chip is incorporated; 
     FIG. 10 is a plan view showing an ion sensing plate of a divided part for an ion sensor used in a fourth embodiment of the present invention; 
     FIG. 11 is a measuring circuit diagram used with this ion sensing plate; 
     FIG. 12 is a perspective view of a fifth embodiment of the ion sensor according to the present invention; and 
     FIG. 13 is a divided part for that ion sensor. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     For the sensitive membrane for the ISFET for which an organic material is used, it is preferred to use a vinyl polymer based compound containing a hydroxyl and/or carboxyl group so as to improve its adhesiveness and uniformity with respect to an electrode. 
     The vinyl polymer based compound containing a hydroxyl and/or carboxyl group may either contain both the hydroxyl and carboxyl (inclusive of an anhydrous carboxylic acid) groups, or be a mixture of the polymer compounds containing them separately. Further, use may be made of polyvinyl chloride, vinyl chloride/vinyl acetate copolymers and vinyl polymer based compounds obtained by the copolymerization of monomers other than hydroxyl and carboxyl groups- More specifically, use may be made of copolymers of vinyl chloride/vinyl acetate/vinyl alcohol, partly saponified products of vinyl chloride/vinyl acetate, copolymers obtained as a result of reaction with unsaturated carboxylic acids being reacted such as copolymers of vinyl chloride/vinyl acetate/maleic acid (or its anhydride) and so on. These vinyl polymer based compounds may further be copolymerized with other monomers. 
     The aforesaid vinyl polymer based compounds may also be used together with additives such as plasticizers, e.g., a plasticizer for vinyl resins. 
     In concrete embodiments, the vinyl polymer based compounds containing a hydroxyl and/carboxyl group are used as the component of ion sensitive membranes in the following manner. 
     By soldering, an FET 5 is first connected and fixed to a source electrode 2, a drain electrode 3 and a gate electrode 4 formed on a glass substrate 1, as illustrated in FIGS. 1 and 2. The gate electrode 4 is then extended to form an extended gate electrode 4a on its extreme end, which is in turn provided there around with a bank body 6 comprising an insulating material such as an epoxy resin, thereby forming a window opening 7 through which a sample under examination is to be added dropwise. Finally, the extended gate electrode 4a facing to the window opening 7 is provided thereon with the following ion sensitive membranes 8. 
     (a) Prepared first was a solution of 0.2 g of polymer powders (#1000 GKT manufactured by Denka, Co., Ltd.) composed of vinyl-chloride/vinyl acetate/vinyl alcohol in a compositional proportion of 91:3:6 and having an average degree of polymerization of 420, 0.25 ml of dioctyl adipate and 2.5 mg of valinomycin in 3 ml of tetrahydrofuran. Ten (10) μl of this solution were added dropwise to the aforesaid electrode 4a for an extended gate and air-dried to form a potassium sensitive film as an ion sensitive membrane 8 having a thickness of about 80 μm. 
     (b) a potassium ion sensitive film was formed as an ion sensitive membrane 8 in a similar manner as stated in (a), except that polymer powders (#1000 CK manufactured by Denka, Co., Ltd.) composed of vinyl chloride/vinyl acetate/maleic acid in a compositional proportion of 86:13:1 and having an average degree of polymerization of 420 was used in place of the polymer powders used in (a). 
     With each of the ion sensor parts provided thereon with these ion sensitive membranes and shown in FIG. 2, a potential on the surface of the potassium ion sensitive membrane was measured by a source follower circuit 10, shown in FIG. 3, using KCl-saturated Ag/AgCl as a reference electrode 11, thereby determining the sensitivity of the potassium ion sensitive membrane at varied potassium ion concentrations of solutions. 
     The sensitivity of the ion sensitive membrane was measured just after its preparation as well as after its 7 and 14-day immersion and storage in 10 mM of KCl. The results are set forth in a table to be described later. 
     Further, whether or not vesicles or blisters were formed between the potassium ion sensitive membrane and the extended gate electrode 4a was visually observed after the lapse of 7 and 14 days. The results are set forth in the same table in which X and O denote the presence and absence of vesicles, respectively. 
     For the purpose of comparison, similar tests as mentioned above were carried out with the aforesaid gate electrode 4a on which an ion sensitive membrane, prepared in a similar manner as mentioned in (a) except that 0.2 g of a vinyl chloride resin manufactured by Aldrich, Co., Ltd. was used for the polymer powders used in (a), was formed in place of the aforesaid ion sensitivity membranes 8. The results are set forth in the following table. T1 -Ion? Sensitivity (mV/pK)? Presence of Vesicles? -Sensitive? Date of? ? ? Date of? ? ? -Mem-? Prepara-? After? After? Prepara-? After? After? -branes? tion? 7 days? 14 days? tion? 7 days? 14 days? -a 52(100) 52(100) 52(100) O O O -(b) 49(100) 49(100) 49(100) O O O -Comp. 52(100) 49(100) 40(77) ? O X X -ex.? - 
     It is noted that the racketed figures indicate the percent (%) with respect to the sensitivity of the ion sensitive membranes on the day on which they were prepared. 
     From the results set forth in the table, it is found that the ion sensitive membranes according to (a) and (b) adhere more closely to the extended gate electrodes, but the adhesion of the comparative membrane is weak (note presence of vesicles after 7 and 14 days). 
     It is appreciated that the composition of the ion sensitive membranes is variable by optimization. 
     While the foregoing has referred to the ion sensitive membranes, it is preferred that the electrode on which they are to be formed be either of a double-layer structure comprising two layers composed mainly of silver chloride and silver, respectively, or contain silver and silver chloride. If this is the case, it is then considered that the following reaction takes place between silver and silver chloride. 
     
         AgCl+e.sup.-⃡Ag+Cl 
    
     On the surface of the electrode, electrons (e - ) enter or leave directly the silver electrode depending upon the concentration of Cl -   ions. When a sensitive membrane for, e.g., potassium or sodium ions is used as the ion sensitive membrane, therefore, the concentration of Cl -  ions fluctuates in correlation with the concentration of these K+ and Na --   ions. As a result, the aforesaid chemical reaction takes place to allow electrons to enter or leave the silver electrode. When a vinyl chloride film is used as the ion sensitive membrane in this case, moisture is allowed to be present on the surface of the AgCl electrode due to its slight water permeation, thus permitting the aforesaid chemical reaction to proceed easily. 
     The present invention will be now explained in more detail with reference to another embodiment. 
     As illustrated in FIG. 4 corresponding to FIG. 2, a copper foil 21a is etched on a glass fiber reinforced epoxy resin substrate 21, and patterning for the formation of a circuit pattern is carried out to form a source electrode 22, a drain electrode 23 (not illustrated) and a gate electrode 24 on the substrate 21. This gate electrode is then extended to form an extended gate electrode 24a. Subsequently, the copper foil is silvered on the surface with a plating solution comprising silver and potassium cyanides. Further, the portion of the extended gate electrode 24a is treated at the surface in 0.1N HCl to form a silver chloride film 24b thereon. 
     Afterwards, a band body 26 comprising an insulating material such as an epoxy resin is provided to form a window opening 27 through which a sample under examination is to be added dropwise, and an FET 25 is soldered to the source electrode 22, drain electrode 23 and gate electrode 24. 
     Finally, a potassium ion sensitive film 28 was formed on the aforesaid extended gate 24a as the ion sensitive membrane in a similar manner as explained for the formation of the aforesaid ion sensitive membrane (a). It is noted that reference numeral 29 stands for an insulating film for an exposed portion of the gate electrode 24. 
     The thus obtained assembly may be combined with a separate reference electrode and used as a ion sensor. This is an example of the completely extended gate FET type structure. 
     With the thus obtained ion sensor part, a potential occurring on the surface of the ion sensitive membrane is measured by a source follower circuit 10, shown in FIG. 3, using KCl-saturated Ag/AgCl as a reference electrode 11, while varying alternately the potassium ion concentration of the solution between 20 mM and 200 mM, and was recorded on a recorder The results are shown in FIG. 5. 
     For the purpose of comparison, an ion sensor as shown in FIG. 4 was prepared in a similar manner as mentioned above, except that the portion of the aforesaid extended gate electrode 24a was not treated on the surface with silver chloride, and was used in combination with the aforesaid comparison electrode for similar measurement as already mentioned. The results are illustrated in FIG. 6. 
     From FIGS. 5 and 6, it was found that the ion sensors having their extended gates treated with silver chloride according to the example of the present invention show no substantial drift (e.g., on the order of 1 mV/min. or lower) while they are immersed in the solutions having the respective potassium ion concentrations, whereas the ion sensor having its extended gate untreated with silver chloride according to the comparison example shows a drift of 4 mV/min. This indicates that the comparative ion sensor is poor in its response rate. 
     While the foregoing has referred to the double layer structure comprising two layers composed mainly of silver chloride and silver, respectively, a mixed layer of silver with silver chloride may be used. Such layers may be provided directly on a substrate, not on a copper foil. It is understood that the wording &#34;composed mainly of silver chloride and silver&#34; includes pure silver chloride and silver. 
     For the silver chloride treatment of an electrode of the double-layer structure comprising two layers composed mainly of silver chloride and silver, respectively, or an electrode containing silver chloride and silver, a silver electrode may be first prepared by plating, vapor deposition, sputtering or like other means and, then, treated with NaCl, FeCl 3  and so on. 
     While the foregoing embodiments have referred to the potassium ion sensors using valinomycin, other ions may be detected with similar effects by the replacement of ion sensitive substances. For instance, a sodium ion sensor may be prepared by using bis-12-crown-4 as an ion sensitive substance. 
     The ion sensitive membranes containing a vinyl polymer based compound having a hydroxy and/or carboxyl group and the electrodes of the double-layer structure comprising two layers composed mainly of silver chloride and silver or the electrodes containing silver chloride and silver according to the present invention may be used for the ion sensors of the structure where said membranes and/or said electrodes are formed independently or in combination on the gate insulating (preferably water-resistant) films of the FET&#39;s . They are also applicable to the ion sensors of the extended gate type FET structure where gates are extended and separated from the FET&#39;s main bodies and reference electrodes are incorporated on the same substrates. That is, as illustrated in FIG. 7, an extended gate electrode 32 and a reference electrode 33 are formed on a glass substrate 31, and a window opening 35, which is not covered with a resin 34 and through which a sample under examination is to be added dropwise, is formed. In this manner, an ion sensing chip 36 is prepared. 
     As illustrated in FIG. 8, on the other hand, reference, source, drain and gate electrode pieces 39, 40, 41 and 42 are formed on the substrate 38, and are connected to said ion sensing chip 36, as shown in FIG. 9. It is noted that reference numeral 37 is an FET. In a similar manner as mentioned above, the ion sensitive membranes and electrodes according to the present invention may further be applied to the ion sensitive membrane sensor plates which are used in combination with the FET&#39;s and in which only electrode portions to come in contact with samples under examination are separated, and the like. That is, as illustrated in FIG. 10, a reference electrode 52 and an extended gate electrode 53 are formed on a alumina substrate 51, and is then surrounded with a bank body 54 comprising a glass layer to form a window opening 55 through which a sample under examination is to be added dropwise. For use, the thus obtained assembly is connected, to leads 55a and 55 for the extended gate and reference electrodes of a circuit 55 shown in FIG. 11. It is noted that reference numeral 56 is an FET. 
     According to the embodiments as described above, it is possible to measure- only one component contained in sample liquids. As illustrated in FIGS. 12 and 13, however, a similar circuit board 33 as illustrated in FIG. 8 may be radially provided thereon with five sets of lead pieces for reference, source, drain and gate electrodes similar to those shown in FIG. 8, and the lead pieces of each set may be connected together in a similar manner as shown in FIG. 9 and provided with FET&#39;s 34&#39;-1, 34&#39;-2, 34&#39;-3, 34&#39;-4 and 34&#39;-5. Then, extended gate pieces (extended gate electrodes) 32&#39;-1, 32&#39;-2, 32&#39;-3, 32&#39;-4 and 32&#39;-5 are provided on a glass substrate 21&#39; with the one ends being centrically concentrated, as shown in FIG. 12, and are soldered to the lead pieces for gate electrodes of the FET&#39;s . These extended gate pieces are provided thereon with a bank body 26&#39; of an epoxy resin to form a window opening 25&#39; through which a sample is to be added dropwise. Further, the extended gate pieces are provided with ion sensitive membranes 24&#39;-1, 24&#39;-1, 24&#39;-3, 24&#39;-4 and 24&#39;-5 carrying thereon such different ionophores as mentioned above. 
     It is noted that reference numeral 23&#39; stands for a reference electrode having its end located in the window opening 25&#39;. It is also noted that reference numerals 30&#39;-2 to 30&#39;-5 indicate source electrode terminals, 31&#39;-1 to 31&#39;-5 drain electrode terminals and 29&#39; a reference electrode terminal. Connecting wires between the FET&#39;s and the reference electrode are coated on their surfaces with an insulating film, although not illustrated. 
     In the foregoing embodiment, the FET&#39;s and the window opening through which a sample is to be added dropwise are formed on the same substrate. However, the assembly of FIG. 13 may be used as a divided part 40&#39;, which is in turn connected to the associated parts, as illustrated in FIG. 12. In that case, the divided part 40&#39; may be connected to the FET&#39;s and reference electrode terminal by insertion or like means. 
     Such a divided part may be replaced separately from the FET&#39;s. 
     In summary, according to the present invention, a vinyl polymer based compound having a hydroxyl and/or carboxyl group is incorporated into an ion sensitive membrane. The adhesiveness of the ion sensitive membrane to an extended gate electrode is thereby improved. Consequently, peeling and the occurrence of vesicles is avoided and the performance of the ion sensitive membrane is stabilized. 
     Further, since at least a surface layer portion of the extended gate electrode is comprised of a multi-layered structure consisting of two layers composed mainly of silver chloride and silver, respectively, or a layer containing silver chloride and silver, it is possible to provide an ion sensor which can obtain an increased current on the basis of a sensitivity value determined by the ion sensitive membrane formed on the extended gate, and reduce its floating capacity to increase its response rate.