Switching device and method of preparing it

Abstract: A swiching device comprises a pair of electrodes (2,5), an organic insulating layer (3) and an inorganic oxide layer (4) which are held between the electrodes. The organic insulating layer is comprised of an organic compound having at least one π-electron level and the inorganic oxide layer is comprised of a metal oxide. Another swiching device comprises a pair of electrodes (2,5), an organic insulating thin film (3) provided between the electrodes and said organic insulating thin film in such a manner that an electrification region between the electrodes may be restricted.

Description: The present invention relates to a switching device with an MIM (metal-insulator-metal) structure, comprising an organic insulating layer, and a method of preparing it. More particularly it relates to a switching device greatly improved in the repetition stability of switching characteristics and the switching speed, and a method of preparing it. Recently, there has been an increasing interest in molecular electronics in which it is attempted to apply functionality of organic molecules to electronic devices, etc., and studies on the Langmuir-Blodgett film (LB film) which may be deemed to be one of the constructive techniques for molecular electronic devices have been made actively. The LB film is formed by laminating organic molecule layers regularly one by one, and the film thickness can be controlled at a level of molecular length. Since a uniform and homogeneous ultra-thin film can therefore be formed, it has been frequently attempted to use this as an insulating film. For example, there are a tunnel bonding element with metal-insulator-metal (MIM) structure [G.L. Larkins et al., "Thin Solid Films", Vol. 99 (1983)]; emission element with metal-insulator-semiconductor (MIS) structure [G.G. Roberts et al., "Electronics Letters", Vol. 20, pp.489 (1984)]; and switching element [N.J. Thomas et al., "Electronics Letters", Vol. 20, pp. 838 (1984)]. It is known to employ thin polymeric insulating film in metal-insulator-metal devices. United States Patent No. 3,519,999 describes such a device having a polymeric thin film formed in a thickness ranging between 10 nm (100Å) and 30 nm (300 Å) and with a molecular structure containing a high ratio of π-electrons to monomer units. Such films have been deposited by low energy glow discharge. MIM devices comprising metal oxide film and insulating film are known from United States Patent No. 4,534,623. In the one example described the device comprises a tantalum metal oxide film 50 nm (500Å) thick and an insulating film (of unspecified material) 340 nm (3400Å) thick. Also described in this specification are single layer devices in which the insulator is selected from one of the inorganic materials Si0₂, Si₃N₄, Al₂0₃ or an organic material, a polyamide. Although devices having satisfactory characteristics have been made, problems remain in reproducibility, stability, variance in the characteristics of each element, and change with lapse of time. Electrodes used in the studies mentioned above have hitherto been formed by vapour deposition using a mask. In instances in which electrodes are formed by vapour deposition using a mask, areas and shapes of electrodes cannot be selected freely, and they are necessarily limited, causing problems, e.g., in preparing integrated circuits. On the other hand, if it is possible to freely select the areas and shapes of electrodes, an improvement would be expected in device characteristics or device-driving methods. The present invention is intended to provide a switching device having improved repetition stability and storage stability. In accordance with the present invention there is provided a switching device comprising a pair of electrodes; and an organic insulating layer which is located between said pair of electrodes and is adjacent to one electrode thereof, which organic insulating layer is comprised of an organic compound having at least one π-electron level and is of a thickness in a range between 0.3 nm(3Å) and 100 nm (1000Å) inclusive; which switching device is characterised by: an inorganic insulating layer of inorganic oxide material located between said organic insulating layer and the other one electrode of the pair of electrodes. The present invention also provides a method of preparing a switching device, comprising the steps of forming an organic insulating thin film by the Langmuir-Blodgett process, and forming by lithography on said organic insulating thin film an inorganic oxide insulating thin film in the form of a pattern. In the accompanying drawings: Fig. 1 is a cross section of a switching device according to an embodiment of the present invention; Fig. 2 illustrates another embodiment of the present invention; Fig. 3 illustrates a measuring circuit used for measuring electrical characteristics of the switching device of the present invention; Fig. 4 is a characteristic diagram to show electrical characteristics (V/I characteristics) of the switching device of the present invention; and Fig. 5 shows electrical characteristics in the on state and off state, confirmed in the switching device of the present invention. To facilitate an understanding of the present invention, preferred embodiments thereof will now be described. The description that follows is given by way of example only. The studies as previously mentioned have hitherto been focused on LB (Langmuir-Blodgette) films of fatty acids, which are relatively easy to handle. Recently, however, even with respect to their thermal resistance and mechanical strength hitherto considered to be poor, organic mmaterials having overcome these are being created one after another. In fact, with respect to the LB films in which such materials are used, the present inventors have prepared a device with the sandwich structure comprising an insulating layer held at its both sides by conductive materials such as metals (commonly called an MIM structure or MIM device in view of its constitution) to observe and measure the physical properties of materials and the characteristics including electrical characteristics and, as a result, have already found a quite new switching phenomenon in electrical conduction. In regard to MIM devices that exhibit such a quite new switching phenomenon, the inventors made further detailed studies on the constitution of the above MIM device in an attempt to lessen the dispersion of its electrical characteristics or improve its stability, and, as a result, completed a switching device having a memory function with greatly superior reliability. In a first embodiment of the present invention, as shown in figure 1, an organic insulating thin film layer containing an organic compound having at least one π-electron level, and an inorganic oxide thin film layer comprising a metal oxide or silicon oxide Si0₂ are laminated. In such a device non-linear current-voltage characteristics which are different from those of conventionally known MIM devices are exhibited by supplying an electric current in the direction parallel to the lamination direction of the organic insulating layer and inorganic oxide layer. A MIM device having a novel switching memory function utilising such characteristics has been realised. A simple constitution of the switching device of the present invention is illustrated in Fig. 1. In the organic insulating layer 3 of the present invention, not only insulating organic compounds but also semiconductive organic compounds can be used so long as they are organic compounds having a π-electron level. Since, in general, almost all organic materials show insulating or semi-insulating properties, the organic materials having a π-electron level, applicable thus to the present invention, including a great variety of materials. Colouring matter structures of a π-electron type suited to the present invention include, for example, colouring matters having a porphyrin structure, such as phthalocyanine and tetraphenylporphyrin; azulene dyes having a squarilium group and a croconic methine group as bonding chains; and cyanine-analogous dyes formed by combining two nitrogen-containing heterocyclic rings such as quinoline, benzothiazole and benzoxazole, with each other through a squarilium group and a croconic methine group; or cyanine dyes; consensed polycyclic aromatics such as anthracene and pyrene; derivatives of tetraquinodimethane or tetrathiafulvalene, as well as analogues thereof and charge transfer complexes thereof; and further metal complex compounds such as ferrocene and a trisbipyridine ruthenium complex. In addition to the low molecular compounds set forth above, it is also possible to use polymeric compounds having π-electrons, such as polyimide, polystyrene, polydiacetylene and polythiophene. Among these, in forming the electrodes described above according to the lithography technique described later, polyimide is particularly preferred because of its excellent thermal resistance and solvent resistance. Suitable polyimides include, for example polyimides obtained by cyclization of polyamic acids, polyamic acid salts or polyamic acid esters, and suitably used are those obtained by cyclization and imidization of a compound having the repeating unit represented by Formula (1) and having a weight-average molecular weight of 20,000 to 3,000,000. In the formula, R₁ represents a tetravalent group containing at least 6 carbon atoms, specifically including, for example, the following: R₂ represents a divalent group containing at least 2 carbon atoms, specifically including, for example, the following: R₃ represents a monovalent group having at least 6 carbon atoms, or a quaternary ammonium salt having at least 6 carbon atoms, having the structure represented by Formula (2). Formula (2) CH₃(CH₂) ℓ -X ℓ = 11 to 21 X = -O-, -N ⊕ (CH₃)₃, -N ⊕ (C₂H₅)₃ It is further possible to use the above organic materials having π-electrons, in combination with other materials. In that instance, if at least one compound has a π-electron level, the remaining compounds may not necessarily have a π-electron level, and it is also possible to use low molecular compounds such as simple fatty acids and lipids, polyacrylic acid derivatives such as polymethacrylic acid, and polymeric compounds such as nylon. Such combination of compounds is carried out mainly for the purpose of increasing stability of the organic compounds having the π-electron level, which lack structural stability. In this case, the both (i.e., the organic compound of a π-electron type and the other mixed compound) may preferably be mixed in the molar ratio ranging from 100:1 to 1:100 (in terms of monomers), and more preferably from 10:1 to 1:10 (in terms of monomers). The organic insulating layer can be formed according to, for example, a vapour deposition process or a cluster-ion beam process. In the present invention, the organic insulating layer 3 has a thickness of not less 0.3 nm (3Å) to not more than 100nm (1,000 Å) and preferably not less than 5 nm 50 Å ) not more than 30 nm (300 Å), and moreover is preferably uniform and homogeneous. As a film-forming technique that satisfies such requirements, the LB process is very suitable among conventional known techniques, from the viewpoints of the controllability, readiness, and reproducibility. The LB process readily can form on a substrate a monomolecular film of the organic compound having a hydrophobic part and a hydrophilic part in its molecule, or a built-up film thereof, and also can stably provide an organic ultra-thin film which has a thickness of the molecular order and is uniform and homogeneous over a large area. The LB process is a process of making a monomolecular film or a built-up film thereof by utilizing the mechanism that when the hydrophilic part and hydrophobic part are appropriately balanced (i,e, amphiphatic balance) in a molecule of the structure having the both parts therein, the molecules form a monomolecular layer by utilizing a phenomenon that hydrophilic groups are aligned downward. The group that constitutes the hydrophobic part includes all sorts of hydrophobic groups such as saturated or unsaturated hydrocarbon groups, condensed polycyclic aromatic groups and chain polycyclic phenyl groups, which are commonly well known in the art. These constitute the hydrophobic part alone or in combination of plural ones. On the other hand, the group most typical as a component for constituting the hydrophilic part may include hydrophilic groups as exemplified by a carboxyl group, an ester group, an acid amido group, an imido group, a hydroxyl group, and further an amino group (primary, secondary, tertiary, and quaternary). These each also constitute the hydrophilic part alone or in combination of plural ones. Colouring matter molecules having these hydrophobic part and hydrophilic part in a good balance and also having a π-electron level with an appropriate magnitude can form a monomolecular film on a water surface, and can serve as materials very suited to the present invention. In the instance where the organic insulating layer 3 is formed using a material comprising the mixture of two or more organic compounds, if at least the compound having the main mixing ratio has a sufficient monomolecular film forming ability, the remaining compounds may have a poor or no monomolecular film forming ability, as previously described. However, from the viewpoint of the stability of the devices finally obtained, all the materials mixed should have the monomolecular film forming ability whether they can form films with ease or difficulty. An inorganic oxide layer 4 is laminated on the organic insulating film 3 thus formed. The above organic insulating layer 3 and inorganic oxide layer 4 are hereinafter referred to as "the insulating layer" for simplification. Inorganic oxides used in the inorganic oxide layer 4 of the present invention may be any of those having insulating or semi-insulating properties, and there can be used, for example, Al₂O₃, BaO, BeO, MgO, SiO₂, CuO, PbO, Fe₂O₃, Cu₂O, V₂O₅, Ta₂O₅, TiO₂, WO₃, Ag₂O and PbTiO₃. As methods for forming the inorganic oxide layer comprising any of these, ordinary vacuum deposition or sputtering can be used. In the instance where the layer is formed by vacuum deposition, it is relatively difficult to directly deposit the oxides, and a metal film may first be formed by deposition followed by a procedure such as air oxidation or anodic oxidation to give an oxide film. In particular, since in the present invention the inorganic oxide layer has a thickness of not less than several tenths nm (Å) to not more than 20 nm (200 Å), and preferably not less than several tenths nm (Å), a metal-deposited film may be subjected to air oxidation (or natural oxidation) to obtain the desired inorganic oxide layer with great ease. On the other hand, in the instance where the oxide layer is formed by sputtering, a metal layer may be provided first as mentioned above, followed by oxidation to obtain the desired inorganic oxide layer, but, as another method, active sputtering may be carried out in a rare gas containing oxygen, so that the inorganic oxide layer can be directly obtained with ease. Methods other than these can also be utilized so long as they are methods fundamentally capable of forming inorganic oxide layers with a film thickness of not more than 20 nm(200 Å). However, since the inorganic oxide layer is formed on the organic insulating layer previously described, it is preferable to employ methods by which the underlying organic materials may be damaged as little as possible. From such a viewpoint, it is preferable in general to employ the procedure such as air oxidation of the metal-deposited film. The electrodes 2 and 5, between which such an insulating layer, prepared as described, is held, may be any of those having a high conductivity, which include, for example, a number of materials including metals such as Au, Pt, Ag, Pd, Al, In, Sn and Pb, or alloys of these, also graphite or silicide, and also conductive oxides such as ITO. These are expected to be applicable in the present invention. As methods of forming the electrodes that employs such materials, conventionally known thin film techniques may suffice for the purpose. However, in either electrode, it is preferred to use conductive materials that may not form any oxides having insulating properties, as exemplified by noble metals and oxide conductive materials such as ITO. Fig. 2 illustrates another embodiment of the switching device of the present invention, where an inorganic oxide layer is provided at a step portion on the electrode for the purpose of stabilizing and improving the device characteristics. In the switching device illustrated in Fig. 2, the electrodes can be formed using a lithographic technique. Hitherto known techniques such as lift-off process and photoetching process are satisfactory for such a lithographic technique. Also, the electrodes can be formed with a width ranging from 10 µm to 1 mm. Since it is possible in the present invention to form the electrodes by lithography the switching speed can be made higher. Such lithography can also be applied to the formation of the inorganic oxide insulating layer. Such an inorganic insulating layer may have a film thickness of from 50 nm (500Å) to 1 µm, and preferably from 100 nm (1,000 Å) to 300 nm (3,000) Å). Methods of forming the inorganic oxide layer need not be limited to vacuum deposition or sputtering so long as they are methods which do not damage easily the above-described organic insulating layer. In regard also to the formation of electrodes, any film forming method capable of forming uniform thin films, can be used and need not be limited to vacuum deposition or sputtering. Moreover, the present invention requires no particular limitations on the materials of substrates or the form thereof. EXAMPLES The present invention will be described below in more detail by referring to Examples. Example 1 On a glass substrate 1 (# 7059, produced by Corning Co.,) having been subjected to hydrophobic treatment by allowing it to stand for 24 hours in a saturated vapor of hexamethyldisilazane (HMDS), Cr was deposited as an underlayer by vacuum deposition with a thickness of 3 nm (30 Å), and Au was further vapour-deposited by the same process (film thickness: 30 nm (300 Å), to form a lower electrode 2 in a stripe with a width of 1 mm. On the resulting substrate made to serve as a carrier, monomolecular films comprising squarilium bis-6-octylazulene (SOAZ) were built up according to the LB process, the build-up procedures of which are detailed below. A benzene solution in which SOAZ was dissolved in a concentration of 0.2 mg/mℓ was spread on an aqueous phase of 20°C of water temperature to form a monomolecular film on the water surface. After removal of the solvent by evaporation, the surface pressure of the monomolecular film thus formed was raised up to 20 mN/m. While keeping constant the surface pressure, the above substrate was gently put in water at a rate of 5 mm/min in the direction traversing the water surface, and thereafter subsequently gently drawn up at a rate of 5 mm/min, thus building up a two-layer Y-type monomolecular film. Such operations were repeated appropriate times to form four kinds of built-up films with 4, 8, 12 or 20 layers on the above substrate, thus providing an organic insulating layer 3. Next, on the surface of the film surface thus formed, Al was vapor-deposited with a thickness of 5 nm (50 Å)in a stripe with a width of 1 mm in such a manner that it may cross at right angle with the lower electrode 2, and thereafter oxygen gas was fed in the vacuum chamber to oxidize the resulting Al-deposited film, thus forming an inorganic oxide layer (Al₂O₃ film) 4. Then, the inside of the chamber was restored again to vacuum (5.3 x 10⁻⁴ Pa (4x 10⁻⁶ Torr) or less), and an Au upper electrode 5 (film thickness: 30 nm (300 Å)) was formed on the above Al₂O₃ film. Using an electric circuit as illustrated in Fig. 3 (in which the numeral 6 denotes an ammeter; 7, a limiting resistor; 8, an input electric source; and 9, a voltmeter), a voltage was applied to between a pair of the upper and lower electrodes of the samples (MIM devices) thus prepared, having the MIM structure, and the current characteristics (VI characteristics) thereby produced were measured to find that the memory-functional switching characteristics as shown in Fig. 4 were obtained. It was further possible to produce a stable on-state (resistance value: several ten Ω) and off-state (resistance value: not less than M (10⁶ order) Ω) as shown in Fig. 5, where the on-to-off switching showed a constant threshold voltage (about 1 to 2 V/20 layers), the off-to-on switching occurred at about -2 to 2 V, and the switching speed was not more than 1 µsec with an on/off ratio (ratio of resistance value in the on-state and off-state) of not less than 5 figures. The 4-layer samples, however, showed somewhat unstable switching characteristics, and the 20-layer samples showed that the off-to-on switching occurred with difficulty. Triangular waves with a peak value of ± 8 V and a alternating electric field frequency of 2 Hz were continuously applied to the 12-layer samples, which showed best switching characteristics, to measure the repeated switching times to find that the switching was repeated at least 10⁷ times, during which a stable reproducibility was seen in the current-voltage characteristics, and moreover no change, e.g., damage was seen in the samples themselves. These 12-layer samples were further maintained for 6 months under conditions of 25°C and 70 % RH and then switching was performed. As a result, no significant difference in the current-voltage characteristics was seen when compared with the samples on which the switching was performed immediately after the preparation of the samples. The repeated switching times were also measured to find that the switching was also repeated 10⁷ times or more with superior stability and reproducibility. On the other hand, samples in which the inorganic oxide layer 4 comprising Al₂O₃ was not laminated on the organic insulating layer 3 formed of the SOAZ 12-layer LB film, and the Au upper electrode 5 (film thickness: 30nm (300Å)) wad directly formed thereon showed somewhat unstable switching characteristics, with greater dispersion thereof between samples. The repeated switching times were also measured to find that the destruction of the devices was observed in about 10⁴ times even at maximum. Next, samples were prepared in which the inorganic oxide layer 4 was also omitted and an Au upper electrode 5 (film thickness:100 nm (1,000 Å)) was directly formed on the SOAZ 12-layer LB film, to evaluate their switching characteristics. As a result, no significant difference in the current-voltage characteristics immediately after the preparation of the sample was seen when compared with the above-mentioned samples having the inorganic oxide layer 4 comprising Al₂O₃, but the repeated switching times were about 7 x 10⁵ times, at which the destruction of the devices was observed. These samples were further stored under conditions of 25°C and 70 % RH and thereafter the switching characteristics was measured, where a number of samples were observed in which the off-to-on switching had become difficult to occur at the time of about 3 months lapsed. It was seen from the above that in the MIM device having the structure that the organic compound is held between electrodes, its switching characteristics can be remarkably improved from the viewpoints of reproducibility and stability when the inorganic oxide layer comprising the inorganic oxide is provided on the organic insulating layer comprising such an organic compound. Example 2 MIM devices were fabricated in entirely the same manner as Example 1 except that the compound used in the organic insulating layer 3 was replaced with a mixture of SOAZ and cadmium arachidate (hereinafter "C₂₀"). Samples having SOAZ:C₂₀ mixing ratios of 4:1, 2:1, 1:1, 1:2, and 1:4 were prepared. Procedures of forming the SOAZ-C₂₀ mixed insulating layer according to the LB process are detailed below. A benzene solution in which SOAZ was dissolved in a concentration of 0.2 mg/mℓ and a benzene solution in which C₂₀ was dissolved in a concentration of 0.5 mg/mℓ were appropriately mixed so as to give the desired mixing ratio (molar ratio) of SOAZ to C₂₀, and thereafter the resulting mixed solution was spread on an aqueous phase comprising an aqueous cadmium chloride solution (water temperature: 20°C) with a concentration of 5 x 10⁻⁴ mol/ℓ, adjusted to pH 6.7 using KHCO₃, to form a SOAZ-C₂₀ mixed monomolecular film on the water surface. After removal of the solvent by evaporation, the surface pressure of the monomolecular film thus formed was raised up to 20 mN/m. While keeping constant the surface pressure, a substrate provided with the lower electrode 2 (Cr/Au), prepared in the same manner as Example 1, was gently put in water at a rate of 5 mm/min in the direction traversing the water surface, and thereafter subsequently gently drawn up at a rate of 5 mm/min, thus building up a two-layer Y-type mixed monomolecular film. Such operations were repeated several times to form a 12-layer mixed built-up film, thus providing the organic insulating layer 3. On the MIM devices thus prepared, the switching characteristics were measured in the same manner as Example 1. As a result, the same switching characteristics as Example 1 were obtained in all the samples. The repeated switching times were 10⁷ times or more for all samples. The samples were further stored for 6 months under conditions of 25°C and 70 % RH, and thereafter after which no changes were seen in the above switching characteristics. Example 3 MIM devices were fabricated in entirely the same manner as Example 1 except that the compound used in the organic insulating layer 3 was replaced with a mixture of SOAZ and polymethyl methacrylate (hereinafter "PMMA"). Samples having SOAZ:PMMA mixing ratios of 1:1, 1:2, 1:4 and 1:10 (molar ratios in terms of monomers) were prepared here. Procedures of forming the SOAZ-PMMA mixed insulating layer according to the LB process are detailed below. A benzene solution in which SOAZ was dissolved in a concentration of 0.2 mg/mℓ and a solution obtained by dissolving PMMA (produced by of Polyscience Co.; syndiotactic; molecular weight: 100,000) in a mixed solution of trichloroethane:benzene = 1:9 (V/V) in a concentration of 0.2 mg/mℓ, were appropriately mixed so as to give the desired mixing ratio, and thereafter the resulting mixed solution was spread on pure water of 20°C in water temperature to form an SOAZ-PMMA mixed monomolecular film on the water surface. After removal of the solvent by evaporation, the surface pressure of the monomolecular film thus formed was raised up to 20 mN/m. While keeping constant the surface pressure, a substrate provided with the lower electrode 2 (Cr/Au), prepared in the same manner as Example 1, was gently put in water at a rate of 5 mm/min in the direction traversing the water surface, and thereafter subsequently gently drawn up at a rate of 5 mm/min, thus building up a two-layer Y-type mixed monomolecular film. Such operations were repeated several times to form a 12-layer mixed built-up film, thus providing the organic insulating layer 3. On the MIM devices thus prepared, the switching characteristics were measured in the same manner as Example 1. As a result, the same switching characteristics as Example 1 were obtained in all the samples. The repeated switching times were 10⁷ times or more for all samples. The samples were further stored for 6 months under conditions of 25°C and 70 % RH, and thereafter which no changes were seen in the above switching characteristics. Example 4 MIM devices were fabricated in entirely the same manner as Example 1 except that the compound used in the organic insulating layer 3 was replaced with a mixture of SOAZ and polyisobutyl methacrylate (hereinafter "PIBM"). Samples having SOAZ:PIBM mixing ratios of 1:1, 1:2, 1:4 and 1:10 (molar ratios in terms of monomers) were prepared here. Procedures of forming the SOAZ-PIBM mixed insulating layer according to the LB process are detailed below. A benzene solution in which SOAZ was dissolved in a concentration of 0.2 mg/mℓ and a solution obtained by dissolving PIBM (Elvacite 2045; produced by Dupont Co.) in a mixed solution of trichloroethane:benzene = 1:9 (V/V) in a concentration of 0.2 mg/mℓ, were appropriately mixed so as to give the desired mixing ratio, and thereafter the resulting mixed solution was spread on pure water of 20°C in water temperature to form an SOAZ-PIBM mixed monomolecular film on the water surface. After removal of the solvent by evaporation, the surface pressure of the monomolecular film thus formed was raised up to 10 mN/m. While keeping constant the surface pressure, a substrate provided with the lower electrode 2 (Cr/Au), prepared in the same manner as Example 1, was gently put in water at a rate of 20 mm/min in the direction traversing the water surface, and thereafter subsequently gently drawn up at a rate of 20 mm/min, thus building up a two-layer Y-type mixed monomolecular film. Such operations were repeated several times to form a 16-layer mixed built-up film, thus providing the organic insulating layer 3. On the MIM devices thus prepared, the switching characteristics were measured in the same manner as Example 1. As a result, the same switching characteristics as Example 1 were obtained in all the samples. The repeated switching times were 10⁷ times or more for all samples. The samples were further stored for 6 months under conditions of 25°C and 70 % RH, and thereafter which no changes were seen in the above switching characteristics. Example 5 MIM devices were fabricated in entirely the same manner as Example 1 except that the organic insulating layer 3 was replaced with a polyimide-LB film. Procedures of forming the polyimide-LB film according to the LB process are detailed below. Polyamide acid represented by the formula (1) was dissolved in an N,N′-dimethylacetamide solvent (at a concentration calculated as monomer: 1 x 10⁻³ M), and thereafter the solution was mixed with a separately prepared 1 x 10⁻³ M solution of N,N′-dimethyloctadecylamine using the same solvent, in the ratio of 1:2 (V/V), to prepare a solution of a polyamide acid octadecylamine salt represented by the formula (2). The solution thus prepared was spread on pure water of 20°C of water temperature, and a 20-layer monomolecular built-up film of the polyamide acid octadecylamine salt was formed on a substrate provided with the lower electrode 2 as described in Example 1. In this occasion, the surface pressure was 25 mN/m., and the substrate was put in and drawn up at a rate of 5 mm/min. Next, this substrate was baked at 300°C for 30 minutes to form the polyamide acid octadecylamine salt into an imide [formula (3)], and a 20-layer polyamide monomolecular built-up film was formed, thus providing the organic insulating layer 3. On the MIM devices thus prepared, the switching characteristics were measured in the same manner as Example 1. As a result, the same switching characteristics as Example 1 were obtained in all the samples. The repeated switching times were 10⁷ times or more for all samples. The samples were further stored for 6 months under conditions of 25°C and 70 % RH, and even no changes were seen in the above switching characteristics. Example 6 In the same manner as Example 5, a polyimide monomolecular built-up film was formed with 20-layer build-up on a substrate provided with the lower electrode (Cr/Au) 2, to form the organic insulating layer 3. Next, on the surface of the film surface thus formed, Ag was deposited by sputtering in a stripe with a width of 1 mm in such a manner that it may cross at right angle with the lower electrode 2, to form an inorganic oxide layer 4 comprising Ag₂O (film thickness: 3 nm(30 Å)). Next, an Au upper electrode 5 (film thickness: 30 nm (300 Å)) was formed on the Ag₂0 layer according to a conventional vacuum deposition method. On the MIM devices thus prepared, the switching characteristics were measured in the same manner as Example 1. As a result, the same switching characteristics as Example 1 were obtained in all the samples. The repeated switching times were 10⁷ times or more for all samples. The samples were further stored for 6 months under conditions of 25°C and 70 % RH, and even thereafter no changes were seen in the above switching characteristics. Example 7 MIM devices were fabricated in entirely the same manner as Example 1 except that the inorganic oxide layer 4 was formed with MgO. The MgO layer was obtained by depositing Mg with a thickness of 4 nm (40 Å) according to a conventional vacuum deposition method, followed by air oxidation. On the MIM devices thus prepared, the switching characteristics were measured in the same manner as Example 1. As a result, the same switching characteristics as Example 1 were obtained for all the samples. The repeated switching times were 107 times or more in all samples. The samples were further stored for 6 months under conditions of 25°C and 70 % RH, and thereafter which no changes were seen in the above switching characteristics. In the above-described Examples, the LB process has been used in forming the insulating layers, but any film formation processes can be used without limitation to the LB process, so long as they can make a very thin, uniform insulating thin film. They specifically include vacuum deposition, electrolytic polymerization, and CVD, thus expanding the range of usable materials. Example 8 Samples having the structure comprising a lower electrode/an insulating thin film/an insulating thin film/an upper electrode (Fig. 2) were prepared according to the following procedures. As a pretreatment, a cleaned glass substrate 1 is subjected to ultrasonic treatment for 5 minutes a cleaned glass substrate 1 by use of butyl acetate, and N₂ is blown thereon, and then the substrate thus treated is put in a 120°C oven to carry out baking for 10 minutes. The resulting substrate is spin-coated with hexamethyldisilazane (HMDS) under conditions of 2,500 rpm and 30 seconds, and baking is carried out for 10 minutes in a 200°C oven. Next, a resist material RD-2000N-10 is spin-coated thereon under conditions of 400 rpm and 1 second for the first coating and 3,000 rpm and 40 seconds for the second coating to give a film thickness of 0.7 µm, and then prebaking is carried out at 80°C for 25 minutes. Subsequently, exposure to light is carried out under conditions of 3.0 counts using a cold mirror (290 nm) of PLA-520F (produced by Canon Inc.) and under hard contact. Thereafter, development is carried out for 100 seconds using a developing solution for its exclusive use, followed by postbaking at 120°C for 20 minutes to make a resist pattern. On the substrate thus treated, Cr was deposited by vacuum deposition with a thickness of 10 nm (100 Å) as an underlayer, and Au was further deposited thereon by the same process (film thickness: 60 nm (6)) Å)). The resulting substrate was subjected to ultrasonic treatment using acetone for 5min., ultrasonic treatment using dimethylformamide (DMF) for 5 min., washing with pure waterfor 5 min., and N₂ blowing, followed by baking for 10 minutes in a 120°C oven, thus forming a lower electrode 2 with a width of 40 µm by the lift-off process. This substrate was left to stand for 24 hours in a saturated vapor of HMDS to carry out hydrophobic treatment. On the substrate thus treated, a 4-, 6-, 12-, 20-, 24-, 30-, 42-, 60-, 200- or 300-layer built-up film (film thickness: 8 nm (80 Å)) of polyimide monomolecular films was formed by the LB process in the same manner as Example 5, thus providing an insulating thin film 3. On this substrate, SiO₂ is deposited by vacuum deposition with a thickness of 300 nm (3,000 Å). Next, a resist material AZ-1370 is coated thereon using a spinner under conditions of 500 rpm and 5 seconds for the first coating and 3,000 rpm and 30 seconds for the second coating to give a film thickness of 1.2 µm. This is then subjected to prebaking at 90°C for 15 minutes, followed by exposure to light under contact for 10 seconds using MA-10, (produced by Mikasa Co.). Next, peeling is carried out under conditions of 15 seconds using a solution of MF312:DI-Water = 1:1 and 15 seconds for washing with pure water, N₂ is blown, and then postbaking is carried out at 120°C for 30 minutes. Thereafter, etching is carried out using a solution of HF:NH₄F = 1:7 to effect patterning of SiO₂, thus providing an inorganic oxide layer 4 which protects the step portion of the electrode. The resulting substrate is subjected to ultrasonic treatment using acetone for 5 minutes, ultrasonic treatment using DMF, and blowing of N₂, and thereafter baking is carried out for 10 minutes in a 120°C oven. On the resulting substrate, Al is deposited by vacuum deposition with a thickness of 100 nm (1,000 Å). Next, a resist material AZ-1370 is coated thereon using a spinner under conditions of 500 rpm and 5 seconds for the first coating and 3,000 rpm and 30 seconds for the second coating to give a film thickness of 1.2 µm. This is then subjected to prebaking at 90°C for 15 minutes, followed by exposure to light under contact for 10 seconds using MA-10 (produced by Mikasa Co.). Next, development is carried out under conditions of 15 seconds using a solution of MF312:DI-Water = 1:1 and 15 seconds for washing with pure water, N₂ is blown, and then postbaking is carried out at 120°C for 30 minutes. Thereafter, Al is etched using a solution of H₃PO₄:HNO₃:CH₃COOH:H₂O = 16:1:2:1 to effect patterning. The resulting substrate is subjected to ultrasonic treatment using acetone for 5 minutes, ultrasonic treatment using DMF for 5 minutes, and washing with pure water for 5 minutes to peel the resist, blowing of N₂ is carried out, and thereafter baking is carried out for 10 minutes in a 120°C oven, thus forming an upper electrode 5. The electrode thus formed was made to have a width of 40 µm. A voltage was applied between the upper and lower electrodes of the samples thus prepared, and current characteristics (VI characteristics) thereby produced were measured. Memory-functional switching characteristics were observed which have been hitherto unknown in other conventional samples (Fig. 4). It was further possible to produce a stable on-state (resistance value: several ten Ω) and off-state (resistance value: not less than M C 106 order Ω) as shown in Fig. 5, where the on-to-off switching showed a constant threshold voltage (about 1 to 2 V/20 layers), the off-to-on switching occurred at about -2 to 2 V, and the switching speed was in the picosecond order with an on/off ratio (ratio of resistance value in the on state and off state) of not less than 5 figures. The threshold voltage was in a tendency to become higher with an increase in the monomolecular film layer number of the insulating layer. As a result, the 4-layer samples and 6-layer samples showed unstable switching characteristics, and the 300-layer samples showed that switching occurred with difficulty. Here, the film thickness per layer of the polyimide monomolecular built-up film measured using ellipsometry was found to be several tenths nm (Å). Also, the repetition stability was evaluated based on the number of the switching times repeated until generation of heat caused destruction of electrodes when triangular waves with a peak value of ± 8 V and an alternating electric field frequency of 2 Hz were continuously applied, to find that the repeated switching times were 2 x 10⁷ times. Example 9 A lower electrode is formed on a substrate by the lift-off process in the same manner as Example 8. A polyimide monomolecular film is also prepared by the LB process in the same manner as Example 8. On the resulting substrate, Al₂O₃ is deposited by vacuum deposition with a thickness of 40,100,200 or 400 nm (400, 1,000, 2,000 or 4,000 Å)or 2 µm. Next, a resist material AZ-1370 is coated thereon using a spinner under conditions of 500 rpm and 5 seconds for the first coating and 3,000 rpm and 30 seconds for the second coating to give a film thickness of 1.2 µm. This is then subjected to prebaking at 90°C for 50 minutes, followed by exposure to light under contact for 10 seconds using MA-10 (produced by Mikasa Co.). Next, development is carried out under conditions of 15 seconds using a solution of MF312:DI-Water = 1:1 and 15 seconds for washing with pure water, N₂ is blown, and then postbaking is carried out at 120°C for 30 minutes. Thereafter, etching is carried out using a solution of HF:NH₄F = 1:7 to effect patterning of Al₂O₃, thus providing a protective layer for the portion of the electrode. On the resulting substrate, the desired upper electrode is formed by photoetching in the same manner as Example 8. The electrode was made to have a width of 20 µm. The current-voltage characteristics of the samples prepared in the above way were measured in the same manner as Example 8 to find that the same memory-functional switching characteristics as Example 8 were observed. The switching speed was in the picosecond order. Also, the on/off ratio was not less than 5 figures, and the repeated switching times were 1 x 10⁷ times. In the instance where the insulating material at the step portion of the electrode had a film thickness of 40 nm (400 Å), it was unable to sufficiently ensure the insulating properties of the insulating material, and in the instance of 2 µm, step breakage was liable to occur at the upper electrode. Example 10 A cleaned glass plate was subjected to hydrophobic treatment by allowing it to stand for 24 hours in a saturated vapor of HMDS. On the substrate thus treated, Cr was deposited as an underlayer by vacuum deposition with a thickness of 10 nm (100 Å), and Au was further deposited by the same process (film thickness: 50 nm (500 Å)). Next, on the resulting substrate, a 24-layer built-up film (film thickness: 9.6 nm (96 Å)) of polyimide monomolecular films was formed using the LB process to provide an insulating thin film. The polyimide monomolecular built-up film was prepared in the same manner as Example 8. On the resulting substrate, Al was deposited as an upper electrode by vacuum deposition (film thickness: 100 nm (1,000 Å)). Next, a resist material AZ-1370 is coated thereon using a spinner under conditions of 500 rpm and 5 seconds for the first coating and 3,000 rpm and 30 seconds for the second coating to give a film thickness of 1.2 µm. This is then subjected to prebaking at 90°C for 15 minutes, followed by exposure to light under contact for 10 seconds using MA-10 (produced by Mikasa Co.). Next, development is carried out under conditions of 15 seconds using a solution of MF312:DI-Water = 1:1 and 15 seconds for washing with pure water, N₂ is blown, and then postbaking is carried out at 120°C for 30 minutes. Thereafter, Al is etched using a solution of H₃PO₄:HNO₃:CH₃COOH:H₂O = 16:1:2:1 to effect patterning. The resulting substrate is subjected to ultrasonic treatment using acetone for 5 minutes, ultrasonic treatment using DMF for 5 minutes, and washing with pure water for 5 minutes to peel the resist under these conditions, blowing of N₂ is carried out, and thereafter baking is carried out for 10 minutes in a 120°C oven, thus forming an upper electrode with an electrode width of 20 µm. On the resulting substrate, SiO₂ is deposited by vacuum deposition with a thickness of 100 nm (1,000 Å). This is subjected to etching of SiO₂ in the same manner as Example 8 to form an opening of the upper electrode. On the resulting substrate, Al is deposited by vacuum deposition with a film thickness of 3,000 Å, followed by the same photoetching as Example 8 to effect patterning of Al, thus providing the desired lead electrode of the upper electrode. The voltage-current characteristics of the samples prepared in the above way were measured in the same manner as Example 8 to find that the same memory-functional switching characteristics as Example 8 were obtained. The switching speed was in the picosecond order, and the repeated switching times were 1 x 10⁷ times. Examples 11 to 18 Using the electrode materials, insulating materials, insulating materials for protecting electrode step portion as shown in Table 1, devices were fabricated with the same device structure as those of Examples 8 and 9. The switching characteristics, switching speed and repeated switching times were measured in the same manner as Examples 8 and 9 to obtain the results as shown in Table 1. Comparative Example 1 A lower electrode was formed and a 20-layer polyimide monomolecular built-up film was formed in the same manner as Example 8. Thereafter an upper electrode was formed without protecting the step portion of the electrode. A voltage was applied between the upper and lower electrode of the samples thus prepared, and the current characteristics thereby produced were measured to find that the same memory-functional switching characteristics as Example 8 were observed. However, when compared with the above Examples, the samples showed lower repetition stability, which was 2 x 10⁶ times. Comparative Example 2 Devices were fabricated in the same manner as Example 8. Here, the electrodes were made to be 150 µm and 1 mm in width. Such electrode widths correspond to the size formed according conventional vapor deposition methods using masks.. The current-voltage characteristics of the devices thus obtained were measured, to find that the same memory-functional switching characteristics as Example 8 were obtained but the switching speed was 30 nanoseconds and 700 nanoseconds, respectively. The present invention effects the following: (i) In the MIM structure device in which the insulating layer is formed of a thin film comprising an inorganic oxide layer laminated on an organic insulating layer, memory-functional switching characteristics are obtained, which have not been seen in conventional devices. (ii) As compared with the instances in which the inorganic oxide layer is not provided in the insulating layer of such MIM devices, great improvements are seen in the repeated switching times and storage stability of devices. (iii) Protecting the step portion of the lower electrode by providing the insulating layer after build-up of LB films can bring about improvements in the repetition stability of the switching characteristics of MIM devices. (iv) The lithographic technique is used in the process of forming the electrodes of the MIM devices in which the polymeric compounds having excellent thermal resistance and solvent resistance are built up by the LB process to form the insulating layer, so that it has become possible to make the electrodes finer and more highly dense. (v) The electrodes can be made finer by using the lithographic technique in the process of forming the electrodes of the MIM devices in which the polymeric compounds having excellent thermal resistance and solvent resistance are built up by the LB process to form the insulating layer, so that it has become possible to make the switching speed higher.

Claims: A switching device comprising: a pair of electrodes (2,5); and an organic insulating layer (3) which is located between said pair of electrodes (2,5) and is adjacent to one electrode (2) thereof, which organic insulating layer (3) is comprised of an organic compound having at least one π-electron level and is of a thickness in a range between 0.3 nm(3Å) and 100 nm (1000Å) inclusive; which switching device is characterised by: an inorganic insulating layer (4) of inorganic oxide material located between said organic insulating layer (3) and the other one electrode (5) of the pair of electrodes (2,5). A switching device as claimed in claim 1, wherein: said organic insulating layer (3) and said other one electrode (5) are separated by said inorganic insulating layer (4) of inorganic oxide material and is of a thickness of not more than 20 nm (200 Å). A switching device as claimed in claim 2 wherein said inorganic insulating layer (4) is of a thickness of not more than 5 nm (50 Å). A switching device as claimed in claim 1 wherein: said organic insulating layer (3) is located at the side of and over said one electrode (2), and has accordingly a stepped structure at the side extremities of said one electrode (2); said organic insulating layer (3) and said other one electrode (5) are in contact through a void portion of said inorganic insulating layer (4); and said inorganic insulating layer (4) extends over the stepped structure of said organic insulating layer (3). A switching device as claimed in claim 4, wherein said inorganic insulating layer (4) has a layer thickness of between 50 nm (500 Å) and 1 µm inclusive. A switching device as claimed in claim 5, wherein said inorganic insulating layer (4) is of a thickness of between 100 nm (1000 Å) to 300 nm (3000 Å) inclusive. A switching device as claimed in any one of the preceding claims 2 to 6 wherein said inorganic insulating layer (4) is comprised of a metal oxide. A switching device according to claim 7, wherein said metal oxide is selected out of Al₂0₃, Ba0, Be0, Mg0, Cu0, Pb0, Fe₂0₃, Cu₂0, V₂0₅, Ta₂0₅, Ti0₂, W0₃, Ag₂0 and PbTi0₃. A switching device as claimed in any one of the preceding claims 2 to 6 wherein said inorganic insulating layer (4) is of silicon oxide Si0₂. A switching device as claimed in any one of the preceding claims, wherein the material of said electrodes (2,5) is selected from the metal elements Au, Pt, Ag, Pd, Al, In, Sn, Pb, alloys thereof and also graphite, silicides and conductive oxides. A switching device as claimed in any one of the preceding claims, wherein said organic compound is selected out of: colouring matters having a porphyrin structure; azulene dyes having a squarilium group and a croconic methine group as bonding chains; cyanine-analogous dyes formed by combining two nitrogen-containing heterocyclic rings; cyanine dyes; condensed polycyclic aromatics; derivatives of tetraquinodimethane or tetrathiafulvalene; and metal complex compounds. A switching device as claimed in any one of the preceding claims 1 to 10, wherein said organic compound is selected out of: polyimide, polystyrene, polydiacetylene and polythiophene. A switching device as claimed in any one of the preceding claims 1 to 10, wherein said organic compound is selected from: a polyimide obtained by cyclic polymerization of a polyamic acid; a polyamic acid salt; or a polyamic acid ester. A switching device as claimed in any one of the preceding claims wherein said organic insulating layer (3) is comprised of an admixture of an organic compound having at least one π-electron level with other material for increasing the stability of said organic compound which compound and material are admixed in a molar ratio in the range of 100:1 to 1:100. A switching device as claimed in claim 14 wherein said ratio is in the range 10:1 to 1:10. A switching device as claimed in any one of the preceding claims 14 to 15 wherein said other material is selected from simple fatty acids, lipids, polyacrylic acid derivatives and polymeric compounds. A switching device as claimed in claim 16 wherein said other material either is a polymethacrylic acid or is of nylon. A switching device as claimed in any one of the preceding claims wherein said organic insulating layer (3) comprises either a monomolecular film or a film built-up therefrom. A switching device as claimed in claim 18, wherein said monomolecular film or film built-up therefrom is a Langmuir-Blodgett film. A method for producing a switching device as claimed in claim 19 which method comprises the steps of: forming an organic insulating thin-film layer (3) by the Langmuir-Blodgett process, and forming a patterned inorganic oxide insulating thin-film layer (4) by lithography on said organic insulating thin-film layer (3). Use of a switching device as claimed in any one of the preceding claims 1 to 19 wherein the resistance value thereof is changed by application of a voltage that is greater than a threshold.

Inventors: KAWADA HARUKI, KAWADE HISAAKI, MATSUDA HIROSHI, MORIKAWA YUKO, SAKAI KUNIHIRO, TAKAMATSU OSAMU, TAKIMOTO KIYOSHI, KAWADA, HARUKI, KAWADE, HISAAKI, MATSUDA, HIROSHI, MORIKAWA, YUKO, SAKAI, KUNIHIRO, TAKAMATSU, OSAMU, TAKIMOTO, KIYOSHI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1989
Main classification: H01L45
IPC codes: H01L49, H01L45, H01L51