PRODUCTION OF THIN-FILM ELECTROLUMINESCENT DEVICE

Abstract: This invention is related to a method of producing a thin-film EL device by laminating sequentially a first electrode consisting of a first conductor layer, a first insulating layer, a light emitting layer, a second insulating layer and a second electrode on a substrate, wherein a pattern of the first conductor layer is formed throughout the formation area of the first electrode and an area in which electrode terminals are to be formed, and is dipped in a plating solution to form selectively the terminal pattern only on the first conductor layer. Accordingly, alignment of the pattern is not necessary and the termi­ nal pattern having high accuracy can be formed by merely dipping the device in the plating solution. Thus, stable charac­ teristics can be maintained for a long period of time without detriment to the device.

Description: The present invention relates to a method of fabricating a thin-film EL (electroluminescent) device and, more particularly, to a method of forming electrodes of such a device. Much attention has been focused on a thin-film EL device using a thin-film phosphor layer instead of a dispersion EL device using zinc sulfide (ZnS) compound phosphor powder, because the former can provide a high luminance. A thin-film EL device, in which a luminous layer is made in the form of a thin film to minimize halation or luminous blur caused by the scattering of externally incident light and of light emitted from the interior of the luminous layer and to thereby offer high sharpness and contrast, has been used as a device for mounting on vehicles, as a display unit such as in a computer terminal, or as an illumination device. In the case where a thin-film EL device is used as a display, the EL device is constructed as a dot matrix type as shown in Figs. 2(a), (b) and (c), in which light-permeable surface and back electrodes 101 and 102 take the forms of patterns of strips mutually spaced at intervals of a predetermined distance, electrodes 101 and 102 mutually intersecting at right angles to each other. A luminous layer 103 is interposed between the surface and back electrodes and each of intersections between the surface and back electrode patterns forming one of the elements of the thin-film EL device. Fig. 2(a) is a perspective view of part of the EL device, Fig. 2(b) is a plan view thereof and Fig. 2(c) is a cross-sectional view thereof. In more detail, first and second insulating layers 104 and 105 are disposed between the luminous layer 103 and the surface and back electrodes 101, 102 respectively. The light emitting process of this thin-film EL device is as follows. First, when a voltage is applied between the patterns of the surface and back electrodes 101 and 102 in response to an input signal, electric fields are induced in the luminous layer 103 at the intersections between these patterns so that electrons so far trapped at the interface level are released therefrom and accelerated, whereby the electrons acquire sufficient energy to bombard with orbital electrons of luminous center impurities, thereby exciting the orbital electrons. When the thus excited luminous center electrons return to the ground or normal state, they emit light. With such a thin-film EL device, the electrodes of the elements of the EL device are provided at their one ends with terminals 106 which are to be connected to an external controller (not shown) through associated lead wires and which terminals are usually made of a nickel (Ni) film. Such terminals 106 have been conventionally formed by an electron beam evaporation process after the sequential formation of the pattern of surface electrodes 101, first insulating layer 104, luminous layer 103, second insulating layer 105 and then the pattern of back electrodes 102. In the patterning process, a nickel thin-film pattern has been made usually by a selective evaporation process, that is, by the electron beam evaporation process using a metal mask. This selective evaporation process, however, has had such a problem that the pattern edge does not correspond exactly to the mask, that is, the shape cannot be sharply defined resulting in a bad pattern accuracy. Such an EL device has also been defective in that, because the electrodes of the device have a high pitch, of about 0.5mm, it is highly difficult to align such a fine pattern with the metal mask and thus production yield is reduced due to positional shift in the pattern. To eliminate such problems, there has been proposed a method in which terminal patterning is carried out by a photolithographic process. In this method, as shown in Fig. 3(a), elements are first formed and then nickel thin films 106' are made in the form of a strip by the electron beam evaporation process. Subsequently, patterning is effected by the photolithographic process to form nickel terminals 106, as shown in Fig. 3(b). This method is advantageous in that the pattern accuracy is improved but disadvantageous in that impurity ions or moisture often cause deterioration of the elements of the EL device in the etching step and further the number of steps in the photolithographic process is large, resulting in that the cost of the associated photo masks is high, and so on. It is generally known that the elements of a thin-film EL device are subject to damage by moisture or impurity ions in the etching step. This phenomenon has been a serious problem in thin-film EL devices, since such devices are operated, in particular, under high electric fields so that frequent use of such a device causes the moisture adsorbed on the device in the electric field to break down and penetrate into the interfaces of the films, thus causing film release and involving a shortened operational life. JP-A-59-23492, JP-A-59-27497 and JP-A-61-61398 disclose thin-film EL devices with matrix-type electrode patterns. US-A-4 614 668 discloses a method of making a thick-film EL device in which a conductive pattern of first electrodes is formed, at locations outside them there being simultaneously formed (a) patches communicating with the first electrodes and serving as a basis for first electrode terminals for the first electrodes and (b) isolated patches which do not communicate with the first electrodes and serve as a basis for second electrode terminals, a communication between the isolated patches and second electrodes being brought about in a step of forming the pattern of second electrodes. According to the present invention, there is provided a method of fabricating a thin-film EL device on a substrate having first and second peripheral portions, the method comprising the steps of: forming on the substrate a conductive pattern comprising, over a first zone, a plurality of first electrodes and first electrode terminal portions for the first electrodes and, over a second zone, a plurality of second electrode terminal portions the first electrode terminal portions being located on the first peripheral portion and the second electrode terminal portions being located on the second peripheral portion; immersing the first and second peripheral portions on which the first and second electrode terminal portions have been formed into a plating solution, without immersing portions of said substrate inboard of said first and second peripheral portions, to form plated layers on said first and second terminal portions only within said first and second peripheral portions by selective plating; forming a first insulating layer over the first electrodes; forming a luminescent layer on the first insulating layer; forming a second insulating layer on the luminescent layer; and forming a plurality of second electrodes on the second insulating layer and edge portions of the plated second electrode terminal portions such that the second electrode terminal portions are connected to respective ones of the second electrodes. The present invention will now be described, by way of example, with reference to the accompanying drawings in which:- Figs. 1(a) to 1(e) show respectively different steps in a process of fabricating a thin-film EL device in accordance with an embodiment of the present invention; Figs. 2(a) to 2(c) are diagrams for explaining the structure of an ordinary thin-film EL device; and Figs. 3(a) and 3(b) show an electrode-terminal forming step in a prior art process of fabricating a thin-film EL device. Referring first to Fig. 1(a), an indium tin oxide (ITO) layer is formed on a glass substrate 1 by a sputtering process and then is subjected to patterning by a photolithographic process to form a light-permeable strip pattern of surface electrodes 2 arranged at intervals of a predetermined spacing and an adjacent terminal-formation pattern 2U provided on one end of the substrate and extending perpendicular to the strip pattern. Referring to Fig. 1(b), a first insulating layer 3 made of tantalum pentoxide (Ta₂O₅) is formed on the substrate obtained in the previous step, by a sputtering process, during which a metal mask is used so as not to cover one ends of the surface electrodes and the entire pattern 2U, that is, to expose them. Referring to Fig. 1(c), the resultant substrate is next immersed a total of four times into an electroless nickel plating solution to a predetermined depth, sequentially from its four sides so that at each immersion of the substrate, the solution reaches level L, whereby nickel plated layers 7 are formed. After this, the nickel plated layers are further subjected similarly to an electroless gold plating application thereon to form gold plated layers 8. As a result, two-layer (nickel and gold) terminal structures are formed (see Figs. 1(d) and 1(e)). In this connection, the nickel plating solution may comprise, for example, 39g/ℓ of NiSO₄.6H₂O, 30g/ℓ of NaH₂PO₂H₂O, 20g/ℓ of NH₂CH₂COOH, 20g/ℓ of Na₃C₆H₅O₇2H₂O, and 2ppm of Pb(NO₃)₂, the pH level and temperature of the solution being adjusted to be 5-6 and 80-90°C respectively. The gold plating solution may comprise, for example, 28g/ℓ of potassium gold cyanide, 60g/ℓ of citric acid, 45g/ℓ of tungstic acid, 16g/ℓ of sodium hydroxide, 3.75g/ℓ of N-N-diethylglycine sodium and 25g/ℓ of potassium phthalate and be adjusted to be 5-6 and 85-93°C in pH and temperature respectively. Following the above steps, a luminous layer 4 made of zinc sulfide (ZnS) containing terbium (Tb) as a luminous centre impurity, i.e. of ZnS:Tb, a second insulating layer 5 made of Ta₂O₅ and a strip-shaped pattern of back electrodes 6 made of aluminum (A1) are formed by an ordinary method to complete such a thin-film EL device of a dot matrix type as shown in Fig. 1(e). The pattern of the back electrodes 6 is arranged to extend perpendicular to the aforementioned surface electrode pattern and also to overlap partly the terminals of the nickel and gold plated layers 7 and 8 to allow ends of the terminals to be exposed and electrically connected to an external device. With the thin-film EL device thus completed, each of the overlapped or intersected parts between the patterns of the surface and back electrodes 2 and 6 corresponds to one of the picture elements of the device, and the supply of power from ones of the terminals corresponding to picture information allows corresponding picture elements to emit light. In accordance with the above embodiment of the present invention, a highly precise terminal pattern can be realized readily, substantially without causing any damage to the elements of the device. Accordingly, there is provided a thin-film EL device which can prevent deterioration of elements in the device due to film release or the like even when the device is used for a long period of time, and therefore can keep the reliability high and the cost low and can prolong the life. Although explanation has been made in connection with the case where the underlying or substrate side has the transparent electrodes in the foregoing embodiment, a method according to the present invention may be applied also to a thin-film EL device of a type having transparent electrodes as the top layer side. In the latter case, when the (back) electrodes and the terminal underlying pattern are made of aluminum, it becomes difficult to plate the terminal pattern with nickel. This is because aluminum is larger in electronegativity than nickel and thus dissolves into the nickel solution. Thus, the terminal pattern should be made of metal other than aluminum, or such material should be used as the material of the (back) electrodes and underlying pattern as does not dissolve into the plating solution to be used, or a special pretreatment should be previously provided so as to allow nickel plating. In the above embodiment, the plating step has been carried out after the formation of the first insulating layer, but it may be effected before the formation of the first insulating layer. Further, it is unnecessary always for the terminals to have a two-layer structure and the terminals may be made to be, for example, of a single nickel layer type. Furthermore, the above description is with reference to a dot matrix type thin-film EL device, but the present invention is not limited to the making of such a device.

Claims: A method of fabricating a thin-film electroluminescent device on a substrate (1) having first and second peripheral portions, the method comprising the steps of: forming on the substrate a conductive pattern comprising, over a first zone, a plurality of first electrodes (2) and first electrode terminal portions for the first electrodes and, over a second zone, a plurality of second electrode terminal portions (2U), the first electrode terminal portions being located on the first peripheral portion and the second electrode terminal portions being located on the second peripheral portion; immersing the first and second peripheral portions on which the first and second electrode terminal portions have been formed into a plating solution, without immersing portions of said substrate inboard of said first and second peripheral portions, to form plated layers on said first and second terminal portions only within said first and second peripheral portions by selective plating; forming a first insulating layer (3) over the first electrodes; forming a luminescent layer (4) on the first insulating layer; forming a second insulating layer (5) on the luminescent layer; and forming a plurality of second electrodes (6) on the second insulating layer and edge portions of the plated second electrode terminal portions such that the second electrode terminal portions are connected to respective ones of the second electrodes. A method according to claim 1, wherein the first electrode terminal portions of said conductive pattern are intergral with respective ones of the first electrodes (2) of the conductive pattern. A method according to claim 1 or 2, wherein said first electrodes (2) are in the form of a plurality of first strips and said second electrodes (6) are in the form of a plurality of second strips substantially perpendicular to the first strips.

Inventors: NIRE, TAKASHI, TANDA, SATOSHI
Applicants: KABUSHIKI KAISHA KOMATSU SEISAKUSHO
Application year: 1988
Main classification: H05B33
IPC codes: H05B33