1. Field of the invention
The present invention relates to a display device and a fabrication method therefor. More particularly, the invention relates to a method for forming a compound film on aluminum electrodes of a display device such as a plasma display panel or a liquid crystal display device for protecting the aluminum electrodes from corrosion and the like, and to a display device fabricated by the method.
2. Related Art
Hitherto known as display devices are liquid crystal display devices, CRTs, plasma display panels (hereinafter referred to as "PDPs") and the like. Among these display devices, the PDPs which have a smaller thickness than CRTs offer installation flexibility. The construction of an exemplary PDP will hereinafter be described.
FIG. 4 is a schematic perspective view of an AC-driven surface discharge PDP. A pair of sustain electrodes (also referred to as "device electrodes") X and Y are formed for each matrix display line L on an interior surface of a front glass substrate 26. The sustain electrodes X and Y each include a transparent electrode 27 and a metal electrode (bus electrode) 28, and are covered with a dielectric layer 29 for AC driving. A protective film 30 of MgO is formed on a surface of the dielectric layer 29 by vapor deposition.
Provided on an interior surface of a rear glass substrate 21 are address electrodes 22, a dielectric layer A, barrier ribs 23 and fluorescent layers 24 of three colors (R, G, B). The barrier ribs 23 each have a linear configuration in plan. The barrier ribs 23 define the discharge space Z as having a predetermined gap, and partition a discharge space Z along a line of the matrix display to define respective subpixels. Each pixel (picture element) for display comprises three subpixels arranged along the line. The subpixel is the discharge cell, where sustain electrodes X and Y intersect an address electrode 22. In the PDP, the barrier ribs 23 are arranged in a so-called stripe pattern and, therefore, the subpixels in each row in the discharge space Z are arranged in sequence across all the lines L. The subpixels in each row are adapted to emit the same color light. Reference numerals 25 and 31 denote a rear substrate and a front substrate, respectively.
FIGS. 5(a) to 5(g) and 6(a) to 6(g) are sectional views for explaining a method for fabricating the PDP shown in FIG. 4. More specifically, FIGS. 5(a) to 5(g) illustrate a method for making the rear substrate, and FIGS. 6(a) to 6(g) illustrate a method for making the front substrate.
The method for making the rear substrate will first be described.
After a glass substrate 21 is cleaned, a metal layer 32 is formed of copper, aluminum or the like on the glass substrate 21 by vacuum evaporation or sputtering (see FIG. 5(a)). The metal layer is formed into address electrodes 22 by the photolithographic method (see FIG. 5(b)).
A low melting point glass is applied onto the resulting substrate by screen printing, and then baked for formation of a dielectric layer A (see FIG. 5(c)).
A barrier rib material layer 33 is formed of a low melting point glass by screen printing, and then dried (see FIG. 5(d)). A mask is formed of a photoresist or the like on the resulting substrate, and then a portion of the barrier rib material layer 33 not covered with the mask is removed by sandblasting. Thereafter, the resulting substrate is baked for formation of barrier ribs 23 (see FIG. 5(e)).
Fluorescent layers are formed between the barrier ribs 23 by screen printing. Thus, the rear substrate 25 is completed (see FIG. 5(f)).
Subsequently, a sealing material is applied on a periphery of the glass substrate 21 and then baked for formation of a sealing member 34 (FIG. 5(g)).
The method for making the front substrate will next be described.
After a glass substrate 26 is cleaned, a transparent conductive film 35 is formed on the glass substrate 26 by sputtering, CVD(chemical vapor deposition), or screen printing (see FIGS. 6(a) and 6(b)).
The transparent conductive film 35 is formed into transparent electrodes 27 each having a predetermined configuration by photolithography (see FIG. 6(c)).
A metal layer 36 is formed on the transparent electrode 27 by vacuum evaporation or sputtering (see FIG. 6(d)), and formed into bus electrodes 28 each having a predetermined configuration by photolithography (see FIG. 6(e)). The bus electrodes 28 serve to reduce the interconnection resistance of the transparent electrodes 27.
A low melting point glass is applied onto the resulting substrate by screen printing, and then baked for formation of a dielectric layer 29 (see FIG. 6(f)).
Thereafter, a protective film 30 of magnesium oxide or the like is formed on the dielectric layer 29 by vacuum evaporation or the like to improve the discharge characteristics of the PDP (see FIG. 6(g)). Thus, the front substrate 31 is completed.
The front substrate 25 and the rear substrate 31 thus fabricated are joined together with the address electrodes 22 arranged perpendicular to the bus electrodes 28. A space defined by the front and rear substrate structures is evacuated, and filled with a discharge gas. Thus, the PDP shown in FIG. 4 is completed. Typically used as the discharge gas is a mixture of rare gases.
In the PDP, the bus electrodes and the address electrodes are each formed of aluminum or comprised of three metal layers of chromium-copper-chromium. The electrodes formed of aluminum present the following three problems.
A first problem is that aluminum diffuses into the low melting point glass when an organic binder contained in the low melting point glass is burned out for removal thereof and the low melting point glass is baked (or once melted) at a softening point thereof (typically not lower than 400.degree. C.) during the process for the formation of the dielectric layer, whereby the aluminum electrodes are eroded. The erosion of the aluminum electrodes results in a loss of metallic luster, so that the illumination from the fluorescent layers cannot efficiently be reflected.
A second problem is a reduction in the transmittance of the sustain electrodes. More specifically, the bus electrodes react with the transparent electrodes during the baking process for the formation of the dielectric layer, so that the transparent electrodes grow black. This leads to a reduction in the transmittance of the sustain electrodes, thereby reducing the illumination efficiency of the PDP. Although transparent electrodes of a NESA film do not suffer from the blacking, the blacking of transparent electrodes of an ITO (indium tin oxide) film is not negligible.
A third problem is that the aluminum electrodes are deformed at the baking of the low melting point glass. The deformation is caused due to grain recrystallization growth in the aluminum electrodes. The recrystallization growth occurs even at a temperature of not higher than a melting point of aluminum. The deformation of the aluminum electrodes starts at the surfaces thereof. When the deformation of the aluminum electrodes reaches the surface of the glass substrate, the deformation is visibly recognized, whereby the display quality of the PDP is degraded. Further, the recrystallization growth may cause stress migration to crack the aluminum electrodes, resulting in breakage thereof.
To solve these problems, there have been proposed a method for forming an aluminum oxide film on the surfaces of the aluminum electrodes by anodizing or thermal oxidation, and a method for coating the surfaces of the aluminum electrodes with a material such as aluminum oxide or silicon nitride having a low diffusion coefficient and a high blocking effect by a film formation process such as sputtering or CVD which is typically employed in the fabrication of semiconductor devices (Japanese Unexamined Patent Publications No. 52-70751 (1987) and No. 53-136957 (1988)).
These methods are characterized in that the surfaces of the aluminum electrodes are coated with an insulating film which does not melt at the baking temperature of the low melting point glass. The aforesaid three problems can be solved by the formation of the insulating film.
However, the aforesaid anodizing process involves a complicated process in which a voltage is applied to each of the electrodes to individually oxidize the surfaces of the electrodes, and takes several minutes to one hour for the application of the voltage, requiring increased costs.
The thermal oxidation process does not involve the application of a voltage, but takes a longer process time than the anodizing process.
Further, the aforesaid film formation processes such as sputtering and CVD which are typically used in the fabrication of semiconductor devices require expensive equipment.
That is, the aforesaid methods require increased process time and/or costs. Therefore, it has been desired to reduce the process time and costs. These problems are associated not only with the PDP but also with an active matrix liquid crystal display device in which signal lines (bus lines) and gate electrodes of thin film transistors (TFTs) for driving liquid crystal cells are formed of aluminum.