Plasma display panels (hereinafter, PDPs) are attracting attention as large-screen display devices because of their high-speed display and easiness of size expansion compared to liquid crystal panels. There has been an increasing development toward higher display quality such as higher definition and higher luminance, and improved reliability.
An AC surface-discharge type PDP generally has a three-electrode structure in which two glass substrates, a front panel and a back panel, are oppositely disposed with a predetermined space therebetween. The front panel includes one of the two glass substrates; display electrodes; a dielectric layer; and a protective layer. The display electrodes consist of scan electrodes and sustain electrodes that are formed in a stripe pattern on the glass substrate. The dielectric layer coats the display electrodes so as to function as a capacitor to store electric charges. The protective layer is about 1 μm thick and formed on the dielectric layer. On the other hand, the back panel includes address electrodes formed on the other glass substrate; an base dielectric layer that coats the address electrodes; barrier ribs formed on the base dielectric layer; and phosphor layers that are applied in display cells formed by the barrier ribs so as to emit red, green, and blue light.
The front panel and the back panel are air-tight sealed with their electrode bearing sides opposed to each other. The barrier ribs form a discharge space filled with a discharge gas containing neon and xenon at a pressure of 400 Torr to 600 Torr. The display electrodes are selectively applied with a video signal voltage so as to discharge a discharge gas. The discharge gas generates ultraviolet light which excites the phosphor layers, allowing them to emit red, green, and blue light so as to display color images.
The protective layer is made of a material highly resistant to sputtering by ion impact so as to protect the dielectric layer from ion sputtering in a discharge. The protective layer functions to emit secondary electrons from its surface so as to reduce the driving voltage at which the discharge gas is discharged. Because of these characteristics, the protective layer is made of a single crystal magnesium oxide (MgO) by using vacuum thin-film coating technology.
The protective layer, however, becomes thinner and changes its secondary-emission characteristics due to ion impact as the lighting time of the PDP gets longer. As a result, a time delay (discharge delay) occurs before generating a discharge after applying a voltage to the display electrodes. This delay causes flickering on the display screen, resulting in significant deterioration of display quality.
The MgO protective layer can have different crystal compositions depending on the manufacturing method, and therefore can change the discharge delay time, and the display quality and lifetime of the PDP. FIG. 6 is a schematic view of a conventional apparatus for forming a protective layer. The following is a conventional method for forming a MgO protective layer with reference to FIG. 6. First, substrate 500 made of high strain-point glass or the like is preheated in preheating chamber 501 and transferred into film-forming chamber 502 in the direction of arrow “F”. In film-forming chamber 502, substrate 500 preheated to about 300° C. is exposed to vapor 503 of evaporated MgO particles blowing from below so as to form a MgO thin film on the surface of substrate 500. Vapor 503 is generated by irradiating an electron beam from Pierce-type electron gun 505 to MgO particle aggregates, which are an evaporation material placed in evaporation source 504 so as to melt and evaporate the MgO. There are provided baffle plates 506 between substrate 500 and evaporation source 504 so as to allow vapor 503 to be formed on the necessary portions of substrate 500.
The application of the electron beam to the MgO particle aggregates used as the evaporation material causes the MgO to decompose, allowing O atoms to escape. As a result, the formed MgO film lacks oxygen. In an attempt to make the composition ratio of the formed MgO film as close to the stoichiometric ratio as possible, oxygen is introduced using oxygen tank 507 or the like. However, the amount and method of introducing oxygen can greatly change the MgO characteristics.
In recent years, PDPs have been demanded to have higher-rate discharge characteristics in line with their improved definition. This makes it an urgent task to reduce the discharge delay due to the MgO composition. The task of reducing the discharge delay has been addressed as follows. In Japanese Patent Unexamined Publication No. 2003-297237, a change is made to the incident angle of evaporated particles. In Japanese Patent, Unexamined Publication No. 2004-031264, Ge or Si is added to MgO used as the evaporation material. In Japanese Patent Unexamined Publication No. 2002-33053, a MgO thin film is formed while being heated in an atmosphere containing hydrogen in an excited or ionized state so as to have sufficiently high sputtering resistance.
In these days, however, display devices have been demanded to have much higher definition and much higher-quality images, making it essential for PDPs to provide high-rate address discharge. When a discharge delay occurs in such a high-rate address discharge, it results in degradation of display quality such as dark defect or lighting failure. On the other hand, when the address time is extended to avoid the discharge delay, it reduces the sustain discharge time, thereby inevitably causing a deterioration in luminance and gradation. Therefore, it is crucial to minimize the discharge delay for the realization of Hi-Vision image display.