Heating – Work chamber having heating means – Having means by which work is progressed or moved mechanically
Reexamination Certificate
1999-10-25
2001-07-17
Lu, Jiping (Department: 3749)
Heating
Work chamber having heating means
Having means by which work is progressed or moved mechanically
C432S128000, C432S011000
Reexamination Certificate
active
06261091
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process and an apparatus for uniformly heat-treating a substrate which has a film-forming composition thereon.
2. Discussion of the Related Art
There are known substrates having films or layers formed thereon of a given material such as a metallic or inorganic material. In the present specification, the term “film” and the term “layer” are used interchangeably, unless otherwise specified. Such substrates include glass substrates made of a glass material, typically, a soda-lime glass, and ceramic substrates made of a ceramic material, typically, alumina. A film or layer, which has a certain function, may be bonded to the substrate by fusion or melting of a glass bonding component or by softening, melting or sintering of the material per se. These substrates may be used for anode plates for vacuum fluorescent displays (VFD), plasma switching boards for plasma address liquid crystal displays (PALC), field-emission displays (FED) and other display devices, thick-film wiring boards, and various electronic devices such as thermal printer heads and image sensors. Generally, the substrates for these electronic devices are subjected to heat treatments at temperatures of about 500-650° C. for the purpose of annealing the substrates per se or forming functional films with a glass material used as a bonding agent. Where the substrates are ceramic, the substrates are heat-treated at about 500-900° C. for forming functional films with a glass material used as a bonding agent or for forming functional films of a metallic material by utilizing the fusion of the metallic material at the interface with the substrates.
Recently, there have been increasing requirements for increasing the number of conductive, resistive, dielectric and other layers or films formed in desired patterns, and for increasing the density of such layers or films. Further, there has been an increasing demand for display devices having a large-sized display screen, and an accordingly increasing requirement for increasing the size of the substrates for such large-sized display devices. To meet these requirements, it is required to form minutely patterned layers or films over a comparatively large area, particularly, on the substrates for the display devices. The substrates for electronic devices described above have patterned functional films having minute cells or cavities. To assure high dimensional and positional accuracy of these minute cells, the functional films should be patterned with an improved degree of uniformity. However, the above heat treatment or firing of the substrates has an influence on the quality of the substrates, which influence increases as the size of the substrates increases. Therefore, the heat treatment causes a variation in the quality of the products using the substrates, and provides some restrictions in the design of the products, or reduces the yield of the products. The quality variation may be a variation in the resistance value of a resistor film, a variation in the withstand voltage of a dielectric film, a variation in the thickness due to uneven ratio of removal of binders by firing of the dielectric film, a variation in the continuity or resistance of a conductive film, a variation in the ease of wire-bonding or sputtering on the conductive film.
Where the substrate suffers from a dimensional change due to expansion or shrinkage of its material upon heat treatment, it is difficult to accurately position the patterned functional films relative to each other, since each function film is fired after it is formed in a predetermined pattern. The uniformity and positioning accuracy of the patterned films tend to be deteriorated with an increase in the density (minuteness) and size of the substrate, whereby the yield ratio of the product is significantly lowered as the density or size of the substrate is increased. In the case of a substrate for a plasma display device having a screen size as large as 40 inches, for example, the causes for lowering the yield ratio may include: insufficient dimensional accuracy of multiple layers which form multiple cells; variation in the height and width dimensions of partition walls; variation in the resistance of resistor cells; variation in the withstand voltage of a dielectric layer; an overall dimensional variation; and inaccurate positioning of front and rear plates which form a discharge cell.
SUMMARY OF THE INVENTION
The present invention was made in the light of the prior art drawbacks described above. It is therefore a first object of this invention to provide a process of firing a substrate with a film-forming composition provided thereon, which permits uniform heating of the substrate to thereby assure a high yield ratio of a product including the substrate.
It is a second object of the invention to provide a firing apparatus suitable for practicing the method indicated above.
The first object may be achieved according to a first aspect of the present invention, which provides a firing process of uniformly heat-treating a substrate having a film-forming composition thereon, comprising the steps of: (a) a first soaking step of holding, for a predetermined first time, said substrate in a first heating chamber whose temperature is maintained at a predetermined first value, so that the temperature within said substrate is held at said first value evenly throughout an entire mass of said substrate; (b) a feeding step of feeding said substrate subjected to said first soaking step, to a second heating chamber whose temperature is maintained at a predetermined second value which is different from said predetermined first value by a predetermined difference; and (c) a second soaking step of holding said substrate in said second heating chamber for a predetermined second time, so that the temperature within said substrate is held at said second value evenly throughout the entire mass of said substrate.
In the present firing process, the substrate having a film-forming composition is heat-treated by first subjecting the substrate to the first soaking step in the first heating chamber in which the substrate is held at the predetermined first temperature value for the predetermined first time for even distribution of the temperature throughout the entire mass of the substrate. Then, the substrate is fed into the second heating chamber whose temperature is maintained at the predetermined second value different from the first value by the predetermined amount, and is subjected to the second soaking step in which the substrate is held at the second value for the predetermined second time for even distribution of the temperature throughout the entire mass of the substrate. The substrate may be subjected to a further soaking step or steps. Thus, the substrate is heat-treated at different temperatures which are different from each other, so that a local variation in the temperature within the substrate and the film-forming composition is minimized. Where the substrate is formed of a glass material and is heat-treated at temperatures higher than the strain point of the glass material, a local dimensional variation or configurational deviation of the substrate can be minimized. Accordingly, the present firing process permits accurate positioning accuracy of films, layers or any other structural features subsequently formed on the substrate, resulting in a considerably increased yield ratio of the product which includes the substrate, even where the substrate has minute or intricate structural patterns or has a relatively large size. The film-forming composition provided on the surface of the substrate may be thick-film dielectric films, dielectric partition walls, thick-film resistor films, electrode films or inorganic pigment films. Since the present, firing process makes it possible to minimize the local temperature variation within the substrate and within such films or layers formed thereon, a glass material contained in the films or layers as a bonding component may be uni
Ichihara Hironobu
Mori Hiroyuki
Oshima Hiroshi
Sakamoto Susumu
Sato Yoji
Lu Jiping
Noritake Co. Ltd.
Oliff & Berridg,e PLC
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