Background of the invention:
a) Field of the invention:
The present invention is in the field of processing for fabrication of the plasma display panel and more precisely in the field of fabricating a plasma display panel with Soda-Lime-Silica glass plates as display substrate and a plasma display panel produced by such method.
b) Prior art of the invention:
Of late plasma display panels (hereinafter called as PDP) have got an increased attention of the scientist and the manufacturing technologist throughout the world in the field of flat panel displays. In the domain of the flat panel displays the two major contenders are the liquid crystal displays (hereinafter called as LCDs) and the PDPs. The LCDs have a larger market share owing to its availability in all the size segments, for example, from very small 1 inch square mobile phone displays to 60 inch and larger diagonal sized televisions. PDPs, on the other hand, maintained a consistent place at 15% of the total market share and showed a slow but steady growth in this field. PDPs are able to remain in the competition because of its superior color rendition, very large viewing angle (close to 180 deg) and high dark room contrast ratio in comparison to the LCDs. However, the manufacturing cost and the operational power consumption of a PDP display set are two major reasons which are working disadvantageously against the growth of the PDPs in the market sphere. Several researchers throughout the world are working towards improving these two aspects of the PDPs.
Fig. 1 shows an overview of one such PDP called a surface discharge alternating current type, among PDPs of this type.
As in figure 1, the front glass substrate (1) and back glass plate (6) are shown. In the front glass substrate (1), display electrodes are made of transparent ITO
electrodes (2). To reduce the resistance of the display electrode, silver bus electrodes (3) are made over the ITO electrodes. The display electrode is covered with a transparent dielectric layer (4) to limit the discharge current. Then the MgO layer (5) is deposited over the transparent dielectric layer (4) to protect the transparent dielectric layer from possible sputtering. On the back glass substrate (6), a plurality of address electrodes (7) are formed with one address electrode (7) formed below each sub-pixel of Red (R ), Green (G) and Blue (B). The address electrodes (7) are covered with a white dielectric layer (8) to limit the discharge current and for reflection of the light formed in the pixels. The straight channel barrier ribs (9) are formed over the white dielectric layer. The R (10a), G (10b), B (10c) phosphor layers are formed in the barrier rib (9) channel spaces. The two plates are sealed with the help of sealing frit and the panel is baked at 350C. After the panel cools down to the room temperature, plasma gas mixture of Ne-Xe is filled in at a particular pressure.
In the product cost distribution of the PDP, the glass substrates constitute 15-19% of the total panel cost. A potential way to reduce the PDP manufacturing cost can be the use of soda-lime-silica (hereinafter called as SLS) glass substrates in place of costlier high-strain-point glass substrates. This way the cost of raw materials such as glass substrates can be reduced by -12%.
Several researches have disclosed methods to fabricate the plasma display panels using SLS glass substrates. However, in all the patents the researchers claimed to have heat treated the glass substrate at a temperature of -580 °C. Strain point is a temperature point in a glass heat treatment cycle, beyond which when a glass substrate is heated and cooled to room temperature, is left with a permanent dimensional change. This is called thermal strain. A soda lime silica glass plate has a strain point of 514 °C. This means that if the glass substrates are heated beyond this temperature it will be subjected to thermal expansion and upon cooling the substrate contracts irreversibly and is thus left with a permanent dimensional strain. The dimensional strain also prohibits
use of SLS glass substrate for HD and FHD applications, which requires an alignment in the range of ± 20 micron. Also misalignment of the add electrodes with reference to the barrier ribs give rise to misfiring of plasma cells. Also when the SLS glass substrate is heated at temperature range of 570°C-580°C, it gives rise to excessive yellowing (a phenomenon caused by red-ox reaction output of Ag+ ion and neutral Ag particles coming from the silver electrodes in the glass substrates and the dielectric glass matrix) in the front and the back panel.
Lee et al in US patent 6,376,398 claimed to have discovered a set of composition for transparent dielectric glass material and proposed to use this on a SLS glass substrate to fabricate a plasma display panel. However, the dielectric material is matured at a temperature range of 550°C-580°C. It is quite evident that the thermal strain resulting from heating the substrate at a temperature range of 550°C-580°C, which is higher than the strain point of SLS glass will not allow a defect free PDP.
Apart from this the low specific resistance log receptivity (log p = 7.5) of the SLS glass is lower than the high strain point glass such as PD 200. During long operation of the display panel a small leakage current may generate between the two adjacent electrode pairs and subsequently there can be loss of current. Some researchers have tried to apply a coating of chemical vapor deposited silica on the SLS glass substrates. Thereby, they tried to avoid a direct contact of the electrode materials and the SLS glass substrate. However, after some time in operation the electrode material diffuse through the silica layer and some residual fringe current starts passing through the SLS glass substrate. Therefore, this coating does not offer any permanent solution to the leakage current problem and the same continues to be a serious problem in this domain. To succeed with the soda lime silica glass in practice above mentioned problems must be addressed.
Object of the present invention:
With a view to the above mentioned problems associated with the usage of soda lime silica glass as substrates for the plasma display panels, the primary object of the present invention is to provide a suitable process to lay array of the horizontal or bus electrodes in the front plate and the vertical or address electrodes on the back plate made of soda-lime-silica glass substrates to fabricate a plasma display panel which can be digitally driven through a ALIS or ADS driving scheme.
Another object of the present invention is to provide a process whereby the electrodes will not give rise to leakage current through the glass substrate leading to failure of the PDP set.
Yet another object of the present invention is to provide a process to lay the array of the electrodes on the front and the back plates without excessive yellowing of the panel.
Yet another object of the present invention is to provide a process to fabricate a plasma display panel with soda lime silica glass based front and back plates without using ITO bus electrodes.
Yet another object of the present invention is to provide a process to fabricate a plasma display panel with soda lime silica glass based front and back plates without using ITO bus electrodes wherein the SLS glass processing steps have processing temperatures at most at 513°C.
Brief description of the drawings
Fig. 1 is a cross sectional view illustrating the basic structure of a conventional PDP relating to the present invention.
Fig. 2 is a cross sectional view illustrating the structure of the front plate of the novel PDP relating to the present invention wherein the foundation layer for the electrodes are applied throughout the front plate of the plasma display panel..
Fig.3 is a cross sectional view illustrating the structure of the front plate of the novel PDP relating to the present invention wherein the foundation layer for the electrodes is applied only below the conducting electrodes of the front plate of the plasma display panel.
Fig.4 is a cross sectional view illustrating the structure of the back plate of the novel PDP relating to the present invention wherein the foundation layer for the electrodes is applied throughout the back plate of the plasma display panel.
Fig. 5 is a cross sectional view illustrating the structure of the back plate of the novel PDP relating to the present invention wherein the foundation layer for the electrodes is applied only below the conducting electrodes of the back plate of the plasma display panel.
Fig.6 is a cross sectional view illustrating the cell structure of the front and the back plate of the PDP assembly.
Fig. 7 is a cross sectional view illustrating the cell structures of the front and the back plate of yet another PDP assembly.
Fig.8 is a flow chart describing the process for fabrication of the novel PDP in the present invention with the Soda Lime Silica glass or a high strain point glass as the substrate.
Description of the preferred embodiments
Hereinafter, the embodiments of the present invention will be described with reference to the drawings. All the drawings of the embodiments in the present invention are cross sectional in nature. The actual full scale of the display will comprise of multiple such unit cells in the direction of x-axis and y-axis. The
actual number of such unit cells arranged in the direction of x-axis and y-axis will depend on the display resolution.
Fig.2 shows the essential part of the first embodiment of the present invention.
As in figure 2, a low melting glass material (2) is coated over the front glass substrate (1). The thickness of the low melting glass coating may be kept at 5 to 15 micron. The coating is applied through screen printing of a suitable glass paste. The glass paste contains 70% of the glass material and the rest 30% organic solvent. The coating may also be made through application of a laminate sheet embedded with the said glass material or through spray of suitable glass material. After the coating is done the glass plate is then fired at 510 °C. The said glass layer works as a buffer layer between the glass plates and the electrodes (3) which will be formed over this layer. After the glass buffer layer coating is applied the bus electrodes (3) are formed. In the present example the electrodes (3) are formed through screen printing process using commercially available conducting silver electrode material for PDP. After formation of the electrodes the plate is fired at 505 °C. However, the electrode (3) formation may well be done through the photolithography process using commercially available photo sensitive conducting silver paste for PDP. After the electrode formation is over, a 40 micron thick transparent dielectric glass coating (4) is done on the electrodes. The thickness of this coating may vary from 30 to 45 micron depending on the dielectric constant of the glass material used. The glass plate is again fired at 500°C. The transparent dielectric coating may well be done either through single coating process or through a double coating screen printing process or a combination of the both. The sputter resistance MgO coating (5) is done through E- Beam evaporation method or a sputtering method. Thus the front plate of the plasma display is ready.
Fig.3 shows the second embodiment of the present invention.
As is Fig. 3 a low melting material (2) is applied on the front plate of the plasma display panel on the predetermined places where the electrodes would be
placed. This arrangement is done on the basis of display resolution and the particular pixel size as defined by the size of the display panel. The application of this first layer before the electrodes are formed is done through screen printing process. The same can well be done through application of a photosensitive paste made from the said low melting material and electrode formation through standard photolithography process. After the layers (2) are formed, the glass plate is dried at 120°C for half an hour and subsequently fired at 510 °C with a peak temperature soaking of approximately half an hour. Thus the foundation layer for the electrodes is made. Thereafter, the silver electrodes (3) are formed by screen printing process using the same screen that was used for the low melting foundation layer (2) printing. The electrode formation can well be made through photo lithography process as described in the preceding section. As described for the screen printing process the photo mask for the photo lithography process will remain same for both the foundation layer and the electrodes for a given PDP front plate. This will ensure that the electrodes (3) are exactly on the foundation layer (2) and there is no misalignment which will allow the electrodes to come in contact with the front glass substrate of the PDP. The subsequent coatings of transparent dielectric and the MgO are applied through the same process as described in the earlier embodiment as detailed in Fig.2.
Fig. 4 shows the third embodiment of the present invention.
As in Fig.4 a low melting material (7) is applied on the back plate of the plasma display panel through screen printing of a suitable paste made from the said low melting material. The thickness of the coating is maintained at 10 micron with a thickness variation of 2 micron through out the back plate of the PDP. The outer periphery of the coating (7) was so designed so that the whole area can offer a under coating to the electrodes (8) which are to be laid after this coating. The said coating (7) is made from a white opaque low melting glass material which is converted into a paste by adding a suitable quantity of organic solvents to it. In the embodiment of this invention, the said white
coating (7) is applied through screen printing process. However, a laminate sheet may well be made using the glass material through a standardized process and the same can well be applied on the back glass substrate (6) of the PDP. After the coating is made, it is dried at 120 °C and subsequently fired at 510°C. Thus the foundation layer for the electrodes is made on the back glass plate of the PDP. Silver electrodes (8) are formed on the said white foundation layer (7) by screen printing process. In absence of a screen printing process the said electrodes array may be made through a photo lithography process, as described earlier, using a commercially available photo sensitive conductive electrode material for PDP. The electrodes are dried at 120°C and subsequently fired at 505°C. Subsequent to this a white dielectric glass coating (9) is applied on the electrode array by screen printing process. As earlier mentioned, the said dielectric coating may well be applied through a suitable laminate process where the glass material is first converted into a laminate by addition of suitable organic solvents and then can be applied on the glass substrate (6) by a hot roller-pressing process followed by firing at suitable temperature of 505°C. Subsequent to this process step, barrier ribs (10) are made on the white dielectric layer (9) by screen printing process followed by drying at 120°C for half an hour and subsequently firing at 500°C. Phosphors (11) are then applied in the barrier rib channels (R, G, B) by screen printing process. The back plate of the plasma display panel thus made is dried at 120°C for half an hour and fired at 500°C for half an hour at peak temperature.
Fig. 5 shows the fourth embodiment of the present invention.
As in Fig. 5 the back plate of the PDP is made through a yet another process. The foundation layer (7) is formed on the back plate of the PDP at places where the electrodes will be laid. The space in between the electrodes is not covered with the white foundation layer and only the areas where the electrodes are there are covered with the foundation layer. The foundation layer (7) is subsequently dried at 120°C and further fired at 505°C. After this layer (7) is made, electrodes are formed on the foundation layers in a process which is
already described in preceding sections. The details are not repeated here again. Subsequent to this the white back dielectric (9), the barrier ribs (10) and the phosphors (11) are deposited in the designated channels following a standard procedure and as described in the earlier sections. Three phosphors such as red phosphor (R), green phosphors (G) and the blue phosphors (B) are deposited in the designated places.
Fig. 6 shows another embodiment of the present invention. The front panel made earlier as described in the Fig. 2 and the back plates made earlier as described in Fig. 4 are taken for further processing. In order to fabricate a PDP, sealing frit is applied to the periphery of the back plate as per usual practice and sealed with the front plate made earlier following conventional ways. A panel thus made is evacuated properly at a temperature of 380°C through a pre-fit tip tube and a mixture of Ne and Xe gas at predetermined pressure is filled into it. The tip tube is sealed and the plasma display panel is ready.
Fig. 7 shows another embodiment of the present invention. In this case, the front plate in the embodiment described in Fig. 3 and the back plate in the embodiment described in Fig. 5 are taken to make the PDP. The panel is sealed and the PDP is made following a process described in the preceding section.
It may be noted here in this point that the plasma display panel could well be made employing sealing of the front plate as described in the embodiment detailed in Fig. 2 and the back plate as described in the embodiment detailed in Fig. 5. And further, it may also be noted that the plasma display panel could well be made employing sealing of the front plate as described in the embodiment detailed in Fig. 3 and the back plate as described in the embodiment detailed in Fig. 4.
The foundation glass coating materials which are applied before the application of the electrodes on the front and the back plate and the glass dielectrics used in the present invention are so chosen that the co efficient of the thermal expansion is in the range of 82-87 X 107 / °C and, therefore, is conforming to
the requirement of both soda lime silica and high strain point glass substrates. The strain point of the transparent dielectric, white back dielectric and the barrier rib glass powder materials used in the present invention are 464°C, 476°C, and 481°C. The strain point of the glass materials used in foundation coating for the electrodes on the front plate and on the back plate in the embodiments of the present invention are 450 °C and 465°C. The dielectric constant of the transparent dielectric used in the front plate and the white back dielectric used in the back plate are 12.5 and 15 respectively. Dielectric materials having different dielectric constants other than that mentioned above may also be used for fabrication of a PDP as described in this invention.
The glass materials (2, & 7) used below the electrodes on the front and the back plate as shown in the embodiments in Fig. 2 to 5 do not contain any monovalent alkali ions such as that of lithium, sodium, and potassium. These glass coatings (2 85 7) will be working as an electrical insulator which will stop any leakage of electrical current flowing from the electrodes through the glass substrates such as that of soda lime silica glass.
Further, these glass materials (2, & 7) will stop the diffusion on the silver electrodes materials to the PDP glass substrates so the there will be no reduction-oxidation reaction possible with the tin atoms available in the glass substrate side with that of the silver ions at higher temperature and during prolonged operation of the PDP.
Further, it may be noted that the glass materials (2, & 7) and the dielectric glass materials mentioned in the embodiments in Fig.2 to 7, in general, do not have any single point melting temperature and they usually show a workable viscosity in the molten stage in a range of temperature. The temperature points mentioned in the embodiments of the present invention may vary with in a range of ± 3°C without causing any detrimental effect on the output quality of the PDP.
Further, the foundation glass coating ( 2) in the embodiments of the present invention as detailed in the Fig. 2 and Fig.3 have transparency in the range of at least 85% in the useful applied thickness. This is so chosen that the out put light from the plasma display panel does not get absorbed and reflected in this layer and, therefore, these layers do not reduce the overall brightness of the PDP.
In the embodiment detailed in the Fig.3, wherein the low melting glass coating (2) below the electrodes are restricted to the area covered by the electrodes, the said low melting glass material may contain oxides of Cobalt (Co), Manganese (Mn) and such black coloring oxides. The said low melting glass with these coloring oxides will be black in color and when applied below the electrodes in front glass plate, will improve the absorption of the visible light from the viewing room falling on the PDP front plate and thus the bright room contrast ratio of the resultant PDP as in embodiment detailed in Fig. 7 of the present invention will improve to a considerable extent.
In the case of the back substrate (6) in Fig. 4 and Fig. 5 the low melting opaque white glass coating may preferentially be on the whole glass instead of only below the electrodes so that this coating may also facilitate better reflection of the visible light which in-turn will improve the display quality of the PDP. Therefore the embodiment in Fig.4 may be preferred to the embodiment in Fig.5.
In the embodiments of the present inventions, the light shielding layers are not shown. However, the light shielding parts may well be fabricated in the inter pixel gaps between the discharge cell. The technique of applying the light shielding arrays is described by Kanazawa et al in the US patents 7,012,370 and Kim et al in the US patent 6,410,214.
Following the methods described in the exemplary embodiments of the present invention front and the back plates were fabricated with 1920 x 1080 pixel resolution in a 42 inch display size using soda lime silica based glass and high
strain point glass based substrates. Standard PDP phosphors such as YAG:Eu3+ as red phosphor, YBO3:Tb3+ as green phosphor and BaMgAhoO17:Eu2+ as blue phosphors were used in the designated channels made by barrier ribs. The front and the back plates thus made were sealed by using a sealing glass paste. The panel was evacuated through the tip tube and Ne+10% Xe gas mixture was filled in the panel at a pressure of 450 torr followed by tip sealing.
Flex connectors were connected to sustain, scan and the address electrodes of the panel. The necessary scan and sustain boards, the power supply sections and the other necessary electronics were added to the plasma display panels thus made and the panel is driven through suitable digitally configured logic programs. Through the employment of suitable video program all the digital signals were displayed in the plasma display panel.
Further it may be noted that the invention is not limited to the above embodiments and various modifications may be made without departing from the spirit and scope of the invention. The method described in the present invention may well be implemented in displays other than plasma displays. Any improvement may be made in part or all of the components by the persons skilled in the art.

What is claimed is:
1. A method of manufacturing a plasma display panel, comprising:
forming a transparent low melting glass coating on the front glass substrate at a temperature less than 514 °C; and
forming a plurality of discharge electrodes arranged on the low melting glass coating at a temperature less than 514 °C, preferably at 510°C ; and then
forming transparent low melting glass dielectric coating on the electrodes array fully covering the display area at a temperature less than 514°C, preferably at 510°C ;and
forming a opaque low melting glass coating on the back glass substrate at a temperature of 514 °C at most; and
forming a plurality of address electrodes arranged on the low melting glass coating at a temperature less than 514 °C, preferably at 510°C ; and then
forming opaque low melting glass white dielectric coating on the electrodes array at a temperature less than 514 °C, preferably at 505°C;
2. A method of manufacturing a plasma display panel, comprising:
forming a transparent low melting glass coating at a temperature less than 514 °C, on the front glass substrate where the application of this low melting glass coating will only be below the array of discharge electrodes and not covering the area beyond the area covered under the electrodes ; and
forming a plurality of discharge electrodes arranged on the low melting glass coating at a temperature less than 514 °C, preferably at 510°C so that the area of the electrodes can come exactly over the low melting glass coating beneath them; and then
forming transparent low melting glass dielectric coating on the electrodes array fully covering the display area at a temperature less than 514 °C, preferably at 505°C; and
forming a opaque low melting glass coating on the back glass substrate at a temperature less than 514 °C; and
forming a plurality of address electrodes arranged on the low melting glass coating at a temperature less than 514 °C, preferably at 510°C; and then
forming opaque low melting glass white dielectric coating on the electrodes array at a temperature less than 514 °C, preferably at 505°C;
3. A method of manufacturing a front plate of a plasma display panel as
claimed in claim 1, wherein the depositing the low temperature
foundation coating for the display electrodes comprises screen printing,
or photolithography or sand blasting.
4. A method of manufacturing a back plate of a plasma display panel as
claimed in claim 2, wherein the depositing the low temperature
foundation coating for the display electrodes comprises screen printing,
or photolithography or sand blasting.
5. A method of manufacturing a front plate of a plasma display panel as
claimed in claim 1, wherein the low temperature foundation coatings
comprising low temperature glass material having co efficient of thermal
expansion in the range of at least 80 X 10-7 / °C to at most 85 X 10 7 / °C
for the display electrodes on the front glass substrate is employed to
work as a yellowing retardant and as an electrical insulator to stop the leakage of the electrical current from one discharge cell to other through the glass substrate.
6. A method of manufacturing a front plate of a plasma display panel as
claimed in claim 2, wherein the low temperature foundation coatings
comprising low temperature glass material having co efficient of thermal
expansion in the range of at least 80 X 107 / °C to at most 85 X 10-7 / °C
for the display electrodes on the front glass substrate may comprise
black coloring oxides of cobalt and manganese and such elements to
work as an absorber of the incident light on the front plate of the display
to improve the bright room contrast ratio of the display.
7. A method of manufacturing a back plate of a plasma display panel as
claimed in claim 1, wherein the low temperature foundation coating for
the address electrodes on the back glass substrate comprises low
temperature glass material having co efficient of thermal expansion in
the range of at least 78X 107 / °C to at most 87 X lO7 / °C.
8. A method of manufacturing a plasma display panel as claimed in claim
1, wherein the both the front and the back glass substrates are heat
treated in any of the multiple heat treatment processes at a maximum of
510 °C which is below the strain point of soda lime silica glass substrate
which is 514°C.
9. A method of manufacturing a plasma display panel as claimed in
claim 1, wherein the both the front and the back glass substrates are
heat treated in any of the multiple heat treatment processes at a
maximum of 510 °C wherein the glass substrates are either both the
front and the back substrates are soda lime silicate based or both are
high strain point glass substrates and both are of similar kinds.
10. A method of manufacturing a front plate of a plasma display panel
as claimed in claim 1, wherein the low temperature foundation coating for the display electrodes on the front glass substrate comprises low temperature glass material having co efficient of thermal expansion in the range of at least 80 X 107 / °C to at most 85 X 107 / °C.