GB2331074A

GB2331074A - Channelled glass substrate

Info

Publication number: GB2331074A
Application number: GB9723030A
Authority: GB
Inventors: John Victor Armstrong; John Michael Bradshaw
Original assignee: PILKINGTON MICRONICS Ltd
Current assignee: PILKINGTON MICRONICS Ltd
Priority date: 1997-11-01
Filing date: 1997-11-01
Publication date: 1999-05-12
Also published as: GB9723030D0

Abstract

A glass substrate for use in a plasma display panel having a number of barrier ribs extending from the base thereof, the barrier ribs being integrated with the base and of the same glass composition. The substrate is manufactured by forming a number of channels in one surface of a glass sheet which define a barrier rib between adjacent channels, wherein the channels are formed by displacing glass from the surface of the sheet of glass, eg by using a cutting wheel, a cutting wire, heat pressing or laser ablation.

Description

Improvements in or relating to the Manufacture of Glass This invention relates to glass substrates for use in plasma display panels, including a method for manufacturing a glass substrate for use in a plasma display panel, a glass substrate manufactured by such a method, and a plasma display panel incorporating such a substrate.

Plasma display panels are used as visual display units in specialized applications, for example flat screen televisions. Such a panel has a large number of cells which are formed between two plates which are conventionally made from glass. In one form of panel each cell is coated with an appropriate phosphor and also contains a low pressure gas which ionises when subjected to an electrical discharge, emitting ultra violet radiation which strikes the phosphor causing it to fluoresce. The cells are isolated from each other by a series of parallel barrier ribs located on the surface of at least one of the glass plates of the panel. These ribs are generally 30-50 pm wide and up to 200 pm high. The barrier ribs not only form cells but also must prevent ultra violet radiation leaking into adjacent cells causing unwanted fluorescence in those adjacent cells.

One method of forming barrier ribs has utilized screen printing technology. However such a process is time consuming as a single printing step forms a coating of at most several tens of microns thick. Thus in order to provide a barrier rib of sufficient dimensions usually about ten printing steps are required and each layer requires drying prior to application of the next layer.

Once all printing steps have been completed the plate must be baked at about 5000 C to harden the barrier ribs formed thereon.

In an alternative method for forming barrier ribs, a surface of a glass plate is coated with a glass frit layer. The frit layer is then masked with an appropriate pattern and the glass plate subjected to sand-blasting which removes the unmasked areas of glass frit. The masking material is then removed and the plate is baked at about 500"C such that barrier ribs are formed from the glass frit material.

Thermal processing of the glass substrate to harden the barrier ribs formed by each of the above methods has led to distortion of the ribs. This is known as fanning and involves distortion along the length of the ribs in the transverse direction so that the ribs are no longer parallel to each other. It is believed that it is the nature of the substrate itself which undergoes localised differences in compaction, possibly due to the difference in cooling rates from the edge of the substrate to its centre, which causes fanning of the ribs. Furthermore it will be appreciated that the barrier ribs and glass plate are of different compositions and therefore subject to differential expansion during the thermal processing to harden the ribs. This differential expansion can enhance the fanning of the ribs. Substrates in which the barrier ribs have suffered from fanning are unsuitable for use in a plasma display panel.

One method used to reduce the occurrences of fanning has been to construct the substrate from a high strain point glass which is thermally stiffer and less prone to localised temperature variations. Such a glass substrate is more expensive than conventional substrates and the barrier ribs still have to be formed by either the screen printing or sand blasting method Furthermore in each of the above methods it may be necessary to polish the barrier ribs so that they are of equal height to ensure the cells of the plasma display panel will be isolated from each other.

In the manufacture of plasma display panels the formation of the barrier ribs is the single most expensive step. Furthermore the thermal processing required to harden the barrier ribs, consisting of heating the whole substrate to a temperature of around 5009C, is expensive.

Accordingly it is an object of the present invention to provide a glass substrate for use in a plasma display panel wherein the formation of the barrier ribs is simplified, and also to reduce the cost of manufacturing such substrates.

According to an aspect of the present invention there is provided a method of manufacturing a glass substrate for use in a plasma display panel, the method including the step of forming a plurality of channels in one surface of a glass sheet which define a barrier rib between adjacent channels wherein the channels are formed by displacing glass from the surface of the sheet of glass.

In this method it is not necessary to subject the body of the substrate to any thermal processing and so the problem of fanning of the barrier ribs is avoided. Furthermore this method simplifies production of glass substrates for plasma display panels as the barrier ribs may be formed by cutting channels out of the glass whereas in previous methods a number of processing steps including thermal processing and polishing were required to form the ribs. By reducing the number of processing steps and eliminating the need to heat the substrate to harden the ribs, the cost of manufacturing such a substrate is accordingly reduced.

Surprisingly, it has proved quite practical to cut channels with appropriate dimensions and spacing using available cutting technology, for example using a cutting wheel or alternatively a cutting wire, and a number of channels may be formed at the same time.

According to a further aspect of the present invention there is provided a glass substrate for use in a plasma display panel comprising a sheet of glass having a plurality of barrier ribs formed therein by displacement of glass from the surface of the sheet.

The invention further provides a glass substrate for use in a plasma display panel having a plurality of barrier ribs extending from the base thereof, the barrier ribs being integral with the base and of the same glass composition.

The barrier ribs may be arranged in a parallel array. They may be up to 500 Rm in height and they may also have a width of at least 10 cut An embodiment of the invention will now be described with reference to the accompanying drawings in which Figure 1 illustrates schematically a sequence of steps for forming barrier ribs on a glass substrate using a screen printing method.

Figure 2 illustrates schematically a sequence of steps for forming barrier ribs on a glass substrate using a sand-blasting method.

Figure 3 is a partial side sectional view of a prior art substrate suitable for use in a plasma display panel.

Figure 4 is an enlarged schematic end view of apparatus suitable for use in the invention Figure 5 is an enlarged partial end sectional view of a glass substrate according to the present invention.

Figure 6 is an enlarged schematic side view of a further form of apparatus suitable for use in the invention.

Figure 7 is a partial sectional side view of a substrate of the present invention suitable for use in a plasma display panel.

Figure 8 is an exploded perspective view of a plasma display panel incorporating a glass substrate according to the present invention.

Figure 1 shows in a simplified manner the steps involved in the formation of barrier ribs 14 using the known screen printing technique. A glass or ceramic composition is screen printed onto a glass sheet 10, the pattern on the screen being such that a number of parallel printed "lines" 12 are built up into barrier ribs 14 by successive printing steps. Usually at least ten printing steps are required but only four have been shown in Figure 1 for simplicity. It will be appreciated that as part of this process, the glass or ceramic composition must be dried between printing steps and that, once the barrier ribs 14 have been formed, the substrate 16 is baked at about 500"C to allow the glass or ceramic composition to fuse and harden.

In Figure 2 a further prior art method of forming barrier ribs is shown. A glass sheet 20 is coated with a thick frit glass layer 22 which is dried. Layer 22 is masked with a coating 24 of a conventional masking material known to the art. Abrasive powder is then sprayed onto the frit glass layer 22 through the gaps in the coating 24 such that the frit is removed down to the glass panel. The masking pattern is in the form of parallel lines and so upon removal of the coating 24 barrier ribs 26 are seen to have been formed which requires baking at about 500QC to harden.

Figure 3 shows part of a conventional substrate which forms a single cell in a conventional plasma display panel. Prior to formation of the ribs by either of the afore-mentioned methods, a series of parallel electrodes 32 are formed on glass sheet 30. The electrodes are covered by a layer of dielectric glass 34 which stores electrical charge and protects the electrode from high velocity ions within the cell when the plasma display panel is in use. The barrier ribs 36 are then formed on the dielectric glass and as has been noted above, are prone to the problem of distortion known as fanning during the baking of the substrate to harden the ribs 36. The distortion or fanning can also affect the electrodes and involves distortion along the ribs 36 and/or electrodes in the transverse direction in such a way that they are no longer parallel to each other. This may cause serious problems in a plasma display panel as a single electrode must run continuously along each channel and fanning may involve distortion to the extent that electrodes are partially or wholly covered by barrier ribs, thus disturbing the proper operation of the plasma display panel. Furthermore, fanning of the barrier ribs may lead to cells of different sizes which is highly undesirable in a plasma display paneL A substrate which displays fanning of the ribs and/or electrodes is usually unsuitable for use in a plasma display paneL In a suitable substrate the walls of the channel formed by adjacent barrier ribs 36 and the dielectric glass 34 are coated with an appropriate phosphor 38.

One type of apparatus used in a specific embodiment of the present invention is shown in Figure 4. A sheet of flat glass 40 is arranged such that a cutting operation may be carried out on it by an appropriate cutting machine. The glass sheet is typically composed of soda-limesilicate float glass but alternatively may be composed of other glass compositions suited for use in plasma display panel applications. Typically the sheet is about 3 mm thick but sheets of other thicknesses are also suitable.

The cutting machine includes a cutting head 42 comprising a number of cutting elements 44 arranged side by side. The cutting elements 44 are known in the art and each typically comprises a rotatable cutting wheel but alternatively may comprise a cutting wire. It will be appreciated that Figure 4 is an enlarged end view and that in practice each cutting element has a width of the order of hundreds of microns and each element is separated by a distance of at least 10 microns from adjacent elements.

In use the cutting elements 42 are brought into contact with the glass sheet 40 and cut into and along a surface thereof in a line parallel to an edge of the glass sheet to form a number of channels 46 by removing glass from the sheet as shown in Figure 5. The glass substrate comprises a base 49 and barrier ribs 48 which are defined between adjacent channels, and as the cutting elements are arranged side by side, are arranged in a substantially parallel array.

Typically the depth of each channel is up to 500 microns and preferably up to 200 microns.

The width of the barrier ribs is typically at least 10 microns and preferably at least 50 microns.

A cutting machine suitable for carrying out the above method is available from Loadpoint Limited from Swindon. With such a machine a number of channels were formed in a sheet of flat glass and a substantially parallel array of barrier ribs were formed. The height of the ribs h was 150 microns, their width w was 50 microns and the spacing between them p was 100 microns.

It will be appreciated that as conventional cutting elements are used in this process thermal processing is not required in the formation of the barrier ribs and so the problem of fanning of the ribs does not arise.

It will also be appreciated that polishing of the ribs to ensure uniform height is not required as flat glass with parallel surfaces has simply had channels of glass removed therefrom and the resulting barrier ribs are of even height.

The invention also encompasses alternative methods of forming channels and their associated barrier ribs in a glass substrate. One particular method is a form of pressing shown in Figure 6 wherein a pressing head 62 includes a number of pressing elements 64 arranged in a similar manner to that described in the afore-mentioned cutting machine. The pressing elements 64 are heated to a temperature sufficient that when the pressing head is pressed into contact with the glass 60, heat is transferred to the glass at the contact point and glass is softened and displaced by the pressing element 64. The pressing head is moved along the surface of the glass in a line parallel to an edge of the glass sheet in the direction of the arrow in Figure 6 to form a plurality of channels and associated barrier ribs.

Preferably the pressing head has a known type of cooling means associated with it (not shown) which assists in cooling the newly formed ribs and prevents them from collapsing ftom a combination of excess heat and their fine dimensions.

Although this embodiment requires a degree of thermal processing it is unlikely that the ribs will suffer from the problem of fanning as the body of the substrate is not subjected to thermal processing. The heating applied is localised and only occurs at the contact point between the pressing elements 64 and the glass. Furthermore, the fact that the ribs and body of the substrate are of the same material and thus have the same coefficient of thermal expansion limits any differential expansion between them resulting from the heat applied to the substrate in this way.

Substrates of the present invention require further processing after formation in order to be usable in a plasma display panel and Figure 7 shows part of a substrate of the present invention which form a single cell in a plasma display panel. An electrode 72 is applied along the channel of the substrate 70 and typically comprises a metal for example silver, chromecopperchrome or gold which may be applied by screen printing or deposition (vacuum or electroless) or photolithography as is weli known in the art.

A layer of dielectric glass frit 74 is then disposed along the channel covering the electrode.

The substrate is then baked at about 500QC to harden the dielectric glass Heating the substrate to this temperature may lead to differential expansion to a certain degree between the glass substrate (including the barrier ribs) and the electrode and also with the dielectric glass.

However, the barrier ribs and body of the glass substrate being made of the same material with the same coefficient of expansion, the differential expansion effects are limited and not likely to lead to fanning and, moreover, the electrodes are in any event trapped in the channels between the ribs and cannot move under the ribs. Thus, when using a glass substrate in accordance with the invention, any heating that may be required after formation of the ribs is unlikely to lead to problems. Once the substrate has been baked the walls of the ribs and the dielectric glass are coated with an appropriate phosphor 76.

As shown in Figure 8 a substrate 80 of the present invention has electrodes 82 protected by a dielectric glass located along the grooves between the barrier ribs with an appropriate phosphors 84 coated within each groove. Reference numerals 84a, 84b and 84c denote red, blue and green phosphors respectively. A conventional front glass substrate 90, which includes a dielectric layer 92 and further electrodes 94, is then secured to substrate 80 to form a plasma display panel which can be used as a visual display unit in, for example, a flat screen television A further method of forming channels and their associated barrier ribs according to the present invention involves the use of the known technique of laser ablation using a conventional excimer laser. A number of laser beams are arranged side by side and directed towards a surface of the glass. The laser arrangement is then moved in a line parallel to an edge of the glass and glass is ablated by the laser beams to form a plurality of channels and associated barrier ribs.

It will be appreciated that as one of the functions of the barrier ribs is to prevent ultra violet radiation leaking into adjacent cells, use of an ultra violet absorbing glass substrate is particularly beneficial in the present invention

Claims

CLAIMS 1. A method of manufacturing a glass substrate for use in a plasma display panel, the method including the step of forming a plurality of channels in one surface of a glass sheet which define a barrier rib between adjacent channels wherein the channels are formed by displacing glass from the surface of the sheet of glass.
2. A method as claimed in claim 1 wherein the channels are formed by a cutting wheeL
3. A method as claimed in claim 1 wherein the channels are formed by a cutting wire.
4. A method as claimed in claim 1 wherein the channels are formed by heated pressing elements in contact with the glass sheet.
5. A method as claimed in claim 1 wherein the channels are formed by laser ablation.
6. A method as claimed in any preceding claim wherein a plurality of channels are formed at the same time.
7. A glass substrate for use in a plasma display panel produced by a method as claimed in any of the preceding claims.
8. A glass substrate for use in a plasma display panel comprising a sheet of glass having a plurality of barrier ribs formed therein by displacement of glass from the surface of the sheet.
9. A glass substrate for use in a plasma display panel having a plurality of barrier ribs extending from the base thereof, the barrier ribs being integrated with the base and of the same glass composition.
10. A glass substrate as claimed in any of claims 7 to 9 wherein the barrier ribs are arranged in a parallel array.
11. A glass substrate as claimed in any of claims 7 to 10 wherein the height of the barrier ribs is up to 500 Rrm
12. A glass substrate as claimed in any of claims 7 to 11 wherein the barrier ribs have a width of at least 10 im
13. A plasma display panel including a glass substrate as claimed in any of claims 7 to 12.
14. A glass substrate for use in a plasma display panel as hereinbefore described with reference to and as illustrated in Figure 5 and Figure 7.
15. A method of manufacturing a glass substrate for use in a plasma display panel as hereinbefore described with reference to Figure 4 and Figure 6.
16. A plasma display panel as hereinbefore described with reference to Figure 8.