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(11) | EP 1 059 655 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Mechanism for maintaining pressure of helium in channels of a palc display panel |
| (57) A channel surface of a glass channel substrate for a PALC panel is loaded with helium
to a sufficient partial pressure that when the channel substrate is used in a PALC
panel with helium as ionizable gas, helium is depleted from the channel at a substantially
lower rate than if the channel surface had not been loaded with helium. |
Cross Referenced to Related Applications
[0001] This application claims the benefit of U.S. Provisional Application No. 60/132,157, filed May 3, 1999.
Background of the Invention
[0002] This invention relates to a mechanism for maintaining pressure of helium in channels of a PALC display panel.
[0003] U.S. Patent 5,077,553 discloses apparatus for addressing data storage elements. A practical implementation of the apparatus shown in U.S. Patent 5,077,553 is illustrated schematically in FIG. 1 of the accompanying drawings.
[0004] The display panel shown in FIG. 1 comprises, in sequence from below, a polarizer 2, a channel member 4, a cover sheet 6 (commonly known as a microsheet), a layer 10 of electro-optic material, an array of parallel transparent data drive electrodes (only one of which, designated 12, can be seen in the view shown in FIG. 1), an upper substrate 14 carrying the data drive electrodes, and an upper polarizer 16. In the case of a color display panel, the panel includes color filters (not shown) between the layer 10 and the upper substrate 14. The panel may also include layers for improving viewing angle and for other purposes. The channel member 4 is typically made of glass and is formed with multiple parallel channels 20 in its upper main face. The channels 20, which are separated by ribs 22, are filled with an ionizable gas, such as helium. An anode 24 and a cathode 26 are provided in each of the channels 20. The channels 20 are orthogonal to the data drive electrodes and the region where a data drive electrode crosses a channel (when viewed perpendicularly to the panel) forms a discrete panel element 28. Each panel element can be considered to include elements of the layer 10 and the lower and upper polarizers 2 and 16. The region of the upper surface of the display panel that bounds the panel element constitutes a single pixel 30 of the display panel.
[0005] When the anode 24 in one of the channels is connected to a reference potential and a suitably more negative voltage is applied to the cathode 26 in that channel, the gas in the channel forms a plasma which provides a conductive path to the reference potential at the lower surface of the cover sheet 6. If a data drive electrode is at the reference potential, there is no significant electric field in the volume element of electro-optic material in the panel element at the crossing of the channel and the data drive electrode and the panel element is considered to be off, whereas if the data drive electrode is at a substantially different potential from the reference potential, there is a substantial electric field in that volume element of electro-optic material and the panel element is considered to be on.
[0006] It will be assumed in the following description, without intending to limit the scope of the claims, that the lower polarizer 2 is a linear polarizer and that its plane of polarization can be arbitrarily designated as being at 0° relative to a reference plane, that the upper polarizer 16 is a linear polarizer having its plane of polarization at 90°, and that the electro-optic material rotates the plane of polarization of linearly polarized light passing therethrough by an angle which is a function of the electric field in the electro-optic material. When the panel element is off, the angle of rotation is 90°; and when the panel element is on, the angle of rotation is zero.
[0007] The panel is illuminated from the underside by an extended light source 34 which emits unpolarized white light. A rear glass diffuser 18 having a scattering surface may be positioned between the light source and the panel in order to provide uniform illumination of the panel. The light that enters a given panel element from the source is linearly polarized at 0° by the lower polarizer 2 and passes sequentially through the channel member 4, the channel 20, the cover sheet 6, and the volume element of the electro-optic material toward the upper polarizer 16 and a viewer 32. If the panel element is off, the plane of polarization of linearly polarized light passing through the volume element of electro-optic material is rotated through 90°, and therefore the plane of polarization of light incident on the upper polarizer element is at 90°. The light is passed by the upper polarizer element and the pixel is illuminated. If, on the other hand, the panel element is on, the plane of polarization of the linearly polarized light is not changed on passing through the volume element of electro-optic material. The plane of polarization of light incident on the upper polarizer element is at 0° and therefore the light is blocked by the upper polarizer element and the pixel is dark. If the electric field in the volume element of electro-optic material is intermediate the values associated with the panel element being off and on, light is passed by the upper polarizer element with an intensity which depends on the electric field, allowing a gray scale to be displayed.
[0008] In a practical implementation of the PALC display panel, the channel member 4 is etched back around the area in which the channels are formed in order to provide a plateau 36 in which the channels 20 are formed, and the cover sheet 6 is secured to the channel member by an endless frit bead 38 in a rabbet 40 extending around the periphery of the plateau. An upper substrate assembly, including the upper substrate 14 and the data drive electrodes 12 carried thereby, is attached to the channel member 4 by means of a glue bead 42.
[0009] When the plasma exists in the channel, charged particles in the channel are accelerated to a high level of kinetic energy. Some of these particles strike the channel surface (the surfaces of the channel substrate and cover sheet that bound the channel) and thereby heat the channel surface.
[0010] It is well known that the permeation rate of helium into glass depends strongly on the temperature of the glass. For example, the permeation rate of helium into glass of the kind that is conventionally used in fabrication of the channel substrate of a PALC panel is approximately 100 times greater at 150° C than at about 18° C. Thus, when the plasma is operating and the channel surface of the channel substrate is heated, helium rapidly permeates into the hot channel surface of the channel substrate toward the cooler bulk.
[0011] When the plasma is removed, the channel surface of the channel substrate cools and some of the helium that previously permeated into the channel substrate tends to permeate back toward the channel. Because the channel surface of the channel substrate cools rapidly, the permeation rate of helium from the bulk of the channel substrate back into the channel when the plasma is off is much lower than the permeation rate toward the bulk when the plasma is operating. Helium is therefore trapped in the channel surface. If sufficient helium is trapped in the channel surface to reduce the pressure of the helium remaining in the channel by a factor of about two, the voltage required to form the plasma will increase. If the voltage increases to such an extent that it exceeds the capability of the drive circuits, the panel becomes inoperable.
Summary of the Invention
[0012] In accordance with a first aspect of the invention there is provided a channel substrate for a PALC panel, comprising a glass substrate having channels formed therein, and wherein a channel surface of the substrate is loaded with helium to a sufficient partial pressure that when the channel substrate is used in a PALC panel with helium as ionizable gas, helium is depleted from the channel at a substantially lower rate than if the channel surface had not been loaded with helium.
[0013] In accordance with a second aspect of the invention there is provided a channel substrate assembly for a PALC panel, comprising a glass substrate having channels formed therein and a glass cover sheet attached to the glass substrate, and wherein a channel surface of the channel substrate assembly is loaded with helium to a sufficient partial pressure that when the channel substrate assembly is used in a PALC panel with helium as ionizable gas, helium is depleted from the channel at a substantially lower rate than if the channel surface had not been loaded with helium.
[0014] In accordance with a third aspect of the invention there is provided a channel substrate for a PALC panel, comprising a glass substrate having channels formed therein, and wherein a surface of the substrate is provided with a coating of a material having a lower permeation rate with respect to helium than the glass of the substrate and having a higher melting point than the glass of the substrate, so that when the coated channel substrate is used in a PALC panel with helium as ionizable gas, helium is depleted from the channel at a substantially lower rate than if the surface had no coating.
[0015] In accordance with a fourth aspect of the invention there is provided a method of manufacturing a channel substrate for a PALC panel, comprising providing a glass substrate having channels formed therein, loading a channel surface of the substrate with helium, subsequently filling the channels with helium and sealing the channels.
[0016] In accordance with a fifth aspect of the invention there is provided a method of processing a channel substrate assembly for a PALC panel, which channel substrate assembly includes a glass substrate formed with channels and a cover sheet attached to the substrate over the channels, said method comprising heating the channel substrate assembly to an elevated temperature, establishing a partial pressure P of helium in the channels of the substrate, and sealing the channels.
[0017] In accordance with a sixth aspect of the invention there is provided a method of manufacturing a channel substrate for a PALC panel, comprising providing a glass substrate formed with channels, and forming a coating on a surface of the substrate, the coating being of a material having a lower permeation rate with respect to helium than the glass of the substrate and having a higher melting point than the glass of the substrate.
Brief Description of the Drawings
[0018] For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which:
FIG. 1 is a partial sectional view of a PALC display panel in accordance with the prior art, and
FIG.2 is a partial sectional view of a channel substrate in accordance with the present invention.
[0019] In this specification, words of orientation and position, such as upper and lower, are used to establish orientation and position relative to the drawings and are not intended to be limiting in an absolute sense. Thus, a surface that is described as upper in the specification may correspond, in a practical implementation of the invention, to a lower surface or a vertical surface, which is neither upper nor lower.
Detailed Description
[0020] The permeation rate of helium into glass from a space bounded by a surface of the glass depends not only on the temperature of the glass but also on the partial pressure of helium in the glass and the partial pressure of helium in the space bounded by the glass.
[0021] In accordance with the invention, the channel surfaces of a PALC display panel are preloaded with helium. Accordingly, the partial pressure of helium in the bulk is increased, reducing the difference between the partial pressure of helium in the channel and the bulk and reducing the permeation rate of helium from the channel into the bulk.
[0022] The channel surfaces can be preloaded with helium by filling and sealing the panel at elevated temperatures and pressure. Conventionally, the panel is filled and sealed at a pressure in the range from about 100 to 500 mb and a temperature in the range from about 20 to 50° C. When the panel is filled and sealed at elevated temperatures and pressures (200-900 mb and 250-400° C), helium permeates into the glass. The helium that remains in the channels is at a sufficient partial pressure for operation of the panel.
[0023] Alternatively, the channel surfaces of the substrate can be preloaded by heating the substrate in a helium atmosphere before filling and sealing but after high temperature processes required for fabrication of the channel substrate assembly have been completed. In this case, the channel substrate can be heated to near 550° C, where the glass is near its softening point.
[0024] A third possibility is to subject the channel substrate assembly (the channel substrate and the cover sheet) to an RF helium plasma. Before the filling and sealing operation, the channel substrate assembly is placed in a cavity which contains helium and in which an RF field suitable to ionize the helium can be generated. By creating a plasma in this manner, helium permeates into the surface layer and is trapped when the plasma is removed. When the channel substrate assembly is then filled and sealed, the temperature is sufficiently low, and the pressure is sufficiently high, that the helium remains trapped in the glass.
[0025] Other possibilities for avoiding the problem of permeation of helium into the glass include providing a surface barrier on the glass and choosing the glass so that the permeation rate of helium into the glass is significantly less than that of the types of glass conventionally used in manufacture of PALC display panels.
[0026] Glasses that are conventionally used in fabrication of PALC display panels are typically borosilicate glasses, such as the glass sold by Schott Glass under the designation D263. Other glasses, such as the alkali-free borosilicate glass sold by Schott Glass under the designation AF45, which has a high concentration of barium oxide and aluminum oxide, are also suitable for fabrication of a PALC display panel and have a significantly lower permeation rate with respect to helium in the temperature ranges involved.
[0027] A suitable surface barrier can be provided by vapor deposition of a material that has a lower permeation rate with respect to helium and a higher melting point than the borosilicate glasses that are conventionally used in fabrication of PALC display panels. Referring to FIG. 2, a barrier layer 46 of such a material is provided on the surfaces bounding the channels of the channel substrate 4 before the channel electrodes are deposited. By providing the barrier layer 46, the relative temperature of the channel surface of the substrate is lower than it would be if the channel were bounded by borosilicate glass. One material having both a lower permeation rate with respect to helium than borosilicate glasses and a higher melting point than borosilicate glasses is sapphire. Techniques for depositing a layer of sapphire on a glass surface are well known.
1. A channel substrate for a PALC panel, comprising a glass substrate having channels
formed therein, and wherein a channel surface of the substrate is loaded with helium
to a sufficient partial pressure that when the channel substrate is used in a PALC
panel with helium as ionizable gas, helium is depleted from the channel at a substantially
lower rate than if the channel surface had not been loaded with helium.
2. A channel substrate assembly for a PALC panel, comprising a glass substrate having
channels formed therein and a glass cover sheet attached to the glass substrate, and
wherein a channel surface of the channel substrate assembly is loaded with helium
to a sufficient partial pressure that when the channel substrate assembly is used
in a PALC panel with helium as ionizable gas, helium is depleted from the channel
at a substantially lower rate than if the channel surface had not been loaded with
helium.
3. A channel substrate for a PALC panel, comprising a glass substrate (4) having channels
(20) formed therein, and wherein a surface of the substrate is provided with a coating
(46) of a material having a lower permeation rate with respect to helium than the
glass of the substrate and having a higher melting point than the glass of the substrate,
so that when the coated channel substrate is used in a PALC panel with helium as ionizable
gas, helium is depleted from the channel at a substantially lower rate than if the
surface had no coating.
4. A channel substrate according to claim 3, wherein said material is sapphire.
5. A method of manufacturing a channel substrate for a PALC panel, comprising:
providing a glass substrate having channels formed therein,
loading a channel surface of the substrate with helium,
subsequently filling the channels with helium and sealing the channels.
6. A method of processing a channel substrate assembly for a PALC panel, which channel
substrate assembly includes a glass substrate formed with channels and a cover sheet
attached to the substrate over the channels, said method comprising:
heating the channel substrate assembly to an elevated temperature,
establishing a partial pressure P of helium in the channels of the substrate, and
sealing the channels.
7. A method of manufacturing a channel substrate for a PALC panel, comprising:
providing a glass substrate (4) formed with channels, and
forming a coating (46) on a surface of the substrate, the coating being of a material having a lower permeation rate with respect to helium than the glass of the substrate and having a higher melting point than the glass of the substrate.
8. A method according to claim 7, wherein said material is sapphire.
9. A method according to claim 7, further comprising filling the channels (20) with helium
and sealing the channels.