EP0715330A1

EP0715330A1 - Radiation cooling apparatus related to display devices

Info

Publication number: EP0715330A1
Application number: EP95308552A
Authority: EP
Inventors: John W. Scoggan; Bruce E. Gnade
Original assignee: Texas Instruments Inc
Current assignee: Texas Instruments Inc
Priority date: 1994-11-28
Filing date: 1995-11-28
Publication date: 1996-06-05
Also published as: JPH08222130A; KR960019424A

Abstract

An apparatus 10 for providing radiation cooling of a workpiece 50 has an evacuated chamber 100 which encloses the workpiece 50, and has a cooling member 140 for cooling the workpiece 50 by radiation. A thermal energy transfer member 150 cools the cooling member 140 by conduction. Methods for radiation cooling the workpiece 50 are also disclosed.

Description

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to field emission flat panel display device production and, more particularly, to radiation cooling of a field emission device anode, cathode, and final assembly during manufacturing.

BACKGROUND OF THE INVENTION

Radiation cooling is the process by which heat is transferred from a body of higher temperature to a body of lower temperature by virtue of its temperature difference, without the aid of an intervening medium. The energy is transferred by electromagnetic radiation. Radiation is the primary means of thermal transport in a vacuum.
The phenomenon of field emission was discovered in the 1950's. Extensive research since then by many individuals has improved the technology to the extent that its prospects for use in the manufacture of inexpensive, low-power, high-resolution, high-contrast, full-color flat displays appears to be promising.
Advances in field emission display (FED) technology are disclosed in U.S. Patent No. 3,755,704, "Field Emission Cathode Structures and Devices Utilizing Such Structures," issued 28 August 1973, to C.A. Spindt et al.; U.S. Patent No. 4,940,916, "Electron Source with Micropoint Emissive Cathodes and Display Means by Cathodoluminescence Excited by Field Emission Using Said Source," issued 10 July 1990 to Michel Borel et al.; U.S. Patent No. 5,194,780, "Electron Source with Microtip Emissive Cathodes," issued 16 March 1993 to Robert Meyer; and U.S. Patent No. 5,225,820, "Microtip Trichromatic Fluorescent Screen," issued 6 July 1993, to Jean-Frédéric Clerc.
The Clerc ('820) patent listed above discloses a field emission device (FED) flat panel color display. This color display has a first glass substrate on which are arranged a matrix of conductors forming the cathode and gate electrodes. The field emission flat panel display also has a second glass substrate facing the first substrate and including regularly spaced parallel conductive stripes coated with phosphor comprising the anode electrode. The image created by the phosphor stripes is observed from the side of the anode glass substrate which is opposite to the phosphor excitation.
With the standard manufacturing process of FED's, the anode and cathode of a field emission display are attached to each other using a glass frit seal or rod at >400°C in air or vacuum. The display is heated by placing it between two heated plates. The time required to heat the device is moderate in order to limit the thermal gradients in the glass from the outside of the device to the inside of the device. Thermal gradients in the glass can cause stress in the glass, resulting in breakage if the gradients are large enough. Thermal gradients cause stress in the glass because of thermal expansion. If the outside of the glass is at a higher temperature than the inside of the display, the outside surface will be in tension.
One area for improvement in the manufacturing of field emission displays of the current technology is in the cooling of the assembled panel after sealing the anode and cathode together. Current manufacturing techniques try to minimize thermal gradients by heating and cooling the panels very slowly. This allows equal convection and conduction cooling of the outside of the panel, and radiation cooling of the inside of the panel. A cooling technique is needed that is less time consuming but will not overly stress the workpiece and will not affect the flatness of the workpiece. Similar cooling techniques would be useful for cooling the individual anode and cathode pieces after they are heated in the manufacturing process.

SUMMARY OF THE INVENTION

An apparatus for providing radiation cooling of a workpiece which has an evacuated chamber which encloses the workpiece, and has a cold plate for facilitating the cooling of the workpiece by radiation cooling. The temperature of the cold surface is maintained by actively cooling the cold plate using a cold fluid, such as liquid nitrogen. Methods for radiation cooling the workpiece are also disclosed.
Because the cooling process is used at least three times during the manufacture of a single FED, a significant manufacturing cycle time reduction is realized. By reducing cycle time in this manner, manufacturing costs are reduced and therefore the FED can be priced more competitively in the marketplace.

BRIEF DESCRIPTION OF THE DRAWING

The foregoing features of the present invention may be more fully understood from the following detailed description, read in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of a first embodiment of a radiation cooling apparatus of the present invention; and
FIG. 2 is a schematic diagram, in cross-sectional view, of a second embodiment of a radiation cooling apparatus of the present invention.

DETAILED DESCRIPTION

Referring initially to FIG. 1, there is shown a schematic diagram of a first embodiment of a radiation cooling apparatus of the present invention. In this embodiment, the radiation cooling apparatus 10 is coupled to a standard radiation heating apparatus 20.
The essential elements of a standard radiation heating apparatus are a quartz lamp 40 and a vacuum chamber 30 which is evacuated by vacuum pump 60 to a pressure of about 1 x 10^-7 Torr. When the quartz lamp 40 is on, a workpiece 50, disposed in the beam of the lamp 40, is heated through radiation heating. Workpiece 50 in the preferred embodiment is either an anode, a cathode, or an anode/cathode assembly. The workpiece 50 enters the chamber 30 through a door 70. A holding apparatus such as holding rods 80 which slide on tracks 90 hold the workpiece 50 in place during radiation heating and also carry the workpiece 50 from heating chamber 30 to cooling chamber 100. Ideally, holding rods 80 are made of a material having a low thermal conductivity. Holding rods 80 made of such a material will be less likely to remove heat by conduction from workpiece 50 at the points of contact between holding rods 80 and workpiece 50.
The purpose for heating the individual FED anode and cathode is to outgas contaminants from workpiece 50. The gaseous contaminants are removed from the ambient by the pumping system. Removing contaminants before and during assembly reduces outgassing after the panel is sealed. This will improve the long term reliability of the panel. This process of removing contaminants, called conditioning, is performed by raising the temperature of the workpiece 50. Next, the workpiece is stabilized at that high temperature to ensure that the conditioning process is complete.
A second reason for heating the FED assembly is to cause a glass frit or rod placed between the anode and cathode to melt and form a glass seal. This sealing operation takes place at >400°C. This seal is used to maintain a vacuum between the anode and cathode.
After workpiece 50 has been conditioned in evacuated heat chamber 30, the quartz lamp 40 is turned off and isolation gate 190, which separates chamber 30 from chamber 100, is opened. Holding rods 80 move the workpiece 50 along tracks 90 into evacuated cooling chamber 100. Isolation gate 190 is now closed and a new workpiece can now be conditioned in the radiation heating apparatus 20 as workpiece 50 is being cooled in the radiation cooling apparatus 10.
Workpiece 50 in the present invention is cooled by radiation cooling. Radiation cooling apparatus 10, shown in FIG. 1 has a cooling chamber 100 which is also evacuated by vacuum pump 60 to approximately 1 x 10^-7 Torr. Located in the chamber 100 are very cold plates 140 which are disposed parallel to workpiece 50. The temperature of cold plates 140 is regulated by the flow of a gas or liquid, such as nitrogen, through pipes 150 within the cold plates 140. The gas or liquid flowing through pipes 150 removes, by conduction, energy collected by the cold plates 140 from workpiece 50. Cooling units 160 contain a compressor 180 and condenser 170 which together refrigerate the liquid in the pipes 150. In the preferred embodiment, the cold plates 140 are maintained at a temperature of around -196°C. Therefore, a very large temperature differential is maintained between the heated workpiece 50 and cold plates 140.
Also in the preferred embodiment the cold plates 140 are made of a high emissivity and high thermal conductivity material such as black anodized aluminum or oxidized copper. The use of a high emissivity material as cold plates 140 facilitates efficient absorption of the thermal radiation emitted by workpiece 50. The use of a high thermal conductivity material facilitates efficient conduction of the heat from cold plates 140 to the circulating fluid in pipes 150. The heat transferred by radiant energy from workpiece 50 is proportional to (1) the difference in the fourth power of the absolute temperatures (T⁴) of the hot source (workpiece 50) and the more-cool receiver of the radiation (cold plates 140) and to (2) a geometric constant which describes the form factors of workpiece 50 and the cold plates 140.
Also as a result of the radiative heat transfer properties described above, the workpiece 50 cools more rapidly at the initial high temperature when the temperature difference between the workpiece and the cold plates is maximum. Because the workpiece 50 is in greatest danger of temperature induced stress at temperatures above 250°C, it is possible to safely remove the workpiece 50 from chamber 100 through door 200 at temperatures somewhere between 250°C and room temperature. In this non-critical temperature range, standard conduction and convection techniques may be used to complete the cooling process of the workpiece 50.
The process of radiative cooling as described is used to cool the FED anode and cathode after conditioning, and to cool a FED after the anode and cathode has been assembled. The radiant cooling process of the present invention as described above can reduce the time for cooling the workpiece 50 from approximately 8 hours to approximately 5 minutes. Even though the present invention reduces the cooling time of workpiece 50, the technique of the present invention does not introduce harmful stress into workpiece 50.
The radiation cooling process of the present invention is used to cool the anode and cathode after conditioning and is also used to cool the anode/cathode assembly after sealing. Because the cooling process is used at least three times during the manufacture of a single FED, a significant manufacturing cycle time reduction is realized. By reducing cycle time in this manner, manufacturing costs are reduced and therefore the FED can be priced more competitively in the marketplace.
In an alternate embodiment of the present invention, shown in FIG. 2, the cold plate 210 encircles the workpiece 50. By encompassing the workpiece 50, a more uniform cooling of workpiece 50 from all sides may be realized.
Several other variations in the above processes, such as would be understood by one skilled in the art to which it pertains, are considered to be within the scope of the present invention. For example, cool plate 110 and cold plates 140 can be chilled by the use of other fluids or gases or other refrigeration techniques. Also other methods may be used to transport the workpiece 50 between chambers 30 and 100; and other methods may be used to hold the workpiece 50 in place during heating and cooling.
Furthermore, the cold plates 140 may be adjustable. Adjustable cold plates 140 could be moved throughout the radiation cooling process to increase or decrease the cooling rate by changing the geometric constant. Additionally, adjustable cold plates 140 could be positioned closer or further apart to accommodate differences in workpiece sizes and shapes.

Claims

Apparatus for providing radiation cooling of a workpiece, comprising:
an evacuated chamber in which said workpiece is disposed; and

cooling means having a surface adjacent said workpiece for cooling said workpiece by radiation cooling.
The apparatus in accordance with Claim 1 further comprising; means for providing conduction cooling of said cooling means.
The apparatus in accordance with Claims 1-2, wherein said surface encloses said workpiece.
The apparatus in accordance with Claims 1-3, wherein said cooling means further comprises a second surface adjacent said workpiece for cooling said workpiece by radiation cooling.
The apparatus in accordance with Claims 1-4, wherein said cooling means is adjustable.
The apparatus in accordance with Claims 1-5, wherein said second surface is adjustable.
The apparatus in accordance with Claims 1-6, further comprising:
a vacuum pump coupled to said chamber to facilitate the evacuation thereof.
The apparatus in accordance with Claims 1-7, further comprising; a thermal energy transfer member for cooling said cooling member by conduction.
The apparatus in accordance with Claims 1-7, further comprising; a thermal energy transfer member for cooling said cooling means by convection.
A method for radiation cooling of a workpiece comprising:
placing said workpiece in an evacuated chamber;

providing a cooling means having a surface adjacent said workpiece; and

cooling said cooling means to thereby provide radiation cooling of said workpiece.
The method in accordance with Claim 10, wherein said cooling step comprises conduction cooling of said cooling means.
The method in accordance with Claim 10, wherein said cooling step comprises convection cooling of said cooling means.