WO2001022463A2

WO2001022463A2 - Thin film tricolor display and method of fabricating the same

Info

Publication number: WO2001022463A2
Application number: PCT/US2000/025385
Authority: WO
Inventors: William C. Powers
Original assignee: Raytheon Company
Priority date: 1999-09-21
Filing date: 2000-09-15
Publication date: 2001-03-29
Also published as: WO2001022463A3

Abstract

A thin film cathodoluminescent anode includes tricolor phosphor elements which are annealed at the same temperature. A moveable mask is used to deposit the different color phosphor elements in a single vacuum cycle, by moving the mask relative to the substrate between deposition of each color phosphor. In a preferred embodiment, the tricolor phosphor elements are comprised of ZnS or ZnS/CdS and are annealed at a temperature between approximately 425-450 °C.

Description

THIN FILM TRICOLOR DISPLAY

BACKGROUND OF THE INVENTION

As is known in the art, modern color displays require higher resolutions than previously achievable, such as resolutions on the order of less than 0.005 inches. Applications requiring such high resolution include helmet displays and global positioning systems (GPSs).

Field emission displays (FEDs) are a type of cathode ray tube which can achieve relatively small display sizes as is also desirable in many modern applications. FEDs include a cathode comprising a dielectric substrate on which an array of field emission elements are formed and a cathodoluminescent anode comprising a dielectric substrate on which an array of phosphor elements is formed. In assembly, the anode and cathode structures are bonded together so that the field emission elements face the phosphor elements and the enclosed structure is evacuated. Driver electronics control the flow of electrons between the field emission elements and the respective pixels. The electrons, according to one accepted theory, are trapped by an activator of the phosphor elements, producing a negatively charged local field. A hole is attracted to this field where it is captured, resulting in an excited state. The return to ground state results in the emission of a photon.

Generally, high resolution displays utilize thin film technology since thin film structures experience less reflection and other phenomena which degrade resolution as compared to conventional crystalline structures. Thin film structures are those in which the phosphor elements are deposited as a continuous film of a homogeneous material. The thin film material is homogeneous in the sense that it is provided as a single crystal, although the material may comprise more than one element. Common thin film deposition techniques include vapor deposition, Metal Oxide Chemical Vapor Deposition (MOCVD), and sputtering and the material or materials comprising the thin film layer may be provided from the same or different sources.

Fabrication of a thin film tricolor cathodoluminescent anode is a multi-step process including adding an activator into the phosphor elements and annealing. The activator is a trace ion which serves to stress the crystal so that it becomes luminescent. Annealing refers to heating the phosphor elements to a temperature sufficient to relieve stresses in the phosphor elements, to allow the activator to diffuse into the phosphor elements, and to recrystalize the material of the phosphor elements. The step of adding an activator to the phosphor material is not necessary in crystalline structures since the powdered, or granular phosphor material of crystalline structures includes an activator. Also, annealing is not necessary in crystalline structures. Rather, crystalline structures are heated to a temperature lower than an annealing temperature for the purpose of removing organics from the structure.

Thin film tricolor anode fabrication includes, for each color, depositing the phosphor material onto a substrate in a vacuum chamber, such as by MOCVD, vapor deposition or sputtering and adding an activator to the phosphor material, such as by ion implantation following deposition of the phosphor material or by co-depositing with the phosphor material. Generally, the phosphor material and activator are deposited over the entire surface of the substrate and the resulting layer is then patterned, such as with the use of conventional photolithography, to provide the discrete phosphor elements of a given color. The structure is then removed from the vacuum chamber and annealed, following which this process is repeated for the phosphor elements of the remaining colors.

Because tricolor phosphors of conventional thin film displays are formed materials having different annealing temperatures, a separate annealing step is required for each color. Illustrative materials include ZincSilicate:Manganese, which require a quartz substrate due to the high annealing temperatures between 800-1000°C. Further, since the structure is removed from the vacuum chamber for annealing following deposition of each color phosphor, three different vacuum cycles are required, one cycle for the deposition of each of the three color phosphors. Multiple annealing steps and vacuum cycles results in a relatively time consuming and expensive fabrication process.

SUMMARY OF THE INVENTION

In accordance with the invention, a thin film cathodoluminescent anode includes a substrate on which are deposited red, blue and green phosphor elements, each having an annealing temperature range characteristic, with the annealing temperature range of the red, blue and green elements overlapping. With this arrangement, fabrication of the anode requires only a single annealing step, thereby reducing the fabrication time and cost as compared to conventional thin film tricolor displays. In one embodiment, the red, blue and green phosphor elements are comprised of ZnS or ZnS/CdS and the annealing temperature range of such phosphor elements is between 400-600°C.

A movable mask is provided for depositing the red, blue and green phosphor elements. With the mask in a first position, first regions of the substrate are exposed for deposition of red phosphor elements, in a second mask position, second regions of the substrate are exposed for deposition of green phosphor elements, and in a third mask position, third regions of the substrate are exposed for deposition of blue phosphor elements.

With this arrangement, the tricolor phosphors can be deposited in a single vacuum cycle. Use of a single vacuum run advantageously further reduces the time and cost associated with fabricating the anode as contrasted to conventional thin film tricolor displays. This is because the substrate need not be removed from the vacuum chamber for photolithographic patterning following deposition of each color phosphor.

An activator is added to the phosphor elements to cause the elements to be luminescent. In one embodiment, the activator is silver and is ion implanted into the tricolor phosphor elements in a single process step. Alternatively, the activator may be co-deposited with the phosphor material of each color.

Also described is a method for fabricating a thin film cathodoluminescent anode including depositing red, green and blue phosphor elements over a substrate and annealing the tricolor elements at the same temperature. The annealing temperature is, preferably, between approximately 425-450°C.

A further method for fabricating a thin film cathodoluminescent anode includes depositing a first phosphor material over a first region of a substrate to form a phosphor element of a first color, depositing a second phosphor material over a second region of the substrate to form a phosphor element of a second color, depositing a third phosphor material over a third region of the substrate to form a phosphor element of a third color, and annealing the phosphor elements of the first, second, and third colors after the tricolor phosphor elements are deposited. Additional process steps include depositing the first, second, and third phosphor materials through apertures of a movable mask and adding an activator to the phosphor elements, such as by ion implanting silver into the phosphors.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of this invention, as well as the invention itself, may be more fully understood from the following description of the drawings in which:

Figure 1 shows a cathodoluminescent anode of a thin film tricolor display during fabrication;

Figure 2 shows the cathodoluminescent anode of Figure 1 during a further stage of fabrication; Figure 3 shows the cathodoluminescent anode of Figure 1 during a further stage of fabrication;

Figure 4 is a flow diagram of a process for fabricating the cathodoluminescent anode of Figure 1; and

Figure 5 shows an illustrative flat panel display utilizing an FED containing the cathodoluminescent anode of Figure 3.

DETAILED DESCRIPTION OF THE INVENTION

Referring to Figures 1-3, a thin film cathodoluminescent anode 10 is shown during three stages of fabrication. The anode 10 is suitable for use in various display types, such as FEDs and cathode ray tubes (CRTs). The anode 10 includes a dielectric substrate 12 on which is deposited red, green, and blue phosphor elements 14, 16, 18, respectively. The substrate 12 may be comprised of various materials, such as borosilicate, soda lime glass, sapphire, or quartz. In the preferred embodiment, the substrate 12 is comprised of borosilicate or soda lime glass. The phosphor elements 14, 16, 18 are deposited on the substrate 12 in a vacuum chamber 24 through apertures 28 of a movable mask 20. More particularly, the mask 20 is controlled by a motor 26, such as a conventional stepper motor, which moves the mask horizontally over the substrate 12 in order to expose different regions of the substrate through apertures 28 during different stages of fabrication. With the movable mask in a first position shown in Figure 1, first regions of the substrate 12 are exposed for deposition of phosphor elements 14 of a first color, such as red. With the movable mask 20 in a second position shown in Figure 2, second regions of the substrate are exposed for deposition of phosphor elements 16 of a second color, such as green, and similarly, when the movable mask 20 is in a third position shown in Figure 3, third regions of the substrate are exposed for deposition of phosphor elements 18 of a third color, such as blue.

According to the invention, each of the red, green and blue phosphor elements 14, 16, 18, respectively, has an annealing temperature range characteristic and the annealing temperature ranges of the three color phosphors overlap. With this arrangement, the tricolor phosphors can be, and preferably are annealed at the same temperature during a single process step, as will be described.

Annealing serves to relieve stresses in the thin film phosphor elements, drive the activator into the phosphor elements, recrystalize the phosphor material and mingle the materials comprising the phosphor elements. The annealing temperature range is the range of temperatures at which the phosphor can be annealed. The temperature at which a material is capable of being annealed is a function of the properties of the material itself, the desired display resolution, and the desired display efficiency. An annealing temperature is selected to optimize these factors for a particular application and may also be limited by the maximum service temperature of the selected substrate material. For example, in the preferred embodiment, in which the substrate is borosilicate or soda lime glass, the maximum service temperature of the substrate is on the order of 500°C for soda lime glass and between 480- 525°C for borosilicate.

The preferred phosphor materials which have suitable color characteristics for forming tricolor elements, and which have overlapping annealing temperature ranges and further have annealing temperature ranges suitable for use with a glass substrate include zinc sulfide (ZnS) and zinc sulfide/cadmium sulfide (ZnS/CdS). The percentages of the elements comprising the phosphor material is varied to form the different color elements. In the preferred embodiment, the red elements 14 are comprised of 21% ZnS and 79% CdS by weight, the green elements 16 are comprised of 58% ZnS and 42% CdS by weight, and the blue elements 18 are comprised of ZnS. The annealing temperature range of the preferred tricolor phosphors is between approximately 400-600°C and thus, not only do such temperature ranges overlap, but they are substantially the same, permitting the phosphors to be annealed at the same temperature during a single process step.

The thin film phosphor elements 14, 16, 18 may be deposited through the mask apertures 28 onto the respective regions of the substrate 12 by various techniques, including MOCVD, vapor deposition and sputtering. In the illustrative embodiment, the mask is a thin metal structure, such as on the order of between 0.001-0.003 inches, having a plurality of photolithographically patterned apertures arranged to provide the desired screen geometry.

The dimensions of the phosphor elements can be varied to provide a desired display resolution. In the illustrative embodiment, the phosphor elements have a thickness between approximately 1-4 microns, a width on the order of 100 microns and spacing between adjacent elements on the order of 20 microns. It will be appreciated by those of ordinary skill in the art that the width can be varied by varying the size of the mask apertures 28 and the phosphor spacing can be varied by varying the distance that the mask is moved between phosphor deposition steps.

Each of the phosphor elements 14, 16, 18 includes an activator, which is added to the phosphor material in order to stress the crystal and make the material luminescent. In the illustrative embodiment, advantageously, the activator is added to each of the red, green and blue elements 14, 16, 18 is comprised of the same material and is added in the same process step. In the illustrative embodiment, the activator is silver and the mole ratio is on the order of 0.012 mol/mol. It will be appreciated by those of ordinary skill in the art however, that the activator material and content may be varied to vary the color of the phosphor elements.

Thus, the different colors of phosphor elements may be formed by varying the percentages of the elements of the phosphor material and/or by varying the material and/or percentage of the activator. The activator may be provided in the phosphor material by ion implantation, by co- depositing with the phosphor material during MOCVD or in a separate vapor deposition step. It will be appreciated by those of ordinary skill in the art that certain materials which may be used to provide the phosphor elements may themselves be cathodoluminescent, such as calcium tungstate, in which case an activator need not be added.

Preferably, the activator is ion implanted into the phosphor elements. Ion implantation advantageously drives the activator deeper into the phosphor material and reduces the necessary annealing temperature, thereby further facilitating use a relatively inexpensive glass substrate. In the illustrative embodiment, the activator is driven into the phosphor elements to a depth on the order of 1 micron.

Referring also to Figure 4, a method for fabricating the thin film cathodoluminescent anode 10 of Figure 3 is illustrated and is described in conjunction with a preferred anode structure. In step 52, a glass substrate 12 is placed into a vacuum chamber 24 and the chamber is evacuated. In step 56, the mask 20 is placed in a first position over the substrate to expose regions on which phosphor elements of a first color, such as red, will be deposited. A phosphor material is deposited over the substrate in step 60 in order to form phosphor elements 14 of the first color.

Steps 56 and 60 are repeated for deposition of the phosphor elements 16, 18 of the remaining two colors, green and blue, respectively. In particular, in step 68, the mask 20 is moved with respect to the substrate 12 to a second position, with the mask apertures 28 exposing regions of the substrate adjacent to the deposited red phosphor elements 14. In step 72, the phosphor material of the second color, such as green, is deposited to form green phosphors 16.

In step 80, the mask 20 is moved again, to a third position to expose further regions of the substrate adjacent to the red phosphor elements 14 and the green phosphor elements 16. In step 84, the phosphor material of the third color, here blue, is deposited through the mask apertures 28 to form blue phosphors 18. It will be appreciated by those of ordinary skill in the art that the order in which the red, green and blue phosphor elements are deposited may be varied.

In a preferred example, the phosphor material of the red elements 14 is 21% ZnS and 79% CdS by weight, the green phosphor elements are comprised of 58% ZnS and 42% CdS by weight, and the phosphor material of the blue elements is comprised of ZnS. As noted above, it will be appreciated by those of ordinary skill in the art that other materials may be suitable for use with the invention and also that the color of the phosphors may be varied by varying the percentages of the elements of the phosphor materials and/or by varying the material and/or percentage of the activator.

In step 88, an activator is implanted into the phosphor elements by ion implantation. As noted above, alternatively, the activator may be deposited by MOCVD or co-deposited with the phosphor material by MOCVD or vapor deposition in steps 60, 72, and 84, in which case step 88 may be eliminated. In the preferred example, the activator is silver and is ion implanted into the phosphor elements to a mole ratio on the order of 0.012 mol/mol.

Once the tricolor phosphor elements 14, 16, 18 are deposited and activated, the structure is removed from the vacuum chamber in step 92 for annealing in step 96. Advantageously, the annealing temperature ranges of the red, green and blue phosphor elements 14, 16, 18, respectively, overlap. In particular, the annealing temperature range of the described phosphors is between approximately 400-600°C. Preferably, the tricolor phosphors are annealed at a temperature on the order of 425-450°C in a single process step. The duration of the annealing step is on the order of 1-5 hours. The annealing step serves to relieve stresses in the phosphor elements, drive the silver activator into the phosphor elements, recrystalize or partially recrystaliize the phosphor material, and mingle the ZnS and CdS of the red and green elements. The emission peak of the red elements 14 occurs at approximately 6400Δ, the emission peak of the green elements 16 occurs at approximately 5300Δ, and the emission peak of the blue elements 18 occurs at approximately 4350Δ.

With the above-described structure and fabrication technique, advantageously, fabrication of the thin film cathodoluminescent anode requires only a single vacuum cycle and only a single annealing step. This is possible due to the use of the movable mask which is moved over the substrate to expose different regions for deposition of the different color elements during a single vacuum run and also by the use of materials for the different color phosphor elements which having at least overlapping, and preferably substantially similar annealing temperature ranges.

Referring also to Figure 5, a flat panel FED 100 is shown to include a thin film cathodoluminescent anode 10 of the type described and shown in conjunction with Figures 1- 3 and a cathode 104. The cathode 104 includes a plurality of field emission elements 108, sometimes referred to as tips, disposed on a dielectric substrate 112. In assembly, the anode 10 and cathode 104 are bonded together and the device is placed in an evacuation chamber which is pumped down to create a vacuum in the interior of the assembled structure.

What is claimed is:

Claims

1. A thin film cathodoluminescent anode comprising: a substrate; and a plurality of red, blue and green phosphor elements disposed on said substrate, each of said plurality of red, blue and green phosphor elements having an annealing temperature range characteristic, wherein said annealing temperature range of each of said plurality of red, blue and green phosphor elements overlap.

2. The thin film cathodoluminescent anode recited in claim 1 wherein said annealing temperature range of each of said plurality of red, blue and green phosphor elements is between approximately 400-600EC.

3. The thin film cathodoluminescent anode recited in claim 1 wherein each of said plurality of red, blue and green phosphor elements is comprised of a material selected from the group consisting of ZnS and ZnS/CdS.

4. The thin film cathodoluminescent anode recited in claim 3 wherein said annealing temperature range of each of said plurality of red, blue and green phosphor elements is between approximately 425-450EC.

5. The thin film cathodoluminescent anode recited in claim 1 wherein each of said plurality of red, blue and green phosphor elements comprises an activator.

6. The thin film cathodoluminescent anode recited in claim 5 wherein said activator is silver.

7. The thin film cathodoluminescent anode recited in claim 1 wherein said substrate is comprised of glass.

8. A thin film cathodoluminescent anode comprising: a substrate; and a plurality of red, blue and green phosphor elements disposed on said substrate, each of said plurality of phosphor elements comprising a material selected from the group consisting of ZnS and ZnS/CdS.

9. The thin film cathodoluminescent anode recited in claim 8 wherein said red phosphor elements are comprised of ZnS/CdS, said blue phosphor elements are comprised of ZnS and said green phosphor elements are comprised of ZnS/CdS.

10. The thin film cathodoluminescent anode recited in claim 8 wherein said material of said plurality of phosphor elements further comprises an activator.

11. The thin film cathodoluminescent anode recited in claim 10 wherein said activator is silver.

12. The thin film cathodoluminescent anode recited in claim 8 wherein said substrate is comprised of glass.

13. A method of fabricating a thin film cathodoluminescent anode, comprising the steps of: depositing a plurality of red, green and blue phosphor elements over a substrate; and annealing said plurality of red, green and blue phosphor elements at the same temperature.

14. The method of claim 13 wherein said annealing temperature is between 400-600°C.

15. The method of claim 14 wherein said annealing temperature is between 425-450°C.

16. A method of fabricating a thin film cathodoluminescent anode, comprising the steps of: depositing a first phosphor material over a first region of a substrate to form a phosphor element of a first color; depositing a second phosphor material over a second region of the substrate to form a phosphor element of a second color; depositing a third phosphor material over a third region of the substrate to form a phosphor element of a third color; and annealing said phosphor elements of said first, second, and third colors after said first, second, and third phosphor materials are deposited over respective regions of said substrate.

17. The method of claim 16 wherein said first phosphor material is ZnS/CdS and said first color is green, said second phosphor material is ZnS and said second color is blue, and said third phosphor material is ZnS/CdS and said third color is red.

18. The method of claim 16 further comprising the step of providing an activator in said phosphor elements of said first, second, and third colors.

19. The method of claim 18 wherein said activator providing step includes ion implanting silver into said phosphor elements of said first, second, and third colors.

20. A method of fabricating a cathodoluminescent anode, comprising the steps of: (a) placing a substrate in a vacuum chamber; (b) positioning a mask over said substrate to expose a first region of said substrate; (c) depositing a first phosphor material over said first region of said substrate to form a phosphor element of a first color; (d) moving said mask relative to said substrate to expose a second region of said substrate; (e) depositing a second phosphor material over said second region of said substrate to form a phosphor element of a second color; (f) moving said mask relative to said substrate to expose a third region of said substrate; (g) depositing a third phosphor material over said third region of said substrate to form a phosphor element of a third color; (h) removing said substrate from said vacuum chamber; and (i) annealing said phosphor elements of the first, second, and third colors.

21. The method of claim 20 wherein said first, second and third phosphor materials are selected from the group consisting of ZnS and ZnS/CdS.

22. The method of claim 20 further comprising the step of providing an activator in said phosphor elements of the first, second, and third colors prior to annealing said phosphor elements.

23. The method of claim 22 wherein said activator providing step includes the step of implanting silver into said phosphor elements of the first, second, and third colors.

24. The method of claim 20 wherein said annealing step includes heating said phosphor elements of said first, second, and third colors to a temperature between approximately 400- 600EC.

25. The method of claim 24 wherein said annealing step includes heating said phosphor elements of said first, second, and third colors to a temperature between approximately 425- 450EC.