WO1998010461A1

WO1998010461A1 - Thin-film phosphors for field emission displays

Info

Publication number: WO1998010461A1
Application number: PCT/US1997/015527
Authority: WO
Inventors: Michael L. Ferris
Original assignee: Saes Getters S.P.A.
Priority date: 1996-09-04
Filing date: 1997-09-03
Publication date: 1998-03-12
Also published as: AU4248897A

Abstract

A field emission display having a screen which includes a plurality of substantially discrete thin film phosphors, and a method for making such a field emission display, is described. The field emission display includes a screen member which is coupled with an electrode support member to define an interior space. The interior surface of the screen member is coupled with a plurality of substantially discrete thin-film phosphor deposits, and the opposing interior surface of the electrode support member is coupled with electrode means such that electronic radiation from said electrode means is effective to excite said thin-film phosphor deposits into luminescent states whereby photons are emitted from the thin film phosphor deposits at angles substantially perpendicular to the exterior surface of said screen member.

Description

THIN-FILM PHOSPHORS FOR FIELD EMISSION DISPLAYS

BACKGROUND OF THE INVENTION

1 . The Field of the Invention

The present invention relates to video devices and solid-state electronics. More specifically, the present invention is related to the production of thin-film phosphors for use in video devices, especially field emission displays (FEDs) and other display devices that include internal vacuums and phosphors.

2. The Relevant Art

Field emission displays (FEDs) are presently under development to provide various video display devices, for example, flat television screens. Using these screens, video displays can be constructed that are much lighter than present cathode ray tubes (CRTs). Because of the great reduction in weight, FEDs can be used to provide video displays having much greater area than present back-projected displays (e.g., CRTs). The general construction and use of FEDs is known to those having skill in the art. See, e.g., European Patent Application Serial Nos. EP 455,162 to Nakayama, published June 1 1, 1991; EP 443,865 to Komatsu, published August 28, 1991 ; EP 572,170 to Kochanski, published December 1, 1993; Spindt, C. A., et al. 1991. Field-Emitter

Arrays for Vacuum Microelectronics. IEEE Transactions on Electron Devices. 38(10): 2355-2363; and Mousa, M. S. 1993. A Study of the Effect of Hydrogen Plasma On Microfabricated Field- Emitter Arrays. Vacuum. 45(2-3): 235-239.

Referring to Figures 1A and IB, which illustrate a FED of the prior art, a FED 100 is generally produced by sealing two parallel, closely spaced, substantially planar members along their perimeters. These members are typically formed of glass. The glass members comprise a screen member 103, having an exterior surface 106 and an interior surface 109, and an electrode support member 112 having an interior surface 115 and an exterior surface 118. Typically, the sealing of the members is performed by melting a glass paste 121 having a low melting point along one or both of the perimeters of the screen and electrode support members and bringing the members together to sealably join them along their perimeters, a method known commonly as "frit sealing". The resulting structure consists of two substantially parallel surfaces separated by an interior space 122 a few hundreds of microns (μm) in width. The interior space of the FED 122 typically is kept under vacuum.

With continued reference to Figure 1 A, on the inner surface of the screen member is deposited a phosphor 124. The opposing interior surface of the electrode support member includes electrode means 127 which typically include a plurality of pointed microcathodes (microtips) 130 made of a metallic or semiconductor material, e.g., molybdenum (Mo), which emit electrons. A plurality of grid electrodes 133 are placed proximate to the cathodes so as to generate a very high electric field. The grid electrodes are supported on dielectric material 136, and the microtips and dielectric material are arranged on a support 139. The electric field created by the arrangement of grid electrodes and microtips ejects electrons from the points of the microtips and accelerates the electrons toward the phosphors, exciting the phosphors into luminescent states. The luminescence intensity of the excited phosphors, and, therefore, the pixel brightness, is proportional to the current emitted by the associated microtips.

Typically, the phosphor 124 is deposited on the surface of the interior surface of the screen member as a slurry of powdered phosphor particles in a binder in substantially the same fashion as phosphors are deposited on the kinescopes of traditional cathode ray tubes (CRTs). This method for providing the phosphor coating has several serious drawbacks, however. First, the large surface area provided by the uneven phosphor surface 142 allows for significant outgassing from the binder which degrades the vacuum required for operation of the FED. Second, particles of phosphor can become dislodged from the binder and fall into the interior space of the FED. This degrades the picture quality of the FED. Furthermore, the combination of phosphor powder and binder adds considerable weight to the FED which diminishes the potential of the FED as a lightweight video monitor.

Some have proposed the use of a substantially continuous thin-film phosphor layer instead of a phosphor slurry to avoid the above-described problems with traditional phosphor slurries. Such an arrangement is illustrated in Figure 2 at 200, where thin-film phosphor 203 is deposited on the interior surface of glass member 103. However, it is generally thought that such thin-film phosphors are unworkable due to the entrapment of emitted photons within the continuous structure of the thin-film phosphor, with emission of the photons occurring only at angles substantially parallel to the thin-film surface — not perpendicular to the thin-film surface as would be required for viewing. In view of this limitation,^' it is presently believed that thin-film phosphor layers are unworkable for field emission display technology.

Thus, it would be beneficial to have a method of depositing thin-film phosphor layers for field emission displays that avoids the above-described problems with traditional phosphor deposition methods and provides a thin-film phosphor that has acceptable photon emission qualities.

SUMMARY OF THE INVENTION

The present invention overcomes the above-described limitations of present phosphors by providing a field emission display screen comprising a plurality of substantially discrete thin film phosphor deposits. As described in greater detail herein, the use of such a plurality of substantially discrete thin film phosphor deposits overcomes the inherent limitations of traditional phosphors and of a continuous thin film phosphor surface.

In one embodiment, the present invention provides a field emission display in which a screen member is coupled with an electrode support member to define an interior space. The interior surface of the screen member is coupled with a plurality of substantially discrete thin-film phosphor deposits. The opposing interior surface of the electrode support member is coupled with electrode means such that electronic radiation from the electrode means is effective to excite the thin-film phosphor deposits into luminescent states whereby photons are emitted from the thin film phosphor deposits at angles substantially perpendicular to the exterior surface of the screen member.

In one embodiment, the phosphor is selected from the group consisting of Zn₂SiO₄:Mn,

Y₃Al₅O₁₂:Ce, Y₂SiO₅:Ce, Y-,Al₅O₁₂Tb, Y₂O₃:Eu, LiAlO₂:Fe, ZnS:Ag, ZnS:Cu, ZnS:Al, CdS:Cu, CdS:Al, Zn_{0 1}Cd₀₉S:Ag, Y₂O₂S:Eu, Gd₂O₂S:Tb, and Y₂O₂S:Tb. In another embodiment, a reflector comprising a thin film of a reflective metal is deposited over the discrete thin film phosphors to allow excitation of the discrete thin-film phosphors by incident electrons and to reflect photons emitted by the excitation such that the photons are emitted at angles substantially perpendicular to the exterior surface of the screen member. In one embodiment, the reflector is selected from the group consisting of Al, Ti, Ni and Cu. In one particular embodiment, the reflector includes Al. In another embodiment, the reflector layer has a thickness on the order of about 100 Angstroms.

In another aspect, the present invention includes a method for forming a field emission display having a plurality of substantially discrete thin-film phosphor deposits thereon. The method of the invention, in one embodiment, comprises providing a screen member having interior and exterior surfaces, and depositing thereon a plurality of substantially discrete thin film phosphor deposits on the interior surface of the screen member. An electrode member having interior and exterior surfaces, in which interior surface of the electrode member includes electrode means coupled thereto is also provided; and the screen member and electrode member are coupled such that electronic radiation from the electrode means is effective to excite the thin-film phosphor deposits into luminescent states whereby photons are emitted from thin film phosphor and through the exterior surface of said screen member at angles substantially perpendicular to the exterior surface of the screen member.

These, and other aspects and advantages of the present invention, will become apparent when the following description is read in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1 A and IB are illustrations of a field emission display according to the prior art. Figure 1 A is a cut-away view of a prior art field emission display. Figure IB is an exterior view of the field emission display shown in Figure 1A.

Figure 2 illustrates a proposed field emission display including a thin film phosphor according to the prior art.

Figures 3A and 3B illustrate a field emission display according to the present invention. Figure 3A is a cut away view showing a field emission display including a plurality of substantially discrete , thin-film phosphor deposits. Figure 3B is an exterior view of the field emission display shown in Figure 3A.

Figure 4 is an illustration of the field emission display shown in Figure 3 A further including a reflective layer deposited over the phosphor deposits.

Figures 5A and 5B illustrate embodiments of an apparatus for creating the field emission display of the present invention. Figure 5A is a cut-away view showing a processing chamber for thermally sputtering phosphor material onto the screen of a field emission display. Figure 5B illustrates the processing chamber of Figure 5 A, but further including a mask for creating discrete phosphor deposits on a screen.

DESCRIPTION OF SPECIFIC EMBODIMENTS

The present invention overcomes the limitations of prior art field emission display technologies by providing a thin film phosphor to be used in a field emission display device. Using the present invention, the limitations currently known in the prior art with respect to the use of thin film phosphor are avoided by the deposition of discrete thin film phosphor deposits upon the interior surface of the display screen of the field emission display as described herein. Thus, it will be seen that the present invention provides for a field emission display that removes the limitations imposed by the use of traditional phosphor slurry deposition in conjunction with field emission display devices.

Figure 3A illustrates one embodiment of the invention at 300. The field emission display includes a screen member 303 having an exterior screen surface 306 and an interior screen surface 309. Screen member 303 is coupled to an electrode member 312 having an interior surface 315 and exterior surface 318 by the use of a sealant shown at 321. The combination of the screen member, electrode member and sealant provides for an interior space 322 into which interior space are arrayed electrode means 327 upon the interior surface 315 of electrode number 312 and a plurality of substantially discrete thin film phosphor deposits 325 on the interior surface 309 of screen member 303. An individual phosphor deposit is shown generally at 330. Electrode means 327 can be any arrangement of electrodes or other electron emitting devices effective to excite phosphors into luminescent states. Various such means will be known to those of skill in the solid state electronics arts.

An exterior view of the field emission display 300 formed an accordance with the present invention is provided in Figure 3B. There, the positions of the phosphor deposits 330 are shown in relation to exterior surface 306 of screen member 303. It will be appreciated that the deposits illustrated are not drawn to scale and that many more such deposits would typically be provided on the interior of the screen member 303. The number density of the discrete phosphor deposits along the interior of screen means 303 will be governed by the desired resolution of the field emission display. Generally, the desired resolution will be a function of the format of the signal input to the field emission display. The materials and methods used in constructing the screen and electrode members of the above-described field emission display, and their coupling, will be familiar to those having skill in the solid state device arts.

The phosphor deposits comprise phosphor compositions well known to those of skill in the art. Representative phosphor compositions include the following:

Phosphor Composition Phosphor Composition Name Name

P-l Zn2Siθ4:Mn P-22 (grn) (Zn, Cd)S:Cu, Al

P-46 Y₃Al₅Oι₂:Ce Zno Cdo.9S:Ag

P-47 Y₂SiO₅:Ce P-22(red) Y₂O₂S:Eu

P-53 Y3Al₅Oι :Tb P-22 Y₂O₂S:Eu

P-56 Y₂O₃:Eu P-43 Gd₂O₂S:Tb

LiAlO^Fe P-45 Y₂O₂S:Tb

P-l l ZnS:Ag

It will be appreciated that those of skill in the art will understand how to prepare and handle the above-described compositions. More generally, phosphors that are particularly useful in the invention are those which have low electronic excitation potentials, generally between about 100 volts (V) and about 200 volts and, in some cases, less than about 100 volts. Useful phosphors are those that have good emission, high luminosity, and high purity of color (i.e., strong red, green, and blue hues). Furthermore, preferred phosphors will be those that are relatively easy to deposit and are stable to electron bombardment (i.e., the phosphors retain good emissive properties over long periods of electron bombardment from electrodes 330). In addition, the phosphors should be vacuum compatible.

In another embodiment, the present invention includes a reflective surface deposited over the substantially discrete thin film phosphors to direct photons through the exterior surface of the screen member at angles that are substantially perpendicular thereto. One such embodiment is illustrated in Figure 4 at 400 wherein a deposit of reflective material 402 is arranged over the discrete phosphors 406 which are deposited on the interior surface of screen 303. In general, materials that are useful for providing the reflective layer 402 include low atomic number metals that are reflective when deposited as thin films and are compatible with vacuum conditions. Such metals include those having an atomic number less than about 30. These metals include Copper (Cu), Nickel (Ni), Aluminum (Al) and Titanium (Ti). In one particular embodiment, the reflector includes Al. In one embodiment, each deposit has a thickness of between about 90 A and about 1000 A. In another embodiment, the deposition thicknesses are between about 100 A and about 1000 A, and, in a more particular embodiment, between about 100 A and about 500 A. In one embodiment, the reflector layer has a thickness on the order of about 100 Angstroms. The deposition of the reflective material can be performed using methods and materials known to those having skill in the art of thin films and solid state electronics.

In one embodiment, the interior of the field emission display of the present invention further includes a getter to maintain vacuum conditions within the interior space of the FED. In one embodiment, the field emission display includes a non-evaporable getter material having an activation temperature greater than about 500 °C.

The formation of the thin film phosphor deposits on the interior surface of the screen member can be performed using methods well known in the art of preparing and depositing thin films and solid state of electronics. Methods of deposition useful with the present invention include, but are not limited to, thermal sputtering, thermal evaporation (either E-beam evaporation or resistive evaporation), laser ablation, plating, and painting. Typically, each method involves preparing a phosphor composition to be deposited on a test screen, such as one of the phosphor compositions described above, performing the deposition operation, and determining the composition of the deposited material. The composition may be adjusted and a dopant may be added to either the phosphor or to the atmosphere of the sputtering chamber to deposit a phosphor having the desired stoichiometry on the screen member. Further adjustments may also be needed in order to achieve the proper surface morphology, i.e., to achieve the correct deposit size and thickness. The optimization of these variables will be recognized as being well known to those of skill in the art of thin films and solid state electronics. The thin film phosphor material may be deposited onto the interior surface of the screen member as discrete phosphor deposits or in larger segments which are then divided using known technology such as scribing or etching, to produce phosphor deposits having the desired dimensions. Alternatively, the deposition may be performed through a mask, such as a lithographic mask or a material mask (e.g. a metal mask), to form the phosphor deposits directly during the deposition operation. Again, it will be recognized that the employment of these techniques is well known to those of skill in the art of thin films and solid state electronics.

Figures 5A and 5B illustrate two embodiments of a method for forming the thin film phosphors of the invention. Figure 5A at 500 illustrates a thermal evaporation deposition apparatus 500 that includes a deposition chamber 503 connected to a pump 506 (designated "P") by a neck 509. Within the chamber, the chamber walls 512 are heated to very high temperatures by a heating element such as shown at 515. Also in the chamber is a material reservoir 518 which comprises the phosphor to be deposited (shown at 519) being held in a container 521. Within the container is a resistive heating element 524. Also in the chamber is a stage formed of a relatively heat insulating material 527 upon which sits the screen member 603 which is arranged so that interior surface 606 is exposed to the chamber atmosphere. In operation, the heating elements 515 and 524 are energized to heat the phosphor 519 and the walls of the chamber 512 to produce thereby a gas of phosphor particles which are deposited upon the relatively cooler surface 606 to form thereon a thin film phosphor coating. Pump 506 is used to control the interior pressure of the pump. As will be apparent to those of skill in the art, additional gases may be added (e.g., dopants) to control the stoichiometry and surface morphology of the deposition. After the thin film has been deposited, the screen is removed from the chamber and processed further to provide the discrete thin film phosphor deposits described above. Examples of such post deposition processing would include scribing or etching. Other methods will be apparent to those of skill in thin film and solid state of electronics art.

A second method for forming the thin film phosphor deposits is illustrated inpigure 5B. Again, the deposition chamber 500 and interior structures 503-527 are substantially the same as described with respect to Figure 5A. However, the chamber further includes a masking structure 530 which comprises a mask 533 supported by supports 535 and 535' above the surface 606 of screen member 603. The deposition operation is performed as described above; however, the presence of mask 533 allows the phosphor particles to be deposited only within discrete regions of surface 606 to provide thereby directly the substantially discrete thin film phosphor coated screen described above.

Without wishing to be bound to any particular theory of action, it is believed that the use of discrete phosphor deposits avoids the limitation of a continuous thin film phosphor deposition by allowing the photons emitted by the excited phosphors to exit the phosphors at the edges of the individual deposits. Thus, photons will be emitted throughout the area of the field emission display that comprises the viewing screen. In contrast, it is believed that a continuous thin film phosphor layer would emit photons only at the very edges of the screen, thus degrading the quality of the projected image.

EXAMPLE

The following is an example of a thin film deposition performed by thermal evaporation in a vaccum deposition chamber in accordance with one embodiment of the present invention. The example is not intended to limit the scope of the invention in any way; rather it is intended to illustrate one method by which a thin film may be deposited.

In a vacuum depositon chamber a phosphor source is provided at a distance of about 1 inch to about 20 inches away from a substrate upon which the phosphor is to be deposited as a thin film of discrete phosphor deposits. A shutter is placed over the source to control the progress and rate of the thin-film phosphor deposition. The chamber is brought to a pressure of between about 10" Ton^* to about 10^"7 Torr before evaporation begins. The phosphor sample is then heated by, for example, resistive heating or E-beam bombardment, to a temperature sufficient to raise the phosphor vapor pressure within the deposition chamber. The substrate is maintained at a temperature sufficient to cause condensation of the phosphor vapors onto the substrate surface to form thereby a thin film of discrete phosphor deposits. During the deposition process, a quartz crystal, whose oscillation rate varies inversely with the thickness of the phosphor deposit monitors the thickness of the phosphor deposition. When the crystal oscillation correlates with the desired phosphor thickness, the deposition process is terminated.

Although certain embodiments and examples have been used to describe the present invention, it will be apparent to those having skill in the art that various changes can be made to those embodiment and/or examples without departing from the scope or spirit of the present invention. For example, it will be appreciated from the foregoing that many different techniques can be used to deposit a plurality of substantially discrete thin film phosphor deposits onto the interior surface of the screen member. Similarly, it will be appreciated that any arrangement of electrodes effective to excite a thin film phosphor is contemplated by the present invention.

Claims

WHAT IS CLAIMED:

1. A field emission display, comprising a screen member coupled with an electrode support member to define an interior space, said screen member and said electrode support member each comprising an interior surface and an exterior surface, the interior surface of said screen member further being coupled with a plurality of substantially discrete thin-film phosphor deposits and the opposing interior surface of said electrode support member further being coupled with electrode means such that electronic radiation from said electrode means is effective to excite said thin-film phosphor deposits into luminescent states whereby photons are emitted from said phosphor deposits at angles substantially perpendicular to the exterior surface of said screen member.

2. The field emission display of claim 1 , wherein said phosphor deposits comprise a phosphor selected from the group consisting of Zn₂SiO₄:Mn, Y₃Al₅O₁₂:Ce, Y₂SiO₅:Cc, Y₃Al₅O₁₂Tb, Y₂O₃:Eu, LiAlO₂:Fe, ZnS:Ag, ZnS:Cu, ZnS:Al, CdS:Cu, CdS:Al, Zn_{0 1}Cd₀₉S:Ag, Y₂O₂S:Eu, Gd₂O₂S:Tb, and Y₂O₂S:Tb.

3. The field emission display of claim 2, wherein said phosphor is Zn₂SiO₄:Mn.

4. The field emission display of claim 2, wherein said phosphor is ZnS: Ag.

5. The field emission display of claim 1 , further including a getter material disposed within said interior space.

6. The field emission display of claim 5, wherein said getter material is a non-evaporable getter material having an activation temperature greater than about 500 °C.

7. The field emission display of claim 1, further including a reflector disposed between said thin-film phosphor and said electrode means, said reflector being effective to allow excitation of said thin-film phosphor by said electrode means and to reflect photons emitted by said excitation such that said photons are emitted at angles substantially perpendicular to the exterior surface of said screen member.

8. The field emission display of claim 7, wherein said reflector comprises a metal having an atomic number of less than about 30.

9. The field emission display of claim 8, wherein said reflector comprises a metal selected from the group consisting of Al, Ti, Ni and Cu.

10. The field emission display of claim 7, wherein said reflector is a metal oxide.

1 1. The field emission display of claim 7, wherein the thickness of said thin-film phosphor deposits is between about 90 A and about lOOOA.

12. The field emission display of claim 1 1, wherein the thickness of said thin-film phosphor deposits is between about 100 A and about 500 A.

13. A method for forming a field emission display having a thin-film phosphor, comprising the steps of:

a) providing a screen member having interior and exterior surfaces;

b) depositing a plurality of substantially discrete thin film phosphor deposits on the interior surface of said screen member;

c) providing an electrode member having interior and exterior surfaces, the interior surface of said electrode member further having electrode means coupled thereto; and

d) coupling said screen member and said electrode member such that electronic radiation from said electrode means is effective to excite said thin-film phosphor deposits into luminescent states whereby photons are emitted from said phosphor and through the exterior surface of said screen member at angles substantially perpendicular to the exterior surface of said screen member.

14. The method of claim 13, wherein said step of depositing comprises depositing said phosphor on the interior surface of said screen member through a mask.

15. The method of claim 14, wherein said mask is a lithographic mask.

16. The method of claim 14, wherein said mask is a physical mask.

17. The method of claim 13, wherein said step of depositing comprises depositing said phosphor on the interior surface of said screen member as at least one substantially continuous layer and dividing said at least one substantially continuous layer into a plurality of phosphor deposits.

18. The method of claim 17, wherein said step of dividing comprises scribing the surface of said thin-film phosphor layer.

19. The method of claim 17, wherein said step of dividing comprises etching the surface of said thin-film phosphor layer.

20. A field emission display produced according to the method of claim 13.

21. A method for forming a field emission display screen having thin-film phosphor, comprising the steps of

a) providing a screen member having interior and exterior surfaces; and

b) depositing a plurality of substantially discrete , thin film phosphor deposits on the interior surface of said screen member.

22. The method of claim 20, wherein said step of depositing comprises depositing said phosphor on the interior surface of said screen member through a mask.

23. The method of claim 21 , wherein said mask is a lithographic mask.

24. The method of claim 21 , wherein said mask is a physical mask.

25. The method of claim 20, wherein said step of depositing comprises depositing said phosphor on the interior surface of said screen member as at least one substantially continuous layer and dividing said at least one substantially continuous layer into a plurality of phosphor deposits.

26. The method of claim 24, wherein said step of dividing comprises scribing the surface of said thin-film phosphor layer.

27. The method of claim 24 wherein said step of dividing comprises etching the surface of said thin-film phosphor layer.

28. A field emission display screen produced according to the method of claim 20.

29. A field emission display produced according to the method of claim 20.