HIGH VOLTAGE COMPATIBLE SPACER COATING

Abstract: A coating material, for coating a spacer structure (300) of a flat panel display, having specific resistivity and secondary emission characteristics is disclosed. The coating material is characterized by formula Psc > 100(Psw) and r < Psw(12/8) wherein Psw is the sheet resistance of a spacer, 1 is height of the spacer, r is the area resistance.

Description: HIGH VOLTAGE COMPATIBLE SPACER COATING TECHNICAL FIELD The present claimed invention relates to the field of flat panel displays. More specifically, the present claimed invention relates to a coating material for a spacer structure of a flat panel display. BACKGROUND ART In some flat panel displays, a backplate is commonly separated from a faceplate using a spacer structure. In high voltage applications, for example, the backplate and the faceplate are separated by spacer structures having a height of approximately 1-2 millimeters. For purposes of the present application, high voltage refers to an anode to cathode potential greater than 1 kilovolt In one embodiment, the spacer structure is comprised of several strips or individual wall structures each having a width of about 50 microns. The strips are arranged in parallel horizontal rows with each strip extending across the width of the flat panel display. The spacing of the rows of strips depends upon the strength of the backplate and the faceplate and the strips Because of this, it is desirable that the strips be extremely strong. The spacer structure must meet a number of intense physical requirements. A detailed description of spacer structures is found in commonly-owned co-pending U.S. Patent Application Serial No. 08/683,789 by Spmdt et al. entitled "Spacer Structure for Flat Panel Display and Method for Operating Same" The Spindt et al application was filed July 18, 1996, and is incorporated herein by reference as background material. In a typical flat panel display, the spacer structure must comply with a long list of characteristics and properties. More specifically, the spacer structure must be strong enough to withstand the atmospheric forces which compress the backplate and faceplate towards each other (In a diagonal 10-mch flat panel display, the spacer structure must be able to withstand as much as a ton of compressing force) Additionally, each of the rows of strips in the spacer structure must be equal in height, so that the rows of strips accurately fit between respective rows of pixels. Furthermore, each of the rows of strips in the spacer structure must be very flat to insure that the spacer structure provides uniform support across the interior surfaces of the backplate and the faceplate. The spacer structure must also have a coefficient of thermal expansion (CTE) which closely matches that of the backplate and faceplate to which the spacer structure is attached (For purposes of the present application, a closely matching CTE means that the CTE of the spacer structure is within approximately 10 percent of the CTE of the faceplate and the backplate to which the spacer structure is attached) The temperature coefficient of resistance (TCR) of the spacer structure must also be low. An acceptable spacer structure must meet all of the above-described physical requirements and must be inexpensive to manufacture with a high yield. Besides the physical requirements set forth above, the conventional spacer structure must also meet several electrical property requirements. Specifically, a spacer structure must have specific resistance and secondary emission characteristics, and have a high resistance to high voltage breakdown. In conventional prior art spacer structures, an insulating material such as alumina is covered with a coating In such prior art spacer structures, the insulating material has a very high sheet resistance, while the coating has a lower sheet resistance Other prior art approaches utilize a spacer structure in which both the insulating material and the overlying coating have a very high sheet resistance Thus, due to the large number of stringent physical requirements on the bulk of the spacer structure (I e , high strength, precise resistivity, low TCR, precise CTE, accurate mechanical dimensions etc ) it is desirable to separate out the additional requirements on the properties of the surface Hence, a need exists for a spacer structure which meets the above-described physical and electrical property requirements without dramatically complicating and/or increasing the cost of the spacer structure manufacturing process DISCLOSURE OF THE INVENTION The present invention eliminates the requirement for a spacer material to meet specific secondary emission characteristics in addition to meeting requirements such as, for example, high strength, precise resistivity, low TCR, precise CTE, accurate mechanical dimensions and the like. The present invention further achieves a spacer structure which meets the above-described physical, electrical, and emission property requirements without dramatically complicating and/or increasing the cost of the spacer structure manufacturing process The present invention achieves the above accomplishments with a coating material which is applied to a spacer body. In addition, the present invention achieves the above accomplishments without stringent CTE, TCR, resistivity, or uniformity requirements on the coating. The present invention also points out advantages of having a spacer body which is resistive, and a spacer coating which has a sheet resistance which is higher than that of the spacer body. Specifically, in one embodiment, the present invention provides a coating material having specific resistivity, thickness, and secondary emission characteristics. The coatmg material of the present embodiment is especially well-adapted for coatmg a spacer structure of a flat panel display. In this embodiment, the coatmg material is characterized by: a sheet resistance, ρ sc , and an area resistance, r, wherein p sc and r are approximately defined by: Psc 100(p sw ) and r < ρ sw (l 2 / 8). In the present embodiment, p sw is the sheet resistance of a spacer structure to which the coating material is adapted to be applied, and 1 is the height of the spacer structure to which the coating material is adapted to be apphed. The bulk sheet resistance p sw is defined here as the resistance of the structure divided by the height and multiplied by the perimeter. In the present embodiment, the sheet resistance, pg W , of said spacer has a value of approximately 10^ to 10^ Q/Γ. By having a coating material with such characteristics, the present invention eliminates the need to place rigorous secondary emission characteristic requirements on the bulk material comprising the spacer structure in a flat panel display In order to avoid stringent requirements on the value or the uniformity of the coatmg, the sheet resistance, p S c, i is desirable to have its value be high compared to Psw> that 1S: Psc approximately 100(p sw ) As in the previous embodiment, p sw is the sheet resistance of the spacer structure to which the coatmg material is adapted to be apphed Additionally, the coating material of the present embodiment has an area resistance, r, wherein r is defined as: ΔV CC / Jc ΔV CC , of the present embodiment is the voltage across the thickness of the coatmg at a charging current j c where the ΔV C c used to characterize r for a typical HV display is in the range of approximately 1-20 volts. In this embodiment, j c is defined as: v j mC (E) (l-δ(E)) dE. In the above relationship, jmc(E) 1S the electron current density, as a function of incident energy E, incident to the coatmg material; and δ is the secondary emission ratio of the coating material as a function of the energy E of electrons incident on the coating material. ΔVcc and j c could be measured by sample currents and energy shifts in peaks using, for example, Auger electron or photoelectron spectroscopy As in the previous embodiment, by having a coatmg material with such characteristics, the present mvention eliminates the need to place rigorous requirements on secondary emission characteristics of the material comprising the spacer structure of a flat panel display. It also allows for tailoring the resistivity and other properties of the spacer without strict requirements on δ, and tailoring of the coatmg without strict requirements on resistivity. These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill m the art after having read the following detailed description of the preferred embodiments which are illustrated m the various drawing figures BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention FIGURE 1 is a graph of a typical secondary emission coefficient (δ) vs incident beam energy (E) impinging on a coating material. FIGURE 2 is a graph of a typical incident current density (jinc) γ s incident beam energy (E) impinging at some height along a spacer structure FIGURE 3 is a side schematic view of a spacer structure including an illustration of charging properties associated with the spacer structure in accordance with the present claimed invention. FIGURE 4 is schematic top plan view of a spacer structure including an illustration of electron attracting properties associated with a spacer structure in accordance with the present claimed invention having a voltage value of HV- ΔV apphed to an adjacent anode. FIGURE 5 is schematic top plan view of a spacer structure including an illustration of electron repelling properties associated with a spacer structure in accordance with the present claimed invention having a voltage value of HV+ ΔV applied to an adjacent anode. FIGURE 6 is a schematic side-sectional view of a spacer structure having a coatmg material applied thereto in accordance with the present claimed invention. FIGURE 7 is a schematic side-sectional view of a spacer structure, including a differential section, dx, having a coatmg material applied thereto in accordance with the present claimed invention DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described m conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included withm the spirit and scope of the invention as defined by the appended claims Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention However, it will be obvious to one of ordinary skill in the art that the present invention may be practiced without these specific details In other instances, well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the present invention Additionally, although the following discussion specifically mentions spacer walls, it will be understood that the present invention is also well suited to the use with various other support structures including, but not limited to, posts, crosses, pins, wall segments, T-shaped objects, and the like Referring now to Figure 1, a typical graph 100 of the secondary emission coefficient (δ) vs the incident beam energy (E) impinging a coatmg material at some angle or angles is shown In order for a spacer structure to remain "electrically invisible" (I e not deflect electrons passing from the row electrode on the backplate to pixel phosphors on the faceplate), the present invention covers the spacer structure with coatmg material having specific resistivity and secondary emission characteristics. Also indicated are the first and second "crossover" energies where δ = 1 (i.e. E l and E2). Referring next to Figure 2, a graph 200 of the incident current density (jinc) vs- the incident beam energy (E) impinging a coating material is shown. As indicated m graph 100, the incident current density varies near the value, E2 This energy distribution will, of course, vary up the wall. The present invention minimizes deleterious charging of the spacer structure. The present invention achieves such an accomplishment by keeping δ at or near the value of 1. However, as shown in graph 200 of Figure 2, δ varies with the incident beam energy, E. Hence, the optimal coating material of the present invention is defined as follows. It is desirable to have a low δ coating which efficiently bleeds charge into the bulk of a resistive spacer, but which does not contribute appreciably to the conductivity of the spacer in the direction parallel to the surface. With reference now to Figure 3, a side schematic view of a spacer structure 300 of the present invention is shown In such a spacer structure, the upper portion 302 of spacer structure 300 (i.e. near the faceplate 304 of the flat panel display) charges sbghtly negative. Conversely, the lower portion 306 of spacer structure 300 (i.e. near the cathode) charges slightly positive. That is, electrons striking upper portion 302 of spacer structure 300 typically strike spacer structure 300 with an energy above level E2 of Figure 2. Because δ(E) < 1, upper portion 302 of spacer structure 300 charges negatively Similarly, electrons striking lower portion 306 of spacer structure 300 strike with energies below level E2 of Figure 2, and, therefore, charge lower portion 306 of spacer structure 300 positively However, when considered in its entirety, an energy distribution of electrons having respective energy levels above and below E tend to cancel the net charging on spacer structure 300 As a result, the nearby pixel deflection as a function of the net electron current is very small. With reference next to Figure 4 a schematic top plan view of spacer structure 300 attracting nearby electrons is shown. As mentioned above, net charging on spacer structure 300 of the present invention is nulled. By decreasing the high voltage (HV) value applied to the anode (1 e. faceplate region of the flat panel display), the charging characteristic of spacer structure 300 of the present invention is altered Specifically, by decreasing HV to HV- ΔV, as shown in Figures 1 and 4, spacer structure 300 becomes increasingly positively charged with increasing anode current As a result, spacer structure 300 of the present invention attracts electrons, typically shown as 402, when a voltage HV- ΔV is applied to the anode. In the present invention, for an HV value of approximately 6000 volts, ΔV typically has a value on the order of 1000 to 2000 volts, or approximately 15-30 percent of the HV value. Although such a value for ΔV is specifically recited above, it will be understood that ΔV could have various other values. By covering a bulk resistive spacer with a less conductive coatmg, other advantages are realized by the present invention. Specifically, the advantages of having the spacer conductivity roughly uniform throughout the bulk as opposed to on the surface are maintained. A detailed description of such advantages is set forth m commonly-owned co-pendmg U.S. Patent Application Serial No. 08/684,270 by Spmdt et al. entitled "Spacer Locator Design for Three-Dimensional Focusing Structures m a Flat Panel Display" The Spmdt et al. application was filed July 17, 1996, and is incorporated herein by reference as background material. Referring now to Figure 5, a schematic top plan view of spacer structure 300 repelling nearby electrons is shown. As mentioned above, net charging on spacer structure 300 of the present invention is approximately nulled. By increasing the high voltage (HV) value applied to the anode, the charging characteristic of spacer structure 300 of the present invention is altered. Specifically, by increasing HV to HV+ ΔV, as shown in Figure 5, spacer structure 300 becomes increasingly negatively charged with increasing anode current As a result, spacer structure 300 of the present invention repels electrons, typically shown as 502, when a voltage HV+ ΔV is apphed to the anode. Therefore, a spacer structure having characteristics described above for the present invention, will either attract or repel electrons depending upon the voltage apphed to the anode As mentioned above, in the present invention, for an HV value of approximately 6000 volts, ΔV typically has a value on the order of 1000 to 2000 volts, or approximately 15-30 percent of the HV value. Referring next to Figure 6, a spacer 600 having a height, 1, is covered by a coating material 602. As stated previously, it is desirable to have a low δ coating which also efficiently bleeds charge into the bulk of a resistive spacer, but which does not contribute appreciably to the conductivity of the spacer in the direction parallel to the surface Although a wall-type spacer structure is shown in Figure 6 for purposes of clarity, the present invention is also well suited for use with various other types of spacer structures. Spacer 600 extends between a backplate 604 and a faceplate 606. For estimation purposes, it is useful to look at a uniform charging current j c Under such conditions and for the case where psc » Psw» the maximum charging voltage, ΔV , is given by: Pswl 2 Jc ΔV W - g (1) where psw 1S the sheet resistivity of the bulk spacer 600. The derivation of the value for ΔV W is given below in conjunction with Figure 7 With reference now to Figure 7, a schematic side sectional view of a spacer structure, including a differential section, dx, 700 is shown. In such a configuration, a mimmum or low voltage occurs at the base (i.e. at the backplate) of spacer 600 with a maximum or high voltage occurring at the top (i.e. at the anode) of spacer 600. Therefore, the current, 1, entering dx 700 is calculated as: ι(x) + Jc dxL = ι(x+dx) (2) where L is the length of the spacer into the page. Using the definition of a derivative, equation 2 becomes Similarly, the voltage drop across dx 700 is found using Ohm's law (Voltage Current x Resistance), i.e. V=IR, to get dx V(x+dx) - V(x) = ι(x) Psc -j (4). Again, using the definition of a derivative, equation (4) can be solved to provide dY _ Psc dx " l(x) L (5) - The derivative of equation (5) substituted into equation (3) gives The solution of equation (6) for the boundary conditions V(l) = high voltage, HV, and V(0) = 0 evaluated at x = 1/2 is given by: 1 , HV Psw Jcl 2 Psw Jc 1 where the term is the charging error Coating 602 of the present invention has a sheet resistivity, ρ SC) which is greater than 100 times the sheet resistivity of spacer 600, p S w > to which coatmg material 602 is apphed. That is, Psc > 100p sw (8). By having the sheet resistivity of coatmg 602 much greater than the sheet resistivity of spacer 600, any deviation of the uniformity of coat g 602 on spacer 600 does not substantially effect the sheet resistance uniformity of the combined spacer material and coatmg structure. For purposes of the present application, umform resistivity is intended to mean a deviation of less than 2 percent. The optimal coating 602 of the present invention is also well suited to having a lesser sheet resistivity value by accordingly increasing the uniformity of optimal coating material 602. As yet another advantage of the present invention, coatmg 602 of the present invention renders the voltage, ΔV CC , across coating 602 for a given charging current, j c , small, compared to the charging voltage, ΔV , (see equation 1) in the bulk of spacer 600. More, specifically, coating 602 of the present invention has a voltage, ΔV CC , across coatmg 602 which is PswJcl ΔV cc < ^ W f- (9) That is, V cc is less than the voltage required to bleed the current out through the bulk of the wall. In a simplified view, sheet resistivity is given by resistivity divided by the thickness, t, of the sheet of material, and the sheet resistance, p sc , of coatmg 602 is defined as follows Psc + T (10) where pc is the resistivity of coatmg material 602 in Ω-cm In practice there are non-uniformity, surface, and interfacial effects such that Psc(z) is not uniform through the coating and p sc D Pc ~ r (the direction of psc( z ) through coatmg 602 is represented by arrow 608 in Figure 6) . Probably even more importantly, fields on the order of 5kV/1.25 mm (i.e. 4V/μm) are apphed to coatmg 602 in the "sheet resistance direction" and fields on the order of 500 V/μm are applied in the "area resistance direction " The VCR of the material will mean that we must use the area resistance, r, (at approximately 10 volts across coating 602) of 500 V/μm, and the sheet resistance, r, (at approximately 5 kilovolts along coating 602) Pc of 4 V/μm, instead of the approximations r=pct and psc + ~ T • With the above in mind, and by considering the unit area through which the charging current, j c , is applied, it can be written that ΔV cc = j C r ≡ (Jc(A)) (11). By combining the results of equations (9), (10), and (11) ΔV CC , of coating material 602 of the present invention is defined as Pswirl ΔV cc = j c r < i ^- (12). As a result, the area resistance of coating material 602 of the present invention is defined to be Psw I 2 , 1 . r < — g — (13). Hence, coating material 602 of the present invention has a sheet resistance, p S c, which is greater than approximately 100(p S w) and an area resistance, r, which is less than approximately psw 0 2 8)- Although such a value for r is recited here, it will be understood that the value of r can vary and, as an example, be approximately r < ρ s (l 2 / 80). Additionally, in the present embodiment, when a combinational spacer structure and coating material structure is formed, the spacer structure has a bulk resistivity value, and a uniform resistivity along the height/length thereof. That is, in the present embodiment, the spacer structure has a uniform resistivity through- its thickness such that the resistivity throughout the thickness of the spacer structure does not vary by more than a factor of 5. Additionally, the spacer structure has a uniform resistivity along its height such that the resistivity does not vary by more than approximately 2 percent along the height of the spacer structure. Furthermore, in the present embodiment, the spacer structure has a height of approximately 1-2 millimeters, and has a coefficient of thermal expansion similar to the coefficient of thermal expansion of a faceplate and a backplate to which the spacer structure is adapted to be attached ( when a wall-type spacer structure is used). In the present embodiment, the faceplate reflects a portion of scattered electrons against the spacer structure It will be understood that the specific coating may vary depending upon the electron backscatter from the faceplate. Although such values and conditions are used in the present embodiment, the present invention is also well suited to using various other values and conditions for the spacer structure. Additionally, in the present invention, coating material 602 is formed of a material having low secondary electron emission such as, for example, cerium oxide material. Although such a material forms coatmg 602 in the present embodiment, the present invention is also well suited to forming coatmg 602 from, for example, chromium oxide material or diamond-like carbon material Also, in the present embodiment, coat g material 602 is apphed to spacer 600 in a layer having a thickness of approximately 200 Angstroms Thus, the present invention eliminates the requirement for a spacer material to meet specific resistivity and secondary emission characteristics in addition to meeting requirements such as, for example, high strength, precise resistivity, low TCR, precise CTE, accurate mechanical dimensions and the like The present invention further achieves a spacer structure which meets the above-described physical and electrical property requirements without dramatically complicating and/or increasing the cost of the spacer structure manufacturing process The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents.

Claims: CLAIMS: 1. A spacer structure and coating combination comprising: a) a spacer having sheet resistance, ρ S w; and b) a coating material apphed to said spacer, said coatmg material having a sheet resistance, ρ S c, wherein p sc is greater than p S w, and having an area resistance r, which is less than approximately ρ sw (l 2 / 8) where 1 is the height of said spacer. 2. The spacer structure and coatmg combination of Claim 1 wherein said sheet resistance, ρ sc , of said coatmg material has a value approximately greater than 100 times said sheet resistance, ρ S w, of said spacer 3. The spacer structure of Claim 1 wherein the spacer structure is of a flat panel display, and wherein p sc > 100(p sw ) and r < p sw ( / 8) 4. The spacer structure and coating combination of Claim 1 or 3 wherein said area resistance, r, is less than approximately pg W (1 / 80) 5. The coating material of Claim 3 wherein said sheet resistance, ρ S c, of said coating material has a value approximately 100 times greater than said sheet resistance, psw > of said spacer 6. A flat panel display apparatus comprising a faceplate; a backplate disposed opposing said faceplate, said faceplate and said backplate connected in a sealed environment such that a low pressure region exists between said faceplate and said backplate; a spacer assembly disposed within said sealed environment, said spacer assembly supporting said faceplate and said backplate against forces acting in a direction towards said sealed environment, said spacer assembly increasingly attracting electrons with increasing anode to cathode current when a first voltage lower than an operating voltage is applied to said faceplate, said spacer assembly increasingly repelling electrons with an increasing anode to cathode current when a second voltage higher than said operating voltage is applied to said faceplate. 7. The flat panel display apparatus of Claim 6 wherein said spacer assembly is comprised of a coating material apphed to a spacer such that a combination spacer and coating material structure is formed. 8. The flat panel display apparatus of Claim 7 wherein said spacer has a sheet resistance, p S w. and said coatmg material has a sheet resistance, p S c, said sheet resistance, p S c, of said coating material being greater than said sheet resistance, ρ S w, of said spacer. 9. The flat panel display apparatus of Claim 8 wherein ρ S c is greater than approximately lOO(psw) and an area resistance, r, is less than approximately PswG /8) where 1 is the height of said spacer. 10 The flat panel display apparatus of Claim 8 wherein p S c is greater than approximately 100(ρ S w) and an area resistance, r, is less than approximately p sw (l2/80) where 1 is the height of said spacer. 11. The flat panel display apparatus of Claim 1, 3 or 8 wherein said sheet resistance, psw, of said spacer has a value of approximately 10 " to 10 ic Ω/r. 12. The flat panel display apparatus of Claim 1 or 7 wherein said spacer has a uniform resistivity through its thickness such that said resistivity throughout said thickness of said spacer does not vary by more than a factor of 5 13. The flat panel display apparatus of Claim 1 or 7 wherein said spacer has a uniform resistivity along said height thereof such that said resistivity does not vary by more than approximately 2 percent along said height of said spacer. 14. The flat panel display apparatus of Claim 1 or 7 wherein said spacer has a height of approximately 1-2 millimeters. 15. The flat panel display apparatus of Claim 1 or 7 wherein said spacer has a coefficient of thermal expansion within approximately 10 percent of the coefficient of thermal expansion of said faceplate and said backplate to which said spacer is adapted to be attached. 16. The flat panel display apparatus of Claim 1, 3 or 7 wherein said coating material apphed to said spacer is selected from the group consisting of cerium oxide material, chromium oxide material, and diamond-like carbon material. 17. The flat panel display apparatus of Claim 1, 3 or 7 wherein said coating material applied to said spacer has a thickness of approximately 200 Angstroms.

Inventors: HOPPLE GEORGE B, SPINDT CHRISTOPHER J, HOPPLE, GEORGE, B., SPINDT, CHRISTOPHER, J.
Applicants: CANDESCENT TECH CORP, CANDESCENT TECHNOLOGIES CORPORATION
Application year: 1998
Main classification: H01J1
IPC codes: H01J5, H01J29, H01J31