Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of pending U.S. patent application Ser. No. 09/653,819, filed Sep. 1, 2000, which is a divisional of pending U.S. patent application Ser. No. 09/085,333, filed May 26, 1998. 
    
    
     GOVERNMENT RIGHTS 
     This invention was made with government support under Contract No. DABT63-93-C-0025 awarded by Advanced Research Projects Agency (ARPA). The government has certain rights in this invention. 
    
    
     TECHNICAL FIELD 
     This invention relates in general to visual displays for electronic devices and in particular to improved focusing electrodes and techniques for field emission displays. 
     BACKGROUND OF THE INVENTION 
     FIG. 1 is a simplified side cross-sectional view of a portion of a field emission display  10  including a faceplate  20  and a baseplate  21  in accordance with the prior art. FIG. 1 is not drawn to scale. The faceplate  20  includes a transparent viewing screen  22 , a transparent conductive layer  24  and a cathodoluminescent layer  26 . The transparent viewing screen  22  supports the layers  24  and  26 , acts as a viewing surface and as a wall for a hermetically sealed package formed between the viewing screen  22  and the baseplate  21 . The viewing screen  22  may be formed from glass. The transparent conductive layer  24  may be formed from indium tin oxide. The cathodoluminescent layer  26  may be segmented into localized portions. In a conventional monochrome display  10 , each localized portion of the cathodoluminescent layer  26  forms one pixel of the monochrome display  10 . Also, in a conventional color display  10 , each localized portion of the cathodoluminescent layer  26  forms a green, red or blue sub-pixel of the color display  10 . Materials useful as cathodoluminescent materials in the cathodoluminescent layer  26  include Y 2 O 3 :Eu (red, phosphor P-56), Y 3 (Al, Ga) 5 O 12 :Tb (green, phosphor P-53) and Y 2 (SiO 5 ):Ce (blue, phosphor P-47) available from Osram Sylvania of Towanda Pa. or from Nichia of Japan. 
     The baseplate  21  includes emitters  30  formed on a planar surface of a substrate  32  that is preferably a semiconductor material such as silicon. The substrate  32  is coated with a dielectric layer  34 . In one embodiment, this is effected by deposition of silicon dioxide via a conventional TEOS process. The dielectric layer  34  is formed to have a thickness that is approximately equal to or just less than a height of the emitters  30 . This thickness is on the order of 0.4 microns, although greater or lesser thicknesses may be employed. A conductive extraction grid  38  is formed on the dielectric layer  34 . The extraction grid  38  may be formed, for example, as a thin layer of polysilicon. An opening  40  is created in the extraction grid  38  having a radius that is also approximately the separation of the extraction grid  38  from the tip of the emitter  30 . The radius of the opening  40  may be about 0.4 microns, although larger or smaller openings  40  may also be employed. 
     In operation, the extraction grid  38  is biased to a voltage on the order of 100 volts, although higher or lower voltages may be used, while the substrate  32  is maintained at a voltage of about zero volts. Signals coupled to the emitters  30  allow electrons to flow to the emitter  30 . Intense electrical fields between the emitter  30  and the extraction grid  38  cause emission of electrons from the emitter  30 . 
     A larger positive voltage, ranging up to as much as 5,000 volts or more but usually 2,500 volts or less, is applied to the faceplate  20  via the transparent conductive layer  24 . The electrons emitted from the emitter  30  are accelerated to the faceplate  20  by this voltage and strike the cathodoluminescent layer  26 . This causes light emission in selected areas, i.e., those areas opposite the emitters  30 , and forms luminous images such as text, pictures and the like. 
     Electrons emitted from each emitter  30  in a conventional field emission display  10  tend to spread out as the electrons travel from the emitter  30  to the cathodoluminescent layer  26  on the faceplate  20 . If the electron emission spreads out too far, it will impact on more than one localized portion of the cathodoluminescent layer  26  of the field emission display  10 . This phenomenon is known as “bleedover.” The likelihood that bleedover may occur is exacerbated by any misalignment between the localized portions of the cathodoluminescent layer  26  and their associated sets of emitters  30 . 
     When the electron emission from an emitter  30  associated with a first localized portion of the cathodoluminescent layer  26  also impacts on a second localized portion of the cathodoluminescent layer  26 , both the first and second localized portions of the cathodoluminescent layer  26  emit light. As a result, the first pixel or sub-pixel uniquely associated with the first localized portion of the cathodoluminescent layer  26  correctly turns on, and a second pixel or sub-pixel uniquely associated with the second localized portion of the cathodoluminescent layer  26  incorrectly turns on. In a color field emission display  10 , this can cause purple light to be emitted from a blue sub-pixel and a red sub-pixel together when only red light from the red sub-pixel was desired. As a result, a degraded image is formed on the faceplate  20  of the field emission display  10 . 
     In a monochrome field emission display  10 , color distortion does not occur, but the resolution of the image formed on the faceplate  20  is reduced by bleedover. In conventional field emission displays  10 , bleedover is alleviated in several ways. A relatively high anode voltage V a  may be applied to the transparent conductive layer  24  of the conventional field emission display  10 , so that the electrons emitted from the emitters  30  are strongly accelerated to the faceplate  20 . As a result, the electron emissions spread out less as they travel from the emitters  30  to the faceplate  20 . A relatively small gap between the faceplate  20  and the baseplate  21  may be used, again reducing opportunity for spreading of the emitted electrons. However, it has been found that these are impractical solutions because too high a voltage applied between the transparent conductive layer  24  and the baseplate  21 , or too small a gap between the faceplate  20  and the baseplate  21  may cause arcing. 
     Another way in which bleedover is reduced in conventional field emission displays  10  is by spacing the localized portions of the cathodoluminescent layer  26  relatively far apart. This is possible because of the relatively low display resolution provided by conventional field emission displays  10 . As a result, the electron emissions impact on the correct localized portion of the cathodoluminescent layer  26 . 
     Another approach to controlling the spatial spread of electrons emitted from a group of the emitters  30  is to surround the area emitting the electrons with a focusing electrode (not illustrated in FIG.  1 ). This allows increased control over the spatial distribution of the emitted electrons via control of the voltage applied to the focusing electrode, which in turn provides increased resolution for the resulting image. One such approach, where each focusing element serves many emitters, is described in U.S. Pat. No. 5,528,103, entitled “Field Emitter With Focusing Ridges Situated To Sides Of Gate”, issued to Spindt et al. 
     There are several disadvantages to these prior art approaches. In most prior art approaches, the focusing electrode is biased by a voltage source that is independent of other bias voltage sources associated with the emitter  30 . As a result, the use of a focusing electrode generally requires another bias voltage source to bias the focusing electrode. This, in turn, leads to problems due to variations in turn on voltage from one emitter  30  to another when a single bias voltage is applied for several focusing electrodes. When a group of emitters  30  are all affected by a single focusing electrode, some of the emitters  30  may exhibit a turn on voltage that differs from that exhibited by other emitters  30 . The effect that the focusing electrode has on the electrons emitted from each of these emitters  30  will differ. Additionally, some of the current through the emitter  30  will be collected by the focusing electrode. This complicates the relationship between the emitter current and light emission because some of the current through the emitter  30  is diverted from the faceplate  20  by the focusing electrode. Further, the effects of the focusing electrode are different for emitters  30  that are closer to the focusing electrode than for emitters  30  that are farther away from the focusing electrode. The lack of control over the amount of light emitted in response to a known emitter current results in poorer imaging characteristics for the display  10 . 
     The problem of bleedover is exacerbated by the trend to higher resolution field emission displays  10 . High resolution field emission displays use fewer emitters  30  per pixel or sub-pixel. This arises for several reasons, one of which is that a smaller pixel or sub-pixel subtends a smaller area in which the emitters  30  can be provided. As display engineers attempt to increase the display resolution of conventional field emission displays  10 , the localized portions of the cathodoluminescent layer  26  are necessarily crowded closer together. As a result, each emitter  30  in a high resolution field emission display makes a greater contribution to the pixel or sub-pixel associated with it. This increases the need to be able to control electron emissions and the spread of electron emissions from each emitter  30 . 
     An approach to focusing electrons emitted from the emitter  30  without requiring a separate bias voltage source to bias the focusing electrode is described in U.S. Pat. No. 5,191,217, entitled “Method and Apparatus for Field Emission Device Electrostatic Electron Beam Focussing,” issued to Kane et al. This approach makes no provision for modifying the focus parameters in response to the amount of current through the emitter  30 . 
     There is, therefore, a need to provide more reliable control of the spatial distribution of the electrons delivered to the faceplate without causing other problems in field emission displays. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention, a field emission display includes a substrate, a plurality of emitters formed on the substrate, and a dielectric layer formed on the substrate having an opening formed about each of the emitters. The field emission display also includes a conductive extraction grid formed substantially in a plane of tips of the plurality of emitters. The extraction grid includes openings each formed about a tip of one of the emitters. In accordance with an aspect of the invention, a focusing electrode that physically confines emitted electrons provides enhanced focusing performance together with reduced circuit complexity compared to prior art approaches. This, in turn, results in superior display performance, especially for high resolution field emission displays. 
     In another aspect of the invention, a focus electrode is formed on the substrate having an opening positioned above the emitter. An impedance element is electrically coupled between the focus electrode and the emitter. The impedance element allows a portion of those electrons that were emitted from the emitter and that were intercepted by the focus electrode to return to the emitter. The current flow through the impedance element produces a voltage that biases the focus electrode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified side cross-sectional view of a portion of a field emission display according to the prior art. 
     FIG. 2 is a simplified side cross-sectional view of a portion of a field emission display including a focusing electrode according to an embodiment of the invention. 
     FIGS. 3A,  3 B and  3 C are a simplified plan views of a portion of a field emission display including a focusing electrode according to embodiments of the invention. 
     FIG. 4 is a simplified schematic view of a field emission display and one emitter and focusing electrode biasing arrangement according to an embodiment of the invention. 
     FIG. 5 is a simplified schematic view of a field emission display and another emitter and focusing electrode biasing arrangement according to another embodiment of the invention. 
     FIG. 6 is a flow chart of a process for manufacturing a focusing electrode according to an embodiment of the present invention. 
     FIG. 7 is a simplified block diagram of a computer including a field emission display using the focusing electrode according to embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 is a simplified side cross-sectional view of a portion of a field emission display  11  including a focusing electrode  62  in accordance with one embodiment of the invention. FIG. 2 is not drawn to scale. Many of the components used in the field emission display  11  shown in FIG. 2 are identical to components used in the field emission display  10  of FIG.  1 . Therefore, in the interest of brevity, these components have been provided with the same reference numerals, and an explanation of them will not be repeated. 
     The pattern made by the emitted electrons when they strike the faceplate  20  is optimized by incorporating focusing electrodes  62  into the circuitry associated with the emitter  30 . This is particularly desirable for high resolution field emission displays  11 . The focusing electrodes  62  may be supported above the extraction grid  38  by a dielectric layer  64  as illustrated or may be placed in the plane of the extraction grid  38  (not illustrated). 
     Significantly, forming the opening in the focusing electrode  62  smaller than the diameter of the beam of electrons that would be emitted from the emitter  30  if the focusing electrode were not present causes the opening in the focusing electrode  62  to act as a pinhole. In other words, placing the focusing electrode  62  such that it physically confines the electrons emitted from the emitter  30  returns a portion of the emitted electrons to the emitter  30 . Under these circumstances, the shape of the electron distribution when the emitted electrons reach the faceplate  20  is determined more by the opening in the focusing electrode  62  than by the geometry of the tip of the emitter  30 . This allows a more uniform image to be displayed despite variations in the tips of the emitters  30 . This effect results from either making the diameter of the opening in the focusing electrode  62  small placing the focusing electrode  62  at a relatively large distance (e.g., up to five to ten microns) above the extraction grid  38  and the emitters  30 . 
     As shown in the simplified plan view of FIG. 3A, a field emission display  11  includes a focusing electrode  62  surrounding a three emitters  30 , grouped in a linear array. Three emitters  30  are shown in FIG. 3A for clarity of explanation and ease of illustration, however, it will be appreciated that more or fewer emitters  30  could be associated with a given focus electrode  62 , with one to ten emitters  30  being desirable, although more may be employed. The emitters  30  may be arranged in a single line, as shown in FIG. 3A, or may be configured in a double line as shown in FIG. 3B or may be staggered in a double line of emitters  30  as shown in FIG. 3C, or may be in some other configuration. In the embodiments shown in FIGS. 3A through 3C, the focusing electrode  62  is preferably spaced laterally (i.e., left to right in FIGS. 3A through 3C) from the emitters  30  by a micron or more. Edge or end effects are reduced if the ends (i.e., top and bottom) of the focusing electrode  62  are several microns away from those emitters  30  that are located at the ends of the groups of emitters  30 . 
     An advantage provided by a linear array of emitters  30  within an oblong focusing electrode  62  is that the focusing electrode  62  provides a more uniform effect on each of the emitters  30  compared to a focusing electrode surrounding a large group of emitters  30  because the emitters  30  in the group are at different distances from the focus electrode. A field emission display using a focusing electrode to surround a group of emitters is described, for example, in U.S. Pat. No. 5,528,103. The uniformity of the linear arrangements shown in FIGS. 3A through 3C renders the focusing electrodes  62  more effective. 
     A linear arrangement is preferred for several reasons. First, emitters in other arrangements may function differently depending upon their location. Furthermore, a focusing electrode optimized for one electrode may not be optimized for other emitters in the group. In contrast, the emitters  30  shown in FIGS. 3A-3C are all the same distance from a focusing electrode  62  and the focus influence thus should be similar for each of the emitters  30 . 
     FIG. 4 is a simplified schematic view of one embodiment of a field emission display  11 ′ in accordance with the invention having the emitter  30  electrically coupled via an optional impedance  66  to the focusing electrode  62 . The focusing electrode  62  is formed above the extraction grid  38  as described above with reference to FIG. 2. A bias voltage is applied to the extraction grid  38  via a power supply  68 , and a bias voltage is supplied to the faceplate  20  via a power supply  70 . In this embodiment, the electrons supplied to the emitter  30  are modulated by a current source  72 , such as a Field Effect Transistor (FET), or other similar devices. 
     By electrically coupling a focusing electrode  62  to the emitter  30 , several different objectives can be met while also simplifying the biasing arrangements for the emitter  30  and ancillary circuitry. One of these objectives is that the current coupled through the emitter  30  by the current source  72  is proportional to the current through the faceplate  20  because any electrons collected by the focusing electrode  62  are automatically resupplied to the emitter  30  through the optional impedance  66 . Many of the prior art arrangements for biasing focusing electrodes permit an undefined amount of the current carried by the emitters to be diverted via the focusing electrodes. This means that the luminosity of the pixel associated with the emitters  30  is not necessarily related to the current that was directed through the emitters  30 . Another of these objectives is that there is no need to adjust the bias voltage on the focusing electrode  62  to compensate for variations in the voltage on the emitter  30 . Further, there is no need for a separate bias voltage source for the focusing electrode  62 . 
     FIG. 5 is a simplified schematic view of another embodiment of a field emission display  11 ″ in accordance with the invention. In the display  11 ″ electrons are supplied to the emitter  30  via a current-limiting element, such as a resistor  73 , that is electrically coupled between the emitter  30  and ground. In this approach, the current through the emitter  30  is ultimately set by a bias voltage applied to the extraction grid  38 . The arrangement of FIG. 5 is used to permit each emitter  30  to be self-biasing and ensures that if one or more of the emitters  30  become short-circuited, e.g., to the extraction grid  38 , the entire pixel is not short-circuited, because the resistor  73  limits the current through any one emitter  30 . 
     In either of the embodiments  11 ′ and  11 ″ of FIGS. 4 and 5, the relationship between the current through the faceplate  20  and the emitter  30  current is simplified compared to the situation where an independent bias voltage source is used to set the voltage on a focusing electrode. In both embodiments  11 ′ and  11 ″, the focusing electrode  62  is electrically coupled to the emitter  30  via the optional impedance  66 . This arrangement ensures that the current through the controlled current source  72  is either directed to the extraction grid  38  or is directed through the opening  40  and is collected by the faceplate  20 . As a result, the focusing electrode  62  does not provide additional path whereby current flowing through the emitter  30  may be diverted. For the case where the optional impedance  66  is simply an interconnection, the effect of the focusing electrode  62  is readily modeled because the potential on the focusing electrode  62  is exactly the same as the potential on the emitter  30 . 
     When the optional impedance  66  comprises a current-limiting element, such as, for example, a high value resistor, the focusing electrode  62  becomes self-biasing because the electrons collected by the focusing electrode  62  bias the focusing electrode  62  negative with respect to the emitter  30 . As the voltage on the focusing electrode becomes more negative, it attracts fewer electrons, thus limiting the voltage on the focusing electrode  62  from becoming even more negative. The use of the impedance  66  does not impair the benefits of not requiring a separate focus power supply and of ensuring that the emitter current corresponds to the luminance. Additionally, a short circuit between the focusing electrode  62  and, for example, the extraction grid  38  (or other structures), need not completely prevent the emitter  30  from functioning, because the impedance  66  isolates the emitter  30  from the focusing electrode  62  to some degree. 
     It will be appreciated that current-limiting elements other than an impedance  66  may be employed, such as constant current elements (e.g., reverse-biased diodes or FETs having the source connected to the gate) or constant voltage elements (e.g., Zener diodes) and the like, to either provide a bias voltage on the focusing electrode  62  that is related to the emitter  30  current or that has a known relationship to the voltage present on the emitter  30 . 
     In the embodiments of FIGS. 3 through 5, the focusing achieved by the focusing electrode  62  is determined by the geometry and placement of the focusing electrode  62  with respect to the other structures, and especially the emitter  30 , forming the field emission display  11 ,  11 ′ or  11 ″. Both the lateral separation of the focusing electrode  62  from the tips of the emitters  30 , typically on the order of one or two micrometers, and the vertical separation of the focusing electrode  62  from the extraction grid  38 , may be varied. The vertical separation may range from zero microns when the focusing electrode  62  is placed in the plane of the extraction grid  38  (not illustrated), to one to five microns or even as much as ten microns or more. 
     FIG. 6 is a flow chart of a process  80  for manufacturing the focusing electrode  62  according to an embodiment of the present invention. The substrate  32  having a plurality of the emitters  30  has been previously formed, and the surface of the substrate  32  and the emitters  30  have been previously coated with the dielectric layer  34 . The extraction grid  38  has also already been formed. The second dielectric layer  64  is formed on the extraction grid  38  in step  82 . A conductive layer is formed on the second dielectric layer  64  in step  84 . The conductive layer is patterned to form the focusing electrode  62  in step  86 . The second dielectric layer is then patterned in step  88  so as to form an opening surrounding each emitter  30  or group of emitters. 
     In one embodiment, the conductive layer is formed as a polysilicon layer, and the second dielectric layer  64  is a layer of silicon dioxide deposited on the extraction grid  38 . This arrangement allows the second dielectric layer  64  to be patterned via the buffered oxide etch using the focusing electrode  62  as a self-aligned mask. The focusing electrode  62  is electrically coupled to the emitter  30  via the optional impedance  66  in step  90 . The process  80  then ends and processing of the field emission display  11 ,  11 ′ or  11 ″ is subsequently completed via conventional fabrication steps. 
     FIG. 7 is a simplified block diagram of a portion of a computer  100  including the field emission display  11 ,  11 ′ or  11 ″ having the focusing electrode  62  as described with reference to FIGS. 2 through 6 and associated text. The computer  100  includes a central processing unit  102  coupled via a bus  104  to a memory  106 , function circuitry  108 , a user input interface  110  and the field emission display  11 ,  11 ′ or  11 ″ including the focusing electrode  62  according to the embodiments of the present invention. The memory  106  may or may not include a memory management module (not illustrated) and does include ROM for storing instructions providing an operating system and a read-write memory for temporary storage of data. The processor  102  operates on data from the memory  106  in response to input data from the user input interface  110  and displays results on the field emission display  11 ,  11 ′ or  11 ″. The processor  102  also stores data in the read-write portion of the memory  106 . Examples of systems where the computer  100  finds application include personal/portable computers, camcorders, televisions, automobile electronic systems, microwave ovens and other home and industrial appliances. 
     Field emission displays  11 ,  11 ′ or  11 ″ for such applications provide significant advantages over other types of displays, including reduced power consumption, improved range of viewing angles, better performance over a wider range of ambient lighting conditions and temperatures and higher speed with which the display can respond. Field emission displays find application in most devices where, for example, liquid crystal displays find application. 
     Although the present invention has been described with reference to a preferred embodiment, the invention is not limited to this preferred embodiment. Rather, the invention is limited only by the appended claims, which include within their scope all equivalent devices or methods which operate according to the principles of the invention as described.

Technology Category: 5