Abstract:
A method of forming an extraction grid for field emitter tip structures is described. A conductive layer is deposited over an insulative layer formed over the field emitter tip structures. The conductive layer is milled using ion milling. Owing to topographical differences along an exposed surface of the conductive layer, ions strike the exposed surface at various angles of incidence. As etch rate from ion milling is dependent at least in part upon angle of incidence, a selectivity based on varying topography of the exposed surface (“topographic selectivity”) results in non-uniform removal of material thereof. In particular, portions of the conductive layer in near proximity to the field emitter tip structures are removed faster than portions of the conductive layer between emitter tip structures. Thus, portions of the insulative layer in near proximity to the field emitter tip structures may be exposed while leaving intervening portions of the conductive layer for forming the extraction grid. Accordingly, such formation of the extraction grid is self-aligned to its associated emitter tip structures.

Description:
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY-SPONSORED AND DEVELOPMENT 
     This invention was made with government support under Contract No. DABT63-97-C-0001 awarded by the Defense Advanced Research Projects Agency (DARPA). The government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to grids and their formation, and more particularly to field extraction grids and their construction for field emission displays. 
     BACKGROUND OF THE INVENTION 
     In the microelectronics industry, there is a movement toward creating flat panel displays. These displays have the advantage of being significantly more compact than cathode ray tube displays, e.g., conventional computer monitors. There are different types of flat panel displays, such as liquid crystal displays (“LCDs”), gas-plasma displays, thin film transistor (“TFT”) displays, and field emission displays (“FEDs”). FEDs are particularly well suited to applications requiring high resolution, low power demand, wide viewing angle, and physical robustness in an operational environment. 
     FEDs are able to achieve high resolution owing in part to the presence of a significant number of emitter tip structures concentrated in a small space. These emitter tip structures, or cold cathode field emitter tip structures, and their formation are described in U.S. Pat. Nos. 5,391,259, 5,372,973, 5,358,908, 5,151,061, 3,755,704, 3,665,241, among others. 
     For emitter tip structures to emit electrons, a voltage bias is applied across the emitter tip structures and an extraction grid to create a potential difference therebetween. In U.S. Pat. No. 5,372,973 to Doan et al., formation of an extraction grid self-aligned to emitter tip structures is described. 
     In Doan et al., after forming emitter tip structures, a silicon nitride layer is deposited over the emitter tip structures. This layer is conformal to the surface upon which it is deposited. Next, boro-phospho-silicate-glass (“BPSG”) is deposited as an insulating layer. The BPSG layer is deposited and re-flowed, such that it does not extend above the silicon nitride layer. In other words, the silicon nitride layer above the emitter tip structures is left exposed after deposition and re-flowing of the BPSG. Next, a conductive layer, such as a layer of polysilicon having impurities (“dopants”), is deposited on the BPSG layer and the exposed regions of the silicon nitride layer. The layer of polysilicon is chemically-mechanically polished to re-expose regions of the silicon nitride layer; specifically, those regions disposed above apexes of the emitter tip structures. Accordingly, the polished conductive layer of polysilicon forms an extraction grid self-aligned to the emitter tips. The assembly may then be etched to pull the silicon nitride and the BPSG away from the emitter tip structures. 
     Though Doan et al. provide a self-aligned process for forming an extraction grid after formation of emitter tip structures, Doan et al. exposes the extraction grid layer to water, chemical-mechanical-polishing (CMP) slurry, and other potentially corrosive materials, some of which must then be cleaned off the assembly with other materials which may be harmful to some emitter structures. 
     A technique known as “etch back” is an alternative to CMP in situations where a blanket flow fill layer is previously deposited. Etch-back typically refers to a blanket plasma (“dry”) etch of such a surface. Etch-back does not have the above-mentioned disadvantages of CMP. However, etch-back uniformly removes material across a surface. Referring to U.S. Pat. No. 5,266,530 to Bagley, et al. (“Bagley”), dielectric layer  24  is etched back to expose a portion of underlying dielectric layer  22 . Dielectric layer  22  may then be etched to pull it away from tip  18 . Gate layer  26  may then be deposited, and subsequently etched to remove a portion of gate layer  26  deposited on tip  18 . In Bagley, uniform removal by etching is employed. However, it would be desirable to define a gate layer with fewer etching steps than Bagley. 
     Accordingly, it would be desirable in the art of manufacturing field emission devices to provide a self-aligned process for forming an extraction grid after forming emitter tip structures with the advantages associated with dry etch with conformal or substantially conformal (with plus or minus 50 nm) deposit material using fewer etch steps than in Bagley. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for forming a grid. In particular, a substrate assembly having one or more emitter tip structures formed thereon or therefrom is provided. An insulative layer is formed on or above the emitter tip structures, as well as on or above an associated emitter layer from which the emitter tip structures protrude. A conductive layer is formed on or above the insulative layer. An exposed surface of the conductive layer thus exhibits topographical variation owing to the presence of the underlying emitter tip structures. The exposed surface is then subjected to particle bombardment from ion milling. These particles are used to remove material from the conductive layer at various etch rates dependent at least in part on angle of incidence thereof. More particularly, portions of the conductive layer in near proximity to the one or more emitter tip structures are removed more rapidly than other portions. Accordingly, the insulative layer may be exposed in near proximity to the one or more emitter tip structures, while leaving a surrounding portion of the conductive layer for forming the grid. 
     In accordance with the present invention, a grid structure may be formed. Such a grid may be used as an anode in a field emitter display device for extracting electrons from emitter tip structures, namely, as an “extraction grid.” Such an “extraction grid” may be formed self-aligned or centered to one or more of the emitter tip structures due to the preferential etching of the conductive layer overlying the emitter tip structure locations. In other words, the extraction grid or portions thereof may have z-axis (an axis traveling up through the center of an emitter tip structure) symmetry with respect to one or more associated emitter tip structures. Stated another way, a portion of the extraction grid in near proximity to an associated emitter tip structure is centered relative to said structure. Owing to performance characteristics dependent upon alignment of an emitter tip structure and its corresponding anode extraction grid section, as well as ease of manufacture, a self-aligned process for forming such an extraction grid is advantageous. Moreover, an extraction grid in accordance with the present invention may be formed in-situ with respect to other portions of the field emission display. Furthermore, ion milling may be used to expose the one or more emitter tip structures for sharpening. Such sharpening may be done in-situ with the ion milling used to expose the emitter tip structures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Features and advantages of the present invention will become more apparent from the following description of the preferred embodiments described below in detail with reference to the accompanying drawings where: 
     FIGS. 1,  2  and  3  are cross-sectional views of exemplary portions of embodiments of FEDs formed in accordance with the present invention. 
     FIGS. 4 and 5 are cross-sectional views of exemplary portions of embodiments of in-process substrate assemblies in accordance with the present invention. 
     FIG. 6 is a cross-sectional view of the substrate assembly of FIG. 5 during ion milling in accordance with the present invention. 
     FIG. 7 is a graphical representation of angle of incidence versus etch rate for ion milling in accordance with the present invention. 
     FIG. 8 is a cross-sectional view of the substrate assembly of FIG. 6 after isotropic etching. 
     FIG. 9 is a cross-sectional view of an exemplary portion of pentode formed in accordance with the present invention. 
     FIG. 10 is a top down view of an exemplary portion of an embodiment of an extraction grid formed in accordance with the present invention. 
    
    
     Reference numbers refer to the same or equivalent parts of embodiment(s) of the present invention throughout the several figures of the drawing. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings which form a part of this disclosure, and which, by way of illustration, are provided for facilitating understanding of specific embodiments in accordance with the present invention described herein. Though the present invention is described in terms of the formation of a portion of an FED, it is to be understood that other embodiments may be practiced without departing from the scope of the present invention. To more clearly describe the present invention, some conventional details with respect to FEDs and systems including an FED have been omitted. 
     Referring to FIGS. 1,  2  and  3 , there are shown cross-sectional views of exemplary portions of embodiments of FED  10  as may be formed through use of the present invention. FED  10  comprises a lower member (“baseplate”)  21 . Baseplate  21  conventionally comprises an electrically insulative body  4 , such as glass, and an electrically conductive body  3 . Conductive body  3  may be patterned to form a grid. 
     Emitter layer  11  extends over conductive body  3 , and is in electrical contact with conductive body  3 . Emitter or resistive layer  11  is electrically conductive and provides a sufficient amount of electrical resistance. Electrically resistive pads  2  of a different electrical resistance than resistive layer  11  may optionally be formed over conductive body  3  in substantial vertical orientation below emitter tip structures  13 , or resistive layer  11  and resistive pads  2  may be formed as a single unit. Power supply  20  is electrically coupled to resistive layer  11  through conductive body  3 . Resistive layer  11  comprises emitter tip structures  13 . Emitter tip structures  13  may be integrally formed as a part of resistive layer  11 , or may be formed from one or more separate layers as illustratively indicated by dashed line  9 . Resistive layer  11  may be made of one or more electrically conductive materials, such as one or more metals or conductively adjusted semiconductors. 
     Spacer  14  extends over resistive layer  11 . Spacer  14  may be made of one or more electrically insulative materials, such as one or more dielectrics, as illustratively shown in FIGS. 4 and 5 with respect to insulative layer  24 . More particularly, spacer  14  may comprise one or more layers of one or more dielectric materials, such as an oxide, a nitride, or like dielectric material. 
     Extraction grid  15  extends over spacer  14 . Extraction grid  15  may be made of one or more electrically conductive materials, such as one or more metals or conductively adjusted semiconductors. By conductively adjusted, it is meant that acceptor and/or donor impurities or defects are intentionally added to a semiconductor to adjust its conductivity. 
     FED  10  further comprises posts  18  and upper member  16  (“faceplate”). Posts  18  aid in defining and maintaining volume  22  between faceplate  16  and emitter tip structures  13 . Volume  22  may be completely or substantially evacuated to further facilitate electron  20  projection  17  from emitter tip structures  13  to phosphors  19  of faceplate  16 . Faceplate  16  may comprise a non-opaque glass  8  having a non-opaque electrically conductive body  7  laminated thereto. Conductive body  7  is conventionally formed of indium tin oxide (“ITO”). 
     Power supply  20  is electrically coupled to resistive layer  1   1 , extraction grid  15  and faceplate  16 . Extraction grid  15  is biased by power supply  20  to be more positive in voltage than resistive layer  11 . By creating a potential difference between emitter tip structures  13  and extraction grid  15 , electrons are ejected or projected from emitter tip structures  13 . To attract and accelerate electrons  17  from emitter tip structures  13  to phosphors  19 , a positive voltage is applied to conductive body  7  of faceplate  16 . As voltage applied to conductive body  7  of faceplate  16  is more positive than that applied to extraction grid  15 , a difference in potential between extraction grid  15  and faceplate  16  exists which facilitates electron attraction. 
     The present invention provides the ability to form extraction grid  15  at different locations with respect to apexes  23  of emitter tip structures  13 . Extraction grid  15  may be formed above apexes  23 , as illustratively shown in FIG.  1 . Alternatively, extraction grid  15  may be formed below apexes  23 , as illustratively shown in FIG.  2 . Alternatively, a portion of extraction grid  15  may be formed coplanar with apexes  23 , as illustratively shown in FIG. 3 with respect to thickness  35  including but not limited to upper surface  6  and lower surface  5  of extraction grid  15 . Moreover, it should be understood from the following detailed description that extraction grid  15  or portions thereof may be formed self-aligned to one or more associated emitter tip structures  13 . 
     Referring to FIGS. 4 and 5, there are shown cross-sectional views of exemplary portions of respective embodiments of in-process FEDs  10  in accordance with the present invention. Notably, in each embodiment emitter tip structures  13  are formed prior to forming extraction grid  15 . 
     Insulative layer  24  is formed adjacent resistive layer  11 . By adjacent it is meant that insulative layer  24  is in near proximity to resistive layer  11  and may or may not be in contact with resistive layer  11 . As illustratively shown in FIG. 4, an intervening layer  29  may exist between insulative layer  24  and resistive layer  11 . Layer  29  may be deposited on resistive layer  11 , may be grown from resistive layer  11 , or may be formed by the interaction of insulative layer  24  and resistive layer  11 . 
     Though insulative layer  24  is shown as conformal or substantially conformal to resistive layer  11 , it need not be. By way of example and not limitation, owing to the contour created by emitter tip structures  13 , insulative layer  24  may be thinner over an upper portion of emitter tip structures  13  as compared to its thickness in valley  26  between emitter tip structures  13 . Moreover, insulative layer may be deposited and ion milled as described in U.S. Patent Application entitled “Structure and Method for Reduced Emitter Tip to Gate Space in Field Emission Devices”, filed Sep. 2, 1998, to Ji Ung Lee and incorporated by reference as though fully set forth herein. In the preferred embodiment, insulative layer  24  is a single layer of a silicon oxide formed by with a low temperature process such as plasma enhanced chemical vapor deposition (“PECVD”) or physical vapor deposition (“PVD”), as illustratively shown in FIG.  5 . 
     Conductive layer  25  is formed adjacent to insulative layer  24 . By adjacent it is meant that conductive layer  25  is in near proximity to insulative layer  24  and may or may not be in contact with insulative layer  24 . Conductive layer  25  is illustratively shown as being in contact with insulative layer  24 . However, conductive layer  25  need not be in contact with insulative layer  24 . By way of example and not limitation, one or more intermediate layers (not shown) may be formed between conductive layer  25  and insulative layer  24 . Intermediate layers may be formed by deposition, growth, or material interaction. The latter type of formation depends at least in part on the materials employed, and such formation includes but is not limited to a silicide, a silicon nitride, a metal oxide, and like combination. 
     Conductive layer  25  comprises one or more layers formed of one or more conductive materials as illustratively shown in FIGS. 4 and 5. In the preferred embodiment, conductive layer  25  is vapor deposited to provide a single layer of amorphous silicon with phosphorous impurities, as illustratively shown in FIG.  5 . Conductive layer  25  need not be conformal as illustratively shown in FIGS. 4 and 5, and preferably it is thinner in near proximity to apexes  23  of emitter tip structures  13  as compared with its thickness in valley  26 . For use of deposited silicon, this thinning may be achieved by adjusting deposition parameters to adjust flow characteristics of the silicon. 
     After formation of conductive layer  25 , extraction grid  15  is formed by ion milling, as illustratively shown in FIG. 6 with respect to particles  27 . With respect to ion milling, an inert or reactive gas environment may be used. By way of example and not limitation, ionized argon (Ar) gas with voltages at or in excess of 100 volts are used in an embodiment for ion milling. Ion milling may be described as ion bombardment of a surface do to effect removal of material therefrom by momentum transfer. 
     By way of example and not limitation, an inductively coupled plasma (ICP) source of a dry or plasma etch tool, such as a Continuum tool from Lam Research Corp. of Fremont, California, with a top and a bottom electrode (dual power chamber) may be used for ion milling. In the Continuum tool, the top and bottom electrodes are not coupled. The top electrode is used to provide a plasma source (“top power”), and the bottom electrode is used to provide a bias voltage (“bottom power”) and a wafer chuck. In the Continuum tool, the bias power is provided as a radio frequency (RF) signal to the bottom electrode. By increasing power of the RF signal, bias voltage increases as applied to the substrate assembly positioned on the bottom electrode. In one embodiment of the present invention, a top electrode power is set at about 2500 Watts (W); a bottom electrode power is set in a range of about 400 to 800 W over a substrate assembly of about 250 by 300 millimeters (about 10 by 12 inches) wide; a gas pressure is set at about 13.16×10 −6  atm (about 10 mTorr); and an argon (Ar) gas flow rate is set at about 200 sccm (standard cubic centimeters per minute; a standard cubic centimeter of gas is conventionally determined at about room temperature at about one atmosphere of pressure). 
     Owing to topographical differences or variations along surface  28  of conductive layer  25  substantially corresponding to locations of underlying emitter tip structures, there is a distribution of angles of incidence, α, of particles  27  impacting on surface  28 . Angle of incidence, α, is defined as angular deviation from normal or perpendicular incidence to a tangential line through a point location at which a particle strikes a surface. Etch rate is dependent at least in part on angle of incidence, α, as illustratively shown in a graph of angle of incidence (x-axis) versus etch rate (y-axis) of FIG.  7 . FIG. 7 indicates that as the angle of incidence increases from 0 degrees toward 90 degrees, etch rate increases. However, just prior to parallel incidence, etch rate dramatically decreases. 
     In accordance with an embodiment of the present invention, particles  27  are directed or projected in a range from substantially perpendicular to perpendicular with respect to substrate assembly  40 . By substrate assembly, it is meant a base member having one or more layers of material formed thereon. 
     Particles  27  impact along surface  28  at a variety of angles of incidence. In valley regions  26 , angles of incidence, α, may range from approximately 0 to 45 degrees inclusive. Along slopes of surface  28  approaching underlying emitter tip structures  13 , angles of incidence, α, may range from approximately 45 to 85 degrees non-inclusive. Along surface  28  disposed above apexes  23 , angles of incidence, α, may range from approximately 85 to 90 degrees inclusive. In the above-described embodiment, etch rate for angle of incidence, α, in a range of approximately 0 to 45 degrees is lower than if it were in a range of approximately 45 to 85 degrees. Accordingly, it should be understood that etch rate is dependent on angle of incidence. This phenomenon also ensures that an extraction grid  15  formed from conductive layer  25  is self-aligned to locations of emitter tip structures  13 , since the portions of conductive layer  25  underlying emitter tip structure  13  are etched most rapidly. Moreover, it should be understood that topography of surface  28  may be tailored to enhance this non-uniform material removal from conductive layer  25 . By way of example and not limitation, geometry of emitter tip structures  13  may be altered to affect angle of incidence in order to effect a change in etch rate. 
     After milling, portions of another layer underlying conductive layer  25  may be exposed. In the preferred embodiment, portions of insulative layer  24  are exposed as illustratively shown in FIG.  6 . The portion of conductive layer  25  remaining after milling forms extraction grid  15  (shown in FIGS. 1,  2 , or  3 ). 
     After milling conductive layer  25 , insulative layer  24  surrounding emitter tip structures  13  may be etched with a plasma (“dry”) or chemical bath (“wet”) process. In the preferred embodiment, a wet etch is used, as illustratively shown in the cross-sectional view of FIG.  8 . Extraction grid  15  may be patterned prior to etching layer  24  so that address lines for extraction grid  15  may be formed. 
     Layers  24  and  25 , as illustratively shown in FIGS. 5,  6  and  8 , may be formed in situ in accordance with the present invention. By in-situ it is meant that all steps may be performed in chamber  50  or a cluster  60  without having to unseal the chamber or the cluster, respectively. By cluster it is meant a plurality of chambers operatively coupled such that vacuum need not be broken when moving a substrate assembly from one chamber to another. Thus, substrate  40  may be placed in chamber  50  or cluster  60  after forming emitter tip structures  13  and prior to forming insulative layer  24 . Chamber  50  or cluster  60  may then be sealed, and layers  24  and  25  may be formed prior to unsealing chamber  50  or cluster  60 , respectively. 
     In a single chamber embodiment, chamber  50  may be a deposition and etch chamber, such as a sputter deposition and etch chamber. In a clustered chambers embodiment, a PECVD or PVD chamber may be used for forming insulative layer  24  and conductive layer  25 , and an etch chamber, such as a “Continuum” tool from Lam Research of Freemont, California, may be used for topographically selectively removing material from conductive layer  25 , and may be used for isotropically dry etching insulative layer  24 . 
     Referring to FIG. 9, there is shown a cross-sectional view of an exemplary portion of pentode  41  in accordance with the present invention. Pentode  41  may be used in a cathode ray tube (CRT) electron gun or in an FED. Pentode  41  comprises a control grid formed by conductive layers  25 ,  25 A, and  25 B, each of which provide a separate anode or grid element. In forming pentode  41 , insulative layers  24 ,  24 A and  24 B, and conductive layers  25 ,  25 A, and  25 B are formed self-alignment to emitter tip structures  13 . 
     Insulative layers  24 ,  24 A, and  24 B may be formed such that each layer is either progressively thinner or thicker than an associated preceding layer. If insulative layers  24 ,  24 A, and  24 B are formed progressively thinner or thicker, then conductive layers  25 ,  25 A, and  25 B may be disposed progressively closer or further, respectively, to or from vertical axis  45  through apexes  23  of emitter tip structures  13 . 
     Referring to FIG. 10, there is shown a top-down view of an exemplary portion of an embodiment of an extraction grid  15  formed in accordance with the present invention. 
     The present invention has been particularly shown and described with respect to certain preferred embodiment(s) and features thereof. It should be readily apparent to those of ordinary skill in the art that various changes and modifications in form and detail may be made without departing from the spirit and scope of the present invention as set forth in the appended claims.