Abstract:
In one aspect, the invention includes a method of forming field emission emitter tips, comprising: a) providing a masking material over a semiconductor substrate to form a masking-material-covered substrate; b) submerging at least a portion of the masking-material-covered semiconductor substrate in a liquid; c) providing particulates suspended on an upper surface of the liquid; d) while the particulates are suspended, moving the submerged masking-material-covered substrate relative to the suspended particulates to form tightly packed monolayer of the particulates supported on the masking material of the masking-material-covered substrate; e) decreasing a dimension of the particulates to leave some portions of the masking material covered by the particulates and other portions of the masking material uncovered by the particulates; f) after decreasing the dimension and while the particulates are supported on the upper surface, exposing the masking-material-covered substrate to first etching conditions which remove uncovered portions of the masking material while leaving covered portions of the masking material over the substrate to define a patterned masking layer; g) removing the particulates; and h) while the patterned masking layer is over the semiconductor substrate, exposing the semiconductor substrate to a second etching conditions to pattern the semiconductor substrate into emitter tips.

Description:
PATENT RIGHTS STATEMENT 
     This invention was made with Government support under Contract No. DABT63-97-C-0001 awarded by Advanced Research Projects Agency (ARPA). The Government has certain rights in this invention. 
    
    
     TECHNICAL FIELD 
     The invention pertains to methods of forming field emission emitter tips. 
     BACKGROUND OF THE INVENTION 
     Field emitters are widely used in display devices, such as, for example, flat panel displays. Clarity, or resolution, of a field emission display is a function of a number of factors, including emitter tip sharpness. Specifically, sharper emitter tips can produce higher resolution displays than less sharp emitter tips. Accordingly, numerous methods have been proposed for fabrication of very sharp emitter tips (i.e., emitter tips having tip radii of 100 nanometers or less). Fabrication of very sharp tips has, however, proved difficult. It has proved particularly difficult to build large areas of sharp emitter tips using photolithography while maintaining resolution and stringent dimensional control over large area substrates used for display manufacture. In light of these difficulties, it would be desirable to develop alternative methods of forming emitter tips. Several methods have been proposed. Some utilize deposited particles to form a non-photolithographic etch mask. A subsequent etching step or series of steps forms the emitter tips. The use of deposited particles on a substrate as an etch mask can reduce complexity of an etching process and improve sharpness of emitter tips relative to photolithographic processing. A difficulty with present methods of using deposited materials as an etch mask is that the materials are frequently non-uniformly deposited on a substrate surface. Accordingly, emitter tips patterned from the deposited materials are not uniformly formed across a substrate surface. It would be desirable to develop alternative methods of using deposited materials as masking layers in emitter tip formation wherein the deposited materials are uniformly deposited across a substrate surface to enable a uniform distribution of emitter tips to be etched into the substrate surface. 
     In methodology understood to be unrelated and never applied to emitter tip formation, Langmuir-Blodgett processing has been developed as a technique for providing a monolayer of particulates over a substrate surface. Langmuir-Blodgett processing in the context of this application involves submerging a substrate in a liquid and providing particulates floating on a surface of the liquid. The particulates are preferably in the form of a monolayer on the liquid surface. The monolayer can be maintained as a tightly packed monolayer by providing a pushing bar to compact the particulates together. The substrate is subsequently pulled through the tightly packed monolayer to form an even coating of particulate materials on the substrate surface. 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention encompasses a method of forming field emission emitter tips. A masking material is provided over a semiconductor substrate to form a masking-material-covered substrate. At least a portion of the masking-material-covered semiconductor substrate is submerged in a liquid. Particulates are provided to be suspended on an upper surface of the liquid. While the particulates are suspended, the submerged masking-material-covered substrate is moved relative to the suspended particulates to form a layer of the particulates supported on the masking material of the masking-material-covered substrate. In a subsequent step, a dimension of the particulates is decreased to leave some portions of the masking material covered by the particulates and other portions of the masking material uncovered by the particulates. After decreasing the dimension and while the particulates are supported on the upper surface, the masking-material-covered substrate is exposed to first etching conditions to remove uncovered portions of the masking material while leaving covered portions of the masking material over the substrate to define a patterned masking layer. The particulates can then be removed. While the so patterned masking layer is over the semiconductor substrate, the semiconductor substrate is exposed to second etching conditions to pattern the semiconductor substrate into emitter tips. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention are described below with reference to the following accompanying drawings. 
     FIG. 1 is a diagrammatic, cross-sectional view of a bead application apparatus of the present invention. 
     FIG. 2 is a top view of the FIG. 1 apparatus. 
     FIG. 3 is a diagrammatic, fragmentary, cross-sectional view of a semiconductor substrate comprising a monolayer of particles formed according to a method of the present invention. 
     FIG. 4 is a view of the FIG. 3 substrate shown at a processing step subsequent to that of FIG.  3 . 
     FIG. 5 is a view of the FIG. 3 substrate shown at a processing step subsequent to that of FIG.  4 . 
     FIG. 6 is a view of the FIG. 3 substrate shown at a processing step subsequent to that of FIG.  5 . 
     FIG. 7 is a view of the FIG. 3 substrate shown at a processing step subsequent to that of FIG.  6 . 
     FIG. 8 is a diagrammatic, cross-sectional view of a second embodiment bead application method of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     This disclosure of the invention is submitted in furtherance of the constitutional purposes of the U.S. Patent Laws “to promote the progress of science and useful arts” (Article 1, Section 8). 
     The invention encompasses methods of forming emitters wherein a monolayer of particles is utilized to define emitter tip regions. A laboratory-test-scaled method for forming a monolayer of particles is as follows. First, a solution of distilled deionized water is prepared and a pH of the water is adjusted to about 3.5. The pH can be adjusted with dilute hydrochloric acid. Two to three drops of the pH 3.5 water are placed on a clean microscope glass slide. Surfactant-free carboxyl latex beads in water (having diameters approximately equal to about 1.7 micrometers) are provided in a clean vial and about 10% (volume to volume) methanol is added to the mixture. A clean pipet is utilized to provide one drop of the bead/water/methanol mixture into the pH 3.5 water on the slide. A two-dimensional array of the beads forms instantly on the water surface, covering an entire surface of the water (about 1 square centimeter). The two-dimensional array typically does not form if the water pH is neutral, or if there is no methanol in the mixture. Also, the latex beads will typically aggregate if the water pH is too low (for example, about pH 2). A factor that can be important for surface array formation is that polystyrene is itself hydrophobic, but can contain a surface charge (from, for example, polymerization initiation or copolymers like acrylic acid) which makes latex particles hydrophilic. Non-ionic initiators can be used to prevent surface charge from being present. However, surfactant should be used in such cases to maintain the stability (prevent aggregation) both during and after particle manufacturing processes. A surfactant (ionic or non-ionic) will make the particle surfaces hydrophilic, which can stabilize a monolayer of the latex beads. Accordingly, it can be preferred to utilize latex particles having some charge groups on the surface, such as, for example, surfactant-free carboxyl groups. 
     It is noted that an effective surface charge can be variable on surfactant-free carboxyl latex particles, enabling surface hydrophobicity to be controlled. Particle hydrophobicity can be controlled, for example, by adjusting a pH to about a surface charge pKa value. A pKa for carboxyl groups is around 4 (in other words, particle effective charge will be reduced and eventually disappear if a pH is adjusted lower than 4). When an effective surface charge drops, particles become increasingly hydrophobic, and it becomes increasingly thermodynamically unfavorable for the particles to be within an aqueous liquid. Accordingly, the particles migrate and aggregate to a water surface, or aggregate within the water to form a clump. Generally, surface aggregation occurs before clumping, and if charge is reduced gradually a monolayer will be formed over the surface. Accordingly, it can be important to carefully adjust pH to avoid having beads clump within an aqueous solution instead of forming a monolayer over the solution. A potentially useful modification of the above-described procedure is to provide a buffering material within the aqueous solution to simply control pH of the solution. 
     Surface aggregation by diffusion can be very slow, and can be unfavored if particles have a density higher than a suspending medium. Accordingly, methanol is utilized in the above-described process. Methanol can propel a particle to surface when it goes into water. 
     FIGS. 1 and 2 illustrate an apparatus  10  utilized in a preferred method for forming a monolayer of particles on a semiconductor substrate. Apparatus  10  comprises a vessel  12  containing a liquid  14 . Liquid  14  can comprise, for example, an aqueous solution having a pH of about 3.5. Such aqueous solution can further comprise an alcohol, such as, for example, methanol, and a buffer to aid in maintaining a pH of liquid  14  at a desired pH. 
     Particulates  16  are provided in liquid  14  and form a monolayer  18  over a surface of liquid  14 . The term “monolayer” refers to a layer of particulates  16  having a thickness of a single particle. In other words, particulates  16  do not overlay one another. Particulates  16  preferably comprise one or more materials phobic relative to liquid  14 , more preferably predominately comprise one or more materials phobic to the liquid (i.e., more than 50% of the particulate, by weight, being phobic materials) and can consist essentially of materials phobic relative to liquid  14 . For instance, if liquid  14  comprises water, particles  16  preferably comprise hydrophobic materials. If liquid  14  comprises water and methanol and a pH of about 3.5, particulates  16  can comprise, for example, latex beads, such as carboxyl latex beads. Particulates  16  can be approximately spherical and have diameters of from about 1.0 to about 2.0 micrometers and most preferably of about 1.7 micrometers. Also preferably, no surfactant is provided at a surface of liquid  14 . Surfactant could cling to a surface of a substrate processed according to a method of the present invention and adversely affect subsequent processing of the substrate. 
     A pusher bar  20  is provided to compress particulates  16  against one another to form a “tightly packed” monolayer. The term “tightly packed” is defined to mean that particulates  16  physically contact one another throughout monolayer  18 , rather than being dispersed from one another in monolayer  18 . In a uniform tightly packed monolayer, all of the particles  16  will be physically against other particulates  16  and will form a single layer over liquid  14 . 
     A parameter that can be important for maintaining monolayer  18  as a tightly packed monolayer is a surface tension at a surface of liquid  14 . Accordingly, a surface tension meter  22  is preferably provided and monitored to ascertain that a constant surface tension over liquid  14  is maintained by suitable application of pressure against beads  16  with pusher bar  20 . Surface tension meter  22  is preferably coupled with circuitry to convert a surface tension to a signal which can be either continuously monitored by appropriate software or monitored by a technician during use of apparatus  10 . Furthermore, the surface tension can be maintained at a value preferable for achieving a monolayer coating by providing feedback circuitry between the surface tension meter  22  and pusher bar  20 . Pusher bar  20  is preferably mechanically joined to an apparatus for pushing pusher bar  20  against beads  16 . A suitable mechanical apparatus can comprise, for example, a step motor configured to push pusher bar  20  in small increments (i.e., increments on the order of 1 to 10 microns). 
     A semiconductor substrate  30  is provided within apparatus  10  and initially at least partially submerged within liquid  14 . To aid in interpretation of this disclosure and the claims that follow, the term “semiconductor substrate” is defined to mean any construction comprising semiconductive material, including, but not limited to, bulk semiconductive materials (either alone or in assemblies comprising other materials thereon), and semiconductive material layers (either alone or in assemblies comprising other materials). The term “substrate” refers to any supporting structure, including, but not limited to, the semiconductor substrates described above. 
     In the shown embodiment, substrate  30  comprises a glass  32 , a semiconductive material layer  34  overlaying glass  32 , and a masking material layer  36  overlaying semiconductive material layer  34 . Semiconductive material layer  34  can comprise, for example, monocrystalline silicon lightly doped with a p-type dopant. Masking layer  36  can comprise, for example, silicon dioxide. Substrate  30  can be inserted in liquid  14  provided before beads  16  are provided at a surface of liquid  14 . Preferably, substrate  30  is initially provided to be only partially submerged within liquid  14  so that substrate  30  effectively forms a dam preventing particulates  16  from migrating to behind substrate  30  and against glass  32 . In the shown embodiment, substrate  30  is contained within a gasket material  40  (FIG. 2) which is provided against both sides of substrate  30  within vessel  12 . Gasket material  40  can provide a seal against substrate  30  to prevent particulates  16  from migrating around substrate  30 . Gasket material  40  can comprise a number of materials known to persons of ordinary skill in the art, including, for example, rubber or plastic. 
     After provision of substrate  30  and particulates  16  within apparatus  10 , substrate  30  is moved relative to an upper surface of liquid  14 . Such moving can comprise, for example, moving substrate  30 , the upper surface of liquid  14 , or both substrate  30  and the upper surface liquid  14 . For instance, liquid  14  can be drained from vessel  12  to move substrate  30  relative to the upper surface of liquid  14 . Alternatively, and as shown, substrate  30  can be lifted from liquid  14  to move substrate  30  relative to an upper surface of liquid  14 . 
     During the moving of substrate  30  relative to the upper surface of liquid  14 , particulates  16  form a monolayer  50  over masking material  36  of substrate  30 . Pusher bar  20  preferably maintains a constant pressure of particulates  16  against masking material  36  as substrate  30  is moved relative to the surface of liquid  14  to form a substantially uniform and tightly packed monolayer  50  over masking material layer  36 . 
     In the shown preferred embodiment, monolayer  50  is formed over a surface of masking material layer  36  that is substantially perpendicular to an upper surface of liquid  14 . It is to be understood, however, that the invention encompasses other embodiments wherein substrate  30  is tilted relative to the upper surface of liquid  14  as substrate  30  is moved relative to the upper surface. For instance, FIG. 8 illustrates an embodiment wherein gaskets  40  are eliminated, and wherein substrate  30  is initially entirely submerged within liquid  14 . An upper surface of submerged masking layer  36  is substantially parallel with an upper surface of liquid  14 . A distance between the upper surface of liquid  14  and an upper surface of making material  36  is then decreased until particulates  16  are deposited on masking layer  36 . During such depositing, the upper surface of masking layer  36  can remain substantially parallel to the upper surface of liquid  14 . 
     In addition to the above-described embodiments, the invention also encompasses embodiments wherein substrate  30  is tilted at an angle such that a surface of masking layer  36  upon which beads  16  are deposited is neither perpendicular nor parallel with an upper surface of liquid  14  as substrate  30  is moved relative to the upper surface of liquid  14 . 
     Referring to FIG. 3, substrate  30  is illustrated after formation of monolayer  50  over masking material  36  and removal of substrate  30  from liquid  14 . Particulates  16  of monolayer  50  can then be exposed to conditions which decrease a dimension of the particulates to expose portions of masking material  36  between the particulates. An example condition for shrinking particulates  16  is exposure to an oxygen plasma to oxidize the beads and shrink them at a controlled rate. Another example method for reducing a dimension of the beads is to heat them to a temperature which evaporates materials from the beads. Yet another example method for reducing a dimension of the beads is to etch them with, for example, a chemical wet etch. A mean diameter of spherical particulates  16  is preferably reduced at least about 20%, and more preferably at least about 50% prior to subsequent process steps. It is to be understood, however, that the invention also encompasses embodiments in which a dimension of particulates  16  is not reduced prior to subsequent process steps. For instance, it can be desired to not reduce a dimension of particulates  16  if a subsequent etch of layer  36  (described below with reference to FIG. 6) is sufficiently isotropic. 
     Referring to FIG. 4, substrate  30  is illustrated after a dimension of particulates  16  has been decreased. Particulates  16  now cover some portions of masking layer  36 , while leaving other portions of masking material between particulates  16  uncovered. Substrate  30  is then exposed to first etching conditions to remove portions of masking material layer  36  exposed between particulates  16 . The first etching conditions are preferably highly anisotropic to remove material of masking layer  36  along a vertical profile without substantially undercutting particulates  16 . Suitable etching processes include plasma etching and reactive ion etching. In embodiments in which particulates  16  comprise latex and masking material layer  36  comprises silicon dioxide, the first etching conditions can comprise, for example, an ion assisted etch utilizing He and one or both of CHF 3  and CF 4 . 
     After etching of masking layer  36 , substrate  30  can be exposed to conditions which remove particulates  16  from over remaining portions of masking material layer  36 . Suitable conditions for removing particulates  16  include, for example, one or more of physical cleaning, chemical cleaning, or dry etching. An example method for removing particulates  16  from masking material  36  is vibration of substrate  30  in an ultrasonic bath. Although particulates  16  are removed in the shown preferred embodiment, it is to be understood that the invention also encompasses embodiments in which particulates are not removed. 
     FIG. 5 shows substrate  30  after exposure to the first etching conditions and after removal of particulates  16 . Etching of masking layer  36  (FIG. 4) has converted the masking layer to a patterned mask  60  which covers portions of semiconductive material  34  and leaves other portions uncovered. 
     Referring to FIG. 6, substrate  30  is exposed to second etching conditions to etch exposed portions of substrate  34  and form conical emitter tips  70  (only some of which are labeled in FIG. 6) under patterned masking layer  60 . In embodiments in which masking layer  60  comprises silicon dioxide and layer  34  comprises monocrystalline, polycrystalline or amorphous silicon, second etching conditions can comprise isotropic etch processes known in the art. 
     Referring to FIG. 7, patterned masking layer  60  is removed from substrate  30  to form an emitter tip array. In embodiments in which masking layer  60  comprises silicon dioxide, it can be removed by, for example, wet etching utilizing buffered hydrofluoric acid. The emitter tip array of FIG. 7 can be incorporated into, for example, a flat panel display device as an emitter assembly. 
     In alternative embodiments (not shown), layer  36  can be eliminated and beads  16  can be utilized directly as the masks for formation of conical tips  70  (FIG.  6 ). Also, although the invention is described with reference to conical tip formation from semiconductive substrates, it is to be understood that the invention can have application to etching of non-semiconductive substrates. 
     In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents.