Patent Publication Number: US-6338938-B1

Title: Methods of forming semiconductor devices and methods of forming field emission displays

Description:
RELATED PATENT DATA 
     This patent is a continuation application of U.S. patent application Ser. No. 09/145,488 which was filed on Sep. 1, 1998, now U.S. Pat. No. 6,037,104. 
    
    
     TECHNICAL FIELD 
     The invention pertains to methods of forming semiconductor devices, and in one aspect pertains to methods of forming field emission displays. 
     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  11  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. In light of these difficulties, it would be desirable to develop alternative methods of forming emitter tips. 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention encompasses a method of forming a semiconductor device. A layer is formed over a substrate and a plurality of openings are formed extending into the layer. Particles are deposited on the layer and collected in the openings. The collected particles are melted and used as a mask during etching of the underlying substrate to define features of the semiconductor device. 
     In another aspect, the invention encompasses a method of forming a field emission display. A silicon dioxide layer is formed over a conductive substrate and a plurality of openings are formed to extend into the silicon dioxide layer. Particles are deposited on the silicon dioxide layer and collected within the openings. The collected particles are utilized as a mask during etching of the conductive substrate to form a plurality of conically shaped emitters from the conductive substrate. A display screen is formed spaced from the emitters. 
    
    
     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, fragmentary, cross-sectional view of a semiconductor substrate at a preliminary process step of a method of the present invention. 
     FIG. 2 is a view of the FIG. 1 substrate shown at a processing step subsequent to that of FIG.  1 . 
     FIG. 3 is a view of the FIG. 1 substrate shown at a processing step subsequent to that of FIG.  2 . 
     FIG. 4 is a view of the FIG. 1 substrate shown at a processing step subsequent to that of FIG.  3 . 
     FIG. 5 is a view of the FIG. 1 substrate shown at a processing step subsequent to that of FIG.  4 . 
     FIG. 6 is a view of the FIG. 1 substrate shown at a processing step subsequent to that of FIG.  5 . 
     FIG. 7 is a view of the FIG. 1 substrate shown at a processing step subsequent to that of FIG.  6 . 
     FIG. 8 is a view of the FIG. 1 substrate shown at a processing step subsequent to that of FIG.  7 . 
     FIG. 9 is a view of the FIG. 1 substrate shown at a processing step subsequent to that of FIG.  8 . 
     FIG. 10 is a schematic, enlarged cross-sectional view showing one embodiment of a field emission display incorporating emitters shown in FIG.  9 . 
    
    
     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). 
     Referring to FIG. 1, a semiconductor substrate  10  is illustrated at a preliminary stage of a processing sequence of the present invention. 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. 
     Substrate  10  comprises a glass plate  12 , a first semiconductive material layer  14  overlying glass plate  12 , a second semiconductive material  16  overlying material  14 , and a silicon dioxide layer  18  overlying second semiconductive material layer  16 . Semiconductive material  14  can comprise either a p-type doped or an n-type doped semiconductive material, and semiconductive material  16  can comprise doped polysilicon material. Materials  12 ,  14  and  16  together comprise a conventional emitter tip starting material. Silicon dioxide layer  18  can be formed over layer  16  by, for example, chemical vapor deposition. 
     Referring to FIG. 2, a patterned masking layer  19  is formed over silicon dioxide layer  18 . Patterned masking layer  19  can comprise, for example, photoresist, and can be patterned by a photolithographic process. Patterned photoresist layer  19  has openings  20  extending therethrough to expose portions of silicon dioxide layer  18 . 
     Referring to FIG. 3, openings  20  are extended into silicon dioxide layer  18 , and subsequently photoresist layer  19  (FIG. 2) is removed. Accordingly, a pattern is transferred from photoresist layer  19  to silicon dioxide layer  18 . Openings  20  can be extended into silicon dioxide layer  18  by, for example, a buffered oxide etch. 
     Referring to FIG. 4, particles  22  are deposited on silicon dioxide layer  18 . Particles  22  can comprise, for example, commercially available microspheres. Such microspheres can be formed of a variety of substances, including polymers such as polystyrene. Microspheres come in a variety of different sizes, with typical sizes being from about 0.01 to about 250 microns in diameter. As used herein, the term “microspheres” refers to small, generally spherical particles of colloidal  11  particle size, and not to any precise geometrical shape. The microspheres may be suspended in a de-ionized water solution or an isopropyl alcohol solution. Suppliers of microspheres include Bangs Laboratories, Inc. of Fishers, Ind. 46038, and Interfacial Dynamics Corp. of Portland, Oreg. 97220. In preferred embodiments of the present invention, particles  22  are microspheres having average diameters of from about 1 to about 2 microns. 
     Referring to FIG. 5, particles  22  are collected within openings  20  and excess particles  22  are removed. Such collection of particles  22  within openings  20  and removal of excess particles  22  can be accomplished by, for example, mechanically urging particles  22  into openings  20  utilizing a squeegee-type technique. Alternatively, microspheres  22  can be positioned within openings  20  by locating them on structure  18  in the form of a concentrated solution and subsequently rinsing a surface of silicon dioxide layer  18  with a spray to remove excess particles  22  and leave particles  22  within openings  20 . 
     In the shown preferred embodiment, silicon dioxide layer  18  has a thickness “A” which is less than an average dimension of particles  22 . For instance, if particles  22  comprise microspheres, thickness “A” is preferably less than an average diameter of microspheres  22 . Accordingly, only one microsphere  22  is provided within any given opening  20 . 
     Referring to FIG. 6, silicon dioxide layer  18  (FIG. 5) is removed, leaving particles  22  as a masking layer over portions of semiconductive material  16 . Silicon dioxide layer  18  is preferably removed with an etch selective for silicon dioxide relative to the silicon material of layer  16 . If layer  16  comprises polysilicon, a suitable etch is an oxide etch utilizing at least one of CF 4  or CHF 3 . 
     As shown, particles  22  remain on polysilicon layer  16  after silicon dioxide layer  18  is removed. A possible mechanism by which particles  22  remain attached to layer  16  is through electrostatic interactions wherein negative charges of the particles interact with positive charges carried by the silicon of layer  16 . It is noted, however, that such mechanism is provided herein merely to possibly aid in understanding of the present invention. The invention is to be limited only by the claims that follow, and not to any particular mechanism, except to the extent that such is specifically recited in the claims. 
     Referring to FIG. 7, particles  22  are melted to transform the spherical particles of FIG. 6 to domed discs. An exemplary method for melting particles  22  comprising is to subject the particles to a “soft bake” at a temperature of about 130° C. for a time of about 5 minutes. 
     Referring to FIG. 8, layer  16  (FIG. 7) is etched while using melted particles  22  as a mask. Such etching forms conically shaped emitters  26  from semiconductive material  16 . In embodiments in which semiconductive material  16  comprises polysilicon, the etching can comprise, for example, a silicon dry etch utilizing SF 6  and helium. 
     Referring to FIG. 9, particles  22  (FIG. 8) are removed. In embodiments in which particles  22  comprise polystyrene, or other organic materials, such removal can comprise, for example, dissolving particles  22  in either an acetone solution, or a piranha (sulfuric acid/hydrogen peroxide) solution. 
     Referring to FIG. 10, emitters  26  can be incorporated into a field emission display  40 . Field emission display  40  includes dielectric regions  28 , an extractor  30 , spacers  32 , and a luminescent screen  34 . Techniques for forming field emission displays are described in U.S. Pat. Nos. 5,151,061; 5,186,670; and 5,210,472; hereby expressly incorporated by reference herein. Emitters  26  emit electrons  36  which charge screen  34  and cause images to be seen by a user on an opposite side of screen  34 . 
     The above-described method of the present invention enables positioning of emitters  26  to be carefully controlled during fabrication of emitters  26 . Such control can enable good electron beam optics to be achieved. Specifically, good electron beam optics from field emitter tips can be achieved if the tips are neither too close to one another, nor too far apart. It is desirable to have a large number of emitter tips per pixel to enhance current and brightness as well as provide redundancy for robustness and lifetime. A trade-off is that emitter tips are  11  preferably far enough away from each other so that they do not adversely effect one another&#39;s electric field. 
     In the above-described processing sequence, it was specified that layer  18  preferably comprises silicon dioxide. The utilization of silicon dioxide for layer  18  can be advantageous over other materials in that it is found that organic microspheres (such as, for example, polystyrene beads) are better transferred to a silicon substrate (such as a polysilicon layer  16 ) when the particles are in apertures formed in silicon dioxide, rather than in apertures formed in other materials. A possible mechanism for the better transfer from apertures formed in silicon dioxide is that silicon dioxide can carry a negative charge which can repel negative charges of particles. Such repulsion can assist in alleviating adhesion of the particles to the silicon dioxide, and ease transfer of the particles to an underlying layer  16 . 
     Another possible mechanism for the improved transfer from apertures formed in silicon dioxide relative to apertures formed in other materials is that the other materials may “stick” to the particles. For instance, if layer  18  comprises photoresist, it can be relatively tacky compared to silicon dioxide. Accordingly, the organic particles can disadvantageously stick to the photoresist layer  18  and be relatively difficult to transfer to an underlying silicon-comprising layer  16 . 
     Although silicon dioxide can be a preferred material for layer  18 , it is to be understood that the invention is not to be limited to any particular material within layer  18  except to the extent that such is specifically expressed in the claims that follow. 
     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.