Patent Document

CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of pending U.S. patent application Ser. No. 09/251,766, filed Feb. 17, 1999. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     This invention was made with United States Government support under contract No. DABT63-97-C-0001 awarded by the Advanced Research Projects Agency (ARPA). The United States Government has certain rights in this invention. 
    
    
     TECHNICAL FIELD 
     This invention relates to the field of microstructure fabrication, and more particularly, to processes and compositions for patterning and depositioning microelectronic substrates. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to masks used for etching of and deposition on substrate surfaces. Microfabrication techniques for making patterns in a surface of a material may be employed in many areas, These techniques are particularly important in the electronics industry which employs microelectronic devices having ever decreasing component size. Microfabrication techniques typically employ fine line lithography to transfer patterns into a resist to form a mask. The patterns are then transferred to the surface of the substrate by etching. 
     It is also known to use a monolayer film of colloidal particles as a mask in the patterning process. The colloidal particles arc formed as polymeric spheres. The mask is formed by coating a substrate with a monolayer of colloidal particles such that the particles are fixed to the substrate. The colloidal particles may be arranged on the surface in either a random or ordered array, Contacting the colloidal suspension with the substrate and rotating the substrate in a horizontal plane about an axis normal to the surface of the substrate at a sufficient speed yields a densely packed ordered monolayer of colloidal particles. The degree of order in the arrays is dependent upon the particle size, the degree of attraction to the substrate and the velocity of the flow along the substrate surface. Adjusting the surface chemistry of the substrate ensures that the colloidal suspension wets the substrate, 
     The array of particles serves a lithographic mask, a suitable etching process transferring the random or ordered array to the substrate. The lithographic mask may also serve as a deposition mask. The current techniques rely on the electrostatic attraction between a surface charge on the colloidal particles and a surface charge on the substrate that is different from the surface charge on the colloidal particles. 
     The use of colloidal particle layer to form a mask is particularly suited for forming field emission tips in a microelectronic substrate. Field emitters are widely used in microscopes, flat panel displays and vacuum microelectronic applications. Cold cathode or field emission based flat panel displays have several advantages over other types of flat panel displays, including low power dissipation, high intensity and low projected cost. In field emission or cold emission displays, a strong electric field liberates electrons from a substance, usually a metal or semiconductor, into a dielectric, usually a vacuum. Field emitters have been extensively studied and are well known in the art. 
     The shape of a field emitter strongly affects its emission characteristics, sharply pointed needles or ends having a smooth, nearly hemispherical shape being the most efficient shapes. The limitations of lithographic equipment has made it difficult to build high performance, large field emitters with more than a few emitter tips per square micron. It is also difficult to perform fine feature photolithography on large area substrates as required by flat panel display type applications, Hence, previous attempts have been made to use colloidal particle masks to fashion field emitter tips in microelectronic substrates. Those attempts have not been very successful due to the tendency of the colloidal particles to aggregate in groups, leading to an undesired distribution of field emitter tips in the microelectronic substrate. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the limitations of the prior art by providing a colloidal suspension containing a plurality of particles in a medium, applying the colloidal suspension to a surface of a substrate such as a microelectronic substrate, and agitating the colloidal suspension to break up any aggregation of particles. In an exemplary embodiment, the colloidal suspension comprises a plurality of polymeric spheres, e.g., polystyrene, polydivinyl Benzene, and polyvinyl toluene, in a suspension medium which comprises deionized water, a resist such as a photoresist, and a solvent such as isopropyl alcohol. The solvent is removed from the suspension medium, such as through evaporative processes, leaving behind a layer of colloidal particles. The particles are well dispersed across the surface of the substrate, the agitation eliminating any aggregations or clumps. The layer of particles serves as a mask for etching or depositioning the substrate. Etching may be performed through conventional chemical means, or through other means such as reactive plasma or ion beam etching. Likewise, depositioning may be performed by conventional depositioning means. 
     Applying a mechanical vibration to the colloidal suspension is one method suitable for breaking up aggregations of colloidal particles. Applying ultrasonic or megasonic acoustic energy are also suitable methods for breaking up any aggregations. 
     Further control over the colloidal particles may be realized by establishing a potential energy gradient across the substrate. Such can be realized by application of a charge to the plurality of particles and the substrate, or through the application of a heat gradient or gravitational gradient across the substrate. 
     The method and composition are suitable for forming micron and submicron structures on a substrate, for example, the forming of field emitter tips in a microelectronic substrate. The method does not rely on the use of electrostatic attraction for causing the particles to adhere to the substrate at the point where they strike the surface. In fact, the process relies on the agitation of the colloidal particles after they have been applied to the substrate to break up any aggregations of the particles. The method achieves a better dispersion of microelectronic components, such as field emitter tips, over the surface of the substrate, resulting in a device having better performance characteristics. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a flow chart showing the steps according to one exemplary method of the invention, 
     FIG. 2 is a isometric view of a colloidal suspension being applied to a substrate according to a first exemplary embodiment of the applying step of FIG.  1 . 
     FIG. 3 is a isometric view of the colloidal suspension of FIG. 2 being dispersed across the surface of the substrate by spinning, according to the first exemplary embodiment of the applying step of FIG.  1 . 
     FIG. 4 is a isometric view showing the application of a colloidal suspension to a substrate according to a second exemplary embodiment of the applying step of FIG.  1 . 
     FIG. 5 is a isometric view of the colloidal suspension covering the substrate after an agitation step. 
     FIG. 6 is a isometric view of the colloidal mask layer formed on the substrate after the removal of the solvent from the colloidal suspension. 
     FIG. 7 is a isometric view of field emitter tips formed on the substrate after performance of an etching step. 
     FIG. 8 is a flow chart of a second exemplary method of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring first to FIG. 1, a first exemplary method of carrying out the invention is shown. The first step  100 , comprises applying a colloidal suspension  10  to a substrate  12 , such as a microelectronic substrate. 
     A first exemplary method of applying the colloidal suspension  10  to the substrate  12  in accordance with step  100  is shown in FIGS. 2 and 3. With reference to FIG. 2, the first exemplary method includes depositing a glob  14  of the colloidal suspension  10  substantially at the center of a surface  16  of the substrate  12 . The colloidal suspension  10  comprises a plurality of colloidal particles  18  suspended in a suspension medium  20 . A conventional device  22  may deposit the colloidal suspension  10  on the surface  16 . 
     With reference to FIG. 3, one method of distributing the colloidal suspension  10  across the surface  16  of the substrate  12  is by rotating or spinning the substrate about a longitudinal axis  24 . The rotational velocity of the substrate  12  is important to achieving proper dispersion of the colloidal suspension  10  across the surface  16  of the substrate  12 . While spinning the substrate  12  causes the colloidal suspension  10  to disperse across the surface  16 , it fails to break up any aggregations or clumps of the colloidal particles  18 . Etching of the substrate  12  through the resulting mask will produce a plurality of field emitter tips, many of which will be clumped together. 
     An second exemplary method of applying the colloidal suspension  10  to the surface  16  of substrate  12 , in accordance with step  100  (FIG. 1) is shown in FIG.  4 . In this case, the colloidal suspension  10  is sprayed over the surface  16 , substantially covering the entire surface  16 . Thus, the spinning or rotating of the substrate  12  about the longitudinal axis  24  may be eliminated. 
     After the application of the colloidal suspension  10  to the substrate  12 , the colloidal suspension  10  is agitated as in accordance with step  102  (FIG.  1 ). There are a variety of methods for agitating the colloidal suspension such that any aggregation of particles is broken up. For example, with reference to FIG. 5, applying a mechanical vibration directly to the colloidal suspension  10  or indirectly to the colloidal suspension  10  through the substrate  12  can sufficiently agitate the colloidal suspension  10 . The mechanical vibration may be along axes  26 ,  28  which are perpendicular to the longitudinal axis  24  The vibration should be of sufficient intensity, duration and period to effectively break up any aggregation of colloidal particles  18  in the suspension medium  20 . Applying ultrasonic or megasonic acoustic energy, having frequencies greater than approximately 16 kHz will also agitate the colloidal suspension  10  sufficiently to raise the effective temperature of the particles to break apart any aggregation of colloidal particles  18  therein. The mechanical or acoustical energy may be of a period and amplitude sufficiently large to set up a standing wave in the colloidal suspension  10 . 
     Further control over the colloidal particles  18  may be realized by establishing a potential energy gradient across the substrate  12 . Such can be realized by application of a charge to the plurality of colloidal particles  18  and the substrate  12 , or through the application of a heat to the substrate  12  to establish a temperature gradient thereacross or, by establishing a gravitational gradient across the substrate  12  by, for example, tilting the substrate  12  with respect to a gravitational vector. 
     In the exemplary embodiment, the colloidal suspension  10  comprises of a plurality of colloidal particles  18  suspended in a suspension medium  20 . The colloidal particles  18  may take the form of beads or spheres of a polymer, such as polystyrene, polydivinyl Benzene, or polyvinyl toluene. The spheres are often made by either suspension or emulsion polymerization. The spheres can be conveniently fabricated in sizes ranging from 0.5 to 5 microns. Suitable spheres are available from Interfacial Dynamics Corporation of Portland, Oreg. and Bangs Laboratories, Incorporated of Fishers, Ind. The suspension medium  20  comprises deionized water, photoresist and a solvent, in the exemplary embodiment. For example, a suitable mixture may comprise: one milliliter of particles in deionized (DI) water combined with 20 milliliters of a photoresist and 5 milliliters of a solvent, such as isopropyl alcohol. The preferred range of for the mixture is approximately 2-20 milliliters of photoresist and approximately 5-50 milliliters of solvent per litter of particles in DI water. 
     In accordance with step  104  (FIG.  1 ), removal of the solvent from the suspension medium  20  occurs after the aggregation of colloidal particles  18  have been broken up. Removal of the solvent may occur through conventional evaporative steps, such as the application of heat to the composition. The removal of the solvent leaves behind a layer of colloidal particles  18  on the surface  16  of the substrate  12 . As shown in FIG. 6, the photoresist fixes the position of each of the particles  18  relative to the surface  16  of the substrate  12 . The colloidal particles  18  serve as a mask for the etching step  106  (FIG.  1 ). Etching may be performed in any known manner such as by chemical means, reactive plasma etching, or ion beam etching. For example, ion beam etching directs a beam of ions at the surface  16  of the substrate  10  through the mask of colloidal particles  18 . The incident ion beam etches away the particles  18  and the surface  16 . The relative etching rates of the particles  18  and the surface  16  determine the configuration of the etched surface  30 , The etching may thereby form microelectronic devices, such as field emitter tips  32 , in the surface  16  of the substrate  12 . 
     A second exemplary method of carrying out the invention is shown in FIG. 8, wherein like numerals correspond to similar elements and steps carried out in the first exemplary method. In the second exemplary embodiment, the colloidal particles  18  left behind on the surface  16  after step  104  serve as a deposition mask. In step  108  material is deposited on the surface  16  of substrate  12  between the colloidal particles  18 . Depositing of the material may be accomplished through conventional means, such as lift off, plating, and ion implanting. 
     Although specific embodiments of, and examples for, the present invention are described herein for illustrative purposes, various equivalent modifications can be made without departing from the spirit and scope of the invention, as will be recognized by those skilled in the relevant art. The teachings provided herein of the present invention can be applied to other substrates to define other microstructures, not necessarily the exemplary microelectronic devices, such as field emission emitter tips, generally described above. 
     These and other changes can be made to the invention in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the invention to the specific embodiments disclosed in the specification and claims, but should be construed to include all substrates and manufacturing of such substrates that operate in accordance with the claims. Accordingly, the invention is not limited by the disclosure, but instead its scope is to be determined entirely by the following claims.

Technology Category: 5