Abstract:
Semiconductor devices with porous insulative materials are disclosed. The porous insulative materials may include a consolidated material with voids dispersed therethrough. The voids may be defined by shells of microcapsules. The voids impart the dielectric materials with reduced dielectric constants and, thus, increased electrical insulation properties.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a divisional of application Ser. No. 10/230,712, filed Aug. 29, 2002, now U.S. Pat. No. 7,153,754, issued Dec. 26, 2006. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to the fabrication of semiconductor devices and, more specifically, to a method of producing an insulator with a low dielectric constant, or “low K dielectric,” for electrically isolating components of an integrated circuit and the resulting insulators. More specifically, the present invention relates to methods for forming porous, low dielectric constant layers or structures in which the insulative material may initially be formed in a substantially solid and structurally sound state and is converted to a porous state once the low dielectric constant layer or structure or one or more layers or structures thereover have been processed, as well as the layers or structures so produced. 
   2. Background of Related Art 
   Integrated circuits (ICs) include transistors and other circuit elements that are configured and interconnected to provide a flow of current. For proper IC operation, the circuit elements and interconnections must be electrically isolated from other circuit elements and interconnections. Such electrical isolation has typically been accomplished by forming insulative layers and structures, or insulators, between the various circuit elements. 
   As consumers continue to demand portable computers with faster operation speeds and electronic devices which are more compact and have more memory, there continues to be a demand for the development of ICs that are smaller and more energy efficient. The densities of ICs generally increase in accordance with Moore&#39;s Law, which states that the number of circuit devices that fit on a chip of given dimensions doubles about every year-and-a-half to two years. As more circuit devices are placed on the chip, the distance between the various circuit devices or circuit elements gets smaller and leads to increased capacitive coupling (crosstalk) and propagation delay. To minimize the problems associated with crosstalk and propagation delay on smaller chips, while also minimizing the sizes of insulative layers and structures, better insulators must be developed. 
   Effective IC insulators should provide low current leakage, good mechanical strength, and low permittivity. The effectiveness of insulators is typically measured in terms of the relative dielectric constant for the material used as the insulator. Generally, a lower dielectric constant for a given material results in the given material being a better insulator. Silicon dioxide (SiO 2 ) has been extensively used as an insulator in IC devices. Silicon dioxide has a dielectric constant of about 4.0. 
   In contrast, air has a dielectric constant of approximately 1.0. Thus, the formation of insulators with air gaps therein (e.g., from porous dielectric materials) is desirable because the presence of the air gaps within the material reduces the overall dielectric constant between adjacent conductive structures. However, the presence of air gaps tends to reduce the mechanical strength and integrity needed by the dielectric material to support various circuit devices and components on the IC. 
   Examples of processes that may be used to form air gaps, or pores, in insulators are the so-called “sol-gel” processes. Sol-gel processes are typically used to fabricate porous, ceramic insulators. Because the silica-containing sol-gel structures shrink upon completion of the sol-gel process, however, relatively high porosities are needed in the initial sol-gel structures to produce an insulator with a suitable dielectric constant. However, the large number of pores present in the high porosity sol-gel structures weakens these insulators and makes them susceptible to crushing, as well as to other types of damage. 
   Other dielectric materials that may be made porous include various organic polymers which have dielectric constants that are less than that (about 4.0) of silicon dioxide. However, many organic polymers have lower mechanical strengths, are softer, and are more malleable than silicon dioxide, making porous insulators that have been formed from organic polymers susceptible to damage during fabrication of the IC. 
   Another example of porous, low dielectric constant materials are the so-called SiLK® (Silicon Low-K) materials that are produced by the Dow Chemical Company of Midland, Mich. While SiLK® purportedly has relatively small (i.e., as small as about 20 nm), closed cell pores which are uniformly distributed therethrough, temperatures on the order of about 400° C or greater are required to cure SiLK® films. The use of such high process temperatures following the fabrication of metal structures is, however, somewhat undesirable, as exposing many of the types of metals that are used in semiconductor device fabrication processes to such high temperatures may stress, fatigue, or damage the layers or structures formed thereby. Moreover, as voids are present in SiLK® films prior to processing thereof or of overlying layers, SiLK® films are still more prone than solid films to being damaged during such processing. 
   An insulating material that may be mechanically processed or structurally support overlying layers during mechanical processing thereof in a substantially solid, nonporous state, then be porified to have a dielectric constant sufficiently low to meet the needs of ever-decreasing device dimensions would thus be an improvement in the art, as would methods for fabricating such a material. 
   SUMMARY OF THE INVENTION 
   The present invention includes methods for fabricating porous low dielectric constant layers and structures, or insulators, in which such insulators may initially be substantially solid and may subsequently be made porous. The low dielectric constant layers and structures that are formed at various stages of the method, as well as semiconductor device structures including such layers or structures, are also within the scope of the present invention. 
   In an exemplary embodiment of the method, a layer of dielectric material is formed over a semiconductor substrate. The dielectric material of the layer is initially formed to be substantially solid and nonporous. Following processing of the layer, pores may be introduced, generated, or otherwise formed in the dielectric material. By way of example only, the porous layer of dielectric material may be produced from a mixture of two materials, at least one of which is initially a liquid. The two materials may be materials that are miscible with one another and that, following mixing thereof, experience the phenomenon known as “Kirkendahl voiding,” which results in the formation of voids therein. Alternatively, the two materials may comprise a first, base material and a second, sacrificial, void-forming material dispersed throughout the base material. Once the two materials are mixed together, one or both of the two materials may be at least partially solidified, then one or both of the two materials may be exposed to a catalyst or catalytic event to effect the formation of voids. 
   Another exemplary embodiment of the method includes forming a layer of dielectric material, or base material, which includes preformed pores, over a semiconductor substrate. The preformed pores may be in the form of hollow or material-filled (e.g., liquid-filled microspheres, dispersed and suspended therethrough. As an example of such a method, a layer of a so-called “sol-gel” with microspheres, microcapsules, or other void-including structures of appropriate size dispersed therethrough may be formed over a semiconductor device structure. The sol-gel may, for example, comprise a mixture that includes an alkoxide, water, and a solvent. The microspheres, microcapsules, or other void-including structures comprise an outer shell which may be hollow or encapsulate a sacrificial filler material. Once the sol-gel has been solidified into a substantially solid matrix and desired processes have been performed thereon or thereover, any filler within the microspheres, microcapsules, or other void-including structures may be removed, resulting in the formation of voids in the solidified sol-gel. 
   Additionally, the present invention includes semiconductor device fabrication processes in which an intermediate, substantially solid insulator layer is formed, the insulator layer or one or more overlying features are processed, and voids are then formed in the insulator layer or a structure that has been formed therefrom. Intermediate and finished semiconductor devices that include insulator layers according to the present invention are also within the scope of the present invention. 
   Other features and advantages of the present invention will become apparent to those of ordinary skill in the art through consideration of the ensuing description, the accompanying drawings, and the appended claims. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The nature of the present invention, as well as other embodiments of the present invention, may be more clearly understood by reference to the following detailed description of the invention, to the appended claims, and to the several drawings herein, wherein: 
       FIGS. 1A through 1D  illustrate various cross-sectional views of a semiconductor device structure fabricated using a dispersion polymerization process of the present invention; 
       FIG. 2  depicts a cross-sectional view of a microcapsule used in an exemplary embodiment of the methods of the present invention; 
       FIGS. 3A and 3B  show two cross-sectional views of a semiconductor device structure manufactured using a seed emulsion process; 
       FIGS. 4A through 4D  illustrate various cross-sectional views of a semiconductor device structure fabricated using a microencapsulated filler suspended in a sol-gel in accordance with the methods of the present invention; and 
       FIGS. 5A through 5D  depict another exemplary method for forming voids in a material layer, by which an ultrafast laser pulse is used to form the voids within a layer or structure of dielectric material. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Generally, the present invention includes porous insulative materials, structures formed from the porous insulative materials, and semiconductor device structures and semiconductor devices including such porous insulative materials. The present invention also includes methods of fabricating the porous insulative materials, methods of fabricating structures that include the porous insulative materials, methods of processing the insulative materials or overlying layers of structures prior to porification thereof, and methods of fabricating semiconductor device structures and semiconductor devices that include the porous insulative materials. While the present invention is described in terms of certain specific, exemplary embodiments, the specific details of these embodiments are merely set forth in order to provide a more thorough understanding of the present invention and not as any limitation of the scope thereof. It will be apparent, however, that the present invention may be practiced in various combinations of the specific, exemplary embodiments presented herein. 
   In describing the following embodiments, the terms “wafer” and “substrate” include any structure having an exposed surface upon which an insulative layer or structure incorporating teachings of the present invention may be formed. The term “substrate” also includes semiconductor wafers. The term “substrate” is further used to refer to semiconductor structures during processing and may include other layers that have been fabricated thereupon. Both “wafer” and “substrate” include doped and undoped semiconductors, epitaxial semiconductor layers supported by a base of a semiconductor or an insulator, as well as other semiconductor structures known to those of ordinary skill in the art. The term “conductor” includes conductively doped semiconductors. The term “insulator” is defined to include any material that is less electrically conductive than the materials referred to herein as “conductors.” The term “circuit element” is understood to include contacts to active regions of individual devices and similar active regions within a substrate or wafer. “Circuit element” also includes metal lines or layers, conductive vias, and similar conductive regions that connect individual devices within an integrated circuit. 
   The present invention provides a low dielectric constant, porous insulator suitable for isolation on any integrated circuit (IC), including, but not limited to, volatile and nonvolatile memory ICs, application-specific ICs, microprocessor ICs, analog ICs, digital ICs, and communication ICs. The insulator provides electrical isolation, such as between circuit elements, between interconnection lines, between circuit elements and interconnection lines, or as a passivation layer overlying both circuit elements and interconnection lines. The insulator may also be used in any other electrical device known to those of ordinary skill in the art where electrical isolation is desired. 
   Referring now to  FIGS. 1A through 1D , there are shown various cross-sectional views of a semiconductor device structure, or semiconductor device, denoted generally at 10, where a low dielectric constant insulative layer  14  is formed by a dispersion polymerization process. As used herein, the term “semiconductor device structure” refers to both intermediate and finished semiconductor devices, with or without the addition of various circuit elements, conductive layers, and insulative layers.  FIGS. 1A through 1D  sequentially illustrate the intermediate semiconductor device structure, or semiconductor device  10 , at various stages of an exemplary fabrication method of the present invention. 
   Referring now to  FIG. 1A , a substrate  12 , such as a substantially complete semiconductor device or a partially formed semiconductor device, is provided. Substrate  12  may comprise a full or partial semiconductor (e.g., silicon, gallium arsenide, indium phosphide, etc.) wafer, or other bulk semiconductor substrate, such as a silicon-on-insulator (e.g., silicon-on-sapphire, silicon-on-glass, silicon-on-ceramic, etc.) structure, but it will be appreciated by those of ordinary skill in the art that the substrate  12  may be any material suitable for semiconductor device  10  formation, such as a semiconductor wafer, and may be doped and/or include an epitaxial layer. 
   In the illustrated embodiment, an insulative layer  14  is formed on a surface, or a portion of the surface, of the substrate  12 . The insulative layer  14  comprises a plurality of microcapsules  18  dispersed throughout and suspended in a liquid, or semi-liquid, insulative material  16 . When the microcapsules  18  are filled with material, as described in further detail hereinafter, the insulative material  16  of insulative layer  14  is said to be in a first, substantially nonporous state. 
   The insulative material  16  comprises a substantially nonvaporizable material under conditions to which the semiconductor device  10  will be exposed and may comprise any electrically nonconductive material, including both polymers and nonpolymers, known to those of ordinary skill in the art and suitable for use as a dielectric layer or structure in a semiconductor device. In the illustrated embodiment, the insulative material  16  is an organic polymer. Polymers that may be used as insulative material  16  include, but are not limited to, polyimide, polybenzoxazole, polyquinoline, polypropylene, polyurethane, nylon, polyethylene, or epoxies as well as any other organic polymer, elastomer, or resin that is nonconductive and known to those of ordinary skill in the art. Nonpolymeric materials that may be used as the insulative material  16  include, but are not limited to, silica aerogels, mesoporous silicas, and other nonconductive nonpolymeric materials that are known to those of ordinary skill in the art. Also within the scope of the present invention is a combination of a polymer with a silica aerogel or mesoporous silica for use as the insulative material  16 . Depending on the type of substrate  12  and insulative material  16  used, binding of the insulative material  16  to the substrate  12  may be augmented with an adhesion layer (not shown) suitable for use with the materials of the substrate  12  and the insulative material  16 , as known to those of ordinary skill in the art. Alternatively, or in addition to the use of an adhesion material, the surface of the substrate  12  may be treated or modified (e.g., roughened by use of an etchant, laser ablation, or otherwise as known in the art) to enhance the adhesion of the insulative material  16  thereto. 
   Referring now to  FIG. 2 , there is shown a cross section of an exemplary microcapsule  18  that may be used in the method depicted in  FIGS. 1A-1D . The microcapsule  18  comprises an outer shell  20  that encapsulates a filler  22 . The outer shell  20  may comprise any material that is substantially nonreactive with the filler  22  and the insulative material  16 . Examples of materials that may be used for the outer shell  20  include, but are not limited to, plastics such as poly-methyl-methacrylate (PMMA) and polyvinyl chloride (PVC). However, it will be apparent to those of ordinary skill in the art that any material which functions the same as, or equivalent to, the plastics described herein are encompassed by the present invention. In the illustrated embodiment, the filler  22  may be in liquid form and comprise a material that is substantially nonvaporizable under selected ambient conditions (e.g., particular temperatures and/or pressures). However, it will be apparent to those of ordinary skill in the art that the filler  22  may be any substance that is nonvaporizable under the selected ambient conditions and does not dissolve or react with the outer shell  20 . Solids that are sublimable under the selected conditions may also be used as the filler  22 , such that the solid possesses the same characteristics as the liquid described herein. Examples of liquid solvents that may be used as the filler  22  include, but are not limited to, water, acetone, N-methylpyrrolidone (NMP), and various alcohols. In an alternative embodiment, two or more different fillers  22  possessing different evaporation temperatures may be used in different or the same shells. Microcapsules  18  may be formed by a variety of methods, such as by known drip or jet coextrusion processes, by miniemulsion polymerization processes, such as those described in Tiarks, F. et al., “Preparation of Polymeric Nanocapsules by Miniemulsion Polymerization,” L ANGMUIR,  17:908-18 (2001) (hereinafter “Tiarks”), by the process described in Nalaskowski, J., et al., “Preparation of Hydrophobic Microspheres from Low Temperature Melting Polymeric Materials,” J. Adhesion Sci. Technol., 13(1):1-17 (1999) (hereinafter “Nalaskowski”), or as otherwise known in the relevant art. The disclosures of Tiarks and Nalaskowski are hereby incorporated herein by this reference in their entireties. 
   Referring again to  FIG. 1A , the insulative layer  14  is formed on the substrate  12  in a liquid or semi-liquid form. In the illustrated embodiment, the insulative layer  14  comprises the insulative material  16  in liquid form with the microcapsules  18  suspended in and dispersed throughout the liquid insulative material  16 . The liquid insulative material  16  and suspended microcapsules  18  are applied to the substrate  12  in a manner known to those of ordinary skill in the art, such as a spin-on technique, mechanical process (e.g., the use of a doctor blade), or any other known processes that may be used to fabricate or form a layer. 
   Once the insulative layer  14  is disposed on the substrate  12 , the insulative material  16  is allowed or caused to set or solidify, such that the insulative layer  14  forms a substantially solid matrix around the microcapsules  18 . In the illustrated embodiment, the selection of the insulative material  16 , outer shell  20 , and filler  22  is based, at least in part, on the temperature at which the insulative material  16  solidifies and the evaporation, or vaporization, temperature of the filler  22 . It will be further appreciated that the selection of insulative materials  16  (e.g., resins and polymers), substrates  12 , metals for circuit elements, and other materials (e.g., plastics for the outer shells  20 ) used in the semiconductor device  10  fabricated herein will be such that the thermal mismatch or differences between coefficients of thermal expansion of the various materials will be minimized. Ideally, the temperature at which the insulative material  16  solidifies is lower than the evaporation point of the filler  22 , such that the insulative material  16  will set into the solid matrix before the filler  22  turns into vapor. For example, if water were used as the filler  22 , the insulative material  16  used would have a solidification temperature that is below the evaporation temperature of water, or 100° C., and also be a temperature compatible with the outer shell  20 . It will be appreciated by those of ordinary skill in the art that, depending on the insulative material  16  used, a soft bake may be used to set the insulative material  16 , wherein the temperature of the soft bake does not vaporize the filler  22 . For example, polyamide may be used as the insulative material  16 , polymethyl methacrylate (PMMA) may be used as the outer shell  20 , and water may be used as the filler  22 . In this example, the polyamide could be soft baked at 65° C. for about 1 to 2 hours. Since 65° C. is a lower temperature than the evaporation temperature of water, the filler  22  in the outer shell  20  will remain intact as a liquid. 
   Referring now to  FIG. 1B , there is shown the semiconductor device  10  after the insulative material  16  has formed the substantially solid matrix. As illustrated in  FIG. 1B , a patterning process may be employed to remove portions  24  of the insulative layer  14  that overlie the substrate  12 , such that isolation regions, circuit elements, or other conductive elements may be formed in the removed portion  24  of the insulative layer  14 . It will be appreciated that any suitable method of patterning an IC component, such as a photolithographic patterning process (if the insulative material  16  is a photoimagable material), a trench-and-fill process, or a mask and etch technique (using an etchant suitable for the insulative material  16 ) may be used to pattern the insulative layer  14  and/or the substrate  12  and not depart from the spirit of the present invention. Alternatively, or in addition, the surface of the insulative layer  14  and/or the surface of the substrate  12  may be planarized using known abrasive planarization techniques, such as mechanical planarization, chemical-mechanical planarization, or chemical-mechanical polishing, to polish and smooth the surface. It will be apparent to those of ordinary skill in the art that the filler  22  in the microcapsules  18  provides mechanical strength to the semiconductor device  10 , such that during such patterning and planarization processes, as well as during fabrication of various circuit elements (e.g., by planarizing and patterning of layers subsequently formed on the semiconductor device  10 ), the insulative layer  14  is able to withstand compressive and other mechanical stresses placed thereon. 
   Referring now to  FIG. 1C , there is shown the semiconductor device  10  after circuitry has been fabricated, as known in the art, at least partially over the insulative layer  14 . A conductive material  26  has been placed in the portion  24  of the insulative layer  14  that was removed overlying the substrate  12 . Other circuit elements and/or layers may be added to the semiconductor device  10  as known to those of ordinary skill, such as the deposition and patterning of a conductive (e.g., polysilicon) layer  28  or another dielectric layer  30 , as known in the art. 
   Referring now to  FIG. 1D , there is shown the semiconductor device  10  after voids  32  have been formed in the insulative material  16  when the insulative material  16  is in a second, porous state. In the illustrated embodiment, the voids  32  are formed by exposing the semiconductor device  10  and the insulative layer  14  to appropriate conditions, referred to herein as “catalysts,” for removing the filler  22  from the outer shells  20 . Of course, the selection of the materials used as insulative material  16  and as outer shells  20  of the microcapsules  18  will correspond to the selection of the filler  22  such that the outer shell  20  and the insulative material  16  will allow the vaporized filler  22  to diffuse out of the microcapsules  18  and out of insulative layer  14 . The type of filler  22  used dictates the “catalyst” that will be used to remove the filler  22 . For example, the “catalyst” for removing a liquid filler  22  may create conditions which vaporize or condense the liquid filler  22 , while the “catalyst” for removing a solid filler  22  may create sublimation conditions. Heat, electromagnetic frequencies such as ultraviolet (UV) light, radio waves produced by a microwave source, or any other known “catalyst” may be employed to remove the filler  22  and create voids  32  within the microcapsules  18  of insulative layer  14 . In the illustrated embodiment, the filler  22  may be vaporized by heating the semiconductor device  10 . When the semiconductor device  10  is heated to at least a vaporization temperature of the filler  22  (e.g., to a temperature of at least about 100° C. when the filler  22  comprises water), the vaporized filler  22  diffuses through the outer shell  20  of the microcapsule  18  and into the surrounding insulative material  16 . Depending on the type of filler  22  used, the filler  22  vapor may diffuse completely out of the semiconductor device  10 , or removal of the filler  22  vapor may be aided by placing the semiconductor device  10  in a vacuum to draw the vapor out of the semiconductor device  10 . 
   The initial process (e.g., a soft bake process) used to substantially solidify the insulative layer  14  may not fully cure the insulative layer  14 , depending upon the type of material used as the insulative material  16 . A final solidification or cure of the insulative material  16  may be accomplished simultaneously when the filler  22  is vaporized for optimum efficiency, or, alternatively, before or after the filler  22  is removed. The filler  22  may more readily escape insulative layer  14  or structures formed therefrom if the insulative material  16  has not yet been fully solidified. In various alternative embodiments and depending upon the type of insulative material  16  used, a hard bake or other known curing process may be used to more fully solidify or cure the insulative layer  14  before completion of the circuitry on the semiconductor device  10 . The final cure may occur before removal of the filler  22 , after the ICs have been completed on the semiconductor device  10 , or at any other appropriate time apparent to those of ordinary skill in the art and consistent with the fabrication processes employed to manufacture the semiconductor device. 
   The selection of the filler  22  and the catalyst used to remove the filler  22  also takes into consideration the substrate  12  and other features of the semiconductor device  10 , such as the various circuit elements and other components thereof, such that the process of vaporizing, or otherwise removing, the filler  22  does not damage any of the circuit elements or other components. For example, if aluminum were used in the semiconductor device  10 , then the temperature selected to vaporize the filler  22  should not exceed 470° C. because the aluminum may oxidize or even melt. Additionally, the catalyst selected to remove the filler  22  should not cause the voids  32  formed in insulative layer  14  to collapse or to be filled with material. 
   In a variation of this embodiment, the outer shells  20  of the microcapsules  18  may comprise a material that deteriorates, loses some structural integrity or otherwise becomes more permeable to the filler  22  after a period of time or when exposed to a particular catalyst or combination of catalysts. If the outer shell  20  starts to or is caused to deteriorate prior to removal of the filler  22 , then the filler  22  may begin to diffuse into the surrounding insulative material  16  before the catalyst is applied, which may make removal of the filler  22  more efficient. 
   Optionally, microcapsules  18  may be substantially hollow and filled with gas or air, in which case it is not necessary to remove material therefrom to create voids  32  within insulative layer  14 . If substantially spherical microspheres are used as microcapsules  18 , an insulative layer  14  which includes such microcapsules  18  may withstand substantial forces exerted thereon during processing (e.g., polishing) thereof, as well as during processing of overlying layers or structures. By way of example only, microcapsules  18  may comprise acrylic microspheres, which are commercially available from a variety of sources and in a variety of sizes. 
   A two-part resin may be used as the insulative material  16 . A first part of the resin may be a UV-curable component of the resin such that the matrix is substantially solidified by UV curing the first part of the resin, while a second part of the resin remains at least semi-liquid. In the final cure, the second part of the resin may be cured using heat, or any other catalyst. 
   An example of this embodiment includes use of a so-called “sol-gel” and is illustrated in  FIGS. 4A through 4D . In  FIGS. 4A through 4D , there are shown various cross-sectional views of a semiconductor device  210  fabricated using another embodiment of the present invention wherein a microencapsulated filler is used to form voids in an insulative layer  214  derived from a sol-gel solution. 
   Referring now to  FIG. 4A , a substrate  212  is provided with a sol-gel solution  216  dispersed thereon. It will be appreciated that methods of sol-gel chemistry used to produce porous films on semiconductor devices are well known to those of ordinary skill in the art. In a typical sol-gel process, a silicon, metal, or metalloid alkoxide is subjected to hydrolysis and condensation reactions to form a gel containing a continuous solid phase of the corresponding silicon, metal, or metalloid oxide. The gel is filled with a solvent and other liquid reactants that are subsequently removed to form a solid matrix, which, in the present invention, may include a plurality of micropores dispersed therethrough. 
   The sol-gel solution  216  used in the present invention may, for example, comprise an insulative base material, such as a silicon oxide (e.g., glass or undoped silicon dioxide), a metal oxide (e.g., a ceramic), or a metalloid alkoxide, as well as water, a solvent, such as alcohol, and a plurality of microcapsules  218  comprising the outer shell  20  encapsulating the filler  22  as shown in  FIG. 2 . The microcapsules  218  are substantially evenly dispersed throughout the sol-gel solution  216 . Metal alkoxides that may be used include, but are not limited to, alkoxysilanes, such as tetramethoxysilane (TMOS) and tetraethoxysilane (TEOS). Other alkoxides that may be used include, but are not limited to, aluminates, titanates, borates, and metalloid alkoxides as known to those of ordinary skill in the art. It will be appreciated by those of ordinary skill in the art that the water in the sol-gel solution  216  causes the hydrolysis reaction and the alcohol causes the condensation reaction. The rate of hydrolysis, condensation, and linking of the particles in the sol-gel solution  216  may be controlled and optimized by varying the pH of the sol-gel solution, the use of a catalyst (e.g., mineral acids and ammonia), varying the amount of water, varying the amount of solvent, and varying the amount of alkoxides. Because the sol-gel reaction may take place at a relatively low temperature (e.g., room temperature), the filler  22  of the microcapsule may comprise a material with a relatively low vaporization point, such as water, a solvent, or an alcohol. In the present embodiment, the outer shell  20  comprises a plastic, such as PMMA or PVC, that is impermeable to the constituents of the sol-gel solution  216  and the filler  22  used in the microcapsule  218 . 
   Referring now to  FIG. 4B , there is shown the semiconductor device  210  once the sol-gel solution  216  has hydrolyzed and condensed such that the particles in the sol-gel solution  216  have formed chemical bonds and are linked together in a substantially solid matrix. The sol-gel process produces porous layers with fine particle sizes (2-10 nm) and porosities of approximately 70% to 99%. Since the microcapsules  218  were suspended in and dispersed throughout the sol-gel solution  216 , the matrix formed by the sol-gel solution  216  is formed around the microcapsules  218 . Once the formation of bonds is complete in the sol-gel solution  216 , an insulative layer  214  is formed comprising the matrix of the sol-gel solution  216  which includes a plurality of micropores formed throughout the matrix, and further includes the embedded microcapsules  218 . 
   The insulative layer  214  may be planarized in any manner known to those of ordinary skill in the art to impart the insulative layer  214  with a desired thickness. The insulative layer  214  may also be patterned, such as by forming damascene trenches (not shown) for the placement of circuit elements or other recesses. It will be apparent to those of ordinary skill in the art that the presence of the microcapsules  218 , filled with the filler  22  ( FIG. 2 ), provides strength and structural integrity to the insulative layer  214  formed from the sol-gel solution  216 . Thus, the filler  22  prevents the insulating layer  214  from being crushed or damaged during the planarizing and patterning thereof. 
   Referring now to  FIG. 4C , there is shown a cross section of the semiconductor device  210  after the insulative layer  214  has been planarized to a desired thickness and the filler  22  of the microcapsules  218  has been removed. When the filler  22  is removed, voids  232 , or pores, are created in the insulative layer  214 . The filler  22  may be removed from the microcapsules  218 , for example, by one of the above-described processes. By way of example only, when the filler  22  is water, the semiconductor device  210  may be heated to vaporize the water. The water will begin to diffuse out of the microcapsule  218  at about 50° C. However, care in heating the semiconductor device  210  when water is used as the filler  22  should be used because if the temperature used to vaporize the water exceeds 90° C. too quickly, the water and, thus, the microcapsules  218  may expand and damage the insulative layer  214 . Once the water has vaporized, a plurality of voids  232  will remain in the insulative layer  214  and further decrease the dielectric constant of the insulative layer  214 . It will be further appreciated that the semiconductor device  210  may be placed in a vacuum to draw the vapor out of the semiconductor device  210  as previously described herein. 
   Referring now to  FIG. 4D , circuit elements of the semiconductor device  210  may be fabricated, such as by depositing a metal coating  240  onto the surface of the insulative layer  214  using methods known to those of ordinary skill in the art, such as by chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), rapid thermal chemical vapor deposition (RTCVD), atomic layer deposition (ALD), or physical vapor deposition (PVD). As known to those of ordinary skill in the art, the metal coating  240  may be subsequently patterned (e.g., by masking and etching) to define various circuit elements therefrom. 
   With continued reference to  FIGS. 4A through 4D , as well as to  FIGS. 1A through 1D , fabrication of the insulative layer  14 ,  214  using the methods described herein provides a finely and evenly distributed plurality of micro-sized voids  32 ,  232  in the insulative layer  14 ,  214 , which substantially lowers the dielectric constant of the insulative layer  14 ,  214 . The effectiveness of the insulative layer  14 ,  214  as a dielectric may be determined based on factors including, but not limited to, the size, number, and distance between the microcapsules  18 ,  218  used to create the voids  32 ,  232  in the insulative material  16 , sol-gel solution  216  and the thickness and type of insulative material  16 , sol-gel solution  216  used in the insulative layer  14 ,  214 . 
   Referring now to  FIGS. 3A and 3B , there is shown a cross-sectional view of a semiconductor device  110  fabricated with an insulative layer  114  produced using another embodiment of the present invention, or a seed emulsion process, which is also referred to herein as an “emulsion process.” In the emulsion process, two nonconductive materials in a liquid, or semi-liquid, state are mixed together to form a mixture. 
   In an example of the emulsion process, depicted in  FIG. 3A , a hydrophilic nonconductive liquid may be mixed with a hydrophobic nonconductive liquid to form an emulsion, wherein globules  132  of one of the hydrophobic nonconductive liquid and the hydrophilic nonconductive liquid are formed in and surrounded by a matrix material  116  which may comprise the other of the hydrophilic and hydrophobic nonconductive liquids, as depicted in  FIG. 3A . By way of example only, the hydrophobic nonconductive liquid may comprise uncured PMMA, while the hydrophilic nonconductive liquid may comprise deionized water or a low molecular weight alcohol (e.g., methanol, ethanol, propanol, etc.). The emulsion is deposited on a substrate  112  in a manner known to those of ordinary skill in the art, such as a spin-on technique, mechanical process (e.g., the use of a doctor blade), or any other known processes that may be used to fabricate or form a layer with a substantially planar surface and having a substantially uniform thickness over the surface of the substrate  112 . After dispersion of the emulsion onto the substrate  112 , the matrix material  116  (e.g., PMMA) is caused to at least partially set or allowed to at least partially set to form a substantially solid matrix that surrounds the globules  132  of hydrophobic liquid which have been substantially evenly dispersed therethrough. Depending on the type of matrix material  116  used to form the emulsion, the matrix material  116  may require a soft bake, a period of time, exposure to a particular wavelength of electromagnetic radiation (e.g., light), exposure to a chemical catalyst (as in a two-part epoxy resin), or exposure to any other catalyst suitable for substantially solidifying the same. It will be further appreciated that the surface of the substrate  112  may be modified, as known in the art, to facilitate adhesion of the insulative layer  114  thereto or that an adhesion layer may be used to augment binding of the insulative layer  114  to the substrate  112  as is known in the art. At this point, the materials of insulative layer  114  are in a first, substantially nonporous state. 
   Referring to  FIG. 3B , there is shown the semiconductor device  110  after the matrix material  116  has set into a substantially solid matrix. After the matrix material  116  has been formed into the substantially solid matrix, the insulative layer  114  and substrate  112  of the semiconductor device  110  may be planarized or patterned in the same manner as previously described herein for the subsequent formation of various IC elements. For example, as shown in  FIG. 3B , a portion  124  of the insulative layer  114  may be removed for the addition of a circuit element (not shown). Structures, such as circuit elements, may also be fabricated over or adjacent to the insulative layer  114 . 
   Once the circuit elements or other structures have been formed, void  132 ′ initiation may be started by applying an appropriate catalyst to the semiconductor device  110 , transforming the material of the insulative layer  114  to a second, porous state. The catalyst may be in the form of certain light frequencies (e.g., UV), radio waves (e.g., use of a microwave), heat, or any other method of removing (e.g., by vaporization, condensation, sublimation, etc.) globules  132 , thereby catalyzing void  132 ′ formation. The catalyzation technique that is used depends, of course, upon the type of material that forms the globules  132 . Diffusion of the material of the globules  132  may be effected by placing the semiconductor device  110  in a negative pressure (i.e., a vacuum) to facilitate drawing out of the material from which the globules  132  are formed. 
   Once the matrix material  116  has solidified, the insulative layer  114  may be planarized and patterned, and IC elements may be fabricated on the semiconductor device  110  as previously described herein. The insulative layer  114  may then be exposed to a second catalyst, such as a hard bake, so voids  132 ′ are produced in the space, or interface, between the filler liquid and the carrier liquid. In the illustrated embodiment, the filler liquid may shrink, or condense, as it cures to cause voids  132 ′ to form in the insulative layer  114 . The temperature for the hard bake is selected such that the matrix material  116  (formed from the solidifying of the carrier liquid) is not heated to too high a temperature to prevent the matrix from collapsing in on the voids  132 ′. Because the carrier liquid was previously solidified to form the matrix material  116 , the shrinking or removal of the filler liquid causes the void  132 ′ formation. 
   In a second example of the seed emulsion process, two nonconductive liquids that are miscible in each other may be combined to form a mixture of the liquids. When the two liquids diffuse into each other, an interface may form between the two liquids. Voids  132 ′ may form at the interface between the two nonconductive materials much like the formation of Kirkendahl voids at an interface between gold and aluminum when gold and aluminum diffuse into each other. By way of example, voids  132 ′ may be formed at the interfaces of globules  132  with matrix material  116 , as at least the matrix material  116  begins to solidify or cure. Of course, the use of miscible material combinations in which void formation may occur is also within the scope of the present invention when such void formation occurs before or after one or both of the matrix material  116  and the material from which the globules  132  are formed begins to solidify or cure, provided that the voids  132 ′ remain dispersed substantially evenly throughout the matrix material  116  once it has become at least semisolid. Subsequent processing of insulative layer  114  or overlying layers or structures may be effected, as described above, prior to the formation of voids  132 ′ in insulative layer  114 . 
   As an alternative to the use of seed emulsion processes to effect the formation of voids at interfaces between miscible materials, and with reference again to  FIGS. 1A  through ID, the insulative material  16  of insulative layer  14  may be miscible with the filler  22  ( FIG. 2 ) of microcapsules  18 . The material from which the outer shells  20  ( FIG. 2 ) of the microcapsules  18  is formed may deteriorate or become or be made permeable (e.g., over time, when exposed to appropriate catalytic conditions, etc.), resulting in contact and, thus, an interface between insulative material  16  and filler  22 . Again, voids  32  may be formed at such an interface. The formation of such voids  32  may occur before, during, or after curing or solidification of one or both of insulative material  16  and filler  22 . 
   Turning now to  FIGS. 5A through 5D , another exemplary embodiment of a method for forming porous dielectric layers and structures is depicted. The method depicted in  FIGS. 5A through 5C  employs known, ultrafast laser pulsing techniques, in which a laser beam which is pulsed at an ultrafast frequency is focused at a location within a layer  314  of dielectric material. 
   In  FIG. 5A , a layer  314  of dielectric material, such as doped silicon dioxide (i.e., a glass, such as borosilicate glass (BSG), phosphosilicate glass (PSG), or borophosphosilicate glass (BPSG)), undoped silicon dioxide, silicon nitride, silicon oxynitride, a dielectric polymer, or the like, in a first, substantially nonporous state is formed over a substrate  12  by known, suitable processes. For example, when layer  314  comprises glass, silicon dioxide, silicon nitride, or silicon oxynitride, any known deposition techniques may be used. As another example, if a dielectric polymer is used to form layer  314 , spin-on processes, the use of a doctor blade, or screen printing processes may be used. It is currently preferred that the material from which layer  314  is formed be transparent at least to the wavelength or wavelengths of electromagnetic radiation that comprise an ultrafast pulsed laser beam  330  ( FIG. 5C ) to be focused therein. 
   As shown in  FIG. 5B , focal points  331 , which are locations at which voids  332  ( FIG. 5C ) are desired, are selected or otherwise determined (e.g., randomly, in a pattern, etc.) within the interior of layer  314 . Each focal point  331  represents a location at which photons will be absorbed and form a plasma within the material of layer  314 . 
     FIG. 5C  schematically depicts use of a so-called ultrafast pulsed laser beam  330  (e.g., pulses having a frequency of about one pulse per femtosecond (10 −‥ second)) to form voids  332  at focal points  331  within layer  314 . Ultrafast pulsed laser beam  330  may be generated by an ultrafast pulsed laser machine  350 , such as the model CPA-2001 femtosecond laser available from the Ultrafast Laser Machining Division of Clark-MXR, Inc., of Ann Arbor, Mich. 
   Since focal points  331  are located within layer  314 , an ultrafast pulsed laser beam  330  may be focused at such internally confined focal points  331  in such a way that the intensity of ultrafast pulsed laser beam  330  does not exceed an intensity threshold of the material from which layer  314  is formed until it reaches each focal point  331 . When ultrafast pulsed laser beam  330  reaches a focal point  331 , however, the intensity thereof reaches or exceeds the intensity threshold for the material of layer  314 , causing the material of layer  314  at that focal point  331  to absorb the energy of ultrafast pulsed laser beam  330 . The absorption of energy by the material of layer  314  at each focal point  331  results in the formation of a plasma at that focal point  331  and, thus, the removal of material of layer  314  and the formation of a void  332  at each focal point  331 , as depicted in  FIG. 5D . The use of ultrafast pulsed laser machine  350  ( FIG. 5C ) in this manner is described in Clark-MXR, Inc., Micromachining Handbook, which is available from Clark-MXR, Inc., the disclosure of which is hereby incorporated herein in its entirety by this reference. When voids  332  are formed in layer  314 , the material of layer  314  is transformed from the first, substantially nonporous state to a second, porous state. 
   Voids  332  of desired size (e.g., diameter) may be formed by use of an ultrafast pulsed laser beam of an appropriate wavelength. By way of example only, an ultrafast pulsed laser beam  330  having a central wavelength of about 0.2 μm, or microns (i.e., about 200 nm), may be used to form voids  332  that measure about 0.02 μm (i.e., about 20 nm or 200 Å) across. 
   Of course, other features, including, without limitation, circuit elements (not shown), may be formed over layer  314  or the structures that have been formed therefrom, as described previously herein with reference to  FIGS. 1D and 4D . Such fabrication may be effected once voids  332  have been formed or, if the materials from which the other features are to be fabricated are substantially transparent to the wavelengths of the ultrafast pulsed laser beam  330  to be used, prior to the formation of voids  332  within layer  314 . 
   The teachings of the present invention are applicable to the fabrication of any dielectric layer or structure of a semiconductor device. At present, these methods are particularly useful for forming insulative structures that will electrically isolate conductive structures, such as redistribution circuitry and redistributed bond pads to be fabricated over the active surfaces of semiconductor devices that have been substantially completely fabricated, from the integrated circuitry of such semiconductor devices. 
   Although the present invention has been shown and described with respect to illustrated embodiments, various additions, deletions and modifications that are obvious to a person of ordinary skill in the art to which the invention pertains, even if not shown or specifically described herein, are deemed to lie within the scope of the invention as encompassed by the following claims.