Abstract:
A void is defined between adjacent wiring lines to minimize RC coupling. The void has a low dielectric value approaching 1.0. For one approach, hollow silicon spheres define the void. The spheres are fabricated to a known inner diameter, wall thickness and outer diameter. The spheres are rigid enough to withstand the mechanical processes occurring during semiconductor fabrication. The spheres withstand elevated temperatures up to a prescribed temperature range. At or above a desired temperature, the sphere walls disintegrate leaving the void in place. For an alternative approach, adjacent wiring lines are “T-topped” (i.e., viewed cross-sectionally). Dielectric fill is deposited in the spacing between lines. As the dielectric material accumulates on the line and substrate walls, the T-tops grow toward each other. Eventually, the T-tops meet sealing off an internal void.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 09/515,519, filed Feb. 29, 2000, now U.S. Pat. No. 6,396,119, issued May 28, 2002, which is a continuation of application Ser. No. 09/207,890, filed Dec. 8, 1998, now U.S. Pat. No. 6,309,946 B1, issued Oct. 30, 2001, which is continuation of application Ser. No. 08/723,263, filed Sep. 30, 1996, now U.S. Pat. No. 5,835,987, issued Nov. 10, 1998; which is a divisional of application Ser. No. 08/550,916, filed Oct. 31, 1995, abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to wiring line formation and inter-line fill processes for a semiconductor substrate, and more particularly, to inter-line fill processes for reducing RC delay between adjacent wiring lines. 
     2. State of the Art 
     Integrated circuit substrates include many different p-type and n-type doped regions. These regions are connected in specific configurations to define desired devices and circuits. Conductive paths are defined on the substrate to connect the various doped regions to form the many devices and circuits. These paths typically are referred to as wires, interconnects, metal stacks, or conductors. The term “wiring line” is used herein to refer to all such conductive paths. 
     As device and circuit densities increase due to advances in technology, it is desirable to decrease wiring line pitches and spacings. A wiring line has a length, a thickness and a width. The non-line area between adjacent lines is referred to as the line spacing. The width and spacing is conventionally referred to as the line pitch. The spacing can be between lines on the same plane of the substrate or between lines on adjacent planes. Conventional line spacing of approximately 1.0 micron is known. There is a desire, however, to decrease line spacing as IC device densities increase. 
     One of the challenges of semiconductor processes is to maintain electrically-independent wiring lines. Electrical coupling between adjacent lines is undesirable. Reliable, uncoupled signals carried along adjacent lines are needed for normal circuit operation. One of the coupling characteristics between adjacent lines is the RC delay (“RC coupling”). Zero delay is ideal. Minimal RC delays are desired. As the spacing between two adjacent lines decreases, the RC coupling tends to increase. One of the physical characteristics defining RC delay (other than spacing) is the dielectric value of the fill material in the spacing between adjacent lines. Currently, dielectric values of approximately 3.0 are common for 1.0 micron line spacing. A dielectric of approximately 3.0 is achieved using tetra ethyl oxy silicate (“TEOS”) as the fill material between adjacent lines. Use of a high density plasma oxide fill at the 1 micron spacing has been found to achieve dielectric values between 2.4 and 2.7. 
     As the line spacings decrease (e.g., below 0.5 microns), new fill processes and materials are needed to avoid RC coupling and achieve minimal RC delays. 
     BRIEF SUMMARY OF THE INVENTION 
     According to the invention, a void is defined between adjacent wiring lines to minimize RC coupling. The void has a low dielectric value approaching 1.0. The void is space absent solid and liquid material. In various embodiments the space is a vacuum or is filled with gaseous substance having desired dielectric properties. 
     According to one method of the invention, a hollow silicon dioxide sphere defines the void. The sphere is fabricated to a known inner diameter, wall thickness and outer diameter. Preferably, the wiring line height is a multiple of the line spacing, or the spacing is a multiple of the wiring line height. Spheres of a unit dimension then fill the spacing to achieve one or more rows (or columns) of spheres. 
     According to one aspect of the method, the spheres are rigid enough to withstand the mechanical processes occurring during semiconductor fabrication. 
     The spheres are held in place during the semiconductor fabrication processes by a binder. According to another aspect of the method, the spheres and binder withstand elevated temperatures up to a prescribed temperature range. At or above a desired temperature, the binder is baked away leaving the sphere intact and in place. 
     According to another method of the invention, the adjacent wiring lines are “T-topped” (i.e., viewed cross-sectionally). In a specific embodiment, the cross section appears as a “T” or as an “I.” Dielectric fill is deposited in the spacing between lines by a chemical vapor deposition (“CVD”) or other deposition process. As the dielectric material accumulates on the wiring line and substrate walls, the T-tops grow toward each other. Eventually, the T-tops meet sealing off an internal void. Using controlled processes, the void is reliably defined to a known size and shape. 
     According to preferred embodiments, a spacing between adjacent wiring lines of a semiconductor substrate includes a first material which defines a void. The void has no solid material or liquid material, but may include a gas. Also, the void is characterized by a dielectric constant which is lesser than the dielectric constant of the first material. In one embodiment, a plurality of discrete hollow objects fill the spacing. Each one of the plurality of objects is a hollow, rigid, silica sphere which defines a void. Each sphere is of substantially the same dimensions. The spacing between adjacent lines is approximately a first multiple of sphere outer diameter. The height of the adjacent wiring lines is approximately a second multiple of sphere outer diameter. Preferably, either one, but not both, of the first multiple and the second multiple are greater than one. 
     According to a preferred embodiment of one method, a void is controllably-defined in spacing between adjacent wiring lines of a semiconductor substrate. At one step, a plurality of discrete hollow silica spheres are applied to the spacing. At another step, excess spheres are removed from areas other than the spacing. At another step, material is deposited over the wiring lines and spheres. For one method, the spheres are applied as part of a film, including a binder. The binder holds the objects in place within the spacing. For one method, the excess spheres are removed by performing a chemical-mechanical polishing (“CMP”) process. Preferably, the deposition step occurs at a temperature sufficient to break down the binder while leaving the spheres in place and intact. 
     According to another preferred embodiment, a void is controllably-defining in spacing between adjacent wiring lines of a semiconductor substrate using an alternative method. At one step, a T-top configuration is etched at each of the adjacent wiring lines. At another step, dielectric material is deposited onto the substrate and adjacent wiring lines. The deposited material accumulates about the T-tops to seal off a void in the spacing. The void forms with dimensions determined by the spacing, wiring line height, and undercut of the T-tops. For various alternatives, the wiring line cross-sections after T-topping resemble an “I” or a “T” configuration. 
    
    
     According to one advantage of the invention, the controllably-defined void(s) reduce the dielectric value in the spacing between adjacent wiring lines. As a result, the RC delay is comparatively reduced. According to another advantage, the reduced dielectric is achieved for conventional (e.g., ≧1.0 microns) or reduced line spacings (e.g., &lt;1.0 microns; &lt;0.5 microns). With sphere outer diameters achieved at 0.1 microns, the method has the advantage of being beneficial for line spacing as low as 0.1 microns. As technologies enable smaller spheres, the method also becomes applicable for smaller line spacings. These and other aspects and advantages of the invention will be better understood by reference to the following detailed description taken in conjunction with the accompanying drawings. 
     DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a partial cross-sectional view of adjacent wiring lines on a substrate having a controllably-defined void in the line spacing according to an embodiment of this invention; 
     FIG. 2 is a partial cross-sectional view of adjacent wiring lines on a substrate having a controllably-defined void in the line spacing according to another embodiment of this invention; 
     FIG. 3 is block diagram of a row of hollow spheres filling the spacing between adjacent wiring lines according to one embodiment of this invention; 
     FIG. 4 is a cross sectional view of a sphere of FIG. 3; 
     FIG. 5 is a cross-sectional view of a substrate receiving a film of spheres according to a step of one method embodiment of this invention; 
     FIG. 6 is a cross-sectional view of a substrate after chemical-mechanical polishing according to a step of one method embodiment of this invention; 
     FIG. 7 is a cross-sectional view of a substrate in which wiring line height is a multiple of line spacing; 
     FIG. 8 is a cross-sectional view of a substrate after a layer is deposited over the wiring lines and spheres of FIG. 6 according to a step of one method embodiment of this invention; 
     FIG. 9 is a cross-sectional view of a substrate with adjacent “T” shaped and “I” shaped metal wiring line stacks according to an embodiment of this invention; 
     FIG. 10 is a cross-sectional view of a “T” shaped metal wiring stack and an “I” shaped metal wiring stack of FIG. 9; 
     FIG. 11 is a cross-sectional view of the substrate of FIG. 9 during dielectric deposition according to a step of a method embodiment of this invention; 
     FIG. 12 is a cross-sectional view of the substrate of FIG. 9 after dielectric deposition according to a step of a method embodiment of this invention; 
     FIG. 13 is a cross-sectional view of the substrate of FIG. 9 after planarizing according to a step of a method embodiment of this invention; and 
     FIG. 14 is a cross-sectional top view along line  90  in FIG. 13, depicting voids between adjacent wiring lines according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Overview 
     FIG. 1 shows a partial view of an integrated circuit (IC)  11  having a void  12  formed between adjacent wiring lines  14 ,  16 . The wiring lines  14 ,  16  are conductively coupled to respective portions of a semiconductor substrate  18 . The substrate  18  includes various n-type and p-type doped regions interconnected by wiring lines, such as lines  14 ,  16 . The interconnected substrate regions define desired semiconductor devices. The wiring lines are formed by one or more layers, including a barrier layer and a metal conductive layer. The barrier layer serves to prevent metal penetration into the substrate  18  during the formation processes. The conductive layer couples a local area of the substrate (e.g ., doped or not doped area) to another area (not shown) or another layer  20 . The spacing between adjacent wiring lines is occupied by fill material  22 ,  24  or is a void  12 . 
     FIG. 2 shows a partial view of an integrated circuit (IC)  30  having a void  32  formed between adjacent wiring lines  34 ,  36  according to an alternative embodiment of this invention. The wiring lines  34 ,  36  are conductively coupled to respective portions of a semiconductor substrate  38 . The substrate  38 , like substrate  18 , includes various n-type and p-type doped regions interconnected by wiring lines. The interconnected substrate regions define desired semiconductor devices. The wiring lines couple a local area of the substrate (e.g., doped or not doped area) to another area (not shown) or layer  40 . The spacing between adjacent wiring lines are occupied by fill material  42  and the void  32 . In the embodiment shown, vias  44  are formed through the fill material  42  and filled with conductive material to respectively couple the wiring lines  34 ,  36  to an adjacent layer  40 . 
     For the various IC embodiments, a void  12 / 32  in the spacing between adjacent wiring lines serves to reduce RC coupling of the lines. RC coupling is reduced by reducing the dielectric constant in the spacing. Specifically, because the dielectric constant of the void (e.g., approximately 1.0) is less than the dielectric constant of conventional fill materials (e.g., approximately 3.0), the dielectric constant in the spacing between lines is reduced. 
     Following are descriptions of alternative methods for controllably-defining the voids  12 ,  32 . 
     Void Defined by Hollow Silicon Spheres 
     Referring to FIG. 3, an integrated circuit  10  having integral devices (not shown) and wiring lines  14 ,  16  formed by known processes receives hollow spheres  50 . As seen in FIG. 4, the spheres  50  have an inner diameter  52 , outer diameter  54  and wall thickness  56  of known dimensions. In a preferred embodiment, the sphere walls  58  are formed of silica. For a given embodiment, each sphere  50  has the same dimensions. Preferably, the line height and the line spacing is a multiple of the sphere outer diameter. Alternatively, the sphere outer diameter is slightly less than a value which makes the spacing or height a multiple of the outer diameter. Although the outer diameter of each sphere  50  is substantially the same for a given embodiment, the outer diameter varies for different embodiments. The outer diameter varies among different embodiments from a value greater 1.0 microns to a value less than 0.5 microns. Spheres as small as 0.1 microns in outer diameter are achievable. 
     The spheres  50  are of sufficient rigidity to withstand the mechanical stresses occurring in fabricating an integrated circuit. In one embodiment, the ratio of outer diameter to wall thickness is approximately 10:1, although greater or lesser ratios are used in other embodiments. 
     At one step, the spheres  50 , together with a binder material and/or dispersion chemical, are applied to the substrate  18  using a spinning process or a monolayer formation process. An exemplary binder material is methyl isobutyl ketone (“MIBK”). The function of the binder is to hold the spheres in place relative to the wiring lines  14 ,  16  and substrate  18 . Exemplary dispersion chemicals include polyethylene oxide or a silanol compound. The function of the dispersion chemicals is to disperse the spheres into the line spacings and over the wiring lines and substrate. A film  60 , formed by the spheres  50 , binder material and/or dispersion chemical accumulates on the substrate  18  and wiring lines  14 ,  16  as shown in FIG.  5 . 
     At another step, the substrate is planarized. A chemical-mechanical polishing (“CMP”) or other planarizing device  59  removes the film  60  from the tops of the wiring lines  14 ,  16  as shown in FIG.  6 . In one embodiment, the wiring lines  14 ,  16  have a height relative to the substrate  18  surface which is a multiple of the sphere  50  outer diameter  54 . For minor variations of height to outer diameter, the wiring lines  14 ,  16  are planed back to be a multiple of sphere  50  outer diameter  54 . For areas  62  not to be filled with the film  60 , an etching process is used to remove the film  60  (see FIG.  6 ). 
     FIG. 6 shows two preferred relations between wiring line  14 ,  16  height and wiring line spacing. In one region  64 , the wiring line height equals the wiring line spacing. In another region  66 , the wiring line spacing is a multiple (e.g., 2) of the wiring line height. Preferably, the ratio of the longer of the height and spacing to the shorter of height and spacing is an integer, (i.e., either the spacing is a multiple of the height or the height is a multiple of the spacing). FIG. 7 shows the height being a multiple of the spacing. For the best mode of the invention, spheres are applied which have an outer diameter substantially equal to (or slightly smaller than) either one or both of the line height or the line spacing. In other embodiments, either one or both of the height and spacing are multiples of the sphere outer diameter. Preferably, both the line height and line spacing are not a multiple greater than 1 relative to the sphere outer diameter. 
     In alternative embodiments, either a dielectric or a plasma oxide layer is applied over the wiring lines  14 ,  16  and spheres  50 . For dielectric layer  68 , low temperature dielectric reflow is deposited on the wiring lines  14 ,  16  and spheres  50 . Reflow improves filling of high aspect-ratio contacts and via openings. Preferably, the deposition process occurs at a temperature high enough to bake off the binder material, but low enough not to alter the structural integrity of the spheres  50 . More specifically, one does not want to collapse or puncture the spheres  50  during the dielectric reflow deposition step. In one embodiment, binder material capable of withstanding temperatures up to a desired temperature (e.g., 200 degrees C.) are used. Above the desired temperature, the binder breaks down and flows out as a vapor, but leaving the spheres in place and intact. 
     Alternatively, for a plasma oxide layer  70 , plasma oxide is deposited over the wiring lines  14 , 16  and the spheres  50 . Preferably, the process occurs at a temperature sufficient to bake off the binder material, while leaving the spheres in place and intact. Further semiconductor processes then occur to fabricate another device level or area of the substrate  18 . 
     Void Defined by Controlled Deposition 
     Referring to FIG. 9, a semiconductor substrate  38  has integral devices (not shown) formed by known processes. Metal stacks  72 ,  73  are formed to define wiring lines  34 ,  36 ,  74 ,  76 . According to alternative embodiments, the stack cross-section appears as a “T” (e.g., stack  72 ) or an “I” shape (e.g., stack  73 ). Of significance is the “T-top” in each embodiment. By depositing a dielectric layer, the T-tops of adjacent wiring lines grow toward each other sealing off a void between adjacent wiring lines (see FIG.  2 ). 
     Referring to FIG. 10, each metal stack includes a barrier layer  78 , a conductive layer  80  and a top layer  82 . A common material for an exemplary barrier layer  78  is titanium, although other elements and alloys are used, (e.g., titanium nitride, titanium tungsten, tantalum nitride). A common material for an exemplary middle layer  80  is aluminum, although other elements and alloys also can be used, (e.g., copper, gold). A common material for an upper layer  82  is titanium nitride, although other materials and alloys are used, (e.g., titanium tungsten, titanium, titanium aluminide, tantalum nitride). 
     In one embodiment, the three layers are deposited, then etched, using a reactive ion etching (RIE) process to achieve a straight metal stack. For an “I” stack  73 , the conductive middle layer  80  is etched using a wet dip process to achieve the “I” configuration. For a “T” stack  72 , both the conductive middle layer  80  and the barrier layer  78  are etched using a wet dip process to achieve the T-top configuration. Alternatively, the barrier layer  78  and middle layer  80  are formed to desired shape by an RIE process. An isotropic overetch then is performed to achieve the “T-top” for either the “T” stack  72  or “I” stack  73 . 
     For each stack  72 ,  73  configuration, the length of undercut  84  is prescribed based upon a desired line resistance, the desired line spacing between adjacent stacks  72  and/or  73  and the size of void desired between adjacent wiring lines  34 / 36 / 74 / 76 . 
     With the stacks formed at desired locations with desired dimensions (e.g., line height, pitch, undercut) and desired line spacings, dielectric material  86  is deposited using a CVD or other deposition process. Exemplary dielectric materials include TEOS, polyamide, Si 3 N 4 , SOG, phosphosilicate glass, and boro-phosphosilicate glass. The dielectric material  86  accumulates on the wiring lines  34 ,  36 ,  74 ,  76  and substrate  38 , as shown in FIG.  11 . As the deposition process continues, the dielectric material accumulating at adjacent “T-tops” seals off an area between the adjacent lines. Such sealed off area is the desired void  32  (see FIGS.  2  and  12 ). The deposition process continues for a prescribed time or a prescribed thickness of dielectric material accumulates above the wiring lines  34 ,  36 ,  74 ,  76 . Thereafter, the substrate is subjected to a chemical-mechanical polishing process or other planarizing process to achieve a dielectric layer of desired thickness, (see FIG.  13 ). 
     For embodiments in which vias  44  (see FIG. 2) are desired, a plasma enhanced chemical vapor deposition of a nitride compound is deposited (e.g., approximately 100 angstroms) prior to dielectric deposition to serve as an etch-stop layer. 
     As seen in FIGS. 12 and 13, the formation of the voids  32  is controlled for a given line spacing by (i) appropriately defining the wiring line height  83  and under cut  84  and (ii) controlling the deposition process (see FIG.  10 ). As a result, the voids  32  occur with known size and shape. Voids  32  formed between adjacent “T” stacks are generally uniform in size and shape. Similarly, voids  32  formed between adjacent “I” stacks are generally uniform in size and shape. The length of each void  32  is determined by the wiring line length of adjacent wiring lines  34 ,  36 ,  74 ,  76 . 
     FIG. 14 depicts a cross-sectional top view along line  90  in FIG. 13, illustrating that the length of each void  32  corresponds with the length of adjacent wiring lines  34 ,  36 ,  74  and  76 . 
     The voids  32  have a dielectric constant of approximately 1.0. The surrounding dielectric material  86  has a higher dielectric value (e.g., TEOS has a dielectric constant of 3.0, high density plasma oxides have a dielectric constant of 2.4-2.7). The net effect of the void is to lower the dielectric constant across the line spacing and thereby reduce RC coupling between adjacent lines. 
     Further semiconductor processes also occur after void formation to fabricate additional devices, levels or other area of the substrate  38 . 
     Meritorious and Advantageous Effects 
     According to one advantage of the invention, the void in the spacing between adjacent wiring lines reduces RC coupling of the lines. RC coupling is reduced by reducing the dielectric constant in the spacing. Specifically, because the dielectric constant of the void (e.g., approximately 1.0) is less than the dielectric constant of conventional fill materials (e.g., approximately 3.0), the dielectric constant in the spacing between lines is reduced. According to another advantage, the reduced dielectric is achieved for conventional or reduced line spacings. 
     Although a preferred embodiment of the invention has been illustrated and described, various alternatives, modifications and equivalents may be used. Therefore, the foregoing description should not be taken as limiting the scope of the inventions which are defined by the appended claims.