Abstract:
A dielectric material is provided having air gaps purposely formed within the dielectric. The dielectric is deposited, and air gaps formed, between respective interconnect lines. The geometries between interconnect lines is purposely controlled to achieve a large aspect ratio necessary to produce air gaps during CVD of the dielectric. Air gaps exist between interconnects to reduce the line-to-line capacitance, and thereby reduce the propagation delay associated with closely spaced interconnects.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to semiconductor fabrication and more particularly to a dielectric material placed between semiconductor interconnect lines such that the dielectric, when deposited, forms an air gap at a midline between the interconnect lines. 
     2. Description of the Relevant Art 
     An integrated circuit includes numerous conductors extending across the topography of a monolithic substrate. A set of interconnect lines (or conductors) which serve to electrically connect two or more components within a system is generally referred to as a “bus”. A collection of voltage levels are forwarded across the conductors to allow proper operation of the components. For example, a microprocessor is connected to memories and input/output devices by certain bus structures. There are numerous types of busses which are classified according to their operation. Examples of well-known types of busses include address busses, data busses and control busses. 
     Conductors within a bus generally extend parallel to each other across the semiconductor topography. The conductors are isolated from each other and from underlying conductive elements by a dielectric, a suitable dielectric being, for example, silicon dioxide (“oxide”). Conductors are thereby lithography patterned across the semiconductor topography, wherein the topography comprises a substrate with a dielectric placed thereon. The topography can also include one or more layers of conductors which are sealed by an upper layer of dielectric material. Accordingly, the layers of conductors overlayed with a dielectric present a topography upon which a subsequent layer of conductors can be patterned. 
     Conductors are made from an electrically conductive material, a suitable material includes Al, Ti, Ta, W, Mo, polysilicon, or a combination thereof. Substrate includes any type of material which can retain dopant ions and the isolated conductivity regions brought about by those ions. Typically, substrate is a silicon-based material which receives p-type or n-type ions. 
     Generally speaking, interconnect lines (or conductors) are fashioned upon the topography and dielectically spaced above an underlying conductor or substrate by a dielectric of thickness T d1 . Each conductor is dialectically spaced from other conductors within the same level of conductors by a distance T d2 . Accordingly, conductor-to-substrate capacitance C LS  is determined as follows: 
     
       
           C   LS   =eW   L   L/T   d1   (Eq. 1) 
       
     
     Further, the conductor-to-conductor capacitance C LL  is determined as follows: 
     
       
           C   LL   ≈eT   c   L/T   d2   (Eq. 2) 
       
     
     where e is the permittivity of the dielectric material (the dielectric material between the conductor and substrate or the dielectric material between conductors), W L  is the conductor width, T c  is conductor thickness, and L is the conductor length. Resistance of the conductor is calculated as follows: 
     
       
           R =( rL )/ W   L   T   c   (Eq. 3) 
       
     
     where r represents resistivity of the conductive material, and T c  is the interconnect (conductor) thickness. Combinations of equations 1 and 3, and/or equations 2 and 3 indicate the propagation delay of a conductor as follows: 
     
       
         
           RC 
           LS 
           ≈reL 
           2 
           /T 
           c 
           T 
           d1 
         
       
     
     
       
         
           RC 
           LL 
           ≈reL 
           2 
           /W 
           L 
           T 
           d2 
         
       
     
     Propagation delay is an important characteristic of an integrated circuit since it limits the speed (frequency) at which the circuit or circuits can operate. The shorter the propagation delay, the higher the speed of the circuit or circuits. It is therefore important that propagation delay be minimized as much as possible given the geometric constraints of the semiconductor topography. 
     Propagation delay is shown to be a function of both line-to-substrate capacitance C LS  as well as line-to-line capacitance C LL . Accordingly, propagation delay is determined by parasitic capacitance values between conductors (C LL ), and parasitic capacitance values between each conductor and substrate (C LS ). As circuit density increases, spacing between conductors decrease and capacitance C LL  becomes predominant relative to C LS . In other words, line-to-line capacitance C LL  increases with decreasing spacing between conductors. FIG. 1 illustrates the effect of line-to-line spacing on C LL . As spacing decreases, C LL  is shown to increase dramatically as compared to C LS . Modern integrated circuits which employ narrow interconnect spacings thereby define C LL  as the primary parasitic capacitance rather than C LS . 
     Increases in C LL  pose two major problems. First, an increase in C LL  generally causes an increase in the time at which a transition on the one end of the conductor occurs at the other end. Increase in transition time (i.e., increase in speed degregation) thereby requires a longer drive period. If the conductor extends along a critical speed path, speed degregation on the line will jeopardize functionality of the overall circuit. Second, a larger C LL  causes an increase in crosstalk noise. A conductor which does not transition, nonetheless receives crosstalk noise from neighboring lines which do. 
     It is therefore important to minimize propagation delay, especially in critical speed paths. Given the constraints of chemical compositions, it is not readily plausible to reduce the resistivity of conductor materials. Geometric constraints make it difficult to increase conductor thickness T c  or dielectric thickness T d1  or T d2 . Still further, instead of reducing length L of a conductor, most modern integrated circuits employ longer interconnect lines which compound the propagation delay problems. Accordingly, a need arises for instituting a reduction in propagation delay but within the chemical and geometric constraints of existing fabrication processes. It is therefore desirable that a fabrication process be derived which can reduce propagation delays by reducing the permittivity e of dielectric material. More specifically, the desired process must be one which reduces permittivity of dielectric material between conductors since the line-to-line capacitance appears a more predominant factor than the line-to-substrate capacitance. As such, it would be desirable to employ a fabrication technique in which dielectrics between conductors achieve low permittivity. 
     SUMMARY OF THE INVENTION 
     The problems outlined above are in large part solved by a fabrication process which produces a low permittivity dielectric between interconnect lines. The process utilizes a deposition technique which purposefully allows formation of an air gap at a midline between closely spaced interconnect lines. It is known that the permittivity of air is less than that of a semiconductor dielectric, such as oxide, oxynitride, spin on polymer, etc. Accordingly, an air gap within the dielectric causes a decrease in overall permittivity between interconnects. Reduction in permittivity results in a reduction in the line-to-line capacitance C LL . Reduction in C LL  is shown above to cause corresponding reduction in propagation delay RC LL . Incorporation of an air gap thereby reduces propagation delay and proves beneficial in meeting speed requirements within critical path interconnect lines, possibly interconnect lines spaced adjacent each other within a bus structure. 
     Broadly speaking, the present invention contemplates a dielectric interposed between a pair of integrated circuit interconnects are. The integrated circuits interconnects arranged upon a semiconductor topography. The topography includes a silicon substrate and one or more layers of dielectric and/or levels of interconnect. The dielectric includes an area therein which is void of dielectric. The voided area is deemed an air gap removed of fluid, gaseous or solid material. If any gaseous material is within the air gap, that gaseous material constitutes an inert gas. The air gap is completely encased within the dielectric near a midline between the pair of integrated circuit interconnects. The air gap area extends a spaced distance from the semiconductor topography and between the pair of interconnects. Further, the air gap area is sealed by the dielectric which flows across the upper regions of the air gap. 
     The present invention further contemplates a method for forming dielectric material between integrated circuit interconnects. The method includes the steps for providing a semiconductor topography and thereafter depositing a layer of metal upon the semiconductor topography. A first dielectric is then deposited upon the layer of metal, and the first dielectric and metal layers are thereafter removed in select regions across the semiconductor topography. Select removal of the first dielectric and the metal allows formation of a spaced set of first dielectric-covered interconnects. A second dielectric material is then deposited from a silane source upon the spaced set of first dielectric-covered interconnects to form an air gap between the dielectric-covered interconnects. 
     The first and second dielectric materials are preferably deposited from a chemical vapor deposition chamber maintained substantially near or slightly below atmospheric pressure. In order to achieve an air gap during second dielectric deposition, the cumulative thickness of the metal and the first dielectric is maintained greater than a distance between the spaced set of first dielectric-covered interconnects. First dielectric thickness is thereby necessary to ensure an aspect ratio (i.e., spacing height versus spacing width) greater than 1.0 by ensuring the cumulative thickness of metal and first dielectric (spacing height) is greater than a distance between the spaced set of dielectric-covered interconnects. Depositing the second dielectric from an atmospheric pressure chemical vapor deposition (APCVD) chamber upon the area of controlled aspect ratio thereby allows formation of voids or air gaps in the regions of interest. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings in which: 
     FIG. 1 is a graph of conductor-to-conductor as well as conductor-to-substrate capacitance plotted as a function of line spacing; 
     FIG. 2 is a partial cross-sectional view of an integrated circuit topography with a conductive layer and first dielectric layer deposited upon the semiconductor topography; 
     FIG. 3 is a processing step subsequent to that shown in FIG. 2, whereby select regions of first dielectric and underlying conductive layer are removed to form a plurality of spaced dielectric-covered interconnects; 
     FIG. 4 is a processing step subsequent to that shown in FIG. 3, whereby a second dielectric is deposited upon and between the dielectric-covered interconnects to form an air gap between each pair of interconnects; and 
     FIG. 5 is a processing step subsequent to that shown in FIG. 4, whereby the second dielectric upper surface is substantially planarized in readiness of an overlying layer of conductors. 
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to the drawings, FIG. 2 illustrates, in partial cross-section, a semiconductor topography  10 . Semiconductor topography  10  includes a dielectric, such as oxide, placed upon a silicon substrate or, alternatively, dielectrically isolated level or levels of interconnects placed upon a silicon substrate. Semiconductor topography  10  includes an upper surface  12  which is preferably planar and conditioned so as to receive a conductive layer  14 . Conductive layer  14  includes any conductive material, such as a metal (e.g., Al, W, Ta, Ti, etc.), polysilicon, or a combination thereof. Conductive layer  14  is deposited, for example, to a thickness between 0.5 to 2.0 microns. Conductive layer  14  is preferably deposited through sputter deposition, chemical vapor deposition, or metal evaporation. Deposited upon conductive layer  14  is a first dielectric layer  16 . First dielectric  16  includes any material having insulative capabilities. A suitable first dielectric  16  includes silicon dioxide (“oxide”) deposited from a chemical vapor deposition chamber. As described below, the thickness at which dielectric  16  is deposited varies depending upon the spacing between dielectric-covered interconnects. 
     Referring now to FIG. 3, a lithography procedure is used to remove first dielectric  16  and underlying conductive layer  14  in select regions across topography  10 . The regions removed of layers  14  and  16  are defined as a space  20  between resulting dielectric-covered interconnects  18 . Each dielectric-covered interconnect  18  comprises a patterned conductive layer  14 ′ and an overlying, patterned first dielectric  16 ′ in which layers  14 ′ and  16 ′ have substantially equal lateral widths. 
     The dimensions of spacing  20  between dielectric-covered interconnects  18  are brought about to achieve a unique aspect ratio. The aspect ratio is defined herein to include a height H greater than width W. Preferably, dimension H is more than twice dimension W so as to produce an aspect ratio of 2.0. The aspect ratio is controlled by varying the thickness of first dielectric  16 . In most instances, thickness of conductive layer  14  is defined based upon the metal sputter process parameters. First dielectric  16  thickness, however, can be changed to produce a unique spacing  20  geometry which is conducive to the formation of air gaps, as described below. 
     Referring now to FIG. 4, a second dielectric  22  is deposited across semiconductor topography  10 , i.e., across and between dielectric-covered interconnects  18 . Second dielectric  22  is deposited in a way which forms air gaps  24  within spacing  20  regions. Air gaps  24  are formed uniformly between closely spaced, dielectric-covered interconnects  18 . More specifically, air gaps  24  are formed in spacing  20  regions which have an aspect ratio exceeding 1.0, and preferably exceeding 2.0. Air gaps  24  are formed during the deposition of second dielectric  22 , from a silane source and from within an atmospheric pressure chemical vapor deposition (APCVD) apparatus. It is preferred that second dielectric  22  not be formed from a TEOS source, or from within a low pressure chemical vapor deposition (LPCVD) chamber or a plasma enhanced chemical vapor deposition (PECVD) chamber. Second dielectric  22  is preferably formed from a nitride source material such that second dielectric  22  comprises an oxynitride, or a layered composite of oxide/nitride/oxide. If second dielectric  22  comprises an oxide, the oxide can encompass phosphorus doping to enhance its cusping and void-formation effect. Accordingly, instead of trying to avoid voids or air gaps as in conventional art, the present process purposely chooses a deposition material and process which ensures second dielectric  22  produces voids or air gaps in regions having large aspect ratios. 
     Each air gap  24  is formed within the CVD chamber, and is thereby dimensioned completely within second dielectric  22 . Air gap  24  is devoid of solid and liquid material, however, it does contain gaseous material resulting from the CVD chamber ambient. Gaseous material, such as argon, hydrogen, and/or oxygen may be entrained within the air of the air gap as a result of the air gap being formed during CVD process, given the fact that CVD does employ such gaseous material. 
     Air gap  24  is completely encompassed within second dielectric  22  at or near the midline between dielectric-covered interconnects  18 . At the upper surface of second dielectric  22  directly above air gaps  24  are recesses or indents  28 . Indents  28  produce localized regions of non-planarity above closely spaced dielectric-covered interconnects  18 . Indents  28  can be subsequently removed, as shown in FIG. 5, by a planarization step. FIG. 5 illustrates, in a processing step subsequent to FIG. 4, removal of indents  28  by selectively etching back initial surface  30  to final surface  32 . Etch back is achieved by depositing, for example, a planarization layer with similar etch characteristics to that of second dielectric  22 . The planarization layer and underlying surface  30  is then blanket etched to produce surface  32 . Alternatively, final surface  32  can be achieved by chemical mechanical polish (CMP) of surface  30  to an elevational level below indents  28 . The finalized surface resulting from selective etch back or CMP is shown in FIG. 5 as reference numeral  32 . 
     It would be appreciated to those skilled in the art having a benefit of this disclosure that this invention is capable of applications with numerous types of MOS-processed circuits. Furthermore, it is to be understood that the form of the invention shown and described is to be taken as presently preferred embodiments. Various modifications and changes may be made to each and every processing step as would be obvious to the person skilled in the art having the benefit of this disclosure. Provided the spacing area is properly configured with a necessary aspect ratio, and provided the second dielectric material is deposited so as to form voids and/or cusps between closely-spaced interconnects, various deposition techniques and deposition processes can be used. Thus, the purpose hereof is to form a reentrant angle near the midline between interconnects during the cumulative deposition of dielectric material comprising the second dielectric layer. The cumulative deposition of second dielectric material presents a void or air gap defined by the reentrant angles. It is intended that the following claims be interpreted to embrace all such modifications and changes and, accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.