Abstract:
Methods of fabricating semiconductor structures on a substrate, where the substrate has transistors formed thereon, are provided. One method includes forming interconnect metallization structures in a plurality of levels. The forming of the interconnect metallization structures includes depositing a sacrificial layer and performing a process to etch trenches, vias, and stubs into the sacrificial layer. The method further includes filling and planarizing the trenches, vias, and stubs that were etched and then etching away the sacrificial layer throughout the plurality of levels of the interconnect metallization structures. The etching leaving a voided interconnect metallization structure that is structurally supported by stubs that are non-electrically functional.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims priority under 35 USC 120 as a divisional of application Ser. No. 11/259,561, filed on Oct. 25, 2005, which is a divisional of application Ser. No. 09/821,415, filed Mar. 28, 2001. The disclosures of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to semiconductor device fabrication technology and techniques for improving the performance of semiconductor devices. Specifically, the present invention relates to implementing sacrificial materials to enhance the performance of semiconductor devices. 
         [0004]    2. Description of the Related Art 
         [0005]    As is well known, the semiconductor industry is moving toward implementing larger substrates having higher density with devices having smaller features. To achieve this task, millions of transistors are connected through multiple layers of interconnect metallization lines, insulating dielectric layers, and conductive via structures and are fabricated on a wafer substrate. Originally, metallization lines and vias were predominantly formed from aluminum, as it is relatively inexpensive, easy to etch, and has relatively low resistivity, while insulators were made predominantly from silicon dioxide. However, due to the decrease in the size of the device features, vias and contact holes as well as the distance between the metallization lines in conjunction with system-on-chip evolution, there is an increased demand to improve semiconductor device performance through changing the materials implemented in semiconductor fabrication. So far, this has been a two-fold task. 
         [0006]    First, rather than aluminum, copper interconnect lines, vias, and contact holes are being implemented. Replacing of aluminum with copper has been favorable, as the latter has lower resistivity and better conductivity and electromigration properties than the former. However, replacing aluminum with copper has been problematic, as it mandates a fundamental change in the metal interconnect formation. Specifically, while aluminum interconnects are formed by etching out unprotected portions of the thin aluminum film overlaid on the surface of the substrate, copper interconnects are formed through depositing copper into via holes and trenches that are etched into dielectric layers. As a result, in a semiconductor device having copper interconnects, a planarization operation must be performed on the substrate surface so as to remove overburden copper from non-trench, via or contact plug areas of the dielectric. 
         [0007]    Second, instead of silicon dioxide, dielectric materials with low dielectric permittivity, or so-called low-K dielectric materials, are being used as insulators. Low-K dielectric materials are preferred because first, they reduce interconnect-to-interconnect capacitance, as the coupling capacitance between two metallization lines placed in close proximity of each other is directly proportional to the dielectric constant of the insulating dielectric material used. Second, low-K dielectric materials reduce cross-talk noise, since the lower is the dielectric constant of a dielectric, the lower is the possibility of cross-metallization line signal interference. By way of example, while the predominantly used dielectric, silicon dioxide, has a dielectric constant of about 4.0, air has the lowest dielectric constant of 1.0, and other low-K dielectric materials ranging from about 1.5 and about 3.5. As air has been recognized to have the lowest dielectric constant, there has been a trend in the semiconductor fabrication technology to manufacture dielectric materials with dielectric constants close to that of air. 
         [0008]    So far, such attempts have resulted in producing highly porous dielectric materials. However, the poor mechanical strength of such porous dielectric materials as well as the current state of the semiconductor fabrication technology hinders their implementation in the semiconductor fabrication process. Particularly, the poor mechanical strength of low-K dielectric materials is problematic during the chemical mechanical planarization (CMP) operation performed on copper metallization lines. As is well known, in a CMP operation, the substrate surface is applied onto a moving polishing pad with force, thus removing the overburden metal from over the substrate surface. However, performing a CMP operation on a semiconductor device having porous low-K dielectric material is complicated as the application of the substrate surface onto the polishing pad may cause regions of the semiconductor structure to collapse or crack, thus hindering performance or requiring that the fabricated semiconductor wafer be discarded. As can be appreciated, when these problems are introduced during semiconductor fabrication processes, the yield of good chips can dramatically decrease, in addition to reducing wafer throughput. 
         [0009]    In view of the foregoing, a need exists for semiconductor structures that can be fabricated using conventional techniques that provide good structural support during CMP operations, while still producing devices having low capacitive delays such as those implementing low-K dielectric materials. 
       SUMMARY OF THE INVENTION 
       [0010]    Broadly speaking, the present invention fills these needs by enabling the fabrication of semiconductor structures using standard dielectric materials that can withstand mechanical stresses and pressures common in chemical mechanical planarization. In one preferred embodiment, a sacrificial material is used to fabricate each layer of copper interconnects of a semiconductor structure which is subsequently etched away and replaced with an insulator having a low dielectric permittivity (low-K). In another implementation, a plurality of supporting stubs is formed within each sacrificial layer, thus defining continuous supporting columns for when the sacrificial material is etched away. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, or a method. Several inventive embodiments of the present invention are described below. 
         [0011]    In one embodiment, a method for making a semiconductor device is provided. The method includes forming transistor structures on a substrate and forming of interconnect metallization structures in a plurality of levels through depositing a sacrificial layer. A dual damascene process is performed to etch trenches and vias, and filling and planarizing the trenches and vias. The sacrificial layer is etched throughout the plurality of levels of the interconnect metallization structures, thus leaving a voided interconnect metallization structure. The voided interconnect metallization structure is filled with low K dielectric material, thus defining a low K dielectric interconnect metallization structure. 
         [0012]    In another embodiment, another method for making a semiconductor device is provided. The method includes forming transistor structures on a substrate and forming interconnect metallization structures in a plurality of levels. The interconnect metallization structures are formed through depositing a sacrificial layer, and performing a dual damascene process to etch trenches, vias, and stubs, and filling and planarizing the trenches, vias, and stubs. The sacrificial layer is etched away throughout the plurality of levels of the interconnect metallization structure leaving a voided interconnect metallization structure and supporting stubs. 
         [0013]    In still another embodiment, a semiconductor device is provided. The semiconductor device includes a substrate having transistor devices and a plurality of copper interconnect metallization lines and conductive vias. The plurality of copper interconnect metallization lines and conductive vias are defined in each of a plurality of interconnect levels of the semiconductor device such that the plurality of copper interconnect metallization lines and conductive vias are isolated from each other by an air dielectric. The semiconductor device further includes a plurality of supporting stubs each of which is configured to form a supporting column that extends through the plurality of interconnect levels of the semiconductor device. 
         [0014]    In yet another embodiment, a semiconductor device is provided. The semiconductor device includes a substrate having transistor devices and a plurality of copper interconnect metallization lines and conductive vias. The plurality of copper interconnect metallization lines and conductive vias are defined in each of a plurality of interconnect levels of the semiconductor device such that the plurality of copper interconnect metallization lines and conductive vias are isolated from each other by a porous dielectric material. The semiconductor device further includes a plurality of supporting stubs each of which is configured to form a supporting column that extends through the plurality of interconnect levels of the semiconductor device. 
         [0015]    The advantages of the present invention are numerous. Most notably, even though the semiconductor structure of the present invention ultimately implements air or low-K dielectric materials as a dielectric, the semiconductor structure of the present invention withstands the structural stresses and pressures that occur during CMP and other operations. In this manner, while inter-metal dielectric capacitance is minimized and faster integrated circuit devices are produced, the shortcomings associated with implementing air or low-K dielectric materials in semiconductor fabrication processes are substantially eliminated. 
         [0016]    Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings, and like reference numerals designate like structural elements. 
           [0018]      FIG. 1A  is a simplified, partial, exploded, cross-sectional view of a semiconductor structure having an interlevel dielectric (ILD) formed over a substrate that has a plurality of active devices, in accordance with one embodiment of the present invention. 
           [0019]      FIG. 1B  is a simplified, partial, exploded, cross-sectional view of the semiconductor structure of  FIG. 1A  further including a first sacrificial layer having a plurality of fabricated metallization lines, vias, and stubs, in accordance with another embodiment of the present invention. 
           [0020]      FIG. 1C  is a simplified, partial, exploded, cross-sectional view of the semiconductor structure of  FIG. 1B  having a plurality of fabricated sacrificial layers, each including a plurality of stubs, in accordance with yet another embodiment of the present invention. 
           [0021]      FIG. 1D  is a simplified, partial, exploded, cross-sectional view of a post-etched multi-layer semiconductor structure having air as a dielectric material, in accordance with yet another embodiment of the present invention. 
           [0022]      FIG. 1E  is a simplified, partial, exploded, cross-sectional view of the post-etched air-dielectric multi-layer semiconductor structure of  FIG. 1D  further including a passivation-capping layer, in accordance with one aspect of the present invention. 
           [0023]      FIG. 1F-1  is a simplified, partial, exploded, cross-sectional view of the post-etched multi-layer semiconductor structure of  FIG. 1D  having been filled with a porous low-K dielectric material, in accordance with another aspect of the present invention. 
           [0024]      FIG. 1F-2  a simplified, partial, exploded, cross-sectional view of the post-etched low-K dielectric filled semiconductor structure of  FIG. 1F  being covered with a passivation-capping layer, in accordance with yet another embodiment of the present invention. 
           [0025]      FIG. 1F-3  is a simplified, partial, exploded, cross-sectional view of a post-etched multi-layer semiconductor structure of  FIG. 1D  having low-K dielectric layers, as covered with a passivation-capping layer, in accordance with still another embodiment of the present invention. 
           [0026]      FIG. 2  is a flow chart diagram of the process operations performed to fabricate an air dielectric semiconductor structure having a plurality of copper metallization lines and supporting stubs, in accordance with another aspect of the present invention. 
           [0027]      FIG. 3  is a generic flow chart method operations process to fabricate a porous low-K dielectric semiconductor structure having a plurality of copper metallization lines, in accordance with yet another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0028]    Embodiments for making semiconductor structures that optimize semiconductor performance by minimizing inter-metal dielectric capacitance are described. In one preferred embodiment, a sacrificial material is used during the fabrication of each layer of copper interconnects and then is etched out and replaced with an insulator having a low dielectric constant. In another embodiment, a plurality of stubs is formed in the sacrificial layer, thus creating nearly continuous supporting columns for when the sacrificial layer is etched away. In preferred implementations, the substantially continuous supporting columns of stubs are configured to extend from the passivation layer to a passivation-capping layer, thus forming a semiconductor structure having high structural integrity with reduced capacitance related delay. In a preferred embodiment, the plurality of stubs is constructed from copper. In another embodiment, the sacrificial layer is a dielectric and the low-K dielectric material is a porous dielectric material. 
         [0029]    In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be understood, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process operations have not been described in detail in order not to unnecessarily obscure the present invention. 
         [0030]      FIG. 1A  is a cross-sectional view of a semiconductor structure  100  having an inter-level dielectric (ILD) formed over a substrate  102  that has a plurality of transistors, in accordance with one embodiment of the present invention. As shown, each of the fabricated transistors includes a plurality of source/drain diffusion regions  103  formed within the substrate  102 . Each of the transistors further includes a conductor polysilicon gate  120 , each of which is separated from the substrate  102  by a dielectric gate oxide  118 . In one embodiment, source/drain regions  103 , which are also known as P-type or N-type regions, may be formed through a doping process that implements impurities such as boron or phosphorous. As shown, the source/drain regions  103  are separated by a plurality of shallow trench isolation regions  104  also formed within the substrate  102 . As designed, the shallow trench isolation regions  104  are made out of a non-conducting material (e.g., silicon dioxide, silicon nitride, etc.). Formed along each of the sidewalls of each of the gate oxides  118  and polysilicon gates  120  are a plurality of dielectric spacers  122 . 
         [0031]    Further shown is the interlevel dielectric (ILD)  106  as it is formed over the substrate  102 . In preferred examples, the ILD  106  is constructed from silicon oxide. However, it must be understood by one of ordinary skill in the art that the ILD  106  may be constructed from any other suitable dielectric material so long as the material is substantially robust and provides ample insulation. The Inter-Level Dielectric layers normally are abbreviated as ILD  1 , ILD  2  etc. Inter-Metal Dielectric (IMD) or Pre-Metal Dielectric (PMD) for the first post-device dielectric layer, are also frequently used to describe the integrated circuit architecture. 
         [0032]    Defined within the ILD  106  are a plurality of contact holes  108  configured to be filled with a conductive material (i.e., forming a plug), thus allowing substantially direct electrical access between the metallization lines and the transistors (i.e., active components). In one implementation, the contact plugs are formed by depositing a layer of tungsten and then planarizing away the overburden tungsten from over the top surface of the ILD  106 . 
         [0033]    Although in this embodiment the contact holes  108  are filled with tungsten, one of ordinary skill in the art should appreciate that the contact holes  108  may be filled with any conductive material so long as their function of providing direct layer to layer electrical access between the metal interconnects and the active components can be achieved. Additionally, although in this example a CMP operation is used to remove the overburden materials, one of ordinary skill in the art must appreciate that any other planarization or material removal operation may be implemented. 
         [0034]    Following the planarization operation, a passivation layer  116  is formed over the ILD  106  so as to protect active components from corrosion and chemical reactions during the subsequent fabrication operations. In one example, the passivation layer  116  is made out of silicon nitride (SiN). 
         [0035]      FIG. 1B  depicts the semiconductor structure  100  of  FIG. 1A  further including a first sacrificial layer  110   a  having a plurality of fabricated metallization lines  115 , via holes  112 , and stub holes  124   a , in accordance with one embodiment of the present invention. As shown, overlaying the passivation layer  116  is the first sacrificial layer  110   a , which in one embodiment, may be formed through a Chemical Vapor Deposition (CVD) method. The first sacrificial layer  110   a  is preferably a silicon dioxide (SiO 2 ) deposited using any proper deposition process. In one embodiment, silicon dioxide may be deposited by decomposing a tetraethylorthosilicate “TEOS” Si(OC 2 H 5 ) 4  reactant using any suitable deposition method. Suitable deposition methods may include a chemical vapor deposition (CVD), a low pressure chemical vapor deposition (LPCVD), an atmospheric pressure chemical vapor deposition (APCVD), sub atmospheric chemical vapor deposition (SACVD), a plasma enhanced chemical vapor deposition (PECVD), etc. Although in this embodiment the sacrificial layer is made out of silicon dioxide, in a different example, the sacrificial layer  110   a  may be made out of any inert to copper material that is mechanically firm to be implemented during the fabrication process. 
         [0036]    Following the formation of the first sacrificial layer  110   a , a copper dual damascene process is performed to form interlayer metallization lines and conductive vias within the first sacrificial layer  110   a . In one implementation, first, the plurality of via holes  112  is formed. This task may be achieved by overlaying the first sacrificial layer  110   a  with a photoresist mask and subsequently etching and removing the unprotected portions of the first sacrificial layer  110   a  down to the surface of the passivation layer  116  and to the contact holes  108 . 
         [0037]    In preferred embodiments, in addition to the via holes  112 , a plurality of stub holes  124   a  are formed within the first sacrificial layer  110   a  implementing almost the same processes as the via holes  112 . That is, a plurality of stub-shaped patterns are masked over the surface of the first sacrificial layer  110   a , which are subsequently etched and removed down to the surface of the passivation layer  116 . Next, implementing the same techniques, a plurality of trenches  114  are formed within the first sacrificial layer  110   a  such that each of the plurality of trenches  114  substantially aligns with a masked via hole  112 . 
         [0038]    At this point, a layer of metal is deposited onto the surface of the first sacrificial layer  110   a  and into the trenches  114 , via holes  112 , and stub holes  124   a . In one embodiment, the trenches  114  are filled with copper through sputtering and electroplating processes, thus defining a plurality of metallization lines  115  and conductive vias. In one implementation, prior to metal deposition, a barrier layer (not shown in this Figure) may be deposited over the surface of the first sacrificial layer  110   a  and into the plurality of via holes  112  and trenches  114 . Exemplary metals that may be used to form a barrier layer typically include a tantalum material or a tantalum nitride material, or a combination of both. Thereafter, a copper seed layer (also not shown in this drawing) maybe deposited on the barrier layer in order to line the inner walls and surfaces within the plurality of via holes  112  and trenches  114 . The seed layer is configured to establish a good electrical contact for subsequent copper electrodeposition process. 
         [0039]    As fabricated, the function of each of the plurality of stubs  125   a  is to support the multi-layer structure of the semiconductor structure  100 . Thus, as opposed to the plurality of conductive vias  113  which are configured to provide electrical connection between different interconnect layers, the function of the stubs  125   a  is to provide a semiconductor structure having high structural integrity. Accordingly, it should be understood that a semiconductor device might have any number of stubs  125  distributed so as to achieve the best structural support arrangement. Furthermore, although in this embodiment the stub holes  124   a  are filled with copper, in a different example, the stub holes  124   a  may be filled with any non-sacrificial material or metal so long as the material used is sufficiently robust to support a subsequently formed passivation-capping layer  118 . 
         [0040]    Following the formation of the metallization lines  115 , conductive vias  113 , and stubs  125   a  a CMP operation is performed on the copper layer that overlies the surface of the first sacrificial layer  110   a  so as to remove overburden copper from the surface of the sacrificial layer  110   a.    
         [0041]    Reference is now made to  FIG. 1C , which depicts the semiconductor structure  100  of  FIG. 1B  having a plurality of fabricated sacrificial layers  110   b - 110   g , each including a plurality of stubs  125   b - 125   g , in accordance with one embodiment of the present invention. In one exemplary embodiment, the second, third, fourth, fifth, sixth, and seventh sacrificial layers  110   b - 110   g  and each of their respective trenches  114 , metallization lines  115 , via holes  112 , conductive vias  113 , stub holes  124   b - 124   g , and stubs  125   b - 125   g  are formed in the same manner and from the same materials as the first sacrificial layer  110   a  and its respective trenches  114 , metallization lines  115 , via holes  112 , conductive vias  113 , stub holes  124   a , and stubs  125   a.    
         [0042]    As illustrated, the plurality of stubs  125   b - 125   g  are formed within the first through seventh sacrificial layers  110   b - 110   g , respectively, such that each of the plurality of stubs  125   b - 125   g  is aligned with one of the plurality of stubs  125   a . That is, each of the plurality of stubs  125   g , together with its respective group of aligned stubs formed within each of the sacrificial layers  110   a - 110   f , defines a substantially contiguous supporting column, as each extends from the surface of the passivation layer  116  to the surface of the seventh sacrificial layer  110   g . In this manner, the stubs  125   a - 125   g  provide adequate support for a subsequently formed passivation-capping layer  118 . 
         [0043]      FIG. 1D  is a cross-sectional view of a post-etched multi-layer semiconductor structure  100 ′ having air as a dielectric material, in accordance with one embodiment of the present invention. As illustrated, the sacrificial layers  110   a - 110   g  of semiconductor  100  depicted in  FIG. 1C  have been etched. For example, the sacrificial layers  110   a - 110   g  may be removed through a wet etch process in which a mixture of hydrofluoric acid (HF) and de-ionized water (i.e., diluted HF (DHF)) is applied to the sacrificial layers  110   a - 110   g  thus removing the sacrificial material surrounding the conductive vias  113 , metallization lines  115 , and stubs  125   a - 125   g . In one implementation, the sacrificial material may be removed by first immersing the semiconductor structure  100  in a tank containing a mixture of hydrofluoric acid and de-ionized water for a specific time. Thereafter, the mixture of hydrofluoric acid and de-ionized water is removed by rinsing the semiconductor structure  100 , which is followed by performing a spin rinsing operation. In another embodiment, the immersion etching maybe enhanced by utilizing heaters and agitation devices (e.g., stirrers, ultrasonic, megasonic, etc.). In an alternate embodiment, rather than immersing the semiconductor structure  100 , the etching operation may be performed through the spraying of the mixture of hydrofluoric acid and de-ionized water onto the semiconductor structure  100 , thus creating a voided interconnect metallization structure. However, in another embodiment, an oxide plasma etch approach may be used to etch out the sacrificial material. 
         [0044]    Preferably, the concentration of the hydrofluoric acid in the mixture of hydrofluoric acid and de-ionized water ranges between approximately about 0.1% to 5.0%. However, one of ordinary skill in the art must appreciate that the concentration of the hydrofluoric acid in the mixture of hydrofluoric acid and de-ionized water may be any appropriate concentration so long as the mixture has the capability to remove the sacrificial layers without effecting the metallization lines. Furthermore, although the mixture of hydrofluoric acid and de-ionized water has been used to perform the etching operation, it must be understood that any appropriate etchant having a suitable concentration may be used so long as the etchant is capable of removing the sacrificial materials. 
         [0045]    It must be noted that the stubs  125   a - 125   g , conductive vias  113 , and the metallization lines  115  are configured not to be effected by the etching operation. That is, the stubs  125   a - 125   g , conductive vias  113 , and the metallization lines  115  are constructed from materials that are substantially inert to the mixture of hydrofluoric acid and de-ionized water. Furthermore, the removal of the sacrificial materials does not effect the electrical performance of the active components, as the sacrificial materials are protected by the passivation layer  116 . Additionally, although in this embodiment air has been used as an insulator, in a different implementation, any gas having a low-K dielectric constant (e.g., nitrogen, etc.) or any inert gas (e.g., neon, argon, etc.) may be used to substitute the sacrificial materials. 
         [0046]    Reference is now made to  FIG. 1E  illustrating the post-etched air-dielectric multi-layer semiconductor structure  100 ′ of  FIG. 1D  further including the passivation-capping layer  118 , in accordance with one embodiment of the present invention. As illustrated, in one example, the passivation-capping layer  118  may include a plurality of concavities  116   b   1  and  116   b   2  substantially formed in the voids created by the removal of the sacrificial layer  125   g.    
         [0047]    As shown, the passivation-capping layer  118  substantially concludes the fabrication of the semiconductor structure  100 ′ and serves two purposes. Besides functioning as a sealing passivation layer, thus preventing corrosion and chemical reaction within the semiconductor structure  100 ′, the passivation-capping layer  118  also functions as a cap for the semiconductor structure  100 ′. As such, the plurality of substantially contiguous columns of stubs  125   a - 125   g  provides sufficient support for the passivation-capping layer  118 . In this manner, as fabricated, the passivation-capping layer  118 , the plurality of stubs  125   a - 125   g , the plurality of metallization lines  115 , the plurality of conductive vias  113 , and the substrate  102  create a semiconductor structure that has high structural integrity with reduced capacitive delay. 
         [0048]      FIG. 1F-1  is a cross-sectional view of the post-etched multi-layer semiconductor structure  100 ′ of  FIG. 1D  having low-K dielectric layers, in accordance with one embodiment of the present invention. As illustrated, the sacrificial layers  110   a - 110   g  have been substituted with dielectric layers  110   a ′- 110   g ′ made out of a low-K dielectric material  111 . The low-K dielectric material  111  is configured to be a highly porous dielectric material preferably having a dielectric constant substantially close to that of air. As such, the low-K dielectric material includes a plurality of air-filled pores  111 ′. In one embodiment, the low-K dielectric material  111  may be Nanoglass™ from AppliedSignal Electronic Materials of Los Gatos, Calif., which is a silicon dioxide material with air-filled pores having a diameter as small as 10 nanometers. In a different embodiment, any porous low-K dielectric material may be implemented (e.g., a spin-on polymer, a CVD deposited organosilicate glass (OSG), a spin-on polymer and a CVD deposited OSG together spin on polymer combined with gas phase evaporation technique, spin on polymer combined with supercritical drying technique, porous silica aerogels, Dow Corning hydrogen silsesquioxan based porous XLK dielectric, silicon evaporation/oxidating deposition in argon/oxygen atmosphere, etc.). For porous materials, the effective dielectric constant ranges between the dielectric constant of air (i.e., 1) and the dielectric constant of the dense material Dow Corning XLK (i.e., 2.2). Hence, in preferred embodiments, the dielectric constant of the porous low-K dielectric material ranges from approximately about 1 to approximately about 4. 
         [0049]    In one exemplary implementation, the post-etched semiconductor structure  100 ′ is filled with the low-K dielectric material  111  through a spin on process or CVD process. Preferably, the low-K dielectric material  111  that is in the form of a liquid is introduced into the post-etched semiconductor structure  100 ′ with pressure. In this manner, the low-K dielectric material  111  penetrates through almost all the etched regions of the semiconductor structure  100 ′ approximately down to the first dielectric layer  110   a ′. In doing so, the post-etched regions of the semiconductor structure  100 ′ can be filled with low-K dielectric material  111  such that substantially all the existing voids are filled with the low-K material  111 . However, it will be understood by one of ordinary skill in the art that depending on the required mechanical strength of the semiconductor structure  100 ″ and the low-K dielectric material, the low-K dielectric material  111  may be introduced into the semiconductor structure  100 ′ such that some void still remains subsequent to the filling operation. For instance, in one aspect, the post-etched semiconductor structure  100 ′ may be filled such that substantially the upper dielectric layers are filled with the low-K dielectric material  111  while the lower dielectric layers remain almost vacant. 
         [0050]    Following the introduction of the low-K dielectric material  111 , as illustrated in  FIG. 1F-2 , the post-etched low-K dielectric semiconductor structure  100 ″ is covered with a passivation-capping layer  118 ′, in accordance with one implementation of the present invention. As shown, the passivation-capping layer  118 ′ functions as a sealing passivation layer as well as a lid. As depicted, the passivation-capping layer  118 ′, the first through seventh dielectric layers  110   a ′- 110   g ′, and the substrate  102  form a semiconductor structure  100 ″ that has high structural integrity with low capacitive delay.  FIG. 1F-3  illustrates the post-etched multi-layer semiconductor structure  100 ′ of  FIG. 1D  having low-K dielectric layers, as covered by the passivation-capping layer  118 ′, in accordance with another embodiment of the present invention. 
         [0051]    Although in these embodiments the sacrificial layers  110   a - 110   g  have been made out of silicon dioxide, it will be known to those skilled in the art that any inert to copper material that is mechanically firm to be implemented during the fabrication process may be used to form the sacrificial layers. It should be noted that the function of the sacrificial layers is to provide good mechanical support during the fabrication of the multi-layer interconnect structures. This mechanical support is needed so that the interconnect structures can withstand the structural stresses and pressures that occur during CMP and other operations. 
         [0052]    Reference is now made to  FIG. 2 , which depicts a flowchart diagram  200  of the process operations performed to fabricate an air dielectric semiconductor structure having a plurality of copper metallization lines and supporting stubs, in accordance to one embodiment of the present invention. The method begins at an operation  202  in which a substrate having active regions is provided. Next, in operation  204 , shallow trench isolation regions are formed in the substrate followed by operation  206  in which transistor structures are formed in the active regions. 
         [0053]    Once the transistors are defined, the method continues on to operation  208  in which the ILD is formed over the substrate surface. Thereafter, in operation  210 , tungsten contact plugs configured to provide direct access between the metallization lines and the transistors are formed through the ILD. The formation of tungsten contact plugs requires the deposition of tungsten onto the surface of the ILD as well as into the vias to form tungsten plugs. Accordingly, in the subsequent operation  212 , the tungsten overlying the ILD surface is planarized thus removing the overburden tungsten. This operation is then followed by operation  214  in which a passivation layer is formed over the ILD to protect the active components of the substrate from contamination. 
         [0054]    At this point, the method continues to operation  216  in which a sacrificial layer is formed over the previously formed layer. Thereafter, in operation  216 , via holes and trenches are formed in the sacrificial layer. Preferably, this is achieved through a via-first, trench-first or a buried-via dual damascene process. Following the formation of via holes and trenches, in operation  220 , stub holes are formed through the sacrificial layer so as to support the multi-layer semiconductor structure. In this manner, the supporting stubs are formed within each sacrificial layer thus providing support for a subsequently formed passivation-capping layer. In some cases, the stub holes can be formed at the same time the via holes are formed. 
         [0055]    Subsequent to the formation of the via holes, trenches, and stub holes, in operation  222 , copper is applied onto the surface of the sacrificial layer and into the via holes, trenches, and stub holes, thus filling the trenches, via holes, and stub holes. As copper is deposited into the via holes, trenches, and stub holes, overburden copper remains on the surface of the sacrificial layer. Accordingly, in operation  224 , the overburden copper is planarized and the substrate surface is cleaned thus removing any contaminants remaining on the substrate surface. Preferably, the overburden copper is planarized through a chemical mechanical planarization (CMP) operation. It is important to note that the interconnect structure is very stable during the CMP operation since sacrificial material is still present. 
         [0056]    Thereafter, the method continues on to operation  226  in which it is determined whether any additional metallization lines need to be formed. If it is determined that additional metallization lines are required, the method returns to operation  216  in which a sacrificial layer is formed over the previously formed layer. In contrast, if there is no need to form additional metallization lines, the method continues on to operation  228  in which the sacrificial layers not protected by the passivation layer are etched and removed. The removal of the sacrificial material is achieved by applying a mixture of HF and de-ionized water to the multi-layer semiconductor structure. Finally, the method continues on to operation  230  in which a passivation-capping layer is formed over the last copper metallization layer and concludes the fabrication process. 
         [0057]    Another embodiment of the present invention can be understood from the flowchart  300  of  FIG. 3  depicting a flowchart diagram  300  of the process operations performed to fabricate a porous low-K dielectric semiconductor structure having copper metallization lines, in accordance to one embodiment of the present invention. This method begins by operation  302  in which a substrate having active regions is provided. Next, in operation  304  shallow trench isolation regions are formed into the substrate followed by operation  306  in which transistor structures are formed in active regions. Subsequent to the formation of the transistor structures, in operation  308 , the ILD is formed over the substrate surface followed by forming of tungsten contact plugs through the ILD in operation  310 . Thereafter, the overburden tungsten overlaying the surface of the ILD is planarized. This is followed by operation  314  in which a passivation layer is formed over the ILD so as to protect the active components of the substrate. 
         [0058]    After the forming of the passivation layer, in operation  316  a sacrificial layer is formed over the previously formed layer which is followed by forming of via holes and trenches through the sacrificial layer in operation  318 . Thereafter, in operation  320 , copper is applied onto the surface of the sacrificial layer thus filling the trenches and via holes. In operation  322 , a planarization and cleaning operation is subsequently performed so as to remove overburden copper and contaminants from over the substrate surface. 
         [0059]    Proceeding to operation  324 , a determination is made as to whether any additional metallization lines need to be formed. If a determination is made that an additional metallization line is required, the method then continues on to operation  316 . Alternatively, the method continues to operation  326  in which sacrificial layers not protected by the passivation layer are etched and removed. The removal of sacrificial layers is achieved by applying a mixture of HF and de-ionized water or any other chemical suitable for dissolving the sacrificial layers to the multi-layer semiconductor structure. Subsequently, in operation  328 , the sacrificial layers are replaced by porous low-K dielectric material. Lastly, in operation  330 , a passivation-capping layer is formed over the last copper metallization layer thus concluding the fabrication process. 
         [0060]    Again, it should be noted that the interconnect structure will be mechanically stable during each of the CMP operations since dense sacrificial material is still present. Once there is no need for further CMP operations, the sacrificial material is removed. Once removed, the interconnect structure can be filled with low K dielectric materials or left as air dielectric. The low K dielectric or air will therefore provide for faster integrated circuit devices. 
         [0061]    Although the foregoing invention has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. For example, embodiments described herein have been primarily directed toward fabrication of semiconductor structures having copper metallization lines; however, it should be understood that the fabrication processes of the present invention are well suited for fabricating semiconductor structures having any type of metallization lines (e.g., aluminum, tungsten and other metals or alloys). Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalents of the appended claims.