Abstract:
An advanced back-end-of-line (BEOL) metallization structure is disclosed. The structure includes a diffusion barrier or cap layer having a low dielectric constant (low-k). The cap layer is formed of amorphous nitrogenated hydrogenated silicon cabride, and has a dielectric constant (k) of less than about 5. A method for forming the BEOL metallization structure is also disclosed, where the cap layer is deposited using a plasma-enhanced chemical vapor deposition (PE CVD) process. The invention is particularly useful in interconnect structure comprising low-k dielectric material for the inter-layer dielectric (ILD) and copper for the conductors.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to the manufacture of high speed semiconductor microprocessors, application specific integrated circuits (ASICs), and other high speed integrated circuit devices. More particularly, this invention relates to an advanced back-end-of-line (BEOL) integration scheme for semiconductor devices using low-k dielectric materials. The invention is specifically directed to an advanced BEOL metallization structure which includes a cap layer having a low dielectric constant (low-k), and a method for forming the BEOL metallization structure.  
         BACKGROUND OF THE INVENTION  
         [0002]    Metal interconnections in very large scale integrated (VLSI) or ultra-large scale integrated (ULSI) circuits typically consist of interconnect structures containing patterned layers of metal wiring. Typical integrated circuit (IC) devices contain from three to fifteen layers of metal wiring. As feature size decreases and device areal density increases, the number of interconnect layers is expected to increase.  
           [0003]    The materials and layout of these interconnect structures are preferably chosen to minimize signal propagation delays, hence maximizing the overall circuit speed. An indication of signal propagation delay within the interconnect structure is the RC time constant for each metal wiring layer, where R is the resistance of the wiring and C is the effective capacitance between a selected signal line (i.e., conductor) and the surrounding conductors in the multilevel interconnect structure. The RC time constant may be reduced by lowering the resistance of the wiring material. Copper is therefore a preferred material for IC interconnects because of its relatively low resistance. The RC time constant may also be reduced by using dielectric materials with a lower dielectric constant, k.  
           [0004]    State-of-the-art dual damascene interconnect structures comprising low-k dielectric material and copper interconnects are described in “A High Performance 0.13 μm Copper BEOL Technology with Low-k Dielectric,” by R. D. Goldblatt et al., Proceedings of the IEEE 2000 International Interconnect Technology Conference, pp. 261-263. A typical interconnect structure using low-k dielectric material and copper interconnects is shown in FIG. 1. The interconnect structure comprises a lower substrate  10  which may contain logic circuit elements such as transistors. A dielectric layer  12 , commonly known as an inter-layer dielectric (WLD), overlies the substrate  10 . In advanced interconnect structures, ILD layer  12  is preferably a low-k polymeric thermoset material such as SiLK™ (an aromatic hydrocarbon thermosetting polymer available from The Dow Chemical Company). An adhesion promoter layer  11  may be disposed between the substrate  10  and ILD layer  12 . A layer of silicon nitride  13  may be disposed on ILD layer  12 . Silicon nitride layer  13  is commonly known as a hardmask layer or polish stop layer. At least one conductor  15  is embedded in ILD layer  12 . Conductor  15  is typically copper in advanced interconnect structures, but may alternatively be aluminum or other conductive material. A diffusion barrier liner  14  may be disposed between ILD layer  12  and conductor  15 . Diffusion barrier liner  14  is typically comprised of tantalum, titanium, tungsten or nitrides of these metals. The top surface of conductor  15  is made coplanar with the top surface of silicon nitride layer  13 , usually by a chemical-mechanical polish (CMP) step. A cap layer  16 , also typically of silicon nitride, is disposed on conductor  15  and silicon nitride layer  13 . Silicon nitride cap layer  16  acts as a diffusion barrier to prevent diffusion of copper from conductor  15  into the surrounding dielectric material.  
           [0005]    A first interconnect level is defined by adhesion promoter layer  11 , ILD layer  12 , silicon nitride layer  13 , diffusion barrier liner  14 , conductor  15 , and cap layer  16  in the interconnect structure shown in FIG. 1. A second interconnect level, shown above the first interconnect level in FIG. 1, includes adhesion promoter layer  17 , ILD layer  18 , silicon nitride layer  19 , diffusion barrier liner  20 , conductor  21 , and cap layer  22 . The first and second levels may be formed by conventional damascene processes. For example, formation of the second interconnect level begins with deposition of adhesion promoter layer  17 . Next, the ILD material  18  is deposited onto adhesion promoter layer  17 . If the ILD material is a low-k polymeric thermoset material such as SiLK™, the ILD material is typically spin-applied, given a post apply hot bake to remove solvent, and cured at elevated temperature. Next, silicon nitride layer  19  is deposited on the ILD. Silicon nitride layer  19 , ILD layer  18 , adhesion promoter layer  17  and cap layer  16  are then patterned, using a conventional photolithography and etching process, to form at least one trench and via. The trenches and vias are typically lined with diffusion barrier liner  20 . The trenches and vias are then filled with a metal such as copper to form conductor  21  in a conventional dual damascene process. Excess metal is removed by a CMP process. Finally, silicon nitride cap layer  22  is deposited on copper conductor  21  and silicon nitride layer  19 .  
           [0006]    However, silicon nitride has a relatively high dielectric constant of about 6 to 7. Fringing electric fields between the copper conductors are known to be present in regions of the copper where a higher-k cap/diffusion barrier film such as silicon nitride is present. When a material having a low dielectric constant of about 2 to 3 is used for the ILD, the effective capacitance of the metal conductors is increased by using a higher-k silicon nitride cap/diffusion barrier layer, resulting in decreased overall interconnect speed. The effective capacitance is also increased by using a higher-k silicon nitride polish-stop layer.  
           [0007]    In addition, interconnect structures using a silicon nitride hardmask layer may suffer from decreased reliability and higher failure rates. Interconnect structures are typically subjected to testing under accelerated stress conditions in order to identify weak points within the structure. Temperatures of about 200 to 300° C. are employed to accelerate the rate of processes leading to failure. One class of tests uses high humidity conditions to accelerate oxidation by water vapor, and another class uses higher current density to accelerate the effects of current flow on the metal interconnect structures. Interconnect structures using low-k dielectric material and copper conductors along with a silicon nitride hardmask layer suffer from unacceptably high failure rates when subjected to these accelerated stress conditions.  
           [0008]    An alternative material for cap layers  16  and  22  is an amorphous hydrogenated silicon carbide material (Si x C y H z ), one example being the material known as Blok™ (an amorphous film composed of silicon, carbon and hydrogen, which is available from Applied Materials, Inc.). Si x C y H z  has a dielectric constant of less than 5, which is much lower than that of silicon nitride. Thus, in an interconnect structure using Si x C y H z  for the cap layer, the effective capacitance of the metal conductors is decreased, and the overall interconnect speed is increased.  
           [0009]    However, it has been discovered that electromigration rates are relatively high in interconnect structures comprising copper conductors and low-k ILD with a cap layer of Si x C y H z . These high electromigration rates often result in rapid failure of the IC chip.  
           [0010]    Thus, there is a need in the art for an interconnect structure utilizing copper or aluminum conductors, a low-k ILD having a dielectric constant of about 2 to 3, and a cap layer which has a dielectric constant of less than about 5, and which also provides effective oxygen barrier properties.  
         SUMMARY OF THE INVENTION  
         [0011]    The problems described above are addressed through use of the present invention, which is directed to an interconnect structure formed on a substrate. In a preferred embodiment, the structure comprises a dielectric layer overlying the substrate; a hardmask layer on the dielectric layer, said hardmask layer having a top surface; at least one conductor embedded in said dielectric layer and having a surface coplanar with the top surface of said hardmask layer; and a cap layer on said at least one conductor and on said hardmask layer, said cap layer having a bottom surface in strong adhesive contact with said conductor, wherein said cap layer is formed of a material including silicon, carbon, nitrogen and hydrogen.  
           [0012]    In an alternative embodiment, the structure comprises a dielectric layer overlying the substrate, said dielectric layer having a top surface; a conductor embedded in said dielectric layer and having a surface coplanar with the top surface of said dielectric layer; and a cap layer on said conductor, wherein said cap layer is formed of a material including silicon, carbon, nitrogen and hydrogen.  
           [0013]    The present invention is also directed to a method of forming an interconnect structure on a substrate. In one embodiment, the method comprises the steps of: depositing a dielectric material on the substrate, thereby forming a dielectric layer, said dielectric layer having a top surface; forming an opening in said dielectric layer; filling said opening with a conductive material, thereby forming a conductor, said conductor having a surface coplanar with the top surface of said dielectric layer; and depositing a cap material on said conductor, said cap material including silicon, carbon, nitrogen and hydrogen, thereby forming a cap layer.  
           [0014]    In another embodiment, the method comprises the steps of: depositing a dielectric material on the substrate, thereby forming a dielectric layer; depositing a hardmask material on said dielectric layer, thereby forming a hardmask layer, said hardmask layer having a top surface; forming an opening in said hardmask layer and said dielectric layer; filling said opening with a conductive material, thereby forming a conductor, said conductor having a surface coplanar with the top surface of said hardmask layer; and depositing a cap material on said conductor, said cap material including silicon, carbon, nitrogen and hydrogen, thereby forming a cap layer.  
           [0015]    The cap layer of the invention has a dielectric constant of less than about 5. When used in combination with a low-k dielectric material having a dielectric constant of less than about 3, and with an optional hardmask layer formed of a material having a dielectric constant less than about 5, the effective capacitance of the interconnect structure is reduced as compared to prior art structures. This lower effective capacitance results in an improvement in overall IC chip speed.  
           [0016]    In addition, the cap layer of this invention provides improved oxygen barrier properties. By providing an effective barrier to oxygen, the cap layer protects the conductor from oxygen diffusion and the formation of oxides on the conductor surface. Elimination of such oxides is believed to inhibit copper transport, thereby lowering electromigration rates and resulting in reduced IC chip failures. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    The features of the invention believed to be novel and the elements characteristic of the invention are set forth with particularity in the appended claims. The drawings are for illustration purposes only and are not drawn to scale. Furthermore, like numbers represent like features in the drawings. The invention itself, however, both as to organization and method of operation, may best be understood by reference to the detailed description which follows, taken in conjunction with the accompanying drawings, in which:  
         [0018]    [0018]FIG. 1 is a schematic cross-sectional view of a partially-fabricated integrated circuit device illustrating a prior art interconnect structure;  
         [0019]    [0019]FIG. 2 is a schematic cross-sectional view of a partially-fabricated integrated circuit device illustrating an interconnect structure in accordance with a preferred embodiment of the invention;  
         [0020]    [0020]FIG. 3 is a schematic cross-sectional view of a partially-fabricated integrated circuit device illustrating an interconnect structure in accordance with an alternative embodiment of the invention; and  
         [0021]    FIGS.  4 ( a )- 4 ( i ) illustrate a method for forming the interconnect structure of FIG. 2. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    The invention will now be described by reference to the accompanying figures. In the figures, various aspects of the structures have been shown and schematically represented in a simplified manner to more clearly describe and illustrate the invention. For example, the figures are not intended to be to scale. In addition, the vertical cross-sections of the various aspects of the structures are illustrated as being rectangular in shape. Those skilled in the art will appreciate, however, that with practical structures these aspects will most likely incorporate more tapered features. Moreover, the invention is not limited to constructions of any particular shape.  
         [0023]    Although certain aspects of the invention will be described with respect to a structure comprising copper, the invention is not so limited. Although copper is the preferred conductive material, the structure of the present invention may comprise any suitable conductive material, such as aluminum.  
         [0024]    Referring to FIG. 2, a preferred embodiment of the interconnect structure of this invention comprises a lower substrate  110  which may contain logic circuit elements such as transistors. A dielectric layer  112 , commonly known as an inter-layer dielectric (ILD), overlies the substrate  110 . An adhesion promoter layer  111  may be disposed between substrate  110  and ILD layer  112 . A hardmask layer  113  is preferably disposed on ILD layer  112 . At least one conductor  115  is embedded in ILD layer  112  and hardmask layer  113 . A diffusion barrier liner  114  may be disposed between ILD layer  112  and conductor  115 . The top surface of conductor  115  is made coplanar with the top surface of hardmask layer  113 , usually by a chemical-mechanical polish (CMP) step. A cap layer  116  is disposed on conductor  115  and hardmask layer  113 .  
         [0025]    A first interconnect level is defined by adhesion promoter layer  111 , ILD layer  112 , hardmask layer  113 , diffusion barrier liner  114 , conductor  115 , and cap layer  116  in the interconnect structure shown in FIG. 2. A second interconnect level, shown above the first interconnect level in FIG. 2, includes adhesion promoter layer  117 , ILD layers  118 , hardmask layer  119 , diffusion barrier liner  120 , conductor  121 , and cap layer  122 .  
         [0026]    ILD layers  112  and  118  may be formed of any suitable dielectric material, although low-k dielectric materials are preferred. Suitable dielectric materials include carbon-doped silicon dioxide (also known as silicon oxycarbide or SiCOH dielectrics); fluorine-doped silicon oxide (also known as fluorosilicate glass, or FSG); spin-on glasses; silsesquioxanes, including hydrogen silsesquioxane (HSQ), methyl silsesquioxane (MSQ) and mixtures or copolymers of HSQ and MSQ; and any silicon-containing low-k dielectric. Examples of spin-on low-k films with SiCOH-type composition using silsesquioxane chemistry include HOSP™ (available from Honeywell), JSR 5109 and 5108 (available from Japan Synthetic Rubber), Zirkon™ (available from Shipley Microelectronics), and porous low k (ELk) materials (available from Applied Materials). For this embodiment, preferred dielectric materials are organic polymeric thermoset materials, consisting essentially of carbon, oxygen and hydrogen. Preferred dielectric materials include the low-k polyarylene ether polymeric material known as SiLK™ (available from The Dow Chemical Company), and the low-k polymeric material known as FLARE™ (available from Honeywell). ILD layers  112  and  118  may each be about 100 nm to about 1000 nm thick, but these layers are each preferably about 600 nm thick. The dielectric constant for ILD layers  112  and  118  is preferably about 1.8 to about 3.5, and most preferably about 2.5 to about 2.9.  
         [0027]    Alternatively, ILD layers  112  and  118  may be formed of an organic polymeric thermoset material containing pores. If ILD layers  112  and  118  are formed of such porous dielectric material, the dielectric constant of these layers is preferably less than about 2.6, and is most preferably about 1.5 to 2.5. It is particularly preferred to use an organic polymeric thermoset material having a dielectric constant of about 1.8 to 2.2.  
         [0028]    Adhesion promoter layers  111  and  117  are preferably about 9 nm thick, and are composed of silicon and oxygen, with a very small carbon content. The adhesion promoter layer is preferably comprises a silane coupling agent, and is preferably prepared from a solution of an alkoxysilane molecule in a suitable solvent, which is then spin-coated onto the substrate. A preferred alkoxysilane molecule is vinyltriacetoxysilane. Other related molecules may also be used, including but not limited to vinyltrimethoxysilane, vinyltriethoxysilane, allyltrimethoxysilane, vinyldiphenylethoxysilane, norborenyltriethoxysilane, trivinyltriethoxysilane and other related silanes containing vinyl or allyl functions. When the preferred adhesion promoter molecule, vinyltriacetoxysilane, is used and the substrate is heated to about 185° C. for about 90 seconds to remove the solvent, a preferred adhesion promoter layer is formed which contains Si—O bonds as detected by infrared spectroscopy (IR) and x-ray photoelectron spectroscopy (XPS). This adhesion promoter layer does not contain acetoxy groups as determined by IR, while the vinyl groups (C═C double bonds) are readily detected by IR. Both the Si—O bonds and the vinyl groups are thermally stable up to 440° C., as determined by IR. Adhesion promoter layers  111  and  117  are preferably about 9 nm thick, although thinner layers of about 0.5 to 9 nm thick may be used within this invention. When an organic polymeric thermoset dielectric is coated onto this adhesion promoter layer, strong adhesion of the dielectric to the substrate is observed. Without this adhesion promoter layer, the adhesion is very weak.  
         [0029]    This embodiment includes hardmask layers  113  and  119 , which are preferably formed of amorphous hydrogenated silicon carbide comprising silicon, carbon and hydrogen. Specifically, these hardmask layers are preferably composed of about 20 to 32 atomic % silicon, about 20 to 40 atomic % carbon, and about 30 to 50 atomic % hydrogen. In other words, hardmask layers  113  and  119  preferably have the composition Si x C y H z , where x is about 0.2 to about 0.32, y is about 0.2 to about 0.4, and z is about 0.3 to about 0.5. A minor amount of oxygen (about 1 to 10 atomic %) may also be present in these hardmask layers. A particularly preferred composition for hardmask layers  113  and  119  is about 24 to 29 atomic % silicon, about 33 to 39 atomic % carbon, and about 34 to 40 atomic % hydrogen. This particularly preferred composition may be expressed as Si x C y H z , where x is about 0.24 to 0.29, y is about 0.33 to 0.39, and z is about 0.34 to 0.4. This Si x C y H z  hardmask layer has a dielectric constant of less than about 5, and preferably about 4.5. Hardmask layers  113  and  119  should be in strong adhesive contact with ILD layers  112  and  118 , respectively. Hardmask layers  113  and  119  are preferably in the range of about 20 to about 100 nm thick, and most preferably in the range of about 25 to about 70 nm thick.  
         [0030]    Conductors  115  and  121  maybe formed of any suitable conductive material, such as copper or aluminum. Copper is particularly preferred as the conductive material, due to its relatively low resistance. Copper conductors  115  and  121  may contain small concentrations of other elements. Diffusion barrier liners  114  and  120  may comprise one or more of the following materials: tantalum, titanium, tungsten and the nitrides of these metals.  
         [0031]    Cap layers  116  and  122  are formed of amorphous nitrogenated hydrogenated silicon carbide comprising silicon, carbon, nitrogen and hydrogen, and have a dielectric constant (k) of less than about 5, and preferably about 4.9. Specifically, these cap layers are preferably composed of about 20 to 34 atomic % silicon, about 12 to 34 atomic % carbon, about 5 to 30 atomic % nitrogen, and about 20 to 50 atomic % hydrogen. In other words, cap layers  116  and  122  preferably have the composition Si x C y N w H z , where x is about 0.2 to about 0.34, y is about 0.12 to about 0.34, w is about 0.05 to about 0.3, and z is about 0.2 to about 0.5. A particularly preferred composition for cap layers  116  and  122  is about 22 to 30 atomic % silicon, about 15 to 30 atomic % carbon, about 10 to 22 atomic % nitrogen, and about 30 to 45 atomic % hydrogen. This particularly preferred composition may be expressed as Si x C y N w H z , where x is about 2.2 to about 3, y is about 1.5 to about 3, w is about 1 to about 2, and z is about 3 to about 4.5. Cap layers  116  and  122  should be in strong adhesive contact with conductors  115  and  121  and hardmask layers  113  and  119 , respectively. Cap layers  116  and  122  are preferably in the range of about 5 to about 120 nm thick, and most preferably in the range of about 20 to about 70 nm thick.  
         [0032]    The cap layers of this invention, such as cap layers  116  and  122 , provide an improved barrier to copper atoms or ions migrating out of the copper conductors, and also provide an improved barrier to diffusion of oxygen species (such as O 2  and H 2 O) moving into the conductor. The latter oxidizing species are believed to be a principal source of failure of interconnect structures under accelerated stress conditions.  
         [0033]    At the interface between the cap layer and the conductor, such as between cap layer  116  and conductor  115 , the cap layer preferably contains less than about 1 atomic % oxygen. The oxygen concentration at this interface may be measured, for example, by Auger Electron Spectroscopy (AES) or by electron energy loss spectroscopy in a Transmission Electron Microscope (TEM). The reliability of the interconnect structure under accelerated stress conditions can be significantly improved by maintaining the oxygen content at this interface at less than about 1 atomic %. This can be achieved by subjecting the surface of the conductor to an ammonia plasma pre-clean step, which is described in more detail below.  
         [0034]    Alternatively, the cap layer may contain a higher nitrogen concentration at the interface between the cap layer and the conductor, such as between cap layer  116  and conductor  115 , than is present in the remainder of the cap layer. In other words, the bottom surface of the cap layer, which is that surface in contact with the conductor, may be enriched with nitrogen as compared to the bulk of the cap layer. The preferred nitrogen concentration at this interface is in the range of about 5 to 20 atomic %, more preferably in the range of about 10 to 15 atomic %. Nitrogen enrichment at this interface results from the ammonia plasma pre-clean step, which is described in more detail below. Nitrogen concentration at the interface may be measured by Auger electron spectroscopy (AES) depth profile, with the signal being calibrated by Rutherford backscattering spectroscopy (RBS).  
         [0035]    The interconnect structure of FIG. 2 may be formed by a damascene or dual damascene process, such as the process shown in FIGS.  4 ( a )- 4 ( i ). The process preferably begins with deposition of adhesion promoter layer  111  on substrate  110 , and is followed by deposition of ILD layer  112  on adhesion promoter layer  111 , as shown in FIG. 4( a ). Adhesion promoter layer  111  and ILD layer  112  may be deposited by any suitable method. For example, if the adhesion promoter layers are prepared from a solution of vinyltriacetoxysilane in a suitable solvent, the solution is spin coated onto the substrate, and the substrate is heated to about 185° C. for about 90 seconds to remove the solvent. If SiLK™ is used for ILD layer  112 , the resin may be applied by a spin-coating process, followed by a baking step to remove solvent and then a thermal curing step.  
         [0036]    Hardmask layer  113  is then deposited on ILD layer  112 , as shown in FIG. 4( a ). Hardmask layer  113  may be deposited by any suitable method, but is preferably deposited by plasma enhanced chemical vapor deposition (PE CVD) directly onto ILD layer  112 . The deposition preferably is performed in a PE CVD reactor at a pressure in the range of about 0.1 to 10 torr, most preferably in the range of about 1 to 10 torr, using a combination of gases that may include, but are not limited to, silane (SiH 4 ), ammonia (NH 3 ), nitrogen (N 2 ), helium (He), trimethyl silane (3MS), tetramethyl silane (4MS), or other methyl silanes and hydrocarbon gases. A typical deposition process uses a flow of 3MS in the range of about 50 to 500 sccm and a flow of He in the range of about 50 to 2000 sccm. The deposition temperature is typically within the range of about 150 to 500° C., most preferably in the range of about 300 to 400° C. The radio-frequency (RF) power is typically in the range of about 100 to 700 watts, and most preferably in the range of about 200 to 500 watts. The final deposition thickness is preferably in the range of about 5 to 100 nm, and most preferably in the range of about 25 to 70 nm. Hardmask layer  113  may function as a patterning layer to assist in later etching of ILD layer  112  to form a trench for conductor  115 . Hardmask layer  113  may also serve as a polish stop layer during a subsequent CMP step to remove excess metal.  
         [0037]    In FIG. 4( b ), at least one trench  115   a  is formed using a conventional photolithography patterning and etching process. In a typical photolithography process, a photoresist material (not shown) is deposited on hardmask layer  113 . The photolithography material is exposed to ultraviolet (UV) radiation through a mask, and then the photoresist material is developed. Depending on the type of photoresist material used, exposed portions of the photoresist may be rendered either soluble or insoluble during development. These soluble portions of the photoresist are then removed, leaving behind a photoresist pattern matching the desired pattern of trenches. Trench  115   a  is then formed by removing hardmask layer  113  and a portion of I)LD layer  112  by, for example, reactive ion etching (RIE), in areas not protected by the photoresist. Hardmask layer  113  may assist in this etching step as follows. Hardmask layer  113  may be etched first in areas not covered by the photoresist, then the photoresist may be removed, leaving behind a patterned hardmask layer  113  matching the photoresist pattern. Then, ILD layer  112  may be etched in areas not covered by hardmask layer  113 .  
         [0038]    Following formation of trench  115   a,  the trench is preferably lined with diffusion barrier liner  114 , and then a conductive material is deposited in trench  115   a  to form conductor  115 . Diffusion barrier liner  114  may be deposited by any suitable method, such as by physical vapor deposition (PVD) or “sputtering,” or by chemical vapor deposition (CVD). A preferred method for depositing diffusion barrier liner  114  is ionized PVD. The diffusion barrier liner may be a multilayer of metals and metal nitrides deposited by PVD and/or CVD. Conductive material  115  may deposited in trench  115   a  by any suitable method, such as by electroplating, PVD or CVD. Electroplating is the most preferred method for depositing copper conductive material  115 .  
         [0039]    Excess liner  114  and conductive material  115  may be removed in a CMP process, in which the top surface of conductor  115  is made coplanar with the hardmask layer  113 . Hardmask layer  113  may serve as a polish-stop layer during this CMP step, thereby protecting ILD layer  112  from damage during polishing.  
         [0040]    Cap layer  116  is then deposited on conductor  115  and hardmask layer  113 , as shown in FIG. 4( d ). Cap layer  116  is preferably deposited using a PE CVD process, in a reactor at a pressure in the range of about 0.1 to 20 torr, most preferably in a range of about 1 to about 10 torr, using a combination of gases that may include, but are not limited to, SiH 4 , NH 3 , N 2 , He, 3MS, 4MS, and other methyl silanes.  
         [0041]    Prior to deposition of cap layer  116 , a plasma cleaning step is preferably performed in the PE CVD reactor. A typical plasma cleaning step uses a source of hydrogen such as NH 3  or H 2  at a flow rate in the range of about 50 to 500 sccm, and is performed at a substrate temperature in the range of about 150 to 500° C., most preferably at a substrate temperature in the range of about 300 to 400° C., for a time of about 5 to 500 seconds and most preferably about 10 to 100 seconds. The RF power is in the range of about 100 to 700 watts, and most preferably in the range of about 200 to 500 watts during this cleaning step. Optionally, other gases such as He, argon (Ar) or N 2  may be added at a flow rate in the range of about 50 to 500 sccm.  
         [0042]    Cap layer  116  is then preferably deposited using 3MS or 4MS at a flow rate in the range of about 50 to 500 sccm, He at a flow rate in the range of about 50 to 2000 sccm, and N 2  at a flow rate in the range of about 50 to 500 sccm. The deposition temperature is preferably in the range of about 150 to 500° C., and most preferably in the range of about 300 to 400° C. The RF power is preferably in the range of about 100 to 700 watts, and most preferably in the range of about 200 to 500 watts. The final deposition thickness is preferably in the range of about 10 to 100 nm, and most preferable in the range of about 25 to 70 nm.  
         [0043]    FIGS.  4 ( a )- 4 ( d ) illustrate the formation of the first interconnect level, which consists of adhesion promoter layer  111 , ILD layer  112 , hardmask layer  113 , diffusion barrier liner  114 , conductor  115  and cap layer  116 . In FIG. 4( e ), the formation of the second interconnect level begins with deposition of adhesion promoter layer  117 , ILD layer  118  and hardmask layer  119 . Adhesion promoter layer  117  maybe deposited using the same method as that for adhesion promoter layer  111 . Likewise, ILD layer  118  may be deposited using the same method as that for ILD layer  112 , and hardmask layer  119  may be deposited using the same method as that for hardmask layer  113 .  
         [0044]    FIGS.  4 ( f ) and  4 ( g ) illustrate the formation of via  121   a  and trench  121   b.  First, at least one via  121  a may be formed in hardmask layer  119 , ILD layer  118 , adhesion promoter layer  117  and cap layer  116 , using a conventional photolithography patterning and etching process, as shown in FIG. 4( f ). Then, at least one trench  121   b  may be formed in hardmask layer  119  and a portion of ILD layer  118 , using a conventional photolithography process, as shown in FIG. 4( g ). Via  121   a  and trench  121   b  may be formed using the same photolithography process as that used to form trench  115   a.    
         [0045]    Alternatively, via  121   a  and trench  121   b  maybe formed by first patterning and etching a trench in hardmask layer  119  and ILD layer  118 , where the trench has a depth equal to the depth of trench  121   b,  but has a length equal to the length of trench  121   b  and the width of via  121   a  combined. Then via  121   a  maybe formed by etching through the remainder of ILD layer  118 , adhesion promoter layer  117  and cap layer  116 .  
         [0046]    Following formation of via  121   a  and trench  121   b,  the via and trench are preferably lined with diffusion barrier liner  120 , and then a conductive material is deposited in the via and trench to form conductor  121 , as shown in FIG. 4( h ). Diffusion barrier liner  120  may be deposited by the same method used for diffusion barrier liner  114 , and conductive material  121  may deposited by the same method used for conductor  115 . Excess liner  120  and conductive material  121  maybe removed in a CMP process, in which the top surface of conductor  121  is made coplanar with the hardmask layer  119 . Hardmask layer  119  may serve as a polish-stop layer during this CMP step, thereby protecting ILD layer  118  from damage during polishing.  
         [0047]    Cap layer  122  is then deposited on conductor  121  and hardmask layer  119 , as shown in FIG. 4( i ). Cap layer  122  may be deposited using the same PE CVD process as that for cap layer  116 .  
         [0048]    In an alternative embodiment shown in FIG. 3, the interconnect structure of this invention is shown without hardmask layers  113  and  119 , and without adhesion promoter layers  111  and  117 . In this embodiment, ILD layers  112  and  118  are preferably formed of a silicon-containing dielectric material, such as carbon-doped silicon dioxide (also known as silicon oxycarbide or SiCOH); fluorine-doped silicon oxide (also known as fluorosilicate glass or FSG); spin-on glasses; and silsesquioxanes. The dielectric material is preferably deposited by a chemical vapor deposition (CVD) process, and has a dielectric constant in the range of about 2.0 to 3.5, and most preferably about 2.5 to 3.2. All other materials in the interconnect structure of this embodiment may be the same as the corresponding materials in the interconnect structure shown in FIG. 2. In other words, ILD layers  112  and  118 , diffusion barrier liners  114  and  120 , conductors  115  and  121  and cap layers  116  and  122  may be formed of the same materials as discussed previously for these layers in the embodiment shown in FIG. 2. Moreover, these layers may be formed using the same processes as discussed previously in relation to FIGS.  4 ( a )- 4 ( i ). Cap layers  116  and  122  should be in strong adhesive contact with conductors  115  and  121  and ILD layers  112  and  118 , respectively.  
         [0049]    While the present invention has been particularly described in conjunction with a specific preferred embodiment and other alternative embodiments, it is evident that numerous alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. It is therefore intended that the appended claims embrace all such alternatives, modifications and variations as falling within the true scope and spirit of the present invention.