Patent Publication Number: US-6699783-B2

Title: Method for controlling conformality with alternating layer deposition

Description:
REFERENCE TO RELATED APPLICATION 
     The present application is a continuation of U.S. patent application Ser. No. 10/237,526, filed Sep. 6, 2002, which is a divisional of U.S. patent application Ser. No. 09/843,518, filed Apr. 26, 2001, issued Nov. 19, 2002 as U.S. Pat. No. 6,482,733, and claims priority under 35 U.S.C. §119(e) to U.S. provisional application No. 60/235,772 of Raaijmakers et al., filed Sep. 26, 2000 and to U.S. Provisional Application No. 60/204,231 of Raaijmakers et al., filed May 15, 2000. In addition, the present application is related to U.S. application Ser. No. 09/644,416, filed Aug. 23, 2000 and entitled CONFORMAL LINING LAYERS FOR DAMASCENE METALLIZATION. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to forming protective layers prior to alternating layer deposition (ALD). More particularly, the invention relates to forming sealing layers prior to high conformality ALD layers over porous layers in integrated circuits. 
     BACKGROUND OF THE INVENTION 
     When fabricating integrated circuits, layers of insulating, conducting and semiconducting materials are deposited and patterned to produce desired structures. “Back end” or metallization processes include contact formation and metal line or wire formation. Contact formation vertically connects conductive layers through an insulating layer. Conventionally, contact vias or openings are formed in the insulating layer, which typically comprises a form of oxide such as borophosphosilicate glass (BPSG) or an oxide formed from tetraethylorthosilicate (TEOS) precursors. The vias are then filled with conductive material, thereby interconnecting electrical devices and wiring above and below the insulating layers. The layers interconnected by vertical contacts typically include horizontal metal lines running across the integrated circuit. Such lines are conventionally formed by depositing a metal layer over the insulating layer, masking the metal layer in a desired wiring pattern, and etching away metal between the desired wires or conductive lines. 
     Damascene processing involves forming trenches in the pattern of the desired lines, filling or overfilling the trenches with a metal or other conductive material, and then etching the excess metal back to the insulating layer. Wires are thus left within the trenches, isolated from one another in the desired pattern. The etch back process avoids the more difficult photolithographic mask and etching processes of conventional metal line definition. 
     In an extension of damascene processing, a process known as dual damascene involves forming two insulating layers, typically separated by an etch stop material, and forming trenches in the upper insulating layer, as described above for damascene processing. After the trenches have been etched, a further mask is employed to etch contact vias downwardly through the floor of the trenches and the lower insulating layer to expose lower conductive elements where contacts are desired. 
     Conductive elements, such as gate electrodes, capacitors, contacts, runners and wiring layers, must each be electrically isolated from one another for proper integrated circuit operation. In addition to providing insulating layers around such conductive elements, care must be taken to prevent diffusion and spiking of conductive materials through the insulating layers, which can cause undesired short circuits among devices and lines. Protective barriers are often formed between via or trench walls and metals in a substrate assembly to aid in confining deposited material within the via or trench walls. Barriers are thus useful for damascene and dual damascene interconnect applications, particularly for small, fast-diffusing elements such as copper. 
     Candidate materials for protective barriers should foremost exhibit effective diffusion barrier properties. Additionally, the materials should demonstrate good adhesion with adjacent materials (e.g., oxide via walls, adhesion layers, etch stop layers and/or metallic materials that fill the vias and trenches). For many applications, a barrier layer is positioned in a current flow path and so must be conductive. Typically, barriers have been formed of metal nitrides (MN X ), such as titanium nitride (TiN), tantalum nitride (TaN), and tungsten nitride (WN), which are dense and adequately conductive for lining contact vias, wiring trenches, and other conductive barrier applications. 
     These lined vias or trenches are then filled with metal by any of a variety of processes, including chemical vapor deposition (CVD), physical vapor deposition (PVD), and electroplating. For effective conductivity and to avoid electromigration during operation, the metal of a contact or wiring layer should fill the via or trench without leaving voids or key holes. Completely filling deep, narrow openings with conductive material is becoming ever more challenging as integrated circuit dimensions are constantly scaled down in pursuit of faster operational processing speeds and lower power consumption. 
     As illustrated in FIGS. 1 and 2, utilizing a conductive barrier layer and/or other liners makes filling the trenches and vias of dual damascene processing even more difficult. FIG. 1 illustrates a dual damascene process in which an upper insulating layer  10  is formed over a lower insulating layer  12 , which is in turn formed over a conductive wiring layer  14 , preferably with an intervening dielectric diffusion barrier  15 . This dielectric barrier  15  serves to prevent copper or other conductive material of the underlying runner  14  from diffusing into the overlying dielectric layer  12 . 
     A mask is employed to pattern and etch trenches  16  in a desired wiring pattern. In the illustrated embodiment, the trench  16  is etched down to the level of an etch stop layer  19 , which is formed between the two insulating layers  10 ,  12 . This etch stop layer  19  is typically patterned and etched, prior to deposition of the upper insulating layer  10 , to form a hard mask that defines horizontal dimensions of desired contact vias that are to extend from the bottom of the trench  16 . Continued etching through the hard mask  19  opens a contact via  20  from the bottom of the trench  16  to the lower conductive wiring layer  14 . FIG. 1 also shows an upper etch stop or chemical mechanical polishing (CMP) stop layer  21  over the upper insulating layer  10  to stop a later planarization step, as will be appreciated by the skilled artisan. 
     Protective liners  22 , preferably formed of conductive material, are then formed on the exposed horizontal and sidewall surfaces. Typically, the liners  22  at least include a metal nitride, and may additionally include adhesion enhancing and seeding layers. For example, the liner  22  can comprise a tri-layer of Ti/TiN/Cu. In such a structure, the titanium layer serves to improve adhesion with exposed oxide sidewalls; the titanium nitride serves as a diffusion barrier; and a thin copper layer serves as a seed for later electroplating of copper. In other examples, the liners  22  can include tantalum nitride or tungsten nitride barriers. The skilled artisan will appreciate that other barrier materials can also be employed. 
     Conformal deposition of the liners  22 , however, is very difficult with conventional processing. For example, physical vapor deposition (PVD), such as sputtering, of a metal layer (for adhesion, barrier and/or seed layer) requires at least about 50 Å over all surfaces of the trench  16  and contact via  20 . Unfortunately, PVD of metal into high aspect ratio voids necessitates much greater deposition on the top surfaces of the workpiece to produce adequate coverage of the via bottom. For example, typical state-of-the-art trench and contact structures for dual damascene schemes require about 500 Å PVD metal in order for 50 Å of metal to reach the bottom and sidewalls of the contact via  20 . 
     This poor step coverage is a result of the high aspect ratio of voids formed for dual damascene processing in today&#39;s integrated circuit designs. The aspect ratio of a contact via is defined as the ratio of depth or height to width. In the case of dual damascene contacts, the trench  16  and contact via  20  together reach through two levels of insulating layers  10 ,  12 , such that the effective aspect ratio of the via  20  is very high. 
     Conventional deposition processes produce very poor step coverage (i.e., the ratio of sidewall coverage to field or horizontal surface coverage) of such high aspect ratio vias for a variety of reasons. Due to the directionality of PVD techniques, for example, deposition tends to accumulate more rapidly at upper corners  26  of the trench  16  and upper corners  28  of the via  20 , as compared to the via bottom  30 . As a result of the rapid build-up of deposited material on the upper surfaces of the structure, the liners occupy much of the conductive line width in the trench  16  and even more, proportionately, of the contact via  20 . These built-up corners  26 ,  28  then cast a shadow into the lower reaches of the structure, such that lower surfaces, and particularly lower corners, are sheltered from further deposition. Although PVD deposition can be directed more specifically to the via bottom, e.g., by collimation or by ionization of the depositing vapor, such additional directionality tends to sacrifice sidewall coverage. 
     Chemical vapor deposition (CVD) processes have been developed for certain metals and metal nitrides. CVD tends to exhibit better step coverage than PVD processes. In order for CVD processes to exhibit good step coverage, the reaction must be operated in the so-called “surface controlled” regime. In this regime, reaction species do not adhere to trench or via walls upon initial impingement. Rather, the species bounce off trench/via surfaces several times (e.g., 10-500 times) before reacting. 
     State-of-the-art CVD processes for depositing barrier layers at temperatures sufficiently low to be compatible with surrounding materials do not operate completely within the surface-controlled regime. Accordingly, even CVD processes tend to deposit far less material at the bottom of a dual damascene contact via  20  then on the upper surfaces and sidewalls of the structure. The upper corners of the trench  16  and the contact via  20  represent a high concentration of surface area to volume. Deposition upon the horizontal upper surfaces and adjacent vertical sidewall surfaces merge together to result in an increased deposition rate near the corners  26 ,  28 . Additionally, flowing reactants diffuse slowly into the confined spaces of the trench  16  and contact via  20 . Accordingly, the concentration of reactants reaching the via bottom  30  is far reduced relative to the concentration of reactants reaching upper surfaces of the structure. Thus, while somewhat improved relative to PVD, CVD step coverage of dual damascene structures remains uneven with most currently known low temperature CVD techniques. 
     In the pursuit of faster operational speeds and lower power consumption, dimensions within integrated circuits are constantly being scaled down. With continued scaling, the aspect ratio of contacts and trenches continues to increase. This is due to the fact that, while the width or horizontal dimensions of structures in integrated circuits continues to shrink, the thickness of insulating layers separating metal layers cannot be commensurately reduced. Reduction of the thickness in the insulating layers is limited by the phenomenon of parasitic capacitance, whereby charged carriers are slowed down or tied up by capacitance across dielectric layers sandwiched by conductive wires. As is known, such parasitic capacitance would become disabling if the insulating layer were made proportionately thinner as horizontal dimensions are scaled down. 
     With reference to FIG. 2, a scaled-down version of FIG. 1 is depicted, wherein like parts are referenced by like numerals with the addition of the suffix “a.” As shown, continued scaling leads to a more pronounced effect of uneven step coverage while lining dual damascene structures. Material build-up at the corners  28   a  of the contact via  20   a  quickly reduces the size of the opening, even further reducing the concentration of reactants that reach into the contact via  20   a . Accordingly, coverage of the via bottom surface  30   a  drops off even faster. Moreover, the percentage of the trench  16   a  occupied by the liner materials is much greater for the scaled down structure of FIG.  2 . Since the lining material is typically less conductive than the subsequent filler metal (e.g., copper), overall conductivity is reduced. Worse yet, cusps at the corners  28   a  of the contact via can pinch off before the bottom  30   a  is sufficiently covered, or during deposition of the filler metal. 
     Independently of efforts to improve barrier film uniformity are efforts to reduce the dielectric or permittivity constant (k) value of the interlevel dielectric (ILD) material. A reduced dielectric constant value results in less parasitic capacitance per unit thickness of the ILD, such that for a given circuit design tolerance for parasitic capacitance, a so-called “low k” material can provide a thinner ILD. “Low k” designates a material with a k value below that of silicon oxide (k≈4), the currently predominant ILD material in integrated circuit fabrication. Accordingly, the aspect ratio of contacts and trenches to be filled can be reduced and lining these openings becomes easier. 
     A variety of materials and techniques are being developed for producing low k films in integrated circuits. Deposition methods currently include spin-on deposition, CVD, plasma enhanced CVD (PECVD) and high density plasma (HDP) CVD, depending upon the characteristics desired. Some of the methods and films have been described by Laura Peters, “Pursuing the Perfect Low-k Dielectric”  Semiconductor International,  Vol. 21, No. 10 (September 1998), and the references cited therein. Some films have a k value from 3 to 3.5 such as hydrogen silsesquioxane (HSQ) and fluorinated oxides. Organic polymers, such as benzoncyclobutene (BCB) and polyarylene ethers (PAE), exhibit even lower k values between 2.5 and 3 range. Other work with polytetrafluoroethylene (PTFE) using spin-on techniques has achieved intrisic k values of about 1.9. Other companies have created nanoporous inorganic-organic hybrids. 
     Use of such low k materials as an ILD in an integrated circuit will considerably reduce the aspect ratios of openings in the ILD. Accordingly, lining such openings with adequate conformality should prove simpler as compared with lining openings with higher aspect ratios. 
     Integrating these new materials with existing technologies, however, introduces its own challenges. Among other requirements, low k films must exhibit high chemical, thermal and mechanical stability in the face of disparate adjacent materials and exposure to a variety of processing environments. ILD materials should be compatible with etching, deposition, cleaning and polishing processes in order to integrate reliably with a manufacturing process. As will be appreciated by the skilled artisan, integration of new materials and processes into established process flows is rarely a straightforward matter, as evidenced by complications arising from the introduction of copper lines into state-of-the-art integrated circuit designs. 
     It would accordingly be advantageous to provide low k material without changing the material characteristics of the ILD with each succeeding generation. One manner in which the k value of a material can be lowered without changing the material properties of the ILD is to make the material porous. In effect, porous dielectrics combine the dielectric strength of air (k≈1.0) with that of the dielectric material in which the pores are formed. Advantageously, the k value of a porous material is “tunable” in the sense that the k value can be altered without introducing new materials by changing the porosity of a material that has already been integrated. 
     Currently silicon oxide (k≈4) is widely used in process flows. Porous versions of silicon oxide or “silica” can have both a low k value and compatibility with current process flows. This has led to the development of classes of porous silica known as nanogels, aerogels, xerogels and mesogels. Similarly, newer low k materials, once integrated into process flows, can have their k values tuned by adjusting porosity of the low k material. With low k materials currently under development, it appears that achieving k values below 2.5 will likely involve providing a porous insulating material. 
     While low k films, and particularly porous low k films, can effectively lower the aspect ratio of openings in an ILD, room remains for improvements in lining technology. Although CVD and PVD may adequately line a low aspect ratio opening, non-conformality of conventional deposition techniques can still be problematic. 
     Accordingly, a need exists for more effective methods of lining openings in integrated circuits, particularly in the context of dual damascene metallization. 
     SUMMARY OF THE INVENTION 
     In satisfaction of this need, methods are provided herein for depositing lining materials into the high-aspect ratio trenches and contact vias of dual damascene metallization schemes. Advantageously, the methods attain high step coverage, such that only the minimum required thickness of the lining layer need be formed on all surfaces. Examples are provided for applying the methods to formation of one or more of adhesion, barrier and electroplating seed layers. 
     In general, the methods comprise cycles of alternating reactant phases, wherein each phase has a self-limiting effect. “Pure” metal layers, for example, can be formed by alternately adsorbing self-saturating halide- or organic-terminated metal monolayers and reducing the metal-containing monolayer. Metal nitrides suitable for conductive diffusion barriers can be formed by alternately adsorbing self-terminated metal-containing monolayers and conducting ligand exchange reactions, substituting nitrogen-containing species for halogen or organic tails on the metal-containing monolayers. Alternatively, the tails of the self-terminated metal-containing monolayer can be reduced or otherwise removed in an intermediate scavenger or getter phase prior to the nitrogen phase. 
     More particularly, the methods are applied to metallization structures formed in porous “low k” materials. Prior to the highly conformal self-saturating processes noted above, a sealing layer is first formed over exposed porous surfaces, blocking the pores. The conformal self-saturating processes cannot then penetrate the pores and the low k dielectric maintains its desired properties. Advantageously, the sealing layer can be metal and subsequent uniformly thick conductive layers can be formed within high-aspect ratio openings (e.g., trenches and vias), desirably as thin as possible consistent with their respective functions, and without risking short circuits through the porous insulating layer. The remaining volume within such openings is thus maximized, facilitating a greater proportionate volume of more highly conductive filler materials, such as copper for metal runners and integral contacts. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects of the invention will be readily apparent to the skilled artisan in view of the description below, the appended claims, and from the drawings, which are intended to illustrate and not to limit the invention, and wherein: 
     FIG. 1 is a schematic cross-section of a dual damascene structure having a conventional barrier layer lining the trench and contact via thereof; 
     FIG. 2 generally illustrates a lined dual damascene structure, similar to FIG. 1, for a scaled-down integrated circuit; 
     FIG. 3 is a flow chart generally illustrating a method of lining high aspect ratio, dual damascene structures prior to filling with a more highly conductive material; 
     FIG. 4 is an exemplary gas flow diagram for depositing a barrier layer, in accordance with a preferred embodiment of the present invention; and 
     FIGS. 5-13 are schematic cross-sections of a partially fabricated integrated circuit, generally illustrating the construction, lining and filling of a trench and via formed in insulating layers above a semiconductor substrate, in accordance with a preferred dual damascene process flow. 
     FIG. 5 shows a first or lower insulating layer  50  over a barrier layer  51  and a conductive element  52  which forms part of a lower interconnect layer. 
     FIG. 6 shows the structure of FIG. 5 after an etch stop layer or hard mask  54  is formed over the insulating layer  50 . 
     FIG. 7 shows the structure of FIG. 6 after a mask and etch process has transferred a pattern of openings  55  to the etch stop  54 . 
     FIG. 8 shows the structure of FIG. 7 after a second or upper insulating layer  56  has been formed over the etch stop layer  54  and a second etch or CMP stop layer  58  have been formed over the upper insulating layer  56 . 
     FIG. 9 a  shows the structure of FIG. 8 after trenches  60  (one shown) and contact vias  62  (one shown) have been etched to form a dual damascene structure. 
     FIGS. 9 b  through  9   e  show non-ideal damascene structures. FIG. 9 b  shows a non-capped dual damascene structure with a higher effective aspect ratio resulting from mask misalignment. 
     FIG. 9 c  shows a contact via  62  that is not fully landed, with a small but very high aspect ratio overetch hole  72 . 
     FIG. 9 d  shows cavities  80  resulting from a selective etch from the via bottom. 
     FIG. 9 e  depicts a damascene structure with a barrel-shaped profile in the trench  60  and via  61  resulting from attack on the insulating layers  50 ,  56  during removal of photoresist. 
     FIG. 10 a  shows the dual damascene structure of FIG. 9 a  after the trench  60  and via  62  have been lined with a sealing layer  148 . 
     FIG. 10 b  is an enlarged view of the via sidewall shown in cross-section, illustrating that the low k material  50  comprises a plurality of pores  120  with in a matrix  122  of insulating material. 
     FIG. 11 shows the dual damascene structure of FIG. 10 a  after additional lining with a lining layer  150  and a seed layer  155 . 
     FIG. 12 shows the dual damascene structure of FIG. 11 filled with a highly conductive metal  160 . 
     FIG. 13 shows the filled damascene structure of FIG. 12 with isolated lines  170  formed by planarization by chemical mechanical planarization (CMP) or other etch back process. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Though described in the context of certain preferred materials, it will be understood, in view of the disclosure herein, that the methods and structures described will have application to a variety of other materials for lining damascene structures. 
     As discussed in the Background section above, lining damascene structures, and particularly dual damascene structures, by physical vapor deposition (PVD) and conventional chemical vapor deposition (CVD) disadvantageously fills a large volume of the trenches and contact vias. Accordingly, less room is left for the highly conductive filler material to follow. Use of a thinner liner material would leave more room for highly conductive filler metals such as copper which would, in turn, increase conductivity and operational signal transmission speeds for the integrated circuit. Conventional methods such as PVD and CVD, by their very nature, produce thicker layers towards the upper end of the damascene structure than at the bottom. While much research has been devoted to obtaining more conformal step coverage of dual damascene trenches and contact vias, it is very difficult to supply the same concentration of reactant species (or PVD sputtered material) to all surfaces of such structures. In particular, it is difficult to supply the same concentration of depositing species at the upper surfaces of such structures as supplied to the bottom of deep, confined contact vias that extend from the bottom of an already-deep trench. 
     By providing almost perfect step coverage, the preferred embodiments advantageously obtain the minimum necessary thickness for the desired liner layers over all surfaces of trenches and contact vias in a dual damascene structure. Desirably, the methods of the preferred embodiment are less dependent upon the relative concentration of reactant species in upper regions as compared to lower regions of the trench and contact via. 
     With reference to FIGS. 5-8, insulating layers are formed over a semiconductor substrate, in accordance with the preferred embodiments. Referring initially to FIG. 5, a first or lower insulating layer  50  is formed over a barrier layer  51  and a conductive element  52 , which forms part of a lower interconnect layer in the illustrated embodiment. As will be appreciated by the skilled artisan, metallization schemes typically employ one metal composition throughout various wiring layers (e.g., copper interconnects or aluminum interconnects). The preferred embodiments are adaptable to many different materials, but certain embodiments are particularly adapted to lining damascene structures wherein the via floor or lower conductive element  52  comprises a highly conductive copper line. The first insulating layer  50  is preferably formed to a thickness sufficient to insulate the lower conductive element  52  from upper wiring structures to be formed. An etch stop layer or hard mask  54  (FIGS. 6-7) is formed over the lower insulating layer  50  and a second or upper insulating layer  56  (FIG. 8) is formed over the etch stop layer  54 . A second etch or CMP stop layer  58  (also known as a shield layer) is preferably also formed over the upper insulating layer  56 . 
     In the illustrated embodiment, each of the lower and upper insulating layers  50 ,  56  comprise dielectric materials with a thickness less than about 1.0 μm, more preferably less than about 0.8 μm, and most preferably less than about 0.6 μm. The skilled artisan will readily appreciate that the insulating layer can comprise any of a number of suitable dielectric materials. For example, dielectric materials have recently been developed that exhibit low permittivity (low k), as compared to conventional oxides. These low k dielectric materials include polymeric materials, porous materials and fluorine-doped oxides. The present methods of lining trenches and contact vias have particular utility in conjunction with such low k materials. 
     The illustrated insulating layers  50 ,  56  thus comprise low k materials, and more particularly porous low k materials, exhibiting a dielectric constant (k) less than about 3.5. Preferably, the k value of the insulating layers is less than about 3.0, more preferably less than about 2.5, and most preferably less than about 2.0. 
     As set forth in the Background section above, increasing porosity effectively lowers dielectric constant. Accordingly, maximum advantage of the low k material&#39;s reduction of parasitic capacitance occurs with maximum porosity. This advantage is balanced against issues of mechanical, chemical and thermal stability during further processing, some of which issues can be resolved by techniques independent of adjusting porosity. While the methods disclosed herein are applicable to insulating layers with any level of porosity, the porosity of the low k films  50 ,  56  is desirably greater than about 50%, more preferably greater than about 70% and most preferably greater than about 75%. 
     An exemplary porous low k material is a spin-on material commercially available under the trade name Nanoglass™ from Honeywell Advanced Microelectronic Materials (AMM) of Sunnyvale, Calif. (formerly Allied Signal). Nanoglass™ is a nanoporous silica with a k value between 2.5 and 1.3 for porosity levels of 50% to 90%. The currently available commercial version of Nanoglass™ has a dielectric constant value of about 2.0 with a porosity of about 70%. A study on an earlier version of Nanoglass™ (Nanoglass K2.2-A10B) found this less porous version of xerogel to have completely connected pores with an average pore size of about 4 nm (40 Å). Ryan et al., “Material property characterization and integration issues for mesoporous silica”  Proceedings of the IEEE  1999  International Interconnect Technology Conference  (1999), pp. 187-189. The skilled artisan will readily appreciate, however, that the methods disclosed herein are applicable a variety of other materials having different levels of porosity. 
     The etch stop layers  54 ,  58  of the illustrated embodiment each comprise a material exhibiting different etch rates relative to the insulating layers  50 ,  56 , allowing better control of etching processes. In the illustrated embodiment, the etch stop layers  54 ,  58  comprise silicon nitride (Si 3 N 4 ), preferably provided to a thickness of between about 100 Å and 700 Å, and more preferably between about 200 Å and 500 Å. The lower barrier layer  51  preferably also comprises Si 3 N 4 . It will be understood that the etch-stop layers  54 ,  58  can also serve to strengthen the underlying porous insulating layers  50 ,  56 . As disclosed in Ryan et al., cited in the previous paragraph, a 1,000 Å CVD oxide cap can be employed to improve resilience during subsequent CMP processing, and can also serve as the end-point when polishing overlying metal. 
     As discussed in the Background section above, after the lower insulating layer  50  and etch stop  54  are formed (FIGS.  5  and  6 ), a mask and etch process transfers a pattern of openings  55  (one shown in FIG. 7) to the etch stop  54 . The second or upper insulating layer  56  and optional CMP stop  58  are then formed over the hard mask  54 . 
     With reference now to FIG. 9 a , the substrate is masked and trenches  60  (one shown) are etched through the upper insulating layer  56 , preferably stopping on exposed portions of the first etch stop layer  54 . As will be understood by the skilled artisan, the trenches  60  are etched across the insulating layer  56  in patterns desired for metal lines, in accordance with an integrated circuit design. In the illustrated embodiment, the width of the trench is less than about 0.35 μm and more preferably less than about 0.25 μm. 
     Continued etching through the hard mask  54  defines contact vias  62  (one shown) extending downwardly from the bottom of the trench and through the lower insulating layer  50  to expose conductive elements below (e.g., metal line  52 ). The contact vias  62  are defined by the openings  55  in the hard mask  54  at discrete locations along the trenches  60 . Desirably, the contact vias  62  have a width of less than about  0 . 35  μm and more preferably between about 0.05 μm and 0.25 μm. The width or the diameter of a contact via  62  can be equal to or slightly less than the line width defined by the trench  60  above. 
     The effective aspect ratio (depth: width) of the contact via  62  is therefore preferably greater than about 1:1. Since the effective depth of the contact via  62  is defined through both insulating layers  50 ,  56 , the effective aspect ratio is more preferably greater than about 2:1, and most preferably between about 2:1 and 4:1. The preferred embodiments will have particular utility in connection with future generation devices, whereby line widths and contact widths will shrink even further. Advantageously, the employment of relatively thin low k dielectrics for the insulating layers  50 ,  56  reduces the aspect ratio relative to an equivalent design using conventional silicon oxides (k≈4). 
     With reference to FIGS. 9 b  to  9   e , the preferred embodiments also have particular utility in conjunction with variations on the dual damascene structure of FIG. 9 a . Parts similar to those of FIG. 9 a  will be referred to by like reference numerals. 
     Referring to FIG. 9 b , a non-capped dual damascene structure is shown. When non-capped vias  62  are allowed by design rules (and they are desirable for obtaining higher circuit densities), mask misalignment can lead to even greater aspect ratios. As one via sidewall is withdrawn from the corresponding edge of the opening  55  defined by the hard mask  54 , the effective contact size decreases, such that aspect ratios can easily be double those listed above for the illustrated embodiment of FIG. 9 a.    
     Referring now to FIG. 9 c , vias that are not fully landed similarly exhibit higher effective aspect ratios. Under such circumstances, the opening  55  of the hard mask  54  overlaps with an edge  70  of the conductive circuit element  52 . Small but very high aspect ratio overetch holes  72  are formed in an insulating or dielectric layer  74  surrounding the circuit element  52 . The depth of the overetched hole  72  will depend, of course, on the etch selectivity between the barrier layer  51  and the surrounding dielectric  74 . 
     FIG. 9 d  illustrates the effect of undercutting the barrier layer  51  during via etch. When the barrier  51  is etched by a selective etch from the via bottom to expose the underlying circuit element  52 , the barrier  51  tends to be laterally recessed. The resultant cavities  80  are very difficult to line by conventional processes. 
     FIG. 9 e  illustrates yet another non-ideal damascene structure. When removing photoresist employed to pattern the structure, insulating layers  50 ,  56  formed of low k dielectrics are susceptible to attack, leaving a barrel-shaped profile in the trench  60  and via  61 . This structure is also difficult to effectively line and fill by conventional processing. 
     Similarly, many other non-ideal conditions can result in other re-entrant profiles, cavities and/or extremely high aspect ratios for dual damascene trenches and vias. Under such circumstances, conventional processing is inadequate for lining and filling these structures without forming voids. The methods of the preferred embodiments, in contrast, can effectively line even the unusual or aberrant structures of FIGS. 9 b  to  9   e . Moreover, the skilled artisan will readily find application for the methods and films disclosed herein beyond the dual damascene context. For example, the methods disclosed herein can also be used effectively for lining trenches in single damascene wiring schemes or for lining conventional contact vias and openings. The methods of lining have particular utility in conjunction with dual damascene process flows employing porous low k materials. 
     Methods of Forming Conformal Liners 
     The damascene structure so formed is thereafter lined with high step coverage. In accordance with the preferred embodiments, lining layers are formed by a periodic process in which each cycle deposits, reacts or adsorbs a layer upon the workpiece in a self-limiting manner. Preferably, each cycle comprises at least two distinct phases, wherein each phase is a saturative reaction with a self-limiting effect, leaving no more than about one atomic monolayer of the desired liner material. 
     FIG. 3 generally illustrates a method of forming damascene lining layers with high step coverage. The preferred method includes at least one process step that is a form of atomic layer deposition (ALD), whereby reactants are supplied to the workpiece in alternating pulses in a cycle. Preferably, each cycle forms no more than about one monolayer of lining material by adsorption and preferably by chemisorption. The substrate temperature is kept within a window facilitating chemisorption. In particular, the substrate temperature is maintained at a temperature low enough to maintain intact bonds between adsorbed species and the underlying surface, and to prevent decomposition of the reactant species. On the other hand, the substrate temperature is maintained at a high enough level to avoid condensation of reactants and to provide the activation energy for the desired surface reactions in each phase. Of course, the appropriate temperature window for any given ALD reaction will depend upon the surface termination and reactant species involved. 
     Each pulse or phase of each cycle is preferably self-limiting in effect. In the examples set forth below, each of the phases are self-terminating (i.e., an adsorbed and preferably chemisorbed monolayer is left with a surface non-reactive with the chemistry of that phase). An excess of reactant precursors is supplied in each phase to saturate the structure surfaces. Surface saturation ensures reactant occupation of all available reactive sites (subject to physical size restraints, as discussed in more detail below), while self-termination prevents excess film growth at locations subject to longer exposure to the reactants. Together, saturation and self-terminating chemistries ensure excellent step coverage. 
     The step coverage is so high that, without precautions, the process could coat the pores of the preferred low k insulating layers  50 ,  56  (FIG. 10 a ) with conductive reactants. Such a result risks conductive paths or short circuits through the insulating layers  50 ,  56 . The inventors have recognized that this risk is particularly high for the preferred reactants set forth in the tables below, especially for metal halides. Accordingly, the process of the preferred embodiments includes blocking pores on the via and trench sidewalls prior to highly conformal ALD processes that line the dual damascene structure with conductive material. 
     As illustrated, the process in accordance with the preferred embodiments begins with formation  100  of a dual damascene structure, such as those illustrated in FIGS. 9 a  to  9   e  and discussed above. As noted, the structure includes porous low k materials. 
     After formation  100 , at least the sidewalls of the vias and trenches are subjected to a process that seals or blocks  101  pores of the low k insulating layer(s). Preferably, blocking  101  comprises a relatively low conformality deposition that pinches off pores prior to a more highly conformal ALD deposition. As set forth in more detail below, the sealing or blocking process  101  can comprise a conventional deposition, such as CVD or PVD, or can itself comprise an alternating deposition process optimized to block the pores prior to significant diffusion of reactants through the porous low k material. 
     The blocking process  101  can form an insulating layer to seal or block the pores, but more preferably deposits a conductive material. Advantageously, when sealing with a conductive material, quality control to determine whether conductive material has excessively penetrated the porous material will simultaneously determine whether the insulating layers remain sufficiently porous to lower the k value of the insulator in accordance with the circuit design. Furthermore, with conductive sealant, limited volume within the openings (vias and trenches) is occupied by conductive material, rather than insulating material, and no further process is required for opening electrical contact to via floor prior to the highly conformal ALD process. 
     If necessary, the exposed surfaces of the dual damascene structure (e.g., the blocking layer over porous trench and via sidewall surfaces, the metal floor shown in FIG. 9 a  in the case of an insulating blocking layer followed by spacer etch, or surfaces of a previously deposited adhesion layer) are terminated  102  to react with the first phase of the ALD process. The first phases of the preferred embodiments (see Tables I to IV) are reactive, for example, with hydroxyl (OH) or ammonia (NH 3 ) termination. Silicon oxide and silicon nitride surfaces do not require a separate termination. Certain metal surfaces, such as a metal blocking layer or the circuit element at the bottom of the via  61  (FIG. 9 a ), if exposed, can be terminated, for example, with ammonia treatment. Where the lining material to be deposited is a metal nitride, surface termination can be considered to include formation of a blocking layer and any further adhesion layer (also by ALD), possibly with an additional surface termination treatment of the blocking layer or of the further adhesion layer. 
     After initial surface termination  102 , if necessary, a first chemistry is then supplied  104  to the workpiece. In accordance with the preferred embodiments, discussed in more detail below with respect to FIG. 4, the first chemistry comprises a metal-containing compound that is reactive with the terminated surfaces left by the previous step  102 . Accordingly, a metal-containing species replaces or adsorbs upon the surface termination. This metal-containing species layer is desirably self-terminating, such that any excess constituents of the first chemistry do not further react with the monolayer formed by this process. Preferably a halide or organic ligand terminates the metal-containing monolayer. 
     The metal-containing reactive species is preferably supplied in gaseous form, and is accordingly referred to hereinbelow as a metal source gas. In some examples, the reactive species actually has a melting point above the process temperature (e.g., in Table V below, CuCl melts at 430° C. while the process is conducted at about 350° C.). Nevertheless, the metal source gas is considered “volatile,” for purposes of the present description, if the species exhibits sufficient vapor pressure under the process conditions to transport the species to the workpiece in sufficient concentration to saturate exposed surfaces. 
     The first chemistry is then removed  106  from the reaction chamber. In the illustrated embodiments, step  106  merely entails stopping the flow of the first chemistry while continuing to flow a carrier gas for a sufficient time to diffuse or purge excess reactants and reactant by-products out of the vias, trenches and the reaction chamber, preferably with greater than about two reaction chamber volumes of the purge gas, more preferably with greater than about three chamber volumes. In the illustrated embodiment, the removal  106  comprises continuing to flow purge gas for between about 0.1 seconds and 20 seconds after stopping the flow of the first chemistry. Inter-pulse purging is described in co-pending U.S. patent application having Ser. No. 09/392,371, filed Sep. 8, 1999 and entitled IMPROVED APPARATUS AND METHOD FOR GROWTH OF A THIN FILM, the disclosure of which is incorporated herein by reference. In other arrangements, the chamber may be completely evacuated between alternating chemistries. See, for example, PCT publication number WO 96/17107, published Jun. 6, 1996, entitled METHOD AND APPARATUS FOR GROWING THIN FILMS, the disclosure of which is incorporated herein by reference. Together, the adsorption  104  and reactant removal  106  represent a first phase  107  in an ALD cycle. 
     When the reactants of the first chemistry have been removed  106  from the chamber, a second chemistry is supplied  108  to the workpiece. The second chemistry desirably reacts with the self-terminated monolayer formed in step  104 . In the illustrated embodiments, described in more detail below with respect to FIG. 4, this reaction comprises supplying a nitrogen source gas to the workpiece. Nitrogen or nitrogen-containing species from the nitrogen source gas preferably reacts with the previously adsorbed metal-containing species to leave a metal nitride in place of the metal-containing monolayer. 
     In other arrangements, the second chemistry may simply scavenge or remove the ligand termination of the adsorbed metal complex monolayer formed in step  104  (e.g., by ligand-exchange, sublimation or reduction) or otherwise prepare the monolayer for deposition/adsorption of a further monolayer and/or reaction with a further chemistry (see, e.g., Tables I, IV and V below). Desirably, the reaction  108  is also self-limiting. Reactants saturate the limited number of reaction sites left by step  104 . Temperature and pressure conditions are preferably arranged to avoid diffusion of reactants from the second chemistry through the monolayer to underlying materials. The second chemistry also leaves a surface termination that operates to limit the deposition in a saturative reaction phase. In the illustrated embodiments of Tables II and III below, nitrogen and NH x  tails terminating a metal nitride monolayer are non-reactive with NH 3  of the second chemistry. 
     After a time period sufficient to completely saturate and react the metal-containing monolayer with the second chemistry, the second chemistry is removed  110  from the workpiece. As with the removal  106  of the first chemistry, this step  110  preferably comprises stopping the flow of the second chemistry and continuing to flow carrier gas for a time period sufficient for excess reactants and reaction by-products from the second chemistry to diffuse out of the vias and trenches of the damascene structure and be purged from the reaction chamber. For example, reactants and reaction by-products can be removed by flowing purge gas after stopping the flow of the second chemistry, preferably with at least about two chamber volumes of purge gas and more preferably with at least about three chamber volumes. In the illustrated embodiment, the removal  110  comprises continuing to flow purge gas for between about 0.1 seconds and 20 seconds after stopping the flow of the second chemistry. Together, the reaction  108  and removal  110  represent a second phase  111  in an ALD cycle. 
     In the illustrated embodiment, where two phases are alternated once the excess reactants and by-products of the second chemistry have diffused out of the vias and trenches, and preferably out of the reaction chamber, the first phase  107  of the ALD process is repeated. Accordingly, again supplying  104  the first chemistry to the workpiece forms another self-terminating monolayer. 
     The two phases  107 ,  111  thus represent a cycle  115  repeated to form monolayers in an ALD process. The first chemistry generally reacts with the termination left by the second chemistry in the previous cycle. If necessary, the cycle  115  can be extended to include a distinct surface preparation, similar to step  102 , as shown in dotted lines in FIG.  3 . The cycle  115  then continues through steps  104  to  110 . This cycle  115  is repeated a sufficient number of times to produce a lining layer within the dual damascene structure of a thickness sufficient to perform its desired function. 
     Though illustrated in FIG. 3 with only first and second chemistries, it will be understood, that, in other arrangements, additional chemistries can also be included in each cycle (see, e.g., Table IV below). Furthermore, though illustrated with an initial metal phase and subsequent nitrogen phase in the examples below, it will be understood that the cycle can begin with the nitrogen phase, depending upon the surfaces and phase chemistries. 
     Blocking Pores Prior to ALD Processes 
     As noted with respect to FIG. 3, the opening (i.e., the via  62  and trench  60 ) in the porous low k layers  50 ,  56  is blocked or sealed at the exposed surfaces of the low k material. The method is selected to block, plug or seal the pores of the porous low k material at the exposed surfaces, without filling the pores to any significant depth into the low k material. Completely filling the pores of the low k material, even with an insulating material, would negate the advantage of the porous low k material by raising the dielectric constant of the material. In the case of blocking with conductive material, significant penetration into the low k material risks forming short circuits through the layers  50 ,  56 . 
     In the illustrated embodiment, blocking is accomplished by lining the opening in the porous low k layers  50 ,  56  with a sealing layer  148  as shown in FIG. 10 a . More particularly, the sealing layer  148  is deposited by a method that does not have high enough conformality to extensively fill the pores through the thickness of the low k insulators  50 ,  56 . Preferably, the deposition fills or plugs pores no more than three pore depths into the low k insulator, where the pore depth is defined by average pore size. More preferably, the deposition fails to continuously line pores beyond three pore depths into the low k insulator. Most preferably, the deposition fills pores no more than one pore depth into the low k material, continuously lines pores no more than two pore depths into the low k material, and insufficiently lines pores three pore depths into the layer to conduct electricity if the deposited material were conductive. 
     With reference to FIG. 10 b , an enlarged view of the via sidewall is shown in cross-section. In accordance with the illustrated embodiment, the low k material  50  comprises a plurality of pores  120  within a matrix  122  of insulating material. The illustrated low k material  50 , comprising Nanoglass™, commerically available from Honeywell Advanced Microelectronic Materials (AMM) of Sunnyvale, Calif. (formerly Allied Signal), has a porosity of about 70% with an average pore size of about 4 nm to achieve a k value of about 2.0. Accordingly, the pores  120  are interconnected, as shown. 
     For purposes of illustration, three consecutive pores are shown with about the average pore size, though the skilled artisan will readily appreciate that the pore size will deviate randomly from the average pore size and that the pore orientations relative to one another will also be random. One pore depth, as defined herein, refers to the distance of the average pore size. In the illustrated artificial example, a first pore  124  extending from the via surface represents the first pore depth, a second pore  126  represents the second pore depth, and a third pore  128  represents the third pore depth. 
     As shown in FIG. 10 b , none of the pores are completely filled. The first pore  124 , open to the via  62  (FIG. 10 a ), is largely filled with the material of the scaling layer  148 . Due to imperfect conformality, however, the sealing layer  148  has pinched off the opening to the first pore  124  before complete filling, leaving a void  130  within the first pore  124 . The second pore  126  is shown with a very thin coating  132  of the pore walls that can in some instances be continuous. The third pore  128  has only non-continuous deposits  134 , if any. Similarly, a fourth pore  136 , which also represents the third pore depth in terms of distance through the pores from the outer (via) surface of the low k material, has only non-continuous deposits, if any. In the illustrated embodiment, a fourth pore  140  and a fifth pore  142 , both representing the fourth pore depth from the surface (along different paths), have no appreciable deposits. 
     If, as preferred, the sealing layer  148  comprises a conductive material, current can conduct only as far as the second pore depth, in the illustrated embodiment. While the discontinuous coatings  138 ,  134  raise some risk of arcing, the risk is minimal and inconsequential for most circuit designs. The skilled artisan will appreciate that some process flows and circuit designs will have very different tolerances (broader or narrower than that illustrated) for the amount of diffusion through the pores that is acceptable. Moreover, regardless of the amount of diffusion allowed during formation of the sealing layer  148 , blocking the pores will be advantageous in avoiding far more extensive diffusion during subsequent conformal ALD processes. 
     In accordance with one embodiment, the sealing layer  148  can comprise an insulating material such as silicon dioxide formed by conventional CVD processes. In this case, deposition of the sealing layer is followed by a selective etch to remove the insulating material from the floor of the via  62  and thereby expose the underlying conductive element. An exemplary selective etch comprises a reactive ion etch, selectively removing horizontal surfaces from the workpiece and tending to straighten the inwardly sloped sidewalls left by the CVD process. 
     In accordance with the illustrated embodiment, however, the sealing layer  148  comprises a conductive material that does not need to be removed prior to filling the via  62  with a contact plug. Conventional CVD of metal, such as tungsten, titanium, etc., can advantageously serve as both a conductive adhesion layer and the sealing layer  148 . If sufficiently conductive, the sealing layer  148  can then remain over all interior surfaces of the opening as a part of the final structure (see FIG.  13 ). 
     Either conductive or insulating sealing layers can also be deposited by PVD (e.g., sputtering or evaporation). Advantageously, PVD typically exhibits lower conformality than CVD and so will likely pinch off the pores more quickly than CVD. This advantage needs to be balanced against the poorer step coverage of the dual damascene structure afforded by PVD. 
     Whether CVD or PVD, conditions for the deposition should be carefully arranged to balance a need for relatively high conformality, to reach the bottom of the via  62  (FIG. 10 a ), with a need for relatively low conformality, to pinch off the pores prior to significant penetration of the sealing layer material though the porous low k material. The skilled artisan can readily optimize deposition conditions, in view of the present disclosure, as a matter of routine experimentation. 
     In another embodiment, the sealing or blocking layer  148  can be formed by an alternating process (using pulses similar to ALD) optimized to block the pores of the low k layers  50 ,  56  before significant penetration into the layers. Previous work has been conducted to determine the conditions under which porous materials can be coated by ALD. See A. W. Ott., J. W. Klaus, J. M. Johnson, S. M. George, K. C. McCarley, J. D. Way, “Modification of Porous Alumina Membranes Using Al 2 O 3  Atomic Layer Controlled Deposition,”  Chem. Mater.  Vol. 9, No. 3 (1997), p. 707-714; and Suvi Haukka, Eeva-Liisa Lakomaa, Tuomo Suntola, “Chemisorption of chromium acetylacetonate on porous high surface area silica,”  Appl. Surf. Sci.  Vol. 75, No. 1-4 (1994), pp. 220-227. The articles noted above are hereby expressly incorporated herein by reference. The skilled artisan will appreciate in view of the present disclosure that, conversely, the conditions for avoiding conformal coating of a porous material can be determined using similar techniques. Advantageously, a “low conformality” alternating process for blocking the pores of the low k material can be followed in situ by high conformality ALD layers, having a minimal, uniform thickness to accomplish the goals of the layers (e.g., adhesion, barrier, electroplating seed layer), thus conserving space for more conductive materials to fill the opening. 
     In one embodiment, the reactants are pulsed into the reaction chamber in an inert carrier gas, similar in sequence to the process described above for ALD. In the first pulse of metal source gas, the surface of the substrate is lined with the metal-containing species. In addition, the metal source gas is able to penetrate into the porous insulating layer by diffusion. If necessary, the first pulse can be lengthened as compared to a subsequent ALD process, ensuring penetration of the metal source gas into the porous insulating layer. 
     Following the first pulse, the metal source gas is purged from the reaction chamber with a pulse of inert gas. In this embodiment of the invention, the purge is insufficient to remove all of the metal source gas from the pores and some remains trapped in the pores of the insulating material. The duration of the purge pulse may be equivalent to that of an ALD process that is optimized to purge reactants from the trenches and vias, but not optimized to purge out the pores. Alternately, the purge pulse may be shortened to ensure that metal source gas remains within the pores of the insulating material. 
     A second chemistry is pulsed into the chamber following the purge. Preferably the second chemistry is a nitrogen source gas. The second chemistry reacts with the monolayer of metal-containing species adsorbed on the surface of the via. Additionally, the second chemistry diffuses into the insulating material where it reacts with the metal source gas that remains within the pores. This is a CVD-type reaction, leading to deposition of much more than one monolayer of metal within the pores. The second chemistry will react with the first metal source gas it encounters and thus its diffusion into the pores will be limited, leading to a depletion effect into the insulating layer. The result of the depletion effect will be the deposition of the most metal at the neck of the first pore, with metal deposition decreasing with distance into the insulating material. This will narrow the neck of the first pore, further limiting diffusion into the porous insulating material during subsequent ALD cycles. 
     Repetition of the ALD cycle will narrow the neck of the first pore further by increasing the thickness of the deposited layer and will eventually lead to a continuous, sealing layer blocking off the pores. The combination of the CVD depletion effect and the resulting decrease in diffusion into the pores, will allow this process to seal the porous insulating material without reducing the insulating properties of the material. The number of repetitions needed to seal off the pores will depend on the pore size and can be determined by the skilled artisan through routine experimentation. This method avoids the poor conformality CVD or PVD provide on the trench and via surfaces yet still provides a conductive sealing layer on which to build a metal adhesion layer or barrier diffusion layer. 
     In the further lining processes of the preferred embodiments, discussed below, very high conformality of ALD risks completely coating the pores of the low k dielectric with a conductive material. This coating could conduct current through the insulator, short circuiting runners and/or devices. The sealing layer  148  (FIG. 10 a ), however, blocks the pores and prevents entry of reactants after the pores have been blocked, particularly before any high conformality ALD process begins. This is particularly significant for the metal and metal nitride deposition chemistries set forth in the tables below. 
     Forming Metal Adhesion Liners 
     Depending upon the exposed materials and desired ALD chemistry, an adhesion layer prior to formation of a barrier diffusion layer may or may not be desired. With TEOS oxides, the inventors have not found the use of an adhesion layer necessary. On the other hand, adhesion layers may be desirable for vias and trenches formed in the preferred spin-on and other low k insulating materials. Conductive adhesion layers may also be desirable to facilitate reaction of the first phase over metal runners or landing pads  52  exposed at the bottom of the via  61  (FIG. 9 a ). 
     Most preferably, the sealing layer  148  (FIG. 10 b ) comprises a metal layer and serves also as an adhesion layer. Accordingly, in the preferred embodiment, no further adhesion layer is required. 
     If an adhesion layer is desired, the adhesion layer preferably comprises a “pure” metal lining layer over oxide, metal and etch stop layers of the dual damascene structures. Prior to forming the preferred barrier layers, therefore, a dual damascene structure similar to those of FIGS. 9 a  to  9   e  is preferably lined with a metal adhesion layer after or simultaneously with forming the sealing layer  148  (FIG. 10 a ). As is known in the art, such adhesion layers can be formed by PVD or CVD. For example, PVD titanium and CVD tungsten processes are well known in the art. Advantageously, such adhesion layers deposited by conventional processes will also serve as the initial protective or blocking layer, as set forth above. 
     It is conceivable that, in other arrangements, the pores of the low k dielectrics can be effectively blocked or plugged without full coverage of the via and trench surfaces. In this case, a further adhesion layer is desirably formed by ALD to provide a minimal thickness with maximum conformality, as exemplified by the process recipe of Table I below. It will be understood that the principles disclosed herein with respect to FIG. 3, and with respect to the particular examples of metal nitrides set forth below, are applicable to the formation of a variety liner materials. For example, a pure metal layer can be deposited by alternately depositing halogen- or organic-terminated monolayers of metal and flowing reduction agents (e.g., H radicals, triethyl boron or other strong reducers) to remove the halogen termination. Removal of the metal monolayer termination by binding and carrying the ligand away can be more generally referred to as “gettering” or “scavenging” the ligand. In the next cycle, therefore, the metal source gas can adsorb upon the underlying metal monolayer in another self-terminating phase. The resultant ALD metal is particularly useful as an adhesion layer prior to barrier layer deposition, and as a seed layer following barrier deposition and preceding electroplating. 
     Accordingly, one of the reactant species preferably includes a metal-containing species with an organic or halide ligand. Exemplary metal precursors include tantalum pentaethoxide, tetrakis(dimethylamino)titanium, pentakis(dimethylamino)tantalum, TaCl 5  and TiCl 4 . In the illustrated embodiment, a tungsten (W) seed layer is formed by ALD, in alternating metal and reducing phases separated by purge steps. In the process recipe of Table I below, tungsten hexafluoride (WF 6 ) is alternated with a scavenger in the form of the reducing agent triethyl boron ((CH 3 CH 2 ) 3 B) or TEB. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE I 
               
               
                   
               
               
                   
                 Carrier 
                   
                 Reactant 
                   
                   
                   
               
               
                   
                 Flow 
                   
                 Flow 
                 Temperature 
                 Pressure 
                 Time 
               
               
                 Phase 
                 (slm) 
                 Reactant 
                 (sccm) 
                 (° C.) 
                 (Torr) 
                 (sec) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 metal 
                 600 
                 WF 6   
                 50 
                 400 
                 10 
                 0.25 
               
               
                 purge 
                 600 
                 — 
                 — 
                 400 
                 10 
                 0.5 
               
               
                 reduce 
                 600 
                 TEB 
                 40 
                 400 
                 10 
                 0.1 
               
               
                 purge 
                 600 
                 — 
                 — 
                 400 
                 10 
                 0.8 
               
               
                   
               
            
           
         
       
     
     Radicals provided by plasma generators can facilitate deposition of metal-containing layers at the low temperatures of ALD processing. Structures and methods of depositing metals and metal-containing layers with radical enhancement are provided in patent application having Ser. No. 09/392,371, filed Sep. 8, 1999 and entitled IMPROVED APPARATUS AND METHOD FOR GROWTH OF A THIN FILM, the disclosure of which is incorporated by reference hereinabove. Another exemplary ALD metal process flow is provided in U.S. Pat. No. 5,916,365 to Sherman, issued Jun. 29, 1999, the disclosure of which is incorporated herein by reference. 
     Methods of Forming Metal Nitride Barrier Liners 
     FIG.  4  and Tables II to IV below illustrate exemplary processes for forming metal nitride barrier layers lining the structures of FIGS. 9A to  9 E. For simplicity, like reference numerals are employed to refer to the phases and steps of the metal nitride examples (FIG. 4) that correspond to the general description of FIG.  3 . 
     With reference now to FIG. 4, a gas flow sequence is represented in accordance with a particular embodiment. In the illustrated example, a conductive nitride, and more particularly a metal nitride, is formed by supplying the workpiece with a metal source gas alternately with a nitrogen source gas. The first or metal phase  107  of each cycle chemisorbs a layer of metal-containing material, desirably in the absence of the nitrogen source gas. The second or nitrogen phase  111  of each cycle reacts or adsorbs a nitrogen-containing material on the deposited metal-containing layer, desirably in the absence of the metal source gas. It will be understood that, in other arrangements, the order of the phases can be reversed, and that the reactant removal or purge steps can be considered part of the preceding or subsequent reactant pulse. 
     Surfaces of the damascene structure upon which the lining material is to be formed are initially terminated to provide a surface that is reactive with the metal source gas. In the embodiment of FIG. 10 a , the exposed surfaces upon which deposition is desired are provided by the sealing layer  148 . The preferred metal layer can be terminated, for example, with NH 3  to facilitate chemisorption of the reactant pulses discussed below. 
     Most preferably, the metal phase  107  is self-limiting, such that no more than about one atomic monolayer is deposited during the first phase. Desirably, a volatile metal source gas is provided in a pulse  104 . Exemplary metal source gases include titanium tetrachloride (TiCl 4 ), tungsten hexafluoride (WF 6 ), tantalum pentachloride (TaCl 5 ), tantalum pentaethoxide, tetrakis(dimethylamino)titanium, pentakis(dimethylamino)tantalum, copper chloride (CuCl) and copper hexafluoroacetylacetonate vinyltrimethylsilane (Cu(HFAC)VTMS). 
     After a sufficient time for the metal source gas to diffuse into the bottom of the dual damascene contact via, shutting off the flow of the metal source gas ends the metal pulse  104 . Preferably, carrier gas continues to flow in a purge step  106  until the metal source gas is purged from the chamber. 
     During the pulse  104 , the metal source gas reacts with exposed and terminated surfaces of the workpiece to deposit or chemisorb a “monolayer” of metal-containing species. While theoretically the reactants will chemisorb at each available site on the exposed layer of the workpiece, physical size of the adsorbed species (particularly with terminating ligands) will generally limit coverage with each cycle to a fraction of a monolayer. In the example of Table II, the ALD process grows metal nitride layers at roughly 0.35 Å/cycle, such that a full monolayer effectively forms from material deposited approximately every 15 cycles for TiN, which has a bulk lattice parameter of about 4.2 Å. Each cycle  115 ,  115   a  is represented by a pair of metal source gas and nitrogen source gas pulses. “Monolayer,” as used herein, therefore refers to a fraction of a monolayer during deposition, referring primarily to the self-limiting effect of the pulse  104 . 
     In particular, the metal-containing species deposited/adsorbed upon the workpiece is self-terminating such that the surface will not further react with the metal source gas. In the examples set forth below, TiCl 4  (Table II) leaves a monolayer of chloride-terminated titanium. WF 6  (Tables III and IV) leaves a monolayer of fluorine-terminated tungsten. Similarly, other volatile metal halides will leave halide-terminated surfaces, and metal organics, such as tantalum pentaethoxide, tetrakis(dimethylamino)titanium, and pentakis(dimethylamino)tantalum, will leave surface terminated with organic ligands. Such surfaces do not further react with the metal source or other constituents of the reactant flow during the metal source gas pulse  104 . Because excess exposure to the reactants does not result in excess deposition, the chemistry during the metal phase  107  of the process is said to be self-limiting or self-saturating. Despite longer exposure to a greater concentration of reactants, deposition over upper surfaces of the workpiece does not exceed deposition over the via floor. 
     In a second phase  111  of the cycle  115 , a pulse  108  of a nitrogen source gas is then provided to the workpiece. In the illustrated examples, the nitrogen source gas comprises ammonia. Preferably, the second phase  111  is maintained for sufficient time to fully expose the monolayer of metal-containing species left by the first phase  107  to the nitrogen source gas. After a sufficient time for the nitrogen source gas to diffuse into the bottom of the dual damascene contact via, shutting off the flow of the metal source gas ends the nitrogen pulse  108 . Preferably, carrier gas continues to flow in a purge step  110  until the nitrogen source gas is purged from the chamber. 
     During the nitrogen pulse  108 , the nitrogen source gas reacts with or chemisorbs upon the self-terminated metal monolayer left by the first phase  107 . In the embodiments of Tables II and III, this chemisorption comprises a saturative ligand-exchange reaction, replacing the halogen termination of the metal monolayer with a nitrogen-containing species. In the embodiment of Table IV, in contrast, an intermediate getter or scavenging phase first removes the halogen termination of the metal monolayer prior to a nitrogen pulse. In this case, in a third phase the nitrogen-containing species reacts with or adsorbs upon the metal left exposed by the getter phase. In either case, a metal nitride is thereby formed, preferably in a single monolayer. Desirably, the process leaves a stoichiometric metal nitride. As discussed with respect to the metal phase  107 , the monolayer need not occupy all available sites, due the physical size of the adsorbed species. However, the second phase  111  also has a self-limiting effect. 
     In particular, the nitrogen source gas reacts with the metal-containing species chemisorbed onto the workpiece surface during the previous pulse of metal source gas. The reaction is also surface terminated, since ammonia during the pulse  108  will not react with nitrogen and NH x  tails terminating the metal nitride monolayer. Moreover, temperature and pressure conditions are arranged to avoid diffusion of ammonia through the metal monolayer to underlying materials. Despite longer exposure to a greater concentration of reactants in this saturative, self-limiting reaction phase  111 , the thickness of the metal nitride formed on upper surfaces of the workpiece does not exceed the thickness of the metal nitride formed on the via floor. 
     The metal phase  107  (including metal source pulse  104  and purge  106 ) and nitrogen phase  108  (including nitrogen source pulse  108  and purge  110 ) together define a cycle  115  that is repeated in an ALD process. After the initial cycle  115 , a second cycle  115   a  is conducted, wherein a metal source gas pulse  104   a  is again supplied. The metal source gas chemisorbs a metal-containing species on the surface of the metal nitride formed in the previous cycle  115 . The metal-containing species readily react with the exposed surface, depositing another monolayer or fraction of a monolayer of metal-containing species and again leaving a self-terminated surface that does not further react with the metal source gas. Metal source gas flow  104   a  is stopped and purged  106   a  from the chamber, and (according to Tables II and III) a second phase  111   a  of the second cycle  115   a  provides nitrogen source gas to nitridize the second metal monolayer. According to the example of Table IV, the nitrogen phase is preceded by an intermediate getter or scavenging phase. 
     The cycle  115   a  is repeated at least about 10 times, and more preferably at least about 20 times, until a sufficiently thick metal nitride is formed to serve a barrier function in the dual damascene structure. Advantageously, layers having a thickness of less than about 200 Å, and more preferably less than about 100 Å, can be formed with near perfect step coverage by the methods of the preferred embodiments. 
     EXAMPLES 
     The tables below provide exemplary process recipes for forming metal nitride layers suitable for barrier applications in dual damascene metallization schemes for ultra large scale integrated processing. Each of the process recipes represents one cycle in a single-wafer process module. In particular, the illustrated parameters were developed for use in the single-wafer ALD module commercially available under the trade name Pulsar 2000™ from ASM Microchemistry Ltd. of Espoo, Finland. 
     Note that the parameters in the tables below (and also in Table I above) are exemplary only. Each process phase is desirably arranged to saturate the via and trench surfaces, and more particularly to saturate sealing layer  148  (FIG. 10 a ) formed to a minimum thickness over the via and trench surfaces. Purge steps are arranged to remove reactants between reactive phases from the vias. The examples herein have been conducted upon planar, unpatterened wafer surfaces in a Pulsar 2000™ reaction chamber, from ASM Microchemistry Ltd. of Finland. Similar ALD processes have been determined to achieve better than 90% step coverage in voids with aspect ratios of greater than about 20. In view of the disclosure herein, the skilled artisan can readily modify, substitute or otherwise alter deposition conditions for different reaction chambers and for different selected conditions to achieve saturated, self-terminating phases at acceptable deposition rates. Due to the previous sealing layer  148 , however, this high conformality ALD process does not reach the pores, which remain largely void and free of conductive coatings. 
     Advantageously, the ALD processes described herein are relatively insensitive to pressure and reactant concentration, as long as the reactant supply is sufficient to saturate the trench and via surfaces. Furthermore, the processes can operate at low temperatures. Workpiece temperature is preferably maintained throughout the process between about 300° C. and 500° C. to achieve relatively fast deposition rates while conserving thermal budgets during the back-end process. More preferably, the temperature is maintained between about 350° C. and 400° C., and most preferably between about 380° C. and 400° C. Pressure in the chamber can range from the milliTorr range to super-atmospheric, but is preferably maintained between about 1 Torr and 500 Torr, more preferably between about 10 Torr and 100 Torr. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE II 
               
               
                   
               
               
                   
                 Carrier 
                   
                 Reactant 
                   
                   
                   
               
               
                   
                 Flow 
                   
                 Flow 
                 Temperature 
                 Pressure 
                 Time 
               
               
                 Phase 
                 (slm) 
                 Reactant 
                 (sccm) 
                 (° C.) 
                 (Torr) 
                 (sec) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 metal 
                 400 
                 TiCl 4   
                  20 
                 400 
                 10 
                 .05 
               
               
                 purge 
                 400 
                 — 
                 — 
                 400 
                 10 
                 0.8 
               
               
                 nitrogen 
                 400 
                 NH 3   
                 100 
                 400 
                 10 
                 0.75 
               
               
                 purge 
                 400 
                 — 
                 — 
                 400 
                 10 
                 1.0 
               
               
                   
               
            
           
         
       
     
     Table II above presents parameters for ALD of a titanium nitride (TiN) barrier into trenches and contact vias of a dual damascene structure. As noted, the metal source gas comprises titanium tetrachloride (TiCl 4 ), the carrier gas comprises nitrogen (N 2 ) and the nitrogen source gas preferably comprises ammonia (NH 3 ). 
     In the first phase of the first cycle, TiC 4  chemisorbs upon the exposed surfaces of the dual damascene trenches and contact vias, particularly upon the sealing layer  148  formed to block pores in the low k material. The metal source gas preferably comprises a sufficient percentage of the carrier flow, given the other process parameters, to saturate the damascene surfaces. A monolayer of titanium complex is left upon the trench and via surfaces, and this monolayer is self-terminated with chloride. 
     Desirably, the reactor includes a catalyst to convert the metal source gas to a smaller and/or more reactive species. In the illustrated embodiment, the preferred reaction chamber comprises titanium walls, which advantageously convert TiCl 4  to TiCl 3 . The smaller species readily diffuse into vias, occupy more reactive sites per cycle and more readily chemisorb onto the active sites. Accordingly, the catalyst enables faster deposition rates. The skilled artisan will readily appreciate that other catalysts can be employed for other chemistries. 
     After the TiCl 4  flow is stopped and purged by continued flow of carrier gas, a pulse of NH 3  is supplied to the workpiece. Ammonia preferably comprises a sufficient percentage of the carrier flow, given the other process parameters, to saturate the surface of the metal-containing monolayer. The NH 3  readily reacts with the chloride-terminated surface of the metal monolayer in a ligand-exchange reaction, forming a monolayer of titanium nitride (TiN). The reaction is limited by the number of available metal chloride complexes previously chemisorbed. Neither ammonia nor the carrier gas further reacts with the resulting titanium nitride monolayer, and the monolayer is left with a nitrogen and NH x  bridge termination. The preferred temperature and pressure parameters, moreover, inhibit diffusion of ammonia through the metal monolayer. 
     In the next cycle, the first phase introduces TiCl 4 , which readily reacts with the surface of the titanium nitride monolayer, again leaving a chloride-terminated titanium layer. The second phase of the second cycle is then as described with respect to the first cycle. These cycles are repeated until the desired thickness of titanium nitride is formed. 
     In the illustrated embodiment, carrier gas continues to flow at a constant rate during both phases of each cycle. It will be understood, however, that reactants can be removed by evacuation of the chamber between alternating gas pulses. In one arrangement, the preferred reactor incorporates hardware and software to maintain a constant pressure during the pulsed deposition. The disclosures of U.S. Pat. No. 4,747,367, issued May 31, 1988 to Posa and U.S. Pat. No. 4,761,269, issued Aug. 2, 1988 to Conger et al., are incorporated herein by reference. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE III 
               
               
                   
               
               
                   
                 Carrier 
                   
                 Reactant 
                   
                   
                   
               
               
                   
                 Flow 
                   
                 Flow 
                 Temperature 
                 Pressure 
                 Time 
               
               
                 Phase 
                 (slm) 
                 Reactant 
                 (sccm) 
                 (° C.) 
                 (Torr) 
                 (sec) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 metal 
                 600 
                 WF 6   
                  50 
                 400 
                 10 
                 0.25 
               
               
                 purge 
                 600 
                 — 
                 — 
                 400 
                 10 
                 0.25 
               
               
                 nitrogen 
                 600 
                 NH 3   
                 100 
                 400 
                 10 
                 0.5 
               
               
                 purge 
                 600 
                 — 
                 — 
                 400 
                 10 
                 1.0 
               
               
                   
               
            
           
         
       
     
     Table III above presents parameters for ALD of tungsten nitride (WN). As noted, the metal source comprises tungsten hexafluoride (WF 6 ), the carrier gas comprises nitrogen (N 2 ) and the nitrogen source gas preferably comprises ammonia (NH 3 ). During each of the reaction phases, the reactants are supplied in sufficient quantity for the given other parameters to saturate the surface. 
     In this case, the metal monolayer formed in the metal phase is self-terminated with fluoride, which does not readily react with WF 6  under the preferred conditions. The preferred nitrogen source gas, however, reacts with or adsorbs upon the fluoride-terminated surface during the nitrogen phase in a reaction limited by the limited supply of tungsten fluoride complexes previously adsorbed. Moreover, nitridation leaves a nitrogen and NH x  termination that does not further react with excess ammonia in the saturative phase. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE IV 
               
               
                   
               
               
                   
                 Carrier 
                   
                 Reactant 
                   
                   
                   
               
               
                   
                 Flow 
                   
                 Flow 
                 Temperature 
                 Pressure 
                 Time 
               
               
                 Phase 
                 (slm) 
                 Reactant 
                 (sccm) 
                 (° C.) 
                 (Torr) 
                 (sec) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 metal 
                 400 
                 WF 6   
                 50 
                 400 
                 10 
                 0.25 
               
               
                 purge 
                 400 
                 — 
                 — 
                 400 
                 10 
                 0.8 
               
               
                 reduce 
                 400 
                 TEB 
                 50 
                 400 
                 10 
                 0.01 
               
               
                 purge 
                 400 
                 — 
                   
                 400 
                 10 
                 0.5 
               
               
                 nitrogen 
                 400 
                 NH 3   
                 100  
                 400 
                 10 
                 0.25 
               
               
                 purge 
                 400 
                 — 
                 — 
                 400 
                 10 
                 0.5 
               
               
                   
               
            
           
         
       
     
     Table IV above presents parameters for another ALD process for forming tungsten nitride (WN). The illustrated embodiment is particularly advantageous for directly depositing a barrier layer upon metal. In the illustrated embodiment, the metal of the preferred sealing layer  148  (FIG. 10 a ) is exposed. In other arrangements, metal can be exposed at the via floor and insulating surfaces of the trench and via, without an intermediate adhesion layer. As noted, the metal source comprises tungsten hexafluoride (WF 6 ), the carrier gas comprises nitrogen (N 2 ) and the nitrogen source gas preferably comprises ammonia (NH 3 ). In this case, a getter or scavenger removes the ligands left by the metal phase. In particular, a strong reducer, comprising TEB (triethyl boron) in the illustrated embodiment, reduces or otherwise removes the halogen-terminated metal complex monolayer. The nitrogen source gas then readily reacts with the reduced metal surface. In other arrangements, the getter can replace the halogen-termination in a ligand-exchange reaction, desirably leaving a surface reactive with a subsequent nitrogen-containing species. The nitrogen phase saturates the reaction sites left by the getter phase (i.e., the exposed tungsten surface in the illustrated embodiment) and leaves a nitrogen and NH x  termination that does not further react with excess ammonia in the saturative phase. 
     The intermediate reduction phase results in a metal nitride crystallinity that exhibits lower resistivity than films formed by the ligand-exchange reaction of Table III. Such lowered resistivity is advantageous for the preferred dual damascene barrier context, where the barrier is placed in the conductive path of integrated circuit wires. 
     Moreover, the intermediate scavenger, as represented by the TEB pulse of Table IV, binds and carries away the halide tails left by the previous metal phase prior to introduction of the ammonia phase. Advantageously, the ammonia phase reacts directly with metal formed in the first phase, rather than liberating hydrogen halides (e.g., HF) in a ligand-exchange reaction. In contrast to HF, the complex produced by binding halides to the getter or scavenger does not corrode sensitive surfaces such as the metal at the bottom of the damascene structure. Accordingly, the metal line  52  or the preferred metal sealing layer  148  (FIG. 10 a ) of the dual damascene structure is protected from corrosive effects of HF or other halide species. The process of Table IV has been found particularly advantageous where, as in the preferred embodiment, the underlying metal comprises copper. Etching of the copper is minimized and uniformity of the blanket metal nitride deposition is thereby improved. The process can be advantageous in a variety of contexts, however, for avoiding the build up of hydrogen halides due to their corrosive effects in general, and particularly due to their tendency to counter the ALD advantage of complete uniformity. 
     Once an initial, thin layer (e.g., about 3-10 nm) of metal nitride (e.g., WN) is formed by the method of Table IV, further deposition of barrier and/or adhesion materials can proceed without the intermediate scavenger phase. Two-phase cycles employing ligand-exchange reactions can more efficiently produce a thicker barrier layer over the initial layer. For example, WN by the method of Table IV can be followed by further deposition of TiN, such as by the method of Table II. The upper TiN surface of a WN/TiN barrier demonstrates better compatibility with some process flows. 
     Methods of Forming Metal Seed Layers 
     After formation of the metal nitride barrier layer, a seed layer may be desirable, depending upon the method to be employed for filling the dual damascene structure and the conductivity of the deposited barrier layer. In the illustrated embodiment, a copper filler is desirably electroplated over the illustrated metal nitride barriers. Accordingly, a highly conductive seed layer is preferably first formed over the barrier layer. 
     As is known in the art, the seed layer preferably comprises a metal layer, more preferably copper, and can be deposited by any of a number of processes. For example, state-of-the-art processing employs PVD or sputtering to form a copper seed layer. In conjunction with high step coverage obtained in forming the prior metal nitride barrier layer by ALD, such methods may be adequate for many dual damascene schemes. 
     More preferably, a CVD process is employed to deposit the seed layer with higher step coverage. Metal organic CVD (MOCVD) techniques are disclosed, for example, by Wolf et al., “Process and equipment simulation of copper chemical vapor deposition using Cu(HFAC)VTMS,” Microelectronic Engineering, Vol. 45, No. 1, pp.15-27 (February 1999), the disclosure of which is incorporated herein by reference. 
     Most preferably, the seed layer is also formed by ALD. The volume saved by high step coverage formation of one or more of the adhesion, barrier and seed layers thus contributes to a higher-conductivity line due to a greater volume available for the more conductive filler metal and increased chance of completely filling the contact vias and trenches. Forming the sealing layer by an alternating process, furthermore, effectively lines the via and trench surfaces by ALD while blocking the pores with CVD reactions. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE V 
               
               
                   
               
               
                   
                 Carrier 
                   
                 Reactant 
                   
                   
                   
               
               
                   
                 Flow 
                   
                 Flow 
                 Temperature 
                 Pressure 
                 Time 
               
               
                 Phase 
                 (slm) 
                 Reactant 
                 (sccm) 
                 (° C.) 
                 Torr) 
                 (sec) 
               
               
                   
               
             
            
               
                 metal 
                 400 
                 CuCl 
                  4 
                 350 
                 10 
                 0.2 
               
               
                 purge 
                 400 
                 — 
                 — 
                 350 
                 10 
                 0.5 
               
               
                 reduce 
                 400 
                 TEB 
                 40 
                 350 
                 10 
                 0.2 
               
               
                 purge 
                 400 
                 — 
                 — 
                 350 
                 10 
                 0.5 
               
               
                   
               
            
           
         
       
     
     Table V above illustrates an ALD pure metal process, similar to that of Table I above. In alternating phases, copper chloride is first adsorbed and then reduced by TEB. Advantageously, copper chloride is a smaller reactive species compared to organic copper species, facilitating rapid and more complete saturation of reactive sites on the workpiece. 
     Resultant Trench and Via Liners 
     Referring now to FIG. 10 a , the dual damascene structure of FIG. 9 a  is illustrated with a blocking or sealing layer  148  over at least the sidewalls of the via  62  and trench  60 . The sealing layer  148  thus blocks the pores of the porous low k first insulating layer  50  and second insulating layer  56 . As discussed above, the sealing layer  148  is preferably formed to the minimum thickness necessary for blocking the pores on the sidewalls prior to forming further lining layers by ALD. The sealing layer may be formed by CVD, PVD, or an alternating process that operates in a CVD mode within the pores, as described above. 
     In the illustrated embodiment, where the sealing layer  148  is formed by conventional deposition processes, such as CVD and PVD, the minimum thickness is formed at the bottom end of the via  62 . The sealing layer  148  at portions of the via  62  and trench  60  will necessarily exceed the minimum necessary thickness for blocking the pores due to the shadow, diffusion and depletion effects noted in the Background section above. Optionally, the excess thickness can be reduced by a directional etch (e.g., reactive ion etch) that preferentially removes the overhanging portion of the sealing layer  148 . In the case of an insulating sealing layer (not shown), this directional etch would at any rate be employed to remove insulating material from the bottom of the via and expose the underlying wire. The skilled artisan will recognize the sealing layer  148  as the result of a CVD or a PVD process, as the case may be, by the layer&#39;s characteristic profile, and particularly by the non-uniformity of this profile. 
     The minimum thickness for a CVD or PVD layer will typically represent between about 50% and 150% of the average pore size, more preferably between about 60% and 75% of the average pore size. PVD will generally require a smaller minimum thickness than CVD, although the required thickness in the field areas (over the insulators) will generally be greater for PVD for a given minimum thickness at the bottom of the via  62 . It will be understood, however, that the actual minimum thickness for blocking the pores will depend upon the structure of the porous low k material (connectivity of the pores, density of the pores, range of deviation from average pore size, etc.) as well as the nature of the deposition. In view of the present disclosure, the skilled artisan can readily optimize deposition of the sealing layer  148  for a given process flow through routine experimentation. 
     In the illustrated embodiment, with an average pore size of about 4 nm, the sealing layer  148  is between about 2 nm and 6 nm in thickness at the bottom end of the via  62 , more preferably between about 2.4 nm and 3 nm in thickness. The skilled artisan can readily determine the amount of deposition required, as measured in the field areas, to achieve the desired minimum thickness at the bottom of the via  62 . Typical dual damascene structures and conventional (CVD, PVD) deposition dictate the field:minimum bottom thickness ratio is about 10:1. For less severe aspect ratios (e.g., single damascene or simple contact structures), the thickness ratio will be reduced significantly. 
     In addition to the illustrated embodiment, the sealing layer may also be formed using an alternating process. In this aspect of the invention, the pulses of reactants may be modified compared to the ALD processes described above. For example, the pulse of metal source gas can be longer than in conventional ALD. In addition, the purge time may be shortened. Both changes are designed to maximize the amount of metal source gas remaining in the pores of the insulating material following the first purge stage. An example of preferable reaction conditions for this method of deposition is provided in Table VI below, whereby residual metal source gas in the pores will react in a CVD-mode reaction with reducing agent from a subsequent phase. However, as will be recognized by one skilled in the art, any ALD method for forming metal nitride or metal layers may be used by modifying the duration of the stages in the cycle to provide for the deposition of a conductive layer that seals the porous low k material. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE VI 
               
               
                   
               
               
                   
                 Carrier 
                   
                 Reactant 
                   
                   
                   
               
               
                   
                 Flow 
                   
                 Flow 
                 Temperature 
                 Pressure 
                 Time 
               
               
                 Phase 
                 (slm) 
                 Reactant 
                 (sccm) 
                 (° C.) 
                 (Torr) 
                 (sec) 
               
               
                   
               
             
            
               
                 metal 
                 600 
                 WF 6   
                 50 
                 400 
                 10 
                 0.5 
               
               
                 purge 
                 600 
                 — 
                 — 
                 400 
                 10 
                 0.5 
               
               
                 reduce 
                 600 
                 TEB 
                 40 
                 400 
                 10 
                 0.2 
               
               
                 purge 
                 600 
                 — 
                 — 
                 400 
                 10 
                 0.8 
               
               
                   
               
            
           
         
       
     
     The minimum thickness for an alternately deposited sealing layer will typically represent between about 50 and 150% of the average pore size, more preferably between 60 and 75% of the average poor size. As for PVD and conventional CVD, the actual minimum thickness will depend on the structure of the porous low k material. However, this embodiment provides for a higher conformality deposition of conductive material, thus eliminating the need for a directional etch and providing for the ability to seal a higher aspect structure. 
     With reference to FIG. 11, if the sealing layer  148  is produced by conventional CVD or PVD, it is followed by a high step coverage lining layer  150 , constructed in accordance with processes set forth above. As previously noted, depending upon the materials of the via and trench structure and the chemistries of the various deposition steps, the liner  150  can comprise an initial metal adhesion layer in addition to a metal nitride barrier layer. The lining layer  150  can comprise, for example, a bilayer of W/TiN, W/WN, Ti/TiN, Ti/WN, and any of a number of other combinations of adhesion film and barrier film. In the example of Table IV, the barrier layer is deposited directly over metal and insulating surfaces of the dual damascene structure, and can optionally comprise a WN/TiN bilayer. Preferably, at least one of the sublayers is formed by a highly conformal ALD process, in accordance with the methods disclosed above. The skilled artisan will recognize the liner  150  (or at least one sub-layer therein) as the result of a highly conformal ALD process by the layer&#39;s characteristic profile, and particularly by the uniformity of this profile. 
     The sealing layer  148 , however, prevents this high conformal process from penetrating into the porous low k material. Most preferably, the underlying sealing layer  148  serves both a pore blocking or plugging function as well the adhesion function. This is the case when an alternating pulse method is used to produce the sealing layer  148 . This method produces a high conformality conductive layer of either metal or metal nitride. In addition, it is possible to use any of these methods to create a metal/metal nitride bilayer that acts to seal the porous low k material. Accordingly, the ALD lining layer  150  preferably consists of a barrier layer alone. 
     In accordance with the barrier needs of dual damascene processing, and particularly in conjunction with fast-diffusing copper metal filler, the metal nitride barrier layer of the lining layer  150  is formed to about the minimal thickness necessary for adequate performance of its barrier function. Accordingly, the metal nitride layer lining the deep trench and via structure preferably has a thickness greater than about 20 Å. At the same time, high step coverage provided by the methods disclosed herein enable formation of the desired thickness uniformly over all surfaces of the trench  60  and contact via  62 , including insulating sidewalls and a conductive via floor. Accordingly, the metal nitride liner within the via  62  is preferably no more than about 200 Å at any point of the structure and at any point during the process. More preferably, the metal nitrides of the preferred embodiments are deposited to a maximum thickness of between about 20 Å and 100 Å, and most preferably between about 40 Å and 80 Å. 
     Under the preferred conditions, material sufficient for a fraction of a monolayer is deposited per cycle, due to the physical size of the chemisorbed species preventing occupation of all available sites, particularly if the adsorbed species include organic ligands. In example of Table II, TiN grows at about 0.35 Å/cycle, such that preferably greater than about 50 cycles, more preferably between about 60 and 300 cycles, and most preferably between about 60 and 200 cycles are performed to produce an adequate TiN barrier layer to prevent copper diffusion. 
     As noted, the methods described herein enable extremely high step coverage (defined as a thickness of the liner on the bottom of the via as a ratio of the thickness of the liner on sidewalls of the via), even of the high aspect ratio trench and via structures of the preferred embodiments. The lining layer  150 , and particularly ALD-formed film(s) within the liner  150 , preferably exhibit step coverage greater than about 90%, more preferably greater than about 93%, and most preferably greater than about 97%. 
     With reference now to FIG. 11, a seed layer  155  is optionally formed over the lining layer  150 . As noted above, such a seed layer  155  is desirable where the filling metal is to be deposited by electroplating and where the lining layer  150  demonstrates inadequate conductivity for effective electroplating. Under such conditions, the seed layer  155  can be deposited by PVD, more preferably by CVD and most preferably by ALD. In the illustrated embodiment, a “pure” copper is employed for the seed layer. In other arrangements, tungsten can be used as an electroplating seed layer. In still other arrangements, no seed layer is employed over the lining layer  150 , such as in process flows preceding a non-electroplating fill procedure or where the barrier layer is adequately conductive (e.g., tungsten nitride), and enables direct nucleation of electroplated copper. 
     Referring now to FIG. 12, the lined damascene structure is then filled with a highly conductive metal  160 . In the illustrated embodiment, where a seeding film is formed over the lining layer  150 , the filler metal  160  preferably comprises electroplated copper. In other arrangements, metal such as aluminum can be deposited under high pressure and/or high temperatures to aid reflow into the deep trench and via structures, as will be appreciated by the skilled artisan. Effective barriers are also important in preventing spiking during the harsh conditions attending such deposition processes. 
     With reference now to FIG. 13, the structures are then planarized by chemical mechanical planarization (CMP) or other etch back process, to leave isolated lines  170  within the trenches  60 , having integral contacts  180  extending downwardly therefrom. Diffusion of the filler metal  160  is prevented both during the fill process as well as during any high temperature processing that follows. 
     Although the foregoing invention has been described in terms of certain preferred embodiments, other embodiments will be apparent to those of ordinary skill in the art. For example, while processes are specifically provided particular lining materials, the skilled artisan will readily appreciate that ALD methods can be applied to lining damascene structures with other materials. Moreover, although illustrated in connection with a particular process flow and structure for dual damascene metallization, the skilled artisan will appreciate variations of such schemes for which the methods disclosed herein will have utility. Additionally, other combinations, omissions, substitutions and modification will be apparent to the skilled artisan, in view of the disclosure herein. Accordingly, the present invention is not intended to be limited by the recitation of the preferred embodiments, but is instead to be defined by reference to the appended claims.