the preferred embodiments are illustrated in the context of metallization processes in an integrated circuit , including contact plug formation and damascene processes , where selective tungsten ( w ) partially fills a void and aluminum is sputtered thereover . the skilled artisan will readily appreciate , however , that the materials and methods disclosed herein will have application in a number of other contexts where contact openings are partially filled with a first conductive layer , desirably followed by a more suitable subsequent conductive layer . fig1 schematically illustrates a process flow for forming a partial contact plug over which further metal can be deposited in accordance with a preferred embodiment of the invention . as shown , the process begins with formation 4 of a dual damascene structure comprising trenches and vias . the trenches and vias may be formed using conventional photolithography and etch technologies . while not shown , the skilled artisan will appreciate that the vias and trenches can at this point be lined with adhesion and / or barrier layers . desirably , any such layers are as thin as possible for their assigned function . typical liners include titanium , titanium nitride , tantalum , tantalum nitride , other refractory metals , metal nitrides , and combinations of the same . subsequently , the via is partially filled 6 with a conductive material , preferably in a batch processing tool , more preferably in a continuous deposition process . in the illustrated embodiment , the tool comprises a hot wall , low - pressure chemical vapor deposition ( lpcvd ) reactor . the deposition comprises a selective or preferential metal deposition described , and more particularly selective tungsten deposition , as described in more detail below . the skilled artisan will readily appreciate , however , that the principles disclosed herein can be applied to other types of selective deposition and tools therefor . as illustrated , the next process step is the formation 8 of a more conductive layer above the selectively deposited tungsten ( w ) layer . the more conductive layer is preferably deposited by physical vapor deposition ( pvd ), such as sputtering , and in the illustrated embodiment the deposition comprises a hot aluminum process . the skilled artisan will readily appreciate , however , that the principles herein can be applied to other types of deposition , such as chemical vapor deposition ( cvd ). the illustrated process flow also includes a subsequent formation 9 of a second conductive filler material over the first material . in the preferred embodiment , forming the second filler material comprises depositing aluminum in a “ cold ” sputter process over the hot deposited aluminum layer . the second filler material can thus be formed in the same pvd sputtering tool with the same process parameters used for the formation 8 of the hot aluminum , except for the temperature range . the next process step , according to fig1 is the planarization 10 of the workpiece surface . in the illustrated embodiment , the planarization comprises a polishing or mechanical abrasion , more preferably chemically aided by slurry provided between the polishing pad and the workpiece . such a planarization process is widely referred to in the industry as a chemical and mechanical polishing ( cmp ) process . in other arrangements , the skilled artisan will appreciate that etchback processes such as reactive ion etching ( rie ) can also effectively remove horizontal surfaces of a workpiece . fig2 - 6 illustrate structures that result from the process described generally above with respect to fig1 . with reference initially to fig2 a partially fabricated integrated circuit is shown on a workpiece or substrate assembly 20 . the structure is formed above a substrate ( not shown ), which may comprise a single - crystal silicon wafer or other semiconductor layer in which active or operable portions of electrical devices are formed . an interlevel dielectric ( ild ) 42 is formed above the substrate . typical ild materials include oxides formed from tetraethylorthosilicate ( teos ), borophosphosilicate glass ( bpsg ), polyamide , etc ., and the illustrated ild 42 comprises bpsg . the ild 42 has a thickness adequate to electrically insulate underlying conductors from overlying conductors , which depends upon circuit design and operational parameters . in the illustrated embodiment , where the substrate assembly 20 represents a static random access memory ( sram ) or dynamic random access memory ( dram ) circuit , the ild 42 preferably has a thickness between about 6 , 000 å and 16 , 000 å , desirably on the order of about 9 , 000 å . as shown in fig2 a dual damascene structure is formed within the ild 42 . the skilled artisan will readily appreciate that the ild 42 thus generally includes two separately formed , planarized insulating layers , typically separated by an etch stop or hard mask layer . the dual damascene structure preferably comprises a first opening that exposes an underlying conductive circuit element . in the illustrated embodiment , the first opening comprises an intermetal contact via 25 , though in other arrangements the opening can comprise a contact opening to the semiconductor substrate . for current state - of - the - art circuit designs , the via 25 has a width of less than about 0 . 25 μm , more preferably less than about 0 . 20 μm , and a depth of about 2 , 000 å to 4 , 000 å . such narrow dimensions result in aspect ratios ( height : width ) greater than about 1 : 1 , and more preferably greater than about 2 : 1 . in the illustrated embodiment , a metal - to - metal contact is to be formed in the via 25 having an aspect ratio between about 1 : 1 and 3 : 1 . aspect ratios in future generation integrated circuits will likely be higher still , since parasitic capacitance concerns limit ild thickness scaling while lateral dimensions continued to be scaled . conventional photolithography techniques can be employed to define the via 25 , and anisotropic etching ( e . g ., reactive ion etching ) is preferred for producing vertical via sidewalls . the dual damascene structure also preferably comprises a second opening . in the preferred embodiment , the second opening comprises a wiring trench 30 that overlies the contact via 25 . similar to the via 25 , conventional photolithography techniques can be employed to define the wiring trench 30 , and anisotropic etching ( e . g ., reactive ion etching ) is preferred for producing vertical via sidewalls . preferably , in the illustrated embodiment , the wiring trench 25 has a width of less than about 1 μm , more preferably less than about 0 . 5 μm . the skilled artisan will readily appreciate many manners of forming the dual damascene structure of fig2 . for example , such structures can be formed by sequential masking steps forming the trenches in wiring patterns followed by vias or openings extending from the bottom of the trenches , or by forming a buried hard mask for the vias and simultaneously etching through a photoresist mask defining the trenches and the hard mask . the illustrated circuit element exposed by the openings comprises a contact landing pad of an underlying conductive runner or wiring layer 40 . the wiring layer 40 preferably comprises copper , aluminum or alloys thereof , though the skilled artisan will appreciate that other conductive materials may be suitable , depending upon the function and desired conductivity of the circuit element . for example , in other arrangements , the lower circuit element can comprise a tungsten or polysilicon feature ( e . g ., local interconnect or contact ). referring to fig3 a , a conductive partial plug 55 is selectively deposited to partially fill the via 25 . as noted with respect to fig1 the preferred embodiments employ a selective or preferential metal deposition , particularly selective tungsten ( w ) deposition , such that more metal is deposited at the bottom surface of the via 25 ( i . e ., over the conductor 40 ) than on insulating surfaces of the dual damascene structure . the partial plug 55 preferably extends from the bottom of the via 25 to between about ⅓ to ⅔ of the via 25 height , more preferably to between about ½ to ⅔ of the via height . in the embodiment of fig3 a , the deposition is shown as completely selective . preferably , the partial plug 55 also occupies between about ⅓ to ⅔ of the via 25 volume , more preferably between about ½ to ⅔ . to provide the desired advantages , however , it is not critical for the partial plug 55 deposition to be completely selective within the via 25 . as is illustrated in fig3 b , therefore , deposition of the partial plug 55 can be accompanied by a lesser deposition of the conductive material over exposed top surfaces of the ild 42 . any such deposition preferably results in less than about 500 å , more preferably less than about 100 å over the insulating surfaces . the ratio of deposition thickness within the via 25 to deposition within the trench 30 to deposition over insulating surfaces is thus preferably greater than about 10 : 1 , more preferably greater than about 20 : 1 . most preferably , the deposition is perfectly selective . generally , parameters that reduce the selectivity of w deposition are relatively higher process temperatures ( i . e ., greater than about 500 ° c . ), relatively higher total and / or wf 6 partial pressure and longer deposition times ( i . e ., higher w thickness ). also , if wafers are not cleaned properly , nucleation of w can take place on surrounding insulation layers such as sio 2 . accordingly , the workpiece is preferably first cleaned by argon or hydrogen plasma and / or wet cleans . selective tungsten deposition in accordance with the illustrated embodiment is conducted in any suitable reactor , and the illustrated embodiment employs a hot - wall lpcvd reactor ( not shown ). the skilled artisan will recognize that other cvd equipment can also be used . the hot - wall reactor comprises resistive heating elements , gas inlet ports for gas mixture entry and outlet ports for residual gases to exit through . the gas inlet ports provide an inert carrier gas , preferably nitrogen gas ( n 2 ), for purging of the reactor chamber before and after wafer processing . additionally , reactant gases for a selective deposition , namely hydrogen ( h 2 ) and tungsten hexafluoride ( wf 6 ) gases in the illustrated embodiment , are introduced to the reactor chamber during selective tungsten deposition . the workpiece 20 is placed in the reactor chamber . in the preferred embodiment , the reactor chamber temperature is ramped between about 200 ° c . and 600 ° c ., and the total chamber pressure is preferably between about 4 torr and 15 torr . hydrogen ( h 2 ) gas is introduced into the reactor chamber at a rate of between about 5 sccm and 300 sccm . tungsten hexafluoride ( wf 6 ) flows at a rate of between about 5 sccm and 300 sccm . deposition under these conditions is preferably conducted for a time period sufficient to form the thickness as described herein above . under the preferred deposition conditions described above , hydrogen ( h 2 ) reduces tungsten hexafluoride ( wf 6 ) selectively at the surface of the conductor 40 to produce the partial plug 55 according the following equation : the skilled artisan will readily appreciate that other methods can be employed to selectively deposit a conductive material over the landing pad 40 , such as to form a partial plug 55 . in still other arrangements , the partial plug can be formed by blanket deposition and directional etchback of a conductive material . referring to fig4 an initial filler metal 60 , preferably more conductive than w , is subsequently deposited on the workpiece 20 . in the illustrated embodiment , the more conductive filler metal 60 comprises hot aluminum , preferably deposited with a thickness to completely fill the remaining portion of the via 25 and partially fill the overlying wiring trench 30 . preferably , the deposited thickness of the hot aluminum layer 60 is between about 4 , 000 å and 12 , 000 å , more preferably between about 6 , 000 å and 8 , 000 å . desirably , the aluminum is deposited to a thickness of about 1 . 5 times the depth of the trench 30 . sputtering in accordance with the illustrated embodiment is conducted in any suitable chamber , and the illustrated embodiment employs a dc magnetron sputtering chamber ( not shown ), such as is commercially available under the trade name endura 5500 pvd ii ™, from applied materials , inc . of santa clara , calif . the skilled artisan will recognize that other sputtering equipment can also be used . the chamber houses a target cathode ( al in the illustrated embodiment ) and a pedestal anode . in other arrangements , the chamber walls can form the anode . a gas inlet into the chamber includes a heavy inert gas , preferably argon ( ar ), for bombarding the target when a plasma is activated within the chamber . additionally , a carrier gas , such as hydrogen gas ( h 2 ) is preferably also introduced into the sputtering chamber to aid in supporting an active plasma within the chamber . exemplary process parameters include argon gas flow ( e . g ., 50 - 100 sccm ), hydrogen gas flow and sputtering power preferably of about 1 - 3 kw . for the preferred hot aluminum process , chamber pressure is preferably maintained at between about 2 mtorr and 8 mtorr , and more preferably between about 3 mtorr to 4 mtorr . the workpiece 20 is heated during sputtering to a process temperature range preferably between about 400 ° c . and 550 ° c ., more preferably between about 450 ° c . and 500 ° c . under such conditions , aluminum is allowed to flow over the hot workpiece surfaces , tending to fill low spots such as the remaining portion of the contact vias 25 . the skilled artisan will readily appreciate that these parameters can be adjusted for sputtering chambers of different volumes , electrode areas and electrode spacing . under the preferred sputtering conditions described above , ar ions strike the al target , liberating aluminum atoms that fall towards and deposit on the workpiece 20 below . deposition under these conditions is preferably conducted for a time period sufficient to at least fill the remaining portion of the via 24 and to partially fill the overlying trench 30 , as shown . fig5 illustrates a second filler metal 70 . the second filler metal 70 of the illustrated embodiment comprises a cold deposited aluminum formed directly over the hot aluminum layer 60 , preferably in the same process chamber in which the hot aluminum layer 60 was formed . the cold aluminum layer 70 is preferably deposited with a thickness to at least completely fill the trench 30 , preferably to a thickness between about 4 , 000 å and 8 , 000 å , more preferably between about 5 , 000 å and 6 , 000 å . step coverage is not as critical towards the top of the trench structure . accordingly , the temperature of the workpiece can be reduced during the second stage deposition to conserve thermal budgets and minimize damage to underlying components . accordingly , the temperature is preferably reduced to between about 200 ° c . and 400 ° c ., more preferably between about 250 ° c . and 350 ° c . deposition under these conditions is preferably conducted for a time period sufficient to adequately fill the remaining portion of the trench 30 , as shown in fig5 . with reference to fig6 the dual damascene structure formation is subsequently planarized to leave an interconnect wire or “ runner ” comprising the filler metal ( s ) 60 , 70 within the trenches 30 . as noted , the structure is preferably planarized by chemical mechanical polishing ( cmp ) or other etchback process to leave isolated wiring lines within the trenches 30 . underlying the trenches 30 are plugs formed of the partial plug 55 and the portion of the filler metal ( s ) 60 , 70 topping off the vias . accordingly , several objects and advantages inhere in the described invention . for example , w partially fills a contact opening , which effectively raises the contact floor of a dual damascene structure . advantageously , the raised contact floor allows for a more conductive metal to be deposited by less conformal deposition , such as physical vapor deposition . tungsten is adequately conductive for part of the contact plug and is minimally ( if at all ) deposited along the walls within the wiring trench . as a result , the remainder of the contact opening and most or all of the wiring trench is advantageously filled with a more conductive metal , such as aluminum . although this invention has been described in terms of a certain preferred embodiment and suggested possible modifications thereto , other embodiments and modifications that may suggest themselves and be apparent to those of ordinary skill in the art are also within the spirit and scope of this invention . accordingly , the scope of this invention is intended to be defined by the claims that follow .