Abstract:
Generally, according to the present invention, the sidewall of the adhesion layer (e.g. TiN 36) in a lower electrode is pre-oxidized after deposition of an unreactive noble metal layer (e.g. Pt 38) but before deposition of an HDC material (e.g. BST 42). An important aspect of the present invention is that the pre-oxidation of the sidewall generally causes a substantial amount of the potential sidewall expansion (and consequent noble metal layer deformation) to occur before deposition of the HDC material. One embodiment of the present invention is a microelectronic structure comprising a supporting layer having a principal surface, and an adhesion layer overlying the principal surface of the supporting layer, wherein the adhesion layer comprises a top surface and an expanded, oxidized sidewall (e.g. TiO 2  40). The structure further comprises a noble metal layer overlying the top surface of the adhesion layer, wherein the noble metal layer comprises a deformed area overlying the oxidized sidewall, and a high-dielectric-constant material layer overlying the noble metal layer. The high-dielectric-constant material layer is substantially free of expansion stress cracks in proximity to the deformed area of the noble metal layer.

Description:
This is a divisional of application Ser. No. 08/283,467 filed Aug. 1, 1994. 
    
    
     CROSS-REFERENCES TO RELATED APPLICATIONS 
     The following related applications were filed concurrently with the instant application: 
     
         __________________________________________________________________________Title                       Inventors                              Ser. No.__________________________________________________________________________Improved High-Dielectric-Constant Material Electrodes                       Summerfelt,                              08/283,881Comprising Thin Platinum Layers                       Beratan,                       Kirlin,                       GnadeImproved Electrodes Comprising Conductive Perovskite-                       Summerfelt,                              08/283,468Seed Layers for Perovskite Dielectrics                       BeratanImproved High-Dielectric-Constant Material Electrodes                       Summerfelt,                              08/283,442Comprising Thin Ruthenium Dioxide Layers                       Beratan,                       Kirlin,                       GnadeHigh-Dielectric-Constant Material Electrodes Comprising                       Nishioka,                              08/283,871Sidewall Spacers            Park,                       Bhattacharya,                       SummerfeltA Conductive Amorphous-Nitride Barrier Layer for High-                       Summerfelt                              08/293,441Dielectric-Constant Material ElectrodesA Conductive Exotic-Nitride Barrier Layer for High-                       Summerfelt                              08/283,873Dielectric-Constant Material ElectrodesA Conductive Noble-Metal-Insulator-Alloy Barrier Layer                       Summerfelt,                              08/283,454for High-Dielectric-Constant Material Electrodes                       Nicolet,                       Reid,                       Kolawa__________________________________________________________________________ 
    
     The following previously filed applications are related to the instant application: 
     
         __________________________________________________________________________                              U.S. Pat.Title                       Inventors                              Ser. No__________________________________________________________________________Improved Electrical Connections to Dielectric Materials                       Gnade, U.S.                       Summerfelt                              5,348,894Improved Electrical Connections to Dielectric Materials                       Gnade, 08/260,149                       SummerfeltLightly Donor Doped Electrodes for High-Dielectric-                       Summerfelt,                              08/040,946Constant Materials          Beratan,                       GnadeLightly Donor Doped Electrodes for High-Dielectric-                       Summerfelt,                              08/276,191Constant Materials          Beratan,                       GnadeImproved Electrode Interface for High-Dielectric-Constant                       Summerfelt,                              08/041,025Materials                   Beratan__________________________________________________________________________ 
    
     FIELD OF THE INVENTION 
     This invention generally relates to improving electrical connections to materials with high-dielectric-constants, such as in the construction of capacitors. 
     BACKGROUND OF THE INVENTION 
     Without limiting the scope of the invention, its background is described in connection with current methods of forming electrical connections to high-dielectric-constant materials, as an example. 
     The increasing density of integrated circuits (e.g. DRAMs) is increasing the need for materials with high-dielectric-constants to be used in electrical devices such as capacitors. Generally, capacitance is directly related to the surface area of the electrode in contact with the capacitor dielectric, but is not significantly affected by the electrode volume. The current method generally utilized to achieve higher capacitance per unit area is to increase the surface area unit area by increasing the topography, such as in trench and stack capacitors using SiO 2  or SiO 2  /Si 3  N 4  as the dielectric. This approach becomes very difficult in terms of manufacturability for devices such as the 256 Mbit and 1 Gbit DRAMs. 
     An alternative approach is to use a high permittivity dielectric material. Many perovskite, ferroelectric, or high-dielectric-constant (hereafter abbreviated HDC) materials such as (Ba,Sr)TiO 3  (BST) usually have much larger capacitance densities than standard SiO 2  --Si 3  N 4  --SiO 2  capacitors. Various metals and metallic compounds, and typically noble metals such as Pt and conductive oxides such as RuO 2 , have been proposed as the electrodes for these HDC materials. To be useful in electronic devices, however, reliable electrical connections should generally be constructed which do not diminish the beneficial properties of these high-dielectric-constant materials. 
     SUMMARY OF THE INVENTION 
     As used herein, the term &#34;high-dielectric-constant&#34; means a dielectric constant greater than about 50 at device operating temperature. The deposition of an HDC material usually occurs at high temperature (generally greater than about 500° C.) in an oxygen containing atmosphere. Many electrode materials oxidize and become insulating or otherwise degrade in this type of environment. An initial electrode structure formed prior to the HDC material deposition should be stable both during and after this deposition, while subsequent electrode structures formed after this deposition need only be stable after this deposition. 
     As mentioned hereinabove, Pt has been suggested for the electrodes of an HDC material layer in standard thin-film (herein defined as generally less than 5 microns (um)) applications. However, although Pt is unreactive with respect to the HDC material, it has been found that it is difficult to use Pt alone as an initial electrode. Pt generally allows oxygen to diffuse through it and hence typically allows neighboring materials to oxidize. In addition, Pt also does not normally stick very well to traditional dielectrics such as SiO 2  or Si 3  N 4 , and Pt can rapidly form a silicide at low temperatures. Thus a Ta or TiN layer has been suggested as an adhesion or buffer layer under the Pt electrode. However, during BST deposition or annealing, oxygen can possibly diffuse through the Pt and oxidize the adhesion layer and make the adhesion layer less conductive. This may be more of a problem on the sides of the adhesion layer than on the top horizontal surface, since the Pt will generally be thicker on the top, and a better diffusion barrier. 
     Conductive oxides such as RuO 2  have also been suggested for the electrodes of an HDC material layer in standard thin-film applications. Again, although RuO 2  is unreactive with respect to the HDC material, it too generally has difficulties. For example, the electrical properties of the structures formed using these oxides are usually inferior to those formed using e.g. Pt. Many thin-film applications require a small leakage-current-density in addition to a large capacitance per unit area. The leakage current is sensitive to many variables such as thickness, microstructure, electrodes, electrode geometry and composition. For example, the leakage current of lead zirconium titanate (PZT) using RuO 2  electrodes is several orders of magnitude larger than the leakage current of PZT using Pt electrodes. In particular it appears that the leakage current is controlled by Schottky barriers, and that the smaller leakage current with Pt electrodes appears to be due to the larger work function. 
     As used herein, the term &#34;unreactive&#34;, when used in reference to a material contacting an HDC material, means a material which provides a stable conductive interface to the HDC material during and after processing. Note that when a conductive oxide such as RuO 2  is used for the unreactive layer (or another part of the electrode), that layer can also contain unoxidized or partially oxidized Ru. For example, an unreactive layer of Ru which is chemically changed by becoming partially or fully oxidized during the HDC deposition process is still considered unreactive since it still provides a stable conductive interface to the HDC material. 
     Other structures which have been proposed for standard thin-film structures include alloys of Pt, Pd, Rh as the electrode and oxides made of Re, Os, Rh and Ir as a sticking layer on single crystal Si or poly-Si. A problem with these electrodes is that these oxides are usually not stable next to Si and that these metals typically rapidly form silicides at low temperatures (generally less than about 450° C). If other associated problems can be avoided or minimized, this type of electrode structure should retain its conductivity even after the deposition of the HDC material if an appropriate adhesion (barrier) layer(s) is used between the conductive oxide and the Si substrate. 
     Thus many of the proposed lower electrode structures comprise the following generic layers: HDC material/unreactive (oxygen stable layer/adhesion (barrier) layer/substrate. In these structures, the adhesion layer typically comprises a conductive material that oxidize under HDC material deposition conditions to provide a conductive oxide. It has been discovered that expansion stress and crack formation in the HDC material can occur due to the oxidizing and consequent expanding of the adhesion layer during HDC material deposition. 
     Although this oxidation/expansion can generally occur at any surface of the adhesion layer, oxidation of the top of the adhesion layer is substantially impeded by the overlying unreactive layer, and oxidation of the bottom of the adhesion layer is substantially impeded by the material surrounding it (e.g. the substrate). Generally, an exposed sidewall would be the most likely surface of the adhesion layer to be oxidized. Since most materials proposed for the adhesion layer experience a volume change when oxidized, the adhesion layer sidewall generally expands and deforms the overlying unreactive layer and causes stress and cracking of the HDC material layer. These cracks can reach from the upper surface of the HDC material layer down to the lower electrode, with detrimental results. For example, if a conductive layer, such as an upper electrode for a capacitor, is deposited on the HDC layer, the capacitor can have substantial leakage or even be shorted between the two electrodes. 
     Generally, according to the present invention, the sidewall of the adhesion layer is pre-oxidized before deposition of the HDC material (but after deposition of an unreactive noble metal layers. An important aspect of the present invention is that the pre-oxidation of the sidewall generally causes a substantial mount of the potential sidewall expansion and consequent noble metal layer deformation to occur before deposition of the HDC material. Potential sidewall expansion is defined as the total amount of expansion that occurs through HDC deposition and annealing (with or without pre-oxidation. 
     According to the present invention, the sidewall of the adhesion layer is substantially oxidized before HDC deposition. In contrast to the present invention, superficial oxidation (e.g. forming a few monolayers of oxide) at various stages of fabrication of a ferroelectric capacitor has apparently been described in the prior art. See European Patent Application 557,937 A1, D. Patel et al., Ramtron International Corp., filed Feb. 23, 1993. The oxidation described by D. Patel et al. is superficial and is apparently in a different portion of the structure and for a different purpose, i.e., better adhesion to the bottom glass layer and to the top ferroelectric material, than the present invention. The adhesion layer sidewall of the present invention must generally be more than merely superficially oxidized, since a superficially oxidized adhesion layer would generally still undergo substantial oxidation and expansion during HDC deposition, to the detriment of the structure. 
     One embodiment of the present invention is a microelectronic structure comprising a supporting layer having a principal surface, and an adhesion layer overlying the principal surface of the supporting layer, wherein the adhesion layer comprises a top surface and an expanded, oxidized sidewall. The structure further comprises a noble metal layer overlying the top surface of the adhesion layer, wherein the noble metal layer comprises a deformed area overlying the oxidized sidewall, and a high-dielectric-constant material layer overlying the noble metal layer. The high-dielectric-constant material layer is substantially free of expansion stress cracks in proximity to the deformed area of the noble metal layer. 
     A method of forming an embodiment of the present invention comprises forming a supporting layer having a principal surface, forming an adhesion layer on the principal surface of the supporting layer, and forming a noble metal layer on a top surface of the adhesion layer, wherein the adhesion layer comprises a substantially unoxidized sidewall. The method further comprises oxidizing the unoxidized sidewall of the adhesion layer to form a pre-oxidized sidewall, and depositing a high-dielectric-constant material layer on the noble metal layer. The preoxidized sidewall minimizes further oxidation and expansion of the adhesion layer adjacent the pre-oxidized sidewall. Expansion stress and cracking of the high-dielectric-constant material layer is minimized during the step of depositing the high-dielectric-constant material layer. 
     These are apparently the first microelectronic structures wherein an electrode to an HDC material comprises an adhesion layer with a pre-oxidized sidewall which impedes the adhesion layer from undergoing substantial oxidation and expansion during HDC material deposition. These structures may also be used for multilayer capacitors and other thin-film devices such as pyroelectric devices (e.g. (uncooled) infrared detectors), non-volatile ferroelectric RAMs (using permanent polarization properties), thin-film piezoelectric and thin-film electro-optic devices. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as other features and advantages thereof, will be best understood by reference to the detailed description which follows, read in conjunction with the accompanying drawings, wherein: 
     FIGS. 1-4 are cross-sectional views showing the progressive steps in the fabrication of a high-dielectric constant material capacitor with an electrode comprising a non-conductive, pre-oxidized sidewall; and 
     FIGS. 5-8 are cross-sectional views showing the progressive steps in the fabrication of a high-dielectric constant material capacitor with an electrode comprising a conductive, pre-oxidized sidewall. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIGS. 1-4, there is shown a method of forming an embodiment of the present invention, a lower electrode structure comprising a preoxidized sidewall. FIG. 1 illustrates an SiO 2  layer 32 overlying silicon semiconductor substrate 30. SiO 2  layer 32 may or may not be capped with a diffusion barrier such as TiO 2  or Si 3  N 4 . TiSi 2  /poly-Si plug 34 provides electrical connection through SiO 2  layer 32. A 100 nm TiN layer 36 has been reactively sputter deposited and patterned etched on the structure. 
     Various other standard processes can be used to form this portion structure, such as sputter depositing Ti on poly-Si followed by an N 2  rapid thermal anneal (700° C. for 30 seconds NH 3  furnace anneal (575° C. for 10 minutes). The TiN is then selectively removed chemically using peroxide to form the patterned TiN layer 36 shown. In an alternate method, a vapor HF clean of the underlying poly-Si is performed immediately prior to the deposition of TiN layers 36, without using TiSi 2 . It is beneficial that the structure not be exposed to a contaminating environment, such as the atmosphere, between the HF clean and the adhesion layer deposition process steps, in order to ensure good adhesion between the poly-Si and the TiN layer. 
     A 100 nm Pt layer 38 is then DC sputter deposited in a 5 mTorr Ar atmosphere using a Pt target with the substrate temperature held at 325° C. Pt layer 38 can also be deposited using e-beam evaporation, chemical vapor deposition (CVD), or metalorganic CVD (MOCVD). The height of Pt layer 38 can vary depending on the desired capacitance density of the HDC material, the total desired capacitance, and the generation of the device. For example, future devices such as 1G DRAMs may generally require taller capacitors to provide more electrode surface area/unit area as compared to 256 DRAM devices, since 1G DRAMs will generally need to provide more capacitance/unit area (due to e.g. increased functionality and shrinking device features). After deposition of Pt layer 38, photoresist is deposited and patterned. Platinum layer 38 is then dry etched in a low-pressure, high-density plasma reactive ion etch (RIE) reactor to form the structure shown in FIG. 1. 
     The structure is then annealed in a diluted oxygen (5% O 2  in N 2 ) gas at 650° l C. to form TiO 2  sidewall 40 as shown in FIG. 2. The substantial deformation of the structure. including Pt layer 38, occurs at this point, before deposition of the HDC material. Alternatively, ozone could be used for annealing. Alternatively, the structure could be annealed at a lower temperature (e.g. 600° C.), allowing Pt layer 38 more time to relax than if the oxidation is performed at full BST deposition temperature. Another benefit of this oxidation anneal process is that Pt layer 38 can rearrange to round any relatively sharp corners present after etching, since sharp corner can cause excess leakage current or even cracks. 
     Then BST layer 42 is formed on the electrode structure using metal organic chemical vapor deposition (MOCVD) at 650° C. in an O 2  /Ar (1/9) mixture gas at a pressure of 10 mTorr, resulting in the structure shown in FIG. 3. Substantial oxidation or expansion of the TiO 2  sidewalls does not occur during BST deposition, thus minimizing expansion stress and cracks in BST layer 42. The deposition may used ionic, photonic, electronic or plasma enhancement. BST layer 42 may also be formed by CVD, sputter or spin coat methods. 
     Upper Pt electrode 44 is them sputter deposited and dry etched to form the capacitor structure shown in FIG. 4. This structure, with both lower and upper electrodes, can again be annealed to improve the capacitor properties. 
     With reference to FIGS. 5-8, there is shown a method of forming another embodiment of the present invention, a capacitor with a lower electrode comprising conductive, pre-oxidized sidewalls. The structure illustrated in FIG. 5 is similar to the structure of FIG. 1, except that ruthenium is deposited for adhesion layer 46, instead of TiN. Since Ru has a conductive oxide, the surface of ruthenium layer 46 is oxidized to form RuO 2  layer 48, before deposition of Pt layer 38. In this embodiment, current will still be able to flow between substrate 30 and Pt layer 38 even though the top surface of the adhesion layer has been oxidized. 
     The structure is then annealed in an oxygen containing atmosphere to form RuO 2  sidewall 50 as shown in FIG. 6. As with the previous embodiment, the substantial deformation of the structure, including Pt layer 38, occurs at this point, before deposition of the HDC material. 
     Then BST layer 42 is formed on the electrode structure using MOCVD as described hereinabove, resulting in the structure shown in FIG. 7. Again, substantial oxidation or expansion of the RuO 2  sidewalls does not occur during BST deposition, thus minimizing expansion stress and cracks in BST layer 42. Upper Pt electrode 44 is them sputter deposited and dry etched to form the capacitor structure shown in FIG. 8. 
     One potential advantage of this embodiment is that the conductive sidewall structure of FIG. 8 generally allows more electrode surface area to be in contact with the HDC material as compared to the non-conductive sidewall structure of FIG. 4. 
     Another potential advantage of this embodiment is that the top surface of Ru layer 46 is oxidized before the deposition of Pt layer 38, so further oxidation of the top surface of the Ru layer 46/RUO 2  layer 48 is minimized. The existing oxide limits the formation of irregular oxidized areas on the top surface of the adhesion layer due to diffusion of oxygen through Pt layer 38 (e.g. along pain boundaries), which can cause hillocking of Pt layer 38. 
     The sole Table, below, provides an overview of some embodiments and the drawings. 
     
                       TABLE______________________________________  Preferred orDrawing  Generic               Other AlternateElement  Examples    Term      Examples______________________________________30     Silicon     Substrate Other single component                        semiconductors                        (e.g. germanium, diamond)                        Compound                        semiconductors (e.g.                        GaAs, InP, Si/Ge, SiC)                        Ceramic substrates32     Silicon dioxide              First level                        Other insulators              insulator (e.g. silicon nitride)                        Doped insudators                        (e.g. BSG, PSG, BPSG)                        May be more than                        one layer                        (e.g. Si.sub.3 N.sub.4                        barrier over SiO.sub.2)                        May or may not                        be used (i.e. first                        level insulator,                        substrate, another                        insulating layer                        or a combination                        thereof may be                        the supporting                        layer for the                        lower electrode)                        Combinations of                        the above materials34     TiSi.sub.2 /poly-Si              Conductive                        Other metal              plug      compounds                        (e.g. nitrides:                        titanium nitride,                        zirconium nitride:                        silicides:                        tantalum silicide,                        tungsten silicide,                        molybdenum silicide,                        nickel silicide;                        carbides: boron                        carbide,                        tantalum carbide;                        borides:                        titanium boride)                        Conductive metals                        (e.g. tungsten,                        tantalum,                        titanium, molybdenum)                        Single component                        semiconductors                        (e.g. single- or                        poly-crystalline                        silicon, germanium)                        Compound                        semiconductors                        (e.g. GaAs, InP,                        Si/Ge, SiC)                        Other silicides                        may be used in a                        composite structure                        (Ni silicide,                        Co silicide, tungsten                        silicide)                        May be multiple                        layers (e.g.                        TiN/TiSi.sub.x /poly-Si)                        Combinations of                        the above materials36     TiN         Adhesion  Other conductive              layer     metal compounds46     Ruthenium             (e.g. oxides:                        ruthenium oxide,                        osmium oxide,                        rhodium oxide,                        iridium oxide,                        indium oxide,                        tin oxide)                        Conductive metals                        (different from                        specific material                        selected for                        drawing element                        38 below)                        (e.g. tungsten,                        tantalum, titanium,                        tin, ruthenium,                        indium, osmium,                        rhodium, iridium)                        Ternary (or greater)                        amorphous nitrides                        (e.g. Ta--Si--N,                        Ti--Si--N, Ta--B--N,                        Ti--B--N)                        Exotic conductive                        nitrides (e.g.                        titanium aluminum                        nitride, Zr nitride,                        Hf nitride, Y nitride,                        Sc nitride, La nitride and                        other rare earth nitrides,                        N deficient Al nitride,                        doped AI nitride,                        Mg nitride,                        Ca nitride,                        Sr nitride,                        Ba nitride)                        Alloys of the                        above exotic                        conductive nitrides                        with common Si                        processing materials                        such as TiN, GaN,                        Ni nitride, Co nitride,                        Ta nitride, W nitride                        (e.g. Ta--Al--N)                        Noble metal                        insulator alloys                        (e.g. Pd--Si--N, Pr--Si--N,                        Pd--Si--O, Pd--Si--O,                        Pd--B--(O,N),                        Pd--Al--N, Ru--Si--(O,N),                        Ir--Si--O, Re--Si--N,                        Rh--Al--O, Au--Si--N,                        Ag--Si--N)                        May be multiple lavers                        (e.g. TiN/TiSi,                        TiN/TiSi/poly-Si)                        May be a layer                        having relatively                        better barrier properties                        over a layer having                        relatively better                        adhesive properties                        (e.g. Ru/TiN)                        Combinations of                        the above materials38     Platinum    Noble metal                        Other oxidation              layer     resistant noble or                        platinum group metals                        (e.g. palladium,                        iridium, rhodium)                        Combinations of                        the above materials                        Layers of the                        above materials40     TiO.sub.2   Pre-Oxidized                        Oxides formed              sidewall  from materials                        used for drawing                        elements 36 and50     RuO.sub.2             46 above (e.g. tantalum                        oxide, titanium oxide,                        tin oxide, indium                        oxide, iridium oxide,                        ruthenium oxide)                        Combinations of                        the above materials42     Barium      Hogh-     Other perovskite,  strontium   dielectric-                        pyroelectric,  titanate    constant  ferroelectric or              material  high-dielectric-              layer     constant oxides                        (e.g. (Ba, Sr, Ca, Pb)                        (Ti, Zr)O.sub.3,                        (Pb, La)(Zr, Ti)O.sub.3,                        bismuth titanate,                        potassium tantalate,                        lead scandium tantalate,                        lead niobate,                        potassium niobate,                        lead zinc niobate,                        lead magnesium niobate                        tantalum pentoxide,                        yttrium oxide)                        Donor, acceptor,                        or donor and                        acceptor doped oxides                        listed above                        Combinations of                        the above materials                        Layers of the                        above materials.44     Platinum    Upper     Conductive metal              electrode compounds (e.g. nitrides:                        titanium nitride,                        ruthenium nitride,                        tin nitride,                        zirconium nitride;                        oxides:                        ruthenium dioxide,                        tin oxide,                        titanium oxide,                        TiON, zinc oxide,                        doped zinc oxide,                        iridium oxide;                        silicides: titanium                        silicide, tantalum                        silicide, tungsten                        silicide, molybdenum                        silicide, nickel silicide;                        carbides: tantalum                        carbide; borides:                        titanium boride)                        Other noble or                        platinum group metals                        (e.g. palladium,                        ruthenium, rhodium,                        gold, iridium, silver)                        Reactive metals                        (e.g. tungsten, tantalum,                        titanium, molybdenum)                        Other common                        semiconductor electrodes                        (e.g. aluminum,                        doped Si or Ge)                        Combinations of                        the above materials                        May contain more                        than one layer48     RuO.sub.2   Conductive                        Other conductive              oxide layer                        oxides (e.g. ruthenium                        oxide, osmium                        oxide, rhodium                        oxide, iridium                        oxide, tin oxide,                        indium oxide)                        Combinations of                        the above materials______________________________________ 
    
     A few preferred embodiments have been described in detail hereinabove. It is to be understood that the scope of the invention also comprehends embodiments different from those described, yet within the scope of the claims. With reference to the structures described electrical connections to such structures can be ohmic, rectifying, capacitive, direct or indirect, via intervening circuits or otherwise. Implementation is contemplated in discrete components or fully integrated circuits in silicon, germanium, gallium arsenide, or other electronic materials families. In general the preferred or specific examples are preferred over the other alternate examples. The scale of the figures is neither to absolute nor relative scale; some thicknesses have been exaggerated for clarity of explanation. Some components of the lower electrode may sometimes be referred to as being part of the electrode and may sometimes be referred to as being interior to, exterior to, inside of, outside of, on, under, etc. the electrode; the structures and methods of the present invention are substantially the same in either case. 
     The adhesion layer may comprise other materials than those listed in the table but which are generally less preferred than those materials in the table. For example, the adhesion layer may comprise other metal compounds such as ruthenium nitride, tin nitride, tungsten nitride, tantalum nitride, titanium oxide, TiON, titanium silicide, tantalum silicide, tungsten silicide, molybdenum silicide, nickel silicide, cobalt silicide, iron silicide, chromium silicide, boron carbide, tantalum carbide, titanium carbide, zirconium carbide, titanium boride or zirconium boride. Alternatively, the adhesion layer may comprise other conductive metals (different from the specific material selected for drawing element 38) such as cobalt, iron, chromium, palladium, rhenium, zirconium, hafnium or molybdenum. Alternatively, the adhesion layer may comprise single component semiconductors such as single- or poly-crystalline silicon or germanium, or compound semiconductors such as GaAs. InP, Si/Ge or SiC. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.