Abstract:
Degradation of organic-doped silica glass low-k inter-layer dielectrics during fabrication is significantly reduced and resolution of submicron features is improved by the formation of dual nature capping/ARC layers on inter-layer dielectric films. The capping/ARC layer is formed in-situ on the organic-doped silica glass inter-layer dielectric. The in-situ formation of the capping/ARC layer provides a strongly adhered capping/ARC layer, formed with fewer processing steps than conventional capping and ARC layers.

Description:
RELATED APPLICATIONS 
     This application contains subject matter similar to subject matter disclosed in copending U.S. patent applications Ser. No. 09/819,987 filed on Mar. 29, 2001 and Ser. No. 09/819,615 filed on Mar. 29, 2001. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a method of manufacturing a semiconductor device exhibiting reduced capacitance loading. The present invention has particular applicability in manufacturing high density, multi-level semiconductor devices comprising sub-micron dimensions and exhibiting high circuit speed. 
     BACKGROUND OF THE INVENTION 
     The escalating requirements for high density and performance associated with ultra large-scale integration (ULSI) semiconductor device wiring are difficult to satisfy in terms of providing sub-micron-sized, low resistance-capacitance (RC) metallization patterns. This is particularly applicable when the sub-micron-features, such as vias, contact areas, lines, trenches, and other shaped openings or recesses have high aspect ratios (depth-to-width) due to miniaturization. 
     Conventional semiconductor devices typically comprise a semiconductor substrate, usually of doped monocrystalline silicon (Si), and a plurality of sequentially formed inter-metal dielectric layers and electrically conductive patterns. An integrated circuit is formed therefrom containing a plurality of patterns of conductive lines separated by interwiring spacings, and a plurality of interconnect lines, such as bus lines, bit lines, word lines, and logic interconnect lines. Typically, the conductive patterns of vertically spaced metallization levels are electrically interconnected by vertically oriented conductive plugs filling via holes formed in the inter-metal dielectric layer separating the metallization levels, while other conductive plugs filling contact holes establish electrical contact with active device regions, such as a source/drain region of a transistor, formed in or on a semiconductor substrate. Conductive lines formed in trench-like openings typically extend substantially parallel to the semiconductor substrate. Semiconductor devices of such type according to current technology may comprise five or more levels of metallization to satisfy device geometry and microminiaturization requirements. 
     A commonly employed method for forming conductive plugs for electrically interconnecting vertically spaced metallization levels is known as “damascene”-type processing. Generally, this process involves forming a via opening in the inter-metal dielectric layer or inter-layer dielectric (ILD) between vertically spaced metallization levels which is subsequently filled with metal to form a via electrically connecting the vertically spaced apart metal features. The via opening is typically formed using conventional lithographic and etching techniques. After the via opening is formed, the via is filled with a conductive material, such as tungsten (W), using conventional techniques, and the excess conductive material on the surface of the inter-metal dielectric layer is then typically removed by chemical-mechanical planarization (CMP). 
     A variant of the above-described process, termed “dual damascene” processing, involves the formation of an opening having a lower contact or via opening section which communicates with an upper trench section. The opening is then filled with a conductive material to simultaneously form a contact or via in contact with a conductive line. Excess conductive material on the surface of the inter-metal dielectric layer is then removed by CMP. An advantage of the dual damascene process is that the contact or via and the upper line are formed simultaneously. 
     High performance microprocessor applications require rapid speed semiconductor circuitry, and the integrated circuit speed varies inversely with the resistance and capacitance of the interconnection pattern. As integrated circuits become more complex and feature sizes and spacings become smaller, the integrated circuit speed becomes less dependent upon the transistor itself and more dependent upon the interconnection pattern. If the interconnection node is routed over a considerable distance, e.g., hundreds of microns or more, as in sub-micron technologies, the interconnection capacitance limits the circuit node capacitance loading and, hence, the circuit speed. As integration density increases and feature size decreases, in accordance with sub-micron design rules, the rejection rate due to integrated circuit speed delays significantly reduces manufacturing throughput and increases manufacturing costs. 
     One way to increase the circuit speed is to reduce the resistance of a conductive pattern. Conventional metallization patterns are typically formed by depositing a layer of conductive material, notably aluminum (Al) or an alloy thereof, and etching, or by damascene techniques. Al is conventionally employed because it is relatively inexpensive, exhibits low resistivity and is relatively easy to etch. However, as the size of openings for vias/contacts and trenches is scaled down to the sub-micron range, step coverage problems result from the use of Al. Poor step coverage causes high current density and enhanced electromigration. Moreover, low dielectric constant polyimide materials, when employed as inter-metal dielectric layers, cause moisture/bias reliability problems when in contact with Al, and these problems have decreased the reliability of interconnections formed between various metallization levels. 
     One approach to improved interconnection paths in vias involves the use of completely filled plugs of a metal, such as W. Accordingly, many current semiconductor devices utilizing VLSI (very large scale integration) technology employ Al for the metallization level and W plugs for interconnections between the different metallization levels. The use of W, however, is attendant with several disadvantages. For example, most W processes are complex and expensive. Furthermore, W has a high resistivity, which decreases circuit speed. Moreover, Joule heating may enhance electromigration of adjacent Al wiring. Still a further problem is that W plugs are susceptible to void formation, and the interface with the metallization level usually results in high contact resistance. 
     Another attempted solution for the Al plug interconnect problem involves depositing Al using chemical vapor deposition (CVD) or physical vapor deposition (PVD) at elevated temperatures. The use of CVD for depositing Al is expensive and hot PVD Al deposition requires very high process temperatures incompatible with manufacturing integrated circuitry. 
     Copper (Cu) and Cu-based alloys are particularly attractive for use in VLSI and ULSI semiconductor devices, which require multi-level metallization. Cu and Cu-based alloy metallization systems have very low resistivities, which are significantly lower than W and even lower than those of previously preferred systems utilizing Al and its alloys. Additionally, Cu has a higher resistance to electromigration. Furthermore, Cu and its alloys enjoy a considerable cost advantage over a number of other conductive materials, notably silver (Ag) and gold (Au). Also, in contrast to Al and refractory-type metals (e.g., titanium (Ti), tantalum (Ta) and W), Cu and its alloys can be readily deposited at low temperatures by well-known “wet” plating techniques, such as electroless and electroplating techniques, at deposition rates fully compatible with the requirements of manufacturing throughput. 
     Electroless plating of Cu generally involves the controlled auto-catalytic deposition of a continuous film of Cu or an alloy thereof on a catalytic surface by the interaction of at least a Cu-containing salt and a chemical reducing agent contained in a suitable solution, whereas electroplating comprises employing electrons supplied to an electrode (comprising the surface(s) to be plated) from an external source (i.e., a power supply) for reducing Cu ions in solution and depositing reduced Cu metal atoms on the plating surface(s). In either case, a nucleation/seed layer is required for catalysis and/or deposition on the types of substrates contemplated herein. A physical vapor deposition technique, such as sputtering, is useful for depositing the nucleation/seed layer. 
     Another technique to increase the circuit speed is to reduce the capacitance of the inter-layer dielectrics. The speed of semiconductor circuitry varies inversely with the resistance (R) and capacitance (C) of the interconnection system. The higher the value of the R×C product, the more limiting the circuit speed. As integrated circuits become more complex and feature sizes and spacings become smaller, the integrated circuit speed becomes less dependent upon the transistor itself and more dependent upon the interconnection pattern. Thus, the performance of multi-level interconnects is dominated by interconnect RC at deep sub-micron regimes, e.g., less than about 0.18 micron. 
     The dielectric constant of materials currently employed in the manufacture of semiconductor devices for an ILD ranges from about 4.0 for dense silicon dioxide to over 8.0 for deposited silicon nitride. The value of the dielectric constant expressed herein is based upon a value of one for a vacuum. In an effort to reduce interconnect capacitance, dielectric materials with lower values of permittivity have been explored. The expression “low-k” material has evolved to characterize materials with a dielectric constant less than about 3.9. 
     One type of low-k material that has been explored is a group of flowable oxides, which are basically ceramic polymers, such as hydrogen silsesquioxane (HSQ). Such polymers and their use are disclosed in, for example, U.S. Pat. Nos. 4,756,977 and 5,981,354. HSQ-type flowable oxides have been considered for gap filling between metal lines because of their flowability and ability to fill small openings. HSQ-type flowable oxides have been found to be vulnerable to degradation during various fabrication steps, including plasma etching. 
     There are several organic low-k materials, typically having a dielectric constant of about 2.0 to about 3.8, which may offer promise for use as an ILD. Organic low-k materials which offer promise are carbon-containing dielectric materials such as FLARE 20™ dielectric, a poly(arylene) ether, available from Allied Signal, Advanced Micromechanic Materials, Sunnyvale, Calif.; BCB (divinylsiloxane bis-benzocyclobutene), and Silk™ or porous Silk™ dielectric, organic polymers, available from Dow Chemical Co., Midland, Mich.; and organic-doped silica glasses (OSG) (also known as carbon-doped glasses) including Black-Diamond™ dielectric available from Applied Materials, Santa Clara, Calif., Aurora™ dielectric available from ASM America, Inc., Phoenix, Ariz., and Coral™ dielectric available from Novellus, San Jose, Calif. 
     In attempting to employ such carbon-containing low-k materials in interconnect technology, as for gap filling or as an ILD, it was found that their dielectric constant became undesirably elevated as a result of processing. For example, the dielectric constant of BCB was found to increase from about 2.6 to greater than about 4. 
     A problem associated with the use of many low-k dielectric materials is that these materials can be damaged by exposure to oxidizing or “ashing” systems, which remove a resist mask used to form openings, such as vias, in the low-k dielectric material. This damage can cause the surface of the low-k dielectric material to become a water absorption site, if and when the damaged surface is exposed to moisture. Subsequent processing, such as annealing, can result in water vapor formation, which can interfere with subsequent filling, with a conductive material, of a via/opening or a damascene trench formed in the dielectric layer. For this reason, the upper surface of the low-k dielectric material is typically protected from damage during removal of the resist mask by a capping layer, such as silicon oxide, disposed over the upper surface. Silicon oxide capping layers, however, require an additional process step of forming the silicon oxide layer. 
     Antireflective coatings (ARC) are frequently employed to help provide sub-micron images during photomicrographic processing. The ARC cuts down on light scattering, which helps in the definition of small images, minimizes standing wave effects, and improves image contrast during photoresist exposure. However, the use of an ARC requires additional processing steps, including spin-on and baking steps. In addition ARC layers, along with capping layers, suffer from poor adhesion to the ILD surface. 
     The term semiconductor devices, as used herein, is not to be limited to the specifically disclosed embodiments. Semiconductor devices, as used herein, include a wide variety of electronic devices including flip chips, flip-chip/package assemblies, transistors, capacitors, microprocessors, and random access memories, etc. In general, semiconductor devices refer to any electrical device comprising semiconductors. 
     SUMMARY OF THE INVENTION 
     There exists a need for methodology enabling the use of low-k carbon-containing dielectric materials as an ILD in high-density, multi-level connection patterns. There exists a particular need for methodology enabling the use of such low-k materials with low-k capping and ARC layers to avoid degradation of the low-k ILD properties while providing sub-micron feature sizes with enhanced resolution. 
     These and other needs are met by embodiments of the present invention, which provide a method of forming a semiconductor device with a first dielectric film on a semiconductor substrate. The first dielectric film is formed by depositing a lower layer of an organic-doped silica glass on a semiconductor substrate. A middle layer comprising a mixture of the organic-doped silica glass and a gradually increasing concentration of SiC is deposited on the lower layer and an upper layer of SiC is deposited on the middle layer. SiC has a lower dielectric constant than prior art ARC layers such as SiON. 
     The earlier stated needs are also met by other embodiments of the instant invention that provide a semiconductor device comprising a first dielectric film formed on a semiconductor substrate. The first dielectric film comprises a lower layer comprising an organic-doped silica glass, a middle layer comprising a mixture of organic-doped silica glass and a gradually increasing concentration of SiC, and an upper layer comprising SiC. 
     This invention address the needs for an improved method of forming capping layers and ARC layers on low-k ILD layers for use in high-density, multi-level interconnection patterns. This invention improves adhesion of capping layers and ARC layers to low-k ILDs. The present invention enables the manufacturing of high reliability, small feature sized semiconductor devices. 
     The foregoing and other features, aspects, and advantages of the present invention will become apparent in the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a semiconductor device with a first dielectric film. 
     FIG. 2 illustrates an opening formed in the first dielectric film. 
     FIG. 3 illustrates filling the opening in the first dielectric film with a metal. 
     FIG. 4 illustrates a planarized semiconductor device with a damascene interconnect structure. 
     FIG. 5 illustrates a semiconductor substrate with first and second dielectric films formed. 
     FIG. 6 illustrates a via formed in the second dielectric film. 
     FIG. 7 illustrates a via and trench formed in the first and second dielectric films. 
     FIG. 8 illustrates the deposition of metal into the via and trench of the first and second dielectric films. 
     FIG. 9 illustrates a dual damascene interconnect structure formed on a semiconductor device. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention enables the production of semiconductor devices with improved performance and reduced feature sizes by depositing a capping/ARC layer in-situ on an ILD during formation of the ILD. These objectives are achieved by strategically varying the flow rates of the ILD precursor gases during ILD formation to gradually convert a low-k dielectric film into a capping/ARC layer. The present invention comprises one or more ILD films formed on a semiconductor substrate. 
     Organic-doped silica glass is particularly useful in forming low-k dielectric films. A particularly useful organic-doped silica glass is SiCOH. SiCOH, which exhibits a dielectric constant of about 2.4 to about 3.1, typically contains carbon in the amount of about 5 to about 20 atomic %, silicon in an amount of about 15 to about 25 atomic %, oxygen in an amount of about 25 to about 35 atomic %, and hydrogen in an amount of about 35 to about 45 atomic %. SiCOH contains SiC, SiH, CH and SiOH bonding. 
     As illustrated in FIG. 1, a layer  14  of an organic-doped silica glass is formed on a semiconductor substrate  12 . SiCOH is deposited by a plasma enhanced chemical vapor deposition process (PECVD) process using a methylsilane and oxygen (O 2 ) as precursor gases and argon or helium as a carrier gas. The preferred methlysilanes include trimethylsilane and tetramethylsilane. For the purpose of describing the invention, trimethylsilane will be exemplified. SiCOH is deposited until a predetermined layer thickness is achieved. Nominal SiCOH layer thicknesses range from about 2,000 Å to about 1.2 μm. 
     Suitable PECVD reactor parameters for SiCOH deposition include a trimethylsilane flow rate of about 400 to about 800 sccm; O 2  flow rate of about 50 to about 3000 sccm; pressure of about 2 to about 8 torr; RF power of about 400 to about 600 watts and a temperature of about 250° C. to about 450° C.; for about 20 or more seconds depending on the desired thickness. 
     In certain embodiments, suitable reactor parameters include an O 2  flow rate of about 50 to about 150 sccm; RF power of about 400 to about 600 watts; a temperature of about 300° C. to about 425° C.; and a spacing distance between the semiconductor and the plasma source of about 0.5 to about 0.9 cm. 
     The decomposition of trimethylsilane in an environment with a substoichiometric amount of oxygen produces SiCOH. As the oxygen flow rate is gradually reduced the amount of SiC formed increases. When the desired SiCOH layer  14  thickness is reached, the flow rate of O 2  is gradually reduced to form a middle layer  16  comprising a mixture of the organic-doped silica glass (SiCOH) and a gradually increasing concentration of SiC. When the O 2  flow rate is reduced to zero, no SiCOH is deposited on the semiconductor substrate  12 . 
     The thickness of the middle layer  16 , which comprises a mixture of SiCOH and SiC, ranges from about 25 Å to about 100 Å. Across the middle layer thickness the SiC concentration ranges from about 0 wt. % at the lower layer/middle layer interface  15  to about 100 wt. % at the middle layer upper surface  17 . 
     As shown in FIG. 1, a SiC dielectric film upper layer  18 , is deposited to a thickness of about 300 Å to about 500 Å. After completing the deposition of the SiC upper layer  18 , a semiconductor device  10 , with a three-layer first dielectric film  11 , is formed. SiC performs several functions. SiC can function as a capping layer for an ILD, an ARC, and an etch/polish stop. In the present invention the SiC dielectric film upper layer  18  functions as a combination capping/ARC layer, and additionally may function as a an etch/polish stop during etching and/or polishing operations. The dual nature of the capping/ARC layer  18  eliminates the necessity of forming an additional ARC layer, including the steps of spinning on the ARC and the ARC baking step. Furthermore, the capping/ARC layer  18  is formed in the same chamber and as part of the same process as the organic-doped glass  14  and middle layer  16  deposition. Therefore, the formation of the three-layer first dielectric film  11  is more economical and time efficient over the prior art processes of forming separate capping layers and ARC layers over an ILD. 
     The gradual transition from the organic-doped silica glass  14  to the SiC upper layer  18  provides strong adhesion of the SiC upper layer  18  to the organic-doped silica glass  14 . 
     Interconnects are formed in the semiconductor device  10  by patterning the first dielectric film  11  to form a trench  20 , as illustrated in FIG.  2 . The trench  20  is formed by conventional photolithographic techniques; applying a photoresist, masking the photoresist, exposing the unmasked portions to actinic radiation, and subsequent development. The pattern formed in the photoresist (not shown) is subsequently transferred into the first dielectric film  11  by conventional anisotropic etching techniques to form the trench  20 . The photoresist is then subsequently removed, as by O 2  ashing. The SiC capping/ARC layer  18  protects the organic-doped silica glass layer  14  from damage during the photoresist removal process especially in the hard mask integration approach. 
     A damascene type interconnect is formed in the trench  20  by depositing a metal  24  into the trench  20 , as shown in FIG.  3 . The interconnect metal  24  can comprise a number of conventional metals including Al, Al alloys, Cu, Cu alloys and W. Cu and Cu alloys are preferred because of the high conductivity of Cu and its alloys. However, Cu is prone to diffusion in silicon based ILD. Therefore, when a metal comprising Cu is used as the interconnect, a barrier layer  22 , as shown in FIG. 3, is first deposited on the dielectric film  11 . Suitable conductive Cu barrier layers include Ta, TaN, Ti, TiN, and WN. 
     After the Cu barrier layer  22  is deposited, the trench  20  is filled with Cu  24  using an electrolytic or electroless plating technique. However, before plating, a seed layer of Cu is deposited using either a chemical vapor deposition (CVD) technique or a physical vapor deposition (PVD) technique, such as sputtering, to form a thin seed layer (not shown) on which the Cu  24  is deposited. In addition to filling the trench  20 , the metal  24  and barrier layer  22  are also deposited on the first dielectric film  11  creating an overfill situation, as shown in FIG.  3 . After metal deposition, the semiconductor device  10  is planarized, as by a chemical-mechanical polishing technique (CMP) to remove the metal  24  overfill and the conductive barrier layer  22  from on top of the dielectric film  11 . The resulting damascene structure is illustrated in FIG.  4 . The SiC layer  18  further functions as a polish stop during CMP processing of the semiconductor device  10 . 
     The present invention is well-suited for forming dual damascene structures on semiconductor substrates. When forming the dual damascene structures, a second dielectric film  13 , similar to the first dielectric film  11 , is formed over the first dielectric film  11 , as shown in FIG.  5 . Second dielectric film  13  includes an organic-doped silica glass lower layer  26 , middle layer  28  comprising a mixture of organic-doped silica glass and SiC, and an upper layer  30  comprising SiC. The second dielectric film lower layer  26 , middle layer  28 , and upper layer  30  are formed in the same manner as the corresponding layers in the first dielectric film  11 . As in first dielectric film middle layer  16 , second dielectric film middle layer  28  comprises a gradually increasing concentration of SiC ranging from 0 weight % of SiC at the lower layer/middle layer interface  27  to about 100 weight % at the middle layer/upper layer interface  29 . The thicknesses of the layers  26 ,  28 , and  30  of second dielectric film  13  are within the same ranges as the corresponding first dielectric film  11  layers. The SiC layer  18  provides improved adhesion for the second dielectric film lower layer  26 . 
     A via opening  32  is formed in second dielectric film  13  using conventional photolithographic and anisotropic etching techniques, as shown in FIG.  6 . In addition to its dual function as a capping and ARC layer, first dielectric film upper layer  18  also functions as an etch stop during formation of the via opening  32  in second dielectric film  13  and subsequent trench formation. 
     As shown in FIG. 7, a trench  34  is formed in the second dielectric film  13  and the via opening  32  is continued through the first dielectric film  11 , exposing a portion of the semiconductor substrate  12 . As with previous patterning steps, conventional photolithographic and etching techniques are used to form the trench/via structure  34 / 32 . For example, a bottom etch stop layer may be provided between the substrate  12  and the lower layer  14 , as known in the art, to protect the substrate  12  during etching of the dielectric material. The bottom etch stop material may be SiN, for example. 
     As shown in FIG. 8, the trench/via structure  34 / 32  is filled with a conductive metal  38 , preferably Cu or Cu alloy. When the metal  38  is Cu or a Cu alloy, barrier layer  36  is deposited prior to filling the combined trench/via  34 / 32  with metal  38 . As discussed earlier, the metal overfill is removed and the conductive material  36 ,  38  is planarized, using a CMP technique, to form a device  40  with a dual damascene interconnect, as shown in FIG.  9 . 
     The method of forming a semiconductor device, as described herein, provides an economical and reliable process of forming high-density, multiple level interconnects. The method and semiconductor device described efficiently utilizes low-k organic-doped silica glass ILD technology. The dual nature of the capping/ARC layer eliminates several process steps and layers, thereby providing a more economical and reliable process. The improved adhesion of the capping/ARC layer to organic-doped silica glass dielectric layers improves the semiconductor device reliability and manufacturing yield. 
     The embodiments illustrated herein are for illustrative purposes only. They should not be construed to limit the scope of the claims. As is clear to one of ordinary skill in the art, the instant disclosure encompasses a wide variety of embodiments not specifically illustrated herein.