Abstract:
An improved semiconductor device structure comprises insertion of a semiconductor wafer into a high-pressure heated chamber and the deposition of a low-melting point aluminum material into a contact hole or via and over an insulating layer overlying a substrate of the wafer. The wafer is heated up to the melting point of the aluminum material and the chamber is pressurized to force the aluminum material into the contact holes or vias and eliminate voids present therein. A second layer of material, comprising a different metal or alloy, which is used as a dopant source, is deposited over an outer surface of the deposited aluminum material layer and allowed to diffuse into the aluminum material layer in order to form a homogenous aluminum alloy within the contact hole or via. A semiconductor device structure made according to the method is also disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a divisional of application Ser. No. 09/146,719, filed Sep. 3, 1998, now U.S. Pat. No. 6,124,205 issued Sep. 26, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to semiconductor devices and, more particularly, to a low temperature method of filling contact holes or vias with a low melting point aluminum material and subsequently depositing a second layer dopant for diffusion into the aluminum-filled contact hole or via to form an alloy therein. 
     2. State of the Art 
     As semiconductor device dimensions shrink, both gap-fill and planarity of the dielectric films become increasingly important. These challenging gap-fill requirements have initiated and stimulated a search for new processes and materials. Many of these devices, such as advanced ultra-large scale integrated (ULSI) devices, utilize elaborate, multi-level metallization schemes to enhance performance and achieve functional integration. As these device dimensions shrink, intra-lead capacitance becomes a major limiting factor in determining the total interconnect capacitance. Use of multi-level metal structures incorporating low dielectric constant materials is therefore necessary to limit the impact of capacitance on power, cross-talk, and RC delay of dense, deep sub-half micron interconnects. 
     Due to the ease of its integration therein, aluminum materials are a preferred material for contact/via resistances, having fewer overall process steps and improved electromigration performance. While aluminum reflow has been used for filling contacts and vias having widths equal to or smaller than 0.5 μm, aluminum reflow processes have not been widely accepted due to the higher deposition temperatures required in comparison to filling processes employing metals or alloys having lower melting-point temperatures than aluminum materials. Additionally, aluminum reflow processes are usually ineffective in completely filling contacts and vias having high aspect ratios, that is, contacts and vias having a high ratio of length or depth of a hole or via in relation to the preplated diameter of the contact or via. 
     Various methods of spreading aluminum or other conductive film on the principal surface to fill the contact holes are already in practical use. These methods include a high temperature sputter method, a bias sputter method, and a reflow after sputter method. A major disadvantage of these conventional aluminum reflow processes is the sensitivity of reflow to surface conditions, hole profile and the type of substrate material. For example, conventional hot sputter deposition and/or reflow processes rely on the diffusive mobility of the atoms. Reflow characteristics are adversely affected by higher contact/via aspect ratios and the typical protrusion of sputtered barrier layers at the hole entrance, making consistent global filling difficult to achieve. Other detriments to complete filling include the presence of spin-on dielectrics and the associated out-gassing from the vias during the reflow process. Global filling is of particular concern for sub-half micron applications since a feasible aluminum reflow technology must be capable of achieving at least an equivalent yield and reliability as compared to conventional technologies, such as a tungsten plug process. 
     To alleviate some of these problems, a high pressure (&gt;700 atm) forced fill Al-plug process has been used for sub-half micron contact and via hole filling. This process typically consists of a bake, soft sputter etch, barrier deposition and aluminum plug formation. The aluminum hole filling is achieved via a two step process. As shown in FIGS. 1 and 2 (representing a section or segment of a semiconductor wafer  30 ), aluminum is applied to insulating layer  24  (typically comprising a dielectric such as SiO, boron nitride, and silicon nitride deposited over a substrate  20 ) through a conventional sputter deposition technique at about 400° C. Prior to the deposition of aluminum, holes or vias  25  are created (e.g. by etching) in insulating layer  24 . The deposited aluminum fills or bridges the mouth of each hole  25  with metal alloy layer  22 . However, due to the high aspect ratio of the formed hole and the inherent surface tension of aluminum alloy layer  22 , void  26  usually forms inside each hole below the filled or bridged mouth. The wafer is then transferred under vacuum to a so-called FORCE FILL™ Module, shown schematically in FIG. 7, consisting of a high pressure chamber  80  with two radiant heaters  82  for controlling the temperature of wafer  84 . Outlet port  88  is connected to a vacuum and controls pressurization and removal of gases from chamber  80 . Inlet port  86  is connected to a pressurized source of gas, such as argon, for pressure regulation within chamber  80  and introduction of a precursor for plasma formation. The deposited aluminum is then forced into the holes by pressurizing the chamber, usually to about 760 atm, with argon while maintaining the temperature at about 400° C. As a result of the forced external pressure (represented by arrows  27  in FIG.  2 ), the aluminum bridge over hole or via  25  is deformed or extruded inwardly to accomplish complete hole filling, as shown in FIG.  2 . 
     For purposes of the forced fill process, use of a low melting-point aluminum alloy (e.g. alloys of aluminum containing between about 10% and about 60% copper), which flows at reduced temperatures, is preferred over pure aluminum or high melting-point aluminum alloys, such as alloys containing 98% aluminum and 2% copper. As a consequence, because lower temperatures can be used for effective hole filling, the respective wafer or substrate containing the hole undergoes less thermal stress, which decreases the potential for damage to the structures, and ultimately the complete devices, being formed on and in the semiconductor material. On the other hand, high melting-point aluminum alloys, such as the Al—Cu alloy referenced above, possess superior electromagnetic and stress migration properties in comparison to low melting-point aluminum alloys and would thus be favored for use in contact/via fill processes if the disadvantages thereof could be reduced or eliminated. 
     Thus, it would be advantageous to provide an aluminum plug fill process which could be carried out at reduced temperatures and which also affords the superior electromagnetic and stress migration properties inherent in high melting-point aluminum alloys. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an improved method for filling contact holes or vias of semiconductor devices and the resulting structures. The improved method begins with insertion of the semiconductor wafer or other substrate of semiconductive material, having one or more contact holes or vias formed in an insulating layer overlying a wafer substrate, into a high-pressure heated chamber. A low-melting point base layer of aluminum material is then deposited over the insulating layer and into the contact holes or vias. During the deposition step, the wafer is heated up to the melting point of the aluminum material to reflow the same into the contact hole or via. Once deposition is completed and while maintaining the elevated temperature, the chamber is pressurized to force the aluminum material into the contact holes or vias and thus eliminate voids present therein under the aluminum material base layer. A second layer of material, comprising a metal or alloy to be used as a dopant source, is then deposited over a top surface of the deposited aluminum material base layer and allowed to diffuse into the aluminum material base layer in order to form a substantially homogenous aluminum alloy within the contact hole or via. The newly formed homogenous aluminum alloy possesses the desirable characteristics of the previously-mentioned high melting-point aluminum alloys, but without the associated difficulties and disadvantages of depositing such alloys in their preformed state. Formation of the homogenous aluminum alloy within the contact holes or vias of the wafer thus improves the strength, stress migration, and electromagnetic properties of the contacts or vias in a viable, economical manner easily applied to existing fabrication methodologies. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     While the specification concludes with claims particularly pointing out and distinctly claiming that which is regarded as the present invention, the advantages of this invention can be more readily ascertained from the following description of the invention when read in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a cross-sectional view of a portion of an integrated circuit structure created through conventional sputter deposition of an aluminum alloy over a via or contact; 
     FIG. 2 is a cross-sectional view of the integrated circuit structure of FIG. 1 illustrating a high pressure forced fill process applied subsequent to the deposition step of FIG. 1; 
     FIG. 3 is a cross-sectional view of a portion of an integrated circuit made in accordance with the present invention after high pressure forced fill of a contact hole or via with the aluminum material base layer; 
     FIG. 4 is a cross-sectional view of the integrated circuit structure of FIG. 3 after deposition of a diffusion layer over the aluminum material base layer; 
     FIG. 5 is a cross-sectional view of the integrated circuit structure of FIG. 3 after the diffusion layer has diffused into the underlying aluminum layer to form an alloy of the two materials; 
     FIG. 6 is a cross-sectional view of a portion of a multilevel wiring structure made in accordance with the principles of the present invention; and 
     FIG. 7 is a schematic representation of a FORCE FILL™ Module used to carry out the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 3, a cross-sectional view of a portion of a wafer or integrated circuit segment  30  is depicted. For purposes of this application, the term “wafer” or “integrated circuit” includes not only traditional wafers, but other substrates of semiconductor materials formed in different manners and specifically contemplated silicon-on-insulator (SOI) structures, silicon-on-ceramic structures, and layers of other semiconductive materials such as gallium arsenide and indium phosphide. For purposes of simplicity, elements common to FIGS. 1 and 2 will hereinafter be numbered identically in subsequent figures. The wafer  30  includes a semiconductive substrate layer  32  and an interlayer isolation or insulation layer  33 . A contact hole or via  37  is defined by sidewall  34 , extending from a principal or top surface  36  of insulation layer  33 , to a bottom wall  35  that is defined by an exposed surface portion of the substrate layer  32 . Contact hole  37  is representative of a plurality of contact holes or vias formed in wafer  30  and associated with the same or other circuit structures. 
     The hole filling process of the invention is suitable for, although not limited to, sub-half micron contact and via hole filling. The method can be applied in the fabrication of a variety of semiconductor devices and ULSI circuits, such as dynamic random access memories (DRAMs), static random access memories (SRAMs), flash memory processors, and application-specific integrated circuits (ASICs). While the diameter of contact hole  37  in most of these devices is typically less than or equal to 50 μm, it can be extended to any diameter in which a substantially complete yield of contact filling is achievable. Where multiple-level metal formation is desired, such as in DRAM generation, contacts and vias with varying diameters can be patterned after interlevel dielectric deposition and planarization. 
     Generally, the hole filling process is initiated by performing the forced fill process, previously described in conjunction with FIG. 2, with a low melting-point aluminum alloy base layer  38  being deposited on top surface  36  of insulation layer  33 , as shown in FIG.  3 . Low melting-point aluminum alloys suitable for use in the hole-filling step of the present invention include any aluminum alloy having a lower melting point than alloys, such as an aluminum alloy containing 98% aluminum and 2% copper, which has a melting point is of about 650° C., which are typically used in hole filling processes. Alternatively, low melting-point aluminum alloy base layer  38  can be selectively deposited over the contact hole  37  areas and not over top surface  36  of insulation layer  33 . This selective deposition step can be facilitated through the use of a masking step or any other method known in the art for selective deposition of materials. 
     The aluminum layer used to fill the top of each hole  37  may be deposited through conventional sputter deposition techniques (also known as physical vapor deposition (PVD)). In this preferred method, a solid slab of a low melting-point aluminum alloy is electrically grounded within a vacuum chamber to form a “target”. A gas, typically argon, is introduced into the chamber and is ionized to a positive charge, thus forming a plasma. The positively charged argon atoms are attracted to the grounded target and accelerate toward the target, eventually striking the target and causing the aluminum atoms to scatter into the vacuum chamber. The sputtered aluminum atoms or molecules scatter in the chamber, with some coming to rest on wafer  30 . Once the initial aluminum alloy layer is deposited, plasma continues to contact and heat aluminum alloy base layer  38 , thus facilitating reflow of aluminum alloy base layer  38  into the holes  37 . Advantageously, heat produced in the aluminum material base layer  38  due to argon ion plasma irradiation dissipates through the wafer  30  towards a wafer support structure (not shown) of the PVD chamber. The dissipation of heat keeps wafer  30  at a sufficiently low temperature capable of preventing an adverse chemical reaction or thermal stress from taking place between aluminum alloy baselayer  38  and both insulating layer  33  and substrate layer  32  of wafer  30 . 
     The sputter deposition technique is preferably conducted at a temperature of about 400° C. A heater  82  (FIG.  7 ), contained within the evacuation chamber  80  (FIG.  7 ), can be used to subsequently heat aluminum alloy layer  38  to a sufficiently high temperature to cause the aluminum alloy layer  38  to reflow into contact hole  37 . Alternatively, it is possible to heat the aluminum alloy layer  38  for reflow simultaneously while irradiation with the plasma is performed, especially when a reduction in the argon ion and plasma energy is desired. 
     A principal feature of the sputtering process is that the “target” material is deposited on the substrate layer  32  over insulating layer  33  without chemical or compositional change, such as seen in the process of chemical vapor deposition (CVD). Deposition of aluminum through sputtering, as opposed to a CVD process, eliminates the need for deposition of TiN, which is required to ensure consistent nucleation of CVD-deposited aluminum prior to such deposition. Another advantage of sputtering over CVD is the conservation of target material composition. 
     Adhesion of the sputtered film to the top surface  36  of the insulation layer  33  is also improved in comparison to evaporation processes (such as electron-beam evaporation and inductive heating evaporation). The higher energy of the arriving aluminum atoms provides better adhesion, and the plasma environment (i.e., the ionized argon gas) inside the chamber has a “scrubbing” action on principal surface  36  and within contact hole  37  surface that cleans these surfaces and thus enhances adhesion. 
     Various sputtering methods can be used in the method of the invention, such as diode sputtering using direct current, diode sputtering using radio frequency, triode sputtering, or magnetron sputtering. Sputter deposition of aluminum according to such processes bridges the top of each contact hole  37  and at least a portion of top surface  36  of the insulation layer  33  with aluminum, usually leaving an underlying void  26  inside hole  37 , as previously described and shown in FIG.  1 . High aspect ratio contacts and vias (i.e., contacts and vias having a high ratio of length or depth of a hole or via in relation to the preplated diameter of the contact or via) are particularly prone to incomplete filling of the hole  37 . 
     According to the principles of the present invention, it is possible to thoroughly fill contact hole  37  with a low melting-point aluminum alloy base layer  38 , even where contact hole  37  has a high aspect ratio, while maintaining semiconductor substrate layer  32  at an appreciably low temperature, such as 400° C. This low temperature process advantageously prevents impurities, usually emanating from insulation layer  33 , from being taken into aluminum alloy base layer  38 , giving aluminum alloy base layer  38  a substantially flat or planar surface which facilitates its working into and alignment with the wirings and surrounding structures. Furthermore, the low temperature process decreases the attendant thermal stress typically seen between substrate layer  32 , insulating layer  33 , and aluminum alloy base layer  38  when using high temperature reflow processes. 
     Removal of the void inside contact hole  37  (already removed in FIG. 3) is accomplished through a forced fill process, as described above. However, because low melting point aluminum alloys are used in place of the aluminum alloys traditionally used in the forced fill process (e.g. aluminum alloy containing 98% Al and 2% Cu, pure Al, or metal and alloys having a melting point greater than pure Al), operating pressures and temperatures may be reduced below conventional levels while still achieving complete hole filling. Alternatively, due to the lower melting point of the selected aluminum alloys, complete hole filling can be accomplished more rapidly when applying conventional operating pressures and temperatures. 
     As shown in FIG. 4, following the deposition and forced fill steps, a second diffusion layer  40  of metal or alloy is deposited onto an exposed or outer surface  39  of the aluminum alloy base layer  38 . Suitable alloys for use as second layer  40  include alloys of aluminum containing from about 10% to about 60% copper, from about 10% to about 70% silver, greater than about 20% zinc, and greater than about 30% tin. In one preferred embodiment, substantially pure copper is used as the diffusion or dopant source and forms the second layer  40 . Alternatively, an Al-Cu alloy can be used as a copper diffusion source. Suitable elements for use as a diffusion or dopant source include any metal or alloy which can be made to diffuse into the underlying aluminum alloy base layer  38  and form a homogeneous aluminum alloy having desired electromagnetic and stress migration properties applicable for ULSI devices. Preferred alloys for use as second layer  40  include alloys of aluminum containing copper, silver, zinc, and tin. Preferred metals for use as second layer  40  include copper, silver, zinc, tin, and magnesium. 
     Where aluminum alloy base layer  38  is selectively deposited over the contact hole  37  areas and not over top surface  36  of insulation layer  33 , as previously described in the alternative embodiment, second diffusion layer  40  of metal or alloy is selectively deposited onto exposed or outer surface  39  of the aluminum alloy base layer  38 . This selective deposition step can be facilitated through the use of a masking step or any other method known in the art for selective deposition of materials. 
     The metals and alloys forming second layer  40  can be deposited through any suitable deposition technique. One preferred deposition technique involves the deposition of copper by an electroless process. Traditional electroless copper plating processes, wherein an alkaline chelated copper reducing solution deposits a thin copper layer (usually 20 to 100 μin) on surfaces, can be employed in the instant process. Generally, the electroless plating process is initiated by combining a source of copper, such as copper sulfate (CuSO 4 ), with a reducing agent (preferably formaldehyde) to reduce the elemental copper (i.e., Cu +2 =2e−&gt;Cu 0 ). Sodium hydroxide is simultaneously combined to maintain the pH between about 11.5 and 12.5 in order to optimize aldehyde reduction. Complexers, such as EDTA and tartrates, hold the copper cations in solution at a high pH. In such a manner, metals such as copper and nickel can be deposited on underlying aluminum alloy base layer  38  to form second layer  40 . Those skilled in the art will recognize and apply the process steps, specific operating conditions, and process controls required to carry out electroless plating of second layer  40  according to the principles of this invention. 
     Vacuum evaporation is another technique which can be used for the deposition of metals on aluminum alloy base layer  38 . Vacuum evaporation takes place inside an evacuated chamber, where a metal is heated to a liquid state so that the atoms or molecules evaporate into the surrounding atmosphere within the chamber. Any known and suitable evaporation method (e.g., filament, electron beam, and flash hot plate evaporation) can be used to evaporate the metals, which will eventually form second layer  40 , in the vacuum system. Vacuum evaporation is preferably performed with pure metals, as alloys are difficult to deposit by this method due to the different evaporation rates at specific temperatures for each element comprising the alloy, which would lead to deposition of second layer  40  having a different composition than the source alloy material. 
     Another preferred deposition technique involves PVD or sputter deposition, as described above with respect to the deposition of aluminum alloy base layer  38 . In contrast to the sputter deposition of aluminum alloy base layer  38 , the target can comprise any suitable or desirable metal (except aluminum) or alloy which makes an effective diffusion or dopant source (e.g., Cu or AlCu). As previously discussed, various sputtering methods can be used, such as diode sputtering using direct current, diode sputtering using radio frequency, triode sputtering, or magnetron sputtering. 
     Sputter deposition is particularly well suited when depositing an alloy as second layer  40 , since sputter deposition does not rely on evaporation of materials having different evaporation rates. For example, in sputtering, an aluminum and 2% copper target material yields a substantially unchanged aluminum and 2% copper alloy second layer  40  over aluminum alloy base layer  38 . 
     As shown in FIG. 5, once the second layer  40  is deposited onto the aluminum alloy base layer  38 , the second layer element(s) diffuse into and form a substantially homogeneous aluminum alloy layer  50 . The second layer element(s)  42 , constituting the material of the dopant source, is uniformly distributed throughout the aluminum alloy base layer  38  by subjecting wafer  30  to elevated temperatures (preferably 400-500° C.), thus forming new alloy layer  50  over insulating layer  33  and within the contact hole  37 . An annealing step can be added to improve dopant distribution and further diffuse the second layer element(s)  42  into the aluminum alloy base layer  38 . 
     In another preferred embodiment of the present invention, second insulation layer  78  can be deposited on homogeneous aluminum alloy layer  50  to create a multilevel wiring structure  70 , as shown in FIG. 6. A third insulation layer  72  can be deposited between the second insulation layer  78  and the homogeneous aluminum alloy layer  50  to provide insulation between wiring structures being formed. Once second insulation layer  78  is deposited, the aforementioned steps (previously described in conjunction with FIGS. 3 through 5) are repeated to form a structure comprising second homogeneous aluminum alloy layer  74  which fills second hole  76  formed within second insulating layer  78 . In carrying out reflow of second homogeneous aluminum alloy layer  74  into second holes  76  formed in the second insulating layer  78 , attention should be directed to avoidance of any disturbance, such as reflow of previously-formed homogeneous aluminum alloy layer  50  of underlying hole  37 . Due to the relatively higher melting point of homogeneous aluminum alloy layer  50  as compared to the low melting-point aluminum material initially being deposited within second hole  76 , use of irradiation, either solely or in combination with heating of the second insulating layer by the heater to a temperature slightly above the melting point of the low melting-point aluminum material, is effective in preventing such reflow of existing hole fill materials. 
     While the hole fill method of the present invention has been described in terms of various preferred embodiments, it is understood that other methods could be adopted by one skilled in the art. For example, various deposition techniques, such as ion deposition, could be employed to deposit the aluminum alloy or second (dopant) layers. Where plasma-dependent deposition is employed, various inert gases could be used for generation of ion plasmas. Where alloys are deposited through PVD techniques, a single target consisting of an alloy can be used or individual targets, each containing individual metals which comprise the alloy, can be used to deposit the selected alloy in the desired constituent ratios. Accordingly, it is understood that the scope of the invention is not to be limited except as otherwise set forth in the claims.