Abstract:
A metal structure ( 600 ) for a bonding pad on integrated circuit wafers, which have interconnecting metallization ( 101 ) protected by an insulating layer ( 102 ) and selectively exposed by windows in the insulating layer. The structure comprises a patterned seed metal layer ( 104 ) positioned on the interconnecting metallization exposed by the window so that the seed metal establishes ohmic contact to the metallization as well as a practically impenetrable seal of the interface between the seed metal and the insulating layer. Further, a metal stud ( 301 ) is formed on the seed metal and aligned with the window. The metal stud is conformally covered by a barrier metal layer ( 501 ) and an outermost bondable metal layer ( 502 ).

Description:
This is a divisional application of application Ser. No. 10/678,709 filed Oct. 3, 2003 now U.S. Pat. No. 6,927,493, the contents of which are herein incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention is related in general to the field of metallurgical systems with application to electronic systems and semiconductor devices and more specifically to methods for sealing and protecting integrated circuit bonding pads. 
     DESCRIPTION OF THE RELATED ART 
     It is a continuing trend in the semiconductor industry to miniaturize integrated circuits (ICs). As a consequence of this trend, the RC time constant of the interconnection between active circuit elements increasingly dominates the achievable IC speed-power product. Consequently, there is a strong need to replace the relatively high impedance of the interconnecting aluminum metallization by the lower impedance of metals such as copper. 
     For IC bond pads made of copper, the formation of thin copper(I)oxide films during the manufacturing process flow severely inhibits reliable attachment of bonding wires, especially for conventional gold-wire ball bonding. In contrast to aluminum oxide films overlying metallic aluminum, copper oxide films overlying metallic copper cannot easily be broken by a combination of thermo-compression and ultrasonic energy applied in the bonding process. As further difficulty, bare copper bond pads are susceptible to corrosion. 
     In order to overcome these problems, several investigations aim at developing a process to cap the clean copper bond pad with one or more metal layers. So far, the process favored by the industry is based on forming a layer of aluminum over the copper bond pad and thus reconstructing the traditional situation of an aluminum pad to be bonded by conventional gold-wire ball bonding. This process, though, has a number of drawbacks due to technical and cost issues. 
     An alternative process is based on the concept of depositing a stack of layers on the copper, wherein the layer closest to the copper acts as a barrier to shield the copper from diffusing into other metals, and the outermost layer is bondable by the conventional gold ball of wire bonding. This process still needs the satisfactory solution of technical problems, such as adhesion among the various metal layers and insulating materials; avoidance of corrosion and chemical undercuts; and increased flexibility in the methods of depositing and removing the metal and insulator films. 
     A need has therefore arisen for a metallurgical bond pad structure suitable for ICs having copper interconnection metallization which combines a low-cost method of fabricating the bond pad structure with high reliability in operating the structure, in particular with reduced possibility of delamination and corrosion. The bond pad structure and the method of fabrication should be flexible enough to be applied for different IC product families and a wide spectrum of design and process variations. Preferably, these innovations should be accomplished while shortening production cycle time and increasing throughput and yield, and without the need of expensive additional manufacturing equipment. 
     SUMMARY OF THE INVENTION 
     One embodiment of the invention is a semiconductor device including a bonding pad having interconnecting metallization protected by an insulating layer; the metallization is selectively exposed by a window in the insulating layer. The device comprises a patterned seed metal layer positioned on the interconnecting metallization exposed by the window and overlapping a portion of the insulating layer around the window. Further, a metal stud is positioned on the seed metal and aligned with the window; the stud includes a top surface and side surfaces. A barrier metal layer covers conformally the top and side surfaces of the stud. An outermost bondable metal layer is over the barrier metal layer. 
     For interconnecting device metallization of copper, the preferred seed metal is a stack of layers: a layer of titanium/tungsten contacting the copper metallization, followed by a layer of copper; alternatively, the seed metal may be a layer of copper. For interconnecting device metallization of aluminum, the seed metal is a stack of titanium/tungsten and copper layers. Further, copper is preferred as the stud metal. In this case, the preferred barrier metal is nickel, the second barrier metal is palladium, and the outermost layer is gold. An alternate stud metal is nickel; in that case, the preferred barrier metal is palladium and the outermost metal is gold. 
     Another embodiment of the invention is a method for fabricating a semiconductor device having interconnecting metallization protected by an insulating layer; the metallization is selectively exposed by windows in the insulating layer. The method comprises the step of depositing a conductive seed layer to cover the metallization exposed in the window and to cover a portion of the insulating layer around the window. The next step forms a metal stud over the conductive seed layer covering the metallization exposed by the window and over the portion of the insulating layer around the window; the metal stud includes a top surface and side surfaces. Next, the conductive seed layer portions not covered by the metal stud are removed. The final steps comprise conformally covering the top and side surfaces of the metal stud with a barrier metal, and depositing an outermost bondable metal layer over the barrier layer. 
     The step of depositing a conductive seed layer can be preceded by a step of cleaning and etching the surface of the wafer, including the insulating layer and the exposed metallization, creating a fresh and activated surface. The seal provided by the prepared insulating layer and the deposited seed metal withstands operational thermomechanical stress. 
     In another embodiment of the invention, the interconnecting chip metallization is an aluminum alloy. The preferred seed metal is an alloy of refractory metals, including titanium and tungsten. 
     In yet another embodiment of the invention, the metal stud is nickel. 
     Embodiments of the present invention are related to wire-bonded IC assemblies, semiconductor device packages, surface mount and chip-scale, packages. It is a technical advantage that the invention offers a low-cost method of sealing the bond pad against moisture and delamination, and thus for protecting the integrated circuit against corrosion and stress-related contact failure. It is an additional technical advantage that the invention offers a methodology to smooth, stable interfaces of the gold wire ball and the modified bond pad, resulting in welds with strong metallic interdiffusion. Further technical advantages include the opportunity to scale the assembly to smaller dimensions, supporting the ongoing trend of IC miniaturization; and the absence of unwanted metals, supporting high reliability of the finished IC assemblies. 
     The technical advantages represented by certain embodiments of the invention will become apparent from the following description of the preferred embodiments of the invention, when considered in conjunction with the accompanying drawings and the novel features set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 6B  are schematic cross sections illustrating certain steps of the process flow for completing the fabrication of a bonding pad on an integrated circuit wafer. 
         FIG. 1A  depicts the deposition of a seed metal layer needed for applying electrical potential in the plating process of a stack of interface layers over the wafer metallization. 
         FIG. 1B  depicts the deposition of a stack of seed metal layers. 
         FIG. 2  illustrates the forming of a patterned photoresist layer. 
         FIG. 3  shows the deposition of a buffer layer over the portion of the seed layer exposed in the photoresist window. 
         FIG. 4  illustrates the removal of the patterned photoresist layer and the underlying portions of the seed metal layer. 
         FIG. 5A  illustrates the electroless deposition of a barrier metal layer and an outermost metal layer over the buffer layer according to an embodiment of the invention. 
         FIG. 5B  illustrates the electroless deposition of two barrier layers and an outermost layer over the buffer layer according to another embodiment of the invention. 
         FIG. 6A  shows a metal wire connected to a bond pad positioned on an integrated circuit according to an embodiment of the invention. 
         FIG. 6B  shows a metal wire connected to a bond pad positioned on an integrated circuit according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is related to U.S. patent application Ser. No. 10/086,117, filed on Feb. 26, 2002 (Bojkov et al., “Waferlevel Method for Direct Bumping on Copper Pads in Integrated Circuits”). 
       FIGS. 1A to 6B  are schematic cross sections illustrating certain steps in the process flow for completing the fabrication of a bonding pad on an integrated circuit (IC) wafer.  FIG. 1A  shows schematically a portion of a semiconductor wafer, generally designated  100 , having interconnections  101  and a protective overcoat  102 . The semiconductor material is preferably silicon, but may alternatively be silicon germanium, gallium arsenide, or another III-V or II-IV semiconductor. The interconnection  101  is preferably copper; alternatively, aluminum or aluminum/copper alloy, or similar materials can be employed. The protective overcoat  102  is preferably silicon nitride, but may alternatively be silicon oxynitride, silicon carbide, silicon dioxide, or a polymer such as polyimide, for example. A window of width  103  in overcoat  102  exposes metallization  101  selectively. 
     The semiconductor wafer  100  including the protective overcoat  102  and the exposed metallization  101  is subjected to a series of cleaning and etching steps as follows: exposing the wafer to organic solvents, thereby removing organic contamination and mechanical particles from the copper contact pads; drying the wafer in dry nitrogen; exposing the wafer to an oxygen and nitrogen/helium/argon plasma, thereby ashing any further organic residues on the copper contact pads and oxidizing the copper surface to a controlled thickness; and, without breaking the vacuum of the plasma chamber, exposing the wafer to a hydrogen and nitrogen/helium/argon plasma, thereby removing the controlled copper oxide from the surface of the exposed copper metallization and passivating the cleaned surface, which is then sputter-etched with energetic ions so that a fresh and activated surface is created. 
     More specifically, the step of exposing the wafer to solvents may be selected from processes such as submerging the wafer in agitated isopropyl alcohol (or, alternatively, in methanol, glycol, or N-methyl pyrrolidone, for example); adding ultrasonic energy to the solvent; spraying the wafer with an organic solvent; and treating the wafer in dry chemical vapor. Between the steps of oxygen plasma and hydrogen plasma, there may be a step of etching the wafer in an aqueous inorganic or organic acid, which helps to remove deep metal defects in the exposed windows  103 . 
     Without delay, a layer  104  of seed metal is then deposited to cover the fresh and activated surface of the wafer, preferably by a sputtering technique without breaking the chamber vacuum. When the chip metallization is copper, the seed metal may be a copper layer  104  as shown in  FIG. 1A . Preferably, however, the seed metal is a stack of a refractory metal layer (for example, titanium/tungsten)  105   a  (facing the chip metallization  101 ) and a copper layer  105   b , as shown in  FIG. 1B . Refractory layer  105   a  and copper layer  105   b  together form seed metal layer  105 . Other options for layer  105   a  include one or more refractory metals such as tantalum, titanium, tungsten, molybdenum, chromium, nickel, vanadium, or an alloy of any of these metals. When the chip metallization  101  is aluminum or an aluminum alloy, the seed metal is a stack of a refractory metal/alloy layer (contacting the aluminum) and a copper layer. 
     Layers  104  and  105  exhibit low electrical resistance, ohmic contact, and low thermo-mechanical stress to the metal pad  101 ; it is, therefore, well suited for the following plating step. Furthermore, layers  104  and  105  exhibit strong adhesion to the chip metallization  101  as well as the protective overcoat  102 . Due to the adhesion, the interface between the seed metals  104  and  105  and the insulating layer  102  provides a practically impenetrable seal against moisture or particles ingress from the ambient environment. 
     Referring now to  FIG. 2 , it illustrates the forming and patterning of a photoresist layer  201 . The photoresist pattern defines the opening  202 , which exposes the seed metal layer over the interconnecting wafer metallization  101  (for simplicity, the seed metal is represented in  FIG. 2  and all following FIGS. only by the single layer  104 ; all considerations under discussion, however, apply equally to the stack of layers  105 ). The process flow continues with the step of exposing the seed metal layer  104  in the opening  202  to a hydrogen and nitrogen/argon plasma, which cleans and passivates the seed metal layer  104  in the photoresist opening  202 . 
     In  FIG. 3 , a buffer metal layer  301  is deposited onto the seed metal layer  104  exposed in the photoresist opening  202 . The preferred material of layer  301  is copper in the thickness range from about 1 to 20 μm. Alternatively, nickel can be used; its thickness is about 0.5 to 10 μm. Because of its relative thickness, buffer layer  301  is frequently referred to as a “stud”. The deposition is executed without exposing the passivated seed metal layer  104  to fresh contamination, whereby two options are available: the preferred method is electrolytic plating; an alternative method is electroless plating. 
     When the buffer layer  301  is deposited as a copper layer by an electroless plating process, it is advantageous to use a plating bath for fine grain, low stress deposits. As an example, a suitable plating bath is commercially available as the “Circuposit” electroless copper 3350 from Shipley Company, Marlborough, Mass., USA. 
     In  FIG. 4 , the photoresist material is stripped; this step exposes the portions of the seed layer  104  outside of the stud  301 . These seed layer portions are then removed by etching, conveniently in a wet etch solution of H 2 SO 4 , H 2 O 2 , and NH 4 OH. It is inconsequential, if small portions of the stud  301  (of the order of 1 to 3 μm thickness) are also removed in this etching process, since there is enough overlap  104   a  (5 to 15 μm) over the protective overcoat  102  around the periphery of window  103 . In this etching process, the stud  301  may receive slightly “rounded” contours. 
       FIG. 5A  illustrates the sequential deposition of a barrier layer and an outermost layer for one embodiment of the invention,  FIG. 5B  shows the analogous deposition of two barrier layers and an outermost layer for another embodiment. In  FIG. 5A , layer  501  is deposited over the stud  301 , preferably by an electroless plating technique. Material and thickness of layer  501  are selected to let layer  501  act as a barrier layer that resists diffusion of the stud metal. If stud  301  is made of copper, barrier layer  501  resists copper diffusion. Usually, the material of layer  501  is less noble than copper and is preferably nickel; other options include tin, lead, cobalt, iron, chromium, zinc, magnesium, aluminum, and alloys thereof. When nickel is selected as barrier metal, layer  501  has a preferred thickness range from 1 to 5 μm. (As commonly practiced, the term “more noble metal” or “less noble metal” refers to the relative position of that particular metal compared to another metal in the table of electrochemical potentials, in which all metals are rank-ordered for their ability to be oxidized; the lower a metal is ranked, the harder it is to be oxidized and thus the more “noble” it is). 
     As  FIG. 5A  shows, barrier layer  501  is deposited over stud  301  so that the exposed sides of stud  301  are conformally covered. There is no undercut in the region  501   a , where barrier layer  501  joins the insulating layer  102 . 
     Next, the outermost layer  502  is deposited over barrier layer  501 , again preferably by an electroless plating technique. As  FIG. 5A  shows, layer  502  covers the exposed sides of barrier layer  501 . The top surface of the outermost layer  502  is smooth; its average surface roughness is less than about 50 nm. The material of layer  502  is preferably more noble than the material of layer  501  and the stud  301 , and is selected so that it is wire bondable (ball, wedge, and ribbon bonding), meaning that a wire or ribbon will reliably adhere to the surface. Due to the surface smoothness of layer  502 , wire bonding provides uniform metal interdiffusion and thus reliable welds. Preferably, layer  502  is made of gold or palladium; other options include silver, platinum, and alloys thereof. When gold is selected, layer  502  has a preferred thickness range from about 50 and 250 nm; in some applications, gold layers up to about 3 μm thickness are used. 
     Referring now to  FIG. 6A , the cross section indicates schematically the attachment of a ball bond  601  to the outermost layer  502  of the bond pad metallization. The preferred metal for the bonding wire  602  is gold; other options include copper, aluminum, silver, and alloys thereof. Due to the low surface roughness  502   a  of less than about 50 nm, the interface  601   a  of the wire ball-to-layer surface is smooth, providing uniform metal interdiffusion at the interface  601   a  and thus a reliable weld. 
     The embodiment of  FIG. 5B  shows two barrier layers  505  and  506 , instead of only one barrier layer  502  as in  FIG. 5A . First barrier layer  505 , positioned directly on metal stud  301 , resists the diffusion of the stud metal (for example, copper), while second barrier layer  506 , positioned on first barrier layer  505 , resists the diffusion of the first barrier metal. When the stud metal is copper, the first barrier material is preferably nickel; other choices include tin, lead, cobalt, iron, chromium, zinc, magnesium, aluminum, and alloys thereof. For nickel, the preferred thickness range of layer  505  is between 1 and 5 μm. The second barrier material is preferably palladium; other options include cobalt, platinum and osmium. For palladium, the preferred thickness range of layer  506  is between 0.1 and 0.5 μm. The deposition method for layers  505  and  506  is preferably electroless plating. 
     The outermost layer  507  is deposited over barrier layer  506 , preferably by electroless plating. The surface of the outermost layer  507  is smooth; its average surface roughness is less than about 50 nm. The material of layer  507  is preferably more noble than the material selected for layers  505  and  506  and stud  301 , and is selected so that it is wire bondable (ball, wedge, and ribbon bonding). Due to the surface smoothness of layer  507 , wire bonding provides uniform metal interdiffusion and thus reliable welds. Preferably, layer  507  is made of gold; other options include platinum, palladium, silver, and alloys thereof. When gold is used, layer  507  is preferably in the thickness range from about 50 to 250 nm. 
     Referring now to  FIG. 6B , the cross section indicates schematically the attachment of a ball bond  611  to the outermost layer  507  of the bond pad metallization. The preferred metal for the bonding wire  612  is gold; other options include copper, aluminum, silver, and alloys thereof. Due to the low surface roughness  507   a  of less than about 50 nm, the interface  611   a  of the wire ball-to-layer surface is smooth, providing uniform metal interdiffusion at the interface  611   a  and thus a reliable weld. 
     While this invention has been described in 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 and embodiments.