Abstract:
A method for reworking integrated circuit (IC) wafers having copper-metallized bond pads exposed in protective overcoat openings and one or more bondable metal layers deposited onto the bond pads by a technology which may produce some parts with off-spec or missing depositions. After identifying the wafer with off-spec metal layers, a layer of glass buffer is deposited over those wafers, which also fill any missing depositions at least partially. The glass-covered surface is then chemically-mechanically polished until the off-spec metal layers and at least portion of the protective overcoat are removed, without damaging the copper metallization. Finally, a fresh layer of protective overcoat is deposited, selectively opened to expose the bond pads, and provided anew with one or more bondable metal layers.  
     The slurry for the chemical-mechanical polishing contains oxidizing or hydroxylating agents as well as mechanical polishing components for metals which are not readily oxidized. In order to minimize undesired scratches of the underlying copper or dielectric layers, a combination of buffers and soft poromeric pads is used. Alternatively, organic buffer layers can be used instead of the glass buffer.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention is related in general to the field of semiconductor devices and processes and more specifically to a method for reworking metal layers mis-processed on bond pads of integrated circuits having copper metallization.  
         DESCRIPTION OF THE RELATED ART  
         [0002]    In integrated circuits (IC) technology, pure or doped aluminum has been the metallization of choice for interconnection and bond pads for more than four decades. Main advantages of aluminum include easy of deposition and patterning. Further, the technology of bonding wires made of gold, copper, or aluminum to the aluminum bond pads has been developed to a high level of automation, miniaturization, and reliability. Examples of the high technical standard of wire bonding to aluminum can be found in U.S. Pat. No. 5,455,195, issued on Oct. 3, 1995 (Ramsey et al., “Method for Obtaining Metallurgical Stability in Integrated Circuit Conductive Bonds”); U.S. Pat. No. 5,244,140, issued on Sep. 14, 1993 (Ramsey et al., “Ultrasonic Bonding Process Beyond 125 kHz”); U.S. Pat. No. 5,201,454, issued on Apr. 13, 1993 (Alfaro et al., “Process for Enhanced Intermetallic Growth in IC Interconnections”); and U.S. Pat. No. 5,023,697, issued on Jun. 11, 1991 (Tsumura, “Semiconductor Device with Copper Wire Ball Bonding”).  
           [0003]    In the continuing trend to miniaturize the ICs, the RC time constant of the interconnection between active circuit elements increasingly dominates the achievable IC speed-power product. Consequently, the relatively high resistivity of the interconnecting aluminum now appears inferior to the lower resistivity of metals such as copper. Further, the pronounced sensitivity of aluminum to electromigration is becoming a serious obstacle. Consequently, there is now a strong drive in the semiconductor industry to employ copper as the preferred interconnecting metal, based on its higher electrical conductivity and lower electromigration sensitivity. From the standpoint of the mature aluminum interconnection technology, however, this shift to copper is a significant technological challenge.  
           [0004]    Copper has to be shielded from diffusing into the silicon base material of the ICs in order to protect the circuits from the carrier lifetime killing characteristic of copper atoms positioned in the silicon lattice. For bond pads made of copper, the formation of thin copper(I)oxide films during the manufacturing process flow has to be prevented, since these films severely inhibit 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 thermocompression and ultrasonic energy applied in the bonding process. As further difficulty, bare copper bond pads are susceptible to corrosion.  
           [0005]    In order to overcome these problems, a process has been disclosed to cap the clean copper bond pad with a layer of aluminum and thus re-construct the traditional situation of an aluminum pad to be bonded by conventional gold-wire ball bonding. A suitable bonding process is described in U.S. Pat. No. 5,785,236, issued on Jul. 28, 1998 (Cheung et al., “Advanced Copper Interconnect System that is Compatible with Existing IC Wire Bonding Technology”). The described approach, however, has several shortcomings.  
           [0006]    First, the fabrication cost of the aluminum cap is higher than desired, since the process requires additional steps for depositing metal, patterning, etching, and cleaning. Second, the cap must be thick enough to prevent copper from diffusing through the cap metal and possibly poisoning the IC transistors. Third, the aluminum used for the cap is soft and thus gets severely damaged by the markings of the multiprobe contacts in electrical testing. This damage, in turn, becomes so dominant in the ever decreasing size of the bond pads that the subsequent ball bond attachment is no longer reliable.  
           [0007]    A low-cost structure and method for capping the copper bond pads of copper-metallized ICs has been disclosed on U.S. Patent Application No. 60/183,405, filed on Feb. 18, 2000. The present invention is related to that application. The structure provides a metal layer electrolessly plated onto the copper, which impedes the up-diffusion of copper. Of several possibilities, nickel is a preferred choice. This layer is topped by a bondable metal layer, which also impedes the up-diffusion of the barrier metal. Of several possibilities, gold is a preferred choice. In a variation of this sequence of metal layers, a second barrier layer such as palladium is electrolessly deposited on the nickel, blocking its up-diffusion, and then followed by a bondable metal such as gold. Metallurgical connections stacked cap of layers can then be performed by conventional wire bonding.  
           [0008]    It is difficult, though, to plate these bond pad caps uniformly in electroless deposition systems, because electroless deoosition is affected by local reactant concentrations and by the agitation velocities of the aqueous solution. Deposition depletes the reactants in areas around the bond pads. Increasing the agitation of the solution only exacerbates the deposition non-uniformity, which is influenced by the flow direction of the solution. Also, a few bond pads may not receive any metal deposition. The problem is further complicated when a whole batch of wafers is to be plated simultaneously in order to reduce cost, since known control methods have been applied only to process single wafers under applied electrical bias. See, for example, U.S. Pat. No. 5,024,746, issued Jun. 18, 1991, and U.S. Pat. No. 4,931,149, issued Jun. 5, 1990 (Stierman et al., “Fixture and a Method for Plating Contact Bumps for Integrated Circuits”).  
           [0009]    An urgent need has arisen for a reliable method of reworking metal caps over copper bond pads which combines minimum fabrication cost with maximum correction control of the IC structure. The reworking method 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 without the need of expensive additional manufacturing equipment.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention discloses a method for reworking integrated circuit (IC) wafers having copper-metallized bond pads exposed in protective overcoat openings and one or more bondable metal layers deposited onto the bond pads by a technology which may produce some parts with off-spec or missing depositions. After identifying the wafer with off-spec metal layers, a layer of glass buffer is deposited over those wafers, which also fill any missing depositions at least partially. The glass-covered surface is then chemically-mechanically polished until the off-spec metal layers and at least portion of the protective overcoat are removed, without damaging the copper metallization. Finally, a fresh layer of protective overcoat is deposited, selectively opened to expose the bond pads, and provided anew with one or more bondable metal layers.  
           [0011]    The slurry for the chemical-mechanical polishing contains oxidizing or hydroxylating agents as well as mechanical polishing components for metals which are not readily oxidized. In order to minimize undesired scratches of the underlying copper or dielectric layers, a combination of buffers and soft poromeric pads is used. Alternatively, organic buffer layers can be used instead of the glass buffer.  
           [0012]    The present invention is related to high density and high speed ICs with copper interconnecting metallization, especially those having high numbers of metallized inputs/outputs, or “bond pads”. These circuits can be found in many device families such as processors, digital and analog devices, logic devices, high frequency and high power devices, and in both large and small area chip categories.  
           [0013]    It is an aspect of the present invention to increase the process yield of wafer fabrication, since it provides a low-cost rework method for wafers misprocessed almost at the end of the process flow. It is towards the completion of the fabrication process when a high amount of work and time has already been invested and the wafers represent high value.  
           [0014]    Another aspect of the invention is to support technologies applicable to bond pad area reduction and thus the shrinking of IC chips. Consequently, the invention helps to alleviate the space constraint of continually shrinking applications such as cellular communication, pagers, hard disk drives, laptop computers and medical instrumentation.  
           [0015]    Another aspect of the invention is to support the fabrication of bond pad metal caps by the self-defining process of electroless deposition, which is preferable over costly photolithographic and alignment techniques.  
           [0016]    Another aspect of the invention is to help advancing the process and reliability of wafer-level multi-probing by eliminating probe marks and subsequent bonding difficulties.  
           [0017]    Another aspect of the invention is to provide materials and process concepts which are flexible so that they can be applied to many families of semiconductor products, and are general so that they can be applied to several generations of products.  
           [0018]    Another aspect of the invention is to use only materials and processes most commonly employed and accepted in the fabrication of advanced IC devices, thus avoiding the cost of new capital investment and using the installed fabrication equipment base.  
           [0019]    These aspects have been achieved by the teachings of the invention concerning selection criteria and process flows suitable for mass production. Various modifications have been successfully employed to satisfy different selections of bonding technologies.  
           [0020]    In the first embodiment of the invention, a glass layer is deposited by a spin-on technique, approximately filling in any bond pad opening having missing or very thin metal cap layers. A slurry for chemical mechanical polishing (CMP) is selected which has nearly equal polishing speeds for the metal layers (such as nickel and palladium) and the protective overcoat (such as silicon oxynitride). The bondable gold layer is thin enough that the CMP slurry does not need to be tailored for it.  
           [0021]    In the second embodiment of the invention, the electrolessly plated metal layers are non-uniform or not thick enough, but all copper-metallized bond pads are covered. A non-selective CMP slurry polishes the protective overcoat and the meal layers deep enough to almost the copper metallization. An auxiliary glass layer may not be necessary.  
           [0022]    In all preferred embodiments, optical reflectivity monitors and/or mechanical roughness sensors indicate the appearance of the borderline to the copper metallization and its surrounding dielectric. Any damage the copper or its surrounding dielectric is thus avoided.  
           [0023]    The technical advances represented by the invention, as well as the aspects thereof, 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  
       [0024]    [0024]FIG. 1 is a schematic and simplified cross section of a portion of an integrated circuit wafer having off-spec bond pad metal layers, identified for reworking according to the first embodiment process of the invention.  
         [0025]    [0025]FIG. 2 is a schematic and simplified cross section of a portion of an integrated circuit wafer having off-spec bond pad metal layers, at the beginning of the rework process according to the first embodiment of the invention.  
         [0026]    [0026]FIG. 3 is a schematic and simplified cross section of a portion of an integrated circuit wafer having off-spec bond pad metal layers, in the rework process according to the first embodiment of the invention.  
         [0027]    [0027]FIG. 4 is a schematic and simplified cross section of a portion of an integrated circuit wafer having off-spec bond pad metal layers, at the end of the rework process according to the first embodiment of the invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]    [0028]FIG. 1 shows in schematic fashion the cross section of a small portion of an integrated circuit (IC) fabricated on the active surface  101  of a semiconductor wafer. The IC has copper interconnecting metallization, which also provides the metallization for the plurality of bond pads  102 . Actually, the copper traces are imbedded in a refractory metal shield (not shown in FIG. 1), which prevents the diffusion of copper into parts of the IC. Typical shield materials include tantalum nitride, tantalum silicon nitride, tungsten nitride, tungsten silicon nitride, titanium, titanium nitride, or titanium tungsten.  
         [0029]    Bond pads  102  are surrounded by dielectric IC portions  103 , only summarily indicated in FIG. 1. These electrically insulating portions may include not only the traditional plasma-enhanced chemical vapor deposited dielectrics such as silicon dioxide, but also newer dielectric materials having lower dielectric constants, such as silicon-containing hydrogen silsesquioxane, organic polyimides, aerogels, and parylenes, or stacks of dielectric layers including plasma-generated or ozone tetraethylorthosilicate oxide. Since these materials are less dense and mechanically weaker than the previous standard insulators, the dielectric under the copper is often reinforced. Examples can be found in U.S. Patent application No. 60/085,876, filed on May 18, 1998 (Saran et al., “Fine Pitch System and Method for Reinforcing Bond Pads in Semiconductors”), and No. 60/092,961, filed Jul. 14, 1998 (Saran, “System and Method for Bonding over Active Integrated Circuits”).  
         [0030]    The surface  101  of the semiconductor wafer is covered by a moisture-impenetrable protective overcoat  104 . This overcoat is usually made of silicon nitride or silicon oxynitride, commonly 0.5 to 1.0 μm thick. Windows  105  are opened in the overcoat in order to expose portion of the copper metallization  102 . The copper exposed by these openings is commonly referred to as the bond pad metallization.  
         [0031]    Since copper is susceptible to corrosion and even thin copper(I)oxide films are difficult to bond to, the U.S. Patent Application No. 60/183,405, filed on Feb. 18, 2000 (to which the present invention is related) provides structures and processes of a cap formed over the exposed copper. According to that application, the cap consists of a metal and has a coordinated thickness such that it satisfies three requirements:  
         [0032]    The cap acts as a barrier against the up-diffusion of copper to the surface of the cap where the copper might impede the subsequent wire bonding operation. Specifically, for the cap the metal selection and thickness are coordinated such that the cap reduces the up-difusion of copper at 250° C. by more than 80% compared with the absence of the barrier metal.  
         [0033]    The cap is fabricated by a technique, which avoids expensive photolithographic steps. Specifically, an electroless process is used to deposit the cap metal layer.  
         [0034]    The cap metal has a surface which is bondable. Specifically, conventional ball and wedge bonding techniques can be used to connect metal wires and other coupling members metallurgically to the bond pad.  
         [0035]    [0035]FIG. 1 depicts examples of bond pad caps designed to be a stack of three metal layers, each layer deposited by an electroless plating process. Bond pad  106  shows a satisfactory cap: all three layers are deposited in their predetermined respective thickness ranges.  
         [0036]    Layer  110  is positioned over copper area  102 , sometimes deposited on a seed metal layer (not shown in FIG. 1). Layer  110  consists of a metal acting as a diffusion barrier against copper. Examples for layer  110  are nickel, cobalt, chromium, molybdenum, titanium, tungsten, and alloys thereof. These metals are inexpensive and can be deposited by electroless plating; however, they are poorly bondable. As mentioned above, in these metals copper has a diffusions coefficient of less than 1×10E-23 cm 2 /s at 250° C. Consequently, these metals are good copper diffusion barriers. The layer thicknesses, required to reduce copper diffusion by more than 80% compared to the absence of the layers, are obtained by diffusion calculations. Generally, a barrier thickness from about 0.5 to 1.5 μm will safely meet the copper reduction criterion.  
         [0037]    Layer  111  is positioned over layer  110  as an effective diffusion barrier against the up-diffusing metal used in layer  110 . The intent is to de-emphasize the barrier function of the outermost layer  112 , and rather emphasize its bondability function. Consequently, the thickness required for the outermost layer  112  can be reduced, thus saving cost. Examples for layer  111  are palladium, cobalt, platinum, and osmium. Examples for layer  112  are gold, platinum, and silver.  
         [0038]    Metals used for layer  111  (such as palladium) have a diffusion coefficient for the metals used in barrier layer  110  (such as nickel) of less than 1×10E-14 cm 2 /s at 250° C. The layer thicknesses required to reduce the diffusion of metal used in layer  110  by more than 80% compared to the absence of layer  111  are obtained from diffusion calculations. Generally, a thickness of layer  111  of about 0.4 to 1.5 μm will safely meet the reduction criterion for metal diffusing from layer  110 .  
         [0039]    The thickness of the bondable outermost layer  112  (such as gold) is in the range from about 0.02 to 0.1 μm.  
         [0040]    The preferred process of depositing layers  110 ,  111  and  112  is by electroless plating. An example of an electroless plating process flow is described in the U.S. Patent Application quoted above. In general, electroless plating is well controlled and offers high process yield. However, it is known that some misprocessing may occasionally happen. As a result, the plated layers may have characteristics outside of the specification window. For instance, in FIG. 1, bond pad  107  shows the correct thickness for layer  110 , however an off-spec thickness for layer  121  (too thin compared to layer  111 , though same material), and again the correct thickness for layer  112 . As another example in FIG. 1, bond pad  108  shows all three layer missing.  
         [0041]    Whenever any layer deposition of the bond pad cap falls outside of specifications, it affects the IC wafer in its process flow close to the finish of a long and expensive series of manufacturing steps, i.e., it affects a wafer of very high value. Consequently, it is economically advantageous to reprocess the wafer in order to eliminate the off-spec layers, repeat the deposition, and obtain bond pad caps within specifications. According to the first embodiment of the invention, the method for reworking of misprocessed metal caps over copper-metallized bond pads comprises the following steps:  
         [0042]    Inspecting Wafers  
         [0043]    After completing the electroless plating process steps for the metal layers capping the bond pad copper metallization, the wafer is inspected for the quality of the deposited layers. Inspection techniques may utilize a high-magnification microscope, X-ray fluorescence equipment, and/or a profilometer based on stylus or reflected light beam. The inspection detects missing, incomplete, non-uniform, defective, or otherwise off-spec layers. Examples are given in FIGS. 1 and 2: The cap metal layers are missing in bond pad  108 , at least one layer is incomplete (too thin) in bond pad  107 .  
         [0044]    Spinning-On Glass  
         [0045]    The off-spec wafers receive a coating with spin-on glass in order to create a substantially planar surface. Several types of glasses may be used, including TEOS (tetraethylorthosilicate). The result is schematically shown in FIG. 2. The spin-on glass  201  covers the well-processed metal layers of bond pad  106  and surrounding surfaces of overcoat  104  with a relatively thin buffer. However, the spin-on glass covers bond pads such as  107 , having cap metal layer too thin, or especially bond pads such  108 , having no cap metal layers at all, with a relatively thick buffer. The spin-on glass is capable to fill-in low spots and voids and thus creates a substantially planar surface across the whole wafer.  
         [0046]    Chemically-Mechanically Polishing  
         [0047]    Subjecting the glass-covered wafer surface to the process of chemical-mechanical polishing (CMP) requires the process steps of:  
         [0048]    Mixing an abrasive slurry so that it polishes the metal layers ( 110 ,  111 ,  112 , and  121  in FIG. 3) at approximately the same rate as the protective overcoat of the wafer ( 104  in FIG. 3). In FIG. 3, the slurry  301  is shown to fill-in the space between the spin-on glass  201  and the polishing plate  302 .  
         [0049]    Applying the slurry  301  for grinding the metal layers as well as the protective overcoat until the appearance of the copper metallization ( 102  in FIG. 3) and its surrounding dielectric  103  is indicated. This appearance is indicated by an optical reflectivity monitor or a mechanical roughness sensor, which are suitable for identifying the boundary  401  of the bond pad copper  102  and its surrounding dielectric  103 . Usually, the boundary  401  is identical with the surface  101  of the wafer discussed in FIG. 1.  
         [0050]    Abrasive slurries with either standardized or customized characteristics are commercially available from suppliers such as Cabot Corporation, Boston, Mass., U.S.A., or Rodel, Inc., Newark, Del., U.S.A. These suppliers are capable of mixing the slurries from components, having oxidizing or hydroxylating characteristics and mechanically abrasive characteristics, in water so that the slurries become suitable for polishing oxidizable and non-oxidizable metals, hard and soft metals, as well as inorganic dielectrics (such as silicon nitride or oxynitride) at approximately the same rate. A number of specific slurry compositions and characteristics have been described in patents, together with their target metals and dielectrics to be polished. Examples are the following:  
         [0051]    U.S. Pat. No. 5,954,997, issued on Sep. 21, 1999 (Kaufman et al, “Chemical Mechanical Polishing Slurry Useful for Copper Substrates”).  
         [0052]    U.S. Pat. No. 5,527,423, Issued on Jun. 18, 1996 (Neville et al., “Chemical Mechanical. Polishing Slurry for Metal Layers”).  
         [0053]    U.S. Pat. No. 5,958,288, issued on Sep. 28, 1999 (Mueller et al., “Composition and Slurry Useful for Metal CMP”).  
         [0054]    U.S. Pat. No. 5,897,375, issued on Apr. 27, 1999 (Watts et al., “Chemical Mechanical Polishing Slurry for Copper and Method of Use in Integrated Circuit Manufacture”).  
         [0055]    U.S. Pat. No. 6,001,730, issued on Dec. 14, 1999 (Farkas et al., “Chemical Mechanical Polishing Slurry for Polishing Copper Interconnects which use Tantalum-Based Barrier Layers”).  
         [0056]    U.S. Pat. No. 6,027,997, issued on Feb. 22, 2000 (Yu et al., “Method for Chemical Mechanical Polishing a Semiconductor Device using Slurry”).  
         [0057]    U.S. Pat. No. 5,225,034, issued on Jul. 6, 1993 (Yu et al., “Method of Chemical Mechanical Polishing Predominantly Copper Containing Metal Layers in Semiconductor Processing”).  
         [0058]    U.S. Pat. No. 5,354,490, issued on Oct. 11, 1994 (Yu et al., “Slurries for Chemical Mechanical Polishing Copper Containing Metal Layers”).  
         [0059]    U.S. Pat. No. 5,567,300, issued on Oct. 22, 1996 (Datta et al., “Electrochemical Metal Removal Technique for Planarization of Surfaces”).  
         [0060]    U.S. Pat. No. 5,840,629, issued on Nov. 24, 1998 (Carpio, “Copper Chemical Mechanical Polishing Slurry Utilizing a Chromate Oxidant”).  
         [0061]    U.S. Pat. No. 5,863,307, issued on Jan. 26, 1999 (Zhou et al., “Method and Slurry Composition for Chemical-Mechanical Polish Planarizing for Copper Containing Conductor Layers”).  
         [0062]    U.S. Pat. No. 5,948,697, issued on Sep. 7, 1999 (Hata, “Catalytic Acceleration and Electric Bias Control of CMP Processing”).  
         [0063]    U.S. Pat. No. 6,030,425, issued on Feb. 29, 2000 (Hata, “Catalytic Acceleration and Electrical Bias Control of CMP Processing”).  
         [0064]    In combination with the slurry, the polishing pad  302  in FIG. 3 is selected so that scratches of copper  102  or dielectrics  103  are minimized. Preferably, a soft poromeric polishing pad is used. After completing the polishing process, the reworked wafer surface may look like shown schematically in FIG. 4, with only small, thin and irregular remnants  402  of the protective overcoat remaining, or none at all. This reworked wafer surface is now ready for the final process steps of the method:  
         [0065]    Depositing a fresh layer of protective overcoat.  
         [0066]    Photolithographically defining the openings for the bond pads.  
         [0067]    Selectively etching the overcoat and exposing the bond pads.  
         [0068]    Repeating the electroless plating steps for depositing the cap metal layers.  
         [0069]    The second embodiment of the invention addresses the reworking of the bond pad metal caps, when the inspection step indicates a misprocessed wafer having some cap metal layers in all bond pad openings (i.e., no bond pad without metal layers), but pads with non-uniform, incomplete, defective, or insufficiently thick layers. In this case, the corrective step of depositing a glass cover by spin-on coating can be omitted. Instead, the CMP slurry, nonselective for the silicon nitride/oxynitride overcoat and the misprocessed cap metal layers, is used directly. As described above, care still needs to be taken to mix the slurry for approximately equal polishing speeds of the overcoat material and the different cap metals, such as nickel and palladium (the gold film is too thin to be of much concern). After polishing and reworking in this somewhat simplified process flow, the reworked wafer looks, similar to the case shown in FIG. 4, and the corrective reprocessing can proceed as described above.  
         [0070]    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. As an example, the invention can be applied to IC bond pad metallizations other than copper, which are difficult or impossible to bond by conventional ball or wedge bonding techniques, such as alloys of refractory metals and noble metals. As another example, the invention may use a hard rather than a soft poromeric polishing pad in order to minimize scratches or other damage to the bond pad metallization or dielectric materials surrounding the bond pads. As another example, the invention applies to any sort of glass buffer or organic buffer layer, independent of the method of depositing the buffer. It is therefore intended that the appended claims encompass any such modifications or embodiments.