Abstract:
A method for forming a top interconnection level and bonding pads for an integrated circuit chip is described. The interconnection level is formed by a damascene type process. Bonding pads are placed above the plane of the wiring channels of the interconnection level. This eliminates the problem of dishing of the relatively large bonding pads which occurs, during chemical mechanical polishing, when the bonding pads are on the same level as the interconnection metallurgy. The interconnection wiring includes a smaller pad base segment upon which the larger bonding pad is then formed. The bonding pad base segments are small enough that dishing during CMP is not a problem. Placing the bonding pads on pad bases provides for a more robust pad. The top level and bonding pad fabrication procedures are applicable with various conductive materials including aluminum, tungsten, and copper.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    (1) Field of the Invention  
           [0002]    The invention relates to processes for the manufacture of semiconductor devices and more particularly to the formation of terminal metal layers and bonding pads.  
           [0003]    (2) Background of the Invention and Description of Prior Art  
           [0004]    Integrated circuits are manufactured by forming discrete semiconductor devices in the surface of silicon wafers. A multi-level metallurgical interconnection network is then formed over the devices, contacting their active elements, and wiring them together to create the desired circuits. The wiring layers are formed by depositing an insulating layer over the discrete devices, patterning and etching contact openings into this layer, and then depositing conductive material into the openings. A conductive layer is applied over the insulating layer and patterned to form wiring interconnections between the device contacts, thereby creating a first level of basic circuitry. The circuits are then further interconnected by utilizing additional wiring levels laid out over additional insulating layers with conductive via pass throughs.  
           [0005]    Depending upon the complexity of the overall integrated circuit, several levels of wiring interconnections are used. On the uppermost level the wiring is terminated at metal pads to which the chip&#39;s external wiring connections are bonded. These bonding pads are generally large in size compared to the interconnection wiring lines, typically measuring larger than about 50×50 microns.  
           [0006]    A method for forming the uppermost or top interconnection layer is the damascene process, whereby openings and trenches, comprising an image of the interconnection pattern are formed in an insulative layer. A metal layer is then deposited into the openings and over the insulative layer. Finally, the metal is polished back to the insulative layer leaving the metal pattern inlaid within the insulative layer. Polishing back of the metal layer is accomplished by CMP (chemical mechanical polishing), a relatively old process which has found new application in planarization of insulative layers and more recently in the damascene process. In a single damascene process a metal line pattern is generated which connects to subjacent vias or contacts. In a dual damascene process, both vias and contacts and an interconnection stripe pattern are formed by a single metal deposition and CMP. A description of both single and dual damascene processes may be found in Chang, C. Y. and Sze, S. M., “ULSI Technology” McGraw-Hill, New York, (1996), p444-445 and in El-Kareh, B., “Fundamentals of Semiconductor Processing Technologies”, Kluwer, Boston(1995), p563-4.  
           [0007]    Carey, et.al., U.S. Pat. No. 5,219,787 shows a method for forming vias and wiring lines in a polyimide base by first forming a trench and via pattern in polyimide layers, depositing a copper seed layer, and then plating copper. The copper is polished back to the polyimide leaving metallization in the trenches and vias. Matsuura, U.S. Pat. No. 5,598,027 cites a metal deposition/polish back (damascene) method for forming interconnection layers using dry etching to form grooves in the insulating layers. After the interconnection material is deposited, the surface is polished back by CMP leaving the conductive pattern in the grooves.  
           [0008]    The metal wiring layers, typically of an aluminum alloy or of an aluminum alloy containing copper and silicon, are deposited by sputtering or by vacuum evaporation. In the damascene process, copper metallization may also be used. The final metal interconnection layer includes the bonding pads which are typically located in the periphery of the integrated circuit. When large area features, such as bonding pads, are included in a damascene processed wiring pattern, a problem of bonding pad dishing arises when the metal is polished back to the insulative layer. Referring to FIG. 1 there is shown a planar view of a portion of the top metallization level of an integrated circuit on a wafer  20 . A bonding pad  24  and wiring lines  26  lie embedded in an insulating layer  22 . FIG. 1 is not drawn to scale the pad  24  is of the order of 50 by 50 microns square or larger and the wiring lines  26  are only of the order one micron or less in breadth.  
           [0009]    A cross section of the region on wafer  20  indicated by the line  2 - 2 ′ is shown in FIG. 2A at the point in the process after a metal layer  25  has been deposited onto the patterned insulative layer  22 . The wide portion of metal  24  in the insulator is to become a bonding pad. When the substrate wafer is polished by CMP, the surfaces of the wide metal bonding pads tend to become dished as illustrated by the curvature  28  in FIG. 2B.  
           [0010]    The dishing weakens the pad, by creating a thin central region. A subsequently attached wire bond will not only be weak mechanically, but also excessively resistive. A passivation layer  29  is applied over the layer  22  and the metal pattern. The passivation layer  29  seals the interconnection metallization on the wafer from contaminants and moisture, and also serves as a scratch protection layer. The passivation layer  29  typically consists of a layer of silicon nitride or a composite layer of phosphosilicate glass (PSG) over silicon oxide. The layer  29  is deposited by plasma enhanced chemical vapor deposition (PECVD). An opening  28  to the bonding pad is patterned and etched in the passivation layer  29  by a plasma etching process.  
           [0011]    Weakening of the bonding pad caused by the CMP dishing is reflected by high yield losses at wafer acceptance testing(WAT) and at subsequent package stress testing. These yield losses also forewarn a reliability degradation. Dummy pads are sometimes added on the interconnection level to counteract CMP dishing. These pads are sacrificial and are not connected to interconnection lines. This awkward fix also lowers the integrity of the interconnection lines.  
           [0012]    An alternative method for forming the top interconnection layer, including the bonding pads is to deposit and pattern the metal layer on the un-patterned surface of the insulative layer  22 . The passivation layer is then deposited over the metal pattern. This is an older method, a predecessor of the damascene method, and is not favorably compatible with current high density multilevel interconnection technology because it produces a higher resistance, and higher defect densities.  
         SUMMARY OF THE INVENTION  
         [0013]    Accordingly, it is an object of this invention to provide a method for forming a top interconnection level of a multilevel integrated circuit including bonding pads.  
           [0014]    It is another object of this invention to provide a method for forming a top interconnection level of a multilevel integrated circuit by a damascene process with bonding pads formed by an etch process.  
           [0015]    It is yet another object of this invention to provide a method for forming a top interconnection level of a multilevel integrated circuit including conductive base segments for bonding pads by a damascene process.  
           [0016]    It is still another object of this invention to provide a method for forming a top interconnection level of a multilevel integrated circuit wherein the bonding pads connected to the level are elevated above the level thereby reducing the impact of environmental or other external electrical disturbances on the interconnection level.  
           [0017]    It is yet another object of this invention to provide a method for forming a top interconnection level of a multilevel integrated circuit without the occurrence of dishing of the bonding pads.  
           [0018]    It is yet another object of this invention to eliminate the need for dummy bonding pads on a top interconnection level of a multilevel integrated circuit.  
           [0019]    These objects are accomplished by first forming the necessary wiring channels of the top interconnection level by patterning trenches and vias in an insulative layer. Bonding pads are not patterned in this insulative layer but segments of interconnection lines which form bases for connecting bonding pads are included. The base segments may be simple terminations of the interconnection wiring or they may be oversized terminations having dimensions somewhat larger than the interconnection line width but much smaller than those of the bonding pads. The interconnection metallurgy is then deposited. In a first embodiment, the interconnection metallurgy is polished back to the insulative layer surface by CMP and a second metal layer is deposited. Bonding pads are then patterned in the second metal layer by plasma etching.  
           [0020]    In a second embodiment, the deposited interconnection metallurgy is partially polished back by CMP to planarize the surface. Bonding pads are then patterned on the residual conductive layer and the layer is etched back to the insulative layer surface, leaving the bonding pads on the surface of the insulative layer, connected to the subjacent wiring level through the access openings. This method is limited to metallurgies which lend themselves to dry plasma etching such as aluminum, aluminum alloys, and tungsten. The bonding pads may then be formed of another metal such as aluminum or tungsten. An advantage of both embodiments is that the thickness of the bonding pads is independent of the thickness of the interconnection level.  
           [0021]    A passivation layer is deposited over the exposed top interconnection level and bonding pads and patterned to form openings over the bonding pads. Bonding pads formed by the methods of the embodiments are robust and flat. Because the bonding pads lie above the top interconnection level the risk of wire bonding damage to the interconnection level is diminished.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1 is a planar view of a portion of a top interconnection level of a multilevel integrated circuit showing a bonding pad and wiring lines.  
         [0023]    [0023]FIG. 2A is a cross section of a portion of a multilevel integrated circuit shown in FIG. 1 showing a metal layer deposited over an arrangement of vias and trenches patterned in an insulative layer.  
         [0024]    [0024]FIG. 2B is a cross section of a portion a multilevel integrated circuit showing the metal layer of FIG. 2. polished back by CMP and covered by a patterned passivation layer.  
         [0025]    [0025]FIG. 3 is a planar view of a portion of a top interconnection level of a multilevel integrated circuit showing a bonding pad base and wiring lines formed by the process taught by the embodiments this invention.  
         [0026]    [0026]FIG. 4A and FIG. 4B are cross sections used to illustrate a first sequence of processing steps leading to the formation of a conductive layer from which a top interconnection level for an integrated circuit is to be formed by the embodiments of the current invention.  
         [0027]    [0027]FIG. 5A and FIG. 5B are cross sections used to illustrate a second sequence of processing steps leading to the formation of a conductive layer from which a top interconnection level for an integrated circuit is to be formed by the embodiments of the current invention.  
         [0028]    [0028]FIG. 6A through FIG. 6D are cross sections illustrating the sequence of processing steps leading to the formation a top interconnection wiring level with bonding pads according to a first embodiment of the current invention.  
         [0029]    [0029]FIG. 7A through FIG. 7B are cross sections illustrating the sequence of processing steps leading to the formation a top interconnection wiring level with bonding pads according to a second embodiment of the current invention.  
         [0030]    [0030]FIG. 8 is a planar view of a portion of a top interconnection level of a multilevel integrated circuit showing a bonding pad and wiring lines covered by a passivation layer formed according to the processes taught by this invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]    In a first embodiment of this invention a top interconnection level is formed by metal deposition onto a insulative layer patterned with trenches. In addition to trenches for top level interconnection lines, the pattern in the insulative layer includes a bonding pad base segment at the termination of one of the interconnection lines. The metal layer is polished back to the insulative layer by CMP leaving a metal pattern inlay in the insulative layer. A planar view of a portion of the in the insulative layer with the inlaid metal pattern is shown in FIG. 3. Two processing paths will be described to form the inlaid metal pattern shown in FIG. 3. The first path will be using a single damascene process over an insulative layer with conductive vias. The second path will be a dual damascene process.  
         [0032]    In FIG. 3 there is shown a line  50  which is a conductor of a top interconnection level on a substrate wafer  30 . The conductive line  52  connects a portion of a subjacent integrated circuit to a bonding pad base segment  54  and is inlaid in an insulative layer  38 . In FIG. 3 a bonding pad base segment  54  is shown at the termination of the conductive line  52  and is described thus in the embodiments. However, it is contemplated that the bonding pad base segment  54  may also be located at any position along the length of the conductive line  52  and thus be considered as a segment of the conductive line  52 . The bonding pad base segment  54  described in the embodiments is rectangular and between about 1 and 10 microns wide and between about 1 and 10 microns long, the length being understood to be along the linear direction of the conductive line. The conductive lines  50 ,  52  are between about 0.2 and 2.0 microns wide. The bonding pad base segment  54  may be of the same width as or wider than the conductive line  52 . In FIGS. 3, 6, and  7  the pad base segment  54  is shown wider than the conductive lines  50 ,  52 . However, it is contemplated in this invention, that the width of the bonding pad base segment  54  may be comparable to the width of the conductive line  52 .  
         [0033]    Referring to the cross section shown in FIG. 4A, a silicon wafer substrate  30  is provided. Integrated circuit devices(not shown) are formed in the of the silicon wafer substrate using conventional state of the art process technology. A plurality of interconnection levels are formed over the wafer  30  separated by insulative layers and interconnected through contacts and vias using integrated circuit processes well known by those skilled in the art. These layers and interconnection levels are not shown in the figures and are understood to be included in the substrate  30 . The upper surface of substrate  30  is insulative and a metal interconnective stripe  32  is formed over the insulative surface. The metal interconnective stripe  32  is a portion of the uppermost of the plurality of interconnection levels, is patterned by conventional methods, and is between about 0.3 and 0.7 microns thick.  
         [0034]    An insulative layer  34  is formed over the metal stripe  32  and a conductive via  36  is formed in an opening in the insulative layer  34 . The insulative layer  34  is deposited to a thickness of between about 0.8 and 1.5 microns. Methods for depositing insulative layers and forming conductive vias are well known in the art. An insulative layer  38  is next deposited over the insulative layer  34 , preferably by PECVD, and a pattern of trenches  40  is etched in the insulative layer  38 , by plasma etching. The trench pattern is an image of the metal pattern shown in FIG. 3. The insulative layer  38  is between about 0.3 and 1.0 microns thick. The via  36  is exposed by the etching. Referring now to FIG. 4B, a conductive layer  42  is deposited over the wafer  30 . The conductive layer  42  used in the embodiment is an aluminum alloy and is between about 4,000 and 20,000 Angstroms. Alternately, other metals for example tungsten or copper, may be used.  
         [0035]    Alternatively, the configuration shown by the cross section of FIG. 4B may be formed by a dual damascene process which is now described. Referring to FIG. 5A a silicon wafer substrate  30  is provided. Integrated circuit devices(not shown) are formed on the silicon wafer substrate using conventional state of the art process technology. A plurality of interconnection levels are formed over the wafer  30  separated by insulative layers and interconnected through contacts and vias using integrated circuit processes well known by those skilled in the art. These layers and interconnection levels, not shown in the figures, are inferred to be included in the substrate  30 . The upper surface of substrate  30  is insulative and a metal interconnective stripe  32  is formed over the insulative surface. The metal interconnective stripe  32  is a portion of the uppermost of the plurality of interconnection levels, is patterned by conventional methods, and is between about 0.3 and 0.7 microns thick.  
         [0036]    An insulative layer  134  is formed over the metal stripe  32  by PECVD. The insulative layer  134  is between about 1.2 and 2.0 microns thick. Methods for depositing insulative layers and forming conductive vias are well known in the art. By dual masking and etching procedures, well known in dual damascene processing, a deep via  40 A is formed over the metal line  32  and shallow trenches  40 ,  40 A are formed to complete the interconnection pattern. The shallow trenches  40 ,  40 A are between about 0.3 and 1.2 microns deep.  
         [0037]    Referring to FIG. 5B, a conductive layer  42  is deposited over the wafer  30 . The conductive layer  42  used in the embodiment is an aluminum alloy and is between about 4,000 and 20,000 Angstroms thick. Alternately, other metals for example tungsten or copper, may be used. The conductive layer  42  fills the deep via  40 A as well as the shallow interconnection lines  40 . The configuration achieved by the single damascene process steps shown in FIG. 4B and that achieved by dual damascene processing steps shown in FIG. 5B are considered equivalent with respect to the subsequent processing steps of the embodiments of this invention. The configuration resulting from the dual damascene process (FIG. 5B) will be used to describe the remainder of the processing steps of the current embodiment, although the configuration shown in FIG. 4B may alternatively be used. The cross sections of FIG. 4B, 5B, and those in FIG. 6A through FIG. 6D are along the line  4 - 4 ′ of the planar view shown in FIG. 3.  
         [0038]    Referring to FIG. 6A, the conductive layer  42  is polished back by CMP to the surface of the insulative layer  134  leaving a metal pattern inlaid in the insulative layer  134 . CMP is the preferred method, particularly if the layer  42  is copper or a copper alloy, Alternately, the conductive layer  42  may, be etched back to the surface of the insulative layer  134  by plasma etching. The metal features  50 ,  52 ,  54  are the cross sections of the corresponding the metal features in FIG. 3.  
         [0039]    Referring to FIG. 6B a silicon oxide etch stop layer  56  is deposited over the insulative layer  134  and an opening  57  is patterned to exposed the bonding pad base segment  54 . The etch stop layer  56  is between about 200 and 1,000 Angstroms thick and is deposited by PECVD. The patterning of the opening  57  is done using photoresist patterned by a block-out mask which does not require critical alignment. Alternately, the etch stop layer  56  may be formed of silicon nitride or of silicon oxynitride.  
         [0040]    A conductive layer  58  is deposited over the etch stop layer  56 . The conductive layer  58  is formed of aluminum and is between about 0.2 and 0.8 microns thick. Alternately the conductive layer  58  may be formed of another conductive material, for example, an aluminum alloy, tungsten, copper or a copper alloy. A photoresist layer  59  is deposited over the conductive layer  58  and patterned to protect a region of the conductive layer  58  which is to become a bonding pad.  
         [0041]    Referring next to FIG. 6C, the conductive layer  58  is etched, preferably by plasma etching. Methods and chemistries for etching aluminum and its alloys by a reactive plasma are well known to those in the art. Etchants containing chlorine or bromine are widely used. Alternately, if the conductive layer  58  comprises a metal such as copper, a wet etching procedure may be preferred for patterning, for example by nitric acid or by etchants containing ammonium or ammonium related ions. The dimensions of the bonding pad  60  are generally of the order of tens of microns and a slight amount of undercutting which occurs during wet etching will have negligible impact on the bonding pad integrity. In the current embodiment the bonding pad is rectangular and between about 40 and 100 microns on a side. The surface of the interconnection pattern is properly protected from the wet etch by the etch stop layer  56 .  
         [0042]    Referring to FIG. 6D, a passivation layer  62  is deposited over the wafer  30  and the bonding pad  60  is exposed by patterning and etching an opening  64  in the passivation layer  62 . The passivation layer  62  is formed as a composite layer of a silicon nitride layer deposited over a silicon oxide layer. The depositions are made by PECVD using precursors and deposition parameters well known by those in the art. The passivation layer  62  is between about 1.0 and 2.0 microns thick. Alternatively a phosphosilicate glass(PSG) layer may be used in place of silicon nitride to form the upper portion of the passivation layer  62 . In practice the passivation layer may be of any composition or form which provides a protective coating over the integrated circuit. The opening  64  is patterned with photoresist and etched by plasma etching using etchants and etching conditions well known to those in the art.  
         [0043]    [0043]FIG. 8 shows a planar view of the completed structure. The dotted line  76  shows the bonding pad  60  extending beneath the passivation layer  62  at the edges of the opening  64 . The interconnection lines  50 ,  52 , beneath the passivation layer  62  are also shown as dotted lines. The cross sections shown in FIG. 6A through FIG. 6D are along the line  6 - 6 ′ in FIG. 8.  
         [0044]    In a second embodiment of this invention a top interconnection level is formed by metal deposition onto a patterned insulative layer. The metal layer is planarized by CMP but not polished entirely back to the insulative layer. Bonding pads are then patterned into the remaining metal layer by plasma etching. Finally a passivation layer is deposited and patterned to form openings to the bonding pads.  
         [0045]    Referring to FIG. 7A a silicon wafer substrate  30  is provided. Integrated circuit devices(not shown) are formed in the surface of the silicon wafer substrate using conventional state of the art process technology. A plurality of interconnection levels are formed over the wafer  30  separated by insulative layers and interconnected through contacts and vias using integrated circuit processes well known by those skilled in the art. These layers and interconnection levels are not shown in the figures but are inferred to be included in the substrate  30 . The upper surface of substrate  30  is insulative and a metal interconnective stripe  32  is formed over the insulative surface. The metal interconnective stripe  32  is a portion of the uppermost of the plurality of interconnection levels, is patterned by conventional methods, and is between about 0.3 and 0.7 microns thick. The wafer  30  is processed in the same manner as in the first embodiment to achieve the configuration shown in cross section by FIGS. 4B or  5 B. The cross sections of FIG. 4B, 5B, and those in FIG. 7A and 7B are all along the line  4 - 4 ′ of the planar view shown in FIG. 3.  
         [0046]    The configuration shown in FIG. 4B and that shown in FIG. 55, achieved by dual masking and etching steps of a single insulative layer are considered equivalent with respect to the subsequent processing steps of the embodiments of this invention. The configuration resulting from the dual masking process (FIG. 58) is used to describe the remainder of the processing steps of the current embodiment, although the configuration shown in FIG. 45 may alternatively be used. The conductive layer  42  used in this embodiment is an aluminum alloy. Alternately, other metals for example tungsten, copper, or alloys of aluminum or copper may be used. Conductive layer  42 , as shown in FIG. 5B, has an as-deposited thickness of between about 4,000 and 20,000 Angstroms.  
         [0047]    Referring now to FIG. 7A, conductive layer  42 , which has been deposited on the patterned insulative layer  134 , is planarized and polished to a thickness d of between about 0.2 and  0 . 8  microns by CMP. Photoresist  70  is deposited and patterned to protect a region of polished conductive layer  42  which is to become a bonding pad. Referring to FIG. 7B, Conductive layer  42  is etched by RIE or by plasma etching to expose the subjacent insulative layer  134 . The photoresist  70  hi is stripped by either plasma ashing or by conventional resist strippers, leaving the completed bonding pad  72 .  
         [0048]    Although the second embodiment forms the bonding pad and the top interconnection level from a single conductive layer deposition, the use of an etch stop layer as exercised in the first embodiment is not permitted. It is therefore critical that excessive over etching is avoided in the conductive layer  42  etch which defines the bonding pad  72 . A suitable etchant must therefore have a high selectivity of metal versus oxide. Such etchants are well known and can be tailored to achieve optimum selectivity. A preferred etchant is one containing Cl 2 , BCl 3 , and CHF 3  in an argon carrier gas. This etchant can achieve an etch rate selectivity of greater than 5:1.  
         [0049]    A passivation layer  62  is deposited over the wafer  30  and the bonding pad  72  is exposed, for subsequent wire bonding, by patterning and etching an opening  64  in the passivation layer  62 . The passivation layer  62 , is formed as a composite layer of a silicon nitride layer deposited over a silicon oxide layer. The depositions are made by PECVD using precursors and deposition parameters well known by those in the art. The passivation layer  62  is between about 1.0 and 2.0 microns thick. Alternatively a phosphosilicate glass(PSG) layer may be used in place of silicon nitride to form the upper portion of the passivation layer  62 . In practice the passivation layer may be of any composition or form which provides a protective coating over the integrated circuit. The opening  64  is patterned with photoresist and etched by plasma etching using etchants and etching conditions well known to those in the art.  
         [0050]    [0050]FIG. 8 shows a plan view of the completed structure. The dotted line  76  shows the bonding pad  72  extending beneath the passivation layer  62  at the edges of the opening  64 . The interconnection lines  50 ,  52 , beneath the passivation layer  62  are also shown as dotted lines. The cross sections shown in FIG. 7A and FIG. 7B are along the line  6 - 6 ′ in FIG. 8.  
         [0051]    While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.