Patent Publication Number: US-6703712-B2

Title: Microelectronic device layer deposited with multiple electrolytes

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the field of microelectronic device fabrication, and more particularly to the deposition of metal onto a semiconductor wafer, and specifically to the electroplating of copper into a cavity formed on a semiconductor wafer. 
     BACKGROUND OF THE INVENTION 
     Electroplating of metals is a well known technology that has recently become more widely used in the manufacture of microelectronic devices such as integrated circuits formed on semiconductor wafers. Electroplating provides a higher deposition rate than thermal deposition processes such as chemical vapor deposition (CVD), physical vapor deposition (PVD) or sputtering, and will do so at a significantly lower temperature. As a result, electroplating is a relatively low cost process for material deposition. 
     Electroplating involves the immersion of a surface to be plated into a solution containing ions of the metal to be deposited. An electrical charge is applied between the solution and the surface, and the metal ions are attracted to the surface where they are reduced to a metal which then plates out onto the surface. For the deposition of copper on a semiconductor wafer surface, the solution may contain, among others, copper sulfate (CuSO 4 ), sulfuric acid (H 2 SO 4 ), other additives and water. The cupper sulfate will break down in this acid solution to form constituent ions. The semiconductor wafer surface is prepared by the deposition of an adhesion/barrier layer and a seed layer. The adhesion/barrier layer may be tantalum, tantalum nitride or other refractory metallic film. The function of the seed layer is to provide a material on which a subsequently deposited material will readily form. The seed layer may be a thin layer (approximately 100 nm) of copper or other suitable material known in the art and applied by PVD or other process known in the art. The wafer and prepared surface are brought in contact with a bath containing the solution, an electrical charge is applied and copper is deposited. 
     FIG. 1 illustrates a prior art semiconductor device  10  having a layer of copper  12  deposited on a silicon wafer  14 . The device  10  includes a barrier layer  16  and a seed layer  18  deposited on the wafer  14  prior to the electro-deposition of the copper layer  12 . Prior to the deposition of the various layers  16 ,  18 ,  12 , there was formed a cavity  20 , such as a trench, via or dual damascene structure, in the top surface of the wafer  14 . One may appreciate that the cavity  20  may be formed directly into the silicon wafer  14  or into one or more layers of dielectric or other materials formed over the silicon, depending upon the requirements of the particular application. The layer of copper  12  may be approximately 0.5-2.5 microns in thickness, while the depth of the cavity  20  may vary from about 0.1 micron to about 1.5 micron. The top portion of the deposited copper is removed by a chemical mechanical polishing (CMP) process or other planarization method to form the final device  22  as illustrated in FIG.  2 . The extra depth of copper is deposited to ensure a full fill in the cavity  20  and to provide sufficient material to ensure that a planar surface can be obtained via the planarization process. 
     One may appreciate that as the depth/width ratio of cavity  20  increases, it becomes increasingly more difficult to ensure that the fill metal is deposited throughout the entire volume of the cavity  20 . Certain additives are commonly used with the electrolyte solution to facilitate a complete bottom-up fill of cavity  20  and/or to provide desired mechanical properties to the as-deposited metal. While the cost of a plating process may be less than alternative thermal deposition processes, the cost of the additives used in the plating solution can be significant. 
     With the demand for microelectronics products being directly responsive to their price in the marketplace, there is continuing pressure to reduce the cost of manufacturing of these devices. Accordingly, a less costly electroplating process is needed. 
     BRIEF SUMMARY OF THE INVENTION 
     A microelectronic device is described at a stage of fabrication as including: a substrate having a surface; a layer of a metal plated onto the substrate surface; and a first region of the layer of metal having a first chemical composition and a second region of the layer of metal plated over the first region and having a second chemical composition different from the first chemical composition. The layer of metal may include a metal and an additive, and wherein the concentration of the additive varies across a depth of the layer of metal. The metal may be copper and the additive may be at least one of the group of a carrier, a leveler and a surfactant. The device may further include: the substrate having a surface with a generally planar field area and a cavity formed therein; and the first region extending from the substrate surface to a level at least filling the cavity. 
     A process for fabricating a microelectronic device is described herein as including: depositing a layer of material on a surface of a semiconductor wafer by contacting the surface with a plating solution; and changing the chemical composition of the plating solution during the step of depositing so that the deposited layer of material comprises a plurality of regions each having a different chemical composition. The process may further include: forming a cavity on the surface of the semiconductor wafer prior to the step of depositing; and changing the chemical composition of the plating solution after the deposited layer of material has been deposited to a predetermined thickness in the cavity. 
     A process for fabricating a microelectronic device, the process comprising: depositing a first region of a layer of material on a surface of a semiconductor wafer in a first plating cell by contacting the surface of the semiconductor wafer with a first plating solution having a first chemical composition; and depositing a second region of the layer of material on the first region in a second plating cell by contacting a surface of the first region with a second plating solution having a second chemical composition different from the first chemical composition. 
     A device for depositing a layer of metal having a plurality of regions with distinct chemical compositions is described as including: a first solution tank and a second solution tank for containing respective first and second plating solutions having distinct chemical compositions; and a bath connected to the first solution tank and to the second solution tank for receiving consecutive volumes of the first and second plating solutions. 
     A device for depositing a layer of metal having a plurality of regions with distinct chemical compositions is further described as including: a first bath for containing a first plating solution having a first chemical composition; a second bath for containing a second plating solution having a second chemical composition different than the first chemical composition; and a handling tool for moving a wafer from the first bath to the second bath. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the present invention will become apparent from the following detailed description of the invention when read with the accompanying drawings in which: 
     FIG. 1 is a partial cross-sectional view of a prior art semiconductor device during a stage of fabrication wherein a cavity has been over-filled with a layer of copper deposited by electroplating. 
     FIG. 2 is the device of FIG. 1 after being subjected to a chemical mechanical polishing process for the removal of a top portion of the layer of copper. 
     FIG. 3 is a partial cross-sectional view of a semiconductor device during a stage of fabrication wherein a cavity has been over-filled with copper deposited in multiple layers, each layer having a different chemistry. 
     FIG. 4 is a plan view of a plating tool having two solution tanks feeding one bath. 
     FIG. 5 is a plan view of a plating tool having two solution tanks feeding two baths. 
     FIG. 6 is a plan view of two plating tools configured together with each tool including one solution tank and one bath. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A method of filling a cavity such as a trench or via or dual damascene structure with a conductive material in accordance with the present invention is described with reference to a resulting structure illustrated in partial cross-sectional view in FIG. 3. A portion  24  of a semiconductor device is illustrated in FIG.  3 . Layer  26  may be a portion of a semiconductor wafer or a layer of dielectric or other material deposited on such a wafer. A cavity  28  such as a via or trench has been formed in the layer  26  by any process known in the art, such as by a selective etching process. A barrier layer  30  has been formed on the surface of layer  26  including the surfaces of cavity  28 . The barrier layer  30  may be a film of tantalum, tantalum nitride, other refractory metal or other known material. A seed layer  32  has been formed on the surface of barrier layer  30  in order to form a layer of material on which a layer of material will form in a subsequent electroplating process. For a subsequent copper layer, the seed layer  32  may be copper or other suitable material known in the art. Both the barrier layer  30  and the seed layer  32  may be deposited by processes known in the art, such as PVD, CVD or sputtering, for example. While the barrier layer  30  and seed layer  32  are commonly used in semiconductor conductor and interconnect fabrication, one may appreciate that one or both of these layers may be omitted in certain applications. 
     FIG. 3 illustrates the semiconductor wafer portion  24  in contact with an electrolyte or electroplating solution  34  for the deposition of a layer  36  of metal such as copper, nickel, lead, tin and/or gold. The layer  36  fills the cavity  28  and extends to a desired level of overfill above the level of the generally planar field area  37  to provide sufficient material for a subsequent planarization process, such as a chemical mechanical polishing process. An anode  38  is placed in the electroplating solution  34  and a cathode  40  is attached to the wafer such as at layer  26 . The layer  36  is formed as metal ions in the electroplating solution  34  are attracted to and plate onto the negatively charged surface of the seed layer  32 , with the thickness of the layer  36  being a function of plating parameters such as plating time, current density, bath concentration, etc. Time lines T 1  and T 2  indicate points in time during the electroplating process when the chemistry of the electroplating solution  34  was changed, resulting in a different chemistry in each of a plurality of regions  42 ,  44 ,  46  of layer  36 . For a layer  36  intended as a precursor structure for a copper conductor in cavity  28 , the bulk material of layer  36  may be copper with each of the plurality of regions  42 ,  44 ,  46  including small amounts of various additives that were included in the electroplating solution  34  for various reasons. 
     Many known electroplating processes incorporate one or more of three types of additives in the electrolyte bath: a carrier, a leveler and a brightener. Carriers are large molecular weight polymers, levelers are surfactants and are usually amine-based, and brighteners are typically acids that have active sulfur groups. Such additives are commercially available, and they include the family of such additives sold by Enthone, Inc. (www.enthone.com) under the trademark CuBath. In one embodiment of the present invention, a first region  42  of copper layer  36  may include an additive used to ensure a complete fill into cavity  28 . A second region  44  may include a dopant known to migrate to the copper grain boundaries upon subsequent thermal processing, thereby pinning the grain boundaries and providing a desired level of strength to the material. A third region  46  may include a surfactant or a brightener. 
     Alternatively, a first region  42 ,  44  may be formed using an electroplating solution  34  containing an additive, while a subsequently deposited region  46  does not contain the additive. This embodiment is especially useful if the additive adds significantly to the cost of the electroplating process. The expensive additive may be used for only the region where the particular benefit that it imparts has value. In other regions, the additive is omitted. Such additive may be included in any one or more of the regions  42 ,  44 ,  46 , depending upon the requirements of the particular application. For high aspect ratio cavities, it is common to use additives that ensure good fill qualities, provide strength to the grain boundaries, and provide a shining surface finish. These features are not needed on the overburden portion of the deposited layer once the cavity has been filled beyond its final, as-planarized, depth. Accordingly, in one embodiment, additives are provided in the electrolyte  34  for a period T 2  until the cavity  28  is filled, then no such additives or a lower concentration of such additives are provided for a time period after T 2 . Thus, the region  42 ,  44  at least filling the cavity  28  has a chemical composition distinct from a region  46  plated over and disposed above the region  42 ,  44 . 
     Commercial electroplating equipment is available from many vendors, including Applied Materials, Inc. (www.appliedmaterials.com), Novellus Systems, Inc. (www.novellus.com), Semitool, Inc. (www.semitool.com), and NuTool, Inc. (www.nutool.com). The change in chemistry between regions  42 ,  44 ,  46  may be accomplished by completely draining and refilling the electroplating solution  34  from a single plating cell. Alternatively, the semiconductor device may be moved from one plating cell to another so that a first region  42  is applied in a first plating cell, with one or more further regions  44 ,  46  being applied in a respective different plating cell having an electrolyte with a different chemistry. A cleaning step may be applied to the top surface of layer  36  prior to beginning the subsequent plating processes. Alternatively, in order to avoid the formation of corrosion products between regions and in order to provide for a more gradual transition of chemistries between regions, it may be preferable that a single plating solution  34  with a changing chemical composition be used. Existing electroplating equipment may have to be modified to accommodate a change in the electroplating solution  34  at times T 1  and T 2 . The dispensing and run-off systems may be modified to permit the chemistry of the solution  34  to be conveniently changed without interrupting the plating process. In one embodiment, an additive is provided in the electroplating solution  34  at a first concentration during the deposition of an initial region  42  of layer  36 , and the concentration of the additive is reduced to essentially zero for a second region  46 . The additive may be actively removed by filtering or flushing of the solution  34 , or the additive may be reduced passively by not being replenished as the plating process progresses. The later approach will result in a first concentration of the additive in the deposited layer of material in a first region  42 , a transition in the concentration of the additive over the depth of a second region  44 , and essentially zero concentration of the additive in a third region  46 . Alternatively, the concentration of an additive in the solution  34  may be gradually increased during the plating process, resulting in a gradual increase in the concentration of the additive in layer  36  across its depth. Furthermore, a first additive may have its concentration increased across the depth of layer  36  while a second additive may have its concentration decreased across the same depth of layer  36 . 
     FIGS. 4-6 illustrate alternative embodiments of plating equipment for depositing a metal layer having a plurality of regions with differing chemistries. FIG. 4 illustrates a single plating tool  50  having two separate solution tanks  52 ,  54  containing two respective plating solutions having distinct chemistries. The two solution tanks  52 ,  54  are connected to a single bath  56  by an appropriate piping arrangement  58  that may include conduits, valves, and pumps as necessary for alternatively supplying bath  56  with consecutive volumes of the solution from either of the tanks  52 ,  54 . Bath  56  may be any style known in the art, including horizontal or vertical configurations. Bath  56  is first filled with a first plating solution from tank  52  for the deposition of a first region of a layer of metal onto a wafer surface. The wafer may then be moved from the bath  56  to a cleaning device such as spin-rinse-dry cell  60  as is known in the art by a handling tool  62  where it is cleansed with de-ionized water and/or other chemicals and dried. The handling tool  62  may be a computer controlled automated machine or a manually controlled pick-and-place device as are known in the art. The first plating solution is drained from the bath  56  and preferably returned to tank  52 , and the bath is filled with a second plating solution from tank  54 . The wafer is returned to the bath  56  and a second region of the layer of metal is deposited. Depending upon the specific chemistries of the two plating solutions, one or more intermediate cleaning/flushing steps may be needed to remove the one solution from bath  56  before the second solution is introduced. 
     FIG. 5 illustrates a single plating tool  64  having two parallel plating trains including solution tanks  66 ,  68  connected to respective baths  70 ,  72 . A first plating solution is contained in the first train  66 ,  70  and a second plating solution is contained in the second train  68 ,  72 . A wafer is processed through the baths  70 ,  72  for the deposition of respective first and second regions of a layer of metal. Handling tool  74  is utilized to move the wafer between the baths  70 ,  72  and to spin-rinse-dry cells  76 ,  78  as appropriate. 
     FIG. 6 illustrates a plating assembly  80  including two separate plating tools  82 ,  84  configured together for the purpose of depositing two regions of a layer of metal onto a wafer surface. Handling tool  86  may be used to move wafers between the tools  82 ,  84 . Each tool  82 ,  84  includes respective solution tanks  86 ,  88 , baths  90 ,  92 , spin-rinse-dry cells  94 ,  96  and handling tools  98 ,  100 . A first region of a layer of metal is deposited onto a wafer surface in the first tool  82 , then after appropriate cleaning and handling steps, a second region of the layer of metal having a distinct chemistry from the first region is deposited in tool  84 . 
     One skilled in the art will appreciate that an electroless process may be used to deposit the layer of metal  36 . Electroless plating uses a reduction reaction to deposit metal layer  36  without the use of an electric current. An electroless plating process can provide a uniform coating thickness over the entire surface of an irregularly shaped object. Electroless plating can also be used to deposit a conductive coating over a non-conductive surface. Electroless technologies involve the reduction of a complexed metal, such as copper or nickel for example, with formaldehyde in an alkaline solution. The reaction may be catalyzed by a palladium seed layer  32 . As with the electroplating processes described above, the plating solution  34  used with an electroless process may be altered during the course of depositing metal layer  36  in order to produce a plurality of regions  42 ,  44 ,  46  having distinct chemical compositions. Expensive additives necessary in a final device may be eliminated from the electroless plating solution  34  once the thickness of the layer  36  completely fills the cavity  28  beyond a point where additional material will be removed by a later planarization step. 
     While the preferred embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those of skill in the art without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.