Abstract:
An electroetching process of the present invention uses a multiphase environment for planarizing a wafer with conductive surface having a non-uniform topography. The multiphase environment includes a high resistance phase and an etching solution phase. The conductive surface to be planarized is placed in the high resistance phase and adjacent a phase interface between the high resistance phase and the etching solution phase. A wiper is used to mechanically move the thin high resistance phase covering the conductive surface so that the raised regions of the non-planar conductive surface is briefly exposed to etching solution phase. The mechanical action of the wiper does not disturb the high resistivity phase filling the rescessed regions of the surface. As the raised surface locations are exposed, the etching solution phase contacts and electroetch the exposed regions of the raised regions until the surface planarized.

Description:
RELATED APPLICATIONS  
       [0001]    This application claims priority from the Provisional Application Serial No. 60/362,513 filed Mar. 6, 2002, (NT-240 P) which is incorporated herein by reference. 
     
    
     
       FIELD  
         [0002]    The present invention relates to manufacture of semiconductor integrated circuits and, more particularly to a method for planar electroetching or etching of conductive layers.  
         BACKGROUND  
         [0003]    Conventional semiconductor devices generally include a semiconductor substrate, usually a silicon substrate, and a plurality of sequentially formed dielectric interlayers such as silicon dioxide and conductive paths or interconnects made of conductive materials. Copper and copper alloys have recently received considerable attention as interconnect materials because of their superior electromigration and low resistivity characteristics. The interconnects are usually formed by filling copper by a deposition process in features or cavities etched into the dielectric interlayers. Although Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD) techniques may also be used, the preferred method of copper deposition process is electroplating. In an integrated circuit, multiple levels of interconnect networks laterally extend with respect to the substrate surface. Interconnects formed in sequential interlayers can be electrically connected using vias or contacts.  
           [0004]    In a typical process, first an insulating dielectric interlayer is formed on the semiconductor substrate. Patterning and etching processes are performed to form features such as trenches and vias in the insulating layer. Then, copper is electroplated to fill all the features. However, the plating process results in a thick copper layer on the substrate some of which need to be removed before the subsequent step. Conventionally, after the copper plating, CMP process is employed to planarize and then reduce the thickness of the copper layer down to the level of the surface of the barrier or insulation layer. In summary, CMP is used to remove all of the conductors from the surface so that copper-filled features electrically isolated from one another. However, CMP process is a costly and time-consuming process that reduces production efficiency. Further, more, although the CMP can be used with the conventional interlayer dielectrics, it may create problems with low-k dielectrics because of the mechanical force applied on the wafer surface during the CMP process. During the CMP step, the low-k materials may be stressed and may delaminate or other defects may form due to the low mechanical strength of the low-k materials.  
           [0005]    Another material removal technique involves well-known electropolishing processes. In the electropolishing, which may also be referred to as “electrochemical etching” or “electroetching,” both the material to be removed and a conductive electrode are dipped into the electro-polishing solution. Typically an anodic (positive) voltage is applied to the material to be removed with respect to the conductive electrode. With the applied voltage, the material is electrochemically dissolved and removed from the wafer surface. However, this technology has a limited use in planarizing non-flat and non-uniform overburden copper layers because, during electroetching, material removal generally progresses in a conformal manner. Conformal nature of the process produces dishing defects in large features with small aspect ratios, which adversely affect wire dimensions.  
           [0006]    This situation can be demonstrated with help of FIGS.  1 A- 1 B. FIG. 1A illustrates a substrate  10  coated with a copper layer  12 , having an overburden to be removed. The substrate is a preprocessed silicon wafer having an insulation layer  14  on top it. The insulation layer  14  is patterned and etched to form features  16 ,  18  such as trenches and field regions  20 . FIG. 1A illustrates a narrow trench  16  with an aspect ratio of greater than 1 and a wide trench  18  with an aspect ratio of less than 1. By aspect ratio, it is meant a ratio of the depth of the trench to the width of the trench. Before the copper plating, the features  16 ,  18  and the field regions  20  are lined with a barrier layer  22  (such as Ta/TaN) and a copper seed layer (not shown). Conventional electrodeposition processes may fill the narrow features  16  in a bottom-up fashion by utilizing additives in electroplating baths and thus yield a flat surface as shown in FIG. 1A. However, copper deposits conformally over the large features  18  and produces recess  23  over such features. As mentioned above, electroetching of the copper layer  12  also progresses conformally during the standard electroetching process. As shown in FIG. 1A, as the etching of the copper layer progresses, the top surface  24  of the copper layer  12  gradually approaches the features  16 ,  18 . This situation is shown by dotted lines  24 ′,  24 ″ and  24 ′″. When the etched surface  24 ′″ is reached, some copper is still on the field regions but the copper in large feature is over etched. As shown in FIG. 1B, as the remaining copper on the field regions is etched away, copper in the large feature  18  is dished because of excessive removal. The large feature in this example may be a trench with a width larger than 10 micrometers.  
           [0007]    Dishing problems originating from the conformal nature of the electroetching process may be alleviated by utilizing methods that partially planarize the overburden layer employing, for example, CMP or another planarization technique prior to the electroetching step. In such approaches, once the overburden is made flat, resulting flat surface is uniformly etched back down to the barrier layer. However, such multi-process approaches are cumbersome and time consuming. Besides, success of such processes strictly depends on the thickness uniformity of the flat layer that the electroetching process is initiated on. If the flat copper layer has slight local or global thickness non-uniformities, i.e., thinner and thicker areas, the features under the thinner copper layer are most likely dished while the thicker copper layer is still being removed from other thicker areas on the wafer. Such thickness non-uniformities can be the result of various reasons such as copper plating tool design, plating chemistry problems, problems with electrical contact to the wafer, plating solution problems, and the like. Alternately, there may be non-uniformities in the electro etching process itself that may cause non-uniform material removal from various parts of the substrate.  
           [0008]    [0008]FIG. 2A illustrates a region of a substrate  30  having a copper layer  32  that is partially planarized and has thickness non-uniformity, which is exaggerated to clarify the point. In this example, t 1  is the measured thickness taken near a first feature  34  and t 2  is the measured thickness taken near a second feature  36 . If t 1  is greater than t 2 , as shown in FIG. 2B, the copper in the second feature  36  will be dished while the copper over the first feature  34  is planarized and leveled with the barrier layer  38 . FIG. 2C shows another area on the substrate  30  where copper layer  32  is thinner with measured thickness t 3  near a third feature  40 , but it is thicker with measured thickness t 4  near a fourth feature  42 . As shown in FIG. 2D, in this case, removal of thin copper layer on the third feature is faster than the removal of the copper layer on the fourth feature  42 . As a result, as the fourth feature  42  is planarized down to the level of barrier layer  38 , the third feature  40  is dished. As described above, due to the thickness non-uniformities of the copper layer, conventional electroetching processes either over etch the copper in all of the features or over etch it in some of features while planarizing it in some features. A process that slows down or stops etching of the areas that begins to dish but accelerates etching of the un-etched areas would overcome above drawbacks.  
           [0009]    Certain variations of electroetching process attempt to alleviate such process drawbacks. In one technique, for example, such recesses are filled or masked with a low ionic conductivity and low diffuisivity material before the wafer is placed into the electroetching system. Use of such material coating on the wafer surface is claimed to slow down the etching of such recess areas during electroetching, and planarize the copper film.  
           [0010]    To this end, however, there is need for alternative etching techniques that etch back even highly non-uniform conductive films with greater efficiency and those that do not cause excessive dishing into the features.  
         SUMMARY  
         [0011]    The present invention uses an etching system utilizing a multi phase process environment in combination with a surface disturbance on the substrate surface.  
           [0012]    An applied surface disturbance such as a sweeping action moves a high resistance solution component of a multiphase solution from surface of a overburden copper instantaneously at the location of the surface disturbance and enables an electroetching solution phase component of the multiphase solution to act upon the substantially exposed surface of the overburden copper at that location. Once the rugged surface is electopolished, the electorpolishing process uniformly proceeds until the desired copper thickness is obtained. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1A is a schematic illustration of a wafer surface having a non uniform copper overburden on top of it;  
         [0014]    [0014]FIG. 1B is a schematic illustration of the wafer surface shown in FIG. 1A, wherein the copper overburden is planarized using a prior art process;  
         [0015]    [0015]FIG. 2A is a schematic illustration of a wafer surface having a copper overburden with non-uniform thickness;  
         [0016]    [0016]FIG. 2B is a schematic illustration of the wafer surface shown in FIG. 2A, wherein the copper overburden is planarized using a prior art process;  
         [0017]    [0017]FIG. 2C is a schematic illustration of another wafer surface having a copper overburden with non-uniform thickness  
         [0018]    [0018]FIG. 2D is a schematic illustration of the wafer shown in FIG. 2C, wherein the copper overburden is planarized using a prior art process;  
         [0019]    [0019]FIG. 3A is a schematic illustration of an exemplary multiphase etching system of the present invention;  
         [0020]    [0020]FIG. 3B is a schematic illustration of the relative positions of a wafer and a solution wiper of the system shown in FIG. 3A;  
         [0021]    [0021]FIG. 4 is a schematic illustration of a wafer surface having a non uniform copper overburden on top of it;  
         [0022]    [0022]FIG. 5 is a schematic illustration of the wafer surface shown in FIG. 4, wherein the surface has been immersed in the dual phase etching environment of the system shown in FIG. 3A;  
         [0023]    [0023]FIG. 6 is a schematic illustration of wafer surface shown in FIG. 5, wherein the surface has been electropolished in the dual phase environment and using a mechanical disturbance;  
         [0024]    FIGS.  7  to  10  are schematic illustrations of the structures from the various stages of the electropolishing process of the present invention;  
         [0025]    [0025]FIG. 11 is a schematic illustration of another exemplary multiphase etching system of the present invention;  
         [0026]    [0026]FIG. 12 is a schematic illustration of a wafer surface, wherein the surface has been immersed in the dual phase etching environment of the system shown in FIG. 12;  
         [0027]    [0027]FIG. 13 is a detail schematic illustration of a recessed region of the surface shown in FIG. 12, wherein the recessed region has been filled with high resistivity phase;  
         [0028]    [0028]FIG. 14 is a schematic illustration of wafer surface shown in FIG. 12, wherein the surface has been electropolished in the dual phase environment and using a mechanical disturbance; and  
         [0029]    [0029]FIG. 15 is a detailed schematic illustration of the surface and the mechanical disturbance on the surface shown in FIG. 13. 
     
    
     DETAILED DESCRIPTION  
       [0030]    The process of the present invention uses an etching system utilizing a multi phase process environment in combination with a mechanical influence on the substrate surface. In one preferred embodiment, multiphase process environment of the present invention is comprised of a dual phase process environment having a liquid phase and a near-surface phase. The dual phase process environment of the present invention is comprised of a dual phase solution having a process solution phase (as the liquid phase) and a high resistance solution phase (as the near-surface phase). In another preferred embodiment, the dual phase process environment of the present invention is comprised of a process solution phase (as the liquid phase) and a near-surface phase comprising solid particles. In one preferred approach, the near-surface phase is comprised of solid particles surrounded by the same process solution that forms the process solution phase. Alternatively, in another approach, the near-surface phase is comprised of solid particles and a surface solution, which is different than the process solution. The dual phase solution may be comprised of an electroetching or electropolishing solution forming the process solution phase and a near-surface solution phase, which is comprised of a high resistance solution. It is understood that the nature of the process solution phase depends strictly on the process that is used. As in the present embodiment if the process is an electrochemical process, the process solution phase comprises an electroetching or electropolishing solution such as phosphoric acid solution. However, if a chemical etching process is considered a conventional copper etching solution such as solutions containing H 2 SO 4  and H 2 O 2  can be used. From here on, to describe the invention, concentration will be on the case where the process solution is an electroetching solution. In this embodiment, the near-surface solution phase is a high resistance solution, which is denser or lighter than the electroetching solution and does not substantially interact with the electroetching solution. In other words, the high resistance solution and the electroetching solution do not substantially mix, dissolve in each other, or chemically react with each other. As a result the solutions substantially stay in their distinct phases such that the high resistance solution forms a separate phase either under or on top of the electroetching solution. During the process, the high resistance solution covers the rugged or non-planar surface of the overburden copper and isolates the electroetching solution from the overburden copper.  
         [0031]    An applied surface disturbance such as a sweeping action moves the high resistance solution from the surface of the overburden copper instantaneously at the location of the surface disturbance and enables electroetching solution to act upon the substantially exposed surface of the overburden copper at that location. As the process progresses, initially upper most bumps, or thicker areas, on the copper layer are exposed to the electroetching solution because the sweeper can only touch them. For a wafer with copper layer facing up, during the electrochemical removal of the thicker areas, thinner areas (valleys) on the rugged upper surface are covered with the high resistance solution and as a result they are not removed. Once the rugged surface is planarized, the removal process uniformly proceeds until the desired copper thickness is obtained.  
         [0032]    In this embodiment, it should be noted that a source of a mechanical disturbance such as a solution wiper should be placed above the surface of the wafer to be processed. This setup is used for the case of employing a high resistance solution that has a higher density than the electroetching solution. If the high resistance solution is lighter than the electroetching solution, a configuration that places the wafer over the sweeper can be used.  
         [0033]    Reference will now be made to the drawings wherein like numerals refer to like parts throughout. FIG. 3A illustrates an exemplary etching system  50  comprising a solution wiper assembly  52  and a wafer holder assembly  54 . Both assemblies  52 ,  54  are placed in a process container  56 . The wafer holder assembly comprises a workpiece holder  58  to retain a workpiece 100 . The sweeper assembly  52  comprises a body  60  having an electrode  62  and a solution wiper  101 . FIG. 3B exemplifies the configuration of the workpiece  100  and the solution wiper  101  during the process of the present invention. Referring back to FIG. 3A, the solution wiper has a surface  63  that contacts the copper layer that is being planarized during the process. The electrode  62  may be placed in the solution wiper if the solution wiper is made of an isolating material and may be shaped as a plate with various shapes or a blade. A preferred material for the solution wiper may be made of a polymeric material with sufficient rigidity. In this embodiment the solution wiper is shaped as a rectangular plate, although it can be round or any other shape. Alternatively, the electrode  62  may be placed on top of the solution wiper  101  or any other place in the system as long as it makes physical contact with the electroetching solution. The electrode and the workpiece  100  are connected to the two terminals of a power supply  63 .  
         [0034]    The solution wiper includes a number of holes or pores  64  extending through, or extending to, the electrode so that electrode is in physical contact with the electroetching solution. The solution wiper  101  can be moved rotationally or laterally or both through a shaft  66  attached to the assembly  52  so as to perform the sweeping function on the workpiece  100 . It should be noted that the same function could also be obtained by keeping the solution wiper stationary and moving the workpiece, or by moving both the wafer and the solution wiper. A dual phase solution  103  having two liquid phases is pooled in the container  56  through a solution delivery system (not shown) such that a first phase  122  covers the workpiece  100 . Further, referring to FIG. 3A, a second phase  124  floats on the first phase  122 . In this embodiment, the first phase is a high resistance solution and the second phase is an electroetching solution. The placement of the phases  122 ,  124  may be done sequentially or together.  
         [0035]    [0035]FIG. 4 exemplifies the work piece  100  as a preprocessed silicon wafer having a conductive layer  102  on it. The conductive layer may be a copper layer that is deposited on the wafer  100  using for example an electroplating process, although it may be formed using any other method such as PVD, CVD or MOCVD. Preprocessing of the wafer  100  may include forming an insulation layer  104  on a top surface  105  of the wafer  100  and patterning and etching it to form features to be filled with copper. The features may be vias  106  with an aspect ratio of greater than 1 and trenches  108  with an aspect ratio of much less than 1. In one application, when filled with copper, such features form the wiring infrastructure of an integrated circuit. Conventionally, before the electroplating process, entire top exposed surface of the wafer is coated with a barrier layer  110  such as a Ta/TaN composite stack, and then a copper seed layer  112 . For the sake of clarification, copper seed layer will not be included in the following figures. As a result, the features  106 ,  108  and top surface  114  of the insulation layer  104  (field regions  114 ) is coated with the barrier and seed layers  110 ,  112 . Referring to FIG. 4, an upper surface  116  of the conductive layer  102  is a non-planar surface including raised regions  118  and recessed regions  120 , or valleys, which will be removed and planarized with the present invention.  
         [0036]    As illustrated in FIG. 5 when a predetermined amount of dual phase solution  103  is delivered into the system shown in FIG. 3A, the high resistance solution  122  covers the conductive layer  102  of the wafer  100  and the electroetching solution floats on top of the high resistance solution. It should be noted that by predetermining the amount of phases in the dual phase solution  103  and pre adjusting the height of the workpiece holder, the copper layer  102  is kept in the high resistance solution throughout the electroetching process. Obviously less high resistance solution could be used and invention could still be practiced as long as all the valleys were substantially filled with the high resistance solution. In this embodiment, the high resistance solution  122  is chosen to be denser than the electroetching solution  124 . Of course, electrical conductivity of the high resistance solution is much smaller than that of the electroetching solution and the high resistance solution does not have the capability to etch or electroetch the copper layer. In this embodiment, the high resistance solution  122  can be delivered to the wafer surface either together with the electroetching solution  124  or they can be separately delivered from different sources.  
         [0037]    The high resistance solution  122  fills the recessed regions  120  of the rugged terrain of the copper layer  102  and effectively protects them from electroetching solution  124  until the raised regions  118  are gradually flattened by exposing them to the electroetching solution  124  with the sweeping function of the solution wiper  101 . Solution wiper sweeps the surface at a fixed elevation and the surface of the solution wiper moves in a plane that is substantially parallel to the surface of the wafer. During the process the solution wiper may be slightly pushed against the wafer with a force in the range of 0.1 to 1 psi. In a given instant, the solution wiper surface cannot touch the valley regions, and therefore cannot remove the high resistance solution from the valleys before touching the thicker regions and exposing them to the electroetching solution. Therefore, the electroetching process of the present invention is self-limiting, i.e., prevents etching of the thinner regions (valleys) before reducing the thickness of the thicker regions to the same thickness level that thinner regions have. As will be described below once the copper layer is made flat at one stage of the process, the process progresses by uniform removal of the copper layer.  
         [0038]    [0038]FIG. 6 illustrates the interaction between the solution wiper  101  and the surface  116  of the conductive layer  102 . In this embodiment, surface  126  of the solution wiper  101  may have a number of sweeping features  128  to enhance the sweeping function of the solution wiper. In this embodiment, tips  129  of the features  128  are in the same plane so that the features do not sag into the recessed regions  120  on the surface  116  of the copper, and remove high resistance solution from the recessed regions. Specifically, as the features  128  of the solution wiper sweeps the surface  116  of the conductive layer  102 , the high resistance solution  122  is instanteniously swept away from the surface  116  so that electroetching solution on top can reach an exposed portion of the surface  116  and etch the surface while an electroetching current is applied between the copper  102  and an electrode contacting the electroetching solution  101 . During the process, the features  128  of the solution wiper are brought to close proximity of the conductive surface  116  and they may becontacted to the upper most end of the surface  116 . Actual physical contact between the top surface of the wafer and the tips  129  may not be necessary to initiate electroetching at the top surface.  
         [0039]    [0039]FIG. 7 shows an intermediate stage at the removal of the conductive layer  102 , which can be an end-product of the process if so desired. As shown in FIG. 7, as the top of the conductive layer  102  is uniformly flattened and removed by the process, a planar conductive layer  102   a  is formed.  
         [0040]    [0040]FIGS. 8 and 9 illustrate a transition stage before the electroetching process that may be terminated in a self-limiting fashion. Referring to FIG. 8, as the electroetching process continues, the planar conductive layer  102   a  is removed from the top of the field regions  114  which are coated with the barrier layer  110 . Therefore, in an instant of the process, a copper surface  116   a  of the copper layer becomes coplanar with the surface of the barrier layer  110  when the barrier layer is exposed. At this stage, the solution wiper can still sweep the surface  116   a  and the barrier layer and cause further etching of the surface  116   a . As shown in FIG. 9, if the sweeping action is continued, an over etch of the conductive layer portions in the features  106 ,  108  may occur before the etching terminates. However, as soon as the over etching happens the high resistance solution fills the over etched surfaces and does not allow further electroetching to occur. Even if the solution wiper is moved and the process continued, it only sweeps the surface of the barrier layer  110  that cannot be etched with the electroetching solution of the process. Since, the features of the solution wiper cannot also sag into the over etched surfaces  116   b , the high resistance solution further protects the over etched surfaces  116   b  and prevents them from being etched. In one example, the over etching depth may be given in the range of the thickness of the barrier layer which is 100 to 300 Angstroms. Accordingly, the etching process advantageously self terminates without needing any endpoint detection system. However, if the process is ran in a constant current mode, an increased voltage may indicate the endpoint of the copper removal. Also, if the process is run in a constant voltage mode, a decreased current value may indicate the end point where the barrier layer on the field regions is exposed.  
         [0041]    As shown in FIG. 10, if the copper is minimally over-etched as shown in FIG. 9, over etched copper is advantageously leveled with the field regions  114  after the removal of the barrier layer  110 . During the barrier layer removal, the copper electroetching solution may be replaced with a specific electroetching solution that etches the barrier layer but not the copper layer. Even if the etching solution has capability to etch both barrier and copper layers, the method of the present invention would still arrest over etching of copper within the features. For example a non-selective solution chemistry that electroetches both the copper and the barrier material may be used. In this case, over etch line  120  (dotted lines in FIG. 10) show the extend of over etch within the features  106 ,  108 . As shown in FIG. 10 once as the electroetching removes the cooper and the barrier layer until the over etch line  120  with a non-selective chemistry solution, the electroetching process stops at the insulator layer. Removal of the barrier layer may also be performed using CMP process. Alternately it is possible to remove the barrier layer with a dry etch process such as reactive ion etch process.  
         [0042]    It should be noted that the present invention would result in the structure of FIG. 9 even if it initially (after the planarization step) had given the structures of FIG. 2A or FIG. 2C instead of the uniform-thickness structure shown in FIG. 7. The reason is the self-limiting nature of the process that does not allow excessive dishing into the features.  
         [0043]    In an alternative embodiment the arrangement of the phases in the process environment can be reversed such that the electroetching solution can be the denser phase and the high resistance solution can be the lighter phase. In such case, a system having the wafer-up position is used such that the lighter high resistance solution always covers the wafer surface by floating on the electroetching solution.  
         [0044]    [0044]FIG. 11 shows an electroetching system  200  having a wafer-up configuration. The system  200  uses an alternative dual phase solution comprising an electroetching solution  202  and a high resistance solution  204 . However, the high resistance solution in this embodiment is lighter than the electroetching solution. Consequently, the high resistance solution floats on the electroetching solution  202  during the process and covers the wafer surface that is being processed. A workpiece carrier  206 , holding a wafer  208 , of the system  200  is moved in the high resistance solution  204 . A solution wiper assembly  210  having a solution wiper  212  and an electrode  214  is placed in a system container  216 . The solution wiper  212  may have a plurality of openings  213  or pores allowing electroetching solution  202  to wet the electrode  214 . The solution wiper assembly  210  is kept in the electroetching solution  202 . The electrode  214  and the wafer are connected to a power supply  218  to apply a voltage difference between the electrode and the copper coated surface of the wafer during the electroetching process. The process is performed in a similar manner described for the above system. This process can also be performed as a chemical etching process, without connecting the system to a power supply and replacing electroetching solution with a conventional copper etching solution.  
         [0045]    Yet in another embodiment, the high resistance phase of the dual phase process environment may preferably be comprised of solid particles and a solution surrounding such particles. The second phase of the dual phase process environment comprises the process solution phase. The solution surrounding the particles may be the same as the process solution, or it may be another solution that does not dissolve in or mix with the process solution. As in the previous embodiment, process of the present embodiment can be also performed without using a potential difference between the electrode and the copper coated wafer surface if the process solution is an etching solution rather than an electroetching solution. In this case, the high resistance phase may be changed with an etch resistant or resistant phase. In other words, in this case, this phase does not have to be electrically high resistance but it has to be chemically resistant to form a barrier to etching of the wafer surface by the etching solution.  
         [0046]    [0046]FIG. 12 illustrates a portion of a wafer  100   a , which is immersed in the dual phase process environment  300 . The surface  116   a  of the copper layer  102   a  has raised and recessed regions  118   a ,  120   a . The dual phase process environment of the present invention comprises a process solution phase  302  and a near-surface phase  304 . The process solution phase  302  is an electroetching solution such as phosphoric acid as in the previous case. However, in this embodiment, the near-surface phase  304  is comprised of a mixture of solid particles  306  that cover the surface of the copper layer  102   a  and liquid electroetching solution  302 . The particles  306  may be less than 500 Angstrom (Å) in size and preferably less than 100Å. Various material powders may be used as the particle material. Materials that are stable in the process solution phase  302  are preferred. Materials with high resistance such as zirconia, alumina, ceria, and high-density polymers may be used as the powder material.  
         [0047]    [0047]FIG. 13 simply shows the functionality of the near-surface phase  304  with respect to electroetching solution  302  and the copper layer  102   a , and for the purpose of clarification, it is highly exaggerated. As exemplified in FIG. 13, the space between the particles  306  is filled with the electroetching solution  302 . If the particles are made of highly resistive materials, they form a high resistance layer over the copper layer  102   a , which layer substantially limits the applied current flow between the copper layer  102   a  and the electrode of the system (see FIG. 3A). For example, referring to FIG. 13, the resistance to current flow in an exposed region  308  of the copper layer  102   a , is much less than the resistance to current flow in a valley region  120  or a slope region  120   a  of the copper layer due to the thickness of the near-surface phase  304  on such regions  120   a  and  120   b.    
         [0048]    In operation, as shown in FIG. 14 and in detail FIG. 15, exposed areas  308  on the copper layer  102   a  can be created instantaneously with the mechanical action of a solution wiper  101   a  that one example of it is described above. As features  128   a  of the wiper  101   a  expose the exposed regions  308  on the raised regions  118   a , the etching solution  302  attacks the exposed regions  308  and electroetches them. Meanwhile, due to the high electrical resistance in the valleys  120   a  and the slopes  120   b , a limited material removal occurs in such areas, resulting in planarization. If process is continued, structures in FIGS. 7, 8,  9  and  10  are obtained as described previously in relation with the above embodiment using high resistance solution phase.  
         [0049]    The use of wiper or wipers in embodiments described previously helps etching of the top portions of the copper layer by exposing these portions to the etching or electroetching solution. Wiper  101   a  also helps removal of etching by-products from the surface and exposing fresh surface for further removal. It helps mass transfer and brings fresh solution to the surface to be etched.  
         [0050]    Alternatively, the dual phase process environment  300  of FIG. 12 may also be used without the wiper. In this case the particles in near-surface phase  304  are surrounded by the electroetching solution  302  and the top of the near-surface phase  304  is substantially parallel to the top surface of the wafer  100   a . An electroetching potential is applied between an electrode (not shown) in contact with the electroetching solution  302  and the copper layer  102   a  making the copper layer anodic. Copper starts to dissolve but since the near-surface phase is thicker over the valley  120   a  or recess in the copper layer  102   a , the resistance to current flow is higher and less electroetching current density passes through that region compared to the higher regions. This automatically starts to planarize the copper layer. As planarization occurs and the copper surface moves down, particles  306  sink down with the copper surface. For this technique to be successful , the size of the particles must be much smaller than the depth of the valleys. Considering the fact that in standard electroplating technique, valley depths in the range of 2000-30000 Å may need to be planarized, a particle size of less than 100 Å is preferred. Since the solution phase surrounding sub-micron particles is expected to be highly resistant to current flow, small increase of the thickness of near-surface phase  304  can induce large resistance changes.  
         [0051]    Although the preferred particle material is made of an inert high-resistance material, conducting powders or conducting powders covered with insulators may also be used in the near-surface phase of this invention. If conductive powders are used, when anodic voltage is applied to the copper surface this voltage would also be communicated to the top surface of the near surface phase. Therefore, electroetching current would want to pass and anodize the conductive material of the particles. If the particles are made of materials such as Ti, Al and Ta, an anodic oxide would grow on the particle surfaces making them electrochemically inactive, and the process would continue as described previously in the case of high-resistivity or insulating particles. Another approach involves selection of a conductive material that is highly stable in the solution but does not form a high resistance oxide upon application of voltage. In this case oxygen may be generated on the surface of such particle materials, however, during the process etching of copper would be preferred since it requires lower voltage. As the sweeper moves and exposes the copper surface, electroetching still continues at the raised regions of the copper surface as described previously.  
         [0052]    In yet another approach particles in near-surface phase may include magnetic particles that are coated with insulators. In such applications core of the particle may be a magnetic material such as cobalt or cobalt alloys. In this approach, once the near surface phase is disposed on the copper layer to be processed, a magnetic field is applied through the backside of the wafer (the side that does not have the copper layer) so as to magnetically attract the particles to the copper surface. After the mechanical action, near-surface phase comprising the magnetic particles quickly forms on the copper surface and prevents electro etching of the copper in the recessed regions. A magnetic field source may be a magnetic film attached or coated to the backside of the wafer.  
         [0053]    One other factor that contributes to the planarization capability of the present invention is the fact that use of electroetching solution such as phosphoric acid itself as part of the near surface phase which also includes high resistance particulates. During the process, when a voltage is first applied to the copper layer concentration of the copper and acid species in the adjacent solution is increased. In this respect, a concentration gradient including such species is established over the copper layer. Acid species from the bulk electroetching solution and copper ions leaving the surface have to diffuse through this surface layer. This surface layer forms overt the entire topography of the copper layer. Due to the concentrated ions and the adjacent high resistivity pariculates in this region, diffusion of ions through the surface layer from either direction is difficult and consequently, etching rate is significantly reduced after the formation of the surface layer. However, as described above use of a mechanical action disturbs the surface layer coating the raised regions and allows bulk electroetching solution to contact the surface of raised regions and electroetch them, while the surface layer in the recessed regions left undisturbed thereby inhibiting electroetching in the valleys.  
         [0054]    In the case of a process solution comprising an etching solution rather than electroetching solution, the only condition for the material of the powder is that it should be stable in the etching solution. Its conductivity does not play any role in the process.  
         [0055]    It should be understood, of course, that the foregoing relates to preferred embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention.