Patent Publication Number: US-2005133374-A1

Title: Method and apparatus for acid and additive breakdown removal from copper electrodeposition bath

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      Embodiments of the invention generally relate to removing organic waste material and acid from semiconductor electrolyte solutions.  
      2. Description of the Related Art  
      Metallization for sub-quarter micron sized features is a foundational technology for present and future generations of integrated circuit manufacturing processes. More particularly, in devices such as ultra large scale integration-type devices, i.e., devices having integrated circuits with more than a million logic gates, the multilevel interconnects that lie at the heart of these devices are generally formed by filling high aspect ratio interconnect features with a conductive material, such as copper or aluminum. Conventionally, deposition techniques such as chemical vapor deposition (CVD) and physical vapor deposition (PVD) have been used to fill these interconnect features. However, as interconnect sizes decrease and aspect ratios increase, void-free interconnect feature fill via conventional metallization techniques becomes increasingly difficult. As a result, plating techniques such as electrochemical plating (ECP) and electroless plating have emerged as viable processes for filling sub-quarter micron sized high aspect ratio interconnect features in integrated circuit manufacturing processes.  
      In an ECP process, sub-quarter micron sized high aspect ratio features formed on a substrate surface may be efficiently filled with a conductive material, such as copper. ECP plating processes are generally two stage processes, wherein a seed layer is first formed over the surface features of the substrate, and then the surface features of the substrate are exposed to an electrolyte solution, while an electrical bias is applied between the substrate and an anode positioned within the electrolyte solution. The electrolyte solution is generally rich in ions to be plated onto the surface of the substrate, and therefore, the application of the electrical bias causes these ions to be urged out of the electrolyte solution and to be plated as a metal on the seed layer. The plated metal, e.g., copper, grows in thickness and forms a copper layer over the seed layer that operates to fill the features formed on the substrate surface. The concentration of chemicals in the electrolyte solution must be maintained within a narrow operation window to achieve void free filling of the features.  
      In order to facilitate and control this plating process, several additives may be utilized in the electrolyte plating solution. For example, a typical electrolyte solution used for copper electroplating may consist of copper sulfate solution, which provides the copper to be plated, having sulfuric acid and copper chloride added thereto. The sulfuric acid may generally operate to modify the acidity and conductivity of the solution. The electrolytic solutions also generally contain various organic molecules, which may be accelerators, suppressors, levelers, brighteners, etc. These organic molecules are generally added to the plating solution in order to facilitate formation of void-free high aspect ratio features and planarized copper deposition. Accelerators, for example, may be sulfide-based molecules that locally accelerate electrical current at a given voltage where they absorb. Suppressors may be polymers of polyethylene glycol, mixtures of ethylene oxides and propylene oxides, or block copolymers of ethylene oxides and propylene oxides which tend to reduce electrical current at the sites where they absorb (the upper edges/corners of high aspect ratio features), and therefore, slow the plating process at those locations, which reduces premature closure of the feature before the feature is completely filled. Levelers may be nitrogen containing, long chain polymers which operate to facilitate planar plating. Additionally, the plating bath usually contains a small amount of chloride, generally between about 20 and about 60 ppm, which provides negative ions needed for adsorption of suppressor molecules on the cathode, while also facilitating proper anode corrosion.  
      Although the various organic additives facilitate the plating process and offer a control element over the interconnect formation process, they also present a challenge since the additives are known to eventually break down and become waste material in the electrolyte solution that is no longer useful and may even be a contaminant. Conventional plating systems traditionally dealt with these organic waste materials via bleed and feed methods (periodically replacing a portion of the electrolyte), extraction methods (filtering the electrolyte with a charcoal filter), photochemical decomposition methods (using UV in conjunction with ion exchange and acid-resistant filters), and/or ozone treatments (dispensing ozone into the electrolyte). However, these conventional methods are known to be inefficient, expensive to implement and operate, bulky, and/or tend to generate hazardous materials or other kinds of contaminants as byproducts.  
      Recently, electrodialysis cells (EDC) have been used to substantially remove all of the organic additives from at least a portion of the electrolyte solution in the plating process as discussed in detail in U.S. patent application Ser. No. 10/074,569, which is herein incorporated by reference in its entirety. Substantially all of the additives are removed since membranes used in the EDC are sufficiently dense such that the additives fail to penetrate through the membranes. The EDC requires an electrical supply and may lack the ability to remove acids. However, it may be desirable to remove acids that accumulate during the plating process and to remove certain organic additives and/or organic waste at a faster rate than other organic additives based on the breakdown rates of the various organic additives. For example, the accelerators breakdown faster than the levelers which breakdown faster than the suppressors. Further, it may be desirable in certain applications to remove only a percentage of the organic additives and/or organic waste from the entire electrolyte solution rather than all of the organic additives from a portion of the electrolyte solution.  
      Therefore, there exists a need for a method and apparatus for removing additive breakdown waste material from semiconductor electroplating baths, wherein the method and apparatus addresses the deficiencies of conventional devices.  
     SUMMARY OF THE INVENTION  
      The invention generally provides a plating cell having an electrolyte inlet and an electrolyte drain, an electrolyte storage unit in fluid communication with the electrolyte inlet, and a diffusion dialysis chamber in fluid communication with the electrolyte drain and the electrolyte storage unit. The diffusion dialysis chamber is generally configured to receive at least a portion of used electrolyte solution and remove waste material therefrom in order to provide a refreshed electrolyte solution to the electrolyte storage unit. The method generally includes supplying an electrolyte solution to a copper plating cell, plating copper onto a substrate in the plating cell with the electrolyte solution, removing used electrolyte solution from the plating cell, and refreshing a portion of the used electrolyte solution with a diffusion dialysis device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      So that the manner in which the above recited features of the invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
       FIG. 1  illustrates an exemplary plating system incorporating a diffusion dialysis device (DDD).  
       FIG. 2  illustrates a schematic view of the DDD in  FIG. 1 .  
       FIG. 3  illustrates an exemplary plating system incorporating the DDD and an electrodialysis cell (EDC).  
       FIG. 4  illustrates a schematic view of the EDC shown in  FIG. 3 .  
       FIG. 5  illustrates a schematic view of an alternative EDC.  
       FIG. 6  illustrates an alternative plating system configuration that incorporates the DDD.  
       FIG. 7  is a graph showing the rate of removal by the DDD of sulfuric acid from an electrolyte solution.  
       FIG. 8  is a graph showing the rate of removal of additives and breakdown products from the additives by the DDD from the electrolyte solution. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      The invention generally relates to removal of organic waste material and acid from an electrolyte solution during a plating process with a diffusion dialysis device (DDD).  FIG. 1  illustrates an exemplary plating system  100  that includes a diffusion dialysis device (DDD)  110  in conjunction with a plating cell  101  such as an electrochemical plating (ECP) cell, an electroless plating cell, or other plating cell configuration. The plating cell  101  includes a fluid inlet  105  configured to deliver an electrolyte solution or plating processing fluid to the plating cell  101  and a fluid outlet or drain  106  configured to receive the electrolyte solution from the plating cell  101 . The electrolyte solution enters the plating cell  101  via the inlet  105  that is in fluid communication with an electrolyte solution storage unit  102 . A fluid pump  104  positioned between the storage unit  102  and the plating cell  101  circulates the electrolyte solution to the plating cell  101 . The fluid outlet  106  of the plating cell  101  returns used electrolyte solution to the storage unit  102  through a fluid conduit  11   1 . The fluid conduit  111  may include a slipstream, bypass, or diverter fluid conduit  112  attached thereto. The diverter fluid conduit  112  receives a portion of or the entire used electrolyte solution returned from the processing cell  101  to the storage unit  102  via the drain  106 . The diffusion dialysis device  110  positioned along the diverter fluid conduit  112  includes an input for accepting the electrolyte solution flowing through the diverter fluid conduit  112  and an output coupled to a fluid conduit  113  for returning the electrolyte solution to the storage unit  102 . In this manner, the electrolyte solution continually circulates through an electrolyte circulation loop.  
      During typical operational periods, the plating cell  101  may receive and/or circulate therethrough approximately  100  liters of electrolyte solution per hour. Thus, the DDD  110  receives any portion of this used electrolyte solution or the entire used electrolyte solution. As the used electrolyte solution passes through the DDD  110 , the DDD  110  removes a portion of the organic additives, waste material from the organic additive breakdown, and acid from the used electrolyte solution to provide a refreshed electrolyte solution. The refreshed electrolyte solution is reintroduced into the fluid storage unit  102  for subsequent use in plating operations. The DDD  110  captures the extracted acids and waste material for disposal. In this manner, the DDD  110  operates to decrease or eliminate the frequency of replacement of the electrolyte solution by retaining copper ions within the electrolyte solution, removing organic additive breakdown waste material, and removing acid accumulated in the electrolyte solution. If needed, acid and additives may be reintroduced to the refreshed electrolyte solution in order to compensate for the loss of these components by the DDD  110 .  
       FIG. 2  illustrates a schematic view of the DDD  110  shown in  FIG. 1 . The DDD  110  includes an outer housing  200  having a plurality of anionic membranes  206  that separate and define electrolyte cells  202  and diluted acid cells  204  within the DDD  110 . The electrolyte cells  202  and the diluted acid cells  204  alternate across the DDD  110 . Although only four electrolyte cells  202  and diluted acid cells  204  are shown, the DDD  110  may include any number of anionic membranes  206  that define any number of alternating electrolyte cells  202  and diluted acid cells  204 . For example, embodiments of the DDD  110  may include between  3  and about  500  total number of electrolyte cells  202  and diluted acid cells  204 . Configuration of the anionic membranes  206  within the housing  200  of the DDD  110  may include any known configuration for diffusion dialysis currently used for free acid recovery.  
      The anionic membranes  206  can be any type of anion-exchange membrane such as any one of many commercially available membranes. For example, Asahi Glass Company produces a wide range of polystyrene based ion-exchange membranes under the trade name Selemion such as anion membranes AMV, AMT, and AMD. Other companies manufacture similar ion-exchange membanes, such as Solvay (France), Sybron Chemical Inc. (USA), Ionics (USA), and FuMA-Tech (Germany) etc. Each anionic membrane  206  comprises a matrix having a positive charge inside and a selected porosity for selectively passing molecules therethrough. In one embodiment, the pore size of the anionic membrane is preferably greater than  50  angstroms and most preferably about  100  angstroms. Thus, the anionic membrane  206  permits water, hydrogen ion, disassociated sulfate ion, and organic additive penetration due to the negative or neutral charge and/or size of these molecules. However, disassociated copper ion penetration is negligible since the copper ions are repelled by the anionic membrane  206  having the same charge. The diffusion rate of the different organic additives through the anionic membranes  206  varies depending on the size and charge of the organic additives. For example, small and negative or neutral charged organic additives such as sulfur containing accelerators and brighteners penetrate through the anionic membrane  206  faster than the organic additives containing nitrogen such as levelers. Further, the polymeric structures of some organic additives such as suppressors substantially lack the ability to pass through the anionic membranes  206  due to their large sizes. Since the contamination material from the various organic additives is caused by their breakdown, the contamination material typically has a smaller chain length than the original organic additive. Thus, the smaller chain length of the contamination material permits the contamination material to penetrate through the anionic membranes  206 .  
      In operation, the conduit  112  supplies used electrolyte solution from the plating cell  100  (shown in  FIG. 1 ) to each of the electrolyte cells  202  through inlets  205  along the housing  200  of the DDD  110 . In one embodiment, the inlets  205  are integral with the housing  200  such that the conduit  112  supplies used electrolyte solution to one location along the housing  200 . The housing  200  includes individual frame chambers that sandwich the anionic membranes  206  between adjacent frame chambers. Passages through the walls of each frame chamber align with apertures in the anionic membranes  206  and passages in adjacent frame chambers to pass the used electrolyte solution across the length of the DDD  110 . Ports connecting the interior of the frame chambers or the electrolyte cells  202  to the appropriate passages in the housing  200  provide the individual inlets  205 . This design may be used for all of the inlets and outlets to the DDD  110  described herein.  
      As shown, the used electrolyte solution includes disassociated copper ions (Cu 2+ ), hydrogen ions (H + ), disassociated sulfate ions (SO 4   2− ), and organic additives and their breakdown products (Org). A diluted acid solution having a higher pH than the electrolyte solution circulates through the diluted acid cells  204 . The diluted acid solution circulates through the DDD  110  by use of a supply tank  208 , a pump  210 , and fluid conduits connecting the supply tank  208  to inlets  201  and outlets  203  disposed in the housing  200  to provide flow through each of the diluted acid cells  204 . SO 4   2− , H + , and Org within the electrolyte cells  202  migrate across the anionic membranes  206  based on diffusion across the concentration gradient between the electrolyte cells  202  and the diluted acid cells  204 . The diffusion of SO 4   2− , H + , and Org from the electrolyte cells  202  to the diluted acid cells  204  effectively removes a portion of the acid and the organic additives from the electrolyte solution while leaving the Cu 2+  min the electrolyte solution. The amount of the various organic additives (e.g. accelerator, leveler, and suppressor) extracted from the electrolyte solution depends on their diffusion rate through the anionic membranes  206 . During operation, the electrolyte solution passes through the electrolyte cells  202  where a portion of the SO 4   2− , H + , and Org is removed prior to the refreshed electrolyte solution exiting the electrolyte cells  202  through outlets  207  along the housing  200  of the DDD  110 .  
      The supply tank  208 , the conduits, the pump  210 , and the diluted acid cells  204  provide a deionized (DI) water loop that circulates through the diluted acid cells  204  of the DDD  110 . To maintain the concentration level of the acid circulating through DI water loop, the supply tank  208  refreshes by draining and discarding the diluted acid solution that contains acids and organic additives extracted from the electrolyte solution. Fresh deionized (DI) water adds to the supply tank to maintain the total volume of the diluted acid solution. In this manner, the concentration of acid within the supply tank  208  and diluted acid cells  204  remains sufficiently low to promote diffusion across the anionic membranes  206 . Preferably, the supply tank  208  refreshes when the acid concentration therein reaches more than about  1  to  10  grams per liter.  
       FIG. 3  illustrates the plating system  100  of  FIG. 1  incorporating the DDD  110  in conjunction with an electrodialysis cell (EDC)  103 . The plating system  100  functions the same as described above except that the refreshed electrolyte solution that exits the DDD  110  first passes through the EDC prior to returning to the storage unit  102 . The DDD  110  removes part of the organic additives and acid as described herein. Next, the EDC  103  completely removes the organic additives and returns the copper sulfate and the remaining acid to the storage tank  102  for reuse. In this manner, a combination of the DDD  110  and the EDC  103  removes a portion of the acid and all the organic additives. Since the DDD  110  substantially lacks the ability to remove some of the levelers and polymeric organic additives such as suppressors, the combination of the DDD  110  and EDC  103  provides for their removal from the electrolyte solution.  
       FIG. 4  shows a schematic view of the EDC  103  for use with the DDD  110  as illustrated in  FIG. 3 . The &#39;569 application that is incorporated by reference and entitled “Apparatus and Method for Removing Contaminants from Semiconductor Copper Electroplating Baths” describes the use of the EDC for removal of waste material. The used electrolyte solution enters the EDC  103  via conduit  408  from the DDD  110 . The conduit  408  supplies the used electrolyte into a plurality of depletion cells or chambers  405  in the EDC  103 . While the used electrolyte is supplied to the depletion cells  405 , a cathode  402  and an anode  403  apply an electrical bias across the EDC  103 . The application of the electrical bias across the EDC  103  operates to urge ions in the used electrolyte solution towards the respective poles, i.e., positive ions urge in the direction of the cathode, while negative ions urge in the direction of the anode. Therefore, the Cu 2+  along with the H +  urge in the direction of the cathode  402 . Similarly, the SO 4   2−  urges in the direction of the anode  403 . However, although the respective ions are urged in the direction of the respective poles, the linear distance the respective ions travel is limited by the positioning of anionic and cationic membranes  409 ,  410 . More particularly, the positive copper and hydrogen ions in depletion cells  405  urge towards cathode  402  and pass into the neighboring concentration chambers  404  since the membranes separating depletion chambers  405  and concentration chambers  404  are cationic membranes  410 . Similarly, the negatively charged sulfate ions urge towards the anode  403  and pass through the anionic membranes  409  into the neighboring concentration chambers  404 . As a result of the alternating positioning of the cationic and anionic membranes  410 ,  409 , positive copper ions and negative sulfate ions diffuse into concentration chambers  404  where these ions combine to form concentrated copper sulfate-sulfuric acid solution (CuSO 4 /H 2 SO 4 ). The electrolyte solution waste material (organic breakdown products, impurities, solid particles, etc.) remain in depletion chambers  405  and are discarded via conduit  413 . The concentrated copper sulfate within the concentration chambers  404  may then be removed via conduit  414  and returned to the storage unit  102  (shown in  FIG. 3 ) for reuse.  
       FIG. 5  shows a schematic view of an alternative EDC  500  which may be used with the DDD  110  as illustrated in  FIG. 3 . Anionic membranes  509  used in the EDC  500  may not possess a sufficiently small porosity to completely prevent the passage of organics such as breakdown products from accelerator and leveler into the purified electrolyte. Unlike the configuration of the EDC  103  shown in  FIG. 4 , at least one cation membrane  510  separates the organic additives within depletion cells  505  from purified electrolyte cells  504  that contain the electrolyte for reuse. Therefore, the EDC  500  operates to provide a purified electrolyte solution based on the non-permeability of cation membranes  510  with respect to the organics such as accelerator and high-molecular weight leveler. A conduit  408  supplies the used electrolyte into a plurality of depletion cells  505  in the EDC  500 . While the used electrolyte is supplied to the depletion cells  505 , a cathode  502  and an anode  503  apply an electrical bias across the EDC  500  to urge ions in the used electrolyte solution towards the respective poles. Therefore, the Cu 2+  along with the H +  urge in the direction of the cathode  502 , and the SO 4   2−  urges in the direction of the anode  503 . The positive copper and hydrogen ions in depletion cells  505  urge towards cathode  502  and pass into the neighboring purified electrolyte cells  504  since the membranes separating depletion cells  505  and purified electrolyte cells  504  are cationic membranes  510 . The negatively charged sulfate ions and some of the organic additives and their breakdown products within the depletion cells  505  urge towards the anode  503  and pass through the anionic membrane  509  into a neighboring waste cell  507 . Acid at a controlled concentration (e.g. 5-50 g/L) and supplied via conduit  508  circulates through acid cells  506  adjacent the purified electrolyte cells  504  and opposite the depletion cells  505 . The sulfate ions within the acid cells  506  pass through the anionic membranes  509  separating the acid cells  506  and the purified electrolyte cells  504  in order to replenish the purified electrolyte. DI water may enter the waste cells  507  and the purified electrolyte cells  504  to aid flow through the EDC  500 . The electrolyte solution waste material (organic breakdown products, impurities, solid particles, etc.) that remains in depletion cells  505  or is transferred to waste cells  507  is discarded via conduit  511 . In this manner, the EDC  500  may return more than  95 % of purified CuSO 4 /H 2 SO 4  for reuse through conduit  414  to the storage unit  102  (shown in  FIG. 3 ).  
       FIG. 6  illustrates an alternative plating system  500  that incorporates the DDD  110 . The plating system  500  generally includes a plating cell  501  configured to fluidly isolate an anode  522  of the plating cell  501  from a cathode  523  or plating electrode of the plating cell  501  via a cation exchange membrane  512  positioned between the substrate being plated and the anode  522  of the plating cell  501 . U.S. patent application Ser. No. 10/187,027, entitled “Electroplating Cell with Copper Acid Correction Module for Substrate Interconnect Formation,” which is herein incorporated by reference in its entirety, describes in detail a plating system using this type of divided plating cell. The plating cell  501  provides a first fluid solution (anolyte) to an anolyte compartment  508 , i.e., the volume between the upper surface of the anode  522  and the lower surface of the membrane  512 , and a second fluid solution (catholyte) to a catholyte compartment  510 , i.e., the volume of fluid positioned above the upper membrane surface. The anode  522  may generally be soluble, e.g., a copper anode, or insoluble, e.g., platinum. The catholyte includes copper sulfate, sulfuric acid, copper chloride, and additives similar to the electrolyte solution described in  FIG. 1 . However, during electrolysis hydrogen ions and copper ions move through the membrane  512  into the catholyte compartment  510 . As a result, the concentration of acid in the catholyte increases and must be removed. Therefore, the use of the DDD  110  as described herein effectively removes the build up of acid in the catholyte along with the build up of waste material from the breakdown of the organic additives.  
       FIG. 7  is a graph showing the rate of removal by the DDD  110  of sulfuric acid from an electrolyte solution. The electrolyte solution used to obtain the graph contained 0.85M CuSO 4  and 0.3M H 2 SO 4 . In operation, one square meter of anionic membrane  206  within the DDD  110  extracts between about 20 and 60 grams of acid from the electrolyte solution per hour. The rate of acid extraction depends on the quality of the anionic membrane  206 , flow rates through the DDD  110 , and the concentration of acid accumulated in the DI water loop. As shown in the graph in  FIG. 7 , acid may need to be added to the electrolyte solution if insufficient acid is not produced during the plating process to compensate for the loss of acid.  
       FIG. 8  is a graph showing the rate of removal of organic additives and their breakdown products by the DDD from the electrolyte solution. The electrolyte solution contained 6.5 milliliters per liter of accelerator, 3 milliliters per liter of suppressor, and 4 milliliters per liter of leveler. As shown, the DDD  110  removes accelerator faster than leveler and leveler faster than suppressor. Therefore, the DDD  110  becomes most effective when the accumulation of accelerator&#39;s breakdown is faster than that of leveler&#39;s and negligibly low for suppressor. During typical plating processes, the breakdown products from accelerator accumulates faster than the breakdown products from leveler, and the breakdown products from suppressor accumulates negligibly within the electrolyte solution. Therefore, the DDD  110  extracts the various organic breakdown products at a rate that mirrors their rate of accumulation within the electrolyte solution.  
      While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.