Abstract:
A system for the treatment of CMP wastewater, including wastewater from a copper CMP process. The wastewater treatment system includes a coagulant supply tank from which an FSC polymer coagulant is directed into a reaction tank that separately receives the untreated wastewater. The coagulant may be mixed with the untreated wastewater in selected ratios to provide a desired dosing quantity of the coagulant in the reaction tank. As the wastewater and the FSC polymer coagulant are mixed in the reaction tank, the coagulant flocs the slurry chemicals in the wastewater and removes the chemicals from solution in the wastewater as a precipitate before the wastewater is directed to a clarifier. The clarifier separates the flocked precipitate from the wastewater, and the flocked particles settle on the bottom of the clarifier to form a sludge. The sludge is re-distributed back into the clarifier to coagulate inert particles in the wastewater.

Description:
FIELD OF THE INVENTION 
     The present invention relates to chemical mechanical polishers used for polishing semiconductor wafers in the semiconductor fabrication industry. More particularly, the present invention relates to a new and improved system and process for treating wastewater from a chemical mechanical polisher used in the polishing of semiconductor wafers. 
     BACKGROUND OF THE INVENTION 
     In the fabrication of semiconductor devices from a silicon wafer, a variety of semiconductor processing equipment and tools are utilized. One of these processing tools is used for polishing thin, flat semiconductor wafers to obtain a planarized surface. A planarized surface is highly desirable on a shadow trench isolation (STI) layer, inter-layer dielectric (ILD) or on an inter-metal dielectric (IMD) layer, which are frequently used in memory devices. The planarization process is important since it enables the subsequent use of a high-resolution lithographic process to fabricate the next-level circuit. The accuracy of a high resolution lithographic process can be achieved only when the process is carried out on a substantially flat surface. The planarization process is therefore an important processing step in the fabrication of semiconductor devices. 
     A global planarization process can be carried out by a technique known as chemical mechanical polishing, or CMP. The process has been widely used on ILD or IMD layers in fabricating modern semiconductor devices. A CMP process is performed by using a rotating platen in combination with a pneumatically-actuated polishing head. The process is used primarily for polishing the front surface or the device surface of a semiconductor wafer for achieving planarization and for preparation of the next level processing. A wafer is frequently planarized one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer can be polished in a CMP apparatus by being placed on a carrier and pressed face down on a polishing pad covered with a slurry of colloidal silica or aluminum. 
     A CMP process is frequently used in the planarization of an ILD or IMD layer on a semiconductor device. Such layers are typically formed of a dielectric material. A most popular dielectric material for such usage is silicon oxide. In a process for polishing a dielectric layer, the goal is to remove typography and yet maintain good uniformity across the entire wafer. The amount of the dielectric material removed is normally between about 5000 A and about 10,000 A. The uniformity requirement for ILD or IMD polishing is very stringent since non-uniform dielectric films lead to poor lithography and resulting window-etching or plug-formation difficulties. The CMP process has also been applied to polishing metals, for instance, in tungsten plug formation and in embedded structures. A metal polishing process involves a polishing chemistry that is significantly different than that required for oxide polishing. 
     Important components used in CMP processes include an automated rotating polishing platen and a wafer holder, which both exert a pressure on the wafer and rotate the wafer independently of the platen. The polishing or removal of surface layers is accomplished by a liquid polishing slurry consisting mainly of colloidal silica suspended in deionized water or KOH solution. The slurry is frequently fed by an automatic slurry feeding system in order to ensure uniform wetting of the polishing pad and proper delivery and recovery of the slurry. For a high-volume wafer fabrication process, automated wafer loading/unloading and a cassette handler are also included in a CMP apparatus. 
     As the name implies, a CMP process executes a microscopic action of polishing by both chemical and mechanical means. While the exact mechanism for material removal of an oxide layer is not known, it is hypothesized that the surface layer of silicon oxide is removed by a series of chemical reactions which involve the formation of hydrogen bonds with the oxide surface of both the wafer and the slurry particles in a hydrogenation reaction; the formation of hydrogen bonds between the wafer and the slurry; the formation of molecular bonds between the wafer and the slurry; and finally, the breaking of the oxide bond with the wafer or the slurry surface when the slurry particle moves away from the wafer surface. It is generally recognized that the CMP polishing process is not a mechanical abrasion process of slurry against a wafer surface. 
     While the CMP process provides a number of advantages over the traditional mechanical abrasion type polishing process, a serious drawback for the CMP process is the difficulty in controlling polishing rates at different locations on a wafer surface. Since the polishing rate applied to a wafer surface is generally proportional to the relative rotational velocity of the polishing pad, the polishing rate at a specific point on the wafer surface depends on the distance from the axis of rotation. In other words, the polishing rate obtained at the edge portion of the wafer that is closest to the rotational axis of the polishing pad is less than the polishing rate obtained at the opposite edge of the wafer. Even though this is compensated for by rotating the wafer surface during the polishing process such that a uniform average polishing rate can be obtained, the wafer surface, in general, is exposed to a variable polishing rate during the CMP process. 
     Recently, a chemical mechanical polishing method has been developed in which the polishing pad is not moved in a rotational manner but instead, in a linear manner. It is therefore named as a linear chemical mechanical polishing process, in which a polishing pad is moved in a linear manner in relation to a rotating wafer surface. The linear polishing method affords a more uniform polishing rate across a wafer surface throughout a planarization process for the removal of a film layer from the surface of a wafer. One added advantage of the linear CMP system is the simpler construction of the apparatus, and this not only reduces the cost of the apparatus but also reduces the floor space required in a clean room environment. 
     Wastewater from the liquid polishing slurry used in the chemical mechanical polishing process must be properly treated for the removal of copper and other chemicals, as well as slurry particles, from the slurry prior to disposal. A typical conventional wastewater treatment system  10  is shown schematically in FIG.  1 . The wastewater treatment system  10  receives the wastewater from a CMP apparatus (not shown) during or after the CMP process. The wastewater treatment system  10  includes one or more wastewater collection tanks  12 , each of which receives the wastewater through an inlet header  11  and wastewater inlet line  13 . Some of the wastewater effluent from the treatment process is distributed into the inlet header  11  through an effluent return line  31  to dilute the wastewater in the collection tank or tanks  12 . The wastewater is distributed from each collection tank  12  through a corresponding wastewater outlet line  14  and valve  16 , and into a reaction tank  18  through a reaction tank inlet line  19 . Sodium hydroxide (NaOH) base may be distributed into the reaction tank  18  through a base infusion line  20 , and sulfuric acid (H 2 SO 4 ) may be distributed into the reaction tank  18  through an acid infusion line  21 , in various proportions to achieve a desired pH of the wastewater in the reaction tank  18 . Selected quantities of PAC (polyaluminum chloride) coagulator are further distributed into the reaction tank  18  from a PAC supply  22 . In the reaction tank  18 , the PAC is rapidly mixed with the wastewater to bind or coagulate with the slurry chemicals in the wastewater and precipitate the chemicals out of solution. A reaction tank outlet line  24  distributes the wastewater, with PAC-bound precipitates, from the reaction tank  18  to a clarifier  25 , which separates the PAC-bound precipitate particles from the wastewater and distributes the purified wastewater effluent to an effluent collection tank  27  through a clarifier outlet line  26 . The PAC-bound slurry particles form a thick sludge which settles in the bottom of the clarifier  25 , and the sludge is periodically removed from the clarifier  25  through a sludge removal line  34 . Finally, the wastewater effluent is distributed to an effluent line  30  through an effluent outlet line  28  and typically through a valve or valves  29 . Excess acid is removed from the effluent line  30  through an acidic waste drain line  32 . Some of the effluent is returned to the inlet header  11  through the effluent return line  31 , to dilute incoming wastewater in the collection tank or tanks  12 , whereas most of the effluent is distributed through an effluent disposal line  33  to a facility disposal system (not shown). 
     While the PAC has been shown to adequately coagulate and precipitate out of solution chemicals in wastewater from slurry used in most chemical mechanical polishing applications, PAC has been found to inadequately precipitate chemicals, particularly copper cations, in wastewater from slurry used in copper CMP processes, due to the particular chemicals used in the Cu-CMP polishing slurry. This results in production of a wastewater effluent having a high copper content and poor wastewater quality. Accordingly, a new system and process is needed for properly precipitating slurry chemicals, particularly copper cations, in CMP wastewater for the proper treatment and disposal of the wastewater. 
     An object of the present invention is to provide a new and improved process for treating CMP wastewater. 
     Another object of the present invention is to provide a new and improved process which effectively removes slurry chemicals from CMP wastewater in the treatment and disposal of the wastewater. 
     Still another object of the present invention is to provide a new and improved system and process for treating CMP wastewater in a variety of CMP applications. 
     A still further object of the present invention is to provide a new and improved system and process which is effective in treating wastewater from a copper CMP process. 
     Yet another object of the present invention is to provide a process which utilizes FSC polymer as a coagulant to remove slurry chemicals from CMP wastewater. 
     Still another object of the present invention is to provide a system and process which mixes CMP wastewater effluent with FSC polymer coagulant to remove slurry chemicals from CMP wastewater. 
     Yet another object of the present invention is to provide a CMP wastewater treatment system which includes a sludge return line for returning sludge removed from CMP wastewater in the a clarifier to wastewater in the clarifier in order to utilize the returned sludge as a coagulator for the removal of inert particles from the wastewater. 
     SUMMARY OF THE INVENTION 
     In accordance with these and other objects and advantages, the present invention is generally directed to a system and process for the treatment of CU-CMP wastewater, including wastewater from a copper CMP process. In a preferred embodiment, the wastewater treatment system includes a coagulant supply tank from which an FSC polymer coagulant is directed into a reaction tank that separately receives the untreated wastewater. The coagulant may first be mixed with the untreated wastewater in selected ratios to provide a desired dosing quantity of the coagulant in the reaction tank. Accordingly, as the wastewater and the FSC polymer coagulant are vigorously mixed in the reaction tank, the coagulant flocs the slurry chemicals, particularly the copper cations, in the wastewater and effectively removes the chemicals from solution in the wastewater as a precipitate before the wastewater is directed to a clarifier. The clarifier separates the flocked precipitate from the wastewater, and the flocked particles settle on the bottom of the clarifier to form a sludge. Some of the sludge is redistributed back into the clarifier to coagulate inert particles in the wastewater. The result is a wastewater effluent which leaves the clarifier with a low copper content and high wastewater quality. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a schematic view of a typical conventional system for the treatment of CMP wastewater; 
     FIG. 2 is a schematic view of a CMP wastewater treatment system of the present invention; and 
     FIG. 3 is a schematic view illustrating a typical dosing system for the coagulant in implementation of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention has particularly beneficial utility in treating wastewater from a chemical mechanical polishing apparatus used in the polishing of semiconductor wafer substrates. However, the invention is not so limited in application, and while references may be made to such chemical mechanical polishing apparatus, the present invention is more generally applicable to treating wastewater in a variety of industrial applications. 
     Referring next to FIG. 2, an illustrative embodiment of the wastewater treatment system of the present invention is generally indicated by reference numeral  70  and includes one or more wastewater collection tanks  72 , each of which is confluently connected to an inlet header  71  through a corresponding wastewater inlet line  73 . The inlet header  71  receives raw or untreated slurry wastewater from a CMP apparatus (not shown). Wastewater outlet lines  74  are provided in fluid communication with a reaction tank  78  through a valve or valves  76  and a reaction tank inlet line  79 . As shown, the wastewater outlet lines  74  may be confluently connected to one of a pair of wastewater lines  44  which connect a flow indicator  42  of a coagulant dosing system  35  to the reaction tank inlet line  79 . A base infusion line  80  may be connected to the reaction tank  78  for the introduction of sodium hydroxide (NaOH) base into the reaction tank  78 . An acid infusion line  81  may be further connected to the reaction tank  78  for the distribution of sulfuric acid (H 2 SO 4 ) into the reaction tank  78 . Accordingly, in application of the system  70  as hereinafter described, the sodium hydroxide and sulfuric acid may be introduced into the reaction tank  78  in various proportions to achieve a desired pH of the wastewater in the reaction tank  78 . A reaction tank outlet line  84  connects the reaction tank  78  to a clarifier  85 , which is connected to an effluent collection tank  87  through a clarifier outlet line  86 . An effluent outlet line  88  connects the effluent collection tank  87  to an effluent line  90 , typically through a pair of valves  89 . An acidic waste drain line  92  may extend from the effluent line  90 . An effluent return line  91  typically extends from the effluent line  90  to the inlet header  71 . An effluent disposal line  93  extends from the effluent line  90 , beyond the acidic waste drain line  92 . 
     Referring to FIGS. 2 and 3, in accordance with the present invention, a coagulant dosing system  35  is provided in the wastewater treatment system  70  for controlled infusion of an FSC polymer coagulant into the reaction tank  78 . As shown in FIG.  2 , the coagulant dosing system  35  includes a coagulant supply tank  36  which contains a supply of the liquid FSC polymer coagulant  41 . The FSC polymer coagulent  41  is a strong cation floculator which is capable of precipitating copper cations out of solution in the wastewater, as hereinafter further described. A polymer flow line  37 , which may be fitted with a valve  37   a , as shown in FIG. 3, connects the coagulant supply tank  36  to a flow controller  38 . The flow controller  38  may be any type of flow controller known by those skilled in the art which is capable of controlling the flow volume of a liquid. A polymer flow line  39 , which may be fitted with a valve  39   a , connects the outlet end of the flow controller  38  to one of two inlets of a liquid mixer  40 . A flow indicator  42  is connected to the reaction tank inlet line  79 , typically through the wastewater lines  44 , as heretofore described and shown in FIG.  2 . The flow indicator  42  may be any type of flow indicator known by those skilled in the art capable of measuring and indicating the rate of flow of a liquid flowing therethrough. An outlet wastewater line  45  connects the outlet of the flow indicator  42  to a second inlet of the liquid mixer  40 . Finally, a polymer entry line  46  extends from the outlet of the mixer  40  and is provided in fluid communication with the reaction tank  78 , as further shown in FIG.  2 . 
     Referring again to FIG. 2, and further in accordance with the present invention, a sludge removal line  94  extends from the bottom of the clarifier  85 . A sludge return line  95  extends from the sludge removal line  94  and is connected to the wastewater inlet area of the clarifier  85 . The sludge removal line  95  is typically fitted with one or a pair of valves  96 . A sludge thickener line  97 , typically fitted with a valve or valves  99 , may further connect the sludge removal line  94  to a thickener supply  98  which contains a supply of copper thickener or other thickener for thickening the sludge to a solid form, typically in conventional fashion. 
     Referring again to FIG. 2, in typical application of the wastewater treatment system  70 , during operation of a CMP apparatus (not shown), wastewater is generated from the polishing slurry as the slurry is used to polish a semiconductor wafer (not shown). The wastewater is distributed from the CMP apparatus to the wastewater treatment system  70 , typically through the inlet header  71 . Each of the wastewater collection tanks  72  receives and collects the raw wastewater  75  from the inlet header  71  through the respective wastewater inlet lines  73 . The wastewater  75  is distributed from each collection tank  72  through the corresponding wastewater outlet line  74 , valve  76  and reaction tank inlet line  79 , respectively, and into the reaction tank  78 . 
     As the raw wastewater  75  is distributed through the reaction tank inlet line  79  into the reaction tank  78 , some of the raw wastewater  75  is distributed through the wastewater lines  44 , through the flow indicator  42  and the outlet wastewater line  45 , respectively, and into the liquid mixer  40  of the coagulant dosing system  35 . Simultaneously, under control by the flow controller  38 , FSC polymer coagulant  41  is distributed from the coagulant supply tank  36  through the polymer flow line  37 , flow controller  38  and polymer flow line  39 , respectively, and into the liquid mixer  40 . The liquid mixer  40  is operated, typically in conventional fashion, to thoroughly mix and disperse the FSC polymer coagulant  41  in the wastewater  75  to define a polymer mixture  47  in the liquid mixer  40 . Preferably, the FSC polymer coagulant  41  is mixed with the wastewater dispersing agent in a concentration of about 0.5% to about 5%, and preferably, about 1%, by weight, of the FSC polymer  41  in the wastewater  75  to define a polymer mixture  47 . The polymer mixture  47  is distributed from the mixer  40 , through the polymer entry line  46  and into the reaction tank  78 . Sodium hydroxide (NaOH) base may be distributed into the reaction tank  78  through the base infusion line  80 , and sulfuric acid (H 2 SO 4 ) may be distributed into the reaction tank  78  through the acid infusion line  81 , in various proportions to achieve a desired pH of the polymer mixture  47  in the reaction tank  78 . A preferred range of pH for the polymer mixture  47  in the reaction tank  78  is 10-11. In the reaction tank  78 , the polymer mixture  47 , which includes the FSC polymer coagulant  41  dispersed in the wastewater  75 , is rapidly mixed and agitated for a period of typically about 5 min. to about 20 min. to flocculate the slurry chemicals, particularly copper cations, in the polymer mixture  47 . Accordingly, the slurry chemicals dissolved in the dispersant wastewater bind to the FSC polymer coagulant molecules and are precipitated out of solution in the polymer mixture  47 . The reaction tank outlet line  84  distributes the flocculated polymer mixture  47 , with FSC-bound slurry chemicals, from the reaction tank  78  to the clarifier  85 . The clarifier  85  separates the FSC-bound chemicals from the wastewater in the polymer mixture  47  and distributes the purified wastewater effluent  48  to the effluent collection tank  87  through the clarifier outlet line  86 . The PAC-bound slurry chemicals form a thick sludge  49  which settles in the bottom of the clarifier  85 , and the sludge  49  flows from the clarifier  85  through the sludge removal line  94 . Some of the sludge  49  is continually recycled back to the intake area of the clarifier  85  through the sludge return line  95  and valve or valves  96 . In the clarifier  85 , the recycled sludge  49  enters the purified wastewater effluent  48 , where the sludge  49  binds inert particles remaining in the purified wastewater effluent  48 . This enhances purification of the wastewater in the clarifier  85  as the sludge  49 , with the inert slurry particles bound thereto, immediately fall to the bottom of the clarifier  85 . The purified wastewater effluent  48  is distributed to the effluent line  90  through an effluent outlet line  88  and the valve or valves  89 . Excess acid may be removed from the purified wastewater effluent  48  in the effluent line  90  through the acidic waste drain line  92 . Some of the purified wastewater effluent  48  may be returned to the inlet header  71  through the effluent return line  91 , to dilute incoming raw wastewater  75  in the collection tank or tanks  72 , as desired. Most of the purified wastewater effluent  48  is typically distributed through the effluent disposal line  93  to a suitable facility disposal system (not shown). 
     It has been shown that the wastewater treatment system  70  of the present invention is capable of removing copper cations and other chemicals from the raw wastewater to form a purified wastewater effluent having a copper content of less than 10 mg/liter. This represents a substantial improvement in the quality of the wastewater as compared to that obtained using conventional wastewater treatment systems. It will be appreciated by those skilled in the art that the FSC polymer coagulant is capable of effectively operating over a wide range of system variations. 
     While the preferred embodiments of the invention have been described above, it will be recognized and understood that various modifications can be made in the invention and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention.