Abstract:
A system and method for providing process control in a CMP system utilizes a vacuum-assisted arrangement for conditioning a wafer polishing pad so that the effluent (i.e., wafer debris, polishing slurry, chemical or other by-products) from the conditioning process is diverted from the waste stream and instead introduced into an analysis module for further processing. The analysis module functions to determine at least one parameter within the effluent and generate a process control signal based upon the analysis. The process control signal is then fed back to the planarization process to allow for the control of various parameters such as polishing slurry composition, temperature, flow rate, etc. The process control signal can also be used to control the conditioning process and/or determining the endpoint of the planarization process itself.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
     This application claims the benefit of U.S. Provisional Application No. 60/539,163, filed Jan. 26, 2004. 
    
    
     TECHNICAL FIELD 
     The present invention relates to chemical mechanical planarization (CMP) and, more particularly, to the analysis of effluent from a CMP conditioning process for controlling the planarization process and providing endpoint detection. 
     BACKGROUND OF THE INVENTION 
     The electronics industry continues to rely upon advances in semiconductor manufacturing technology to realize higher-functioning devices while improving their reliability and cost. For many applications, the manufacture of such devices is complex, and maintaining cost-effective manufacturing processes while concurrently maintaining or improving product quality is difficult to accomplish. As the requirements for device performance and cost become more demanding, realizing a successful manufacturing process becomes more difficult. 
     Indeed, as the level of circuit integration increases, more layers are required to be formed upon the silicon starting wafer. The use of multiple layers results in problems associated with surface non-planarity, impacting both yield and chip performance. Indeed, one of the most crucial processing steps today is related to restoring a planar surface to the wafer between the formation of each layer, as well as “planarizing/polishing” the final wafer structure before it is diced into separate components. Extreme care must be taken during this planarization process, since a significant amount of time and money has been invested in transforming the wafer from a uniform silicon slab into a complicated electronic circuit by the time the final planarization process is performed. 
     Within the past decade or so, a process known as chemical mechanical planarization (CMP) has evolved as a preferred technique for planarizing a wafer surface. CMP involves the use of a polishing pad affixed to a polishing table, with a separate holder used to present the silicon wafer “face down” against the rotating polishing pad. A polishing slurry containing both abrading particulates and chemical additives is dispensed onto the surface of the polishing pad and used to carefully remove irregularities from the wafer surface. The abrading particulates provide for the “mechanical” aspect of the planarization process, while specific chemical additives are used to selectively oxidize or etch the non-planar material from the wafer surface. When the surface layer of the wafer is, for example, a dielectric material, potassium hydroxide or another base oxidizer may be used as the chemical additive. When the surface layer of the wafer comprises copper (as discussed further below, metal CMP is becoming more prevalent), the chemical additive may comprise hydrogen peroxide. In any case, the combination of the abrading particulates and the chemical additive(s) in the polishing slurry results in planarizing the wafer surface as it moves against the polishing pad. 
     One area of concern with the CMP process is the changes that occur to the polishing pad over time. That is, if the polishing pad is not cleaned on a regular basis, the surface of the pad begins to accumulate spent polishing slurry abrasive particulates, removed wafer material and chemical or other by-products of the polishing process. This deposited debris, in combination with polishing heat effects, causes the polishing pad to become matted down and wear unevenly (often referred to in the art as the “glazing effect”). Thus, it becomes necessary to restore the polishing pad surface to a state suitable for continued polishing. 
     “Pad conditioning” or “pad dressing” is a process known in the art that is used to restore the surface of the polishing pad and remove the glazing by dislodging particulates and spent polishing slurry from the pad. Pad conditioning also planarizes the pad by selectively removing pad material, and roughens the surface of the polishing pad. Pad conditioning may be performed “ex-situ” (i.e., by conditioning the polishing pad between wafer polishing cycles), or “in-situ” (i.e., by conditioning the polishing pad currently with, or during, a wafer polishing cycle). In a typical prior art “in-situ” pad conditioning process, a fixed abrasive that functions to remove a small amount of pad material and debris is applied to the pad surface, thus creating new asperities for allowing the polishing slurry to flow freely. The removed pad material and debris thereafter combine with the slurry flow stream from the polishing process and are carried away from the pad and the wafer being polished by normal slurry transport mechanics. Ultimately, these materials are flushed at the end of the polishing cycle with rinse water, and collected in the central drain of the polisher. 
     During a conventional CMP process, the removal rate of the surface material will change as a function of various factors including, but not limited to, applied pressure, rotational speed, flow rate of the polishing slurry, temperature of the polishing slurry, size and/or concentration of particulates in the polishing slurry and chemistry of the polishing slurry, as well as the amount of material remaining on the surface of the wafer to be planarized. At times, it is difficult to control the planarization process so that “overpolishing” (referred to as “dishing”) or “underpolishing” (not clearing the entire film) does not occur. One prior art arrangement utilizes a multiple number of polishing stations within the CMP apparatus to attempt to control the planarization process. In particular, a first station may be used to perform a “rough” planarization to remove the bulk amount of the unwanted material, perhaps depending on a specific time period to determine when to stop the rough planarization process. A second station may then be used to perform a “finer” planarization step, perhaps including some means of “endpoint detection” to determine when the appropriate amount of unwanted material has been removed. Lastly, a third station may be used as a “buffing” station to apply a final polishing to the wafer. Each of these stations can then be separately controlled to provide the greatest degree of care for the overall process. When performing metal CMP, different polishing stations may be used to selectively remove different types of material from the wafer surface. For example, a first station may be used to remove the overburden copper, a second station to remove the barrier metal (e.g., tantalum), and a third station to achieve final planarity and protect the copper from corrosion. 
     Since various other parameters associated with the polishing slurries, polishing pad and wafer will affect each of these stations, it remains difficult to accurately and efficiently control the planarization process in any type of multi-step CMP process. 
     SUMMARY OF THE INVENTION 
     The various needs of the prior art are addressed by the present invention, which relates to a conditioning process for CMP wafer polishing that utilizes a portion of the debris or effluent removed during conditioning to control the various steps in the planarization operation (including, but not limited to, endpoint detection). 
     In accordance with the present invention, a CMP system includes an abrasive conditioning disk with an apertured/open structure that is used to dislodge debris from the polishing pad surface and evacuate the dislodged debris through the apertured surface by applying a vacuum force through the conditioning disk. The debris, as it is being created during the polishing process, is therefore pulled through the conditioning disk and evacuated into an analysis system. Various flushing agents (either ultra-pure water (UPW) or a liquid with a particular chemistry) may be introduced through the conditioning apparatus onto the polishing pad surface to assist in the debris removal process. The evacuated debris (also referred to hereinafter as “effluent”) is then directed into an analyzer that can determine the various materials present in the effluent (or specific properties of these materials), perhaps in terms of the concentration of each component. This information is then fed back to the polishing slurry delivery apparatus, the polisher mechanical controller and/or the conditioning system, where it is used to control the planarization process. 
     In one instance, the information fed back to the planarization process may be used to modify the material removal rate as a function of the measured concentration of various materials analyzed in the effluent. For example, if the particular concentration of conditioning process effluent is lower than desired, the control signal fed back to the polishing slurry delivery apparatus may be used to adjust the flow rate of the polishing slurry, the temperature of the polishing slurry, the concentration/size of the abrasive particulate, etc. Indeed, there are a significant number of planarization process and/or conditioning process parameter variations that may be utilized to provide CMP process control in accordance with the present invention. 
     In another instance, the information fed back to the planarization process may be used to determine the endpoint of the planarization process itself. For example, when used with copper CMP, the concentration of copper ions in the conditioning effluent will rapidly decrease upon onset of the “endpoint”. Thus, by monitoring the copper concentration (or conductivity of the effluent), the planarization process may be stopped when the predetermined “endpoint concentration” or other appropriate parameter is obtained. 
     Various arrangements may be used to perform the analysis on the evacuated conditioning effluent. For example, the conductivity of the effluent may be measured and used as a feedback signal. The pH of the conditioning effluent may be measured and used in an alternative arrangement. In a more sophisticated system, Raman spectroscopy may be used to analyze the concentration of various components within the effluent. An electrochemical cell may alternatively be used to determine the ion concentration of a metal as it is being removed during a metal CMP process. The particular method of effluent analysis is not of concern, as long as an understanding of certain characteristics of various effluent components can be elicited and used by the CMP system to control the planarization process. 
     Indeed, other and further aspects of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the drawings, where like numerals represent like parts in several views: 
         FIG. 1  illustrates an exemplary CMP system including a conditioning apparatus feedback arrangement for controlling a planarization process in accordance with the present invention; 
         FIG. 2  is a top view of the arrangement of  FIG. 1 ; and 
         FIG. 3  contains a graph of an exemplary planarization process. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an exemplary CMP system  10  that may be used to perform in-situ conditioning and planarization process control in accordance with the present invention. CMP system  10  is shown as comprising a polishing pad  12  that is secured to a platen  13 . While platen  13  is illustrated here as being circular, it is to be understood that other systems may use a linear platen, an orbital platen, or any other geometry appropriate for performing the planarization process on a semiconductor wafer surface. A wafer carrier (not shown) is used to secure a wafer-to-be-polished  11  “face down” onto polishing pad  12 . A polisher mechanical controller  20  is used to apply a controlled, downward force on wafer  11  to adjust, as necessary, the pressure applied by surface  11 A of wafer  11  against surface  12 A of polishing pad  12 . A polishing slurry from a dispensing arrangement  14  is dispensed onto surface  12 A of polishing pad  12 . 
     A conditioning apparatus  15  is used, in accordance with the present invention, to evacuate debris, polishing slurry and conditioning agents (hereinafter referred to as “conditioning process effluent”) from polishing pad surface  12 A and perform an analysis on at least a portion of the conditioning process effluent to generate a feedback signal that is sent to at least one of dispensing arrangement  14 , a polisher mechanical controller  20  and/or conditioning apparatus  15 , the feedback signal used to control the planarization process. As described in our co-pending application Ser. No. 10/447,373 filed May 29, 2003 and assigned to the current assignee, a conditioning disk within conditioning apparatus  15  is formed of an abrasive material and contains a number of apertures/openings through the disk. The abrasive material serves to dislodge the debris as it collects on polishing pad surface  12 A. Conditioning “agents”, such as ultra-pure water (UPW) or other flushing liquids, gasses or other types of solid conditioners (including specifically-chosen chemicals) may be dispensed from dispensing arrangement  14  and through conditioning apparatus  15  onto polishing pad surface  12 A to assist in the debris removal process. 
     Referring to the top view of  FIG. 2 , the exemplary CMP system  10  is illustrated as utilizing a motorized effector arm  16  to sweep conditioning apparatus  15  across surface  12 A of polishing pad  12  so as to dislodge the collected debris, while also imparting a predetermined downward force and rotational movement to the conditioning disk. A motor  17  is used in this particular embodiment to both pivot end effector arm  16  in arc AB (or through any other appropriate translational movement) about a fixed shaft  18 , while simultaneously providing rotational motion and applying a downward force to the conditioning disk. Alternatively, a pad conditioner within apparatus  15  may be formed to cover the entire pad radius and not require the use of a motor or the pivoting of an end effector arm to provide across-pad conditioning. As will be discussed below, a “mechanical system” feedback signal from the analysis unit of the present invention may be applied to the various components of conditioning apparatus  15 , polisher mechanical controller  20 , platen  13  or other elements of CMP system  10  so as to control the applied downward force, rotational movement, translational movement and various other mechanical properties of the polishing and conditioning processes. 
     A first hose  21  is illustrated in both  FIGS. 1 and 2  as attached to a vacuum outlet port  22  on conditioning apparatus  15 , such that a vacuum force may be applied through first hose  21  and used to pull the conditioning process effluent from polishing pad surface  12 A. A second hose  23 , attached to an inlet port  19  of conditioning apparatus  15  is coupled to dispensing arrangement  14  and may be used to dispense flushing liquids, UPW or other conditioning agents onto polishing pad surface  12 A. The collected effluent traveling through first hose  21  is then directed into an analysis unit  30 , which is used in accordance with the present invention to evaluate predetermined characteristics of the effluent (for example, determining the concentration of one or more elements within the conditioning process effluent). The output from analysis unit  30 , in the form of an electrical feedback signal, is then applied as an input to a control unit  32 , where control unit  32  generates at least one control signal used to adjust the operation of one or more components of CMP system  10 . For example, a first control signal may be sent to dispensing arrangement  14  and used to control the selection of various polishing slurries and/or conditioning agents, control the flow rate of a dispensed material, control the temperature of a dispensed material, etc. A second control signal may be sent to condition apparatus  15  and perhaps applied as an input to motor  17  of conditioning apparatus  15  so as to control mechanical properties of the conditioning process, such as applied downforce, rotational speed of the abrasive disk, translation speed of effector arm  16 , etc. Other control signals may be applied to, as mentioned above, platen  13  and/or polisher mechanical controller  20 . 
     In general, feedback signal(s) from the analysis of the conditioning effluent is thus used by control unit  32  to adjust the actual planarization process, by varying one or more chemical parameters associated with the delivery of the polishing slurry and/or conditioning agents to the surface of the polishing pad, and/or varying one or more mechanical parameters such as rotational velocity, pressure applied by the conditioner or wafer, vacuum pull through the conditioning disk, etc. For example, the flow rate of the polishing slurry (or a secondary component, such as an oxidizer) may be modified in response to a control signal. Alternatively (or additionally), the temperature of the slurry may be adjusted, the concentration of the abrasive particulate (and/or the size of the actual particulate material) may be changed, the vacuum pressure applied to conditioning apparatus  15 , and/or the downforce applied by wafer  11  against polishing pad  12  may be altered, etc. The temperature of applied conditioning fluids may be modified in response to a signal received by control unit  32  in order to maintain a stable temperature at surface  12 A. Alternatively, a control signal associated with the chemistry of the analyzed effluent may be used by control unit  32  and dispensing arrangement  14  to control the application of a neutralizing agent to overcome reactions associated with a prior-applied polishing slurry. 
     As mentioned above, a significant aspect of the present invention is that the concentration measurement of the conditioning process effluent may be used to perform endpoint detection of the planarization process and actually turn “off” the planarization process.  FIG. 3  contains a graph of an exemplary planarization process where the conductivity of the effluent was measured during a copper CMP process to perform endpoint detection. As shown the conductivity has a first peak C (conductivity of approximately 350 μS) after about 60 seconds of wafer polishing. The conductivity of the effluent then drops a bit, then reaches a second peak D (a conductivity of approximately 508 μS) after about 150 seconds of wafer polishing. After this second peak, the conductivity is seen to rapidly fall off, indicating that the overburden copper has been removed—and that the “endpoint” of the copper planarization process has been reached. 
     As mentioned above, an output signal from control unit  32  may be applied to motor  17  of conditioning apparatus  15  to modify the downforce applied by the conditioning disk against polishing pad surface  12 A, as illustrated in  FIG. 2 . Indeed, this particular control signal may request that the abrasive disk be removed from the conditioning process (i.e., “zero downforce”) if the measured conductivity or concentration of an exemplary effluent component were too high. An adjustment system attached to effector arm  16  is considered to provide the desired precise vertical movement of effector arm  16  in the presence of various force considerations. The adjustment system may include a linear actuator in the form of a double-acting cylinder driven by pressure differential in the cylinder chambers. The double-acting feature enables both sides of a piston flange (not shown) to be alternately pressurized and thereafter translated into bi-directional powered motion of end effector  16  in the vertical direction. Control unit  32  may further receive as an input a force signal corresponding to the linear force measured by the adjustment system. Thus, control unit  32  may use, as part of the second control signal transmitted to conditioning apparatus  15 , a force adjustment signal to control the conditioning pressure applied by the conditioning disk against pad surface  12 A. A separate control signal may used to adjust the position of the double-acting cylinder. Alternatively, the rotational speed of the abrasive disk and/or the translational movement of effector arm  16  may be controlled to either increase or decrease (as desired) the concentration of a particular component within the recovered effluent. Another control signal, applied to platen  13 , can be used to control the rotational speed of platen  13  with respect to the wafer being polished. The mechanical aspects of the polishing process itself (e.g., downforce of the wafer against the polishing pad, rotational velocity of the wafer, etc.) may also be controlled via a signal applied to polisher mechanical controller  20 . 
     It is to be understood that these various examples of potential process control for both the planarization process and conditioning process are exemplary only. Any number of process variations may be made by virtue of studying the effluent collected by the conditioning process, in accordance with the teachings of the present invention. 
     Additionally, there are various arrangements that may be used to implement analysis unit  30 . In one case, an arrangement for measuring the pH of the effluent may be used. For example, when performing planarization of a dielectric layer, potassium hydroxide may be used as the chemical additive in the slurry, where the hydroxide will create water as a by-product of the oxidation phase of the planarization process. Inasmuch as the presence of excess water will affect the pH of the effluent, a measurement of the pH can be used to determine the proper amount of consumed hydroxide so as to allow for a controlled, uniform oxidation-reduction during planarization of the dielectric layer on the wafer. Alternatively, the oxidation potential of the conditioning process effluent may be measured and used to generate a feedback signal. In a further example, particle size within the effluent may be measured and used to generate a feedback signal to adjust the vacuum force or pressure being applied by conditioning apparatus  15 . 
     When using the inventive CMP control process in a metal CMP system (for example), an electrochemical analyzer may be used as analysis unit  30 . An electrochemical analyzer functions to distinguish metal ions of interest from the remaining elements in the effluent, according to a predetermined reduction-oxidation potential, then quantifies the redox potential and metal ion concentration based on predetermined calibration curves. In particular, as the planarization process begins, the amount of metal ions in the effluent will rapidly increase, then reach a plateau value. During a subsequent “soft landing” polishing step (designed to remove the last vestiges of the unwanted metal), the concentration of metal ions in the effluent will be reduced by at least an order of magnitude. At the point where the unwanted metal has been completely removed from the wafer surface, the concentration will again rapidly decrease. Thus, by being able to measure when these changes in concentration occur, the arrangement of the present invention can accurately determine the “endpoint” of the planarization process. An appropriate feedback signal from analysis unit  30  can then be applied to control unit  32  and used to generate a “halt” signal to stop the planarization process and lessen the chance of over-polishing and dishing into the wafer surface. This “halt” control signal may be applied, for example, to dispensing arrangement  14 , polisher mechanical controller  20 , or both. 
     In the case where the surface layer of the semiconductor wafer contains more than one material (such as, for example, an interconnect metal (e.g., copper) and a barrier metal (e.g., tantalum)), a particular embodiment of the present invention can be used to provide control and monitoring of the planarization of each of these materials. In particular, a Raman spectrometer can be used as analysis unit  30  to ascertain the concentration of each material in the effluent. During the planarization process, the relative concentrations of the two metals will change as a function of time. For example, at the beginning of the process, a large amount of copper will begin to be removed from the wafer surface, with virtually no tantalum being present in the wafer debris. Thus, the concentration of copper in the evacuated effluent will be relatively high, with essentially no tantalum being detected. As the process continues, the tantalum will begin to be exposed and the relative concentrations of copper and tantalum in the collected effluent will change accordingly. The feedback output from the Raman spectrometer can then be used by control unit  32  to generate control signals for performing system adjustments, such as adjusting the down pressure applied by the wafer against the polishing pad, or alternatively, changing the chemistry of the slurry once the copper has been removed, modifying the polishing slurry flow rate, temperature, abrasive particulate morphology, etc., as discussed above. Alternatively, the conductivity of the collected effluent may be measured and used as a feedback signal. In any case, by virtue of the collection of effluent occurring in real time (and before it enters the common waste stream), the concentration of various materials in the effluent remain relatively high (on the order of 20–80 times greater than if allowed to combine with the remainder of the waste stream). This higher concentration allows for a more precise analysis of the debris, with a much-improved signal-to-noise ratio over other waste analysis systems of the prior art. 
     While the foregoing description of the implementation of a control path based on collected conditioning process effluent has been described in terms of preferred embodiments, it is to be understood that there exist various modifications that may be made by those skilled in the art that will fall within the scope of the present invention. For example, various other techniques may be used to analyze the conditioning process effluent and control the planarization process. The control signal may also be used as a feedback to the conditioning process itself, modifying parameters such as conditioning agents, vacuum force, abrasive conditioning disk down force, etc. All of these variations are considered to be within the realm of one skilled in the art and the subject matter of the present invention will be limited only by the scope of the claims appended hereto.