Abstract:
A method and apparatus for optical multi-angle in situ CMP endpoint detection include a sensor block having light emitting channels, light receiving channels and an opening where the light emitting channels terminate and the light receiving channels originate and means for determining endpoint based on the amount of reflected light that is received from the light receiving channels. At least a portion of the sensor block is embedded in a polishing pad backer such that the light emitting channels can emit light through a polishing pad window to the surface of a wafer and the light receiving channels can receive light reflected from the wafer surface through the polishing pad window. Connectors may be used to connect a light source to the light emitting channels and a light detector to the light receiving channels. Further, fiber optic cables may be used between the light emitting channels and the light receiving channels and their respective connectors in order to facilitate transporting emitted and reflected light to the light source and light detector which are each positioned.

Description:
FIELD OF INVENTION  
         [0001]    The present invention generally relates to a method and apparatus for optical in situ endpoint detection during chemical mechanical planarization (CMP). More particularly, the present invention relates to a method and apparatus for optical multi-angle in situ endpoint detection for CMP.  
         BACKGROUND OF THE INVENTION  
         [0002]    Planar surfaces are required in the manufacture of semiconductors because the size of the devices and interconnects used to build semiconductors continue to rapidly decrease. Therefore, if the surface of a wafer is not planar during the semiconductor fabrication process, the risk of producing failed devices increases. The chemical mechanical planarization of workpieces, and particularly wafers, usually involves pressing the surface of a wafer against a polishing surface, typically a polishing pad, that is attached to a rotating or orbiting platen in the presence of a slurry. During the planarization process, data relating to the condition of the wafer&#39;s surface is often recorded in order to determine endpoint, i.e. when the polishing should be stopped or interrupted. In-situ systems are generally preferred in determining endpoint.  
           [0003]    A common optical technique for determining endpoint involves the process of reflecting light off the surface of a wafer and capturing the reflected light by a properly positioned receptor. The receptor transmits the reflected light through a fiber optic cable to a metrology instrument that analyzes data. Examples of optical endpoint methods measuring in-situ reflectivity can be seen in U.S. Pat. No. 5,433,651 and PCT International Publication Number WO 99/23449.  
           [0004]    However, optical endpoint detection does have several drawbacks or disadvantages. First, wafers are typically planarized face down on a polishing pad which makes it difficult to achieve direct optical communication with the surface of the wafer. Therefore, in order to detect the wafer so that measurements can be taken, holes, windows or transparent areas must be manufactured into the polishing pad, or alternatively, the wafer must travel over the edge of the polishing pad. Both of these options possess the risk of introducing undesirable variables.  
           [0005]    Second, slurry is generally used in CMP and in some cases the slurry may totally block the optical signal. Third, the constant relative motion between the wafer and the polishing pad makes it difficult to take repeatable measurements at the same point on a wafer&#39;s surface. Fourth, measurements of the wafer&#39;s surface must be taken and analyzed quickly in order to utilize results.  
           [0006]    Optical systems including sources and detectors are typically positioned in the platen of a CMP apparatus to perform endpoint detection through a window contained in the polishing pad. To reduce downtime in the event of an error or malfunction in the optical system, it is desirable to place the optical system in a position within the CMP apparatus that is more easily accessible for repair and replacement. Furthermore, given the limitations that exist with respect to optical endpoint determination, there is a need to optimize accuracy and increase efficiency of endpoint detection using optics. One such method for optimizing endpoint detection includes the use of a multiple emitter-detector sensor assembly which involves emitting multiple angles of light on a wafer surface and detecting multiple angles of light reflected from the wafer surface. Positioning this type of optimal sensor in a position within the CMP apparatus and close to the wafer surface where it is easily accessible further optimizes the endpoint detection process. An example of one such location within the CMP apparatus is within a polishing pad backer.  
           [0007]    Accordingly, there is a need for a method and apparatus which involves positioning a multiple angle sensor within a polishing pad backer without adversely affecting process performance and without requiring costly redesigns of the polishing pad backer.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention enhances “platen based” endpoint detection, where platen refers to a traditional hard platen or flexible platen, to provide state of the art equipment in through the table endpoint detection. In order to carry this out, a multi-angle sensor assembly is inserted or positioned within a polishing pad backer that utilizes a polishing pad having a window for endpoint detection. The present invention experiences negligible adverse impact on process performance. In addition, the present invention may take the form of an integrated sensor assembly that can be easily manufactured and which produces repeatable performance.  
           [0009]    The entire endpoint detection assembly is shrunk in an on-lay process to fit a multi-angle sensor assembly within a limited amount of space within the polishing pad backer and thereby avoids costly redesigns of the polishing pad backer. More specifically, optical channels are formed within a sensor block and, using an in-lay process, optical waveguides are formed that couple light from a light emitting means, to a through-hole window in a polishing pad, and then to a light detecting means via light emitting channels and light detecting channels positioned in the sensor block.  
           [0010]    One exemplary embodiment of the apparatus of the present invention for CMP in situ endpoint detection includes a sensor block positioned in a polishing pad backer which has at least one light emitting channel and at least one light receiving channel and means for determining the CMP endpoint based on the amount of light received by the light receiving channel.  
           [0011]    The sensor block may be formed by molding a housing assembly having two halves where each half contains a set of grooves and then securing the two halves together so that their grooves lie adjacent to one another to form the two types of channels, namely the light emitting channels and the light receiving channels.  
           [0012]    In one aspect of the invention, the sensor block includes an opening where the light emitting channels terminate and the light receiving channels begin. The opening may be filled with an optically clear material and is placed directly beneath the window contained in a polishing pad during polishing.  
           [0013]    In another aspect of the invention, the light emitting and receiving channels are fiber optic cables or, alternatively, they are coated with a reflective coating and filled with an optically clear material. Connectors may also be used for connecting means for emitting light to the light emitting channels and means for detecting light to the light receiving channels.  
           [0014]    In still another aspect of the invention, fiber optic cables embedded in the polishing pad backer are used to connect the light emitting channels with the light emitting means and the light receiving channels with the light detecting means.  
           [0015]    In another exemplary embodiment of the endpoint detection apparatus of the present invention, a sensor assembly is formed from a single mold where one portion of the sensor assembly is embedded in the polishing pad backer directly beneath the polishing pad window and another portion of the sensor assembly which contains light emitting and light receiving channels is positioned behind the polishing pad backer such that it lies between the polishing pad backer and the backing plate.  
           [0016]    In an exemplary method of the present invention for in situ endpoint detection during CMP, a sensor block having at least one emitting and detecting channel is positioned in a polishing pad backer, a polishing pad having a window is positioned over the pad backer so that ends of the light emitting and detecting channels lie adjacent the window, a light is emitted through the light emitting channel during CMP, light reflected from the wafer surface during CMP is detected from the light detecting channels, and endpoint is determined based on the light reflected from the wafer surface. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    A more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in connection with the following illustrative Figures. In the following Figures, like reference numbers refer to similar elements throughout the Figures.  
         [0018]    [0018]FIG. 1 is a side view of a sensor block embedded in a polishing pad backer in accordance with the apparatus of the present invention;  
         [0019]    [0019]FIG. 2 is a cross-sectional view of an exemplary embodiment of a sensor block in accordance with the apparatus of the present invention;  
         [0020]    [0020]FIG. 3 is a top view of a molded, partially formed sensor block in accordance with the exemplary embodiment of a sensor block of the present invention shown in FIG. 2;  
         [0021]    [0021]FIG. 4 is a schematic of an exemplary embodiment of the apparatus of the present invention for in situ endpoint detection during CMP;  
         [0022]    [0022]FIG. 5 is a schematic of another exemplary embodiment of the apparatus of the present invention for in situ endpoint detection during CMP; and  
         [0023]    [0023]FIG. 6 is a flowchart showing an exemplary method for making an apparatus of the present invention for in situ endpoint detection during CMP. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0024]    It should be understood that the particular embodiments shown and described are illustrative of the invention and its best mode and are not intended to otherwise limit the scope of the present invention in any way. The apparatus of the present invention includes a sensor block which is embedded in a polishing pad backer. FIG. 1 shows a side view of a sensor block  12  embedded in a polishing pad backer  14  that is contained between a polishing pad  16  and a backing plate  18 . Sensor block  12  includes a first side  20  which houses channels for transporting emitted light (see FIG. 2) and a second side  22  which houses channels for transporting light reflected from the wafer surface (see FIG. 2). An opening  24  in sensor block  12  traverses both sides  20  and  22  and is placed directly beneath the window  26  contained in polishing pad  16  during CMP.  
         [0025]    A cross-section of sensor block  12  in FIG. 1 is shown in FIG. 2. FIG. 2 shows that sensor block  12  is a nine emitter-detector sensor in that it includes nine light emitting channels  28  in side  20  and nine light receiving or detecting channels  30  in side  22 . Light emitting channels  28  transport light to opening  24  in sensor  12 . Accordingly, light emitting channels  28  terminate at opening  24  and the light transported through emitting channels  28  travels through opening  24  and is directed through window  26  of polishing pad  16  to the wafer surface (not shown). Similarly, light receiving channels  30  transport light reflected from the wafer surface to a light detector (not shown). Light reflected from the wafer surface is directed through opening  24  of sensor  12  and into light receiving channels  30 .  
         [0026]    Various methods may be used to form sensor block  12 . One exemplary method involves molding a housing assembly having first and second halves containing grooves. The halves are then aligned and secured to one another to form two different types of channels, namely light emitting channels  28  and light receiving channels  30  as shown in FIG. 2.  
         [0027]    [0027]FIG. 3 shows a top view of a molded, partially formed nine emitter-detector sensor of sensor block  12  shown in FIG. 2. A housing assembly  40  having first and second halves  41  and  43  is molded from one piece. The one piece housing assembly mold is preferably comprised of a plastic material such as modified acrylics, urethanes, Teflon fluoropolymers and other like materials that are lightweight and flexible, and possess an ANSI durometer 40-80 Shore D. However, it should be noted that the hardness of the plastic material should be comparable to that of the surrounding material. For example, if the housing is embedded in a rigid platen, the plastic material may be considerably harder than that used if the housing were embedded in a pad backer. The one piece housing assembly  40  is molded so that grooves  45  are formed in both first and second halves  41  and  43  for forming channels  28  and  30  (see FIG. 2). Fiber optic cables  47  may be placed in the grooves  45  of either first and second halves  41  and  43 . The dark lines in FIG. 3 represent fiber optic cables  47  that have been placed in grooves contained in first half  41 . Grooves  45  contained in second half  43  of housing  40  then function as mating grooves for the fiber optic cables  47 . Each of first and second halves  41  and  43  of housing  40  contain grooves that will form both light emitting channels and light receiving channels. Once the fiber optic cables  47  are secured in grooves contained in one half of housing  40 , the entire housing  40  is folded so that first and second halves  41  and  43  are adjacent one another. The resulting sensor block  12  includes light emitting channels in side  20  and light receiving channels in side  22 . Dotted lines  49  indicate the directionality of the lines  47 .  
         [0028]    In another exemplary embodiment of the sensor block  12  of the apparatus of the present invention, grooves  45  are formed in both first and second halves  41  and  43  as in the previously described exemplary embodiment, but unlike the previously described exemplary embodiment, fiber optic cables are not placed into grooves  45 . In stead, grooves  45  contained in both first and second halves  41  and  43  are coated with a reflective coating such as aluminum, gold, aluminum coated with a dielectric such as SiO 2 , copper coated with an oxidation inhibitor or film stacks with the property of total internal reflection, and like materials or combinations of materials that function to reflect light from the walls of grooves  45 . Once coated, grooves  45  are then filled with an optically clear plastic material such as epoxy, polycarbonate, silicone, acrylic polycarbon, polymethylmethacrylate (PMMA) or like materials.  
         [0029]    Turning now to FIG. 4, a schematic of an exemplary embodiment  50  of the apparatus of the present invention for in situ endpoint detection is shown. FIG. 4 shows an embedded integrated sensor block  52  in a polishing pad backer  54  with two sets of nine fiber optic cables  55  leading to bulk connectors  57  and  59  which are positioned on opposite sides of the pad backer  54 . Sensor block  52  is embedded in pad backer  54  and, like pad backer  54 , is sandwiched between polishing pad  56  and backing plate  58 .  
         [0030]    Sensor block  52  comprises a nine emitter-detector sensor like that previously described with reference to FIGS. 2 and 3. Sensor block  52  includes first and second sides  60  and  62  which each include nine light emitting channels  68  and nine light receiving channels  70 , respectively. Sensor block  52  also includes an opening  64  traversing both first and second sides  60  and  62  such that light emitting channels  68  terminate at opening  64  and light receiving channels  70  begin at opening  64 . When the in situ endpoint detection system of the present invention is employed, waveguides are formed which couple light from a light source  72 , to connector  57 , to a first set of fiber optic cables  55 , to light emitting channels  68  in sensor block  52 , to opening  64  in sensor block  52 , through window  66  in polishing pad  56 , and onto a wafer surface that is being polished, and also couple light reflected from the wafer surface back through window  66  in polishing pad  56 , back through opening  64  in sensor block  60 , into light receiving channels  70  contained in sensor block  52 , into a second set of fiber optic cables  55 , to connector  59  and to light detector  75 .  
         [0031]    Fiber optic cables  55  can be placed in trenches in the pad backer  54  but are preferably encased in a compliant, protective sheath that terminates in either connector  57  or  59  located just outside the pad backer  54 . Connectors  57  and  59  are typically comprised of hard plastics and/or non-corrosive metal for a CMP environment. Many different types of connectors can be obtained from AMP, Amphenol, and Molex corporations. Light source  72  preferably comprises a semiconductor laser or other light source capable of providing monochromatic or whitelight while light detector  75  preferably comprises a laser detector or whitelight detector.  
         [0032]    A schematic of another exemplary embodiment  80  of the apparatus of the present invention for in situ endpoint detection is shown in FIG. 5. Exemplary embodiment  80  is directed to an integrated sensor assembly  82  which includes connectors  57  and  59 , fiber optic cables  55 , and sensor block  52  which contains light emitting channels  68 , opening  64  and light receiving channels  70 . (see FIGS. 2 and 4) This integrated sensor assembly  82  is formed from a single mold and inserted into the back of the pad backer  54  prior to mounting the backing plate  58  to the integrated sensor assembly and pad backer  54 . A portion  84  of integrated sensor assembly  82  which includes part of sensor block  52  having opening  64  is embedded in pad backer  54  immediately underneath window  66  in polishing pad  56 . The rest of integrated sensor assembly  82  is positioned behind pad backer  54 . Fiber optic cables  55 , light emitting channels  68  and light receiving channels  70  may all be formed like the light emitting channels and light receiving channels previously described with reference to FIGS. 2, 3 and  4 . Light source  72  and light detector  75  are positioned outside of the polishing pad backer  54  .  
         [0033]    [0033]FIG. 6 shows a flowchart for an exemplary method  100  for making the CMP in situ endpoint apparatus of the present invention. First, in step  102 , a housing assembly is molded from a single piece of material that has first and second halves with each half having grooves formed within it. Next, the first and second halves of the molded single piece are folded together in step  104  after placing fiber optic cables in the grooves or coating them with a reflective coating. The halves are secured to one another so that the grooves contained in each of the halves are adjacent to one another thereby forming channels within the single molded piece. The channels formed in the single molded piece include both light emitting channels and light receiving channels. Next, in step  106 , at least a portion of the assembled housing is embedded in a polishing pad backer such that ends of the light emitting channels and ends of the light receiving channels are positioned near a window in a polishing pad such that light emitted from the light emitting channels can be directed to a wafer surface and light reflected from the wafer surface can be received by the light receiving channels. A light source is then connected to the light emitting channels in step  108  so that light can be emitted through the light emitting channels and a light detector is connected to the light receiving channels in step  110  so that light reflected from the wafer surface can be directed to the light detector through the light receiving channels.  
         [0034]    Alternatively, instead of steps  108  and  110 , the following steps may be performed subsequent to embedding at least a portion of the assembled housing into a polishing pad backer in step  106 : a) the light emitting channels are secured to a first connector positioned outside of the pad backer in step  112 , b) a light source is connected to the first connector in step  114  such that light emitted from the light source is transported to the wafer surface through the first connector and the light emitting channels, c) the light receiving channels are connected to a second connector positioned outside of the pad backer in step  116 , and d) the second connector is connected to the light receiving channels in step  118  so that light reflected from the wafer surface is transported to the light detector through the light receiving channels and the second connector.  
         [0035]    The present invention has been described above with reference to exemplary embodiments. However, those skilled in the art having read this disclosure will recognize that changes and modifications may be made to the exemplary embodiments without departing from the scope of the present invention. these and other changes or modifications are intended to be included within the scope of the present invention, as expressed in the following claims.