Abstract:
A manufactured chemical mechanical polishing pad is annealed after manufacture to improve its operating characteristics. Annealing can stabilize the operational properties of the pad, such as coefficient of thermal expansion and compressibility. In one embodiment, annealing partially or fully completes a curing process that was incomplete after the pad was manufactured.

Description:
BACKGROUND  
         [0001]    1. Technical Field  
           [0002]    An embodiment of the invention relates generally to integrated circuit manufacturing, and in particular relates to devices used for polishing operations during integrated circuit manufacturing.  
           [0003]    2. Description of the Related Art  
           [0004]    During the fabrication of integrated circuits, chemical mechanical polishing (CMP), sometimes also referred to as chemical mechanical planarization, is used to remove controlled levels of surface material from a semiconductor wafer (e.g., partially remove a metal layer previously deposited on the semiconductor wafer) while retaining the required levels of smoothness and flatness on the wafer surface. CMP is typically performed by polishing the wafer with a CMP pad and a chemical slurry. The chemical slurry serves as both a polishing compound and a chemical agent that reacts with the material being polished. To meet sub-micron tolerances in the polishing operation, the pad must exert a certain pressure on the wafer during the polishing. The stability of this pressure is affected by the coefficient of thermal expansion (CTE) of the pad, and by the pad compressibility. Pad compressibility is related to the mechanical modulus of the pad material. (Mechanical modulus is a measure of the stress/strain ratio in a material, a characteristic sometimes loosely referred to as hardness. Compressibility indicates how easily a material may be compressed under pressure. The two are inversely related, i.e., a low mechanical modulus indicates high compressibility, indicating that a given amount of compression requires only a relatively small amount of force.)  
           [0005]    Many CMP pads are formed from a “cake” (a bulk quantity of material with a predetermined size and/or shape) of a polymer-based material, such as polyurethane filled with solid fibers and/or fillers. The cake is sliced into individual pads after the material has been cured. As used herein, “cure” refers to non-reversible chemical and/or physical changes in the material. Unfortunately, the curing process that takes place during manufacture is typically incomplete when the bulk material is sliced into pads, when the pads are sold, and when the pads are used. The degree of cure may vary from one cake to another, and may even vary between pads from the same cake, depending on where in the cake that a given pad was produced. During operational use of the pads, the degree of cure affects the stability of the relevant properties of CTE and compressibility. This results in multiple quality control problems for the pad users: 1) The pads can continue to slowly cure after manufacture, so the relevant properties change while the pads are still on the shelf, making the operational properties of any particular pad unpredictable; and 2) the operating temperature of the pad during use varies due to friction and other effects, and the CTE and mechanical modulus of an uncured pad can vary considerably with temperature, causing unpredictable variations in the polishing operation (e.g., as the CTE changes with temperature, if the rate of expansion/contraction of the pad changes by an undetermined amount, the polishing pressure also changes by an undetermined amount, and the frictional heating due to the polishing pressure will cause undetermined temperature changes and CTE changes, etc.); 3) as the mechanical modulus and compressibility of the pad material vary with temperature, those two parameters change by an undetermined amount, resulting in further unpredictability in pressure and effectiveness; etc.  
           [0006]    Conventional ways of addressing these problems are: 1) to reduce the shelf life of the pads to reduce the range of post-manufacturing curing, 2) to increase the frequency of pad replacement (thereby reducing a pad&#39;s operational lifetime), and/or 3) to reduce the operating cycle time of each pad to minimize operation-induced temperature effects. Each of these solutions is wasteful and expensive. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    The invention may best be understood by referring to the following description and accompanying drawings that are used to illustrate embodiments of the invention. In the drawings:  
         [0008]    [0008]FIG. 1 shows CMP pads at various stages of the manufacturing and annealing process, according to one embodiment of the invention.  
         [0009]    [0009]FIG. 2 shows a flow chart of a method to produce CMP pads, according to one embodiment of the invention.  
         [0010]    [0010]FIG. 3 shows a graph of the affect of annealing temperature on the width of the operational temperature range within which CTE is low, according to one embodiment of the invention.  
         [0011]    [0011]FIG. 4 shows a graph of the affect of annealing time on the width of the operational temperature range within which CTE is low, according to one embodiment of the invention.  
         [0012]    [0012]FIG. 5 shows a graph of the affect of annealing temperature on mechanical modulus, according to one embodiment of the invention.  
         [0013]    [0013]FIG. 6 shows a flow chart of a process to anneal CMP pads in an oven, according to one embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0014]    In the following description, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known circuits, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.  
         [0015]    References to “one embodiment”, “an embodiment”, “example embodiment”, “various embodiments”, etc., indicate that the embodiment(s) of the invention so described may include a particular feature, structure, or characteristic, but not every embodiment necessarily includes the particular feature, structure, or characteristic. Further, repeated use of the phrase “in one embodiment” does not necessarily refer to the same embodiment, although it may.  
         [0016]    In various embodiments of the invention, CMP pads that have already been manufactured are annealed to fully or more fully complete the curing process, resulting in more stable operating characteristics for the pads. The operating characteristics that are more fully stabilized in this manner may include one or both of CTE and compressibility.  
         [0017]    [0017]FIG. 1 shows CMP pads at various stages of the manufacturing and annealing process, according to one embodiment of the invention. FIG. 1 shows a CMP cake  110 , which is sliced into manufactured CMP pads  120 . Annealing is performed in an oven  130  under the control of controller  135 , to produce annealed CMP pads  140 . FIG. 2 shows a flow chart of a method to produce CMP pads, according to one embodiment of the invention. In the following text, FIGS. 1 and 2 are sometimes described in relation to each other. However, it is understood that the embodiment of FIG. 1 may be implemented without using the embodiment of FIG. 2, and the embodiment of FIG. 2 may be implemented without using the embodiment of FIG. 1.  
         [0018]    In the exemplary embodiment described in flow chart  200  of FIG. 2, a CMP cake is produced at block  210 . In one operation, a fluid form of the pad material is poured into a mold and processed until the material solidifies. In the exemplary embodiment of FIG. 1 the cake is cylindrically shaped, but in other embodiments the cakes may have other shapes. The cake may be made of various materials, and may undergo various processes suited to those materials. The cake formation process may use any number of techniques to solidify the material, including one or more of time, heat, and chemical reaction. While in one embodiment a basic material in the cake is polyurethane, in alternate embodiments the basic material may be something else (e.g., polycarbonate). In some embodiments, additional substances are introduced into the basic material to provide the necessary abrasive qualities. In one embodiment solid polymer fibers and/or fillers (material in a non-fiber shape) are added to the basic material while the basic material is in a liquid state to provide an abrasive surface on the finished pad. In another embodiment, gas micro bubbles are introduced into the basic material when the basic material is in the liquid state. The resulting cavities produce a rough surface when the material is hardened and sliced into pads, exposing the bubble cavities. Still other embodiments may use other techniques.  
         [0019]    Returning to FIG. 2, but with reference to FIG. 1, at block  220  the cake is sliced into individual pads  120 . At block  230  the polishing surfaces on each pad are prepared, to produce the necessary roughness and other surface characteristics. In various embodiments, preparation may include one or more of operations including but not limited to sanding, grinding, embossing, grooving, etc. to achieve the desired surface characteristics on the pad&#39;s polishing surfaces. In a particular embodiment in which only one surface of the pad is to be used for polishing, the preparation may be limited to that one surface. If the method of slicing produces a surface with the necessary qualities, preparation of the polishing surfaces may be unnecessary and block  230  may be skipped.  
         [0020]    While in one embodiment the pads are considered manufactured after the slicing operation, in another embodiment the pads are considered manufactured after the polishing surfaces are prepared.  
         [0021]    At block  240 , the pads  120  are annealed by placing them into an oven  130  and heating the pads  120  at a predetermined temperature for a predetermined time. Controller  135  may be used to control the temperature and/or time of heating in oven  130 . Operation of controller  135  may include, but is not limited to, one or more of mechanical, electrical, electronic, and programmable means to control the temperature and/or time of annealing. After annealing, the annealed CMP pads  140  are provided for CMP usage at block  250 .  
         [0022]    [0022]FIG. 3 shows a graph of the affect of annealing temperature on the width of the operational temperature range within which CTE is low, according to one embodiment of the invention. The effect of annealing on the CTE of manufactured pads is affected by the annealing temperature and the time at which the pad is exposed to that temperature. These parameters are further affected by the material used in the pad and by the amount of cure that previously took place when the cake was formed. The measurements shown for the example embodiments of FIG. 3- 5  are based on commercially available polyurethane pads with solid non-organic fillers, but other embodiments with other pad materials may produce different characteristic curves. For example, other embodiments may include organic fibers, organic fillers, and/or non-organic fibers. Fillers may be considered material in which the length-to-width ratio of individual particles is approximately ‘1’, while individual particles of fiber may have a length-to-width ratio greater than that value.  
         [0023]    In the example embodiments of FIGS. 1 and 2, CTE is very low, and may even approach zero, within a range (a ‘thermal window’) of the operating temperatures that the pad will likely experience during a polishing operation. Such operating temperatures typically fall between about 25 and 75° C. A pad with a wider thermal window can operate over a wider portion of these typical operating temperatures without experiencing the detrimental effects caused by excessive expansion/contraction of the pad. As shown in FIG. 3, the width of this thermal window for a given pad varies with the amount of annealing. To provide a common standard for pads of different sizes and manufacturing techniques, FIGS. 3 and 4 show the percentage change in the width of the thermal window (as compared with the width for a non-annealed pad) rather than the measured width in ° C. In the exemplary embodiment of FIG. 3, measurements were taken for pads that were annealed for one hour at 70° C., 110° C., 150° C., and 190° C., as well as for a non-annealed pad kept at room temperature (RT). The remaining portion of the curve was extrapolated from these data points.  
         [0024]    In the example embodiment of FIG. 3, annealing the pads at a temperature of approximately 110° C. maximizes the range of operating temperatures within which CTE remains low. In some embodiments, annealing at approximately 110° C. produces pads having a low CTE across the entire operating range of 25° C. -75° C. Other embodiments may include pads that have other optimum annealing temperatures.  
         [0025]    [0025]FIG. 4 shows a graph of the affect of annealing time on CTE, according to one embodiment of the invention. The annealing temperature for these pads was 110 ° C. Like FIG. 3, the vertical axis shows a percentage change in the width of the thermal window of operating temperatures in which CTE is acceptably low. In the example embodiment, the maximum thermal window is achieved with an annealing time of about 8 hours, and annealing beyond that time gradually decreases the width of the thermal window.  
         [0026]    [0026]FIG. 5 shows a graph of the effect of annealing temperature on mechanical modulus, according to one embodiment of the invention. The vertical axis of the graph represents the maximum variation in mechanical modulus as the operating temperature of the pads varies across the range of normal operating temperatures. Actual tests were conducted on pads that had been annealed for one hour at 70° C., 110° C., 150° C. and 190° C., as well as a non-annealed pad kept at room temperature (RT) until the test. Intermediate annealing temperatures have been extrapolated from these points. As can be seen from the graph, the lowest variation in mechanical modulus occurred in pads that were annealed at a temperature of approximately 80° C., with pads annealed at temperatures above that optimum temperature also showing lower variation than non-annealed pads.  
         [0027]    As seen from FIGS. 3, 4 and  5 , a single combination of annealing time/temperature may not optimize both CTE and compressibility. But depending on the overall requirements for polishing performance, an annealing time/temperature to produce best overall results for each type of pad may be determined with minimal testing. In the example embodiments of FIGS. 3, 4, and  5 , a variation of plus or minus 10° C. in temperature and plus or minus one hour in time does not produce significantly different results, so one embodiment uses an annealing temperature of between approximately 100-120 degrees ° C., and an annealing time of between approximately 7-9 hours. Other embodiments may use other times and/or temperatures.  
         [0028]    The invention may be implemented in one or a combination of hardware, firmware, and software. The invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by at least one processor to perform operations described herein. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computer). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other form of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others.  
         [0029]    [0029]FIG. 6 shows a flow chart of a process to anneal CMP pads in an oven, according to one embodiment of the invention. The process of flow chart  600  may be implemented through any number of techniques, such as but not limited to one or more of: 1) manual control of the oven, 2) automated control of the oven (e.g., with a computerized oven controller), 3) a combination of manual and automated controls, etc.  
         [0030]    Different types of pads (e.g., different materials, different manufacturing processes, different sizes, etc.) may produce different characteristic curves than those shown in FIGS. 3, 4 and  5 . Based on this information, the preferred parameters of annealing time and annealing temperature for different pads may be determined through testing and stored for reference. The correct parameters may then be retrieved and used as needed. At block  610 , parameters are obtained that identify the annealing characteristics for the particular CMP pads to be annealed. While in one embodiment these time and temperature parameters are provided by a human operator through a keyboard or other input device, in another embodiment these time and temperature parameters are provided through an input port from a device external to the oven controller. Blocks  620 ,  622  and  624  pertain to three different types of identifying parameters. In block  620 , time and temperature parameters for the oven are input directly.  
         [0031]    In block  622 , one or more ‘type’ parameters are obtained, identifying the type of pad to be annealed. This information may be in the form of brand name, model number, or other parameter that identifies a particular type of pad. At block  632 , the type parameter may be used to look up a corresponding entry in a table that contains the time and temperature parameters for various types of pads.  
         [0032]    In block  624 , indirect pad parameters are obtained. These parameters may define such things as the material used in the pad, the pad&#39;s dimensions, and other material characteristics that affect the desired annealing parameters, regardless of the brand or model of the pads. At block  634 , these indirect parameters may be used to look up a corresponding entry in a table that contains the time and temperature parameters for pads with various characteristics. Alternately, indirect parameters may also be used to calculate time and temperature parameters using one or more predetermined algorithms.  
         [0033]    The approaches described in blocks  620 ,  622 / 632 , and  624 / 634  represent three different ways of obtaining time/temperature setting for the annealing process. While in one embodiment an oven controller is designed to receive parameters in only one of these methods, in another embodiment two or more of these may be incorporated into a single oven controller that permits multiple forms of input. At block  640 , the temperature parameter derived from the appropriate combination of blocks  620 ,  622 / 632 , and  624 / 634  is applied to heat up the oven.  
         [0034]    Although not shown in flow chart  600 , the pads may be placed in the oven at any time prior to block  650 . While in one embodiment this is done manually by a human operator, in another embodiment the pads are placed into the oven through automated means.  
         [0035]    At block  650  a timer is started to determine when the pads have been annealed for the proper amount of time, with the expiration of the annealing time determined at block  660 . While in one embodiment a mechanical timer is used, in another embodiment an electronic or software timer is used. While in one embodiment the annealing time includes predetermined heatup and cool down times, in another embodiment the pads are placed into, and removed from, the oven while the oven is at the indicated annealing temperature. In still another embodiment, the annealing process includes multiple sequential annealing stages, each with its own predetermined time and temperature parameters. Each stage may include predetermined heat up and cool down times. When multiple stages are used, multiple timeouts may be experienced at block  660 . Regardless of whether one or multiple annealing stages are used, when the final timeout occurs, the annealing process for the current pads is ended at block  670 .  
         [0036]    In another embodiment not shown, instead of placing a fixed number of pads into a stationary oven, a conveyer belt may move a continuous supply of pads through a tunnel-shaped oven. The conveyor speed may be set to control the annealing time by controlling how long it takes a given pad to travel through the oven. Different stages may be achieved by using thermal dividers to segment the oven into different temperature zones having different lengths.  
         [0037]    The foregoing description is intended to be illustrative and not limiting. Variations will occur to those of skill in the art. Those variations are intended to be included in various embodiments of the invention, which are limited only by the spirit and scope of the appended claims.