Abstract:
An apparatus and method for conditioning a polishing pad used in a chemical mechanical polishing (CMP) process. The polishing pad is conditioned by the application of a conditioning device to the surface of the rotating polishing pad. The amount of force which is applied to the conditioning device is directly controlled by a force control mechanism so as to make the conditioning process more consistent.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for conditioning polishing pads for polishing semiconductor wafers. 
     DESCRIPTION OF THE RELATED ART 
     Chemical mechanical polishing (CMP) is an essential process in the production of integrated circuits (ICs). CMP is used to refine the surfaces of semiconductor wafers during fabrication. This process, known as planarization, serves to remove any excess or unwanted material from the surface of the wafer, and thus allows more circuits to be created on each wafer. The polishing is typically accomplished by applying the semiconductor wafer to a rotating polishing pad. The wafer is typically attached to a stationary shaft which is driven against a rotating polishing platen which has the polishing pad affixed to its upper surface. In alternative configurations, the drive shaft and semiconductor wafer may also be rotated. In most conditioning processes, a slurry (i.e. chemical liquid) is added to the surface of the polishing pad in order to assist in the polishing process. The slurry usually contains a polishing agent, such as alumina or silica, as well as various other chemicals which serve to etch or oxidize specific portions of the wafer during polishing. 
     A principal problem which occurs during the polishing process is the phenomenon known as “glazing.” Glazing occurs when abrasive particles from the polishing slurry and the semiconductor wafers become embedded in the surface of the polishing pad. Often glazing results in a significant reduction in the efficiency of the polishing pad. 
     In addition to glazing, the polishing pad also often becomes worn in certain areas due to extended use. This wear also impacts on the effectiveness of the polishing process. Since the slurry is held by small depressions in the surface of the polishing pad, when areas become worn, the slurry is no longer effectively held, and the polishing process suffers. 
     In order to restore the polishing pad to its optimum condition, various “conditioning” processes are employed in the prior art. Conditioning is a process by which the polishing pad is treated with a conditioning device to increase its lifetime. Most conditioning processes use a conditioning head pressed against the polishing pad to accomplish the conditioning. The conditioning head usually includes an abrasive surface, for example, diamonds embedded in a nickel plating. The abrasive surface of the conditioning head is driven against the polishing pad in much the same way as the semiconductor wafers are during polishing. The conditioning head removes excess particulate material from the surface of the polishing pad and roughens (i.e. places new depressions) in worn areas, thereby restoring the polishing pad to its optimum condition. 
     There are two basic types of conditioning processes: in-situ and ex-situ. In-situ conditioning processes condition the polishing pad at the same time that wafers are being polished. Essentially, two separate heads, one for polishing and one for conditioning, overlie the polishing pad simultaneously. An ex-situ conditioning process takes place in between wafer polishings. In an ex-situ-process, only one of the conditioning and polishing heads overlies the polishing pad at any one time. Generally, in-situ conditioning processes are favored because valuable polishing time is not wasted on conditioning. However, in-situ processes often experience problems because particulate material removed by the conditioning head often strays onto the polishing portion of the polishing pad, thereby interfering with the polishing process. 
     One of the main problems experienced by both in-situ and ex-situ conditioning processes is a lack of consistency in the amount of pressure applied to the conditioning head during conditioning. The amount of pressure applied to the conditioning head is directly related to the amount of conditioning which is accomplished. Thus, the more pressure that is applied to the conditioning head, the more vigorous the conditioning process will be, and vice versa. Too much or too little conditioning can result in decreased lifetime for polishing pads. Therefore, there is currently a need for a conditioning process which accurately and efficiently controls the amount of conditioning which the polishing pad experiences. 
     FIG. 1 shows a prior art conditioning device generally designated by reference numeral  100 . The device  100  is an example of an ex-situ conditioning device, however, the following explanation applies equally as well to an in-situ conditioning device. The device  100  includes a polishing platen  110 , a polishing pad  140 , a conditioning head  130 , conditioner  170 , and a support arm  190 . The conditioning head  130  is supported by a first rotatable shaft  137 , which is rotated about axis A 1 , by first drive motor  180 . The polishing platen  110  is supported by a second rotatable shaft  120 , which is rotated about axis A 2  by second drive motor  150 . The conditioner  170  is held to the conditioning head  130  by a retaining member (not shown), such as bolts, glue, or magnets. Preferably, the conditioner  170  is held to the conditioning head with bolts, so that the conditioner  170  may be easily changed or replaced. The support arm  190  performs a dual function, it serves to rotate the conditioning head  130  onto and off of the polishing pad  140 , and it also serves to force the conditioning head  130  against the polishing pad  140 . Since the device  100  is an ex-situ device, the conditioning head  130  only overlies the polishing pad  140  when conditioning is required. The conditioning head  130  is rotated on and off of the polishing pad  140  by rotation of the support arm  190  about axis A 3 . If the device  100  were in-situ, the conditioning head  130  would overly the polishing pad at all times, even during polishing. 
     The forcing of the conditioning head  130  against the polishing pad is accomplished by displacing a first shaft portion  192  of the support arm  190  in the vertical direction. Note that the first shaft portion  192  lies inside a second shaft portion  194  of the support arm  190 . The second shaft portion  194  allows the first shaft portion  192  to be displaced within the second shaft portion  194  to thereby force the conditioning head  130  against the polishing pad  140 . A control circuit (not shown) controls the vertical displacement of the first shaft portion  192  in the holder  194 . This displacement of the first shaft portion  192  causes the conditioning head  130  to either be pressed against the polishing pad  140  or removed from it, depending on the direction of displacement. For example, an upward movement of the first shaft portion  192  moves the conditioning head  130  away from the polishing pad  140 , whereas a downward movement moves the conditioning head  130  closer to the polishing pad  140 . The amount of displacement directly controls the amount of conditioning which the polishing pad  140  will experience. Thus, as the conditioning head  130  is pressed more firmly against the polishing pad  140 , more particles are cleared away and more depressions are formed in the polishing pad. In order to optimize the conditioning process, it is necessary to accurately control the force applied to the conditioning head  130 . 
     SUMMARY OF THE INVENTION 
     The present invention is a method and apparatus for improving the process for conditioning a polishing pad. A stationary support arm and a force control mechanism accomplish the conditioning. The force control mechanism is attached to the conditioning head and is used to raise and lower the head with respect to the polishing pad. The force control mechanism comprises a force control mechanism, such as a piston or magnet, which accurately controls the amount of force applied to the conditioning head. 
     The above and other advantages and features of the present invention are better understood from the following detailed description of the preferred embodiments of the invention which is provided in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a prior art conditioning apparatus. 
     FIG. 2 illustrates the present invention where a force control mechanism is designated generically. 
     FIG. 3 illustrates a first embodiment of the present invention. 
     FIG. 4 illustrates a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Since the prior art conditioning devices use a “translated” force (i.e. the force of the displacement of a shaft translated through a support arm to the conditioning head) to press the conditioning head against the polishing pad, they are often not consistent in their conditioning of the polishing pad. The present inventors have discovered that the pressure applied to the conditioning head can be more accurately controlled by a force control mechanism coupled to the conditioning head. By controlling the conditioning head pressure directly, a more consistent conditioning process can be achieved. 
     The present invention comprises an apparatus and method for conditioning a polishing pad for polishing semiconductor wafers. The present invention utilizes a force control mechanism coupled to a conditioning head to apply the head to the polishing pad in a consistent manner. According to a first embodiment, the force is controlled by a hydraulic or pneumatic mechanism attached to the conditioning head. In a second embodiment, the force is controlled by oppositely polarized magnets located in the conditioning head and the polishing platen, respectively. 
     FIG. 2 shows a conditioning device  200  according to the present invention. For the ease of discussion, the force control mechanism is designated generically by reference numeral  260 . Examples of the pneumatic/hydraulic and magnetic force control mechanisms are shown in FIGS. 3-4. The conditioning device  200  includes a polishing platen  210 , a polishing pad  240 , a conditioning head  230 , a support shaft  290 , and a force control mechanism  260 . The conditioning head  230  is supported by a first rotatable shaft  237 , which is rotated about axis A 1  by first drive motor  280 . The polishing platen  210  is supported by a second rotatable shaft  220 , which is rotated about axis A 2  by second drive motor  250 . A conditioner  270  for conditioning the surface of the polishing pad  240  may be formed of a disc of diamond impregnated nickel material. The conditioner  270  is held to the conditioning head  230  by a retaining member (not shown), such as bolts, glue, or magnets. Preferably, the conditioner  270  is held to the conditioning head  230  with bolts, so that various conditioners (e.g. brushes, discs made of different materials) may be easily inserted and replaced. The force control mechanism  260  is coupled to the conditioning head  230  so that force created therein propels the conditioning head against the polishing pad  240 . In contrast to the prior art conditioning device  100  shown in FIG. 1, the conditioning device  200  includes a stationary support shaft  290  which does not allow displacement in the vertical direction. The force control mechanism  260  instead controls the force exerted on the conditioning head  230 . 
     The operation of the conditioning device  200  is next described with respect to a ex-situ conditioning process, however, conditioning processes as according to the present invention could also be performed in-situ. Typically, in an ex-situ conditioning process, the conditioning head  230  is kept off the polishing pad  240  until conditioning is required, at which point the conditioning head is brought into contact with the polishing pad to perform the conditioning. The support shaft  290  is rotatable about an axis A 3  to move the conditioning head  230  on and off the polishing pad  240 . Thus, in the present invention, when conditioning of the polishing pad  240  is required, the conditioning head  230  is rotated to a position over the polishing pad  240  by rotation of the support arm  290  about axis A 3 . When the conditioning head  230  lies overtop the polishing pad  240 , as shown in FIG. 2, the conditioning process is ready to begin. At this point a control circuit (not shown) sends control signals to the force control mechanism  260  to cause the force control mechanism to create a downward force  235  on conditioning head  230 . This downward force  235  pushes the conditioner  270  of head  230  into contact with the rotating polishing pad  130  to begin the conditioning process. The abrasive surface of the conditioner  270  (e.g. diamond impregnated nickel) causes extraneous particles located on the surface of the polishing pad  240  to be stripped away. The abrasive surface of the conditioner  270  also creates depressions in areas of the polishing pad  240  which are worn. The creation of these depressions allows the polishing pad  240  to hold more polishing slurry (not shown) and to perform improved polishing. Although diamond impregnated nickel is a preferred material for the conditioner  270 , other materials such as silicon carbide and the like are also usable. Further, alternatively to the diamond impregnated disc described above, a brush or other abrasive object may also be used for conditioner  270 . In fact, any abrasive equivalent means known to those skilled in the art may be used for conditioner  270 . 
     In order to condition the entire surface of the polishing pad  240 , the polishing platen  210  is displaced in different directions while it is rotating by movement of rotatable shaft  220 . Once conditioning of the entire polishing pad  240  has been completed, control signals are sent to the force control mechanism  260  to create an upward force to draw the conditioning head  230  away from the polishing pad  240 . Finally, the conditioning head  230  is rotated away from the polishing pad  240  by rotation of support shaft  290  so that wafers can again be polished. 
     The force control mechanism  260  can be formed in many different ways, and by many different combinations of elements. For example, according to a first embodiment of the present invention, the mechanism  260  comprises a pneumatic or hydraulic device such as a piston. In a second embodiment, the force control mechanism  260  comprises a magnetic device. 
     FIG. 3 shows a first embodiment of the present invention where the force control mechanism  260  comprises a pneumatic or hydraulic mechanism  262 . The device  200 ′ shown in FIG. 3 has similar components to the device  200  shown in FIG. 2, and like reference numerals denote like elements. In order to force the conditioning head  230  against the polishing pad  240 , the mechanism  262  creates a force which is translated directly to the conditioning head. The forcing of air or hydraulic fluid into chamber  263  causes a portion of the mechanism  262  to force conditioning head  230  down onto the polishing pad  240 . Conversely, the removal of such air or fluid causes the conditioning head  230  to retract away from the polishing pad  240 . A control system (not shown) sends control signals to the mechanism  262  in order to control the operation of the mechanism  262 . In this way, the pressure applied to the conditioning head  230  is accurately controlled, and the consistency of the conditioning process is significantly increased. 
     The force control mechanism can also comprise a set of oppositely polarized magnetic regions. FIG. 4 shows such a conditioning device  300  according to a second embodiment of the present invention. The device  300  has similar components to the device  200  shown in FIG. 2, and like reference numerals denote like elements. The device  300  comprises first  331  and second  311  magnetic regions defining a force control mechanism. A portion  332  of the first magnetic region  331  which lies directly above a conditioning head  330  is of a specific polarity (e.g. north), and a portion  312  of the second magnetic region  311  which lies directly below a polishing pad  340  is of a specific polarity which is opposite to that of the first portion (e.g. south). The opposing polarity portions  332 ,  312  cause the conditioning head  330  and the polishing platen  310  to be attracted to one another. A current source (not shown) varies the current through magnetic regions  331 , 311  in order to control the degree of attraction. By controlling the degree of attraction between the magnetic regions, the force exerted on conditioning head  330  can be effectively controlled, and the consistency of the conditioning process can be improved. 
     Although the above discussion with reference to FIG. 4 emphasized magnets which were located in the conditioning head and the polishing platen, it should be noted that this is not the only method of implementing the second embodiment. The invention may also be constructed with a single magnetic region in the conditioning head or polishing platen, with the opposing region being made of a magnetically responsive material, such as steel. Similarly, other embodiments can be constructed where both the oppositely polarized magnetic regions are located in one or the other of the conditioning head and the polishing platen. 
     Although the invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claim should be construed broadly, to include other variants and embodiments of the invention which may be made by those skilled in the art without departing from the scope and range of equivalents of the invention.