Abstract:
A method and system for cleaning the wafer after CMP is disclosed. A brush module having at least two brushes placed adjacent to each other and having the wafer placed in between. A dummy roller is in contact with an edge of the wafer and follows a rotation of the wafer, wherein when the wafer is rotated, the brushes clean both sides of the wafer, and the dummy roller detects a rotation speed and a rotation direction of the wafer for adjusting the rotation of the wafer.

Description:
BACKGROUND  
       [0001]     The present invention relates generally to an improved system for cleaning wafers after going through chemical mechanical polish (CMP).  
         [0002]     During manufacturing stages of an integrated circuit (IC), chemical mechanical polish (CMP) is commonly used for removing any unnecessary materials. CMP is considered to be a unique fabrication process, because a wafer is deliberately introduced into an environment with a high particle count. During the CMP process, a polishing slurry that is full of abrasive particles in the nanometer to micrometer scale is used. Once the wafer has finished polishing, work must be done to carefully remove those particles from the wafer surface or they will affect the further fabrication steps.  
         [0003]     In order to remove those particles from the wafer surface, such that the possibility of defects can be minimized, a cleaning process, typically referred to as post-CMP cleaning, is necessary. After CMP, the wafer must be kept wet prior to cleaning because once the slurry is allowed to dry on the wafer, it becomes hard to remove. Due to electrostatic attraction forces, a simple water rinse may remove little, if any, of these particles after polishing. Consequently, conventional systems using a set of wafer brush cleaners were developed to clean and dry the wafer after CMP. These conventional systems typically use a set of PVA brushes to mechanically wipe the surfaces of the wafer and remove the abrasive particles. In the conventional systems, the wafer is typically cleaned by placing the wafer between a pair of spinning PVA brushes while the wafer itself is being rotated by a set of roller caps which are placed on the bottom of a brush module. The wafer in the brush module has forces exerted on it in two directions: one is exerted by the pair of rotating PVA brushes; the other is applied by the rotating roller caps. A friction force is exerted on the surface of the wafer by the pair of PVA brushes as they scrub in the down direction. This force will push and press the wafer down to contact the roller caps. Other tangent forces are applied by the rotating roller caps at the bottom of the brush module to make the wafer rotate by itself. These tangent forces must be greater than the friction force on the wafer surface exerted by the PVA brushes. Otherwise, the wafer will not rotate smoothly, and may stop at any time, thereby causing the slurry to reside in a certain area. This defect is known as a fan type defect. Since no tools are implemented in such a conventional system to measure the rotation speed and direction of the wafer and PVA brushes, fan type defects and an unstable etch rate can occur during the post-CMP cleaning process, thereby lowering the yield and efficiency in reducing defects.  
         [0004]     It is therefore desirable to have a new improved system for post-CMP cleaning such that the yield and efficiency can be improved and that the abnormal wafer recovery success rate may be increased.  
       SUMMARY  
       [0005]     In view of the foregoing, this invention provides an improved system used for post-CMP cleaning of wafers for defect reduction.  
         [0006]     In one embodiment of the present invention, a brush module is included having at least two brushes placed adjacent to each other and having the wafer placed in between. A dummy roller is in contact with an edge of the wafer and follows a rotation of the wafer, wherein when the wafer is rotated, the brushes clean both sides of the wafer, and the dummy roller detects a rotation speed and a rotation direction of the wafer for adjusting the rotation of the wafer.  
         [0007]     The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  illustrates a conventional system designed for cleaning a wafer after going through a CMP process.  
         [0009]      FIG. 2A  illustrates a vertical-type brush module used for cleaning a wafer after going through a CMP process in accordance with one embodiment of the present invention.  
         [0010]      FIG. 2B  illustrates a horizontal-type brush module used for cleaning a wafer after going through a CMP process in accordance with one embodiment of the present invention.  
         [0011]      FIG. 3  illustrates an optical encoder that can be implemented on a dummy roller for making rotation speed and direction measurements of a wafer in accordance with one embodiment of the present invention.  
         [0012]      FIG. 4A  illustrates a contact area between the edge of a wafer and the trench of a conventional roller cap.  
         [0013]      FIG. 4B  illustrates an improvement that is made to the conventional roller caps used for rotating the wafer in a brush module in accordance with one embodiment of the present invention.  
         [0014]      FIG. 5  is a diagram showing a pair of adjustable PVA brushes used for post-CMP cleaning of a wafer in accordance with one embodiment of the present invention. 
     
    
     DESCRIPTION  
       [0015]     The following will provide a detailed description of an improved system for cleaning a wafer after going through a chemical mechanical polish (CMP) process to reduce defects and to improve reliability and efficiency.  
         [0016]      FIG. 1  is a diagram  100  of a conventional system designed for cleaning a wafer after going through a CMP process. The diagram  100  shows a front view of a brush module. The brush module within the diagram  100  is a vertical system. In other words, a wafer  102  is cleaned by rotating it between two PVA brushes  104  in an upright position. Since the diagram  100  only shows the front view of the brush module, only one of the two PVA brushes  104  is shown in the diagram  100 . The wafer  102  is rotated by a set of roller caps  106 , which are controlled by a roller motor  108  via a timing belt  110 . In the diagram  100 , the roller motor  108 , the timing belt  110 , and the roller caps  106  all turn in a same direction, thereby causing the wafer  102  to rotate in an opposite direction. As the wafer  102  rotates, the PVA brushes  104  are designed to spin and clean off all defects on both sides of the wafer  102 .  
         [0017]     As mentioned, the tangent force on the wafer  102  applied by the roller caps  106  has to be greater than the friction force on the wafer surface exerted by the brushes  104 ; otherwise, the wafer  102  cannot rotate smoothly or can even stop, and it would cause a fan-type defect in the wafer  102 , resulting in an unstable etch rate. However, since the conventional system is a vertical system, the rotation of the wafer  102  may not be detected easily. The rotation of the PVA brushes  104  also cannot be detected in the conventional method. Without such measurements and corresponding adjustments, defects such as a fan-type defect and an unstable etch rate can occur.  
         [0018]      FIG. 2A  is a diagram  200  showing a vertical-type brush module used for cleaning a wafer after going through a CMP process in accordance with one embodiment of the present invention. Since the brush module within the diagram  200  is a vertical system, a wafer  202  is cleaned by rotating it between two PVA brushes  204  vertically. It is understood that since this is a top view, one of the PVA brushes  204  is hidden behind the wafer  202  because both “faces” of the wafer  202  are to be cleaned. The wafer  202 , much like the wafer  102  of  FIG. 1 , is rotated by a set of roller caps  206  that are controlled by a roller motor  208  via a timing belt  210 . The combination of the set of roller caps  206 , the roller motor  208  and the timing belt  210  can be seen as a means for rotating the wafer. A dummy roller  212  equipped with a set of optical sensors is implemented along with the roller caps  206  to determine rotation speed and direction of the wafer  202 . The dummy roller  212  can provide feedback so the wafer rotation status can be measured in real time. As shown, the dummy roller  212  is not connected to the timing belt  210  such that it can only be turned if the wafer  202  rotates. In the diagram  200 , since the roller motor  208 , the timing belt  210 , and the roller caps  206  all turn in the same direction, the wafer  202  rotates in a direction opposite to that direction. Because of the rotation of the wafer  202 , the dummy roller  212  will also turn in the same direction as the roller caps  206 . As the wafer  202  rotates, the PVA brushes  204  are designed to spin and clean off all defects on both sides of the wafer  202 .  
         [0019]     Note that the start torque of the dummy roller  212  need to be minimized in order to make an accurate measurement of the wafer rotation. For this purpose, a set of high-speed bearings is used in this embodiment, while the materials and the shape of the bearing shafts are also changed to stainless steel. This allows the inertia of the dummy roller  212  to decrease to 10% and the start torque to decrease to about 5%. The placement location of the dummy roller  212  is also important since the brush module operates at best when the normal force applied on the dummy roller  212  is the largest. Since the friction force exerted on the wafer surface by the PVA brushes  204  is downward and the tangent force applied by the roller caps  206  is in a horizontal direction, a combination force will be directed in the lower right hand direction. Therefore, the best placement location for the dummy roller  212  is in the lower right hand corner as shown in the figure. By using the high speed bearings, the light-start-torque shafts, and the perfect positioning of the dummy roller, the safety margin and reliability of the operation can be improved.  
         [0020]     In order for the dummy roller  212  to make accurate measurements of the rotation speed and direction of the wafer  202 , implementation of a means for detecting rotation speed and direction, which will be introduced in  FIG. 3 , is required.  
         [0021]      FIG. 2B  is a diagram  214  showing a horizontal-type brush module used for cleaning a wafer after going through a CMP process in accordance with one embodiment of the present invention. This horizontal system is designed such that a wafer  216  is laid flat and cleaned by a PVA brush  218  that is placed above the surface of the wafer  216 . The wafer  216  is rotated by a set of roller caps  220  which are placed all around the wafer. Like the vertical systems shown in  FIGS. 1 and 2 A, various roller caps  220  are used to turn the wafer  216 , and it is contemplated that they be controlled by a roller motor via a timing belt, which are not shown in the figure. A dummy roller  222  is implemented in between two of the roller caps  220  to help determine the rotation speed of the wafer  216 .  
         [0022]     Like the vertical system shown in  FIG. 1A , the rotation speed and direction of the wafer  202  can be determined accurately by using the dummy roller  222  along with a set of optical sensors that will be introduced in  FIG. 3 . With accurate rotating speed and direction measurements, the yield, efficiency, and the success rate of defect reduction can be improved.  
         [0023]      FIG. 3  illustrates an example of a means for detecting the rotation speed and direction of the wafer. As shown, an optical encoder  300  can be implemented with the dummy roller  212  of  FIG. 2A  or the dummy roller  222  of  FIG. 2B  for making rotation speed and direction measurements of a wafer in accordance with one embodiment of the present invention. By implementing optical encoder  300  with a dummy roller, accurate measurements of rotation speed and direction for the wafer within the brush module can be taken. The optical encoder  300  is composed of two optical sensors  302  and  304  that are placed above a light interdict plate  306 . The light interdict plate  306  is attached to one side of the dummy roller so that when the dummy roller turns, the light interdict plate  306  rotates as well. The optical sensors  302  and  304  operate by bouncing light onto the rotating light interdict plate  306  and sensing a return signal. In order for the optical sensors  302  and  304  to be able to determine if the light interdict plate  306  is rotating, a set of trenches  308  is etched onto the surface of the light interdict plate  306  precisely so that each trench  308  is separated by the same distance. The optical sensors  302  and  304  are also designed to be 90 degrees off-phase in order to detect the rotation speed and direction. As the dummy roller rotates, the optical sensors  302  and  304  will detect light bouncing off the light interdict plate  306  to provide necessary information for a controller to analyze. For the controller, the frequency will determine the rotation speed, while the sign of the frequency indicates the rotation direction.  
         [0024]      FIG. 4A  is a diagram  400  showing a contact area between the edge of a wafer and the trench of a conventional roller cap. As explained in  FIGS. 2A and 2B , during the post-CMP cleaning process, a wafer is rotated in a brush module by a set of roller caps that are controlled by a roller motor via a timing belt. In the diagram  400 , a vertical view of a roller cap  402  is presented with the edge of a wafer  404  inside the trench of the roller cap  402 . The trench width of the roller cap  402  is approximately 1.5 mm which is much larger than the width of a typical wafer such as the wafer  404 . Because of this difference in width, the contact area (shown at the “valley” of the trench between the trench of the roller cap  402  and the edge of the wafer  404  may be less than desired. With such small contact area, the possibility of error in rotating the wafer is much higher.  
         [0025]      FIG. 4B  is a diagram  406  showing an improvement that is made to the conventional roller caps used for rotating the wafer in a brush module in accordance with one embodiment of the present invention. For the conventional roller cap  402  shown in  FIG. 4A , it has a trench approximately 1.5 mm wide for the wafer  404  to rotate on even though the wafer  404  is much thinner than the width of the trench of roller cap  402 . The diagram  406  shows an improved design of the roller caps that can be used for rotating the wafer by simply making the trench of a roller cap narrower, allowing a better fit for the wafer. The width of the trench is reduced to be about the thickness of the wafer plus an additional space for keeping the wafer rotating. The additional space provided is determined by the smoothness of the wafer rotation. As long as the rotation of the wafer is not interfered, the width of the trench should be kept at a minimum. For example, a roller cap  408  can have a trench width of around 0.8 mm, close to half the width of the trenches of the roller caps used in conventional methods. This allows the roller cap  408  to have much more contact area with the edge of the wafer  410 . With the new roller caps, the contact surface can be increased by 200% compared with the conventional method, thereby reducing the margin of error while improving the reliability of operation.  
         [0026]      FIG. 5  illustrates a diagram  500  showing a pair of adjustable PVA brushes  502  and  504  used for post-CMP cleaning of a wafer in accordance with one embodiment of the present invention. A brush gap  506  between the two PVA brushes  502  and  504  is designed to be adjustable by using a step motor  508  to move the two PVA brushes  502  and  504 . Though not shown, it is understood that a wafer will be placed between the two PVA brushes  502  and  504  during actual cleaning. A controller, which is not shown in the figure, is used to control a step motor  508  to create an automatic feed-forward rotation-controller. The brush gap  506  is designed to control the speed of the rotation by tightening the PVA brushes  502  and  504 , thereby creating friction, when rotation speed is too high, and loosening the PVA brushes  502  and  504 , thereby reducing friction, when it is too slow. Along with the optical encoder of  FIG. 3  that is implemented with the dummy roller of  FIG. 2A , the rotation speed can be determined and sent to a signal process control board, or the controller where it decides if the rotation of the wafer is fast, slow, or normal and if the brush gap  506  needs to be adjusted. The controller is pre-programmed with an algorithm to determine how much action the step motor  508  should take. For example, if the rotation speed is too fast where a speed feedback over the algorithm setting is greater than 30%, the controller will make the step motor  508  tighten the brush gap  506  by 2 mm. If the rotation speed is slightly faster than normal where the speed feedback over the algorithm setting is greater than 15% and less than 30%, the brush gap  506  will be tightened by lmm. If the rotation speed is normal where the speed feedback over the algorithm setting is less than 15% while a speed feedback under the algorithm setting is also less than 15%, no action will be taken and the brush gap  506  will not change. If the rotation speed is slightly slower where the speed feedback under the algorithm setting is between 15% and 30%, the step motor  508  will loosen the brush gap  506  by 1 mm. If the rotation speed is too slow where the speed feedback under the algorithm setting is over 30%, the brush gap  506  will be loosened by 2mm.  
         [0027]     With this automatic feed-forward rotation-controller, the rotation speed of the wafer  506  can be controlled and the margin of error is lessened, thereby improving the reliability of the brush module. With accurate measurements of the rotation speed and direction from the dummy roller and the optical encoder and the implementation of the step motor to control the speed of the rotation of the wafer, a higher number of the unwanted particles can be removed in the clean-up process.  
         [0028]     Specifically, an extra dummy roller and a set of optical encoders are implemented in this new design for making measurements of the rotation speed and direction of the wafer. The measurements can then be used by a control board to control a step motor that would adjust the width of the gap between the two PVA brushes so the rotation speed of the wafer can be controlled.  
         [0029]     The above illustration provides many different embodiments or embodiments for implementing different features of the invention. Specific embodiments of components and processes are described to help clarify the invention. These are, of course, merely embodiments and are not intended to limit the invention to that described in the claims.  
         [0030]     Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.