Wafer polishing and endpoint detection

Abstract

In a chem-mech polishing process for planarizing insulators such as silicon oxide and silicon nitride, a pool of slurry is utilized at a temperature between 85° F.-95° F. The slurry particulates (e.g. silica) have a hardness commensurate to the hardness of the insulator to be polished. Under these conditions, wafers can be polished at a high degree of uniformity more economically (by increasing pad lifetime), without introducing areas of locally incomplete polishing.

We claim:  
     
       1. In a process of chem-mech polishing an insulator layer arranged on a semiconductor wafer, wherein the wafer is brought into contact with a rotating polish wheel having a polishing pad disposed thereon, the improvement comprising: 
       providing sufficient slurry to form a pool defined by a dam disposed about the outer periphery of the polish wheel, wherein said polishing pad is immersed in said pool during polishing, and wherein said slurry comprises a liquid suspension of solid particulates having a hardness commensurate to the hardness of the insulator layer; and  
       elevating the temperature of said slurry to at least approximately 85° F.  
     
     
       2. The process as recited in  claim 1 , wherein said slurry is at a temperature between approximately 85° F.-95° F. 
     
     
       3. The process as recited in  claim 1 , wherein said solid particulates have an average size of at least 0.02 microns. 
     
     
       4. The process is  as recited in  claim 1 , wherein said solid particulates constitute approximately 10%-15% by weight of said slurry. 
     
     
       5. The process as recited in  claim 4 , wherein said particulates are comprised of SiO 2 . 
     
     
       6. The process as recited in  claim 1 , wherein said polish wheel rotates at a speed no greater than approximately 30 RPM. 
     
     
       7. The process as recited in  claim 6 , wherein said polish wheel rotates at a speed between approximately 15 RPM-20 RPM. 
     
     
       8. The process as recited in  claim 6 , wherein a plurality of semiconductor wafers are simultaneously polished by being brought into contact with the same polish wheel. 
     
     
       9. The process as recited in  claim 8 , wherein at least one of said wafers is a dummy wafer comprising a silicon substrate with a discrete layer thereon having a thickness commensurate to the amount of material to be removed from the remaining wafers. 
     
     
       10. The process as recited in  claim 9 , further comprising: 
       monitoring said dummy wafer during polishing to indicate when the polishing process is completed.  
     
     
       11. In a process of chem-mech polishing an insulator layer arranged on a semiconductor wafer, wherein the wafer is brought into contact with a rotating polish wheel having a polishing pad disposed thereon, the improvement comprising: 
       providing sufficient slurry to form a pool defined by a dam disposed about the outer periphery of the polish wheel, wherein said polishing pad is immersed in said pool during polishing, and wherein said slurry comprises a suspension of silica particulates having an average size greater than 0.006 microns, said silica particles comprising approximately 10%-15% by weight of said slurry; and  
       elevating the temperature of said slurry to at least approximately 85° F.  
     
     
       12. The process as recited in  claim 11 , wherein said silica particulates have an average size of approximately 0.01 microns. 
     
     
       13. The process as recited in  claim 12 , wherein said polish wheel rotates at a speed no greater than approximately 30 RPM. 
     
     
       14. The process as recited in  claim 13 , wherein said polish wheel rotates at a speed between approximately 15 RPM-20 RPM. 
     
     
       15. The process as recited in  claim 13 , wherein a plurality of semiconductor wafers are simultaneously polished by being brought into contact with the same polish wheel. 
     
     
       16. The method as recited in  claim 15 , wherein at least one of said wafers is a dummy wafer comprising a silicon substrate with a discrete layer thereon having a thickness commensurate to the amount of material to be removed from the remaining wafers. 
     
     
       17. The method as recited in  claim 16 , further comprising: 
       monitoring said dummy wafer during polishing to indicate when the polishing process is completed.  
     
     
       18. In a process of chem-mech polishing a layer of silicon oxide disposed on a semiconductor substrate, wherein the substrate is supported by a quill in contact with a rotating polish wheel having a polishing pad disposed thereon, the improvement comprising: 
       providing an amount of slurry sufficient to form a pool defined by a dam disposed about the polish wheel, wherein said polishing pad is immersed in said pool during polishing, and wherein said slurry comprises a suspension of approximately 10%-15% silica particulates by weight, said particulates having an average diameter of at least approximately 0.02 microns; and  
       elevating the temperature of said slurry to at least approximately 85° F.; and wherein  
       said polish wheel rotates at a speed no greater than approximately 30 RPM.  
     
     
       19. The process as recited in  claim 18 , wherein said slurry is at a temperature between approximately 85° F.-95° F. and said polish wheel rotates at a speed between approximately 15 RPM-20 RPM. 
     
     
       20. The process as recited in  claim 19 , wherein: 
       a plurality of semiconductor wafers are simultaneously polished by being brought into contact with the same polish wheel; and  
       at least one of said wafers is a dummy wafer comprising a silicon substrate with a discrete layer thereon having a thickness commensurate to the amount of material to be removed from the remaining wafers;  
       and further comprising:  
       monitoring said dummy wafer during polishing to indicate when the polishing process is completed.  
     
     
       21. In a process of chem- mech polishing a layer of oxide disposed on a semiconductor substrate, wherein the substrate is supported by an electric motor driven quill in contact with a rotating polish wheel having a polishing pad thereon, the improvement comprising:    
       
         monitoring the process endpoint by detecting current changes in the motor driving the quill. 
       
     
     
       22. The process of  claim 21  wherein the substrate is silicon and the monitoring process comprises ( a )  setting a motor controller, coupled to the motor driven quill, to allow the quill rotation speed to decrease, when the polishing pad has increased drag thereon by encountering the silicon substrate, and  ( b )  sensing an increase in current in the motor caused by the decrease in the rotational speed of the quill.   
     
     
       23. The process of  claim 22  wherein said setting step is performed so that the increase in the current occurs as a current spike. 
     
     
       24. In a chem- mech polishing apparatus having a rotating polishing wheel with a quill assembly for supporting an oxide coated semiconductor substrate in contact with the polishing wheel, said quill assembly being driven by an electric motor, the improvement comprising:    
       
         means for determining a polishing endpoint, said means comprising a current detector for detecting a change in current drawn by the electric motor driving the quill assembly. 
       
     
     
       25. The apparatus of  claim 26  further comprising means for stopping polishing when said current detector detects said change in current. 
     
     
       26. A method for planarizing a semiconductor wafer comprising: 
       
         a. holding a semiconductor wafer in contact with a polishing platen in the presence of a chemical slurry;  
       
       
         b. rotating the wafer with respect to and against the polishing platen with an electric drive motor; and  
       
       
         c. sensing the change in friction between the wafer and the polishing platen to detect the planar endpoint on said wafer with a current meter for the drive motor which detects a change in amperage through the drive motor. 
       
     
     
       27. The process as claimed in  claim 26  and wherein: 
       
         sensing the planar endpoint occurs when an oxide coating on the wafer is planarized and a surface of the wafer including a different material is contacted by the polishing platen. 
       
     
     
       28. The process as claimed in  claim 26  and wherein; 
       
         the current meter measures a current flow to the drive motor which is proportional to the torque output of the drive motor divided by a multiplying factor equal to the distance between the wafer and the center of the polishing platen. 
       
     
     
       29. The process as claimed in  claim 28  and wherein: 
         torque  ( T )  on the motor is equated to the force  ( F )  exerted on the wafer by the polishing platen in a direction of relative motion of the wafer and the polishing platen and to the radius  ( r )  of the wafer from the center of the polishing platen by the formula: T=F×r.   
     
     
       30. A method of detecting a planar endpoint on a semiconductor wafer during a chem- mech planarization process comprising:    
       
         a. rotating the wafer with respect to the polishing platen by an electric motor; and  
       
       
         b. sensing a change in friction between the wafer and polishing platen by detecting a change in current in a current meter for the motor. 
       
     
     
       31. The method as recited in  claim 30  and wherein: 
       
         the planar endpoint occurs when coating on the wafer is removed to expose a surface formed with a different material. 
       
     
     
       32. The method as recited in  claim 31  and wherein: 
       
         both the wafer and polishing platen are moved. 
       
     
     
       33. The method as recited in  claim 30  and wherein: 
         the coefficient of friction between the wafer and polishing platen is related to the torque  ( T )  on the motor according to the equation T=F×r, wherein “F” is a force exerted by the polishing platen on the wafer in a direction of relative motion of the polishing platen and the wafer and is a function of the coefficient of friction between the wafer and the polishing platen, and “r” is a radius from the center of the polishing platen to the center of the wafer.   
     
     
       34. In a chem- mech planarization apparatus for a wafer having a polishing head for holding and moving the wafer against a movable polishing platen in a polishing slurry, an endpoint detection apparatus comprising:    
       
         means for determining a polishing endpoint by detecting a current change to a drive motor for moving the polishing head, wherein said change occurs with a change in friction between the wafer and the polishing platen. 
       
     
     
       35. Endpoint detection apparatus as claimed in  claim 34  and wherein: 
       
         said change in friction occurs when a coating of the wafer is removed and a surface formed of a different material is contacted. 
       
     
     
       36. Endpoint detection apparatus as claimed in  claim 34  further comprising: 
       
         control means responsive to said determining means for controlling the planarization apparatus. 
       
     
     
       37. Endpoint detection apparatus as claimed in  claim 36  and wherein: 
       
         both the polishing head and the polishing platen are rotated and the wafer is moved across the polishing platen. 
       
     
     
       38. Endpoint detection apparatus as claimed in  claim 36  and wherein: 
         said control means measures a distance “r” from the center of the polishing head to the center of the polishing platen, to be used by the control means as a multiplying factor for determining a torque  ( T )  on the drive motor according to the formula T=F×r, where  ( F )  is a force exerted by the polishing platen against the wafer in a direction of relative movement of the polishing platen and the wafer.   
     
     
       39. Apparatus for chem- mech planarizing a semiconductor wafer and for detecting a planar endpoint of the wafer comprising:    
       
         a. holding means for holding and moving the wafer including a polishing head rotated by a first electric drive motor;  
       
       
         b. polishing means including a polishing platen rotated by a second electric drive motor and a polishing agent for contact with the wafer and with the polishing platen; and  
       
       
         c. means for determining a planar endpoint, comprising a current meter for measuring current in the first electric drive motor, whereby a change in friction between the wafer and the polishing platen is detected by a change in motor current and equated to the planar endpoint. 
       
     
     
       40. Apparatus as recited in  claim 39  and wherein: 
       
         the planar endpoint occurs when an oxide coating is removed from the wafer and a surface including a different material is exposed. 
       
     
     
       41. Apparatus as recited in  claim 39  and wherein: 
       
         the polishing head and the polishing platen are rotated in the same direction. 
       
     
     
       42. Apparatus as recited in  claim 39  and further comprising: 
       
         control means, responsive to said determining means, for controlling the apparatus.