Abstract:
A method of cleaning polybenzoxazole (PBO) from a semiconductor wafer coated with PBO includes baking a PBO-coated semiconductor wafer, and then exposing the semiconductor wafer with ultraviolet light through a patterned mask to soften selected regions of PBO on the semiconductor wafer. PBO is then dissolved in an edge region of the semiconductor wafer with solvent. After dissolving PBO in the edge region, the semiconductor wafer is chemically developed to dissolve the elected softened regions of PBO on the semiconductor wafer and to dissolve PBO remaining in the edge region of the semiconductor wafer that was left behind after the step of dissolving the PBO in the edge region with the solvent.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/454,857 filed Mar. 21, 2011 for “Edge Bead Removal for Polybenzoxazole (PBO)” by Roger Carroll. 
     
    
     INCORPORATION BY REFERENCE 
       [0002]    U.S. Provisional Application No. 61/454,857 is hereby incorporated by reference in its entirety. 
       BACKGROUND 
       [0003]    The present invention relates to semiconductor wafer processing, and more specifically to a process for effectively cleaning polybenzoxazole (PBO) from the edge of a wafer. 
         [0004]    In the process of developing a semiconductor wafer that is patterned with PBO, it is often important to clean the wafer edge via a process known as edge bead removal (EBR). EBR is typically performed by spraying a solvent on the edge region of the wafer, to dissolve PBO on the wafer edge so that the edge region is cleaned. Existing processes of performing EBR can leave residual amounts of PBO in the edge region of the wafer, which can have an undesirable effect on bond pads and/or wire bonds at the edge region of the wafer. 
       SUMMARY 
       [0005]    The present invention is directed to a method of cleaning polybenzoxazole (PBO) from a semiconductor wafer coated with PBO, and to a semiconductor wafer selectively coated with PBO. A PBO-coated semiconductor wafer is baked, and then exposed with ultraviolet light through a patterned mask to soften selected regions of PBO on the semiconductor wafer. PBO is dissolved in an edge region of the semiconductor wafer with solvent. After dissolving PBO in the edge region, the semiconductor wafer is chemically developed to dissolve the elected softened regions of PBO on the semiconductor wafer and to dissolve PBO remaining in the edge region of the semiconductor wafer that was left behind after the step of dissolving the PBO in the edge region with the solvent. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a flow diagram illustrating a process of effectively cleaning PBO from the edge of a semiconductor wafer according to an embodiment of the present invention. 
           [0007]      FIGS. 2A ,  3 A,  4 A and  5 A are perspective views, and  FIGS. 2B ,  3 B,  4 B and  5 B are section views, of a semiconductor wafer processed according to the process of  FIG. 1 . 
           [0008]      FIG. 6  is a flow diagram illustrating a prior art process of cleaning PBO from the edge of a semiconductor wafer. 
           [0009]      FIGS. 7A ,  8 A,  9 A and  10 A are perspective views, and  FIGS. 7B ,  8 B,  9 B and  10 B are section views, of a semiconductor wafer processed according to the process of  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION 
       [0010]      FIG. 1  is a flow diagram illustrating a process of effectively cleaning PBO from the edge of a semiconductor wafer according to an embodiment of the present invention. Initially, in step  10 , the wafer is coated with PBO. Next, at step  12 , the PBO-coated wafer is baked, which drives off excess solvents and improves the photo-development characteristics of the PBO for future steps. Next, at step  14 , the PBO on the wafer is exposed with ultraviolet (UV) light through a patterned mask, softening the exposed regions of PBO. Next, at step  16 , EBR is performed by spraying the edge of the semiconductor wafer with solvent, dissolving the PBO in the edge region of the wafer. Next, at step  18 , the PBO on the wafer is chemically developed, which dissolves the exposed PBO to form the desired pattern on the wafer, and also dissolves any residual (unexposed) PBO left at the edge region of the wafer after the EBR step. This unexposed PBO at the edge region is dissolved because the chemical development step, in addition to dissolving exposed PBO, also dissolves unexposed PBO at a smaller rate (about 20-30% in most cases) than it dissolves the exposed PBO. Finally, at step  20 , the PBO is cured and any remaining solvents are driven off of the wafer. This process results in a clean edge region of the wafer, avoiding the presence of residual PBO that can have a detrimental effect on the bond pads and wire bonds that may be located in the edge region of the wafer. 
         [0011]      FIGS. 2A ,  3 A,  4 A and  5 A are perspective views, and  FIGS. 2B ,  3 B,  4 B and  5 B are section views, of a semiconductor wafer processed according to the process of  FIG. 1 . In  FIGS. 2A and 2B , wafer  30  is coated with PBO layer  32  and baked (steps  10  and  12 ,  FIG. 1 ). In  FIGS. 3A and 3B , PBO layer  32  on wafer  30  is exposed with UV light through patterned mask  34  to produce unexposed regions  36  and exposed regions  38  (step  14 ,  FIG. 1 ). In  FIGS. 4A and 4B , EBR is performed by spraying solvent on the edge region of wafer  30 , dissolving the outer portion of unexposed regions  36  (and leaving residual PBO in outer portions  40  of wafer  30 , due to the imperfect EBR process) (step  16 ,  FIG. 1 ). This leaves a central region of semiconductor wafer  30  with unexposed regions  36  and exposed regions  38 . In  FIGS. 5A and 5B , wafer  30  is developed, which removes the PBO in exposed regions  38  and also removes residual PBO from outer portions  40  of wafer  30 , due to the development process dissolving unexposed PBO at a slower rate than exposed PBO, which is sufficient to dissolve the thin amounts of residual PBO at outer portions  40  of wafer  30  (step  18 ,  FIG. 1 ). 
         [0012]      FIG. 6  is a flow diagram illustrating a prior art process of cleaning PBO from the edge of a semiconductor wafer. Initially, in step  50 , the wafer is coated with PBO. Next, at step  52 , the PBO-coated wafer is baked, which drives off excess solvents and improves the photo-development characteristics of the PBO for future steps. Next, at step  54 , the PBO on the wafer is exposed with ultraviolet (UV) light through a patterned mask, softening the exposed regions of PBO. Next, at step  56 , the PBO on the wafer is chemically developed, which dissolves the exposed PBO and forms the desired pattern on the wafer. Next, at step  58 , EBR is performed by spraying the edge of the semiconductor wafer with solvent, dissolving the PBO in the edge region of the wafer. Finally, at step  60 , the PBO is cured and any remaining solvents are driven off of the wafer. 
         [0013]    The process shown in  FIG. 6  can result in residual amounts of PBO being left on the outer regions of the semiconductor wafer, which can have detrimental effects on the subsequent back grinding process and on the bond pads and/or wire bonds of the wafer. This may occur because the EBR dissolves the PBO at the wafer edge which flows out into the edge region that the EBR is attempting to clean, and/or because droplets of EBR solvent may splash onto the interior of the wafer, locally dissolving PBO that may flow over a bond pad and contaminate it. The process described above with respect to  FIG. 1  offers improved performance by developing the PBO after the EBR step has been performed, to ensure that any residual PBO left by the EBR step is dissolved. 
         [0014]      FIGS. 7A ,  8 A,  9 A and  10 A are perspective views, and  FIGS. 7B ,  8 B,  9 B and  10 B are section views, of a semiconductor wafer processed according to the process of  FIG. 6 . In  FIGS. 7A and 7B , wafer  70  is coated with PBO layer  72  and baked (steps  50  and  52 ,  FIG. 6 ). In  FIGS. 8A and 8B , PBO layer  72  on wafer  70  is exposed with UV light through patterned mask  74  to produce unexposed regions  76  and exposed regions  78  (step  54 ,  FIG. 6 ). In  FIGS. 9A and 9B , wafer  70  is developed, which removes the PBO in exposed regions  78  (step  56 ,  FIG. 6 ). In  FIGS. 10A and 10B , EBR is performed by spraying solvent on the edge region of wafer  70 . This leaves residual PBO in outer portions  80  of wafer  70 , due to the imperfect EBR process (step  58 ,  FIG. 6 ). As discussed above, the residual PBO in outer portions  80  of wafer  70  can have undesirable effects. 
         [0015]    As can be seen from the above description, a more effective process of cleaning the edge region of a semiconductor wafer can be achieved by performing EBR prior to chemically developing the wafer, contrary to the accepted wisdom which suggests that EBR should be performed last (that is, after exposure and development of the wafer). 
         [0016]    The above description of the present invention, with reference to  FIGS. 1 ,  2 A,  2 B,  3 A,  3 B,  4 A,  4 B,  5 A and  5 B, is intended to provide an example of the concept of the invention without necessarily limiting the invention to the scope of the example. For instance, the process described indicates that PBO is exposed by UV light through a patterned mask to soften the exposed regions of PBO on the wafer. In some embodiments, it may be possible to reverse this process so that exposed regions of material are hardened rather than softened, with the pattern of the mask reversed, while proceeding with other steps of the invention essentially as described. Other modifications and permutations of the exemplary steps disclosed herein will be apparent to a person of ordinary skill in the art. 
         [0017]    While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. 
         [0018]    Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.