Systems and methods for using white light interferometry to measure undercut of a bi-layer structure

Abstract

Systems and methods for using white light interferometry to measure undercut of a bi-layer structure are provided. One such method involves performing a first scan of a first bi-layer structure with a microscope using a first scan range, where the microscope is configured for white light interferometry, generating a first interferogram using data from the first scan, performing a second scan of the first bi-layer structure with the microscope using a second scan range, generating a second interferogram using data from the second scan, determining a first distance between features of the first interferogram, determining a second distance between features of the second interferogram, and calculating a width of the undercut based on the first distance and the second distance. One such system involves using the microscope and/or a computer to perform one or more actions of this method.

What is claimed is: 
     
         1 . A system for measuring an undercut of bi-layer structures on a wafer, the system comprising:
 a microscope configured for white light interferometry, wherein the microscope is configured to:
 perform a first scan of a first bi-layer structure using a first scan range; 
 perform a second scan of the first bi-layer structure using a second scan range; 
 generate a first interferogram using data from the first scan; and 
 generate a second interferogram using data from the second scan; and 
   a computer coupled to the microscope and configured to:
 determine a first distance between features of the first interferogram; 
 determine a second distance between features of the second interferogram; and 
 calculate a width of the undercut based on the first distance and the second distance. 
   
     
     
         2 . The system of  claim 1 , wherein the first scan range is less than the second scan range, wherein the first and second ranges extend in a direction that is about normal to a top surface of the first bi-layer structure. 
     
     
         3 . The system of  claim 2 :
 wherein the features of the first interferogram comprise two edges each corresponding to a portion of the first interferogram having a maximum slope; and   wherein the features of the second interferogram comprise two peaks each corresponding to a point of the second interferogram having a peak amplitude.   
     
     
         4 . The system of  claim 3 :
 wherein the first bi-layer structure comprises an upper layer on a lower layer, where an area of the lower layer is less than that of the upper layer;   wherein the two edges of the first interferogram correspond in position to outer edges of the upper layer; and   wherein the two peaks of the second interferogram correspond in position to outer edges of the lower layer.   
     
     
         5 . The system of  claim 3 :
 wherein the first bi-layer structure comprises an upper layer on a lower layer, where an area of the lower layer is less than that of the upper layer;   wherein the first bi-layer structure comprises a centrally disposed hole extending through both the upper layer and the lower layer;   wherein the two edges of the first interferogram correspond in position to edges of the upper layer defining the hole; and   wherein the two peaks of the second interferogram correspond in position to edges of the lower layer defining the hole.   
     
     
         6 . The system of  claim 2 :
 wherein the features of the first interferogram comprise four edges each corresponding to a portion of the first interferogram having a maximum slope;   wherein the features of the second interferogram comprise four peaks each corresponding to a point of the second interferogram having a peak amplitude;   wherein the microscope is configured to perform the first scan of the first bi-layer structure and a second bi-layer structure using the first scan range; and   wherein the microscope is configured to perform the second scan of the first bi-layer structure and the second bi-layer structure using the second scan range.   
     
     
         7 . The system of  claim 6 :
 wherein the first bi-layer structure comprises an upper layer on a lower layer, where an area of the lower layer of the first bi-layer structure is less than that of the upper layer of the first bi-layer structure;   wherein the second bi-layer structure comprises an upper layer on a lower layer, where an area of the lower layer of the second bi-layer structure is less than that of the upper layer of the second bi-layer structure;   wherein a first two edges of the four edges of the first interferogram correspond in position to outer edges of the upper layer of the first bi-layer structure;   wherein a second two edges of the four edges of the first interferogram correspond in position to outer edges of the upper layer of the second bi-layer structure;   wherein a first two peaks of the four peaks of the second interferogram correspond in position to outer edges of the lower layer of the first bi-layer structure; and   wherein a second two peaks of the four peaks of the second interferogram correspond in position to outer edges of the lower layer of the second bi-layer structure.   
     
     
         8 . The system of  claim 2 :
 wherein the features of the first interferogram comprise four edges each corresponding to a portion of the first interferogram having a maximum slope;   wherein the features of the second interferogram comprise four peaks each corresponding to a point of the second interferogram having a peak amplitude;   wherein the first bi-layer structure comprises an upper layer on a lower layer, where an area of the lower layer of the first bi-layer structure is less than that of the upper layer of the first bi-layer structure;   wherein the first bi-layer structure comprises a first hole and a second hole, each extending through both the upper layer and the lower layer;   wherein a first two edges of the four edges of the first interferogram correspond in position to edges of the upper layer of the first bi-layer structure defining the first hole;   wherein a second two edges of the four edges of the first interferogram correspond in position to edges of the upper layer of the first bi-layer structure defining the second hole;   wherein a first two peaks of the four peaks of the second interferogram correspond in position to edges of the lower layer of the first bi-layer structure defining the first hole; and   wherein a second two peaks of the four peaks of the second interferogram correspond in position to edges of the lower layer of the first bi-layer structure defining the second hole.   
     
     
         9 . The system of  claim 1 , wherein the second scan range is less than the first scan range, wherein the first and second ranges extend in a direction that is about normal to a top surface of the first bi-layer structure. 
     
     
         10 . The system of  claim 1 , wherein if the width of the undercut is outside of a preselected range, then a fabrication process for forming the first bi-layer structure is modified. 
     
     
         11 . The system of  claim 10 , wherein the fabrication process is modified by adjusting a duration of a development sub-process used in forming the undercut of the first bi-layer structure.