Electroplating process using a proximity mask with an overhang and an apparatus for performing the same

Abstract

A metallic seed layer may be deposited on a top surface of a wafer. The wafer may be mounted on a wafer holder using a wafer clamp assembly. The wafer clamp assembly includes a wafer clamp frame, a dielectric seal ring, and a proximity mask. An electroplating process may be performed in an electroplating bath while the wafer holder and the wafer are immersed in an electroplating solution. Electrical field within a predominant fraction of an entire volume of an annular region between an annular backside surface of the proximity mask and the wafer has a distribution of a tilt angle relative to the axial direction that is greater than arctangent (0.1).

What is claimed is: 
     
         1 . A method of depositing a metal, the method comprising:
 depositing a metallic seed layer on a top surface of a wafer including a two-dimensional array of dies;   mounting the wafer on a wafer holder using a wafer clamp assembly, wherein the wafer clamp assembly comprises a wafer clamp frame having an inner sidewall that is laterally offset from a symmetry axis passing through a geometrical center of the wafer and is perpendicular to a backside surface of the wafer by a first radial distance that equals a radius of the wafer less a first edge offset distance, a dielectric seal ring interposed between the wafer clamp frame and the wafer, and a proximity mask comprising a conductive material and having a cylindrical inner sidewall that is laterally offset from the symmetry axis by a second radial distance that equals the radius of the wafer less a second edge offset distance; and   performing an electroplating process in an electroplating bath while the wafer holder and the wafer are immersed in an electroplating solution and while the proximity mask is electrically biased such that electrical field within a predominant fraction of an entire volume of an annular region has a distribution of a tilt angle relative to an axial direction that is greater than arctangent (0.1), wherein the annular region is defined as a region that is radially offset from the symmetry axis by a radial distance that is greater then the first radial distance and is less than the second radial distance, and is bounded by the wafer and by the proximity mask along an axial direction that is parallel to a direction of the symmetry axis.   
     
     
         2 . The method of  claim 1 , wherein the proximity mask is in contact with a front surface of the wafer clamp frame. 
     
     
         3 . The method of  claim 1 , wherein the second edge offset distance is greater than the first edge offset distance by a radial distance difference that is in a range from 1% to 20% of the radius of the wafer. 
     
     
         4 . The method of  claim 3 , wherein the proximity mask comprises an annular backside surface that faces a front surface of the wafer and is spaced from the front surface of the wafer by an axial gap distance. 
     
     
         5 . The method of  claim 4 , wherein a ratio of the radial distance difference to the axial gap distance is in a range from 1 to 5. 
     
     
         6 . The method of  claim 4 , wherein the axial gap distance is greater than 1.0% of the radius of the wafer and is less than 10% of the radius of the wafer. 
     
     
         7 . The method of  claim 1 , wherein an annular backside surface of the proximity mask is exposed to the electroplating bath and faces a front side of the wafer. 
     
     
         8 . The method of  claim 1 , further comprising forming a patterned photoresist layer over the metallic seed layer prior to mounting the wafer to the wafer holder, wherein the dielectric seal ring contacts the patterned photoresist layer. 
     
     
         9 . The method of  claim 1 , wherein:
 the wafer comprises a two-dimensional array of redistribution structures located on a carrier substrate; and   each of the redistribution structures comprises at least one respective redistribution dielectric layer and respective redistribution wiring interconnects.   
     
     
         10 . The method of  claim 1 , wherein:
 the wafer comprises a two-dimensional array of semiconductor dies; and   each of the semiconductor dies comprises a respective set of semiconductor devices and a respective set of metal interconnect structures.   
     
     
         11 . A method of depositing a metal, the method comprising:
 depositing a metallic seed layer on a top surface of a wafer including a two-dimensional array of dies;   mounting the wafer on a wafer holder using a wafer clamp assembly, wherein the wafer clamp assembly comprises a wafer clamp frame, a dielectric seal ring interposed between the wafer clamp frame and the wafer, and a proximity mask comprising a conductive material and having a cylindrical inner sidewall that radially protrudes inward from an inner sidewall of the wafer clamp frame such that an annular region is provided between an annular backside surface of the proximity mask and a front side of the wafer; and   performing an electroplating process in an electroplating bath while the wafer holder and the wafer are immersed in an electroplating solution and while the proximity mask is electrically biased such that such that electrical field within a predominant fraction of an entire volume of the annular region has a distribution of a tilt angle relative to an axial direction that is greater than arctangent(0.1), wherein the axial direction is parallel to a symmetry axis of the wafer that is perpendicular to a backside surface of the wafer and passes through a geometrical center of the wafer.   
     
     
         12 . The method of  claim 11 , wherein an entire area of a die selected from the two-dimensional array of dies has an areal overlap with the proximity mask and does not have any areal overlap with a circular opening in the proximity mask in a plan view along the axial direction. 
     
     
         13 . The method of  claim 11 , further comprising:
 electrically connecting a cathode contact lead to the metallic seed layer; and   providing an anode that is exposed to the electroplating solution, wherein the proximity mask is electrically biased at a more negative voltage relative to the cathode contact lead during the electroplating process.   
     
     
         14 . The method of  claim 11 , wherein:
 the wafer clamp frame comprises an inner sidewall that is laterally offset from the symmetry axis a first radial distance; and   the proximity mask comprises a cylindrical inner sidewall that is laterally offset from the symmetry axis by a second radial distance that is less than the first radial distance by a radial distance difference that is in a range from 1% to 20% of a radius of the wafer.   
     
     
         15 . The method of  claim 11 , wherein a spacing along the axial direction between the proximity mask and the wafer is less than a spacing along the axial direction between the proximity mask and the anode during the electroplating process. 
     
     
         16 . An electroplating apparatus comprising:
 a wafer holder comprising a flat front surface;   a wafer clamp assembly that comprises a wafer clamp frame which is configured to press against a wafer toward the front flat surface and has an inner sidewall that is laterally offset from a symmetry axis that is perpendicular to the flat front surface by a first radial distance, a dielectric seal ring located on a backside of the wafer clamp frame and facing the flat front surface, and a proximity mask comprising a conductive material and having a cylindrical inner sidewall that is laterally offset from the symmetry axis by a second radial distance that is less than the first radial distance;   an electroplating bath configured to hold an electroplating solution, a wafer, the wafer clamp assembly, and an anode therein;   a cathode contact lead configured to press against a peripheral front surface of the wafer; and   a process controller configured to electrically bias the anode, the proximity mask, and the cathode contact lead such that electrical field within a predominant fraction of an entire volume of an annular region has a distribution of a tilt angle relative to an axial direction that is greater than arctangent(0.1) under a condition in which a conductive material layer is present on a front side of the wafer, wherein the annular region is defined as a region that is radially offset from the symmetry axis by a radial distance that is greater then the first radial distance and is less than the second radial distance, and is bounded by the wafer and by the proximity mask along an axial direction that is parallel to a direction of the symmetry axis.   
     
     
         17 . The electroplating apparatus of  claim 16 , wherein the proximity mask is in contact with a front surface of the wafer clamp frame. 
     
     
         18 . The electroplating apparatus of  claim 16 , wherein the first radial distance is greater than the second radial distance by a radial distance difference that is in a range from 1% to 20% of a radial distance between a tip of the cathode contact lead and the symmetry axis. 
     
     
         19 . The electroplating apparatus of  claim 16 , wherein the electroplating apparatus is configured to position a backside surface of the proximity mask within a two-dimensional plane that is uniformly spaced from the flat front surface of the wafer holder by a distance that equals a sum of an axial gap distance and a thickness of the wafer. 
     
     
         20 . The electroplating apparatus of  claim 19 , wherein a difference between the first radial distance and the second radial distance is greater than the axial gap distance.