Process for wafer backside polymer removal with wafer front side gas purge

Abstract

A process is provided for removing polymer from a backside of a workpiece. The process includes supporting the workpiece on the backside in a vacuum chamber while leaving a peripheral annular portion of the backside exposed. Gas flow is confined at the edge of the workpiece within a gap at the edge of the workpiece, the gap configured to be on the order of about 1% of the diameter of the chamber, the gap defining a boundary between an upper process zone containing the front side and a lower process zone containing the backside. The process further includes evacuating the lower process zone, generating a plasma in an external chamber from a polymer etch precursor gas, and introducing a by-product from the plasma into the lower process zone. The process further includes pumping a purge gas into the upper process zone to remove polymer etch species from the upper process zone.

1 . A process for removing polymer from a backside of a workpiece, comprising:
 supporting said workpiece on the backside in a vacuum chamber while leaving a peripheral annular portion of the backside exposed;   confining gas flow at the edge of said workpiece within a gap at the edge of said workpiece, said gap configured to be on the order of about 1% of the diameter of the chamber, said gap defining a boundary between an upper process zone containing said front side and a lower process zone containing said backside;   evacuating said lower process zone;   generating a plasma in an external chamber from a polymer etch precursor gas, and introducing a by-product from said plasma into said lower process zone; and   pumping a purge gas into said upper process zone to remove polymer etch species from said upper process zone.   
   
   
       2 . The process of  claim 1  further comprising confining said upper process zone between the wafer front side and a ceiling of the reactor to an upper process zone height on the order of 1% of the diameter of the chamber. 
   
   
       3 . The process of  claim 1  further comprising heating the workpiece to a temperature on the order of 300 degrees C. 
   
   
       4 . The process of  claim 2  wherein said gap and said upper process zone height are both about 2 mm. 
   
   
       5 . The process of  claim 1  wherein said polymer etch species comprises oxygen gas. 
   
   
       6 . The process of  claim 5  wherein said purge gas comprises nitrogen or an inert gas. 
   
   
       7 . The process of  claim 5  wherein said purge gas comprises a scavenger of said polymer etch species. 
   
   
       8 . The process of  claim 7  wherein said scavenger comprises hydrogen gas. 
   
   
       9 . The process of  claim 7  wherein said scavenger comprises carbon monoxide. 
   
   
       10 . The process of  claim 7  wherein said scavenger comprises a species whose reaction products with said polymer etch species are less reactive with ultra low-K film than said polymer etch species. 
   
   
       11 . The process of  claim 2  wherein the upper process zone height is sufficiently small to limit residency time of gas in said upper process zone to less than 1% to 5% of the residency time of gases in said lower process zone. 
   
   
       12 . The process of  claim 1  wherein the step of pumping the non-reactive purge gas into the upper process zone comprises pumping said purge gas at a sufficiently high flow rate so as to limit residency time of species in said upper process zone to less than 1% to 5% of the residency time of gases in said lower process zone. 
   
   
       13 . The process of  claim 1  wherein the step of confining is a carried out by one of: (a) defining said gap between the wafer edge and the sidewall of the chamber, (b) defining said gap between the wafer edge and a ring surrounding the wafer. 
   
   
       14 . The process of  claim 1  further comprising:
 directing the flow of the plasma by-products from the plasma of said external plasma source through axial conduits underlying the backside, and directing said by-products from said conduits through nozzles aimed at the backside edge of the workpiece.   
   
   
       15 . A process for removing polymer from a backside of a workpiece, comprising:
 supporting said workpiece on the backside in a vacuum chamber while leaving a peripheral annular portion of the backside exposed;   confining the front side of said workpiece by a ceiling of said chamber to establish a height of an upper process zone that is less than 1% of the diameter of said workpiece, said upper process zone corresponding to the portion of the chamber between the wafer and the ceiling of the chamber;   evacuating a lower process zone of the chamber said lower process zone corresponding to the chamber portion on the side of the wafer opposite said upper process zone;   generating a plasma in an external chamber from a polymer etch precursor gas, and confining a by-product from said plasma into said lower process zone through one or more conduits terminating near and aimed at the wafer backside edge; and   removing polymer etch species from said upper process zone by pumping a purge gas into said upper process zone.   
   
   
       16 . The process of  claim 13  further comprising heating said workpiece to a temperature on the order of 300 degrees C. 
   
   
       17 . The process of  claim 13  wherein said polymer etch species comprises oxygen gas. 
   
   
       18 . The process of  claim 13  wherein said purge gas comprises a scavenger of said polymer etch species. 
   
   
       19 . A process for removing polymer from a backside of a workpiece, comprising:
 supporting said workpiece on the backside in a vacuum chamber while leaving a peripheral annular portion of the backside exposed, said workpiece corresponding to a boundary between an upper process zone and a lower process zone of said chamber;   confining a front side of said workpiece by a ceiling of said chamber to as to establish a height of an upper process zone that is sufficiently small to limit residency time of gas in said upper process zone to less than about 1% to about 5% of the residency time of gases in said lower process zone;   evacuating said lower process zone;   generating a plasma in an external chamber from a polymer etch precursor gas, and confining by-products from said plasma into said lower process zone through one or more conduits terminating near and aimed at the wafer backside edge; and   removing polymer etch species from said upper process zone by pumping a purge gas into said upper process zone.   
   
   
       20 . The process of  claim 5  wherein said purge gas comprises one of: (a) an inert gas, (b) nitrogen, (c) a scavenger of said polymer etch species.