Apparatus and method for shielding a wafer from charged particles during plasma etching

Abstract

A plasma etching system having a wafer chuck with a magnet that applies a magnetic field over a wafer to shield the wafer from charged particles. The magnetic field is parallel with the wafer, and is strongest near the wafer surface. The magnetic field may be straight, or circular. In operation, electrons are deflected from the wafer by the Lorentz force, the wafer acquires a positive charge, and ions are deflected by electrostatic repulsion. Neutral species are allowed through the magnetic field, and they collide with the wafer. Neutral species generally provide more isotropic and material-selective etching than charged particles, so the present magnetic field tends to increase etch isotropy and material selectivity. Also, the magnetic field can protect the wafer from seasoning processes designed to clean unwanted films from the chamber surface as seasoning processes typically rely on etching by charged particles.

What is claimed is:  
     
         1 . An apparatus for plasma etching comprising: 
 a) a wafer chuck;    b) a volume for plasma above the wafer chuck;    c) a magnet in the wafer chuck for producing a magnetic field parallel to a wafer surface of a wafer disposed on the wafer chuck, wherein the magnetic field is strongest near the wafer, and decreases toward the plasma such that the magnetic field reflects electrons traveling from the plasma to the wafer.    
     
     
         2 . The plasma etching apparatus of  claim 1  wherein more than 50% of electrons from the plasma traveling toward the wafer are reflected away from the wafer by the magnetic field.  
     
     
         3 . The plasma etching apparatus of  claim 1  wherein the magnet produces a round magnetic field with a clockwise direction as viewed from above the wafer chuck.  
     
     
         4 . The plasma etching apparatus of  claim 1  wherein the magnet produces a round magnetic field with a counter-clockwise direction as viewed from above the wafer chuck.  
     
     
         5 . The plasma etching apparatus of  claim 1  wherein the magnet is an electromagnet.  
     
     
         6 . The plasma etching apparatus of  claim 1  wherein the magnet is a permanent magnet.  
     
     
         7 . The plasma etching apparatus of  claim 1  wherein the magnetic field is linear.  
     
     
         8 . The plasma etching apparatus of  claim 1  wherein the magnetic field has a strength in the range of 5-50 Gauss at the wafer.  
     
     
         9 . The plasma etching apparatus of  claim 1  wherein the magnetic field decreases in strength by at least about 75% from the wafer to the volume for plasma.  
     
     
         10 . The plasma etching apparatus of  claim 1  wherein the magnetic field reflects electrons having an energy of 2 eV or less traveling toward the wafer in a Z-direction.  
     
     
         11 . The plasma etching apparatus of  claim 1  wherein the magnetic field reflects electrons having an energy of 10 eV or less traveling toward the wafer in a Z-direction.  
     
     
         12 . A method for shielding a wafer from charged particles in a plasma, comprising the steps of: 
 a) producing a plasma above the wafer;    b) creating a magnetic field between the wafer and plasma, wherein the magnetic field is parallel to the wafer, and wherein the magnetic field decreases in strength with distance above the wafer such that the magnetic field reflects electrons traveling from the plasma to the wafer.    
     
     
         13 . The method of  claim 12  wherein more than 50% of electrons from the plasma traveling toward the wafer are reflected away from the wafer by the magnetic field.  
     
     
         14 . The method of  claim 12  wherein chemistry of the plasma is selected such that shielding the wafer from charged particles causes an increase in isotropy of an etch process.  
     
     
         15 . The method of  claim 14  wherein the magnetic field reflects electrons having an energy of 2 eV or less traveling toward the wafer in a Z-direction.  
     
     
         16 . The method of  claim 12  wherein chemistry of the plasma is selected such that the plasma provides chamber seasoning.  
     
     
         17 . The method of  claim 16  wherein the magnetic field reflects electrons having an energy of 10 eV or less traveling toward the wafer in a Z-direction.  
     
     
         18 . The method of  claim 16  wherein the magnetic field reflects electrons having an energy of 4 eV or less traveling toward the wafer in a Z-direction.  
     
     
         19 . A method for chamber seasoning with a wafer in-situ, comprising the steps of: 
 a) producing a chamber seasoning plasma above the wafer;    b) creating a magnetic field between the wafer and seasoning plasma, wherein the magnetic field is parallel to the wafer, and wherein the magnetic field decreases in strength with distance above the wafer, and wherein the magnetic field reflects electrons having an energy of 100 eV or less traveling toward the wafer in a Z-direction.