Patent Number: 
Section: claims

1. Ion implantation apparatus having an evacuated housing and, contained in said housing, a) a holder for holding a substrate for implantation;  b) a source of positive ions for implanting in said substrate;  c) a beam of said ions being formed which is directed at said substrate;  d) an electron flood source to supply low energy electrons to the beam in front of said substrate for neutralizing positive charge build up on said substrate;  e) a magnetic filter located adjacent to said substrate holder between said substrate holder and said electron flood source providing a magnetic field extending across the beam immediately in front of said substrate, said field having a strength selected to deflect out of the region containing the beam secondary electrons emitted from the substrate with energies above 15 eV, but to allow lower energy electrons including those supplied by said flood source to diffuse across said field without being deflected out of said beam region; and  f) a conductive element located out of said beam region to be impacted by and absorb said deflected secondary electrons;  said magnetic filter being located substantially at a substrate end of the conductive element. 2. Apparatus as claimed in  claim 1 , wherein said magnetic filter provides said magnetic field with a field strength selected to deflect secondary electrons with energies above 5 eV to impact said conductive element. claim 1 3. Apparatus as claimed in  claim 1 , wherein said magnetic filter includes a pair of opposite magnetic poles located on opposite sides of said beam region. claim 1 4. Apparatus as claimed in  claim 1 , wherein said conductive element comprises a conductive tube surrounding said beam in front of said substrate holder. claim 1 5. Apparatus as claimed in  claim 4 , wherein said electron flood source is located to supply said low energy electrons into said conductive tube. claim 4 6. A method of implanting ions in a substrate comprising, holding the substrate in an evacuated housing;  directing a beam of desired positive ions at the substrate for implanting therein;  flooding the beam in front of the substrate with low energy electrons from an electron flood source to neutralize positive charge build up on the substrate;  applying a magnetic filtering field extending across the beam immediately in front of said substrate between the substrate and the electron flood source, said field having a strength selected to deflect out of the region containing the ion beam secondary electrons emitted from the substrate with energies above 15 eV but to allow lower energy electrons, including those from the flood source, to diffuse across said magnetic filtering field without being deflected out of said beam region,  and absorbing said secondary electrons deflected by said magnetic filtering field on a conductive element located out of said beam region. 7. A method of implanting as claimed in  claim 6 , wherein the field strength of the magnetic filtering field is selected to deflect said secondary electrons with energies above 5 eV out of said region for absorbing on said conductive element. claim 6 8. A method of implanting as claimed in  claim 6 , wherein said magnetic filtering field is a dipole field. claim 6 9. A method of implanting as claimed in  claim 6 , wherein the low energy electrons from said flood source are contained around the ion beam region by a conductive tube surrounding the beam in front of the substrate, and said deflected secondary electrons are absorbed on said tube. claim 6 10. A method of preventing negative charge build up on a substrate being implanted with positive ions comprising absorbing higher energy secondary electrons emitted from the substrate at a located proximate said substrate, said higher energy secondary electrons being those emitted with energies above a predetermined value, whereby the population density of said higher energy secondary electrons immediately in front of said substrate is reduced. 11. A method as claimed in  claim 10 , wherein said higher energy secondary electrons are absorbed by deflecting them with a magnetic field extending across the surface of the substrate being implanted to impact and be absorbed on a conductive element. claim 10 12. A method as claimed in  claim 11 , wherein a beam of positive ions is directed at the substrate and said magnetic field extends across said beam to deflect said higher energy secondary electrons to be absorbed on said conductive element located out of the beam region. claim 11 13. A method as claimed in  claim 11 , including the step of flooding the region in front of said magnetic field across the substrate with low energy electrons having energies below said predetermined value, whereby said low energy electrons diffuse through said magnetic field and are absorbed on the substrate to prevent positive charge build up thereon. claim 11