Charge neutralization apparatus for ion implantation system

• US 5,136,171 A
• Filed: 01/25/1991
• Issued: 08/04/1992
• Est. Priority Date: 03/02/1990
• Status: Expired due to Term

• Current Assignee: Varian Semiconductor Equipment Associates Incorporated (Applied Materials Incorporated)
• Original Assignee: Varian Associates, Inc.
• Inventors: Williams, Malcom D., Leung, Ka-Ngo, McKenna, Charles M., Kunkel, Wulf B.
• Primary Examiner(s): Berman, Jack I.
• Application Number:
  US07/646,361
• Time in Patent Office: 557 Days
• Field of Search: 250/492.2, 250/492.21, 250/492.3, 250/398, 250/251, 335/210, 335/300
• US Class Current: 250/492.2
• CPC Class Codes: H01J 37/026 Means for avoiding or neutralising unwanted electrical charges on tube components
• CPC Class Codes: H01J 37/3171 for ion implantation plasma immersion ion implantation H01J37/32412

Abstract

Methods and apparatus for neutralization of a workpiece such as a semiconductor wafer in a system wherein a beam of positive ions is applied to the workpiece. The apparatus includes an electron source for generating an electron beam and a magnetic assembly for generating a magnetic field for guiding the electron beam to the workpiece. The electron beam path preferably includes a first section between the electron source and the ion beam and a second section which is coincident with the ion beam. The magnetic assembly generates an axial component of magnetic field along the electron beam path. The magnetic assembly also generates a transverse component of the magnetic field in an elbow region between the first and second sections of the electron beam path. The electron source preferably includes a large area lanthanum hexaboride cathode and an extraction grid positioned in close proximity to the cathode. The apparatus provides a high current, low energy electron beam for neutralizing charge buildup on the workpiece.

Claims

1. Apparatus for neutralization of a workpiece in a system wherein a beam of positive ions is applied to the workpiece, comprising:

an electron source for generating an electron beam having a relatively large cross-sectional area; and

p1 means for transporting said electron beam from said electron source to said workpiece so as to provide substantially uniform distribution of said electron beam over said workpiece, said means for transporting comprising magnetic means for generating a magnetic field along a prescribed electron beam path between said electron source and said workpiece for guiding said electron beam to said workpiece, said magnetic field having an axial component parallel to said electron beam path along substantially the entire length of said electron beam path, and a supply of positive ions along said electron beam path for space charge neutralization of said electron beam.

2. Apparatus as defined in claim 1 wherein said magnetic means includes a steering coil extending generally between said electron source and said workpiece for generating said axial component of magnetic field.

3. Apparatus as defined in claim 1 wherein said magnetic means comprises a steering coil including a generally helical conductor surrounding said electron beam and means for providing an electrical current through said steering coil for generating an axial component of said magnetic field generally parallel to said beam path.

4. Apparatus as defined in claim 3 wherein said steering coil includes a first section and a second section, said steering coil having an opening between said first and second sections for receiving said ion beam, said electron beam and said ion beam being coincident in said second section.

5. Apparatus as defined in claim 4 wherein said steering coil extends from a region adjacent to said electron source to a region adjacent to said workpiece.

6. Apparatus as defined in claim 4 wherein said magnetic means further includes means for generating a transverse component of said magnetic field in an elbow region between said first and second sections that is perpendicular to said electron beam.

7. Apparatus as defined in claim 6 wherein said means for generating a tranverse components of said magnetic field comprises coil means positioned above and below said elbow region.

8. Apparatus as defined in claim 7 wherein said magnetic means further includes first and second trim coils in said elbow region between said first and second sections for modifying the axial component of said magnetic field, said first trim coil having a plane perpendicular to said ion beam and said second trim coil having a plane parallel to said ion beam.

9. Apparatus as defined in claim 7 wherein said magnetic means further includes a magnetically permeable element between said workpiece and said electron source for completing a magnetic circuit with said steering coil.

10. Apparatus as defined in claim 1 wherein the electrons in said electron beam have energies of 30 electron volts or less.

11. Apparatus as defined in claim 1 wherein said electron source comprises a large area cathode and means for extracting low energy electrons from said cathode.

12. Apparatus as defined in claim 11 wherein said cathode comprises lanthanum hexaboride.

13. Apparatus as defined in claim 10 wherein said magnetic field has an axial component in the direction of said electron beam less than 100 gauss.

14. Apparatus as defined in claim 12 wherein said cathode has an area that is approximately equal to the area of the workpiece.

15. A method for neutralization of a workpiece in a system wherein a beam of positive ions is applied to the workpiece, said method comprising the steps of:

generating an electron beam having a relatively large cross-sectional area with an electron source; and

transporting said electron beam from said electron source to said workpiece so as to provide substantially uniform distribution of said electron beam over said workpiece, the step of transporting said electron beam including generating a magnetic field along a prescribed electron beam path between said electron source and said workpiece for guiding said electron beam to said workpiece, said magnetic field having an axial component parallel to said electron beam path along substantially the entire length of said electron beam path, and providing a supply of positive ions along said electron beam path for space charge neutralization of said electron beam.

16. A method as defined in claim 15 wherein the step of generating a magnetic field includes providing a steering coil extending generally between said electron source and said workpiece for generating said axial component of magnetic field.

17. A method as defined in claim 16 wherein said electron beam path includes a first section and a second section that are generally perpendicular to each other, said first and second sections intersecting in an elbow region, and wherein the step of generating a magnetic field further includes the step of generating a transverse component of said magnetic field perpendicular to said electron beam path in said elbow region.

18. A method as defined in claim 15 wherein the step of generating an electron beam includes extracting low energy electrons from a large area lanthanum hexaboride cathode with an extraction grid positioned in close proximity to said cathode.

19. Ion implantation apparatus comprising:

means for generating a beam of positive ions for treatment of a semiconductor wafer;

an electron source for generating an electron beam having a relatively large cross-sectional area; and

means for transporting said electron beam from said electron source to said semiconductor wafer so as to provide substantially uniform distribution of said electron beam over said semiconductor wafer, said means for transporting comprising magnetic means for generating a magnetic field along a prescribed electron beam path between said electron source and said semiconductor wafer for guiding said electron beam to said semiconductor wafer, said magnetic field having an axial component parallel to said electron beam path along substantially the entire length of said electron beam path, and a supply of positive ions along said electron beam path for space charge neutralization of said electron beam.

20. Ion implantation apparatus as defined in claim 19 wherein said magnetic means comprises a steering coil including a generally helical conductor surrounding said electron beam and means for providing an electrical current through said steering coil for generating said axial component of said magnetic field generally parallel to said beam path.

21. Ion implantation apparatus as defined in claim 20 wherein said steering coil includes a first section and a second section, said steering coil having an opening between said first and second sections for receiving said ion beam, said electron beam and said ion beam being coincident in said second section.

22. Ion implantation apparatus as defined in claim 21 wherein said steering coil extends from a region adjacent to said electron source to a region adjacent to said workpiece.

23. Ion implantation apparatus as defined in claim 21 wherein said magnetic means further includes means for generating a transverse component of said magnetic field in an elbow region between said first and second sections that is perpendicular to said electron beam.

24. Ion implantation apparatus as defined in claim 23 wherein said means for generating a transverse component of said magnetic field comprises coil means positioned above and below said elbow region.

25. Ion implantation apparatus as defined in claim 19 wherein said electron source comprises a large area cathode and means for extracting low energy electrons from said cathode.

26. Ion implantation apparatus as defined in claim 25 wherein said cathode comprises lanthanum hexaboride.

27. Ion implantation apparatus comprising:

means for generating an ion beam of positive ions and for directing the ion beam along an ion beam path to a semiconductor wafer;

an electron source for generating an electron beam having a relatively large cross-sectional area;

means for transporting said electron beam from said electron source to said semiconductor wafer so as to provide substantially uniform distribution of said electron beam over said semiconductor wafer, said means for transporting comprising magnetic means for generating a magnetic field along a prescribed electron beam path between said electron source and said semiconductor wafer for guiding said electron beam to said semiconductor wafer, said magnetic field having an axial component parallel to said electron beam path along substantially the entire length of said electron beam path, and a supply of positive ions along said electron beam path for space charge neutralization of said electron beam,

a vacuum chamber enclosing said ion beam path, said electron beam path and said semiconductor wafer, said vacuum chamber defining a high vacuum region; and

means for vacuum pumping of said vacuum chamber.

28. Ion implantation apparatus as defined in claim 27 wherein said electron source is remotely located from said ion beam path, and further including means for selectively isolating said electron source from said high vacuum region.

29. Ion implantation apparatus as defined in claim 28 wherein said means for selectively isolating said electron source includes vacuum valve means positioned between said electron source and said high vacuum region.

30. Ion implantation apparatus as defined in claim 29 wherein said magnetic means comprises a steering coil including a generally helical conductor surrounding said electron beam and means for providing an electrical current through said steering coil.

31. Ion implantation apparatus as defined in claim 27 wherein said ion beam and said electron beam intersect at an angle in a range from slightly greater than 0°

to slightly less than 180°

.

32. Ion implantation apparatus as defined in claim 27 wherein said ion beam and said electron beam intersect at an angle in a range from about 90°

to slightly less than 180°

.

34. Ion implantation apparatus as defined in claim 27 further including cooling means disposed along said electron beam path for removing thermal energy generated by said electron source and for condensing materials emitted by said electron source.

35. Ion implantation apparatus as defined in claim 34 wherein said magnetic means comprises a steering coil including a generally helical conductor surrounding said electron beam and means for providing an electrical current through said steering coil.

36. Ion implantation apparatus as defined in claim 35 wherein said steering coil comprises a tubular conductor and wherein said cooling means comprises means for circulating a cooling fluid through said tubular conductor.

37. Ion implantation apparatus as defined in claim 36 wherein cooling means further comprises a nonmagnetic tube located within said steering coil and thermally connected to said steering coil.

38. Ion implantation apparatus as defined in claim 27 further including a Faraday charge measurement system positioned between said means for generating an ion beam and said semiconductor wafer, said electron beam being guided into said Faraday charge measurement system.

39. Ion implantation apparatus as defined in claim 38 wherein said magnetic means comprises a steering coil including a generally helical conductor surrounding said electron beam and means for providing an electrical current through said steering coil.

40. Ion implantation apparatus as defined in claim 39 further including cooling means disposed along said electron beam path for removing thermal energy generated by said electron source and for condensing materials emitted by said electron source.

41. Ion implantation apparatus as defined in claim 40 wherein said cooling means comprises a nonmagnetic, thermally conductive tube located within said steering coil and thermally connected to said steering coil, said thermally conductive tube extending from a region adjacent to said electron source to said Faraday charge measurement system.

42. Ion implantation apparatus as defined in claim 41 wherein said steering coil comprises a tubular conductor and wherein said cooling means comprises means for circulating a cooling fluid through said tubular conductor.

43. Ion implantation apparatus as defined in claim 27 wherein said electron source comprises a large area cathode and means for extracting low energy electrons from said cathode.

44. Ion implantation apparatus as defined in claim 43 wherein said cathode comprises lanthanum hexaboride.

45. Ion implantation apparatus as defined in claim 27 wherein said magnetic means further includes means for generating a tranverse component of said magnetic field in an elbow region where said electron beam intersects said ion beam, said transverse component being perpendicular to said electron beam and said ion beam.

46. Ion implantation apparatus as defined in claim 45 wherein said means for generating a transverse component of said magnetic field comprises Helmholtz coils positioned on opposite sides of said ion beam.

47. Ion implantation apparatus as defined in claim 43 wherein said large area cathode has an area that is approximately equal to the area of the semiconductor wafer.

48. Ion implantation apparatus as defined in claim 27 wherein said electron source generates an electron beam having a current of up to 100 milliamps.

33. Ion implantation apparatus comprising:

means for generating an ion beam of positive ions and for directing the ion beam along an ion beam path to a semiconductor wafer;

an electron source for generating an electron beam;

magnetic means for generating a magnetic field along a prescribed electron beam path between said electron source and said semiconductor wafer for guiding said electron beam to said semiconductor wafer, said magnetic field having an axial component parallel to said electron beam path along substantially the entire length of said electron beam path;

a vacuum chamber enclosing said ion beam path, said electron beam path and said semiconductor wafer, said vacuum chamber defining a high vacuum region; and

means for vacuum pumping of said vacuum chamber, said ion beam and said electron beam intersecting at an angle in a range from greater than 90°

to slightly less than 180°

such that contaminants from the electron source are directed away from the semiconductor wafer.