Ion implantation surface charge control method and apparatus

• US 4,804,837 A
• Filed: 01/11/1988
• Issued: 02/14/1989
• Est. Priority Date: 01/11/1988
• Status: Expired due to Term

• Current Assignee: Axcelis Technologies Incorporated
• Original Assignee: Eaton Corp. (Wisconsin) (Eaton Corporation PLC.)
• Inventors: Farley, Marvin
• Primary Examiner(s): Anderson, Bruce C.
• Application Number:
  US07/142,365
• Time in Patent Office: 400 Days
• Field of Search: 250/251, 250/492.21, 250/398, 315/111.81
• US Class Current: 250/251
• CPC Class Codes: G21K 1/14 using charge exchange devices, e.g. for neutralising or changing the sign of the electrical charges of beams producing or accelerating neutral particle beams H05H3/00
• CPC Class Codes: H01J 2237/31701 Ion implantation
• CPC Class Codes: H01J 37/026 Means for avoiding or neutralising unwanted electrical charges on tube components

Abstract

An ion beam neutralizer. High energy electrons are directed through an ion beam neutralizing zone or region containing an ionizable gas. As the high energy electrons collide with the gas molecules, they ionize the gas molecules and produce low energy electrons which are trapped by a positively charged ion beam. As high energy electrons pass out of the neutralizing zone they are deflected back to the neutralizing zone by a cylindrical conductor biased to deflect the high energy electrons and an accelerating grid for accelerating the electrons back through the beam neutralizing zone.

Claims

1. In an ion beam implantation system that includes an ion source for producing a positively charged ion beam and for directing said ion beam along a path to impinge on a workpiece, apparatus for controlling ion beam charging of the workpiece comprising:

(a) source means for directing high energy electrons through a beam neutralizing region that is intersected by said ion beam;

(b) means for providing an ionizable gas in the beam neutralizing region that collide with said high energy electrons to provide low energy secondary electrons that are trapped by the ion beam to reduce the charge of said ion beam;

(c) repeller means for deflecting high energy electrons that pass through said beam neutralizing region back toward said beam neutralizing region; and

(d) grid means for accelerating electrons deflected from the repeller means back into the beam neutralizing region.

2. The apparatus of claim 1 where the repeller means comprises a conductive elongated cylinder which bounds said beam neutralizing region, said cylinder having an inlet opening and an outlet opening at opposite ends of the cylinder to allow said ion beam to pass through said cylinder.

3. The apparatus of claim 1 where the source means comprises a filament wire, a power supply to energize the filament wire, an electrode to accelerate electrons emitted by the wire to the beam neutralizing region, and a power supply to bias the electrode relative the filament wire.

4. The apparatus of claim 3 wherein the grid means comprises a wire mesh that forms a cylinder inside the conductive elongate cylinder of said repeller means and which is maintained at the same electric potential as the electrode which accelerates electrodes leaving the filament wire.

5. A method for controlling ion beam charging of a workpiece placed in an ion beam path comprising the steps of:

(a) directing the ion beam along a beam path to cause said ion beam to impinge upon a workpiece;

(b) injecting high energy electrons through a volume intersected by the ion beam containing ionizable gas prior to ion beam impingement on the workpiece to ionize gas molecules in the volume and produce secondary low energy electrons which are attracted to the ion beam and neutralize the charge of said beam to reduce charge build up on the workpiece; and

(c) changing a trajectory of high energy electrons that pass through the volume containing said ionizable gas and then accelerating re-directed electrons back into the volume containing the ionizable gas to ionize additional gas molecules in the volume.

6. The method of claim 6 wherein high energy electrons are emitted from a filament and accelerated to the high energy by an accelerating electrode.

7. In an ion beam implantation system comprising:

(a) a source for producing a positively charged ion beam and for directing said ion beam along a path to impinge on a workpiece;

(b) wafer positioning means for positioning a workpiece within the ion beam;

(c) electron producing means positioned along an ion beam travel path for directing high energy electrons through a beam neutralizing region that is intersected by said ion beam;

(d) means for providing an ionizable gas in the beam neutralizing region that collide with said high energy electrons to provide low energy secondary electrons that are trapped by the ion beam to reduce a net charge of said ion beam;

(e) repeller means for deflecting high energy electrons that pass through said beam neutralizing region without colliding with gas molecules back toward said beam neutralizing region;

(f) grid means for accelerating electrons deflected from the repeller means back into the beam neutralizing region; and

(g) power supply means for biasing the repeller means at a potential below the grid means.

8. The system of claim 7 wherein the repeller means comprises a metal cylinder closed in at an entrance and exit by annular aperture defining disks.

9. The system of claim 8 wherein the grid means comprises a wire tantalum mesh supported inside the cylinder and having an mesh entrance and mesh exit coaxial with the repeller entrance and exit.

10. The system of claim 7 wherein the electron producing means comprises a wire filament, a low voltage adjustable power supply for energizing the filament, a metal electrode for accelerating electrons from the filament and a power supply for biasing the filament below the electrode.