System and method for producing oscillating magnetic fields in working gaps useful for irradiating a surface with atomic and molecular ions

• US 5,311,028 A
• Filed: 02/28/1992
• Issued: 05/10/1994
• Est. Priority Date: 08/29/1990
• Status: Expired due to Term

• Current Assignee: Nissin Ion Equipment Co., LTD. (Sumitomo Corporation)
• Original Assignee: Nissin Electric Company Limited (Sumitomo Corporation)
• Inventors: Glavish, Hilton F.
• Primary Examiner(s): Berman, Jack I.
• Application Number:
  US07/843,391
• Time in Patent Office: 802 Days
• Field of Search: 250/396 ML, 250/398, 250/396 R, 250/492.21, 250/492.3, 335/210, 335/213
• US Class Current: 250/492.21
• CPC Class Codes: G21K 1/093 by magnetic means
• CPC Class Codes: H01F 3/02 made from sheets
• CPC Class Codes: H01F 7/202 Electromagnets for high magnetic field strength for superconducting electromagnets H01F6/00; for transformers or inductances without a magnetic core H01F30/08
• CPC Class Codes: H01J 2237/31701 Ion implantation
• CPC Class Codes: H01J 29/76 Deflecting by magnetic fields only
• CPC Class Codes: H01J 3/32 by magnetic fields only
• CPC Class Codes: H01J 37/147 Arrangements for directing or deflecting the discharge along a desired path H01J37/045 take precedence; lenses H01J37/10
• CPC Class Codes: H01J 37/1475 magnetic
• CPC Class Codes: H01J 37/3007 Electron or ion-optical systems
• CPC Class Codes: H01J 37/3171 for ion implantation

Abstract

Deflection apparatus is shown for high perveance ion beams, operating at 20 Hz fundamental and substantially higher order harmonics, having a magnetic structure formed of laminations with thickness in range between 0.2 and 1 millimeter. Additionally, a compensator is shown with similar laminated structures with resonant excitation circuit, operating at 20 Hz or higher, in phase locked relationship with the frequency of the previously deflected beam. Furthermore, features are shown which have broader applicability to producing strong magnetic field in magnetic gap. Among the numerous important features shown are special laminated magnetic structures, including different sets of crosswise laminations in which the field in one lamination of one set is distributed into multiplicity of laminations of the other set of coil-form structures, field detection means and feedback control system, cooling plate attached in thermal contact with number of lamination layers. Surfaces on the entry and exit sides of the compensator magnetic structure have cooperatively selected shapes to increase the length of path exposed to the force field dependently with deflection angle to compensate for contribution to deflection angle caused by higher order components. The entry and exit surfaces of the magnetic scanner and compensator structures cooperating to produce desired beam profile and desired limit on angular deviation of ions within the beam. Also shown is an accelerator comprising a set of accelerator electrodes having slotted apertures, a suppressor electrode at the exit of the electrostatic accelerator, a post-accelerator analyzer magnet having means for adjusting the angle of incidence by laterally moving the post-accelerator analyzer magnet, and a magnet to eliminate aberration created by the post-accelerator analyzer magnet. In the case of use of a spinning substrate carrier for scanning in one dimension, the excitation wave form of the scanner relates changes in scan velocity in inverse dependence with changes in the radial distance of an implant point from the rotation axis. Also an oxygen implantation method is shown with 50 mA ion beam current, the ion beam energy above 100 KeV, and the angular velocity of a rotating carrier above 50 rpm.

Claims

1. A magnetic scanning system for rapidly sweeping a high perveance beam of atomic or molecular ions over a selected surface, said beam initially propagating in a predetermined direction, said scanning system comprising

a magnetic scanning structure and an associated scanning excitation circuit for repeatedly sweeping said ion beam in one dimension in response to an oscillating magnetic field having a fundamental frequency and higher order harmonics induced by excitation current from said circuit, and

a separate magnetic compensating structure and associated compensating excitation circuit spaced from said scanning structure along the beam axis, for continuously deflecting said ion beam after it has been swept by said scanning structure, to re-orient the beam to a direction substantially parallel to an output axis of said system,

said scanning circuit and said compensating circuit having separate power sources having wave forms at the same fundamental frequency for their respective scanning and compensating functions, said circuits being in constant phase relationship with a predetermined phase angle difference.

2. The magnetic scanning system of claim 1 having means to produce different, complementary wave forms for said scanning and compensating circuits.

3. The magnetic scanning system of claim 1 in which the ratio of the lateral width of the gap at the entrance of said magnetic scanning structure to the gap spacing is of the order of 3 to 1.

4. The magnetic scanning system of claim 1 wherein the lateral width of pole pieces of said scanning structure and said compensating structure increase along the length of the beam axis to accommodate a progressively wider beam sweep.

5. The magnetic scanning system of claim 1 wherein said magnetic scanning structure comprises a plurality of laminations of high magnetic permeability material having thickness in the range between about 0.2 and 1 millimeter, said laminations being separated by relatively thin electrically insulating layers, said laminations forming a yoke connecting pole pieces that define a gap through which said ion beam passes, said laminations providing a low reluctance magnetically permeable path for said fundamental frequency and higher order harmonic components of said magnetic field, the laminations serving to confine induced eddy currents to limited values in local paths in respective laminations.

6. The magnetic scanning system of claim 2, 3, 4 or 5 wherein said magnetic compensating structure comprises a plurality of laminations of high magnetic permeability material having thickness in the range between 0.2 and 1 millimeter, the laminations being separated by relatively thin electrically insulating layers, said laminations forming a yoke connecting pole pieces that define a gap through which said ion beam passes, said laminations providing a low reluctance magnetically permeable path for said fundamental frequency of said magnetic field, the laminations serving to confine induced eddy currents to limited values in local paths in respective laminations.

7. The magnetic scanning system of claim 1 including means arranged to provide said high perveance ion beam with a ribbon-profile entering said scanning structure, said scanning structure arranged to deflect said ion beam in the long direction of said ribbon profile.

8. The apparatus of claim 1 wherein said compensating excitation circuit is constructed to operate in resonance with the fundamental frequency of oscillation of the previously deflected beam.

9. The magnetic scanning system of claim 8 further comprising a synthesizer for producing said fundamental frequency.

10. The system of claim 8 wherein said excitation circuit comprises

excitation coils connected in series,

a tank capacitance connected in parallel with said excitation coils,

a coupling capacitance having a value related to the inductance of the circuit to achieve resonant operation, and

a power amplifier driven by said fundamental frequency signal for delivering power to said circuit to compensate for the energy losses in said excitation circuit and said magnetic structure.

11. The magnetic scanning system of claim 1 further comprising means to drive said magnetic scanning structure by an excitation current of a generally triangular wave formed by said fundamental frequency and superimposed higher order harmonics, and means to drive said magnetic compensating structure by excitation current in resonance with said fundamental frequency.

12. The magnetic scanning system of claim 11 wherein said generally triangular wave form is selected to compensate for a component of scanning motion of the surface being irradiated.

13. The system of claim 11 or 12 including a dynamic feedback control comprising

magnetic field detection means for detecting a magnetic field of said magnetic scanning system and producing signals representing said magnetic field and,

a feedback control system for controlling the excitation current producing a prescribed magnetic field in a closed loop arrangement with the respective magnetic excitation circuit and magnetic field detection means.

14. The system of claim 13 wherein said field detection means comprise at least one inductive coil inside of said magnetic circuit.

15. The system of claim 14 wherein said feedback control system further comprises

a signal conditioner connected to said inductive coil and adapted to eliminate circuitry-induced distortions of said detected signal by adjusting the gain and shape of said detected signal,

a phase compensator adapted to receive said signal from said signal conditioner and to correct said detected signal for a phase shift arising from electronic delay times and finite permeability of the yoke material,

a differential amplifier adapted to generate and to amplify an error voltage which is a difference between a reference signal and said detected signal conditioned and compensated by said conditioner and said compensator, respectively, and

said power amplifier of said resonant excitation circuit adapted to receive said error voltage and generate excitation current to said excitation coil proportional to said error voltage to produce said prescribed magnetic field.

16. The magnetic scanning system of claim 13 including a respective said dynamic feedback control for each said magnetic scanning structure and said magnetic compensating structure.

17. The magnetic scanning system of claim 1 or 7 wherein said magnetic compensating structure comprises pole pieces having surfaces on the beam entry and exit sides of cooperatively selected shape to increase, dependently with deflection angle of the beam, the length of the ion beam path exposed to the force-field of said magnetic compensating structure to compensate for the contribution to said deflection angle caused by said higher order harmonics of said scanning structure excitation current so that said ion beam is re-oriented by said magnetic compensating structure to a direction substantially parallel to an exit axis of said magnetic compensating structure.

18. The magnetic scanning system of claim 1 or 8 wherein the surfaces on the beam entry and beam exit sides of said magnetic scanning structure and of said magnetic compensating structure are shaped in a predetermined cooperative relationship relative to the profile of said beam entering said system to establish cooperating magnetic fringing fields for both said magnetic structures that cooperate to produce a desired profile of said beam and desired limitation on the angular deviation of ions within said beam.

19. The magnetic scanning system of claim 1 having, prior to said magnetic scanning structure, an electrostatic-magnetostatic system for accelerating and focusing a high perveance beam of atomic or molecular ions comprising

a set of acceleration electrodes forming an electrostatic accelerating system charged by a power source, said electrodes adapted to electrostaticly focus said ion beam in one dimension, said electrodes adapted to supply electrostatic energy to accelerate said ion beam in the direction corresponding to the potential difference across said electrodes,

a suppressor electrode for maintaining electrons within said beam, said electrode having an aperture and being located at the exit port of the electrostatic accelerating system, and

a post-acceleration analyzer magnet arranged to focus said beam in the other direction, said magnet having means for adjusting the angle of incidence of said ion beam into the entrance region of said post-acceleration magnet.

20. The magnetic scanning system of claim 19 wherein said electrodes form a slotted aperture, said electrodes adapted to focus said beam in the direction of the short dimension of said slotted aperture.

21. The magnetic scanning system of claim 19 and 20 including means arranged to provide said high perveance ion beam with a ribbon-profile entering said accelerating system, said accelerating system arranged to focus said beam in the short direction of said ribbon profile.

22. The magnetic scanning system of claim 19 or 20 wherein said set of acceleration electrodes comprises two relatively movable electrodes for electrostatic focusing of said ion beam.

23. The magnetic scanning system of claim 19 or 20 wherein said set of acceleration electrodes comprises three fixed electrodes adapted to electrostaticly focus said ion beam by varying the electric potential on the center electrode.

24. The magnetic scanning system of claim 19 or 20 wherein said means for adjusting the angle of incidence of said ion beam into the entrance region of said post-acceleration magnet comprises means for laterally moving said post-acceleration magnet relative to the beam path, thereby focusing said beam while removing neutral particles and ions of unwanted momentum ions from said ion beams.

25. The magnetic scanning system of claim 24 further comprising a multipole magnet with a static magnetic field arranged to eliminate aberration of said ion beam created by said post-acceleration analyzer magnet.

26. The system of claim 25 wherein said multipole magnet is a sextupole.

27. The system of claim 1 further comprising a reciprocating carrier for repeatedly carrying said selected surface mounted on said carrier under said beam to effect scanning in one dimension,

said magnetic scanning system arranged for rapidly sweeping said ion beam in the orthogonal dimension.

28. The system of claim 1 further comprising

a driven rotatable carrier for repeatedly carrying said selected surface mounted on said carrier under said beam to effect scanning in one dimension,

said magnetic scanning system arranged for rapidly sweeping said ion beam in the radial direction of said rotatable carrier to effect scanning in the other dimension,

and means for producing said excitation wave form applied to said magnetic scanning system to govern its sweeping action, the functional dependence of said excitation wave form relating changes in the scan velocity in inverse dependence with change in the radial distance between the instantaneous location of the center of the beam and the axis of rotation of said rotatable carrier.

29. The system of claim 1 including a focusing device for determining the dimension of the beam in the direction of magnetic scanning and normal thereto.

30. The system of claim 29 wherein said focusing device comprises an adjustable accelerator having slotted apertures, the narrow dimension of the apertures corresponding to the direction normal to the magnetic scanning dimension.

31. A method of rapidly sweeping over a selected surface a high perveance beam of atomic or molecular ions initially propagating in a predetermined direction, comprising the steps of:

(a) introducing said ion beam to a magnetic scanning structure,

(b) sweeping said ion beam in one dimension by an oscillating magnetic field having a fundamental frequency of at least 20 Hz and higher order harmonics induced by a scanning excitation current generated by a scanning excitation circuit, to produce a constantly changing deflection angle and

(c) continuously re-deflecting said deflected ion beam using a separate magnetic compensating structure to re-orient said ion beam to a direction substantially parallel to an exit axis of the compensating structure by driving said magnetic compensating structure with a compensating excitation current having the same fundamental frequency as said scanning excitation current, said compensating excitation current being generated by a separate compensating circuit operating in a phase-locked mode with said scanning excitation circuit.

32. The method of claim 31 including generating said compensating excitation current in resonance with said fundamental frequency of said scanning excitation current.

33. The method of claim 32 wherein the length of the ion beam path exposed to the force-field of said magnetic compensating structure is varied to compensate for the contribution to said deflection angle caused by higher order harmonics of said scanning excitation current.

34. A method of implanting a uniform dose of oxygen ions into a silicon wafer to form a buried oxide layer, using an ion implantation system, comprising the steps of:

(a) accelerating a high perveance oxygen ion beam of above 50 mA current to energy above 100 keV,

(b) rotating said silicon wafer mounted on a carrier at above 50 rpm,

(c) with a magnetic scanning system, scanning said ion beam in the radial direction of said carrier at a frequency above 50 Hz while maintaining a substantially constant incident angle of said beam to the surface of said silicon wafer,

(d) controlling said system to obtain substantially uniform dose across the wafer under substantially uniform temperature conditions,

said scanning step including deflecting said beam in an oscillating pattern using a magnetic scanner drive by a substantially triangular wave form and said controlling said system including applying compensating magnetic fields to reorient the deflected beam to a desired direction, said compensating step being performed with a dynamic magnetic deflecting compensator system driven at the same frequency in phase locked relationship with said scanning.

35. The method of claim 34 wherein said compensator is driven in resonance with said scanning.