Method and apparatus for producing high density plasma using whistler mode excitation

• US 5,225,740 A
• Filed: 03/26/1992
• Issued: 07/06/1993
• Est. Priority Date: 03/26/1992
• Status: Expired due to Fees

• Current Assignee: General Atomics, Inc.
• Original Assignee: General Atomics, Inc.
• Inventors: Ohkawa, Tihiro
• Primary Examiner(s): LaRoche, Eugene R.
• Assistant Examiner(s): YOO, DO H
• Application Number:
  US07/857,981
• Time in Patent Office: 467 Days
• Field of Search: 315/111.21, 315/111.41, 315/111.71, 315/111.81, 313/231.31, 204/298.38, 204/298.37
• US Class Current: 315/111.41
• CPC Class Codes: H01J 37/32192 Microwave generated discharge H01J37/32357, H01J37/32366, H01J37/32394, H01J37/32403 take precedence
• CPC Class Codes: H01J 37/32247 Resonators
• CPC Class Codes: H01J 37/3266 Magnetic control means
• CPC Class Codes: H05H 1/18 wherein the fields oscillate at very high frequency, e.g. in the microwave range , e.g. using cyclotron resonance
• CPC Class Codes: H05H 1/46 using applied electromagnetic fields, e.g. high frequency or microwave energy H05H1/26 takes precedence

Abstract

High density plasma is produced in a long cylindrical cavity by the excitation of a high frequency whistler wave within the cavity. The cylindrical cavity, and hence the plasma, is imbedded in a high magnetic field, with magnetic lines of force passing axially (longitudinally) through the cavity. The magnetic field has an electron cyclotron frequency associated therewith that is much greater than the wave frequency associated with the whistler wave. In one embodiment, electromagnetic energy is coupled axially into the cylindrical cavity using a resonant cavity, which coupled energy excites the whistler wave. In another embodiment, electromagnetic energy is coupled radially into the cylindrical cavity using a slow wave structure. The plasma is created without using electrodes; and the excitation of the whistler wave is achieved at a high Q value. Various configurations are used to couple between the resonant cavity or circuit and the plasma, thereby allowing the invention to be used for numerous applications, such as plasma processing, e.g., plasma etching, stripping or deposition; or the excitation of high power lasers; ion sources; or sputtering guns.

Claims

1. Plasma production apparatus comprising:

a substantially cylindrical plasma chamber having a longitudinal axis, said plasma chamber having a prescribed gas therein;

magnetic field generating means for generating a magnetic field with an associated electron cyclotron frequency ω

_cc and magnetic lines of force that axially traverse said plasma chamber; and

resonant excitation means for exciting a whistler wave having a frequency ω

in said plasma chamber using resonant electromagnetic energy, where ω

_cc <

ω

, said whistler wave ionizing said prescribed gas, whereby ionized particles exist in said plasma chamber having a collisional frequency associated therewith, said ionized particles comprising a plasma;

said resonant excitation means comprising a resonant cavity positioned adjacent a first end of said plasma chamber, said resonant cavity being substantially axially aligned with said longitudinal axis so that electromagnetic energy resonating in said resonant cavity axially couples into the plasma within said plasma chamber.

2. The plasma production apparatus as set forth in claim 1 further including plasma delivery means for controlling the delivery of plasma from said plasma chamber to a workpiece.

3. The plasma production apparatus as set forth in claim 2 wherein said workpiece is positioned adjacent a second end of said plasma chamber, said plasma delivery means directing plasma from said plasma chamber to said workpiece, whereby plasma processing may be carried out on said workpiece.

4. Plasma production apparatus comprising:

a substantially cylindrical plasma chamber having a longitudinal axis, said plasma chamber having a prescribed gas therein;

magnetic field generating means for generating a magnetic field with an associated electron cyclotron frequency ω

_cc and magnetic lines of force that axially traverse said plasma chamber; and

resonant excitation means for exciting a whistler wave having a frequency ω

in said plasma chamber using resonant electromagnetic energy, where ω

_cc <

ω

, said whistler wave ionizing said prescribed gas, said resonant excitation means comprising;

a multiplicity of annular conductive rings within said plasma chamber, said rings being centered about and uniformly spaced along the longitudinal axis of the plasma chamber so as to form a slow wave structure inside of said plasma chamber, and

means for causing an electrical current to flow through said conductive rings so as to excite resonant electromagnetic energy that is radially coupled into said plasma, said radially coupled electromagnetic energy exciting said whistler wave within said plasma chamber;

whereby ionized particles exist in said plasma chamber having a collisional frequency associated therewith, said ionized particles comprising the plasma.

5. The plasma production apparatus as set forth in claim 4 wherein the electrical current flowing in said conductive rings excites said whistler wave using a TE₀₁ mode.

6. The plasma production apparatus as set forth in claim 4 wherein the electrical current flowing in said conductive rings excites said whistler wave using a TE₁₁ mode.

7. Apparatus for exciting a gas to produce a laser comprising:

a substantially cylindrical plasma chamber having a longitudinal axis, said plasma chamber having a prescribed gas therein;

magnetic field generating means for generating a mangetic field with an associated electron cyclotron frequency ω

_cc and magnetic lines of force that axially traverse said plasma chamber; and

resonant excitation means for exciting a whistler wave having a frequency ω

in said plasma chamber using resonant electromagnetic energy, where ω

_cc <

ω

, said whistler wave ionizing said prescribed gas;

said plasma chamber being closed at both ends;

said closed plasma chamber and the plasma confined therein comprising part of an optical system that allows coherent light to reflect between two reflective surfaces, one placed at each end of said plasma chamber;

said resonant excitation means including a resonant cavity that surrounds at least a portion of said plasma chamber;

said resonant excitation means including means for exciting electromagnetic energy within said resonant cavity that is radially coupled into said plasma chamber so as to excite said whistler wave, said whistler wave, in turn, ionizing said prescribed gas and producing coherent light that reflects between said two reflective surfaces, thereby producing the laser.

8. The apparatus as set forth in claim 7 wherein said resonant cavity comprises a slotted resonant cavity surrounding said plasma chamber, said slotted resonant cavity having an inner wall at a first radius from said longitudinal axis, said slotted resonant cavity having periodically slotted gaps, the spacing of said gaps being determined by the wavelength of the excited whistler wave, and a solid outer wall spaced at a second radius from said longitudinal axis, said second radius being greater than said first radius.

9. Plasma production apparatus comprising:

a cylindrical cavity having a prescribed gas therein;

a magnetic field generator that generates a magnetic field having magnetic lines of force that axially traverse said cylindrical cavity; and

an electromagnetic launcher that generates and launches resonant electromagnetic energy into the gas held in said cylindrical cavity, said electromagnetic energy exciting a whistler wave that axially propagates through said cylindrical cavity following said magnetic lines of force and ionizes said prescribed gas, thereby producing a plasma;

said electromagnetic launcher comprising;

a resonant cavity positioned adjacent a first end of said cylindrical cavity, said resonant cavity being substantially axially aligned with a longitudinal axis of said cylindrical cavity, and

means for exciting the resonant electromagnetic energy in said resonant cavity, said resonant electromagnetic energy being coupled axially into said cylindrical cavity.

10. Plasma production apparatus comprising:

a cylindrical cavity having a prescribed gas therein;

a magnetic field generator that generates a magnetic field having magnetic lines of force that axially traverse said cylindrical cavity; and

an electromagnetic launcher that generates and launches electromagnetic energy into the gas held in said cylindrical cavity, said electromagnetic energy exciting a whistler wave that axially propagates through said cylindrical cavity following said magnetic lines of force and ionizing said prescribed gas, thereby producing a plasma;

said electromagnetic launcher comprising

a multiplicity of annular conductive rings within said cylindrical cavity, said rings being centered about and uniformly spaced along a longitudinal axis of the cylindrical cavity so as to form a slow wave structure inside of said cylindrical cavity, and

means for exciting an electrical current in said conductive rings so as to excite resonant electromagnetic energy within said cylindrical cavity, said resonant electromagnetic energy being coupled radially into the gas contained within said cylindrical cavity.

11. A method of producing plasma comprising:

(a) selectively injecting a specified gas into a cylindrical cavity, said cylindrical cavity having a longitudinal axis;

(b) generating a magnetic field having an associated electron cyclotron frequency ω

_cc and magnetic lines of force that are substantially parallel to said longitudinal axis; and

(c) generating resonant electromagnetic energy in a resonant cavity, said resonant cavity being positioned adjacent said cylindrical cavity; and

(d) exciting a whistler wave in said cylindrical cavity by coupling the resonant electromagnetic energy from said resonant cavity into said cylindrical cavity, said whistler wave propagating through said cylindrical cavity following said magnetic lines of force and ionizing said prescribed gas, thereby producing a plasma, said whistler wave having a frequency ω

, where ω

_cc >

ω

.

12. The method as set forth in claim 11 wherein step (c) further includes axially coupling the resonant electromagnetic energy into said cylindrical cavity.

13. The method as set forth in claim 11 wherein step (c) further includes radially coupling the resonant electromagnetic energy into said cylindrical cavity.

14. The method as set forth in claim 11 wherein said plasma is a high density plasma.