Plasma reactor with overhead RF source power electrode having a resonance that is virtually pressure independent

• US 7,141,757 B2
• Filed: 02/13/2004
• Issued: 11/28/2006
• Est. Priority Date: 03/17/2000
• Status: Expired due to Term

• Current Assignee: Applied Materials Incorporated
• Original Assignee: Applied Materials Incorporated
• Inventors: Buchberger, Douglas A. Jr., Hoffman, Daniel, Yang, Jang Gyoo, Burns, Douglas
• Primary Examiner(s): PASCHALL, MARK H
• Application Number:
  US10/778,620
• Publication Number:
  US 20040159287A1
• Time in Patent Office: 1,019 Days
• Field of Search: 219/121.4, 219/121.41, 219/121.43, 219/121.57, 219/121.59, 156/345.43, 156/345.44, 156/345.47, 315/111.21, 315/111.51, 118/723.I, 118/723.R
• US Class Current: 219/121.43
• CPC Class Codes: H01J 37/32082 Radio frequency generated discharge H01J37/32357, H01J37/32366, H01J37/32394 and H01J37/32403 take precedence
• CPC Class Codes: H01J 37/32091 the radio frequency energy being capacitively coupled to the plasma
• CPC Class Codes: H01J 37/32183 Matching circuits
• CPC Class Codes: H01J 37/3244 Gas supply means
• CPC Class Codes: H01J 37/32623 Mechanical discharge control means
• CPC Class Codes: H01J 37/3266 Magnetic control means

Abstract

A plasma reactor operable over a very wide process window of pressure, source power and bias power includes a resonant circuit consisting of an overhead electrode having a first impedance, a wafer support pedestal having a second impedance and a bulk plasma having a third impedance and generally lying in a process zone between the overhead electrode and the wafer support pedestal, the magnitudes of the impedances of the overhead electrode and the wafer support pedestal being within an order of magnitude of one another, the resonant circuit having a resonant frequency determined by the first, second and third impedances.

Claims

1. A plasma reactor, comprising:

a chamber enclosure and a wafer support pedestal within said chamber enclosure;

an overhead electrode facing said wafer support pedestal and having a capacitance to ground;

an RF plasma source power generator and a fixed impedance match element coupled to said RF plasma source power generator and to said overhead electrode;

an RF plasma bias power generator and an impedance match circuit coupled to said RF plasma bias power generator;

said wafer support pedestal comprising;

(a) a conductive RF feed layer coupled to said impedance match circuit;

(b) a grounded base layer;

(c) a dielectric gap separating said grounded base layer from said conductive RF feed layer to form a capacitor across the dielectric gap, said capacitor having a capacitance to ground that is within an order of magnitude of the capacitance to ground of said overhead electrode.

2. The apparatus of claim 1 wherein said capacitance to ground of said capacitor is within a factor of two of the capacitance to ground of said overhead electrode.

3. A plasma reactor comprising:

a resonant circuit comprising an overhead electrode having a first impedance, a wafer support pedestal having a second impedance and a bulk plasma having a third impedance and generally lying in a process zone between said overhead electrode and said wafer support pedestal, the magnitudes of the impedances of said overhead electrode and said wafer support pedestal being within an order of magnitude of one another, said resonant circuit having a resonant frequency determined by said first, second and third impedances;

an RF plasma source power generator having a frequency at least nearly matching said resonant frequency; and

a fixed impedance match element coupled to said RF plasma source power generator and to said overhead electrode.

4. The apparatus of claim 3 wherein the magnitudes of the impedances of said overhead electrode and said wafer support pedestal are within a factor of two of one another.

5. The plasma reactor of claim 3 wherein said first and second impedances have capacitive reactances and said third impedance has an inductive reactance.

6. The apparatus of claim 3 further comprising a bias power RF generator and an impedance match circuit coupled to said bias power RF generator and to said wafer support pedestal.

7. The apparatus of claim 3 wherein said fixed impedance match element has a resonant frequency at least nearly matching said resonant frequency.

8. The apparatus of claim 3 wherein the impedance of said bulk plasma changes with gas pressure inside said reactor, and wherein said impedance of said wafer support pedestal near said resonant frequency is sufficient to maintain a VSWR at said RF plasma source power generator not exceeding 3:

1 for variations in said pressure from 10 mT to 900 mT.

9. The apparatus of claim 6 wherein said wafer support pedestal comprises a conductive RF feed layer and a grounded base layer separated from said conductive RF feed layer across a dielectric gap to form a cathode capacitor across said dielectric gap, the length of said gap being sufficient, in combination with the dielectric constant of said dielectric gap and the area of said wafer support pedestal, to provide a cathode capacitance constituting said second impedance.

10. The apparatus of claim 9 further comprising:

an elongate RF conductor coupled at one end to said impedance match circuit, said conductive RF feed layer having a centrally located feed point coupled to an opposite end of said elongate RF conductor;

a dielectric sleeve assembly coaxially surrounding at least a portion of said elongate RF conductor and having a reactance that renders an impedance at said feedpoint closer to an impedance of a peripheral portion of said RF feed layer at said resonant frequency.

11. The apparatus of claim 10 wherein said dielectric sleeve assembly comprises plural dielectric sleeves coaxial with said elongate RF conductor, said plural sleeves having respective lengths and respective dielectric constants, said respective lengths and dielectric constants being such that said dielectric sleeve assembly brings the impedance of said feedpoint closer to the impedance of said peripheral portion of said conductive RF feed layer at both the frequency of said RF plasma source power generator and at the frequency of said RF plasma bias power generator.

12. The apparatus of claim 11 wherein the impedance of said peripheral portion corresponds to said cathode capacitance.

13. The apparatus of claim 11 wherein:

said fixed impedance match element comprises a coaxial tuning stub having a near end coupled to said overhead electrode and a far end, said tuning stub being shorted at said far end.

14. The apparatus of claim 13 wherein:

the impedance through said tuning stub to ground at the frequency of said RF plasma bias power generator is sufficiently low to render said overhead electrode the lowest impedance path to ground for current from said RF plasma bias power generator; and

the impedance of said feedpoint at the frequency of said RF plasma source power generator is sufficiently low to render said wafer support pedestal the lowest impedance path to ground for current from said RF plasma source power generator.

15. The apparatus of claim 11 wherein said resonant frequency and the frequency of said RF plasma source power generator are VHF frequencies and the frequency of said RF plasma bias power generator is an HF frequency.

16. A plasma reactor comprising:

a resonant circuit comprising an overhead electrode having a first impedance, a wafer support pedestal having a second impedance and a bulk plasma having a third impedance and generally lying in a process zone between said overhead electrode and said wafer support pedestal, said resonant circuit having a resonant frequency determined by said first, second and third impedances;

an RF plasma source power generator having a frequency at least nearly matching said resonant frequency; and

a fixed impedance match element coupled to said RF plasma source power generator and to said overhead electrode,

wherein the impedance of said bulk plasma changes with gas pressure inside said reactor, and wherein said impedance of said wafer support pedestal near said resonant frequency is sufficient to maintain a VSWR at said RF plasma source power generator not exceeding 3;

1 for fluctuations in said pressure from 10 mT to 900 mT.

17. A method of operating a plasma reactor, comprising:

providing a resonant circuit comprising an overhead electrode having a first impedance, a wafer support pedestal having a second impedance and a bulk plasma having a third impedance and generally lying in a process zone between said overhead electrode and said wafer support pedestal, said resonant circuit having a resonant frequency determined by said first, second and third impedances;

providing an RF plasma source power generator having a frequency at least nearly matching said resonant frequency; and

providing a fixed impedance match element coupled to said RF plasma source power generator and said overhead electrode.

18. The method of claim 17 further comprising:

setting the impedance of said wafer support pedestal at a level sufficient to maintain a VSWR at said RF plasma source power generator not exceeding 3;

1 for changes in said pressure from 10 mT to 900 mT, in said source power generator from about 5 to 2000 Watts and in said bias power generator from about 5 to 2000 Watts;

varying said pressure, said source power generator and said bias power generator within a window of 10 mT to 900 mT, 5 to 2000 Watts of source power and 5 to 2000 Watts of bias power without extinguishing said plasma.

19. In a plasma reactor having a generally cylindrical chamber, a method of radially confining a plasma within a process zone, said method comprising:

providing a resonant circuit comprising an overhead electrode having a first impedance, a wafer support pedestal having a second impedance and a bulk plasma having a third impedance and generally lying in a process zone between said overhead electrode and said wafer support pedestal, said resonant circuit having a resonant frequency determined by said first, second and third impedances;

coupling RF plasma source power at a frequency at least nearly matching said resonant frequency to the overhead electrode through a fixed impedance match element;

coupling RF plasma bias power through an impedance match circuit to the wafer support pedestal;

providing a path to ground in the fixed impedance match element having a sufficiently low impedance at the frequency of said RF plasma bias power to render the overhead electrode a low impedance path to ground for the RF plasma bias power; and

providing a uniform impedance across a surface of the wafer support pedestal at the frequencies of both the RF plasma source power and the RF plasma bias power simultaneously, said uniform impedance being sufficiently low at the frequency of the RF plasma source power to render the wafer support pedestal the lowest impedance path to ground for the RF plasma source power.

20. The method of claim 19 wherein the fixed impedance match element comprises a coaxial tuning stub coupled at one end to the overhead electrode, and wherein the step of providing a low impedance path to ground at the bias power frequency comprises providing a short circuit at the opposite end of the coaxial tuning stub.

21. The method of claim 20 wherein:

the step of coupling RF plasma bias power comprises coupling the RF plasma bias power through a central conductor to a feedpoint of a conductive RF feed layer in the wafer support pedestal that is separated from a grounded base layer by a dielectric gap;

the step of providing a uniform impedance across the wafer support pedestal comprises bringing the impedance at the feedpoint closer to an impedance of a peripheral portion of said RF feed layer.

22. The apparatus of claim 21 wherein the step of bringing the impedance at the feedpoint closer to an impedance of the peripheral portion of the wafer support pedestal comprises providing plural dielectric sleeves coaxially surrounding respective portions of the RF conductor, the plural sleeves having respective lengths and respective dielectric constants.