System for characterizing AC properties of a processing plasma

• US 5,523,955 A
• Filed: 03/19/1992
• Issued: 06/04/1996
• Est. Priority Date: 03/19/1992
• Status: Expired due to Term

• Current Assignee: Advanced Energy Industries Incorporated
• Original Assignee: Advanced Energy Industries Incorporated
• Inventors: Heckman, Randy L.
• Primary Examiner(s): ZANELLI, MICHAEL J
• Application Number:
  US07/854,804
• Time in Patent Office: 1,538 Days
• Field of Search: 364/550, 364/481, 364/483, 364/552, 324/76.76, 204/298.03, 204/298.08, 204/298.32, 204/192.1, 204/192.13, 204/192.32, 204/192.33, 315/111.21, 315/111.31, 333/32, 156/627, 313/231.31
• US Class Current: 702/64
• CPC Class Codes: H01J 2237/334 Etching
• 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 37/32935 Monitoring and controlling tubes by information coming from the object and/or discharge
• CPC Class Codes: H01J 37/32963 End-point detection
• CPC Class Codes: H01J 37/3299 Feedback systems
• CPC Class Codes: H05H 1/0081 by electric means

Abstract

A probe to be inserted in-line in an AC plasma processing system allows accurate, real time determination of plasma parameters such as power and complex impedance over a broad dynamic range. Any need to know power output from a source is avoided and signals are selected to optimize accuracy such that only two alternating signals need be sensed for many applications. Signals are selected such that magnitudes of simple alternating signals can be easily measured as scalar values in a fashion that affords the use of these values to completely characterize the power actually delivered to the processing plasma and its complex impedance in real time. Plasma characterization can be limited to only specific frequencies for more accurate determination. Microstrip directional couplers are used to sense signals from the power transmission. These signals are then utilized to derive simple alternating signals representative of power or voltage. Three or more scalar values representative of the magnitude of the alternating signals and their combination serve as the variable inputs to determine complex reflection coefficient or impedance using known formulas. Use of a sign bit detector or assumptions with respect to the processing plasma is disclosed for complete characterization of the plasma in an efficient manner.

Claims

1. A method of characterizing a plasma being subjected to an alternating power transmission comprising the steps of:

a. accepting said alternating power transmission as an input;

b. sensing at least two signals from said alternating power transmission;

c. deriving at least three alternating signals from said sensed signals;

d. measuring at least three scalar values from said alternating signals wherein each of said scalar values are representative of the magnitude of the alternating signals;

e. manipulating said measured scalar values as variable inputs to characterize the plasma; and

f. passing a substantial portion of said alternating power transmission through to said plasma.

2. A method of characterizing a plasma being subjected to an alternating power transmission as described in claim 1 and further comprising the step of limiting the content of said scalar values to that of only a specific frequency.

3. A method of characterizing a plasma being subjected to an alternating power transmission as described in claim 2 wherein said step of limiting the content of said scalar values to that of only a specific frequency is accomplished prior to measuring said scalar values.

4. A method of characterizing a plasma being subjected to an alternating power transmission as described in claim 1 or 3 wherein said step of manipulating said measured scalar values as variable inputs to characterize the plasma comprises the step of ascertaining the power delivered to said plasma.

5. A method of characterizing a plasma being subjected to an alternating power transmission as described in claim 1 or 3 wherein said alternating power transmission created by a power source having an output level, wherein said power transmission is both accepted and passed through by a probe which also accomplishes the step of manipulating said measured scalar values as variable inputs to characterize the plasma and wherein the level of said output level is unknown to said probe.

6. A method of characterizing a plasma being subjected to an alternating power transmission as described in claim 1 or 3 wherein said plasma is characterized by a parameter having a positive or negative sign and wherein said step of manipulating said scalar signals to characterize said plasma comprises the steps of:

a. calculating the absolute value of said parameter of the plasma through utilization of said scalar values; and

b. determining the sign of said parameter of said plasma through assuming the sign of said parameter of the plasma.

10. A method of characterizing a plasma being subjected to an alternating power transmission as described in claim 1 wherein said step of sensing at least two signals from said alternating power transmission comprises the step of sensing three signals.

7. A method of characterizing a plasma being subjected to an alternating power transmission comprising the steps of:

a. accepting said alternating power transmission;

b. sensing a first signal from said alternating power transmission;

c. deriving a first alternating signal from said first signal;

d. measuring a first scalar value from said first alternating signal wherein said first scalar value is representative of the magnitude of said first alternating signal;

e. sensing a second signal from said alternating power transmission;

f. deriving a second alternating signal from said second signal;

g. measuring a second scalar value from said second alternating signal wherein said second scalar value is representative of the magnitude of said second alternating signal;

h. combining said first and said second alternating signals to create a third alternating signal;

i. measuring a third scalar value representative of the magnitude of said third alternating signal;

j. manipulating said first, second, and third scalar values to characterize said plasma; and

k. passing a substantial portion of said alternating power transmission through to said plasma.

8. A method of characterizing a plasma being subjected to an alternating power transmission as described in claim 7 and further comprising the step of limiting the content of said scalar values to that of only a specific frequency.

9. A method of characterizing a plasma being subjected to an alternating power transmission as described in claim 7 wherein said first signal is representative of the sum of a voltage across said plasma and a multiple of the current passing through to said plasma, and wherein said second alternating signal is representative of the difference between said voltage across said plasma and said multiple of the current passing through to said plasma.

11. A method of characterizing a plasma being subjected to an alternating power transmission comprising the steps of:

a. accepting said alternating power transmission;

b. sensing a first signal from said alternating power transmission wherein said first signal is representative of the sum of a voltage across said plasma and a multiple of the current passing through to said plasma;

c. deriving a first alternating signal from said first signal;

d. limiting the content of said first alternating signal to that of only a specific frequency;

e. measuring a first scalar value from said first alternating signal wherein said first scalar value is representative of the magnitude of said first alternating signal;

f. sensing a second signal from said alternating power transmission wherein said second alternating signal is representative of the difference between said voltage across said plasma and said multiple of the current passing through to said plasma;

g. deriving a second alternating signal from said second signal;

h. limiting the content of said second alternating signal to that of only said specific frequency;

i. measuring a second scalar value from said second alternating signal wherein said second scalar value is representative of the magnitude of said second alternating signal;

j. ascertaining a third alternating signal;

k. measuring a third scalar value from said third alternating signal wherein said third scalar value is representative of the magnitude of said third alternating signal;

l. manipulating said first, second, and third scalar values to characterize said plasma; and

m. passing a substantial portion of said alternating power transmission through to said plasma.

12. A method of characterizing a plasma being subjected to an alternating power transmission as described in claim 11 wherein said step of ascertaining said third alternating signal comprises the step of combining said first and said second alternating signals to create a third alternating signal.

13. A method of characterizing a plasma being subjected to an alternating power transmission as described in claim 12 and further comprising the step of limiting the content of said third alternating signal to that of only said specific frequency.

14. A method of characterizing a plasma being subjected to an alternating power transmission as described in claim 13 wherein said step of limiting the content of said third alternating signal comprises the step of combining said first and second alternating signals after accomplishing said step of limiting the content of said first and second alternating signals.

15. A method of characterizing a plasma being subjected to an alternating power transmission as described in claim 13 wherein said step of combining said first and second alternating signals is accomplished before accomplishing said step of limiting the content of said first and second alternating signals.

16. A method of characterizing a plasma being subjected to an alternating power transmission as described in claim 11 wherein said step of ascertaining said third alternating signal comprises the steps of:

a. sensing a third signal from said alternating power transmission; and

b. deriving a third alternating signal from said third signal.

17. A method of characterizing a plasma being subjected to an alternating power transmission as described in claim 16 and further comprising the step of limiting the content of said third signal to that of only said specific frequency.

18. A method of characterizing a plasma being subjected to an alternating power transmission as described in claim 9 or 11 wherein said alternating power transmission has a voltage and wherein each of said first and second scalar values are proportional to said voltage.

19. A method of characterizing a plasma being subjected to an alternating power transmission as described in claim 9 or 11 wherein said first and second signals have an angle between them and wherein said step of manipulating said first, second, and third scalar values to characterize said plasma comprises the step of ascertaining the angle between said first and second signals.

20. A method of characterizing a plasma being subjected to an alternating power transmission as described in claim 9 or 11 wherein said plasma has a complex impedance and wherein said step of manipulating said first, second, and third scalar values to characterize said plasma comprises the step of ascertaining the complex impedance of said plasma.

21. A method of characterizing a plasma being subjected to an alternating power transmission as described in claim 9 or 11 wherein said plasma has a reflection coefficient and wherein said step of manipulating said first, second, and third scalar values to characterize said plasma comprises the step of ascertaining the reflection coefficient of said plasma.

22. A method of characterizing a plasma being subjected to an alternating power transmission as described in claim 21 and further comprising the step of manipulating said first and second scalar values to ascertain the power delivered to said plasma.

23. A method of characterizing a plasma being subjected to an alternating power transmission as described in claim 9 or 11 wherein said power transmission is created by a power source having an output level, wherein said manipulation is accomplished by a probe and wherein said output level by the power source is unknown to said probe.

24. A method of characterizing a plasma being subjected to an alternating power transmission as described in claim 11 wherein said step of limiting the content of said first signal to that of only a specific frequency comprises the step of filtering said first signal prior to measuring said first scalar value and wherein said step of limiting the content of said second signal to that of only a specific frequency comprises the step of filtering said second signal prior to measuring said second scalar value.

25. A method of characterizing a plasma being subjected to an alternating power transmission as described in claim 9 or 11 wherein said plasma has a variable parameter having a positive or negative sign and wherein said step of manipulating said first, second, and third scalar values to characterize said plasma comprises the step of:

a. calculating the absolute value of said parameter of the plasma; and

b. be determining the sign of said parameter of said plasma through assuming the sign of said parameter of the plasma.

26. A method of characterizing a plasma being subjected to an alternating power transmission as described in claim 3 or 11 wherein said step of limiting the content comprises the step of filtering, converting to a different signal through utilization of a coherent oscillator, converting to a different signal through utilization of a non-coherent oscillator and then filtering, or synchronously detecting values.

27. A circuit to characterize a plasma being subjected to an alternating power transmission comprising:

a. a means for accepting a power transmission;

b. a means for sensing at least two signals from said power transmission provided by said means for accepting;

c. a means for deriving at least three alternating signals from said signals provided by said means for sensing;

d. a means for measuring at least three scalar values from said alternating signals wherein said scalar values are representative of the magnitude of the alternating signals;

e. a means for manipulating said measured scalar values as variable inputs to characterize the plasma; and

f. a means for passing a substantial portion of said power transmission from said means for accepting to said plasma.

28. A circuit to characterize a plasma being subjected to an alternating power transmission as described in claim 27 and further comprising a means for limiting the content of said sensed scalar values to that of only a specific frequency.

29. A circuit to characterize a plasma being subjected to an alternating power transmission as described in claim 28 wherein said means for sensing comprises:

a. a means for sensing a first signal; and

b. a means for sensing a second signal;

and wherein said means for deriving alternating signals comprises;

a. a means for deriving a first alternating signal from said first signal provided by said means for sensing said first signal;

b. a means for deriving a second alternating signal from said second signal provided by said means for sensing said second signal; and

c. a means for deriving a third alternating signal;

and wherein said means for measuring scalar values representative of the magnitude of the sensed signals comprises;

a. a means for measuring a first scalar value representative of the magnitude of the first alternating signal;

b. a means for measuring a second scalar value representative of the magnitude of the second alternating signal; and

c. a means for measuring a third scalar value representative of the magnitude of the third alternating signal.

30. A circuit to characterize a plasma being subjected to an alternating power transmission as described in claim 29 wherein said means for deriving the third alternating signal comprises a means for combining said first and second alternating signals.

31. A circuit to characterize a plasma being subjected to an alternating power transmission as described in claim 29 and further comprising a means for sensing a third signal.

32. A circuit to characterize a plasma being subjected to an alternating power transmission as described in claim 29 wherein the first alternating signal is representative of the sum of a voltage across said plasma and a multiple of the current passing through to said plasma, and wherein said second alternating signal is representative of the difference between said voltage across said plasma and said multiple of the current passing through to said plasma.

34. A circuit to characterize a plasma being subjected to an alternating power transmission as described in claim 27 wherein said means for sensing comprises:

a. a means for sensing a first signal;

b. a means for sensing a second signal; and

c. a means for sensing a third signal;

and wherein said means for deriving alternating signals comprises;

a. a means for deriving a first alternating signal from said first signal provided by said means for sensing said first signal;

b. a means for deriving a second alternating signal from said second signal provided by said means for sensing said second signal; and

c. a means for deriving a third alternating signal from said third signal provided by said means for sensing said third signal;

and wherein said means for measuring scalar values representative of the magnitude of the sensed signals comprises;

a. a means for measuring a first scalar value representative of the magnitude of the first alternating signal;

b. a means for measuring a second scalar value representative of the magnitude of the second alternating signal; and

c. a means for measuring a third scalar value representative of the magnitude of the third alternating signal.

37. A circuit to characterize a plasma being subjected to an alternating power transmission as described in claim 28 wherein said means for limiting the content of said scalar signals to that of only a specific frequency comprises a filter, a coherent oscillator, a non-coherent oscillator and a filter, or a synchronous detector.

39. A circuit to characterize a plasma being subjected to an alternating power transmission as described in claim 27, 28, or 33 wherein said means for manipulating is contained within a probe, and wherein said power transmission has an output level and wherein said output level is unknown to said probe.

40. A circuit to characterize a plasma being subjected to an alternating power transmission as described in claim 29, 31, or 33 and further comprising:

a. a means for sensing a fourth signal from said power transmission;

b. a means for deriving a fourth alternating signal from said fourth signal; and

c. a means for measuring a fourth scalar value representative of the magnitude of said fourth alternating signal.

41. A circuit to characterize a plasma being subjected to an alternating power transmission as described in claim 40 wherein the aspect which said fourth scalar value represents is selected to overcome any ambiguity in characterizing said plasma.

42. A circuit to characterize a plasma being subjected to an alternating power transmission as described in claim 34 and further comprising:

a. a means for limiting the content of said first signal to a specific frequency wherein said means for limiting said first signal is connected between said means for sensing said first signal and said means for measuring said first scalar value;

b. a means for limiting the content of said second signal to said specific frequency wherein said means for limiting said second signal is connected between said means for sensing said second signal and said means for measuring said second scalar value;

c. a means for limiting the content of said third signal to said specific frequency wherein said means for limiting said third signal is connected between said means for sensing said third signal and said means for measuring said third scalar value;

d. a means for sensing a fourth signal from said power transmission;

e. a means for deriving a fourth alternating signal from said fourth signal;

f. a means for limiting the content of said fourth signal to said specific frequency wherein said means for limiting said fourth signal is connected between said means for sensing said fourth signal and said means for measuring said fourth scalar value;

g. a means for measuring a fourth scalar value representative of the magnitude of said fourth alternating signal.

45. A circuit to characterize a plasma being subjected to an alternating power transmission as described in claim 29, 33 or 43 wherein said plasma is contained within a processing chamber having an input, wherein said means for sensing a first signal is responsive to said input, and wherein said means for sensing a second signal is responsive to said input.

47. A circuit to characterize a plasma being subjected to an alternating power transmission as described in claim 27 wherein at least two of said sensed signals have an angle between them, and wherein said means for manipulating said measured scalar values as variable inputs to characterize the plasma comprises a means for determining the angle between said two sensed signals.

48. A circuit to characterize a plasma being subjected to an alternating power transmission as described in claim 47 wherein said means for determining the angle between said two signals comprises a sign detector.

49. A circuit to characterize a plasma being subjected to an alternating power transmission as described in claim 48 wherein said sign detector comprises a preset input.

50. A circuit to characterize a plasma being subjected to an alternating power transmission as described in claim 47 wherein said means for determining said angle comprises two in-phase splitters, a zero crossing detector and a D flip-flop, or a phase detector.

33. A circuit to characterize a plasma being subjected to an alternating power transmission comprising:

a. a means for accepting a power transmission;

b. a means for sensing a first signal from said power transmission provided by said means for accepting;

c. a means for deriving a first alternating signal from said first signal provided by said means for sensing said first signal;

d. a means for measuring a first scalar value representative of the magnitude of said first alternating signal;

e. a means for sensing a second signal from said power transmission provided by said means for accepting;

f. a means for deriving a second alternating signal from said second signal provided by said means for sensing said second signal;

g. a means for measuring a second scalar value representative of the magnitude of said second alternating signal;

h. a means for combining said first and said second alternating signals to create a third alternating signal;

i. a means for measuring a third scalar value representative of the magnitude of said third alternating signal;

j. a means for manipulating said measured scalar values as variable inputs to characterize the plasma; and

k. a means for passing a substantial portion of said power transmission from said means for accepting to said plasma.

35. A circuit to characterize a plasma being subjected to an alternating power transmission as described in claim 33 or 34 wherein the first alternating signal is representative of the sum of a voltage across said plasma and a multiple of the current passing through to said plasma, and wherein said second alternating signal is representative of the difference between said voltage across said plasma and said multiple of the current passing through to said plasma.

36. A circuit to characterize a plasma being subjected to an alternating power transmission as described in claim 35 wherein said means for limiting the content comprises:

a. a means for limiting the content of said first scalar value to that of only a specific frequency; and

b. a means for limiting the content of said second scalar value to that of only a specific frequency.

38. A circuit to characterize a plasma being subjected to an alternating power transmission as described in claim 36 wherein said means for limiting the content of said scalar signals to that of only a specific frequency comprises a filter, a coherent oscillator, a non-coherent oscillator and a filter, or a synchronous detector.

43. A circuit to characterize a plasma being subjected to an alternating power transmission as described in claim 33 wherein said means for sensing a first signal and said means for sensing a second signal each comprise a directional coupler.

44. A circuit to characterize a plasma being subjected to an alternating power transmission as described in claim 43 wherein said means for combining comprises an in-phase combiner connected to said first and said second alternating signals.

46. A circuit to characterize a plasma being subjected to an alternating power transmission as described in claim 43 wherein said third scalar value is representative of a plasma voltage and wherein said means for combining comprises two substantially equal high impedance resistors connected after said means for sensing said first and said second signals.

51. A circuit to characterize a plasma being subjected to an alternating power transmission as described in claim 33 or 43 wherein at least two of said sensed signals have an angle between them, and wherein said means for manipulating said measured scalar valued as variable inputs to characterize the plasma comprises a means for determining the angle between said two sensed signals.

52. A circuit to characterize a plasma being subjected to an alternating power transmission as described in claim 51 wherein said means for determining the angle between said two signals comprises a sign detector.

53. A circuit to characterize a plasma being subjected to an alternating power transmission as described in claim 52 wherein said sign detector comprises a preset input.

54. A circuit to characterize a plasma being subjected to an alternating power transmission as described in claim 51 wherein said means for determining said angle comprises two in-phase splitters, a zero crossing detector and a D flip-flop, or a phase detector.

55. A circuit to characterize a plasma being subjected to an alternating power transmission as described in claim 51 wherein said means for combining comprises a node connected to said first and second alternating signals.

56. A circuit to characterize a plasma being subjected to an alternating power transmission as described in claim 54 wherein said means for combining comprises a node connected to said first and second alternating signals.