Electrical conductivity tester and methods thereof for accurately measuring time-varying and steady state conductivity using phase shift detection

• US 5,508,610 A
• Filed: 12/03/1992
• Issued: 04/16/1996
• Est. Priority Date: 12/03/1992
• Status: Expired due to Fees

• Current Assignee: Georgia Tech Research Corporation (University System of Georgia)
• Original Assignee: Georgia Tech Research Corporation (University System of Georgia)
• Inventors: Rohatgi, Ajeet, Hertling, David R., Feeney, Robert K.
• Primary Examiner(s): Snow, Walter E.
• Application Number:
  US07/985,635
• Time in Patent Office: 1,230 Days
• Field of Search: 324/233, 324/719, 324/165 R, 324/158 D, 324/750, 324/765, 324/641, 324/709, 324/227
• US Class Current: 324/233
• CPC Class Codes: G01N 27/023 where the material is placed in the field of a coil
• CPC Class Codes: G01R 27/02 Measuring real or complex resistance, reactance, impedance, or other two-pole characteristics derived therefrom, e.g. time constant by measuring phase angle only G01R25/00
• CPC Class Codes: G01R 31/2648 Characterising semiconductor materials testing of materials or semi-finished products G01R31/2831; testing during manufacture H01L22/00

Abstract

An electrical conductivity tester accurately measures the time-varying electrical conductivity σ(t) and steady-state electrical conductivity σ_ss of a test material. In a first embodiment, the transmission phase of a probe circuit is monitored to determine the conductivity of a test material. In the first embodiment, an oscillator circuit generates a reference oscillator signal. A probe circuit receives the reference oscillator signal, magnetically couples to the test material, and modifies the reference oscillator signal via electromagnetic induction to derive a modified transmission phase signal. Finally, a phase detector circuit derives a transmission phase signal by combining the reference oscillator signal and the modified transmission phase signal, the transmission phase signal being directly convertible to the conductivity. In a second embodiment, an amplifier is connected to the probe circuit to form an oscillator circuit. The oscillator circuit generates an oscillator signal in response to the magnetic coupling of the probe circuit with the test material. A frequency discriminator generates a frequency signal from the oscillator signal, the frequency signal being convertible to the conductivity.

Claims

1. An electrical conductivity tester for accurately measuring the time-varying conductivity and steady-state conductivity of a test material, comprising:

an oscillator means for generating a reference oscillator signal;

a single probe coil means for receiving said reference oscillator signal, for magnetically coupling to said test material, and for modifying said reference oscillator signal via electromagnetic induction to derive a modified transmission phase signal;

a phase detector means for deriving a transmission phase signal indicative of a phase difference by comparing said reference oscillator signal to said modified transmission phase signal, said transmission phase signal being convertible to said time varying conductivity; and

means for averaging said transmission phase signal over time and for generating an average phase signal, said average phase signal being convertible to steady-state conductivity of said test material.

2. The electrical conductivity tester of claim 1, wherein said probe means comprises a combination of two series-connected capacitors in parallel with an inductor, said combination connected in series with an amplifier.

3. The electrical conductivity tester of claim 1, wherein said probe means comprises a cavity resonator.

4. The electrical conductivity tester of claim 1, wherein said probe means comprises a helical resonator.

5. The electrical conductivity tester of claim 1, wherein said test material comprises a semiconductor.

6. The electrical conductivity tester of claim 1, further comprising a phase shifter means for administering a phase shift to said reference oscillator signal before comparing said reference oscillator signal to said modified transmission phase signal.

7. The electrical conductivity tester of claim 1, further comprising an adjustable attenuator and an amplifier in series with said oscillator means, said adjustable attenuator for permitting adjustment of the power level of said reference oscillator signal.

8. The electrical conductivity tester of claim 1, further comprising a light source for directing light against said test material.

9. The electrical conductivity tester of claim 6, wherein said averaging means comprises feedback means for filtering said transmission phase signal to derive said average phase signal and for forwarding said average phase signal to said phase shifter means, said feedback means and said phase shifter means for adjusting a phase of said reference oscillator signal so that said transmission phase signal is reduced to zero with a steady-state conductivity.

10. The electrical conductivity tester of claim 6, wherein said phase shift is approximately ninety degrees when said transmission phase signal is equal to zero.

14. The electrical conductivity tester of claim 1, wherein said time-varying conductivity is a transient conductivity induced in said solid test material by a short-duration light source.

11. A method for accurately determining the time varying conductivity and steady-state conductivity of a test material, the method comprising the steps of:

sending a reference oscillator signal to a single probe coil means;

magnetically coupling said reference oscillator signal and said test material with said single probe coil means to derive a modified transmission phase signal;

comparing said reference oscillator signal to said modified transmission phase signal to derive a transmission phase signal indicative of a phase difference, said transmission phase signal being convertible to said time-varying conductivity; and

averaging said transmission phase signal to determine an average phase signal, said average phase signal being convertible to said steady-state conductivity of said test material.

12. The method of claim 11, further comprising the steps of exciting said test material with light and measuring a time-varying transient conductivity of said test material based upon said transmission phase signal.

13. The method of claim 11, wherein said step of averaging comprises the steps of:

filtering said transmission phase signal to derive said average phase signal; and

using said average phase signal to adjust a phase of said reference oscillator signal so that said transmission phase signal is reduced to zero with a steady-state conductivity.

15. The method of claim 11, further comprising the step of removing said solid test material from the light source in order to determine the time-varying conductivity recognized as transient conductivity.

16. An electrical conductivity tester for accurately measuring the conductivity of a test material, comprising:

an oscillator means for generating a reference oscillator signal;

a probe means for receiving said reference oscillator signal, for magnetically coupling to said test material, and for modifying said reference oscillator signal via electromagnetic induction to derive a modified transmission phase signal;

a phase detector means for deriving a transmission phase signal by comparing said reference oscillator signal to said modified transmission phase signal, said transmission phase signal being convertible to said conductivity; and

a feedback means for filtering said transmission phase signal to derive an average transmission phase signal and for forwarding said average transmission phase signal to said phase detector means, said feedback means and said phase detector means for adjusting a phase of said reference oscillator signal so that said transmission phase signal is reduced to zero with a steady-state conductivity.

17. The electrical conductivity tester of claim 16, wherein said probe means comprises a combination of two series connected capacitors in parallel with an inductor, said combination connected in series with an amplifier.

18. The electrical conductivity tester of claim 16, wherein said probe means comprises a cavity resonator.

19. The electrical conductivity tester of claim 16, wherein said probe means comprises a helical resonator.

20. The electrical conductivity tester of claim 16, wherein said test material comprises a semiconductor.

21. The electrical conductivity tester of claim 16, further comprising a phase shifter means for administering a phase shift to said reference oscillator signal before comparing said reference oscillator signal to said modified transmission phase signal.

22. The electrical conductivity tester of claim 16, further comprising an adjustable attenuator in an amplifier in series with said oscillator means, said adjustable attenuator for permitting adjustment of the power level of said reference oscillator signal.

23. The electrical conductivity tester of claim 16, further comprising a light source for directing light against said test material.

24. The electrical conductivity tester of claim 21, wherein said phase shift is approximately 90°

when said average transmission phase signal is equal to zero.

25. The electrical conductivity tester of claim 16, wherein said probe means comprises a single probe coil configured to magnetically couple to said test material.

26. A method for accurately determining the conductivity of a test material, the method comprising the steps of:

sending a reference oscillator signal to a probe circuit;

magnetically coupling said reference oscillator signal and said test material to derive a modified transmission phase signal;

comparing said reference oscillator signal to said modified transmission phase signal to derive a transmission phase signal, said transmission phase signal being convertible to said conductivity;

filtering said transmission phase signal to derive an average transmission phase signal; and

using said average transmission phase signal to adjust a phase of said reference oscillator signal so that said transmission phase signal is reduced to zero with a steady-state conductivity.

27. The method of claim 26, further comprising the step of exciting said test material with light.

28. The method of claim 26, further comprising the step of using a cavity resonator in said probe circuit for magnetically coupling said reference oscillator signal and said test material.

29. The method of claim 26, further comprising the step of using a helical resonator in said probe circuit for magnetically coupling said reference oscillator signal and said test material.

30. The method of claim 26, wherein said step of magnetically coupling said reference oscillator signal and said test material comprises the step of coupling said reference oscillator signal to a semiconductor material.

31. The method of claim 26, further comprising the step of administering a phase shift to said reference oscillator signal before comparing said reference oscillator signal with said modified transmission phase signal.

32. The method of claim 26, further comprising the step of adjusting the power level of said reference oscillator signal.

33. The method of claim 26, further comprising the step of directing light against said test material.

34. The method of claim 26, further comprising the step of filtering said transmission phase signal to derive an average transmission phase signal.

35. The method of claim 26, further comprising the step of adjusting a phase of said reference oscillator signal so that said transmission phase signal is reduced to zero with a steady-state conductivity.