Steady state method for measuring the thickness and the capacitance of ultra thin dielectric in the presence of substantial leakage current

• US 6,680,621 B2
• Filed: 05/08/2001
• Issued: 01/20/2004
• Est. Priority Date: 01/26/2001
• Status: Active Grant

• Current Assignee: Semiconductor Diagnostics, Inc. (SEMILAB Félvezeto Fizikai Laboratórium Zrt.)
• Original Assignee: Semiconductor Diagnostics, Inc. (SEMILAB Félvezeto Fizikai Laboratórium Zrt.)
• Inventors: Wilson, Marshall D., Lagowski, Jacek, Jastrzebski, Lubomir L., Dʼamico, John, Savtchouk, Alexander
• Primary Examiner(s): Cuneo, Kamand
• Assistant Examiner(s): PATEL, PARESH H
• Application Number:
  US09/851,291
• Publication Number:
  US 20020125900A1
• Time in Patent Office: 987 Days
• Field of Search: 324/765, 324/767, 324/455, 324/158.1, 324/678, 324/71.1, 324/71.6, 324/452
• US Class Current: 324/750.02
• CPC Class Codes: G01B 7/085 for measuring thickness of coating
• CPC Class Codes: G01R 31/2648 Characterising semiconductor materials testing of materials or semi-finished products G01R31/2831; testing during manufacture H01L22/00

Abstract

A method is described for measuring the capacitance and the equivalent oxide thickness of an ultra thin dielectric layer on a silicon substrate in which the dielectric layer is uniform or patterned. The surface of a dielectric layer is electrically charged by a flux on ions from a corona discharge source until a steady state is reached when the corona flux is balanced by the leakage current across a dielectric. The flux is abruptly terminated and the surface potential of a dielectric is measured versus time. The steady state value of the surface potential is obtained by extrapolation of the potential decay curve to the initial moment of ceasing the corona flux. The thickness of a dielectric layer is determined by using the steady state potential or by using the value of the surface potential after a predetermined time.

Claims

1. A method of determining the thickness of a dielectric layer deposited on a semiconducting wafer, the method comprising:

depositing an ionic charge onto a surface of the dielectric layer disposed on the semiconducting wafer, with an ionic current sufficient to cause a steady state condition;

measuring a voltage decay on the dielectric surface as a function of time; and

determining the thickness of the dielectric layer based upon the measured voltage decay.

2. The method of claim 1, wherein measuring the voltage decay includes using a non-contact probe.

3. The method of claim 1 further including terminating the deposition of ionic charge after causing the steady state condition, wherein the voltage decay is measured after terminating the deposition of ionic charge.

4. The method of claim 1, wherein the dielectric layer has a thickness of about 40 Å

or less.

5. The method of claim 1, wherein the steady state condition results when the ionic current substantially equals a leakage current flowing from the semiconducting wafer and across the dielectric layer.

6. The method of claim 1, wherein the step of determining the thickness of the dielectric layer includes determining an initial steady state surface potential, V₀, on the dielectric layer from the measured voltage decay.

7. The method of claim 6, further including terminating the deposition of ionic charge after causing the steady state condition, wherein the initial surface potential is determined by extrapolating the measured voltage decay back to a time at which the deposition of ionic charge is terminated, i.e. the time t=0.

8. The method of claim 6, wherein the step of determining the thickness of the dielectric layer further includes using the steady state initial surface potential, V₀, in a linear expression to calculate an equivalent oxide thickness, T, of the dielectric layer, the linear expression given by the relationship V₀=aT+b.

9. The method of claim 8, wherein the coefficients a and b in the linear expression are determined by a calibrating procedure.

10. The method of claim 9, wherein the calibrating procedure comprises recording a decay voltage on a plurality of semiconducting wafers each having a known dielectric layer thickness, and determining from each measured voltage decay an initial surface potentials.

11. The method of claim 8, wherein the semiconductor wafer is p-type silicon having a doping of about 1×

10¹⁵ cm³, the dielectric layer is SiO₂, the corona charge has positive polarity, a potential measuring reference electrode is made of platinum, the thickness of the dielectric layer is about 40 Å

or less, a is about 88 mV per Å

, and b is about −

550 mV.

12. The method of claim 11, wherein further including rescaling the coefficient b by adding the value Δ

b, where Δ

b[mV]=−

26 ln(N_A2/N_A1) in which N_A1 is a dopant concentration in a calibrating semiconducting wafer having a known dielectric layer thickness and N_A2 is a dopant concentration in the semiconducting wafer being measured.

13. The method of claim 1, wherein the step of determining the thickness of the dielectric layer includes determining the surface potential on the dielectric surface at a time greater than t=0 from the measured voltage decay.

14. The method of claim 13, wherein the surface potential is determined at a time of about 1 second after t=0.

15. The method of claim 13, wherein the step of determining the thickness of the dielectric layer includes using the surface potential at a time greater than t=0, V_D, to calculate a dielectric thickness, T, via the expression V_D=cT+d, in which the coefficients c and d are derived from a calibrating procedure.

16. The method of claim 15, wherein the calibrating procedure includes measuring a voltage decay on a plurality of semiconducting wafers each having a known dielectric layer thickness, and determining from each measured voltage decay the surface potential, V_D, at the same time in the decay, the time being greater than t=0.

17. The method of claim 1, wherein the steps of depositing a charge onto a surface of the dielectric layer, measuring the voltage, V₀, and determining the thickness of the dielectric layer all occur in less than about 7 seconds.

18. The method of claim 15, wherein the steps of depositing a charge onto a surface of the dielectric layer, measuring the voltage, V_D, and determining the thickness of the dielectric layer all occur in less than about 7 seconds.

19. The method of claim 13, wherein the step of determining the thickness of the dielectric layer includes using the surface potential at a first time greater than t=0, V_D1, and a second time greater than t=0 and different than the first time, V_D2, to calculate a dielectric thickness, T.

20. The method of claim 19, wherein T is determined via the expression V_D1−

V_D2=cT+d, in which the coefficients c and d are derived from a calibrating procedure.

21. The method of claim 1, further including determining the capacitance of the dielectric layer deposited on the semiconducting wafer.

22. The method of claim 21, wherein the capacitance is obtained from the relationship C_OX=J_C/R, where J_C the ionic current at the steady state condition, R is the initial voltage decay rate, dV/dt|_t=0, derived from the measured voltage decay.

23. The method of claim 1, wherein depositing ionic charge, measuring the voltage decay, and determining the thickness are performed on the measurement area smaller than a total surface area of the semiconducting wafer.

24. The method of claim 1, further including depositing a precharging ionic charge on the dielectric layer on a precharge area larger than an area for which the dielectric thickness is determined.

25. The method of claim 24, wherein the precharging ionic charge is of the same polarity as the charge deposited to achieve the steady state.

26. The method of claim 1 further including illuminating the dielectric surface.

27. The method of claim 1 further including performing the steps of depositing ionic charge, measuring voltage decay, and determining the dielectric thickness on a plurality of measurement sites on the dielectric layer.

28. The method of claim 1, wherein the ionic charge has a positive polarity.

29. The method of claim 1, wherein the ionic charge has a negative polarity.

30. A method of determining the thickness of a patterned dielectric layer deposited on a semiconducting wafer, the method comprising:

depositing an ionic charge onto a surface of the patterned dielectric layer disposed on the semiconducting wafer with an ionic current sufficient to cause substantially a steady state condition;

measuring a voltage decay on the patterned dielectric surface as a function of time; and

determining the thickness of the dielectric layer based upon the measured voltage decay, wherein the patterned dielectric layer includes at least one thick region of dielectric material and at least one thin region of dielectric material in which the thickness of the thick region is greater than the thin region.

31. The method of claim 30, wherein measuring the voltage decay includes using a non-contact probe.

32. The method of claim 30, wherein depositing the ionic charge on the surface of the patterned dielectric layer to cause a steady state condition includes generating a substantially equipotential surface between thick and thin regions of the patterned dielectric layer via electro-migration of the ionic charge.

33. The method of claim 30 further including terminating the deposition of ionic charge after causing the steady state condition, wherein the voltage decay is measured after terminating the deposition of ionic charge.

34. The method of claim 30, wherein the thin region of the patterned dielectric layer has a thickness of about 40 Å

or less.

35. The method of claim 30, wherein the dielectric material of the thin region comprises a different material than the dielectric material of the thick region.

36. The method of claim 30, wherein the thin region comprises more than one dielectric material.

37. The method of claim 30, wherein the steady state condition results when the ionic current equals a leakage current flowing from the semiconducting wafer and across the thin region of the patterned dielectric layer.

38. The method of claim 30, wherein the step of determining the thickness of the dielectric layer includes determining an initial steady state surface potential, V₀, on the patterned dielectric layer from the measured voltage decay.

39. The method of claim 38, further including terminating the deposition of ionic charge after causing the steady state condition, wherein the initial surface potential is determined by extrapolating the measured voltage decay back to a time at which the deposition of ionic charge is terminated, i.e. the time t=0.

40. The method of claim 38, wherein the step of determining the thickness of the dielectric layer further includes using the initial steady state surface potential, V₀, in a linear expression to calculate an equivalent oxide thickness, T, of the dielectric layer, the linear expression given by the relationship V₀=aT+b.

41. The method of claim 40, wherein the coefficients a and b in the linear expression are determined by a calibrating procedure.

42. The method of claim 41, wherein the calibrating procedure comprises depositing an ionic charge onto a plurality of semiconducting wafers each having a known dielectric layer thickness to generate an ionic current, J_C, recording a decay voltage on the plurality of semiconducting wafers, and determining from each measured voltage decay an initial surface potentials.

43. The method of claim 42, wherein depositing an ionic charge onto a surface of the patterned dielectric layer disposed on the semiconducting wafer with an ionic current sufficient to cause a steady state condition includes depositing a reduced ionic current given by the relationship J_C/(1+S_Thick/S_Thin), where J_C is the ionic current generated in the calibrating procedure and S_Thick/S_Thin is the ratio of the surface thick region surface area relative to the thin region surface area.

44. The method of claim 42, wherein each of the plurality of semiconducting wafers includes a patterned dielectric layer having thick and thin regions of dielectric material.

45. The method of claim 30, wherein the step of determining the thickness of the dielectric layer includes determining the surface potential on the dielectric surface at a time greater than t=0 from the measured voltage decay.

46. The method of claim 45, wherein the surface potential is determined at a time of about 1 second after t=0.

47. The method of claim 45, wherein the step of determining the thickness of the dielectric layer includes using the surface potential at a time greater than t=0, V_D, to calculate a dielectric thickness, T, via the expression V_D=cT+d, in which the coefficients c and d are derived from a calibrating procedure.

48. The method of claim 47, wherein the calibrating procedure includes measuring a voltage decay on a plurality of semiconducting wafers each having a known dielectric layer thickness, and determining from each measured voltage decay the surface potential, V_D, at the same time in the decay, the time being greater than t=0.

49. The method of claim 30, wherein the steps of depositing a charge onto a surface of the dielectric layer, measuring the voltage, V₀, and determining the thickness of the dielectric layer all occur in less than about 7 seconds.

50. The method of claim 47, wherein the steps of depositing a charge onto a surface of the dielectric layer, measuring the voltage, V_D, and determining the thickness of the dielectric layer all occur in less than about 7 seconds.

51. The method of claim 45, wherein the step of determining the thickness of the dielectric layer includes using the surface potential at a first time greater than t=0, V_D1, and a second time greater than t=0 and different than the first time, V_D2, to calculate a dielectric thickness, T.

52. The method of claim 51, wherein the surface potential at the second time is recorded at a time relative to t=0 that is larger than the surface potential recorded at the first time.

53. The method of claim 51, wherein T is determined via the expression V_D1−

V_D2=cT+d, in which the coefficients c and d are derived from a calibrating procedure.

54. The method of claim 30, further including determining the capacitance of the dielectric layer deposited on the semiconducting wafer.

55. The method of claim 54, wherein the capacitance is obtained from the relationship

C_OX=J_C/R, where J_C the ionic current at the steady state condition, R is the initial voltage decay rate, dV/dt|_t=0, derived from the measured voltage decay.

56. The method of claim 30, further including cleaning the patterned dielectric layer with a pre-cleaning solution prior to depositing the ionic charge onto the surface of the patterned dielectric layer.

57. The method of claim 30, further including depositing a precharging ionic charge on the patterned dielectric layer on a precharge area larger than an area for which the dielectric thickness is determined.

58. The method of claim 57, wherein the precharging ionic charge is of the same polarity as the charge deposited to achieve the steady state.

59. The method of claim 30 further including illuminating the patterned dielectric surface.

60. The method of claim 30 further including performing the steps of depositing ionic charge, measuring voltage decay, and determining the dielectric thickness on a plurality of measurement sites on the patterned dielectric layer.

61. The method of claim 30, wherein the ionic charge has a positive polarity.

62. The method of claim 30, wherein the ionic charge has a negative polarity.