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

• US 20020130674A1
• Filed: 03/16/2001
• Published: 09/19/2002
• Est. Priority Date: 01/26/2001
• Status: Active Grant

• Current Assignee: Semilab Semiconductor Physics Laboratory Co., Ltd. (Euroventures Capital Tanacsad Kft)
• Original Assignee: Semiconductor Diagnostics, Inc. (SEMILAB Félvezeto Fizikai Laboratórium Zrt.)
• Inventors: Savtchouk, Alexander, Wilson, Marshall D., Lagowski, Jacek
• Granted Patent: US 6,597,193 B2
• Time in Patent Office: Days
• US Class Current: 324/750
• 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 non-contact measuring the capacitance and the equivalent oxide thickness of ultra thin dielectric layer on a silicon substrate. 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. The method produces highly accurate results for oxide thickness below 40 Å with a demonstrated repeatability of a 0.03 Å in a series of 10 measurements. Alternatively, the rate of surface potential decay is calculated at the initial moment providing a measure of the charge dissipation on a dielectric capacitor. The capacitance of a dielectric layer is calculated from this rate.

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, via a non-contact probe, 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 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.

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

or less.

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

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

6. The method of claim 5, 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.

7. The method of claim 5, wherein the step of determining the thickness of the dielectric layer further includes using the 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.

8. The method of claim 7, wherein the coefficients a and b in the linear expression are determined by a calibrating procedure.

9. The method of claim 8, 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.

10. The method of claim 7, 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.

11. The method of claim 10, 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.

12. 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.

13. The method of claim 12, wherein the surface potential is determined at a time of about 1 second after t=0.

14. The method of claim 12, 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.

15. The method of claim 14, 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 a surface potential, V_D, at the same time in the decay, the time being greater than t=0.

16. 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.

17. The method of claim 14, 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.

18. The method of claim 12, 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.

19. The method of claim 18, 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.

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

21. The method of claim 20, 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.

22. 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.

23. The method of claim 1, further including depositing a precharging ionic charge on the dielectric layer on area larger than the area of thickness measuring cite.

24. The method of claim 23, wherein the precharging ionic charge is of the same polarity as the charge deposited to achieve the steady state.

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

26. 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.

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

illuminating the dielectric surface;

depositing a precharging ionic charge on the dielectric layer;

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

ceasing ionic charging after establishing the steady state condition;

measuring, via a non-contact probe, a voltage decay on the semiconducting wafer as a function of time after ceasing the ionic charging;

analyzing the voltage decay to determine a characteristic of the measured voltage decay, the characteristic of the measured voltage decay being selected from the group consisting of an initial surface potential, V₀, a surface potential at a time greater than t=0, V_D, and an initial rate of voltage decay, dV/dt|_t=0; and

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