Thin film temperature measurement using optical absorption edge wavelength

• US 8,786,841 B2
• Filed: 06/21/2010
• Issued: 07/22/2014
• Est. Priority Date: 06/19/2009
• Status: Active Grant

• Current Assignee: k-Space Associates, Inc.
• Original Assignee: k-Space Associates, Inc.
• Inventors: Barlett, Darryl, Taylor, Charles A. II, Wissman, Barry D.
• Primary Examiner(s): Toatley, Gregory J
• Assistant Examiner(s): DECENZO, SHAWN H
• Application Number:
  US13/378,788
• Publication Number:
  US 20120133934A1
• Time in Patent Office: 1,492 Days
• Field of Search: 374/161, 356/326, 356/504, 356/72, 356/446
• US Class Current: 356/72
• CPC Class Codes: H01L 21/67248 Temperature monitoring
• CPC Class Codes: H01L 22/12 for structural parameters, e.g. thickness, line width, refractive index, temperature, warp, bond strength, defects, optical inspection, electrical measurement of structural dimensions, metallurgic measurement of diffusions electrical measurement of diffusions H01L22/14

Abstract

A technique for determining the temperature of a sample including a semiconductor film 20 having a measurable optical absorption edge deposited on a transparent substrate 22 of material having no measurable optical absorption edge, such as a GaN film 20 deposited on an Al2O3 substrate 22 for blue and white LEDs. The temperature is determined in realtime as the film 20 grows and increases in thickness. A spectra based on the diffusely scattered light from the film 20 is produced at each incremental thickness. A reference division is performed on each spectra to correct for equipment artifacts. The thickness of the film 20 and an optical absorption edge wavelength value are determined from the spectra. The temperature of the film 20 is determined as a function of the optical absorption edge wavelength and the thickness of the film 20 using the spectra, a thickness calibration table, and a temperature calibration table.

Claims

1. A method for determining temperature of a semiconductor film having a measurable optical absorption edge deposited on an optically transparent substrate material having no measurable optical absorption edge, said method comprising the steps of:

a) providing an optically transparent substrate of material having no measurable optical absorption edge,

b) depositing a film of a semiconductor material having a measurable optical absorption edge and a measurable thickness on the substrate,

c) interacting light with the film deposited on the substrate to produce diffusely scattered light,

d) collecting the diffusely scattered light from the film,

e) producing a spectra indicating optical absorption of the film based on the diffusely scattered light from the film,

f) determining a thickness of the film,

g) determining the optical absorption edge wavelength of the film based on the spectra, and

h) determining a temperature of the film at the film thickness as a function of the film thickness and the optical absorption edge wavelength.

2. The method of claim 1 wherein said step h) includes accounting for the dependence of the optical absorption of the film on the film thickness.

3. The method of claim 1 including re-determining the temperature of the film at incremental thicknesses to detect changes in temperature due to said step b).

4. The method of claim 1 including providing a temperature calibration table indicating optical absorption edge wavelength versus temperature at a constant thickness of the film, providing a thickness calibration table indicating optical absorption edge wavelength versus thickness at a constant temperature of the film, and

wherein said step h) includes determining a difference between an optical absorption edge wavelength at the thickness determined by said step f) and the optical absorption edge wavelength at the thickness of the temperature calibration table using the thickness calibration table to obtain a wavelength difference,

subtracting the wavelength difference from the optical absorption edge wavelength determined by said step g) to provide an adjusted optical absorption wavelength value, and

using the adjusted optical absorption wavelength value to determine the temperature of the film at the thickness determined by said step f).

5. The method of claim 4 wherein said step of providing the temperature calibration table and said step of providing the thickness calibration table includes identifying the semiconductor material of the film and providing the tables for the identified semiconductor material.

6. The method of claim 1 wherein said step g) includes accounting for the semiconductor material and the thickness of the film.

7. The method of claim 1 wherein at least one of said step g) and said step h) includes employing the following equation:

I(d)/I(0)=1−

exp(−

α

d)

wherein d is the thickness of the film, I(O) is the intensity of diffusely scattered light collected from the substrate without the film, a is the absorption coefficient of the semiconductor, and I(d) is the intensity of the diffusely scattered light collected from the film at the film thickness (d).

8. The method of claim 1 including adjusting the thickness of the film and re-determining the temperature of the film at the adjusted thickness.

9. The method of claim 1 wherein said step h) includes determining the dependence of the optical absorption edge wavelength of the film on the film thickness.

10. The method of claim 9 wherein the step of determining the dependence of the optical absorption edge wavelength on the thickness includes preparing an optical absorption edge versus thickness calibration table at a constant temperature.

11. The method of claim 10 wherein said step of preparing a thickness calibration table includes depositing the film on the substrate at a constant temperature and measuring the optical absorption edge wavelength of the film at the constant temperature and a plurality of thicknesses.

12. The method of claim 1 including normalizing the spectra.

13. The method of claim 12 wherein said step of normalizing the spectra includes:

i) interacting light with the substrate prior to step b) to produce diffusely scattered light,

j) collecting the diffusely scattered light from the substrate of step i),

k) producing a reference spectra based on the diffusely scattered light from the substrate of step j),

l) normalizing the reference spectra of step k),

m) dividing the normalized spectra of claim 12 by the normalized reference spectra of step l) to produce a resultant spectra, and

n) normalizing the resultant spectra of step m).

14. The method of claim 13 wherein said step m) includes employing oscillations of the spectra of claim 12 below the optical absorption edge region of the spectra.

15. The method of claim 14 including the step of employing a derivative analysis of the oscillations.

16. The method of claim 14 wherein said step m) includes employing the following equation:

17. The method of claim 1 wherein said step g) includes determining the optical absorption edge wavelength as a function of the film thickness and band gap energy of the semiconductor material.

18. The method of claim 1 wherein said step b) includes using at least one of a chemical vapor deposition process, a molecular deposition process and a sputtering process.

19. The method of claim 1 wherein said step b) includes transporting the substrate along a conveyor as the film is being deposited.

20. An apparatus for determining an optical absorption edge of a semiconductor film having a measurable optical absorption edge deposited on an optically transparent material having no measurable optical absorption edge comprising:

a detector for collecting diffusely scattered light from a film of a semiconductor material having a measurable optical absorption edge and a measurable thickness after it has been deposited on an optically transparent substrate of material having no measurable optical absorption edge,

a spectrometer for producing a spectra from the diffusely scattered light, and

a software program for determining a temperature of the film as a function of film thickness and optical absorption edge wavelength of the film using spectra produced by the spectrometer.

21. A system for determining an optical absorption edge of a semiconductor film having a measurable optical absorption edge deposited on an optically transparent material having no measurable optical absorption edge comprising:

a substrate of optically transparent material having no measurable optical absorption edge,

a film of a semiconductor material having a measurable optical absorption edge and a measurable thickness deposited on said substrate,

a means for depositing the film on said substrate,

a light source for interacting light with said film deposited on said substrate,

a detector for collecting diffusely scattered light from said film,

a spectrometer for producing a spectra from the diffusely scattered light, and

a software program for determining a temperature of the film as a function of film thickness and optical absorption edge wavelength using spectra produced by the spectrometer.

22. A system as set forth in claim 21 wherein said substrate includes aluminum oxide (Al₂O₃) and said film includes gallium nitride (GaN).