Methods and apparatus for characterizing a surface

• US 5,625,451 A
• Filed: 11/27/1995
• Issued: 04/29/1997
• Est. Priority Date: 11/27/1995
• Status: Expired due to Term

• Current Assignee: Schmitt Measurement Systems, Inc. (Schmitt Industries, Inc.)
• Original Assignee: Schmitt Measurement Systems, Inc. (Schmitt Industries, Inc.)
• Inventors: Schiff, Tod F., Bernt, Marvin L.
• Primary Examiner(s): Pham, Hoa Q.
• Application Number:
  US08/562,876
• Time in Patent Office: 519 Days
• Field of Search: 356/236, 356/371, 356/445, 356/446, 250/228, 359/387
• US Class Current: 356/236
• CPC Class Codes: G01B 11/303 using photoelectric detection means
• CPC Class Codes: G01N 21/474 Details of optical heads therefor, e.g. using optical fibres

Abstract

A system and method for characterizing a surface are disclosed. The system includes a light source and source optics which direct a beam of light toward the surface. A first optical integrating device is positioned and configured to receive a first portion of the scattered light which corresponds to a first range of spatial frequencies. A second optical integrating device is positioned and configured to receive a second portion of the scattered light corresponding to a second range of spatial frequencies. In one embodiment, an integrating sphere is employed as the first optical integrating device. The sphere includes a sampling aperture which is surrounded by a light absorption region on the interior of the sphere. Total integrated scatter data is generated for each range of spatial frequencies and is used to approximate the spectral scatter function of the surface. RMS roughness is then approximated for any range of spatial frequencies.

Claims

1. A method for characterizing a surface, comprising the steps of:

selecting a plurality of bands of spatial frequencies, each having an upper and a lower limit;

directing a beam of light having a known wavelength onto the surface at a known incident angle with respect to normal of the surface;

collecting the total scatter of light corresponding to each of the selected bands of spatial frequencies;

measuring the total integrated scatter of each of the selected bands of spatial frequencies by detecting the intensity of the collected light;

using the total integrated scatter data and the upper and lower limits of spatial frequency for each of the selected bands to estimate the spectral integrated scatter function; and

approximating the total integrated scatter of the surface over desired spatial frequency limits from the spectral integrated scatter function.

2. A method for approximating the total integrated scatter of a surface as defined in claim 1, wherein the step of selecting a plurality of bands of spatial frequencies comprises selecting a first band having an upper limit (f₁) and a lower limit (f₂) and selecting a second band having an upper limit (f₃) and a lower limit (f₄), and wherein f₁, f₂, f₃, and f₄ are selected such that the following relationship is satisfied:

##EQU16##

3. A method for characterizing a surface as defined in claim 2, wherein the step of using the total integrated scatter data and the upper and lower limits of spatial frequency for each of the selected bands to estimate the spectral integrated scatter function comprises assuming the spectral integrated scatter function is linear in log-log space and determining the slope (n) of the spectral integrated scatter function according to the following equation:

##EQU17## where TIS₁ is the total integrated scatter of the first selected band of spatial frequencies and TIS₂ is the total integrated scatter of the second selected band of spatial frequencies.

4. A method for approximating the total integrated scatter of a surface as defined in claim 3, wherein f₂ and f₃ are selected such that f₂ equals f₃.

5. A method for characterizing a surface as defined in claim 3, wherein the step of approximating the total integrated scatter of the surface over desired spatial frequency limits comprises determining the total integrated scatter (TIS_eff) over desired spatial frequency limits according to the following equation:

##EQU18## where f_min and f_max are the desired spatial frequency limits.

6. A method for characterizing a surface as defined in claim 5, further comprising the step of ascertaining the RMS roughness (σ

_eff) of the surface over the desired spatial frequency limits for surfaces having a roughness less than the wavelength of the beam of light (λ

) by calculating the RMS roughness of the surface according to the following equation;

##EQU19## where θ

_i is the incident angle of the beam of light with respect to the normal of the surface.

7. A method for characterizing a surface as defined in claim 5, wherein the step of directing a beam of light onto the surface comprises directing a beam of light onto the surface at an incident angle (θ

_i) less than about 10 degrees with respect to the normal of the surface and further comprising the step of ascertaining the RMS roughness (σ

_eff) of the surface over the desired spatial frequency limits for surfaces having a roughness less than the wavelength of the beam of light (λ

) by calculating the RMS roughness of the surface according to the following equation;

##EQU20## where TIS_eff is the total integrated scatter over desired spatial frequency limits.

8. A method for approximating the total integrated scatter of a surface as defined in claim 1, wherein the step of collecting the total scatter of light corresponding to each of the selected bands of spatial frequencies comprises using a plurality of optical integrating devices.

9. A system for characterizing a surface, comprising:

a light source capable of producing a beam of light at a predetermined wavelength (λ

);

source optics for directing the beam of light toward the surface at an incident angle (θ

_i) with respect to the normal of the surface to thereby create a reflected specular beam and scattered light;

a first optical integrating device positioned and configured to receive a first portion of the scattered light, the first portion of the scattered light extending from a first scatter angle (θ

₁) to a second scatter angle

a first detector positioned to detect the intensity of the first portion of the scattered light;

a second optical integrating device positioned and configured to receive a second portion of the scattered light, the second portion of the scattered light extending from a third scatter angle (θ

₃) to a fourth scatter angle (θ

₄);

a second detector positioned to detect the intensity of the second portion of the scattered light; and

a specular detector positioned to detect the intensity of the specular beam.

10. A system for characterizing a surface as defined in claim 9, wherein the scatter angles (θ

_n) are selected to correspond to predetermined spatial frequencies (f_n) of the surface such that the following relationship is satisfied;

θ

_n =sin^-1 (sinθ

_i -f_n λ

).

11. A system for characterizing a surface as defined in claim 9, wherein the first optical integrating device comprises

a hollow sphere having a radius (R_s), the hollow sphere configured with an input aperture, a sampling aperture, and an output aperture,

the light source, source optics, and sphere are positioned for directing the beam of light through the input aperture, through the sampling aperture, and onto the surface and such that the second portion of the scattered light and the specular beam are directed out of the sphere through the output aperture, and

wherein the second optical integrating device comprises a focusing mirror configured with an output aperture for allowing the specular beam to pass, the focusing mirror positioned to receive and reflect the second portion of the scattered light into the second detector.

12. A system for characterizing a surface as defined in claim 11, wherein the input aperture of the hollow sphere has a circular perimeter with a radius (r_i), the sampling aperture of the hollow sphere has a circular perimeter with a radius (r_s), the output aperture of the hollow sphere has a circular perimeter with a radius (r_o), the focusing mirror has a circular perimeter with a radius (r₂₀), and the output aperture of the focusing mirror has a circular perimeter with a radius (r_2i).

13. A system for characterizing a surface as defined in claim 12, wherein the radius of the perimeter of the sampling aperture (r_s) is determined as a function of the first scatter angle (θ

₁).

14. A system for characterizing a surface as defined in claim 13, wherein the sphere has an interior surface, the incident angle of the beam of light (θ

_i) is less than about 10 degrees, the first scatter angle (θ

₁) is greater than about 45 degrees, and the radius of the perimeter of the sampling aperture (r_s) is approximately equal to (h/tanθ

₁), where h is the distance from the surface to the interior surface of the hollow sphere at the sampling aperture along a line normal to the surface.

15. A system for characterizing a surface as defined in claim 12, wherein the radius of the perimeter of the output aperture of the hollow sphere (r_o) is determined as a function of the second scatter angle (θ

₂).

16. A system for characterizing a surface as defined in claim 15, wherein the incident angle of the beam of light (θ

_i) is less than about 10 degrees and the second scatter angle (θ

₂) is less than about 20 degrees and wherein the radius of the perimeter of the output aperture of the hollow sphere (r_o) is approximately equal to 2R_s tanθ

².

17. A system for characterizing a surface as defined in claim 12, wherein the incident angle of the beam of light (θ

_i) is less than about 10 degrees and the third scatter angle (θ

₃) is less than about 20 degrees, and wherein the minimum radius of the perimeter of the focusing mirror (r₂₀) is approximately equal to (tanθ

₃)(b+2R_s), where b equals the distance along the path of the specular beam from the output aperture of the hollow sphere to the focusing mirror.

18. A system for characterizing a surface as defined in claim 12, wherein the incident angle of the beam of light (θ

_i) is less than about 10 degrees, the second scatter angle (θ

₂) is less than about 20 degrees, and the second scatter angle (θ

₂) equals the third scatter angle (θ

₃), and wherein the radius of the perimeter of the focusing mirror (r₂₀) is defined by the following relationship;

##EQU21## where b equals the distance along the path of the specular beam from the output aperture of the hollow sphere to the focusing mirror.

19. A system for characterizing a surface as defined in claim 12, wherein the incident angle of the beam of light (θ

_i) is less than about 10 degrees and the fourth scatter angle (θ

₄) is less than about 20 degrees, and wherein the radius of the perimeter of output aperture of the focusing mirror (r_2i) is approximately equal to (tanθ

₄)(b+2R_s), where b equals the distance along the path of the specular beam from the output aperture of the hollow sphere to the focusing mirror.

20. A system for characterizing a surface as defined in claim 12, wherein the source optics are configured such that the incident beam has a circular cross section with a radius (r₁) as it leaves the source optics and wherein the radius of the perimeter of the output aperture of the focusing mirror (r_2i) is defined by the following relationship:

##EQU22## where a equals the minimum distance along the path of the incident beam from the source optics to the input aperture of the hollow sphere and b equals the distance along the path of the specular beam from the output aperture of the hollow sphere to the focusing mirror.

21. A system for characterizing a surface as defined in claim 12, wherein the source optics are configured such that the specular beam is positioned within the output aperture of the focusing mirror.

22. A system for characterizing a surface as defined in claim 21, wherein the source optics are configured such that the specular beam is focused at the output aperture of the focusing mirror.

23. A system for characterizing a surface as defined in claim 22, wherein the source optics are configured such that the incident beam has a circular cross section with a radius (r₁) as it leaves the source optics and wherein the radius of the perimeter of the input aperture of the hollow sphere (r_i) is defined by the following relationship:

##EQU23## for values of K≧

1, where a equals the distance along the beam of light from the source optics to the input aperture of the hollow sphere and b equals the distance along the path of the specular beam from the output aperture of the hollow sphere to the focusing mirror.

24. A system for characterizing a surface as defined in claim 23, wherein the light source produces a laser beam and K≧

√

2.5.

25. A system for characterizing a surface as defined in claim 12, wherein the first optical integrating device further comprises an interior surface including an absorption region surrounding the sampling aperture, the interior surface of the sphere outside the absorption region comprising a reflective region, the absorption region having a reflectance less than the reflectance of the reflective region at the wavelength of the light source.

26. A system for characterizing a surface as defined in claim 25, wherein the absorption region of the interior surface of the sphere has a circular outside diameter and is positioned concentric with the sampling aperture and wherein the focusing mirror and the second detector are positioned such that the outside diameter of the absorption region is imaged at the maximum field of view of the second detector.

27. A system for characterizing a surface as defined in claim 25, wherein θ

₁, θ

₂, θ

₃, and θ

₄ are selected such that the following relationship is satisfied;

##EQU24##

28. A system for characterizing a surface as defined in claim 25, wherein θ

₂ and θ

₃ are selected such that θ

₂ equals θ

₃.

29. A system for characterizing a surface as defined in claim 25, wherein θ

_i is less than about 10 degrees.

30. A system for characterizing a surface as defined in claim 29, wherein the second scatter angle (θ

₂) is less than about 20 degrees, the second scatter angle (θ

₂) equals the third scatter angle (θ

₃), and the radius of the perimeter of the focusing mirror (r₂₀) is defined by the following relationship;

##EQU25## where b equals the distance along the path of the specular beam from the output aperture of the hollow sphere to the focusing mirror.

31. A system for characterizing a surface as defined in claim 25, wherein the light source comprises a laser beam.

32. A system for characterizing a surface as defined in claim 11, wherein the first detector is positioned within the integrating sphere such that the angle (θ

_D) between the specular beam and the line between the sampling aperture and the first detector is greater than θ

₁.

33. A system for characterizing a surface as defined in claim 9, wherein the first optical integrating device is configured with an aperture having a circular perimeter for allowing the second portion of the scattered light to pass and wherein the second optical integrating device is configured with an aperture having a circular perimeter for allowing the specular beam to pass.