Rough surface profiler and method

• US 5,204,734 A
• Filed: 05/06/1992
• Issued: 04/20/1993
• Est. Priority Date: 06/12/1991
• Status: Expired due to Term

• Current Assignee: Bruker Nano, Inc. (Bruker Corporation)
• Original Assignee: WYKO, Inc.
• Inventors: Brophy, Chris P., Cohen, Donald K., Caber, Paul J.
• Primary Examiner(s): Turner, Samuel A.
• Assistant Examiner(s): KEESEE, LACHARLES
• Application Number:
  US07/880,083
• Time in Patent Office: 349 Days
• Field of Search: 356/359, 356/381, 356/360, 356/376, 356/377, 250/561, 382/8
• US Class Current: 356/497
• CPC Class Codes: G01B 11/2441 using interferometry

Abstract

A method of profiling a rough surface of an object includes moving the object along a z axis so that a highest point of the rough surface is optically aligned with and outside of the focus range of a solid-state imaging array. An interferogram of the rough surface then is produced by means of a two beam interferometer. The solid-state imaging array is operated to scan the rough surface along x and y axes to produce intensity data for each pixel of the solid-state imaging array for a plurality of frames each shifted from the other by a preselected phase difference. The modulation for each pixel is computed from the intensity data. The most recently computed modulation of each pixel is compared with a stored prior value of modulation of that pixel. The prior value is replaced with the most recently computed value if the most recently computed value is greater. The object is incrementally moved a selected distance along the z axis, and the foregoing procedure is repeated until maximum values of modulation and corresponding relative height of the rough surface are obtained and stored for each pixel.

Claims

1. A method of profiling a rough surface of an object, comprising the steps of:

(a) positioning the object along an optical axis so that a predetermined feature of the rough surface is optically aligned with an imaging device;

(b) producing an interference pattern of the rough surface by means of an interferometer;

(c) varying an optical path difference between the object and a reference surface of the interferometer;

(d) operating an imaging device to scan the rough surface to produce intensity data for each pixel of an image of the rough surface for a plurality of frames each shifted from the other by a preselected phase difference;

(e) extracting a modulation envelope from the intensity data for each pixel;

(f) locating a preselected characteristic of the modulation envelope for each pixel;

(g) correlating the preselected characteristic of the modulation envelope to a relative height of the rough surface for each pixel,

wherein step (e) includes

(1) eliminates a constant or slow-charging component from the intensity data, as the optical path difference is varied, to produce a first digital signal;

(2) rectifying the first digital signal to produce a second digital signal; and

(3) separating the modulation envelope from the second digital signal, the fist and second digital signals and the modulation envelope being successive values of digital words.

2. The method of claim 1 wherein step (1) is performed by means of a digital high pass filter.

3. The method of claim 2 wherein step (2) is performed by mathematical squaring of the first digital signal.

4. The method of claim 3 wherein step (3) is performed by means of a digital low pass filter.

5. A method of profiling a surface of an object, comprising the steps of:

(a) moving the object along an optical axis so that a predetermined feature of the surface is optically aligned with an imaging device;

(b) producing an interference pattern of the surface by means of an interferometer;

(c) varying an optical path difference between the object and a reference surface at the interferometer;

(d) operating an imaging device of the interferometer to scan the surface to produce intensity data for each pixel of an image of the surface for a plurality of frames each shifted from the other by a preselected phase difference;

(e) computing phase information from the intensity data for each pixel;

(f) determining a relative height corresponding to the phase information for each pixel, and using the relative heights as the profile of the surface if the surface is of a predetermined smoothness;

(g) extracting a modulation envelope from the intensity data for each pixel;

(h) locating a preselected characteristic of the modulation envelope for each pixel;

(i) correlating the preselected characteristic of the modulation envelope to a relative height of the rough surface for each pixel, and using the relative heights as the profile of the surface if the surface is of roughness greater than a predetermined high roughness; and

(j) combining the phase information with the relative height of the peak to obtain a relative height of improved accuracy for the surface at each pixel, and using the relative heights as the profile of the surface if the surface is of a predetermined intermediate roughness,

wherein step (g) includes

(1) eliminating a slow-charging component from the intensity data as the optical path difference is varied to produce a first digital signal;

(2) rectifying the first digital signal to produce a second digital signal; and

(3) separating the modulation envelope from the second digital signal, the first and second digital signals and the modulation envelope being successive values of digital words.

6. The method of claim 5 wherein step (1) is performed by means of a digital high pass filter.

7. The method of claim 6 wherein step (2) is performed by mathematical squaring of the first digital signal.

8. The method of claim 7 wherein step (3) is performed by means of a digital low pass filter.

9. A device for profiling a rough surface of an object, comprising in combination:

(a) an imaging device;

(b) means for moving the object along an optical axis so that a highest point of the rough surface is optically aligned with the imaging device;

(c) an interferometer, and means for producing an interference pattern of the rough surface by means of the interferometer;

(d) means for operating an imaging device of the interferometer to scan the rough surface to produce intensity data for each pixel of an image of the rough surface for a plurality of frames each shifted from the other by a preselected phase difference;

(e) means for extracting modulation envelope from the intensity data for each pixel;

(f) means for locating a preselected characteristic of the modulation envelope for each pixel ;

and

(g) means for correlating the preselected characteristic of the modulation envelope to a relative height of the rough surface at the peak for each pixel to obtain the profile of the rough surface.

10. The device of claim 9 wherein the modulation envelope extracting means includes

(1) means for stripping a DC component from the intensity data as the optical path difference is varied to produce a first digital signal;

(2) means for rectifying the first digital signal to produce a second digital signal; and

(3) means for separating the modulation envelope from the second digital signal, the first and second digital signals and the modulation envelope being successive values of digital words.

11. The device of claim 10 wherein the stripping means includes a digital high pass filter, wherein the rectifying means performs mathematical squaring of the first digital signal, and the separating means includes a digital low pass filter.