Rough surface profiler and method

• US 5,355,221 A
• Filed: 10/25/1993
• Issued: 10/11/1994
• Est. Priority Date: 06/12/1991
• Status: Expired due to Term

• Current Assignee: Bruker Nano, Inc. (Bruker Corporation)
• Original Assignee: WYKO, Inc.
• Inventors: Cohen, Donald K., Brophy, Chris P., Caber, Paul J.
• Primary Examiner(s): Turner, Samuel A.
• Assistant Examiner(s): KEESEE, LACHARLES
• Application Number:
  US08/143,372
• Time in Patent Office: 351 Days
• Field of Search: 356/359, 356/381, 356/360, 356/376, 356/377, 382/8, 250/561
• 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 surface of an object, comprising the steps of:

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

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

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

(d) operating the imaging device to scan the object surface to produce intensity data for each pixel of an image of the object surface;

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

(f) rectifying the first digital signal to produce a second digital signal including a high frequency component and a low frequency component;

(g) digitally filtering the second digital signal to eliminate the high frequency component from the second digital signal to thereby produce a third digital signal;

(h) locating a preselected characteristic of the third digital signal for each pixel; and

(i) correlating the preselected characteristic of the third digital signal to a relative height of the object surface for each pixel.

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

(a) an imaging device;

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

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

(d) means for operating the imaging device to scan the object surface to produce intensity data for each pixel of an image of the object surface

i. means for measuring a position value corresponding to the relative distance between the object surface and a reference surface in the interferometer and producing a position signal representing the position value,

ii. means for applying the position signal as position feedback to a control circuit driving a translator connected to vary the relative distance between the object surface and the reference surface, and

iii. means for operating the control circuit in response to the position signal to precisely linearly adjust the relative distance between the object surface and the reference surface;

(e) means for computing information representative of variations in amplitude of intensity for each pixel from the intensity data;

(f) means for comparing a most recently computed value of the information for each pixel with a stored prior value of the information for that pixel;

(g) means for replacing the prior value of the information with the most recently computed value of the information if the most recently computed value of the information is greater than the prior value of the information;

(h) means for obtaining and storing a corresponding relative height of the object surface if the most recently computed value of the information is greater than the prior value of the information; and

(i) means for varying the relative distance between the object surface and the reference surface until maximum values of the information and corresponding relative heights of the object surface are obtained and stored for each pixel, respectively.

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

(a) an imaging device;

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

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

(d) means for operating the imaging device to scan the object surface to produce intensity data for each pixel of an image of the object surface, including

i. means for measuring a position value corresponding to the relative distance between the object surface and a reference surface in the interferometer and producing a position signal representing the position value,

ii. means for applying the position signal as position feedback to a control circuit driving a translator connected to vary the relative distance between the object surface and the reference surface, and

iii. means for operating the control circuit in response to the position signal to precisely linearly adjust the relative distance between the object surface and the reference surface;

(e) data processing circuitry adapted to remove a constant or slow-changing component from the intensity data, as the optical path difference is varied, to produce a first digital signal;

(f) means for rectifying the first digital signal to produce a second digital signal including a high frequency component and a low frequency component;

(g) means for digitally filtering the second digital signal to eliminate the high frequency component from the second digital signal to thereby produce a third digital signal;

(h) means for locating a preselected characteristic of the third digital signal for each pixel; and

(i) means for correlating the preselected characteristic of the third digital signal to a relative height of the object surface for each pixel.

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

(a) an imaging device;

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

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

(d) means for operating the imaging device to scan the object surface to produce intensity data for each pixel of an image of the object surface, including

i. means for measuring a position value corresponding to the relative distance between the object surface and a reference surface in the interferometer and producing a position signal representing the position value,

ii. means for applying the position signal as position feedback to a control circuit driving a translator connected to vary the relative distance between the object surface and the reference surface, and

iii. means for operating the control circuit in response to the position signal to precisely linearly adjust the relative distance between the object surface and the reference surface;

(e) means for determining a preselected characteristic of the intensity data for each pixel; and

(f) means for correlating the preselected characteristic to a relative height of the object surface for each pixel.

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

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

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

(c) operating an imaging device to scan the object surface to produce intensity data for each pixel of an image of the object surface, by

i. measuring a position value corresponding to the relative distance between the object surface and a reference surface in the interferometer and producing a position signal representing the position value,

ii. applying the position signal as position feedback to a control circuit driving a translator connected to vary the relative distance between the object surface and the reference surface, and

iii. operating the control circuit in response to the position signal to precisely linearly adjust the relative distance between the object surface and the reference surface;

(d) computing information representative of variations in amplitude of intensity for each pixel from the intensity data;

(e) comparing a most recently computed value of the information for each pixel with a stored prior value of the information for that pixel;

(f) replacing the prior value of the information with the most recently computed value of the information if the most recently computed value of the information is greater than the prior value of the information;

(g) obtaining and storing a corresponding relative height of the object surface if the most recently computed value of the information is greater than the prior value of the information; and

(h) varying the relative distance between the object surface and the reference surface until maximum values of the information and corresponding relative heights of the object surface are obtained and stored for each pixel, respectively.

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

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

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

(c) operating an imaging device to scan the object surface to produce intensity data for each pixel of an image of the object surface, by

i. measuring a position value corresponding to the relative distance between the object surface and a reference surface in the interferometer and producing a position signal representing the position value,

ii. applying the position signal as position feedback to a control circuit driving a translator connected to vary the relative distance between the object surface and the reference surface, and

iii. operating the control circuit in response to the position signal to precisely linearly adjust the relative distance between the object surface and the reference surface;

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

(e) rectifying the first digital signal to produce a second digital signal including a high frequency component and a low frequency component;

(f) digitally filtering the second signal to eliminate the high frequency component from the second digital signal to thereby produce a third digital signal;

(g) locating a preselected characteristic of the third digital signal for each pixel; and

(h) correlating the preselected characteristic of the third digital signal to a relative height of the object surface for each pixel.

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

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

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

(c) operating an imaging device to scan the object surface to produce intensity data for each pixel of an image of the object surface, by

i. measuring a position value corresponding to the relative distance between the object surface and a reference surface in the interferometer and producing a position signal representing the position value,

ii. applying the position signal as position feedback to a control circuit driving a translator connected to vary the relative distance between the object surface and the reference surface, and

iii. operating the control circuit in response to the position signal to precisely linearly adjust the relative distance between the object surface and the reference surface;

(d) determining a preselected characteristic of the intensity data for each pixel; and

(e) correlating the preselected characteristic to a relative height of the object surface for each pixel.

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

(a) an imaging device;

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

(c) a microcontroller adapted to operate the imaging device to scan the object surface to produce intensity data for each pixel of an image of the object surface, the microcontroller including

i. a position transducer adapted to measure the relative distance between the object surface and a reference surface in the interferometer and producing a position signal representing the relative distance,

ii. a circuit receiving the position signal, producing position information in response to the position signal, and utilizing the position information to control a translator control circuit driving the translator in order to precisely linearly adjust the relative distance between the object surface and the reference surface;

(d) the microcontroller being adapted to correlate a preselected characteristic of the intensity data to a relative height of the object surface for each pixel.

9. The device of claim 8 wherein the transducer includes a linear variable differiential transducer.

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

(a ) an imaging device;

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

(c) a translator adapted to move the object along an optical axis of the interferometer to vary an optical path difference between the object surface and a reference surface;

(d) a control circuit adapted to operate the imaging device to scan the object surface to produce intensity data for each pixel of an image of the object surface;

(e) digital processing circuitry adapted to perform computations on the intensity data to remove a constant or slowchanging component from the intensity data as the optical path difference is varied, to produce a first digital signal;

(f) a rectifier rectifying the first digital signal to produce a second digital signal including a high frequency component and a low frequency component;

(g) digital processing circuitry adapted to perform computations on the second digital signal to eliminate the high frequency component from the second digital signal to thereby produce a third digital signa, to locate a preselected characteristic of the third digital signal for each pixels and to correlate the preselected characteristic of the third digital signal to a relative height of the object surface for each pixel.