Patent Application
20070024866
The field of the invention is the field of interferometric measurement of the surface topography of objects.
It is an object of the invention to produce a method of combining a plurality of interferograms of an object into a single interferogram to measure, for example, the surface topography of objects too large to be measured without moving the objects in the optical system.
U.S. Pat. No. 5,907,404 by Marron, et al. entitled “Multiple wavelength image plane interferometry” issued May 25, 1999;
U.S. Pat. No. 5,926,277 by Marron, et al. entitled “Method and apparatus for three-dimensional imaging using laser illumination interferometry” issued Jul. 20, 1999
U.S. patent application Ser. No. 10/893052 filed Jul. 16, 2004 entitled “Object imaging system using changing frequency interferometry method” by Michael Mater
U.S. patent application Ser. No. 10/349651 filed Jan. 23, 2003 entitled “Interferometry method based on changing frequency” by Michael Mater,
The above identified patents and patent applications are assigned to the assignee of the present invention.
If both the reference mirror 14 and the object 15 are flat mirrors aligned perpendicular to the incoming light from beam 12, and the light path traversed by the light from the light source to the image receiver is identical, the light from both the reference mirror and the object mirror will be in phase, and the image receiver will show a uniformly bright image. Such devices were the bane of undergraduate optics students before the advent of lasers, since the distances had to be equal to within a small part of the wavelength of light and the mirrors had to be aligned within microradians. Even with the advent of lasers, such devices are subject to vibration, thermal drift of dimensions, shocks, etc.
However, the Michelson interferometer design of
Another inspection technique which is very useful is when the Michalson interferometer of
One problem with the above description is that there are no numbers telling the difference between a depression and a hill, or in which direction the slope runs. However, if the reference mirror is moved, the lines will move, and, for example, the circles on a hill will shrink and a depression will expand for a particular direction of travel.
Interferometric techniques work very well for optical surface inspection to check whether the surface is flat, or curved to within a certain specification. However, for many surfaces which are rough on the scale of the wavelength of visible light, or have height variations or steep slopes, the “lines” of equal phase (or height) of the interferogram will be very close together. Any disturbances, noise, or other variation will make it difficult or impossible to “count” the fringes and thus measure the “height” of the various features. As an analogy, the result would be like trying to hike using a topographic map with lines every inch in height difference!
U.S. Pat. Nos. 5,907,404 and 5,926,277, assigned to the assignee of the present invention, show that a number of such interferograms taken with various phase delays in the reference beam and various wavelengths of the light source 10 may be recorded and computer analyzed to construct a “synthetic interferogram”, which is an interferogram which one would measure if one had a light source of much different wavelength from the wavelengths from the light source 10. Thus, the “lines” on the interferogram could show height differences of, say, 100 microns instead of 0.4 micron height differences, so the lines would be much further apart and much easier to keep track of. The advantage, of course, is that lasers of 200 micron wavelength are hard to find, and electronic imaging equipment for such wavelengths is even harder to find, and spatial resolution of such a detector, if available, could not possibly match the resolution of detectors for visible and near infra-red light.
Manual inspection of results from a megapixel imaging device of course is difficult for humans, but easy for a computer programmed with a fast Fourier transform (FFT) program or other statistical analysis program. The FFT of a perfect sine wave gives a delta function telling the frequency of the wave, and in the case of a sine wave displaced from the origin also gives a “phase”, as well as the amplitude A and average intensity I0. Since the “frequency” of the results from all the pixels is the same, the relative “phase” for each pixel can be recorded from sufficient measurements of pixel intensity as the reference mirror is moved to change the phase of the reference beam. The multiple measurements remove much of the “noise” which would complicate the interpretation of an interferogram taken with an object fixed with respect to the reference mirror, as the maximum height peak of the FFT is easily identified and lower height peaks introduced by noise are ignored. The recorded measurements of phase and amplitude are sometimes called a digital hologram. The phase, amplitude, or other measurements so recorded as images are called, for the purposes of this specification, as synthetic “phase images”, and can be printed out as a two dimensional image where brightness or color is directly related to phase, intensity, etc. I0 can be printed out, and looks similar to the image which would be recorded in absence of the reference beam or a normal photographic or digital image of the object.
When the field of view of the optical system is too small to “see” the entire surface of the object 25, one could translate the object a known distance in a known direction perpendicular to the object beam, and record a new interferogram, and combine the interferograms. Unfortunately, systems to hold and transport objects macroscopic distances, and place them within a small part of a wavelength in position without introducing errors and microradian tilts are extremely expensive and delicate.
The method of the invention records at least two digital phase images of different parts of the surface of an object, each interferogram recording at least one overlapping image of the same portion of the surface of the object. The at least one of thetranslation vector of the relative motion of the two images, the relative phase, the tilt angle, and the direction difference between the two digital phase images are then calculated using attributes of the measurements, such as phases, amplitudes, intensities, or other statistical information generated from the data recorded to produce the recorded phase images, and the corresponding pixels of the two images are identified. Then, at least one of the phase images is corrected to account for the relative translation vector, phase, tilt angle, and tilt angle direction differences in the two images. The two phase images are then combined into one digital phase image.
The number of phases and the number of frequencies used for the measurements are interrelated. For a single frequency, the number of phases required to make measurements such as shown in
The number of wavelengths of the interfering light may be as small as one, for surfaces which have no discontinuities or slopes which would give more than a change of phase of about 90 degrees per pixel of the image on the image receiver. However, if there are such discontinuities and slopes, preferably at least three different wavelengths of light are used to record the synthetic phase image. More preferably, at least 5 wavelengths are used, and most preferably more than 7 wavelengths are used. 16 wavelengths ensures multiple redundancies in the data, and can be used for especially “noisy” results.
For interferometers with unequal object and reference arm path lengths, changing the frequency also changes the relative phase of the interfering light beams at the detector. Thus, measurements such as shown by
The corresponding pixels of the overlapping sections of each portion of the object surface are identified most easily if there are features on the surface of the object which give good contrast in the reflected intensity of light in absence of the reference beam. I0, as shown in
The images are most preferably segmented so that only the overlapping portions of the images are used in the calculations.
Once the phase images have been remapped to make the correspondence between each pixel in the overlapping images, the phase images may be corrected. A relative phase difference, tilt angle and tilt direction is chosen as a starting point, and one of the phase images has phase added to each pixel to account for the change of height and tilt introduced when the object is moved. Then, the phase differences between the images is minimized for each pixel, for example by minimizing the square of the differences as the chosen relative phase and tilt angle and direction are varied.
Once the best measure of the relative phase, relative tilt angle and direction has been found, appropriate phase can be added to the phases recorded for each of the synthetic phase images for each portion of the surface, and the phase images are combined to give one phase image of the entire measured portion of the surface of the object.
All patents, patent publications, and publications referred to herein are included by reference in their entirety, including included references.
Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.
