Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2002-06-03
2004-08-24
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
C356S511000, C356S512000, C356S450000
Reexamination Certificate
active
06781700
ABSTRACT:
FIELD OF THE INVENTION
In general, this invention relates to the field of interferometry and, in particular, to the high accuracy measurement of aspherical surfaces and wavefronts in an absolute manner.
BACKGROUND OF THE INVENTION
Aspherical surfaces have become more and more important in modern optical systems because they offer a higher number of parameters for simplifying systems while optimizing their performance. This can lead to systems with less surfaces, less weight, smaller dimensions and higher states of correction, to mention only a few advantages. This is especially true in fields where a high number of optical surfaces are not practical, like in astronomical telescopes or normal incidence reflecting surfaces for the EUV wavelength of 13.6 nm used for lithography tools where it is mandatory to keep the number of surfaces as low as possible. In such cases, there is no choice but to use aspherical surfaces. With demands for high quality performance for complete systems operating in the EUV-regime, the surface errors of reflecting surfaces within such a system must be kept below 0.1 nm, and the measuring accuracy and precision for such errors must be even higher to be able to produce the surfaces in a deterministic manner. In addition, lens surfaces in multielement lithography lenses operating at wavelengths of 193 nm and 157 nm are made aspherical to lower the number of elements made, which are of rare and expensive materials. In these cases, the departures from a best fitting sphere can be as large as 1000 &mgr;m, and the dimensions of such lens surfaces have increased to nearly 500 mm.
In an optical system, the function of any its lens elements is to modify the wavefront transmitted by the individual lens elements according to the optical design of the whole system. If a spherical wave or a plane wave enters such a lens, an aspherical wavefront with a very high departure from the best fitting sphere is produced, depending on the conjugates used in the particular test-configuration. So, even the fundamental single lens element, with either spherical or aspherical surfaces can only be tested properly if one is able to deal with aspherical wavefronts in a test set-up. Moreover, this ability is very important for testing wavefronts transmitted through lens elements because inhomogeneity of the lens material itself can deteriorate the wavefront, even when the surfaces are otherwise free of error.
The measurement of aspherical surfaces and wavefronts has been very difficult because of the large departure from the best fitting sphere. With interferometric measurements, high precision is possible by making the dynamic range of the measurement very small, and for this purpose, the wavefront of the reference wavefront, against which the aspherical wavefront is compared, has to be made aspherically as well to ideally fit the wavefront to be measured completely. In prior art, this has been done either by refractive systems, so called “null-lenses”, or with diffractive elements, so called “computer generated holograms”, which alter a wave of known and measurable shape (spherical or preferably plane wave) as it transits the compensation element to fit the design aspherical surface at the location where it is placed in the test-set up by design.
In all these cases, the compensation element must be tested to be sure that the correct wavefront is delivered for comparison. But, it is obvious that the same difficulties exist for this type of testing because, again, an aspherical wavefront is produced. Therefore, only indirect test methods are applied by, for instance, measuring the surface of each lens element used in a null system, which is exclusively built with the help of spherical surfaces. Also, the refractive index of the lens material, the lens thickness and the air-spacing of the lenses are measured carefully. Nevertheless, the final accuracy is questionable because of accumulation of measurement errors and the uncertainty of the homogeneity within the lens material.
There are many methods and apparatus in the prior art for measuring aspherical optical surfaces as, for example: 1. Contacting and non-contacting stylus based profilers; 2. Contacting and non-contacting stylus based coordinate measuring machines; 3. Spherical wavefront interferometers; 4. Lateral and radial shearing interferometers; 5. Interferometers with null lenses in the measurement path; 6. Scanning spherical wave interferometers; 7. Scanning white light interferometers; 8. Sub-aperture stitching interferometers; 9. Interferometers using computer generated holograms-CGHs; 10. Point diffraction interferometers-PDIs; 11. Longer wavelength interferometry; and 12. Two wavelength interferometry. While these techniques have utility for many applications, they are limited in their operational capabilities or precision compared with those needed for today's evolving lithography applications.
Contacting and non-contacting stylus based profilers mechanically scan the aspherical surface under test, and therefore, are slow because they measure only a few data points at a time. Slow techniques are very susceptible to measurement errors due to temperature variations during the measurement. The same limitations apply to contacting and non-contacting stylus based coordinate measuring machines.
Spherical wavefront interferometers usually require the spacing between the element generating the spherical wavefront and the aspherical surface under test to be scanned thereby increasing the measurement time for the entire surface under test thus introducing another parameter which must be measured, usually by another measurement device, and means, commonly known as stitching, for connecting the data from the various zones which fit as the spacing is scanned.
Scanning white light interferometers have many of the same limitations as spherical wavefront interferometers. Lateral and radial shearing interferometers usually measure the slope of the surface under test and thereby introduce measurement errors during the reconstruction of the surface under test via integration of the slopes. This latter type of limitation applies to differential types of profiling techniques as well.
Sub-aperture stitching interferometers introduce serious measurement errors in the stitching process. Interferometers using computer generated holograms are susceptible to errors introduced by the CGH and stray Moiré patterns. They are also difficult to calibrate, i.e., know the calibration of the CGH. Point diffraction interferometers are a class of spherical wavefront interferometers, and therefore, have many of the same limitations, as well as poor lateral spatial resolution.
None of the prior art approaches is entirely satisfactory since each involves a trade-off that places long lead times on the design of the measurement apparatus and method, requires additional fabrication, increases the difficulty of using and calibrating the measurement apparatus, decreases the accuracy and precision, and greatly increases the cost and delivery time of the aspherical optical element.
As a result of certain deficiencies in prior approaches to measuring aspheres, it is a principle object of the present invention to provide a method(s) and apparatus for high accuracy absolute measurement of aspherical surfaces or aspherical wavefronts, either the surface of the final optical part or the wavefront of the final optical lens element in transmission, or by absolutely qualifying the compensation elements for the measurement of aspheres, being either of the refractive, diffractive of reflective type, therefore enabling other, more productive methods for the measurement of the components to be produced in volume.
It is another object of the present invention to provide method(s) and apparatus for measuring aspherical surfaces and wavefronts with large aspherical departures and surface slopes
It is another object of the present invention to provide method(s) and apparatus for measuring aspherical surfaces and wavefronts with large diameters and clear aperture.
It is yet a
Brown Khaled
Caufield Francis J.
LandOfFree
Scanning interferometer for aspheric surfaces and wavefronts does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Scanning interferometer for aspheric surfaces and wavefronts, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Scanning interferometer for aspheric surfaces and wavefronts will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3357043