Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2002-05-21
2004-08-03
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
C356S329000, C356S489000, C356S495000, C356S513000, C356S514000
Reexamination Certificate
active
06771375
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates to the field of interferometry and, in particular, to apparatus and method(s) for the high accuracy measurement of aspherical optical surfaces and wavefronts using the interference of light.
BACKGROUND OF THE INVENTION
A single aspherical optical surface can be used to replace several conventional spherical or plano optical surfaces or elements in an optical system to improve the system's optical performance by reducing optical aberrations and increasing optical transmission, for example. In lithography tools where use is made of shorter wavelengths to make integrated circuits, the limited choice of materials from which refractive optical elements can be made necessitates the use of refractive and reflective aspherical optical surfaces to achieve the ever increasing performance demands for this application. Surface errors for optical surfaces needed for extreme ultraviolet (EUV) lithography tools operating at wavelengths of 13.6 nm, for example, must be less than 0.1 nm for reflective aspherical surfaces. Not only are the tolerances for the measurement accuracy increasing, but the magnitude of the aspherical departure from a reference sphere is increasing, for example, to nearly 1000 micrometers for some applications. As the integrated circuit line widths shrink, the size (diameter) of the optical elements is increasing to nearly 500 millimeters.
The measurement of aspherical surfaces and wavefronts has been very difficult because of the large departure from a best-fit reference sphere thereby producing an interferogram with many fringes that are very closely spaced. Prior art techniques have used some sort of aspherical null to mitigate this problem. Also, the Nyquist condition which requires at least two pixels per fringe, imposes a severe limit on what can be measured with available 2-D cameras. Since fringe spatial density is proportional to the slope of the aspherical surface, or wavefront, even weakly aspherical surfaces and wavefronts will violate the Nyquist condition. Typically, prior art systems are limited to surfaces with no more than 10-20 waves of aspherical departure.
In addition, for many of the prior art techniques, the wavefront difference measured in the interferogram is not simply the difference between the test and reference wavefronts. An aspherical measurement system requires that the entire interferometric measurement system, including all of the interferometer's optics, be ray traced for the aspherical surface under test. This requirement understandably complicates the calibration of the measurement system and also reduces the accuracy of the measurements.
While high accuracy aspherical optical surfaces have been difficult and expensive to make, thereby limiting their use, there have been improvements in the fabrication of aspherical surfaces which will make them more prevalent. With fabrication improvements in, for example, magneto-rheological finishing, ion figuring, and computer controlled polishing, there is a concomitant need for improved aspherical measurement methods and apparatus to provide the error maps for these manufacturing methods to enable them to produce the aspherical optical surfaces needed to meet the requirements of integrated circuit lithography tools such a steppers and scanners operating at wavelengths of 193 nm, 157 nm, and 13.6 nm, for example.
There are many methods and apparatus in the prior art for measuring aspherical optical surfaces, 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. It is 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
Consequently, it is a primary object of the present invention to provide a method and apparatus for the high accuracy measurement of aspherical optical surfaces and wavefronts.
It is another object of the present invention to provide methods and apparatus for accurately measuring surfaces and wavefronts with large aspherical departures and surface slopes.
Yet another object of the present invention is to provide methods and apparatus for accurately measuring aspheric surfaces and wavefronts with large diameters (clear apertures).
Still another object of the present invention is to provide interferometric methods and apparatus for accurately measuring aspheric surfaces and wavefronts with no refractive optics in the interferometer (measurement) cavity.
Yet another object of the present invention is to provide methods and apparatus for accurately measuring aspheric surfaces and wavefronts with reduced sensitivity to temperature changes.
It is yet another object of the present invention is to provide methods and apparatus for accurately measuring aspheric surfaces and wavefronts with reduced sensitivity to turbulence of the gas in the interferometer (measurement) cavity.
Yet another object of the present invention is to provide methods and apparatus for accurately measuring aspheric surfaces and wavefronts with high speed.
Still another object of the present invention is to provide methods and apparatus for accurately measuring aspheric surfaces and wavefronts while meeting the Nyquist condition for the most challenging applications
And another object of the present invention is to provide high spatial
Brown Khaled
Caufield Francis J.
Font Frank G.
Zygo Corporation
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