Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2001-06-25
2003-09-23
Bruce, David V. (Department: 2882)
Optics: measuring and testing
By light interference
For dimensional measurement
Reexamination Certificate
active
06624894
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is related in general to the field of scanning interferometry and, in particular, to a new approach for correcting motion nonlinearities and other scanning errors occurring during interferometric measurements.
2. Description of the Related Art
Conventional scanning interferometry utilizes a light source, such as white light or a laser beam, to produce interference fringes at a light detector as the optical path difference (OPD) between a test surface and a reference surface is varied during a vertical scan. All measurement techniques based on phase shifting interferometry (PSI), vertical scanning interferometry (VSI, also referred to as white-light interferometry), combinations of VSI and PSI, phase shifting interferometry on-the-fly (PSIOTF), and lateral scanning interferometry (LSI, disclosed in Ser. No. 09/569,131) rely on an analysis of the interference between two beams of light. One beam (the object beam) is reflected from the sample; the other beam (the reference beam) is reflected from the reference mirror; and the two beams are recombined to create an interference pattern (the interferogram) at each scanning step. A detector (usually a CCD camera) registers the interferogram in a number of frames while the optical path difference (OPD) between the interfering beams is changing in a predefined fashion, which is realized either by moving the object, or by scanning the interferometric objective or the reference mirror at a preferably constant speed. The resultant shape of the sample object is calculated based on the intensity patterns in the interferograms and can be used to describe the height profile of the sample.
PSI is preferably used for measurements of smooth surfaces with small changes in profile (see K. Creath, “Temporal Phase Measurement Methods,” Interferogram Analysis, Institute of Physics Publishing Ltd., Bristol, 1993, pp. 94-140). VSI is generally used to measure smooth and/or rough surfaces with large interpixel height ranges (K. G. Larkin, “Efficient Nonlinear Algorithm for Envelope Detection in White Light Interferometry,” J. Opt. Soc. Am., A/Vol. 13, 832-843 (1996). The combination of VSI and PSI has been used, for example, to measure large steps with PSI precision (C. Ai, U.S. Pat. No. 5,471,303). The PSIOTF technique, which is a particular case of VSI and PSI combination, improves measurements of smooth surfaces in the larger height range (A. Harasaki et al., “Improved Vertical Scanning Interferometry,” Appl. Opt. 39, 2107-2115, 2000).
In VSI, interference fringes are localized only in a small region around the focus because of the low coherence source and/or the high numerical aperture of the microscope objective typically employed. While scanning through focus, fringes for different parts of the sample surface are produced and analyzed resulting in an unambiguous measurement of the object shape. As is well understood in the art, the height information from a low-coherence interferogram can be retrieved in many ways, such as, for example, by peak detection of the coherence envelope or fringe, by calculation of the centroid of the intensity signal, or by determination of the slope of the phase of the average wavelength. All these methods of analysis estimate the best vertical-scan focus position for a given pixel by sensing the coherence peak and utilize this information to determine the relative height difference between pixels (that is, a relative height profile of the test surface).
In PSI techniques, the interference fringes are examined only around a single focus position using a quasi-monochromatic light source such as a laser. PSI was introduced to the discipline of optics in 1974 from the telecommunications field (see J. H. Bruning et al., “Digital Wavefront Measuring Interferometer for Testing Optical Surfaces and Lenses,” Appl. Opt. 13, 2693-2703, 1974). Since then many applications of PSI have been developed for optics, resulting in a large number of reliable and robust algorithms (see, for example, P. Hariharan et al., “Digital Phase Shifting Interferometry: a Simple Error-Compensating Phase Calculation Algorithm,” Appl. Opt. 26, 2504-2505, 1987; and J. Schwider et al., “Digital Wave-Front Measuring Interferometry: Some Systematic Error Sources,” Appl. Opt. 22, 3421-3432, 1983). These algorithms produce phase measurements that may be ambiguous (because of so-called 2&pgr; ambiguities) and therefore require further processing to remove the ambiguity by unwrapping the phase data. Because of this limitation, only objects with relatively small inter-pixel height differences can be measured using conventional PSI analysis. On the other hand, the advantage of PSI over VSI techniques is that PSI allows for more precise measurements.
The PSIOTF technique affords the high precision of PSI combined with the lack of ambiguity of VSI measurements. Accordingly, this combined technique is often utilized for determining the shape of smooth surfaces with moderate to large height differences (that is, surfaces with steep profiles). The procedure first involves finding a coarse focus position for each pixel using a VSI technique; then a PSI algorithm is applied to the intensity data collected at frames around this focus position to achieve a high precision measurement where the 2&pgr; ambiguity has already been resolved by VSI. The PSIOTF technique can be used with a low-coherence light source or with a combination of both low- and high-coherence sources. In addition, the technique can be applied to each pixel individually or to groups of pixels in separate areas, as in the case of a stepped sample surface with separate smooth and flat regions.
All of these interferometric applications rely on accurate calibration of the scanning device used for changing the optical path difference (OPD) between the test object and the reference surface. Accordingly, these procedures cannot tolerate material departures from the calibrated condition. Scanner miscalibration, nonlinearities and vibrations affect the surface-profile measurements produced by PSI, VSI and PSIOTF algorithms by attributing an incorrect size to each scanner step, which in turn yields erroneous relative height measurements. Thus, the rate of change in OPD, normally referred to in the art as the “phase step,” needs to be well calibrated in order to achieve an accurate measurement. The most commonly used algorithms for the interferometric techniques used in the art require a predefined, nominal phase step which, once the scanner is calibrated, is assumed to remain constant along the whole scan for each measurement. However, this is not always the case and the errors in scanning speed influence the final result and need to be accounted for to achieve accurate measurements.
Calibration of the scanner is usually carried out as part of instrument maintenance and is assumed to remain unchanged during subsequent measurement runs. However, in fact the calibration may change, or the scanner can exhibit motion errors due to other influences such as friction, back-lash, etc. In such cases, these errors are carried forward into the measurement results in ways that depend on the type and magnitude of the phase-shift errors, and on the algorithm used to perform the interferometric analysis.
In view of the foregoing, it is recognized in the art that the advantage of high sensitivity afforded by optical interferometry is offset in part by its vulnerability to environmental influences, such as air convection, vibrations, scanner miscalibration and higher harmonics in the detected signal. A typical system operating at an average wavelength of 632 nm requires that scanning-step errors be confined to nanometer level; accordingly, all interferometric systems utilize some sort of vibration isolation, such as provided by air tables, or employ rather complicated active compensation circuits. This results in significant equipment cost and in limited mobility of such systems. In addition, these problems influence the range of applications and confine the us
Olszak Artur
Schmit Joanna
Artman Thomas R.
Bruce David V.
Durando Antonio R.
Durando Birdwell & Janke PLC
Veeco Instruments Inc.
LandOfFree
Scanning interferometry with reference signal 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 interferometry with reference signal, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Scanning interferometry with reference signal will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3037213