The maximum measurable range of a spectral interference microscope depends on the coherence length of the light transmitted by its tunable spectral filter. To achieve a large range in step-height measurement we have developed a new tunable spectral filter that uses tandem liquid-crystal Fabry-Perot interferometers (LC-FPIs), which can simultaneously attain both a high spectral resolution and a large tuning range. Fringe visibility measurements were carried out, and it was found that the coherence length of the light transmitted through tandem LC-FPIs is two times larger than that transmitted through a single LC-FPI. Using this novel tunable spectral filter, we developed a new spectral interference microscope for the measurement of three-dimensional shapes of discontinuous objects. Experimental results of step-height measurements both with a single LC-FPI and with tandem LC-FPIs are presented for a combination of standard steel gauge block sets with 1-, 99-, and 100-microm steps. A large range (1-100 microm) of measurement with submicrometer resolution was achieved with tandem LC-FPIs that was not possible with our previous system in which a single LC-FPI was used.
A nonmechanical scanning Mirau-type spectral interference microscope has been developed for the measurement of three-dimensional surface profiles of discontinuous objects. An acousto-optic tunable filter (AOTF) is used as a high-resolution spectral filter, which scans the optical frequency of the broadband light emitted from a superluminescent diode. To generate spectral fringes that make full use of the limited coherence length of the filtered light we unbalanced the Mirau interferometric system by positioning the reference mirror nearly halfway between the top and the bottom of the step height. When the frequency of the broadband light source is scanned by an AOTF, the interference fringes move in opposite directions on the top and the bottom of the object. To uniquely determine the sign of the fringe movement over the large area of the object, we developed a three-dimensional Fourier-transform technique, and from the detected sign of the fringe movement and phase information, we determined the three-dimensional step height. Experimental results of the measurement of 100-microm step height are presented. The main advantages of the proposed system are that it provides nonmechanical scanning and a large measurement range without ambiguity in the sign of the phase.
A noncontact, nonmechanical scanning, wide-field spectral interference microscope is developed for simultaneous measurement of three-dimensional step-height of discontinuous objects and tomographic imaging. A superluminescent diode (SLD) is used as a broadband light source and a liquid-crystal Fabry-Perot interferometer (LC-FPI) as a frequency-scanning device. By means of changing the injection current to the SLD, the spectral profile of the SLD is equalized, and a constant light input to the interferometer is achieved over the entire frequency-scan range. The Fourier-transform technique is used to determine both the amplitude and the phase of spectral fringe signals. Three-dimensional height distribution of a discontinuous object is obtained from the phase information, whereas optically sectioned images of the object are obtained either from the amplitude information alone or from the combination of both the amplitude and phase information. Experimental results with submicrometer resolution are presented for both step-height measurement and tomographic sectioning.
We propose a non-mechanical scanning Mirau-type spectral interference microscopic imaging system for the measurement of three-dimensional step-height of discontinuous objects. In this system a superluminescent diode is used as a broad-band light source, and an acousto-optic tunable filter (AOTF) as a high-resolution frequency scanning device. The inteiferometric system was made unbalanced by putting the reference mirror position exactly half-way between the top and bottom of the total step-height of the discontinuous object. While scanning the frequency of the broad-band light source using AOTF, the interference fringes move in opposite directions on the top and bottom of the object, respectively. A two-dimensional Fourier transform method was used for the unique determination of the sign of fringe movement over a large area of the object without any photo-detectors and fringe counters. From the detected sign of the fringe movement and phase information, the three-dimensional step-height is measured. Experimental results of the measurement of 1OOim step-height are presented. The main advantages of the proposed system are non-mechanical scanning and large measurement range without ambiguity in the sign of phase.
A spectral interference microscope using superluminescent diode as light source and a liquid-crystal Fabry-Perot etalon as a frequency tunable device is developed for three-dimensional imaging microprofilometry. By changing the current to the superluminescent diode the constant light input to the interferometer was achieved throughout the measurement. A high speed Fourier transform technique was developed for the analysis of the data which takes few seconds to measure the three-dimensional absolute height distribution of a discontinuous object. Experimental results with improved resolution and speed of measurement are presented
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