Computerized Optical Profiler

Vibration insensitive extended range interference microscopy

Using a simultaneous phase sensor, the proposed instrument performs highly repeatable measurements over an extended range in the presence of vibration common to a laboratory setting. Measurement of a 4.5 μm step standard in the presence of vibration amplitudes of 40 nm produces a repeatability of 1.5nm RMSwith vertical scanning data acquired at 400 nmintervals. The outlined method demonstrates the potential to tolerate larger vibration amplitudes up to or beyond a quarter wavelength and to increase the data acquisition step size to that approaching the depth of field of standard microscope imaging systems.

Computerized interferometric surface measurements [invited]

The addition of electronics, computers, and software to interferometry has enabled enormous improvements in optical metrology. This paper discusses four areas in which computerized interferometric measurement improvements have been made in the measurement of surface shape and surface roughness: (a) The use of computer-generated holograms for the testing of aspheric optics, (b) phase-shifting interferometry for getting interferometric data into a computer so the data can be analyzed, (c) computerized interference microscopes, including multiple-wavelength and coherence scanning, for the precision measurement of surface microstructure, and (d) vibration-insensitive dynamic interferometers for enabling precise measurements in noncontrolled environments.

Optical admittance monitor through a dynamic interferometer

A way to increase the optical monitoring sensitivity in quarterwave stack fabrication by modified optical admittance loci was demonstrated. The optical admittance value was obtained from an in-situ dynamic interferometer combing with a photodetector, and the result was compared with ellipsometer measurement.

Offset of coherent envelope position due to phase change on reflection

Different materials with different phase changes on reflection affect the surface-height measurement when interferometric techniques are employed for testing objects constructed of different materials that are adjacent to one another. We test the influence of this phase change on reflection when vertical scanning interferometry with a broadband source is used. We show theoretically and experimentally that the strong linear dependence of the dispersion of the phase change on reflection preserves the shape of the coherence envelope of the fringes but shifts it along the optical axis by approximately 10–40 nm for metallic surfaces.

Fringe modulation skewing effect in white-light vertical scanning interferometry

An interference fringe modulation skewing effect in white-light vertical scanning interferometry that can produce a batwings artifact in a step height measurement is described. The skewing occurs at a position on or close to the edge of a step in the sample under measurement when the step height is less than the coherence length of the light source used. A diffraction model is used to explain the effect.

Improved vertical-scanning interferometry

We describe a method that combines phase-shifting and coherence-peak-sensing techniques to permit measurements with the height resolution of phase-shifting interferometry without the interval-slope limitation of l/4 per data sample of phase-shifting interferometry. A five-frame algorithm is used to determine both the best-focus frame position and the fractional phase from the best-focus frame of the correlogram acquired through vertical scanning. The two surface profiles retrieved from the phase and the modulation contrast of the correlograms are compared in the phase-unwrapping process to remove fringe-order ambiguity.

Large field of view, high spatial resolution, surface measurements

  • James C. Wyant and Joanna Schmit
  • Int. J. Mach Tools Manufact., Vol 38, 691-698, (1998)
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It is difficult in interferometric metrology to maintain high spatial resolution over a large field of view. Interferometric microscope measurements yield high resolution, but only over a small area. Other conventional interferometric systems can measure large areas, but they fail to provide the necessary spatial resolution. High spatial resolution over a large field-of-view (FOV) can be obtained by stitching together multiple high spatial resolution measurements of adjacent areas of a measured surface. The measurements can be fit together in a global sense, or by matching the piston and tilt over the overlap region. Care must be taken in the stitching process to make sure the measurements are precisely overlapped to minimize errors. The larger the overlap the easier it is to match data sets, but of course more data sets are required to get a given field of view. This paper shows that a 20 percent overlap gives a good trade off between having good repeatability and obtaining a large field of view with a minimum number of data sets. Typical measurement results are shown for stitching as many as 285 sub-regions.

Computerized interferometric measurement of surface microstructure

  • James C. Wyant and Joanna Schmit
  • Proceedings of SPIE Vol. 2782 (SPIE, Bellingham, WA), pages 26-37, 1996
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The addition of modern electronics, computers, and software to an interference microscope greatly increases the surface height measurement capability of the interference microscope. The RMS repeatability of surface microstructure measured using a computerized phase-shifting interference microscope can be less than 0.1 nanometer. While phase-shifting interferometry having sub-nanometer height precision has limited dynamic range, the dynamic range of an interference microscope can be extended to hundreds, or even thousands, of microns by using vertical scanning coherence peak sensing techniques. This paper describes the measurement capabilities of an interference microscope employing both phase-shifting phase measurement capability and coherence peak sensing. Typical measurements obtained using phase-shifting and coherence peak sensing are illustrated. Techniques for extending the measurement capability of computerized interference microscopes are discussed.

Computerized interferometric measurement of surface microstructure

  • James C. Wyant
  • Proceedings of SPIE Vol. 2576 (SPIE, Bellingham, WA), pages 122-130, 1995
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Nearly all modern high-quality measuring instruments now use micro computers for the collection and analysis of data. This paper describes a computerized interferometric microscope system for the measurement of surface microstructure. For the instrument described in this paper the surface microstructure can be measured at data array sizes as large as 739 x 484 points for measurement fields ranging from 30 x 25 microns to 8.2 x 6. 1 mm. A repeatability of the surface height measurements of less than 0. 1 nm can be obtained for smooth surfaces. Surfaces having height variations as large as 500 microns can be measured to within an accuracy of a few nanometers.

Advances in interferometric optical profiling

  • James C. Wyant and Katherine Creath
  • Int. J. Mach Tools Manufact., Vol 32, 5-10, (1992)
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This paper discusses some advances in non-contact, interferometric optical profilers. Topics discussed include: (i) The advantages of using a white light source, rather than a laser source, (ii) The tradeoff between the use of Michelson, Mirau, and Linnik interferometers for different fields of view and different lateral resolutions, and (iii) Techniques for removing errors in the reference surface enabling a person to measure sub-Angstrom surface microstructure in the presence of a much rougher reference surface.

Comparison of surface roughness measured with an optical profiler and a scanning probe microscope

  • Jay Jahanmir and James C. Wyant
  • Proceedings of SPIE Vol. 1720 (SPIE, Bellingham, WA), pages 111-118, 1992
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The surface topography of various samples has been measured using an optical profiler and a scanning probe microscope (SPM). Optical profilers offer fast and accurate measurements of surface topography but are limited in their lateral resolution by the wavelength of light used. SPMs extend the lateral resolution down to atomic dimensions. Topography measurements are used to obtain surface roughness data. We find that for a scan size of 50×50 tm, the roughness data obtained from the optical profiler agree with the SPM measurements. The roughness data do not vary significantly when higher magnification images are taken with the SPM on surfaces that lack high frequency components. But for surfaces that have rough features that are smaller than the resolution of the optical profiler, roughness data calculated from higher magnification images by SPM can vary significantly.

Absolute measurement of surface roughness

In an interferometer which uses a reference surface, the measured surface heights correspond to the difference between the test and reference surfaces. To accurately determine the rms roughness of supersmooth surfaces, the effects of the reference surface roughness needs to be removed. One technique for doing this involves averaging a number of uncorrelated measurements of a mirror to generate a reference surface profile which can then be subtracted from subsequent measurements so that they do not contain errors due to the reference surface. The other technique provides an accurate rms roughness of the surface by taking two uncorrelated measurements of the surface. These two techniques for measurement of supersmooth surfaces are described in detail, and results of the measurement of a 0.7-Å rms surface roughness mirror are presented. The expected error in the rms roughness measurement of a supersmooth mirror due to instrument noise is 0.02 Å.

Absolute measurement of surface roughness

  • James C. Wyant and Katherine Creath
    Proceedings of SPIE Vol. 1319 (SPIE, Bellingham, WA), pages 568-569, 1990
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A new three-dimensional non-contact digital optical profiler

  • Bharat Bhushan, James C. Wyant, and John Meiling
  • Wear, Vol. 122, pages 301-312, 1988
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New features of a three-dimensional non-contact digital optical profiler are described. This instrument uses a Michelson interferometer for 1.5X, 2.5X, and 5X objective magnifications, a Mirau interferometer for 10X, 20X, and 40X magnifications, and a Linnik objective for 100X, 150X, and 200X magnifications. The instrument is capable of a lateral resolution of 0.4 um and a vertical resolution as small as 0.1 nm. The software can calculate various statistical roughness parameters for flat, cylindrical, and spherical surfaces. The radii of cylindrical and spherical surfaces can be measured with high accuracy. Flatness, crown, camber, twist, taper angle, pole-tip gap recession, and edge quality of magnetic slider surfaces can also be measured.

Development of a three-dimensional noncontact digital optical profiler

  • J. C. Wyant, C. L. Koliopoulos, B. Bhushan, and D. Basila
  • Journal of Tribology, Vol. 108, page 1, January 1986
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A noncontact three-dimensional optical profiler for measuring surface roughness is described. The system consists of a reflection microscope, Mirau interferometer with a reference surface mounted on a piezoelectric transducer, CID detector array, frame grabber, and micro-computer. Interferometric phase-shifting techniques are used to obtain surface height information. The height measurements are processed by a computer to obtain topographical statistical parameters, which are useful in predicting tribological and magnetic performances of the head-media interface in magnetic storage systems. Sample data are presented for magnetic media (tape, floppy disk, and rigid disk), a magnetic head, a silicon wafer, and a glass slide.

Direct comparison of mechanical and optical measurements of the finish of precision machined optical surfaces

  • E. L. Church, T. V. Vorburger, and J. C. Wyant
  • Optical Engineering, Vol. 24, page 388, May/June 1985
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This paper compares two methods of measuring the finish of precision machined optical surfaces: the older, well-established mechanical stylus gauge and a recently developed optical gauge using interference microscopy. Results are found to be in good quantitative agreement for both random and periodic surface features, provided that appropriate filtering procedures are included in the data analysis to account for the differing transfer functions and bandwidths of the two measurement techniques. These results affirm the use of these techniques for the quantitative measurement and specification of machined optical surfaces.

Measurement of surface topography of magnetic tapes by Mirau interferometry

Stylus-profiling techniques cannot be used for surface characterization of polymeric surfaces, such as magnetic tapes, because of their relatively low hardness An interferometric-optical-profiling microscope system was used to obtain high-accuracy surface profiles of magnetic media, rapidly and without physical contact with the sample. The profilometer consists of a conventional, reflection-type optical microscope with a Mirau two-beam interferometer attachment. The interference patterns of the surface can be observed through the eyepieces and can be detected with a solid-state linear array of 1024 detector elements. By translating the reference surface of the interferometer with a piezoelectric transducer while taking consecutive measurements, accurate surface-height measurements can be obtained from each detector element. The microscope system is controlled by a microcomputer, which communicates with a desk-top computer for further analysis of the surface-profile data. A computer-controlled specimen stage is added to increase the sample size. The reasons for selecting the Mirau two-beam interferometry are also discussed. Sample data of magnetic tapes are presented. Experimental data presented in the paper show that optimization of surface roughness is necessary to obtain optimum magnetic amplitude. friction, and wear properties.

An optical profilometer for surface characterization of magnetic media

  • James C. Wyant, Chris Koliopoulos, Bharat Bhushan, and Orrin E. George
    ASLE Trans, Vol. 27, pages 101-113, 1984
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Conventional surface-characterization techniques either are not sophisticated enough to provide complete surface-topographical data or cannot be employed because of the relatively low hardness of magnetic media. An optical profilometer has been developed which provides a noncontact method of obtaining surface characteristics from a magnetic medium. The system consists of a standard Leitz reflection microscope, a Mirau interferometer controlled by a piezoelectric transducer, a linear array of photodiode detectors, and a microcomputer. The combination yields a system that measures the optical-height variations of surfaces to a high degree of precision. This height variation is processed by a computer to provide surface-topographical statistical parameters, which are useful to predict tribological and magnetic performances of the head-media interface. Sample data of magnetic media (tape, floppy disk, and rigid disk) are presented.

 

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