White light scanner

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Figure 1. Lunate cells of Nepenthes khasiana visualized by Scanning White Light Interferometry (SWLI). Nepenthes khasiana lunate cells.jpg
Figure 1. Lunate cells of Nepenthes khasiana visualized by Scanning White Light Interferometry (SWLI).

A white light scanner (WLS) is a device for performing surface height measurements of an object using coherence scanning interferometry (CSI) with spectrally-broadband, "white light" illumination. Different configurations of scanning interferometer may be used to measure macroscopic objects with surface profiles measuring in the centimeter range, to microscopic objects with surface profiles measuring in the micrometer range. For large-scale non-interferometric measurement systems, see structured-light 3D scanner.

Contents

Description

Figure 2. File:Twyman-Green interferometer set up as a white light scanner Twyman-Green interferometer set up as white light scanner.svg
Figure 2. File:Twyman-Green interferometer set up as a white light scanner

Vertical scanning interferometry is an example of low-coherence interferometry, which exploits the low coherence of white light. Interference will only be achieved when the path length delays of the interferometer are matched within the coherence time of the light source. VSI monitors the fringe contrast rather than the shape of the fringes.

Fig. 2 illustrates a Twyman–Green interferometer set up for white light scanning of a macroscopic object. Light from the test specimen is mixed with light reflected from the reference mirror to form an interference pattern. Fringes appear in the CCD image only where the optical path lengths differ by less than half the coherence length of the light source, which is generally on the order of micrometers. The interference signal (correlogram) is recorded and analyzed as either the specimen or reference mirror is scanned. The focus position of any particular point on the surface of the specimen corresponds to the point of maximum fringe contrast (i.e. where the modulation of the correlogram is greatest).

Fig. 3 illustrates a white light interferometric microscope using a Mirau interferometer in the objective. Other forms of interferometer used with white light include the Michelson interferometer (for low magnification objectives, where the reference mirror in a Mirau objective would interrupt too much of the aperture) and the Linnik interferometer (for high magnification objectives with limited working distance). [1] The objective (or alternatively, the sample) is moved vertically over the full height range of the sample, and the position of maximum fringe contrast is found for each pixel. [2] [3]

The chief benefit of low-coherence interferometry is that systems can be designed that do not suffer from the 2 pi ambiguity of coherent interferometry, [4] [5] [6] and as seen in Fig. 1, which scans a 180 μm × 140 μm × 10 μm volume, it is well suited to profiling steps and rough surfaces. The axial resolution of the system is determined by the coherence length of the light source and is typically in the micrometer range. [7] [8] [9] Industrial applications include in-process surface metrology, roughness measurement, 3D surface metrology in hard-to-reach spaces and in hostile environments, profilometry of surfaces with high aspect ratio features (grooves, channels, holes), and film thickness measurement (semi-conductor and optical industries, etc.). [10]

Technical

Figure 3. White light interferometric microscope White light interferometric microscope.gif
Figure 3. White light interferometric microscope

White-light interferometry scanning (WLS) systems capture intensity data at a series of positions along the vertical axis, determining where the surface is located by using the shape of the white-light interferogram, the localized phase of the interferogram, or a combination of both shape and phase. The white light interferogram actually consists of the superposition of fringes generated by multiple wavelengths, obtaining peak fringe contrast as a function of scan position, that is, the red portion of the object beam interferes with the red portion of the reference beam, the blue interferes with the blue, and so forth. In a WLS system, an imaging interferometer is vertically scanned to vary the optical path difference. During this process, a series of interference patterns are formed at each pixel in the instrument field of view. This results in an interference function, with interference varying as a function of optical path difference. The data are stored digitally and processed in a variety of ways depending on the system manufacturer, including being Fourier-transformed into frequency space, subject to cross-correlation methods, or analysis in the spatial domain.

If a Fourier transform is used, the original intensity data are expressed in terms of interference phase as a function of wavenumber. Wavenumber k is a representation of wavelength in the spatial frequency domain, defined by k = 2π/λ. If phase is plotted versus wavenumber, the slope of the function corresponds to the relative change in group-velocity optical path difference DG by Dh = DG/2nG where nG is group-velocity index of refraction. If this calculation is performed for each pixel, a three-dimensional surface height map emerges from the data.

In the actual measuring process, the optical path difference is steadily increased by scanning the objective vertically using a precision mechanical stage or piezoelectric positioner. Interference data are captured at each step in the scan. In effect, an interferogram is captured as a function of vertical position for each pixel in the detector array. To sift through the large amount of data acquired over long scans, many different techniques can be employed. Most methods allow the instrument to reject raw data that do not exhibit sufficient signal-to-noise. The intensity data as a function of the optical path difference are processed and converted to height information of the sample.

Related Research Articles

Wave interference Phenomenon

In physics, interference is a phenomenon in which two waves superpose to form a resultant wave of greater, lower, or the same amplitude. Constructive and destructive interference result from the interaction of waves that are correlated or coherent with each other, either because they come from the same source or because they have the same or nearly the same frequency. Interference effects can be observed with all types of waves, for example, light, radio, acoustic, surface water waves, gravity waves, or matter waves. The resulting images or graphs are called interferograms.

Interferometry Measurement method using interference of waves

Interferometry is a technique in which waves are superimposed to cause the phenomenon of interference, which is used to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber optics, engineering metrology, optical metrology, oceanography, seismology, spectroscopy, quantum mechanics, nuclear and particle physics, plasma physics, remote sensing, biomolecular interactions, surface profiling, microfluidics, mechanical stress/strain measurement, velocimetry, optometry, and making holograms.

In physics, two wave sources are perfectly coherent if their frequency and waveform are identical and their phase difference is constant. Coherence is an ideal property of waves that enables stationary interference. It contains several distinct concepts, which are limiting cases that never quite occur in reality but allow an understanding of the physics of waves, and has become a very important concept in quantum physics. More generally, coherence describes all properties of the correlation between physical quantities of a single wave, or between several waves or wave packets.

Mach–Zehnder interferometer

In physics, the Mach–Zehnder interferometer is a device used to determine the relative phase shift variations between two collimated beams derived by splitting light from a single source. The interferometer has been used, among other things, to measure phase shifts between the two beams caused by a sample or a change in length of one of the paths. The apparatus is named after the physicists Ludwig Mach and Ludwig Zehnder; Zehnder's proposal in an 1891 article was refined by Mach in an 1892 article. Demonstrations of Mach-Zehnder interferometry with particles other than photons had been demonstrated as well in multiple experiments.

Michelson interferometer

The Michelson interferometer is a common configuration for optical interferometry and was invented by Albert Abraham Michelson. Using a beam splitter, a light source is split into two arms. Each of those light beams is reflected back toward the beamsplitter which then combines their amplitudes using the superposition principle. The resulting interference pattern that is not directed back toward the source is typically directed to some type of photoelectric detector or camera. For different applications of the interferometer, the two light paths can be with different lengths or incorporate optical elements or even materials under test.

Optical coherence tomography Imaging technique

Optical coherence tomography (OCT) is an imaging technique that uses low-coherence light to capture micrometer-resolution, two- and three-dimensional images from within optical scattering media. It is used for medical imaging and industrial nondestructive testing (NDT). Optical coherence tomography is based on low-coherence interferometry, typically employing near-infrared light. The use of relatively long wavelength light allows it to penetrate into the scattering medium. Confocal microscopy, another optical technique, typically penetrates less deeply into the sample but with higher resolution.

Surface metrology is the measurement of small-scale features on surfaces, and is a branch of metrology. Surface primary form, surface fractality and surface roughness are the parameters most commonly associated with the field. It is important to many disciplines and is mostly known for the machining of precision parts and assemblies which contain mating surfaces or which must operate with high internal pressures.

Profilometer

A profilometer is a measuring instrument used to measure a surface's profile, in order to quantify its roughness. Critical dimensions as step, curvature, flatness are computed from the surface topography.

Holographic interferometry (HI) is a technique which enables static and dynamic displacements of objects with optically rough surfaces to be measured to optical interferometric precision. These measurements can be applied to stress, strain and vibration analysis, as well as to non-destructive testing and radiation dosimetry. It can also be used to detect optical path length variations in transparent media, which enables, for example, fluid flow to be visualised and analyzed. It can also be used to generate contours representing the form of the surface.

Interferometric synthetic-aperture radar

Interferometric synthetic aperture radar, abbreviated InSAR, is a radar technique used in geodesy and remote sensing. This geodetic method uses two or more synthetic aperture radar (SAR) images to generate maps of surface deformation or digital elevation, using differences in the phase of the waves returning to the satellite or aircraft. The technique can potentially measure millimetre-scale changes in deformation over spans of days to years. It has applications for geophysical monitoring of natural hazards, for example earthquakes, volcanoes and landslides, and in structural engineering, in particular monitoring of subsidence and structural stability.

Point diffraction interferometer

A point diffraction interferometer (PDI) is a type of common-path interferometer. Unlike an amplitude-splitting interferometer, such as a Michelson interferometer, which separates out an unaberrated beam and interferes this with the test beam, a common-path interferometer generates its own reference beam. In PDI systems, the test and reference beams travel the same or almost the same path. This design makes the PDI extremely useful when environmental isolation is not possible or a reduction in the number of precision optics is required. The reference beam is created from a portion of the test beam by diffraction from a small pinhole in a semitransparent coating. The principle of a PDI is shown in Figure 1.

ISO 25178: Geometric Product Specifications (GPS) – Surface texture: areal is an International Organisation for Standardisation collection of international standards relating to the analysis of 3D areal surface texture.

Fourier-transform infrared spectroscopy technique to analyze the infrared spectrum of matter

Fourier-transform infrared spectroscopy (FTIR) is a technique used to obtain an infrared spectrum of absorption or emission of a solid, liquid or gas. An FTIR spectrometer simultaneously collects high-resolution spectral data over a wide spectral range. This confers a significant advantage over a dispersive spectrometer, which measures intensity over a narrow range of wavelengths at a time.

Angle-resolved low-coherence interferometry (a/LCI) is an emerging biomedical imaging technology which uses the properties of scattered light to measure the average size of cell structures, including cell nuclei. The technology shows promise as a clinical tool for in situ detection of dysplastic, or precancerous tissue.

The N-slit interferometer is an extension of the double-slit interferometer also known as Young's double-slit interferometer. One of the first known uses of N-slit arrays in optics was illustrated by Newton. In the first part of last century, Michelson described various cases of N-slit diffraction.

Air-wedge shearing interferometer

The air-wedge shearing interferometer is probably the simplest type of interferometer designed to visualize the disturbance of the wavefront after propagation through a test object. This interferometer is based on utilizing a thin wedged air-gap between two optical glass surfaces and can be used with virtually any light source, even with non-coherent white light.

A common-path interferometer is a class of interferometers in which the reference beam and sample beams travel along the same path. Examples include the Sagnac interferometer, Zernike phase-contrast interferometer, and the point diffraction interferometer. A common-path interferometer is generally more robust to environmental vibrations than a "double-path interferometer" such as the Michelson interferometer or the Mach–Zehnder interferometer. Although travelling along the same path, the reference and sample beams may travel along opposite directions, or they may travel along the same direction but with the same or different polarization.

White light interferometry

As described here, white light interferometry is a non-contact optical method for surface height measurement on 3-D structures with surface profiles varying between tens of nanometers and a few centimeters. It is often used as an alternative name for coherence scanning interferometry in the context of areal surface topography instrumentation that relies on spectrally-broadband, visible-wavelength light.

Coherence scanning interferometry

Coherence scanning interferometry (CSI) refers to a class of optical surface measurement methods wherein the localization of interference fringes during a scan of optical path length provides a means to determine surface characteristics such as topography, transparent film structure, and optical properties. CSI is currently the most common interference microscopy technique for areal surface topography measurement. The term "CSI" was adopted by the International Organization for Standardization (ISO).

References

  1. ↑ Schmit, J.; Creath, K.; Wyant, J. C. (2007). "Surface Profilers, Multiple Wavelength, and White Light Intereferometry". Optical Shop Testing. p. 667. doi:10.1002/9780470135976.ch15. ISBN   9780470135976.
  2. ↑ Harasaki, A.; Schmit, J.; Wyant, J. C. (2000). "Improved vertical-scanning interferometry" (PDF). Applied Optics. 39 (13): 2107–2115. Bibcode:2000ApOpt..39.2107H. doi:10.1364/AO.39.002107. hdl: 10150/289148 . PMID   18345114 . Retrieved 21 May 2012.
  3. ↑ "HDVSI - Introducing High Definition Vertical Scanning Interferometry for Nanotechnology Research from Veeco Instruments". Veeco. Retrieved 21 May 2012.
  4. ↑ Plucinski, J.; Hypszer, R.; Wierzba, P.; Strakowski, M.; Jedrzejewska-Szczerska, M.; Maciejewski, M.; Kosmowski, B.B. (2008). "Optical low-coherence interferometry for selected technical applications" (PDF). Bulletin of the Polish Academy of Sciences. 56 (2): 155–172. Retrieved 8 April 2012.
  5. ↑ Yang, C.-H.; Wax, A; Dasari, R.R.; Feld, M.S. (2002). "2π ambiguity-free optical distance measurement with subnanometer precision with a novel phase-crossing low-coherence interferometer" (PDF). Optics Letters. 27 (2): 77–79. Bibcode:2002OptL...27...77Y. doi:10.1364/OL.27.000077. PMID   18007717.
  6. ↑ Hitzenberger, C. K.; Sticker, M.; Leitgeb, R.; Fercher, A. F. (2001). "Differential phase measurements in low-coherence interferometry without 2pi ambiguity". Optics Letters. 26 (23): 1864–1866. Bibcode:2001OptL...26.1864H. doi:10.1364/ol.26.001864. PMID   18059719.
  7. ↑ Wojtek J. Walecki, Kevin Lai, Vitalij Souchkov, Phuc Van, SH Lau, Ann Koo Physica Status Solidi C Volume 2, Issue 3 , Pages984 - 989
  8. ↑ W. J. Walecki et al. "Non-contact fast wafer metrology for ultra-thin patterned wafers mounted on grinding and dicing tapes" Electronics Manufacturing Technology Symposium, 2004. IEEE/CPMT/SEMI 29th International Volume , Issue , July 14–16, 2004 Page(s): 323 - 325
  9. ↑ "Services". www.zebraoptical.com.
  10. ↑ "Metrology Applications: Surface Roughness Measurement, Thickness, Volume Loss". www.novacam.com.
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