Test & measurement
MAKING THE INVISIBLE
MEASURABLE:
WHITE LIGHT INTERFEROMETRY FOR ULTRA-THIN LAYERS
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Based on highest resolution and a non-contact, non-destructive operating principle, interferometry measurement technology is used in numerous industries for quality assurance in high-tech areas. It reliably delivers high precision distance and thickness values, for example, in the production of semiconductors, optical components and precision mechanics, says Glenn Wedgbrow, business development manager at Micro-Epsilon UK.
nterferometry measurement is used in numerous industry sectors where it helps to optimise processes, reduce waste and create innovative products, while fulfilling the highest performance requirements.
In interferometry, a distinction is made between laser and white light interferometers. Unlike laser interferometers, white light interferometers measure absolute distances without having to use a reference. They deliver extremely precise and clear measurement results in the sub-nanometre range. To do so, they use polychromatic (white) light with a short coherence length. The interference of light waves and the analysis of the received, superimposed waves can be used to precisely determine distances and intervals.
AUTOMOTIVE, SEMICONDUCTORS AND METALS
The non-contact measuring principle with remote electronics makes the systems insensitive to harsh environments such as high temperatures, shock or vibrations. Typical applications range from wafer and mask measurements in semiconductor production to the measurement of lenses, mirrors and glass surfaces. Sectors such as automotive, medical technology, metals and packaging industries also benefit from the outstanding properties of the technology.
So what are the advantages of absolute measuring white light interferometers over conventional laser interferometers, and what kind of applications are they used in?
THE MEASURING PRINCIPLE OF INTERFEROMETRY
Interferometry has proven itself in numerous applications such as measuring layer thicknesses and distances. In interferometry measurements, a beam splitter divides a continuous spectrum of light waves into two partial beams, a reference beam and a measurement beam, which each take different paths. The light source itself can be either a laser or a superluminescent diode (SLD), which emits polychromatic (white) light with a short coherence length.
The measuring principle of interferometry is based on emitting and receiving wavelengths of poly- chromatic light. A measured variable is assigned to the received signal by means of a Fourier transformation.
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The two partial beams emitted by the light source hit the object to be measured, are reflected and then overlap. Depending on the measuring object, the phase shift of the partial beams varies with the wavelength. Due to this variation, constructive interference occurs at certain
wavelengths and destructive interference at others. If the intensity of this interference signal is plotted against the wavelength, alternating minima and maxima appear.
The resulting periodic intensity signal in the spectrum of the reflected light is assigned to a distance or a thickness by means of Fourier transformation. This enables the precise determination of a measured value that reflects the exact thickness of an object or the distance from a surface. If the interferometry method is used to measure thicknesses, the two beams reflected from the front and back of the layer can interfere with each other. This means that the measurement result is independent of the distance from the measuring object, which provides great flexibility for industrial measurement tasks.
USING THE ADVANTAGES OF WHITE LIGHT INTERFEROMETRY White light interferometers that work with a superluminescent diode use an extended wavelength spectrum. This is how they acquire more distance information than conventional laser interferometers. This means that white light interferometers can measure absolutely without having to use a reference – which is crucial for many industrial measurement applications. This absolute measurement method is particularly favourable in the case of signal interruptions that can be caused by steps, holes, false reflections or structured surfaces, for example. After a signal interruption, users immediately receive a measurement value, whereas laser interferometers first have to
June 2026 Instrumentation Monthly
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