Phase Shift Interferometry
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Phase Shift Interferometry (PSI) is a technique for areal surface characterisation that focuses on digitising the interference data acquired during a controlled phase shift, usually introduced by the mechanical oscillation of an interference objective.
The technique provides full 3D images with typical height measurement repeatability of less than 1 nm, independent of field size. It is a powerful method that allows analysing interferograms to recover phase information. Traditionally, interferograms were measured by locating the centre of a fringe and then tracing along it. Phase shift interferometry avoids the need to track the location of fringes and enables a point-by-point reconstruction of the wavefront.
PSI offers significantly lower measurement noise, potentially as low as 0.01 nm. It is the preferred technique for measuring the surface roughness of super-smooth surfaces.
Principle of Operation
[edit]Interferometers utilize the wave properties of light to analyse surface characteristics, particularly surface height variations. For evaluating areal surface topography, interferometers separate the source light so that it follows two independent paths: one includes a reference surface and the other the object surface. The separated light beams then recombine and are directed to a digital camera that measures the resultant light intensity over multiple image points simultaneously.
PSI acquires a sequence of images with a precisely controlled phase change between them. When a few fringes are visible on the surface, this manifests as a shift in fringe position between the images captured by a camera. The phase shifting is almost always generated by mechanical motion of the interference objective, which allows for fast, non-contact metrology.
In traditional interferometry, the phase information is encoded in the interference pattern. However, extracting this information can be challenging due to the ambiguity in determining the sign of the phase. PSI overcomes this limitation by introducing a known phase shift between the interfering waves.
By acquiring multiple interferograms with different phase shifts, PSI enables the accurate determination of phase information.
Applications
[edit]PSI is widely used in high-precision manufacturing for surface metrology and quality control. Common applications include:
- Optical component testing
- Precision engineering
- Semiconductor manufacturing