Coherence Scanning Interferometry (CSI), also known as White Light Interferometry or Vertical Scanning Interferometry, is a non-contact optical metrology technique used to measure 3D surface topography.
It works with broadband light (typically white light) and takes advantage of its short coherence length. By scanning the optical path length, the system identifies the position where the coherence envelope of the interference signal reaches its maximum.
This makes CSI particularly suitable for measuring smooth to moderately rough surfaces, with vertical resolution down to the nanometer scale.
HOW DOES CSI WORK?
The operating principle of CSI can be described in the following steps:
Broadband illumination: A white light source is split into two beams; one directed to the reference mirror and the other to the sample.
Scanning: The objective or the sample is moved vertically, gradually changing the optical path length.
Interference generation: Interference fringes appear only when the path lengths of both beams are nearly identical, due to the short coherence length of white light.
Envelope detection: At each scanning position, the system records interferograms. The maximum of the interference envelope indicates the exact height for each pixel.
3D reconstruction: By repeating this process across the whole field of view, a detailed 3D surface map is generated with nanometric precision.
CSI APPLICATIONS
Coherence Scanning Interferometry is widely used in fields that require precise surface measurements across a broad range of roughness levels:
Semiconductors and microelectronics: Measurement of wafers, MEMS devices, and layered structures.
Materials science: Characterization of coatings, composites, and moderately rough materials.
Engineering and manufacturing: Quality control of precision components, machined parts, and industrial surfaces.
Optics: Inspection of lenses, mirrors, and other optical components.
Research and development: Surface analysis where nanometer-level resolution is needed over relatively large height variations.
