Coherence Scanning Interferometry (CSI)
Coherence Scanning Interferometry (CSI, a type of white-light interferometer), is a highly effective technique for measuring surface roughness, owing to its non-contact nature, data clarity, high precision, and ability to capture both smooth and rough surface textures. It employs a broadband light source to generate interference patterns, which are used to determine surface heights down to the sub-nanometer scale. This capability is particularly valuable throughout many industry sectors like; semiconductor manufacturing, precision optics, automotive, medical, aerospace and material science, where surface quality is critical.
Broadband light and surface coverage
The use of white-light, consisting of a broad range of wavelengths (for Polytec central wavelength band is 525nm), a Coherence Scanning Interferometer as optical profiler can be used to thouroughly measure a wide variety of surface features: For smooth surfaces, CSI detect minute deviations in height with nanometer-level accuracy. On rough surfaces, the broadband nature of white-light ensures that the profiler can still distinguish small details with clarity, even when the surface consists of larger, irregular features. This broad wavelength range enables Coherence Scanning Interferometers to accommodate the full spectrum of surface roughness, from smooth, polished surfaces to those with significant texture variations, for example; surface peening or additive manufactured surfaces.
Working principle of Coherence Scanning Interferometry
In a Coherence Scanning Interferometry optical profiler, white-light passes through a beam splitter, which directs the light to both a sample surface and to a reference mirror. When the light reflects back from these two surfaces, it is recombined, and a pattern of interference “fringes” forms. Maximum fringe contrast occurs at the best focus position for each point on the sample. Either the test sample or the measurement head is scanned/ translated vertically (Z direction) in such a way that each individual camera pixel point on the surface passes through focus. In general, a math algorithm is applied. It should be noted that the better the math, the greater the clarity of the focus can be defined. For Polytec as Coherence Scanning Interferometry specialist, they have a math algorithm called the “correlogram”. This correlogram signal processing algorithm uses the interference signals to find the best/ sharpest focus point, operating beyond the diffraction barrier of light to provide sub-nanometer Z sensitivity.
High Z sensitivity with large Z range
When using this CSI driven correlogram algorithm with a long-scanning mechanical setup, this possibility enables high Z sensitivity to be applied over large Z-translation ranges, while maintaining good noise performance and high surface sensitivity irrespective of long or short scanning range. The series of data sampling collected can precisely map the height of each surface point with sub – nanometer resolution over the full Z scan range with no sacrifice of range to resolution limit, other than that of the working distance of the interferometric lens (up to 30 mm). Polytec refers to this approach as its CST Continuous Scanning Technology. All data is deterministic in Z in nature (not focus driven via different lens options) and always has good noise performance and clarity of surface information.
This CST Continuous Scanning Technolgy and data processing allow for precise measurements even in the presence of super smooth, rough or irregular surface textures and combinations.
Non-contact scanning of height data (Z) on XY
One of the major advantages of CSI Coherence Scanning Interferometers is that it does not require direct contact with the surface being measured. This eliminates the risk of surface damage, which can occur with tactile measurement techniques like stylus profilometry. The non-contact nature of CSI is particularly beneficial for fragile or sensitive surfaces, such as delicate optics, flexible electronics or thin films, where physical contact could alter the surface structure. In addition, non-contact measurement allows CSI to capture data over a larger area in a short amount of time. By scanning an entire single field of view (FOV) all at once or even using multiple FOVs and image stitching to extending the measurement area. This stitched data is patched together in a controlled way with advanced data combining software, allowing the visualization of texture over larger surface areas to be viewed. CSI offers a faster alternative to traditional point-by-point techniques, while maintaining high precision and data clarity.
Precision data from smooth & rough surfaces
CSI can effectively measure surface roughness on both smooth and rough textures due to its vertical resolution, which can reach the sub-nanometer scale. On smooth surfaces, CSI provides detailed height information, detecting even the smallest variations in texture. It can also measure rough surfaces with high accuracy, as the technique can handle large variations in height without losing resolution and allowing an operator to access a large spatial frequency bandwidth of data, especially when supported by image stitching, the result is numerical data with good stability, ideal for the control and monitoring of the most subtle changes of a surface, plus, the visual pleasing imagery that has clarity and integrity, ideal for being used as a helpful failure analysis tool.
Measure on difficult surface materials
The ability of CSI to handle a broad range of surface textures is further enhanced by its ability to automatically adjust to different surface characteristics. This makes it highly versatile for applications requiring measurements across different types of materials, whether metals, ceramics, glass, silicon, paper or polymers, with varying degrees of roughness, contrast and reflectivity. Reflectivity performance is from 0.05% to 100%.
Comprehensive 3D surface data
Unlike traditional 2D profilometry methods, CSI provides comprehensive three-dimensional surface data. By capturing detailed height information across the entire surface, it allows for a more complete analysis of surface roughness. The resulting 3D maps enable engineers and scientists to assess the overall texture and uniformity of the surface, which is essential for applications requiring precise control of surface quality, such as in optical lenses, coatings, and microelectronics.
Vivid color mode adding value information
One area where traditional white-light interferometers (WLI) have proved less suitable is that typically WLI are signal processing grey scale image data, therefore, camera image captures are normally only supplied as grey scale images. This makes WLI system much less intuitive for applications like tribology and some material sciences, where added color images can add to the sense of a surface.
However, Polytec has developed a method to gather the RGB intensity information from a surface using three light sources (red, green and blue). Collecting this additional surface information enables a vivid color representation of the surface in line with the original areal data set, offering the end operator a supreme vasal experience unique to the Polytec CSI profilers featuring Continuous Scanning Technology.
Color surface imaging reveals surface details that would be difficult to see. People like to see 3D height color maps or with a vivid graphic representation for enhanced viewing and reliable numerical representation in order to getting…
The Answer of A Surface