Optical Profilometry

Introduction to Optical Profilometry

Optical Profilometry is a non-destructive, interferometric-based non-contact technique to measure and study surface topography of materials. This is also known as White Light Interferometry (WLI). OP is used to measure roughness, flatness and curvature of a surface along with measuring coating thickness variation and thin-film coating stress of material samples.

Optical Profilometry Principle and Methodology

A profilometer is used to perform this technique. A diamond stylus is used to scan the sample surface in a contact mode profilometer, where its vertical movement is recorded. Whereas, a non-contact mode profilometer uses a laser or optical beam. The light beam is split with one half directed to the surface and another half to the mirror. When these beams are recombined, the wavelength scale path difference produced by the topography variations causes interference. The contour information obtained is extrapolated from the interpretation of the interference patterns.

Laser confocal microscopy, patterned light and wide area 3D measurements and white light interferometry are the other techniques related to OP.

An Optical Profilometer

An optical profilometer, also known as an optical profiler, is used to scan a wide range of scientific and industrial samples. An OP is used to evaluate roughness statistics and feature dimensions of a sample through 2D and 3D images. 3D optical profilometry is the latest aspect of this technology. Performance of a profilometer depends on the following parameters-

1. Field of view: This is the whole area of the material surface that the profilometer can measure.
2. Resolution in space: This property varies in the longitudinal and lateral directions for different profilers. The range of this spatial resolution depends upon the user requirements and required accuracy levels.
3. Picture acquisition time: This correlates to the needed image type, image frame rates, and accuracy.

Uses of Optical Profilometry

Optical profilometry is used (a) to measure roughness profile for automotive parts, (b) to perform curvature analysis for thin-film stress measurement, adhesion, etc., (c) to measure coating surface characteristics, (d) to study corrosion on the metal surfaces, (e) to measure fabric surface morphology, (f) to image cracks and other surface defects, (f) to measure flip chip and other semiconductor packaging flatness and solder bumps, and (g) to measure the radius of curvature for optical parts and nanometric surface features, among many other uses across different industries. 

Strengths and Limitations of Optical Profilometry

As its advantages, this technique does not require a vacuum or special chamber, can accommodate very large to small samples and offers surface roughness precision in the sub-nanometer range.

Limitations of OP include considerable variation in the surface height hard to measure and any variation in the refractive index of films can produce erroneous thickness estimations.