Engineers and material scientists use Optical Profilometry (OP) as a non-destructive, interferometric-based non-contact technique to measure and study the surface topography of materials. Additionally, researchers also know this technique as White Light Interferometry (WLI). Furthermore, OP measures roughness, flatness, and curvature of a surface, along with measuring coating thickness variation and thin-film coating stress of material samples.
Optical Profilometer Principle and Methodology
Researchers perform this technique using a profilometer. In a contact mode profilometer, a diamond stylus scans the sample surface, and the instrument records its vertical movement. In contrast, a non-contact mode profilometer uses a laser or optical beam. Specifically, the system splits the light beam: one half is directed toward the surface, and the other half goes to a mirror. Then, when the beams recombine, the topography variations produce a wavelength-scale path difference, which causes interference. Finally, technicians extrapolate the contour information by interpreting the resulting 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 Profilometry
Scientists and industrial technicians use an optical profilometer, also known as an optical profiler, to scan a wide range of scientific and industrial samples. Specifically, they use an OP to evaluate roughness statistics and feature dimensions of a sample through 2D and 3D images. Moreover, 3D optical profilometry represents the latest aspect of this technology. Finally, the following parameters determine the performance of a profilometer.-
- Field of view: This is the whole area of the material surface that the profilometer can measure.
- 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.
- Picture acquisition time: This correlates to the needed image type, image frame rates, and accuracy.
Uses of Optical Profilometer
Researchers and engineers use optical profilometry for a wide range of applications across different industries. For example, it measures the roughness profile of automotive parts. Additionally, it performs curvature analysis for thin-film stress measurement, adhesion studies, and similar evaluations. Furthermore, the technique measures coating surface characteristics and studies corrosion on metal surfaces. Moreover, it measures fabric surface morphology and images cracks along with other surface defects. In addition, optical profilometry measures flip chip and other semiconductor packaging flatness, as well as solder bumps. Finally, it measures the radius of curvature for optical parts and nanometric surface features.
- Step height and dimensional measurement
- Characterizing wear and friction of mechanical parts
- Assessing bow on coated/processed wafers, e.g. MEMS fabrication
- Measuring the radius of curvature of microfluidic channels, optics, etc.
- Quantifying thickness of continuous, transparent films (i.e. no physical step is needed)
- Determining consistency of solder bump heights, e.g. on flip chips and other advanced packaging
- Correlating roughness measurements with materials properties, e.g. adhesion, corrosion, appearance
OP, Optical Profilometry Technical Specifications
- Lateral Resolution: 0.5 μm (best)
- Maximum feature height: 10 mm
- Minimum image size: 0.06×0.047 mm2
- Height Resolution: 0.01 nm (PSI) or 6 nm (VSI)
- Film Thickness: Between 150 µm and 2 µm thick
- Maximum sample size: 300 mm diameter, 100 lbs weight, 4″/100 mm height
- Maximum image size: 2.2×1.1 mm2 (single image); 70×70 mm2 (stitched images)
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.
Strengths
- wide range of possible analysis areas, feature heights and roughness statistics.
- for non-destructive/non-contact applications.
- it is fast, and therefore good for multiple repeat measurements.
- it accommodates very large and very small sample sizes.
