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Surface metrology, which can otherwise be referred to as surface measurement, involves the investigation of the texture, structural characterization, roughness, shape and finish of any precision surface. Some of the most common technologies that are used to obtain surface measurements include stylus profilers and interferometry, as well as both three-dimensional (3D) and optical microscopy.
Digital vs. Optical Microscopy
Although optical microscopy was initially developed in the 17th century, this analytical tool has remained the gold standard for most scientific industries around the world. Since then, a great deal of technological advancements has been made in an effort to enhance the resolution of previous microscopy techniques and ultimately lead to the development of modern digital microscopy.
Compared to optical microscopes, which are limited to a single plane of focus on a sample, 3D digital microscopes can instead scan through multiple focal planes of a given sample in order to create a focused 3D image. In doing so, researchers are able to visualize specific features of a sample at a much greater depth of field.
3D Digital Microscopy Parameters
The use of 3D digital microscopy for surface metrology analysis can be found in a wide range of industrial sectors. Compared to traditional metrology techniques, 3D surface measurements allow users to better quantify and differentiate surfaces that may exhibit similar or even identical average surface roughness values. Since their original development in 1991, 3D surface measurement parameters have been refined to include amplitude, spatial, hybrid and functional measurements.
The 3D surface measurement parameter of amplitude encompasses several different surface characteristics, including kurtosis, which is defined as the degree of peakedness of a surface height distribution, the averages of both the highest and lowest points on a given sample’s surface, the root-mean-square of height distribution and skewness.
Alternatively, spatial parameters of a surface are instead based on the given frequencies of its features such as texture and density. Whereas hybrid parameters are a combination of both the height and frequency parameters, functional parameters instead focus on the applicability of specified surface functions.
There are several limitations associated with the use of optical microscopes for surface metrology purposes. These limitations include an inability to generate a 3D image and a deficient lateral resolution due to the wavelength of light used by these imaging devices as well as their tip geometry. Additionally, optical profilers have the potential to cause damage onto the surface of a sample by their high forces during imaging.
Even so, optical profilers are still widely used to assess the surface roughness of various sample types. Optical profilers are based on interference microscopy technology that allows these devices to measure the roughness of a sample’s surface with assistance from the wavelength of light.
More specifically, the wavelength properties are used to compare the optical path difference between a sample’s surface and that of a reference surface in order to generate a precise measurement of not only the roughness of a sample’s surface, but also its finish and shape.
Some of the key advantages that support the use of optical profilers for this purpose include their feasibility to scan large areas rapidly, accurate measurement capabilities and theirnon-contact requirements during imaging. As a result of these advantages, several different industries, some of which include optics and data storage, utilize optical profiling for their surface measurement purposes.
References and Further Reading