Surface Texture Measurement for the Unseen World

Source: Mahr Inc.

Unbeknownst to us, we are actively measuring surface texture all the time. Maybe not actually measuring the surface, but rather reacting to how it’s supposed to perform as part of our interaction with it.

Surfaces are all around us and make our everyday tasks work well. Whether the surface is used to reflect light or interact with another surface, a lot of technology goes into creating it. Operating elements and design surfaces have special textures achieved through blasting, polishing, etching or using a laser. The spectrum ranges from finely structured metal housing surfaces with a homogeneous appearance to rough plastic or leather patterns with distinct embossed structures.

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For that specially created surface to look, feel or perform as it’s supposed to, it must be measured during development as well as for quality control of the process to ensure the proper surface is maintained. We used to rely on well-proven 1D surface roughness, profiling and waviness systems to inspect these surfaces. However, with newer structures frequently requiring nanometer-scale dimensions, more surface analysis is required, including 3D surface roughness, profiling and waviness. Additionally, there are often requirements for isotropy (surface uniformity in all orientations), structural evaluation, pore and particle analysis, fault detection, structure height, layer thickness and even geometry and microgeometry at levels of nanometers.

None of these requirements can be met with a customary contact probing system. The only way to really see what is going on is to look at it optically. Optical noncontact surface analysis can provide the magnification required for these measurements during development and for in-process measurements. And just as there is such a wide range of surface measurement controls, there are many different optical systems that can provide the fastest and most accurate means of evaluation. So, whether your particular need for roughness is more critical or the geometry, dimensions and locations, choosing the correct optical measuring technology or a combination of technologies, must be considered for the application.

Today, a number of 3D area-based optical technologies are used to measure and analyze surfaces. Each one has its own capabilities, strengths and weaknesses, and in some cases, a combination of technologies may be used within the same instrument to achieve the desired result. However, by understanding the specific testing requirements along with the capabilities of the technology, one can achieve the proper testing criteria.

The most common technologies include:

A white light interferometer (WLI), which provides the highest vertical resolution and enables the detection of topographies in the sub-nanometer range. WLI combines the power of interferometry and confocal microscopy, enabling highly detailed 3D surface images, often used to measure electronic and optical components. Laboratories and quality assurance processes can thus determine the finest roughness, step heights or planes in the nanometer range in just a few seconds.

Confocal microscopy is an advanced optical imaging technique that enhances resolution and contrast in microscope images by using a spatial pinhole to block out-of-focus light. Capturing multiple 2D images at different depths in a sample enables the reconstruction of 3D structures. This technique is often used for semiconductor inspection and material evaluations.

It would not be uncommon to combine variations of both technologies into a measuring system that, depending on the part requirements, provides the measurement needed in one station. While WLI and confocal microscopy have been used for decades, what has made them practical for part development and process control in industry has been more powerful computing and intelligent software for data analysis.

Using high-performance algorithms that search for the best correlation by comparing every single pixel results in calculated height values that are very precise and robust, with minimum noise, for unparalleled data quality. This, combined with the ability to put together multiple images, allows for the creation of very large surface profiles for area analysis. User interfaces must be powerful yet easy enough to operate, providing fast measurement, data collection, analysis and evaluation in a single, complete package.

As we look at the functions of surfaces all around us, it is reassuring that there are technologies available today that can help qualify them, so you will never notice them doing their job.