Performing 3D Optical Profiling Surface Measurements That Correlate To Traceable Standards
This article explains the benefits of the non-contact inspection method used by 3D optical profilers, and outlines the best practices and measurement results for some specialized PTB (Physikalish-Technische Bundesanstalt) traceable roughness standards and other low-cost fingernail roughness gages.
The correlation results are based on measurement factors that should be understood and taken into consideration when imaging and analyzing surface textures that range in roughness from a few nanometers to micrometers in scale.
3D optical profilers that utilize coherence scanning interferometry, also called white light interferometry (WLI), deliver fast, accurate surface measurements over large areas to determine various properties about surfaces of interest. They are being increasingly employed in research, engineering, and production process control for a variety of markets, including materials science, aerospace, automotive, data-storage, solar, semiconductor, and MEMS.
Understanding how this technology correlates to conventional 2D techniques and standards, and how the increase of measurement data can be determined and utilized is vital to fully exploit the capabilities of today’s top-performing 3D optical profilers.
Advantages of 3D Optical Profiling over Other Measurement Technologies
2D stylus profilers were used initially for surface roughness characterization in the early 1930s and were adopted as the industry standard until the development of 3D metrology instruments decades later. 3D optical profiler measurement systems have many advantages, which have prompted the international metrology community to develop new measurement standards to fully exploit this superior technology.
Modern sophisticated surface profilers have industry-leading speed and accuracy, while maintaining the same “nanometer” Z accuracy at all magnifications. Such systems are capable of measuring a very wide range of surface parameters, including pitch, roughness, curvature, step heights, waviness, and lateral displacement, all in a single measurement and on almost any surface.
Based on white light interferometry shown in Figure 1, this measurement technique is capable of rapidly determining 3D surface shape over large lateral areas - up to 8 mm - in a single measurement. Surface areas larger than this value can be measured by applying stitching algorithms to enable multiple lateral images to be captured and merged into one image for analysis.
Although several other technologies provide fast speeds, good resolution, or larger areas of measurement, they each have their own limitations as well. For instance, stylus profiling provides scans up to hundreds of millimeters in length, but each scan is just a trace along one probe-tip-wide line, limiting the area that can be analyzed without taking multiple traces, which makes acquisition time slow for larger areas.
Figure 1. Basic white light interferometry schematic with Bruker’s self-calibration HeNe laser.
Similarly, confocal microscopes provide reasonable Z resolution at very high magnifications, but have much slower data acquisition time because of the scanning technology used to capture the Z height data. Finally, optical focusing techniques are employed for more coarse manufacturing surface finishes, but are not generally able to achieve the Z resolution of an interference-based 3D optical profiler, especially for surface texture on finely machined structures.
These other techniques also have other downsides when it comes to measuring surface topography and quantifying texture. A major disadvantage of contact stylus measurement is that the stylus tip has to run perpendicular to the predominant surface pattern or surface lay of the measurement surface. If this is not the case, the tip may follow the surface structure and deliver false surface texture results, similar to a record player needle following the grooves in a record.
Another disadvantage of stylus measurements is the limitation in Z height measurement range. A stylus system needs to use a skid plate to extend the measurement range, allowing it to measure over larger steps but then limiting its ability to measure waviness or stepped features precisely as the skid plate must track the surface of interest. This produces a sort of mechanical filtering of the surface representation.
Finally, as very hard materials, such as diamond, are used to make most stylus tips in order to reduce wear and increase tip life, carrying out scans using them can cause damage to the surface of interest and deliver false readings, as shown in Figure 2.
Figure 2. Stylus damage to reference standard.
Confocal microscopy finds the height at each pixel location by calculating the center of mass of the intensity distribution around the focus position or by detecting the peak intensity. The intensity envelope is very narrow for high-magnification objectives, but becomes wider at lower magnifications due to the lower numerical aperture (NA) of the objective, which increases the depth of field.
This large depth of field impairs a confocal system's ability to repeatedly detect the centroid and peak intensity, and as a result, deteriorates the Z accuracy and resolution. High-magnification objectives (20x and above) must typically be used to gain Z accuracy, but this limits the lateral field of view.
Previously, the ability to measure steep angles was the key advantage of using a confocal microscope. However, the development of higher magnification objectives for 3D optical profilers as well as the improved lateral resolution of high-resolution cameras enables non-confocal systems to measure steep angles approaching 90° on non-mirror surfaces.
Modern 3D optical profilers do not have any limitations to surface structure orientation and eliminate surface damage, as they are based fully on a non-contact measurement technique. Additionally, 3D optical profilers are generally not Z-height limited, and are capable of measuring up to 10 mm in height.
The fringe envelope of a 3D optical profiler remains very narrow at all magnifications, from 0.75x to 230x, and as a result, the profiler maintains the same high Z resolution across the field of view at any magnification