Low-Noise Interferometry Enables Precision Three-Dimensional Surface Characterization

Optical profilers that employ white light interferometry are one of the most accurate and flexible metrology tools for precision three-dimensional surface characterization. They are instrumental in an incredibly diverse range of industrial applications, from the measurement of data storage read-write heads or the cylinder walls of engines to the characterization of the drying rates of paint and adhesives, semiconductor linewidths and spacing analysis, and medical devices metrology.

Challenges of High Slopes and Roughness

One of the challenges with any optical measurement is characterizing anything with steep angles. The larger the field of view of an optical system, the lower its numerical aperture will be. Light that strikes a surface through the microscope objective must be collected again for focus onto a camera in order to process the information and create the desired three-dimensional surface map.

Light reflected from surfaces of higher angle than accepted by the microscope objective is not collected by the optical system, making accurate measurement impossible. This metrology limitation affects results on both steep and very rough surfaces. Steep surfaces, such as lenses, gratings, microfluidics devices, and ball bearings may have large slope areas that do not reflect light back into the optics. Rough surfaces also contain many local slopes and, generally, have fewer locations that are flat with respect to the optics. Therefore, much of the light is never collected by the objective, and resulting data is either noisy or completely missing. Table 1 shows theoretical maximum collection angles for a variety of common interferometric microscope objectives.

Table 1. Typical magnifications, numerical apertures, and slopes on smooth surfaces for interferometric objectives

Fortunately, the limits in Table 1 apply strictly for very smooth surfaces, where all of the light that strikes the sample from a single direction reflects away in a single direction. Such surfaces would typically appear visually smooth, and numerically have surface roughness less than 10 nanometers. Many surfaces, particularly machined metal surfaces, are not that smooth, and the light that strikes them from one angle is reflected at a variety of angles. The scattered light can be collected by the microscope on these rougher surfaces, and, if it exceeds the signal-to-noise ratio of the system, accurate and quantitative surface metrology is possible.

Figure 1 diagrams the light path for steep smooth and rough surfaces. For a smooth surface, the light exiting the microscope leaves at the same relative angle as the incoming light with equal intensity. For a rough surface, some of the exiting light leaves at the same relative angle as the incoming light, but at a lesser intensity since much of the light is scattered across many other angles.

Figure 1. A smooth surface (left) will reflect all incident light at the same relative angle as it strikes the surface. A rough surface (right) will reflect some of the light this way, but also scatter a significant amount of light at other angles.

Maximizing the Signal

Bruker's latest generation of optical profilers take advantage of advanced LED light sources, superior cameras, precision scanners, and sophisticated noise-reduction electronics and algorithms to maximize their signa lto- noise ratio. Current systems have a noise floor more than two times lower than previous generation products. This allows even very small amounts of light collected by the microscope objective to lead to good measurement performance on a surface.

Figure 2 shows a measurement of a screw thread with a 20X objective with a theoretical slope limit of about 17 degrees. Multiple fields have been stitched together to obtain a length of more than 4 millimeters along the screw thread in a single image. Threads over 63 degrees in slope are accurately measured because the low noise of the instrument allows the small amount of scattered light to be efficiently collected for accurate metrology. With this kind of configuration, 50X objectives can achieve slopes over 70 degrees, while even a 5X objective can achieve most data up to 30 degrees and some data at upwards of 50 degrees.

Figure 2. Top: Measurements of a 10mm pitch screw showing slopes of more than 63 degrees can be measured even with a 20X magnification objective. Bottom: Measurement of a machined cylinder using 5X objective with over 1mm2 field of view; near continuous data up to about 30 degrees and sporadic data up to 50 degrees slope.

For rough surfaces, the improved signalto- noise capability means that more data is collected even on extremely rough surfaces. Figure 3 demonstrates the data achievable on two very rough surfaces, a ceramic with approximately 4 microns average roughness and a CMP polishing pad with more than 7 microns average roughness. A 20X objective and 0.55X field conversion lens were used to achieve a field of view of approximately 0.5mm on a side. More than 95% of the ceramic and more than 70% of the pad surface area achieved valid measurement data, allowing rapid, accurate and repeatable characterization over fairly large fields. The 2D traces in Figure 4 illustrate how local slopes can significantly exceed the theoretical maximum slope of 16.7 degrees maximum angle. Angles upwards of 50 degrees are measured.

Figure 3. Rough surface metrology of ceramic (top) and polishing pad (bottom), with fields of view of 0.42mm x 0.56mm.

Figure 4. 2D traces of the ceramic part showing local slopes of more than 50 degrees can be characterized, which is much higher than the stated limit of 17 degrees for this magnification objective.

Conclusion

Interferometric optical profilers can achieve high lateral and vertical resolution measurements over very large ranges. With a low-noise system, the scattered light from steep or very rough surfaces may be collected and used for accurate surface characterization. Slopes upwards of 70 degrees are measurable and surfaces with roughness on the order of 10 microns with very high local slopes can also be characterized. Flexible configurations and a variety of objective magnifications and options enable these instruments to be configured to best fit needs from research to full-scale production part qualification. In terms of slope, speed, repeatability and accuracy, interferometric optical profilers are the most flexible and effective metrology tools available today.

This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.

For more information on this source, please visit Bruker Nano Surfaces.