Metrology Applications of 3D Optical Microscopy

3D optical microscope is a key metrology technique used in a myriad of industries. White light interferometry (WLI) and confocal microscopy or laser scanning confocal microscopy (LSCM) are the other two key techniques providing a 3D surface representation from a microscope image.

Each technique has its own advantages and disadvantages based on the principle of operation. This article discusses the unique metrology advantages of ContourGT 3D optical microscopes from Bruker over confocal microscopes in some applications.

One of the key advantages of ContourGT is its capability to maintain 0.1 nm RMS repeatability and subnanometer vertical resolution irrespective of magnification or field of view.

Principles of Measurement

In confocal microscope, the sample is progressed vertically in steps and a tiny aperture is positioned in front of the detector to allow light from a single point as it traverses focus. LSCM can measure only one point at a time and requires raster scanning in the X, Y and Z directions to get information for each point on the surface. However, it takes more time to capture data over a large field of view.

A 3D microscope based on WLI allows the vertical scanning along the Z-axis so that every point on the test surface crosses focus and captures the X and Y data with a single acquisition at each step along the Z-axis. This technique is faster than confocal techniques where it is necessary to scan each point in the both X-Y and Z as shown in Figure 1.

Figure 1. Diagram outlining different scanning techniques utilized by confocal microscopes and 3D microscopes.

The WLI-based 3D optical microscope involves the splitting of light approaching the sample and directing it partly at a high-quality reference surface and at the sample, followed by the recombination of the reflected light from these two surfaces. The light interacts at points where the sample is in close proximity with focus to create a pattern of bright and dark lines that trace the surface shape.

The vertical scanning of the specialized microscope objective with respect to the surface is performed so that each point on the surface passes through focus. The location of the optimum contrast in the bright and dark lines represents the best focus position for each pixel and the surface’s full 3D surface map within the field of view of the microscope is created.

Onboard software is then used to study these data to measure different parameters of interest, including surface roughness, texture, or other critical geometric dimensional information.

Vertical Resolution

Vertical resolution is the most basic performance feature in the surface profile measurement application. It is limited in confocal microscopy by the axial point spread function. As depicted in Figure 2, confocal microscopes do not provide highly accurate data with 5x, 10x and even 20x objectives, and achieve acceptable vertical resolution with an objective 50x magnification or beyond, which restricts the field of view. To map large areas, data stitching is needed, thus considerably increasing measurement time.

Figure 2. Confocal microscopes produce a wider, weaker signal for lower magnification objectives.

The fringe envelope continues to be very narrow at all magnifications in the WLI-based 3D microscope, as shown in Figure 3. This feature together with fringe phase detection allows sub-nanometer vertical resolution, which, in turn, enables use of lower magnification objectives without compromising vertical resolution for true metrology applications. The lower magnification objectives offer larger fields of view, thus providing much higher throughput together with better Z accuracy measurements.

Figure 3. WLI microscopes provide a constant, narrow signal for all objectives.

Existing WLI-based microscopes like Bruker's ContourGT are suitable for comprehensive analysis of surface profiles with low-magnification lenses. It is possible to use rapid stitching for areas greater than 5x5 mm, as shown in Figure 4.

Figure 4. Comparison of a stitched measurement covering 3 mm taken from a 13 mm radius sphere (shape and form removed) measured with a confocal and a WLI 3D microscope using a 50x objective. The 3D microscope based on WLI provided better stitching and smaller aberrations, indicated by the 100 nm Peak to Valley (PV) residual, a factor of six improved over the confocal based 3D image.

Lateral Resolution

Pixel-limited resolution and diffraction are two potential limits to the lateral resolution of an optical system. Bruker's exclusive AcuityXR enhancement for ContourGT 3D optical microscopes utilizes an algorithm for the reconstruction of the object that has been imaged by an optical system devoid of diffraction.

Through system modeling and multiple scans, AcuityXR generates a measurement with double the number of pixels in both the X and Y directions when compared to a typical interferometric measurement. AcuityXR uses a patent-pending iterative technique with feedback obtained from the metrology hardware to decrease system noise and reduce blurring effects attributable to diffraction on the final measured surface height.

A measurement taken with a WLI 3D microscope equipped with ActuityXR enhancement (right) and the one taken with a standard WLI 3D microscope (left) are shown in Figure 5.

Figure 5. 350 nm linewidth measurements taken with a standard 3D microscope based on WLI(left) and the same microscope with AcuityXR enhancement (right) shows how the latter provides high levels of feature differentiation.

WLI-based 3D microscopes demonstrate scan speeds of up to 100 microns per second vertically, with imaging of the entire field of view in a fraction of a second. The comparison of best resolution data estimates for equivalent inspection areas measured by LSCM and WLI-based 3D microscopes is shown in Table 1.

Table 1. LSCM and WLI Comparison of time to data for best metrology (vertical) resolution (0.3 MP/sec. image for LSCM, interline transfer image acquisition for WLI).

WLI-based systems produce higher quality data in much faster timescale. Current WLI-based 3D optical microscopes can measure steep angles of up to 60° easily, as illustrated in Figure 6. Latest models can measure up to 87°.

Figure 6. Lead angle of screw threads can easily be measured at 60 degrees with a WLI-based 3D optical microscope.

WLI-based 3D optical microscope’s capability to provide high-speed precision metrology measurements of steep angles is shown in Figure 7.

Figure 7. Patterned Sapphire Substrate image taken showing capability of WLI 3D microscopes to provide high-speed precision metrology measurements of steep angles.

Measuring Samples over a Wide Range of Reflectances

WLI-based 3D optical microscopes can be utilized on samples with a broad range of reflectances, as depicted in Figure 8. They can generate data within 30 seconds on even steeply angled samples without tilt correction, as shown in Figure 9.

Figure 8. WLI 3D microscopes can accurately measure samples with a wide range of reflectances.

Figure 9. 5x thread image generated on a WLI 3D microscope, just by optically focusing and measuring.

WLI-based 3D optical microscopes are now capable of generating color images as shown in Figure 10.

Figure 10. WLI microscopes can now produce color images, as seen here for wire bonding characterization.

Conclusion

Quality of production processes in many different industries can be ensured through the measurement of surface topography and the shape and size of microscopic surface features. For this purpose, different instruments have been used, including confocal microscopes and WLI-based 3D microscopes each have their own benefits and drawbacks.

The latest generation Bruker ContourGT 3D microscopes demonstrate high-speed operation, workability in factory environments, and unprecedented accuracy, which includes sub-nanometer resolution in the vertical (Z) axis.

Because of these advantages, they have been increasingly utilized in a myriad of applications, from characterization of the tooling utilized in the production of contact and intraocular lenses and inspection of critical wear of industrial surfaces to measurement of PSS utilized in HB-LEDs.

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.