Integrating AFM Metrology with 3D Optical Microscopy

In the LED, semi-conductor, automotive and medical sectors, surface metrology and characterization is an important criterion to ensure product performance in a wide range of applications. These industries employ 3D optical microscopes for fast and accurate process monitoring, research, and product development. However, these imaging systems tend to have certain drawbacks. To address this issue, Bruker Nano Surfaces has launched an advanced atomic force microscope (AFM) module that can be placed directly onto a 3D optical microscope to deliver unparalleled flexibility for surface metrology. In this application note, the basic system operation and application of the NanoLens AFM on a ContourGT 3D optical microscope are discussed in detail.

Figure 1. NanoLens AFM Module installed on a bench-top ContourGT-I 3D Optical Microscope.

NanoLens AFM Module

The NanoLens AFM is integrated with an AFM scan head and an electronics control box that can be placed onto a Bruker 3D optical microscope turret. Using a HDMI cable connector and USB connection, the electronics control box is linked to the scan head and the microscope computer, respectively. A micro fabricated silicon chip featuring a cantilever probe is accommodated by the scan head. This probe contacts the surface in the course of measurement. The sharp tip of the probe is made to contact the surface of the sample and is held there with the help of a force-based feedback loop. The probe is then examined across the measurement surface.

Basic Operation of AFM Module

Two main imaging modes are included in the NanoLens. The first is the static imaging mode, wherein the probe is made to contact the surface at a known pressure so that a given amount of laser beam is deflected by the cantilever and then rastered over the surface. A surface map is simulated by deviations from the so-called pressure deflection. The second is the dynamic imaging mode, which is generally employed to reduce surface damage in fragile or softer surfaces. In this mode, the NanoLens AFM cantilever is swung to the surface close to the probe’s resonance frequency so as to reduce contact force and remove shear force in the sample. Additionally, a force-based feedback loop is employed in both these imaging modes to control the probe’s contact to the surface under test.

Bruker's 3D optical microscope capabilities are suitably complimented by the NanoLens AFM module, which can also be used for co-aligned operation on the microscope system to promote rapid inspection of a specific area. In addition, users can employ the integrated 8x objective lens to view the interaction between the sample and cantilever through the 3D microscope camera to detect the measurement area. A quick-release adapter promotes rapid exchange of cantilever with micron-level position reproducibility. A cantilever groove design as shown in the figure below allows this reproducibility.

Figure 2. Top view of cantilever chip seated in kinematic mounting interface.

Defect Inspection Using NanoLens AFM

The NanoLens AFM delivers exceptional surface information with below 10 nm lateral resolution and sub-nanometer height resolution. The subsequent example demonstrates how the 3D optical microscope combined with NanoLens AFM can be employed for identification and high-resolution inspection.

Figure 3. Intensity image of CD ROM surface with contaminants shown at 16X magnification through integrated 8X optics and 2X FOV modifier lens.

Figure 4. 50X interferometric objective intensity image near same site.

The above figure shows images of a surface area on a CD ROM disk. The tiny black square specifies a region for NanoLens AFM inspection. Once a desired region is identified, 3D surface measurements at that site can be performed using the Bruker 3D microscope and NanoLens AFM toolset. For instance, in certain area, optical inspection at 50x magnification may prove useful.

Figure 5. 3D representation from 50x objective inspection on CD ROM surface. The black square represents area of interest for extended NanoLens AFM inspection.

Combination of 3D Microscope and AFM Systems

The full benefit of the NanoLens AFM can be achieved once optical measurement is performed at a specific region. The NanoLens can be adjusted into position and the area of interest can be determined directly without actually moving the sample.

Figure 6. NanoLens AFM inspection of area of interest in progress.

The image given below was created through dynamic force measurement using the NanoLens AFM. The etching pits and also the defects can be seen clearly. These depressions were imaged utilizing the NanoLens AFM. This is an additional benefit provided by the NanoLens while measuring samples of these levels. This, the NanoLens AFM is capable of correcting any errors that could arise as a result of narrow line width spacing. Users can employ the turret-mounted setup to easily switch back to optical system functionality

Figure 7. High-resolution NanoLens AFM image data of area of interest.

Conclusion

The Bruker 3D optical microscope integrated with NanoLens AFM delivers multiple metrology capabilities. The 1000x lens removes different thin film and material issues and extends lateral resolution. It offers instant capability for parcentric operation of AFM and optical technologies. The combination of these two systems helps improve metrology capabilities as well as the overall efficiency of the measurement. This results in a user-friendly system that offers excellent precision and resolution for present research and production applications.

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.