Universal Surface Measurements with 3D Optical Microscopy

The introduction of VXI measurement mode from Bruker facilitates nearly universal measurement results on a variety of surfaces. This unique features is only installed on Bruker 3D optical microscopes, providing improved processing of the complete optical signal, fully automated, self-sensing of the surface being analyzed, and the most accurate measurement of surface topography.

This article discusses the application of this capability in industries, including display, MEMS and LED.

VXI Measurement Mode

Input and modulation of many different parameters are essential for other competitive ‘single measurement mode’ methods to produce better results on various surface textures in the same field of view and on surfaces with low and high reflectivity.

In contrast, VXI measurement mode can be set to sense the surface type automatically, providing unprecedented 3D optical metrology representation of the surface.

Figure 1. VXI 115X magnification 3D image showing areas of smooth and rough topography on a wafer during fabrication process steps.

VXI offers a single measurement mode capable of providing sub-nanometer vertical resolution on virtually any surface with vertical range as high as 10mm. Wafer measurement during one of the processes for LED production by means of automatic resolution settings is illustrated in Figure 1. The results clearly show both smooth and pitted/rough areas.

Broad Applicability of VXI Measurement Mode

VXI offers the following operational resolution modes that can selected from a simple pull-down menu based on the surface texture expected (Figure 2).

Figure 2. VXI measurement settings dialogue.

• Standard — This is a phase-based resolution mode with a speed- enhanced algorithm to process the surface measurement signal.
• High Fidelity — This is another phase-based algorithm providing the absolute best vertical resolution to the application even on rough or diffuse surfaces.
• Auto — This mode enables the 3D microscope system to detect the surface texture properties automatically and change the resolution setting accordingly. This setting will switch to High Fidelity for relatively smooth surfaces. For less smooth surfaces, Standard resolution will be set for processing the surface signal data into a topography map with a balance between speed and resolution.
• High Speed — This is a center of mass (COM) enhanced algorithm with improved precision over earlier vertical scanning algorithms. This mode will provide the quickest VXI measurement results, and can be used when surfaces have long steps to scan for measurement, are known to be relatively rough, or where high vertical resolution is trivial.

Automatic sensing is preferred for most cases, as it ensures both optimum data quality and acquisition speed. Besides the resolution setting dropdown menu, VXI provides a SNR threshold setting to reject low or limited quality data.

Bruker also offers the sophisticated AcuityXR measurement method that provides the absolute highest lateral resolution for a specific magnification and field of view combination.

AcuityXR improves edge detection and enhances lateral repeatability three to five folds when compared to traditional microscope imaging for equivalent measurement parameters. The SNR threshold setting rejects data with average modulation below the set value. The default setting of ‘0’ rejects no data and is handy in most cases (Figure 3).

Figure 3. SNR threshold setting showing default setting = 0.

Application Example: MEMS Inertial and Position Sensors

VXI mode is ideally suited to measure rough, smooth and even stepped surfaces. This flexibility can be observed in the measurement of a MEMS sensor that has step heights on the order of ≥ 25µm. Flat areas also featured in the microscope field of view (Figure 4).

Figure 4. A 2.5X magnification image of MEMS structures (courtesy Sandia National Laboratory).

The data quality provided by Bruker’s VXI technology is apparent in the 3D interactive view of the MEMS structure at 50X magnification (Figure 5). The resultant surface data have very low noise and precisely characterize this relatively large area of interest with highest fidelity.

Figure 5. 3D view of 50X image of MEMS structure.

The zoomed area of the field of view shows the clean surface data devoid of diffraction effects that would be seen in other optical measurements of similar stepped structures that have sharp edges and smooth surfaces (Figure 6). VXI conveniently measures this surface topography, providing high-quality data of the surface being analyzed with high accuracy.

Figure 6. Zoomed area of MEMS structures and cursor analysis showing high-quality height information.

The measurement of a transparent coated glass substrate for fabrication of Active Matrix OLED (AMOLED) devices using VXI demonstrates its capability to provide an accurate large area measurement on a very smooth surface. Here, VXI was utilized to image a defect-free smooth area and an area with a minor pit defect roughly 100nm deep (Figure 7).

Figure 7. A 2.2mm x 1.7mm area of an AMOLED substrate showing <1nm Ra imaged with VXI.

Figure 8 shows the ability of the VXI measurement to show angstrom-level roughness, while handling the pit deviation from the ideal surface.

Figure 8. Zoomed view of AMOLED display substrate with ~100nm pit imaged with VXI.

Conclusion

With VXI measurement mode, Bruker 3D optical microscope users are able to access the most accurate 3D surface topography measurements possible for a variety of surfaces with a push of a button.

The results discussed in this article are achieved with a single measurement in VXI. This innovation in 3D microscope metrology generates the most easily acquired and most accurate depiction of different technical surfaces.

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.