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3D-SEM characterization of tactile probing tips

Categories:
  • Division 5
  • Metrology for Economy
22.12.2020

At PTB, long and narrow piezoresistive silicon microprobes with integrated probe tips have been used for years for precise and fast surface measurements, including roughness measurements within microstructures with high aspect ratio [1]. Unfortunately, the fast measurement on hard surfaces leads to significant wear of the integrated silicon tips. Since the tactilely measured topography is always convoluted with the shape of the probe tip, accurate surface measurement also requires a fast and validated method for probe tip characterization.

A four-quadrant detector system (point electronic GmbH) has now been put into operation at the Verios G4 electron microscope (FEI, Thermo-Fischer-Scientific) in PTB's Clean Room Center, which can be used to reconstruct the 3D shape of a structure to be measured by means of the so-called "shape from shading" method [2,3]. With the help of calibration structures [4], influences such as sample rotation, tilting and the working distance could be systematically investigated. It was found that sample rotation is not critical if the substrate is flat in the edge regions of the images and tilting up to 2° does not affect the calibration factors for 3D reconstruction. The optimal working distance of 5.4 mm of the detector system could be determined by an intensity maximum in the signal of the backscattered electrons. Figure 1(a) shows the reconstructed 3D images of a sharp probing tip and an edge-like tip. The corresponding line profiles of the reconstructed tips, which can be used to determine 2D geometric parameters such as cone angle and diameter are shown in Figure 1(b).

Compared to other available methods for tip characterization, including the use of the tip testing standard developed in FB 5.1 [1], 3D electron microscopy is now already a new, particularly fast way to provide a complete 3D reconstruction of the front region of probing tips for qualitative evaluation. However, especially for steep flanks above 50°, shadowing effects occur and the scaling factor for the z-axis is systematically too small. To better understand this behavior and the limitations of the method, measurements are currently being performed on very precisely fabricated tungsten carbide microspheres. Since spheres cover the full angular range, the reconstruction behavior and 3D representation of steep structures can be studied. This should allow not only to use 3D reconstructions of silicon microprobes directly for deconvolution of areal tactile measurements, but also to better interpret the representation of unknown samples with varying surface angle range.



Fig. 1(a) 3D reconstructions of two different silicon stylus tips, the last 20 µm of the up to 100 µm high stylus tips can be seen. In (b) the z-profiles of a 2D section running through the center of the styli are shown.

Literatur
[1] U. Brand et al., “Long Slender Piezo-Resistive Silicon Microprobes for Fast Measurements of Roughness and Mechanical Properties inside Micro-Holes with Diameters below 100 µm,” Sensors, vol. 19, no. 6, 2019, doi: 10.3390/s19061410.

[2] D. Berger, „Characterization of a 4-Quadrant-Large-Angles BSE-Detector for in-situ 3D Quantitative Analysis of the Sample Morphology in SEM”, Microscopy and Microanalysis, 13, 74-75, 2007, doi:10.1017/S1431927607080373.

[3] L. Reimer et al., „Shape from Shading using multiple detector signals in Scanning Electron Microscopy”, Scanning Electron Microscopy, vol. 1, no. 3, pp. 963–973, 1987.

[4] M. Ritter et al. "A versatile 3D calibration object for various micro-range measurement methods." Proceedings XXth ISPRS Congress Istanbul. Vol. 25. 2004.

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