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Production sequence of Si-spheres and interferometrical determination of the sphere volume

A novel true 3D-AFM head


Requirements for measuring devices in the field of dimensional nanometrology for the determination of surface geometry are becoming increasingly three-dimensional (3D) and complex. Previous critical dimension atomic force microscopes (CD/3D-AFM) played a supporting role in these measurements. However, these AFMs, which were developed specifically for CD measurements, are only suitable for 3D measurements to a limited extent, since the sensor can only detect 2D. The hardware-based reason for this lies in the mechanical properties of the probe, the CD cantilever with a cylindrical CD tip which is flared at the apex. This shows a mechanical crosstalk for displacements of the CD tip in horizontal y- and vertical z-direction, which leads to deviations of the measured surface from the real surface. Furthermore, the stiff CD cantilever in combination with the compliant and slender CD tip shows low detectable displacements or angular changes, thus low sensitivity. Usually, the movements of a cantilever are detected with an optical lever and a proportional relationship of the bending angle of the CD cantilever to the displacement of the CD tip is assumed. However, this detection method also shows a crosstalk in y-, z-direction for the CD cantilevers used so far, which leads to tip breakage under unfavorable circumstances. Therefore, the previous detection method in combination with the CD cantilever is unsuitable for a true 3D measurement.

A newly developed 3D AFM head in combination with a 3D-Nanoprobe now makes the 3D displacements of the CD tip measurable for the first time [1, 2]. The principle is illustrated in figure 1. Optimized solid-state joints (FH1 and FH2) divide the 3D-Nanoprobe into stiff and flexible regions for an isotropic stiffness ratio. This is matched to the mechanical properties of the CD tip to improve sensitivity. The front region of the 3D-Nanoprobe (HS2) is optimized to show an angular change (α2, β2) only under horizontal (x, y) displacements of the CD tip. This angular change is detected by an optical lever. The z-displacement of the CD tip is detected by a differential interferometer, which measures the z-displacements of the head region 1 (HS1). A selectivity of the probing directions of 50:1 in 3D is achieved by calibrating a sensitivity matrix that also compensates for misalignments and manufacturing deviations of the 3D-Nanoprobe. A measurement of two orthogonally oriented reference standards of type IVPS100-PTB with perpendicular line structures of silicon is shown in Figure 2. Repeat measurements of this CD structure show a decrease in structure width of -0.33 pm and -5.4 pm per line in the x-direction and y-direction, presumably predominantly due to tip wear [2].





[1] Thiesler, J., Tutsch, R., Fromm, K., and Dai, G., “True 3D-AFM sensor for nanometrology”, Measurement Science and Technology, 31, 7, 2020. doi:10.1088/1361-6501/ab7efd
[2] Thiesler, J., Ahbe, T., Tutsch, R., and Dai, G., “True 3D Nanometrology: 3D-Probing with a cantilever-based sensor”, Sensors, 2022, 22(1), 314. doi:10.3390/s22010314



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