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In-situ Characterization of Tip-Surface-Interactions of Microprobes during High-Speed Surface Scans


High-speed surface scans using piezoresistive microprobes enable new industrial applications for direct measurements in processing machines [1]. For this application, microprobes with long beams are desirable, since they allow for smaller contact forces and a larger linear deflection range. Also, their larger maximum deflection before breakage occurs is beneficial for rough industrial measurement conditions. However, investigations of high-speed surface scans carried out with 5 mm long, slender, piezoresistive microprobes, reveal phenomena such as resonance oscillations and tip-flight to be present in the acquired data [2], which deteriorate the achievable measurement uncertainty.

Topography measurements acquired with high bandwidth were carried out to investigate the tip-surface interaction during high-speed surface scans for different contact forces. The electrical signal from the piezoresistive bridge was acquired by a HBM QuantumX MX410B universal amplifier at 196 kSample/s.

Figure 1(a) depicts a typical topography measurement result of a 1 µm step-height silicon standard measured using a microprobe with 5 mm beam length at different scan velocities. Spatially resolved spectral analysis of the piezoresistive deflection signal at tip velocity vtip = 15 mm/s as depicted in Fig. 1(b) shows the dominating frequency components (f0‘, f1)at 14.7 kHz und 45.7 kHz. These values correspond well to the theoretical resonance frequencies of 15.17 kHz und 45.9 kHz for this beam configuration with a clamped and a hinged beam end as boundary conditions.

Using a narrow FIR band-stop filter, as also depicted in Fig. 1(a), the profile can be correctly reconstructed even at scan velocities of 15 mm/s. The remaining waviness in the signal compared to a 1 mm/s scan can be attributed to the mechanical stages of the setup.

This experimental investigation of dynamic tip-surface interactions with special emphasis on the behaviour during high-speed topography scans, such as surface induced beam oscillations of microprobes, will help to deduce strategies for active and passive damping. Additional damping will enable further applications of these microprobes for precision high-speed surface scans.

Profilometry measurements of a 1 µm step height standard. Depicted is the deflection signal in µm over the measurement distance in mm shown starting at 4.15 mm and ending at 4.55 mm. The purple curve is measured at 1 mm/s and shows the original profile undistorted and without spurious signals. The blue curve shows the profile measured at 15 mm/s measurement speed. It shows damped oscillations with an initial amplitude of 1 µm starting right after the step and decaying completely over a distance of 0.3 mm. Also visible is a waviness with 0.25 µm amplitude over the complete measurement range, caused by the mechanical stage. The yellow curve shows the profile measured at 15 mm/s measurement speed with additional band-stop filtering around the mid-frequencies of 14.7 kHz and 45.7 kHz. It shows small and quickly decaying damped oscillations with an initial amplitude of 0.25 µm starting right after the step and decaying completely over a distance of only 0.02 mm. Also visible is a waviness with 0.25 µm amplitude over the complete measurement range, caused by the mechanical stage.
Fig. 1(a) Example of the measured topography of a 1 µm step-height standard using a 5 mm microprobe at different scan speeds. The scan at vtip = 15 mm/s shows strong oscillations in the deflection signal.

Depicted is a spectrogram of the measurements of Fig.1(a), showing the spatially resolved frequency components as false color map. The frequency components of 14.7 kHz and 45.7 kHz occurring directly after the step are visible. The 14.7 kHz component is much more pronounced and decays over the whole remaining distance range of 0.3 mm. the 45.7 kHz component is of smaller amplitude, decays more quickly and is only visible directly after the step before it vanishes in the background signal.
Fig. 1(b) Spectrogram for the investigation of surface-induces oscillations in the beam of a microprobe, scanning at vtip = 15 mm/s. The dominating frequencies at 14.7 and 45.7 kHz correspond well to the beam resonance frequencies, which are excited by the 1 µm step at 4.14 mm.



[1] Brand, U.; Xu, M.; 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 2019, 19, 1410.; doi.org/10.3390/s19061410

[2] Xu, M.; Li, Z.; Fahrbach, M.; Peiner, E.; Brand, U. Investigating the Trackability of Silicon Microprobes in High-Speed Surface Measurements. Sensors 2021, 21, 1557. doi.org/10.3390/s21051557



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