To investigate the microprobe silicon tip degradation in roughness measurements

Surface roughness measurement requires a measurement technology that combines high accuracy with high throughput. Due to their superior dynamics, microprobes are promising candidates for high-speed roughness measurements [1]. However, for many measurement tasks, the radius of the microprobe's silicon tip increases very rapidly during roughness measurement, and so does the measurement uncertainty. To investigate this tip variation, experiments were performed using a probe tip characterization standard TSPN [1] developed at PTB to characterize the tip geometry.

The influences of tip wear, probing force, and relative motion of the tip with respect to the roughness standard were investigated. The tip wear rate was recorded by using a rotary stage to set a constant relative velocity between the tip and the rotating target and a low probing force of 50 µN to avoid tip breakage. The volume loss caused by tip wear was so small that the tip radius of the microprobe was almost unchanged after 260 m of measurement travel. When the probing force was increased to 150 µN, tip breakage occurred, and tip volume loss increased more than tenfold.

If the relative movement between tip and target is realized by a conventional scanning of the tip with accelerations and decelerations, the volume loss rate of the tip increases ten thousand times, despite a low probing force of 50 µN. At this volume loss rate, a tip radius of 2 µm is reached after only several meters of measurement (see Fig. 1).

The results indicate that tip breakage and not tip wear is the main reason for the sharp increase in tip radius. Accelerations and decelerations of the tip during the measurements result in larger dynamic loads, which then lead to tip fractures because silicon has a relatively low fracture toughness. Maintaining a constant tip-sample relative speed to avoid lateral dynamic loads can serve to effectively protect the tip.

Fig.1: In scanning mode, tip breaks cause the tip radius to exceed 2 µm after only several meters of measuring length. The SEM image shows the shape of the original tip. The colored curves are tip profiles measured with the standard TSPN.

Reference

[1] Brand, U.; Xu, M.; Doering, L.; Langfahl-Klabes, J.; Behle, H.; Bütefisch, S.; Ahbe, T.; Peiner, E.; Völlmeke, S.; Frank, T.; Mickan, B.; Kiselev, I.; Hauptmannl, M.; Drexel, M. 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