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E-Modulmessung in Mikrobohrungen mittels der Kontaktresonanzmethode

06.12.2017

To analyse the performance of high-aspect-ratio microstructures (HARMS), it is not only necessary to measure the form and surface topography, but also to characterize the material mechanical properties quantitatively and nondestructively. The Profilscanner with the slender piezoresistive microprobe enables performing traceable roughness measurements inside small holes with diameters down to 50 µm. Contact resonance (CR) technology makes it possible to quantitatively map the mechanical properties of materials on a nanoscale. Therefore, we apply the contact resonance technology to the Profilscanner and the slender piezoresistive cantilever microprobe to investigate the feasibility of quantitatively determining the material mechanical properties inside HARMS.

At first, the spring constant of the piezoresistive microprobe is determined based on the Sader method. Then the tip at the free end of the microprobe is brought into contact with the sample surface. As the result, the microprobe resonant frequencies and the shape and width of the resonant curves are changed. The Young’s modulus of the sample surface can be determined through measuring these frequency changes and by comparing with reference materials.

Using a polycarbonate standard sample with a Young’s modulus of 2.7 GPa as reference material and a 5 mm long microprobe with diamond tip, the indentation modulus of a photoresist SU-8 sample could be obtained to be 3.41 GPa using the contact resonance method. This result is by 20 % smaller than the literature value of 4.25 ± 0,73 GPa.

The experimental results indicate that the utilization of the contact resonance technology with the piezoresistive microprobe is possible. However, the modelling and the theoretical analysis have to be improved to increase the precision and accuracy of the method.



Fig. 1. The 1st and 2nd free resonant frequencies of the microprobe are about 3.2 kHz and 19.5 kHz respectively (upper part). The resonant frequencies are shifted to 14.5 kHz and 44.5 kHz on the Polycarbonate surface (middle part), while they are shifted to 14.9 kHz and 46 kHz on the SU-8 surface (lower part).