Creation of Microstructures in Silicon for Acoustic Applications by Proton Beam Writing

Dipl.-Phys. Uwe Scholz

Micromachined Si structures have been produced for acoustic applications like mode converters. Different arrays of upright and inclined rods with heights ranging from 10 µm - 50 µm and different array sizes and distances between the rods were created by Proton Beam Writing (PBW) and subsequent electrochemical etching of Si. These structures were investigated using an ultrasonic microscope at frequencies of 86 MHz in holography mode. The comparison of measured and simulated holographic patterns show significant differences between structured and unstructured silicon surfaces indicating a transverse/longitudinal mode conversion of acoustic waves at the structured surfaces.

Figure 1: (a) REM image of micromachined 30° inclined walls with a lateral distance of 10 µm in a Si sample. (b) Transients of the magnitude obtained by scanning the micromachined ~430 µm thick sample in tomography mode at different places on the sample.

In order to increase this effect of mode conversion, arrays of silicon walls with different inclination angles q have been produced (Fig. 1(a)). Acoustic tomography measurements reveal different intensities of acoustic transmission which can be ascribed to mode conversion depending on q (Fig. 1(b)).