Laser-vibrometric ultrasonic characterization of resonant modes and quality factors of Ge membranes

Abstract

The vibrations of a single-crystal germanium (Ge) membrane are studied in air and vacuum using laser vibrometry, in order to determine mechanical properties such as Q-factors, tensile stress, anisotropy, and robustness to shock. Resonance modes up to 3:2 are identified, giving a residual stress measurement of 0.22 GPa, consistent with the value obtained from x-ray relaxation studies. The membrane is found to be isotropic, with Q-factors ranging from around 40 at atmospheric pressure to over 3200 at mbar, significantly lower than those found in polycrystalline Ge micromechanical devices. The robustness to shock is explained through the high resonance mode frequencies and the dissipation mechanism into the substrate, which is a direct consequence of having a high quality film with low residual tensile stress, giving the potential for such films to be used in optoelectronic devices.

Laser ultrasonic characterization of membranes for use as micro-electronic mechanical systems (MEMS)

“…Previous research has confirmed that the membranes resonate in the stress-governed regime [5]. The stress can be measured by performing a sweep of frequency and identifying resonances from the increased vibrational amplitudes.…”

“…A pair of linear stages were used to produce a two-dimensional scanning system, onto which the IOS laser head was mounted. Frequency scans at fixed positions were used to find resonant frequencies, at positions chosen in regions where high displacements are expected for certain modes, for example at the centre and along the diagonal of the membrane [5]. Following this, positional scans were performed using a raster-scan pattern at fixed frequencies to identify the modes.…”

“…These materials offer the potential for producing devices on a single chip, taking the technology beyond the current state of the art for silicon (Si) devices [4,5]. By producing a suspended membrane of the material, thus decoupling the Ge or SiC from a silicon substrate, a more rapid and higher sensitivity sensor can be produced, and devices can be built which have a good level of thermal isolation from the environment [4][5][6]. Such membranes may be fragile, and hence a testing technique is required which will ensure that the devices produced are suitable for the environment in which they are to be used.…”

“…However, as membranes are made thinner, or with more complicated structures, in order to improve thermal isolation and give a larger vibrational response, such methods can be destructive. A non-contact method will have significant benefits; the alternative is to measure the vibrational properties, identifying resonances of the membrane [5,9].…”

“…Optical techniques can also be optimised to ensure that spatial resolution is sufficient for measuring membranes with sub-mm lateral dimensions. We have recently published work using laser ultrasound to perform linear scans of membranes, showing that the properties can be inferred from the displacement profiles obtained [5]. This paper takes the work further, giving full two-dimensional scans of the membranes to ensure that resonant modes are correctly identified and account for mode-mixing.…”