Ultrasonic Inspection and Self-Healing of Ge and 3C-SiC Semiconductor Membranes

Abstract

Knowledge of the mechanical properties and stability of thin film structures is important for device operation. Potential failures related to crack initiation and growth must be identified early, to enable healing through e.g. annealing. Here, three square suspended membranes, formed from a thin layer of cubic silicon carbide (3C-SiC) or germanium (Ge) on a silicon substrate, were characterised by their response to ultrasonic excitation. The resonant frequencies and mode shapes were measured during thermal cycling over a temperature range of 20-100 • C. The influence of temperature on the stress was explored by comparison with predictions from a model of thermal expansion of the combined membrane and substrate. For an ideal, non-cracked sample the stress and Q-factor behaved as predicted. In contrast, for a 3C-SiC and a Ge membrane that had undergone vibration and thermal cycling to simulate extended use, measurements of the stress and Q-factor showed the presence of damage, with the 3C-SiC membrane subsequently breaking. However, the damaged Ge sample showed an improvement to the resonant behaviour on subsequent heating. Scanning electron microscopy showed that this was due to a self-healing of sub-micrometer cracks, caused by expansion of the germanium layer to form bridges over the cracked regions, with the effect also observable in the ultrasonic inspection. [2020-0017] Index Terms-Laser ultrasound, vibration, microelectromechanical systems (MEMS), thin films. I. INTRODUCTION S OLID thin films are used in a wide variety of engineering systems, including development of micro-electromechanical systems (MEMS) and nano-electro-mechanical systems (NEMS) [1]-[4]. The suspended square thin film is a simple MEMS structure and can be used in pressure sensors, or as a platform for integrating devices for applications such as near-infrared photo-detectors, flow sensors, photonic modulators, lasers, and enhancing silicon light emission via carrier injection [5]-[10]. The use of tensile strained membranes, such as Germanium (Ge) on Silicon (Si), can change the electronic or optoelectronic properties by reducing the energy band gap [11], [12]. However, the residual stress in thin films, especially within a multilayer structure, is temperature dependent. Changes in temperature can induce mechanical Manuscript

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“…Aside the articulated structures like those discussed above, materials with intrinsic self-healing properties are also reported in literature. The work in [117] discusses restoration of the mechanical resonant properties of cubic silicon carbide and germanium thin-films. Such 3D layers are intentionally altered by ultrasonic excitation and temperature cycling stress, and then recovered by means of heating.…”