Issue 47

P. Gallo et alii, Frattura ed Integrità Strutturale, 47 (2019) 408-415; DOI: 10.3221/IGF-ESIS.47.31 409 Electro-Mechanical Systems (MEMS, NEMS), but gives us an insight into the breakdown of the continuum fracture mechanics. For these reasons, fracture nanomechanics has sparked the increasing interest of the scientific community. While effort has been primarily devoted to determining Young’s modulus and tensile strength of silicon and other materials [6,7], the fracture properties and fracture process at the nanoscale have been marginally treated. Recent results suggest that at micro and nanoscale the material still fails by nucleation and propagation of cracks [2] and therefore some local approaches, commonly employed at the macroscale, can be directly scaled down [8]. However, there is a limit below which the continuum assumption of the LEFM has to face the discrete nature of atoms [9]. In the above background, the experimental characterization of fracture behavior at the nanoscale and the investigation of the low limit of the continuum theory are at the present relevant and challenging aspects. Therefore, this contribution presents a synthesis of experimental tests on the evaluation of single crystal silicon fracture toughness at the nanoscale [10,11]. First, results obtained from pre-cracked samples are reviewed and crack propagation is commented. Later, results from notched nano-cantilever beams by using the theory of critical distances (TCD) are presented. At the end, the procedures are compared to highlight the pros and cons. The work also provides an insight into the micro-mechanisms of crack propagation in brittle materials. F ABRICATION OF THE SPECIMENS he specimen fabrication is depicted in Fig. 1. The pre-cracked samples and the notched nano-cantilever beams have similar procedures of fabrication with substantial differences only in the final steps. First, a block is generally carved out from a single-crystal Si (100) plate. The block is then deposited on a flat end of a gold (Au) wire and stabilized by tungsten vapor deposition. In this phase, the orientation of the sample can be re-arranged if needed. The block is then processed by FIB (focused ion beam) to obtain the final specimen. In the case of pre-cracked specimens, a trapezoidal plate is generated, and V-shape grooves are introduced in the vertical direction. These grooves are used as a guide for the generation of the pre-crack. A notch is introduced at the top surface, and a wedge-shaped indenter is employed to apply an opening displacement until crack propagation is detected. Finally, the top part of the specimen can be cut to obtain the desired crack-length. The process of pre-crack generation is in general quite difficult and time-consuming since precise control of the geometry with the FIB is problematic at this small scale. In the case of notched specimens, the cantilever beam is cut from the block in several steps, and a notch is finally introduced. In the generation of the notch, attention should be paid to obtain a good level of symmetry on the two lateral surfaces. This aim is achieved by tilting the stage to correct the beam angle. It should be pointed out that also in this case the control on the notch root radius is very difficult, especially when the generation of sharp notches is needed. Because of the extremely small sizes of the samples, even the smallest radius currently achievable with FIB could not be simplified and considered as “zero”. The procedure is, however, faster than the fabrication of pre-cracked specimens and, as is shown in next sections, accurate control of the notch geometry is not needed when applying the TCD. Figure 1 : Example of fabrication of specimens. T