Size Affected Toughening and Strain Rate Sensitivity of Silicon

Designing ever stronger nanostructured materials that can even withstand harsh environments has become a major goal in the last decades. However, the increase in strength commonly comes hand in hand with a severe reduction in ductility, which at the same time also limits the toughness of these materials. In the present work we show an outstanding example where size reduction does the opposite for a brittle material, it increases the fracture toughness.<br/>Addressing the most abundantly used functional material in semiconductor technology, we studied the fracture properties of Si at sub-micron to nanoscale dimensions by advanced nanomechanical testing and local strain mapping, both conducted simultaneously in-situ in the transmission electron microscope. In fact, using well annealed FIB fabricated fracture beams with extremely sharp pre-cracks, we examine the intrinsic room temperature fracture toughness of Si for dimensions ranging from hundreds of nanometers down to ~80 nm. An astonishing size effect sets in below ~250 nm in dimension, causing the fracture toughness to increase up to about 300%, in conjunction with the emergence of a rate sensitivity of the brittle Silicon.<br/>The origins for this unexpected toughening of Si at ambient temperatures are explained by in-situ observation of dislocation nucleation in Si, in conjunction with the analysis of the local stress state using nanodiffraction mapping and an assessment of the size dependent shear and cleavage stresses in the crack tip region using a digital twin.