Size-dependent plasticity and activation parameters of lithographically-produced silicon micropillars

Silicon is brittle at ambient temperature and pressure, but using micro-scale samples fabricated by focused ion beam (FIB) plasticity has been observed. However, typical drawbacks of this methodology are FIB-damage and surface amorphization. In this study, lithographic etching was employed to fabricate a large number of (100)-oriented Si pillars with various diameters in the micro-scale. This allowed quantitative study of plasticity and the size effect of FIB-free Si in the brittle temperature range (25-500 degrees C) by conducting monotonic and transient microcompression in situ in the scanning electron microscope (SEM). Lithographic pillars achieved the ideal strength in temperature range of 25-100 degrees C and displayed significantly higher strengths (30-60%) than FIB-machined pillars because of the undamaged surface and the oxide layer confinement. The activation energy of deformation revealed a transition in dislocation mechanisms as a function of temperature. Strain rate sensitivity and activation volume measured from strain rate jump and stress relaxation tests indicated the surface nucleation of kink-pairs associated with the constricted dislocation motion in Si during deformation at temperatures below the brittle-ductile transition. A modified analytical model is proposed to accurately evaluate the size-dependent strength of covalent crystalline Si. The weak size effect observed in Si is attributed to the surface nucleation of dislocations and high lattice friction during their motion. (C) 2020 The Authors. Published by Elsevier Ltd.