Paper SE+TF+NC-ThA11
Improving Stiffness of Silicon Chevrons by 1.2 MeV Ar⁺⁸ Ion Irradiation

Thursday, October 23, 2008, 5:20 pm, Room 204

Five-armed silicon chevrons were grown by oblique angle ion beam sputter deposition on Si(111) substrates pre-patterned with 260 nm diameter silica spheres and 500 nm diameter polystyrene spheres. The patterning spheres self-assemble in hexagonal network, act as nucleation sites, and control the dimensions as well as the distance between the growing chevrons. The chevron arms are about 500 nm long, 350 nm wide and 200 nm thick with odd numbered arms making an angle of about 45° and even numbered arms an angle of about 55° from the vertical. In order to tailor the mechanical strength of the chevron coatings, the samples were irradiated with 1.2 MeV Ar⁺⁸ ions at liquid nitrogen temperature at different fluences of 10¹⁵, 10¹⁶ and 10¹⁷ ions/cm². The range of Ar ions in the amorphous Si matrix as determined by the simulation code SRIM is about 1.1 µm. Therefore, the Ar ions traverse the complete length of the chevrons before getting implanted in the Si(111) substrate. The spring constant of the chevrons was determined by force-distance spectroscopy using an atomic force microscope. Results show that the spring constant of the irradiated Si chevrons grown on 260 nm diameter silica spheres increases from 480 N/m for pristine to 1320 N/m for a fluence of 10¹⁶ ions/cm². For chevrons grown on 500 nm diameter polystyrene spheres, an increase from 370 N/m for pristine to 1325 N/m for fluence of 10¹⁷ ions/cm² is observed. Thus, irradiated chevrons are stiffer as compared to pristine ones. Finite element analysis of chevrons provides an insight into the dynamics of the resulting deformations when the chevrons are loaded. The modeling establishes that for these odd-armed chevrons a vertically downward load results in a torque about the base of the chevrons. The load produces a significantly larger lateral stress/displacement relative to the vertical, thus making them more robust to the loading force. Investigations by micro-Raman and glancing angle X-ray diffraction suggest that the Ar irradiation fosters a transformation from amorphous to poly-crystalline Si. The improved mechanical strength of the chevrons indicates that ion irradiated chevron coatings are capable of withstanding higher loads as compared to pristine coatings without getting delaminated.