The birefringence and polarization effects of amorphous Ge and Si gratingsby focused-ion-beam

The polarization and birefringence effects in amorphous germanium (a-Ge) an d amorphous silicon (a-Si) gratings formed by focused-ion beam (FIB) were i nvestigated by using a linearly polarized He-Ne laser beam (632.8nm). The a -Si and a-Ge thin films were deposited on quartz substrates by plasma-enhan ced-chemical-vapor deposition (PECVD) and thermal evaporation, respectively . In order to obtain the optimum grating arrays, an inorganic a-Se75Ge25 re sist, top layer, was prepared on each films with the thickness (Z(min)) opt imized by Monte Carlo (MC) simulation. As the results of MC simulation, the Z(min) of a-Se75Ge25 resist was estimated to be about 600 Angstrom. The do uble layers prepared were milled by Cat-FIB with an accelerating energy of 50 keV and then etched by reactive-ion etching (RTE). Eventually, grating a rrays fabricated had linewidth and its separation of 0.5 mu m and 0.7 mu m, respectively. The maximum diffraction efficiencies, which were measured at an incident angles (about 16.7 degrees) to satisfy the Bragg condition, ex hibited very large values to be about 41% and 44% for a-Si and a-Ge, respec tively. Also, the birefringence dn obtained by a simple formula were estima ted to be approximately 0.200 and 0.148 for a-Si and a-Ge, respectively.