Low energy ion beam irradiation of silicon

A focused ion beam system with a liquid metal ion source has been used to irradiate monocrystalline silicon with various ions (Ga, Si, Au, Au clusters) at energies from 60 keV down to 40 eV. The fluence was between 10(12) and 10(16) atoms/cm(2). A structure size of approximately 20 mu m by 20 mu m has been chosen to facilitate optical measurements. The reflectivity at 785 nm wavelength and laser induced modulated reflectivity have been used to characterise the optical properties of the samples after irradiation. With the beginning of the amorphisation process at fluences around 10(14) to 10(15) atoms/cm(2) for keV energies, an abrupt increase in reflectivity and modulated reflectivity has been found. Silicon irradiated by silicon ions shows a smaller increase in reflectivity, since there is no enhanced optical absorption due to implantation of metallic ions. Irradiation at energies below 1 keV results in a three orders of magnitude lower defect production in comparison to higher energies. The modulated reflectivity of silicon irradiated with Au clusters at 6 keV/atom shows no dependence on cluster size.