Optical Performance of laser-patterned high-resistivity Silicon Wafer in the frequency range of 0.1-4.7 THz

Abstract

Direct laser ablation (DLA) is a mask-less technology used for the research and development of optical components of various materials [1]. The relevance of the DLA technology is verified demonstrating the functional optical components including multilevel phase Fresnel lenses on silicon and Soret zone plates developed on a free standing metal-foil [2]. In order to reduce the reflection losses, the anti-reflection structures on a back side of silicon wafer can be patterned by the same DLA technology as this has been proposed recently [3]. In this work we studied optical transmission of laser patterned high resistivity silicon wafers used for development of the diffractive optics in the frequency range of 0.1 – 4.7 THz. The samples were prepared on a 500 µm thick, both-sides polished, high resistivity silicon wafer varying the surrounding environment as well as the DLA parameters in order to modify the composition and roughness of the surface modified. Most of the samples were fabricated in an ambient air, while others were developed in an argon-rich atmosphere at the pressure of 1 atm and 2 atm. Stylus profiler and scanning electron microscope were employed to characterize the samples morphology. Optical performances were studied measuring with a Golay cell detector the transmittance of the THz beam of a quantum cascade laser (QCL) operating at 2.5, 3.1, and 4.7 THz. The dielectric constants dispersion for each sample was also obtained by a THz time domain spectroscopy (TDS). Dependence of transmittance on surface roughness at different THz frequency allowed us to identify the critical value Ra at which the transmittance dropped by 20%. For example data presented in Fig. 1 indicates that the critical Ra value at frequency 4.7 THz is of about 1.9 m. We will discuss a nonlinear dependence of the critical Ra value on the THz radiation frequency. The impact of silicon processing in an oxygen-free environment to the transmittance performance will also be demonstrated and discussed.