Infrared spectroscopy study of a poly-silicon film for optimizing the boron-implanting dose

An experiment was carried out for recording the infrared transmission between 3 and 16 μm for a free-standing poly-silicon (Si) thin film as a function of the boron-ion implanting. The optical constants were extracted in order to optimize the postdeposition implanting of the membranes in a surface micromachined Fabry-Perot interferometer for the thermal infrared. The free-carrier concentration must not degrade the refractive index n significantly since a high and constant value over the application wavelength range is desired. Moreover, an increase in the extinction coefficient k is detrimental for the interferometer performance. On the other hand, poly-Si electrical conductivity must be doped high enough to avoid static charging and to prevent any layout-dependent distribution of the applied control voltage. We applied the variable-angle transmission spectrometry for recording the data of suspended poly-Si membranes, doped with different levels of implanting dose. The Drude model dispersion formula was exploited for extracting the optical constants n(λ) and k(λ). The optical constants are presented as a function of the dopant concentration and the electrical resistivity.