Ruthenium thin films: terahertz generation and direct laser patterning

Ruthenium is emerging as a potential replacement for other conventionally used materials in semiconductor manufacturing. In this context, it is crucial to develop a detailed understanding of its material and electronic properties. This was done via experiments on the emission of coherent terahertz (THz) electromagnetic radiation from Ru thin films. It is demonstrated that Ru emits THz radiation as a result of illumination with a near-infrared femtosecond laser pulse through a second-order nonlinear optical process. It is also shown that laser-induced oxidation can occur and that the amount of oxidation, resulting either from laser exposure or from thermal annealing, correlates with the emission-strength of the THz radiation. This result also shows that THz radiation can be used as a probe for the degree of oxidation of Ru samples. Laser-induced oxidation, followed by the removal of the un-oxidized Ru through a NaClO solution is then presented as a method for producing conductive patterns consisting of Ru with a thin layer of Ru-oxide on top. A proof of concept is provided by producing near-circular Ru/Ru-oxide islands with an as-small-as possible diameter. Remarkably, islands with a diameter as small as 500 nm were obtained using a 1030 nm wavelength exposure laser, focused to a spot with a 2 micron diameter. These islands, therefore, have a diameter below the optical diffraction limit of the optical system used for exposure. This patterning process is further studied by identifying the key parameters which influence its performance via experimental methods and numerical simulations.