Development of a magnetron sputtering apparatus equipped with superconducting bulk magnets for the preparation of optical multilayer films

We have constructed the two-cathode superconducting permanent magnet bearing magnetron sputtering apparatus with the aim at fabricating Mo/Si multilayer films to serve as an optical mirror at extreme ultraviolet wavelength of 10-14 nm. By using the magnetized bulk superconductor, the magnetic field generated immediately above the target can be increased 20 times as strong as that in a conventional magnetron sputtering. This strong magnetic field enables us to perform sputtering in a low gas pressure and with a long throw distance. In the present study, we deposited about 10 nm thick Si and then 10 nm thick Mo layer on the Si wafer and studied the effect of deposition conditions on an interdiffusion layer formed between Si and Mo layers by measuring the grazing incident X-ray reflectivity. As a result, we could successfully reduce the interdiffusion layer thickness down to 0.6 nm, when the deposition was made at the Xe gas pressure of 0.02 Pa, a throw distance of 450 mm and discharge voltage of 2 kV. To the best of our knowledge, this is the lowest thickness of an interdiffusion layer of Mo-Si film ever reported. This promises us to increase the reflectivity towards 70%, when a multilayer film is ready to be synthesized in the present superconducting magnetron sputtering apparatus.