A new apparatus for fabricating narrow-bandpass multilayer filters applicable to wavelength-division multiplexing and the development of this apparatus are described. Electron-cyclotron-resonance (ECR) sputtering with two plasma sources is used in the apparatus. The plasma is produced from a mixture of Ar and O-2 gas and, with a metallic target, generates stable "metal-mode" deposition. That is, the target is sputtered in a metallic state, and this is followed by oxide-film formation, which is enhanced by exposure of the substrate to the ECR plasma. SiO2 and Ta2O5 films formed by ECR sputter-deposition in metal-mode are suitable for optical multilayer filters because this form of deposition provides surface smoothness, low loss, and stability of refractive index. Uniformity of deposition is good, with variations in thickness over a 200 mm diameter within +/- 0.43% for SiO2 and +/-0.27% for Ta2O5. The refractive indices of the SiO2 and Ta2O5 films are 1.46 and 2.10 at a wavelength of 1550 nm. Surfaces are very smooth, with typical root-mean-square roughness values of only 0.13 nm for the Ta2O5 film and 0.19 nm for the SiO2 film. We describe how we deposited multilayered structures consisting of 43 layers on an optical glass substrate. alternately SiO2 and Ta2O5, and thus fabricated a narrow-band bandpass filter. We demonstrate a thin full width at half maximum of only 0.224 nm and bandpass filtering uniformity within +/-0.1% over the whole 100 mm diameter substrate. (C) 2004 American Vacuum Society.