We report better spectral uniformity in Si-wire-based multistage delayed Mach-Zehnder interferometer (DMZI) type optical multiplexers/demultiplexers (MUX/DeMUX). Based on a 300 mm silicon-on-insulator wafer-scale ArF-immersion lithography process that guarantees higher phase controllability for low insertion loss, spectral flatness, and low crosstalk, we were able to further improve spectral uniformity across the wafer by optimizing the waveguide width within the optical path of each DMZI region. Irrespective of channel spacing in the wavelength domain, it was experimentally demonstrated that the filter spectral center wavelength deviation of the proposed DeMUXs was three times smaller than that of conventional ones. The proposed device scheme would also be attractive for arbitrary channel spacing and channel count, which makes the proposed scheme more practical in high-performance wavelength division multiplexing optical transceivers.