Mask substrates for advanced semiconductor microlithography at 157 nm and 13 nm (EUV) require a surface flatness of 50 nm P-V. Production polishing methods require the surface metrology accuracy to be about 1110th this tolerance, or 5nm. Interferometric measurement accuracy is affected by the following factors: fringe pattern from the backside of the plane-parallel substrate, accuracy of the reference flat surface, retrace error of the interferometer optics, and coherent noise (speckle) associated with the coherent source. This paper describes the performance of a new wavelength-shifting Fizeau interferometer that is capable of simultaneously measuring the surface height of the front and back surfaces of an EUVL mask blank substrate to an absolute accuracy of 5 nm and an RMS repeatability of <0.01 nm. Unlike conventional Fizeau interferometers, it can simultaneously measure front surface height, back surface height, and relative thickness of a plane-parallel transparent glass plate while minimizing multi-surface fringe print-through artifacts. Surfaces with substrates as thin as 2 mm can be measured by this instrument, although the technology is readily extendable to thinner substrates. Back side fringe print-through is shown to be in the range of 0.1-0.5 nm P-V. Additional features of this new interferometer include a 300 mm diameter aperture, a set-up configuration suitable for automated part handling, an optical design optimized for low retrace error, a wavelength-tunable AlGaAs laser diode operating at 850 nm, a fiducialized transmission flat surface that is calibrated to 3 nm absolute accuracy, and a spatially incoherent source configuration for low speckle noise performance. Measurement data as well as an accuracy budget are presented.