Previously we described an x-ray mask alignment system, capable of nanometer-level superposition precision, whose alignment signal did not appear to be adversely affected by overlayers of resist, polysilicon, or metal [E. E. Moon, P. N. Everett, and H. I. Smith, J. Vac. Sci. Technol. B 13, 2648 (1995)]. The system, called interferometric broad-band imaging (IBBI), employs grating and grid type alignment marks on mask and substrate, respectively. These are viewed at an inclined angle, through the mask, using f/10 optics with a working distance of 110 mm. The inclined angle and long working distance avoid interruption of the x-ray beam. Using a charge-coupled-device camera, misalignment is measured from two identical sets of interference fringes (similar to 50 mu m period) that move in opposite directions as the mask is moved relative to the substrate. Alignment corresponds to matching the spatial phases of the two sets of fringes. Here we demonstrate that the same alignment optics and grating type alignment mark on the mask can be used to measure the mask-sample gap, with a sensitivity similar to 50 nm, via a diffractive-Michelson interferometer. No mark is required on the substrate. X-ray exposures were carried out with the IBBI system. The disparity between (a) the misalignment as measured by the IBBI system and (b) the postprocessing misalignment, as measured on the substrate by a related moire technique, was evaluated. Mean disparities of 0.1, -0.4, and -0.3 nm were measured for alignment marks of 1, 2, and 4 mu m period, respectively. The 3-sigma values of the distributions were 2.4, 7.3, and 5.9 nm. The larger values for the 2 mu m period case are attributed to multiple defects in the grating alignment marks.