We describe the use of holographic phase-shifting interferometry (HPSI) to measure in-plane distortion of a substrate in which a shallow grating, produced by interferometric lithography, has been etched. The diffractive metrology inherent in HPSI should enable one to study process-induced distortions down to the level of a few nanometers. We used HPSI to investigate distortion introduced by the anodic bonding of silicon nitride membranes to Pyrex frames in x-ray lithography masks. This was part of an effort to develop an inverted x-ray mask configuration. The HPSI technique gave quantitative measurements of the linear and nonlinear components of distortions in one-dimension. The high levels of distortion found were presumably introduced by the high temperatures used in anodic bonding, combined with the asymmetric manner in which the membranes were brought into contact with the Pyrex frame. The comprehensive, quantitative measurement of distortion provided by HPSI should enable us to modify the mask making process so as to avoid steps that introduce distortion.