We describe here a means of improving pattern placement accuracy in the electron-beam lithographic manufacture of membrane masks. Our method is based on collection of the beam transmitted through the membrane with a detector fixed to the mask assembly. A fiducial grid overlaid onto the detector is used to provide a global reference for measurement of the beam position during lithographic patterning. We give results from-recent experiments with components of such a system, and estimate I sigma accuracy in our method to be on the order of 1.5 nm. We discuss the effect of beam scattering on our technique, and possible improvements in our implementation. [S0734-211X(98)03606-3].