Electron beam nanolithography tools readily permit the fabrication of ultra small structures. However, the disadvantage of such a system is that the depth of field is very small, making it difficult to maintain the focus of the system as the sample is moved due to sample topography, tilt, and stage run out. This presents a serious Limitation to nanofabrication. To overcome this problem, we have built and installed a two color, heterodyne common path interferometer on a converted JEOL 100 CXII scanning transmission electron microscope. The interferometer compensates for the very small depth of field of the system by detecting variations in the sample working distance with respect to the electron optical objective lens, and dynamically correcting the lens current. The advantage of this system is that optical access to the sample can be made in a microscope with a very small working distance or sample space. To demonstrate this system, we have fabricated 38 nm period gratings on a silicon substrate. The process was a conventional one using a single 30 nm layer of 350 k molecular weight poly methyl methacrylate resist. The grating was exposed with a linear dose of 1.6 nC/cm and developed in 3:1 IPA:MIBK for 30 s at 23 degrees C. Patterns were transferred by lift off of an evaporated Ni/Au film. The gratings have a 1:1 mark:space ratio and a Line-width variation of 5 nn at 4 sigma. The stage can be moved over large areas and from chip to chip, and the lithography repeated successfully. This system therefore permits reliable state-of-the-art nanolithography.