Pattern placement errors pose a serious problem in the manufacture of masks for proximity x-ray lithography. Many of these errors are attributable to long term drifts in beam position relative to external fiducials. To address this problem we have developed a technique based on through-the-membrane monitoring of the electron beam position. This technique uses as a detector a reverse biased Schottky diode with high bandwidth and gain. In use this detector is mounted near the back surface of the membrane. An accurately patterned overlayer on the detector provides the fiducial reference. The overlayer is designed to modulate the electron-hole pair current generated in the diode by absorbing the incident beam. Position information is obtained by analyzing the image created from recording the digitized diode current during patterning. The phase in a Fourier transform of the data at the spatial frequency of the patterned absorber gives a measure of the position of the incident beam. Changes in the observed phase from one frame to the next can then be used to correct position errors of the beam in real time. We report results from tests of various components of this system. Initial results indicate that the system will be sufficiently fast and accurate for use in pattern placement correction for 130 nm ground rule mask production.