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The authors present the basic concept and some preliminary experimental data on a novel method for measuring critical dimensions on masks used for X-ray lithography. The method uses a scanning electron microscope (SEM) in a transmitted-electron imaging mode and can achieve nanometer precision. Use of this technique in conjunction with measurement algorithms derived from electron-beam interaction modeling may ultimately enable measurements to these masks to be made to nanometer accuracy. Furthermore, since a high contrast image results, this technique lends itself well to automated mask defect recognition and inspection. It is concluded that this method has the potential advantage of avoiding or at least minimizing the basic limitations imposed by the electron-beam interaction effects normally encountered in conventional methods of dimensional metrology in the SEM.