A fiducial beam monitor includes a patterned diode layer on a semiconducting substrate. An electrical field between the diode (or diodes) on the diode layer and the semiconducting substrate modulates the size of the depletion layer formed by the diode or diode. A high energy beam incident upon a diode on the diode layer produces a greater current than a high energy beam incident upon a non-diodic region of the same layer. In use, the beam monitor is typically fixed to the backside of a workpiece such as a semitransparent membrane being patterned by a focused high energy beam that is translated with respect to workpiece and attached monitor. The changes in current during translation are then correlated with the position of the beam with respect to the pattern on the diode layer.