Marks for SCALPEL((R)) tool optics optimization

A method for optimizing the electron optics of the SCALPEL exposure tool is described. The method uses the SCALPEL mark detection method with a grating mark as an aerial image monitor. The root-mean-square deviation of the recorded backscattered electron signal from an ideal triangle waveform was used as a measure of the image fidelity, scale and orientation. The resolution of the technique is limited only by signal-to-noise and the fidelity of the marks. Experiments were performed using 2 mu m period grating marks that were fabricated in a SiO2/WSi2 structure using SCALPEL lithography and plasma processing. The projector lenses and magnification/rotation coils were optimized. For these experiments the measured resolutions for determining focus (delta f), magnification (delta M), and rotation (delta theta) of a 250 mu m X 250 mu m field were delta f similar to +/- mu m, delta M similar to +/- 15 ppm and delta theta similar to +/- 0.1 mrad. A straightforward path to improving these results is described.