SCALPEL aerial image monitoring: Principles and application to space charge

We present an approach to real time direct aerial image monitoring which ut ilizes the information contained in an alignment signal generated by scanni ng the image of a mask grating over a corresponding wafer grating and detec ting the backscatter electron signal. The basic principles of this measurem ent technique are described. The effect of noise and other common errors, s uch as magnification and rotation on the signal quality, are derived and us ed to set requirements on signal contrast and noise level for obtaining blu r values accurate to approximately I nm. The application of this approach t o measuring space charge blur is described and preliminary data illustratin g the concept is presented. The potential for this technique to Form the ba sis of an automated self-calibration system on SCALPEL tools is clear. (C) 2000 American Vacuum Society. [S0734-211X(00)01605-X].