Direct, in-scanner, aerial image sensing

With feature sizes now less than one half of the wavelength of the actinic radiation, determining the intensity distribution in the aerial image has become both more critical because of the low k1 factors used and more challenging because of the limitations of scalar diffraction theory. Here we describe ways of measuring in situ the intensity distribution generated at the wafer surface by the wafer exposure tool (‘scanner’ or ‘stepper’) and that eliminate the delays and vagaries of resist processing. Some early results show a 2D image generated by an aerial image sensor loaded into an unmodified 193 nm scanner. Nominal feature sizes are down to 67 nm. One additional advantage of this technique over the slower alternative of SEM viewing of the resist image is the gray-scale of the intensity image from the sensor. Applications can include assurance of the correctness of the stepper-mask combination, verification and development of the modeling of the aerial image (including the effect of resolution enhancing techniques) and the qualification of incoming masks.