Detection of focus and spherical aberration by use of a phase grating

A phase only grating consisting of equal lines of 0 and 90 degree phase is imaged into a highly absorbing photoresist forming a surface relief grating that is measured with a dark- field optical microscope and a CCD array. The resulting images are analyzed to determine spherical aberration, +-0.001 wave RMS, and focus variations of +-2.0 nm. This method of measurement and analysis is applied to both 248 and 193 nm photolithography lenses.