Application of blazed gratings for determination of equivalent primary azimuthal aberrations

Aberrations are determined ti om wafers exposed using a test reticle having blazed gratings with orientations from 0 to 337.5 degrees in increments of 22.5 degrees. The image of the phase grating is micro stepped through focus on the surface of high absorption photoresist. A second blanket exposure is applied to the latent images to make the developed resist images have a surface relief linearly related to the aerial image intensity. The first order diffraction efficiency of this surface grating is directly proportional to the first harmonic of the surface relief. This diffraction efficiency is recorded as a digitized dark-field image and processed to evaluate the aberrations. These images are simulated and matched to those recorded in the photoresist by adjusting the aberrations to achieve a best match. The primary azimuthal aberration contributions are separated by Fourier analysis into coma, astigmatism, 3 leaf clover, etc. and are used to indicate lens quality. Several grating frequencies are required to separate higher order components of each azimuthal aberration. Only one grating frequency is used in the data reported here and all orders of each azimuthal aberration are lumped into equivalent primary azimuthal aberrations.