The standard ASML distortion measurement technique uses 8 m lines. For distortion of relatively large structures, only the sensitivities for the coma terms have a significant value. These aberrations induce overlay errors and left-right asymmetries. The current reproducibility is 2 nm. It uses a circular phase object, with a 180 phase shift, which prints as a ring in resist.
Aberrations induce a shape change in the ring, which depends on the focus position.
The inner and outer contours of the ring are written as a Fourier series, and the order of the Fourier harmonics corresponds to the angular order of the Zernike polynomials. By measuring at multiple illumination modes, the Fourier components can be separated into the full set of Zernike coefficients. The exposure, measurement and analysis time is about eight hours. The actual measurements are done on a SEM and take about six hours. Current reproducibility is 2 nm.