Experimental and simulation studies of alignment marks

The roles of alignment mark structure, geometry, film materials and scanned illumination image on dark-field alignment signal quality are investigated through simulation interpreted experiments. The illumination of the alignment system consists of a low numerical aperture 1.0 micrometers image of a slit at 488.0 and 514.5 nm which is scanned across the alignment mark. The light scattered into a dark-band protected cone is collected. The structures consist of 0.5 to 1.2 micrometers wide features including trenches in silicon coated with 1.34 micrometers of photoresist, and trenches in nitride coated silicon with 1.00 micrometers of photoresist. For simulation, the illumination image is calculated using SAMPLE and input to a massively parallel finite-difference time-domain scattering analysis program called TEMPEST at various lateral shifts to simulate scanning. The diffracted orders output from TEMPEST are used to calculate total diffracted energy in various angular bands and to view dark field instantaneous images. Studied of depth, width, thin-film stack and coating thickness are used to identify key phenomena such as the role of interference between reflection from the alignment mark and substrate surfaces. Simulations are used to interpret the experimental observations and provide guidelines for topography control, and illumination and collection optics. The alignment signal is shown to be a strong function of trench depth, and only a weak function of trench width. An order of magnitude signal strength reduction occurs when the effective optical depth of the trench is a multiple of (lambda) resist/2 compared to the surrounding area.