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Resist coating on alignment marks can be asymmetrical due to the spinning motion. Alignment errors may result from this asymmetrical coating because the alignment signal can contain asymmetry when the optics detects the mark through the resist. The amount of signal asymmetry depends on the mark topography, resist thickness, spin speed, location on the wafer and the alignment technique. We have proposed a Brewster angle illumination technique (BAIT) as an effective means to greatly reduce the resist-induced alignment error by reducing alignment signal reflections from the surface of the resist. In this paper, we present the experimental results. We built an interferometric alignment system to implement BAIT. By scanning a resist-coated mark in close proximity to an exposed reference mark, drift and other system offsets are minimized, allowing an offset detectability of 6 nm. Resist-induced alignment errors with a radial dependence on mark position and magnitudes up to 25 nm have been measured when BAIT is not used. However, when BAIT is employed the resist-induced errors at all positions on the wafer are reduced to less than 10 nm and are close to the resolution limit of the alignment setup. The validity of the data was established by changing the polarization of the light from TM to TE, thereby destroying the BAIT condition while holding all other parameters constant. A study of the dependence of resist-induced alignment error on spin speed reveals that the amount of asymmetry in the resist profile increases monotonically as the spin speed increases. This effect is due to two factors: that the resist is thinner and that the spin speed is also faster. Further study using a thinner to dilute the resist isolated these factors and showed that the resist-induced alignment errors depend more sensitively on spin speed than the resist thickness. In most of the cases, the BAIT alignment system is immune to resist coating asymmetry and the error magnitude stays below 10 nm. However, the BAIT cannot work effectively when resist thickness becomes thinner than 0.6 micrometer. This limitation of BAIT technique was attributed to the ripples in the resist profile causing local impinging angles to deviate significantly from the Brewster angle.
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