LIGHT DIFFRACTION BASED OVERLAY MEASUREMENT

Optical overlay measurement methods are very effective since they are rapid and non-destructive. Imaging techniques need sophisticated image processing and suffer from the wave-optical resolution drawback. Presently, leading edge devices are offered with 5 through 10 nm measuring accuracy. In this paper a method is proposed that relies on the diffraction of a probing laser beam at a periodic reference pattern. This special pattern is implemented in the circuit layout. After the resist patterning of the second of two consecutive layers, the diffraction at the resulting net grating is measured. If an appropriate grating design is chosen, the misalignment error can be directly extracted from the diffraction efficiency. In order to obtain a strong diffraction signal and thus a sufficient signal-to-noise ratio optimum grating designs have been computed by means of rigorous diffraction modeling. Experimental results supported by rigorous modeling suggest that this technique could have the potential to meet next generation overlay accuracy requirements.