Rough film wafer sensitivity improvement using light scattering inspection system

Improvements in the sensitivity of commercially available laser based surface scanning inspection systems can be achieved by adding an aperture to the optical path so that the rough film haze effect is spatially filtered. The current approach to optimizing the shape and dimensions of the aperture is time consuming and requires measurement equipment other than the laser based surface scanning inspection system for which the aperture is being optimized. The authors´ approach determines the optimal optical aperture using only haze and defect data collected directly on the inspection system. Two test wafers deposited with aluminum blanket films are used to demonstrate sensitivity improvements with designed optimal optical aperture. Aluminum blanket films are known to be rough films that generate significant background scatter, i.e. haze, that limits sensitivity by adversely impacting SNR. These wafers were taken from the deposition process for a key interconnect layer used in the back end of line (BEOL) fabrication of dynamic random access memory (DRAM) and logic devices. The proposed approach to optimizing optical apertures resulted in a 49% sensitivity improvement when detecting polystyrene latex (PSL) spheres deposited on one test wafer and a 32% increase in natural defect count on a second test wafer. In addition to the improvement in sensitivity, the optimized optical aperture enabled the differentiation of two more defect types: small particles and flakes.