Monitoring defects using low thresholds is key to manufacturing yield. For inspecting SOI wafers, UV light overcomes the limitations of visible light. Here’s why.
With visible-light inspection tools, the scattering behavior of defects on SOI structures depends on silicon and oxide thicknesses. Because of buried interfaces, transmitted visible light is sent back to the surface after coherent reflection, and can interfere with incoming light. Phase shift of the reflected beam is driven by silicon and buried oxide thickness, resulting in constructive or destructive interferences, increasing or decreasing reflectivity compared to a reference bulk-silicon reflectivity. Scattering intensity on the surface is proportional to the apparent illumination, and depends on structure thicknesses.
Therefore, the only way to implement robust defectivity monitoring using visible light sources is to generate calibration curves for each product generation and SOI thickness. But this is not sufficiently aggressive for sub-90nm technologies.
Using UV light, on the other hand, the transmitted light has to be absorbed before interfering with incoming light at the surface, resulting in constant reflectivity regardless of SOI thickness. Bulk siliconlike metrology can then be implemented on SOI, without additional calibration work for specific SOI films thickness combinations.
With UV defectivity inspection, SOI behaves like bare silicon regardless of the silicon and oxide layer thicknesses. Aggressive thresholds are demonstrated, closing the gaps with industry roadmaps. SOI substrates can be inspected using standard inspection strategies. Similar yields, which have been reported for microprocessor device processing on SOI and bulk, can then be confirmed when benchmarking similar defectivity levels on SOI and epi material, using the same high sensitivity recipe.