The paper details a critical innovation for scaling optical probing to access the small feature sizes on advanced silicon process technologies. By using the liquid immersion principle to increase the numerical aperture of the microscope objective, a focused laser spot size of 0.50 μm is achieved for the first time. The liquid immersion objective is the first known application of the immersion principle to backside probing. This system improvement allows optical probing to be used in geometrically scaled processes that would not be accessible without it, and thus will extend the usefulness of laser probing for at least one more generation.
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