24 November 2016 Tip/tilt-compensated through-focus scanning optical microscopy
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Abstract
Through-Focus Optical Microscopy (TSOM), with nanometer scale lateral and vertical sensitivity matching those of scanning electron microscopy, has been demonstrated to be utilized for 3D inspection and metrology. There have been sensitivity and instability issues in acquiring through-focus images because TSOM 3D information is indirectly extracted by differentiating a target TSOM image from reference TSOM images. This paper first reports on the optical axis instability that occurs during the scanning process of TSOM when implemented in an existing patterned wafer inspection tool by moving the wafer plane; this is followed by quantitative confirmation of the optical/mechanical instability using a new TSOM tool on an optical bench with a Shack-Hartmann wavefront sensor and a tip/tilt sensor. Then, this paper proposes two tip/tilt compensated TSOM optical acquisition methods that can be applied with adaptive optics. The first method simply adopts a tip/tilt mirror with a quad cell in a simple closed loop, while the second method adopts a highorder deformable mirror with a Shack-Hartmann sensor. The second method is able to correct high-order residual aberrations as well as to perform through-focus scanning without z-axis movement, while the first method is easier to implement in pre-existing wafer inspection systems with only minor modification.
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Jun Ho Lee, Jun Hyung Park, Dohwan Jeong, Eun Ji Shin, Chris Park, "Tip/tilt-compensated through-focus scanning optical microscopy", Proc. SPIE 10023, Optical Metrology and Inspection for Industrial Applications IV, 100230P (24 November 2016); doi: 10.1117/12.2247807; https://doi.org/10.1117/12.2247807
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