Through-focus scanning optical  microscopy (TSOM) with adaptive optics

Jun Ho Lee; Gyunam Park; Junhee Jeong; Chris Park

doi:10.1117/12.2282527

13 March 2018 Through-focus scanning optical microscopy (TSOM) with adaptive optics

Jun Ho Lee, Gyunam Park, Junhee Jeong, Chris Park

Proceedings Volume 10585, Metrology, Inspection, and Process Control for Microlithography XXXII; 105850C (2018) https://doi.org/10.1117/12.2282527
Event: SPIE Advanced Lithography, 2018, San Jose, California, United States

Abstract

Through-focus optical microscopy (TSOM) with nanometer-scale lateral and vertical sensitivity levels matching those of scanning electron microscopy has been demonstrated to be useful both for 3D inspections and metrology assessments. In 2014, funded by two private companies (Nextin/Samsung Electronics) and the Korea Evaluation Institute of Industrial Technology (KEIT), a research team from four universities in South Korea set out to investigate core technologies for developing in-line TSOM inspection and metrology tools, with the respective teams focusing on optics implementation, defect inspection, computer simulation and high-speed metrology matching. We initially confirmed the reported validity of the TSOM operation through a computer simulation, after which we implemented the TSOM operation by throughfocus scanning of existing UV (355nm) and IR (800nm) inspection tools. These tools have an identical sampling distance of 150 nm but have different resolving distances (310 and 810 nm, respectively). We initially experienced some improvement in the defect inspection sensitivity level over TSV (through-silicon via) samples with ~ 6.6 μm diameters. However, during the experiment, we noted sensitivity and instability issues when attempting to acquire TSOM images. As TSOM 3D information is indirectly extracted by differentiating a target TSOM image from reference TSOM images, any instability or mismatch in imaging conditions can result in measurement errors. As a remedy to such a situation, we proposed the application of adaptive optics to the TSOM operation and developed a closed-loop system with a tip/tilt mirror and a Shack-Hartmann sensor on an optical bench. We were able to keep the plane position within in RMS 0.4 pixel by actively compensating for any position instability which arose during the TSOM scanning process along the optical axis. Currently, we are also developing another TSOM tool with a deformable mirror instead of a tip/tilt mirror, in which case we will not require any mechanical scanning.

Conference Presentation

Citation Download Citation

Jun Ho Lee, Gyunam Park, Junhee Jeong, and Chris Park "Through-focus scanning optical microscopy (TSOM) with adaptive optics", Proc. SPIE 10585, Metrology, Inspection, and Process Control for Microlithography XXXII, 105850C (13 March 2018); https://doi.org/10.1117/12.2282527

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