Nanometer inspection by deep-ultraviolet reflective Fourier ptychographic microscopy

Reflective Fourier ptychographic microscopy (rFPM) has high potential for semiconductor metrology and inspection due to its ability to achieve high resolution and wide field-of-view (FOV) simultaneously. While ultraviolet (UV) wavelength sources are required to improve resolution below 200 nm, this leads to low signal-to-noise ratio signals, which causes degradation in phase measurement accuracy. Low SNR signals also challenge iterative computational techniques like FPM, as they require significant time to find the global optimal solution. To address this problem, we introduce the Momentum Fourier Ptychographic Topography (MFPT) algorithm for nanometer inspection. This method has been proven to overcome convergence to local minima in simulation even under noisy conditions and acquire accurate phase and amplitude, ensuring high accuracy in height measurements. MFPT also eliminates the need for total variation regulation in the iterative process, saving substantial computational time. We conducted actual experiments with the MFPT algorithm using a 365 nm UV source rFPM system to inspect the United States Air Force (USAF) 1951 target and two step-height samples. The combination of UV source and MFPT algorithm rapidly achieved a resolution of 173 nm on the USAF target and was able to inspect step-height samples of 30 nm and 70 nm.