Non-destructive and accurate characterization of high aspect ratio (HAR) and composite micro-trenches is essential for semiconductor inspection in fields like microelectromechanical systems (MEMS) and micro-optoelectronic systems, offering valuable insights for production guidance and yield improvement. However, the strong optical modulation inherent in HAR microstructures poses significant challenges for morphological measurement. As a result, most optical techniques are restricted to retrieving only statistical averages of trench dimensions. Coherence scanning interferometry (CSI), a high-precision topographic tool, can provide three-dimensional (3D) topography of thick objects. Nevertheless, its application to HAR and composite microstructures is limited by low signal-to-noise ratio (SNR) and restricted lateral resolution.
In a new paper published in Light: Science & Applications , a team of scientists, led by Professor Zhishan Gao from the School of Electronic and Optical Engineering, Nanjing University of Science and Technology, China, and co-workers have presented Fourier ptychographic coherence scanning interferometry (FP-CSI), the first transmissive CSI framework that harnesses the complementary strengths of FPM and CSI. FP-CSI integrates two complementary ideas: the aperture synthesis strategy of Fourier ptychographic microscopy (FPM) and the quantitative phase-resolved capability of interferometry. In FP-CSI, multiple illumination angles provide dense spatial frequency coverage, and the corresponding information is fused in the Fourier domain to form a high-resolution quantitative phase map. Unlike conventional FPM workflows that rely on intensity-only measurements and iterative phase retrieval, FP-CSI directly recovers quantitative phase maps from interferograms, which helps avoid convergence instability and reduces computational overhead. This approach enables robust, high-resolution 3D morphology of HAR microstructures without iterative phase retrieval or prior information.
A key design choice is the transmissive interferometric configuration. Compared with reflective interferometric architectures, the transmissive setup minimizes sample-induced optical modulation, preserving signal-to-noise ratio and enhancing interference contrast. This is particularly valuable for characterizing deep trenches, vertical vias, and multilayer MEMS devices, where strong modulation in reflective arrangements can degrade fringe visibility. By combining the transmissive configuration with angular spectrum scanning and frequency-domain fusion, FP-CSI alleviates the CSI trade-off between lateral resolution and signal quality.
Using FP-CSI, the team demonstrates precise 3D morphology measurements of a high aspect ratio micro-trench with an aspect ratio of 30:1, as well as multilayer MEMS devices exhibiting aspect ratios ranging from 6:1 to 20:1. This method achieves lateral resolution up to the incoherent diffraction limit and maintains that performance even at the bottoms of the trenches.
“FP-CSI operates in transmission mode at the probing wavelength, which necessitates that samples be transparent. This characteristic makes the method particularly suitable for quality control prior to metallization. Looking ahead, we expect to see increased throughput and automation. The result includes enhancements such as faster illumination steering, which could be achieved by substituting mechanical motion with digital micromirror devices, reduced sampling through established FPM acceleration strategies, and the use of GPU-accelerated reconstruction pipelines.” the scientists forecast.
Fourier ptychographic coherence scanning interferometry for 3D morphology of high aspect ratio and composite micro-trenches