Lensless Through-Silicon Microscopy System for Precise Alignment in Photonic Integration Processes

The ongoing evolution of information technologies - driven by AI, cloud computing, and Big Data -has created a growing demand for compact, scalable photonic systems. To meet these needs, the photonics industry is shifting from discrete component solutions to highly integrated platforms. This transition places increasing emphasis on precise alignment, efficient assembly, and reproducible packaging processes compatible with high-throughput manufacturing. In this work, we present a compact lensless Through-Silicon Microscopy (TSM) system that utilizes coaxial bottom illumination to exploit silicon's transparency at wavelengths above <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1 \mu ~\mathrm{m}$</tex>. The system captures interference patterns through the silicon substrate and reconstructs high-contrast images using computational phase retrieval algorithms. Compared to conventional amplitude-based microscopy, the proposed method achieves a substantial contrast improvement - from 13 % to 28 % - when imaging photonic structures and bonding interfaces. Experimental results confirm the system's capability to resolve micrometer-scale features and enhance structural visibility, enabling accurate post-assembly inspection. Owing to its simplicity, compactness, and robustness, the approach is well suited for in-line verification in heterogeneous photonic integration workflows. The demonstrated solution provides a cost-effective and scalable tool for advanced silicon photonics packaging and supports future automation in photonic device manufacturing.