Conference Agenda

This work compares signal formation in focus variation microscopy and coherence scanning interferometry using a unified experimental setup and numerical modeling. Simulated intensity cross-sections are compared with experimental FVM data and validated by CSI measurements. The results underline the trade-offs in selecting suitable modeling and experimental approaches.

Hybrid integration of III–V light sources on silicon photonic circuits demands sub-micrometer alignment, low thermal distortion, and reliable post-bond inspection. We present a compact through-silicon optical metrology platform integrated with bottom-irradiation laser-assisted bonding (LAB) for precision assembly of silicon photonic devices. The approach combines near- and short-wave infrared imaging with localized solder reflow, enabling both alignment control before bonding and non-destructive quality assessment after joining. Unlike global-heating methods such as thermo-compression bonding, the proposed process concentrates heat at the bonding interface, reaching approximately 300 °C in less than 1 s while minimizing thermal load on the overall assembly. In experiments with multichannel III–V laser-diode chips bonded onto silicon photonic circuits, full solder reflow was achieved across all four pads with post-bond surface misalignment below 0.3 µm. Mechanical characterization showed shear forces above 7 N, with the best samples reaching above 10 N (23 MPa). Through-silicon microscopy further revealed clear differences between bonding modes, with thermo-compression-bonded references showing stronger signatures of solder spreading, delamination, and stress-related fringe formation. These results demonstrate that through-silicon optical metrology can serve as an in-tool enabler of alignment-sensitive, quality-aware, and scalable hybrid photonic integration.

Advanced packaging processes such as hybrid bonding require defect-free surfaces with nanometre-level cleanliness. Conventional bright-field and dark-field inspection struggle on patterned substrates: bright-field contrast is dominated by the nominal pattern, while dark-field is overwhelmed by diffracted orders from periodic structures. We apply digital holography to this problem, using differential-phase processing to suppress the nominal substrate geometry and isolate defect-induced optical path differences with sub diffraction-limit sensitivity. We present detection results, demonstrating that weak (transparent or sub-resolution) defects invisible in intensity imaging are revealed in the phase channel.

High-accuracy characterization of optical grating pitch is increasingly important for the quality control of diffractive optical elements, particularly in the emerging augmented reality optics. To address the limitations of conventional measurement methods, such as AFM and diffractometry, we present a snap-shot interferometric method utilising polarization camera, that provides high-density wide-area pitch data in rapid acquisition, eliminating the need for sample scanning. Phase sensitive interferometer utilises a polarizer and wave-plates with polarizing beam splitter to control the polarization of the beams in the test and reference arms. A polarization camera records 4 interferograms with consecutive phase shifts of 90° without any moving components. The interferograms allow for calculation of the phase map of the beam diffracted from the sample. When measured with calibrated rotary table in Littrow configuration for diffraction orders m = ±1, the phase maps can be used to calculate the traceable accurate pitch information across the field of view.

Calibration and alignment of scanning beam interference lithography (SBIL) systems remain major challenges due to the high sensitivity of the interference pattern to multiple degrees of freedom. In this contribution, we present a fringe observation system as a quantitative tool for the calibration and alignment of SBIL writing heads. It consists of a compact microscope mounted on a nanopositioning machine, enabling direct imaging of the three-dimensional aerial writing pattern onto a camera. Fringe motion is quantified using the carrier-frequency method. Based on this approach, a comprehensive calibration protocol was developed to characterize key degrees of freedom, including fringe orientation, fringe period, pattern tilt, and positioning errors during the scan-and-stitch process. The writing pattern was aligned parallel to the scan direction to ensure optimal exposure contrast. The fringe period and pattern tilts were determined with nanometer precision, and scan-and-stitch positioning errors were evaluated along a classical SBIL trajectory. The calibration procedure was validated by exposure tests in positive photo resist, demonstrating high structural quality and excellent agreement between structures and optical measurements. The presented fringe observation system provides a powerful tool for SBIL calibration and forms the basis for future implementation of compensation strategies to further improve fabrication accuracy.