Out-of-focus 3D alignment for digital holography

Abstract

Image alignment plays a crucial role in the semiconductor industry. However, achieving high alignment accuracy with fine image resolution while maintaining the depth of field of optics can be challenging. A novel method utilizing digital holography is proposed to address this issue. This method allows for object retrieval without the need to adjust the focus of the optics, specifically targeting out-of-focus targets. The research builds upon the original phase correlation method applied to the extracted hologram and introduces a phase unwrapping algorithm on the phase difference buffer for curve fitting. This enhancement significantly improves accuracy in the axial direction while preserving lateral accuracy. Through simulation cases, the algorithm demonstrates subpixel accuracy in translation, rotation, and tilt measurements, showcasing its robustness against various sources of error. In the experimental phase, the algorithm’s accuracy is evaluated using a carefully designed setup and real samples. By aligning holograms with different x and z positions, a total of 1110 cases are processed. Results indicate an overall alignment accuracy of 0.1μm in x and y translation and 2μm in the z-axis. The experimental results demonstrate that hologram alignment can achieve higher accuracy than is currently demanded by the industry, highlighting promising applications. Although certain sources of error, such as distortion, need to be addressed, the study suggests satisfactory performance of the algorithm. By tackling these factors, further improvements can be made to enhance the alignment algorithm and broaden its potential applications in various industries.