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Abstract
High-precision aligned wafer bonding is essential to heterogeneous integration, with the device dimension reduced continuously. To get the alignment more accurately and conveniently, we propose a moiré-based alignment method using centrosymmetric grating marks. This method enables both coarse and fine alignment steps without requiring additional conventional cross-and-box alignment marks. Combined with an aligned wafer bonding system, alignment accuracy better than 300 nm (3σ) was achieved after bonding. Furthermore, the working principle of the moiré-based alignment for the backside alignment system was proposed to overcome the difficulty in bonding of opaque wafers. We believe this higher alignment accuracy is feasible to satisfy more demanding requirements in wafer-level stacking technologies.
Highlights
Wafer bonding is regarded as one of the most promising technologies for three-dimensional (3D) heterogeneous integration [1,2,3,4]
A variety of wafer alignment tools were developed for high-precision wafer bonding, in which the alignment accuracy was improved to sub-μm range, even using conventional alignment marks [8,9]
The alignment and bonding chamber is the main part of the wafer bonding system, which consists of a wafer transfer pin lift shaft, electrostatic chucks holding the top and bottom wafers, a piezoelectric walking table serving as the alignment table, IR camera/table for detection of alignment marks, and a bonding head to load the pressure
Summary
Wafer bonding is regarded as one of the most promising technologies for three-dimensional (3D) heterogeneous integration [1,2,3,4]. Conventional alignment marks (e.g., “cross-and-box” or “cross-on-cross” structures) are pre-fabricated in the top and bottom wafers. Optical or infrared (IR) microscopes are used to detect the alignment marks. Complex sub-pixel peak estimation algorithms were employed in the image detection process. A variety of wafer alignment tools were developed for high-precision wafer bonding, in which the alignment accuracy was improved to sub-μm range, even using conventional alignment marks [8,9]. Moiré fringes generated by the overlaying of two sets of gratings with slightly different periods have extremely high alignment sensitivity more than just optics [10,11]. A variety of moiré fringe measurement methods were developed for alignment in proximity optical, x-ray, and nanoimprint lithography [12,13,14]
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