Asymmetric Microlens for Phase Detection Autofocus

The microlens is designed with an asymmetric height profile where the curvature radius in the first direction (R1) differs from the curvature radius in the second direction (R2), with R1 < R2. This asymmetric geometry creates different focal points in orthogonal directions, enabling the lens to differentiate between light rays arriving at different angles. The asymmetric structure transforms the microlens into an astigmatic lens that can resolve angular information while maintaining manufacturability through standard photolithography and reflow processes.

The microlens employs differential curvature where the first direction (along the line connecting the two photodiodes) has a larger curvature radius (R1) and the second direction (perpendicular to the first direction) has a smaller curvature radius (R2). This local quality variation optimizes the lens performance: the direction with larger R1 provides better angular discrimination by creating more separated focal points, while the direction with smaller R2 maintains sufficient focusing power to converge light effectively onto the photodiodes.

The microlens is fabricated by modifying the reflow parameters and block geometry to achieve the desired asymmetric curvature. A rectangular or square block of photoresist material is formed with dimensions that, when thermally reflowed, produce the asymmetric spherical cap shape with R1 < R2. The reflow temperature and duration are optimized to achieve the target curvature radii, transforming a simple manufacturing process into one that produces the complex asymmetric geometry required for rapid autofocus.