Process study and optimization of ICP etching chemistries for continuous curved-profile transfer of InP microlenses

Microlens arrays can redistribute incident light and concentrate it onto the effective photosensitive region, making them essential for achieving high optical coupling efficiency in large-format, small-pixel InP-based photodetectors. In this work, InP microlens arrays were fabricated by photoresist thermal reflow followed by inductively coupled plasma etching. Three etching chemistries, CH 4 /H 2 /Cl 2 , Cl 2 , and Cl 2 /BCl 3 , were systematically compared for continuous curved-profile transfer, with emphasis on mask stability, etch selectivity, and surface quality. By optimizing the Cl 2 /BCl 3 gas flow ratio, the InP-to-photoresist etch-rate ratio was adjusted to the range required for proportional profile transfer. An intermittent cyclic etching process was then introduced to reduce particle-like surface residues, mitigate cumulative byproduct accumulation, and limit local heat buildup during prolonged etching. Under the optimized conditions, the surface roughness of the microlens was reduced from 12.7 nm to 2.9 nm. The fabricated InP microlens array exhibited an aperture of 95.4 μ m, with a fill factor of 91%. The sag-height error and focal-length error were approximately 1.6% and 0.2%, respectively, at 1064 nm. These results demonstrate that the combination of Cl 2 /BCl 3 chemistry and cyclic etching provides a reproducible and high-fidelity route for fabricating InP microlens arrays, which is promising for integration into next-generation small-pixel InP-based photodetectors.