Microfabrication and characterization of single-mask silicon microlens arrays for the IR spectra

Microlens Arrays in silicon are suitable for an important wavelength range within the IR spectrum, since silicon features relatively high refractive index and is transparent at the aimed wavelengths, leading to microlenses with a focal length short enough to allow compact systems, offering an alternative for applications where miniaturization and reduction of alignment and packaging costs are necessary. The microlenses are meant to sample and focus an IR beam on a focal plane array, which might be an image sensor, or a dedicated IR sensor, as for instance lab-on-chip, or a selective gas detection system. Nowadays refractive microlenses are manufactured using sophisticated techniques with relatively high costs and complexity of well controlled steps, like thermal reflow, and grayscale lithography. We hereby propose an alternative solution for microfabrication of silicon microlens arrays, with a single-mask step using KOH anisotropic etching of Si. The proposed technique solves many current demands, like achieving high reproducibility, fill-factor close to 100%, and higher precision of focal axes alignment. We have made optical profilometric measurements to estimate the shape, roughness and the focal distance. We have also observed the focal points imaging in the IR spectrum, proving that the silicon microlenses actually yield the results expected.