Fabrication of a SiO₂ optical window for controlling light transmission

This paper focuses on the compressive stress reduction in a SiO₂ optical window. The design and fabrication of the optical window for an optical modulator toward image sensing applications are reported. The optical window consists of micrometer-order SiO₂ capillaries (porous solids) that can modulate the transmission light intensity by moving a liquid in and out of the porous solid. A high optical transmittance can be achieved due to refractive index matching when the liquid is penetrated into the porous solid. Otherwise, its light transmittance is low because of light reflection and scattering by air holes and capillary walls. Silicon capillaries fabricated by deep reactive ion etching process are completely oxidized to form the SiO₂ capillaries. A large compressive stress of the oxide causes bending of the capillary structure, which is reduced by using thin supporting beams. A 7.2 mm × 9.6 mm optical window area toward a fully integrated with the image sensor is successfully fabricated, and a light modulation effect dependent on liquid penetration is clearly demonstrated in visible region (wavelength range from 450 to 650 nm).