Design and fabrication of a large area freestanding compressive stress SiO₂ optical window

This paper reports the design and fabrication of a 7.2 mm × 9.6 mm freestanding compressive stress SiO₂ optical window without buckling. An application of the SiO₂ optical window with and without liquid penetration has been demonstrated for an optical modulator and its optical characteristic is evaluated by using an image sensor. Two methods for SiO₂ optical window fabrication have been presented. The first method is a combination of silicon etching and a thermal oxidation process. Silicon capillaries fabricated by deep reactive ion etching (deep RIE) are completely oxidized to form the SiO₂ capillaries. The large compressive stress of the oxide causes buckling of the optical window, which is reduced by optimizing the design of the device structure. A magnetron-type RIE, which is investigated for deep SiO₂ etching, is the second method. This method achieves deep SiO₂ etching together with smooth surfaces, vertical shapes and a high aspect ratio. Additionally, in order to avoid a wrinkling optical window, the idea of a Peano curve structure has been proposed to achieve a freestanding compressive stress SiO₂ optical window. A 7.2 mm × 9.6 mm optical window area without buckling integrated with an image sensor for an optical modulator has been successfully fabricated. The qualitative and quantitative evaluations have been performed in cases with and without liquid penetration.