Initial deflection of silicon-on-insulator thin membrane micro-mirror and fabrication of varifocal mirror

Thin membranes fabricated from silicon-on-insulator (SOI) wafer are valuable for deformable mirrors. The mirror is controlled to generate a specific wave-front with precision smaller than a wavelength. Here, we investigate quantitatively the initial deflection of the thin membrane mirrors fabricated from SOI wafer, which are often used in micro-electro-mechanical systems. A 1-μm-thick and 450-μm-diameter mirror fabricated from SOI wafer deflects upward around the circumference at an angle of 0.12°. The maximum deflection of the mirror is 320 nm at the center. The stress conditions of the mirrors are analyzed on the basis of material strength theory. The deflection is explained by the residual stress of the buried oxide layer of SOI wafer. The in-plane stresses of the micro-mirrors of diameters from 450 μm to 860 μm range from compressive stress of 1.2 MPa to tensile stress of 2.1 MPa. Furthermore, based on the above experimental and theoretical analyses, a 1-μm-thick varifocal micro-mirror of the diameter of 400 μm is fabricated. The focus of the mirror is varied from -28 mm to 21 mm with the deviation smaller than 4 nm from parabola in the mirror central region.