SU-8 2002 Surface Micromachined Deformable Membrane Mirrors

This paper describes two surface micromachining processes, i.e., one using a wet-etch release and the other using a dry-etch release, to create deformable membrane mirrors made from a thin film of low-stress SU-8 2002. The mirrors are designed for electronic focus and aberration control in imaging systems, and exhibit a large range of motion and high optical quality. The processes result in free-standing membrane mirrors with in-plane film stress as low as 12.5 MPa while attaining well-defined lithographic features as small as 3 μm in a 2.5-μm-thick film. We achieved a maximum deflection of 14.8 μm for a 3-mm × 4.24-mm elliptical boundary mirror, limited by electrostatic pull-in. Using a 3-mm × 4.24-mm mirror, which is designed for operation with a circular beam at 45° incidence angle, we demonstrate focus control while compensating spherical aberration in an optical microscope over a depth of 137 μm using a 50× 0.4-NA objective lens.