Surface micro-machined SU-8 2002 deformable membrane mirrors
Imaging systems are continually decreasing in size, especially in applications such as microscopy and cell phone cameras. Much research is being done to increase focus control capabilities of these instruments. This paper describes a wet-etch release and a dry-etch release fabrication technique for SU-8 2002 surface micro-machined deformable mirrors for focus control and compensation of focus-induced spherical aberration. Producing a good quality SU-8 2002 membrane layer proved difficult and a detailed discussion of the recipe development is presented. A thorough review of both release processes is also included. The dry-etch release process has high yield and realizes larger mirrors with greater than a two-fold improvement in stroke, relative to the wet-etch release. This paper presents deflection vs. voltage plots, mechanical frequency response measurements, surface roughness and flatness, and intrinsic stress for the 750 micron - 4.24 mm nominal dimension circular and elliptical boundary membrane mirrors. The use of a 3 mm x 4.24 mm elliptical boundary mirror for 45° incidence focus control in microscopy is also demonstrated. The intrinsic stress of the film indicates that devices of similar size should be capable of 30 micron displacement in the future. This would allow for sufficient focus control in high NA microscopy applications, while the mirror or mechanism for such control could fit into an endoscopic sized imaging system.