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Piezoelectric unimorph deformable mirror concept by wafer transfer for ultralarge space telescopes

Future concepts of ultra large space telescopes include the telescopes with segmented silicon mirrors and inflatable polymer mirrors. Primary mirrors for these systems cannot meet optical surface figure requirements and are likely to generate over several microns of wavefront errors. In order to correct for these large wavefront errors, high stroke optical quality deformable mirrors are required. The deformable mirror in this paper consists of an optical quality membrane mirror backed by a piezoelectric unimorph actuator array. A fabrication technology has been recently developed for transferring an entire wafer-level mirror membrane from one substrate to another. A thin membrane, 100 mm in diameter, has been successfully transferred without using adhesives or polymers. The measured peak-to-valley surface figure error of a wafer-level transferred and patterned membrane (1 mm × 1 mm × 0.016 mm) is only 9 nm. The mirror element actuation principle is based on a piezoelectric unimorph. A voltage applied to the piezoelectric layer induces stress in the longitudinal direction causing the film to deform and pull on the mirror connected to it. The advantage of this approach is that the small longitudinal strains obtainable from a piezoelectric material at modest voltages are thus translated into large vertical displacements. Modeling is performed for unimorph membranes consisting of clamped rectangular membranes with piezoelectric layers with variable dimensions. The membrane transfer technology combined with the piezoelectric unimorph actuator concept constitutes the successful demonstration of high quality, compact, large stroke continuous deformable mirror devices, resulting in compact AO systems for use in ultra large space telescopes.