Thin-Film Piezoelectric Unimorph Actuator-Based Deformable Mirror With a Transferred Silicon Membrane

This paper describes a proof-of-concept deformable mirror (DM) technology, with a continuous single-crystal silicon membrane reflecting surface, based on PbZr_0.52Ti_0.48O₃ (PZT) unimorph membrane microactuators. A potential application for a terrestrial planet finder adaptive nuller is also discussed. The DM comprises a continuous, large-aperture, silicon membrane "transferred" onto a 20times20 piezoelectric unimorph actuator array. The actuator array was prepared on an electroded silicon substrate using chemical-solution-deposited 2-mum-thick PZT films working in a d₃₁ mode. The substrate was subsequently bulk-micromachined to create membrane structures with residual silicon acting as the passive layer in the actuator structure. A mathematical model simulated the membrane microactuator performance and aided in the optimization of membrane thicknesses and electrode geometries. Excellent agreement was obtained between the model and the experimental results. The resulting piezoelectric unimorph actuators with patterned PZT films produced large strokes at low voltages. A PZT unimorph actuator, 2.5 mm in diameter with optimized PZT/silicon thickness and design showed a deflection of 5.7 mum at 20 V. A DM structure with a 20-mum-thick silicon membrane mirror (50 mm times50 mm area) supported by 400 PZT unimorph actuators was successfully fabricated and optically characterized. The measured maximum mirror deflection at 30 V was approximately 1 mum. An assembled DM showed an operating frequency bandwidth of 30 kHz and an influence function of approximately 30%