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Multifunctional LTCC Substrates for Thermal Actuation of Tunable Micro Lenses Made of Aluminum Nitride Membranes.
Keywords: tunable micro optics, integrated actuation, aluminum nitride
Thin membranes of polymers are widely used for tunable micro-lenses, where the membrane defines the surface of a subjacent liquid. Applying a pressure deflects the membrane and forms a lens in conjunction with the liquid. In many cases, a macroscopic pump generates the pressure. Only a few publications deal with the integrated pressure generation using piezoelectric or thermal actuation, though. In our approach we use a multifunctional LTCC substrate, which consists of two cavities: an optical cavity and a driving cavity for applying the pressure. A micro-fluidic network allows for separate filling with different fluids. For the actuation, the driving cavity is filled with a fluid (air, water and oil) and the optical cavity is filled with immersion oil. If the driving cavity is heated with screen-printed resistors, the fluid expands and a pressure is transmitted to the optical cavity via a micro-fluidic channel. The optical cavity is covered with a glass chip and a membrane chip. We use 500 nm thick membranes of Aluminum Nitride, which are 3 mm in diameter and fabricated with technologies of silicon based micro-electro-mechanical systems. With a power of 3 W a heating of 200 K within the driving cavity is achieved. The generated pressure causes a membrane deflection in the optical cavity of about 30 µm at its center. Using thermal actuation, thermal gradients along the membrane, which involve gradients in refractive index, have to be avoided. Hence, thermal actuation is investigated using thermal imaging. By heating the driving cavity, the optical cavity is heated as well, but no temperature gradients along the silicon chip are measured. Optical experiments show the imaging capability of the lens and tunable refractive power of 0..20 dpt. We gratefully acknowledge financial support of the German Research Foundation (DFG) within the priority program 1337 for “Tunable micro-optics” (HO2284/1-1).
Steffen Leopold,
Chair for Micromechanical Systems, Ilmenau University of Technology
Ilmenau, Thuringia

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