Multifunctional LTCC Substrates for Thermal Actuation of Tunable Micro-Lenses Made of Aluminum Nitride Membranes

Abstract

Thin membranes are widely used for tunable micro-lenses, where the membrane, usually made from polymers, defines the surface of a subjacent liquid. If the liquid is pressurized, the membrane deflects and forms a lens. In many cases, a macroscopic pump is used to generate the pressure. Here, we use a multifunctional LTCC substrate, which consists of two cavities. A micro-fluidic network allows their independent filling. For the actuation, the actuating cavity is filled with air and the optical cavity is filled with immersion oil. If the actuating cavity is heated by 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, respectively. We use 500 nm thick membranes of aluminum nitride (AlN), which have a 3 mm diameter and are fabricated using technologies of silicon based micro-electro-mechanical systems. With 6 W electrical heating power a temperature increase of 100 K within the actuating cavity is achieved. The generated pressure causes a membrane deflection in the optical cavity of about 30 μm. The thermal actuation is investigated using thermal imaging. By heating the actuating cavity, the optical cavity also heats up, but no temperature gradients along the silicon chip is found. Optical experiments show the imaging capability of the lens and confirm a tunable refractive power of 0..17 dpt.