Fabricating Biologically Inspired Artificial Haircell Sensors

June 1, 2009

Air Force Research Laboratory

As electronics packaging and equipment have decreased in size and weight, so have the potential dimensions of unmanned aircraft. Specifically, a classification of micro air vehicles (MAVs) has emerged that limits the scale of the aircraft to approximately 6" (15 cm). A project is underway aimed at developing artificial haircell flow sensors following biological inspiration of insect flow sensors, and demonstrate the potential of these sensors for controlling the flight of MAVs.

Photo of a packaged Haircell Sensor. Electrical leads are provided by wire bonding.

Artificial haircell sensors were developed based on both polymer and silicon materials. A scanning electron micrograph of a silicon haircell sensor consists of a silicon cantilever with a vertical hair at its distal end. The vertical hair is made of photodefinable epoxy material (SU-8). Fluid flow acting on the vertical hair introduces drag force to it and develops a bending torque on the cantilever. The torque in turn develops surface stress at the base of the cantilever.

Individual silicon sensors are then packaged to provide robustness and electrical access. The cantilever beam is 200 μm long, 20 μm wide, and 2 μm thick. The hair is typically 100-750 μm tall. The height of the hair is controlled by the thickness of the deposited layer. The resonant frequency of the cantilever is approximately 1-5 kHz, depending on the dimensions of the cantilever and the hair. A high-yield process for fabricating the sensors has been invented. The sensor is fabricated using a silicon-on-insulator wafer (SOI), with the top silicon layer’s thickness being the thickness of the silicon cantilever.

First, the SOI wafer is selectively doped to form piezoresistors for strain sensing. Secondly, the wafer is photolithographically defined using photoresist and etched with plasma etching until the underlying silicon oxide insulator layer is exposed. The backside of the wafer is then patterned and etched using deep reactive ion etching method to define the backside cavity. A front side deposition of photodefinable epoxy is carried out, followed by photo exposure to define the hairs. Development of photosensitive polymer is not performed immediately. The oxide is removed by backside etching. After this step, the wafer is diced into chips and then the photo definable epoxy is developed.

The sensors are packaged and placed on aerodynamic airfoils with flush mounting. Towards this end, the airfoil structures are first machined and then grooves of controlled dimensions are produced, which match those of the haircell chip. The sensorized airfoil is then placed in a flow tunnel. The wing is attached to a six-degrees-of-freedom balance that measures the mo ment and force applied to the wing.

This work was done by Chang Liu of the University of Illinois at Urbana-Champaign for the Air Force Research Laboratory. AFRL-0124