A new precision bonding process developed by university researchers has been used in the design and fabrication of piezoelectric microactuators. With the largest unit only a hair’s breadth wide, the microactuators, built by researchers at Pennsylvania State University (University Park, PA; www.psu.edu), are based on pairing commercially available materials with existing micromachining technology.
The microactuators provide controlled force, high resolution, and large displacements. Potential medical product applications include microfluidic pumps and valves, such as those found in insulin pumps, diagnostic equipment, and portable oxygen generators.
“These new piezoelectric microactuators are the first to use microfabrication methods, which are used to make computer chips and micromachines from silicon-based materials,” says Srinivas A. Tadigadapa, associate professor of electrical engineering and developer of the bonding process and microactuators. “Our new low-temperature wafer-bonding techniques, which make the actuators possible, can also be used for precision integration of dissimilar materials in other microelectromechanical systems.”
The microactuators are made from flat strips of bulk PZT, a commercially available piezoelectric material that shrinks slightly when voltage is applied. A micromachined silicon beam is bonded at both ends to a relatively thick PZT substrate using solder bonding techniques. When the PZT substrate is actuated by applying an electric field across it, the resulting compressive stress on the silicon beam causes it to buckle and results in a convex deflection.
For the bonding process utilized in making the microactuators, the researchers used photolithography and low-temperature solders to produce the required bridge shape. “The PZT loses polarity if you heat it too high, so the temperature is crucial,” says Tadigadapa. As a result, the researchers used a low-temperature solder bonding process at 200° C.
Using the new bonding process, the researchers have built actuators with dimensions ranging from 350 to 600 µm long, 50 to 100 µm wide, and 5 to 6 µm thick. The bandwidth of an actuator was measured at 265 KHz. In tests, the actuators showed good repeatability with a large amplitude stroke of about 8 µm when actuated using –100 to 100 V.
The actuators and bonding process are described in a paper, “Fabrication and Performance of a Flextensional Microactuator,” which appears in the October issue of Journal of Micromechanics and Microengineering. The paper was written by Tadigadapa, Jongpil Cheong, Abhijat Goyal, and Christopher D. Rahn.