Low-Temp Fabrication of Piezoelectric Microactuators Demonstrated

Anne L. Fischer

Scientists at Pennsylvania State University in University Park have produced flextensional microactuators the width of a human hair using a low-temperature bonding process. The devices feature a precision micromachined silicon beam coupled to lead zirconate titanate, a common piezoelectric material.

Tiny actuators that achieve large displacements with controlled force and high resolution are in demand for use in radio-frequency and optical switches for such products as cell phones and wide-screen televisions. Piezoelectric films often are employed in such microscale actuators, but the electromechanical coefficients of the films are much lower than those of the bulk material. Micromachining techniques for the bulk piezoelectric are not yet well developed.

The researchers were able to integrate precision micromachined silicon structures and piezoelectric substrates, but it was not until they demonstrated a new, low-temperature, solder bonding technique that they realized the full potential of micromachining piezoelectrics. Their process involves deep reactive ion etching and In/Sn solder bonding at 200 °C. They have fabricated bridge-shaped actuators 350 to 600 µm long, 50 to 100 µm wide and 5 to 6 µm thick.

The lead zirconate titanate shrinks slightly when voltage is applied, and this small in-plane strain is translated into a large out-of-plane displacement of the bonded silicon beam. With a bandwidth of 265 kHz, the devices showed good repeatability and a large amplitude stroke of about 8 µm when driven at –100 to 100 V.

The researchers believe that the low-temperature wafer bonding technique also can be used for the integration of other dissimilar materials in microelectromechanical systems, and that they may find their way into microfluidic pumps and valves, micromanipulators and atomic force microscope drives.