| AVS 54th International Symposium | |
| MEMS and NEMS | Monday Sessions |
| Session MN-MoA |
| Session: | Materials Processing, Characterization and Fab Aspects |
| Presenter: | W.-C. Tian, General Electric Co. |
| Authors: | X. Wang, General Electric Co. Y. Fan, General Electric Co. W.-C. Tian, General Electric Co. H. Kwon, General Electric Co. S. Kennerly, General Electric Co. G. Claydon, General Electric Co. A. May, General Electric Co. |
| Correspondent: | Click to Email |
We report a capacitive micromachined ultrasound transducer (CMUT) for air-coupled, noncontact, nondestructive evaluation (NDE) applications. Air-coupled ultrasound is an attractive inspection technique for materials or structures that are not suitable for contact or immersion ultrasound inspections, such as the honeycomb composites used in aircraft structures. In the past CMUTs have been extensively studied for water-coupled, immersion applications.1 In principle, CMUTs have better acoustic impedance match with air compared with piezoelectric ultrasound transducers. This makes them ideally suited for air-coupled NDE. However, they are not widely used in air inspection due to large air attenuation of acoustic power. To overcome this problem, CMUTs’ transduction efficiency must be improved. We have successfully developed a large gap air-coupled CMUT. The device has a 1MHz operation frequency and can function both as ultrasound transmitter and receiver. The CMUT structure employs a large gap and a specially patterned SiO2 layer to provide large acoustic output while avoiding dielectric charging. A wafer-bonding process was used in fabrication.2-4 In this process, a Si substrate was first etched to create cavities and oxidized. The SiO2 on the cavity floor was patterned to form pillar structures, which prevented arcing and charging in actuation. An SOI wafer was then bonded to the substrate wafer followed by handle wafer removal and metallization steps to complete the device. In our tests, CMUTs with patterned SiO2 insulation layers showed consistent operation over time. In comparison, control devices with un-patterned blanket SiO2 layers suffered from unsynchronized cell vibrations and fast signal decay due to dielectric charging. Acoustic though-transmission tests with paired CMUTs showed a loop gain of -51dB. As a comparison, a pair of state-of-the-art commercial air-coupled ultrasound transducers showed a -65 dB loop gain, 14 dB lower than the CMUTs reported here. This clearly indicates that improvement of air-coupled CMUTs is achievable and can lead to wide use of CMUTs for air inspections.
1 X. Jin, et al., J. Microelectromech. Syst., 8, 100 (1999).
2 Y. Huang, et al., J. Microelectromech. Syst., 12,128 (2003).
3 Y. Huang, et al., IEEE T. Ultrason. Ferr., 52, 578 (2005).
4 W-C. Tian, et al., US Patent Publication #2006/0004289 (2006).