Sacrificial-Etch Fabrication Process for CMUTs

Fabricating CMUTs with wafer-bonding is relatively new [1]. In contrast to surface micromachining, the cavity and membrane are defined on separate wafers. Wafer-bonding, considered as a bulk process, is widely used in micromachining, and older than surface micromachining techniques. There are three basic wafer bonding techniques: anodic bonding, fusion bonding and adhesive bonding. Among these, silicon fusion bonding has earned a stable position in today's technology, and is now used for various applications: bond-and-etchback silicon-on-insulator (SOI) wafers, SMART-CUT SOI wafers, power devices, and many silicon microstructures, such as pressure sensors and accelerometers. Silicon fusion bonding has many important advantages over surface micromachining techniques. Along with stability, wafer-bonding allows easier fabrication of complex structures (e.g., membranes) out of single crystal silicon. Single-crystal silicon as a mechanical material has been extensively studied, and is very well characterized. Its mechanical and electrical properties are consistent on a wafer, from wafer to wafer, and from time to time; important characteristics which surface micromachining does not exhibit.

The fabrication of cMUTs using the wafer-bonding technique benefit from the advantages associated with silicon fusion bonding. The process begins with two wafers: a prime quality silicon wafer and a SOI wafer. The cavity is produced on a prime quality, highly conductive silicon wafer using thermal oxidation and photolithographic patterning. Thermal oxidation is a very well characterized and controlled process, thus the cavity depth, which is the gap height, can be controlled precisely. The membrane is the active silicon layer of an SOI wafer subsequently bonded to the other wafer in vacuum. The backside of the SOI wafer and the buried oxide layer, called the handle and BOX respectively, are then removed leaving the membrane bonded to the prime wafer with vacuum gaps inside the cavities. Thus, the membrane thickness is determined by the very well controlled active layer thickness of the SOI wafer. The process ends with metal deposition over the membranes, patterning, and definition of the array elements. The number of processing steps is less than that of sacrificial release process, and the lead time is significantly reduced. The basic fabrication steps are shown in Fig. 1. Better control over process parameters and physical dimensions of the transducers, faster lead time, and higher yield make wafer-bonding a very viable production process. In recent years we have fabricated one-dimensional CMUT arrays with very high yield and uniformity using this process. One example is a 172-element low-frequency under-water imaging array designed to work from 0.6 to 1.2 MHz. The measured bandwidth was between 0.4 to 3 MHz, significantly wider than required. We have also fabricated high-frequency (10-50 MHz) linear and ring arrays using the wafer-bonding process [2]. Some of these arrays were also thinned down to demonstrate the feasibility of wafer thinning for applications that have tight packaging requirements. A detailed review of CMUT fabrication processes can be found in [3].

FIGURE 1. Fabricating cMUTs using the wafer-bonding technique: (a) First thermal oxidation step and cavity definition with photolithography and etch. (b) Second thermal oxidation to create the insulation layer. (c) Silicon direct bonding of the patterned prime wafer to the un-patterned SOI wafer. (d) Removal of the handle and the BOX of the SOI wafer to release the membranes. (e) Ground contact definition, top electrode deposition and patterning. (f) Element definition by photolithography and silicon etch.

Resources

[1] Huang Y, Ergun AS, Haeggström E, Badi MH, and Khuri-Yakub BT, "Fabricating Capacitive Micromachined Ultrasonic Transducers with Wafer-Bonding," IEEE/ASME Journal of Microelectromechanical Systems, vol. 12, no.2, pp. 128-137, Apr. 2003.

[2] Yeh D, Oralkan Ö, Ergun AS, Zhuang X, Wygant IO, Cheng CH, Huang Y, Yaralioglu GG, and Khuri-Yakub BT, “High-frequency CMUT arrays for high-resolution medical imaging,” Proc. SPIE Medical Imaging Conference, pp. 87-98, 2005.

[3] S. Ergun, Y. Huang, X. Zhuang, Ö. Oralkan, G. G. Yaralioglu, and B. T. Khuri-Yakub, “Capacitive micromachined ultrasonic transducers: fabrication technology,” IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 52, no. 12, pp. 2242-2258, Dec. 2005.

Acknowledgements

This work was supported by the U.S Office of Naval Research and National Institutes of Health.