Methods of fabricating these devices as well as microfabricated ultrasonic transducers integrated with complementary metal oxide semiconductor (CMOS) substrates are described. Fabrication may involve two separate wafer bonding steps. Wafer bonding can be used to fabricate cavities 306 sealed within the substrate 302. Wafer bonding can also be used to bond the substrate 302 to another substrate 304, such as a CMOS wafer. At least the second wafer bonding can be performed at a low temperature.
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