Microfabricated ultrasonic transducers and related apparatus and methods
Micromachined ultrasonic transducers integrated with complementary metal oxide semiconductor (CMOS) substrates are described, as well as methods of fabricating such devices. Fabrication may involve two separate wafer bonding steps. Wafer bonding may be used to fabricate sealed cavities in a substrate. Wafer bonding may also be used to bond the substrate to another substrate, such as a CMOS wafer. At least the second wafer bonding may be performed at a low temperature.
1. A capacitive micromachined ultrasonic transducer (CMUT) substrate, comprising:
a first substrate having a plurality of ultrasonic transducer cavities formed in a first thermal oxide layer, the plurality of ultrasonic transducer cavities bounded on at least one side by a flexible membrane; and
a second substrate having a second thermal oxide layer, bonded to the first thermal oxide layer, defining an oxide-to-oxide bond that seals the plurality of cavities, the plurality of ultrasonic transducer cavities having an internal pressure from about 1×10 −3 Torr to about 1×10 −5 Torr.
2. The CMUT substrate of claim 1 , wherein at least one of the first substrate or the second substrate is a silicon-on-insulator (SOI) substrate.
3. The CMUT substrate of claim 1 , wherein at least one of the first substrate or the second substrate is a bulk silicon substrate.
4. The CMUT substrate of claim 1 , wherein the first substrate is a silicon-on-insulator (SOI) substrate, and the second substrate is a bulk silicon substrate.
5. The CMUT substrate of claim 4 , wherein the second substrate includes a thinned bulk silicon layer that comprises a bottom electrode for the ultrasonic transducer cavities.
6. The CMUT substrate of claim 5 , wherein the flexible membrane comprises a silicon device layer.
7. The CMUT substrate of claim 6 , further comprising a plurality of isolation structures formed in the bottom electrode so as to electrically isolate sections of the bottom electrode corresponding to individual ultrasonic transducer cavities.
8. The CMUT substrate of claim 7 , wherein the isolation structures extend through the thinned bulk silicon layer comprising the bottom electrode.
9. The CMUT substrate of claim 8 , wherein the isolation structures comprise trenches within the thinned bulk silicon layer that are filled with an insulating layer.
10. The CMUT substrate of claim 8 , wherein the bottom electrode comprises doped sections of the thinned bulk silicon layer and the isolation structures comprise undoped sections of the thinned bulk silicon layer.