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 method of fabricating an ultrasound device, the method comprising:
forming, in a first wafer having a first silicon device layer and a dielectric layer, a plurality of cavities by completely etching through a first thickness of the dielectric layer and by partially etching through a second thickness of the first silicon device layer, wherein at least one of the plurality of cavities comprises an inner cavity portion and an outer cavity portion that at least partially surrounds the inner cavity portion, the outer cavity portion having a greater depth than the inner cavity portion, and wherein forming the plurality of cavities comprises forming the outer cavity portion by completely etching through the first thickness of the dielectric layer and by partially etching through the second thickness of the first silicon device layer and forming the inner cavity portion by completely etching through the first thickness of the dielectric layer until at least a portion of the first silicon device layer is uncovered; and
bonding a second wafer, having a second silicon device layer, with the first wafer so that the plurality of cavities are disposed between the first silicon device layer and the second silicon device layer.
2. The method of claim 1 , wherein bonding the second wafer with the first wafer comprises annealing the bonded first and second wafers at a temperature greater than 500° C.
3. The method of claim 1 , wherein bonding the second wafer with the first wafer comprises forming a Si—SiO 2 bond or a SiO 2 —SiO 2 bond.
4. The method of claim 1 , further comprising thinning a portion of the second silicon device layer.
5. The method of claim 1 , further comprising bonding a CMOS wafer with the bonded first and second wafers at a temperature that is less than 450° C.
6. The method of claim 5 , further comprising placing a bonding material in contact with the second silicon device layer, and wherein bonding the CMOS wafer with the bonded first and second wafers comprises placing a metallization layer of the CMOS wafer electrically in contact with the bonding material.
7. The method of claim 1 , wherein the first wafer is a silicon-on-insulator (SOI) wafer and the second wafer is a bulk silicon wafer.
8. The method of claim 1 , wherein the outer cavity portion has a width that is between 100 nm and 500 μm.
9. The method of claim 8 , wherein the inner cavity portion has a width that is less than 400 μm.
10. The method of claim 1 , further comprising forming a passivation layer to cover the plurality of cavities.