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. An apparatus comprising:
a wafer comprising first and second silicon device layers;
a dielectric layer disposed between the first and second silicon device layers; and
a plurality of sealed cavities formed between the first and second silicon device layers, wherein a first sealed cavity of the plurality of sealed cavities comprises a recess in the first silicon device layer and extends through the dielectric layer disposed between the first and second silicon device layers, wherein a recessed surface of the recess in the first silicon device layer represents a first sealed end of the first sealed cavity.
2. The apparatus of claim 1 , wherein the first and second silicon device layers are doped with at least 10 15 dopants/cm 3 .
3. The apparatus of claim 1 , further comprising a metallization layer electrically connected to the second silicon device layer through a bonding material.
4. The apparatus of claim 1 , wherein the first sealed cavity comprises an outer cavity portion extending through the dielectric layer and in part through the first silicon device layer, and an inner cavity portion extending through the dielectric layer but not through the first silicon device layer.
5. The apparatus of claim 4 , wherein the outer cavity portion has a greater depth than the inner cavity portion.
6. The apparatus of claim 4 , wherein the outer cavity portion at least partially surrounds the inner cavity portion.
7. The apparatus of claim 6 , wherein the outer cavity portion has a width that is between 100 nm and 500 μm.
8. The apparatus of claim 7 , wherein the inner cavity portion has a width that is less than 400 μm.
9. The apparatus of claim 1 , further comprising a clear out region extending through the first silicon device layer, the dielectric layer, and the second silicon device layer.
10. The apparatus of claim 1 , further comprising a passivation layer disposed over the first silicon device layer and covering the first sealed cavity of the plurality of sealed cavities.