Ultrasonic transducer, ultrasonic probe and method for fabricating the same
View Patent ↗In an ultrasonic transducer including a gap between an upper electrode and a lower electrode on a silicon substrate, it is made possible to reduce or adjust warpage of an above-gap membrane vibrated by electrostatic actuation due to internal stress. A fourth insulating film and a fifth insulating film of films positioned above the gap which is a cavity required for transmitting and receiving ultrasonic are respectively a silicon oxide film for compression stress and a silicon nitride film for tensile stress. Therefore, compression stress and tensile stress cancel each other, so that warpage of the above-gap membrane is reduced. An amount of warpage can be adjusted by adjusting a film thickness of the fourth insulating film and a film thickness of the fifth insulating film.
1. An ultrasonic transducer for transmitting and receiving ultrasonic comprising:
a semiconductor substrate;
a first insulating film provided on the semiconductor substrate, wherein the first insulating film comprises silicon oxide;
a lower electrode provided on the first insulating film;
a second insulating film provided on the lower electrode, wherein the second insulating film comprises silicon oxide;
a gap provided above the second insulating film;
a third insulating film provided on the gap, wherein the third insulating film comprises silicon oxide;
an upper electrode provided above the third insulating film;
a fourth insulating film provided above the upper electrode, wherein the fourth insulating film is a compression stress layer, the fourth insulating film comprising silicon oxide; and
a fifth insulating film provided on the fourth insulating film, wherein the fifth insulating film is a tensile stress layer,
wherein the third insulating film, the upper electrode, the fourth insulating film and the fifth insulating film above the gap comprise a vibratable membrane.
2. The ultrasonic transducer according to claim 1 ,
wherein the fifth insulating film is a silicon nitride film.
3. The ultrasonic transducer according to claim 1 ,
wherein the thickness of either the fourth insulating film or the fifth insulating film is greater at the peripheral portion of the gap than at a central portion of the gap.
4. The ultrasonic transducer according to claim 1 ,
wherein an area of the upper electrode is at least 70% of an area of a horizontal face of the gap.
5. The ultrasonic transducer according to claim 1 ,
wherein an end portion of the upper electrode is positioned beyond an end portion of the gap.
6. An ultrasonic probe including the ultrasonic transducer according to claim 1 .
7. A method for fabricating an ultrasonic transducer for transmitting and receiving ultrasonic comprising these steps:
a step of forming a silicon oxide film and a lower electrode on a semiconductor substrate;
a step of forming a sacrificial layer for forming a gap on the silicon oxide film;
a step of forming a third insulating film on the sacrificial layer, wherein the third insulating film comprises silicon oxide;
a step of forming an upper electrode on the sacrificial layer and the third insulating film;
a step of forming a fourth insulating film on the upper electrode, the fourth insulating film having compressive stress, wherein the fourth insulating film comprises silicon oxide;
a step of forming a through-hole extending to the sacrificial layer in the third insulating film and the fourth insulating film;
a step of removing the sacrificial layer; and
a step of forming a fifth insulating film on the fourth insulating film and filling the through-hole with the fifth insulating layer, the fifth insulating film having tensile stress,
wherein the third insulating film, the upper electrode, the fourth insulating film and the fifth insulating film above the gap comprise a vibratable membrane.