Anti-stiction bottom cavity surface for micromachined ultrasonic transducer devices
A method of forming an ultrasonic transducer device includes forming an insulating layer having topographic features over a lower transducer electrode layer of a substrate; forming a conformal, anti-stiction layer over the insulating layer such that the conformal layer also has the topographic features; defining a cavity in a support layer formed over the anti-stiction layer; and bonding a membrane to the support layer.
1. A method of forming an ultrasonic transducer device, the method comprising:
forming an insulating layer having topographic features over a lower transducer electrode layer of a substrate;
forming a conformal, anti-stiction layer over the insulating layer such that the conformal layer also has the topographic features;
defining a cavity in a support layer formed over the anti-stiction layer; and
bonding a membrane to the support layer.
2. The method of claim 1 , further comprising forming the topographic features, the topographic features corresponding to a region of contact between the conformal, anti-stiction layer and the membrane in a collapse mode of transducer operation.
3. The method of claim 1 , wherein forming the insulating layer having topographic features further comprises:
forming a first type layer over the substrate and a second type sacrificial layer over the first type layer;
lithographically patterning and removing portions of the second type sacrificial layer;
forming a third type layer over the patterned second type sacrificial layer and first type layer;
planarizing the third type layer to a top surface of the patterned second type sacrificial layer; and
removing remaining portions of the patterned second type sacrificial layer.
4. The method of claim 3 , wherein:
the first type layer comprises SiO 2 ;
the second type sacrificial layer comprises SiN; and
the third type layer comprises SiO 2 .
5. The method of claim 3 , wherein the conformal, anti-stiction layer comprises a thin film aluminum oxide layer formed by atomic layer deposition (ALD).
6. The method of claim 5 , wherein the thin film aluminum oxide layer is formed at a thickness of about 20-40 nm.
7. The method of claim 3 , wherein:
the first type layer is formed at a thickness of about 10-30 nm;
the second type sacrificial layer is formed at a thickness of about 30-70 nm; and
the third type layer is formed at a thickness of about 400-700 nm.
8. The method of claim 1 , wherein forming the insulating layer having topographic features further comprises:
forming a first type layer over the substrate;
lithographically patterning and removing portions of the first type layer to temporarily expose portions of the lower electrode layer and define the topographic features; and
conformally depositing additional first type layer material over the first type layer in a manner that maintains the topographic features.
9. The method of claim 8 , wherein forming the conformal, anti-stiction layer further comprises conformally depositing a second type layer over the additional first type layer material.
10. The method of claim 9 , further comprising:
conformally depositing third type layer over the second type layer, the third type layer comprising a chemical mechanical polishing (CMP) stop layer;
depositing a fourth type layer over the third type layer, the fourth type layer comprising a CMP buffer layer; and
planarizing the fourth type layer to a top surface of the third type layer.
11. The method of claim 10 , wherein defining the cavity in the support layer further comprises:
depositing a membrane support layer over the CMP stop layer;
lithographically patterning and etching the membrane support layer, the CMP stop layer, and any remaining portions of the CMP buffer layer to expose the anti-stiction layer.
12. The method of claim 11 , wherein:
the first type layer comprises SiO 2 ;
the second type layer comprises a thin film aluminum oxide layer formed by atomic layer deposition (ALD);
the third type layer comprises SiN; and
the fourth type layer comprises SiO 2 .
13. The method of claim 12 , wherein the thin film aluminum oxide layer is formed at a thickness of about 20-40 nm.
14. The method of claim 13 , wherein:
the first type layer is formed at a thickness of about 10-30 nm;
the additional first type layer material is formed at a thickness of about 10-30 nm;
the third type layer is formed at a thickness of about 20-50 nm; and
the fourth type layer is formed at a thickness of about 400-700 nm.
15. An ultrasonic transducer device, comprising:
an insulating layer having topographic features disposed over a lower transducer electrode layer of a substrate;
a conformal, anti-stiction layer disposed over the insulating layer such that the conformal layer also has the topographic features;
a cavity defined in a support layer formed over the anti-stiction layer; and
a membrane bonded to the support layer.
16. The ultrasonic transducer device of claim 15 , wherein the conformal, anti-stiction layer comprises a thin film aluminum oxide layer.
17. The ultrasonic transducer device of claim 16 , wherein the thin film aluminum oxide layer is formed at a thickness of about 20-40 nm.
18. The ultrasonic transducer device of claim 15 , wherein the topographic features correspond to a region of contact between the conformal, anti-stiction layer and the membrane in a collapse mode of transducer operation.
19. The ultrasonic transducer device of claim 15 , further comprising a chemical mechanical polishing (CMP) stop layer disposed between the anti-stiction layer and the support layer.
20. The ultrasonic transducer device of claim 19 , wherein the CMP stop layer comprises SiN.
21. The ultrasonic transducer device of claim 15 , wherein the topographic features of the insulating layer comprise:
a lower elevation region; and
a plurality of posts defined within the lower elevation region, wherein top surfaces of the plurality of posts are disposed at a higher elevation than the lower elevation region.
22. The ultrasonic transducer device of claim 21 , wherein the plurality of posts are circular, having a first diameter and the lower elevation region has a second diameter.
23. The ultrasonic transducer device of claim 22 , wherein an area of the lower elevation region corresponds to an area of the membrane that comes into contact with the insulating layer during a collapse mode of operation.
24. The ultrasonic transducer device of claim 22 , wherein the first diameter is about 3 microns (μm), and the plurality of posts are spaced at a pitch of about 3 μm with respect to another.
25. The ultrasonic transducer device of claim 24 , wherein the second diameter is about 40 μm and a diameter of the cavity is about 200 μm.
26. The ultrasonic transducer device of claim 24 , wherein the second diameter is about 140 μm and a diameter of the cavity is about 200 μm.