Adaptive cavity thickness control for micromachined ultrasonic transducer devices

1. A method of forming an ultrasonic transducer device including a substrate and a film stack over the substrate and having a metal electrode layer and a chemical mechanical polishing (CMP) stop layer formed over the metal electrode layer, the method comprising:

forming a membrane support layer over the film stack, the membrane support layer having a thickness dependent upon a measured thickness of the CMP stop layer, such that a combined thickness of the CMP stop layer and the membrane support layer represents a transducer cavity depth.

2. The method of claim 1 , further comprising measuring a thickness of the CMP stop layer after planarizing an insulation layer to the CMP stop layer, wherein measuring the thickness provides the measured thickness of the CMP stop layer.

3. A method of forming an ultrasonic transducer device, the method comprising:

forming and patterning a film stack over a substrate, the film stack comprising a metal electrode layer and a chemical mechanical polishing (CMP) stop layer formed over the metal electrode layer;

forming an insulation layer over the patterned film stack;

planarizing the insulation layer to the CMP stop layer;

measuring a remaining thickness of the CMP stop layer; and

forming a membrane support layer over the patterned film stack, wherein the membrane support layer is formed at thickness dependent upon the measured remaining thickness of the CMP stop layer, such that a combined thickness of the CMP stop layer and the membrane support layer corresponds to a desired transducer cavity depth.

4. The method of claim 3 , further comprising:

etching a cavity in the membrane support layer and the CMP stop layer; and

bonding a membrane to the membrane support layer to seal the cavity.

5. The method of claim 4 , wherein the CMP stop layer comprises a dielectric material.

6. The method of claim 5 , wherein the CMP stop layer comprises SiN.

7. The method of claim 4 , wherein the metal electrode layer comprises titanium (Ti).

8. The method of claim 7 , wherein the metal electrode layer has a thickness of about 100 nm to about 300 nm.

9. The method of claim 4 , wherein the insulation layer comprises SiO 2 .

10. The method of claim 9 , wherein the SiO 2 layer is formed to an initial thickness of about 400 nanometers (nm) to about 900 nm before planarizing.

11. The method of claim 4 , wherein the film stack further comprises a bottom cavity layer disposed between the metal electrode layer and the CMP stop layer.

12. The method of claim 11 , wherein the bottom cavity layer comprises a chemical vapor deposition (CVD) SiO 2 layer and an atomic layer deposition (ALD) Al 2 O 3 layer formed on the SiO 2 layer.

13. The method of claim 12 , wherein the CVD SiO 2 layer has a thickness of about 10 nm to about 30 nm and the ALD Al 2 O 3 layer has a thickness of about 20 nm to about 40 nm.

14. The method of claim 4 wherein the membrane support layer comprises SiO 2 and the membrane comprises doped silicon.

15. The method of claim 14 , wherein the membrane support layer has a thickness of about 100 nm to about 300 nm, and the membrane has a thickness of about 2 microns (μm) to about 10 μm.

16. A method of forming ultrasonic transducer devices, the method comprising:

forming and patterning a film stack over a first wafer, the film stack comprising a metal electrode layer and a chemical mechanical polishing (CMP) stop layer formed over the metal electrode layer;

forming an insulation layer over the patterned film stack;

planarizing the insulation layer to the CMP stop layer;

measuring a remaining thickness of the CMP stop layer;

forming a membrane support layer over the patterned film stack, wherein the membrane support layer is formed at thickness dependent upon the measured remaining thickness of the CMP stop layer, such that a combined thickness of the CMP stop layer and the membrane support layer corresponds to a desired transducer cavity depth; and

using the measured remaining thickness of the CMP stop layer from the first wafer as CMP parameter for forming subsequent transducer devices on one or more additional wafers.