Sealed force sensor with etch stop layer

1. A microelectromechanical system (MEMS) device, comprising:

a substrate; and

a MEMS force sensor mounted on the substrate, the MEMS force sensor comprising:

a sensor die configured to receive an applied force, the sensor die comprising a first substrate and a second substrate, wherein at least a portion of the second substrate defines a deformable membrane;

a cavity formed in the first substrate;

a sensing element on a surface of the second substrate, the sensing element operable to convert a strain on the surface of the second substrate to an analog electrical signal that is proportional to the strain;

a protective layer at least partially covering the surface of the second substrate; and

a conductive pad on the protective layer and electrically coupled to the sensing element, wherein:

the MEMS force sensor is mounted on the substrate using an under bump metal (UBM) layer that is on the conductive pad and an electrical connector that is between the UBM layer and the substrate; and

the conductive pad is electrically coupled to the substrate through the UBM layer and the electrical connector.

2. The MEMS device of claim 1 , further comprising an etch stop layer between the first substrate and the second substrate.

3. The MEMS device of claim 1 , wherein the substrate comprises a package substrate.

4. The MEMS device of claim 1 , wherein the substrate comprises one of a printed circuit board or a flexible printed circuit board.

5. The MEMS device of claim 1 , wherein the protective layer at least partially covers the sensing element or the conductive pad.

6. The MEMS device of claim 1 , further comprising a mesa in the cavity and formed from the first substrate.

7. The MEMS device of claim 6 , further comprising an etch stop layer at least partially covering the deformable membrane of the second substrate.

8. The MEMS device of claim 1 , wherein the conductive pad, the UBM layer, and the electrical connector overlap fully with a mesa in the cavity.

9. The MEMS device of claim 1 , further comprising a mesa in the cavity and over the deformable membrane, wherein the conductive pad, the UBM layer, and the electrical connector do not overlap with the mesa and the deformable membrane.

10. The MEMS device of claim 1 , wherein the sensing element is a piezoresistive or piezoelectric sensing element.

11. The MEMS device of claim 1 , further comprising a cap substrate, wherein:

the cap substrate is bonded to the first substrate; and

the cavity is sealed between the cap substrate and the first substrate.

12. A method of manufacturing a microelectromechanical system (MEMS) device, the method comprising:

providing an etch stop layer between a first substrate and a deformable membrane of a second substrate;

etching the first substrate to remove a portion of the first substrate to form a cavity in the first substrate;

providing a sensing element on a surface of the deformable membrane;

providing a conductive pad over the surface of the second substrate and electrically coupling the conductive pad to the sensing element;

providing an under bump metal (UBM) layer on the conductive pad;

providing an electrical connector on the UBM layer; and

mounting a substrate on the electrical connector, wherein the conductive pad is electrically coupled to the substrate through the UBM layer and the electrical connector.

13. The method of claim 12 , further comprising providing a protective layer over the surface to at least partially cover the surface of the second substrate, wherein the conductive pad is on the protective layer.

14. The method of claim 13 , wherein the protective layer at least partially covers the sensing element or the conductive pad.

15. The method of claim 12 , wherein:

an etch rate of the etch stop layer is different than an etch rate of the first substrate; and

an etch rate ratio between the etch rate of the first substrate and the etch rate of the etch stop layer is between 50 and 150.

16. A microelectromechanical system (MEMS) force sensor, comprising:

a sensor die configured to receive an applied force, the sensor die comprising a first substrate bonded to a second substrate, wherein at least a portion of the second substrate defines a deformable membrane;

a cavity in the first substrate;

an etch stop layer between the first substrate and the second substrate, wherein the etch stop layer at least partially covers the deformable membrane of the second substrate;

a sensing element on a surface of the second substrate, the sensing element operable to convert a strain on the surface of the second substrate to an analog electrical signal that is proportional to the strain;

a protective layer at least partially over the surface of the second substrate; and

a conductive pad on the protective layer and electrically coupled to the sensing element.

17. The MEMS force sensor of claim 16 , further comprising a cap substrate bonded to the first substrate, wherein the cavity is sealed between the cap substrate and the first substrate.

18. The MEMS force sensor of claim 17 , further comprising:

a mesa in the cavity and formed from the first substrate; and

a gap between the mesa and the cap substrate.

19. The MEMS force sensor of claim 16 , further comprising:

an under bump metal (UBM) layer over the conductive pad;

an electrical connector over the UBM layer, wherein the MEMS force sensor is mounted to a substrate using the UBM layer and the electrical connector.

20. The MEMS force sensor of claim 16 , wherein the electrical connector is one of a solder bump or a copper pillar.