MEMS process and device

1. A method of fabricating a micro-electrical-mechanical system (MEMS) transducer, the method comprising:

forming a membrane on a first side of a substrate; and

forming a back-volume through the substrate from a second side;

wherein the step of forming a back-volume in the substrate comprises the steps of forming a first back-volume portion and a second back-volume portion, the back-volume having a step in a sidewall of the back-volume, said step defining a separation between the first back-volume portion from the second back-volume portion; and

wherein the step of forming the first back-volume portion further comprises the steps of:

applying a first mask layer to the second side of the substrate, the first mask layer having an opening corresponding substantially to the cross-sectional area of the first back-volume portion;

etching the substrate to a first depth;

enlarging the opening of the first mask layer, such that said opening corresponds substantially to the cross-sectional area of the second back-volume portion; and

etching the substrate to a second depth.

2. A method as claimed in claim 1 , wherein the step in the sidewall comprises substantially a right angle.

3. A method as claimed in claim 1 , wherein the step in the sidewall comprises a discontinuity in the cross-sectional area of the back-volume in a plane parallel to the substrate.

4. A method as claimed in claim 1 , wherein the step in the sidewall comprises a discontinuity in a rate of change of cross-sectional area of the back-volume in a plane parallel to the substrate with distance from the membrane.

5. A method as claimed in claim 1 , wherein the step in the sidewall comprises a change in the gradient of the sidewall.

6. A method as claimed in claim 1 , wherein the step in the sidewall comprises two or more changes in the gradient of the sidewall.

7. A method as claimed in claim 1 , wherein the step of etching the substrate to a second depth comprises the step of etching the substrate until the first back-volume portion reaches the membrane.

8. A method of fabricating a micro-electrical-mechanical system (MEMS) transducer, the method comprising:

forming a membrane on a first side of a substrate; and

forming a back-volume through the substrate from a second side;

wherein the step of forming a back-volume in the substrate comprises the steps of forming a first back-volume portion and a second back-volume portion, the back-volume having a step in a sidewall of the back-volume, said step defining a separation between the first back-volume portion from the second back-volume portion;

wherein the step of forming the back-volume comprises the step of:

applying a mask layer to the second side of the substrate, the mask layer having a first mask region and a second mask region,

wherein the first mask region allows the substrate to be etched at a first rate, and

wherein the second mask region allows the substrate to be etched at a second rate.

9. A method as claimed in claim 8 , wherein the first mask region has a cross-sectional area corresponding substantially to the cross-sectional area of the first back-volume portion.

10. A method as claimed in claim 8 , wherein the first mask region comprises an opening in the mask layer.

11. A method as claimed in claim 8 , wherein the second mask region comprises a plurality of apertures in the mask layer, the plurality of apertures controlling the rate at which the second region is etched.

12. A method as claimed in claim 1 , wherein the step of forming a first back-volume portion comprises a dry etch process.

13. A method as claimed in claim 1 , wherein the step of forming a first back-volume portion comprises a wet etch process.

14. A method as claimed in claim 1 , wherein the step of forming a second back-volume portion comprises a dry etch process.

15. A method as claimed in claim 1 , wherein the step of forming a second back-volume portion comprises a wet etch process.

16. A method as claimed in claim 1 , further comprising the step of forming a plurality of MEMS transducers on a wafer.

17. A method as claimed in claim 1 , further comprising the step of forming a third back-volume portion in the back-volume.

18. A method as claimed in claim 17 , wherein the step of forming the third back-volume portion comprises the step of forming the third back-volume portion between the first back-volume portion and the membrane.

19. A method as claimed in claim 17 , wherein the third back-volume portion is formed having a cross sectional area or diameter that is larger than the cross-sectional area or diameter of the first back-volume portion, but smaller than the cross-sectional area or diameter of the membrane.

20. A method as claimed in claim 17 , wherein the third back-volume portion is formed by etching away a portion of sacrificial material.

21. A method as claimed in claim 17 , wherein the third back-volume portion is etched using a different etching process to the etching of the first and/or second back-volume portions.

22. A method of fabricating a micro-electrical-mechanical system (MEMS) transducer, the method comprising:

forming a membrane on a first side of a substrate; and

forming a back-volume through the substrate from a second side;

wherein the step of forming a back-volume in the substrate comprises the steps of forming a first back-volume portion and a second back-volume portion, the back-volume having a step in a sidewall of the back-volume, said step defining a separation between the first back-volume portion from the second back-volume portion; and

wherein the step of forming the first back-volume portion further comprises the steps of:

applying a first mask layer to the second side of the substrate, the first mask layer having an opening corresponding substantially to the cross-sectional area of the first back-volume portion;

etching the substrate to a first depth;

applying a second mask layer to the second side of the substrate, the second mask layer having an opening corresponding substantially to the cross-sectional area of the second back-volume portion; and

etching the substrate to a second depth.

23. A method as claimed in claim 22 , wherein the first mask layer is removed prior to applying the second mask layer.

24. A method as claimed in claim 22 , wherein the step in the sidewall comprises substantially a right angle.

25. A method as claimed in claim 22 , wherein the step in the sidewall comprises a discontinuity in the cross-sectional area of the back-volume in a plane parallel to the substrate.

26. A method as claimed in claim 22 , wherein the step in the sidewall comprises a discontinuity in a rate of change of cross-sectional area of the back-volume in a plane parallel to the substrate with distance from the membrane.

27. A method as claimed in claim 22 , wherein the step in the sidewall comprises a change in the gradient of the sidewall.

28. A method as claimed in claim 22 , wherein the step in the sidewall comprises two or more changes in the gradient of the sidewall.

29. A method as claimed in claim 22 , wherein the step of etching the substrate to a second depth comprises the step of etching the substrate until the first back-volume portion reaches the membrane.

30. A method as claimed in claim 22 , wherein the second mask region comprises a ring, the ring having an inner boundary corresponding to an outer boundary of the first back-volume portion, and the ring having an outer boundary corresponding to the cross-sectional area of the second back-volume portion.

31. A method as claimed in claim 22 , wherein the second mask region comprises a plurality of apertures in the mask layer, the plurality of apertures controlling the rate at which the second region is etched.