MEMS device and process
A MEMS capacitive transducer with increased robustness and resilience to acoustic shock. The transducer structure includes a flexible membrane supported between a first volume and a second volume, and at least one variable vent structure in communication with at least one of the first and second volumes. The variable vent structure includes at least one moveable portion which is moveable in response to a pressure differential across the moveable portion so as to vary the size of a flow path through the vent structure. The variable vent may be formed through the membrane and the moveable portion may be a part of the membrane, defined by one or more channels, that is deflectable away from the surface of the membrane. The variable vent is preferably closed in the normal range of pressure differentials but opens at high pressure differentials to provide more rapid equalisation of the air volumes above and below the membrane.
1. A MEMS transducer comprising:
a flexible membrane; and
at least one variable vent structure wherein said variable vent structure provides a flow path between a first volume on one side of the flexible membrane and a second volume on the other side of the flexible membrane, the vent structure having a size that varies according to a non-linear relationship with pressure differential across the membrane.
2. A MEMS transducer as claimed in claim 1 , wherein said variable vent structure comprises at least one moveable portion which is moveable in response to a pressure differential across the moveable portion so as to vary the size of a flow path through the vent structure and wherein the equilibrium position of the at least one moveable portion corresponds to a minimum size of flow path.
3. A MEMS transducer as claimed in claim 2 , wherein the at least one moveable portion is configured such that there is substantially no movement of the moveable portion from the equilibrium position at pressure differentials below a first threshold.
4. A MEMS transducer as claimed in claim 2 , wherein the first threshold is greater than 150 Pa.
5. A MEMS transducer as claimed in claim 2 , wherein the first threshold is greater than 1 kPa.
6. A MEMS transducer as claimed in claim 1 , wherein at least one said variable vent structure provides substantially no significant variation in flow path size for pressure differentials in the range of 0 Pa-200 Pa.
7. A MEMS transducer as claimed in claim 2 , wherein the at least one moveable portion is configured such that there is substantial movement of the moveable portion from the equilibrium position at pressure differentials above a second threshold.
8. A MEMS transducer as claimed in claim 7 , wherein the second threshold is lower than 100 kPa.
9. A MEMS transducer as claimed in claim 1 , wherein at least one said variable vent structure provides substantially a significant increase in flow path size for pressure differentials in the range of 100 kPa-200 kPa, compared to the flow path size at equilibrium.
10. A MEMS transducer as claimed in claim 1 , wherein at least one said variable vent structure comprises at least one moveable portion which is moveable in response to a pressure differential across the moveable portion of at least 100 kPa.
11. A MEMS transducer as claimed in claim 2 , wherein the equilibrium position of the at least one moveable portion corresponds to the flow path being substantially closed.
12. A MEMS transducer as claimed in claim 1 , wherein at least one said variable vent structure is formed in the flexible membrane and the flow path is a path through the flexible membrane.
13. A MEMS transducer as claimed in claim 12 , wherein the at least one moveable portion comprises a portion of the flexible membrane which is able to be deflected away from the surface of the rest of the flexible membrane.
14. A MEMS transducer as claimed in claim 13 , wherein said moveable portion of the flexible membrane is connected to the rest of the flexible membrane via a beam structure.
15. A MEMS transducer as claimed in claim 14 , wherein said beam structure is able to bend and/or twist to allow said moveable portion of the flexible membrane to be deflected away from the surface of the rest of the flexible membrane.
16. A MEMS transducer as claimed in claim 1 , wherein at least one said variable vent structure comprises at least two moveable portions of the flexible membrane, the at least two moveable portions being able to be deflected away from the surface of the rest of the flexible membrane to expose a hole in the flexible membrane.
17. A MEMS transducer as claimed in claim 1 , wherein at least one said variable vent structure is formed with a flow-path that bypasses the flexible membrane.
18. A MEMS transducer as claimed in claim 17 , wherein the flow-path that bypasses the flexible membrane runs through at least part of a sidewall of a transducer structure that supports the flexible membrane.
19. A MEMS transducer as claimed in claim 1 , comprising a back-plate structure wherein the flexible membrane is supported with respect to said back-plate structure.
20. A MEMS transducer as claimed in claim 1 , wherein the transducer is a capacitive microphone.