Micromechanical detection structure for a MEMS acoustic transducer and corresponding manufacturing process
A micromechanical structure for a MEMS capacitive acoustic transducer, has: a substrate made of semiconductor material, having a front surface lying in a horizontal plane; a membrane, coupled to the substrate and designed to undergo deformation in the presence of incident acoustic-pressure waves; a fixed electrode, which is rigid with respect to the acoustic-pressure waves and is coupled to the substrate by means of an anchorage structure, in a suspended position facing the membrane to form a detection capacitor. The anchorage structure has at least one pillar element, which is at least in part distinct from the fixed electrode and supports the fixed electrode in a position parallel to the horizontal plane.
1. A micromechanical structure comprising:
a semiconductor substrate having a first surface;
an anchorage structure including a plurality of pillar elements separated from each other by a conductive material; and
a detection capacitor including:
a membrane coupled to said substrate and configured to deform in response to acoustic-pressure waves; and
a fixed electrode that is rigid with respect to said acoustic-pressure waves and is coupled to said substrate by the anchorage structure, the anchorage structure being configured to support the fixed electrode in a suspended position facing said membrane.
2. A micromechanical structure comprising:
a semiconductor substrate having a first surface;
an anchorage structure including a plurality of pillar elements; and
a detection capacitor including:
a membrane coupled to said substrate and configured to deform in response to acoustic-pressure waves; and
a fixed electrode that is rigid with respect to said acoustic-pressure waves and is coupled to said substrate by the anchorage structure, the anchorage structure being configured to support the fixed electrode in a suspended position facing said membrane
a rigid plate coupling the fixed electrode to the anchorage structure, the fixed electrode including a conductive layer facing said membrane; and
wherein said plurality of pillar elements include a same material as the conductive layer, and at least some of the pillar elements including prolongations that extend in a direction that is transverse with respect to said first surface of the substrate and terminate at a distance from said first surface of said substrate.
3. The structure according to claim 1 , wherein said plurality of pillar elements together form a closed perimeter that surrounds said membrane.
4. The structure according to claim 2 , wherein said anchorage structure is configured to exert on said fixed electrode a compensation force that compensates for a deformation force applied to said fixed electrode.
5. The structure according to claim 1 , further comprising a rigid plate coupling the electrode to the anchorage structure, wherein each pillar element comprises:
wall portions that extend transversely with respect to said first surface of said substrate and include the conductive material; and
a body portion enclosed between said wall portions, said rigid plate extending over said body portion.
6. The structure according to claim 5 , wherein each pillar element includes insulative material and conductive material.
7. The structure according to claim 5 , wherein:
said rigid plate includes a conductive layer having prolongations that extend from said rigid plate forming a top part of said wall portions; and
said wall portions have a bottom part that are distinct from said prolongations, the bottom part extending between the first surface of said substrate and said prolongations.
8. The structure according to claim 1 , wherein each of said plurality of pillar elements includes a conductive portion and an insulative portion surrounded by the conductive material.
9. The structure according to claim 8 wherein two adjacent pillar elements share the same insulative portion.
10. The structure according to claim 1 , wherein:
said fixed electrode comprises a conductive layer facing said membrane and an insulating layer located over said conductive layer; and
said insulating layer couples said conductive layer to said anchorage structure.
11. A process for manufacturing a micromechanical structure, the process comprising:
forming a detection capacitor by:
forming a membrane coupled to a first surface of a semiconductor substrate, the membrane being configured to deform in response to acoustic-pressure waves;
forming a fixed electrode that is rigid with respect to said acoustic-pressure waves and is located in a suspended position facing said membrane; and
forming an anchorage structure that includes a plurality of pillar elements and a conductive material, the anchorage structure coupling said fixed electrode to said substrate, and wherein said conductive material of said anchorage structure is distinct from said fixed electrode and separates the plurality of pillar elements from each other, and wherein said anchorage structure is configured to support said fixed electrode parallel to said first surface of the substrate.
12. The process according to claim 11 , wherein forming the membrane comprises:
forming a first conductive layer above a first sacrificial layer that is located above said first surface of said substrate; and
forming said membrane by removing an underlying portion of said first sacrificial layer,
and wherein forming the anchorage structure comprises:
forming a plurality of first openings in the sacrificial layer; and
filling said plurality of first openings while forming said first conductive layer of the conductive material and forming a bottom part of wall portions of said pillar elements.
13. The process according to claim 12 , wherein said forming a fixed electrode comprises:
forming a second conductive layer above a second sacrificial layer located over said first conductive layer; and
releasing said fixed electrode portion by removing an underlying portion of said second sacrificial layer,
and wherein said forming the anchorage structure further comprises:
forming a plurality of second openings in said second sacrificial layer; and
filling said plurality of second openings while forming said second conductive layer.
14. The process according to claim 13 , wherein said second conductive layer that fills the plurality of second openings is the same material that is used to form the fixed electrode.
15. The process according to claim 11 , wherein:
said steps of forming the membrane and forming the fixed electrode comprise a common step of removing layers of sacrificial material; and
said anchorage structure includes a plurality of pillar elements, each including wall portions of the conductive material that extend in a first direction that is transverse with respect to said first surface of said substrate and a body portion that is enclosed between said wall portions, said body portion including sacrificial material that is removable by chemical etching, and said wall portions forming the conductive portion that is insensitive to said chemical etching.
16. The process according to claim 11 , wherein said pillar elements form a closed perimeter that surrounds said membrane.
17. The process according to claim 11 , wherein forming said anchorage structure comprises forming a plurality of pillar elements coupled to the first surface of the substrate.
18. An electronic device comprising:
a microprocessor; and
a micromechanical structure coupled to the microprocessor, the micromechanical structure including:
a semiconductor substrate having a first surface;
an anchorage structure that includes a plurality of pillar elements that include a conductive portion and an insulating portion, each of the pillar elements being separated from each other by conductive material;
a detection capacitor:
a membrane flexibly coupled to the semiconductor substrate and configured to deform in response to pressure; and
a rigid plate that includes a fixed electrode spaced apart from the membrane, the rigid plate being fixedly coupled to the semiconductor substrate by the anchorage structure, the fixed electrode being electrically isolated from the pillar elements of the anchorage structure.
19. The electronic device according to claim 18 , wherein the anchorage structure extends around the entire perimeter of the membrane.
20. The electronic device according to claim 18 , wherein the conductive portion of the pillar elements include two layers of conductive material and the insulating portion of the pillar elements include at least two layers of insulating material.
21. The electronic device according to claim 18 , wherein the electronic device is at least one of a mobile phone, a PDA, a notebook, a voice recorder, a reader of audio files with voice-recording capacity, a hydrophone, and a wearable device.
22. The electronic device according to claim 18 , wherein the conductive portion of the pillar elements is of a same conductive material as the fixed electrode.